Involvement of D1 and D2 Dopamine Receptors in the Control of Horizontal Cell Electrical Coupling in the Turtle Retina

We studied the actions of D1 and D2 dopamine agonists and antagonists on the coupling of horizontal cell axons in the turtle retina by a combination of pharmacological and electrophysiological methods. Both D1 and D2 receptors were identified in membrane fractions by radioligand binding using [3H]-SCH 23390 and [3H]-spiperone, respectively. The KD of both receptor classes were identical (0.21 nM) but D1 receptor density exceeded that of D2 receptors by more than four-fold. D1 agonists increased the activity of adenylate cyclase in a dose-dependent manner, whereas D2 agonists were without significant effect by themselves, nor did D2 antagonists block the D1-mediated increase in adenylate cyclase activity. Intracellular recordings and Lucifer Yellow dye injections were used to characterize the modifications of the receptive field profile of horizontal cell axons (H1AT) exposed to different pharmacological agents. Dopamine or D1 agonists (0.05 - 10 microM) induced a marked constriction of the H1AT receptive field, whereas D2 agonists elicited a small expansion of the receptive field. However, in the presence of a D1 antagonist, as well as IBMX to inhibit phosphodiesterase, D2 agonists (10 - 70 microM) induced a marked increase in the receptive field profile. These results indicate that both D1 and D2 dopamine receptors play a role in shaping the receptive field profile of the horizontal cell axon terminal in the turtle retina.     

Endogenous dopaminergic regulation of horizontal cell coupling in the mammalian retina

Horizontal cells in an isolated wholemount preparation of the mouse retina were injected with Lucifer yellow and neurobiotin to characterize both the pattern of gap junctional connectivity and its regulation by dopamine. The injected horizontal cells had a uniform morphology of a round cell body, a compact dendritic tree, and an axon, which could sometimes be traced to an expansive terminal system. The dendro-dendritic gap junctions between neighboring cells mediated both weak Lucifer yellow dye coupling and strong neurobiotin tracer coupling. The extent of the tracer coupling was decreased by either exogenous dopamine (100 microM) or cyclic adenosine monophosphate (cAMP) analogs and was significantly increased by the D1 antagonist SCH 23390 (10 microM). These results provide the first evidence in the mammalian retina that the gap junctions between horizontal cells are endogenously regulated by dopamine, which acts through D1 receptors to increase the intracellular cAMP. It has been proposed that the gap junctional coupling between horizontal cells is mediated by connexin 32 (Cx32), but the pattern and dopaminergic regulation of horizontal cell coupling were unaffected in Cx32-knockout mice, ruling out the possible involvement of Cx32. Every tracer-coupled horizontal cell showed calbindin immunoreactivity, and vice versa, providing strong evidence that the horizontal cells in the mouse retina comprise a single cell type. Like the axonless horizontal cells in other mammalian retinas, the axon-bearing horizontal cells in the mouse retina are coupled by gap junctions that are permeable to Lucifer yellow and dopamine sensitive, suggesting that the mouse horizontal cells have hybrid properties to compensate for the absence of axonless horizontal cells.     

Intracellular cyclic-amp suppresses the permeability of gap junctions between retinal amacrine cells

Gap junctions are intercellular channels composed of subunit protein connexin and subserve electrotonic transmission between connected neurons. Retinal amacrine cells, as well as horizontal cells of the same class, are homologously connected by gap junctions. The gap junctions between these neurons extend their receptive fields, and may increase the inhibitory postsynaptic effects in the retina. In the present study, we investigated whether gap junctions between the neurons are modulated by internal messengers. The permeability of gap junctions was examined by the diffusion of intracellularly injected biotinylated tracers, biocytin or Neurobiotin, into neighboring cells since gap junctions are permeable to these molecules freely. 4% Lucifer Yellow and 6% biocytin or Neurobiotin were injected intracellularly into horizontal cells and amacrine cells in isolated retinas of carp and goldfish and Japanese dace following electrophysiological identification. In the control condition, the tracer spread into many neighboring cells from the recorded cells. Superfusion of retinas with dopamine (100 microM) suppressed diffusion of the tracer into the neighboring horizontal cells, but not in the case of amacrine cells. Intracellular injection of cyclic AMP (300 mM) completely blocked diffusion of the tracer into neighboring horizontal cells and amacrine cells. However, superfusion of retinas with 8-bromo-cyclic AMP (2 mM), membrane permeable cyclic AMP analog, permitted the tracer to diffuse into the neighboring horizontal cells or amacrine cells. Intracellular injection of cyclic GMP (300 mM) blocked the diffusion between neighboring horizontal cells, but did not suppress the diffusion between amacrine cells. These results show that the permeability of gap junctions between amacrine cells is regulated by high concentration of intracellular cyclic AMP level, but not for intracellular cyclic GMP or applied dopamine or extracellularly applied low concentrations of intracellular cyclic AMP level. The present study suggests that these laterally oriented inhibitory interneurons, horizontal cells and amacrine cells, express different connexins which may be differentially regulated by intercellular messengers.     

Local nonuniformities in the syncytium of the horizontal cells of the retina in the turtle

Electrical coupling between horizontal cells of the turtle retina was investigated by means of two microelectrodes (current and recording ones) penetrating neighbouring cells at a fixed distance from each other. The morphological coupling was revealed by means of fluorescent dye Lucifer Yellow. The electrical coupling was confirmed between elements of similar type (L1--axonal terminals, or L2--cell bodies, or R/G type cells) and no coupling was found between elements of different types, though L1 and L2 are directly connected through thin axons. In the L1 syncytium the electrical coupling at small (less than or equal to 50 microns) but fixed distances between microelectrodes could differ several times depending on the minimal displacement of microelectrodes. This local nonuniformity of coupling can be explained on the basis of structural nonuniformities in the L1 (axon terminal) network. It is unlikely however that the structural nonuniformities can influence the functional properties of horizontal cell network when the retina is stimulated adequately (by light).     

Dopaminergic mechanisms underlying the reduction of electrical coupling between horizontal cells of the turtle retina induced by d-amphetamine, bicuculline, and veratridine

Previous studies have shown that dopamine, bicuculline, or d-amphetamine reduce the electrical and dye-coupling between the axon terminals of the horizontal cells of the turtle retina (see Piccolino et al., 1984). In the present study we observed similar effects following the application of veratridine. The actions of all these drugs were prevented by dopamine antagonists acting on D1 receptors such as flupenthixol and SCH 23390. However, in contrast to dopamine, the actions of d-amphetamine, bicuculline, and veratridine were attenuated or abolished by pharmacological agents (such as 6-OH-dopamine, alpha-methyl-p-tyrosine, or reserpine) known to reduce the release of dopamine from dopaminergic neurons. Moreover, the actions of veratridine and bicuculline were prevented by tetrodotoxin, indicating that one or more neurons in the dopamine pathway are spike-generating. We conclude that d-amphetamine, bicuculline, and veratridine reduce electrical coupling between the axon terminals of the turtle horizontal cells by promoting the release of endogenous dopamine from the dopaminergic amacrine cells previously identified (Witkovsky et al., 1984). Electron-microscopic observations revealed that 6-OH-dopamine selectively attacked this population of amacrine cells. No degenerating terminals were found adjacent to the horizontal cell axon terminals. On this basis, we postulate that dopamine reaches the horizontal cell by diffusion through the extracellular space.     

Connexin50 couples axon terminals of mouse horizontal cells by homotypic gap junctions

Horizontal cells in the mouse retina are of the axon‐bearing B‐type and contribute to the gain control of photoreceptors and to the center‐surround organization of bipolar cells by providing feedback and feedforward signals to photoreceptors and bipolar cells, respectively. Horizontal cells form two independent networks, coupled by dendro‐dendritic and axo‐axonal gap junctions composed of connexin57 (Cx57). In Cx57‐deficient mice, occasionally the residual tracer coupling of horizontal cell somata was observed. Also, negative feedback from horizontal cells to photoreceptors, potentially mediated by connexin hemichannels, appeared unaffected. These results point to the expression of a second connexin in mouse horizontal cells. We investigated the expression of Cx50, which was recently identified in axonless A‐type horizontal cells of the rabbit retina. In the mouse retina, Cx50‐immunoreactive puncta were predominantly localized on large axon terminals of horizontal cells. Electron microscopy did not reveal any Cx50‐immunolabeling at the membrane of horizontal cell tips invaginating photoreceptor terminals, ruling out the involvement of Cx50 in negative feedback. Moreover, Cx50 colocalized only rarely with Cx57 on horizontal cell processes, indicating that both connexins form homotypic rather than heterotypic or heteromeric gap junctions. To check whether the expression of Cx50 is changed when Cx57 is lacking, we compared the Cx50 expression in wildtype and Cx57‐deficient mice. However, Cx50 expression was unaffected in Cx57‐deficient mice. In summary, our results indicate that horizontal cell axon terminals form two independent sets of homotypic gap junctions, a feature which might be important for light adaptation in the retina. J. Comp. Neurol. 523:2062–2081, 2015. © 2015 Wiley Periodicals, Inc.     

Gap junctions among the perikarya, dendrites, and axon terminals of the luminosity-type horizontal cell of the turtle retina

Gap junctions of the H1 horizontal cell of the turtle retina (Leeper, '78) were studied in thin-sectioned material and in freeze-fracture replicas. Perikaryal gap junctions were extremely restricted, 0.02-0.07 micron2 in in area, whereas those of axon terminals were much larger, most being 0.1-1.0 micron2. Both varieties, however, had the usual seven-layered appearance in thin section and measured 15 +/- 1 nm in overall width between cytoplasmic faces. Freeze-fractured views of the perikaryal junctions revealed roughly circular patches of P-face 9-nm particles and E-face pits. The axon terminal gap junctions were seen as large areas of P-face particles and E-face pits containing occasional islands of unspecialized membrane. Particle densities varied from 1,455 to 2,448 microns-2. A serial reconstruction was made of a portion of the axon terminal network in order to measure the surface areas of the axons contained therein and the fraction occupied by gap junctions. These data demonstrated that the fractional area occupied by gap junctions was roughly in inverse proportion to the area of the axon region (tuberous core vs. terminal process). It is argued that this constitutes an impedance matching device to ensure adequate current flow through the axon processes. Assuming that each P-face particle represents a connection having a conductance of 10(-10) S and given the P-face particle density and gap junctional areas determined in this report, we calculated that the gap junction distribution is adequate to account for the spatial properties of the horizontal cell axon network (Lamb, '76).     

Renewal of electrotonic synapses in teleost retinal horizontal cells

In teleost retinas, the somata of same-type cone horizontal cells are electrically coupled via extensive gap junctions, as are the axon terminals of same-type cells. This coupling persists throughout the animal's life and is modulated by dopamine and conditions of light- vs. dark-adaptation. Gap junction particle density in goldfish horizontal cell somata has also been shown to change under these conditions, indicating that these junctions are dynamic. We have used electron microscopy to examine gap junctions in bass horizontal cells with a fixation method that facilitates detection of gap junctions. Annular gap junction profiles were observed in the somatic cytoplasm of all cone horizontal cell types in both light- and dark-adapted animals. Serial sections showed that most profiles represented gap junction vesicles free within the cytoplasm; the remainder represented vesicles still attached to extensive plasma membrane gap junctions by a thin cytoplasmic neck, suggestive of an intermediate stage in endocytosis. Observations of gap junction vesicles containing fragments of gap junctional membrane and/or fused with lysosomal bodies further supported this hypothesis. Because gap junctions persist between the horizontal cells, we propose that gap junction endocytosis and lysosomal degradation are balanced by addition of new junctions. While endocytosis has been widely demonstrated to serve in programmed removal of gap junctions (without subsequent replacement), from both nonneuronal cells and developing neurons, this study indicates that it can also function in the renewal of electrical synapses in the adult teleost retina, where gap junction elimination is not the goal.     

Gap junction particle density of horizontal cells in goldfish retinas lesioned with 6-OHDA

The I1 dopaminergic interplexiform cells of the fish retina are believed to modulate horizontal cell coupling by increasing gap junction resistance. Dopamine also modulates the morphology of horizontal cell gap junctions and mimics the effects of light adaptation. To determine whether the light-dependent changes in gap junction morphology are due to endogenous dopamine release, horizontal cell gap junctions were studied in goldfish retinas lacking dopaminergic neurons. Dopaminergic interplexiform cells were destroyed by intraocular injections of 6-hydroxydopamine in both eyes. After lesioning, fish were treated in one of four ways: (1) light-adapted, (2) dark-adapted (1 hour), (3) light-adapted and given an intraocular injection of dopamine, or (4) dark-adapted (1 hour) and injected with dopamine. The effectiveness of lesioning was evaluated by autoradiographic detection of [3H]-dopamine uptake in the retina of one eye. Retinas in which lesioning of the contralateral eye was deemed effective were processed for freeze-fracture electron microscopy and the particle density of horizontal cell gap junctions determined. Lesioned retinas, whether light- or dark-adapted, had elevated horizontal cell soma gap junction particle densities compared to lesioned retinas treated with dopamine. These results demonstrate that high soma gap junction particle densities can be correlated with the absence of dopamine and low densities associated with the presence of dopamine. The differences in gap junction particle density between lesioned and lesioned + dopamine-treatment were similar to differences between nonlesioned dark-adapted (1 hour) and light-adapted retinas, respectively. Therefore, the particle density of light- and dark-adapted soma gap junctions suggests a greater release of dopamine in light-adapted fish than in 1 hour dark-adapted fish.(ABSTRACT TRUNCATED AT 250 WORDS)     

Choline acetyltransferase is found in terminals of horizontal cells that label with GABA, nitric oxide synthase and calcium binding proteins in the turtle retina

In this study, we discriminated the various types of horizontal cell in the turtle retina on their content of neuroactive substances. Double label immunocytochemistry was performed on sectioned and wholemount retina using antisera to neural- and endothelial-nitric oxide synthase (nNOS, and eNOS), calretinin (CR), calbindin (CB), gamma-aminobutyric acid (GABA) and choline acetyltransferase (ChAT). H1 cells and their axon terminals label with CR, CB and GABA. Only H1 axon terminals label with eNOS. H2 cells contain CB, CR, nNOS and GABA maybe in their dendrites. H3 cells label only with nNOS. The localization of nNOS in the H2 and H3 cells is a novel finding. None of these antibodies labels H4 cells. The photoreceptor subtypes have been differentiated by different intensity of labeling with CB. The accessory member of the double cone is less intensely labeled with CB than the principal member and rods and blue cones do not appear to label at all. ChAT-IR is located in terminal boutons of H1 and H2 horizontal cells and H1 axon terminals and these boutons contact rods and all spectral types of cones. Clearly, GABA is present in H1 horizontal cells and may be used in neurotransmission between horizontal cells and possibly for feedback pathways to photoreceptors. The evidence of nNOS immunoreactivity in H2 and H3 horizontal cells, combined with available physiological evidence, suggests that NO may be involved in electrical coupling and/or modulation of synaptic input to these types of cells. Furthermore, our results raise the possibility that cholinergic synaptic transmission may occur from horizontal cell processes to photoreceptors in the outer plexiform layer of the turtle retina.     

Dopaminergic regulation of horizontal cell gap junction particle density in goldfish retina

Light- or dark-adapted goldfish (Carassius auratus) retinas were treated with dopamine, which is believed to uncouple horizontal cells via D1 receptors, or with the dopamine antagonist haloperidol. Aldehyde-fixed retinas were freeze-fractured and the replicas examined by electron microscopy to identify horizontal gap junctions. The density (number per micron2) of intra-membrane particles of horizontal cell soma gap junctions was significantly lower in light-adapted and dopamine-treated retinas than in dark-adapted and haloperidol-treated retinas. There was no statistically significant difference between gap junction particles densities in (I) light-adapted (untreated) and in dopamine-treated (light- or dark-adapted) retinas, or between (II) dark-adapted (untreated) and haloperidol-treated (light- or dark-adapted). These results suggest that the uncoupling of horizontal cell somas by dopamine is accompanied by a decrease in gap junction particle density and that there is a greater release of dopamine during light-adaptation than dark-adaptation. Unlike horizontal cell somas, horizontal cell axon terminals did not show consistent changes in gap junction particle density with light- or dark-adaptation. Although the data suggests that there may be a reduction in axon terminal gap junction particle density with dopamine treatment, this effect is not reversible with haloperidol treatment. Our results suggest that the regulation of gap junctions may differ at two sites within the same cell.     

Connexin57 is expressed in dendro‐dendritic and axo‐axonal gap junctions of mouse horizontal cells and its distribution is modulated by light

Mouse horizontal cells are coupled by gap junctions composed of connexin57. These gap junctions are regulated by ambient light via multiple neuromodulators including dopamine. In order to analyze the distribution and structure of horizontal cell gap junctions in the mouse retina, and examine the effects of light adaptation on gap junction density, we developed antibodies that detect mouse retinal connexin57. Using immunohistochemistry in retinal slices, flat‐mounted retinas, and dissociated retinal cells, we showed that connexin57 is expressed in the dendrites and axon terminal processes of mouse horizontal cells. No staining was found in retinas of connexin57‐deficient mice. Significantly more connexin57‐positive puncta were found in the distal than in the proximal outer plexiform layer, indicating a higher level of expression in axon terminal processes than in the dendrites. We also examined the gap junctions using immunoelectron microscopy and showed that connexin57 does not form hemichannels in the horizontal cell dendritic tips. Light adaptation resulted in a significant increase in the number of connexin57‐immunoreactive plaques in the outer plexiform layer, consistent with previously reported effects of light adaptation on connexin57 expression in the mouse retina. This study shows for the first time the detailed location of connexin57 expression within mouse horizontal cells, and provides the first ultrastructural data on mouse horizontal cell gap junctions. J. Comp. Neurol. 513:363–374, 2009. © 2009 Wiley‐Liss, Inc.     

Horizontal cell receptive fields are reduced in connexin57-deficient mice

Horizontal cells are coupled by gap junctions; the extensive coupling of the horizontal cells is reflected in their large receptive fields, which extend far beyond the dendritic arbor of the individual cell. In the mouse retina, horizontal cells express connexin57 (Cx57). Tracer coupling of horizontal cells is impaired in Cx57-deficient mice, which suggests that the receptive fields of Cx57-deficient horizontal cells might be similarly reduced. To test this hypothesis we measured the receptive fields of horizontal cells from wildtype and Cx57-deficient mice. First, we examined the synaptic connections between horizontal cells and photoreceptors: no major morphological alterations were found. Moreover, horizontal cell spacing and dendritic field size were unaffected by Cx57 deletion. We used intracellular recordings to characterize horizontal cell receptive fields. Length constants were computed for each cell using the cell's responses to concentric light spots of increasing diameter. The length constant was dependent on the intensity of the stimulus: increasing stimulus intensity reduced the length constant. Deletion of Cx57 significantly reduced horizontal cell receptive field size. Dark resting potentials were strongly depolarized and response amplitudes reduced in Cx57-deficient horizontal cells compared to the wildtype, suggesting an altered input resistance. This was confirmed by patch-clamp recordings from dissociated horizontal cells; mean input resistance of Cx57-deficient horizontal cells was 27% lower than that of wildtype cells. These data thus provide the first quantification of mouse horizontal cell receptive field size and confirm the unique role of Cx57 in horizontal cell coupling and physiology.     

Electrical coupling, receptive fields, and relative rod/cone inputs of horizontal cells in the tiger salamander retina

Light responses, dendritic/axonal morphology, receptive field diameters, patterns of dye coupling, and relative rod/cone inputs of various types of horizontal cells (HCs) were studied using intracellular recording and Lucifer yellow/neurobiotin dye injection methods in the flatmount tiger salamander retina. Three physiologically and morphologically distinct types of HC entities were identified. 1) The A-type HCs are somas that do not bear axons, with average (+/-SE) soma diameters of 20.01 +/- 0.59 microm, relatively sparse and thick dendrites, and they resemble the A-type HC in mammals. The average receptive field diameter of these cells is 529.6 +/- 10.87 microm and they receive inputs predominantly from cones. 2) The B-type HCs are broad-field somas that bear thin and long axons, with average soma diameters of 17.67 +/- 0.38 microm, thinner dendrites of higher density, and they resemble the B-type HC in mammals. The average receptive field diameter of these cells is 1,633.55 +/- 37.34 microm and they receive mixed inputs from rods and cones. 3) The B-type HC axon terminals are broad-field, coarse axon terminal processes and they resemble the B-type HC axon terminal in rabbits. The average receptive field diameter of these axon terminals is 1,291.67 +/- 24.02 microm and they receive mixed inputs from rods and cones. All these types of HC are dye-coupled with adjacent HCs of the same type. Additionally, B-type HCs and axon terminals are dye-coupled with subpopulations of bipolar cells whose axon terminals ramify in the proximal half of the inner plexiform layer, raising the possibility that these HCs may send feedforward antagonistic surround responses to depolarizing bipolar cells through electrical synapses.     

Dark- and light-induced changes in coupling between horizontal cells in mammalian retina

Retinal horizontal cells exhibit large receptive fields derived from their extensive electrical coupling by means of gap junctions. The conductance of these gap junctions seems to be regulated by dopamine acting through a cAMP-mediated cascade. There is now abundant evidence that extracellular dopamine levels vary with changes in ambient light intensity, suggesting that changes in the dark/light adaptational state of the retina can modulate coupling between horizontal cells. We studied this question in the mammalian retina by determining the effects of ambient light levels, in the form of changing background light intensity, on the coupling profiles of A- and B-type horizontal cells in the rabbit. Changes in coupling were assessed by measurements of the space constants of the syncytium formed by horizontal cells and the intercellular spread of the biotinylated tracer Neurobiotin. Our results indicate that dark-adapted horizontal cells show relatively weak coupling. However, presentation of background lights as dim as one-quarter log unit above rod threshold resulted in increases in both the averaged extent of tracer coupling and space constants of A- and B-type horizontal cells. Coupling expanded further as background light intensities were increased by 1-1.5 log units, after which additional light adaptation brought about an uncoupling of cells. Coupling reached its minimum at light intensities about 3 log units above rod threshold, after which, with further light adaptation, it stabilized at levels close to those seen in dark-adapted retinas. Our results indicate that electrical coupling between mammalian horizontal cells is modulated dramatically by changes in the adaptational state of the retina: coupling is maximized under dim ambient light conditions and diminishes as the retina is dark or light adapted from this level.     

Light- and dopamine-regulated receptive field plasticity in primate horizontal cells

Center-surround antagonistic receptive fields (CSARFs) are building blocks for spatial vision and contrast perception. Retinal horizontal cells (HCs) are the first lateral elements along the visual pathway, and are thought to contribute to receptive field surrounds of higher order neurons. Primate HC receptive fields have not been found to change with light, and dopaminergic modulation has not been investigated. Recording intracellularly from HCs in dark-adapted macaque retina, we found that H1-HCs had large receptive fields (λ = 1,158 ± 137 μm) that were reduced by background light (-45%), gap junction closure (-53%), and D1 dopamine receptor activation (-48%). Tracer coupling was modulated in a correlative manner, suggesting that coupling resistance plays a dominant role in receptive field formation under low light conditions. The D1 antagonist SCH23390 increased the size of receptive fields (+13%), suggesting tonic dopamine release in the dark. Because light elevates dopamine release in primate retina, our results support a dopaminergic role in post-receptoral light adaptation by decreasing HC receptive field diameters, which influences the center-surround receptive field organization of higher-order neurons and thereby spatial contrast sensitivity.     

Axons connecting somata and axon terminals of luminosity-type horizontal cells in the turtle retina: receptive field studies and intracellular injections of HRP

Luminosity (L)-type horizontal cell responses to small and large spots of light in the turtle (Geoclemys) retina were recorded intracellularly, and each cell so studied was filled with horseradish peroxidase (HRP) by ionophoretic injection. Following histochemical reaction of the HRP with diaminobenzidine (DAB), light microscopic inspection demonstrated that this type of horizontal cell has a cell body connected to a tuberous axon terminal by a fine axon. By examining the location of the cell in relation to the linear array of electrode penetrations in each preparation, it was possible to determine whether a given recording was made in the cell body or in the axon terminal. The size of the receptive field of the cell body was significantly smaller than that of the axon terminals. Although the center-to-center distances between the cell body and the axon terminal of some L-type horizontal cells were as short as 50 micron, the length of the intervening axon was invariably longer and was at least 250 micron. Unless signals are amplified by some active mechanisms during their propagation along the axon, it is unlikely that current generated in the cell body spreads to the axon terminal through the thin axon, or in the opposite direction. The difference in receptive field sizes of cell body and of axon terminal may therefore indicate that these two structural subunits are electrically isolated.     

Spatial orientation of horizontal cell axon terminals in the carp retina

In flatmounts of the carp (Cyprinus carpio) retina, 646 horizontal cells were singly marked by intracellular Lucifer yellow CH (LY) in the presence of dopamine or amphetamine, agents which were useful for restricting LY to single injected cells. Most axon terminals of cone-connected horizontal cells have a tendency to orient either radially or tangentially in the retinal field with respect to the optic disc as a center. Although the fluorescent cellular (dendritic field) area greatly varied depending upon the cell type (L-, RG- and YRB-type), the lengths of the axonal processes (axon plus terminal) were all comparable (400-600 microns). A few cells (4.1% of cells with visible axons) possessed a bifurcate axon with two axon terminals. Axons were not observed on rod-connected horizontal cells. The cellular area and the axonal length of L- and RG-type cells appeared to be smaller in the central than in the intermediate region of the retinal field.     

Modulation of cone horizontal cell activity in the teleost fish retina. III. Effects of prolonged darkness and dopamine on electrical coupling between horizontal cells

The effects of prolonged darkness and dopamine on the coupling between horizontal cells in the isolated, superfused white perch retina were studied. Two assays of coupling were employed; area versus amplitude relationships (area-response curves) and the diffusion of the fluorescent dye Lucifer yellow from intracellularly injected cells to neighboring cells. In prolonged dark-adapted retinas, area-response curves were difficult to determine because of the small light responses; however, light-evoked responses did not increase in size when light spots were larger than 0.8 mm in diameter. Following the presentation of dim background illumination that partially sensitized the retina, responses to light spots of various sizes were enhanced and an area-response curve could be constructed. Subsequent presentation of moderate background illumination that more fully sensitized the retina resulted in reduced responses to small spots (less than 1.6 mm in diameter) and enhanced responses to large spot or full-field stimuli. In retinas exposed to moderate background illumination, Lucifer yellow injected intracellularly into cone horizontal cells diffused into many neighboring horizontal cells. The coupled cells were very similar in morphology, suggesting they were of the same type. In prolonged dark-adapted retinas, on the other hand, the dye was usually restricted to the injected cell and a few adjacent cells. These results indicate that coupling between cone horizontal cells is modulated by prolonged darkness and background illumination. Following dopamine (50 microM) application, in both 6-OHDA-treated and untreated retinas, changes in area-response curves of cone horizontal cells were observed just opposite to those that occurred when prolonged dark-adapted retinas were exposed to background illumination. That is, following 5 min application of dopamine to the retina, responses to small spots (less than 2 mm in diameter) increased in size while responses to larger spots decreased in amplitude compared with control responses. Following 20 min of superfusion with dopamine, the recorded responses were very small, and an accurate area-response curve could not be determined. Following dopamine application to light-sensitized retinas, Lucifer yellow was restricted to the injected cells or to the injected cell and a few neighboring cells. The results suggest that the modulation of coupling between cone horizontal cells by prolonged darkness and background illumination may be mediated by dopamine. Spatial properties of rod horizontal cells were also examined.(ABSTRACT TRUNCATED AT 400 WORDS)