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Kinetics of the GABA A and GABA C receptor currents. Long GABA puff applications (1 mM, 6 sec) were delivered in the absence or presence of SR95531 (100 M) and TPMPA (50 M). The SR95531resistant or GABA C receptor current remained sustained during the time of application, whereas the TPMPA-resistant or GABA A receptor current rapidly decreased in amplitude during the puff.
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Protein MAP1B was recently reported to link GABA(C) receptors to the cytoskeleton at neuronal synapses. This interaction was demonstrated in the mammalian retina, where GABA(C) receptors were thought to be exclusively expressed in bipolar cells. Our previous studies on cultured photoreceptors suggested however the presence of GABA(C) receptors in c...
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Context 1
... further assess the kinetics of the GABA response, GABA was applied for long duration (6 sec). When the GABA C receptor component was suppressed by TPMPA application (50 M; Fig. 6), the remaining GABA A response desensitized during the 6 sec application by 84% of its initial amplitude. By contrast, the GABA C receptor component isolated in the presence of SR95531 (100 M) was only reduced by 24% during the 6 sec application (Fig. 6). These different kinetics are consistent with the expression of de- sensitizing ...
Context 2
... (6 sec). When the GABA C receptor component was suppressed by TPMPA application (50 M; Fig. 6), the remaining GABA A response desensitized during the 6 sec application by 84% of its initial amplitude. By contrast, the GABA C receptor component isolated in the presence of SR95531 (100 M) was only reduced by 24% during the 6 sec application (Fig. 6). These different kinetics are consistent with the expression of de- sensitizing GABA A receptors and nondesensitizing GABA C re- ceptors in cone ...
Citations
... In neonatal rabbit retina, cone photoreceptors transiently express GABA A receptor subunits α1 and β2/3 (Mitchell and Redburn, 1996;Mitchell et al., 1999), when GABA and GAD67 levels are high in horizontal cells (Schnitzer and Rusoff, 1984). Cone terminals of pig and rat were reported to show GABA A ρ subunit (ρ subunit) immunoreactivity suggesting the presence of a GABA A ρ receptor (Picaud et al., 1998b;Pattnaik et al., 2000). However, Deniz et al. (2019) reported bicuculinesensitive, but not TPMPA-sensitive, GABA evoked currents in mouse cone photoreceptors in retinal slices, suggesting the presence of ionotropic GABA A receptors, but not those comprising ρ-subunits. ...
Feedback inhibition by horizontal cells regulates rod and cone photoreceptor calcium channels that control their release of the neurotransmitter glutamate. This inhibition contributes to synaptic gain control and the formation of the center-surround antagonistic receptive fields passed on to all downstream neurons, which is important for contrast sensitivity and color opponency in vision. In contrast to the plasmalemmal GABA transporter found in non-mammalian horizontal cells, there is evidence that the mechanism by which mammalian horizontal cells inhibit photoreceptors involves the vesicular release of the inhibitory neurotransmitter GABA. Historically, inconsistent findings of GABA and its biosynthetic enzyme, L-glutamate decarboxylase (GAD) in horizontal cells, and the apparent lack of surround response block by GABAergic agents diminished support for GABA's role in feedback inhibition. However, the immunolocalization of the vesicular GABA transporter (VGAT) in the dendritic and axonal endings of horizontal cells that innervate photoreceptor terminals suggested GABA was released via vesicular exocytosis. To test the idea that GABA is released from vesicles, we localized GABA and GAD, multiple SNARE complex proteins, synaptic vesicle proteins, and Cav channels that mediate exocytosis to horizontal cell dendritic tips and axonal terminals. To address the perceived relative paucity of synaptic vesicles in horizontal cell endings, we used conical electron tomography on mouse and guinea pig retinas that revealed small, clear-core vesicles, along with a few clathrin-coated vesicles and endosomes in horizontal cell processes within photoreceptor terminals. Some small-diameter vesicles were adjacent to the plasma membrane and plasma membrane specializations. To assess vesicular release, a functional assay involving incubation of retinal slices in luminal VGAT-C antibodies demonstrated vesicles fused with the membrane in a depolarization- and calcium-dependent manner, and these labeled vesicles can fuse multiple times. Finally, targeted elimination of VGAT in horizontal cells resulted in a loss of tonic, autaptic GABA currents, and of inhibitory feedback modulation of the cone photoreceptor Cai, consistent with the elimination of GABA release from horizontal cell endings. These results in mammalian retina identify the central role of vesicular release of GABA from horizontal cells in the feedback inhibition of photoreceptors.
... Many physiological recordings in the normal mammalian retina do not show a direct action of GABAergic agents on cones (Verweij et al., 2003;Crook et al., 2009;Kemmler et al., 2014;Szikra et al., 2014;Grove et al., 2019), but there is evidence for GABAR subunit expression by mammalian photoreceptors (Greferath et al., 1993;Grigorenko and Yeh, 1994;Picaud et al., 1998;Vardi et al., 1998;Chaffiol et al., 2017). However, in cultured retinal explants (and possibly in rd1 mice lacking rods), cones may be reprogrammed and respond to GABA application (Picaud et al., 1998;Pattnaik et al., 2000), and one report indicates GABA activation of TPMPAinsensitive GABAR Cl − channels in wild-type mouse cones (Deniz et al., 2019). Further complicating the functional role for photoreceptor Cl − flux, during surround light stimulation that hyperpolarizes horizontal cells, the resulting disinhibition of Ca V channels in cones in fish (Verweij et al., 1996) and macaque (Verweij et al., 2003) is accompanied by an increase in Ca 2+ -activated Cl − conductance. ...
How neurons in the eye feed signals back to photoreceptors to optimize sensitivity to patterns of light appears mediated by one or more unconventional mechanisms. Via these mechanisms, horizontal cells control synaptic gain and enhance key aspects of temporal and spatial light adaptation, including center-surround receptive field antagonism. After the transduction of light energy into an electrical signal in photoreceptors, a major task in visual processing is transmission of an optimized signal to the follower neurons in the retina. For this to happen, the release of the excitatory neurotransmitter glutamate from photoreceptors is carefully regulated via feedback from horizontal cells, which acts like a thermostat to keep synaptic transmission in an optimal range during changes in light patterns and intensities. A recently described model that casts a classical neurotransmitter system together with ion transport mechanisms to adjust the alkaline milieu outside the synapse, is emphasized here. We review the new evidence showing how this novel inter-neuronal messaging system carries the feedback inhibition using two separate, but interwoven, regulated systems. It may be the complex interplay between these two signaling modalities, creating synaptic modulation-at-a-distance, that has made its definition difficult.
The foundations of our understanding of the feedback mechanism from horizontal cells to photoreceptors have been long established: Horizontal cells have broad receptive fields, suitable for providing surround inhibition, their membrane potential regulates inhibition of photoreceptor voltage-gated calcium channels, and strong artificial pH buffering eliminates this action. This report compares and contrasts models of how these foundations are linked, focusing on a recent report of a novel action of GABA in mammals that shows tonic horizontal cell release of GABA activating chloride and bicarbonate permeable GABA autoreceptors. The membrane potential of horizontal cells provides the driving force for GABAR-mediated bicarbonate efflux, alkalinizing the cleft when horizontal cells are hyperpolarized by light or adding to their depolarization cells in darkness by contributing to cleft acidification via NHE-mediated proton efflux. This model reverses interpretations of earlier studies that were considered to rule out a role for GABA in feedback to cones.
... Many physiological recordings in the normal mammalian retina do not show a direct action of GABAergic agents on cones (Verweij et al., 2003;Crook et al., 2009;Kemmler et al., 2014;Szikra et al., 2014;Grove et al., 2019), but there is evidence for GABAR subunit expression by mammalian photoreceptors (Greferath et al., 1993;Grigorenko and Yeh, 1994;Picaud et al., 1998;Vardi et al., 1998;Chaffiol et al., 2017). However, in cultured retinal explants (and possibly in rd1 mice lacking rods), cones may be reprogrammed and respond to GABA application (Picaud et al., 1998;Pattnaik et al., 2000), and one report indicates GABA activation of TPMPAinsensitive GABAR Cl − channels in wild-type mouse cones (Deniz et al., 2019). Further complicating the functional role for photoreceptor Cl − flux, during surround light stimulation that hyperpolarizes horizontal cells, the resulting disinhibition of Ca V channels in cones in fish (Verweij et al., 1996) and macaque (Verweij et al., 2003) is accompanied by an increase in Ca 2+ -activated Cl − conductance. ...
How neurons in the eye feed signals back to photoreceptors to optimize sensitivity to patterns of light appears to be mediated by one or more unconventional mechanisms. Via these mechanisms, horizontal cells control photoreceptor synaptic gain and enhance key aspects of temporal and spatial center-surround receptive field antagonism. After the transduction of light energy into an electrical signal in photoreceptors, the next key task in visual processing is transmission of an optimized signal to the follower neurons in the retina. For this to happen, the release of the excitatory neurotransmitter glutamate from photoreceptors is carefully regulated via horizontal cell feedback, which acts like a thermostat to keep synaptic transmission in an optimal range during changes to light patterns and intensities. Novel findings of a recently described model that casts a classical neurotransmitter system together with ion transport mechanisms to adjust the alkaline milieu outside the synapse, are reviewed. This novel inter-neuronal messaging system carries the feedback inhibition using two separate, but interwoven, regulated systems. The complex interplay between these two signaling modalities, creating synaptic modulation-at-a-distance, have obscured its being defined.
The foundations of our understanding of the feedback mechanism from horizontal cells to photoreceptors have been long established: Horizontal cells have broad receptive fields, suitable for providing surround inhibition, their membrane potential, a function of stimulus intensity and size, regulates inhibition of photoreceptor voltage-gated Ca2+ channels, and strong artificial pH buffering eliminates this action. This review compares and contrasts models of how these foundations are linked, focusing on a recent report in mammals that shows tonic horizontal cell release of GABA activating Cl− and HCO3− permeable GABA autoreceptors. The membrane potential of horizontal cells provides the driving force for GABAR-mediated HCO3− efflux, alkalinizing the cleft when horizontal cells are hyperpolarized by light or adding to their depolarization in darkness and contributing to cleft acidification via NHE-mediated H+ efflux. This model challenges interpretations of earlier studies that were considered to rule out a role for GABA in feedback to cones.
... Many physiological recordings in the normal mammalian retina do not show a direct action of GABAergic agents on cones (Verweij et al., 2003;Crook et al., 2009;Kemmler et al., 2014;Szikra et al., 2014;Grove et al., 2019), but there is evidence for GABAR subunit expression by mammalian photoreceptors (Greferath et al., 1993;Grigorenko and Yeh, 1994;Picaud et al., 1998;Vardi et al., 1998;Chaffiol et al., 2017). However, in cultured retinal explants (and possibly in rd1 mice lacking rods), cones may be reprogrammed and respond to GABA application (Picaud et al., 1998;Pattnaik et al., 2000), and one report indicates GABA activation of TPMPAinsensitive GABAR Cl − channels in wild-type mouse cones (Deniz et al., 2019). Further complicating the functional role for photoreceptor Cl − flux, during surround light stimulation that hyperpolarizes horizontal cells, the resulting disinhibition of Ca V channels in cones in fish (Verweij et al., 1996) and macaque (Verweij et al., 2003) is accompanied by an increase in Ca 2+ -activated Cl − conductance. ...
How neurons in the eye feed signals back to photoreceptors to optimize sensitivity to patterns of light appears to be mediated by one or more unconventional mechanisms. Via these mechanisms, horizontal cells control photoreceptor synaptic gain and enhance key aspects of temporal and spatial center-surround receptive field antagonism. After the transduction of light energy into an electrical signal in photoreceptors, the next key task in visual processing is transmission of an optimized signal to the follower neurons in the retina. For this to happen, the release of the excitatory neurotransmitter glutamate from photoreceptors is carefully regulated via horizontal cell feedback, which acts like a thermostat to keep synaptic transmission in an optimal range during changes to light patterns and intensities. Novel findings of a recently described model that casts a classical neurotransmitter system together with ion transport mechanisms to adjust the alkaline milieu outside the synapse, are reviewed. This novel inter-neuronal messaging system carries the feedback inhibition using two separate, but interwoven, regulated systems. The complex interplay between these two signaling modalities, creating synaptic modulation-at-a-distance, have obscured its being defined.
The foundations of our understanding of the feedback mechanism from horizontal cells to photoreceptors have been long established: Horizontal cells have broad receptive fields, suitable for providing surround inhibition, their membrane potential, a function of stimulus intensity and size, regulates inhibition of photoreceptor voltage-gated Ca2+ channels, and strong artificial pH buffering eliminates this action. This review compares and contrasts models of how these foundations are linked, focusing on a recent report in mammals that shows tonic horizontal cell release of GABA activating Cl− and HCO3− permeable GABA autoreceptors. The membrane potential of horizontal cells provides the driving force for GABAR-mediated HCO3− efflux, alkalinizing the cleft when horizontal cells are hyperpolarized by light or adding to their depolarization in darkness and contributing to cleft acidification via NHE-mediated H+ efflux. This model challenges interpretations of earlier studies that were considered to rule out a role for GABA in feedback to cones.
... a 1 and r subunits were not detected in mouse cones by immunoelectron microscopy (Kemmler et al., 2014). A GABA-evoked current was detected in only a fraction of cones in macaque flat mount retina, without pharmacological characterization (Verweij et al., 2003); the presence of both GABA A and GABA C receptors was reported by combining electrophysiology and immunohistochemistry in porcine cones in culture (Picaud et al., 1998) and in putative mouse cones from flat mount rd1 retinas or dissociated from wild-type C57BL/6J retinas (Pattnaik et al., 2000). These apparent discrepancies may be due to variations between species, a difficult path for GABA to reach the cone terminals in retinal flat mounts, low levels of expression of GABA receptor subunits in mammalian cones, difficulty in identifying dissociated cones, and/or changes in expression during retinal degeneration or in culture. ...
... No pharmacology was performed to determine if those responses could be due to the activation of GABA receptors. Large GABA A and GABA C currents were recorded both in primary culture of pig cones (Picaud et al., 1998) and in putative mouse cones, either on flat mount rd1 mouse retinas or on dissociated from C57BL/6J retinas (Pattnaik et al., 2000). , from a holding potential of 274 mV, with 100 ms steps to potentials ranging from 146 mV to 2134 mV by 20 mV decrements (E Cl 5 229 mV intrapipette solution). ...
... The apparent discrepancy between our results and the reported presence of both GABA A and GABA C currents in cultured pig cones (Picaud et al., 1998) and in mouse cones from flat mount rd1 retinas or freshly dissociated from C57BL/6J retinas (Pattnaik et al., 2000) can be tentatively explained. In the former case, it could come either from a difference between mouse and pig cones or a partial dedifferentiation of pig cones after 2-10 days in culture. ...
At the first retinal synapse, horizontal cells (HCs) contact both photoreceptor terminals and bipolar cell dendrites, modulating information transfer between these two cell types to enhance spatial contrast and mediate color opponency. The synaptic mechanisms through which these modulations occur are still debated. The initial hypothesis of a GABAergic feedback from HCs to cones has been challenged by pharmacological inconsistencies. Surround antagonism has been demonstrated to occur via a modulation of cone calcium channels through ephaptic signaling and pH changes in the synaptic cleft. GABAergic transmission between HCs and cones has been reported in some lower vertebrates, like the turtle and tiger salamander. In these reports, it was revealed that GABA is released from HCs through reverse transport and target GABA receptors are located at the cone terminals. In mammalian retinas, there is growing evidence that HCs can release GABA through conventional vesicular transmission, acting both on autaptic GABA receptors and on receptors expressed at the dendritic tips of the bipolar cells. The presence of GABA receptors on mammalian cone terminals remains equivocal. Here, we looked specifically for functional GABA receptors in mouse photoreceptors by recording in the whole-cell or amphotericin/gramicidin-perforated patch clamp configurations. Cones could be differentiated from rods through morphological criteria. Local GABA applications evoked a Cl ⁻ current in cones but not in rods. It was blocked by the GABA A receptor antagonist bicuculline methiodide and unaffected by the GABA C receptor antagonist TPMPA [(1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid]. The voltage dependency of the current amplitude was as expected from a direct action of GABA on cone pedicles but not from an indirect modulation of cone currents following the activation of the GABA receptors of HCs. This supports a direct role of GABA released from HCs in the control of cone activity in the mouse retina.
... Only THRB-or GNAT2-expressing cones were found in the early cohort, while NRL-expressing rods were captured in the late cohort. Comparison of these electrophysiological profiles confirmed that the early tdTomato+ cell recordings were predominantly weighted by cone responses, whereas late recordings were weighted by rods ( Figure 1I), consistent with prior mouse studies [19]. ...
Cell type-specific investigations commonly employ gene reporters or single-cell (sc) analytical techniques. However, reporter line development is arduous and generally limited to a single gene of interest, while scRNA-seq frequently yields equivocal results that preclude definitive cell identification. To examine gene expression profiles of multiple retinal cell types derived from human pluripotent stem cells (hPSCs), we performed scRNA-seq on optic vesicle-like structures (OVs) cultured under cGMP-compatible conditions. However, efforts to apply traditional scRNA-seq analytical methods based on unbiased algorithms were unrevealing. Therefore, we developed a simple, versatile, and universally applicable approach that generates gene expression data akin to those obtained from reporter lines. This method ranks single cells by expression level of a bait gene and searches the transcriptome for genes whose cell-to-cell rank order expression most closely matches that of the bait. Moreover, multiple bait genes can be combined to refine datasets. Using this approach, we provide further evidence for the authenticity of hPSC-derived retinal cell types. This article is protected by copyright. All rights reserved.
... Several studies show evidence of GABA receptors in the cones of various vertebrates (Yazulla et al. 1989;Liu et al. 2005). In mammals, GABA receptors within the photoreceptor terminal invagination are found in bipolar cell processes, especially opposite contacts with horizontal cell processes (Greferath et al. 1994;Vardi and Sterling 1994;Vardi et al. 1998), and in cone terminals (although the evidence is more limited; Vardi et al. 1998;Pattnaik et al. 2000). Thus, horizontal cell processes can be presynaptic to two different structures in the invagination: bipolar cell processes and the photoreceptor terminal membrane of the invagination (Fig. 7). ...
Typically, presynaptic terminals form a synapse directly on the surface of postsynaptic processes such as dendrite shafts and spines. However, some presynaptic terminals invaginate-entirely or partially-into postsynaptic processes. We survey these invaginating presynaptic terminals in all animals and describe several examples from the central nervous system, including giant fiber systems in invertebrates, and cup-shaped spines, electroreceptor synapses, and some specialized auditory and vestibular nerve terminals in vertebrates. We then examine mechanoreceptors and photoreceptors, concentrating on the complex of pre- and postsynaptic processes found in basal invaginations of the cell. We discuss in detail the role of vertebrate invaginating horizontal cell processes in both chemical and electrical feedback mechanisms. We also discuss the common presence of indenting or invaginating terminals in neuromuscular junctions on muscles of most kinds of animals, and especially discuss those of Drosophila and vertebrates. Finally, we consider broad questions about the advantages of possessing invaginating presynaptic terminals and describe some effects of aging and disease, especially on neuromuscular junctions. We suggest that the invagination is a mechanism that can enhance both chemical and electrical interactions at the synapse.
... Voltage steps ranged from -100 mV to 40 mV (10 mV increments). Below -60 mV, only weak linear inward currents were observed, and the cells typically exhibited a large outward current at potentials above -40 mV, as expected for cone photoreceptors (Fig. 5B, C) (32). ...
Aims:
Rod-derived cone viability factor long (RdCVFL) is an enzymatically active thioredoxin encoded by the nucleoredoxin-like-1 (Nxnl1) gene. The second product of the gene, RdCVF, made by alternative splicing is a novel trophic factor secreted by rods that protects cones in rodent models of retinitis pigmentosa, the most prevalent inherited retinal disease. It acts on the cones by stimulating aerobic glycolysis through its interaction with a complex containing basigin-1 and the glucose transport GLUT1. We studied the role of Nxnl1 in cones after its homologous recombination using a transgenic line expressing Cre recombinase under the control of a cone opsin promoter.
Results:
We show that the cones of these mice are dysfunctional and degenerate by 8 months of age. The age-related deficit in cones is exacerbated in young animals by exposure to high level of oxygen. In agreement with this phenotype, we found that the cones express only one of the two Nxnl1 gene products, the thioredoxin RdCVFL. Administration of RdCVFL to the mouse carrying a deletion of the Nxnl1 gene in cones reduces the damage produced by oxidative stress. Silencing the expression of RdCVFL in cone-enriched culture reduces cell viability showing that RdCVFL is a cell-autonomous mechanism of protection.
Innovation:
This novel mode of action is certainly relevant for the therapy of retinitis pigmentosa since the delivery into cones of the rd10 mouse, a recessive model of the disease, rescues cones.
Conclusion:
Our work highlights the duality of the Nxnl1 gene which protects the cones by two distinct mechanisms.
... GABA C Rs are expressed in many different retinal neurons, including horizontal cells (Blanco et al. 1996;Dong and Werblin 1996;Enz et al. 1996;Qian and Dowling 1993), cone photoreceptors (Pattnaik et al. 2000), a subset of ganglion cells (Rotolo and Dacheux 2003), and, most notably, bipolar cells (Feigenspan et al. 1993;Lukasiewicz and Werblin 1994;Matthews et al. 1994;Pan and Lipton 1995). GABA C Rs do not appear to be expressed by amacrine cells and do not mediate inhibition within amacrine cell networks (Lukasiewicz and Shields 1998;Zhou and Fain 1995). ...
Neuronal microcircuits - small, localized signaling motifs involving two or more neurons - underlie signal processing and computation in the brain. Compartmentalized signaling within a neuron may enable it to participate in multiple, independent microcircuits. Each A17 amacrine cell in the mammalian retina contains within its dendrites hundreds of synaptic feedback microcircuits that operate independently to modulate feedforward signaling in the inner retina. Each of these microcircuits comprises a small (<1 μm) synaptic varicosity that typically receives one excitatory synapse from a presynaptic rod bipolar cell (RBC) and returns two reciprocal inhibitory synapses back onto the same RBC terminal. Feedback inhibition from the A17 sculpts the feedforward signal from the RBC to the AII, a critical component of the circuitry mediating night vision. Here, we show that the two inhibitory synapses from the A17 to the RBC express kinetically distinct populations of GABA receptors: rapidly activating GABAARs are enriched at one synapse while more slowly activating GABACRs are enriched at the other. Anatomical and electrophysiological data suggest that macromolecular complexes of voltage-gated (Cav) channels and Ca(2+)-activated K(+) channels help to regulate GABA release from A17 varicosities and limit GABACR activation under certain conditions. Finally, we find that selective elimination of A17-mediated feedback inhibition reduces the signal to noise ratio of responses to dim flashes recorded in the feedforward pathway (i.e. the AII amacrine cell). We conclude that A17-mediated feedback inhibition improves the signal to noise ratio of RBC-AII transmission near visual threshold, thereby improving visual sensitivity at night.
Copyright © 2015, Journal of Neurophysiology.
... The majority of the studies discussed in this section derive from interactomics approaches, with MAP1B interacting partners listed in Table 2. MAP1B interacts with several ligand-gated ion channels or transmembrane receptors and shows different physiological effects in each case. The q1 and q2 subunits of the ionotropic Cl 2 -permeable GABAcR interact with MAP1B HC, anchoring the channel subunits to microtubules, modifying channel activity and reducing its sensitivity (Hanley et al., 1999;Billups et al., 2000;Pattnaik et al., 2000). ...
The functions of microtubule-associated protein 1B (MAP1B) have historically been linked to the development of the nervous system, based on its very early expression in neurons and glial cells. Moreover, mice in which MAP1B is genetically inactivated have been used extensively to show its role in axonal elongation, neuronal migration and axonal guidance. In the last few years, it has become apparent that MAP1B has other cellular and molecular functions that are not related to its microtubule-stabilizing properties in the embryonic and adult brain. In this review, we present a systematic review of the canonical and novel functions of MAP1B and propose that, in addition to regulating the polymerization of microtubule and actin microfilaments, MAP1B also acts as a signaling protein involved in normal physiology and pathological conditions in the nervous system. © 2014 Wiley Periodicals, Inc. Develop Neurobiol, 2014.
