Synaptic organization of the cone horizontal cells in the catfish retina

Glycogen in retinal horizontal cells of the African mud catfish Clarias gariepinus (Burchell, 1822) and its physiological significance

This paper reports on glycogen store in the retinal horizontal cells (HC) of the African mud catfish Clarias gariepinus, as seen by histochemical reaction with periodic acid Schiff (PAS) and transmission electron microscopy in light- as well as dark-adapted state. Glycogen is abundant in the large somata and less in their axons, characterised ultrastructurally by many microtubules and extensive gap junctions interconnecting them. There was no apparent difference in glycogen content in HC somata between light- and dark adaptation, but the axons clearly showed absence of glycogen in dark condition. The HC somata (presynaptic) make synapses with dendrites in the outer plexiform layer. Müller cell inner processes, which contain more densely packed glycogen, invest the HC. Other cells of the inner nuclear layer do not show any appreciable content of glycogen. Rods, but not cones, contain abundant glycogen in their inner segments and synaptic terminals. It is likely that glycogen is used as energy substrate in hypoxia for this species that dwell muddy aquatic environment with low oxygen content. They appear to have high energy demand, and a high glycogen content in HC could act as a ready source to fulfil physiological processes, like microtubule-based transport of cargo from the large somata to axons and the maintenance of electrical activities across the gap junctions between the axonal processes. It is also likely that they can supplement glucose to the neighbouring inner nuclear layer neurons, which are clearly devoid of glycogen.

Super-resolution STED imaging in the inner and outer whole-mount mouse retina

Since its invention, super-resolution microscopy has become a popular tool for advanced imaging of biological structures, allowing visualisation of subcellular structures at a spatial scale below the diffraction limit. Thus, it is not surprising that recently, different super-resolution techniques are being applied in neuroscience, e.g. to resolve the clustering of neurotransmitter receptors and protein complex composition in presynaptic terminals. Still, the vast majority of these experiments were carried out either in cell cultures or very thin tissue sections, while there are only a few examples of super-resolution imaging in deeper layers (30 - 50 µm) of biological samples. In that context, the mammalian whole-mount retina has rarely been studied with super-resolution microscopy. Here, we aimed at establishing a stimulated-emission-depletion (STED) microscopy protocol for imaging whole-mount retina. To this end, we developed sample preparation including horizontal slicing of retinal tissue, an immunolabeling protocol with STED-compatible fluorophores and optimised the image acquisition settings. We labelled subcellular structures in somata, dendrites, and axons of retinal ganglion cells in the inner mouse retina. By measuring the full width at half maximum of the thinnest filamentous structures in our preparation, we achieved a resolution enhancement of two or higher compared to conventional confocal images. When combined with horizontal slicing of the retina, these settings allowed visualisation of putative GABAergic horizontal cell synapses in the outer retina. Taken together, we successfully established a STED protocol for reliable super-resolution imaging in the whole-mount mouse retina at depths between 30 and 50 µm, which enables investigating, for instance, protein complex composition and cytoskeletal ultrastructure at retinal synapses in health and disease.

Vesicular Release of GABA by Mammalian Horizontal Cells Mediates Inhibitory Output to Photoreceptors

Feedback inhibition by horizontal cells regulates rod and cone photoreceptor calcium channels that control their release of the neurotransmitter glutamate. This inhibition contributes to synaptic gain control and the formation of the center-surround antagonistic receptive fields passed on to all downstream neurons, which is important for contrast sensitivity and color opponency in vision. In contrast to the plasmalemmal GABA transporter found in non-mammalian horizontal cells, there is evidence that the mechanism by which mammalian horizontal cells inhibit photoreceptors involves the vesicular release of the inhibitory neurotransmitter GABA. Historically, inconsistent findings of GABA and its biosynthetic enzyme, L-glutamate decarboxylase (GAD) in horizontal cells, and the apparent lack of surround response block by GABAergic agents diminished support for GABA's role in feedback inhibition. However, the immunolocalization of the vesicular GABA transporter (VGAT) in the dendritic and axonal endings of horizontal cells that innervate photoreceptor terminals suggested GABA was released via vesicular exocytosis. To test the idea that GABA is released from vesicles, we localized GABA and GAD, multiple SNARE complex proteins, synaptic vesicle proteins, and Cav channels that mediate exocytosis to horizontal cell dendritic tips and axonal terminals. To address the perceived relative paucity of synaptic vesicles in horizontal cell endings, we used conical electron tomography on mouse and guinea pig retinas that revealed small, clear-core vesicles, along with a few clathrin-coated vesicles and endosomes in horizontal cell processes within photoreceptor terminals. Some small-diameter vesicles were adjacent to the plasma membrane and plasma membrane specializations. To assess vesicular release, a functional assay involving incubation of retinal slices in luminal VGAT-C antibodies demonstrated vesicles fused with the membrane in a depolarization- and calcium-dependent manner, and these labeled vesicles can fuse multiple times. Finally, targeted elimination of VGAT in horizontal cells resulted in a loss of tonic, autaptic GABA currents, and of inhibitory feedback modulation of the cone photoreceptor Cai, consistent with the elimination of GABA release from horizontal cell endings. These results in mammalian retina identify the central role of vesicular release of GABA from horizontal cells in the feedback inhibition of photoreceptors.

Activation of retinal glial (Müller) cells by extracellular ATP induces pronounced increases in extracellular H+ flux

Small alterations in extracellular acidity are potentially important modulators of neuronal signaling within the vertebrate retina. Here we report a novel extracellular acidification mechanism mediated by glial cells in the retina. Using self-referencing H+-selective microelectrodes to measure extracellular H+ fluxes, we show that activation of retinal Müller (glial) cells of the tiger salamander by micromolar concentrations of extracellular ATP induces a pronounced extracellular H+ flux independent of bicarbonate transport. ADP, UTP and the non-hydrolyzable analog ATPγs at micromolar concentrations were also potent stimulators of extracellular H+ fluxes, but adenosine was not. The extracellular H+ fluxes induced by ATP were mimicked by the P2Y1 agonist MRS 2365 and were significantly reduced by the P2 receptor blockers suramin and PPADS, suggesting activation of P2Y receptors. Bath-applied ATP induced an intracellular rise in calcium in Müller cells; both the calcium rise and the extracellular H+ fluxes were significantly attenuated when calcium re-loading into the endoplasmic reticulum was inhibited by thapsigargin and when the PLC-IP3 signaling pathway was disrupted with 2-APB and U73122. The anion transport inhibitor DIDS also markedly reduced the ATP-induced increase in H+ flux while SITS had no effect. ATP-induced H+ fluxes were also observed from Müller cells isolated from human, rat, monkey, skate and lamprey retinae, suggesting a highly evolutionarily conserved mechanism of potential general importance. Extracellular ATP also induced significant increases in extracellular H+ flux at the level of both the outer and inner plexiform layers in retinal slices of tiger salamander which was significantly reduced by suramin and PPADS. We suggest that the novel H+ flux mediated by ATP-activation of Müller cells and of other glia as well may be a key mechanism modulating neuronal signaling in the vertebrate retina and throughout the brain.

Invaginating Presynaptic Terminals in Neuromuscular Junctions, Photoreceptor Terminals, and Other Synapses of Animals

Typically, presynaptic terminals form a synapse directly on the surface of postsynaptic processes such as dendrite shafts and spines. However, some presynaptic terminals invaginate-entirely or partially-into postsynaptic processes. We survey these invaginating presynaptic terminals in all animals and describe several examples from the central nervous system, including giant fiber systems in invertebrates, and cup-shaped spines, electroreceptor synapses, and some specialized auditory and vestibular nerve terminals in vertebrates. We then examine mechanoreceptors and photoreceptors, concentrating on the complex of pre- and postsynaptic processes found in basal invaginations of the cell. We discuss in detail the role of vertebrate invaginating horizontal cell processes in both chemical and electrical feedback mechanisms. We also discuss the common presence of indenting or invaginating terminals in neuromuscular junctions on muscles of most kinds of animals, and especially discuss those of Drosophila and vertebrates. Finally, we consider broad questions about the advantages of possessing invaginating presynaptic terminals and describe some effects of aging and disease, especially on neuromuscular junctions. We suggest that the invagination is a mechanism that can enhance both chemical and electrical interactions at the synapse.

Lateral interactions in the outer retina

Lateral interactions in the outer retina, particularly negative feedback from horizontal cells to cones and direct feed-forward input from horizontal cells to bipolar cells, play a number of important roles in early visual processing, such as generating center-surround receptive fields that enhance spatial discrimination. These circuits may also contribute to post-receptoral light adaptation and the generation of color opponency. In this review, we examine the contributions of horizontal cell feedback and feed-forward pathways to early visual processing. We begin by reviewing the properties of bipolar cell receptive fields, especially with respect to modulation of the bipolar receptive field surround by the ambient light level and to the contribution of horizontal cells to the surround. We then review evidence for and against three proposed mechanisms for negative feedback from horizontal cells to cones: 1) GABA release by horizontal cells, 2) ephaptic modulation of the cone pedicle membrane potential generated by currents flowing through hemigap junctions in horizontal cell dendrites, and 3) modulation of cone calcium currents (I(Ca)) by changes in synaptic cleft proton levels. We also consider evidence for the presence of direct horizontal cell feed-forward input to bipolar cells and discuss a possible role for GABA at this synapse. We summarize proposed functions of horizontal cell feedback and feed-forward pathways. Finally, we examine the mechanisms and functions of two other forms of lateral interaction in the outer retina: negative feedback from horizontal cells to rods and positive feedback from horizontal cells to cones.

Calcium-permeable AMPA receptors in the retina

The retina transforms light entering the eye into a sophisticated neural representation of our visual world. Specialized synapses, cells, and circuits in the retina have evolved to encode luminance, contrast, motion, and other complex visual features. Although a great deal has been learned about the cellular morphology and circuitry that underlies this image processing, many of the synapses in the retina remain incompletely understood. For example, excitatory synapses in the retina feature the full panoply of glutamate receptors, but in most cases specific roles for different receptor subtypes are unclear. In this brief review, I will discuss recent progress toward understanding how Ca²⁺-permeable AMPA receptors (CP-GluARs) contribute to synaptic transmission and newly discovered forms of synaptic plasticity in the retina.

Morphological types and connectivity of horizontal cells found in the adult zebrafish (Danio rerio) retina

We describe here different types of horizontal cells in the zebrafish retina and how they connect to photoreceptors. To label horizontal cells, crystals of DiI were placed onto the tips of pulled glass pipettes and inserted into the inner nuclear layer of fixed whole-mount retinas. The DiI-labeled horizontal cells were imaged by confocal microscopy and analyzed according to dendritic arborization, cell depth, dendritic terminal morphology, and connectivity with photoreceptors. Three types of horizontal cells were unequivocally identified: two cone-connecting (H1/2 and H3) and one rod-related cell. H1/2 cells have dendritic terminals that are arranged in "rosette" clusters and that connect to cone photoreceptors without any apparent specificity. H3 cells are larger and have dendritic terminal doublets arranged in a rectilinear pattern. This pattern corresponds to the mosaic of the single cones in the zebrafish photoreceptor mosaic and indicates that H3 cells connect specifically to either the blue-sensitive (long-single) or ultraviolet-sensitive (short-single) cones. Thus, H3 cells are likely to be chromaticity-type cells that process specific color information, whereas H1/2 cells are probably luminosity-type cells that process luminance information. Rod horizontal cells were identified by their shape and dendritic pattern, and they connect with numerous rod photoreceptors via small spherical terminals.

Transport-mediated synapses in the retina

Most synapses rely on regulated exocytosis for determining the concentration of transmitter in the synaptic cleft. However, this mechanism may not be universal. Several synapses in the retina appear to use a synaptic machinery in which transmitter transporters play an essential role. Two types of transport-mediated synapses have been proposed. These synapses have been best observed in horizontal cells and cones of nonmammalian retinas. Horizontal cells use a transporter to mediate a bidirectional shuttle, whose balance point is set by ion concentrations and voltage. Nonmammalian cones combine exocytosis and the activity of a transporter. Because exocytosis is voltage independent over most of a cone's physiological voltage range, a voltage-dependent transporter determines the concentration of transmitter in the synaptic cleft. These two synapses may be models for transport-mediated synapses that operate in other parts of the brain.

Synapse formation is arrested in retinal photoreceptors of the zebrafish nrc mutant

We describe the effects of a recessive mutation on visual behavior, the electroretinogram (ERG), and photoreceptor structure in zebrafish. At 6 d post-fertilization (dpf), no optokinetic reflex could be elicited in no optokinetic response c (nrc) mutant animals under any test condition. The animals exhibited ERG responses at 5-7 dpf that were markedly abnormal and could be categorized into two groups. The first showed an initial negative a-wave followed by a delayed positive b-wave of small amplitude. Often a second ERG-like response was recorded after the initial b-wave. The second group showed only a large negative a-wave; an initial b-wave was not evident. In most recordings additional oscillatory waves varying in number, amplitude, and time course were observed. Multiple responses at the cessation of long-duration flashes were also observed. Light and electron microscopy revealed that the cone photoreceptor pedicles of nrc fish were highly abnormal. Although the appropriate number of synaptic ribbons formed in these terminals, they "floated" in the terminal, unassociated with postsynaptic processes or arciform densities. The few processes invaginating the nrc pedicles resembled those of horizontal cells. Invaginating bipolar cell processes were rare, but basal contacts were observed on pedicle surfaces. The severity of the mutation did not change between 6 and 8 dpf, showing that there is neither a delay in development nor a degeneration of the terminals; rather, nrc pedicle development appears arrested. Bipolar cell terminals in the inner plexiform layer made normal ribbon synapses; thus, the mutation appears to affect only the outer retina.

The GABA(C) receptor is present in cone-horizontal cell axon terminals isolated from catfish retina

Whole cell voltage-clamp recordings were performed on isolated terminals and somata from catfish retina to compare the distribution of excitatory and inhibitory receptors in both structures. Saturating concentrations of glutamate or kainate produced small currents in axon terminals, averaging less than 8% of the current evoked in the soma. In contrast, application of high concentrations of gamma-aminobutyric acid (GABA) produced approximately similar current amplitudes in both structures. Based on estimates of membrane surface area, GABA-induced current densities were around 0.05 pA/microm2 for both structures. The GABA-activated current in the axon terminal was not blocked by bicuculline or SR95531, but was completely inhibited by picrotoxin. Baclofen did not mimic the GABA effect, but trans-4-aminocrotonic acid (TACA, 300 microM) and muscimol (1 mM) elicited currents of 100 and 40 pA, respectively. These results suggest that the axon terminals of cone-horizontal cells possess GABA(C) receptors at a high density, do not possess GABA(A) or GABA(B) receptors, and have few glutamate receptors. The GABA(C) receptors could function as postsynaptic receptors in the inner plexiform layer or as autoreceptors.

Origins of horizontal cell spectral responses in the retina of marine teleosts (Centropomus and Mugil sp)

Intracellular recording and marking of horizontal cells (HCs) were carried out in isolated retinas of marine teleosts (Centropomus and Mugil sp.) to identify the cellular origins of different spectral types of S-potential. The spectral responses recorded under mesopic conditions were classified into four types: photopic L, biphasic C(R/G), biphasic C(G/R), and scotopic L(sL). Intracellular marking with Lucifer yellow (LY) or Procion yellow (PY) revealed that in the Centropomus retina an L-type response was recorded from H1 and H2 cells, C(R/G)-type from H3 cells, and sL-type from H4 cells. However, in 20% of microelectrode penetrations from the photoreceptor surface, the sequential order of response appearances was found to be L-, sL-, and C(R/G)-types. In this species, thick dendritic processes of H3 and H4 cells are distributed at the same level as the cell body. In the Mugil retina, on the other hand, L-type response was recorded from H1 cells, C(G/R)-type from H2 cells, C(R/G)-type from H3 cells, and sL-type from H4 cells, this sequential arrangement being very regular. In both species, the cone-connected H1 cells are small and possess an axon, while the two other cone HCs as well as rod-connected H4 cells are axonless. Earlier (Negishi et al., 1988) and present findings indicate that there is a wide variety of HC morphology and functional organization in the teleost retina.

GABA transport and calcium dynamics in horizontal cells from the skate retina

1. Changes in intracellular calcium concentration [Ca2+]i in response to extracellularly applied gamma-aminobutyric acid (GABA) were studied in isolated horizontal cells from the all-rod skate retina. 2. Calcium measurements were made using fura-2 AM, both with and without whole-cell voltage clamp. Superfusion with GABA, in the absence of voltage clamp, resulted in an increase in [Ca2+]i; the threshold for detection was approximately 50 microM GABA, and a maximal response was elicited by 500 microM GABA. 3. The rise in [Ca2+]i was not mimicked by baclofen nor was it blocked by phaclofen, picrotoxin or bicuculline. However, the GABA-induced [Ca2+]i increase was completely abolished when extracellular sodium was replaced with N-methyl-D-glucamine. 4. With the horizontal cell voltage clamped at -70 mV, GABA evoked a large inward current, but there was no concomitant change in [Ca2+]i. Nifedipine, which blocks L-type voltage-gated Ca2+ channels, suppressed the GABA-induced increase in [Ca2+]i. These findings suggest that the calcium response was initiated by GABA activation of sodium dependent electrogenic transport, and that the resultant depolarization led to the opening of voltage-gated Ca2+ channels, and a rise in [Ca2+]i. 5. The GABA-induced influx of calcium appears not to have been the sole source of the calcium increase. The GABA-induced rise in [Ca2+]i was reduced by dantrolene, indicating that internal Ca2+ stores contributed to the GABA-mediated Ca2+ response. 6. These observations demonstrate that activation of the GABA transporter induces changes in [Ca2+]i which may have important implications for the functional properties of horizontal cells.

Synaptic feedback, depolarization, and color opponency in cone photoreceptors

For some 20 years, synaptic feedback from horizontal cells to cones has often been invoked, more or less convincingly, in discussions of retinal action and vision. However, feedback in cones has proved to be rather complex and difficult to study experimentally. The mechanisms and consequences of feedback are therefore still only partly understood. This review attempts to assess the knowns and unknowns. The limitations of the evidence for feedback are reviewed to support the position that unequivocal evidence still largely rests on intracellular recording from cones. Of the three distinct types of depolarization observed in cones, the graded depolarization is taken as the fundamental manifestation of feedback. The evidence for the hypothesis that GABA is the neurotransmitter for feedback appears reasonably strong but several complications will have to be resolved to make the hypothesis more secure. There is evidence that feedback contributes to aspects of light adaptation and spatiotemporal processing of visual information. The contributions seem modest in magnitude. The role of feedback in shaping the color-opponent responses of retinal neurons is evaluated with particular emphasis on pharmacological studies, spatial and temporal aspects of the response of chromatic horizontal cells, and the enigmatic nature of depolarizations in blue- and green-sensitive cones. On this and other evidence, it is suggested that feedback may impress some detectable wavelength dependency in some cones but the dominant mechanisms for color opponency probably reside beyond the photoreceptors.

Development of color vision in goldfish: selective delayed maturation of blue vision

Wavelength discriminations in juvenile and adult goldfish were measured among three sensitivity maxima (450, 525 and 625 nm) of goldfish cone photoreceptors using a "go/no-go" (electroshock avoidance) task. The ability of juveniles to discriminate was significantly poorer in 450/525 nm and 450/625 nm discriminations than that of adults. While only a few juveniles acquired clear discriminative responses, there was a greater proportion of adults. In contrast to those discriminations, the ability of juveniles to discriminate between 525 and 625 nm was similar to that of adults. These results suggest that juveniles, in contrast to adults, have selective delayed development of the blue-sensitive mechanism. Some possible explanations for the poor blue discrimination of juveniles are proposed.

Analysis of the horizontal cell contribution to the receptive field surround of ganglion cells in the rabbit retina

1. The influence of horizontal cells on ganglion cells, the output neuron of the retina, was examined in an in vitro rabbit eyecup preparation. The extracellular spike activity of ganglion cells was monitored while pulsatile DC or sinusoidally modulated current was injected intracellularly into nearby horizontal cells. Interactions between the effects of light stimulation and horizontal cell current injections on ganglion cell responses were also examined. 2. Horizontal cells were found to contribute to the receptive field surround of ganglion cells. In particular, horizontal cells contributed to surround excitability and to surround antagonism of the centre light response. 3. Brisk, sluggish and direction-selective ganglion cells were all affected by current injections into horizontal cells. However, brisk ganglion cells responded to lower amplitude currents than did sluggish or direction-selective cells. 4. Horizontal cells with receptive fields that overlap those of ganglion cells were able to affect ganglion cell discharge. Moreover, the closer a horizontal cell was to the receptive field centre of a ganglion cell, the more effective were current injections in modulating ganglion cell discharge rate. The length constant of the horizontal cell contribution to the ganglion cell receptive field was approximately 200 microns. These results indicate that horizontal cells which are located within or outside of a ganglion cell's receptive field centre can influence that ganglion cell's activity. 5. The influence of horizontal cells on ganglion cell discharges was relatively weak at low temporal frequencies of sinusoidally modulated current. 6. Application of 2-amino-4-phosphonobutyrate (APB), a glutamate analogue, blocked the modulation of spike activity of on-centre ganglion cells that was induced by sinusoidally modulated current injected into nearby horizontal cells. The spike activity of off-centre ganglion cells was not blocked. 7. These findings suggest that horizontal cells contribute to the surround of ganglion cells and bipolar cells primarily through a feedback pathway onto cone photoreceptor cells.

Organization and development of horizontal cells in the goldfish retina, I: The use of monoclonal antibody AT101

We have produced and characterized a monoclonal antibody, AT101, which selectively labels both viable and formaldehyde-fixed horizontal cell axon terminals, but not their somas or axons, of the goldfish (Carassius auratus) retina. The antigen recognized by AT101 appears to be a cell surface glycoprotein with a molecular weight of about 35,000 Daltons, and is present exclusively or predominantly in nervous tissues of all vertebrate species examined. We have used AT101 as a probe to analyze immunocytochemically the organization of horizontal cell axon terminals (HCATs) in the adult goldfish retina, and the emergence and maturation of these terminals during retinal development. Because of continued growth at the retinal margin in adult goldfish, there is a peripheral-to-central gradient in the age of cells, with the most mature in the center and the youngest in the periphery. In the center and near periphery of the adult retina, HCATs have a fusiform morphology and form a dense network in the middle and proximal part of the inner nuclear layer. In the far peripheral retina, the axon terminals appear round or ellipsoid. The retina closest to the retinal margin is devoid of AT101 staining, indicating that either HCATs are absent or the antigen recognized by AT101 is not present on HCATs at this stage. A similar sequence of changes in staining pattern is seen during development. Although AT101 staining can first be demonstrated in the larval retina at 1 month after hatching, it appears mostly as punctate structures. At a later stage, there are round or ellipsoid structures that resemble in morphology and location (in the inner nuclear layer) those found in the far peripheral adult retina. Double-labeling experiments with AT101 and antiserum against tubulin also indicate that AT101 labels the HCATs when they first appear during development. These data suggest that the emergence and maturation of HCAT is a late event in retinal development.

Glial and neuronal markers in bass retinal horizontal and Müller cells

Retinal horizontal cells (HCs) are second-order neurons that integrate information from photoreceptors over large retinal areas, mediating the lateral spread of visual signals in the distal retina. The 'glial' vs. 'neuronal' nature of the HC has been widely debated. For example, carbonic anhydrase (CA), glutamine synthetase (GS), and glial fibrillary acidic protein (GFAP) are considered 'glial' markers, yet both CA and GFAP have been previously reported in HCs of the teleost retina in species-specific patterns. In contrast, the neurofilament triplet (NFT) proteins are considered 'neuronal' markers; these proteins have been immunolocalized to a mammalian HC, but are absent from teleost HCs. We have studied these cytochemical characteristics in HCs from the white bass, by immunolabeling both cryosections of intact retina and freshly isolated, identified cells attached to coverslips. We found that both HCs (neurons) and Müller cells (MCs; glia) immunolabeled with antisera to CA. Both type 1 (external) HCs and MCs immunolabeled with an antibody to vimentin. Only MCs immunolabeled with antisera to GS and GFAP. Neither HC perikarya (and their major dendrites) nor MCs immunolabeled with an antibody to the 160-kDa subunit of NFT protein. Thus, bass HCs and MCs share the presence of CA and vimentin epitopes and absence of the NFT 160-kDa epitope. Moreover, retinal cell isolation, by itself, does not affect cell-type specific immunolabeling patterns in identified cells, except for what may be lost with the finer processes of the various cells. Isolated cell studies can aid in interpreting immunolabeling patterns observed in the intact retina, especially in retinal layers where several cell types may be present.

Horizontal cell axon terminals in growing goldfish

In the retina of teleost fish, cone horizontal cell axons penetrate the inner nuclear layer, where they enlarge into fusiform terminal swellings. The present study shows that horizontal cell axon terminals enlarge disproportionately during postembryonic growth of the retina in juvenile and adult goldfish: the relative volume of axon terminals increases almost 20-fold, while the volume of the entire retina increases only about fourfold during a 2-3-yr period. The enlarging axon terminals fill in the gaps created as the numerical density of nuclei in the inner nuclear layer falls. Horizontal cell axon terminals are thought to participate in cone-dominated visual pathways, although their precise role is unclear. The results of this study suggest that a comparison of horizontal cell function in small and large fish might help to resolve this issue.

Telodendrial contact of HRP-filled photoreceptors in the turtle retina: pathways of photoreceptor coupling

Synaptic contacts of photoreceptors in the turtle retina were studied by intracellular injection of horseradish peroxidase (HRP) and electron microscopy. Both cone and rod photoreceptors radiated basal processes (telodendria) from their terminal endings. These telodendria ran laterally in the outer plexiform layer. The telodendria of cones gave rise to many fine branches that penetrated synaptic cavities of several neighboring cones. Tips of these branches terminated near the walls of synaptic cavities. Some of the telodendrial contact formed two types of basal junction: symmetrical and punctate. The distribution of cones that made telodendrial contacts with the HRP-filled cone were quantitatively investigated. Green-sensitive cones (n = 3) made telodendrial contacts with neighboring red- and blue-sensitive cones, blue-sensitive cones (n = 4) with red- and green-sensitive cones, and red-sensitive cones (n = 9) with red- and green-sensitive cones. In contrast to these cone connections, rod telodendria did not penetrate neighboring photoreceptors. Direct synaptic contacts were not found between rods and cones. Our results clarify the variety of cone couplings in turtle retina: the three chromatic classes of cones are selectively coupled by the basal junctions at the ends of telodendrial processes.

Quantitative analysis of cone photoreceptor-horizontal cell connectivity patterns in the retina of a cyprinid fish: electron microscopy of functionally identified and HRP-labelled horizontal cells

Horizontal cells generating photopic luminosity and biphasic/chromaticity-type S-potentials were identified and intracellularly labelled with horseradish peroxidase in the retina of the roach. The synaptic connectivity patterns of the horizontal cell dendrites within pedicles of different spectral types of cone were then quantitatively studied by electron microscopy. Luminosity-type responses were generated by H1-like horizontal cells contacting similar numbers of red- and green-sensitive cones and very few blue-sensitive cones. Most dendritic contacts were lateral to synaptic ribbons. Central contacts with ribbons were made almost exclusively within red-sensitive cone pedicles. Biphasic/chromaticity-type S-potentials were generated by H2-like horizontal cells. The dendrites of the latter contacted green- and blue-sensitive cones, both at central and lateral sites at synaptic ribbons. An attempt was made to correlate cone ribbon connectivity patterns and spectral characteristics of the horizontal cells according to several hypotheses, some proposed in earlier studies.

Junctions form between catfish horizontal cells in culture

Cone horizontal cells from the catfish retina extend out processes after a few days in culture that sometimes contact adjacent cone horizontal cells. Two types of specialized junctions were observed by electron microscopy along the newly formed contact areas. One junctional type consisted of prominent electron-dense material along and just under the plasma membrane of one or both of the contacting elements. Sometimes vesicle clusters were associated with these junctions. The other type of junction showed some electron-dense material along the membranes of both processes and patchy areas of close membrane apposition resembling gap junctions. In about half of the cases tested, electrical coupling was detected between cone horizontal cells that had made contact in culture. In no case was the coupling as tight as is typically found between horizontal cells that had formed gap junctions in vivo.

Plasticity of cone horizontal cell functioning in cyprinid fish retina: effects of background illumination of moderate intensity

In electrophysiological experiments involving intracellular recording from horizontal cells in the isolated retina of the roach, light adaptation of the retina has been shown to result in potentiation both of (1) the depolarizing component of biphasic chromaticity type S-potentials, and (2) the temporal frequency transfer functions of photopic luminosity type horizontal cells. Under identical light adaptation conditions, the number of spinules on horizontal cell dendrites positioned laterally at cone pedicle ribbon synapses, increase by some threefold. The latter effect occurs equally in pedicles of red- and green-sensitive cones. Thus, horizontal cells are 'plastic' in both structural and electrophysiological respects. Furthermore, since the two electrophysiological parameters studied depend on negative feedback from horizontal cells onto cones, the results suggest that it is the inhibitory synapses that are plastic and that spinules may be sites of the negative feedback interaction. Physiological and behavioural aspects of light-dependent horizontal cell plasticity are also discussed.

Ultrastructural evidence that horizontal cell axon terminals are presynaptic in the human retina

The organization of the rod spherule and of the horizontal cell axon terminals within the invagination of the rod spherule in the human retina was examined in serial sections by electron microscopy. Twenty-one rod spherules were reconstructed in this study. Axon terminal processes of type I horizontal cells consistently make one or two small punctate synapses onto each rod spherule within the invagination. In addition, these axon terminal processes make distinct synapses upon rod bipolar dendrites outside the spherule before both processes enter the invagination. This is the first positive description of a synapse from a horizontal cell axon terminal process onto a photoreceptor terminal and the first identification of a synapse from a horizontal cell to a rod bipolar cell in the mammalian outer plexiform layer. We speculate that the axon terminal-to-rod synapse is responsible for feedback while the synapse upon the rod bipolar cell is feed-forward and serves to expand the receptive field of the rod bipolar cell beyond its dendritic field. Alternatively, the latter may contribute to a center-surround organization of the rod bipolar's receptive field.

Depolarization without calcium can release gamma-aminobutyric acid from a retinal neuron

Calcium influx is often an essential intermediate step for the release of neurotransmitter. However, some retinal neurons appear to release transmitter by a mechanism that does not require calcium influx. It was uncertain whether depolarization released calcium from an intracellular store or released transmitter by a mechanism that does not require calcium. The possibility that voltage, and not calcium, can regulate the release of transmitter was studied with pairs of solitary retinal neurons. Horizontal and bipolar cells were isolated from fish retinas and juxtaposed in culture. Communication between them was studied with electrophysiological methods. A horizontal cell released its neurotransmitter, gamma-aminobutyric acid, when depolarized during conditions that buffered the internal calcium concentration and prohibited calcium entry. The speed and amount of material released were sufficient for a contribution to synaptic transmission.

Synapses of cone horizontal cell axons in goldfish retina

The axon terminals of cone horizontal cells in the goldfish retina form typical chemical synaptic contacts in the middle of the inner nuclear layer. Approximately 60% of the identified postsynaptic elements were perikarya, axons and dendrites of bipolar cells. The other identified postsynaptic elements were perikarya and processes of interplexiform cells. We propose that the horizontal cell axon terminal contribute to the antagonistic surround responses of the bipolar cells and that they modulate inputs to the outer plexiform layer conveyed by interplexiform cells. Output synapses from horizontal cell axons to unidentified neuronal processes as well as occasional input synapses to the axons from interplexiform cell processes and unidentified perikarya were also observed in the same region of the inner nuclear layer.