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An anterograde tracing study of the vagal innervation of rat liver, portal vein and biliary system
Abstract
In order to investigate the distribution and structure of the vagal liver innervation, abdominal vagal afferents and efferents were selectively labeled by injecting WGA-HRP or Dil into the nodose ganglia, and DiA into the dorsal motor nucleus, respectively. Vagal afferent fibers produced characteristic terminal-like structures at three locations in the liver hilus: 1. Fine varicose endings preferentially surrounding, but not entering, the numerous peribiliary glands in the larger intra and extrahepatic bile ducts 2. Large, cup-shaped terminals in almost all paraganglia 3. Fine varicose endings in the portal vein adventitia. No fibers and terminals were found in the hepatic parenchyma. While about two thirds of the vagal afferent fibers that originate in the left nodose ganglion, and are contained in the hepatic branch, bypass the liver hilus area on their way to the gastroduodenal artery, a significant number (approx. 10% of the total) of vagal afferents that do innervate the area, originates from the right nodose ganglion, and projects to the periarterial plexus of the common hepatic artery and liver pedicle most likely through the dorsal celiac branch. Varicose vagal efferent fibers were present within the fascicles of the vagal hepatic branch and fine terminal-like structures in a small fraction of the paraganglia. No efferents were found to terminate in the hepatic parenchyma or on the few neurons embedded in nerves or paraganglia. In contrast to the paucity of vagal terminals in the hepatic parenchyma, an abundance of vagal efferent and afferent fibers and terminals with distinctive distribution patterns and structural characteristics was present in esophagus and gastrointestinal tract.(ABSTRACT TRUNCATED AT 250 WORDS)
... 2,3,9,62 HPV-to-Brain Pathways for Food Intake Control How are signals from HPV glucose sensors transmitted to the brain to exert effects on food intake? The HPV is innervated by both vagal 63,64 and spinal 65 afferents. Accordingly, it is thought that glucose sensors in the HPV likely signal via the vagus nerve 66,67 or through spinal afferents 68 to the brain and ultimately manifest physiological or behavioral effects. ...
... 70 In contrast, signals traveling through the vagus nerve synapse only with brainstem neurons in the DVC. 63,66,74 Vagotomy does not abolish, but rather increases, LH firing, suggesting that the vagus nerve is not required for, but may play a modulatory role in, communicating HPV glucose signals to the brain. 70 A recent study applied a transsynaptic virus to the HPV and revealed labeling within the DVC, most notably within the dorsal motor nucleus of the vagus (DMX) with only sparse labeling in the nucleus of the solitary tract (NTS). ...
... Tracing studies using DiI injected in to the nodose ganglia find fine varicose endings terminating in the tunica adventitia. 63,65 Similarly, CGRP (a marker for sensory afferents) is found in the tunica adventitia, 65 and also in the smooth and longitudinal muscle layers, and sparsely in the tunica interna. 80 Garcia-Luna et al 75 examined the depth of sensory innervation of the HPV by systematically performing injections of a transsynaptic anterograde tracer on the surface, within the muscular wall, and in the lumen of the HPV. ...
The detection of nutrients in the gut influences ongoing and future feeding behavior as well as the development of food preferences. In addition to nutrient sensing in the intestine, the hepatic portal vein plays a considerable role in detecting ingested nutrients and conveying this information to brain nuclei involved in metabolism, learning, and reward. Here, we review mechanisms underlying hepatic portal vein sensing of nutrients, particularly glucose, and how this is relayed to the brain to influence feeding behavior and reward. We additionally highlight several gaps where future research can provide new insights into the effects of portal nutrients on neural activity in the brain and feeding behavior.
... Originally thought to mediate nutrient-induced satiation signals, they now are implicated in nutrient-induced appetition signaling (36). In addition to the gut itself, the associated hepatic portal vein (37,38) and pancreatic β cells (39,40) are also innervated by vagal afferents. ...
... [45][46][47][48][49][50][51][52][53][54]. Unfortunately, most of these analyses seem unaware of the projection targets of vagal afferent and efferent fibers in the common hepatic branch, as identified in neuronal tracing and multiorgan functional analyses with electrical stimulation (37,41,55). These studies in rats have collectively demonstrated that a majority of vagal fibers passing through the (easily accessible) hepatic branch innervate targets in the proximal small intestine, pylorus, antrum, and pancreas, while a minority innervate the hepatic portal vein, bile ducts, and the liver hilum (37). ...
... Unfortunately, most of these analyses seem unaware of the projection targets of vagal afferent and efferent fibers in the common hepatic branch, as identified in neuronal tracing and multiorgan functional analyses with electrical stimulation (37,41,55). These studies in rats have collectively demonstrated that a majority of vagal fibers passing through the (easily accessible) hepatic branch innervate targets in the proximal small intestine, pylorus, antrum, and pancreas, while a minority innervate the hepatic portal vein, bile ducts, and the liver hilum (37). That is, fibers in the common hepatic branch join the dense plexus surrounding the common hepatic artery, and the majority continue along the gastroduodenal artery. ...
Given the crucial role of the gastrointestinal tract and associated organs in handling nutrient assimilation and metabolism, it has long been known that its communication with the brain is important for the control of ingestive behavior and body weight regulation. It is also clear that gut-brain communication is bidirectional and utilizes both rapid neural and slower humoral mechanisms and pathways. However, progress in understanding these mechanisms and leveraging them for the treatment of obesity and metabolic disease has been hindered by the enormous dimension of the gut mucosa, the complexity of the signaling systems, and lack of specific tools. With the ascent of modern neurobiological technology, our understanding of the role of vagal afferents in gut-brain communication has begun to change. The first function-specific populations of vagal afferents providing nutritional feedback as well as feed-forward signals have been identified with genetics-guided methodology, and it is hoped that extension of the methodology to other neural communication pathways will follow soon. Currently, efficient clinical leveraging of gut-brain communication to treat obesity and metabolic disease is limited to a few gut hormones, but a more complete understanding of function-specific and projection-specific neuronal populations should make it possible to develop selective and more effective neuromodulation approaches.
... It has been described that only a limited number of vagal sensory neurons in the left nodose ganglion project to the liver in rats (13). The peripheral nerve terminals of the vagal sensory neurons innervating the liver are primarily present in the bile ducts and portal veins, whereas no afferent terminals are observed in the hepatic parenchyma of rats (14). Although the central projection sites of vagal sensory neurons innervating the liver remain unknown, the central axon terminals of vagal sensory neurons are generally localized to the nucleus of the tractus solitarius (NTS), area postrema (AP), and dorsal motor nucleus of the vagus (DMV) (15). ...
... vagal sensory neurons in the left nodose ganglion would play a role in the vagovagal reflex pathways.The peripheral nerve terminals of vagal sensory neurons innervating the liver were observed primarily in the bile ducts and portal veins, whereas no afferent terminals were observed in the hepatic parenchyma of rats(14). In our preparations, the periportal region had dense innervation of vagal sensory neurons innervating the liver. ...
Background and Aims
The visceral organ-brain axis, mediated by vagal sensory neurons in the vagal nerve ganglion, is essential for maintaining various physiological functions. In this study, we investigated the impact of liver-projecting vagal sensory neurons on energy balance, hepatic steatosis, and anxiety-like behavior in mice under obesogenic conditions.
Methods
We performed single-nucleus RNA sequencing of vagal sensory neurons innervating the liver. Based on our snRNA-Seq results, we used the Avil CreERT2 strain to identify vagal sensory neurons that innervate the liver.
Results
A small subset of polymodal sensory neurons innervating the liver was located in the left and right ganglia, projecting centrally to the nucleus of the tractus solitarius, area postrema, and dorsal motor nucleus of the vagus, and peripherally to the periportal areas in the liver. Male and female control mice developed diet-induced obesity (DIO) during high-fat diet feeding. Deleting liver-projecting advillin-positive vagal sensory neurons prevented DIO in male and female mice, and these outcomes are associated with increased energy expenditure. Although males and females exhibited improved glucose homeostasis following disruption of liver-projecting vagal sensory neurons, only male mice displayed increased insulin sensitivity. The loss of liver-projecting vagal sensory neurons limited the progression of hepatic steatosis in male and female mice fed a steatogenic diet. Finally, mice lacking liver-innervating vagal sensory neurons exhibited less anxiety-like behavior compared to the control mice.
Conclusions
The liver-brain axis contributes to the regulation of energy balance, glucose tolerance, hepatic steatosis, and anxiety-like behavior depending on the nutrient status in healthy and obesogenic conditions.
... The hepatic portal vein and liver are in a strategically compelling position to monitor fluxes of absorbed nutrients and some of their metabolic derivatives [86], and both are clearly innervated by DRG afferents that also connect with hindbrain neurons in the NTS or even the dorsal motor complex for potential spinal-vagal reflexes [107,108]. While the hepatic portal vein and bile ducts are also clearly innervated, the hepatic parenchyma is only sparsely innervated by vagal afferents, at least in the rat [109]. DRG afferents originating in the hepatic portal vein were speculated to be mediating the suppressive effects of intragastric glucose on basomedial hypothalamic AGRP neuron activity and subsequent effects on food intake based on celiac/sup. ...
... This is particularly true for the gastrointestinal tract and associated organs, which are innervated via mixed nerve plexuses along blood vessels (Fig. 3). A typical example is the sumptuous nerve plexus surrounding the celiac artery and its offshoots, the left gastric, hepatic, and splenic arteries, where vagal afferent and efferent axons are intimately intermingled with DRG and postganglionic sympathetic axons from the celiac and superior mesenteric ganglia [109]. At this anatomical level any attempt for selective manipulation such as transection or electrical stimulation is extremely difficult if not impossible (see also discussion in Section 4). ...
Interoception plays an important role in homeostatic regulation of energy intake and metabolism. Major interoceptive pathways include gut-to-brain and adipose tissue-to brain signaling via vagal sensory nerves and hormones, such as leptin. However, signaling via spinal sensory neurons is rapidly emerging as an additional important signaling pathway. Here we provide an in-depth review of the known anatomy and functions of spinal sensory pathways and discuss potential mechanisms relevant for energy balance homeostasis in health and disease. Because sensory innervation by dorsal root ganglia (DRG) neurons goes far beyond vagally innervated viscera and includes adipose tissue, skeletal muscle, and skin, it is in a position to provide much more complete metabolic information to the brain. Molecular and anatomical identification of function specific DRG neurons will be important steps in designing pharmacological and neuromodulation approaches to affect energy balance regulation in disease states such as obesity, diabetes, and cancer.
... We analyzed the expression pattern of ChAT in liver tissues of reporter mice expressing green fluorescent protein (GFP) under the control of transcriptional regulatory elements for ChAT (Chat-GFP reporter mice). In contrast to the extensive cholinergic neural fibers and plexuses in the small intestine, we found no ChAT-expressing neural fibers in either the parenchyma of normal liver or in HCC (Extended Data Fig. 2a), findings in line with earlier reports [15][16][17] . However, we observed accumulation of lymphocyte-like ChAT-expressing cells in HCC (Extended Data Fig. 2a). ...
... Despite some discrepancies among studies, sympathetic and parasympathetic neural markers have been detected in regions of the hepatic artery, portal vein and bile ducts in the majority of species investigated 14 . However, the liver parenchyma of rodents and humans appears to be devoid of vagal or cholinergic innervation, as determined by immunohistochemistry, retrograde tracing and advanced three-dimensional imaging [15][16][17] . Thus, whether and how cholinergic signaling plays a role in HCC regulation remains an open question. ...
Cholinergic nerves are involved in tumor progression and dissemination. In contrast to other visceral tissues, cholinergic innervation in the hepatic parenchyma is poorly detected. It remains unclear whether there is any form of cholinergic regulation of liver cancer. Here, we show that cholinergic T cells curtail the development of liver cancer by supporting antitumor immune responses. In a mouse multihit model of hepatocellular carcinoma (HCC), we observed activation of the adaptive immune response and induction of two populations of CD4 ⁺ T cells expressing choline acetyltransferase (ChAT), including regulatory T cells and dysfunctional PD-1 ⁺ T cells. Tumor antigens drove the clonal expansion of these cholinergic T cells in HCC. Genetic ablation of Chat in T cells led to an increased prevalence of preneoplastic cells and exacerbated liver cancer due to compromised antitumor immunity. Mechanistically, the cholinergic activity intrinsic in T cells constrained Ca ²⁺ –NFAT signaling induced by T cell antigen receptor engagement. Without this cholinergic modulation, hyperactivated CD25 ⁺ T regulatory cells and dysregulated PD-1 ⁺ T cells impaired HCC immunosurveillance. Our results unveil a previously unappreciated role for cholinergic T cells in liver cancer immunobiology.
... supposed to be derived from the nodose ganglion. 41 Nodose ganglion neurons then project to the medullary nucleus of the solitary tract. The common hepatic branch of the vagus bifurcates to the larger gastroduodenal branch, targeting the pancreas and gut, while the minor hepatic branch proper targets the liver hilum. ...
... Regarding vagal afferents, fibers were noticed around larger portal triads in the hilum of different liver lobes, around the extrahepatic part of the portal vein, and around almost all paraganglia adjacent to the hepatic branch of the vagus and the hilum. 41,44 Overall, (mainly afferent) parasympathetic hepatic innervations may be restricted to hilar structures, while a direct hepatic cholinergic innervation is doubtful. Some authors propose an indirect vagal control of the liver, by affecting the sympathetic celiac ganglion or microganglia around the celiac artery. ...
Abbreviations: VMN/PVN, hypothalamic ventromedial nucleus/paraventricular nucleus; VLM/VMM, ventrolateral medulla/ventromedial medulla; SMG/CG, superior mesenteric ganglion/caeliac ganglia; NTS, nucleus of the solitary tract; NG, nodose ganglion.
Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disorder. Increased sympathetic (noradrenergic) nerve tone has a complex role in the etiopathomechanism of NAFLD, affecting the development/progression of steatosis, inflammation, fibrosis, and liver hemodynamical alterations. Also, lipid sensing by vagal afferent fibers is an important player in the development of hepatic steatosis. Moreover, disorganization and progressive degeneration of liver sympathetic nerves were recently described in human and experimental NAFLD. These structural alterations likely come along with impaired liver sympathetic nerve functionality and lack of adequate hepatic noradrenergic signaling. Here, we first overview the anatomy and physiology of liver nerves. Then, we discuss the nerve impairments in NAFLD and their pathophysiological consequences in hepatic metabolism, inflammation, fibrosis, and hemodynamics. We conclude that further studies considering the spatial-temporal dynamics of structural and functional changes in the hepatic nervous system may lead to more targeted pharmacotherapeutic advances in NAFLD.
... and parasympathetic nerves whose branches, through the splanchnic and vagal nerves, form two discrete perivascular plexuses in the portal region. 2,6,8,[10][11][12][13] Most of the knowledge regarding the morphological and functional features of the rat hepatic nerves comes from specific histochemical staining for neuronal markers, 10 from neuronal tracing, 12,[14][15][16] and chemical and surgical denervation. 10,[17][18][19] Thus, in the rat, as in other mammals, and humans, the liver is supplied with both efferent and afferent nerve fibers. ...
... These observations are in line with previous studies demonstrating the role of the vagus nerve in the hepatic cholinergic innervation. 2,8,10,12,15,[17][18][19]39,40 The inconstant ...
Background and aims:
To contribute to the knowledge of the autonomic innervation in liver regeneration, here we investigate the distribution of tyrosine hydroxylase (TH)- and choline acetyltransferase (ChAT)- like immunoreactive (LI) nerve fibers, to indicate noradrenergic and cholinergic nerves, respectively, in rats under different conditions of liver damage and repair.
Methods:
By immunohistochemistry and assessment of nerve fiber density, three models of induced hepatic regeneration were examined: the carbon tetrachloride (CCl4 ) intoxication, with two treatment periods of 14 weeks (wk) and 18 wk; the partial hepatectomy (PH); the thyroid hormone (T3) treatment.
Results:
TH- and ChAT-LI nerve fibers were detectable mostly in the portal spaces, the TH-LI ones occurring only around blood vessels while the ChAT-LI nerve fibers were also associated with secretory ducts. The density of TH-like immunoreactivity in the portal areas decreased after the CCl4 14 wk treatment and PH and increased after T3. By contrast, ChAT-LI nerve fibers appeared particularly abundant around the neoductal elements in the CCl4 rats and were rare to absent in the PH and T3-treated groups. The ChAT-LI nerve fiber density within the portal areas revealed an increase in the CCl4 -treated rats while showing no change in the PH and T3-treated rats.
Interpretation:
The changes in the density of perivascular TH- and ChAT-containing nerve fibers suggest a finely tuned autonomic modulation of hepatic blood flow depending on the type of subacute/chronic induced hyperplasia, while the characteristic occurrence of the periductal cholinergic innervation after the CCl4 treatment implies a selective parasympathetic role in regulating the physiopathological regenerative potential of the rat liver. This article is protected by copyright. All rights reserved.
... Consequently, the branch consists of heterogeneous nerve fibers with respect to the projections of their peripheral axons. In rats, vagal afferent terminals are not found in the hepatic parenchyma (Berthoud et al., 1992;Berthoud and Neuhuber, 2000). It is inferred that the CHBV plays pivotal roles in interoception based on its high content of afferent fibers. ...
... The enhancement of IEG expression by Arg in the insular cortex and CCK in the hypothalamus was abolished by CHBV vagotomy. While it has already been shown that the CHBV contains neural fibers projecting to various peripheral organs (Berthoud et al., 1992;Berthoud and Neuhuber, 2000;Puizillout, 2005), there is little information on the CNS regions activated by this branch. Our results indicate that the CHBV activates different brain regions depending on the type of peripheral stimulation. ...
Information from the peripheral organs is thought to be transmitted to the brain by humoral factors and neurons such as afferent vagal or spinal nerves. The common hepatic branch of the vagus (CHBV) is one of the main vagus nerve branches, and consists of heterogeneous neuronal fibers that innervate multiple peripheral organs such as the bile duct, portal vein, paraganglia, and gastroduodenal tract. Although, previous studies suggested that the CHBV has a pivotal role in transmitting information on the status of the liver to the brain, the details of its central projections remain unknown. The purpose of the present study was to investigate the brain regions activated by the CHBV. For this purpose, we injected L-arginine or anorexia-associated peptide cholecystokinin-8 (CCK), which are known to increase CHBV electrical activity, into the portal vein of transgenic Arc-dVenus mice expressing the fluorescent protein Venus under control of the activity-regulated cytoskeleton-associated protein (Arc) promotor. The brain slices were prepared from these mice and the number of Venus positive cells in the slices was counted. After that, c-Fos expression in these slices was analyzed by immunohistochemistry using the avidin-biotin-peroxidase complex method. Intraportal administration of L-arginine increased the number of Venus positive or c-Fos positive cells in the insular cortex. This action of L-arginine was not observed in CHBV-vagotomized Arc-dVenus mice. In contrast, intraportal administration of CCK did not increase the number of c-Fos positive or Venus positive cells in the insular cortex. Intraportal CCK induced c-Fos expression in the dorsomedial hypothalamus, while intraportal L-arginine did not. This action of CCK was abolished by CHBV vagotomy. Intraportal L-arginine reduced, while intraportal CCK increased, the number of c-Fos positive cells in the nucleus tractus solitarii in a CHBV-dependent manner. The present results suggest that the CHBV can activate different brain regions depending on the nature of the peripheral stimulus.
... Therefore, liver glycogen, through maintaining hepatic ATP levels, contributed to decreasing appetite and this effect was triggered by the vagus nerve. In rats, it has been shown that sensory vagal fibres do not provide substantial direct innervation of hepatocytes, but rather with portal triads in proximity to branches of the hepatic artery, portal vein and bile ducts [33]. However, neuroanatomical data for the vagus in mice are limited. ...
... Nonetheless, we cannot exclude the possibility that the mechanism described in our study depends on sensors located in the portal region. Another factor that could complicate the interpretation of the data is that cutting the common hepatic vagal branch denervates not only the liver but also parts of the gastrointestinal tract [33]. Despite these neuroanatomical difficulties, the evidence presented here highlight the liver as a powerful metabolic sensor, and a crucial link in the brain regulation of energy metabolism [37]. ...
Aims/hypothesisLiver glycogen plays a key role in regulating food intake and blood glucose. Mice that accumulate large amounts of this polysaccharide in the liver are protected from high-fat diet (HFD)-induced obesity by reduced food intake. Furthermore, these animals show reversal of the glucose intolerance and hyperinsulinaemia caused by the HFD. The aim of this study was to examine the involvement of the hepatic branch of the vagus nerve in regulating food intake and glucose homeostasis in this model. Methods
We performed hepatic branch vagotomy (HBV) or a sham operation on mice overexpressing protein targeting to glycogen (PtgOE). Starting 1 week after surgery, mice were fed an HFD for 10 weeks. ResultsHBV did not alter liver glycogen or ATP levels, thereby indicating that this procedure does not interfere with hepatic energy balance. However, HBV reversed the effect of glycogen accumulation on food intake. In wild-type mice, HBV led to a significant reduction in body weight without a change in food intake. Consistent with their body weight reduction, these animals had decreased fat deposition, adipocyte size, and insulin and leptin levels, together with increased energy expenditure. PtgOE mice showed an increase in energy expenditure and glucose oxidation, and these differences were abolished by HBV. Moreover, PtgOE mice showed an improvement in HFD-induced glucose intolerance, which was suppressed by HBV. Conclusions/interpretationOur results demonstrate that the regulation of food intake and glucose homeostasis by liver glycogen is dependent on the hepatic branch of the vagus nerve.
... The hypothesis is that nutrients absorbed from the small intestine travel to the liver via the portal vein, and are sensed by vagal afferent fibres innervating this region. This concept is supported by anatomical evidence identifying vagal fibres terminating on hepatic triads and portal vein, which receives absorbed nutrients directly from the gut, and the innervation of the bile duct, which is important in fat digestion (Berthoud et al. 1992). Furthermore, portal infusion of nutrients (D-glucose, L-lysine and glutamate) has been reported to activate afferent fibres of the hepatic vagal branch in situ (Niijima, 1982(Niijima, , 2000Torii & Niijima, 2001). ...
... Furthermore, portal infusion of nutrients (D-glucose, L-lysine and glutamate) has been reported to activate afferent fibres of the hepatic vagal branch in situ (Niijima, 1982(Niijima, , 2000Torii & Niijima, 2001). However, interpretation of studies performing common hepatic branch vagotomy are complicated by the fact that the majority of the fibres in this branch also supply the antrum, pyloric sphincter, proximal duodenum and pancreas (Berthoud et al. 1992). Thus, it is difficult to attribute the physiological phenotype of common hepatic branch vagotomy to the vagus nerve innervating the liver (Yi et al. 2010). ...
This review highlights evidence for a role of the vagus nerve in the development of obesity and how targeting the vagus nerve with neuromodulation or pharmacology can be used as a therapeutic treatment of obesity. The vagus nerve innervating the gut plays an important role in controlling metabolism. It communicates peripheral information about the volume and type of nutrients between the gut and the brain. Depending on the nutritional status, vagal afferent neurons express two different neurochemical phenotypes that can inhibit or stimulate food intake. Chronic ingestion of calorie-rich diets reduce sensitivity of vagal afferent neurons to peripheral signals and constitutively express orexigenic receptors and neuropeptides. This disruption of vagal afferent signalling is sufficient to drive hyperphagia and obesity. Furthermore neuromodulation of the vagus nerve can be used in the treatment of obesity. Although the mechanisms are poorly understood, vagal nerve stimulation (VNS) prevents weight gain in response to high fat diet. In small clinical studies, in patients with depression or epilepsy, VNS has been demonstrated to promote weight loss. Vagal blockade, that inhibits the vagus nerve, results in significant weight loss. VBLOC is proposed to inhibit aberrant orexigenic signals arising in obesity as a putative mechanism of VBLOC-induced weight loss. Approaches and molecular targets to develop future pharmacotherapy targeted to the vagus nerve for the treatment obesity are discussed. In conclusion there is strong evidence that the vagus nerve is involved in the development of obesity and is proving to be an attractive target for the treatment of obesity. This article is protected by copyright. All rights reserved.
... Following the example of nerve-fiber migration from the olfactory bulb, [291] and the study on injected Mn, [283] a likely path is the vagus nerve, which has extensive innervation in the liver, with 10 times as many afferent nerves as efferent nerves, and particularly concentrated on the outer surface of the bile ducts. [30] MRI abnormalities indicative of Mn toxicity in the globi pallidi and substantia nigra were noted in three cases of patients with liver disease. [120] PD is associated with nonmotor symptoms that often precede the movement impairment aspects. ...
... Berthoud et al. [30] proposed that the onset of PD may be associated with impairment of the vagus nerve, and subsequent functional inhibition of the dopamine system. Clinical trials have revealed pathological alteration of the vagus nerve in PD patients. ...
Manganese (Mn) is an often overlooked but important nutrient, required in small amounts for multiple essential functions in the body. A recent study on cows fed genetically modified Roundup-Ready feed revealed a severe depletion of serum Mn. Glyphosate, the active ingredient in Roundup , has also been shown to severely deplete Mn levels in plants. Here, we investigate the impact of Mn on physiology, and its association with gut dysbiosis as well as neuropathologies such as autism, Alzheimer's disease (AD), depression, anxiety syndrome, Parkinson's disease (PD), and prion diseases. Glutamate overexpression in the brain in association with autism, AD, and other neurological diseases can be explained by Mn deficiency. Mn superoxide dismutase protects mitochondria from oxidative damage, and mitochondrial dysfunction is a key feature of autism and Alzheimer's. Chondroitin sulfate synthesis depends on Mn, and its deficiency leads to osteoporosis and osteomalacia. Lactobacillus, depleted in autism, depend critically on Mn for antioxidant protection. Lactobacillus probiotics can treat anxiety, which is a comorbidity of autism and chronic fatigue syndrome. Reduced gut Lactobacillus leads to overgrowth of the pathogen, Salmonella, which is resistant to glyphosate toxicity, and Mn plays a role here as well. Sperm motility depends on Mn, and this may partially explain increased rates of infertility and birth defects. We further reason that, under conditions of adequate Mn in the diet, glyphosate, through its disruption of bile acid homeostasis, ironically promotes toxic accumulation of Mn in the brainstem, leading to conditions such as PD and prion diseases.
... Although this might be true, one also has to consider the pathways by which the neurons travel and the surgery technique. Berthoud and colleagues (8) have recently reported that the HVB afferents were not found in association with liver parenchymal cells, but rather innervated the biliary system. They further noted that two-thirds of the afferents bypass the liver and course over the hepatic portal vein on their way to innervate the hepatic portal vein, pancreas, stomach, pylorus, or the duodenum. ...
... As previously noted (3), if continued protein synthesis was not stimulated or of a normal magnitude in the liver of the denervated animals in response to the influx of the amino acids in the imbalanced mix, the dramatic reduction of the limiting amino acid would not be as great and the imbalanced feeding response would be blunted. This is possible following hepatic denervation [but see (8)] because the hypothalamus, via autonomic innervation to the liver, has been implicated in protein and DNA synthesis (i.e., parasympathetic efferents increase and sympathetic efferents decrease both protein and DNA synthesis) (16,17,33,35). Autonomic efferents to the liver have also been shown to affect the activity of hepatic enzymes including: tryptophan pyrrolase, tyrosine transaminase, aspartate transcarbamoylase, and thymidine kinase (9,16,20,25,31). ...
Rats presented with an 1MB show reduced food intake (FI) within 3h, however this suppression can be attenuated by ventral vagal branch transection above (VENT), but not below (VENT-B) the hepatic vagal branch (HVB). The present study had seven groups (grps, n=7-16) with the following cuts: VENT, bilateral subdiaphragmatic vagotomy (TVAG), VENT-B, HVB + accessory coeliac (AC), HVB + ventral gastric branch (VGB), HVB, and sham. Diets were milk (3d), casein gel (5d), and low protein basal diet (8d). The last two days on the basal diet starting at lights out FI was recorded at 3, 6, 12, and 24h intervals. The rats were then presented with an 1MB with interval measurements and then daily measurements for 7d. The first day on 1MB the VENT grp had the highest FI and the VENT-B group the lowest. The body weight (BW, as a % of starting BW) of the VENT-B remained low and was (P<0.05) lower than all grps by day 7 on the 1MB. The TVAG had the highest BW which was higher than all grps by day 7. BW was elevated at an intermediate level in the VENT and HVB+VGB grps . The data show that the AC branch probably doesn't play a role in the FI response to the 1MB, while combination cut of the HVB and VGB enhances BW recovery.
... Afferent signals arrive from peripheral organs, including the gastrointestinal tract where nutrients are absorbed, or from other tissues such as adipose tissue, liver, and pancreas. One important area for nutrients, including glucose sensing, is the hepatic portal vein, which is innervated by both spinal and vagal afferents (5)(6)(7). Sensory signals through the dorsal root ganglia or the left nodose ganglion are relayed to the dorsal vagal complex. A recent study conducted in rats used herpes simplex virus-1 (an anterograde, transsynaptic viral tracer) to define sensory pathways from the hepatic portal and superior mesenteric veins (8). ...
The prevalence of metabolic disorders, including type 2 diabetes mellitus, continues to increase worldwide. Although newer and more advanced therapies are available, current treatments are still inadequate and the search for solutions remains. The regulation of energy homeostasis including glucose metabolism, involves an exchange of information between the nervous systems and peripheral organs and tissues; therefore, developing treatments to alter central and/or peripheral neural pathways could be an alternative solution to modulate whole-body metabolism. Liver glucose production and storage are major mechanisms controlling glycemia, and the autonomic nervous system plays an important role in the regulation of hepatic functions. Autonomic nervous system imbalance contributes to excessive hepatic glucose production, and thus to the development and progression of type 2 diabetes mellitus. At cellular levels, change in neuronal activity is one of the underlying mechanisms of autonomic imbalance; therefore, modulation of the excitability of neurons involved in autonomic outflow governance has the potential to improve glycemic status. Tissue-specific subsets of pre-autonomic neurons differentially control autonomic outflow, therefore, detailed information about neural circuits and properties of liver-related neurons is necessary for the development of strategies to regulate liver functions via the autonomic nerves. This review provides an overview of our current understanding of the hypothalamus - ventral brainstem - liver pathway involved in the sympathetic regulation of the liver, outlines strategies to identify organ-related neurons, and summarizes neuronal plasticity during diabetic conditions with particular focus on liver-related neurons in the paraventricular nucleus.
... The liver is innervated by the hepatic branch from the anterior vagal trunk Neuhuber 2000a, 2000b). The vagal fibers are mostly present at the porta hepatis, which consists of the portal vein, hepatic artery, and bile duct (Berthoud et al. 1992;Neuhuber 2000a, 2000b). However, some studies have also suggested vagal innervation at the liver parenchyma (Forssmann and Ito 1977;Metz and Forssmann 1980;Tiniakos et al. 1996). ...
The vagus nerve is involved in the autonomic regulation of physiological homeostasis, through vast innervation of cervical, thoracic and abdominal visceral organs. Stimulation of the vagus with bioelectronic devices represents a therapeutic opportunity for several disorders implicating the autonomic nervous system and affecting different organs. During clinical translation, vagus stimulation therapies may benefit from a precision medicine approach, in which stimulation accommodates individual variability due to nerve anatomy, nerve-electrode interface or disease state and aims at eliciting therapeutic effects in targeted organs, while minimally affecting non-targeted organs. In this review, we discuss the anatomical and physiological basis for precision neuromodulation of the vagus at the level of nerve fibers, fascicles, branches and innervated organs. We then discuss different strategies for precision vagus neuromodulation, including fascicle- or fiber-selective cervical vagus nerve stimulation, stimulation of vagal branches near the end-organs, and ultrasound stimulation of vagus terminals at the end-organs themselves. Finally, we summarize targets for vagus neuromodulation in neurological, cardiovascular and gastrointestinal disorders and suggest potential precision neuromodulation strategies that could form the basis for effective and safe therapies.
... By preferentially innervating the small and large intestines [215], including the Roux and common limbs, the celiac branches are in a position to pick up any signaling originating from the rearranged gut. Because surgical transection of the common hepatic branch, which supplies vagal sensory innervation to the hepatic portal vein [216] does not affect the outcome of RYGB in rats [217], it is unlikely that GLP-1 sensing mechanisms within the hepatic portal vein play an important role. Given that these surgical subdiaphragmatic vagal branch cuts included both sensory and motor fibers, the findings are consistent with, but do not unequivocally prove, a role for vagal afferents. ...
Background
Bariatric or weight loss surgery is currently the most effective treatment for obesity and metabolic disease. Unlike dieting and pharmacology, its beneficial effects are sustained over decades in most patients, and mortality is among the lowest for major surgery. Because there are not nearly enough surgeons to implement bariatric surgery on a global scale, intensive research efforts have begun to identify its mechanisms of action on a molecular level in order to replace surgery with targeted behavioral or pharmacological treatments. To date, however, there is no consensus as to the critical mechanisms involved.
Scope of Review
The purpose of this non-systematic review is to evaluate the existing evidence for specific molecular and inter-organ signaling pathways that play major roles in bariatric surgery-induced weight loss and metabolic benefits, with a focus on Roux-en-Y gastric bypass (RYGB) and vertical sleeve gastrectomy (VSG), in both humans and rodents.
Major Conclusions
Gut-brain communication and its brain targets of food intake control and energy balance regulation are complex and redundant. Although the relatively young science of bariatric surgery has generated a number of hypotheses, no clear and unique mechanism has yet emerged. It seems increasingly likely that the broad physiological and behavioral effects produced by bariatric surgery do not involve a single mechanism, but rather multiple signaling pathways. Besides a need to improve and better validate surgeries in animals, advanced techniques, including inducible, tissue-specific knockout models, and the use of humanized physiological traits will be necessary. State-of-the-art genetically-guided neural identification techniques should be used to more selectively manipulate function-specific pathways.
... The rat common hepatic vagal branch contains both afferents and efferents (and even some nonvagal nerve fibers [57], and projects primarily to the proximal duodenum, pylorus, and pancreas via the gastroduodenal artery. It also innervates the portal hepatic vein, and only a small fraction actually innervates the liver itself along the hepatic artery [58]. Therefore, this complicates the interpretation of the functional effects of common hepatic branch vagotomy, particularly when looking at longer-term effects. ...
Omnivores, including rodents and humans, compose their diets from a wide variety of potential foods. Beyond the guidance of a few basic orosensory biases such as attraction to sweet and avoidance of bitter, they have limited innate dietary knowledge and must learn to prefer foods based on their flavors and postoral effects. This review focuses on postoral nutrient sensing and signaling as an essential part of the reward system that shapes preferences for the associated flavors of foods. We discuss the extensive array of sensors in the gastrointestinal system and the vagal pathways conveying information about ingested nutrients to the brain. Earlier studies of vagal contributions were limited by nonselective methods that could not easily distinguish the contributions of subsets of vagal afferents. Recent advances in technique have generated substantial new details on sugar- and fat-responsive signaling pathways. We explain methods for conditioning flavor preferences and their use in evaluating gut–brain communication. The SGLT1 intestinal sugar sensor is important in sugar conditioning; the critical sensors for fat are less certain, though GPR40 and 120 fatty acid sensors have been implicated. Ongoing work points to particular vagal pathways to brain reward areas. An implication for obesity treatment is that bariatric surgery may alter vagal function.
... Given this evidence, the existence of a vagal sensory circuit monitoring blood glucose, communicating this to the brain and driving both appropriate autonomic and behavioral responses seems possible. Vagal sensory innervation of the liver and pancreas have been described in classical studies performed in rats (34,35). With the platform already developed for functional and genetic investigation of vagal sensory subtypes (28)(29)(30)(31)(32), this represents an area ripe for investigation using contemporary neuroscience techniques. ...
Tight regulation of blood glucose is essential for long term health. Blood glucose levels are defended by the correct function of, and communication between, internal organs including the gastrointestinal tract, pancreas, liver, and brain. Critically, the brain is sensitive to acute changes in blood glucose level and can modulate peripheral processes to defend against these deviations. In this mini-review we highlight select key findings showcasing the utility, strengths, and limitations of model organisms to study brain-body interactions that sense and control blood glucose levels. First, we discuss the large platform of genetic tools available to investigators studying mice and how this field may yet reveal new modes of communication between peripheral organs and the brain. Second, we discuss how rats, by virtue of their size, have unique advantages for the study of CNS control of glucose homeostasis and note that they may more closely model some aspects of human (patho)physiology. Third, we discuss the nascent field of studying the CNS control of blood glucose in the zebrafish which permits ease of genetic modification, large-scale measurements of neural activity and live imaging in addition to high-throughput screening. Finally, we briefly discuss glucose homeostasis in drosophila, which have a distinct physiology and glucoregulatory systems to vertebrates.
... It also decreases mean arterial blood pressure, and increases left and right ventricular end diastolic pressure (4). Neuronal cell bodies residing in the DMN and nucleus ambiguus project axons within the vagus nerve, the longest nerve in the body, which innervates and coordinates organ responses to threat (5)(6)(7)(8). ...
Significance
Electronic devices that stimulate electrical activity in the vagus nerve are being studied for clinical use in rheumatoid arthritis, inflammatory bowel disease, and other inflammatory syndromes because vagus nerve signals inhibit inflammation and cytokine production. A vagus nerve mechanism, termed the inflammatory reflex, has been widely studied, but the origin and functional neuroanatomy of vagus nerve fibers controlling inflammation were previously unknown. Here we reveal cholinergic neurons in the brainstem dorsal motor nucleus (DMN) of the vagus projecting to the celiac-superior mesenteric ganglia and transmitting cytokine-inhibiting signals to the splenic nerve. By combining optogenetics, anatomical and functional mapping, and measurement of TNF production, our data show the DMN is an important brainstem locus controlling anti-inflammatory signals in the inflammatory reflex.
... Other studies performing portal glucose infusions in rodents report effects during infusion and have led to the identification of glucose sensors in the portal vein itself and connected to the brain via afferent hepatic branches of the vagus nerve (Berthoud et al. 1992). The portal vein drains visceral adipose tissue and the intestine, and carries blood rich in nutrients, metabolites and factors derived from the diet, microbiota and adipose to the liver. ...
Poor nutrition plays a fundamental role in the development of insulin resistance, an underlying characteristic of type 2 diabetes. We have previously shown that high-fat diet-induced insulin resistance in rats can be ameliorated by a single glucose meal, but the mechanisms for this observation remain unresolved. To determine if this phenomenon is mediated by gut or hepatoportal factors, male Wistar rats were fed a high-fat diet for three weeks before receiving one of five interventions: high-fat meal, glucose gavage, high-glucose meal, systemic glucose infusion or portal glucose infusion. Insulin sensitivity was assessed the following day in conscious animals by a hyperinsulinemic-euglycemic clamp. An oral glucose load consistently improved insulin sensitivity in high-fat fed rats, establishing the reproducibility of this model. A systemic infusion of a glucose load did not affect insulin sensitivity, indicating that the physiological response to oral glucose was not due solely to increased glucose turnover or withdrawal of dietary lipid. A portal infusion of glucose produced the largest improvement in insulin sensitivity, implicating a role for the hepatoportal region rather than the gastrointestinal tract in mediating the effect of glucose to improve lipid-induced insulin resistance. These results further deepen our understanding of the mechanism of glucose mediated regulation of insulin sensitivity and provide new insight into the role of nutrition in whole body metabolism.
... anterograde tracing from the DMV labels axons in the liver (Berthoud, Kressel, & Neuhuber, 1992). Hence, an insulin-activated circuit from the DMV to the liver could also help to raise blood glucose in response to insulin-induced hypoglycemia. ...
Low blood glucose activates brainstem adrenergic and cholinergic neurons, driving adrenaline secretion from the adrenal medulla and glucagon release from the pancreas. Despite their roles in maintaining glucose homeostasis, the distributions of insulin‐responsive adrenergic and cholinergic neurons in the medulla are unknown. We fasted rats overnight and gave them insulin (10 IU/kg i.p.) or saline after 2 weeks of handling. Blood samples were collected before injection and before perfusion at 90 min. We immunoperoxidase‐stained transverse sections of perfused medulla to show Fos plus either phenylethanolamine N‐methyltransferase (PNMT) or choline acetyltransferase (ChAT). Insulin injection lowered blood glucose from 4.9+0.3 mmol/L to 1.7+0.2 mmol/L (mean+SEM; n=6); saline injection had no effect. In insulin‐treated rats, many PNMT‐immunoreactive C1 neurons had Fos‐immunoreactive nuclei, with the proportion of activated neurons being highest in the caudal part of the C1 column. In the rostral ventrolateral medulla, 33.3%±1.4% (n=8) of C1 neurons were Fos‐positive. Insulin also induced Fos in 47.2%±2.0% (n=5) of dorsal medullary C3 neurons and in some C2 neurons. In the dorsal motor nucleus of the vagus (DMV), insulin evoked Fos in many ChAT‐positive neurons. Activated neurons were concentrated in the medial and middle regions of the DMV beneath and just rostral to the area postrema. In control rats, very few C1, C2 or C3 neurons and no DMV neurons were Fos‐positive. The high numbers of PNMT‐immunoreactive and ChAT‐immunoreactive neurons that express Fos after insulin treatment reinforce the importance of these neurons in the central response to a decrease in glucose bioavailability.
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... Recent studies have revealed that the multipotent stem/progenitor cells exist in the PBGs, and they expressed Sox17 in human and mouse (Cardinale et al. 2011;Carpino et al. 2012). On the other hand, the gene expression of the PBGs in rats remains unclear, while the obvious PBGs are found among the biliary wall in the rats (Berthoud et al. 1992). Considering the development of the biliary tract of rats is free from the Sox17 regulation, Sox17 might not be expressed in the PBGs. ...
The gallbladder is the hepatobiliary organ for storing and secreting bile fluid, and is a synapomorphy of extant vertebrates. However, this organ has been frequently lost in several lineages of birds and mammals, including rodents. Although it is known as the traditional problem, the differences in development between animals with and without gallbladders are not well understood. To address this research gap, we compared the anatomy and development of the hepatobiliary systems in mice (gallbladder is present) and rats (gallbladder is absent). Anatomically, almost all parts of the hepatobiliary system of rats are topographically the same as those of mice, but rats have lost the gallbladder and cystic duct completely. During morphogenesis, the gallbladder–cystic duct domain (Gb–Cd domain) and its primordium, the biliary bud, do not develop in the rat. In the early stages, SOX17, a master regulator of gallbladder formation, is positive in the murine biliary bud epithelium, as seen in other vertebrates with a gallbladder, but there is no SOX17-positive domain in the rat hepatobiliary primordia. These findings suggest that the evolutionary loss of the Gb–Cd domain should be translated simply as the absence of a biliary bud at an early stage, which may correlate with alterations in regulatory genes, such as Sox17, in the rat. A SOX17-positive biliary bud is clearly definable as a developmental module that may be involved in the frequent loss of gallbladder in mammals.
... We found that a few labelled cells in the DMN were observed in animals with the section of the dorsal sub-diaphragmatic trunk compared to VgX, indicating that the certain selectivity is recognized. Second, some of the afferent vagal innervations of the liver originate in the right nodose ganglion and project through the celiac branch and the periarterial plexus of the common hepatic arteries 22 . This projection cannot be easily cut in a selective manner, which cannot be completely ignored, although the contribution of afferent fibres to the liver by this route is only approximately 10%. ...
We examined whether glucagon-like peptide-1 (GLP-1) affects β-cell mass and proliferation through neural pathways, from hepatic afferent nerves to pancreatic efferent nerves via the central nervous system, in high-fat diet (HFD)-induced obese rats. The effects of chronic administration of GLP-1 (7–36) and liraglutide, a GLP-1 receptor agonist, on pancreatic morphological alterations, c-fos expression and brain-derived neurotrophic factor (BDNF) content in the hypothalamus, and glucose metabolism were investigated in HFD-induced obese rats that underwent hepatic afferent vagotomy (VgX) and/or pancreatic efferent sympathectomy (SpX). Chronic GLP-1 (7–36) administration to HFD-induced obese rats elevated c-fos expression and BDNF content in the hypothalamus, followed by a reduction in pancreatic β-cell hyperplasia and insulin content, thus resulting in improved glucose tolerance. These responses were abolished by VgX and SpX. Moreover, administration of liraglutide similarly activated the hypothalamic neural pathways, thus resulting in a more profound amelioration of glucose tolerance than native GLP-1 (7–36). These data suggest that GLP-1 normalizes the obesity-induced compensatory increase in β-cell mass and glucose intolerance through a neuronal relay system consisting of hepatic afferent nerves, the hypothalamus, and pancreatic efferent nerves.
... To this purpose, they were first anesthetized with an intraperitoneal injection of diazepam (30 mg/kg) and ketamine hydrochloride (45 mg/kg) and then, under aseptic conditions, a 5% solution of the tracer Dil (1,1 0 -dioctadecyl 3,3,3 0 ,3 0 -tetramethyl-indocarbocyanine perchlorate, Molecular Probes, Eugene, OR) was unilaterally injected into the whisker pad using a Hamilton micro-syringe (1 lL) at a positive pressure (50 lL/min). This tracer was chosen because it is specifically used to label nerve fibers (Tamamaki 1997;Woodhams and Terashima 2000) and axon terminals (Honig and Hume 1989;Berthoud et al. 1992) and also because we have successfully used it in previous studies regarding the macrovibrissae system (Mameli et al. 2008(Mameli et al. , 2009(Mameli et al. , 2010(Mameli et al. , 2014(Mameli et al. , 2016. ...
It has been recently shown in rats that spontaneous movements of whisker pad macrovibrissae elicited evoked responses in the trigeminal mesencephalic nucleus (Me5). In the present study, electrophysiological and neuroanatomical experiments were performed in anesthetized rats to evaluate whether, besides the whisker displacement per se, the Me5 neurons are also involved in encoding the kinematic properties of macrovibrissae movements, and also whether, as reported for the trigeminal ganglion, even within the Me5 nucleus exists a neuroanatomical representation of the whisker pad macrovibrissae. Extracellular electrical activity of single Me5 neurons was recorded before, during, and after mechanical deflection of the ipsilateral whisker pad macrovibrissae in different directions, and with different velocities and amplitudes. In several groups of animals, single or multiple injections of the tracer Dil were performed into the whisker pad of one side, in close proximity to the vibrissae follicles, in order to label the peripheral terminals of the Me5 neurons innervating the macrovibrissae (whisking‐neurons), and therefore, the respective perikaria within the nucleus. Results showed that: (1) the whisker pad macrovibrissae were represented in the medial‐caudal part of the Me5 nucleus by a single cluster of cells whose number seemed to match that of the macrovibrissae; (2) macrovibrissae mechanical deflection elicited significant responses in the Me5 whisking‐neurons, which were related to the direction, amplitude, and frequency of the applied deflection. The specific functional role of Me5 neurons involved in encoding proprioceptive information arising from the macrovibrissae movements is discussed within the framework of the whole trigeminal nuclei activities.
... Sympathetic afferent fibers originating in the liver enter into the dorsal root ganglion, which then terminate in the dorsal horn of the spinal cord [3]. In contrast, for parasympathetic signaling, hepatic vagal afferents enter into the nodose ganglion and then extend into the nucleus of the solitary tract [7] (Fig. 1b). Both the afferent and efferent signaling pathways of the liver play a role in the regulation of hepatic lipid and lipoprotein metabolism. ...
Purpose of review:
Hepatic lipid and lipoprotein metabolism is an important determinant of fasting dyslipidemia and the development of fatty liver disease. Although endocrine factors like insulin have known effects on hepatic lipid homeostasis, emerging evidence also supports a regulatory role for the central nervous system (CNS) and neuronal networks. This review summarizes evidence implicating a bidirectional liver-brain axis in maintaining metabolic lipid homeostasis, and discusses clinical implications in insulin-resistant states.
Recent findings:
The liver utilizes sympathetic and parasympathetic afferent and efferent fibers to communicate with key regulatory centers in the brain including the hypothalamus. Hypothalamic anorexigenic and orexigenic peptides signal to the liver via neuronal networks to modulate lipid content and VLDL production. In addition, peripheral hormones such as insulin, leptin, and glucagon-like-peptide-1 exert control over hepatic lipid by acting directly within the CNS or via peripheral nerves. Central regulation of lipid metabolism in other organs including white and brown adipose tissue may also contribute to hepatic lipid content indirectly via free fatty acid release and changes in lipoprotein clearance.
Summary:
The CNS communicates with the liver in a bidirectional manner to regulate hepatic lipid metabolism and lipoprotein production. Impairments in these pathways may contribute to dyslipidemia and hepatic steatosis in insulin-resistant states.
... The nerve endings of vagal afferents have been extensively characterized in the GI tract, especially the esophagus and stomach, following injections of neuronal tracers into nodose and jugular ganglia (1,2,(22)(23)(24). However, much less was known about spinal afferent endings in the esophagus (17,20) and stomach of mammals. ...
Spinal afferent neurons play a major role in detection and transduction of painful stimuli from internal (visceral) organs. Recent technical advances have made it possible to visualize the endings of spinal afferent axons in visceral organs. Although it is well known that the sensory nerve cell bodies of spinal afferents reside within dorsal root ganglia (DRG), identifying their endings in internal organs has been especially challenging because of a lack of techniques to distinguish them from endings of other extrinsic and intrinsic neurons (sympathetic, parasympathetic and enteric). We recently developed a surgical approach in live mice that allows selective labelling of spinal afferent axons and their endings, revealing a diverse array of different types of varicose and non-varicose terminals in visceral organs, particularly the large intestine. In total, 13 different morphological types of endings were distinguished in the mouse distal large intestine, originating from lumbosacral DRGs. Interestingly, the stomach, esophagus, bladder and uterus had less diversity in their types of spinal afferent endings. Taken together, spinal afferent endings (at least in the large intestine) appear to display greater morphological diversity than vagal afferent endings that have previously been extensively studied. We discuss some of the new insights that these findings provide.
... Then, following a small craniotomy, a second tracer Dil (1,1′-dioctadecyl 3,3,3′,3′-tetramethyl-indocarbocyanine perchlorate, Molecular Probes, Inc., solubilized in 5 % N,N-dimethylformamide) was bilaterally injected (0.05 µL) at a positive pressure (50 nL/min) into the Me5 nucleus and, in particular, into its medial-caudal part ( Fig. 1) where the neurons activated by both spontaneous and artificial movements of the macrovibrissae were found, and whose peripheral axons directly targeted these special sensory detectors (Mameli et al. 2010(Mameli et al. , 2014. The tracer Dil was chosen because it is specifically used as an anterograde tracer to label nerve fibers (Tamamaki 1997;Woodhams and Terashima 2000) and axon terminals (Honig and Hume 1989;Berthoud et al. 1992), and also because we successfully used it in previous studies regarding the macrovibrissae system (Mameli et al. 2008(Mameli et al. , 2010. The microinjection of the tracer [coordinates: anterior-posterior = −9.80; ...
Previous studies performed in rats showed that the whisker-pad motor innervation involves not only the facial nerve, but also some hypoglossal neurons whose axons travel within the trigeminal infraorbital nerve (ION) and target the extrinsic muscles surrounding the whisker-pad macrovibrissae. Furthermore, the electrical stimulation of the ION induced an increase in the EMG activity of these muscles, while the hypoglossal nucleus stimulation elicited evoked potentials and single motor unit responses. However, the existence of a neural network able to involve the XIIth nucleus in macrovibrissae whisking control was totally unknown until now. Since other recent experiments demonstrated that: (1) the mesencephalic trigeminal nucleus (Me5) neurons respond to both spontaneous and artificial movements of macrovibrissae, and (2) the Me5 peripheral terminals provide a monosynaptic sensory innervation to the macrovibrissae, the present study was aimed at analyzing a possible role of the Me5 nucleus as a relay station in the sensory-motor loop that involves the XIIth nucleus neurons in rhythmic whisking control. Two tracers were used in the same animal: Fluoro Gold, which was injected into the whisker pad to retrogradely label the hypoglossal whisker-pad projection neurons, and Dil, which was instead injected into the Me5 to label its projections to these hypoglossal neurons. Results demonstrated that terminals of the Me5 neurons monosynaptically target the hypoglossal whisker-pad projection neurons. The functional role of this sensory-motor connection is discussed, with particular regard to a hypothesized proprioceptive reflex in whisker-pad extrinsic muscles that can be elicited by the activation of the Me5 macrovibrissae receptors.
... In mice, two major nerve populations supply the biliary tract: nerves from the celiac and myenteric plexuses of the duodenum. Because these two populations mostly contain the sympathetic nerves and vagus nerves (parasympathetic), it is expected that the sympathetic nerves and vagus regulate the murine biliary system, similar to what occurs in many mammals (Burnett et al., 1964;Hopton, 1973;Padbury et al., 1993a) and rats, which lack a gallbladder (Berthoud et al., 1992). Because the nerves from the celiac and myenteric plexuses make multiple anastomoses, the cystic nerve of the gallbladder may contain axons from both nerve populations. ...
The biliary tract is a well-branched ductal structure that exhibits great variation in morphology among vertebrates. Its function is maintained by complex constructions of blood vessels, nerves, and smooth muscles, the so-called hepatobiliary system. Although the mouse (Mus musculus) has been used as a model organism for humans, the morphology of its hepatobiliary system has not been well documented at the topographical level, mostly because of its small size and complexity. To reconcile this, we conducted whole-mount anatomical descriptions of the murine extrahepatic biliary tracts with related blood vessels, nerves and smooth muscles using a recently developed transparentizing method, CUBIC. Several major differences from humans were found in mice; (1) Among the biliary arteries, the arteria gastrica sinistra accessoria was commonly found, which rarely appears in humans; (2) The sphincter muscle in the choledochoduodenal junction is un-separated from the duodenal muscle; (3) The pancreatic duct opens to the bile duct without any sphincter muscles because of its distance from the duodenum. This state is identical to a human congenital malformation, an anomalous arrangement of pancreaticobiliary ducts (APBDs). However, other parts of the murine hepatobiliary system (such as the branching patterns of the biliary tract, blood vessels and nerves) presented the same patterns as humans and other mammals topologically. Thus the mouse is useful as an experimental model for studying the human hepatobiliary system. This article is protected by copyright. All rights reserved.
How the gastrointestinal tract communicates with the brain, via sensory nerves, is of significant interest for our understanding of human health and disease. Enterochromaffin (EC) cells in the gut mucosa release a variety of neurochemicals, including the largest quantity of 5‐hydroxytryptamine (5‐HT) in the body. How 5‐HT and other substances released from EC cells activate sensory nerve endings in the gut wall remains a major unresolved mystery. We used in vivo anterograde tracing from nodose ganglia to determine the spatial relationship between 5‐HT synthesizing and peptide‐YY (PYY)‐synthesizing EC cells and their proximity to vagal afferent nerve endings that project to the mucosa of mouse small intestine. The shortest mean distances between single 5‐HT‐ and PYY‐synthesizing EC cells and the nearest vagal afferent nerve endings in the mucosa were 33.1 ± 14.4 µm ( n = 56; N = 6) and 70.3 ± 32.3 µm ( n = 16; N = 6). No morphological evidence was found to suggest that 5‐HT‐ or PYY‐containing EC cells form close morphological associations with vagal afferents endings, or varicose axons of passage. The large distances between EC cells and vagal afferent endings are many hundreds of times greater than those known to underlie synaptic transmission in the nervous system (typically 10–15 nm). Taken together, the findings lead to the inescapable conclusion that communication between 5‐HT‐containing EC cells and vagal afferent nerve endings in the mucosa of the mouse small intestinal occurs in a paracrine fashion, via diffusion.
New and Noteworthy None of the findings here are consistent with a view that close physical contacts occur between 5‐HT‐containing EC cells and vagal afferent nerve endings in mouse small intestine. Rather, the findings suggest that gut–brain communication between EC cells and vagal afferent endings occurs via passive diffusion. The morphological data presented do not support the view that EC cells are physically close enough to vagal afferent endings to communicate via fast synaptic transmission.
Background
Recent research suggests that microbial molecules translocated from the intestinal lumen into the host's internal environment may play a role in various physiological functions, including sleep. Previously, we identified that butyrate, a short-chain fatty acid, produced by intestinal bacteria, and lipoteichoic acid, a cell wall component of gram-positive bacteria induce sleep when their naturally occurring translocation is mimicked by direct delivery into the portal vein. Building upon these findings, we aimed to explore the sleep signaling potential of intraportally administered lipopolysaccharide, a primary component of gram-negative bacterial cell walls, in rats.
Results
Low dose of lipopolysaccharide (1 µg/kg) increased sleep duration and prolonged fever, without affecting systemic lipopolysaccharide levels. Interestingly, administering LPS systemically outside the portal region at a dose 20 times higher did not affect sleep, indicating a localized sensitivity within the hepatoportal region, encompassing the portal vein and liver, for the sleep and febrile effects of lipopolysaccharide. Furthermore, both the sleep- and fever-inducing effects of LPS were inhibited by indomethacin, a prostaglandin synthesis inhibitor, and replicated by intraportal administration of prostaglandin E2 or arachidonic acid, suggesting the involvement of the prostaglandin system in mediating these actions.
Conclusions
These findings underscore the dynamic influence of lipopolysaccharide in the hepatoportal region on sleep and fever mechanisms, contributing to a complex microbial molecular assembly that orchestrates communication between the intestinal microbiota and brain. Lipopolysaccharide is a physiological component of plasma in both the portal and extra-portal circulation, with its levels rising in response to everyday challenges like high-fat meals, moderate alcohol intake, sleep loss and psychological stress. The increased translocation of lipopolysaccharide under such conditions may account for their physiological impact in daily life, highlighting the intricate interplay between microbial molecules and host physiology.
Maintaining blood glucose at an appropriate physiological level requires precise coordination of multiple organs and tissues. The vagus nerve bidirectionally connects the central nervous system with peripheral organs crucial to glucose mobilization, nutrient storage, and food absorption, thereby presenting a key pathway for the central control of blood glucose levels. However, the precise mechanisms by which vagal populations that target discrete tissues participate in glucoregulation are much less clear. Here we review recent advances unraveling the cellular identity, neuroanatomical organization, and functional contributions of both vagal efferents and vagal afferents in the control of systemic glucose metabolism. We focus on their involvement in relaying glucoregulatory cues from the brain to peripheral tissues, particularly the pancreatic islet, and by sensing and transmitting incoming signals from ingested food to the brain. These recent findings - largely driven by advances in viral approaches, RNA sequencing, and cell-type selective manipulations and tracings - have begun to clarify the precise vagal neuron populations involved in the central coordination of glucose levels, and raise interesting new possibilities for the treatment of glucose metabolism disorders such as diabetes.
The biliary tract includes the entire biliary excretion route from the liver to the duodenum. Bile is secreted from hepatocytes into the bile canaliculi. It is eventually excreted into the duodenum after passing through the biliary tract. The main components of bile include bile acids, phospholipids, cholesterols, and bilirubin. Some of these substances are reabsorbed in the intestine and returned to the liver via the portal vein, a cycle termed enterohepatic circulation. Enterohepatic circulation involves physiologically active substances and ensures its effective use. The function of the biliary tract is controlled by the autonomic nervous system. Branches from the hepatic plexus, which is formed by the sympathetic nerves and the vagus nerve, are distributed across the biliary tract. The sphincter of Oddi at the papilla and gallbladder play important roles in the control of the bile efflux, and dysfunctions can occur when their motility is inhibited. The gallbladder dysfunction is a motility disorder and causes pain similar to chronic cholecystitis. The sphincter of Oddi regulates the excretion of bile and prevents the regurgitation of duodenal juice. In papillary dysfunction, the sphincter of Oddi is excessively contracted, which inhibits the excretion of bile and pancreatic juice. Pancreaticobiliary maljunction is a congenital malformation in which the pancreatic duct and the bile duct join outside the duodenal wall. In this condition, the sphincter of Oddi does not regulate the confluence of the pancreatic duct and bile duct, resulting in bidirectional regurgitation of bile and pancreatic juice and various complications in the bile duct and the pancreas.KeywordsBiliary tractBiliary tract disordersBile acidsEnterohepatic circulation
The liver is the largest organ in the human body and is responsible for the metabolism and storage of the three principal nutrients: carbohydrates, fats, and proteins. In addition, the liver contributes to the breakdown and excretion of alcohol, medicinal agents, and toxic substances and the production and secretion of bile. In addition to its role as a metabolic centre, the liver has recently attracted attention for its function in the liver-brain axis, which interacts closely with the central nervous system via the autonomic nervous system, including the vagus nerve. The liver-brain axis influences the control of eating behaviour in the central nervous system through stimuli from the liver. Conversely, neural signals from the central nervous system influence glucose, lipid, and protein metabolism in the liver. The liver also receives a constant influx of nutrients and hormones from the intestinal tract and compounds of bacterial origin via the portal system. As a result, the intestinal tract and liver are involved in various immunological interactions. A good example is the co-occurrence of primary sclerosing cholangitis and ulcerative colitis. These heterogeneous roles of the liver-brain axis are mediated via the vagus nerve in an asymmetrical manner. In this review, we provide an overview of these interactions, mainly with the liver but also with the brain and gut.
This article is part of the special Issue on ‘Cross Talk between Periphery and the Brain’.
Known as the gas exchange organ, the lung is also critical for responding to the aerosol environment in part through interaction with the nervous system. The diversity and specificity of lung innervating neurons remains poorly understood. Here, we interrogated the cell body location, molecular signature and projection pattern of lung innervating sensory neurons. Retrograde tracing from the lung coupled with whole tissue clearing highlighted neurons primarily in the vagal ganglia. Centrally, they project specifically to the nucleus of the solitary tract in the brainstem. Peripherally, they enter the lung alongside branching airways. Labeling of nociceptor Trpv1+ versus peptidergic Tac1+ vagal neurons showed shared and distinct terminal morphology and targeting to airway smooth muscles, vasculature including lymphatics, and alveoli. Notably, a small population of vagal neurons that are Calb1+ preferentially innervate pulmonary neuroendocrine cells, a demonstrated airway sensor population. This atlas of lung innervating neurons serves as a foundation for understanding their function in lung.
Due to its pivotal role in autonomic networks, the vagus attracts continuous interest from both basic scientists and clinicians. In particular, recent advances in vagus nerve stimulation strategies and their application to pathological conditions beyond epilepsy provide a good opportunity to recall basic features of vagal peripheral and central anatomy. In addition to the “classical” vagal brainstem nuclei, i.e., dorsal motor nucleus, nucleus ambiguus and nucleus tractus solitarii, the spinal trigeminal and paratrigeminal nuclei come into play as targets of vagal afferents. On the other hand, the nucleus of the solitary tract receives and integrates not only visceral but also somatic afferents. Thus, the vagus system participates significantly in what may be defined as “somato-visceral interface”.
Résumé
Le glucose est la source principale d’énergie métabolique pour le cerveau. Prévenir le développement d’hypoglycémies est donc nécessaire pour la survie de l’individu. L’insuline et le glucagon, des hormones produites, respectivement, par les cellules ß et α des îlots de Langerhans et agissant sur le foie, les muscle et le tissu adipeux, jouent un rôle essentiel dans le maintien de l’homéostasie glucidique. Cependant, ces cellules endocrines, de même que leurs tissus cibles, sont sous le contrôle du système nerveux autonome. Celui-ci est régulé par le glucose, par le biais de neurones sensibles au glucose, activés soit par l’hyperglycémie, soit par l’hypoglycémie. Élucider les propriétés de ces neurones, les circuits neuronaux qu’ils forment, et les fonctions physiologiques qu’ils contrôlent, est un but important de la recherche sur les maladies métaboliques.
Vagal and spinal sensory endings in the wall of the hepatic portal and superior mesenteric veins (PMV) provide the brain with chemosensory information important for energy balance and other functions. To determine their medullary neuronal targets, we injected the transsynaptic anterograde viral tracer HSV-1 H129-772 (H129) into the PMV wall or left nodose ganglion (LNG) of male rats, followed by immunohistochemistry (IHC) and high-resolution imaging. We also determined the chemical phenotype of H129-infected neurons, and potential vagal and spinal axon terminal appositions in the dorsal motor nucleus of the vagus (DMX) and the nucleus of the solitary tract (NTS). PMV wall injections generated H129-infected neurons in both nodose ganglia and in thoracic dorsal root ganglia (DRGs). In the medulla, cholinergic preganglionic parasympathetic neurons in the DMX were virtually the only targets of chemosensory information from the PMV wall. H129-infected terminal appositions were identified on H129-infected somata and dendrites in the DMX, and on H129-infected DMX dendrites that extend into the NTS. Sensory transmission via vagal and possibly spinal routes from the PMV wall therefore reaches DMX neurons via axo-somatic appositions in the DMX and axo-dendritic appositions in the NTS. However, the dearth of H129-infected NTS neurons indicates that sensory information from the PMV wall terminates on DMX neurons without engaging NTS neurons. These previously underappreciated direct sensory routes into the DMX enable a vago-vagal and possibly spino-vagal reflexes that can directly influence visceral function.
Energy homeostasis requires precise measurement of the quantity and quality of ingested food. The vagus nerve innervates the gut and can detect diverse interoceptive cues, but the identity of the key sensory neurons and corresponding signals that regulate food intake remains unknown. Here, we use an approach for target-specific, single-cell RNA sequencing to generate a map of the vagal cell types that innervate the gastrointestinal tract. We show that unique molecular markers identify vagal neurons with distinct innervation patterns, sensory endings, and function. Surprisingly, we find that food intake is most sensitive to stimulation of mechanoreceptors in the intestine, whereas nutrient-activated mucosal afferents have no effect. Peripheral manipulations combined with central recordings reveal that intestinal mechanoreceptors, but not other cell types, potently and durably inhibit hunger-promoting AgRP neurons in the hypothalamus. These findings identify a key role for intestinal mechanoreceptors in the regulation of feeding.
With few effective treatments available, the global rise of metabolic diseases, including obesity, type 2 diabetes mellitus, and cardiovascular disease, seems unstoppable. Likely caused by an obesogenic environment interacting with genetic susceptibility, the pathophysiology of obesity and metabolic diseases is highly complex and involves crosstalk between many organs and systems, including the brain. The vagus nerve is in a key position to bidirectionally link several peripheral metabolic organs with the brain and is increasingly targeted for neuromodulation therapy to treat metabolic disease. Here, we review the basics of vagal functional anatomy and its implications for vagal neuromodulation therapies. We find that most existing vagal neuromodulation techniques either ignore or misinterpret the rich functional specificity of both vagal efferents and afferents as demonstrated by a large body of literature. This lack of specificity of manipulating vagal fibers is likely the reason for the relatively poor beneficial long‐term effects of such therapies. For these therapies to become more effective, rigorous validation of all physiological endpoints and optimization of stimulation parameters as well as electrode placements will be necessary. However, given the large number of function‐specific fibers in any vagal branch, genetically guided neuromodulation techniques are more likely to succeed.
The gut sensory vagus transmits a wide range of meal-related mechanical, chemical and gut peptide signals from gastrointestinal and hepatic tissues to the central nervous system at the level of the caudal brainstem. Results from studies using neurophysiological, behavioral physiological and metabolic approaches that challenge the integrity of this gut-brain axis support an important role for these gut signals in the negative feedback control of energy availability by limiting food intake during a meal. These experimental approaches have now been applied to identify important and unanticipated contributions of the vagal sensory gut-brain axis to the control of two additional effectors of overall energy balance: the feedback control of endogenous energy availability through hepatic glucose production and metabolism, and the control of energy expenditure through brown adipose tissue thermogenesis. Taken together, these studies reveal the pleiotropic influences of gut vagal meal-related signals on energy balance, and encourage experimental efforts aimed at understanding how the brainstem represents, organizes and coordinates gut vagal sensory signals with these three determinants of energy homeostasis.
Extra-adrenal paragangliomas are a group of neoplasms that derive from extra-adrenal sympathetic and parasympathetic paraganglia and related neuroendocrine cell systems in various organs. The biological behavior of these neoplasms ranges from a relatively benign course to frankly malignant behavior. Primary hepatic paraganglioma is a very rare neoplasm that is usually situated within the liver substance and is nonfunctional. The tumors can grow to a size exceeding 5 cm in diameter, may show a fibrous capsule, and can undergo cystic change. Hepatic paraganglioma can metastasize to locoregional lymph nodes. Histologically, the leading cell type is the large eosinophilic cell that forms paraganglia. These cells are strongly reactive for chromogranin A and synaptophysin. The complex stroma of paragangliomas contains S100 protein-positive sustentacular cells. Paraganglioma can also develop in extrahepatic bile ducts and cause biliary obstruction. Exceptionally, paraganglioma synchronously involves liver and bile ducts. The tumor was also observed in the gallbladder and hepatic ligaments.
Experiments on vagal afferents are often hampered by a lack of information about the precise distributions, as well as the specialized terminal architectures, of the peripheral processes of the neurons. As discussed elsewhere,³⁰ full analysis of an afferent system and its transduction mechanisms is impractical under such conditions. When the precise locations of the peripheral terminals are unknown, when the structural characteristics of the terminals are uninvestigated, and when any juxtapositions of the neurites with accessory cells or tissues are unknown, physiological investigations are significantly curtailed. This principle is clear in the abstract: Where would visual or auditory neuroscience be if such information about the retina and the cochlea were missing? Nonetheless, experimental approaches to the vagus frequently proceed under such handicaps.
In visceral organs of mammals, most noxious (painful) stimuli as well as innocuous stimuli are detected by spinal afferent neurons, whose cell bodies lie in dorsal root ganglia (DRG). One of the major unresolved questions is the location, morphology and neurochemistry of the nerve endings of spinal afferents that actually detect these stimuli in the viscera. In the upper gastrointestinal (GI)-tract, there have been many anterograde tracing studies of vagal afferent endings, but none on spinal afferent endings. Recently, we developed a technique which now provides selective labeling of only spinal afferents. We used this approach to identify spinal afferent nerve endings in the upper GI-tract of mice. Animals were anesthetized and injections of dextran-amine made into thoracic DRGs(T8-T12). Seven days post-surgery, mice were euthanized, the stomach and esophagus removed, fixed and stained for Calcitonin Gene Related Peptide (CGRP). Spinal afferent axons were identified which ramified extensively through many rows of myenteric ganglia and formed nerve endings in discrete anatomical layers. Most commonly, intraganglionic varicose endings (IGVEs) were identified in myenteric ganglia of the stomach and varicose "simple-type" endings in the circular muscle and mucosa. Less commonly, nerve endings were identified in internodal strands, blood vessels, submucosal ganglia and longitudinal muscle. In the esophagus, only IGVEs were identified in myenteric ganglia. No intraganglionic lamellar endings (IGLEs) were identified in stomach or esophagus. We present the first identification of spinal afferent endings in the upper GI-tract. Eight distinct types of spinal afferent endings were identified in the stomach; most were CGRP-immunoreactive. This article is protected by copyright. All rights reserved.
Communication between the brain and internal organs is likely utilized to maintain homeostasis in the body of an animal. Consistently, converging evidence suggests that the brain receives information not only from the outside but also from the inside of the body. The perception of information from the inside of the body is called “interoception,” which includes a sense of the condition of internal organs such as the lungs, the heart, and the organs of the digestive system, as well as a sense of internal milieu such as pH and temperature of body fluid. This perception and subsequent response of the brain provide one of the processes in brain–internal organ communications. Recent findings have shown that interoception plays more pivotal roles than have been thought. For example, it has been revealed that interoception of the condition of the liver plays pivotal roles in health and disease. The liver is the body’s center of metabolism, and the condition of this organ is likely to influence brain functions. The main purpose of this chapter is to understand how the brain and internal organs interact, mainly from the point of view of interoception. This is the reason why the liver is taken as an example of an internal organ and knowledge of the interaction between the brain and this organ is mainly described.
Recent progress in behavioral neuroscience indicates that peripheral neural and endocrine feedback signals are crucial controls for the initiation, maintenance, and termination of meals, and, therefore, for total food intake and the maintenance of body weight (Campfield and Smith 1990; Scharrer and Langhans 1988; Smith and Gibbs 1992; Blundell 1991). I review here evidence that glucagon is one such signal. Results from tests of administration of glucagon or glucagon antagonists suggest that, under many conditions, glucagon released from the pancreas during meals acts in the liver to initiate a neural signal that is conveyed by vagal afferents to the brain, where it contributes to the termination of the meal.
The vascular bed of the gastrointestinal tract is supplied by the celiac artery and the superior and the inferior mesenteric artery. Gastrointestinal blood flow is controlled by four classes of vasomotor neurons: sympathetic vasoconstrictor, parasympathetic vasodilator, enteric vasodilator, and primary afferent vasodilator neurons. The principal transmitters of sympathetic vasoconstrictor neurons are adenosine triphosphate, norepinephrine, and neuropeptide Y. Acetylcholine and vasoactive intestinal polypeptide are the primary transmitters of enteric and, possibly, parasympathetic vasodilator neurons, while calcitonin gene-related peptide figures prominently as a transmitter of primary afferent vasodilator neurons, although substance P, neurokinin A, and nitric oxide may also play a transmitter role. Appropriate regulation of gastrointestinal hemodynamics is achieved by a balanced interaction between the vasoconstrictor and vasodilator neurons. This is exemplified by the neural regulation of postprandial hyperemia, the neurogenic rise of mucosal blood flow in the face of pending injury, and the disturbed function of vasomotor neurons in inflammatory bowel disease.
The efferent innervation and some characteristics of nerve fibers of the liver lobule in the tree shrew, a primate, are described. Nerve endings on hepatocytes were encountered regularly and were determined to be efferent adrenergic nerves. Transmission electron microscopy revealed nerve endings and varicosities in close apposition to the hepatocytes adjacent to the connective tissue of the triads as well as within the liver lobule in the space of Disse. Fluorescence microscopy indicated the existence of adrenergic nerves with a similar distribution. Autoradiography of the avid uptake of exogenous [3H]norepinephrine indicated that all intralobular nerves are potentially norepinephrinergic (adrenergic). Chemical sympathectomy with 6-OH-dopamine resulted in the degeneration of all intralobular liver nerve fibers as revealed by fluorescence microscopy and electron microscopy. Substantial regeneration occurred after 60-90 days but was not completed by that time. Some nerves were also observed in close association with von Kupffer cells and endothelial cells. The functional significance of the efferent liver innervation is discussed.
Wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) was injected into nodose ganglia of rats. In the esophagus and cardia, dense networks of anterogradely labeled fibers and beaded terminal-like arborisations were observed around myenteric ganglia after combined histochemistry for HRP and acetylcholinesterase. The muscularis externa and interna proper were free of label except for a few traversing fibers. Submucosal and mucosal labeling was rather sparse except for the most oral part of the esophagus, where a dense mucosal innervation was found. Control experiments including WGA-HRP injections into the cervical vagus nerve, nodose ganglion injections after supranodose vagotomy, and anterograde [3H]leucine tracing from the nodose ganglion indicated that all labeled fibers in the esophagus and cardia originated from sensory neurons in the nodose ganglion. Electron microscopy revealed that labeled vagal sensory terminals related to myenteric ganglia were mostly large, mitochondria-rich profiles located predominantly on the surface of the ganglia. Specialized membrane contacts connected sensory terminals with other unlabeled profiles possibly derived from intrinsic neurons. The polarity of these contacts suggested the vagal sensory terminals to be presynaptic to intrinsic neurons of the myenteric ganglia. A hypothesis is formulated postulating a mechanoreceptive role for 'myenteric' vagal sensory terminals, providing both the brainstem (via the vagus nerve) and, by synaptic action upon intrinsic neurons, the myenteric plexus with information on tension and motility of the esophagus and cardia.
Recently, evidence has accumulated to support an osmoregulatory mechanism utilizing hepatic receptors as indicators of extracellular fluid concentration. The vagus nerve is the centripetal trunk carrying this information to the brain and the release of antidiuretic hormone (ADH) from hypothalamic nuclei is one regulatory response to the hepatic stimulus. Although this general mechanism is now accepted, the specific details of the connections between the vagal sensory system and the hypothalamic neuroendocrine cells responsible for ADH release have, until recently, been unclear. By utilizing physiological methods which allow the specific activation of hepatic sodium and osmoreceptors, single-neuron recording methods and sensitive retrograde anatomical tracing techniques, a pathway between the nucleus of the solitary tract and the supraoptic nucleus has been established which may be responsible for this hepato-neuroendocrine reflex. The application of these same methods of experimentation may lead to an improved understanding of other visceral afferent functions such as those related to feeding and drinking behavior.
Analyse de la regulation du taux de glucose dans le foie par les nerfs sympathiques. Effet de l'activation des nerfs sympathiques sur la liberation du glucose, activation des nerfs parasympathiques et utilisation du glucose
1.
An wachen Ratten mit einoperierten Gefkathetern werden gleichzeitig und mit gleicher Geschwindigkeit Wasser in die V. portae und 1,8% NaCl-Lsung in die V. cava inf. infundiert, um eine hepatogene Diurese zu induzieren; bei den Kontrollversuchen werden die Infusionsorte vertauscht. (Sogenannte Doppelinfusionstechnik zur Beschrnkung der osmotischen nderungen auf den Portalkreislauf.)
2.
Der Leberast des rechten Vagus wird am wachen Tier mit Hilfe eines feinen Kunststoffadens durchgerissen, der vorher operativ um den Nerven gelegt wurde.
3.
Anschlieen wird die Doppelinfusion wiederholt. Die typische Antwort einer hepatogenen Diurese ist nicht mehr auslsbar.
4.
Dieses Resultat gibt einen indirekten Beweis dafr, da der afferente Weg der hepatischen Osmoreceptoren im N. vagus verluft.
1.
In unanaesthetized rats with indwelling catheters simultaneous infusions of water into the portal vein and 1.8% NaCl solution into the caval vein are performed at equal rates to induce a hepatogenous diuresis. In the control experiments the infusion sites are reversed. (Double infusion technique in order to confine osmotic changes to the hepatic portal circulation.)
2.
In the conscious animal the hepatic branch of the right vagus is cut by pulling a loop of a fine rayon thread previously placed around the nerve.
3.
Now the double infusion is repeated but fails to provoke the typical hepatogenous diuretic responses.
4.
This result gives indirect proof that the afferent pathway of hepatic osmoreceptors is to be found in the vagal nerve.
Several reports have suggested that the mammalian liver contains neural receptors, innervated by the vagus nerve, that monitor the sodium concentration and osmolarity of the portal circulation. These reports have been concerned primarily with either the neurophysiological identification of these receptors or their role in the short term control of urine output. Inasmuch as relatively little is known about the role of these receptors to consummatory behavior, we investigated the effects of hepatic vagotomy in rats on sodium intake as well as on sodium output. Hepatic vagotomized (HV) rats drank less NaCl solution (0.03, 0.1, 0.3M) in 24 hr during a two-bottle test with water than sham operated rats. Comparable differences in the intakes of either water, KCl or glucose solutions were not found. The two groups of rats did not differ in their intakes of water or 0.3M NaCl after an injection of either an osmotic load (IP, 2 M NaCl, 1% BW), deoxycorticosterone acetate (SC, 5 mg) along with furosemide (SC, 10 mg), or after 10 days of sodium deprivation. Urinary sodium output was reduced in HV rats during sodium deprivation but not when the rats had adequate levels of sodium in their diet. Because circadian patterns of water and food intake as well as body weight growth of hepatic vagotomized rats were similar to those of control rats, general malaise due to surgery and generalized deficits in motivation were ruled out as explanations for the depressed daily drinking of NaCl solutions. These findings support the existence of hepatic sodium receptors and their possible involvement to the control of sodium regulation.
In a recent study (Skaaring and Bierring, 1976) we found cholinesterase-positive nerve-like structures in the lobules of rat liver, and scanning electron microscopy revealed cords having a distribution pattern similar to that of the cholinesterase-positive structures. To obtain further evidence for an intralobular nerve supply the methods of cobalt and Procion Yellow nerve staining (Stretton and Kravitz, 1968; Iles and Mulloney, 1971; Pitman, Tweedle and Cohen, 1972) were adapted, iontophoretic introduction of the dyes being attempted through cut axonal ends in the surface of small excised blocks of rat liver.
In this paper we critically review anatomic, electrophysiological, physiological, and behavioral evidence for neural receptors in the liver. Several lines of evidence suggest that the afferent innervation of the liver may be substantial, although few anatomic studies have directly addressed the question of an hepatic sensory supply. On the other hand, there is convincing functional evidence for a variety of hepatic sensory receptors. Hepatic osmo-, ion, and baroreceptors, sensitive to changes in the osmolarity and electrolyte concentrations in blood and to variations in portal venous pressure, modulate diuresis and natriuresis. Metabolic receptors, for which the adequate stimuli have not yet been specified, influence feeding behavior and gastric acid secretion in response to alterations in hepatic metabolism associated with changes in the supply of metabolic fuels. Directions for future research are suggested and general conclusions about the role of hepatic receptors in homeostasis are discussed.
1. The terminal microcirculation in the transilluminated ventral margin of the rat liver was observed and recorded by a video-microscope system. The volumetric flow rate in a liver sinusoid was calculated from the observed diameter of the sinusoid and the intra-sinusoid erythrocyte flow velocity. 2. The topographic distribution of liver sinusoids within an arbitrary boundary of a microscopic field of terminal liver microcirculation was observed and the total inflow and outflow in the field were determined. 3. Both vagus nerves at the lower end of the oesophagus were stimulated at supramaximal voltage. Vagal stimulation dilated the calibre of liver sinusoids and paradoxically diminished the erythrocyte flow velocity in each individual liver sinusoid, but the total volumetric flows in a microscopic field remained unchanged. 4. Vagal stimulation also increased the number of liver sinusoids in a microscopic field by opening previously closed liver sinusoids. This recruitment contributed two-thirds of the total increase of the sinusoidal capacity while the other one third was the result of distension of existing liver sinusoids.
Sympathetic nerves to the intact liver of the cat were selectively destroyed by injection of 6-hydroxydopamine into the portal vein 1 week prior to the experiment. Glucose output was calculated from the product of the arterial-venous glucose difference and hepatic blood flow. Hepatic blood flow was monitored by an electromagnetic flowmeter using a hepatic venous long-circuit. Stimulation of the parasympathetic nerves isolated from around the common hepatic artery produced a rapid reduction in hepatic glucose output to one-quarter of control levels by 2 min and to zero by 10 min of nerve stimulation. The data show that hepatic glucose balance is readily influenced by the hepatic parasympathetic nerves.
Rat liver was perfused with Ringer solution through the portal vein by use of a perfusion system which was designed to switch from standard Ringer solution to hypertonic or hypotonic Ringer solution. Neural responses to the osmotic change in the perfusion solutions were analyzed. They showed that two different types of osmosensitive afferent fibers exist in the hepatic vagus; one is characterized by increasing the frequency of spike discharges responding to higher osmotic pressure, while the other shows the same response to lower osmotic pressure. Behavioral changes caused by hepatic vagotomy were also observed. Though no differences could be detected in routine behavior (e.g., daily intakes of food and water, body-weight increase) between the vagotomized and the sham-operated rats, the former lost the ability to adjust urine concentration immediately in response to osmotic changes in the internal environment. These results provide evidence for the hypothesis that hepatic osmoreceptors exist in the rat.
The abdominal vagal paraganglia of the rat consist of small groups of cells, interspersed by blood vessels and nerve bundles and lying close to, or within, the vagus nerve or its branches. Each cell group consists of 2–10 Type I cells incompletely invested by 1–3 satellite cells. Type I cells are characterised by the presence of numerous dense-cored vesicles in their cytoplasm and may exhibit ‘synaptic’-like contact with each other.
Small efferent nerve endings make synaptic contacts with Type I cells. Larger cup-shaped afferent nerve endings also make synaptic contacts of two kinds with Type I cells. Nerve-nerve synapses are often seen within or close to paraganglia.
Attention is drawn to the close similarity of fine structure of abdominal vagal paraganglia, carotid body and small intensely fluorescent cells of the superior cervical ganglion in rats. Possible functional implications of this morphological similarity are discussed.
Intraarterial injection into the carotid artery or intravenous injection of glucose caused a decrease in efferent discharge rate of the adrenal nerve and an increase in discharge rate of the pancreatic branch of the vagus nerve. Decrease in blood sugar level following administration of insulin increased adrenal nerve activity and caused a decrease in the activity of the pancreatic branch of the vagus nerve. Administration of 2-DG resulted in an increase in adrenal nerve activity and a decrease in pancreatic nerve activity. Glucose also caused a decrease in discharge rate of afferent nerve fibers from glucoreceptors in the liver. The results suggest the existence of a nervous regulatory mechanism of blood sugar levels.
Although the gastric tension receptor has been characterized behaviorally and electrophysiologically quite well, its location and structure remains elusive. Therefore, the vagal afferents to the rat fundus (forestomach or nonglandular stomach) were anterogradely labeled in vivo with injections of the carbocyanine dye Dil into the nodose ganglia, and the nerves and ganglia of the enteric nervous system were labeled in toto with intraperitoneal Fluorogold injection. Dissected layers and cryostat cross sections of the fundic wall were mounted in glycerin and analyzed by means of conventional and laser scanning confocal microscopy.
Particularly in the longitudinal, and to a lesser extent in the circular, smooth muscle layers, Dil‐labeled fibers and terminals were abundant. These processes, which originated from fibers coursing through the myenteric ganglia and connectives, entered either muscle coat and then ran parallel to the respective muscle fibers, often for several millimeters. They ran in close association with the Fluorogold‐labeled network of interstitial cells of Cajal, upon which they appeared to form multiple spiny appositions or varicosities.
In the myenteric plexus, two different types of afferent vagal structures were observed. Up to 300 highly arborizing endings forming dense accumulations of small puncta similar to the esophageal intraganglionic laminar endings (Rodrigo et al., '75 Acta Anat. 92 :79–100) were found in the fundic wall ipsilateral to the injected nodose ganglion. They often covered small clusters of myenteric neurons or even single isolated ganglion cells (mean = 5.8 neurons) and tended to extend throughout the neuropil of the ganglia. In a second pattern, fine varicose fibers with less profuse arborizations innervated mainly the central regions of myenteric ganglia.
Camera lucida analyses established that single vagal afferent fibers had separate collaterals in both a smooth muscle layer and the myenteric ganglia. Finally, Dil‐labeled afferent vagal fibers were also found in the submucosa and mucosa. Control experiments in rats with supranodose vagotomy as well as rats with Dil injections directly in the distal cervical vagus ruled out the possibility of colabeling of efferent fibers of passage. In triple labeling experiments, in conjunction with Dil labeling of afferents and Fluorogold labeling of enteric neurons, the carbocyanine dye DiA was injected into the dorsal motor nucleus of the vagus to anterogradely label the efferent vagal fibers and terminals. The different distributions and morphological characteristics of the vagal afferents and efferents could be simultaneously compared. In some instances the same myenteric ganglion was apparently innervated by an afferent laminar ending and an efferent terminal.
Given their locations, distributions, and morphology, we suggest that the intramuscular afferents are gastric tension receptors. It seems probable that their intramuscular collaterals associated with the interstitial cell network are responsible for detecting tension of the gut wall. The function of the highly arborizing intramyenteric vagal afferent endings is not known. We suggest that they are collaterals of intramuscular tension receptors with either an efferent function, to serve local reflex action, or a chemoreceptive function to detect blood borne factors (such as CCK) or messengers released locally by myenteric ganglia. © 1992 Wiley‐LIss, Inc.
The vagal innervation of the different layers of the rat gastrointestinal wall was identified with the fluorescent carbocyanine dye Dil, injected into the dorsal motor nucleus of the vagus (dmnX). Multiple, bilateral injections were used to label all dmnX preganglionic motoneurons, and as a consequence, most of the vagal primary afferents that terminate in the adjacent nucleus of the solitary tract (nts) were also retrogradely and transganglionically labeled. With Fluorogold used to label the enteric nervous system completely and specifically, the Dil-labeled vagal profiles could be visualized and quantified in their anatomical relation to the neurons of the myenteric and submucous ganglia. In the myenteric plexus, vagal fibers and terminals were found throughout the gastrointestinal tract as far caudal as the descending colon, but there was a general decreasing proximodistal gradient in the density of vagal innervation. All parts of the gastric myenteric plexus (fundus, corpus, antrum), as well as the proximal duodenum, were extremely densely innervated, with vagal fibers and terminals in virtually every ganglion and connective. Further caudally, both the percentage of innervated myenteric ganglia and the average density of label within the ganglia rapidly decreased, with the exception of the cecum and proximal colon, where up to 65% of the ganglia were innervated. In the gastric and duodenal submucosa very few and in the mucosa no vagal fibers and terminals were found. With both normal epifluorescence and laser scanning confocal microscopy, highly varicose or beaded terminal structures of various size and geometry could be identified. The Dil injections, which impregnated the dmnX as well as the adjacent nts, resulted in retrograde and anterograde labeling of all the previously reported forebrain connections with the dorsal vagal complex. We conclude that the myenteric plexus is the primary target of vagal innervation throughout the gastrointestinal tract, and that its innervation is more complete than previously assumed. In contrast, vagal afferent (and efferent) innervation of mucosa and submucosa seems conspicuously sparse or absent. Furthermore, the use of more focal injections of Dil offers the prospect to simultaneously identify specific subsets of vagal preganglionics and their central nervous inputs.
Intraperitoneal injection of an aqueous solution of the fluorescent marker Fluoro-gold provides an extremely simple, reliable, and versatile protocol for selectively and completely labeling the enteric nervous system and other autonomic ganglia of the rat.
Vagal efferent innervation of the pancreas was labeled by anterograde transport of Dil injected into the dorsal motor nucleus (dmnX). While over the entire organ only 19 +/- 3 (or 8 +/- 1%) of the 231 +/- 17 interlobular ganglia received Dil-labeled vagal fibers and terminals, the proximal duodenal lobe (or head) was significantly more densely innervated. Laser scanning confocal microscopy revealed further morphological details of the vagal terminals and their target ganglion cells. No vagal fibers or terminals were found in islets and acinar tissue.
Neuropeptide Y-containing nerve fibers were identified by light and electron microscopic immunocytochemistry in the human liver, gallbladder, and pancreas. In the liver, neuropeptide Y-containing nerve fibers were distributed richly in Glisson's sheath and were prominent around the walls of the interlobular vein, interlobular hepatic artery, and hepatic bile duct. The fibers also formed a dense network surrounding the hepatocytes. The nerve terminals were found close to the endothelial cells of blood vessels, as well as being distributed in Disse's space, where they appeared to terminate. Occasionally these terminals contacted directly the membrane of a hepatocyte. In the gallbladder, neuropeptide Y fibers were found in each layer, with an especially dense network in the lamina propria. The fibers also ran close to the epithelium and parallel to the muscle bundles. Blood vessels throughout the gallbladder were well supplied with such nerve fibers. In the pancreas, neuropeptide Y fibers were found mainly near blood vessels and partly in gaps between exocrine glands, seeming to terminate on certain endocrine cells. Nerve terminals were located in the vascular walls and adjacent to the surface of exocrine acinar cells. These studies provide a basis for correlating the neuropeptide Y distribution with pharmacological and physiological studies in humans.
We determined the site at which the fructose analogue 2,5-anhydro-D-mannitol (2,5-AM) acts to increase food intake in rats. Rats began eating sooner and ate more food during hepatic portal than during jugular infusions of 2,5-AM (50, 100, or 150 mg/h). After rats were intubated with 2,5-[14C]AM (1.15 microCi in 200 mg/kg), significant quantities of radioactivity were found in liver but not in brain. Hepatic vagotomy prevented the eating response to 200 mg/kg 2,5-AM without altering the effect of the analogue on plasma fuels. These results indicate that low doses of 2,5-AM act in the liver to increase food intake and suggest that the signal for feeding generated in the liver is transmitted to the brain through the hepatic vagus nerve. Taken together, this work provides the strongest evidence to date that a signal initiating feeding behavior originates in the liver.
The gastrointestinal territories innervated by the gastric, celiac, and hepatic abdominal vagi were identified in rats with selective branch vagotomies by means of 1) anterograde tracing with the carbocyanine dye DiI injected into the dorsal motor nucleus and 2) measurement of cervical vagal stimulation-induced motility responses throughout the gut axis. Presence of DiI-labeled vagal terminals in the myenteric plexus and evoked motility responses were well correlated across the sampled gastrointestinal (GI) sites. In animals with only the two gastric branches intact, the entire stomach and the most proximal duodenum showed significant motility responses and were densely innervated, having DiI-labeled vagal terminals in almost every ganglion. The hepatic branch was found to primarily innervate the duodenum, with minor projections to the distal antral stomach and the intestines. The two celiac branches were found to almost exclusively innervate the jejunum, ileum, cecum and entire colon, and, together with the other vagal branches, the duodenum. Therefore, while there is some degree of specific innervation by the abdominal vagal branches of the oral-to-anal gut axis, which could be called "viscerotopic," the considerably overlapping innervation of the duodenum does not satisfy a viscerotopy criterion and needs further functional analysis.
The brainstem location and peripheral course of the vagal preganglionic fibers that stimulate gastric acid secretion were identified using electrical stimulation combined with retrograde (True Blue; Dr. K. G. Illing, Gross Umstadt, Germany) and anterograde (Dil; Molecular Probes) fluorescent neural tracers in rats with various selective vagotomies. Animals with only one or both gastric branch(es) spared had normal, large gastric acid responses to electrical stimulation of the ipsilateral cervical vagus and showed an abundance of Dil-labeled vagal fibers and terminals in the gastric myenteric plexus. Rats with only the unpaired hepatic branch spared had a much smaller but significant gastric acid response and a few labeled vagal profiles in the antral region of the stomach. In contrast, rats with only one or both celiac branch(es) intact had neither a gastric acid response, nor evidence for Dil transport to the stomach. Retrograde transport of True Blue through the spared vagal axons to the brainstem indicated that the cell bodies of the preganglionics that send their axons through the acid-positive gastric and hepatic branches occupy the medial longitudinal columnar subnuclei of the dorsal motor nucleus. It is concluded that besides the long-recognized gastric branches, which are the major access route to the parietal cells, the hepatic branch contains a small number of fibers that most likely reach the antrum through the right gastroepiploic artery along the greater curvature and/or the right gastric artery.
We reported the existence of vagal arginine sensors in the liver which modulate arginine-induced pancreatic hormone secretion. The present study was carried out to examine the possible existence of other amino acid sensors such as L-alanine and L-leucine in the hepato-portal system in rats using an electrophysiological approach. Afferent discharges were recorded from fine filaments dissected from the peripheral cut end of the hepatic branch of the vagus nerve. Administration of 0.1, 1, and 10 mM L-alanine and L-leucine solution (0.1 ml) into the portal vein caused an increase in the discharge rate of hepatic vagal afferents in a dose-dependent manner. The results suggest the existence of vagal amino acid sensors which are sensitive to alanine and leucine in the hepato-portal system.
The possible presence of neural sprouting in the afferent neurons of regenerated rat liver after hepatectomy was investigated by retrograde transport of horseradish peroxidase. This experiment was carried out to see if the increase in hepatic parenchyma could provide an adequate stimulus for the sprouting process. The study was limited to the vagal afferents, particularly the left ones, because they are the principal contributors to hepatic afferent innervation in the rat. The results show that neural sprouting does not occur in regenerated rat liver after 3 weeks. In fact, the number of intensely labeled neurons in the left nodose ganglia of hepatectomized rats was significantly smaller than in controls. This could be due to a lessened availability of horseradish peroxidase to nerve terminals because of the increased non-innervated hepatic mass. There was no difference between right nodose ganglia neurons in hepatectomized and control animals. This could be a consequence of their possible distribution in hepatic areas not involved in the regenerative process.
The present study provides a LM and EM inventory of the fibers of the rat abdominal vagus, including dorsal and ventral trunks and the five primary branches. Whole mounts (n = 15) were prepared to characterize the branching patterns. A set of EM samples consisting of both trunks and all branches (i.e. dorsal and ventral gastric, dorsal and accessory celiac, and hepatic) were then obtained from each of six additional animals. A complete cross-sectional montage (x 10000) was prepared from each sample. All axons were counted, and >10% of them were evaluated morphometrically. The means of unmyelinated axon diameters for each of the five branches were similar (0.75–0.83 μm). However, the shapes of the fiber size distributions, as summarized by their skew coefficients, revealed that the two gastric branches differed significantly from the two celiac branches; furthermore, the hepatic size distribution differed from all others. Most of the myelinated fibers (85%) in all branches were The structural profiles observed (i.e. unmyelinated and myelinated fibers size distributions, presence of extrinsic fascicles, glomus tissue content, etc.) differentiate the vagal branches into three morphologically distinct sets: a gastric pair, a celiac pair, and a hepatic branch. The fiber counts, when considered with observations of the numbers of efferents and adventitial fibers in the nerve, suggest that the percentage of efferent fibers is much higher than in all the widely accepted estimates found in the literature: efferent fibers may represent over a quarter of the total number of fibers.
Selective injection of horseradish peroxidase (HRP) into the left and median lobes (LM) and into the right and caudate (RC) lobes of the liver is followed by labeling of neuronal somata in the right and left nodose ganglia. The size distribution of the labeled neuronal population shows that the afferent neurons from the two parts of the liver can be grouped in two corresponding classes; a third class is apparent following injection into the LM lobes. Small neurons are more numerous after injection into the LM lobes, whereas large ones are labeled in the left nodose ganglion after injection into the RC lobes. It is suggested that the two parts of the liver may have a different functional role in conveying afferent signals.
The contribution of the vagus nerves to the innervation of the liver has been studied with the cobaltous chloride impregnation method. With this method we have demonstrated that the fiber plexus in the rat hepatic parenchyma, that we had previously described and stained for acetylcholinesterase, is of a nervous nature and of vagal origin. Our results show that branches from the vagus spread abundantly with the connective tissue at the capsule. From this peripheral location, the fibres expand deeply through the parenchyma in close contact with the hepatocytes towards the central veins. Other branches run with the interlobular connective tissue, distributing to the portal veins, hepatic arteries and biliary ducts. They also have lateral branches which penetrate into the parenchyma.
Calcitonin gene-related peptide immunoreactivity was localized immunohistochemically in nerve fibers innervating the biliary pathway and liver of the guinea-pig. Immunoreactive fibers are present in all layers of the gallbladder and biliary tract and are particularly numerous around blood vessels. In the liver, immunoreactive processes are usually restricted to the interlobular space and porta hepatis, and only a few, very thin, beaded processes were observed in the hepatic parenchyma. A rich innervation is also associated with the vena portae. Positive ganglion cell bodies were not visualized within the ganglionated plexus of the biliary system, whereas they were found in the myenteric and submucosal plexus in the cranial portion of the duodenum corresponding to the sphincter of Oddi. The vast majority, if not all, of calcitonin gene-related peptide-immunoreactive fibers contain substance P immunoreactivity; however, there are some substance P-containing fibers lacking calcitonin gene-related peptide immunoreactivity. The lack of co-occurrence of calcitonin gene-related peptide and substance P immunoreactivities in intrinsic ganglion cells suggests that these two peptides are coexpressed in the extrinsic component of the innervation of the hepatobiliary system.
Anterograde tracing from the nodose ganglion with wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) was utilized to investigate vagal afferent innervation of the rat pancreas. Labelled afferent fibers were consistently detected in islets in all animals. Only about 10% of islets were labelled even in best cases. In only 5 out of 16 rats, acini and excretory ducts received afferent innervation. Injections into the right nodose ganglion resulted in labelling preferentially within the splenic lobe, whereas injections into the left ganglion labelled fibers predominantly in the duodenal lobe. These results point to a non-random distribution of vagal afferent fibers within the pancreas, and suggest a role for these afferents in the regulation of endocrine pancreatic function.
Although a well-developed plexus of nerves and ganglia is known to be present in the wall of the gallbladder, little has previously been learned about the function or organization of this innervation. The current study was undertaken in order to evaluate the hypothesis that the ganglionated plexus of the gallbladder is analogous to elements of the enteric nervous system (ENS). The ganglionated plexus of the gallbladder was found to resemble closely the submucosal plexus of the small intestine in its organization into two irregular anastomosing and interwoven networks of ganglia, in the numbers of neurons per ganglion, and in the manifestation of histochemically demonstrable acetylcholinesterase activity in virtually all ganglion cells. In common with enteric ganglia, laminin immunoreactivity was observed to be excluded from the interiors of gallbladder ganglia, which were surrounded by a periganglionic laminin-immunoreactive sheath. As in the submucosal plexus, intrinsic substance P-, vasoactive intestinal polypeptide (VIP)-, and neuropeptide Y (NPY)-immunoreactive neurons were seen in the ganglionated plexus of the gallbladder. Extrinsic nerves in the gallbladder that degenerated following chemical sympathectomy with 6-hydroxydopamine (6-OHDA), and which contained NPY, tyrosine hydroxylase (TH), and dopamine-beta-hydroxylase (DBH) immunoreactivities, formed a perivascular plexus closely associated with blood vessels. Endogenous catecholamines could also be demonstrated in these perivascular nerves by aldehyde-induced histofluorescence. In addition to perivascular nerves, paravascular nerve bundles were observed that were loosely associated with vessels, did not degenerate following administration of 6-OHDA, and contained NPY immunoreactivity. Other paravascular nerves, probably visceral sensory axons, coexpressed substance P and calcitonin-gene-related peptide (CGRP) immunoreactivities. The ganglionated plexus of the gallbladder resembled enteric ganglia in having intrinsic 5-hydroxytryptamine (5-HT)-immunoreactive cells and highly varicose nerve fibers. The 5-HT-immunoreactive gallbladder axons were, like those of the gut, resistant to 6-OHDA, and separate from fibers that expressed TH immunoreactivity. Differences between the ganglionated plexus of the gallbladder and enteric ganglia of the small intestine included in the gallbladder are 1) the presence of TH-immunoreactive cells that contain an endogenous catecholamine, but not DBH; 2) DBH-immunoreactive neurons, some of which coexpress substance P immunoreactivity, but which contain neither a catecholamine nor TH immunoreactivity; 3) an apparent absence of CGRP-immunoreactive cell bodies.(ABSTRACT TRUNCATED AT 400 WORDS)
Paraganglia are associated with every branch of the rat vagus nerve except the pharyngeal branch. Some of the paraganglia closely resemble the glomus caroticum, whereas others appear like small, intensely fluorescent (SIF) cells of autonomic ganglia. The paraganglionic cells of SIF cell-like bodies (SLB) store catecholamines (the most abundant is probably noradrenaline) and in some cases neurotensin. The innervation pattern of SLB is variable and their physiological role remains unclear.
Paraganglionic cells of glomus-like bodies (GLB) predominantly store dopamine and probably also to a lesser extent noradrenaline. These putative chemoreceptor organs receive sensory innervation from nodose ganglion neurons as revealed by degeneration experiments and by anterograde neuronal tracing. Substance P- and calcitonin gene-related peptide-immunoreactive fibres seen in the region of vascular entry into the GLB may account for some of these sensory fibres, but the peptide/classical transmitter stored in sensory terminals synapsing on paraganglionic cells is unknown. Ultrastructural immunocytochemistry revealed vasoactive intestinal polypeptide (VIP)-immunoreactive fibres lying in the interstitial space between paraganglionic cells and large capillaries. These fibres may originate from VIP-immunoreactive neurons, being frequently attached to GLB. The major difference between GLB and the glomus caroticum concerns their blood supply and related innervation: Arteries and arterioles do not penetrate into GLB and, accordingly, noradrenaline- and neuropeptide Y-containing nerve fibres are lacking within GLB. This peculiar arrangement of paraganglionic parenchyma and arterial blood supply may be one of the reasons for the different physiological properties of vagal and carotid arterial chemoreceptors.
The distribution, possible origins and fine structure of neuropeptide Y (NPY)-containing nerve fibers in the rat liver were investigated by immunohistochemistry, nerve transection and immunoelectron microscopy. Light-microscopic immunohistochemistry showed NPY fibers forming a complex network in and around the walls (tunica adventitia and tunica media) of hepatic vessels. They were also closely associated with interlobular bile ducts. The NPY fibers in the liver were almost completely eliminated by transection of the greater splanchnic nerves just distal to the celiac ganglion. Transection of the greater splanchnic nerves just proximal to the celiac ganglion resulted in a marked decrease in NPY fibers, but a significant number remained intact. Under electron microscopy. NPY terminals without a covering of glial processes were seen not only in proximity to smooth muscle cells within the tunica media of hepatic vessels but also in the subendothelial areas of the tunica intima. Some NPY axon terminals devoid of glial ensheathment were located close to the basal lamina of interlobular bile ducts. Occasionally, single axon terminals with NPY were found in the vicinity of or in contact with hepatic cells. There was a small number of NPY fibers that had lost their glial sheaths while running toward lymphatic capillaries. These findings suggest that hepatic NPY arises from the celiac ganglion and paravertebral sympathetic ganglia, and that it is involved in more complex physiological processes than the previously described neuropeptides in the liver, which are localized exclusively to hepatic vessel walls.
Innervation of the intrahepatic biliary tree was examined in human normal livers, extrahepatic biliary obstruction and hepatolithiasis. Nerve fibers were immunohistochemically identified on formalin-fixed and paraffin-embedded sections by antibodies to S-100 protein (S-100) and neuron specific enolase (NSE). S-100 and NSE-immunoreactive nerve fibers were present in the walls of intrahepatic large, medium-sized and septal bile ducts as well as in peribiliary glands. Some nerve fibers were in close contact with epithelia of bile ducts and peribiliary glands. Serial section observations showed that the nerve fibers arising from nerve bundles approached, and came to lie in close contact with epithelia of the bile ducts and peribiliary glands. Nerve fibers were sparse around the interlobular bile ducts and bile ductules. These immunoreactive nerve fibers of the intrahepatic biliary tree were rather sparse in normal livers, intermediate in extrahepatic biliary obstruction and dense in hepatolithiasis. These findings suggest that intrahepatic bile ducts and peribiliary glands are innervated and biliary functions are regulated in part by these nerve fibers. Increased nerve fibers may have altered effects on biliary functions in hepatolithiasis.
By utilizing a horseradish peroxidase (HRP) axonal flow technique for retrograde tracing, and electron microscopic techniques, attempts were made in adult Wistar rats to elucidate the central organization of the parasympathetic hepatic innervation. The results were as follows: 1) cells of origin directly innervating the liver were located mainly in the left dorsal motor nucleus of the vagus (DMV). Their mean number was 175 with a standard deviation of 14 (n = 14). 2) the hepatic branch of the vagus consisted mostly of unmyelinated nerve fibers. The mean number of unmyelinated fibers was 2072 with a standard of 137, while that of myelinated fibers was only 13 with a standard deviation of 4 (n = 10). 3) nerve terminals were found in two different regions; one adjacent to the vasculature in the portal area, and the other in the space of Disse among hepatic parenchymal cells. These nerve terminals contained several synaptic vesicles different both in their form and size.
To test the hypothesis that the sodium pump of hepatocytes is involved in the control of food intake, we investigated the effect of ouabain, an inhibitor of the sodium pump, on feeding in intact and hepatic vagotomized rats. Ouabain (2 mg/kg b.wt.), injected intraperitoneally during the bright phase of the lighting cycle, stimulated feeding in intact and sham-vagotomized rats, but not in hepatic vagotomized rats. Atropinization did not block ouabain's hyperphagic effect. Ouabain did not affect portal blood glucose level. Rats started to eat sooner than normal when ouabain was injected, while their meal size and duration was unchanged. The results are consistent with a role of the sodium pump of hepatocytes in the control of food intake.
To test the possibility that vagally mediated signals derived from hepatic fatty acid oxidation affect feeding, we investigated the influence of selective hepatic vagotomy on the acute hyperphagic effect of the fatty acid oxidation inhibitor 2-mercaptoacetate (MA) in rats kept on a medium fat diet (18% fat). An i.p. injection of MA (400 mumol/kg b.wt.) stimulated feeding in sham-vagotomized rats clearly more than in vagotomized rats. A high dose of MA (800 mumol/kg b.wt.) initially increased food intake in sham-vagotomized but not in vagotomized rats and later decreased food intake similarly in both surgical groups. MA retained its potency to stimulate feeding after an i.p. injection of atropine methylnitrate (5 mg/kg b.wt.). These results indicate that hepatic fatty acid oxidation provides a satiety signal that is mediated by vagal afferents.
To identify the distribution of central preganglionics associated with each branch of the subdiaphragmatic vagus, the fluorescent tracer True Blue (TB) was administered intraperitoneally to rats with 4 out of 5 branches cauterized, and then, after 72 h, the animals were sacrificed for histological analysis. Each vagal branch contained the axons of a topographically distinct column of cells within the dorsal motor nucleus of the vagus (DMN). The columns representing the 4 branches with the largest numbers of efferents are organized as paired, bilaterally symmetrical, longitudinal distributions on either side of the medulla. Each DMN side contains a column occupying the medial two-thirds or more of the nucleus and corresponding to one of the gastric branches (left DMN, anterior gastric; right DMN, posterior gastric). Also on each side, the lateral pole of the DMN consists of a coherent cell column corresponding to one of the celiac branches (left DMN, accessory celiac; right DMN, celiac). The fifth branch, the hepatic, is represented by a limited number of somata forming a diffuse column largely coextensive with that representing the anterior gastric branch. At some levels of the DMN, the columns overlap. Labeled cells observed in the reticular formation were correlated in number, left-right ratios and response to vagotomy with those in the DMN, which suggests that they are displaced cells of the nucleus. Distributions of labeled cells in the nucleus ambiguus and the retrofacial nucleus were not tightly correlated with those of the DMN. An analysis of cell counts obtained for each of the individual branches suggests that vagal axons do not generally send collaterals through more than one branch.
1.1. Electrical stimulation of the vagus nerve of rabbits caused a marked increase in the activity of liver glycogen synthetase (UDPG:α-1,4-glucan α-4-glucosyltransferase, EC 2.4.1.11). Stimulation of the splanchnic nerve resulted in a decrease in enzymic activity.2.2. On vagal stimulation the glycogen synthetase activity increased to nearly the maximal level within 10 min, and its change was closely correlated with the time course of decrease in the UDPG and Glc-6-P levels in the liver.3.3. Increase in enzymic activity on vagal stimulation is not due to a change in the total activity of the enzyme, but appears to be due to transformation of the enzyme to a form with higher affinities for UDPG and Glc-6-P. Vagal stimulation for 10 min lowered the apparent Km for UDPG from 2.9 · 10−3 to 4.8 · 10−4 M (determined in the presence of a physiological concentration of Glc-6-P, without causing a significant change in vmax. The apparent Ka for Glc-6-P was also lowered from 7.1 · 10−4 to 2.2 · 10−4 M by vagal stimulation. These effects of vagal stimulation were not affected by removal of the pancreas.4.4. Splanchnic-nerve stimulation caused a slight increase in the apparent Km for UDPG of glycogen synthetase, without affecting the value of vmax.5.5. These results indicate that vagal stimulation results in activation of glycogen synthetase, whereas splanchnic-nerve stimulation causes inactivation of the enzyme. The possible mechanisms and physiological significance for the actions of these two autonomic nerves on the regulation of glycogen synthetase and glycogen metabolism in the liver are discussed.
Typical vagal paraganglia of Syrian hamsters are encapsulated in connective tissue and consist of groups of epithelial cells. Ganglion cells, a few fenestrated capillaries, and bundles of unmyelinated nerve fibers are intermingled among the parenchymal cells. The parenchymal cells are of two types: chief or paraganglion and sustentacular or supporting cells. The processes of the supporting cells partly or completely surround the paraganglion cells. In addition to the nucleus, Golgi complex, mitochondria, parallel-arrayed granular endoplasmic reticulum, and lipofuscin pigment, the chief cells are characterized by the presence of numerous membrane-bound, electron opaque granules. After an injection of 3H-dopa, labelings were concentrated over the chief cells and were associated predominantly with the granules. Following glutaraldehyde-dichromate treatment the granules gave a positive reaction for unsubstituted amines. These results suggest that the chief cells contain catecholamines in the electron opaque granules.
Several neuroanatomical and neurophysiological experiments suggest that the hepatic portal vein is not only richly innervated with sympathetic efferents, but also it is an important source of afferent information. By combining retrograde tracing (with True Blue as a marker) and immunological techniques, the cell bodies for the substance P-containing nerves that surround the portal vein and the hepatic artery of the rat have been localized to the spinal sensory ganglia (T8-T13). Since dorsal root rhizotomy abolished all substance P immunoreactive material from nerve fibres that surround these blood vessels, and since no double-labelled cells were detected in the nodose ganglia, an exclusive spinal origin for the substance P-containing sensory nerves is suggested.
The hepatic vagus nerve contains various thermosensitive afferent fibers which are widely varied in their sensitivity. Their Q10 values lie between 4 and 16. The discharge rate is positively correlated with increase of liver temperature (warm fiber type). The result supports the existence of a thermosensitive structure in the liver which may possibly contribute to maintain thermal homeostasis. Neural responses to the osmotic changes in the perfusion solution have been analyzed. It was found that two different types of osmosensitive afferent fibers exist in the hepatic vagus; one is characterized by increasing the frequency of spike discharges in response to higher osmotic pressure, while the other shows the same response to lowered levels. Behavioral changes caused by hepatic vagotomy were observed. These results provide evidence for the existence of an osmoreceptor mechanism. The role of these hepatic afferent nerves in homeostasis are briefly discussed.
The innervation of the gall bladder and the biliary pathways was studied in guinea-pigs by means of histochemical methods for catecholamines and for acetylcholinesterase on whole mount preparations, on cryostat sections and on sections of plastic-embedded tissues. The gall bladder contains on average 367 neurons in a ganglionated plexus which lies at the outer surface of the muscle coat. The overall appearance of this plexus is rather similar to that of the submucosal plexus of the duodenum. From the gall bladder the plexus extends into the cystic duct, the hepatic duct and the common bile duct, but from the middle portion of the common bile duct downwards, it is positioned at or near the inner surface of the muscle coat. Concurrently with the marked increase in muscle thickness in the lower parts of the common bile duct, another ganglionated plexus appears, which is truly intramuscular. The latter plexus is highly developed, lies usually between longitudinal and circular muscle and resembles in appearance the myenteric plexus of the duodenum, with which it is in continuity. Throughout the biliary system, the extent of the ganglionated plexus is roughly related to the extent of the musculature. An exchange of adrenergic fibres between the ganglionated plexus and perivascular nerves is observed in the gall bladder. Another nerve plexus, without ganglia but rich in adrenergic and acetylcholinesterase-positive fibres, lies between the mucosa and the muscle coat. Very few nerve fibres run into the musculature of the gall bladder. On the other hand, in the thick musculature of the lower portion of the common bile duct, several intramuscular nerve fibres are found.
Action potentials evoked by the stimulation of the hepatic branch of the splanchnic nerve were recorded from the peripheral cut end of the dorsal celiac branch of the vagus nerve in the rat. Action potentials were clearly demonstrated after averaging over 100 times by a computer. The observations indicate the existence of a nervous pathway from the hepatic branch of the splanchnic nerve to the dorsal celiac branch of the vagus nerve in the rat.
The hepatic branch of the vagus nerve has been implicated as an important source of afferent input controlling both physiological and behavioral homeostasis. In addition, it is clear that parasympathetic efferents to the liver can significantly alter hepatic functions. In order to begin physiological studies on the nature of hepatic afferent and efferent relations, it will be necessary to understand the central anatomical organization of the components of this small visceral nerve. By carefully exposing and dissecting the hepatic branch of the vagus and applying crystalline horseradish peroxidase (HRP) to it, we were able to elucidate a predominant pattern of afferent terminations within the left subnucleus gelatinosus, the medial division of the left solitary nucleus and the left lateral edge of the area postrema. Efferent nuclei were concentrated in the left dorsal motor nucleus of the vagus (DMN) with a few scattered neurons located in the right DMN as well as the left anterior nucleus ambiguous.
Afferent discharges were recorded from the hepatic branch of the vagus nerve. It was revealed that some afferent fibers were sensitive to an increase in liver temperature and that their Q10 values lay between 4 and 16. These thermosensitive fibers failed to respond to mechanical stimulations applied to the liver. The other types of afferent fibers in this nerve such as an osmosensitive one did not respond to thermal stimulation. These data support the existence of a thermosensitive structure in the liver which may possibly contribute to maintain thermal homeostasis.