![Major pathways by which glutamate regulates cerebral blood flow.Pathways from astrocytes and neurons (left) that regulate blood flow by sending messengers (arrows) to influence the smooth muscle around the arterioles that supply oxygen and glucose to the cells (right, shown as the vessel lumen surrounded by endothelial cells and smooth muscle). In neurons, synaptically released glutamate acts on N-methyl-D-aspartate receptors (NMDAR) to raise [Ca2+]i, causing neuronal nitric oxide synthase (nNOS) to release NO, which activates smooth muscle guanylate cyclase. This generates cGMP to dilate vessels. Raised [Ca2+]i may also (dashed line) generate arachidonic acid (AA) from phospholipase A2 (PLA2), which is converted by COX2 to prostaglandins (PG) that dilate vessels. Glutamate raises [Ca2+]i in astrocytes by activating metabotropic glutamate receptors (mGluR), generating arachidonic acid and thus three types of metabolite: prostaglandins (by COX1/3, and COX2 in pathological situations) and EETs (by P450 epoxygenase) in astrocytes, which dilate vessels, and 20-HETE (by ω-hydroxylase) in smooth muscle, which constricts vessels. A rise of [Ca2+]i in astrocyte endfeet may activate Ca2+-gated K+ channels (gK(Ca)), releasing K+, which also dilates vessels.](https://www.researchgate.net/profile/Alastair-Buchan/publication/47743037/figure/fig1/AS:279174842470425@1443571877174/Major-pathways-by-which-glutamate-regulates-cerebral-blood-flowPathways-from-astrocytes_Q320.jpg)
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Glial and neuronal control of brain blood flow
Abstract and Figures
Blood flow in the brain is regulated by neurons and astrocytes. Knowledge of how these cells control blood flow is crucial for understanding how neural computation is powered, for interpreting functional imaging scans of brains, and for developing treatments for neurological disorders. It is now recognized that neurotransmitter-mediated signalling has a key role in regulating cerebral blood flow, that much of this control is mediated by astrocytes, that oxygen modulates blood flow regulation, and that blood flow may be controlled by capillaries as well as by arterioles. These conceptual shifts in our understanding of cerebral blood flow control have important implications for the development of new therapeutic approaches.
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... 14 tPA, a serine-protease renowned for its role in fibrinolysis, [16][17][18] plays a key role in the full expression of functional hyperemia. 13,14,19,20 tPA, released by synaptic activity, [21][22][23][24] is required for the production of the vasodilator nitric oxide (NO) from neuronal NO synthase (nNOS) upon NMDA receptor (NMDAR) activation. [25][26][27][28] Thus, tPA is required for the full expression of the NMDAR-nNOS dependent component of functional hyperemia. ...
... nNOS-NO-dependent component of the resulting hyperemic response. 13,14,20 tPA is released from active neurons 23,24,[52][53][54] and is required for the full expression of the CBF increase triggered by neural activity. 13,14,19 These neurovascular effects are mediated by promoting the phosphorylation of nNOS and resulting in NO production during NMDA receptor activation. ...
INTRODUCTION
Amyloid beta (Aβ) impairs the cerebral blood flow (CBF) increase induced by neural activity (functional hyperemia). Tissue plasminogen activator (tPA) is required for functional hyperemia, and in mouse models of Aβ accumulation tPA deficiency contributes to neurovascular and cognitive impairment. However, it remains unknown if tPA supplementation can rescue Aβ‐induced neurovascular and cognitive dysfunction.
METHODS
Tg2576 mice and wild‐type littermates received intranasal tPA (0.8 mg/kg/day) or vehicle 5 days a week starting at 11 to 12 months of age and were assessed 3 months later.
RESULTS
Treatment of Tg2576 mice with tPA restored resting CBF, prevented the attenuation in functional hyperemia, and improved nesting behavior. These effects were associated with reduced cerebral atrophy and cerebral amyloid angiopathy, but not parenchymal amyloid.
DISCUSSION
These findings highlight the key role of tPA deficiency in the neurovascular and cognitive dysfunction associated with amyloid pathology, and suggest potential therapeutic strategies involving tPA reconstitution.
Highlights
Amyloid beta (Aβ) induces neurovascular dysfunction and impairs the increase of cerebral blood flow induced by neural activity (functional hyperemia).
Tissue plasminogen activator (tPA) deficiency contributes to the neurovascular and cognitive dysfunction caused by Aβ.
In mice with florid amyloid pathology intranasal administration of tPA rescues the neurovascular and cognitive dysfunction and reduces brain atrophy and cerebral amyloid angiopathy.
tPA deficiency plays a crucial role in neurovascular and cognitive dysfunction induced by Aβ and tPA reconstitution may be of therapeutic value.
... Indeed, lactate has been shown to increase oxygen consumption during network rhythms likely reflecting enhanced oxidative metabolism in mitochondria 9,24 . Dynamic adaptations in brain metabolism and blood flow are indeed necessary to distribute energy substrates and oxygen to areas where neuronal activity is increased 3,4 . This process is important for synchronized network activities, such as gamma oscillations, which are exquisitely sensitive to disturbances in oxygen and www.nature.com/scientificreports/ ...
Microglia, brain-resident macrophages, can acquire distinct functional phenotypes, which are supported by differential reprogramming of cell metabolism. These adaptations include remodeling in glycolytic and mitochondrial metabolic fluxes, potentially altering energy substrate availability at the tissue level. This phenomenon may be highly relevant in the brain, where metabolism must be precisely regulated to maintain appropriate neuronal excitability and synaptic transmission. Direct evidence that microglia can impact on neuronal energy metabolism has been widely lacking, however. Combining molecular profiling, electrophysiology, oxygen microsensor recordings and mathematical modeling, we investigated microglia-mediated disturbances in brain energetics during neuroinflammation. Our results suggest that proinflammatory microglia showing enhanced nitric oxide release and decreased CX3CR1 expression transiently increase the tissue lactate/glucose ratio that depends on transcriptional reprogramming in microglia, not in neurons. In this condition, neuronal network activity such as gamma oscillations (30–70 Hz) can be fueled by increased ATP production in mitochondria, which is reflected by elevated oxygen consumption. During dysregulated inflammation, high energy demand and low glucose availability can be boundary conditions for neuronal metabolic fitness as revealed by kinetic modeling of single neuron energetics. Collectively, these findings indicate that metabolic flexibility protects neuronal network function against alterations in local substrate availability during moderate neuroinflammation.
... ; focusing on the brain endothelium and microglia/macrophages. These two components are encompassed in neurovascular unit (NVU) [11] and play a critical role in maintaining brain homeostasis [12]. Endothelial cells within the blood brain barrier (BBB) and NVU express CD31, which can serve as an indicator of endothelial BBB integrity [13]. ...
Purpose: To investigate ultra-high-dose rate helium ion irradiation and its potential FLASH sparing effect with the endpoint acute brain injury in preclinical in vivo settings.
Material and methods: Raster-scanned helium ion beams were administered to explore and compare the impact of dose rate variations between standard dose rate (SDR at 0.2 Gy/s) and FLASH (at 141 Gy/s) radiotherapy (RT). Irradiation-induced brain injury was investigated in healthy C57BL/6 mice via DNA damage response kinetic studies using nuclear gamma H2AX as a surrogate for double-strand breaks (DSB). The integrity of the neurovascular and immune compartments was assessed via CD31+ microvascular density and microglia/macrophages activation. Iba1+ ramified and CD68+ phagocytic microglia/macrophages were quantified, together with the expression of inducible nitric oxide synthetase (iNOS).
Results: Helium FLASH RT significantly prevented acute brain tissue injury compared with SDR. This was demonstrated by reduced levels of DSB and structural preservation of the neurovascular endothelium after FLASH RT. Moreover, FLASH RT exhibited reduced activation of neuroinflammatory signals compared with SDR, as detected by quantification of CD68+ iNOS+ microglia/macrophages.
Conclusion: To our knowledge, this is the first report on the FLASH-sparing neuroprotective effect of raster scanning helium ion radiotherapy in vivo.
... Neurovascular coupling (NVC) is the process by which neuronal activity regulates blood flow in the brain, ensuring that adequate oxygen and nutrients are delivered to active regions Iadecola [25], Iadecola [26], Dienel and Hertz [10], Attwell et al. [3]. This process is essential for normal brain function, and disturbances in NVC have been implicated in various neurological disorders Jennings et al. [27], Krainik et al. [32], Mentis et al. [36], Pineiro et al. [41]. ...
Neurovascular coupling is a fundamental concept that elucidates how neuronal electrical activity regulates vascular function, ensuring optimal blood flow to active regions of the brain and optimizing neural function. In this article, we have constructed a novel model of the endothelial Kir2.1 channel that senses potassium ions in the perivascular space and integrated it into a neurovascular coupling model. Our computational framework accounts for the dynamics of neurons, astrocytes, endothelial cells, and smooth muscle cells at the cellular level. By incorporating recent experimental findings, we successfully demonstrate that capillary endothelial cells can detect localized fluctuations in extracellular \(\mathrm {K^+}\) concentration through the Kir2.1 channel on their membranes. Subsequently, these cells transmit this signal to smooth muscle cells, initiating the release of vasoactive substances and facilitating vascular motion. This sequence of events forms the core of our model. Furthermore, our simulations support the inclusion of a temperature term in our model, allowing us to accurately replicate experimental observations. This accounts for variations in neuronal electrical activity due to increased temperature, leading to vasoconstriction. In conclusion, our dynamic model serves as a valuable tool for investigating the mechanisms underlying neurovascular coupling and the role of capillary \(\mathrm {K^+}\)-sensing in regulating vascular activity. It enhances our understanding of brain function and provides insights for developing therapeutic interventions for neurovascular diseases.
... As a consequence of astrocytic Ca 2+ elevations, gliotransmitters are released, acting on neighboring neurons influencing the neuronal network (Araque et al., 1998;Bezzi et al., 1998;Perea & Araque, 2007). Additionally, astrocytic Ca 2+ signals are crucial to adjust local blood flow to neuronal activity, fueling the cellular network with metabolites (Attwell et al., 2010;Beiersdorfer et al., 2019;Gordon et al., 2007; CC-BY-NC-ND 4.0 International license made available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprint this version posted June 13, 2024. ...
The prefrontal cortex (PFC) is a cortical brain region whose multifaceted functions are based on a complex interplay between excitatory pyramidal neurons, inhibitory GABAergic interneurons and astrocytes maintaining a fine-tuned excitation/inhibition balance (E/I balance). The regulation of the E/I balance in cortical network is crucial as the disruption leads to impairments in PFC-associated behavior and pathologies. Astrocytes express specific GABA receptors that mediate intracellular Ca 2+ signaling upon stimulation by γ-aminobutyric acid (GABA), resulting in the release of gliotransmitters directly impacting information processing. However, the signaling pathway leading to GABA-induced Ca 2+ signaling in astrocytes of the PFC is not well understood. Here we took advantage of GLAST-promoter driven GCaMP6s expression in astrocytes to study GABAergic Ca 2+ signaling in PFC astrocytes by confocal microscopy. The results show that GABA induces Ca 2+ signaling via the stimulation of the metabotropic GABAB receptor in astrocytes. GABAB receptor-mediated Ca 2+ signals greatly depend on intracellular Ca 2+ stores rather than on extracellular Ca 2+. Additionally, antagonists of the PLC/IP3-signaling cascade significantly reduced GABAB receptor-mediated Ca 2+ signaling in astrocytes, suggesting that astrocytic GABAB receptors in the PFC are coupled to the Gq-GPCR signaling pathway.
Cerebrovascular resistance (CVR) regulates blood flow in the brain, but little is known about the vascular resistances of the individual cerebral territories. We present a method to calculate these resistances and investigate how CVR varies in the hemodynamically disturbed brain. We included 48 patients with stroke/TIA (29 with symptomatic carotid stenosis). By combining flow rate (4D flow MRI) and structural computed tomography angiography (CTA) data with computational fluid dynamics (CFD) we computed the perfusion pressures out from the circle of Willis, with which CVR of the MCA, ACA, and PCA territories was estimated. 56 controls were included for comparison of total CVR (tCVR). CVR were 33.8 ± 10.5, 59.0 ± 30.6, and 77.8 ± 21.3 mmHg s/ml for the MCA, ACA, and PCA territories. We found no differences in tCVR between patients, 9.3 ± 1.9 mmHg s/ml, and controls, 9.3 ± 2.0 mmHg s/ml (p = 0.88), nor in territorial CVR in the carotid stenosis patients between ipsilateral and contralateral hemispheres. Territorial resistance associated inversely to territorial brain volume (p < 0.001). These resistances may work as reference values when modelling blood flow in the circle of Willis, and the method can be used when there is need for subject-specific analysis.
Hyperbaric Oxygen Therapy (HBOT), the use of pure oxygen (100% O 2 ) at high pressure (2–3 ATM), is gaining prominence as a tool for managing persistent post-concussive symptoms, otherwise known as post-concussion syndrome (PCS). Recent research has emerged that elucidates the mechanisms by which HBOT improves PCS. This article reviews the progression and pathophysiology of PCS, challenges in diagnosis, and novel imaging solutions. It also delves into recent advancements in the understanding of HBOT mechanisms and the benefits observed from HBOT in PCS patients. The discussion concludes with an examination of innovative imaging techniques, novel biomarkers, the potential role of data sharing, machine learning, and how these developments can advance the use of HBOT in the management of PCS.
Background
Studies of hyperbaric oxygen therapy (HBOT) treatment of mild traumatic brain injury persistent postconcussion syndrome in military and civilian subjects have shown simultaneous improvement in posttraumatic stress disorder (PTSD) or PTSD symptoms, suggesting that HBOT may be an effective treatment for PTSD. This is a systematic review and dosage analysis of HBOT treatment of patients with PTSD symptoms.
Methods
PubMed, CINAHL, and the Cochrane Systematic Review Database were searched from September 18 to November 23, 2023, for all adult clinical studies published in English on HBOT and PTSD. Randomized trials and studies with symptomatic outcomes were selected for final analysis and analyzed according to the dose of oxygen and barometric pressure on symptom outcomes. Outcome assessment was for statistically significant change and Reliable Change or Clinically Significant Change according to the National Center for PTSD Guidelines. Methodologic quality and bias were determined with the PEDro Scale.
Results
Eight studies were included, all with < 75 subjects/study, total 393 subjects: seven randomized trials and one imaging case-controlled study. Six studies were on military subjects, one on civilian and military subjects, and one on civilians. Subjects were 3-450 months post trauma. Statistically significant symptomatic improvements, as well as Reliable Change or Clinically Significant changes, were achieved for patients treated with 40-60 HBOTS over a wide range of pressures from 1.3 to 2.0 ATA. There was a linear dose-response relationship for increased symptomatic improvement with increasing cumulative oxygen dose from 1002 to 11,400 atmosphere-minutes of oxygen. The greater symptomatic response was accompanied by a greater and severe reversible exacerbation of emotional symptoms at the highest oxygen doses in 30-39% of subjects. Other side effects were transient and minor. In three studies the symptomatic improvements were associated with functional and anatomic brain imaging changes. All 7 randomized trials were found to be of good-highest quality by PEDro scale scoring.
Discussion
In multiple randomized and randomized controlled clinical trials HBOT demonstrated statistically significant symptomatic improvements, Reliable Changes, or Clinically Significant Changes in patients with PTSD symptoms or PTSD over a wide range of pressure and oxygen doses. The highest doses were associated with a severe reversible exacerbation of emotional symptoms in 30-39% of subjects. Symptomatic improvements were supported by correlative functional and microstructural imaging changes in PTSD-affected brain regions. The imaging findings and hyperbaric oxygen therapy effects indicate that PTSD can no longer be considered strictly a psychiatric disease.
The prostanoid-synthesizing enzyme cyclooxygenase-2 (COX-2) is expressed in selected cerebral cortical neurons and is involved in synaptic signaling. We sought to determine whether COX-2 participates in the increase in cerebral blood flow produced by synaptic activity in the somatosensory cortex. In anesthetized mice, the vibrissae were stimulated mechanically, and cerebral blood flow was recorded in the contralateral somatosensory cortex by a laser-Doppler probe. We found that the COX-2 inhibitor NS-398 attenuates the increase in somatosensory cortex blood flow produced by vibrissal stimulation. Furthermore, the flow response was impaired in mice lacking the COX-2 gene, whereas the associated increase in whisker-barrel cortex glucose use was not affected. The increases in cerebral blood flow produced by hypercapnia, acetylcholine, or bradykinin were not attenuated by NS-398, nor did they differ between wild-type and COX-2 null mice. The findings provide evidence for a previously unrecognized role of COX-2 in the mechanisms coupling synaptic activity to neocortical blood flow and provide an insight into one of the functions of constitutive COX-2 in the CNS.
The role of nitric oxide (NO) in cerebral blood flow-metabolism coupling was assessed in SV-129 wild-type (WT) and neuronal (type I) NO synthase (NOS) knockout mice (Kn). Regional cerebral blood flow (rCBF; laser-Doppler flowmetry) was measured over the contralateral cortical barrel field during unilateral mechanical vibrissal deflection (2-3 Hz, 60 s) under urethan anesthesia. The rCBF response was similar in WT and Kn and did not differ when recorded over the intact skull or closed cranial window preparations. Whisker stimulation increased rCBF by 41 ± 8% (maximum) and 27 ± 6% (mean) in WT (n = 6) and 41 ± 7% (maximum) and 26 ± 6% (mean) in Kn (n = 6) when recorded through a closed cranial window. After superfusion with topical Nω-nitro-L-arginine (L-NNA; 1 mM), the rCBF response was inhibited by ∼45% in WT mice (P < 0.05), whereas there was no inhibition in Kn. Endothelium-dependent relaxation, assessed by pial vessel dilation in response to topical acetylcholine (100 μM) and inhibition by L-NNA (1 mM), was the same in both groups. Our results suggest that 1 ) endothelial NO production does not mediate the rCBF coupling to neuronal activity in Kn, 2) the inhibitory effect of L-NNA on the rCBF response to whisker stimulation in WT is a consequence of type I (neuronal) NOS inhibition, and 3) NO-independent mechanisms couple rCBF and metabolism during whisker stimulation in mice lacking expression of neuronal NOS.
We investigated the role of nitric oxide (NO)/cGMP in the coupling of neuronal activation to regional cerebral blood flow (rCBF) in α-chloralose-anesthetized rats. Whisker deflection (60 s) increased rCBF by 18 ± 3%. NO synthase (NOS) inhibition by Nω-nitro-l-arginine (l-NNA; topically) reduced the rCBF response to 9 ± 4% and resting rCBF to 80 ± 8%. NO donors [ S-nitroso- N-acetylpenicillamine (SNAP; 50 μM), 3-morpholinosydnonimine (10 μM)] or 8-bromoguanosine 3',5'-cyclic-monophosphate (8-BrcGMP; 100 μM)] restored resting rCBF andl-NNA-induced attenuation of the whisker response in the presence ofl-NNA, whereas the NO-independent vasodilator papaverine (1 mM) had no effect on the whisker response. Basal cGMP levels were decreased to 35% byl-NNA and restored to 65% of control by subsequent SNAP superfusion. Inhibition of neuronal NOS by 7-nitroindazole (7-NI; 40 mg/kg ip) or soluble guanylyl cyclase by 1 H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 100 μM) significantly reduced resting rCBF to 86 ± 8 and 92 ± 10% and whisker rCBF response to 7 ± 4 and 12 ± 3%, respectively. ODQ reduced tissue cGMP to 54%. 8-BrcGMP restored the whisker response in the presence of 7-NI or ODQ. We conclude that NO, produced by neuronal NOS, is a modulator in the coupling of neuronal activation and rCBF in rat somatosensory cortex and that this effect is mainly mediated by cGMP.l-NNA-induced vasomotion was significantly reduced during increased neuronal activity and after restoration of basal NO levels, but not after restoration of cGMP.
The haemodynamic responses to neural activity that underlie the blood-oxygen-level-dependent (BOLD) signal used in functional magnetic resonance imaging (fMRI) of the brain are often assumed to be driven by energy use, particularly in presynaptic terminals or glia. However, recent work has suggested that most brain energy is used to power postsynaptic currents and action potentials rather than presynaptic or glial activity and, furthermore, that haemodynamic responses are driven by neurotransmitter-related signalling and not directly by the local energy needs of the brain. A firm understanding of the BOLD response will require investigation to be focussed on the neural signalling mechanisms controlling blood flow rather than on the locus of energy use.
Individual inhibition of nitric oxide (NO) synthase and cytochrone P450 (CYP) epoxygenase activity attenuates cortical functional hyperemia evoked by whisker stimulation. The objectives of the present study were to determine (1) if administration of epoxygenase inhibitors attenuates cortical functional hyperemia by using a different modality of sensory activation (i.e., electrical stimulation of the rat forepaw), (2) if epoxygenase inhibition has ail additive effect with NO synthase inhibition on the flow response, and (3) the cellular localization of the epoxygenase CYP20C1 1 in cerebral cortex. In six groups of anesthetized rats, the cortical surface was superfused for 90 minutes with (1) vehicle; (2) 1-mmol/L Nomega-nitro-L-arginine (L-NNA), to inhibit NO synthase activity; (3) 20-mumol/L N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MS-PPOH), a substrate inhibitor of P450 epoxygenase; (4) MS-PPOH Plus L-NNA; (5) 20-mumol/L miconazole, a reversible inhibitor at the heme site of P450 epoxygenase and (6) miconazole Plus L-NNA. The percent increases in laser-Doppler perfusion over primary sensory cortex during 20-second fore-paw stimulation were reduced by 44% to 64% in all drug-treated groups. The addition of L-NNA to MS-PPOH produced no additional reduction (64%) compared with MS-PPOH alone (64%) or L-NNA alone (60%). The addition of L-NNA to miconazole also produced no additional reduction in the flow response. In situ hybridization of CYP2C1 1 mRNA showed localization in astrocytes. including those adjacent to blood vessels. Thus, activity of both epoxygenase, presumably localized in astrocytes, and NO synthase is required for generating a complete cortical hyperemic response evoked by electrical forepaw stimulation. The lack of additional blood flow attenuation with the combination of the NO synthase and the distinct epoxygenase inhibitors suggests that the signaling pathways do not act in a simple parallel fashion and that other mediators may be involved in coupling cortical blood flow to neuronal activation.
