Synaptic contacts of the horizontal cells in the retina of the marine teleost, Callionymus lyra L

Gap junctional coupling in the vertebrate retina: variations on one theme?

Gap junctions connect cells in the bodies of all multicellular organisms, forming either homologous or heterologous (i.e. established between identical or different cell types, respectively) cell-to-cell contacts by utilizing identical (homotypic) or different (heterotypic) connexin protein subunits. Gap junctions in the nervous system serve electrical signaling between neurons, thus they are also called electrical synapses. Such electrical synapses are particularly abundant in the vertebrate retina where they are specialized to form links between neurons as well as glial cells. In this article, we summarize recent findings on retinal cell-to-cell coupling in different vertebrates and identify general features in the light of the evergrowing body of data. In particular, we describe and discuss tracer coupling patterns, connexin proteins, junctional conductances and modulatory processes. This multispecies comparison serves to point out that most features are remarkably conserved across the vertebrate classes, including (i) the cell types connected via electrical synapses; (ii) the connexin makeup and the conductance of each cell-to-cell contact; (iii) the probable function of each gap junction in retinal circuitry; (iv) the fact that gap junctions underlie both electrical and/or tracer coupling between glial cells. These pan-vertebrate features thus demonstrate that retinal gap junctions have changed little during the over 500 million years of vertebrate evolution. Therefore, the fundamental architecture of electrically coupled retinal circuits seems as old as the retina itself, indicating that gap junctions deeply incorporated in retinal wiring from the very beginning of the eye formation of vertebrates. In addition to hard wiring provided by fast synaptic transmitter-releasing neurons and soft wiring contributed by peptidergic, aminergic and purinergic systems, electrical coupling may serve as the 'skeleton' of lateral processing, enabling important functions such as signal averaging and synchronization.

Large retinal ganglion cells in the pipid frog Xenopus laevis form independent, regular mosaics resembling those of teleost fishes

Population-based studies of retinal neurons have helped to reveal their natural types in mammals and teleost fishes. In this, the first such study in a frog, labeled ganglion cells of the mesobatrachian Xenopus laevis were examined in flatmounts. Cells with large somata and thick dendrites could be divided into three mosaic-forming types, each with its own characteristic stratification pattern. These are named alpha a, alpha ab, and alpha c, following a scheme recently used for teleosts. Cells of the alpha a mosaic (approximately 0.4% of all ganglion cells) had very large somata and trees, arborizing diffusely within sublamina a (the most sclerad). Their distal dendrites were sparsely branched but achieved consistent coverage by intersecting those of their neighbors. Displaced and orthotopic cells belonged to the same mosaic, as did cells with symmetric and asymmetric trees. Cells of the alpha ab mosaic (approximately 1.2%) had large somata, somewhat smaller trees that appeared bistratified at low magnification, and dendrites that branched extensively. Their distal dendrites arborized throughout sublamina b and the vitread part of a, tessellating with their neighbors. All were orthotopic; most were symmetric. Cells of the alpha c mosaic (approximately 0.5%) had large somata and very large, sparse, flat, overlapping trees, predominantly in sublamina c. All were orthotopic; some were asymmetric. Nearest-neighbor analyses and spatial correlograms confirmed that each mosaic was regular and independent, and that spacings were reduced in juvenile frogs. Densities, proportions, sizes, and mosaic statistics are tabulated for all three types, which are compared with types defined previously by size and symmetry in Xenopus and potentially homologous mosaic-forming types in teleosts. Our results reveal strong organizational similarities between the large ganglion cells of teleosts and frogs. They also demonstrate the value of introducing mosaic analysis at an early stage to help identify characters that are useful markers for natural types and that distinguish between within-type and between-type variation in neuronal populations.

Gap junctions in the vertebrate retina

The vertebrate retina is a highly laminated assemblage of specialized neuronal types, many of which are coupled by gap junctions. With one interesting exception, gap junctions are not directly responsible for the 'vertical' transmission of visual information from photoreceptors through bipolar and ganglion cells to the brain. Instead, they mediate 'lateral' connections, coupling neurons of a single type or subtype into an extended, regular array or mosaic in the plane of the retina. Such mosaics have been studied by several microscopic techniques, but new evidence for their coupled nature has recently been obtained by intracellular injection of biotinylated tracers, which can pass through gap junctional assemblies that do not pass Lucifer Yellow. This evidence adds momentum to an existing paradigm shift towards a population-based view of the retina, which can now be envisaged both as an array of semi-autonomous vertical processing modules, each extending right through the retina, and as a multi-layered stack of interacting planar mosaics, bearing some resemblance to a set of interleaved neural networks. Junctional conductance across mosaics of horizontal cells is known to be controlled dynamically with a circadian rhythm, and other dynamically-regulated conductance changes are also likely to make important contributions to signal processing. The retina is an excellent system in which to study such changes because many aspects of its structure and function are already well understood. In this review, we summarize the microscopic appearance, coupling properties and functions of gap junctions for each cell type of the neural retina, the regulatory properties that could be provided by selective expression of different connexin proteins, and the evidence for gap junctional coupling in retina development.

Three types of GABA-immunoreactive cone horizontal cells in teleost retina

Peroxidase-anti-peroxidase immunocytochemistry, applied on serial semithin epoxy resin sections, was used to examine the localization of endogenous GABA in horizontal cells in the retina of a marine teleost, the dragonet (Callionymus lyra L.). The immunostaining shows that not only the external H1 cone horizontal cells label with antibodies against GABA, but also the H2 and H3 cone horizontal cells in the inner nuclear layer. The distribution of the H1 cells corresponds to that of the single cones. They are square-patterned and in the dorsal retina their density equals 20,000 cells/mm2. The estimated density of the immunostained H2 and the H3 cells in the dorsal retina is 9500 and 1300 cells/mm2, respectively. The H2 and H3 cells are not geometrically arranged, but nearest-neighbor analysis shows that these horizontal cell types do have a very regular disposition. We suggest that GABA is the likely neurotransmitter substance used by all cone horizontal cell types in teleost retina.

Regular mosaics of large displaced and non-displaced ganglion cells in the retina of a cichlid fish

Large retinal ganglion cells in the tilapid cichlid fish Oreochromis spilurus (standard length 15-54 mm) were filled with horseradish peroxidase and studied in flatmounts. Three types, with distinct patterns of dendritic stratification, formed spatially independent, nonrandom mosaics. One type (about 0.3% of all ganglion cells) resembled the outer (off) alpha cells of mammals. They were very large, with thick primary dendrites and large, sparsely branched planar trees in the outer part of the inner plexiform layer (IPL). About 300 were arrayed regularly across each retina, their exact number and spacing depending on its size. Their somata were often displaced into the IPL, even where neighbours in the mosaic were orthotopic. Another type (0.8%) resembled the inner (on) alpha cells of mammals. These had slightly smaller somata that were never displaced and smaller trees in the middle layers of the IPL. About 800 were arrayed uniformly and regularly across each retina. A rarer type (0.06-0.08%) had two planar trees: one forming a coarse mosaic in the outer part of the inner plexiform layer (co-planar with the trees of outer alpha-like cells) and another in the outer plexiform layer. These "biplexiform" cells were smaller and rounder than alpha-like cells and always displaced. The dendrites were finer and less tapered. Cells in which we could identify an outer plexiform tree failed to cover the retina completely, but were nonrandomly distributed. We draw three main conclusions: (1) some nonmammalian vertebrates have separate inner and outer mosaics of large ganglion cells like those of mammals, (2) the vertical displacement of ganglion cell somata can vary widely within a single mosaic and may thus be functionally irrelevant, and (3) biplexiform ganglion cells exist in fish but differ in morphology from the biplexiform types described in some other vertebrates.

Glutamate-like immunoreactivity in the retina of a marine teleost, the dragonet

The localisation of endogenous glutamate in the dragonet retina was investigated by light microscopic postembedding silver-enhanced immunogold labeling after incubation with an anti-glutamate antiserum. Rod and cone inner segments and synaptic terminals, as well as the inner plexiform layer, are moderately labeled. Bipolar cells and ganglion cell bodies show strong labeling. In the dorsal inner plexiform layer, the levels with square-patterned bipolar synaptic boutons can be identified by their prominent glutamate-immunoreactivity. These results support the idea that the majority of the neurons that constitute the direct, centripetal pathways through the retina use glutamate as their neurotransmitter.

Identification of pedicles of putative blue-sensitive cones in the human retina

Cone photoreceptor pedicles from midperipheral regions of the human retina (6 mm from the foveal center) have been studied by light and electron microscopy. Three areas of cone pedicle mosaic were serially thin-sectioned, in the tangential plane, from the inner border of the outer plexiform layer to the emergence of the cone axons from the cone pedicles. Semithin sections were then collected from the cone axon level through the cone cell bodies to the cone inner segment level. Two hundred twenty-one cone pedicles were followed by this means to their respective inner segments. Eight percent of the cone pedicles were from cones with inner segment characteristics of the blue cones. All 221 cone pedicles were reconstructed by tracing images from electron micrographs. The cone pedicle locations, surface areas, telodendrial projections, and synaptic ribbons could then be measured by morphometry and analyzed by statistical methods. Some selected cone pedicles were reconstructed by computer graphics methods. The cone pedicles identified as belonging to the blue cone type could be distinguished from the surrounding longer wavelength types on the following morphological criteria: 1) they were smaller (50% the area of the surrounding pedicles), 2) they contained shorter synaptic ribbons, 3) they exhibited essentially no telodendrial contact to neighboring cone pedicles, 4) they were positioned slightly more vitread in the outer plexiform layer than neighboring pedicles, and 5) their irregular occurrence in the cone mosaic coincided with the distribution criteria established in our previous paper (Ahnelt et al: J. Comp. Neurol. 255:18-34, '87) for putative blue sensitive cones in midperipheral human retina.

Structural and functional properties of two types of horizontal cell in the skate retina

Two morphologically distinct types of horizontal cell have been identified in the all-rod skate retina by light- and electron-microscopy as well as after isolation by enzymatic dissociation. The external horizontal cell is more distally positioned in the retina and has a much larger cell body than does the internal horizontal cell. However, both external and internal horizontal cells extend processes to the photoreceptor terminals where they end as lateral elements adjacent to the synaptic ribbons within the terminal invaginations. Whole-cell voltage-clamp studies on isolated cells similar in appearance to those seen in situ showed that both types displayed five separate voltage-sensitive conductances: a TTX-sensitive sodium conductance, a calcium current, and three potassium-mediated conductances (an anomalous rectifier, a transient outward current resembling an A current, and a delayed rectifier). There was, however, a striking difference between external and internal horizontal cells in the magnitude of the current carried by the anomalous rectifier. Even after compensating for differences in the surface areas of the two cell types, the sustained inward current elicited by hyperpolarizing voltage steps was a significantly greater component of the current profile of external horizontal cells. A difference between external and internal horizontal cells was seen also in the magnitudes of their TEA-sensitive currents; larger currents were usually obtained in recordings from internal horizontal cells. However, the currents through these K+ channels were quite small, the TEA block was often judged to be incomplete, and except for depolarizing potentials greater than or equal to +20 mV (i.e., outside the normal operating range of horizontal cells), this current did not provide a reliable indicator of cell type. The fact that two classes of horizontal cell can be distinguished by their electrophysiological responses, as well as by their morphological appearance and spatial distribution in the retina, suggests that they may play different roles in the processing of visual information within the retina.

Morphological and physiological studies of rod-driven horizontal cells with special reference to the question of whether they have axons and axon terminals

Rod-driven (intermediate) horizontal cells were examined in the carp retina to determine whether they bear axons and axon terminals. These cells were injected with HRP after physiological identification of the response type; which consisted of a higher sensitivity to light and a slower response time course than cone-driven (external) horizontal cells, and spectral sensitivity peaking at 520 nm. The labeled cells were further identified morphologically by tracing their dendrites to rod photoreceptors by light and electron microscopy. About two-thirds of the labeled cells (18/30) had a slender, axonlike process (less than 1 micron in diameter, 70-300 micron in length) running horizontally from the soma. No axonlike process was found in the remaining cells. Unlike the axons of external horizontal cells, this axonlike process was short and did not form a long fusiform expansion. No membrane specialization was found along the axonlike process. Since it has been reported that the syncytium made of axon terminals of external horizontal cells serves as a signal bypass of the syncytium made of the somata, it was asked, in separate experiments, whether the intermediate horizontal cells also had such a double syncytial layer. Response amplitudes to a slit of light were measured by placing the slit at various distances from the recording electrode. The response amplitude decayed with distance with a single exponential function, indicating that the syncytium of intermediate horizontal cells consists of a monolayer. These physiological data are consistent with the morphological observations.

Displaced small amacrine cells in the retina of the marine teleost Callionymus lyra L

The displaced small amacrine cells (DSA cells) in the dorsal pure cone part of the retina of the marine teleost Callionymus lyra have been analysed in a combined light and electron microcopical study. These unistratified cells have their dendritic arborization at 70% of the depth of the inner plexiform layer (P5 level). The DSA cells constitute a dense population and have variable dendritic field sizes. The bipolar input occurs in the P5,1 and the P5,2 pattern layer. The short, central DSA dendrites make ribbon synapses with midget mixed di-cone bipolar cells and with two types of pure cone bipolar cells. The amacrine input and output occurs in the fibrous layer that separates both pattern layers. The dendritic arborization is most extensive and the dendrites of neighbouring DSA cells are interconnected. The thick, central DSA dendrites are presynaptic to adjacent DSA cells and possibly to large bistratified and diffuse ganglion cells. The fine, peripheral DSA dendrites receive input from neighbouring DSA cells and probably from large uni- and bistratified and diffuse amacrine cells. A matching population of regularly placed small amacrine cells (RSA cells) has been observed. Their unistratified dendritic arborization is situated at 20% of the depth of the inner plexiform layer. The synaptic relations of RSA cells have not yet been completely analysed in detail. However, results up till now indicate that they most probably receive input from two bipolar cell types, one of which may be a pure cone type. In addition, the large bistratified amacrine and ganglion cells may be synaptically connected to the RSA cells as well as to the DSA cells.

Synaptic contacts between bipolar and photoreceptor cells in the retina of Callionymus lyra L

A combined light and electron microscopic study of Golgi-impregnated retinas of the marine teleost Callionymus lyra L. revealed mixed bipolar cells (M types) contacting rods and cones and pure cone bipolar cells (C types). Five types of mixed bipolar cells can be differentiated on the basis of their synaptic contacts. Two out of the five mixed bipolar cell types contact double cones, single cones, and rods (mixed, dark, pale, single [Mdps and midget-Mdps]). Their endbuds make narrow cleft junctions, with each type of photoreceptor, and in addition, two endbuds end centrally in the synaptic ribbon complexes of the dark and pale double-cone pedicles. Three types of mixed bipolar cells contact only double cones and rods. The endbuds of one type (mixed, dark, pale, ribbon [Mdpr]) end centrally in the synaptic ribbon complexes of the dark and pale double-cone pedicles as well as of the rod spherules. The endbuds of two types (Mdp and midget-Mdp) make wide cleft junctions in dark and pale double-cone pedicles and in rod spherules. All pure cone bipolar cell types contact cones exclusively with narrow cleft junctions. Four types are seen: a type that contacts predominantly pale double-cone pedicles but also a few dark double-cone pedicles (Cp), a type that is connected with dark and pale double-cone pedicles in about equal numbers (Cdp), a type that makes synaptic contacts with pale double-cone pedicles and single-cone pedicles (Cps), and a type that is connected with both types of double cones and to single-cone pedicles (Cdps). A resemblance between the ultrastructural features of mixed bipolar cell synapses in Callionymus and in Carassius auratus is noted.

Stratification and square pattern arrangements in the dorsal inner plexiform layer in the retina of Callionymus lyra L

The dorsal inner plexiform layer in the retina of Callionymus lyra consists of successive fibrous F layers and geometrically organized P layers. In the F layers no pattern arrangement of the bipolar synaptic buttons was observed. In the P layers, on the contrary, the bipolar synaptic buttons are grouped by three distinct square patterns. The alpha pattern measures 5 micron and corresponds to the square pattern of the photoreceptor cells. The beta and the gamma pattern measure 7 micron. One of both patterns corresponds to the single cone arrangement, while the other coincides with the crossings of the rows of double cone pairs. The alternation of F and P layers results in the stratified appearance of the inner plexiform layer.

A second type of horizontal cell in the monkey retina

A second type of horizontal cell with a distinctly different appearance from the previously known horizontal cell has been seen in Golgi-impregnated monkey retinas. The new HII cell has a different dendritic branching pattern from the commonly described HI cell type, and it has a short convoluted axon with collaterals bearing small clusters of terminals. Comparisons of the two cell types at different retinal eccentricities show that HII cells have larger dendritic trees than HI cells in the foveal region but smaller dendritic trees than HIs in peripheral retina. Dendritic terminals of the new HII cell, like the HI cell, contact cone pedicles as lateral elements of the ribbon synapses. The isolated terminals or clusters of terminals borne by the short axon of the HII cell also contact cone pedicles as lateral elements rather than contacting rod spherules as lateral elements like HI terminal arborizations. Thus, the monkey like other vertebrates has at least two horizontal cell types which differ in morphology and synaptic connections.