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

ON and OFF starburst amacrine cells are controlled by distinct cholinergic pathways

Cholinergic signaling in the retina is mediated by acetylcholine (ACh) released from starburst amacrine cells (SACs), which are key neurons for motion detection. SACs comprise ON and OFF subtypes, which morphologically show mirror symmetry to each other. Although many physiological studies on SACs have targeted ON cells only, the synaptic computation of ON and OFF SACs is assumed to be similar. Recent studies demonstrated that gene expression patterns and receptor types differed between ON and OFF SACs, suggesting differences in their functions. Here, we compared cholinergic signaling pathways between ON and OFF SACs in the mouse retina using the patch clamp technique. The application of ACh increased GABAergic feedback, observed as postsynaptic currents to SACs, in both ON and OFF SACs; however, the mode of GABAergic feedback differed. Nicotinic receptors mediated GABAergic feedback in both ON and OFF SACs, while muscarinic receptors mediated GABAergic feedback in ON SACs only in adults. Neither tetrodotoxin, which blocked action potentials, nor LY354740, which blocked neurotransmitter release from SACs, eliminated ACh-induced GABAergic feedback in SACs. These results suggest that ACh-induced GABAergic feedback in ON and OFF SACs is regulated by different feedback mechanisms in adults and mediated by non-spiking amacrine cells other than SACs.

Spatial organization of the mouse retina at single cell resolution by MERFISH

The visual signal processing in the retina requires the precise organization of diverse neuronal types working in concert. While single-cell omics studies have identified more than 120 different neuronal subtypes in the mouse retina, little is known about their spatial organization. Here, we generated the single-cell spatial atlas of the mouse retina using multiplexed error-robust fluorescence in situ hybridization (MERFISH). We profiled over 390,000 cells and identified all major cell types and nearly all subtypes through the integration with reference single-cell RNA sequencing (scRNA-seq) data. Our spatial atlas allowed simultaneous examination of nearly all cell subtypes in the retina, revealing 8 previously unknown displaced amacrine cell subtypes and establishing the connection between the molecular classification of many cell subtypes and their spatial arrangement. Furthermore, we identified spatially dependent differential gene expression between subtypes, suggesting the possibility of functional tuning of neuronal types based on location.

Light responses and amacrine cell modulation of morphologically-identified retinal ganglion cells in the mouse retina

By analyzing light-evoked spike responses, cation currents (ΔI_C) and chloride currents (ΔI_Cl) of over 100 morphologically-identified retinal ganglion cells (GCs) in dark-adapted mouse retina, we found there are at least 14 functionally- and morphologically-distinct types of RGCs. These cells can be divided into 5 groups based on their patterns of spike response to whole field light steps (SRWFLS), a GC identification scheme commonly used in studies with extracellular recording techniques. We also found that all GCs in the mouse retina express strychnine-sensitive glycine receptors, and receive light-elicited chloride current (ΔI_Cl) accompanied by a conductance increase from narrow-field, glycinergic amacrine cells. As the dark membrane potential of RGC are near the chloride-equilibrium potential, mouse GCs' spike responses are mediated primarily by bipolar cells inputs, and modulated by "shunting inhibition" from narrow-field amacrine cells. Analysis of strychnine actions on light-evoked cation current ΔI_C (bipolar cell inputs) in GCs suggests that narrow-field amacrine cells modulate GCs by sending ON-OFF crossover feedback signals to presynaptic bipolar cell axon terminals via sign-inverting glycinergic synapses, and the feedback signals are synergistic to the bipolar cell light responses. Therefore narrow-field amacrine cells enhance light-evoked bipolar cell inputs to GCs by presynaptic "synergistic addition", besides the abovementioned postsynaptic "shunting inhibition" in GCs.

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.

Gain control by sparse, ultra-slow glycinergic synapses

In the retina, ON starburst amacrine cells (SACs) play a crucial role in the direction-selective circuit, but the sources of inhibition that shape their response properties remain unclear. Previous studies demonstrate that ∼95% of their inhibitory synapses are GABAergic, yet we find that the light-evoked inhibitory currents measured in SACs are predominantly glycinergic. Glycinergic inhibition is extremely slow, relying on non-canonical glycine receptors containing α4 subunits, and is driven by both the ON and OFF retinal pathways. These attributes enable glycine inputs to summate and effectively control the output gain of SACs, expanding the range over which they compute direction. Serial electron microscopic reconstructions reveal three specific types of ON and OFF narrow-field amacrine cells as the presumptive sources of glycinergic inhibition. Together, these results establish an unexpected role for specific glycinergic amacrine cells in the retinal computation of stimulus direction by SACs.

Mouse Retinal Cell Atlas: Molecular Identification of over Sixty Amacrine Cell Types

Amacrine cells (ACs) are a diverse class of interneurons that modulate input from photoreceptors to retinal ganglion cells (RGCs), rendering each RGC type selectively sensitive to particular visual features, which are then relayed to the brain. While many AC types have been identified morphologically and physiologically, they have not been comprehensively classified or molecularly characterized. We used high-throughput single-cell RNA sequencing to profile >32,000 ACs from mice of both sexes and applied computational methods to identify 63 AC types. We identified molecular markers for each type and used them to characterize the morphology of multiple types. We show that they include nearly all previously known AC types as well as many that had not been described. Consistent with previous studies, most of the AC types expressed markers for the canonical inhibitory neurotransmitters GABA or glycine, but several expressed neither or both. In addition, many expressed one or more neuropeptides, and two expressed glutamatergic markers. We also explored transcriptomic relationships among AC types and identified transcription factors expressed by individual or multiple closely related types. Noteworthy among these were Meis2 and Tcf4, expressed by most GABAergic and most glycinergic types, respectively. Together, these results provide a foundation for developmental and functional studies of ACs, as well as means for genetically accessing them. Along with previous molecular, physiological, and morphologic analyses, they establish the existence of at least 130 neuronal types and nearly 140 cell types in the mouse retina.SIGNIFICANCE STATEMENT The mouse retina is a leading model for analyzing the development, structure, function, and pathology of neural circuits. A complete molecular atlas of retinal cell types provides an important foundation for these studies. We used high-throughput single-cell RNA sequencing to characterize the most heterogeneous class of retinal interneurons, amacrine cells, identifying 63 distinct types. The atlas includes types identified previously as well as many novel types. We provide evidence for the use of multiple neurotransmitters and neuropeptides, and identify transcription factors expressed by groups of closely related types. Combining these results with those obtained previously, we proposed that the mouse retina contains ∼130 neuronal types and is therefore comparable in complexity to other regions of the brain.

A retinal circuit for the suppressed-by-contrast receptive field of a polyaxonal amacrine cell

Amacrine cells are a diverse population of interneurons in the retina that play a critical role in extracting complex features of the visual world and shaping the receptive fields of retinal output neurons (ganglion cells). While much of the computational power of amacrine cells is believed to arise from the immense mutual interactions among amacrine cells themselves, the intricate circuitry and functions of amacrine-amacrine interactions are poorly understood in general. Here we report a specific interamacrine pathway from a small-field, glutamate-glycine dual-transmitter amacrine cell (vGluT3) to a wide-field polyaxonal amacrine cell (PAS4/5). Distal tips of vGluT3 cell dendrites made selective glycinergic (but not glutamatergic) synapses onto PAS4/5 dendrites to provide a center-inhibitory, surround-disinhibitory drive that helps PAS4/5 cells build a suppressed-by-contrast (sbc) receptive field, which is a unique and fundamental trigger feature previously found only in a small population of ganglion cells. The finding of this trigger feature in a circuit upstream to ganglion cells suggests that the sbc form of visual computation occurs more widely in the retina than previously believed and shapes visual processing in multiple downstream circuits in multiple ways. We also identified two different subpopulations of PAS4/5 cells based on their differential connectivity with vGluT3 cells and their distinct receptive-field and luminance-encoding characteristics. Moreover, our results revealed a form of crosstalk between small-field and large-field amacrine cell circuits, which provides a mechanism for feature-specific local (<150 µm) control of global (>1 mm) retinal activity.

Glycinergic and GABAergic interneurons shift the location and differentially alter the size of ganglion cell receptive field centers in the mammalian retina

By using the multi-electrode array (MEA) recording technique in conjunction with white-noise checkerboard stimuli and reverse correlation methods, we studied modulatory actions of glycinergic and GABAergic interneurons on spatiotemporal profiles of ganglion cells (GCs) in dark-adapted mouse retinas. We found that application of 2 µM strychnine decreased receptive field center radii of GCs by a mean value of 11%, and shifted the GC receptive field (RF) centers by a mean distance of 28.3 µm. On the other hand, 200 µM picrotoxin + 100 µM bicuculline + 50 µM TPMPA increased GC receptive field center radii by a mean value of 19%, and shifted the GC RF centers by a mean distance of 53.7 µm. Glycinergic neurons in the mouse retina are narrow-field amacrine cells that have been shown to mediate ON-OFF crossover inhibitory synapses within the RGs' RF center, therefore they may increase the size and shift the location of GC RF center by synergistic addition to bipolar cell inputs to GCs. GABAergic neurons are wide-field amacrine cells and horizontal cells that are known to mediate antagonistic surround responses of GCs, and thus they decrease the GCs' RF center size. Our results suggest that a major global function of glycinergic and GABAergic interneurons in the mammalian retina is to provide the flexibility for adjusting the size and location of GCs' RF centers. The apparent shifts of GC RF centers suggest that the synergistic addition by GlyACs and the surround inhibition by GABAergic interneurons are not spatially symmetrical within GC RFs.

Principal Neurons in the Anteroventral Cochlear Nucleus Express Cell-Type Specific Glycine Receptor α Subunits

Signal processing in the principal neurons of the anteroventral cochlear nucleus (AVCN) is modulated by glycinergic inhibition. The kinetics of IPSCs are specific to the target neurons. It remains unclear what glycine receptor subunits are involved in generating such target-specific IPSC kinetics in AVCN principal neurons. We investigated the expression patterns of glycine receptor α (GlyRα) subunits in AVCN using immunohistochemical labeling of four isoforms of GlyRα subunits (GlyRα₁-α₄), and found that AVCN neurons express GlyRα₁ and GlyRα₄, but not GlyRα₂ and GlyRα₃ subunits. To further identify the cell type-specific expression patterns of GlyRα subunits, we combined whole-cell patch clamp recording with immunohistochemistry by recording from all three types of AVCN principal neurons, characterizing the synaptic properties of their glycinergic inhibition, dye-filling the neurons, and processing the slice for immunostaining of different GlyRα subunits. We found that AVCN bushy neurons express both GlyRα₁ and GlyRα₄ subunits that underlie their slow IPSC kinetics, whereas both T-stellate and D-stellate neurons express only GlyRα₁ subunit that underlies their fast IPSC kinetics. In conclusion, AVCN principal neurons express cell-type specific GlyRα subunits that underlie their distinct IPSC kinetics, which enables glycinergic inhibition from the same source to exert target cell-specific modulation of activity to support the unique physiological function of these neurons.

GlyRα2, not GlyRα3, modulates the receptive field surround of OFF retinal ganglion cells

Receptive fields (RFs) of most retinal ganglion cells (RGCs) consist of an excitatory center and suppressive surround. The RF center arises from the summation of excitatory bipolar cell glutamatergic inputs, whereas the surround arises from lateral inhibitory inputs. In the retina, both gamma amino butyric acid (GABA) and glycine are inhibitory neurotransmitters. A clear role for GABAergic inhibition modulating the RGC RF surround has been demonstrated across species. Glycinergic inhibition is more commonly associated with RF center modulation, although there is some evidence that it may contribute to the RF surround. The synaptic glycinergic chloride channels are formed by three homomeric β and two homomeric α subunits that can be glycine receptor (GlyR) α1, α2, α3, or α4. GlyRα composition is responsible for currents with distinct decay kinetics. Their expression within the inner plexiform laminae and neuronal subtypes also differ. We studied the role of GlyR subunit selective modulation of RGC RF surrounds, using mice lacking GlyRα2 (Glra2 -/-), GlyRα3 (Glra3 -/-), or both (Glra2/3 -/-). We chose this molecular genetic approach instead of pharmacological manipulation because there are no subunit selective antagonists and strychnine blocks all GlyRs. Comparisons of annulus-evoked responses among wild type (WT) and GlyRα knockouts (Glra2 -/-, Glra3 -/- and Glra2/3 -/-) show that GlyRα2 inhibition enhances RF surround suppression and post-stimulus excitation in only WT OFF RGCs. Similarities in the responses in Glra2 -/- and Glra2/3 -/- RGCs verify these conclusions. Based on previous and current data, we propose that GlyRα2-mediated input uses a crossover inhibitory circuit. Further, we suggest that GlyRα2 modulates the OFF RGC RF center and surround independently. In summary, our results define a selective GlyR subunit-specific control of RF surround suppression in OFF RGCs.

Defects of the Glycinergic Synapse in Zebrafish

Glycine mediates fast inhibitory synaptic transmission. Physiological importance of the glycinergic synapse is well established in the brainstem and the spinal cord. In humans, the loss of glycinergic function in the spinal cord and brainstem leads to hyperekplexia, which is characterized by an excess startle reflex to sudden acoustic or tactile stimulation. In addition, glycinergic synapses in this region are also involved in the regulation of respiration and locomotion, and in the nociceptive processing. The importance of the glycinergic synapse is conserved across vertebrate species. A teleost fish, the zebrafish, offers several advantages as a vertebrate model for research of glycinergic synapse. Mutagenesis screens in zebrafish have isolated two motor defective mutants that have pathogenic mutations in glycinergic synaptic transmission: bandoneon (beo) and shocked (sho). Beo mutants have a loss-of-function mutation of glycine receptor (GlyR) β-subunit b, alternatively, sho mutant is a glycinergic transporter 1 (GlyT1) defective mutant. These mutants are useful animal models for understanding of glycinergic synaptic transmission and for identification of novel therapeutic agents for human diseases arising from defect in glycinergic transmission, such as hyperekplexia or glycine encephalopathy. Recent advances in techniques for genome editing and for imaging and manipulating of a molecule or a physiological process make zebrafish more attractive model. In this review, we describe the glycinergic defective zebrafish mutants and the technical advances in both forward and reverse genetic approaches as well as in vivo visualization and manipulation approaches for the study of the glycinergic synapse in zebrafish.

Serotonin receptor 2C regulates neurite growth and is necessary for normal retinal processing of visual information

Serotonin (5HT) is present in a subpopulation of amacrine cells, which form synapses with retinal ganglion cells (RGCs), but little is known about the physiological role of retinal serotonergic circuitry. We found that the 5HT receptor 2C (5HTR2C) is upregulated in RGCs after birth. Amacrine cells generate 5HT and about half of RGCs respond to 5HTR2C agonism with calcium elevation. We found that there are on average 83 5HT+ amacrine cells randomly distributed across the adult mouse retina, all negative for choline acetyltransferase and 90% positive for tyrosine hydroxylase. We also investigated whether 5HTR2C and 5HTR5A affect RGC neurite growth. We found that both suppress neurite growth, and that RGCs from the 5HTR2C knockout (KO) mice grow longer neurites. Furthermore, 5HTR2C is subject to post-transcriptional editing, and we found that only the edited isoform's suppressive effect on neurite growth could be reversed by a 5HTR2C inverse agonist. Next, we investigated the physiological role of 5HTR2C in the retina, and found that 5HTR2C KO mice showed increased amplitude on pattern electroretinogram. Finally, RGC transcriptional profiling and pathways analysis suggested partial developmental compensation for 5HTR2C absence. Taken together, our findings demonstrate that 5HTR2C regulates neurite growth and RGC activity and is necessary for normal amplitude of RGC response to physiologic stimuli, and raise the hypothesis that these functions are modulated by a subset of 5HT+/ChAT-/TH+ amacrine cells as part of retinal serotonergic circuitry. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 419-437, 2017.

Mapping Synaptic Input Fields of Neurons with Super-Resolution Imaging

As a basic functional unit in neural circuits, each neuron integrates input signals from hundreds to thousands of synapses. Knowledge of the synaptic input fields of individual neurons, including the identity, strength, and location of each synapse, is essential for understanding how neurons compute. Here, we developed a volumetric super-resolution reconstruction platform for large-volume imaging and automated segmentation of neurons and synapses with molecular identity information. We used this platform to map inhibitory synaptic input fields of On-Off direction-selective ganglion cells (On-Off DSGCs), which are important for computing visual motion direction in the mouse retina. The reconstructions of On-Off DSGCs showed a GABAergic, receptor subtype-specific input field for generating direction selective responses without significant glycinergic inputs for mediating monosynaptic crossover inhibition. These results demonstrate unique capabilities of this super-resolution platform for interrogating neural circuitry.

Response Properties of a Newly Identified Tristratified Narrow Field Amacrine Cell in the Mouse Retina

Amacrine cells were targeted for whole cell recording using two-photon fluorescence microscopy in a transgenic mouse line in which the promoter for dopamine receptor 2 drove expression of green fluorescent protein in a narrow field tristratified amacrine cell (TNAC) that had not been studied previously. Light evoked a multiphasic response that was the sum of hyperpolarizing and depolarization synaptic inputs consistent with distinct dendritic ramifications in the off and on sublamina of the inner plexiform layer. The amplitude and waveform of the response, which consisted of an initial brief hyperpolarization at light onset followed by recovery to a plateau potential close to dark resting potential and a hyperpolarizing response at the light offset varied little over an intensity range from 0.4 to ~10^6 Rh*/rod/s. This suggests that the cell functions as a differentiator that generates an output signal (a transient reduction in inhibitory input to downstream retina neurons) that is proportional to the derivative of light input independent of its intensity. The underlying circuitry appears to consist of rod and cone driven on and off bipolar cells that provide direct excitatory input to the cell as well as to GABAergic amacrine cells that are synaptically coupled to TNAC. Canonical reagents that blocked excitatory (glutamatergic) and inhibitory (GABA and glycine) synaptic transmission had effects on responses to scotopic stimuli consistent with the rod driven component of the proposed circuit. However, responses evoked by photopic stimuli were paradoxical and could not be interpreted on the basis of conventional thinking about the neuropharmacology of synaptic interactions in the retina.

Identification of AⅡ amacrine, displaced amacrine, and bistratified ganglion cell types in human retina with antibodies against calretinin

Antibodies against calretinin are markers for one type of rod pathway interneuron (AⅡ amacrine cell) in the retina of some but not all mammalian species. The AⅡ cells play a crucial role in night-time (scotopic) vision and have been proposed as a target for optogenetic restoration of vision in retinal disease. In the present study we aimed to characterize the AⅡ cells in human retina. Postmortem human donor eyes were obtained with ethical approval and processed for calretinin immunofluorescence. Calretinin-positive somas in the inner nuclear and the ganglion cell layer were filled with the lipophilic dye DiI. The large majority (over 80%) of calretinin-immunoreactive cells is located in the inner nuclear layer, is immunopositive for glycine transporter 1, and shows the typical morphology of AⅡ amacrine cells. In addition, a small proportion of calretinin-positive cells in the inner nuclear layer and in the ganglion cell layer is glutamic acid decarboxylase-positive and shows the morphology of widefield amacrine cells (stellate, semilunar, and thorny amacrine cells). About half of the calretinin cells in the ganglion cell layer are bistratified ganglion cells resembling the small bistratified (presumed blue-ON/yellow-OFF) and the G17 ganglion cell previously described in primates. We conclude that in human retina, antibodies against calretinin can be used to identify AⅡ amacrine cells in the inner nuclear layer as well as widefield amacrine and small bistratified ganglion cells in the ganglion cell layer.

Morphology and connectivity of the small bistratified A8 amacrine cell in the mouse retina

Amacrine cells comprise ∼ 30 morphological types in the mammalian retina. The synaptic connectivity and function of a few γ-aminobutyric acid (GABA)ergic wide-field amacrine cells have recently been studied; however, with the exception of the rod pathway-specific AII amacrine cell, the connectivity of glycinergic small-field amacrine cells has not been investigated in the mouse retina. Here, we studied the morphology and connectivity pattern of the small-field A8 amacrine cell. A8 cells in mouse retina are bistratified with lobular processes in the ON sublamina and arboreal dendrites in the OFF sublamina of the inner plexiform layer. The distinct bistratified morphology was first visible at postnatal day 8, reaching the adult shape at P13, around eye opening. The connectivity of A8 cells to bipolar cells and ganglion cells was studied by double and triple immunolabeling experiments by using various cell markers combined with synaptic markers. Our data suggest that A8 amacrine cells receive glutamatergic input from both OFF and ON cone bipolar cells. Furthermore, A8 cells are coupled to ON cone bipolar cells by gap junctions, and provide inhibitory input via glycine receptor (GlyR) subunit α1 to OFF cone bipolar cells and to ON A-type ganglion cells. Measurements of spontaneous glycinergic postsynaptic currents and GlyR immunolabeling revealed that A8 cells express GlyRs containing the α2 subunit. The results show that the bistratified A8 cell makes very similar synaptic contacts with cone bipolar cells as the rod pathway-specific AII amacrine cell. However, unlike AII cells, A8 amacrine cells provide glycinergic input to ON A-type ganglion cells.

Adeno-associated virus-RNAi of GlyRα1 and characterization of its synapse-specific inhibition in OFF alpha transient retinal ganglion cells

In the central nervous system, inhibition shapes neuronal excitation. In spinal cord glycinergic inhibition predominates, whereas GABAergic inhibition predominates in the brain. The retina uses GABA and glycine in approximately equal proportions. Glycinergic crossover inhibition, initiated in the On retinal pathway, controls glutamate release from presynaptic OFF cone bipolar cells (CBCs) and directly shapes temporal response properties of OFF retinal ganglion cells (RGCs). In the retina, four glycine receptor (GlyR) α-subunit isoforms are expressed in different sublaminae and their synaptic currents differ in decay kinetics. GlyRα1, expressed in both On and Off sublaminae of the inner plexiform layer, could be the glycinergic isoform that mediates On-to-Off crossover inhibition. However, subunit-selective glycine contributions remain unknown because we lack selective antagonists or cell class-specific subunit knockouts. To examine the role of GlyRα1 in direct inhibition in mature RGCs, we used retrogradely transported adeno-associated virus (AAV) that performed RNAi and eliminated almost all glycinergic spontaneous and visually evoked responses in PV5 (OFFα(Transient)) RGCs. Comparisons of responses in PV5 RGCs infected with AAV-scrambled-short hairpin RNA (shRNA) or AAV-Glra1-shRNA confirm a role for GlyRα1 in crossover inhibition in cone-driven circuits. Our results also define a role for direct GlyRα1 inhibition in setting the resting membrane potential of PV5 RGCs. The absence of GlyRα1 input unmasked a serial and a direct feedforward GABA(A)ergic modulation in PV5 RGCs, reflecting a complex interaction between glycinergic and GABA(A)ergic inhibition.

ON-pathway-dominant glycinergic regulation of cholinergic amacrine cells in the mouse retina

Direction selectivity in the retina has been studied as a model of dendritic computation of neural circuits. Starburst amacrine cells (SACs) have been examined as a model system of dendritic computation as they play a pivotal role in the formation of direction selectivity. Because the difference of anatomical location inside the retina made ON-SACs an easier target to record, the biophysical properties of ON-SACs have been used to predict those of OFF-SACs. In this study, we systematically compared the responses of ON- and OFF-SACs to the two principal neurotransmitters, glycine and glutamate. We found that responses to glycine were significantly larger in ON-SACs than in OFF-SACs. In contrast, ON- and OFF-SACs responded similarly to glutamate. The amplitude of glycine responses in ON-SACs increased after eye opening and the largest amplitude was observed at postnatal day 28. On the other hand, no increase in the amplitude of glycine responses in OFF-SACs was observed until postnatal day 28. Glycine-evoked currents were inhibited by the application of strychnine. Glutamate-evoked currents were mimicked by the application of AMPA or kainite, and responses to N-methyl-d-aspartate were observed in the absence of Mg(2+) block. Glutamate-evoked currents produced an increase in the frequency of GABAergic inhibitory postsynaptic currents. Our results suggest that signal processing in ON-SACs cannot be directly used to understand the properties of OFF-SACs. Therefore fully defining the physiological properties of OFF-SACs will be critical to understanding and modelling direction selectivity in the retina.

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.

Inputs underlying the ON-OFF light responses of type 2 wide-field amacrine cells in TH::GFP mice

In the mammalian retina, two types of catecholaminergic amacrine cells have been described. Although dopaminergic type 1 cells are well characterized, the physiology of type 2 cells is, so far, unknown. To target type 2 cells specifically, we used a transgenic mouse line that expresses green fluorescent protein under the control of the tyrosine hydroxylase promoter. Type 2 cells are GABAergic and have an extensive dendritic arbor, which stratifies in the middle of the inner plexiform layer. Our data suggest that type 2 cells comprise two subpopulations with identical physiological properties: one has its somata located in the inner nuclear layer and the other in the ganglion cell layer. Immunostaining with bipolar cell markers suggested that type 2 cells receive excitatory inputs from type 3 OFF and type 5 ON bipolar cells. Consistently, patch-clamp recordings showed that type 2 cells are ON-OFF amacrine cells. Blocking excitatory inputs revealed that different rod and cone pathways are active under scotopic and mesopic light conditions. Blockade of inhibitory inputs led to membrane potential oscillations in type 2 cells, suggesting that GABAergic and glycinergic amacrine cells strongly influence type 2 cell signaling. Among the glycinergic amacrine cells, we identified the VGluT3-immunoreactive amacrine cell as a likely candidate. Collectively, light responses of type 2 cells were remarkably uniform over a wide range of light intensities. These properties point toward a general function of type 2 cells that is maintained under scotopic and mesopic conditions.

Glycinergic transmission in the Mammalian retina

Glycine and γ-aminobutyric acid (GABA) are the major inhibitory neurotransmitters in the retina. Approximately half of the amacrine cells release glycine at their synapses with bipolar, other amacrine, and ganglion cells. Glycinergic amacrine cells are small-field amacrine cells with vertically oriented dendrites and comprise more than 10 different morphological types. The retinal distributions of glycine receptor (GlyR) α1, α2, α3 and α4 subtypes have been mapped with subunit-specific antibodies. GlyRs were clustered at postsynaptic hot spots which showed selective distributions for the different subunits. As a rule, only one α subunit was expressed at a given postsynaptic site. The kinetic properties of GlyRs were measured by recording spontaneous inhibitory postsynaptic currents (sIPSCs) from identified retinal neurons in wild-type, Glra1^spd-ot, Glra2 and Glra3 knockout mice. From observed differences of sIPSCs in wild-type and mutant mice, the cell-type specific subunit composition of GlyRs could be defined. OFF-cone bipolar cells and A-type ganglion cells receive prominent glycinergic input with fast kinetics that is mainly mediated by α1β GlyRs (decay time constant τ ∼ 5 ms). By contrast, AII amacrine cells express α3β GlyRs with medium fast kinetics (τ ∼ 11 ms). Narrow-field (NF) and wide-field amacrine cells contain predominantly α2β GlyRs with slow kinetics (τ ∼ 27 ms). Lastly, ON-starburst, narrow-field and wide-field amacrine cells in Glra2 knockout mice express α4β GlyRs with very slow kinetics (τ ∼ 70 ms).