The synaptic complex of cones in the fovea and in the periphery of the macaque monkey retina

Regional variation in the organization and connectivity of the first synapse in the primate night vision pathway

Sensitivity of primate daylight vision varies across the visual field. This is attributed to regional variations in cone photoreceptor density and synaptic connectivity of the underlying circuitry. In contrast, we have limited understanding of how synapse organization of the primate night vision pathway changes across space. Using serial electron microscopy, we reconstructed the first synapse of the night vision pathway between rod photoreceptors and second-order neurons, at multiple locations from the central part of the primate retina, fovea, to the periphery. We find that most facets of the rod synapse connectivity vary across retinal regions. However, rod synaptic divergence and convergence patterns do not change in the same manner across locations. Moreover, patterns of rod synapse organization are tightly correlated with photoreceptor density. Such regional heterogeneities revise the connectivity diagram of the primate rod synapse which will shape synapse function and sensitivity of the night vision pathway across visual space.

Dye coupling of horizontal cells in the primate retina

In the monkey retina, there are two distinct types of axon-bearing horizontal cells, known as H1 and H2 horizontal cells (HCs). In this study, cell bodies were prelabled using 4',6-diamidino-2-phenylindole (DAPI), and both H1 and H2 horizontal cells were filled with Neurobiotin™ to reveal their coupling, cellular details, and photoreceptor contacts. The confocal analysis of H1 and H2 HCs was used to assess the colocalization of terminal dendrites with glutamate receptors at cone pedicles. After filling H1 somas, a large coupled mosaic of H1 cells was labeled. The dendritic terminals of H1 cells contacted red/green cone pedicles, with the occasional sparse contact with blue cone pedicles observed. The H2 cells were also dye-coupled. They had larger dendritic fields and lower densities. The dendritic terminals of H2 cells preferentially contacted blue cone pedicles, but additional contacts with nearly all cones within the dendritic field were still observed. The red/green cones constitute 99% of the input to H1 HCs, whereas H2 HCs receive a more balanced input, which is composed of 58% red/green cones and 42% blue cones. These observations confirm those made in earlier studies on primate horizontal cells by Dacey and Goodchild in 1996. Both H1 and H2 HCs were axon-bearing. H1 axon terminals (H1 ATs) were independently coupled and contacted rod spherules exclusively. In contrast, the H2 axon terminals contacted cones, with some preference for blue cone pedicles, as reported by Chan and Grünert in 1998. The primate retina contains three independently coupled HC networks in the outer plexiform layer (OPL), identified as H1 and H2 somatic dendrites, and H1 ATs. At each cone pedicle, the colocalization of both H1 and H2 dendritic tips with GluA4 subunits close to the cone synaptic ribbons indicates that glutamate signaling from the cones to H1 and H2 horizontal cells is mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors.

Mechanisms of simultaneous linear and nonlinear computations at the mammalian cone photoreceptor synapse

Neurons enhance their computational power by combining linear and nonlinear transformations in extended dendritic trees. Rich, spatially distributed processing is rarely associated with individual synapses, but the cone photoreceptor synapse may be an exception. Graded voltages temporally modulate vesicle fusion at a cone's ~20 ribbon active zones. Transmitter then flows into a common, glia-free volume where bipolar cell dendrites are organized by type in successive tiers. Using super-resolution microscopy and tracking vesicle fusion and postsynaptic responses at the quantal level in the thirteen-lined ground squirrel, Ictidomys tridecemlineatus, we show that certain bipolar cell types respond to individual fusion events in the vesicle stream while other types respond to degrees of locally coincident events, creating a gradient across tiers that are increasingly nonlinear. Nonlinearities emerge from a combination of factors specific to each bipolar cell type including diffusion distance, contact number, receptor affinity, and proximity to glutamate transporters. Complex computations related to feature detection begin within the first visual synapse.

Size variations in synaptic terminals among different types of photoreceptors and across the zebrafish retina

Photoreceptor synaptic terminals are responsible for transmitting visual information to downstream neurons. In vertebrate retinas, photoreceptor synaptic terminals are of different sizes and structures. The molecular mechanisms that underlie photoreceptor synaptic development are not clearly understood. Here, we have systematically examined the size variations in the synaptic terminals of cone and rod photoreceptors in the adult zebrafish retina. We reveal that the average cone pedicle sizes expand in the order of UV, blue, green, and red cones, echoing the increasing maximally sensitive wavelengths of the opsins expressed in the corresponding cone types. In addition, rod spherules are smaller than all cone pedicles. The terminals of each photoreceptor type also display distinct regional variations across the retina and between males and females. These findings establish the basis for using the zebrafish retina to study the molecular mechanisms that regulate the sizes and structures of photoreceptor terminals for proper visual functions.

Multiple Invagination Patterns and Synaptic Efficacy in Primate and Mouse Rod Synaptic Terminals

Purpose

Optical retina images are scaled based on eye size, which results in a linear scale ratio of 10:1 for human versus mouse and 7:1 for macaque monkey versus mouse. We examined how this scale difference correlates with the structural configuration of synaptic wiring in the rod spherule (RS) between macaque and mouse retinas compared with human data.

Methods

Rod bipolar cell (BC) dendrites and horizontal cell (HC) axonal processes, which invaginate the RS to form synaptic ribbon-associated triads, were examined by serial section transmission electron microscopy.

Results

The number of rod BC invaginating dendrites ranged 1∼4 in the macaque RS but only 1∼2 in the mouse. Approximately 40% of those dendrites bifurcated into two central elements in the macaque, but 2% of those dendrites did in the mouse. Both factors gave rise to 10 invagination patterns of BC and HC neurites in the macaque RS but only two in the mouse. Five morphological parameters: the lengths of arciform densities and ribbons, the area of the BC-RS contact, and the surface areas of BC and HC invaginating neurites, were all independent of the invagination patterns in the macaque RS. However, those parameters were significantly greater in the macaque than in the mouse by ratios of 1.5∼1.8.

Conclusions

The primate RS provides a more expansive BC-RS interface associated with the longer arciform density and more branched invaginating neurites of BCs and HCs than the mouse RS. The resulting greater synaptic contact area may contribute to more efficient signal transfer.

Development and maintenance of vision's first synapse

Synapses in the outer retina are the first information relay points in vision. Here, photoreceptors form synapses onto two types of interneurons, bipolar cells and horizontal cells. Because outer retina synapses are particularly large and highly ordered, they have been a useful system for the discovery of mechanisms underlying synapse specificity and maintenance. Understanding these processes is critical to efforts aimed at restoring visual function through repairing or replacing neurons and promoting their connectivity. We review outer retina neuron synapse architecture, neural migration modes, and the cellular and molecular pathways that play key roles in the development and maintenance of these connections. We further discuss how these mechanisms may impact connectivity in the retina.

Transmission at rod and cone ribbon synapses in the retina

Light-evoked voltage responses of rod and cone photoreceptor cells in the vertebrate retina must be converted to a train of synaptic vesicle release events for transmission to downstream neurons. This review discusses the processes, proteins, and structures that shape this critical early step in vision, focusing on studies from salamander retina with comparisons to other experimental animals. Many mechanisms are conserved across species. In cones, glutamate release is confined to ribbon release sites although rods are also capable of release at non-ribbon sites. The role of non-ribbon release in rods remains unclear. Release from synaptic ribbons in rods and cones involves at least three vesicle pools: a readily releasable pool (RRP) matching the number of membrane-associated vesicles along the ribbon base, a ribbon reserve pool matching the number of additional vesicles on the ribbon, and an enormous cytoplasmic reserve. Vesicle release increases in parallel with Ca²⁺ channel activity. While the opening of only a few Ca²⁺ channels beneath each ribbon can trigger fusion of a single vesicle, sustained release rates in darkness are governed by the rate at which the RRP can be replenished. The number of vacant release sites, their functional status, and the rate of vesicle delivery in turn govern replenishment. Along with an overview of the mechanisms of exocytosis and endocytosis, we consider specific properties of ribbon-associated proteins and pose a number of remaining questions about this first synapse in the visual system.

Light responses of mammalian cones

Cone photoreceptors provide the foundation of most of human visual experience, but because they are smaller and less numerous than rods in most mammalian retinas, much less is known about their physiology. We describe new techniques and approaches which are helping to provide a better understanding of cone function. We focus on several outstanding issues, including the identification of the features of the phototransduction cascade that are responsible for the more rapid kinetics and decreased sensitivity of the cone response, the roles of inner-segment voltage-gated and Ca²⁺-activated channels, the means by which cones remain responsive even in the brightest illumination, mechanisms of cone visual pigment regeneration in constant light, and energy consumption of cones in comparison to that of rods.

Optimized Signal Flow through Photoreceptors Supports the High-Acuity Vision of Primates

The fovea is a neural specialization that endows humans and other primates with the sharpest vision among mammals. This performance originates in the foveal cones, which are extremely narrow and long to form a high-resolution pixel array. Puzzlingly, this form is predicted to impede electrical conduction to an extent that appears incompatible with vision. We observe the opposite: signal flow through even the longest cones (0.4-mm axons) is essentially lossless. Unlike in most neurons, amplification and impulse generation by voltage-gated channels are dispensable. Rather, sparse channel activity preserves intracellular current, which flows as if unobstructed by organelles. Despite these optimizations, signaling would degrade if cones were lengthier. Because cellular packing requires that cone elongation accompanies foveal expansion, this degradation helps explain why the fovea is a constant, miniscule size despite multiplicative changes in eye size through evolution. These observations reveal how biophysical mechanisms tailor form-function relationships for primate behavioral performance.

Elevated energy requirement of cone photoreceptors

We have used recent measurements of mammalian cone light responses and voltage-gated currents to calculate cone ATP utilization and compare it to that of rods. The largest expenditure of ATP results from ion transport, particularly from removal of Na⁺ entering outer segment light-dependent channels and inner segment hyperpolarization-activated cyclic nucleotide-gated channels, and from ATP-dependent pumping of Ca²⁺ entering voltage-gated channels at the synaptic terminal. Single cones expend nearly twice as much energy as single rods in darkness, largely because they make more synapses with second-order retinal cells and thus must extrude more Ca²⁺ In daylight, cone ATP utilization per cell remains high because cones never remain saturated and must continue to export Na⁺ and synaptic Ca²⁺ even in bright illumination. In mouse and human retina, rods greatly outnumber cones and consume more energy overall even in background light. In primates, however, the high density of cones in the fovea produces a pronounced peak of ATP utilization, which becomes particularly prominent in daylight and may make this part of the retina especially sensitive to changes in energy availability.

Molecular mechanisms underlying selective synapse formation of vertebrate retinal photoreceptor cells

In vertebrate central nervous systems (CNSs), highly diverse neurons are selectively connected via synapses, which are essential for building an intricate neural network. The vertebrate retina is part of the CNS and is comprised of a distinct laminar organization, which serves as a good model system to study developmental synapse formation mechanisms. In the retina outer plexiform layer, rods and cones, two types of photoreceptor cells, elaborate selective synaptic contacts with ON- and/or OFF-bipolar cell terminals as well as with horizontal cell terminals. In the mouse retina, three photoreceptor subtypes and at least 15 bipolar subtypes exist. Previous and recent studies have significantly progressed our understanding of how selective synapse formation, between specific subtypes of photoreceptor and bipolar cells, is designed at the molecular level. In the ON pathway, photoreceptor-derived secreted and transmembrane proteins directly interact in trans with the GRM6 (mGluR6) complex, which is localized to ON-bipolar cell dendritic terminals, leading to selective synapse formation. Here, we review our current understanding of the key factors and mechanisms underlying selective synapse formation of photoreceptor cells with bipolar and horizontal cells in the retina. In addition, we describe how defects/mutations of the molecules involved in photoreceptor synapse formation are associated with human retinal diseases and visual disorders.

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.

Structural consequences of arrested foveal development in preterms with persisting signs of immaturity

Purpose

To evaluate the impact of structural changes in a limited sample of adult preterms with foveal immaturity from optical coherence tomography (OCT) B-scan images and to estimate layer displacement and changes in areal and volume magnification within the inner fovea.

Subjects and methods

Layer thickness was measured in conventional and directional OCT scans from eight preterms with different degrees of foveal immaturity (24–33 weeks of gestation, 22–33 years of age) and five controls (20–33 years of age). We obtained reflectivity profiles of the outer plexiform layer (OPL) and manual segmentation data of the inner nuclear layer (INL) and the combined ganglion cell layer (GCL) and inner plexiform layer (IPL) at specified eccentricities from 300 to 900 µm. Displacement of cumulative thickness curves of preterms compared with that of the controls was used to estimate retardation of layer displacement. Changes in areal magnification and layer thickness were used to construct a structural model of redistribution within the fovea of preterms.

Results

Retardation of centrifugal layer displacement of OPL and all inner retinal layers (IRL) was marked in both preterm groups with foveal immaturity, whereas retardation was marginal in the preterm group without clinical signs of immaturity. Retarded displacement within the IRL and OPL had a major impact on available space within the central fovea.

Conclusions

A marked retardation of displacement was demonstrated for all IRL within the immature fovea of preterms with decreased areal and volume magnification and reduced space available for synaptic communication coupled to the degree of immaturity.

Stem cell-based retina models

From the early days of cell biological research, the eye-especially the retina-has evoked broad interest among scientists. The retina has since been thoroughly investigated and numerous models have been exploited to shed light on its development, morphology, and function. Apart from various animal models and human clinical and anatomical research, stem cell-based models of animal and human cells of origin have entered the field, especially during the last decade. Despite the observation that the retina of different species comprises endogenous stem cells, most stem cell-related research in the human retina is now based on pluripotent stem cell models. Herein, systems of two-dimensional (2D) cultures and co-cultures of distinctly differentiated retinal subtypes revealed a variety of cellular aspects but have in many aspects been replaced by three-dimensional (3D) structures-the so-called retinal organoids. These organoids not only contain all major retinal cell subtypes compared to the physiological situation, but also show a distinct layering in close proximity to the in vivo morphology. Nevertheless, all these models have inherent advantages and disadvantages, which are expounded and summarized in this review. Finally, we discuss current application aspects of stem cell-based retina models and the specific promises they hold for the future.

The clinical relevance of visualising the peripheral retina

Recent developments in imaging technologies now allow the documentation, qualitative and quantitative evaluation of peripheral retinal lesions. As wide field retinal imaging, capturing both the central and peripheral retina up to 200° eccentricity, is becoming readily available the question is: what is it that we gain by imaging the periphery? Based on accumulating evidence it is clear that findings in the periphery do not always associate to those observed in the posterior pole. However, the newly acquired information may provide useful clues to previously unrecognised disease features and may facilitate more accurate disease prognostication. In this review, we explore the anatomy and physiology of the peripheral retina, focusing on how it differs from the posterior pole, recount the history of peripheral retinal imaging, describe various peripheral retinal lesions and evaluate the overall relevance of peripheral retinal findings to different diseases.

The primate fovea: Structure, function and development

A fovea is a pitted invagination in the inner retinal tissue (fovea interna) that overlies an area of photoreceptors specialized for high acuity vision (fovea externa). Although the shape of the vertebrate fovea varies considerably among the species, there are two basic types. The retina of many predatory fish, reptilians, and birds possess one (or two) convexiclivate fovea(s), while the retina of higher primates contains a concaviclivate fovea. By refraction of the incoming light, the convexiclivate fovea may function as image enlarger, focus indicator, and movement detector. By centrifugal displacement of the inner retinal layers, which increases the transparency of the central foveal tissue (the foveola), the primate fovea interna improves the quality of the image received by the central photoreceptors. In this review, we summarize ‒ with the focus on Müller cells of the human and macaque fovea ‒ data regarding the structure of the primate fovea, discuss various aspects of the optical function of the fovea, and propose a model of foveal development. The "Müller cell cone" of the foveola comprises specialized Müller cells which do not support neuronal activity but may serve optical and structural functions. In addition to the "Müller cell cone", structural stabilization of the foveal morphology may be provided by the 'z-shaped' Müller cells of the fovea walls, via exerting tractional forces onto Henle fibers. The spatial distribution of glial fibrillary acidic protein may suggest that the foveola and the Henle fiber layer are subjects to mechanical stress. During development, the foveal pit is proposed to be formed by a vertical contraction of the centralmost Müller cells. After widening of the foveal pit likely mediated by retracting astrocytes, Henle fibers are formed by horizontal contraction of Müller cell processes in the outer plexiform layer and the centripetal displacement of photoreceptors. A better understanding of the molecular, cellular, and mechanical factors involved in the developmental morphogenesis and the structural stabilization of the fovea may help to explain the (patho-) genesis of foveal hypoplasia and macular holes.

Connectivity map of bipolar cells and photoreceptors in the mouse retina

In the mouse retina, three different types of photoreceptors provide input to 14 bipolar cell (BC) types. Classically, most BC types are thought to contact all cones within their dendritic field; ON-BCs would contact cones exclusively via so-called invaginating synapses, while OFF-BCs would form basal synapses. By mining publically available electron microscopy data, we discovered interesting violations of these rules of outer retinal connectivity: ON-BC type X contacted only ~20% of the cones in its dendritic field and made mostly atypical non-invaginating contacts. Types 5T, 5O and 8 also contacted fewer cones than expected. In addition, we found that rod BCs received input from cones, providing anatomical evidence that rod and cone pathways are interconnected in both directions. This suggests that the organization of the outer plexiform layer is more complex than classically thought.

ON Bipolar Cells in Macaque Retina: Type-Specific Synaptic Connectivity with Special Reference to OFF Counterparts

To date, 12 macaque bipolar cell types have been described. This list includes all morphology types first outlined by Polyak (1941) using the Golgi method in the primate retina and subsequently identified by other researchers using electron microscopy (EM) combined with the Golgi method, serial section transmission EM (SSTEM), and immunohistochemical imaging. We used SSTEM for the rod-dense perifoveal area of macaque retina, reconfirmed ON (cone) bipolar cells to be classified as invaginating midget bipolar (IMB), diffuse bipolar (DB)4, DB5, DB6, giant bipolar (GB), and blue bipolar (BB) types, and clarified their type-specific connectivity. DB4 cells made reciprocal synapses with a kind of ON-OFF lateral amacrine cell, similar to OFF DB2 cells. GB cells contacted rods and cones, similar to OFF DB3b cells. Retinal circuits formed by GB and DB3b cells are thought to substantiate the psychophysical finding of fast rod signals in mesopic vision. DB6 cell output synapses were directed to ON midget ganglion (MG) cells at 70% of ribbon contacts, similar to OFF DB1 cells that directed 60% of ribbon contacts to OFF MG cells. IMB cells contacted medium- or long-wavelength sensitive (M/L-) cones but not short-wavelength sensitive (S-) cones, while BB cells contacted S-cones but not M/L-cones. However, IMB and BB dendrites had similar morphological architectures, and a BB cell contacting a single S-cone resembled an IMB cell. Thus, both IMB and BB may be the ON bipolar counterparts of the OFF flat midget bipolar (FMB) type, likewise DB4 of DB2, DB5 of DB3a, DB6 of DB1, and GB of DB3b OFF bipolar type. The ON DB plus GB, and OFF DB cells predominantly contacted M/L-cones and their outputs were directed mainly to parasol ganglion (PG) cells but also moderately to MG cells. BB cells directed S-cone-driven outputs almost exclusively to small bistratified ganglion (SBG) cells. Some FMB cells predominantly contacted S-cones and their outputs were directed to OFF MG cells. Thus, two-step synaptic connections largely narrowed down the S-cone component to SBG and some OFF MG cells. The other OFF MG cells, ON MG cells, and ON and OFF PG cells constructed M/L-cone dominant pathways.

Mechanism of High-Frequency Signaling at a Depressing Ribbon Synapse

Ribbon synapses mediate continuous release in neurons that have graded voltage responses. While mammalian retinas can signal visual flicker at 80-100 Hz, the time constant, τ, for the refilling of a depleted vesicle release pool at cone photoreceptor ribbons is 0.7-1.1 s. Due to this prolonged depression, the mechanism for encoding high temporal frequencies is unclear. To determine the mechanism of high-frequency signaling, we focused on an "Off" cone bipolar cell type in the ground squirrel, the cb2, whose transient postsynaptic responses recovered following presynaptic depletion with a τ of ∼0.1 s, or 7- to 10-fold faster than the τ for presynaptic pool refilling. The difference in recovery time course is caused by AMPA receptor saturation, where partial refilling of the presynaptic pool is sufficient for a full postsynaptic response. By limiting the dynamic range of the synapse, receptor saturation counteracts ribbon depression to produce rapid recovery and facilitate high-frequency signaling.

OFF bipolar cells in macaque retina: type-specific connectivity in the outer and inner synaptic layers

OFF bipolar cells in the macaque retina were recently classified into five types: flat midget bipolar (FMB) and diffuse bipolar (DB) 1, 2, 3a, and 3b. We examined all parallel pathways from cone photoreceptors via OFF bipolar cells to parasol and midget ganglion cells by serial section transmission electron microscopy. Basal contacts of OFF bipolar cells to cone pedicles were previously categorized as triad-associated (TA) and non-TA (NTA). The latter was further divided into two groups located in the middle and marginal areas of the pedicle at the present eccentricity of 15°. We then mapped the distributions of all three basal contacts of the five OFF bipolar cell types in the same area of cone pedicles. TA contacts were more numerous than NTA contacts in FMB (93%), DB1 (67%), and DB3a (81%) cells, but less in DB2 (30%) and DB3b (21%) cells. Cluster analysis of these contact parameters reconfirmed five distinct OFF bipolar cell types and showed these positional configurations of basal synapses to be cell type-specific. This architecture is thought to provide a spatial framework for the interstitial diffusion and local uptake of the neurotransmitter (glutamate) that spills over from ribbon synapses. All five OFF bipolar cell types formed ribbon-synaptic contacts to both parasol and midget ganglion cells. DB2 and 3a, DB1 and 3b, and FMB predominantly, moderately, and negligibly contacted parasol ganglion cells, respectively. FMB almost exclusively contacted midget ganglion cells, to which DB1 provided dominant output (58%), and DB2, 3a, and 3b provided between 3% and 10% of their output. Consequently, the cone signal sampling routes of a midget ganglion cell consisted of two substructures: the narrow (mainly 2-3 cones) FMB pathway and the wide (mainly 10 cones) DB pathway, where connection strength was four-fold greater in the FMB than DB pathway. The narrow and strong FMB pathway may confer the highest spatial resolution and sporadically may include blue cone signals.

Functional architecture of the retina: development and disease

Structure and function are highly correlated in the vertebrate retina, a sensory tissue that is organized into cell layers with microcircuits working in parallel and together to encode visual information. All vertebrate retinas share a fundamental plan, comprising five major neuronal cell classes with cell body distributions and connectivity arranged in stereotypic patterns. Conserved features in retinal design have enabled detailed analysis and comparisons of structure, connectivity and function across species. Each species, however, can adopt structural and/or functional retinal specializations, implementing variations to the basic design in order to satisfy unique requirements in visual function. Recent advances in molecular tools, imaging and electrophysiological approaches have greatly facilitated identification of the cellular and molecular mechanisms that establish the fundamental organization of the retina and the specializations of its microcircuits during development. Here, we review advances in our understanding of how these mechanisms act to shape structure and function at the single cell level, to coordinate the assembly of cell populations, and to define their specific circuitry. We also highlight how structure is rearranged and function is disrupted in disease, and discuss current approaches to re-establish the intricate functional architecture of the retina.

Retinal representation of the elementary visual signal

The propagation of visual signals from individual cone photoreceptors through parallel neural circuits was examined in the primate retina. Targeted stimulation of individual cones was combined with simultaneous recording from multiple retinal ganglion cells of identified types. The visual signal initiated by an individual cone produced strong responses with different kinetics in three of the four numerically dominant ganglion cell types. The magnitude and kinetics of light responses in each ganglion cell varied nonlinearly with stimulus strength but in a manner that was independent of the cone of origin after accounting for the overall input strength of each cone. Based on this property of independence, the receptive field profile of an individual ganglion cell could be well estimated from responses to stimulation of each cone individually. Together, these findings provide a quantitative account of how elementary visual inputs form the ganglion cell receptive field.

OFF bipolar cells express distinct types of dendritic glutamate receptors in the mouse retina

Parallel representations of the visual world are already established at the very first synapse of the visual system. Cone photoreceptors, which hyperpolarize in response to light, forward the visual signal onto distinct types of ON and OFF cone bipolar cells (BCs). In the case of OFF BCs, the glutamatergic cone input is integrated by ionotropic glutamate receptors, giving rise to a sign-preserving mode of synaptic transmission. The combination of glutamate receptor (GluR) subunits, i.e. AMPA or kainate subunits, importantly contributes to shaping the OFF bipolar cells' distinct response properties. The mouse is one of the few mammals in which the (most likely) complete set of (five) retinal OFF BC types is identified. However, it is not clear which GluR subtypes are expressed by the different mouse OFF BC types. We addressed this question by combining immunolabeling, electrical whole-cell recordings and pharmacology, and present evidence that the different types of OFF BCs express distinct types of glutamate receptors: Type 1 BCs exclusively expressed AMPA receptors, whereas type 2 and type 3a BCs expressed kainate receptors of different subunit compositions. Additionally, we found that two OFF BC types (3b and 4) very likely express both AMPA and kainate receptors but, interestingly, the two receptor subunits were not co-localized at the same dendritic site. The complex, BC type-specific expression pattern of GluRs we describe here supports their essential role in establishing parallel pathways at the first synapse of the mouse visual system.

Structure and function of a complex sensory synapse

Vision is the most important of the senses for humans, and the retina is the first stage in the processing of light signals in the visual system. In the retina, highly specialized light-sensing neurons, the rod and cone photoreceptors, convert light into neural signals. These signals are extensively processed and filtered in the subsequent retinal network before transmitted to the higher visual centres in the brain, where the perception of viewed objects and scenes is finally constructed. A key feature of signal processing in the mammalian retina is parallel processing. Visual information is segregated in parallel pathways already at the rod and cone photoreceptor terminals, which provide multiple output synapses for the faithful encoding and transfer of the visual signals to the post-receptoral retinal network. This review aims at highlighting the current knowledge about the structural and functional pre- and post-synaptic specializations of rod and cone photoreceptor ribbon synapses, which belong to the most complex chemical synapses in the central nervous system.

Photoreceptor coupling mediated by connexin36 in the primate retina

Photoreceptors are coupled via gap junctions in many mammalian species. Cone-to-cone coupling is thought to improve sensitivity and signal-to-noise ratio, while rod-to-cone coupling provides an alternative rod pathway active under twilight or mesopic conditions (Smith et al., 1986; DeVries et al., 2002; Hornstein et al., 2005). Gap junctions are composed of connexins, and connexin36 (Cx36), the dominant neuronal connexin, is expressed in the outer plexiform layer. Primate (Macaca mulatta) cone pedicles, labeled with an antibody against cone arrestin (7G6) were connected by a network of fine processes called telodendria and, in double-labeled material, Cx36 plaques were located precisely at telodendrial contacts between cones, suggesting strongly they are Cx36 gap junctions. Each red/green cone made nonselective connections with neighboring red/green cones. In contrast, blue cone pedicles were smaller with relatively few short telodendria and they made only rare or equivocal Cx36 contacts with adjacent cones. There were also many smaller Cx36 plaques around the periphery of every cone pedicle and along a series of very fine telodendria that were too short to reach adjacent members of the cone pedicle mosaic. These small Cx36 plaques were closely aligned with nearly every rod spherule and may identify sites of rod-to-cone coupling, even though the identity of the rod connexin has not been established. We conclude that the matrix of cone telodendria is the substrate for photoreceptor coupling. Red/green cones were coupled indiscriminately but blue cones were rarely connected with other cones. All cone types, including blue cones, made gap junctions with surrounding rod spherules.

Glutamate spillover between mammalian cone photoreceptors

Cone photoreceptors transmit signals at high temporal frequencies and mediate fine spatial vision. High-frequency transmission requires a high rate of glutamate release, which could promote spillover to neighboring cells, whereas spatial vision requires that cones within a tightly packed array signal light to postsynaptic bipolar cells with minimal crosstalk. Glutamate spread from the cone terminal is thought to be limited by presynaptic transporters and nearby glial processes. In addition, there is no ultrastructural evidence for chemical synapses between mammalian cones, although such synapses have been described in lower vertebrate retinas. We tested for cone-cone glutamate diffusion by recording from adjacent cone pairs in the ground squirrel retina, and instead found that the glutamate released by one cone during electrical stimulation activates glutamate transporter Cl(-) conductances on neighboring cones. Unlike in other systems, where crosstalk is diminished by increasing the temperature and by moving to a more intact preparation, glutamate spread persisted at physiological temperatures (37°C) and in retinal flat mounts. The glutamate-gated anion conductance in cones has a reversal potential of ∼-30 mV compared with a cone resting potential of ∼-50 mV; thus, crosstalk should have a depolarizing effect on the cone network. Cone-cone glutamate spread is regulated by the physiological stimulus, light, and under physiological conditions can produce a response of ∼2 mV, equivalent to 13-20% of a cone's light response. We conclude that in the absence of discrete chemical synapses, glutamate flows between cones during a light response and may mediate a spatially distributed positive feedback.

Cone synapses in macaque fovea: I. Two types of non-S cones are distinguished by numbers of contacts with OFF midget bipolar cells

L and M cones, divided into two groups by absorption spectra, have not been distinguished by structure. Here, we report what may be such a difference. We reconstructed the synaptic terminals of 16 non-S cones and the dendritic arbors of their ON and OFF midget bipolar cells from high-magnification electron micrographs of serial thin sections of a small region of macaque fovea. Each cone terminal contacted a similar number (~16) of invaginating central elements provided by its ON midget bipolar cell. By contrast, the numbers of connections between a cone terminal and its OFF midget bipolar cell were grouped into two clusters: 30-37 versus 43-50 basal contacts in the triad-associated position and 41-47 versus 61-74 Outer Densities within those basal contacts. The coefficients of variation of these distributions were all in the range of 10% or lower, characteristic of single populations. If these two clusters correspond to M- and L-cone circuits, the results reveal structural differences between M and L cones and between their corresponding OFF midget bipolar cells.

Cone synapses in macaque fovea: II. Dendrites of OFF midget bipolar cells exhibit Inner Densities similar to their Outer synaptic Densities in basal contacts with cone terminals

As described in the companion paper, the synaptic terminal of a cone photoreceptor in macaque monkey makes an average of 35 or 46 basal contacts with the tips of the dendrites of its OFF midget bipolar cell. Each basal contact has one or more symmetrically thickened dense regions. These “Outer Densities,” averaging 48 or 67 in number, harbor clusters of ionotropic glutamate receptors and are ~0.8μm (and ~1-ms diffusion time) from active zones associated with synaptic ribbons. Here, we show similarly appearing “Inner Densities,” averaging 53 or 74 in number, located more proximally on the dendrites of these OFF midget bipolar cells, ~0.4μm inward from the tips of the dendrites and out of contact with the basal surface of the cone terminal. Compared to desmosome-like junctions, Inner Densities are closer to the terminal and are less dense and less thick. Each Inner Density is shared with another cell, the partners including diffuse bipolar cells, ON midget bipolar cells, and horizontal cells. Given the diversity of the partners, the OFF midget bipolar cells are unlikely to be in a synaptic relationship with the partners. Instead, Inner Densities are near enough to the active zones associated with synaptic ribbons to receive pulses of glutamate at concentrations effective for glutamate receptors. The role of Inner Densities is not known, but they might represent additional clusters of glutamate receptors.

Retinal connectivity and primate vision

The general principles of retinal organization are now well known. It may seem surprising that retinal organization in the primate, which has a complex visual behavioral repertoire, appears relatively simple. In this review, we primarily consider retinal structure and function in primate species. Photoreceptor distribution and connectivity are considered as are connectivity in the outer and inner retina. One key issue is the specificity of retinal connections; we suggest that the retina shows connectional specificity but this is seldom complete, and we consider here the functional consequences of imprecise wiring. Finally, we consider how retinal systems can be linked to psychophysical descriptions of different channels, chromatic and luminance, which are proposed to exist in the primate visual system.

How robust is a neural circuit?

Design in engineering begins with the problem of robustness-by what factor should intrinsic capacity exceed normal demand? Here we consider robustness for a neural circuit that crosses the retina from cones to ganglion cells. The circuit's task is to represent the visual scene at many successive stages, each time by modulating a stream of stochastic events: photoisomerizations, then transmitter quanta, then spikes. At early stages, the event rates are high to achieve some critical signal-to-noise ratio and temporal bandwidth, which together set the information rate. Then neural circuits concentrate the information and repackage it, so that nearly the same total information can be represented by modulating far lower event rates. This is important for spiking because of its high metabolic cost. Considering various measurements at the outer and inner retina, we conclude that the "safety factors" are about 2-10, similar to other tissues.

OFF midget bipolar cells in the retina of the marmoset, Callithrix jacchus, express AMPA receptors

Recent studies suggested that different types of OFF bipolar cells express specific types of ionotropic (AMPA or kainate) glutamate receptors (GluRs) at their contacts with cone pedicles. However, the question of which GluR type is expressed by which type of OFF bipolar cell in primate retina is still open. In this study, the expression of AMPA and kainate receptor subunits at the dendritic tips of flat (OFF) midget bipolar (FMB) cells was analyzed in the retina of the common marmoset, Callithrix jacchus. We used preembedding electron microscopy and double immunofluorescence with subunit-specific antibodies. The FMB cells were labeled with antibodies against the carbohydrate epitope CD15. Cone pedicles were identified with peanut agglutinin. Immunoreactivity for the GluR1 subunit and for CD15 is preferentially located at triad-associated flat contacts. Furthermore, the large majority of GluR1 immunoreactive puncta is localized at the dendritic tips of FMB cells. These results suggest that FMB cells express the AMPA receptor subunit GluR1. In contrast, the kainate receptor subunit GluR5 is not colocalized with the dendritic tips of FMB cells or with the GluR1 subunit. Immunoreactive puncta for the GluR1 subunit are found at all M/L-cone pedicles but are only rarely associated with S-cone pedicles. This is consistent with our recent findings in marmoset retina that FMB cells do not contact S-cone pedicles. The presence of GluR5 clusters at S-cone pedicles indicates that in primate retinas OFF bipolar cells expressing kainate receptor subunits receive some S-cone input.

Connections of diffuse bipolar cells in primate retina are biased against S-cones

In mammalian retina, each diffuse bipolar type stratifies in a distinct layer of the inner plexiform layer. Thus, different types of bipolar cells provide output to distinct visual pathways. Here, the question of whether diffuse bipolar cell types differ with respect to their contacts with short wavelength-sensitive (S-) cones was investigated in the retinas of a New World monkey, Callithrix jacchus, and an Old World monkey, Macaca fascicularis. Subpopulations of OFF bipolar cells were labeled with antibodies to the glutamate transporter Glt-1 and ON bipolar cells were labeled with antibodies to the alpha subunit of the Go protein (Goalpha). Two types of diffuse ON bipolar cells, DB4 and DB6, were identified with antibodies to protein kinase Calpha and CD15, respectively. Cone pedicles were labeled either with peanut agglutinin coupled to fluorescein or with antibodies to the ribbon protein, C-terminus binding protein 2. We found that immunoreactivity for Glt-1 (OFF bipolar cells) is reduced at S-cones in comparison to medium/long wavelength-sensitive (M/L-) cones. Immunoreactivity for Goalpha (ON bipolar cells) is comparable at all cone types. Nearly all M/L-cone pedicles contact the diffuse ON bipolar types DB4 and DB6, but only between 60% and 75% of the S-cone pedicles make contact. Furthermore, the number of dendritic tips of DB4 and DB6 cells at S-cone pedicles is lower than that at M/L-cone pedicles. These results suggest that there is a bias in the S-cone connectivity of diffuse bipolar cells.

Microcircuitry for two types of achromatic ganglion cell in primate fovea

Synaptic circuits in primate fovea have been quantified for midget/parvocellular ganglion cells. Here, based on partial reconstructions from serial electron micrographs, we quantify synaptic circuits for two other types of ganglion cell: the familiar parasol/magnocellular cell and a smaller type, termed "garland." The excitatory circuits both derive from two types of OFF diffuse cone bipolar cell, DB3 and DB2, which collected unselectively from at least 6 +/- 1 cones, including the S type. Cone contacts to DB3 dendrites were usually located between neighboring triads, whereas half of the cone contacts to DB2 were triad associated. Ribbon outputs were as follows: DB3, 69 +/- 5; DB2, 48 +/- 4. A complete parasol cell (30 microm dendritic field diameter) would collect from approximately 50 cones via approximately 120 bipolar and approximately 85 amacrine contacts; a complete garland cell (25 microm dendritic field) would collect from approximately 40 cones via approximately 75 bipolar and approximately 145 amacrine contacts. The bipolar types contributed differently: the parasol cell received most contacts (60%) from DB3, whereas the garland cell received most contacts (67%) from DB2. We hypothesize that DB3 is a transient bipolar cell and that DB2 is sustained. This would be consistent with their relative inputs to the brisk-transient (parasol) ganglion cell. The garland cell, with its high proportion of DB2 inputs plus its high proportion of amacrine synapses (70%) and dense mosaic, might correspond to the local-edge cell in nonprimate retinas, which serves finer acuity at low temporal frequencies. The convergence of S cones onto both types could contribute S-cone input for cortical areas primary visual cortex and the middle temporal area.

Progressive cone and cone-rod dystrophies: phenotypes and underlying molecular genetic basis

The cone and cone-rod dystrophies form part of a heterogeneous group of retinal disorders that are an important cause of visual impairment in children and adults. There have been considerable advances made in recent years in our understanding of the pathogenesis of these retinal dystrophies, with many of the chromosomal loci and causative genes having now been identified. Mutations in 12 genes, including GUCA1A, peripherin/RDS, ABCA4 and RPGR, have been described to date; and in many cases detailed functional assessment of the effects of the encoded mutant proteins has been undertaken. This improved knowledge of disease mechanisms has raised the possibility of future treatments for these disorders, for which there are no specific therapies available at the present time.

Cone-rod dystrophy, intrafamilial variability, and incomplete penetrance associated with the R172W mutation in the peripherin/RDS gene

PURPOSE

To determine the underlying molecular genetic basis of a retinal dystrophy identified in a 5-generation family, and to examine the phenotype and degree of intrafamilial variability.

DESIGN

Family genetic study.

PARTICIPANTS

Nine affected individuals from a nonconsanguineous British family.

METHODS

Ophthalmologic examination, color vision testing, fundus photography, autofluorescence imaging, and electrophysiological assessment were performed. The clinical notes of 2 additional deceased affected family members were also reviewed. Blood samples were taken for DNA extraction, with linkage analysis being performed, and subsequent mutation screening of the peripherin/RDS gene.

RESULTS

Linkage analysis established a disease interval on chromosome 6p, which harbored the retinal candidate gene, peripherin/RDS. The 3 coding exons of the peripherin/RDS gene were subsequently screened for mutations in affected and unaffected family members. A nonconservative missense substitution, Arg172Trp (R172W), segregated uniquely in all affected subjects. The majority of subjects carrying the R172W peripherin/RDS mutation complained of reduced central vision starting in the second or third decade, with subsequent gradual deterioration of visual acuity and color vision. Three affected individuals complained of nyctalopia. A range of macular appearances were seen, varying from a typical granular appearance to extensive macular atrophy. Autofluorescence imaging in the majority of individuals identified a highly characteristic speckled macular appearance. All affected subjects had abnormal pattern electroretinograms (ERGs) consistent with macular dysfunction and 4 subjects showed additional full-field ERG abnormalities, providing evidence of generalized retinal dysfunction. There was marked variation in the clinical phenotype in those individuals who carried the R172W peripherin/RDS mutation, ranging from severe cone-rod dystrophy to asymptomatic individuals with normal retinal function.

CONCLUSIONS

The Arg172Trp (R172W) peripherin/RDS mutation has been previously reported to cause a fully penetrant progressive macular dystrophy with high intrafamilial and interfamilial consistency of phenotype. This is the first report describing marked intrafamilial variation associated with this mutation, including nonpenetrance. These findings are clinically important in relation to advice on prognosis and accurate genetic counseling.

Mutation in the Gene GUCA1A, Encoding Guanylate Cyclase-Activating Protein 1, Causes Cone, Cone-Rod, and Macular Dystrophy

PURPOSE

To determine the underlying molecular genetic basis of a retinal dystrophy identified in a 4-generation family and to examine the phenotype and the degree of intrafamilial variability.

DESIGN

Prospective case series.

PARTICIPANTS

Six affected individuals from a nonconsanguineous British family.

METHODS

Detailed ophthalmologic examination, color fundus photography, autofluorescence imaging, and electrophysiologic assessment were performed. Blood samples were taken for DNA extraction, and mutation screening of GUCA1A, the gene encoding guanylate cyclase-activating protein 1 (GCAP1), was undertaken.

RESULTS

All affected subjects complained of mild photophobia and reduced central and color vision. Onset was between the third and fifth decade, with subsequent gradual deterioration of visual acuity and color vision. Visual acuity ranged between 6/9 and counting fingers. Color vision was either absent or markedly reduced along all 3 color axes. A range of macular appearances was seen, varying from mild retinal pigment epithelial disturbance to extensive atrophy. Electrophysiologic testing revealed a range of electrophysiologic abnormalities: isolated cone electroretinography abnormalities, reduced cone and rod responses (with cone loss greater than rod), and isolated macular dysfunction. The 4 coding exons of GUCA1A were screened for mutations in affected and unaffected family members. A single transition, A319G, causing a nonconservative missense substitution, Tyr99Cys, segregated uniquely in all affected subjects.

CONCLUSIONS

The Tyr99Cys GUCA1A mutation has been previously shown to cause autosomal dominant progressive cone dystrophy. This is the first report of this mutation also causing both cone-rod dystrophy and isolated macular dysfunction. The phenotypic variation described here exemplifies the intrafamilial heterogeneity of retinal dysfunction that can be observed in persons harboring the same mutation and chromosomal segment.

Evidence that each S cone in macaque fovea drives one narrow-field and several wide-field blue-yellow ganglion cells

A rule of retinal wiring is that many receptors converge onto fewer bipolar cells and still fewer ganglion cells. However, for each S cone in macaque fovea, there are two S-cone ON bipolar cells and two blue-yellow (BY) ganglion cells. To understand this apparent rule reversal, we reconstructed synaptic patterns of divergence and convergence and determined the basic three-tiered unit of connectivity that repeats across the retina. Each foveal S cone diverges to four S-cone ON bipolar cells but contacts them unequally, providing 1-16 ribbon synapses per cell. Next, each bipolar cell diverges to two BY ganglion cells and also contacts them unequally, providing approximately 14 and approximately 28 ribbon synapses per cell. Overall, each S cone diverges to approximately six BY ganglion cells, dominating one and contributing more modestly to the others. Conversely, of each pair of BY ganglion cells, one is dominated by a single S cone and one is diffusely driven by several. This repeating circuit extracts blue/yellow information on two different spatiotemporal scales and thus parallels the circuits for achromatic, spatial vision, in which each cone dominates one narrow-field ganglion cell (midget) and contributes some input to several wider-field ganglion cells (parasol). Finally, because BY ganglion cells have coextensive +S and -(L+M) receptive fields, and each S cone contributes different weights to different BY ganglion cells, the coextensive receptive fields must be already present in the synaptic terminal of the S cone. The S-cone terminal thus constitutes the first critical locus for BY color vision.

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.

Inner S-cone bipolar cells provide all of the central elements for S cones in macaque retina

Synaptic terminals of cones (pedicles) are presynaptic to numerous processes that arise from the dendrites of many types of bipolar cell. One kind of process, a central element, reaches deeply into invaginations of the cone pedicle just below an active zone associated with a synaptic ribbon. By reconstruction from serial electron micrographs, we show that L- and M-cone pedicles in macaque fovea are presynaptic to approximately 20 central elements that arise from two types of inner (invaginating) bipolar cell, midget and diffuse. In contrast, S-cone pedicles, with more synaptic ribbons, active zones/ribbon, and central elements/active zone, are presynaptic to approximately 33 central elements. Moreover, all of these arise from one type of bipolar cell, previously described by others, here termed an inner S-cone bipolar cell. Each provides approximately 16 central elements. Thirty-three is twice 16; correspondingly, these bipolar cells are twice as numerous as S cones. (Specifically, each S cone is presynaptic to four inner S-cone bipolar cells; in turn, each bipolar cell provides central elements to two S cones.) These bipolar cells are presynaptic to an equal number of small-field bistratified ganglion cells, giving cell numbers in 2G:2B:1S ratios. Each ganglion cell receives input from two or more inner S-cone bipolar cells and thereby collects signals from three or more S cones. This convergence, along with chromatic aberration of short-wavelength light, suggests that S-cone contributions to this ganglion cell's coextensive blue-ON/yellow-OFF receptive field are larger than opponent L/M-cone contributions via outer diffuse bipolar cells and that opponent L/M-cone signals are conveyed mainly by inner S-cone bipolar cells.

The eye of the african mole-rat Cryptomys anselli: to see or not to see?

In an attempt to clarify its possible physiological role, we studied the eye of the Zambian mole rat Cryptomys anselli by light, electron and confocal microscopy using conventional staining as well as immunolabelling with rod and cone cell markers. The small eyes of Cryptomys are located superficially and display all features typical of sighted animals: iris, pupil and well-developed lens, separating the anterior chamber and the vitreous. The retina shows a well stratified organization and the folds described in blind subterranean or nocturnal mammals were not observed. The major population of the photoreceptor cells in the Cryptomys retina consists of rod cells, again with a morphology quite similar to that found in sighted animals. The relatively short outer segments contain numerous well-stacked disks and show a strong rod-opsin as well as transducin immunoreaction. Synapses were evident in the spherules, the round basal processes of the rod cell, but they lacked the precise organization reported for sighted mammals. Cone cells were present as well, as indicated by peanut lectin staining, but no immunolabelling with polyclonal M/L-opsin antisera was detectable. The presence of cone cells was also suggested by some basal processes at the outer plexiform layer which displayed several synaptic active sites and irregular contours. While the other retinal layers also showed an organization typical of sighted mammals, there were signs of less tightly preserved morphology as well. Displaced rods and amacrine and/or ganglion cells were observed, and some sparse rod spherules penetrated into the inner nuclear layer. A major reduction was observed in the number of ganglion cells, estimated from the number of axons in the optic nerve, that was very low (approximately 1000 per retina on average) relative to sighted mammals. The data we have suggest a slow, ongoing loss of cells with ageing. Apoptotic nuclei, mainly corresponding to photoreceptor cells and ganglion cells, were detected in young individuals, and an overall reduction in the thickness of the retina was observed in older animals. The morphological data presented here allow some first speculations on the physiological role of the Cryptomys eye and will hopefully trigger detailed studies on the chronobiology and the anatomy of the retinal projections and of the visual cortex of this remarkable species.

Two ribbon synaptic units in rod photoreceptors of macaque, human, and cat

The rod photoreceptor's synaptic terminal (or spherule) uses an elaborate synaptic structure to signal absorption of one or more photons to its postsynaptic targets. This structure includes one or two synaptic ribbons inside the terminal and a pouch-like "invagination" outside the terminal, into which enter a widely variable number of incoming fibers and postsynaptic targets-central elements supplied by rod bipolar cells and lateral elements supplied by horizontal cells. Nonetheless, our three-dimensional reconstructions of this synaptic structure in foveal retina of macaque monkey and peripheral retina of human and cat reveal several features that are highly conserved across species and with eccentricity: 1). every spherule has one invagination; 2). with rare exceptions, every spherule has two ribbon synaptic units with these features: a). on the presynaptic side, each ribbon synaptic unit has a ribbon or part of a ribbon and one trough-shaped arciform density that demarcates its active zone; b). on the postsynaptic side, each ribbon synaptic unit has two apposed lateral elements and one or more central elements; 3). the volume of the extracellular space in the single invagination is small, approximately 0.1 microm(3); and 4). the largest distance from active zone to receptor regions on bipolar cells is small, less than approximately 1.5 microm. With such small dimensions, release of one quantum of transmitter can pulse glutamate to a concentration comparable to the EC(50) of the metabotropic glutamate receptors on the central elements associated with both synaptic units. We speculate that two ribbon synaptic units are required to sustain the high quantal release rate needed to signal a single photon.

How Müller glial cells in macaque fovea coat and isolate the synaptic terminals of cone photoreceptors

A cone synaptic terminal in macaque fovea releases quanta of glutamate from approximately 20 active zones at a high rate in the dark. The transmitter reaches approximately 500 receptor clusters on bipolar and horizontal cell processes by diffusion laterally along the terminal's 50 microm(2) secretory face and approximately 2 microm inward. To understand what shapes transmitter flow, we investigated from electron photomicrographs of serial sections the relationship between Müller glial processes and cone terminals. We find that each Müller cell has one substantial trunk that ascends in the outer plexiform layer below the space between the "footprints" of the terminals. We find exactly equal numbers of Müller cell trunks and foveal cone terminals, which may make the fovea particularly vulnerable to Müller cell dysfunction. The processes that emerge from the single trunk do not ensheathe a single terminal. Instead, each Müller cell partially coats two to three terminals; in turn, each terminal is completely coated by two to three Müller cells. Therefore, the Müller cells that coat one terminal also partially coat the surrounding ( approximately six) terminals, creating a common environment for the cones supplying the center/surround receptive field of foveal midget bipolar and ganglion cells. Upon reaching the terminals, the trunk divides into processes that coat the terminals' sides but not their secretory faces. This glial framework minimizes glutamate transporter (EAAT1) beneath a terminal's secretory face but maximizes EAAT1 between adjacent terminals, thus permitting glutamate to diffuse locally along the secretory face and inward toward inner receptor clusters but reducing its effective spillover to neighboring terminals.

ON-pathway disturbance in two siblings

PURPOSE

To present two clinical cases diagnosed with predominant cone dystrophy and demonstrating early disturbance in the on-centre bipolar cells (ON-pathway).

METHODS

Electrophysiological findings are presented in two siblings with predominant cone dystrophy. The subjects showed no remarkable ophthalmoscopic or fluorescein angiographic retinal changes, but demonstrated progressive visual disturbance during their 20s.

RESULTS

The electroretinograms (ERGs) showed reduced dark-adapted responses but the positive component of the photopic ERG was absent. Response to 30 Hz flicker was severely reduced. Electroretinograms elicited by long-duration stimuli showed a loss of the b-wave, and the off-response was slightly reduced. In both patients, multifocal ERGs (m-ERGs) were more reduced within the central 10 degrees, where the ON-pathway is normally a major contributor.

CONCLUSION

We conclude that these patients may be affected by an abnormality of the synapses of the cone receptors and that their decrease in vision might, at least initially, be due to selective ON-pathway dysfunction.

Effect of glutamate analogues on red-green opponent interaction in monkey electroretinograms

The effect of glutamate analogues on red-green opponent interaction was electrophysiologically investigated in anesthetized cynomolgus monkeys (Macaca fascicularis). Two approaches were employed: amplitude measurement and principal component analysis. Electroretinograms were recorded for 23 monochromatic stimuli (400-700 nm) at an equal energy with white light adaptation before and after treatment with the glutamate analogues, 2-amino-4-phosphonobutyric acid, cis -2,3-piperidine-dicarboxylic acid, or both. Before treatment, although spectral amplitude curves of the a- and d-waves showed single, broad peaks at about 550 nm, the b-wave curve had three peaks at about 460, 540 and 600 nm, indicating the occurrence of the red-green opponent interaction. Principal component analysis performed on these waveforms extracted three components with short, middle, and long wavelength peaks, well defined characteristics of the red-green opponency. After vitreal injection of 2-amino-4-phosphonobutyric acid, the a- and d-wave amplitudes were enhanced while the b-wave amplitude was almost completely diminished. However, principal component analysis showed basically similar characteristics to those before drug, suggesting that the red-green opponency was not affected. In contrast, after application of cis -2,3-piperidine-dicarboxylic acid, the a- and d-waves were diminished and the b-wave was enhanced as expected, however the enhancement was observed only in the short and middle wavelengths. As a result of this partial enhancement, the b-wave spectral amplitude curve showed only a single peak, unlike in the control. In addition, principal component analysis revealed a quite different result from the control; only two components with short and middle wavelength peaks and the component with long wavelength peak disappeared. Similar two components were also separated after the conjunction of both drugs. These results demonstrate that red-green opponency is greatly inhibited by cis -2,3 piperidine-dicarboxylic acid, and thus suggest that horizontal cells are related to a generation of the red-green opponency through a cone type selective or nonselective negative feedback.

Localization of kainate receptors at the cone pedicles of the primate retina

In the macaque monkey retina cone pedicles, the output synapses of cone photoreceptors, contain between 20 and 45 ribbon synapses (triads), which are the release sites for glutamate, the cone transmitter. Several hundred postsynaptic dendrites contact individual cone pedicles, and we studied the glutamate receptors expressed and clustered at these contacts, particularly the kainate receptor subunits GluR5, GluR6/7, and KA2. Pre- and postembedding immunocytochemistry and electron microscopy were used to localize GluR5 and GluR6/7 to specific synaptic contacts at the cone pedicle base. The GluR5 subunit was aggregated at bipolar cell flat contacts. The GluR6/7 subunit was aggregated at bipolar cell flat contacts and at the desmosome-like junctions formed by horizontal cell processes underneath the cone pedicles. KA2 immunoreactivity was observed at the invaginating dendritic tips of ON-cone and rod bipolar cells, which we interpret as a cross-reactivity of the KA2 antiserum with some other, unknown protein of the monkey retina. Kainate receptors are preferentially expressed by OFF-cone bipolar cells and to a lesser extent by horizontal cells. We also performed double-labeling experiments with the ribbon-specific marker bassoon and with antibodies against GluR5 and GluR6/7 in order to define the position of the flat bipolar cell contacts with respect to the triads. There was a tendency of GluR6/7 clusters to represent triad-associated contacts, whereas GluR5 clusters represented non-triad-associated contacts. The GluR5 and GluR6/7 subunits were clustered at different bipolar cell contacts. We studied a possible cone-selective expression of the kainate receptor subunits by double labeling cone pedicles for the S-cone opsin and for the different receptor subunits. We observed a reduced expression of both GluR5 and GluR6/7 at the S-cone pedicles. The reduced expression of GluR6/7 was analyzed in more detail and it appears to be a consequence of a horizontal cell-specific expression: H1 horizontal cells express GluR6/7, whereas H2 horizontal cells, which preferentially innervate S-cones, show no expression of GluR6/7.