Segregation and integration of visual channels: layer-by-layer computation of ON-OFF signals by amacrine cell dendrites

The co-expression of the depolarizing and hyperpolarizing mechanosensitive ion channels in mammalian retinal neurons

Introduction

The elevation of the intraocular and extraocular pressures is associated with various visual conditions, including glaucoma and traumatic retinal injury. The retina expresses mechanosensitive channels (MSCs), but the role of MSCs in retinal physiology and pathologies has been unclear.

Methods

Using immunocytochemistry, confocal microscopy, and patch-clamp recording techniques, we studied the co-expression of K+-permeable (K-MSCs) TRAAK and big potassium channel BK with the epithelial sodium channel ENaC and transient receptor potential channel vanilloid TPRV4 and TRPV2 favorably permeable to Ca2+ than Na+ (together named N-MSCs), and TRPV4 activity in the mouse retina.

Results

TRAAK immunoreactivity (IR) was mainly located in Müller cells. Photoreceptor outer segments (OSs) expressed BK and ENaCα intensively and TRAAK, TRPV2, and TRPV4 weakly. Somas and axons of retinal ganglion cells (RGCs) retrograde-identified clearly expressed ENaCα, TRPV4, and TRPV2 but lacked TRAAK and BK. Rod bipolar cells (RBCs) showed TRPV4-IR in somas and BK-IR in axonal globules. Horizontal cells were BK-negative, and some cone BCs lacked TRPV4-IR. TRPV4 agonist depolarized RGCs, enhanced spontaneous spikes and excitatory postsynaptic currents, reduced the visual signal reliability (VSR = 1-noise/signal) by ~50%, and resulted in ATP crisis, which could inactivate voltage-gated sodium channels in RGCs.

Conclusion

Individual neurons co-express hyperpolarizing K-MSCs with depolarizing N-MSCs to counterbalance the pressure-induced excitation, and the level of K-MSCs relative to N-MSCs (RK/N ratio) is balanced in the outer retina but low in RGCs, bringing out novel determinants for the pressure vulnerability of retinal neurons and new targets for clinical interventions.

TRPV4 affects visual signals in photoreceptors and rod bipolar cells

Introduction

Mechanical sensitive channels expressed in mammalian retinas are effectors of elevated pressure stresses, but it is unclear how their activation affects visual function in pressure-related retinal disorders.

Methods

This study investigated the role of the transient potential channel vanilloid TRPV4 in photoreceptors and rod bipolar cells (RBCs) with immunohistochemistry, confocal microscopy, electroretinography (ERG), and patch-clamp techniques.

Results

TRPV4 immunoreactivity (IR) was found in the outer segments of photoreceptors, dendrites and somas of PKCα-positive RBCs and other BCs, plexiform layers, and retinal ganglion cells (RGCs) in wild-type mice. TRPV4-IR was largely diminished in the retinas of homozygous TRPV4 transgenic mice. Genetically suppressing TRPV4 expression moderately but significantly enhanced the amplitude of ERG a- and b-waves evoked by scotopic and mesopic lights (0.55 to 200 Rh*rod-1 s-1) and photopic lights (105-106 Rh*rod-1 s-1) compared to wild-type mice in fully dark-adapted conditions. The implicit time evoked by dim lights (0.55 to 200 Rh*rod-1 s-1) was significantly decreased for b-waves and elongated for a-waves in the transgenic mice. ERG b-wave evoked by dim lights is primarily mediated by RBCs, and under voltage-clamp conditions, the latency of the light-evoked cation current in RBCs of the transgenic mice was significantly shorter compared to wild-type mice. About 10% of the transgenic mice had one eye undeveloped, and the percentage was significantly higher than in wild-type mice.

Conclusions

The data indicates that TRPV4 involves ocular development and is expressed and active in outer retinal neurons, and interventions of TRPV4 can variably affect visual signals in rods, cones, RBCs, and cone ON BCs.

Interpreting the retinal neural code for natural scenes: From computations to neurons

Understanding the circuit mechanisms of the visual code for natural scenes is a central goal of sensory neuroscience. We show that a three-layer network model predicts retinal natural scene responses with an accuracy nearing experimental limits. The model's internal structure is interpretable, as interneurons recorded separately and not modeled directly are highly correlated with model interneurons. Models fitted only to natural scenes reproduce a diverse set of phenomena related to motion encoding, adaptation, and predictive coding, establishing their ethological relevance to natural visual computation. A new approach decomposes the computations of model ganglion cells into the contributions of model interneurons, allowing automatic generation of new hypotheses for how interneurons with different spatiotemporal responses are combined to generate retinal computations, including predictive phenomena currently lacking an explanation. Our results demonstrate a unified and general approach to study the circuit mechanisms of ethological retinal computations under natural visual scenes.

Dual-Cell Patch-Clamp Recording Revealed a Mechanism for a Ribbon Synapse to Process Both Digital and Analog Inputs and Outputs

A chemical synapse is either an action potential (AP) synapse or a graded potential (GP) synapse but not both. This study investigated how signals passed the glutamatergic synapse between the rod photoreceptor and its postsynaptic hyperpolarizing bipolar cells (HBCs) and light responses of retinal neurons with dual-cell and single-cell patch-clamp recording techniques. The results showed that scotopic lights evoked GPs in rods, whose depolarizing Phase 3 associated with the light offset also evoked APs of a duration of 241.8 ms and a slope of 4.5 mV/ms. The depolarization speed of Phase 3 (Speed) was 0.0001–0.0111 mV/ms and 0.103–0.469 mV/ms for rods and cones, respectively. On pairs of recorded rods and HBCs, only the depolarizing limbs of square waves applied to rods evoked clear currents in HBCs which reversed at −6.1 mV, indicating cation currents. We further used stimuli that simulated the rod light response to stimulate rods and recorded the rod-evoked excitatory current (rdEPSC) in HBCs. The normalized amplitude (R/R_max), delay, and rising slope of rdEPSCs were differentially exponentially correlated with the Speed (all p < 0.001). For the Speed < 0.1 mV/ms, R/R_max grew while the delay and duration reduced slowly; for the Speed between 0.1 and 0.4 mV/ms, R/R_max grew fast while the delay and duration dramatically decreased; for the Speed > 0.4 mV/ms, R/R_max reached the plateau, while the delay and duration approached the minimum, resembling digital signals. The rdEPSC peak was left-shifted and much faster than currents in rods. The scotopic-light-offset-associated major and minor cation currents in retinal ganglion cells (RGCs), the gigantic excitatory transient currents (GTECs) in HBCs, and APs and Phase 3 in rods showed comparable light-intensity-related locations. The data demonstrate that the rod-HBC synapse is a perfect synapse that can differentially decode and code analog and digital signals to process enormously varied rod and coupled-cone inputs.

Generators of Pressure-Evoked Currents in Vertebrate Outer Retinal Neurons

(1) Background: High-tension glaucoma damages the peripheral vision dominated by rods. How mechanosensitive channels (MSCs) in the outer retina mediate pressure responses is unclear. (2) Methods: Immunocytochemistry, patch clamp, and channel fluorescence were used to study MSCs in salamander photoreceptors. (3) Results: Immunoreactivity of transient receptor potential channel vanilloid 4 (TRPV4) was revealed in the outer plexiform layer, K⁺ channel TRAAK in the photoreceptor outer segment (OS), and TRPV2 in some rod OS disks. Pressure on the rod inner segment evoked sustained currents of three components: (A) the inward current at <-50 mV (I_pi), sensitive to Co²⁺; (B) leak outward current at ≥-80 mV (I_po), sensitive to intracellular Cs⁺ and ruthenium red; and (C) cation current reversed at ~10 mV (I_pc). Hypotonicity induced slow currents like I_pc. Environmental pressure and light increased the FM 1-43-identified open MSCs in the OS membrane, while pressure on the OS with internal Cs⁺ closed a Ca²⁺-dependent current reversed at ~0 mV. Rod photocurrents were thermosensitive and affected by MSC blockers. (4) Conclusions: Rods possess depolarizing (TRPV) and hyperpolarizing (K⁺) MSCs, which mediate mutually compensating currents between -50 mV and 10 mV, serve as an electrical cushion to minimize the impact of ocular mechanical stress.

Adaptation of Inhibition Mediates Retinal Sensitization

In response to a changing sensory environment, sensory systems adjust their neural code for a number of purposes, including an enhanced sensitivity for novel stimuli, prediction of sensory features, and the maintenance of sensitivity. Retinal sensitization is a form of short-term plasticity that elevates local sensitivity following strong, local, visual stimulation and has been shown to create a prediction of the presence of a nearby localized object. The neural mechanism that generates this elevation in sensitivity remains unknown. Using simultaneous intracellular and multielectrode recording in the salamander retina, we show that a decrease in tonic amacrine transmission is necessary for and is correlated spatially and temporally with ganglion cell sensitization. Furthermore, introducing a decrease in amacrine transmission is sufficient to sensitize nearby ganglion cells. A computational model accounting for adaptive dynamics and nonlinear pathways confirms a decrease in steady inhibitory transmission can cause sensitization. Adaptation of inhibition enhances the sensitivity to the sensory feature conveyed by an inhibitory pathway, creating a prediction of future input.

Color Processing in Zebrafish Retina

Zebrafish (Danio rerio) is a model organism for vertebrate developmental processes and, through a variety of mutant and transgenic lines, various diseases and their complications. Some of these diseases relate to proper function of the visual system. In the US, the National Eye Institute indicates >140 million people over the age of 40 have some form of visual impairment. The causes of the impairments range from refractive error to cataract, diabetic retinopathy and glaucoma, plus heritable diseases such as retinitis pigmentosa and color vision deficits. Most impairments directly affect the retina, the nervous tissue at the back of the eye. Zebrafish with long or short-wavelength color blindness, altered retinal anatomy due to hyperglycemia, high intraocular pressure, and reduced pigment epithelium are all used, and directly applicable, to study how these symptoms affect visual function. However, many published reports describe only molecular/anatomical/structural changes or behavioral deficits. Recent work in zebrafish has documented physiological responses of the different cell types to colored (spectral) light stimuli, indicating a complex level of information processing and color vision in this species. The purpose of this review article is to consolidate published morphological and physiological data from different cells to describe how zebrafish retina is capable of complex visual processing. This information is compared to findings in other vertebrates and relevance to disorders affecting color processing is discussed.

Physiological and morphological characterization of ganglion cells in the salamander retina

Retinal ganglion cells (RGCs) integrate visual information from the retina and transmit collective signals to the brain. A systematic investigation of functional and morphological characteristics of various types of RGCs is important to comprehensively understand how the visual system encodes and transmits information via various RGC pathways. This study evaluated both physiological and morphological properties of 67 RGCs in dark-adapted flat-mounted salamander retina by examining light-evoked cation and chloride current responses via voltage-clamp recordings and visualizing morphology by Lucifer yellow fluorescence with a confocal microscope. Six groups of RGCs were described: asymmetrical ON-OFF RGCs, symmetrical ON RGCs, OFF RGCs, and narrow-, medium- and wide-field ON-OFF RGCs. Dendritic field diameters of RGCs ranged 102-490 μm: narrow field (<200 μm, 31% of RGCs), medium field (200-300 μm, 45%) and wide field (>300 μm, 24%). Dendritic ramification patterns of RGCs agree with the sublamina A/B rule. 34% of RGCs were monostratified, 24% bistratified and 42% diffusely stratified. 70% of ON RGCs and OFF RGCs were monostratified. Wide-field RGCs were diffusely stratified. 82% of RGCs generated light-evoked ON-OFF responses, while 11% generated ON responses and 7% OFF responses. Response sensitivity analysis suggested that some RGCs obtained separated rod/cone bipolar cell inputs whereas others obtained mixed bipolar cell inputs. 25% of neurons in the RGC layer were displaced amacrine cells. Although more types may be defined by more refined classification criteria, this report is to incorporate more physiological properties into RGC classification.

Imaging retinal ganglion cells: enabling experimental technology for clinical application

Recent advances in clinical ophthalmic imaging have enhanced patient care. However, the ability to differentiate retinal neurons, such as retinal ganglion cells (RGCs), would advance many areas within ophthalmology, including the screening and monitoring of glaucoma and other optic neuropathies. Imaging at the single cell level would take diagnostics to the next level. Experimental methods have provided techniques and insight into imaging RGCs, however no method has yet to be translated to clinical application. This review provides an overview of the importance of non-invasive imaging of RGCs and the clinically relevant capabilities. In addition, we report on experimental data from wild-type mice that received an in vivo intravitreal injection of a neuronal tracer that labelled RGCs, which in turn were monitored for up to 100 days post-injection with confocal scanning laser ophthalmoscopy. We were able to demonstrate efficient and consistent RGC labelling with this delivery method and discuss the issue of cell specificity. This type of experimental work is important in progressing towards clinically applicable methods for monitoring loss of RGCs in glaucoma and other optic neuropathies. We discuss the challenges to translating these findings to clinical application and how this method of tracking RGCs in vivo could provide valuable structural and functional information to clinicians.

Morphological and physiological properties of enhanced green fluorescent protein (EGFP)-expressing wide-field amacrine cells in the ChAT-EGFP mouse line

Mammalian retinas comprise a variety of interneurons, among which amacrine cells represent the largest group, with more than 30 different cell types each exhibiting a rather distinctive morphology and carrying out a unique function in retinal processing. However, many amacrine types have not been studied systematically because, in particular, amacrine cells with large dendritic fields, i.e. wide-field amacrine cells, have a low abundance and are therefore difficult to target. Here, we used a transgenic mouse line expressing the coding sequence of enhanced green fluorescent protein under the promoter for choline acetyltransferase (ChAT-EGFP mouse) and characterized a single wide-field amacrine cell population monostratifying in layer 2/3 of the inner plexiform layer (WA-S2/3 cell). Somata of WA-S2/3 cells are located either in the inner nuclear layer or are displaced to the ganglion cell layer and exhibit a low cell density. Using immunohistochemistry, we show that WA-S2/3 cells are presumably GABAergic but may also release acetylcholine as their somata are weakly positive for ChAT. Two-photon-guided patch-clamp recordings from intact retinas revealed WA-S2/3 cells to be ON-OFF cells with a homogenous receptive field even larger than the dendritic field. The large spatial extent of the receptive field is most likely due to the extensive homologous and heterologous coupling among WA-S2/3 cells and to other amacrine cells, respectively, as indicated by tracer injections. In summary, we have characterized a novel type of GABAergic ON-OFF wide-field amacrine cell which is ideally suited to providing long-range inhibition to ganglion cells due to its strong coupling.

Light-evoked currents in retinal ganglion cells from dystrophic RCS rats

PURPOSE

To study the electrophysiological properties of the light-evoked currents in ganglion cells in situations of retinal degeneration.

METHODS

We investigated light-evoked currents in ganglion cells by performing whole-cell patch-clamp recordings from ganglion cells using a retina-stretched preparation from Royal College of Surgeons (RCS) rats, a model of retinal degeneration and congenic controls at different ages. Pharmacological inhibitors of the AMPA receptor (NBQX), GABA receptor (BMI), and sodium channels (TTX) were used to identify the components of the light-evoked currents in ON, OFF and ON-OFF retinal ganglion cells.

RESULTS

We found that the light-evoked currents in ganglion cells from control rats were inhibited by NBQX, BMI and TTX, suggesting that AMPA receptors, GABA receptors and sodium channels contribute to these currents in ganglion cells. However, only AMPA receptor-mediated currents were recorded in RCS rats. Light-evoked inward currents were absent in the majority of ganglion cells from RCS rats, particularly at the later stages of retinal degeneration. At earlier stages of retinal degeneration, we found that both the timing and amplitude of light-evoked currents are significantly different in ganglion cells from RCS and control rats.

CONCLUSIONS

Our study furthers the understanding of the electrophysiological characteristics of retinal ganglion cells during retinal degeneration, and provides insight into the optimal timing for the treatment of retinal degeneration.

Transformation of visual signals by inhibitory interneurons in retinal circuits

One of the largest mysteries of the brain lies in understanding how higher-level computations are implemented by lower-level operations in neurons and synapses. In particular, in many brain regions inhibitory interneurons represent a diverse class of cells, the individual functional roles of which are unknown. We discuss here how the operations of inhibitory interneurons influence the behavior of a circuit, focusing on recent results in the vertebrate retina. A key role in this understanding is played by a common representation of the visual stimulus that can be applied at different stages. By considering how this stimulus representation changes at each location in the circuit, we can understand how neuron-level operations such as thresholds and inhibition yield circuit-level computations such as how stimulus selectivity and gain are controlled by local and peripheral visual stimuli.

Responses of starburst amacrine cells to prosthetic stimulation of the retina

Recent advances in the design and development of retinal implants have made these devices a promising therapeutic strategy for restoring sight to the blind. Over the last decade a plethora of studies have investigated the responses of the retinal ganglion cells (RGCs) to electrical stimulation under a variety of stimulus configurations. Similar to the RGCs, the amacrine cells also survive in large numbers following retinal neural degeneration. However, with the exception of two previous reports, where the responses of the amacrine cells were measured indirectly, these cells have thus far received little attention in the context of prosthetic stimulation. In this study we focused on the starburst amacrine cells (SACs), a particularly well-characterized amacrine cell among the approximately two-dozen types known to exist in the retina. Using whole-cell patch clamp recordings in the whole-mount rabbit retina, we investigated the temporal responses of the SACs following subretinal biphasic pulse stimulation. These cells responded to the stimuli with oscillatory membrane potentials that lasted for tens to hundreds of milliseconds, with the response amplitude increasing as a function of stimulus strength. Furthermore, the SAC responses originated primarily from the presynaptic inputs they receive, rather than through direct activation of these cells by the electrical stimuli.

Physiological characterization and functional heterogeneity of narrow-field mammalian amacrine cells

Light-evoked responses of 106 morphologically identified narrow-field amacrine cells (ACs) were studied in dark-adapted mouse retinal slices. Forty-five cells exhibit AIIAC morphology, 55% of which show characteristic AIIAC physiological properties (AIIAC1s) and the remaining 45% display different physiological responses, suggesting that AIIACs are functionally heterogeneous. Moreover, we found that 42 cells exhibit morphology that resembles the seven morphological types of glycine-positive ACs (GlyAC1-7) reported in the rat retina, and for the first time assigned light response and function properties to these morphological types of glycinergic ACs in the mouse retina. In addition, five narrow-field ACs exhibited morphology resembling that of the GlyAC5 or GlyAC7 but with different physiological responses (GlyAC5(#) and GlyAC7(#)). Therefore, the eight morphological types of narrow-field ACs exhibit 12 classes of physiological responses. Furthermore, we found ACs whose physiological responses were indistinguishable from those of GlyAC3 or GlyAC4s but with different morphology (GlyAC3* or GlyAC4*). These observations suggest that although the majority of narrow-field mammalian ACs forms discrete functional groups that correlate with their morphology, a significant number of these cells with similar morphology do not display the same light responses, and some with similar light responses do not exhibit the same morphology.

Disinhibitory gating of retinal output by transmission from an amacrine cell

Inhibitory interneurons help transform the input of a neural circuit into its output. Such interneurons are diverse, and most have unknown function. To study the function of single amacrine cells in the intact salamander retina, we recorded extracellularly from a population of ganglion cells with a multielectrode array, while simultaneously recording from or injecting current into single Off-type amacrine cells that had linear responses. We measured how visual responses of the amacrine cell interacted both with other visual input to the ganglion cell and with transmission between the two cells. We found that on average, visual responses from Off-type amacrine cells inhibited nearby Off-type ganglion cells. By recording and playing back the light-driven membrane potential fluctuations of amacrine cells during white noise visual stimuli, we found that paradoxically, increasing the light-driven modulations of inhibitory amacrine cells increased the firing rate of nearby Off-type ganglion cells. By measuring the correlations and transmission between amacrine and ganglion cells, we found that, on average, the amacrine cell hyperpolarizes before the ganglion cell fires, generating timed disinhibition just before the ganglion cell spikes. In addition, we found that amacrine to ganglion cell transmission is nonlinear in that increases in ganglion cell activity produced by amacrine hyperpolarization were greater than decreases in activity produced by amacrine depolarization. We conclude that the primary mode of action of this class of amacrine cell is to actively gate the ganglion cell response by a timed release from inhibition.

Effects of histamine on light responses of amacrine cells in tiger salamander retina

Using immunofluorescence, we showed that histamine receptor 1 is expressed by horizontal cell axons and a subset of amacrine cells in the tiger salamander retina. The effects of histamine on light responses of amacrine cells were studied in slice preparations. Histamine modulated the light responses of many salamander amacrine cells, depending upon the morphological type. The most pronounced effects of histamine were decreases in the light responses of broadly stratified amacrine cells, particularly those having medium-sized dendritic field diameters. To determine whether the effects of histamine were direct, Co(++) was substituted for Ca(++) in the extracellular medium to block synaptic transmission. Histamine still affected broadly stratified amacrine cells, but not narrowly stratified amacrine cells under these conditions. Taken together, these findings suggest that inhibitory interactions between strata of the IPL and within the classical receptive fields of the ganglion cells would be particularly sensitive to histamine released from retinopetal axons.

Responses and receptive fields of amacrine cells and ganglion cells in the salamander retina

Retinal amacrine cells (ACs) and ganglion cells (GCs) have been shown to display large morphological diversity, and here we show that four types of ACs and three types of GCs exhibit physiologically-distinguishable properties. They are the sustained ON ACs; sustained OFF ACs; transient ON-OFF ACs; transient ON-OFF ACs with wide receptive fields; sustained ON-center/OFF-surround GCs; sustained OFF-center/ON-surround GCs and transient ON-OFF GCs. By comparing response waveforms, receptive fields and relative rod/cone inputs of ACs and GCs with the corresponding parameters of various types of the presynaptic bipolar cells (BCs), we analyze how different types of BCs mediate synaptic inputs to various ACs and GCs. Although more types of third-order retinal neurons may be identified by more refined classification criteria, our observations suggest that many morphologically-distinct ACs and GCs share very similar physiological responses.

Functional localization of the nitric oxide/cGMP pathway in the salamander retina

Nitric oxide (NO) is a gaseous neuromodulator that has physiological functions in every cell type in the retina. Evidence indicates that NO often plays a role in the processing of visual information in the retina through the second messenger cyclic guanosine monophosphate (cGMP). Despite numerous structural and functional studies of this signaling pathway in the retina, none have examined many of the elements of this pathway within a single study in a single species. In this study, the NO/cGMP pathway was localized to specific regions and cell types within the inner and outer retina. We have immunocytochemically localized nitric oxide synthase, the enzyme that produces NO, in photoreceptor ellipsoids, four distinct classes of amacrine cells, Müller and bipolar cells, somata in the ganglion cell layer, as well as in processes within both plexiform layers. Additionally, we localized NO production in specific cell types using the NO-sensitive dye diaminofluorescein. cGMP immunocytochemistry was used to functionally localize soluble guanylate cyclase that was activated by an NO donor in select amacrine and bipolar cell classes. Analysis of cGMP and its downstream target, cGMP-dependent protein kinase II (PKGII), showed colocalization within processes in the outer retina as well as in somata in the inner retina. The results of this study showed that the NO/cGMP signaling pathway was functional and its components were widely distributed throughout specific cell types in the outer and inner salamander retina.

gamma-Protocadherins regulate neuronal survival but are dispensable for circuit formation in retina

Twenty-two tandemly arranged protocadherin-gamma (Pcdh-gamma) genes encode transmembrane proteins with distinct cadherin-related extracellular domains and a common intracellular domain. Genetic studies have implicated Pcdh-gamma genes in the regulation of neuronal survival and synapse formation. Because mice lacking the Pcdh-gamma cluster die perinatally, we generated conditional mutants to analyze roles of Pcdh-gamma genes in the development and function of neural circuits. Retina-specific deletion of Pcdh-gammas led to accentuation of naturally occurring death of interneurons and retinal ganglion cells (RGCs) during the first two postnatal weeks. Nonetheless, many neuronal subtypes formed lamina-specific arbors. Blocking apoptosis by deletion of the pro-apoptotic gene Bax showed that even neurons destined to die formed qualitatively and quantitatively appropriate connections. Moreover, electrophysiological analysis indicated that processing of visual information was largely normal in the absence of Pcdh-gamma genes. These results suggest that Pcdh-gamma genes are dispensable for elaboration of specific connections in retina, but play a primary role in sculpting neuronal populations to appropriate sizes or proportions during the period of naturally occurring cell death.

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.

Dscam and Sidekick proteins direct lamina-specific synaptic connections in vertebrate retina

Synaptic circuits in the retina transform visual input gathered by photoreceptors into messages that retinal ganglion cells (RGCs) send to the brain. Processes of retinal interneurons (amacrine and bipolar cells) form synapses on dendrites of RGCs in the inner plexiform layer (IPL). The IPL is divided into at least 10 parallel sublaminae; subsets of interneurons and RGCs arborize and form synapses in just one or a few of them. These lamina-specific circuits determine the visual features to which RGC subtypes respond. Here we show that four closely related immunoglobulin superfamily (IgSF) adhesion molecules--Dscam (Down's syndrome cell adhesion molecule), DscamL (refs 6-9), Sidekick-1 and Sidekick-2 (ref. 10)--are expressed in chick by non-overlapping subsets of interneurons and RGCs that form synapses in distinct IPL sublaminae. Moreover, each protein is concentrated within the appropriate sublaminae and each mediates homophilic adhesion. Loss- and gain-of-function studies in vivo indicate that these IgSF members participate in determining the IPL sublaminae in which synaptic partners arborize and connect. Thus, vertebrate Dscams, like Drosophila Dscams, play roles in neural connectivity. Together, our results on Dscams and Sidekicks suggest the existence of an IgSF code for laminar specificity in retina and, by implication, in other parts of the central nervous system.

Retinal ganglion cells can rapidly change polarity from Off to On

Retinal ganglion cells are commonly classified as On-center or Off-center depending on whether they are excited predominantly by brightening or dimming within the receptive field. Here we report that many ganglion cells in the salamander retina can switch from one response type to the other, depending on stimulus events far from the receptive field. Specifically, a shift of the peripheral image—as produced by a rapid eye movement—causes a brief transition in visual sensitivity from Off-type to On-type for approximately 100 ms. We show that these ganglion cells receive inputs from both On and Off bipolar cells, and the Off inputs are normally dominant. The peripheral shift strongly modulates the strength of these two inputs in opposite directions, facilitating the On pathway and suppressing the Off pathway. Furthermore, we identify certain wide-field amacrine cells that contribute to this modulation. Depolarizing such an amacrine cell affects nearby ganglion cells in the same way as the peripheral image shift, facilitating the On inputs and suppressing the Off inputs. This study illustrates how inhibitory interneurons can rapidly gate the flow of information within a circuit, dramatically altering the behavior of the principal neurons in the course of a computation.

The eye communicates to the brain all the information needed for vision in the form of electrical pulses, or spikes, on optic nerve fibers. These spikes are produced by retinal ganglion cells, the output neurons of the retina. In a popular view of retinal function, each ganglion cell responds to a small region of interest in the visual image, known as its receptive field, and is specialized for certain image features within that window. When a cell encounters that image feature, the neuron responds by firing one or more spikes. Different neurons are tuned to different features. For example, some ganglion cells fire when light dims, others when it brightens. Here we show that a rapid shift in the image on the retina can cause a dramatic change in a neuron's preferred feature: For example, a dimming-detector can briefly turn into a brightening-detector. We explore the mechanisms that implement such a switch of feature tuning, and the consequences it might have for visual processing.

A peripheral image shift produces a transient switch in retinal ganglion cell responses from Off-dominated to On-dominated. This modulation is exerted at least in part presynaptically, presumably at the bipolar cell synaptic terminal.

Synchronized firing among retinal ganglion cells signals motion reversal

We show that when a moving object suddenly reverses direction, there is a brief, synchronous burst of firing within a population of retinal ganglion cells. This burst can be driven by either the leading or trailing edge of the object. The latency is constant for movement at different speeds, objects of different size, and bright versus dark contrasts. The same ganglion cells that signal a motion reversal also respond to smooth motion. We show that the brain can build a pure reversal detector using only a linear filter that reads out synchrony from a group of ganglion cells. These results indicate that not only can the retina anticipate the location of a smoothly moving object, but that it can also signal violations in its own prediction. We show that the reversal response cannot be explained by models of the classical receptive field and suggest that nonlinear receptive field subunits may be responsible.

Narrow and wide field amacrine cells fire action potentials in response to depolarization and light stimulation

Action potentials in amacrine cells are important for lateral propagation of signals across the inner retina, but it is unclear how many subclasses of amacrine cells contain voltage-gated sodium channels or can fire action potentials. This study investigated the ability of amacrine cells with narrow (< 200 μm) and wide (> 200 μm) dendritic fields to fire action potentials in response to depolarizing current injections and light stimulation. The pattern of action potentials evoked by current injections revealed two distinct classes of amacrine cells; those that responded with a single action potential (single-spiking cells) and those that responded with repetitive action potentials (repetitive-spiking cells). Repetitive-spiking cells differed from single-spiking cells in several regards: Repetitive-spiking cells were more often wide field cells, while single-spiking cells were more often narrow field cells. Repetitive-spiking cells had larger action potential amplitudes, larger peak voltage-gated NaV currents lower action potential thresholds, and needed less current to induce action potentials. However, there was no difference in the input resistance, holding current or time constant of these two classes of cells. The intrinsic capacity to fire action potentials was mirrored in responses to light stimulation; single-spiking amacrine cells infrequently fired action potentials to light steps, while repetitive-spiking amacrine cells frequently fired numerous action potentials. These results indicate that there are two physiologically distinct classes of amacrine cells based on the intrinsic capacity to fire action potentials.

Timing and Computation in Inner Retinal Circuitry

In the vertebrate inner retina, the second stage of the visual system, different components of the visual scene are transformed, discarded, or selected before visual information is transmitted through the optic nerve. This review discusses the connections between higher-level functions of visual processing, mathematical descriptions of the neural code, inner retinal circuitry, and visual computations. In the inner plexiform layer, bipolar cells deliver spatially and temporally filtered input to approximately ten anatomical strata. These layers receive a unique combination of excitation and inhibition, causing cells in different layers to respond with different kinetics to visual input. These distinct temporal channels interact through amacrine cells, a diverse class of inhibitory interneurons, which transmit signals within and between layers. In particular, wide-field amacrine cells transmit transient inhibition over long distances within a layer. These mechanisms and properties are combined into computations to detect the presence of differential motion and suppress the visual effects of eye movements.

Form and Function of on-off Amacrine Cells in the Amphibian Retina

on-off amacrine cells were studied with whole cell recording techniques and intracellular staining methods using intact retina-eyecup preparations of the tiger salamander ( Ambystoma tigrinum) and the mudpuppy ( Necturus maculosus). Morphological characterization of these cells included three-dimensional reconstruction methods based on serial optical sections obtained with a confocal microscope. Some cells had their detailed morphology digitized with a computer-assisted tracing system and converted to compartmental models for computer simulations. The dendrites of on-off amacrine cells have spines and numerous varicosities. Physiological recordings confirmed that on-off amacrine cells generate both large- and small-amplitude impulses attributed, respectively, to somatic and dendritic generation sites. Using a multichannel model for impulse generation, computer simulations were carried out to evaluate how impulses are likely to propagate throughout these structures. We conclude that the on-off amacrine cell is organized with multifocal dendritic impulse generating sites and that both dendritic and somatic impulse activity contribute to the functional repertoire of these interneurons: locally generated dendritic impulses can provide regional activation, while somatic impulse activity results in rapid activation of the entire dendritic tree.

Functional organization of ganglion cells in the salamander retina

Recently, we reported a novel technique for recording all of the ganglion cells in a retinal patch and showed that their receptive fields cover visual space roughly 60 times over in the tiger salamander. Here, we carry this analysis further and divide the population of ganglion cells into functional classes using quantitative clustering algorithms that combine several response characteristics. Using only the receptive field to classify ganglion cells revealed six cell types, in agreement with anatomical studies. Adding other response measures served to blur the distinctions between these cell types rather than resolve further classes. Only the biphasic off type had receptive fields that tiled the retina. Even when we attempted to split these classes more finely, ganglion cells with almost identical functional properties were found to have strongly overlapping spatial receptive fields. A territorial spatial organization, where ganglion cell receptive fields tend to avoid those of other cells of the same type, was only found for the biphasic off cell. We further studied the functional segregation of the ganglion cell population by computing the amount of visual information shared between pairs of cells under natural movie stimulation. This analysis revealed an extensive mixing of visual information among cells of different functional type. Together, our results indicate that the salamander retina uses a population code in which every point in visual space is represented by multiple neurons with subtly different visual sensitivities.

Targeting of amacrine cell neurites to appropriate synaptic laminae in the developing zebrafish retina

Cellular mechanisms underlying the precision by which neurons target their synaptic partners have largely been determined based on the study of projection neurons. By contrast, little is known about how interneurons establish their local connections in vivo. Here, we investigated how developing amacrine interneurons selectively innervate the appropriate region of the synaptic neuropil in the inner retina, the inner plexiform layer (IPL). Increases (ON) and decreases (OFF) in light intensity are processed by circuits that are structurally confined to separate ON and OFF synaptic sublaminae within the IPL. Using transgenic zebrafish in which the majority of amacrine cells express fluorescent protein, we determined that the earliest amacrine-derived neuritic plexus formed between two cell populations whose somata, at maturity, resided on opposite sides of this plexus. When we followed the behavior of individual amacrine cells over time, we discovered that they exhibited distinct patterns of structural dynamics at different stages of development. During cellular migration, amacrine cells exhibited an exuberant outgrowth of neurites that was undirected. Upon reaching the forming IPL, neurites extending towards the ganglion cell layer were relatively more stable. Importantly, when an arbor first formed, it preferentially ramified in either the inner or outer IPL corresponding to the future ON and OFF sublaminae, and maintained this stratification pattern. The specificity by which ON and OFF amacrine interneurons innervate their respective sublaminae in the IPL contrasts with that observed for projection neurons in the retina and elsewhere in the central nervous system.

STRUCTURAL AND FUNCTIONAL PROPERTIES OF HOMOLOGOUS ELECTRICAL SYNAPSES BETWEEN RETINAL AMACRINE CELLS

Retinal amacrine cells regulate activities of retinal ganglion cells, the output neurons to higher visual centers, through cellular mechanism of lateral inhibition in the inner plexiform layer (IPL). Electrical properties of gap junction networks between amacrine cells in the IPL were investigated using combined techniques of intracellular recordings, Lucifer yellow and Neurobiotin injection, dual patch-clamp recordings and high voltage electron microscopy in isolated retinas of cyprinid fish. Six types of gap-junctionally connected amacrine cells were classified after their light-evoked responses to light flashes were recorded. Among them, gap junction networks of three types of amacrine cells were studied with structure-function correlation analysis. Cellular morphology of intercellular connections between three homologous cell classes was characterized. The interconnections between laterally extending dendrites in the IPL were localized at dendritic tip terminals. Three types of cells presented the dendrodendritic connections of tip-contact manner in the homologous cell population. High voltage as well as conventional electron microscopy revealed gap junctions between the dendritic tips of Neurobiotin-coupled cells. Receptive field properties of these amacrine cells were examined, displacing a slit of light along the distance from recording sites in the dorsal intermediate region of the retina. Receptive field size, space length constant, response latency and conduction velocity were measured. Spatial and temporal properties of receptive fields were symmetric along horizontally expanding dendrites in the dorsal retina. Simultaneous dual patch-clamp recordings revealed that the lateral gap junction connections between homologous amacrine cells expressed bidirectional electrical synapses passing Na(+) spikes. These results demonstrate that bidirectional electrical transmission in gap junction networks of these amacrine cells is symmetric along the lateral gap junction connections between horizontally extending dendrites. Lateral inhibition regulated by amacrine cells in the IPL appears to be associated with the directional extension of the dendrites and the orientation of dendrodendritic gap junctions.

Redundancy in the population code of the retina

We have explored the manner in which the population of retinal ganglion cells collectively represent the visual world. Ganglion cells in the salamander were recorded simultaneously with a multielectrode array during stimulation with both artificial and natural visual stimuli, and the mutual information that single cells and pairs of cells conveyed about the stimulus was estimated. We found significant redundancy between cells spaced as far as 500 mum apart. When we used standard methods for defining functional types, only ON-type and OFF-type cells emerged as truly independent information channels. Although the average redundancy between nearby cell pairs was moderate, each ganglion cell shared information with many neighbors, so that visual information was represented approximately 10-fold within the ganglion cell population. This high degree of retinal redundancy suggests that design principles beyond coding efficiency may be important at the population level.

Signal transmission from cones to amacrine cells in dark- and light-adapted tiger salamander retina

Amacrine cells (ACs) are third-order interneurons in the retina that mediate antagonistic surround inputs to retinal ganglion cells and motion-related signals in the inner retina. Previous studies have revealed that rod-to-AC signals in dark-adapted retina are mediated by a nonlinear high-gain synaptic pathway. In this study, we investigated how cone signals are transmitted to ACs under dark- and light-adapted conditions. By using the spectral subtraction method, we found that the voltage gain of the cone-AC synaptic pathway in dark-adapted salamander retina (GD) is between 28 and 72, which is about one order of magnitude lower than the voltage gain of the rod-AC pathway. This suggests that, in darkness, rod signals are more efficiently transmitted to the ACs than cone signals. The voltage gain of the cone-AC synaptic pathway in the presence of 500 nm/-2.4 background light, GL, ranges between 28 and 56. Linear regression analysis indicates that GD and GL are strongly, positively, and linearly correlated. The average GL/GD ratio is 0.73, suggesting that, on average, GL in any given AC is about 73% of GD. This adaptation-induced change in cone-AC voltage gain exemplifies use-dependent modulations of synaptic transmission in the retina, and possible mechanisms underlying light-mediated alterations of retinal synaptic function are discussed.

Identification and morphological classification of horizontal, bipolar, and amacrine cells within the zebrafish retina

Horizontal, bipolar, and amacrine cells in the zebrafish retina were morphologically characterized using DiOlistic techniques. In this method, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-coated microcarriers are shot at high speed onto the surfaces of living retinal slices where the DiI then delineates axons, somata, and dendrites of isolated neurons. Zebrafish retinal somata were 5-10 microm in diameter. Three horizontal cell types (HA-1, HA-2, and HB) were identified; dendritic tree diameters averaged 25-40 microm. HA somata were round. Cells classified as HA-2 were larger than HA-1 cells and possessed an axon. HB somata were flattened, without an axon, although short fusiform structure(s) projected from the soma. Bipolar cells were separated into 17 morphological types. Dendritic trees ranged from 10 to 70 microM. There were six B(on) types with axon boutons only in the ON sublamina of the inner plexiform layer (IPL), and seven B(off) types with axon boutons or branches only in the OFF sublamina. Four types of bistratified bipolar cells displayed boutons in both ON and OFF layers. Amacrine cells occurred in seven types. A(off) cells (three types) were monostratified and ramified in the IPL OFF sublamina. Dendritic fields were 60-150 microM. A(on) pyriform cells (three types) branched in the ON sublamina. Dendritic fields were 50-170 microM. A(diffuse) cells articulated processes in all IPL strata. Dendritic fields were 15-90 microM. These findings are important for studies examining signal processing in zebrafish retina and for understanding changes in function resulting from mutations and perturbations of retinal organization.

Vesicular glutamate transporter 3 expression identifies glutamatergic amacrine cells in the rodent retina

Synaptic transmission from glutamatergic neurons requires vesicular glutamate transporters (VGLUTs) to concentrate cytosolic glutamate in synaptic vesicles. In retina, glutamatergic photoreceptors and bipolar cells exclusively express the VGLUT1 isoform, whereas ganglion cells express VGLUT2. Surprisingly, the recently identified VGLUT3 isoform was found in presumed amacrine cells, generally considered to be inhibitory interneurons. To investigate the synaptic machinery and conceivable secondary neurotransmitter composition of VGLUT3 cells, and to determine a potential functional role, we further investigated these putative glutamatergic amacrine cells in adult and developing rodent retina. Reverse transcriptase-PCR substantiated VGLUT3 expression in mouse retina. VGLUT3 cells did not immunostain for ganglion or bipolar cell markers, providing evidence that they are amacrine cells. VGLUT3 colocalized with synaptic vesicle markers, and electron microscopy showed that VGLUT3 immunostained synaptic vesicles. VGLUT3 cells were not immunoreactive for amacrine cell markers gamma-aminobutyric acid, choline acetyltransferase, calretinin, or tyrosine hydroxylase, although they immunostain for glycine. VGLUT3 processes made synaptic contact with ganglion cell dendrites, suggesting input onto these cells. VGLUT3 immunostaining was closely associated with the metabotropic glutamate receptor 4, which is consistent with glutamatergic synaptic exocytosis by these cells. In the maturing mouse retina, Western blots showed VGLUT3 expression at postnatal day 7/8 (P7/8). VGLUT3 immunostaining in retinal sections was first observed at P8, achieving an adult pattern at P12. Thus, VGLUT3 function commences around the same time as VGLUT1-mediated glutamatergic transmission from bipolar cells. Furthermore, a subset of VGLUT3 cells expressed the circadian clock gene period 1, implicating VGLUT3 cells as part of the light-entrainable retina-based circadian system.

Immuocytochemical analysis of spatial organization of photoreceptors and amacrine and ganglion cells in the tiger salamander retina

In the present study, using double- or triple-label immunocytochemistry in conjunction with confocal microscopy, we aimed to examine the population and distribution of photoreceptors, GABAergic and glycinergic amacrine cells, and ganglion cells, which are basic but important parameters for studying the structure–function relationship of the salamander retina. We found that the outer nuclear layer (ONL) contained 82,019 ± 3203 photoreceptors, of which 52% were rods and 48% were cones. The density of photoreceptors peaked at ∼8000 cells/mm2 in the ventral and dropped to ∼4000 cells/mm2 in the dorsal retina. In addition, the rod/cone ratio was less than 1 in the central retina but larger than 1 in the periphery. Moreover, in the proximal region of the inner nuclear layer (INL3), the total number of cells was 50,576 ± 8400. GABAergic and glycinergic amacrine cells made up approximately 78% of all cells in this layer, including 43% GABAergic, 32% glycinergic, and 3% GABA/glycine colocalized amacrine cells. The density of these amacrine cells was ∼6500 cells/mm2 in the ventral and ∼3200 cells/mm2 in the dorsal area. The ratio of GABAergic to glycinergic amacrine cells was larger than 1. Furthermore, in the ganglion cell layer (GCL), among a total of 36,007 ± 2010 cells, ganglion cells accounted for 65.7 ± 1.5% of the total cells, whereas displaced GABAergic and glycinergic amacrine cells comprised about 4% of the cells in this layer. The ganglion cell density was ∼1800 cells/mm2 in the ventral and ∼600 cells/mm2 in the dorsal retina. Our data demonstrate that all three major cell types are not uniformly distributed across the salamander retina. Instead, they exhibit a higher density in the ventral than in the dorsal retina and their spatial arrangement is associated with the retinal topography. These findings provide a basic anatomical reference for the electrophysiological study of this species.

Stratum-by-stratum projection of light response attributes by retinal bipolar cells of Ambystoma

The visual system processes light images by projecting various representations of the visual world to segregated regions in the brain through parallel channels. Retinal bipolar cells constitute the first parallel channels that carry different light response attributes to different parts of the inner plexiform layer (IPL). Here we present a systematic study on detailed axonal morphology and light response characteristics of over 200 bipolar cells in dark-adapted salamander retinal slices by the whole-cell voltage clamp and Lucifer yellow fluorescence (with a confocal microscope) techniques. Four major groups of bipolar cells were identified according to the patterns of axon terminal ramification in the IPL: 36% were narrowly monostratified (whose axon terminals ramified in one of the 10 strata of the IPL), 27% were broadly monostratified, 19% were multistratified, and 18% bore pyramidally branching axons. By analysing the bipolar cells with narrowly monostratified axon terminals in each of the 10 strata of the IPL, we found that several key light response attributes are highly correlated with the strata in which the cells' axon terminals ramify. The 10 IPL strata appear to be the basic building blocks for attributes of light-evoked signal outputs in all bipolar cells, and several general stratum-by-stratum rules were identified by analysing the broadly monostratified, multistratified and pyramidally branching cells. These rules not only uncover mechanisms by which third-order retinal cells integrate and compute bipolar cell signals, but also shed considerable light on how bipolar cells in other vertebrates process visual information and how physiological signals may shape the morphology and projection of output synapses of visual neurones during development.

Classification of retinal ganglion cells: a statistical approach

Numerous studies have shown that retinal ganglion cells exhibit an array of responses to visual stimuli. This has led to the idea that these cells can be sorted into distinct physiological classes, such as linear versus nonlinear or on versus off. Although many classification schemes are widely accepted, few studies have provided statistical support to favor one scheme over another. Here we test whether some of the most widely used classification schemes can be statistically verified, using the mouse retina as the model system. We used a cluster analysis approach and focused on 4 standard response parameters: 1) response latency, 2) response duration, 3) relative amplitude of the on and off responses, and 4) degree of nonlinearity in the stimulus-to-response transformation. For each parameter, we plotted its distribution and tested quantitatively, using a bootstrap method, whether it divided into distinct clusters. Our analysis showed that mouse ganglion cells clustered into several groups based on response latency, duration, and relative amplitude of the on and off responses, but did not cluster into more than one group based on degree of nonlinearity-the latter formed a single, large, continuous group. Thus while some well-known schemes for classifying ganglion cells could be statistically verified, others could not. Knowledge of which schemes can be confirmed is important for building models of how retinal output is processed and how retinal circuits are built. Finally, this cluster analysis approach is general and can be used to test other classification proposals as well, both physiological and anatomical.

Neural remodeling in retinal degeneration

Mammalian retinal degenerations initiated by gene defects in rods, cones or the retinal pigmented epithelium (RPE) often trigger loss of the sensory retina, effectively leaving the neural retina deafferented. The neural retina responds to this challenge by remodeling, first by subtle changes in neuronal structure and later by large-scale reorganization. Retinal degenerations in the mammalian retina generally progress through three phases. Phase 1 initiates with expression of a primary insult, followed by phase 2 photoreceptor death that ablates the sensory retina via initial photoreceptor stress, phenotype deconstruction, irreversible stress and cell death, including bystander effects or loss of trophic support. The loss of cones heralds phase 3: a protracted period of global remodeling of the remnant neural retina. Remodeling resembles the responses of many CNS assemblies to deafferentation or trauma, and includes neuronal cell death, neuronal and glial migration, elaboration of new neurites and synapses, rewiring of retinal circuits, glial hypertrophy and the evolution of a fibrotic glial seal that isolates the remnant neural retina from the surviving RPE and choroid. In early phase 2, stressed photoreceptors sprout anomalous neurites that often reach the inner plexiform and ganglion cell layers. As death of rods and cones progresses, bipolar and horizontal cells are deafferented and retract most of their dendrites. Horizontal cells develop anomalous axonal processes and dendritic stalks that enter the inner plexiform layer. Dendrite truncation in rod bipolar cells is accompanied by revision of their macromolecular phenotype, including the loss of functioning mGluR6 transduction. After ablation of the sensory retina, Müller cells increase intermediate filament synthesis, forming a dense fibrotic layer in the remnant subretinal space. This layer invests the remnant retina and seals it from access via the choroidal route. Evidence of bipolar cell death begins in phase 1 or 2 in some animal models, but depletion of all neuronal classes is evident in phase 3. As remodeling progresses over months and years, more neurons are lost and patches of the ganglion cell layer can become depleted. Some survivor neurons of all classes elaborate new neurites, many of which form fascicles that travel hundreds of microns through the retina, often beneath the distal glial seal. These and other processes form new synaptic microneuromas in the remnant inner nuclear layer as well as cryptic connections throughout the retina. Remodeling activity peaks at mid-phase 3, where neuronal somas actively migrate on glial surfaces. Some amacrine and bipolar cells move into the former ganglion cell layer while other amacrine cells are everted through the inner nuclear layer to the glial seal. Remodeled retinas engage in anomalous self-signaling via rewired circuits that might not support vision even if they could be driven anew by cellular or bionic agents. We propose that survivor neurons actively seek excitation as sources of homeostatic Ca(2+) fluxes. In late phase 3, neuron loss continues and the retina becomes increasingly glial in composition. Retinal remodeling is not plasticity, but represents the invocation of mechanisms resembling developmental and CNS plasticities. Together, neuronal remodeling and the formation of the glial seal may abrogate many cellular and bionic rescue strategies. However, survivor neurons appear to be stable, healthy, active cells and given the evidence of their reactivity to deafferentation, it may be possible to influence their emergent rewiring and migration habits.

Goalpha labels ON bipolar cells in the tiger salamander retina

By using double-label immunocytochemistry and confocal microscopy, we studied rod and cone synaptic contacts, photoreceptor-bipolar cell convergence, and patterns of axon terminal ramification of ON bipolar cells in the tiger salamander retina. An antibody to recoverin, a calcium-binding protein found in photoreceptors and other retinal neurons in various vertebrates, differentially labeled rods and cones by lightly staining rod cell bodies, axons, and synaptic pedicles and heavily staining cone cell bodies and pedicles. An antibody to G(oalpha) labeled most ON bipolar cells, with axon terminals ramified mainly in strata 6-9 and a minor band in stratum 3 of the inner plexiform layer (IPL). Stratum 10 of the IPL was G(oalpha) negative, and previous studies showed that axon terminals of rod-dominated ON bipolar cells are monostratified in that stratum. The axonal morphology of G(oalpha)-positive cells resembled that of the cone-dominated (DBC(C)) or mixed rod and cone ON (DBC(M)) bipolar cells. The G(oalpha)-positive dendritic processes made close contact with all cone pedicles and superficial contact with some rod pedicles, consistent with the idea that G(oalpha) subunits are present in DBC(C)s and DBC(M)s. The size and density of these cells were analyzed, and their spatial distributions were determined. To our knowledge, this is the first study to characterize photoreceptor inputs and axon terminal morphology of a population of ON bipolar cell with the use of a G(oalpha) antibody as an immunomarker in the salamander retina.

Differential modulation of light-evoked on- and off-EPSCs by paired-pulse stimulation in salamander retinal ganglion cells

Short-term plasticity of On- and Off-EPSPs, and its potential role in regulation of signal processing was studied in salamander retinal On-Off ganglion cells by whole-cell recording. Paired-pulse light stimulation resulted in a depression of On-, and an enhancement of Off-EPSCs. Recovery from depression and enhancement was exponential and complete by 20 s. Paired-pulse enhancement, but not depression, was abolished with increasing stimulus duration. Blockade of On-EPSC by L-2-amino-4-phosphonobutyrate (AP-4), an agonist at group III mGluRs, significantly increased Off-EPSCs evoked by short (<2 s) duration conditioning light stimuli, resulting in a reversal of the paired-pulse enhancement to depression. The acetylcholinesterase inhibitor eserine reduced Off-EPSC1 and increased the ratio of enhancement. An opposite effect was observed in the presence of the nACh receptor antagonist d-tubocurarine. AP-7, an antagonist of NMDA receptors attenuated the enhancement of Off-EPSCs. In current clamp mode paired-pulse stimulation resulted in a modulation of light evoked, as well as the depolarization-induced spike firing pattern of ganglion cells. The present study suggests that paired light stimulation differently modulates On and Off EPSPs, and the light-evoked spike firing pattern of On-Off ganglion cells.

Morphological and functional organization of ON and OFF pathways in the adult newt retina

Morphological and functional organization of ON and OFF pathways in the adult newt retina were examined by intracellular recording and staining techniques and immunohistochemistry. Synaptotagmin immunoreactivity discriminated three broad bands within the IPL: the distal band (sublamina I), the middle band (sublamina II) consisting of two dense punctate bands (sublaminae II(a) and II(b)), and proximal band (sublamina III). The Lucifer-yellow labeled OFF amacrine and ganglion cells send their processes mainly in sublamina I and/or II(a) where OFF bipolar cells extend their axon terminals, while ON amacrine and ganglion cells send their processes in sublamina III and/or II(b) where ON bipolar cells extend their axon terminals. Processes of ON-OFF amacrine and ganglion cells ramify broadly in the whole thickness of the IPL. Many bipolar cells responded to light spot with a transient hyperpolarization at both light onset and offset. They are probably subtypes of ON bipolar cells, because their axon terminals branch mainly in sublaminae III and/or II(b), although a few cells ramified the axon at both sublaminae II(a) and III. Two immunohistochemical markers for bipolar cells, PKC and RB-1, identified axon terminals in sublaminae III and/or II(b). From the ramification pattern of axon terminal, they are probably subtypes of ON bipolar cells. ChAT-ir amacrine cells ramified their dendrites in either sublamina I or II(b). Altogether, present studies support the general idea of segregation of ON and OFF pathways in sublaminae a and b of the IPL.