Glycinergic input of small-field amacrine cells in the retinas of wildtype and glycine receptor deficient mice

Nitric oxide modulates contrast suppression in a subset of mouse retinal ganglion cells

Neuromodulators have major influences on the regulation of neural circuit activity across the nervous system. Nitric oxide (NO) has been shown to be a prominent neuromodulator in many circuits and has been extensively studied in the retina. Here, it has been associated with the regulation of light adaptation, gain control, and gap junctional coupling, but its effect on the retinal output, specifically on the different types of retinal ganglion cells (RGCs), is still poorly understood. In this study, we used two-photon Ca2+ imaging and multi-electrode array (MEA) recordings to measure light-evoked activity of RGCs in the ganglion cell layer in the ex vivo mouse retina. This approach allowed us to investigate the neuromodulatory effects of NO on a cell type-level. Our findings reveal that NO selectively modulates the suppression of temporal responses in a distinct subset of contrast-suppressed RGC types, increasing their activity without altering the spatial properties of their receptive fields. Given that under photopic conditions, NO release is triggered by quick changes in light levels, we propose that these RGC types signal fast contrast changes to higher visual regions. Remarkably, we found that about one-third of the RGC types, recorded using two-photon Ca2+ imaging, exhibited consistent, cell type-specific adaptational response changes throughout an experiment, independent of NO. By employing a sequential-recording paradigm, we could disentangle those additional adaptational response changes from drug-induced modulations. Taken together, our research highlights the selective neuromodulatory effects of NO on RGCs and emphasizes the need of considering non-pharmacological activity changes, like adaptation, in such study designs.

Functional properties of GABAA receptors of AII amacrine cells of the rat retina

Amacrine cells are a highly diverse group of inhibitory retinal interneurons that sculpt the responses of bipolar cells, ganglion cells, and other amacrine cells. They integrate excitatory inputs from bipolar cells and inhibitory inputs from other amacrine cells, but for most amacrine cells, little is known about the specificity and functional properties of their inhibitory inputs. Here, we have investigated GABAA receptors of the AII amacrine, a critical neuron in the rod pathway microcircuit, using patch-clamp recording in rat retinal slices. Puffer application of GABA evoked robust responses, but, surprisingly, spontaneous GABAA receptor-mediated postsynaptic currents were not observed, neither under control conditions nor following application of high-K+ solution to facilitate release. To investigate the biophysical and pharmacological properties of GABAA receptors in AIIs, we therefore used nucleated patches and a fast application system. Both brief and long pulses of GABA (3 mM) evoked GABAA receptor-mediated currents with slow, multi-exponential decay kinetics. The average weighted time constant (τw) of deactivation was ~163 ms. Desensitization was even slower, with τw ~330 ms. Non-stationary noise analysis of patch responses and directly observed channel gating yielded a single-channel conductance of ~23 pS. Pharmacological investigation suggested the presence of α2 and/or α3 subunits, as well as the γ2 subunit. Such subunit combinations are typical of GABAA receptors with slow kinetics. If synaptic GABAA receptors of AII amacrines have similar functional properties, the slow deactivation and desensitization kinetics will facilitate temporal summation of GABAergic inputs, allowing effective summation and synaptic integration to occur even for relatively low frequencies of inhibitory inputs.

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.

Distinct Developmental Mechanisms Act Independently to Shape Biased Synaptic Divergence from an Inhibitory Neuron

Neurons often contact more than one postsynaptic partner type and display stereotypic patterns of synaptic divergence. Such synaptic patterns usually involve some partners receiving more synapses than others. The developmental strategies generating "biased" synaptic distributions remain largely unknown. To gain insight, we took advantage of a compact circuit in the vertebrate retina, whereby the AII amacrine cell (AII AC) provides inhibition onto cone bipolar cell (BC) axons and retinal ganglion cell (RGC) dendrites, but makes the majority of its synapses with the BCs. Using light and electron microscopy, we reconstructed the morphology and connectivity of mouse retinal AII ACs across postnatal development. We found that AII ACs do not elaborate their presynaptic structures, the lobular appendages, until BCs differentiate about a week after RGCs are present. Lobular appendages are present in mutant mice lacking BCs, implying that although synchronized with BC axonal differentiation, presynaptic differentiation of the AII ACs is not dependent on cues from BCs. With maturation, AII ACs maintain a constant number of synapses with RGCs, preferentially increase synaptogenesis with BCs, and eliminate synapses with wide-field amacrine cells. Thus, AII ACs undergo partner type-specific changes in connectivity to attain their mature pattern of synaptic divergence. Moreover, AII ACs contact non-BCs to the same extent in bipolarless retinas, indicating that AII ACs establish partner-type-specific connectivity using diverse mechanisms that operate in parallel but independently.

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.

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.

Development of glycinergic innervation to the murine LSO and SPN in the presence and absence of the MNTB

Neurons in the superior olivary complex (SOC) integrate excitatory and inhibitory inputs to localize sounds in space. The majority of these inhibitory inputs have been thought to arise within the SOC from the medial nucleus of the trapezoid body (MNTB). However, recent work demonstrates that glycinergic innervation of the SOC persists in Egr2; En1^CKO mice that lack MNTB neurons, suggesting that there are other sources of this innervation (Jalabi et al., 2013). To study the development of MNTB- and non-MNTB-derived glycinergic SOC innervation, we compared immunostaining patterns of glycine transporter 2 (GlyT2) at several postnatal ages in control and Egr2; En1^CKO mice. GlyT2 immunostaining was present at birth (P0) in controls and reached adult levels by P7 in the superior paraolivary nucleus (SPN) and by P12 in the lateral superior olive (LSO). In Egr2; En1^CKO mice, glycinergic innervation of the LSO developed at a similar rate but was delayed by one week in the SPN. Conversely, consistent reductions in the number of GlyT2⁺ boutons located on LSO somata were seen at all ages in Egr2; En1^CKO mice, while these numbers reached control levels in the SPN by adulthood. Dendritic localization of GlyT2+ boutons was unaltered in both the LSO and SPN of adult Egr2; En1^CKO mice. On the postsynaptic side, adult Egr2; En1^CKO mice had reduced glycine receptor α1 (GlyRα1) expression in the LSO but normal levels in the SPN. GlyRα2 was not expressed by LSO or SPN neurons in either genotype. These findings contribute important information for understanding the development of MNTB- and non-MNTB-derived glycinergic pathways to the mouse SOC.

The Lgr5 transgene is expressed specifically in glycinergic amacrine cells in the mouse retina

Retinal amacrine cells are a diverse set of interneurons within the inner nuclear layer. The canonical Wnt pathway is highly active within mature amacrine cells, but its role remains unclear. Leucine-rich repeat containing G-protein receptor 5 (Lgr5) is a newly identified component of the Wnt receptor complex that potentiates beta-catenin signaling. In multiple epithelial organs Lgr5 marks adult tissue stem cells. We investigated the expression of this gene using Lgr5-eGFP-IRES-CreER transgenic reporter mice. In the eye, Lgr5 was exclusively expressed in glycinergic amacrine cells in adult mice. Amacrine cells are post-mitotic and represent the first neuronal and non-stem cell lineage to express Lgr5. We further interrogated the spatiotemporal labeling of individual amacrine cells with controlled fluorophore expression. This "fluorofilling" technique provides a tool to study amacrine morphology and dissect neural networks.

Postnatal development of glycine receptor subunits α1, α2, α3, and β immunoreactivity in multiple brain stem respiratory-related nuclear groups of the rat

The respiratory system is immature at birth and significant development occurs postnatally. A critical period of respiratory development occurs in rats around postnatal days 12-13, when enhanced inhibition dominates over suppressed excitation. The mechanisms underlying the heightened inhibition are not fully understood. The present study tested our hypothesis that the inhibition is marked by a switch in glycine receptor subunits from neonatal to adult form around the critical period. An in-depth immunohistochemical and single neuron optical densitometric study was undertaken on four respiratory-related nuclear groups (the pre-Bötzinger complex, nucleus ambiguus, hypoglossal nucleus, and ventrolateral subnucleus of solitary tract nucleus), and a non-respiratory cuneate nucleus in P2-21 rats. Our data revealed that in the respiratory-related nuclear groups: (1) the expressions of GlyRα2 and GlyRα3 were relatively high at P2, but declined after 1-1½ weeks to their lowest levels at P21; (2) the expression of GlyRα1 increased with age and reached significance at P12; and (3) the expression of GlyRβ rose from P2 to P12 followed by a slight decline until P21. No distinct increase in GlyRα1 at P12 was noted in the cuneate nucleus. Thus, there is a switch in dominance of expression from neonatal GlyRα2/α3 to the adult GlyRα1 and a heightened expression of GlyRα1 around the critical period in all respiratory-related nuclear groups, thereby supporting enhanced inhibition at that time. The rise in the expression of GlyRβ around P12 indicates that it plays an important role in forming the mature heteropentameric glycine receptors in these brain stem nuclear groups.

Light adaptation alters the source of inhibition to the mouse retinal OFF pathway

Sensory systems must avoid saturation to encode a wide range of stimulus intensities. One way the retina accomplishes this is by using both dim-light-sensing rod and bright-light-sensing cone photoreceptor circuits. OFF cone bipolar cells are a key point in this process, as they receive both excitatory input from cones and inhibitory input from AII amacrine cells via the rod pathway. However, in addition to AII amacrine cell input, other inhibitory inputs from cone pathways also modulate OFF cone bipolar cell light signals. It is unknown how these inhibitory inputs to OFF cone bipolar cells change when switching between rod and cone pathways or whether all OFF cone bipolar cells receive rod pathway input. We found that one group of OFF cone bipolar cells (types 1, 2, and 4) receive rod-mediated inhibitory inputs that likely come from the rod-AII amacrine cell pathway, while another group of OFF cone bipolar cells (type 3) do not. In both cases, dark-adapted rod-dominant light responses showed a significant contribution of glycinergic inhibition, which decreased with light adaptation and was, surprisingly, compensated by an increase in GABAergic inhibition. As GABAergic input has distinct timing and spatial spread from glycinergic input, a shift from glycinergic to GABAergic inhibition could significantly alter OFF cone bipolar cell signaling to downstream OFF ganglion cells. Larger GABAergic input could reflect an adjustment of OFF bipolar cell spatial inhibition, which may be one mechanism that contributes to retinal spatial sensitivity in the light.

Inhibitory glycine receptors: an update

Strychnine-sensitive glycine receptors (GlyRs) mediate synaptic inhibition in the spinal cord, brainstem, and other regions of the mammalian central nervous system. In this minireview, we summarize our current view of the structure, ligand-binding sites, and chloride channel of these receptors and discuss recently emerging functions of distinct GlyR isoforms. GlyRs not only regulate the excitability of motor and afferent sensory neurons, including pain fibers, but also are involved in the processing of visual and auditory signals. Hence, GlyRs constitute promising targets for the development of therapeutically useful compounds.

Glycine receptor mutants of the mouse: what are possible routes of inhibitory compensation?

Defects in glycinergic inhibition result in a complex neuromotor disorder in humans known as hyperekplexia (OMIM 149400) with similar phenotypes in rodents characterized by an exaggerated startle reflex and hypertonia. Analogous to genetic defects in humans single point mutations, microdeletions, or insertions in the Glra1 gene but also in the Glrb gene underlie the pathology in mice. The mutations either localized in the α (spasmodic, oscillator, cincinnati, Nmf11) or the β (spastic) subunit of the glycine receptor (GlyR) are much less tolerated in mice than in humans, leaving the question for the existence of different regulatory elements of the pathomechanisms in humans and rodents. In addition to the spontaneous mutations, new insights into understanding of the regulatory pathways in hyperekplexia or glycine encephalopathy arose from the constantly increasing number of knock-out as well as knock-in mutants of GlyRs. Over the last five years, various efforts using in vivo whole cell recordings provided a detailed analysis of the kinetic parameters underlying glycinergic dysfunction. Presynaptic compensation as well as postsynaptic compensatory mechanisms in these mice by other GlyR subunits or GABA_A receptors, and the role of extra-synaptic GlyRs is still a matter of debate. A recent study on the mouse mutant oscillator displayed a novel aspect for compensation of functionality by complementation of receptor domains that fold independently. This review focuses on defects in glycinergic neurotransmission in mice discussed with the background of human hyperekplexia en route to strategies of compensation.

Electrical synapses between AII amacrine cells in the retina: Function and modulation

Adaptation enables the visual system to operate across a large range of background light intensities. There is evidence that one component of this adaptation is mediated by modulation of gap junctions functioning as electrical synapses, thereby tuning and functionally optimizing specific retinal microcircuits and pathways. The AII amacrine cell is an interneuron found in most mammalian retinas and plays a crucial role for processing visual signals in starlight, twilight and daylight. AII amacrine cells are connected to each other by gap junctions, potentially serving as a substrate for signal averaging and noise reduction, and there is evidence that the strength of electrical coupling is modulated by the level of background light. Whereas there is extensive knowledge concerning the retinal microcircuits that involve the AII amacrine cell, it is less clear which signaling pathways and intracellular transduction mechanisms are involved in modulating the junctional conductance between electrically coupled AII amacrine cells. Here we review the current state of knowledge, with a focus on the recent evidence that suggests that the modulatory control involves activity-dependent changes in the phosphorylation of the gap junction channels between AII amacrine cells, potentially linked to their intracellular Ca(2+) dynamics. This article is part of a Special Issue entitled Electrical Synapses.

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.

Selective glycine receptor α2 subunit control of crossover inhibition between the on and off retinal pathways

In the retina, the receptive fields (RFs) of almost all ganglion cells (GCs) are comprised of an excitatory center and a suppressive surround. The RF center arises from local excitatory bipolar cell (BC) inputs and the surround from lateral inhibitory inputs. Selective antagonists have been used to define the roles of GABA(A) and GABA(C) receptor-mediated input in RF organization. In contrast, the role of glycine receptor (GlyR) subunit-specific inhibition is less clear because the only antagonist, strychnine, blocks all GlyR subunit combinations. We used mice lacking the GlyRα2 (Glra2(-/-)) and GlyRα3 (Glra3(-/-)) subunits, or both (Glra2/3(-/-)), to explore their roles in GC RF organization. By comparing spontaneous and visually evoked responses of WT with Glra2(-/-), Glra3(-/-) and Glra2/3(-/-) ON- and OFF-center GCs, we found that both GlyRα2 and GlyRα3 modulate local RF interactions. In the On pathway, both receptors enhance the excitatory center response; however, the underlying inhibitory mechanisms differ. GlyRα2 participates in crossover inhibition, whereas GlyRα3 mediates serial inhibition. In the Off pathway, GlyRα2 plays a similar role, again using crossover inhibition and enhancing excitatory responses within the RF center. Comparisons of single and double KOs indicate that GlyRα2 and GlyRα3 inhibition are independent and additive, consistent with the finding that they use different inhibitory circuitry. These findings are the first to define GlyR subunit-specific control of visual function and GlyRα2 subunit-specific control of crossover inhibition in the retina.

Prolonged zymosan-induced inflammatory pain hypersensitivity in mice lacking glycine receptor alpha2

Glycinergic synapses play a major role in shaping the activity of spinal cord neurons under normal conditions and during persistent pain. However, the role of different glycine receptor (GlyR) subtypes in pain processing has only begun to be unraveled. Here, we analysed whether the GlyR alpha2 subunit might be involved in the processing of acute or persistent pain. Real-time RT-PCR and in situ hybridization analyses revealed that GlyR alpha2 mRNA is enriched in the dorsal horn of the mouse spinal cord. Mice lacking GlyR alpha2 (Glra2(-/-) mice) demonstrated a normal nociceptive behavior in models of acute pain and after peripheral nerve injury. However, mechanical hyperalgesia induced by peripheral injection of zymosan was significantly prolonged in Glra2(-/-) mice as compared to wild-type littermates. We conclude that spinal GlyRs containing the alpha2 subunit exert a previously unrecognized role in the resolution of inflammatory pain.

Six different roles for crossover inhibition in the retina: correcting the nonlinearities of synaptic transmission

Early retinal studies categorized ganglion cell behavior as either linear or nonlinear and rectifying as represented by the familiar X- and Y-type ganglion cells in cat. Nonlinear behavior is in large part a consequence of the rectifying nonlinearities inherent in synaptic transmission. These nonlinear signals underlie many special functions in retinal processing, including motion detection, motion in motion, and local edge detection. But linear behavior is also required for some visual processing tasks. For these tasks, the inherently nonlinear signals are "linearized" by "crossover inhibition." Linearization utilizes a circuitry whereby nonlinear ON inhibition adds with nonlinear OFF excitation or ON excitation adds with OFF inhibition to generate a more linear postsynaptic voltage response. Crossover inhibition has now been measured in most bipolar, amacrine, and ganglion cells. Functionally crossover inhibition enhances edge detection, allows ganglion cells to recognize luminance-neutral patterns with their receptive fields, permits ganglion cells to distinguish contrast from luminance, and maintains a more constant conductance during the light response. In some cases, crossover extends the operating range of cone-driven OFF ganglion cells into the scotopic levels. Crossover inhibition is also found in neurons of the lateral geniculate nucleus and V1.

Three forms of spatial temporal feedforward inhibition are common to different ganglion cell types in rabbit retina

There exist more than 30 different morphological amacrine cell types, but there may be fewer physiological types. Here we studied the amacrine cell outputs by measuring the temporal and spatial properties of feedforward inhibition to four different types of ganglion cells. These ganglion cells, each with concentric receptive field organization, appear to receive a different relative contribution of the same three forms of feed-forward inhibition, namely: local glycinergic, local sustained GABAergic, and broad transient GABAergic inhibition. Two of these inhibitory components, local glycinergic inhibition and local sustained GABAergic inhibition were localized to narrow regions confined to the dendritic fields of the ganglion cells. The third, a broad transient GABAergic inhibition, was driven from regions peripheral to the dendritic area. Each inhibitory component is also correlated with characteristic kinetics expressed in all ganglion cells: broad transient GABAergic inhibition had the shortest latency, local glycinergic inhibition had an intermediate latency, and local sustained GABAergic inhibition had the longest latency. We suggest each of these three inhibitory components represents the output from a distinct class of amacrine cell, mediates a specific visual function, and each forms a basic functional component for the four ganglion cell types. Similar subunits likely exist in the circuits of other ganglion cell types as well.

Electrical Coupling and Passive Membrane Properties of AII Amacrine Cells

AII amacrine cells in the mammalian retina are connected via electrical synapses to on-cone bipolar cells and to other AII amacrine cells. To understand synaptic integration in these interneurons, we need information about the junctional conductance ( gj), the membrane resistance ( rm), the membrane capacitance ( Cm), and the cytoplasmic resistivity ( Ri). Due to the extensive electrical coupling, it is difficult to obtain estimates of rm, as well as the relative contribution of the junctional and nonjunctional conductances to the total input resistance of an AII amacrine cell. Here we used dual voltage-clamp recording of pairs of electrically coupled AII amacrine cells in an in vitro slice preparation from rat retina and applied meclofenamic acid (MFA) to block the electrical coupling and isolate single AII amacrines electrically. In the control condition, the input resistance ( Rin) was ∼620 MΩ and the apparent rm was ∼760 MΩ. After block of electrical coupling, determined by estimating gj in the dual recordings, Rin and rm were ∼4,400 MΩ, suggesting that the nongap junctional conductance of an AII amacrine cell is ∼16% of the total input conductance. Control experiments with nucleated patches from AII amacrine cells suggested that MFA had no effect on the nongap junctional membrane of these cells. From morphological reconstructions of AII amacrine cells filled with biocytin, we obtained a surface area of ∼900 μm2 which, with a standard value for Cm of 0.01 pF/μm2, corresponds to an average capacitance of ∼9 pF and a specific membrane resistance of ∼41 kΩ cm2. Together with information concerning synaptic connectivity, these data will be important for developing realistic compartmental models of the network of AII amacrine cells.

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).

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.

Amacrine-to-amacrine cell inhibition in the rabbit retina

We studied the interactions between excitation and inhibition in morphologically identified amacrine cells in the light-adapted rabbit retinal slice under patch clamp. The majority of on amacrine cells received glycinergic off inhibition. About half of the off amacrine cells received glycinergic on inhibition. Neither class received any GABAergic inhibition. A minority of on, off, and on-off amacrine cells received both glycinergic on and GABAergic off inhibition. These interactions were found in cells with diverse morphologies having both wide and narrow processes that stratify in single or multiple layers of the inner plexiform layer (IPL). Most on-off amacrine cells received no inhibition and have monostratified processes confined to the middle of the IPL. The most common interaction between amacrine cells that we measured was "crossover inhibition," where off inhibits on and on inhibits off. Although the morphology of amacrine cells is diverse, the interactions between excitation and inhibition appear to be relatively limited and specific.

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.