GABAergic and glycinergic IPSCs in ganglion cells of rat retinal slices

Inhibitory inputs to an inhibitory interneuron: Spontaneous postsynaptic currents and GABAA receptors of A17 amacrine cells in the rat retina

Amacrine cells constitute a large and heterogenous group of inhibitory interneurons in the retina. The A17 amacrine plays an important role for visual signaling in the rod pathway microcircuit of the mammalian retina. It receives excitatory input from rod bipolar cells and provides feedback inhibition to the same cells. However, from ultrastructural investigations, there is evidence for input to A17s from other types of amacrine cells, presumably inhibitory, but there is a lack of information about the identity and functional properties of the synaptic receptors and how inhibition contributes to the integrative properties of A17s. Here, we studied the biophysical and pharmacological properties of GABAergic spontaneous inhibitory postsynaptic currents (spIPSCs) and GABA_A receptors of A17 amacrines, using whole-cell and outside-out patch recordings from rat retinal slices. The spIPSCs displayed fast onsets (10-90% rise time ~740 μs) and double-exponential decays (τ_fast ~4.5 ms [43% of amplitude]; τ_slow ~22 ms). Ultrafast application of brief pulses of GABA (3 mM) to patches evoked responses with deactivation kinetics best fitted by a triple-exponential function (τ₁ ~5.3 ms [55% of amplitude]; τ₂ ~48 ms [32% amplitude]; τ₃ ~187 ms). Non-stationary noise analysis of spIPSCs and patch responses yielded single-channel conductances of ~21 and ~25 pS, respectively. Pharmacological analysis suggested that the spIPSCs are mediated by receptors with an α1βγ2 subunit composition and the somatic receptors have an α2βγ2 and/or α3βγ2 composition. These results demonstrate the presence of synaptic GABA_A receptors on A17s, which may play an important role in signal integration in these cells.

Orexin-A differentially modulates inhibitory and excitatory synaptic transmission in rat inner retina

In this work, modulation by orexin-A of the release of glutamate and GABA from bipolar and amacrine cells respectively was studied by examining the effects of the neuropeptide on miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs) of rat retinal ganglion cells (GCs). Using RNAscope in situ hybridization in combination with immunohistochemistry, we showed positive signals for orexin receptor-1 (OX₁R) mRNA in the bipolar cell terminals and those for orexin receptor-2 (OX₂R) mRNA in the amacrine cell terminals. With whole-cell patch-clamp recordings in rat retinal slices, we demonstrated that application of orexin-A reduced the interevent interval of mEPSCs of GCs through OX₁R. However, it increased the interevent interval of mIPSCs, mediated by GABA_A receptors, through OX₂R. Furthermore, orexin-A-induced reduction of mEPSC interevent interval was abolished by the application of PI-PLC inhibitors or PKC inhibitors. In contrast, orexin-A-induced increase of GABAergic mIPSC interevent interval was mimicked by 8-Br-cAMP or an adenylyl cyclase activator, but was eliminated by PKA antagonists. Finally, application of nimodipine, an L-type Ca²⁺ channel blocker, increased both mEPSC and mIPSC interevent interval, and co-application of orexin-A no longer changed the mEPSCs and mIPSCs. We conclude that orexin-A increases presynaptic glutamate release onto GCs by activating L-type Ca²⁺ channels in bipolar cells, a process that is mediated by an OX₁R/PI-PLC/PKC signaling pathway. However, orexin-A decreases presynaptic GABA release onto GCs by inhibiting L-type Ca²⁺ channels in amacrine cells, a process that is mediated by an OX₂R/cAMP-PKA signaling pathway.

Intravitreal Delivery of Melatonin Is Protective Against the Photoreceptor Loss in Mice: A Potential Therapeutic Strategy for Degenerative Retinopathy

Melatonin is a circadian hormone with potent cytoprotective effects. Retinitis pigmentosa (RP) comprises a heterogeneous group of inherent retinopathies that characterized by the photoreceptor death in bilateral eyes. The N-methyl-N-nitrosourea (MNU) administered mouse is a type of chemically induced RP model with rapid progressive rate. We intend to study the melatonin mediated effects on the MNU administered mice. Melatonin was delivered into the vitreous body of the MNU administered mice. Subsequently, the melatonin treated mice were subjected to histological analysis, optokinetic behavior tests, spectral-domain optical coherence tomography (SD-OCT), and electroretinogram (ERG) examination. Multi-electrodes array (MEA) was used to analyze the status of visual signal transmission within retinal circuits. Biochemical analysis was performed to quantify the expression levels of antioxidative enzymes, oxidative stress markers, and apoptotic factors in the retinas. The intravitreal injection of melatonin ameliorated effectively the MNU induced photoreceptor degeneration. Melatonin therapy mitigated the spontaneous firing response, and preserved the basic configurations of visual signal pathway in MNU administered mice. MEA is effective to evaluate the pharmacological effects on retina. Of note, the cone photoreceptors in degenerative retinas were rescued efficiently by melatonin therapy. Melatonin afforded these protective effects by modulating the apoptotic cascades and alleviating the oxidative stress. These findings suggest that melatonin could act as an alternative treatment for degenerative retinopathy. Melatonin might be used in combination with other therapeutic approaches to alleviate the photoreceptor loss and preserve the visual function of RP patients.

Rac1 Modulates Excitatory Synaptic Transmission in Mouse Retinal Ganglion Cells

Ras-related C3 botulinum toxin substrate 1 (Rac1), a member of the Rho GTPase family which plays important roles in dendritic spine morphology and plasticity, is a key regulator of cytoskeletal reorganization in dendrites and spines. Here, we investigated whether and how Rac1 modulates synaptic transmission in mouse retinal ganglion cells (RGCs) using selective conditional knockout of Rac1 (Rac1-cKO). Rac1-cKO significantly reduced the frequency of AMPA receptor-mediated miniature excitatory postsynaptic currents, while glycine/GABA_A receptor-mediated miniature inhibitory postsynaptic currents were not affected. Although the total GluA1 protein level was increased in Rac1-cKO mice, its expression in the membrane component was unchanged. Rac1-cKO did not affect spine-like branch density in single dendrites, but significantly reduced the dendritic complexity, which resulted in a decrease in the total number of dendritic spine-like branches. These results suggest that Rac1 selectively affects excitatory synaptic transmission in RGCs by modulating dendritic complexity.

Activation of group I metabotropic glutamate receptors regulates the excitability of rat retinal ganglion cells by suppressing Kir and I h

Group I metabotropic glutamate receptor (mGluR I) activation exerts a slow postsynaptic excitatory effect in the CNS. Here, the issues of whether and how this receptor is involved in regulating retinal ganglion cell (RGC) excitability were investigated in retinal slices using patch-clamp techniques. Under physiological conditions, RGCs displayed spontaneous firing. Extracellular application of LY367385 (10 µM)/MPEP (10 µM), selective mGluR1 and mGluR5 antagonists, respectively, significantly reduced the firing frequency, suggesting that glutamate endogenously released from bipolar cells constantly modulates RGC firing. DHPG (10 µM), an mGluR I agonist, significantly increased the firing and caused depolarization of the cells, which were reversed by LY367385, but not by MPEP, suggesting the involvement of the mGluR1 subtype. Intracellular Ca²⁺-dependent PI-PLC/PKC and calcium/calmodulin-dependent protein kinase II (CaMKII) signaling pathways mediated the DHPG-induced effects. In the presence of cocktail synaptic blockers (CNQX, D-AP5, bicuculline, and strychnine), which terminated the spontaneous firing in both ON and OFF RGCs, DHPG still induced depolarization and triggered the cells to fire. The DHPG-induced depolarization could not be blocked by TTX. In contrast, Ba²⁺, an inwardly rectifying potassium channel (Kir) blocker, and Cs⁺ and ZD7288, hyperpolarization-activated cation channel (I _h) blockers, mimicked the effect of DHPG. Furthermore, in the presence of Ba²⁺/ZD7288, DHPG did not show further effects. Moreover, Kir and I _h currents could be recorded in RGCs, and extracellular application of DHPG indeed suppressed these currents. Our results suggest that activation of mGluR I regulates the excitability of rat RGCs by inhibiting Kir and I _h.

Temporal properties of dual-peak responses of mouse retinal ganglion cells and effects of inhibitory pathways

Dual-peak responses of retinal ganglion cells (RGCs) are observed in various species, previous researches suggested that both response peaks were involved in retinal information coding. In the present study, we investigated the temporal properties of the dual-peak responses recorded in mouse RGCs elicited by spatially homogeneous light flashes and the effect of the inhibitory inputs mediated by GABAergic and/or glycinergic pathways. We found that the two peaks in the dual-peak responses exhibited distinct temporal dynamics, similar to that of short-latency and long-latency single-peak responses respectively. Pharmacological studies demonstrated that the application of exogenous GABA or glycine greatly suppressed or even eliminated the second peak of the cells' firing activities, while little change was induced in the first peak. Co-application of glycine and GABA led to complete elimination of the second peak. Moreover, application of picrotoxin or strychnine induced dual-peak responses in some cells with transient responses by unmasking a second response phase. These results suggest that both GABAergic and glycinergic pathways are involved in the dual-peak responses of the mouse RGCs, and the two response peaks may arise from distinct pathways that would converge on the ganglion cells.

GABAergic neurotransmission and retinal ganglion cell function

Ganglion cells are the output retinal neurons that convey visual information to the brain. There are ~20 different types of ganglion cells, each encoding a specific aspect of the visual scene as spatial and temporal contrast, orientation, direction of movement, presence of looming stimuli; etc. Ganglion cell functioning depends on the intrinsic properties of ganglion cell's membrane as well as on the excitatory and inhibitory inputs that these cells receive from other retinal neurons. GABA is one of the most abundant inhibitory neurotransmitters in the retina. How it modulates the activity of different types of ganglion cells and what is its significance in extracting the basic features from visual scene are questions with fundamental importance in visual neuroscience. The present review summarizes current data concerning the types of membrane receptors that mediate GABA action in proximal retina; the effects of GABA and its antagonists on the ganglion cell light-evoked postsynaptic potentials and spike discharges; the action of GABAergic agents on centre-surround organization of the receptive fields and feature related ganglion cell activity. Special emphasis is put on the GABA action regarding the ON-OFF and sustained-transient ganglion cell dichotomy in both nonmammalian and mammalian retina.

Orexin-A differentially modulates AMPA-preferring responses of ganglion cells and amacrine cells in rat retina

By activating their receptors (OX1R and OX2R) orexin-A/B regulate wake/sleeping states, feeding behaviors, but the function of these peptides in the retina remains unknown. Using patch-clamp recordings and calcium imaging in rat isolated retinal cells, we demonstrated that orexin-A suppressed α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)-preferring receptor-mediated currents (AMPA-preferring currents) in ganglion cells (GCs) through OX1R, but potentiated those in amacrine cells (ACs) through OX2R. Consistently, in rat retinal slices orexin-A suppressed light-evoked AMPA-preferring receptor-mediated excitatory postsynaptic currents in GCs, but potentiated those in ACs. Intracellular dialysis of GDP-β-S or preincubation with the Gi/o inhibitor pertussis toxin (PTX) abolished both the effects. Either cAMP/the protein kinase A (PKA) inhibitor Rp-cAMP or cGMP/the PKG blocker KT5823 failed to alter the orexin-A effects. Whilst both of them involved activation of protein kinase C (PKC), the effects on GCs and ACs were respectively eliminated by the phosphatidylinositol (PI)-phospholipase C (PLC) inhibitor and phosphatidylcholine (PC)-PLC inhibitor. Moreover, in GCs orexin-A increased [Ca(2+)]i and the orexin-A effect was blocked by intracellular Ca(2+)-free solution and by inositol 1,4,5-trisphosphate (IP3) receptor antagonists. In contrast, orexin-A did not change [Ca(2+)]i in ACs and the orexin-A effect remained in intracellular or extracellular Ca(2+)-free solution. We conclude that a distinct Gi/o/PI-PLC/IP3/Ca(2+)-dependent PKC signaling pathway, following the activation of OX1R, is likely responsible for the orexin-A effect on GCs, whereas a Gi/o/PC-PLC/Ca(2+)-independent PKC signaling pathway, following the activation of OX2R, mediates the orexin-A effect on ACs. These two actions of orexin-A, while working in concert, provide a characteristic way for modulating information processing in the inner retina.

Activity-dependent modulation of inhibitory synaptic kinetics in the cochlear nucleus

Spherical bushy cells (SBCs) in the anteroventral cochlear nucleus respond to acoustic stimulation with discharges that precisely encode the phase of low-frequency sound. The accuracy of spiking is crucial for sound localization and speech perception. Compared to the auditory nerve input, temporal precision of SBC spiking is improved through the engagement of acoustically evoked inhibition. Recently, the inhibition was shown to be less precise than previously understood. It shifts from predominantly glycinergic to synergistic GABA/glycine transmission in an activity-dependent manner. Concurrently, the inhibition attains a tonic character through temporal summation. The present study provides a comprehensive understanding of the mechanisms underlying this slow inhibitory input. We performed whole-cell voltage clamp recordings on SBCs from juvenile Mongolian gerbils and recorded evoked inhibitory postsynaptic currents (IPSCs) at physiological rates. The data reveal activity-dependent IPSC kinetics, i.e., the decay is slowed with increased input rates or recruitment. Lowering the release probability yielded faster decay kinetics of the single- and short train-IPSCs at 100 Hz, suggesting that transmitter quantity plays an important role in controlling the decay. Slow transmitter clearance from the synaptic cleft caused prolonged receptor binding and, in the case of glycine, spillover to nearby synapses. The GABAergic component prolonged the decay by contributing to the asynchronous vesicle release depending on the input rate. Hence, the different factors controlling the amount of transmitters in the synapse jointly slow the inhibition during physiologically relevant activity. Taken together, the slow time course is predominantly determined by the receptor kinetics and transmitter clearance during short stimuli, whereas long duration or high frequency stimulation additionally engage asynchronous release to prolong IPSCs.

Niemann-Pick type C1 patient-specific induced pluripotent stem cells display disease specific hallmarks

Background

Niemann-Pick type C1 disease (NPC1) is a rare progressive neurodegenerative disorder caused by mutations in the NPC1 gene. In this lysosomal storage disorder the intracellular transport and sequestration of several lipids like cholesterol is severely impaired, resulting in an accumulation of lipids in late endosomes and lysosomes. The neurological manifestation of the disease is caused by dysfunction and cell death in the central nervous system. Several animal models were used to analyze the impaired pathways. However, the underlying pathogenic mechanisms are still not completely understood and the genetic variability in humans cannot be reflected in these models. Therefore, a human model using patient-specific induced pluripotent stem cells provides a promising approach.

Methods

We reprogrammed human fibroblasts from a NPC1 patient and a healthy control by retroviral transduction with Oct4, Klf4, Sox2 and c-Myc. The obtained human induced pluripotent stem cells (hiPSCs) were characterized by immunocytochemical analyses. Neural progenitor cells were generated and patch clamp recordings were performed for a functional analysis of derived neuronal cells. Filipin stainings and the Amplex Red assay were used to demonstrate and quantify cholesterol accumulation.

Results

The hiPSCs expressed different stem cell markers, e.g. Nanog, Tra-1-81 and SSEA4. Using the embryoid body assay, the cells were differentiated in cells of all three germ layers and induced teratoma in immunodeficient mice, demonstrating their pluripotency. In addition, neural progenitor cells were derived and differentiated into functional neuronal cells. Patch clamp recordings revealed voltage dependent channels, spontaneous action potentials and postsynaptic currents. The accumulation of cholesterol in different tissues is the main hallmark of NPC1. In this study we found an accumulation of cholesterol in fibroblasts of a NPC1 patient, derived hiPSCs, and neural progenitor cells, but not in cells derived from fibroblasts of a healthy individual. These findings were quantified by the Amplex Red assay, demonstrating a significantly elevated cholesterol level in cells derived from fibroblasts of a NPC1 patient.

Conclusions

We generated a neuronal model based on induced pluripotent stem cells derived from patient fibroblasts, providing a human in vitro model to study the pathogenic mechanisms of NPC1 disease.

Inner retinal inhibition shapes the receptive field of retinal ganglion cells in primate

Abstract The centre–surround organisation of receptive fields is a feature of most retinal ganglion cells (RGCs) and is critical for spatial discrimination and contrast detection. Although lateral inhibitory processes are known to be important in generating the receptive field surround, the contribution of each of the two synaptic layers in the primate retina remains unclear. Here we studied the spatial organisation of excitatory and inhibitory synaptic inputs onto ON and OFF ganglion cells in the primate retina. All RGCs showed an increase in excitation in response to stimulus of preferred polarity. Inhibition onto RGCs comprised two types of responses to preferred polarity: some RGCs showed an increase in inhibition whilst others showed removal of tonic inhibition. Excitatory inputs were strongly spatially tuned but inhibitory inputs showed more variable organisation: in some neurons they were as strongly tuned as excitation, and in others inhibitory inputs showed no spatial tuning. We targeted one source of inner retinal inhibition by functionally ablating spiking amacrine cells with bath application of tetrodotoxin (TTX). TTX significantly reduced the spatial tuning of excitatory inputs. In addition, TTX reduced inhibition onto those RGCs where a stimulus of preferred polarity increased inhibition. Reconstruction of the spatial tuning properties by somatic injection of excitatory and inhibitory synaptic conductances verified that TTX-mediated inhibition onto bipolar cells increases the strength of the surround in RGC spiking output. These results indicate that in the primate retina inhibitory mechanisms in the inner plexiform layer sharpen the spatial tuning of ganglion cells.

Excitatory and inhibitory contributions to receptive fields of alpha-like retinal ganglion cells in mouse

The ON and OFF pathways that emerge at the first synapse in the retina are generally thought to be streamed in parallel to higher visual areas, but recent work shows cross talk at the level of retinal ganglion cells. The ON pathway drives inhibitory inputs onto some OFF ganglion cells, such that these neurons show "push-pull" convergence of OFF-excitation and ON-disinhibition. In this study we measure the spatial receptive field of excitatory and inhibitory inputs to OFF-sustained (OFF-S) retinal ganglion cells of mouse, establish how contrast adaptation modulates excitatory and inhibitory synaptic inputs, and show the pharmacology of the inhibitory inputs. We find that the spatial tuning properties of excitatory and inhibitory inputs are sufficient to determine the spatial profile of the spike output and that high spatial acuity may be particularly reliant on disinhibitory circuits. Contrast adaptation reduced excitation to OFF-S ganglion cells, as expected, and also unmasked an asymmetry in inhibitory inputs: disinhibition at light-off was immune to contrast adaptation, but inhibition at light-on was substantially reduced. In pharmacological experiments we confirm that inhibitory inputs are partly mediated by glycine, but our measurements also suggest a substantial role for GABA. Our observations therefore reveal functional diversity in the inhibitory inputs to OFF ganglion cells and suggest that in addition to enhancing operational range these inputs help shape the spatial receptive fields of ganglion cells.

Receptor targets of amacrine cells

Amacrine cells are a morphologically and functionally diverse group of inhibitory interneurons. Morphologically, they have been divided into approximately 30 types. Although this diversity is probably important to the fine structure and function of the retinal circuit, the amacrine cells have been more generally divided into two subclasses. Glycinergic narrow-field amacrine cells have dendrites that ramify close to their somas, cross the sublaminae of the inner plexiform layer, and create cross talk between its parallel ON and OFF pathways. GABAergic wide-field amacrine cells have dendrites that stretch long distances from their soma but ramify narrowly within an inner plexiform layer sublamina. These wide-field cells are thought to mediate inhibition within a sublamina and thus within the ON or OFF pathway. The postsynaptic targets of all amacrine cell types include bipolar, ganglion, and other amacrine cells. Almost all amacrine cells use GABA or glycine as their primary neurotransmitter, and their postsynaptic receptor targets include the most common GABA(A), GABA(C), and glycine subunit receptor configurations. This review addresses the diversity of amacrine cells, the postsynaptic receptors on their target cells in the inner plexiform layer of the retina, and some of the inhibitory mechanisms that arise as a result. When possible, the effects of GABAergic and glycinergic inputs on the visually evoked responses of their postsynaptic targets are discussed.

Human neural progenitor cells show functional neuronal differentiation and regional preference after engraftment onto hippocampal slice cultures

The transplantation of stem cells offers potential therapies for many neurodegenerative disorders that currently have limited or no treatment options. However, relatively little is known about how the host environment affects the development and integration of these cells. In this study we have engrafted immortalized human midbrain neural progenitor cells (NPCs) onto rat hippocampal brain slice cultures to examine the influence of a neural environment on differentiation. Patch clamp recordings revealed that the transplanted progenitor cells could express neuronal-type voltage-gated currents and rapidly receive synaptic input from the hippocampal brain slice. The distribution of progenitor cells across the hippocampal slices was strongly influenced by the neural architecture, with most cells located in the fissural regions and sending processes parallel to the laminar structure, while in contrast, cells located in the dentate gyrus showed no organized pattern. Almost no cells were found in the stratum radiatum or pyramidal cell layers. Together, these results demonstrate the potential for the architecture of the host environment to regulate the integration of transplanted cells, and highlight the utility of coculture systems for studying the mechanisms underlying the migration, integration, and differentiation of human NPCs in structured neural environments.

Colocalization of hyperpolarization-activated, cyclic nucleotide-gated channel subunits in rat retinal ganglion cells

The current-passing pore of mammalian hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels is formed by subunit isoforms denoted HCN1-4. In various brain areas, antibodies directed against multiple isoforms bind to single neurons, and the current (I(h)) passed during hyperpolarizations differs from that of heterologously expressed homomeric channels. By contrast, retinal rod, cone, and bipolar cells appear to use homomeric HCN channels. Here, we assess the generality of this pattern by examining HCN1 and HCN4 immunoreactivity in rat retinal ganglion cells, measuring I(h) in dissociated cells, and testing whether HCN1 and HCN4 proteins coimmunoprecipitate. Nearly half of the ganglion cells in whole-mounted retinae bound antibodies against both isoforms. Consistent with colocalization and physical association, 8-bromo-cAMP shifted the voltage sensitivity of I(h) less than that of HCN4 channels and more than that of HCN1 channels, and HCN1 coimmunoprecipitated with HCN4 from membrane fraction proteins. Finally, the immunopositive somata ranged in diameter from the smallest to the largest in rat retina, the dendrites of immunopositive cells arborized at various levels of the inner plexiform layer and over fields of different diameters, and I(h) activated with similar kinetics and proportions of fast and slow components in small, medium, and large somata. These results show that different HCN subunits colocalize in single retinal ganglion cells, identify a subunit that can reconcile native I(h) properties with the previously reported presence of HCN4 in these cells, and indicate that I(h) is biophysically similar in morphologically diverse retinal ganglion cells and differs from I(h) in rods, cones, and bipolar cells.

Genetic targeting and physiological features of VGLUT3+ amacrine cells

Amacrine cells constitute a diverse class of interneurons that contribute to visual signal processing in the inner retina, but surprisingly, little is known about the physiology of most amacrine cell subtypes. Here, we have taken advantage of the sparse expression of vesicular glutamate transporter 3 (VGLUT3) in the mammalian retina to target the expression of yellow fluorescent protein (YFP) to a unique population of amacrine cells using a new transgenic mouse line. Electrophysiological recordings made from YFP-positive (VGLUT3+) amacrine cells provide the first functional data regarding the active membrane properties and synaptic connections of this recently identified cell type. We found that VGLUT3+ amacrine cells receive direct synaptic input from bipolar cells via both N-methyl-d-aspartate receptors (NMDARs) and non-NMDARs. Voltage-gated sodium channels amplified these excitatory inputs but repetitive spiking was never observed. VGLUT3+ amacrine cells responded transiently to both light increments (ON response) and decrements (OFF response); ON responses consisted exclusively of inhibitory inputs, while OFF responses comprised both excitatory and inhibitory components, although the inhibitory conductance was larger in amplitude and longer in time course. The physiological properties and anatomical features of the VGLUT3+ amacrine cells suggest that this bistratified interneuron may play a role in disinhibitory signaling and/or crossover inhibition between parallel pathways in the retina.

Cannabinoids modulate spontaneous synaptic activity in retinal ganglion cells

The endocannabinoid (ECB) system has been found throughout the central nervous system and modulates cell excitability in various forms of short-term plasticity. ECBs and their receptors have also been localized to all retinal cells, and cannabinoid receptor activation has been shown to alter voltage-dependent conductances in several different retinal cell types, suggesting a possible role for cannabinoids in retinal processing. Their effects on synaptic transmission in the mammalian retina, however, have not been previously investigated. Here, we show that exogenous cannabinoids alter spontaneous synaptic transmission onto retinal ganglion cells (RGCs). Using whole-cell voltage-clamp recordings in whole-mount retinas, we measured spontaneous postsynaptic currents (SPSCs) in RGCs in adult and young (P14-P21) mice. We found that the addition of an exogenous cannabinoid agonist, WIN55212-2 (5 μM), caused a significant reversible reduction in the frequency of SPSCs. This change, however, did not alter the kinetics of the SPSCs, indicating a presynaptic locus of action. Using blockers to isolate inhibitory or excitatory currents, we found that cannabinoids significantly reduced the release probability of both GABA and glutamate, respectively. While the addition of cannabinoids reduced the frequency of both GABAergic and glutamatergic SPSCs in both young and adult mice, we found that the largest effect was on GABA-mediated currents in young mice. These results suggest that the ECB system may potentially be involved in the modulation of signal transmission in the retina. Furthermore, they suggest that it might play a role in the developmental maturation of synaptic circuits, and that exogenous cannabinoids are likely able to disrupt retinal processing and consequently alter vision.

Direct and indirect control of orexin/hypocretin neurons by glycine receptors

Hypothalamic hypocretin/orexin (hcrt/orx) neurons promote arousal and reward seeking, while reduction in their activity has been linked to narcolepsy, obesity and depression. However, the mechanisms influencing the activity of hcrt/orx networks in situ are not fully understood. Here we show that glycine, a neurotransmitter best known for its actions in the brainstem and spinal cord, elicits dose dependent postsynaptic Cl⁻ currents in hcrt/orx cells in acute mouse brain slices. The effect was blocked by the glycine receptor (GLyR) antagonist strychnine and mimicked by the GlyR agonist alanine. Postsynaptic GlyRs on hcrt/orx cells remained functional during both early postnatal and adult periods, and gramicidin-perforated patch-clamp recordings revealed that they progressively switch from excitatory to inhibitory during the first two postnatal weeks. The pharmacological profile of the glycine response suggested that developed hcrt/orx neurons contain α/β-heteromeric GlyRs that lack α2-subunits, whereas α2-subunits, whereas α2-subunits are present in early postnatal hcrt/orx neurons. All postsynaptic currents (PSCs) in developed hcrt/orx cells were blocked by inhibitors of GABA and glutamate receptors, with no evidence of GlyR-mediated PSCs. However, the frequency but not amplitude of miniature PSCs was reduced by strychnine and increased by glycine in ~50% of hcrt/orx neurons. Together, these results provide the first evidence for functional GlyRs in identified hcrt/orx circuits and suggest that the activity of developed hcrt/orx cells is regulated by two GlyR pools: inhibitory extrasynaptic GlyRs located on all hcrt/orx cells and excitatory GlyRs located on presynaptic terminals contacting some hcrt/orx cells.

Melatonin potentiates glycine currents through a PLC/PKC signalling pathway in rat retinal ganglion cells

In vertebrate retina, melatonin regulates various physiological functions. In this work we investigated the mechanisms underlying melatonin-induced potentiation of glycine currents in rat retinal ganglion cells (RGCs). Immunofluorescence double labelling showed that rat RGCs were solely immunoreactive to melatonin MT(2) receptors. Melatonin potentiated glycine currents of RGCs, which was reversed by the MT(2) receptor antagonist 4-P-PDOT. The melatonin effect was blocked by intracellular dialysis of GDP-beta-S. Either preincubation with pertussis toxin or application of the phosphatidylcholine (PC)-specific phospholipase C (PLC) inhibitor D609, but not the phosphatidylinositol (PI)-PLC inhibitor U73122, blocked the melatonin effect. The protein kinase C (PKC) activator PMA potentiated the glycine currents and in the presence of PMA melatonin failed to cause further potentiation of the currents, whereas application of the PKC inhibitor bisindolylmaleimide IV abolished the melatonin-induced potentiation. The melatonin effect persisted when [Ca(2+)](i) was chelated by BAPTA, and melatonin induced no increase in [Ca(2+)](i). Neither cAMP-PKA nor cGMP-PKG signalling pathways seemed to be involved because 8-Br-cAMP or 8-Br-cGMP failed to cause potentiation of the glycine currents and both the PKA inhibitor H-89 and the PKG inhibitor KT5823 did not block the melatonin-induced potentiation. In consequence, a distinct PC-PLC/PKC signalling pathway, following the activation of G(i/o)-coupled MT(2) receptors, is most likely responsible for the melatonin-induced potentiation of glycine currents of rat RGCs. Furthermore, in rat retinal slices melatonin potentiated light-evoked glycine receptor-mediated inhibitory postsynaptic currents in RGCs. These results suggest that melatonin, being at higher levels at night, may help animals to detect positive or negative contrast in night vision by modulating inhibitory signals largely mediated by glycinergic amacrine cells in the inner retina.

Caffeine inhibition of ionotropic glycine receptors

We found that caffeine is a structural analogue of strychnine and a competitive antagonist at ionotropic glycine receptors (GlyRs). Docking simulations indicate that caffeine and strychnine may bind to similar sites at the GlyR. The R131A GlyR mutation, which reduces strychnine antagonism without suppressing activation by glycine, also reduces caffeine antagonism. GlyR subtypes have differing caffeine sensitivity. Tested against the EC(50) of each GlyR subtype, the order of caffeine potency (IC(50)) is: alpha2beta (248 +/- 32 microm) alpha3beta (255 +/- 16 microm) > alpha4beta (517 +/- 50 microm) > alpha1beta(837 +/- 132 microm). However, because the alpha3beta GlyR is more than 3-fold less sensitive to glycine than any of the other GlyR subtypes, this receptor is most effectively blocked by caffeine. The glycine dose-response curves and the effects of caffeine indicate that amphibian retinal ganglion cells do not express a plethora of GlyR subtypes and are dominated by the alpha1beta GlyR. Comparing the effects of caffeine on glycinergic spontaneous and evoked IPSCs indicates that evoked release elevates the glycine concentration at some synapses whereas summation elicits evoked IPSCs at other synapses. Caffeine serves to identify the pharmacophore of strychnine and produces near-complete inhibition of glycine receptors at concentrations commonly employed to stimulate ryanodine receptors.

Glycinergic input of widefield, displaced amacrine cells of the mouse retina

Glycine receptors (GlyRs) of displaced amacrine cells of the mouse retina were analysed using whole cell recordings and immunocytochemical staining with subunit-specific antibodies. During the recordings the cells were filled with a fluorescent tracer and 11 different morphological types could be identified. The studies were performed in wild-type mice and in mutant mice deficient in the GlyRalpha1 (Glra1(spd-ot), 'oscillator' mouse), the GlyRalpha2 (Glra2(-/-)) and the GlyRalpha3 subunit (Glra3(-/-)). Based on their responses to the application of exogenous glycine in the retinas of wild-type and mutant mice, the cells were grouped into three major classes: group I cells (comprising the morphological types MA-S5, MA-S1, MA-S1/S5, A17, PA-S1, PA-S5 and WA-S1), group II cells (comprising the morphological types PA-S4, WA-S3 and WA-multi) and ON-starburst cells. For further analysis, spontaneous inhibitory postsynaptic currents (sIPSCs) were measured both in wild-type and mutant mouse retinas. Glycinergic sIPSCs and glycine induced currents of group I cells remained unaltered across wild-type and the three mutant mice (mean decay time constant of sIPSCs, tau approximately 25 ms). Group II cells showed glycinergic sIPSCs and glycine induced currents in wild-type, Glra1(spd-ot) and Glra3(-/-) mice (tau approximately 25 ms); however, glycinergic currents were absent in group II cells of Glra2(-/-) mice. Glycine induced currents and sIPSCs recorded from ON-starburst amacrine cells did not differ significantly between wild-type and the mutant mouse retinas (tau approximately 50-70 ms). We propose that GlyRs of group II cells are dominated by the alpha2 subunit; GlyRs of ON-starburst amacrine cells appear to be dominated by the alpha4 subunit.

Signals and noise in an inhibitory interneuron diverge to control activity in nearby retinal ganglion cells

Information about sensory stimuli is represented by spatiotemporal patterns of neural activity. The complexity of the central nervous system, however, frequently obscures the origin and properties of signals and noise that underlie these activity patterns. We minimized this constraint by examining mechanisms governing correlated activity in mouse retinal ganglion cells (RGCs) under conditions in which light-evoked responses traverse a specific circuit, the rod bipolar pathway. Signals and noise in this circuit produced correlated synaptic input to neighboring On and Off RGCs. Temporal modulation of light intensity did not alter the degree to which noise in the input to nearby RGCs was correlated, and action potential generation in individual RGCs was largely insensitive to differences in network noise generated by dynamic and static light stimuli. Together, these features enable noise in shared circuitry to diminish simultaneous action potential generation in neighboring On and Off RGCs under a variety of conditions.

Long-term dark rearing induces permanent reorganization in retinal circuitry

Recent data challenged the assumption that light has little effect on retina development. Here, we report evidence that dark rearing permanently changes the synaptic input to GCs. A reduced spontaneous postsynaptic currents (SPSCs) frequency was found in retinal GCs from rats born and raised in the dark for three months. Glutamate antagonists (CNQX and AP-5) reversibly reduced SPSCs frequency in control and dark-reared (DR) retinae. The GABA antagonist picrotoxin (PTX) reduced SPSCs frequency in control retinas, but increased SPSCs frequency in DR, mainly by presynaptic action on excitatory currents. In DR animals exposed to normal cyclic light for 3 months, SPSCs frequency remained lower then in control rats and increased following PTX, suggesting that long-term dark rearing induces permanent modifications of the retinal circuitry. Our results strongly support the idea that light stimulation plays a role in establishing normal synaptic input to GCs.

Hyperpolarization-activated cation current is involved in modulation of the excitability of rat retinal ganglion cells by dopamine

Modulation of membrane properties and excitability of retinal ganglion cells (RGCs) by dopamine was investigated in rat retinal slices, using whole cell patch clamp techniques. Application of dopamine (10 microM) caused a small depolarization of the membrane potential, a reduction of the input resistance and a decrease in the number of current-evoked action potentials of RGCs, and these effects were blocked by a D1 antagonist (SCH23390, 10 microM), but not by a D2 antagonist (sulpiride, 10 microM). SKF38393 (10 microM), a D1 agonist, but not quinpirole (10 microM), a D2 agonist, mimicked the effects of dopamine on RGCs. Like dopamine, 8-Br-cAMP, a membrane-permeable analog of cAMP, produced similar changes in the membrane properties and the excitability of RGCs. All these results suggest that these effects of dopamine are likely mediated by D1 receptors. Pre-application of KT5720, an inhibitor of protein kinase A (PKA), blocked the dopamine effects, indicating that the effects were PKA-dependent. Possible involvement of hyperpolarization-activated cation currents (I(h)) in the dopamine effects was tested. Inward currents were induced by voltage steps, with an activation threshold of around -70 mV, in the presence of TTX, Cd(2+), TEA-Cl and 4-AP. These currents, with a reversal potential of -33.2 mV, displayed inward rectification and were blocked by ZD7288, a specific I(h) channel blocker. These results are indicative of the presence of I(h) in rat RGCs. Dopamine increased the amplitude of I(h) and shifted the activation curve of I(h) to a range of more positive potentials. SKF38393 and 8-Br-cAMP increased the amplitude of I(h), which was blocked by KT5720. The dopamine effects were abolished when the preparations were pre-incubated by ZD7288. These data strongly suggest that the dopamine effects on rat RGCs may be, at least in part, mediated by modulation of I(h) through the cAMP- and PKA-dependent pathway.

Glycine receptors of A-type ganglion cells of the mouse retina

A-type ganglion cells of the mouse retina represent the visual channel that transfers temporal changes of the outside world very fast and with high fidelity. In this study we combined anatomical and physiological methods in order to study the glycinergic, inhibitory input of A-type ganglion cells. Immunocytochemical studies were performed in a transgenic mouse line whose ganglion cells express green fluorescent protein (GFP). The cells were double labeled for GFP and the four alpha subunits of the glycine receptor (GlyR). It was found that most of the glycinergic input of A-type cells is through fast, alpha1-expressing synapses. Whole-cell currents were recorded from A-type ganglion cells in retinal whole mounts. The response to exogenous application of glycine and spontaneous inhibitory postsynaptic currents (sIPSCs) were measured. By comparing glycinergic currents recorded in wildtype mice and in mice with specific deletions of GlyRalpha subunits (Glra1spd-ot, Glra2-/-, Glra3-/-), the subunit composition of GlyRs of A-type ganglion cells could be further defined. Glycinergic sIPSCs of A-type ganglion cells have fast kinetics (decay time constant tau = 3.9 +/- 2.5 ms, mean +/- SD). Glycinergic sIPSCs recorded in Glra2-/- and Glra3-/- mice did not differ from those of wildtype mice. However, the number of glycinergic sIPSCs was significantly reduced in Glra1spd-ot mice and the remaining sIPSCs had slower kinetics than in wildtype mice. The results show that A-type ganglion cells receive preferentially kinetically fast glycinergic inputs, mediated by GlyRs composed of alpha1 and beta subunits.

Studying properties of neurotransmitter receptors by non-stationary noise analysis of spontaneous postsynaptic currents and agonist-evoked responses in outside-out patches

Chemical synaptic transmission depends on neurotransmitter-gated ion channels concentrated in the postsynaptic membrane of specialized synaptic contacts. The functional characteristics of these neurotransmitter receptor channels are important for determining the properties of synaptic transmission. Whole-cell recording of postsynaptic currents (PSCs) and outside-out patch recording of transmitter-evoked currents are important tools for estimating the single-channel conductance and the number of receptors contributing to the PSC activated by a single transmitter quantum. When single-channel activity cannot be directly resolved, non-stationary noise analysis is a valuable tool for determining these parameters. Peak-scaled non-stationary noise analysis can be used to compensate for quantal variability in synaptic currents. Here, we present detailed protocols for conventional and peak-scaled non-stationary noise analysis of spontaneous PSCs and responses in outside-out patches. In addition, we include examples of computer code for individual functions used in the different stages of non-stationary noise analysis. These analysis procedures require 3-8 h.

Synaptic inputs to retinal ganglion cells that set the circadian clock

Melanopsin-containing retinal ganglion cells (RGCs) project to the suprachiasmatic nuclei (SCN) and mediate photoentrainment of the circadian system. Melanopsin is a novel retinal-based photopigment that renders these cells intrinsically photosensitive (ip). Although genetic ablation of melanopsin abolishes the intrinsic light response, it has a surprisingly minor effect on circadian photoentrainment. This and other non-visual responses to light are lost only when the melanopsin deficiency is coupled with mutations that disable classical rod and cone photoreceptors, suggesting that melanopsin-containing RGCs also receive rod- and cone-driven synaptic inputs. Using whole-cell patch-clamp recording, we demonstrate that light triggers synaptic currents in ipRGCs via activation of ionotropic glutamate and gamma-aminobutyric acid (GABA) receptors. Miniature postsynaptic currents (mPSCs) were clearly observed in ipRGCs, although they were less robust and were seen less frequently than those seen in non-ip cells. Pharmacological treatments revealed that the majority of ipRGCs receive excitatory glutamatergic inputs that were blocked by DNQX and/or kynurenic acid, as well as inhibitory GABAergic inputs that were blocked by bicuculline. Other ipRGCs received either glutamatergic or GABAergic inputs nearly exclusively. Although strychnine (Strych)-sensitive mPSCs were evident on many non-ipRGCs, indicating the presence of glycinergic inputs, we saw no evidence of Strych-sensitive events in ipRGCs. Based on these results, it is clear that SCN-projecting RGCs can respond to light both via an intrinsic melanopsin-based signaling cascade and via a synaptic pathway driven by classical rod and/or cone photoreceptors. It remains to be determined how the ipRGCs integrate these temporally distinct inputs to generate the signals that mediate circadian photoentrainment and other non-visual responses to light.

Spontaneous IPSCs and glycine receptors with slow kinetics in wide-field amacrine cells in the mature rat retina

The functional properties of glycine receptors were analysed in different types of wide-field amacrine cells, narrowly stratifying cells considered to play a role in larger-scale integration across the retina. The patch-clamp technique was used to record spontaneous IPSCs (spIPSCs) and glycine-evoked patch responses from mature rat retinal slices (4-7 weeks postnatal). Glycinergic spIPSCs were blocked reversibly by strychnine (300 nM). Compared to previously described spIPSCs in AII amacrine cells, the spIPSCs in wide-field amacrine cells displayed a very slow decay time course (tau(fast) approximately 15 ms; tau(slow) approximately 57 ms). The kinetic properties of spIPSCs in whole-cell recordings were paralleled by even slower deactivation kinetics of responses evoked by brief pulses of glycine (3 mm) to outside-out patches from wide-field amacrine cells (tau(fast) approximately 45 ms; tau(slow) approximately 350 ms). Non-stationary noise analysis of patch responses and spIPSCs yielded similar average single-channel conductances (approximately 31 and approximately 34 pS, respectively). Similar, as well as both lower- and higher-conductance levels could be identified from directly observed single-channel gating during the decay phase of spIPSCs and patch responses. These results suggest that the slow glycinergic spIPSCs in wide-field amacrine cells involve alpha2beta heteromeric receptors. Taken together with previous work, the kinetic properties of glycine receptors in different types of amacrine cells display a considerable range that is probably a direct consequence of differential expression of receptor subunits. Unique kinetic properties are likely to differentially shape the glycinergic input to different types of amacrine cells and thereby contribute to distinct integrative properties among these cells.

Network variability limits stimulus-evoked spike timing precision in retinal ganglion cells

Visual, auditory, somatosensory, and olfactory stimuli generate temporally precise patterns of action potentials (spikes). It is unclear, however, how the precision of spike generation relates to the pattern and variability of synaptic input elicited by physiological stimuli. We determined how synaptic conductances evoked by light stimuli that activate the rod bipolar pathway control spike generation in three identified types of mouse retinal ganglion cells (RGCs). The relative amplitude, timing, and impact of excitatory and inhibitory input differed dramatically between On and Off RGCs. Spikes evoked by repeated somatic injection of identical light-evoked synaptic conductances were more temporally precise than those evoked by light. However, the precision of spikes evoked by conductances that varied from trial to trial was similar to that of light-evoked spikes. Thus, the rod bipolar pathway modulates different RGCs via unique combinations of synaptic input, and RGC temporal variability reflects variability in the input this circuit provides.

Dissociated GABAergic retinal interneurons exhibit spontaneous increases in intracellular calcium

Early in development, before the retina is responsive to light, neurons exhibit spontaneous activity. Recently it was demonstrated that starburst amacrine cells, a unique class of neurons that secretes both GABA and acetylcholine, spontaneously depolarize. Networks comprised of spontaneously active starburst cells initiate correlated bursts of action potentials that propagate across the developing retina with a periodicity on the order minutes. To determine whether other retinal interneurons have similar “pacemaking” properties, we have utilized cultures of dissociated neurons from the rat retina. In the presence of antagonists for fast neurotransmitter receptors, distinct populations of neurons exhibited spontaneous, uncorrelated increases in intracellular calcium concentration. These increases in intracellular calcium concentration were sensitive to tetrodotoxin, indicating they are mediated by spontaneous membrane depolarizations. By combining immunofluorescence and calcium imaging, we found that 44% of spontaneously active neurons were GABAergic and included starburst amacrine cells. Whole cell voltage clamp recordings in the absence of antagonists for fast neurotransmitters revealed that after 7 days in culture, individual retinal neurons receive bursts of GABA-A receptor mediated synaptic input with a periodicity similar to that measured in spontaneously active GABAergic neurons. Low concentrations of GABA-A receptor antagonists did not alter the inter-burst interval despite significant reduction of post-synaptic current amplitude, indicating that pacemaker activity of GABAergic neurons was not influenced by network interactions. Together, these findings indicate that spiking GABAergic interneurons can function as pacemakers in the developing retina.

Functional properties of spontaneous IPSCs and glycine receptors in rod amacrine (AII) cells in the rat retina

AII amacrine cells play a crucial role in retinal signal transmission under scotopic conditions. We have used rat retinal slices to investigate the functional properties of inhibitory glycine receptors on AII cells by recording spontaneous IPSCs (spIPSCs) in whole cells and glycine-evoked responses in outside-out patches. Glycinergic spIPSCs displayed fast kinetics with an average 10-90% rise time of approximately 500 mus, and a decay phase best fitted by a double-exponential function with tau(fast) approximately 4.8 ms (97.5% amplitude contribution) and tau(slow) approximately 33 ms. Decay kinetics were voltage dependent. Ultrafast application of brief ( approximately 2-5 ms) pulses of glycine (3 mm) to patches, evoked responses with fast deactivation kinetics best fitted with a double-exponential function with tau(fast) approximately 4.6 ms (85% amplitude contribution) and tau(slow) approximately 17 ms. Double-pulse experiments indicated recovery from desensitization after a 100-ms pulse of glycine with a double-exponential time course (tau(fast) approximately 71 ms and tau(slow) approximately 1713 ms). Non-stationary noise analysis of spIPSCs and patch responses, and directly observed channel gating yielded similar single-channel conductances ( approximately 41 to approximately 47 pS). In addition, single-channel gating occurred at approximately 83 pS. These results suggest that the fast glycinergic spIPSCs in AII cells are probably mediated by alpha1beta heteromeric receptors with a contribution from alpha1 homomeric receptors. We hypothesize that glycinergic synaptic input may target the arboreal dendrites of AII cells, and could serve to shunt excitatory input from rod bipolar cells and transiently uncouple the transcellular current through electrical synapses between AII cells and between AII cells and ON-cone bipolar cells.

Characterization of the glycinergic input to bipolar cells of the mouse retina

Glycine and gamma-aminobutyric acid (GABA) are the major inhibitory transmitters of the mammalian retina, and bipolar cells receive GABAergic and glycinergic inhibition from multiple amacrine cell types. Here we evaluated the functional properties and subunit composition of glycine receptors (GlyRs) in bipolar cells. Patch-clamp recordings were performed from retinal slices of wild-type, GlyRalpha1-deficient (Glra1(spd-ot)) and GlyRalpha3-deficient (Glra3(-/-)) mice. Whole-cell currents following glycine application and spontaneous inhibitory postsynaptic currents (IPSCs) were analysed. During the recordings the cells were filled with Alexa 488 and, thus, unequivocally identified. Glycine-induced currents of bipolar cells were picrotoxinin-insensitive and thus represent heteromeric channels composed of alpha and beta subunits. Glycine-induced currents and IPSCs were absent from all bipolar cells of Glra1(spd-ot) mice, indicating that GlyRalpha1 is an essential subunit of bipolar cell GlyRs. By comparing IPSCs of bipolar cells in wild-type and Glra3(-/-) mice, no statistically significant differences were found. OFF-cone bipolar (CB) cells receive a strong glycinergic input from AII amacrine cells, that is preferentially based on the fast alpha1beta-containing channels (mean decay time constant tau = 5.9 +/- 1.4 ms). We did not observe glycinergic IPSCs in ON-CB cells and could elicit only small, if any, glycinergic currents. Rod bipolar cells receive a prominent glycinergic input that is mainly mediated by alpha1beta-containing channels (tau = 5.5 +/- 1.6 ms). Slow IPSCs, the characteristic of GlyRs containing the alpha2 subunit, were not observed in bipolar cells. Thus, different bipolar cell types receive kinetically fast glycinergic inputs, preferentially mediated by GlyRs composed of alpha1 and beta subunits.

Quantal Encoding of Information in a Retinal Ganglion Cell

A retinal ganglion cell receives information about a white-noise stimulus as a flickering pattern of glutamate quanta. The ganglion cell reencodes this information as brief bursts of one to six spikes separated by quiescent periods. When the stimulus is repeated, the number of spikes in a burst is highly reproducible (variance < mean) and spike timing is precise to within 10 ms, leading to an estimate that each spike encodes about 2 bits. To understand how the ganglion cell reencodes information, we studied the quantal patterns by repeating a white-noise stimulus and recording excitatory currents from a voltage-clamped, brisk-sustained ganglion cell. Quanta occurred in synchronous bursts of 3 to 65; the resulting postsynaptic currents summed to form excitatory postsynaptic currents (EPSCs). The number of quanta in an EPSC was only moderately reproducible (variance = mean), quantal timing was precise to within 14 ms, and each quantum encoded 0.1–0.4 bit. In conclusion, compared to a spike, a quantum has similar temporal precision, but is less reproducible and encodes less information. Summing multiple quanta into discrete EPSCs improves the reproducibility of the overall quantal pattern and contributes to the reproducibility of the spike train.

Extrasynaptic localization of glycine receptors in the rat supraoptic nucleus: further evidence for their involvement in glia-to-neuron communication

Neurons of the rat supraoptic nucleus (SON) express glycine receptors (GlyRs), which are implicated in the osmoregulation of neuronal activity. The endogenous agonist of the receptors has been postulated to be taurine, shown to be released from astrocytes. We here provide additional pieces of evidence supporting the absence of functional glycinergic synapses in the SON. First, we show that blockade of GlyRs with strychnine has no effect on either the amplitude or frequency of miniature inhibitory postsynaptic currents recorded in SON neurons, whereas they were all suppressed by the GABA(A) antagonist gabazine. Then, double immunostaining of sections with presynaptic markers and either GlyR or GABA(A) receptor (GABA(A)R) antibodies indicates that, in contrast with GABA(A)Rs, most GlyR membrane clusters are not localized facing presynaptic terminals, indicative of their extrasynaptic localization. Moreover, we found a striking anatomical association between SON GlyR clusters and glial fibrillary acidic protein (GFAP)-positive astroglial processes, which contain high levels of taurine. This type of correlation is specific to GlyRs, since GABA(A)R clusters show no association with GFAP-positive structures. These results substantiate and strengthen the concept of extrasynaptic GlyRs mediating a paracrine communication between astrocytes and neurons in the SON.

Characterization of inhibitory postsynaptic currents in rod bipolar cells of the mouse retina

The synaptic terminals of mammalian rod bipolar cells are the targets of multiple presynaptic inhibitory inputs arriving from glycinergic and GABAergic amacrine cells. To investigate the contribution of these different inhibitory receptor types, we have applied the patch-clamp technique in acutely isolated slices of the adult mouse retina. By using the whole-cell configuration, we measured and analyzed the spontaneous postsynaptic currents (PSCs) in rod bipolar cells. The spontaneous synaptic activity of rod bipolar cells was very low. However, when amacrine cells were depolarized by AMPA or kainate, the PSC frequency in rod bipolar cells increased significantly. These PSCs comprised several types that could be distinguished by pharmacological and kinetic criteria. Strychnine-sensitive, glycinergic PSCs were characterized by a mean peak amplitude of -43.5 pA and a weighted decay time constant (tauw) of 10.9 ms. PSCs that persisted in the presence of strychnine, but were completely inhibited by bicuculline, were mediated by GABAARs. They had a mean peak amplitude of -20.0 pA and a significantly faster tauw of 5.8 ms. Few PSCs remained in the presence of strychnine and bicuculline, suggesting that they were mediated by GABACRs. These PSCs were characterized by much smaller amplitudes (-6.2 pA) and a significantly slower decay kinetics (tauw=51.0 ms). We conclude that rod bipolar cells express at least three types of functionally different inhibitory receptors, namely GABAARs, GABACRs, and GlyRs that may ultimately regulate the Ca2+ influx into rod bipolar cell terminals, thereby modulating their glutamate release.

Effects of GABA receptor antagonists on retinal glycine receptors and on homomeric glycine receptor alpha subunits

Glycinergic and GABAergic inhibition are juxtaposed at one retinal synaptic layer yet likely perform different functions. These functions have usually been evaluated using receptor antagonists. In examining retinal glycine receptors, we were surprised to find that commonly used concentrations of GABA antagonists blocked significant fractions of the glycine current. In retinal amacrine and ganglion cells, the competitive GABAA receptor antagonists (bicuculline and SR95531) were also competitive GlyR antagonists. Picrotoxinin produced a noncompetitive inhibition of retinal GlyRs. [1,2,5,6-tetrahydropyridine-4-yl] methylphosphinic acid, the GABACR antagonist, did not inhibit glycine receptors. All three GABAA receptor antagonists were competitive inhibitors of homomeric alpha1 or alpha2 GlyRs expressed in human embryonic kidney cells (HEK293) cells. Interestingly, bicuculline was much more effective at alpha2 GlyRs and might be used to separate glycine receptor subtypes. Thus commonly used concentrations of GABA antagonists do not unambiguously differentiate GABA and glycine pathways. Picrotoxinin inhibition of GABAC receptors requires two amino acids in the second transmembrane region (TM2): 2' serine and 6' threonine. Although TM2 regions in GABA and glycine receptors are highly homologous, neither 2' serine nor 6' threonine is essential for picrotoxinin sensitivity in glycine receptors.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Light signaling in scotopic conditions in the rabbit, mouse and rat retina: a physiological and anatomical study

In the dark, light signals are conventionally routed through the following circuit: rods synapse onto rod bipolar (RB) cells, which in turn contact AII amacrine cells. AII cells segregate the light signal into the on and off pathways by making electrical synapses with on cone bipolar (CB) cells and glycinergic inhibitory chemical synapses with off CB cells. These bipolar cells synapse onto their respective ganglion cells, which transfer on and off signals to the visual centers of the brain. Two alternative pathways have recently been postulated for the signal transfer in scotopic conditions: 1) electrical coupling between rods and cones, and 2) a circuit independent of cone photoreceptors, implying direct contacts between rods and off CB cells. Anatomical evidence supports the existence of both these circuits. To investigate the contribution of these alternative pathways to scotopic vision in the mammalian retina, we have performed patch-clamp recordings from ganglion cells in the dark-adapted retina of the rabbit, mouse, and rat. Approximately one-half of the ganglion cells in the rabbit retina received off signals through a circuit that was independent of RB cells. This was shown by their persistence in the presence of the glutamate agonist 2-amino-4-phosphonobutyric acid (APB), which blocks rod-->RB cell signaling. Consistent with this result, strychnine, a glycine receptor antagonist, was unable to abolish these off responses. In addition, we were able to show that some off cone bipolar dendrites terminate at rod spherules and make potential contacts. In the mouse retina, however, there seems to be a very low proportion of off signals carried by an APB-resistant pathway. No ganglion cells in the rat retina displayed APB- and strychnine-resistant responses. Our data support signaling through flat contacts between rods and off CB cells as the alternative route, but suggest that the significance of this pathway differs between species.

Changes on the properties of glycine receptors during neuronal development

Glycine receptors (GlyRs) play a major role in the excitability of spinal cord and brain stem neurons. During development, several properties of these receptors undergo significant changes resulting in major modifications of their physiological functions. For example, the receptor structure switches from a monomeric alpha or heteromeric alpha 2 beta in immature neurons to an alpha 1 beta receptor type in mature neurons. Together with these changes in receptor subunits, the postsynaptic cluster size increases with development. Parallel to these modifications, the apparent receptor affinity to glycine and strychnine, as well as that of Zn(2+) and ethanol increases with time. The mature receptor is characterized by a slow desensitizing current and high sensitivity to modulation by protein kinase C. Also, the high level of glycinergic transmission in immature spinal neurons modulates neuronal excitability causing membrane depolarization and changes in intracellular calcium. Due to these properties, chronic inhibition of glycinergic transmission affects neurite outgrowth and produces changes in the level of synaptic transmission induced by GABA(A) and AMPA receptors. Finally, the high level of plasticity found in immature GlyRs is likely associated to changes in cytoskeleton dynamics.

Molecular structure and function of the glycine receptor chloride channel

The glycine receptor chloride channel (GlyR) is a member of the nicotinic acetylcholine receptor family of ligand-gated ion channels. Functional receptors of this family comprise five subunits and are important targets for neuroactive drugs. The GlyR is best known for mediating inhibitory neurotransmission in the spinal cord and brain stem, although recent evidence suggests it may also have other physiological roles, including excitatory neurotransmission in embryonic neurons. To date, four alpha-subunits (alpha1 to alpha4) and one beta-subunit have been identified. The differential expression of subunits underlies a diversity in GlyR pharmacology. A developmental switch from alpha2 to alpha1beta is completed by around postnatal day 20 in the rat. The beta-subunit is responsible for anchoring GlyRs to the subsynaptic cytoskeleton via the cytoplasmic protein gephyrin. The last few years have seen a surge in interest in these receptors. Consequently, a wealth of information has recently emerged concerning GlyR molecular structure and function. Most of the information has been obtained from homomeric alpha1 GlyRs, with the roles of the other subunits receiving relatively little attention. Heritable mutations to human GlyR genes give rise to a rare neurological disorder, hyperekplexia (or startle disease). Similar syndromes also occur in other species. A rapidly growing list of compounds has been shown to exert potent modulatory effects on this receptor. Since GlyRs are involved in motor reflex circuits of the spinal cord and provide inhibitory synapses onto pain sensory neurons, these agents may provide lead compounds for the development of muscle relaxant and peripheral analgesic drugs.

Diversity of glycine receptors in the mouse retina: localization of the alpha2 subunit

Gamma-aminobutyric acid (GABA) and glycine are the major inhibitory neurotransmitters in the retina, glycine being produced in approximately half of all amacrine cells. Whereas retinal cell types expressing the glycine receptor (GlyR) alpha1 and alpha3 subunits have been mapped, the role of the alpha2 subunit in retinal circuitry remains unclear. By using immunocytochemistry, we localized the alpha2 subunit in the inner plexiform layer (IPL) in brightly fluorescent puncta, which represent postsynaptically clustered GlyRs. This was shown by doubly labeling sections for GlyR alpha2 and bassoon (a presynaptic marker) or gephyrin (a postsynaptic marker). Synapses containing GlyR alpha2 were rarely found on ganglion cell dendrites but were observed on bipolar cell axon terminals and on amacrine cell processes. Recently, an amacrine cell type has been described that is immunopositive for glycine and for the vesicular glutamate transporter vGluT3. The processes of this cell type were presynaptic to GlyR alpha2 puncta, suggesting that vGluT3 amacrine cells release glycine. Double labeling of sections for GlyR alpha1 and GlyR alpha2 subunits showed that they are clustered at different synapses. In sections doubly labeled for GlyR alpha2 and GlyR alpha3, approximately one-third of the puncta were colocalized. The most abundant GlyR subtype in retina contains alpha3 subunits, followed by those containing GlyR alpha2 and GlyR alpha1 subunits.

Distribution of 5-hydroxytriptamine2C receptor mRNA in rat retina

There are several factors that suggest serotonin [5-hydroxytryptamine (5-HT)] plays a role as a neurotransmitter/neuromodulator within the retina. The presence of mRNAs encoding 5-HT receptors (5-HTR) of the types 5-HT2CR and 5-HT5AR within the rat retina was investigated using in situ hybridization of digoxigenin-labeled probes. The 5HT5AR probe produced no labeling, whereas the 5HT2CR probe hybridized in cells scattered in the inner nuclear and ganglion cell layers. Thus, the 5HT2CR gene is expressed by retinal neurons, some of which represent third-order neurons, either amacrine or ganglion cells. This suggests that 5-HT may modulate the outgoing signal from the retina.

Characterization of receptors for glutamate and GABA in retinal neurons

Glutamate and gamma-aminobutyric acid (GABA) are major excitatory and inhibitory neurotransmitters in the vertebrate retina, "a genuine neural center" (Ramón y Cajal, 1964, Recollections of My Life, C.E. Horne (Translater) MIT Press, Cambridge, MA). Photoreceptors, generating visual signals, and bipolar cells, mediating signal transfer from photoreceptors to ganglion cells, both release glutamate, which induces and/or changes the activity of the post-synaptic neurons (horizontal and bipolar cells for photoreceptors; amacrine and ganglion cells for bipolar cells). Horizontal and amacrine cells, which mediate lateral interaction in the outer and inner retina respectively, use GABA as a principal neurotransmitter. In recent years, glutamate receptors and GABA receptors in the retina have been extensively studied, using multi-disciplinary approaches. In this article some important advances in this field are reviewed, with special reference to retinal information processing. Photoreceptors possess metabotropic glutamate receptors and several subtypes of GABA receptors. Most horizontal cells express AMPA receptors, which may be predominantly assembled from flop slice variants. In addition, these cells also express GABAA and GABAC receptors. Signal transfer from photoreceptors to bipolar cells is rather complicated. Whereas AMPA/KA receptors mediate transmission for OFF type bipolar cells, several subtypes of glutamate receptors, both ionotropic and metabotropic, are involved in the generation of light responses of ON type bipolar cells. GABAA and GABAC receptors with distinct kinetics are differentially expressed on dendrites and axon terminals of both ON and OFF bipolar cells, mediating inhibition from horizontal cells and amacrine cells. Amacrine cells possess ionotropic glutamate receptors, whereas ganglion cells express both ionotropic and metabotropic glutamate receptors. GABAA receptors exist in amacrine and ganglion cells. Physiological data further suggest that GABAC receptors may be involved in the activity of these neurons. Moreover, responses of these retinal third order neurons are modulated by GABAB receptors, and in ganglion cells there exist several subtypes of GABAB receptors. A variety of glutamate receptor and GABA receptor subtypes found in the retina perform distinct functions, thus providing a wide range of neural integration and versatility of synaptic transmission. Perspectives in this research field are presented.

Availability of low-threshold Ca2+ current in retinal ganglion cells

Spiking in central neurons depends on the availability of inward and outward currents activated by depolarization and on the activation and priming of currents by hyperpolarization. Of these processes, priming by hyperpolarization is the least described. In the case of T-type Ca2+ current availability, the interplay of hyperpolarization and depolarization has been studied most completely in expression systems, in part because of the difficulty of pharmacologically separating the Ca2+ currents of native neurons. To facilitate understanding of this current under physiological conditions, we measured T-type current of isolated goldfish retinal ganglion cells with perforated-patch voltage-clamp methods in solutions containing a normal extracellular Ca2+ concentration. The voltage sensitivities and rates of current activation, inactivation, deactivation, and recovery from inactivation were similar to those of expressed alpha1G (CaV3.1) Ca2+ channel clones, except that the rate of deactivation was significantly faster. We reproduced the amplitude and kinetics of measured T currents with a numerical simulation based on a kinetic model developed for an alpha1G Ca2+ channel. Finally, we show that this model predicts the increase of T-type current made available between resting potential and spike threshold by repetitive hyperpolarizations presented at rates that are within the bandwidth of signals processed in situ by these neurons.

Diversity of glycine receptors in the mouse retina: localization of the alpha3 subunit

Glycine receptors (GlyRs) and their role in retinal circuitry were analyzed immunocytochemically in wild-type and GlyR alpha3 subunit-deficient (Glra3(-/-)) mouse retinae. GlyRs are localized in the inner plexiform layer in brightly fluorescent puncta, which are likely to represent postsynaptically clustered GlyRs. Approximately one third of the clusters were found to contain the alpha1 subunit, and half possessed the alpha3 subunit. However, these two GlyR isoforms were localized at different glycinergic synapses. In the Glra3(-/-) mouse, alpha3 subunit clusters were completely eliminated, although the total number of GlyR clusters was only slightly reduced. This finding indicates that other GlyR subunits (such as alpha2 or alpha4) may have compensated for the loss of the alpha3 subunit. Characteristic expression patterns of the alpha1 and alpha3 subunits within the synaptic circuits of the retina were revealed by double labeling sections for GlyRs and markers that define specific retinal neurons. The alpha1 subunit mediates signal transfer in the rod pathway between AII amacrine cells and OFF-cone bipolar cells. In contrast, the alpha3 subunit appears to be predominantly involved with the cone pathways. Thus, expression of different GlyR alpha subunit genes correlates with anatomically defined connectivities.