The rod circuit in the rabbit retina

Classification and analysis of retinal interneurons by computational structure under natural scenes

Inhibitory neurons are diverse across the brain, but for the visual system we lack the ability to functionally classify these neurons under complex natural stimuli. Here we take the approach of classifying retinal amacrine cell responses to natural scenes using optical recording and an interpretable neural network model. We fit mouse amacrine cell responses to a two-layer convolutional neural network model of a class shown previously to accurately capture salamander ganglion cell responses to natural scenes. Using an approach from interpretable machine learning, we determined for each stimulus the model interneurons that generated each amacrine response, analogous to the set of bipolar cells that target the amacrine population. From this analysis we clustered amacrine cells not by their natural scene responses, but by the model presynaptic neurons that constructed those responses, conservatively finding approximately seven groups by this approach. By analyzing the set of model presynaptic input neurons for each amacrine cluster, we find that distributed rather than dedicated inputs generate natural scene responses for different amacrine cell types. Additional analyses revealed distinct transient and sustained modes exhibited by the network during the response to simple flashes. These results give insight into the computational structure of how the diverse amacrine cell population responds to natural scenes, and generate multiple quantitative hypotheses for how synaptic inputs generate those responses.

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.

Spectral Domain Optical Coherence Tomography in Awake Rabbits Allows Identification of the Visual Streak, a Comparison with Histology

Purpose

To evaluate visual streak (VS) identification on spectral-domain optical coherence tomography (SD-OCT) scans in awake rabbits. To report thickness measurements in the VS and adjacent retina on OCT B-scans and histologic sections and to assess inter-method bias, precision and repeatability between OCT and histology.

Methods

Vertical SD-OCT B-scan images through the optic nerve head and VS were acquired from 16 awake, ophthalmologically healthy experimental rabbits. Scans were acquired from both eyes, which were later enucleated and processed for light microscopy. Inner retina, inner nuclear layer, outer nuclear layer, outer retina (OR) and photoreceptor outer segment (PROS) thickness were measured on OCT images and digitalized microscopy slides in- and outside of the VS, and compared using linear mixed effects models.

Results

Both SD-OCT and histology allowed retinal layer identification and measurement. On OCT, OR and PROS were thickest in the central VS and thinnest outside the VS. Histology mirrored OCT results for central outer retinal layers but shows discrepancies for other layers likely because of postmortem processing artifacts. The method comparison demonstrated better repeatability for OCT measurements compared with histology.

Conclusions

Increased OR and PROS thickness compared with the adjacent retina allowed identification of the VS on SD-OCT in awake rabbits. OCT allows measurements devoid of processing artifacts in contrast to histology.

Translational Relevance

SD-OCT is possible in awake rabbits. Easy and reliable identification of the VS may facilitate the positioning and use of rabbits as model species in human macular and generalized retinal disease research.

Cell types and cell circuits in human and non-human primate retina

This review summarizes our current knowledge of primate including human retina focusing on bipolar, amacrine and ganglion cells and their connectivity. We have two main motivations in writing. Firstly, recent progress in non-invasive imaging methods to study retinal diseases mean that better understanding of the primate retina is becoming an important goal both for basic and for clinical sciences. Secondly, genetically modified mice are increasingly used as animal models for human retinal diseases. Thus, it is important to understand to which extent the retinas of primates and rodents are comparable. We first compare cell populations in primate and rodent retinas, with emphasis on how the fovea (despite its small size) dominates the neural landscape of primate retina. We next summarise what is known, and what is not known, about the postreceptoral neurone populations in primate retina. The inventories of bipolar and ganglion cells in primates are now nearing completion, comprising ~12 types of bipolar cell and at least 17 types of ganglion cell. Primate ganglion cells show clear differences in dendritic field size across the retina, and their morphology differs clearly from that of mouse retinal ganglion cells. Compared to bipolar and ganglion cells, amacrine cells show even higher morphological diversity: they could comprise over 40 types. Many amacrine types appear conserved between primates and mice, but functions of only a few types are understood in any primate or non-primate retina. Amacrine cells appear as the final frontier for retinal research in monkeys and mice alike.

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.

Coincidence detection of single-photon responses in the inner retina at the sensitivity limit of vision

BACKGROUND

Vision in starlight relies on our ability to detect single absorbed photons. Indeed, the sensitivity of dark-adapted vision approaches limits set by the quantal nature of light. This sensitivity requires neural mechanisms that selectively transmit quantal responses and suppress noise. Such mechanisms face an inevitable tradeoff because signal and noise cannot be perfectly separated, and rejecting noise also means rejecting signal.

RESULTS

We report measurements of single-photon responses in the output signals of the primate retina. We find that visual signals arising from a few absorbed photons are read out fundamentally differently by primate On and Off parasol ganglion cells, key retinal output neurons. Off parasol cells respond linearly to near-threshold flashes, retaining sensitivity to each absorbed photon but maintaining a high level of noise. On parasol cells respond nonlinearly due to thresholding of their excitatory synaptic inputs. This nonlinearity reduces neural noise but also limits information about single-photon absorptions.

CONCLUSIONS

The long-standing idea that information about each photon absorption is available for behavior at the sensitivity limit of vision is not universally true across retinal outputs. More generally, our work shows how a neural circuit balances the competing needs for sensitivity and noise rejection.

Real-time imaging of rabbit retina with retinal degeneration by using spectral-domain optical coherence tomography

BACKGROUND

Recently, a transgenic rabbit with rhodopsin Pro 347 Leu mutation was generated as a model of retinitis pigmentosa (RP), which is characterized by a gradual loss of vision due to photoreceptor degeneration. The purpose of the current study is to noninvasively visualize and assess time-dependent changes in the retinal structures of a rabbit model of retinal degeneration by using speckle noise-reduced spectral-domain optical coherence tomography (SD-OCT).

METHODOLOGY/PRINCIPAL FINDINGS

Wild type (WT) and RP rabbits (aged 4-20 weeks) were investigated using SD-OCT. The total retinal thickness in RP rabbits decreased with age. The thickness of the outer nuclear layer (ONL) and between the external limiting membrane and Bruch's membrane (ELM-BM) were reduced in RP rabbits around the visual streak, compared to WT rabbits even at 4 weeks of age, and the differences increased with age. However, inner nuclear layer (INL) thickness in RP rabbits did not differ from that of WT during the observation period. The ganglion cell complex (GCC) thickness in RP rabbits increased near the optic nerve head but not around the visual streak in the later stages of the observation period. Hyper-reflective change was widely observed in the inner segments (IS) and outer segments (OS) of the photoreceptors in the OCT images of RP rabbits. Ultrastructural findings in RP retinas included the appearance of small rhodopsin-containing vesicles scattered in the extracellular space around the photoreceptors.

CONCLUSIONS/SIGNIFICANCE

In the current study, SD-OCT provided the pattern of photoreceptor degeneration in RP rabbits and the longitudinal changes in each retinal layer through the evaluation of identical areas over time. The time-dependent changes in the retinal structure of RP rabbits showed regional and time-stage variations. In vivo imaging of RP rabbit retinas by using SD-OCT is a powerful method for characterizing disease dynamics and for assessing the therapeutic effects of experimental interventions.

Retinal parallel processors: more than 100 independent microcircuits operate within a single interneuron

Most neurons are highly polarized cells with branched dendrites that receive and integrate synaptic inputs and extensive axons that deliver action potential output to distant targets. By contrast, amacrine cells, a diverse class of inhibitory interneurons in the inner retina, collect input and distribute output within the same neuritic network. The extent to which most amacrine cells integrate synaptic information and distribute their output is poorly understood. Here, we show that single A17 amacrine cells provide reciprocal feedback inhibition to presynaptic bipolar cells via hundreds of independent microcircuits operating in parallel. The A17 uses specialized morphological features, biophysical properties, and synaptic mechanisms to isolate feedback microcircuits and maximize its capacity to handle many independent processes. This example of a neuron employing distributed parallel processing rather than spatial integration provides insights into how unconventional neuronal morphology and physiology can maximize network function while minimizing wiring cost.

Evolution of vertebrate retinal photoreception

Recent findings shed light on the steps underlying the evolution of vertebrate photoreceptors and retina. Vertebrate ciliary photoreceptors are not as wholly distinct from invertebrate rhabdomeric photoreceptors as is sometimes thought. Recent information on the phylogenies of ciliary and rhabdomeric opsins has helped in constructing the likely routes followed during evolution. Clues to the factors that led the early vertebrate retina to become invaginated can be obtained by combining recent knowledge about the origin of the pathway for dark re-isomerization of retinoids with knowledge of the inability of ciliary opsins to undergo photoreversal, along with consideration of the constraints imposed under the very low light levels in the deep ocean. Investigation of the origin of cell classes in the vertebrate retina provides support for the notion that cones, rods and bipolar cells all originated from a primordial ciliary photoreceptor, whereas ganglion cells, amacrine cells and horizontal cells all originated from rhabdomeric photoreceptors. Knowledge of the molecular differences between cones and rods, together with knowledge of the scotopic signalling pathway, provides an understanding of the evolution of rods and of the rods' retinal circuitry. Accordingly, it has been possible to propose a plausible scenario for the sequence of evolutionary steps that led to the emergence of vertebrate photoreceptors and retina.

Biocytin wide-field bipolar cells in rabbit retina selectively contact blue cones

The biocytin wide-field bipolar cell in rabbit retina has a broad axonal arbor in layer 5 of the inner plexiform layer and a wide dendritic arbor that does not contact all cones in its dendritic field. The purpose of our study was to identify the types of cones that this cell contacts. We identified the bipolar cells by selective uptake of biocytin, labeled the cones with peanut agglutinin, and then used antibodies against blue cone opsin and red-green cone opsin to identify the individual cone types. The biocytin-labeled cells selectively contacted cones whose outer segments stained for blue cone opsin and avoided cones that did not. We conclude that the biocytin wide-field bipolar cell is an ON blue cone bipolar cell in the rabbit retina and is homologous to the blue cone bipolar cells that have been previously described in primate, mouse, and ground squirrel retinas.

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.

Rod and cone input to horizontal cells in the rabbit retina

In the rabbit retina, there are two types of horizontal cell (HC). The A-type HC is axonless and extensively coupled. The B-type HC is axon bearing; the somatic dendrites are radially symmetric and form a second coupled network, while the axon branches expansively to form a complex terminal structure. The B-type axon terminals (ATs) are independently coupled to form a third network in the outer plexiform layer. We have modified our dye-injection methods to obtain detailed fills of the three different horizontal cell networks for analysis via confocal microscopy. We have confirmed that A-type HCs and the somatic dendrites of B-type HCs receive input exclusively from cones, whereas the B-type ATs receive input only from rods. A single B-type AT may receive input from as many as 1,000 rods, but, surprisingly, our data reveal only one end terminal per rod spherule. The somatic dendrites of A- and B-type HCs form clusters at each cone pedicle coincident with GluR2/3 and GluR4 glutamate receptor subunits. The B-type ATs have GluR2/3- or GluR4-labeled glutamate receptors in two locations: small puncta on the end terminals within the rod spherule invagination and large clusters on the terminal stalks, approximately 1.5 microm from the rod synaptic ribbon. We conclude that AMPA receptors of the same or similar composition mediate photoreceptor input to all types of HCs.

Temporal response properties of the primary and secondary rod-signaling pathways in normal and Gnat2 mutant mice

Multiple signaling pathways have been proposed for rod vision in the mammalian retina. The primary and secondary rod pathways have been characterized in humans with the scotopic 15-Hz flicker electroretinogram (ERG). The purpose of this study was to determine whether the response properties of these pathways in the mouse are similar to those of humans. C57BL/6J and Gnat2(cpfl3) mutant mice lacking functional cones were used in these experiments. Standard ERG recording techniques were employed. Response functions were obtained for a range of flash intensities (-4.7logcd-s/m(2) to -0.2logcd-s/m(2)) and temporal modulation frequencies (1-30Hz). The mouse intensity-response functions to 15-Hz flickering stimuli possessed the same features as that of humans - a local amplitude minimum and a rapid phase change in the intensity region where the primary and secondary pathways are mutually inhibitory. However, the secondary pathway in the mouse did not achieve the same level of sensitivity as previously shown for humans, suggesting inter-species differences in post-receptoral signal processing. In Gnat2(cpfl3) mutant mice, the secondary pathway was completely abolished. Measurements of temporal acuity indicated that the primary and secondary rod pathways could mediate temporal frequencies as high as 30 and 50Hz, respectively. The response functions for mice are similar to those of humans, although the evidence suggests that the primary rod pathway dominates all rod-mediated signal processing in the mouse. Nevertheless, these results demonstrate the feasibility of measuring non-invasively the performance characteristics of the primary and secondary rod retinal pathways in the mouse and provide a mechanism for testing hypotheses about the action of disease where post-receptoral cells are differentially affected.

Postsynaptic calcium feedback between rods and rod bipolar cells in the mouse retina

Light-evoked currents were recorded from rod bipolar cells in a dark-adapted mouse retinal slice preparation. Low-intensity light steps evoked a sustained inward current. Saturating light steps evoked an inward current with an initial peak that inactivated, with a time constant of about 60-70 ms, to a steady plateau level that was maintained for the duration of the step. The inactivation was strongest at hyperpolarized potentials, and absent at positive potentials. Inactivation was mediated by an increase in the intracellular calcium concentration, as it was abolished in cells dialyzed with 10 mM BAPTA, but was present in cells dialyzed with 1 mM EGTA. Moreover, responses to brief flashes of light were broader in the presence of intracellular BAPTA indicating that the calcium feedback actively shapes the time course of the light responses. Recovery from inactivation observed for paired-pulse stimuli occurred with a time constant of about 375 ms. Calcium feedback could act to increase the dynamic range of the bipolar cells, and to reduce variability in the amplitude and duration of the single-photon signal. This may be important for nonlinear processing at downstream sites of convergence from rod bipolar cells to AII amacrine cells. A model in which intracellular calcium rapidly binds to the light-gated channel and reduces the conductance can account for the results.

Comparative Study on Calretinin Immunoreactivity in Gerbil and Rat Retina

Expression of calretinin in retina has been ascribed to multiple biological and functional aspects in the visual system. In this study, we examined the distribution patterns of calretinin immunoreactivity in gerbil and rat retina. In the gerbil, calretinin immunoreactivity was present in bipolar and amacrine cells of the inner nuclear layer and in neurones of the ganglion cell layer. In the rat, amacrine and ganglion cells showed calretinin immunoreactivity, but bipolar cells did not contain calretinin immunoreactivity. In both species, calretinin immunoreactivity was absent in cones, cone bipolars, and horizontal cells. In conclusion, gerbil as well as rat has a rod-dominant retina. The differences in calretinin expression between rat and gerbil require further investigations under various functional and developmental conditions.

Transmission of scotopic signals from the rod to rod-bipolar cell in the mammalian retina

Mammals can see at low scotopic light levels where only 1 rod in several thousand transduces a photon. The single photon signal is transmitted to the brain by the ganglion cell, which collects signals from more than 1000 rods to provide enough amplification. If the system were linear, such convergence would increase the neural noise enough to overwhelm the tiny rod signal. Recent studies provide evidence for a threshold nonlinearity in the rod to rod bipolar synapse, which removes much of the background neural noise. We argue that the height of the threshold should be 0.85 times the amplitude of the single photon signal, consistent with the saturation observed for the single photon signal. At this level, the rate of false positive events due to neural noise would be masked by the higher rate of dark thermal events. The evidence presented suggests that this synapse is optimized to transmit the single photon signal at low scotopic light levels.

Functional expression of connexin57 in horizontal cells of the mouse retina

Horizontal cells are interneurons of the vertebrate retina that exhibit strong electrical and tracer coupling but the identity of the channel-forming connexins has remained elusive. Here we show that horizontal cells of the mouse retina express connexin57 (Cx57). We have generated Cx57-deficient mice by replacing the Cx57 coding region with a lacZ reporter gene, expressed under control of the endogenous Cx57 promoter. These mice were fertile and showed no obvious anatomical or behavioural abnormalities. Cx57 mRNA was expressed in the retina of wild-type littermates but was absent from the retina of Cx57-deficient mice. Previously reported results that the Cx57 gene was very weakly expressed in several other mouse tissues turned out to be unspecific. Cx57 mRNA is abundantly expressed in the retina and weakly in the thymus of adult mice but absent in all other adult tissues tested, including brain. Furthermore, Cx57 is expressed in embryonic kidney at E16.5 to E18.5 days post-conception, as indicated by the pattern of lacZ expression. Within the retina, lacZ signals were assigned exclusively to horizontal cells based on co-localization with cell-type-specific marker proteins. Microinjection of Neurobiotin into horizontal cells of isolated retinae revealed less than 1% of tracer coupling in Cx57-deficient retinae compared with wild-type controls. Cx57 is the first connexin identified in mammalian horizontal cells and the first connexin whose expression is apparently restricted to only one type of neuron.

Physiological response properties of displaced amacrine cells of the adult ferret retina

The ganglion cell layer (GCL) of the mammalian retina contains a large number of neurons called displaced amacrine cells (DACs) that do not project to the optic nerve. However, with the exception of the rabbit starburst amacrine cell little is known regarding the function of this large population due to the difficulty experienced in making physiological recordings from these neurons. We have overcome these difficulties and have used whole-cell patch-clamp techniques to examine the intrinsic membrane properties of DACs in the ferret retina. Our results indicate a large degree of diversity in their intrinsic membrane properties. In response to maintained depolarizing current injection, DACs responded with graded depolarization or by eliciting either transient or sustained bursts of spiking activity. At the resting membrane potential, 10% of the DACs generated spontaneous spikes in either an apparently random manner or at the peak of intrinsic waves of depolarization. The resting membrane activity of the remaining DACs recorded could be classified into three groups that were quiescent (28%), had robust uncorrelated synaptic activity (30%), or underwent slow waves of depolarization (42%). Diversity was also revealed in the membrane currents recorded in voltage-clamp where some DACs were quiescent (19%), or exhibited robust nonrhythmic synaptic events (42%). The remaining DACs exhibited waves of oscillatory activity (39%), characterized by either rhythmic bursts of synaptic events (17%) or slow inward currents (22%). Bath application of 50 μM biccuculine or 150 μM picrotoxin had no effect on the waves of activity, however, the gap junction blocker, carbenoxolone (100 μm), blocked both oscillatory patterns. By including Lucifer yellow and biocytin in the recording pipette, it was possible to determine the morphology of recorded neurons and group them based on dendritic extent as small-, medium-, or large-field DACs. There were few relationships between these morphologically defined groups and their intrinsic membrane properties. The present study provides the first in-depth examination of the intrinsic membrane properties of DACs in the ferret retina and provides new insights into the potential roles these neurons play in the processing of visual information in the mammalian retina.

Effects of dopamine on human retinal vessel diameter and its modulation during flicker stimulation

We performed a randomized, subject-blinded, placebo and time-controlled, two-way crossover study in 12 healthy male subjects. Placebo or dopamine was administered on two separate study days. After saline infusion, dopamine hydrochloride was infused in three consecutive doses (5, 10, and 15 microg x kg(-1) x min(-1)). Plasma levels of dopamine were determined at each perfusion step. Arterial and venous retinal vessel diameters were measured with the use of a Zeiss retinal vessel analyzer. Diffuse luminance flicker stimuli of 8 Hz were applied for 60 s. Blood pressure and pulse rate were monitored continuously. Flicker stimulation (8 Hz) increased retinal vessel diameters under basal conditions. The response to 8-Hz flicker light was significantly reduced by dopamine administration. In addition, dopamine slightly but significantly increased retinal vessel diameters. Dopamine hydrochloride significantly increased systolic but not diastolic or mean arterial pressure. The present study indicates that dopamine has a distinct effect on retinal vessel diameters also attenuating the flicker-induced response reactivity of retinal vessels. This implies a role of dopamine in retinal blood flow hemodynamics.

Feedback inhibition in the inner plexiform layer underlies the surround-mediated responses of AII amacrine cells in the mammalian retina

Intracellular recordings were made from narrow-field, bistratified AII amacrine cells in the isolated, superfused retina-eyecup of the rabbit. Pharmacological agents were applied to neurons to dissect the synaptic pathways subserving AII cells so as to determine the circuitry generating their off-surround responses. Application of the GABA antagonists, picrotoxin, bicuculline and 1,2,5,6-tetrahydropyridine-4-yl methylphosphinic acid (TPMPA) all increased the on-centre responses of AII amacrine cells, but attenuated the off-surround activity. At equal concentrations, picrotoxin was approximately twice as effective as bicuculline or TPMPA in modifying the response activity of AII amacrine cells. These results indicate that the mechanism underlying surround inhibition of AII amacrine cells includes activation of both GABA(A) and GABA(C) receptors in an approximately equal ratio. Application of the GABA antagonists also increased the size of on-centre receptive fields of AII amacrine cells. Again, picrotoxin was most effective, producing, on average, a 54 % increase in the size of the receptive field, whereas bicuculline and TPMPA produced comparable 34 and 33 % increases, respectfully. Application of the voltage-gated sodium channel blocker TTX produced effects on AII amacrine cells qualitatively similar to those of the GABA blockers. Intracellular application of the chloride channel blocker 4,4'-dinitro-stilbene-2,2'-disulphonic acid (DNDS) abolished the direct effects of GABA on AII amacrine cells. Moreover, DNDS increased the amplitude of both the on-centre and off-surround responses. The failure of DNDS to block the off-surround activity indicates that it is not mediated by direct GABAergic inhibition. Taken together, our results suggest that surround receptive fields of AII amacrine cells are generated indirectly by the GABAergic, reciprocal feedback synapses from S1/S2 amacrine cells to the axon terminals of rod bipolar cells.

Glutamate receptors in the rod pathway of the mammalian retina

Rod bipolar (RB) cells of the mammalian retina release glutamate in a graded, light-dependent fashion from 20 to 40 ribbon synapses (dyads). At the dyads, two classes of amacrine cells, the AI and AII cells, are the postsynaptic partners. We examined the glutamate receptors (GluRs) that are expressed by AI and AII cells using immunocytochemistry with specific antibodies against GluR subunits. Sections of macaque monkey and rabbit retina were examined by confocal microscopy. AII amacrine cells were selectively labeled for calretinin, and AI cells in rabbits were labeled for 5-HT uptake. Thus, double- and triple-labeling for these markers and GluR subunits was possible. Electron microscopy using postembedding immunocytochemistry and double-labeling was applied to show the synaptic expression of GluRs. We also studied the synaptic localization of the two postsynaptic density proteins PSD-95 and glutamate receptor-interacting protein (GRIP). We found that AII amacrine cells express the AMPA receptor subunits GluR2/3 and GluR4 at the RB cell dyads, and they are clustered together with PSD-95. In contrast, AI amacrine cells express the delta1/2 subunits that appear to be associated with kainate receptor subunits and to be clustered together with GRIP. The RB cell dyad is therefore a synapse that initiates two functionally and molecularly distinct pathways: a "through conducting" pathway based on AMPA receptors and a modulatory pathway mediated by a combination of delta1/2 subunits and kainate receptors.

Microcircuits for night vision in mouse retina

Because the mouse retina has become an important model system, we have begun to identify its specific neuron types and their synaptic connections. Here, based on electron micrographs of serial sections, we report that the wild-type mouse retina expresses the standard rod pathways known in other mammals: (1) rod --> cone (via gap junctions) to inject rod signals into the cone bipolar circuit; and (2) rod --> rod bipolar --> AII amacrine --> cone bipolar --> ganglion cell. The mouse also expresses another rod circuit: a bipolar cell with cone input also receives rod input at symmetrical contacts that express ionotropic glutamate receptors (Hack et al., 1999, 2001). We show that this rod-cone bipolar cell sends an axon to the outer (OFF) strata of the inner plexiform layer to form ribbon synapses with ganglion and amacrine cells. This rod-cone bipolar cell receives direct contacts from only 20% of all rod terminals. However, we also found that rod terminals form gap junctions with each other and thus establish partial syncytia that could pool rod signals for direct chemical transmission to the OFF bipolar cell. This third rod pathway probably explains the rod responses that persist in OFF ganglion cells after the well known rod pathways are blocked (Soucy et al., 1998).

Visual transmission deficits in mice with targeted disruption of the gap junction gene connexin36

In the mammalian retina, rods feed into the cone pathway through electrotonic coupling, and recent histological data suggest the involvement of connexin36 (Cx36) in this pathway. We therefore generated Cx36 null mice and monitored the functional consequences of this deficiency on early visual transmission. The homozygous mutant mice had a normally developed retina and showed no changes in the cellular organization of the rod pathway. In contrast, the functional coupling between AII amacrine cells and bipolar cells was impaired. Recordings of electroretinograms revealed a significant decrease of the scotopic b-wave in mutant animals and an increased cone threshold that is compatible with a distorted, gap junctional transmission between AII amacrine cells and cone bipolar cells. Recordings of visual evoked potentials showed extended latency in mutant mice but unaffected ON and OFF components. Our results demonstrate that Cx36-containing gap junctions are essential for normal synaptic transmission within the rod pathway.

Amacrine cells of the anuran retina: morphology, chemical neuroanatomy, and physiology

Amacrine cells are third-order retinal interneurons, projecting their processes into the inner plexiform layer. Historically, they were not considered as neurons first. By the middle of the 20th century, their neuronal nature was confirmed, and their enormous diversity established. Amacrine cells have been most successfully subdivided into morphological categories based on two parameters: diameter of the dendritic field and ramification pattern in the inner plexiform layer. Works combining anatomy, physiology, and neurochemistry are scarce and in the case of the anuran retina, the situation is even worse. Correlation between morphology, neurochemistry, and physiology is little studied. Here we try to build up a database and pinpoint some of the missing data. Obtaining those could help to better understand retinal function. Sporadic attempts did not make it possible to develop a comprehensive catalog of morphologically distinct amacrine cell types in the anuran retina. The number of morphologically identified amacrine cells currently stands at 16. The list of neurochemically identified distinct cell types can be given as follows: five types GABA-containing cell types with secondary markers and at least one without; two glycinergic cell types and one interplexiform cell where glycine colocalizes with somatostatin; one dopaminergic amacrine cell and also a variant of this with interplexiform morphology; two types of serotoninergic cells; three NADPHdiaphorase-positive cells, one substance P-positive cell type without identified second marker; one CCK-positive cell type without identified second marker and the calbindin positive cells (at least one but potentially more types). This adds up to 19 cell types, out of which two are interplexiform in character. This is more than that could be identified by purely morphological means. Out of Cajal's original 13 amacrine cell types described in the frog retina, 5 parallel unequivocally with neurons defined by neurochemistry. Three others have one close match each, but their exact identity is uncertain. The remaining amacrine cells have more than one potential matches. At the same time, on one hand the amacrine cell named two-layered by Cajal so far has no match among the neurochemically identified amacrine cells. On the other hand, the interplexiform subtype of the dopaminergic cell, the somatostatin-containing glycinergic interplexiform cell, the starburst cell, and the bistratified neuropeptide Y-immunoreactive cell have no match among Cajal's cells. All in all, the number of known amacrine and interplexiform cells now stands at at least 21 in the anuran retina. Physiological characterization of amacrine cells shows that their general features seem to be rather similar to those described in tiger salamander retina. In Xenopus retina, morphologically and physiologically identified amacrine cells responded to light stimulation most frequently with ON-OFF characteristics. Immunhistochemical identification of the recorded and dye injected cells showed that amacrine cells of the "same physiological type" might have different morphology. In other words, amacrine cells with different morphology can respond similarly to illumination. Even so, small differences between almost identical responses may reflect that the cell they stem from indeed belongs to different cell types.

Somatostatin and somatostatin subtype 2A expression in the mammalian retina

This review discusses the expression and cellular localization of the neuropeptide somatostatin (SRIF) and one of the SRIF subtype (sst) receptors, sst(2A) in the mammalian retina. SRIF immunoreactivity is predominantly localized to a sparse population of amacrine and displaced amacrine cells in the ganglion cell layer in several mammalian retinas including the rat, rabbit, cat, and primate. These cells, characterized by multiple processes, form a sparse network in the inner plexiform layer (IPL) in all retinal regions. Very few processes are also in the outer plexiform layer. In contrast to the predominant distribution of SRIF processes to the IPL, there is a widespread distribution of sst(2A) immunoreactivity to both the inner and outer retina in all mammalian retinas studied to date. In rabbit retina, sst(2A) immunoreactivity is predominant in rod bipolar cells and in sparse wide-field amacrine cells. In the rat retina, sst(2A) immunoreactivity is localized to several neuronal cell types-cone photoreceptors, horizontal cells, rod and cone bipolar cells, and amacrine cells. Reverse-transcriptase-polymerase chain reaction analysis found that sst(2A) mRNA is expressed in the rat retina, while sst(2B) mRNA is not detected. Finally, in the primate retina sst(2) immunoreactivity is predominant in cone photoreceptors, with additional immunostained cell bodies and processes in the inner retina. These findings indicate that SRIF may modulate several neuronal cell types in the retina, and that it has a broad influence on both scotopic and photopic visual pathways.

Somatostatin receptor subtype 2A expression in the rat retina

Somatostatin is mainly expressed by sparsely occurring amacrine and interplexiform cells in the retina. In this study, we characterized the expression and cellular localization of one of the somatostatin subtype (sst) receptors, sst2A, in the rat retina. The presence of sst2A receptor messenger RNA in retinal extracts was demonstrated by reverse transcription-polymerase chain reaction using specific primers to detect the sst2 receptor and its isoforms, sst2A and sst2B. Specific sst2A receptor immunoreactivity was mainly localized to the plasma membrane of several neuronal cell types. In the outer retina, immunoreactivity was localized to cone photoreceptors, horizontal cells, and rod and cone bipolar cells. Double-label experiments showed the co-localization of sst2A receptor and protein kinase C (alpha and beta), a rod bipolar cell marker, and of sst2A receptor and Calbindin-D28k, a horizontal cell marker. In the inner retina, sst2A receptor immunoreactivity occurred in tyrosine hydroxylase-positive amacrine cells; most were of medium to large size. These findings indicate that somatostatin may act at a distance, in a paracrine manner, on several cell types that express the sst2A receptor, and therefore exert a broad modulatory influence on both scotopic and photopic visual pathways.

Postnatal development of parvalbumin immunoreactive amacrine cells in the rabbit retina

In the adult rabbit, rat and cat retina, parvalbumin (PV) immunoreactivity is primarily localized to a population of narrow-field, bistratified amacrine cells, the AII amacrine cells-major interneurons of the rod pathway. This investigation examines the postnatal development of PV immunoreactivity in order to better understand the ontogeny of the AII amacrine cell population and the formation of the rod pathway. Rabbit retinas at various postnatal ages were processed for immunohistochemistry using a monoclonal antibody directed to PV and analyzed morphometrically. On the day of birth, PV immunoreactive cell bodies are numerous in the proximal inner nuclear layer (INL) in all retinal regions. These cells have a primary process directed towards the inner plexiform layer (IPL). At postnatal day (PND) 2, a few faint immunoreactive processes are observed in the IPL. At PND 4, well-stained processes are observed to ramify mainly in the proximal IPL. At PND 6, strongly immunoreactive processes are present in both the distal and proximal IPL, and at PND 10 they form a continuous, dense plexus in both levels of the IPL. By PND 10, the morphology of PV immunoreactive cells is similar to PV immunoreactive cells in adult retinas. The density of PV immunoreactive cells in the proximal INL increases from PND 2 to PND 5, then it gradually decreases to adult values, while the total number of PV immunoreactive cell bodies increases until PND 10. PV immunoreactive amacrine cells at PND 2, as in the adult, are nonrandomly distributed across the retinal surface. These studies show that PV immunoreactive amacrine cells have a developmental profile that is similar to several other amacrine cell types. This includes the elaboration of processes in the IPL during the first postnatal week and a mature appearance towards the end of the second week of life, about the time of eye opening. These observations indicate that the AII amacrine cell may participate in the processing of visual information at eye opening.

Immunocytochemical localization of calretinin containing neurons in retina from rabbit, cat, and dog

Calcium homeostasis is critical for many neuronal functions, yet the distribution of calcium-binding protein is not always conserved among species, even between closely related species. We decided therefore to study the distribution of one of these calcium-binding proteins calretinin, in retina from rabbit, cat, and dog. Calretinin was localized using antibody immunocytochemistry. Calretinin immunoreactivity was found in numerous cell bodies in the ganglion cell layer in all three animals. These cells had small to medium-sized somas. Large ganglion cells, however, were not labeled using antiserum against calretinin. In the inner nuclear layer, calretinin immunoreactivity was found in many neurons in all three species. The regular distribution of neurons, the inner marginal location of their cell bodies in the inner nuclear layer, and the distinctive bilaminar morphologies of their dendritic arbors in the inner plexiform layer suggested that these calretinin-positive cells were AII amacrine cells. Calretinin immunoreactivity was observed in both A- and B-type horizontal cells in cat and dog retina. However, horizontal cells in the rabbit retina were not labeled by this antibody. Neurons in the photoreceptor cell layer were not labeled by this antibody. The present study suggests that calretinin immunoreactivity is present in several populations in the retina. In particular, calretinin labels AII amacrine cells and a subpopulation of ganglion cells in all three animals. Horizontal cells, however, were not labeled in rabbit.

Expression of the somatostatin subtype 2A receptor in the rabbit retina

In the retina, somatostatin influences neuronal activity likely by acting at one or more somatostatin subtype (sst) receptors. Somatostatin and somatostatin-binding sites are distributed predominantly to the inner retina. The present study has investigated the cellular expression of one of the sst receptors, the sst2A receptor isoform, in the rabbit retina. These studies have used a new polyclonal antibody directed to the predicted C-terminus of mouse sst2A(361-369) receptor. Antibody specificity was tested by preadsorption of the primary antibody with a peptide corresponding to sst2A(361-369). sst2A Receptor immunoreactivity was localized mainly to the plasma membrane of rod bipolar cells and to sparsely occurring, wide-field amacrine cells. Immunostaining in rod bipolar cells was strongest in the axon and axon terminals in lamina 5 of the inner plexiform layer (IPL) and was weakest in the cell body and dendrites. Double-labeling experiments using a monoclonal antibody against protein kinase C (PKC; alpha and beta), a rod bipolar cell-selective marker, showed complete colocalization. In horizontal sections of retina, immunostained bipolar cell bodies had a dense distribution, which is in agreement with the reported distribution of rod bipolar cell bodies. Immunoreactive amacrine cell bodies were located at the border of the inner nuclear layer and the IPL, and thin varicose processes ramified mainly in laminae 2 and 4 of the IPL. These observations indicate that somatostatin influences visual information processing in the retina 1) by acting presynaptically on rod bipolar cell axon terminals and b) by influencing the activity of sparsely occurring amacrine cells.

The DAPI-3 amacrine cells of the rabbit retina

In the rabbit retina, the nuclear dye, 4,6,diamidino-2-phenylindole (DAPI), selectively labels a third type of amacrine cell, in addition to the previously characterized type a and type b cholinergic amacrine cells. In this study, these "DAPI-3" amacrine cells have been characterized with respect to their somatic distribution, dendritic morphology, and neurotransmitter content by combining intracellular injection of biotinylated tracers with wholemount immunocytochemistry. There are about 100,000 DAPI-3 amacrine cells in total, accounting for 2% of all amacrine cells in the rabbit retina, and their cell density ranges from about 130 cells/mm2 in far-peripheral retina to 770 cells/mm2 in the visual streak. The thin varicose dendrites of the DAPI-3 amacrine cells form a convoluted dendritic tree that is symmetrically bistratified in S1/S2 and S4 of the inner plexiform layer. Tracer coupling shows that the DAPI-3 amacrine cells have a fivefold dendritic-field overlap in each sublamina, with the gaps in the arborization of each cell being occupied by dendrites from neighboring cells. The DAPI-3 amacrine cells consistently show the strongest glycine immunoreactivity in the rabbit retina and they also accumulate exogenous [3H]-glycine to a high level. By contrast, the AII amacrine cells, which are the best characterized glycinergic cells in the retina, are amongst the most weakly labelled of the glycine-immunopositive amacrine cells. The DAPI-3 amacrine cells costratify narrowly with the cholinergic amacrine cells and the On-Off direction-selective ganglion cells, suggesting that they may play an important role in movement detection.

Two classes of bipolar cell in the retina of the skate Raja erinacea

We have used immunoreactions against serotonin and protein kinase C to visualize two distinct classes of bipolar cell in the all-rod retina of the skate, Raja erinacea. To enhance the immunoreaction in serotonin-accumulating bipolar cells, prior to fixation, some retinas were incubated in Ringer's solution containing serotonin and pargyline. We found the somata of serotonin-accumulating bipolar cells to be located slightly distal to the midline of the inner nuclear layer. With increasing eccentricity from the visual streak, the size of the perikarya increases, concomitant with a decline in density of their distribution. Dendrites emanate from stout primary stalks and branch out before reaching the outer plexiform layer. Axons are bistratified within the inner plexiform layer with ramifications at the border of strata 1 and 2 and in stratum 4. The overall morphology of serotonin-accumulating bipolar cells is similar to that of serotonin-accumulating OFF bipolar cells of other non-mammalian vertebrates. Protein kinase C immunoreactive cells display the typical appearance of rod bipolar cells. Somata of protein kinase C immunoreactive bipolar cells are spindle-shaped and located distal to the serotonin-accumulating bipolar cells. Dendrites of these bipolars do not ramify before reaching the outer plexiform layer. Thin axons of protein kinase C immunoreactive bipolar cells end in large, club-shaped terminals in stratum 5 of the inner plexiform layer, bearing a striking similarity to axon terminals of mammalian ON rod bipolar cells. Our findings suggest that the all-rod retina of the skate contains at least two distinct vertical pathways including an OFF bipolar cell pathway in addition to a classical rod ON bipolar pathway.

Pharmacology of the skate electroretinogram indicates independent ON and OFF bipolar cell pathways

Organization of afferent information into parallel ON and OFF pathways is a critical feature of the vertebrate visual system. All afferent visual information in the vertebrate retina reaches the inner plexiform layer (IPL) via bipolar cells. It is at the bipolar cell level that separation of ON and OFF information first appears for afferent information from cones. This may also hold true for the rod pathway of cold-blooded vertebrates, but not for mammals. The all-rod retina of the skate presents an opportunity to examine such pathways in a retina having but a single class of photoreceptor. Immunocytochemical evidence suggests that both ON and OFF bipolar cells are present in the skate retina. We examined the pharmacology of the skate electroretinogram (ERG) to test the hypothesis that independent ON and OFF bipolar cell pathways are functional as rod afferent pathways from outer to inner plexiform layer in the skate. 100 microM 2-amino-4-phosphonobutyric acid (APB) reversibly blocked the skate ERG b-wave. A small d-wave-like OFF component of the ERG revealed by DC recording of response to a prolonged (10 s) flash of light was reduced or blocked by 5 mM kynurenic acid (KYN). We found that addition of 200 microM picrotoxin to the Ringer's solution revealed prominent ON and OFF components of the skate ERG while reducing the c-wave. These ON and OFF components were reversibly blocked by 100 microM APB and 5 mM KYN, respectively. Reversible block of the OFF component by KYN was also accomplished in the presence of 500 microM N-methyl-DL-aspartate. From these findings, we conclude that ON and OFF bipolar cells are likely to be functional as parallel afferent interplexiform pathways in the all-rod retina of the skate.

Retinal neurons of the California ground squirrel, Spermophilus beecheyi: a Golgi study

Although the optic nerve fibers of the cone-dominant ground squirrel retina have been well studied physiologically, the morphological details of the retinal neurons have not. To that end, retinal neurons of the California ground squirrel have been studied in Golgi-impregnated wholemounts. Two types of horizontal cell have been identified: H1 has an axon and axon terminal, whereas H2 is axonless. The dendritic field of H1 cells enlarges in a nonuniform manner with increasing displacement from the central retina. The smallest examples lie centrally in the visual streak, and the largest occur in the superior periphery. Eight types of bipolar cell are distinguished by morphological differences in dendritic branching pattern and field size in the outer plexiform layer, cell body size, and layering within the inner nuclear layer and by the morphology and stratification of axon terminals in the inner plexiform layer. A large bistratified bipolar cell (B8) is introduced here; the other 7 types closely resemble those in the retinas of other sciurid species described by R.W. West (1976, J. Comp. Neurol. 168:355-378; 1978, Vision Res. 18:129-136). The B1 type is proposed as a blue cone bipolar cell. Amacrine cells are classified into 27 cell types. Six of these occur as mirror-image pairs across the inner plexiform layer, the soma of one of each pair being "displaced" to the ganglion cell layer. The best described of these pairs is the very elaborate starburst amacrine cell, A5, which stains regularly in these wholemounted retinas. Changes in dendritic field size of both A5 subtypes with retinal location are quantified. The morphology of three amacrine cell types identified in Spermophilus beecheyi suggests that their possible counterparts in S. mexicanus (West, 1976) were, as displaced amacrine cells, misidentified as ganglion cells. Amacrine cell types that may play roles in the rod pathway, the blue cone pathway, and ganglion cell directional selectivity are discussed. No type of interplexiform cell was observed. Ganglion cells are classified into 19 cell types, 9 of which probably correspond to the ganglion cells described by West (1976) in the Mexican ground squirrel. The bistratified G11 cell is proposed as an ON-OFF directionally selective type.

A differential effect of APB on ON- and OFF-center ganglion cells in the dark adapted rabbit retina

The glutamate analog, 2-amino-4-phosphonobutyric acid (APB) is a proven tool in exploring the retinal circuit; it has been shown to interfere specifically with the transmission from photoreceptor to depolarizing bipolar cell. Consequently, in photopic retinae, the application of APB disrupts the ON-channel leaving the OFF-channel undisturbed; on the other hand, in the scotopic state, APB application blocks all ganglion cell responses. In this paper, we will show that the ON- and OFF-channels have a differential sensitivity to application of APB. That is to say, APB blocks center responses in ON-ganglion cells at mean concentration of 22 +/- 5.1 microM (mean +/- standard error of the mean; n = 15) and in OFF-ganglion cells at mean concentration of 91 +/- 15.5 microM (n = 16). Since considerable data rule out direct effects of APB on ganglion cells, we hypothesize that this effect is due to a difference in the synaptic gain of ON and OFF pathways in the inner retina.

An alternative pathway for signal flow from rod photoreceptors to ganglion cells in mammalian retina

Rod signals in the mammalian retina are thought to reach ganglion cells over the circuit rod-->rod depolarizing bipolar cell-->AII amacrine cell-->cone bipolar cells-->ganglion cells. A possible alternative pathway involves gap junctions linking the rods and cones, the circuit being rod-->cone-->cone bipolar cells-->ganglion cells. It is not clear whether this second pathway indeed relays rod signals to ganglion cells. We studied signal flow in the isolated rabbit retina with a multielectrode array, which allows the activity of many identified ganglion cells to be observed simultaneously while the preparation is stimulated with light and/or exposed to drugs. When transmission between rods and rod depolarizing bipolar cells was blocked by the glutamate agonist 2-amino-4-phosphonobutyric acid (APB), rod input to all On-center and briskly responding Off-center ganglion cells was dramatically reduced as expected. Off responses persisted, however, in Off-center sluggish and On-Off direction-selective ganglion cells. Presumably these responses were generated by the alternative pathway involving rod-cone junctions. This APB-resistant pathway may carry the major rod input to Off-center sluggish and On-Off direction-selective ganglion cells.

Receptive field properties of starburst cholinergic amacrine cells in the rabbit retina

Patch-clamp recordings were made from ON starburst cholinergic amacrine cells with somas located in the ganglion cell layer of an isolated, dark-adapted rabbit retina preparation. Light responses were analysed and cell identify was confirmed anatomically. The centre light response had a linear current-voltage relation with a reversal potential close to 0 mV. The receptive field size was similar to the dendritic field size. Cholinergic amacrine cells displayed significant surround inhibition. The receptive field profile consisted of a central excitatory region flanked by an inhibitory surround. The surround attenuated the central response to 36% of the maximum. The surround was probably mediated by a combination of presynaptic and postsynaptic inhibition. Starburst amacrine cells did not display action potentials and the presence of a large, voltage-dependent outward current limited depolarizing responses to a maximum potential of about -40 mV. Light responses were completely suppressed during application of 100 microM D,L-2-amino-4-phosphonobutyric acid (APB), consistent with activation exclusively through rod bipolar cells (on) and ON-cone bipolar cells. In darkness the cells displayed a tonic inward current that could be blocked by 100 microM APB and 2 microM CNQX.

Müller glial cells of the tree shrew retina

The tree shrew is one of the few mammalian species whose retinae are strongly cone dominated, which is usually the case in reptilian and avian retinae. Müller cells of the tree shrew (Tupaia belangeri) retina were studied by transmission electron microscopy of tissue sections and freeze-fracture replicas, by immunolabeling of the intermediate filament protein vimentin in radial paraffin sections and in whole retinae, as well as by intracellular dye injection in slices of retinae. In addition, enzymatically isolated cells were stained by Pappenheim's panoptic staining method. The cells showed an ultrastructure that is similar to other mammalian Müller cells with two exceptions: Due to the extensive lateral fins of cone inner segments, the apical microvilli of Müller cells are arranged in peculiar palisades, and the basket-like Müller cell sheaths around neuronal somata in both nuclear layers consist of unusual multilayered membrane lamellae. Unlike Müller cells in other mammalian species studied thus far, but similar to reptilian and avian Müller cells, those of tree shrews commonly have two or more vitread processes rather than one main trunk. Müller cell densities range between some 13,000 mm-2 in the periphery and about 20,000 mm-2 in the retinal center. Neuron:(Müller)glial cell ratios were estimated to be 7.9:1 in the center and 6.2:1 in the periphery. For each Müller cell, about 1.5 (cone) photoreceptor cells, four or five interneurons of the inner nuclear layer, and about one cell of the ganglion cell layer were counted. This is a much lower number of neurons per Müller cell than in most other mammals studied.

Light-induced c-fos expression in amacrine cells in the rabbit retina

Retinal neurons that express the immediate early gene c-fos after light exposure were characterized by neurotransmitter content using histochemical and immunocytochemical staining. In Northern blots the amount of c-fos mRNA peaked at 30 min, but remained detectable 60 min following light stimulation. Fos proteins were seen in the inner nuclear and ganglion cell layers, and the staining was most intense two and three hours after beginning the light exposure. In the ganglion cell layer 30-40% of Fos-immunoreactive cells were cholinergic displaced amacrine cells and 3-5% were ganglion cells. In the inner nuclear layer 24% of Fos-immunoreactive cells were Type I and 7% Type II NADPH-diaphorase-reactive (nitric oxide synthase) amacrine cells, 11% were tyrosine hydroxylase-containing cells, and 10-15% cholinergic amacrine cells. No Fos immunoreactivity was seen in serotoninergic, somatostatin- or VIP-immunoreactive cells, bipolar, horizontal or photoreceptor cells. Nicotine, kainic acid, NMDA and SCH 38393, a dopamine D1 receptor agonist, induced Fos immunostaining in the inner nuclear and ganglion cell layers, but administration of the corresponding receptor blockers mecamylamine, kynuretic acid, MK-801, haloperidol and SCH 23990 did not prevent light-induced Fos expression.

Baclofen enhancement of acetylcholine release from amacrine cells in the rabbit retina by reduction of glycinergic inhibition

1. The mechanism by which the GABAB-receptor agonist, baclofen, enhances the light-evoked release of [3H]acetylcholine (ACh) from cholinergic amacrine cells was studied using an eye-cup preparation in anaesthetized rabbits and isolated retinas. 2. When applied locally to the rabbit retina, baclofen increased the release of ACh evoked by a flickering light (3 Hz) by over 40%. 3. In isolated retinas, baclofen strikingly inhibited the K(+)-evoked release of glycine but had no effect on GABA release. 4. In the rabbit eye cup, strychnine enhanced the light-evoked release of ACh to a similar degree to that produced by baclofen. The effects of baclofen and strychnine on the light-evoked release of ACh were not additive. In contrast, bicuculline did not affect the enhancing action of baclofen on the light-evoked release of ACh. 5. In order to see whether the glycinergic amacrine cells might be stimulated by ACh, isolated rat and rabbit retinas were exposed to muscarine. This cholinergic agonist potentiated the K(+)-evoked release of glycine by 54%. 6. We suggest that baclofen enhances the light-evoked release of ACh from amacrine cells by inhibiting glycine release from glycinergic amacrine cells which are stimulated by ACh and form an inhibitory feedback loop to the cholinergic neurones.

Rate of quantal transmitter release at the mammalian rod synapse

Under scotopic conditions, the mammalian rod encodes either one photon or none within its integration time. Consequently the signal presented to its synaptic terminal is binary. The synapse has a single active zone that releases neurotransmitter quanta tonically in darkness and pauses briefly in response to a rhodopsin isomerization by a photon. We asked: what minimum tonic rate would allow the postsynaptic bipolar cell to distinguish this pause from an extra-long interval between quanta due to the stochastic timing of release? The answer required a model of the circuit that included the rod convergence onto the bipolar cell and the bipolar cell's signal-to-noise ratio. Calculations from the model suggest that tonic release must be at least 40 quanta/s. This tonic rate is much higher than at conventional synapses where reliability is achieved by employing multiple active zones. The rod's synaptic mechanism makes efficient use of space, which in the retina is at a premium.

Development of the rabbit retina. V. The question of 'columnar units'

A qualitative and quantitative description of the columnar units in the mammalian retina, and a discussion of their ontogeny and putative functions is given. Columnar arrangements of cells exist in the developing retina which can be observed by means of scanning electron microscopy. In the adult retina, each Müller cell ensheaths a columnar group of neuronal cells. Counting the number of cells in radial H/E stained sections at various developmental stages reveals a constant ratio of neuronal cells per Müller cell, independent of the developmental stage (after postnatal day 9), and independent of the retinal topography. Such groups of cells always consist of one Müller cell, 11 rod photoreceptor cells, about 2 bipolar cells, and 1 to 2 amacrine cells. Retinal ganglion cells, cone photoreceptor cells, and horizontal cells are more sparsely distributed in the retina than these units; since they are known to arise earlier in the ontogenesis than other cell types they are considered to exist independently of the columnar units. It is suggested that the units arise by migration of groups of preneurons along a common Müller (precursor) cell; these preneurons and the corresponding Müller cell may be clonally related. In the adult retina, such columns might constitute metabolic and functional units.

Distribution and coverage of A- and B-type horizontal cells stained with Neurobiotin in the rabbit retina

Both A- and B-type horizontal cells in the rabbit retina were labeled by brief in vitro incubations of the isolated retina in the blue fluorescent dye 4,6-diamino-2-phenylindole. Intracellular injection of Lucifer Yellow into the somata revealed the morphology of the individual cells. Dye-coupling with Lucifer Yellow was seen only between A-type horizontal cells. By contrast, injection of the tracer Neurobiotin showed dye-coupling between both A- and B-type horizontal cells. There also appeared to be coupling between the axon terminals of B-type horizontal cells. The extensive dye-coupling seen following injection of Neurobiotin into a single horizontal cell soma can be used to obtain population counts of each cell type. Staining of large numbers of each cell type across the retina showed that each type increased in number and declined in dendritic diameter as the visual streak was approached, such that relatively constant coverage across the retina was maintained. In the visual streak, A-type horizontal cells numbered 555 cells/mm2 and averaged 120 microns in diameter, compared to 1375 cells/mm2 and 100 microns for B-type horizontal cells. In the periphery, the A- and B-types numbered 250 cells/mm2 and 400 cells/mm2, respectively. The average diameters of the dendritic trees at these locations were 225 microns for the A-type and 175 microns for the B-type. Coverage across the retina averaged almost six for A-type horizontal cells and 8-10 for the B-type. A-type horizontal cells in the visual streak whose elliptical dendritic fields were shown by Bloomfield (1992) to correlate physiologically with orientation bias were shown to be dye-coupled to cells with symmetrical dendritic fields.