The synaptic organization of the dopaminergic amacrine cell in the cat retina

Summary The dopaminergic amacrine cells of the cat retina have been stained by immunocytochemistry using an antibody to tyrosine hydroxylase (Toh). The complete population of Toh+cells has been studied by light microscopy of retinal wholemounts to evaluate morphological details of dendritic structure and branching patterns. Selected Toh+amacrine cells have been studied by serial-section electron microscopy to analyse synaptic input and output relationships. The majority of Toh+amacrine cells occur in the amacrine cell layer of the retina and have their dendrites ramifying and forming the characteristic rings in stratum 1 of the inner plexiform layer. A minority of Toh+cells have cell bodies displaced to the ganglion cell layer but their dendrites also stratify in stratum 1. All Toh+cells have some dendritic branches running in stratum 2 as well as in stratum 1, and frequently they have long axon-like processes (500–1000 m long) dipping down to run in stratum 5 before passing up to rejoin the major dendritic arbors in stratum 1. In addition Toh+stained processes follow blood vessels in the inner plexiform layer and in the ganglion cell layer. A population of Toh+cells found in the inferior retina appears to give rise to stained processes that pass to the outer plexiform layer and therein to run for as far as one millimeter.Electron microscopy reveals that Toh+amacrine cells are postsynaptic to amacrine cells and a few bipolar cell terminals in stratum 1 of the inner plexiform layer and are primarily presynaptic to All amacrine cell bodies and lobular appendages, and to another type of amacrine cell body and amacrine dendrites hypothesized to be the A17 amacrine cell. The Toh+dendrites in stratum 2 are presynaptic to All lobular appendages primarily. Stained axon-like processes running in stratum 5 prove to be presynaptic to All amacrine dendrites as they approach the rod bipolar axon terminals and they may also be presynaptic to the rod bipolar terminal itself. The Toh+stained dendrites that have been followed in the outer plexiform layer run along the top of the B-type horizontal cell somata and may have small synapses upon them. The only clear synapses seen in the outer plexiform layer are from the Toh+profiles upon vesicle filled amacrine-like profiles that are in turn presynaptic to bipolar cell dendrites in the outer plexiform layer. We presume the cells postsynaptic to the Toh+dendrites in the outer plexiform layer are interplexiform cells. Finally the Toh+profiles that course along blood vessel walls and in the ganglion cell layer appear to end either against the basal lamina of the blood vessel or at intercellular channels of vesicle-laden Muller cell end-feet.     

Light and electron microscopic observations on the inner plexiform layer of the rabbit retina

By comparison of electron micrographs with light microscopical specimens impregnated with the Golgi technique, the large endings of the rod bipolar cells have been identified in the innermost region of the inner plexiform layer of the rabbit retina. The rod bipolar endings contain ribbons and synaptic vesicles, do not synapse with the perikaryon of the ganglion cells, are presynaptic to ganglion cell dendrites and to nerve processes which contain synaptic vesicles but lack ribbons. In these synaptic contacts a ribbon is closely associated with the presynaptic membrane and a dense web of fuzzy material is adherent to the cytoplasmic aspect of the postsynaptic membrane. Commonly, one of these synaptic contacts involves a rod bipolar ending and two postsynaptic processes. The postsynaptic process which is provided with synaptic vesicles is often, in turn, presynaptic to the same rod bipolar ending. This synaptic contact is characterized by the presence of a cluster of vesicles closely related to the presynaptic membrane, whereas the postsynaptic membrane lacks a definite subsynaptic web. In the intermediate and scleral regions of the inner plexiform layer endings containing ribbons and synaptic vesicles show with neighboring nerve processes a synaptic pattern similar to the rod bipolar endings. Nerve processes containing synaptic vesicles but lacking ribbons are presynaptic to the perikaryon and dendrites of the ganglion cells; the synaptic contact shows a cluster of vesicles adherent to the presynaptic membrane. Bipolar cells are proposed as the source of the ribbon containing processes while amacrine cells are proposed as the source of the processes devoid of ribbons and presynaptic to both bipolar endings and ganglion cell dendrites and perikarya.     

Localization of glycine uptake and receptors in the cat retina

Immunocytochemistry using a monoclonal antibody against glycine receptors revealed that these receptors in cat retina are confined to the inner plexiform layer (IPL). The outer half of that layer showed strong patchy labelling with some indication of two bands corresponding to the on- and off-sublamina. High-affinity uptake of [3H]glycine followed by autoradiography labelled amacrine cells, bipolar cells and the outer portion of the IPL. Silver grains in the IPL were patchily distributed. These results indicate in the cat retina a close match between presynaptic glycinergic elements labelled by high-affinity uptake and the postsynaptic receptor sites for glycine revealed by immunocytochemistry.     

Glycinergic synaptic transmission to bullfrog retinal bipolar cells is input-specific

Glycinergic inhibitory postsynaptic currents (IPSCs) focally elicited at the dendrites and axon terminals were recorded from bipolar cells in the bullfrog retinal slice, using the whole-cell clamp technique. IPSCs driven by input from interplexiform cells at bipolar cell dendrites (ipc-IPSCs) had a much slower decay time constant (25.2 +/- 7.8 ms) than IPSCs driven by input from amacrine cells at bipolar cell axon terminals (ac-IPSCs) (14.7 +/- 5.5 ms). Furthermore, peak-scaled non-stationary noise analysis revealed that the weighted mean single-channel conductance of the glycine receptors underlying bipolar cell dendritic ipc-IPSCs (20.8 +/- 6.6 pS) was significantly larger than that of those underlying bipolar cell axon terminal ac-IPSCs (12.9 +/- 2.9 pS). These results demonstrate that glycinergic synaptic transmission with different properties at bipolar cell dendrites and axon terminals differentially mediates intraretinal centrofugal signal transfer from the inner retina to the outer retina provided by interplexiform cells and lateral inhibition offered by amacrine cells in the inner retina.     

Neuronal expression of P2X3 purinoceptors in the rat retina

P2X3 purinoceptors are involved in fast, excitatory neurotransmission in the nervous system, and are expressed predominantly within sensory neurons. In this study, we examined the cellular and synaptic localization of the P2X3 receptor subunit in the retina of the rat using immunofluorescence immunohistochemistry and pre-embedding immunoelectron microscopy. In addition, we investigated the activity of ecto-ATPases in the inner retina using an enzyme cytochemical method. The P2X3 receptor subunit was expressed in the soma of a subset of GABA immunoreactive amacrine cells, some of which also expressed protein kinase C-alpha. In addition, punctate immunoreactivity was observed within both the inner and outer plexiform layers of the retina. Double labeling studies showed that P2X3 receptor puncta were associated with both rod and cone bipolar cell axon terminals in the inner plexiform layer. Ultrastructural studies indicated that P2X3 receptor subunits were expressed on putative A17 amacrine cells at sites of reciprocal synaptic input to the rod bipolar cell axon terminal. Moreover, we observed P2X3 immunolabeling on amacrine cell processes that were associated with cone bipolar cell axon terminals and other conventional synapses. In the outer retina, P2X3 immunoreactivity was observed on specialized junctions made by putative interplexiform cells. Ecto-ATPase activity was localized to the inner plexiform layer on the extracellular side of all plasma membranes, but was not apparent in the ganglion cell layer or the inner nuclear layer, suggesting that ATP dephosphorylation occurs exclusively in synaptic regions of the inner retina. These data provide further evidence that purines participate in retinal transmission, particularly within the rod pathway.     

Spike-dependent GABA inputs to bipolar cell axon terminals contribute to lateral inhibition of retinal ganglion cells

The inhibitory surround signal in retinal ganglion cells is usually attributed to lateral horizontal cell signaling in the outer plexiform layer (OPL). However, recent evidence suggests that lateral inhibition at the inner plexiform layer (IPL) also contributes to the ganglion cell receptive field surround. Although amacrine cell input to ganglion cells mediates a component of this lateral inhibition, it is not known if presynaptic inhibition to bipolar cell terminals also contributes to surround signaling. We investigated the role of presynaptic inhibition by recording from bipolar cells in the salamander retinal slice. TTX reduced light-evoked GABAergic inhibitory postsynaptic currents (IPSCs) in bipolar cells, indicating that presynaptic pathways mediate lateral inhibition in the IPL. Photoreceptor and bipolar cell synaptic transmission were unaffected by TTX, indicating that its main effect was in the IPL. To rule out indirect actions of TTX, we bypassed lateral signaling in the outer retina by either electrically stimulating bipolar cells or by puffing kainate (KA) directly onto amacrine cell processes lateral to the recorded cell. In bipolar and ganglion cells, TTX suppressed laterally evoked IPSCs, demonstrating that both pre- and postsynaptic lateral signaling in the IPL depended on action potentials. By contrast, locally evoked IPSCs in both cell types were only weakly suppressed by TTX, indicating that local inhibition was not as dependent on action potentials. Our results show a TTX-sensitive lateral inhibitory input to bipolar cell terminals, which acts in concert with direct lateral inhibition to give rise to the GABAergic surround in ganglion cells.     

Expression of the neurokinin 1 receptor in the rabbit retina

Substance P is the preferred ligand for the neurokinin 1 (NK1) receptor. In vertebrate retinas, substance P is expressed by amacrine, interplexiform and ganglion cells. Substance P influences the activity of amacrine and ganglion cells and it is reported to evoke dopamine release. We investigated NK1 receptor expression in the rabbit retina using affinity-purified NK1 receptor antibodies. NK1 receptors were expressed by two distinct populations of retinal neurons. One is a population of ON-type bipolar cells characterized by axonal arborizations that ramified in the inner plexiform layer near the ganglion cell layer. Double-label studies showed that NK1 receptor-expressing bipolar cells were distinct from rod bipolar cells and from other immunocytochemically identified types of cone bipolar cells. Their density was about 2250 cells/mm2 in the visual streak and 1115 cells/mm2 in ventral mid-periphery. They were distributed in a non-random pattern. In the outer plexiform layer, the dendrites of these bipolar cells converged into heavily immunostained clusters having a punctate appearance. The density of these clusters in mid-peripheral ventral regions (about 13000 clusters/mm2) was similar to the reported cone density [Famiglietti and Sharpe (1995) Vis. Neurosci. 12, 1151-1175], suggesting these dendrites contact all cone photoreceptors. The second NK1 receptor expressing cell population corresponds to the tyrosine hydroxylase-containing amacrine cell population. NK1 receptor immunostaining was localized to the cell body and processes, but not to the processes that form the 'rings' that are known to encircle somata of AII amacrine cells. These findings show that NK1 receptor immunoreactivity is localized to a population of ON-type cone bipolar cells and to dopaminergic amacrine cells, suggesting that substance P acting on NK1 receptors influences multiple retinal circuits in the rabbit retina.     

Histology of the ferret retina

The retina of the adult ferret, Mustelo furo, was studied with light and transmission electron microscopy to provide an anatomical basis for use of the ferret as a model for retinal research. The pigment epithelium is a simple cuboidal layer of cells characterized by a zone of basal folds, apical microvilli, and pigment granules at various stages of maturation. The distinction between rod and cone photoreceptor cells is based on their location, morphology, heterochromatin pattern and the electron density of their inner segments. The round, light-staining cone cell nuclei occupy the layer of perikarya along the apical border of the outer nuclear layer. The remainder of the outer nuclear layer consists of oblong, deeply-stained rod cell nuclei. Ribbon type synaptic complexes involving photoreceptor cell axons, horizontal cell processes, and bipolar cell dendrites characterize the outer plexiform layer. The inner nuclear layer is comprised of horizontal, bipolar, and amacrine cell perikarya as well as the perikarya of the Müller cells. The light-staining horizontal cell nuclei are prominent along the apical border of the inner nuclear layer. The light-staining amacrine cell nuclei form a more or less continuous layer along the basal border of the inner nuclear layer. Both conventional and ribbon-type synapses characterize the inner plexiform layer. The ganglion cells form a single cell layer. The optic fiber layer contains bundles of axons surrounded by Müller cell processes. Small blood vessels and capillaries are present in the basal portion of the retina throughout the region extending from the internal limiting membrane to the outer plexiform layer. The adult one-year-old retina is compared with the retina at the time of eye opening.     

Tyrosine hydroxylase immunoreactive interplexiform cells in the lamprey retina

The distribution of tyrosine hydroxylase immunoreactivity was investigated in retinae of metamorphic, postmetamorphic and adult lampreys. Immunoreactive cell bodies were located mainly in the innermost part of the inner nuclear layer, with a few cells scattered throughout the inner plexiform layer. The processes of these neurons ran preferentially in the inner plexiform layer. Additionally, dense plexus of labelled processes were observed in the outer plexiform and nuclear layers. These findings suggest that most of the tyrosine hydroxylase-immunoreactive cells in the lamprey retina are interplexiform cells.     

Electron microscopy of glutamate decarboxylase (GAD) immunoreactivity in the inner plexiform layer of the rhesus monkey retina

Summary With indirect immunofluorescence, glutamate decarboxylase (GAD), the GABA synthesizing enzyme, was localized to cell bodies in the inner half of the inner nuclear layer and a few in the outer tier of the ganglion cell layer in the rhesus monkey retina. In the inner plexiform layer there were three strongly GAD-immunoreactive laminae separated by two less immunoreactive laminae. Electron microscopy demonstrated that the GAD was contained in amacrine cells and these GAD-immunoreactive amacrines were primarily pre- and postsynaptic to biopolar cell axon terminals. The GAD-containing processes possessed small synaptic vesicles and formed synapses that could be characterized as symmetrical. Large, dense-cored vesicles were often found in the cell bodies and synaptic processes of the GAD-immunoreactive amacrine cells. As the vast majority of the synaptic input and output of the GAD-containing amacrine cells was to and from bipolar cells and the strongest GAD-immunoreactivity correlated with the endings of bipolar cells that connect with a single cone, the functional effects of GABA in the primate retina are likely to be found in the responses of single cone pathways in the inner plexiform layer.     

Synaptic organization of the outer plexiform layer of the turtle retina: an electron microscope study of serial sections

Summary Neural connections in the outer plexiform layer of thePseudemys turtle retina have been studied by electron microscopy of serial ultrathin sections. While the distinguishing features of the photoreceptors have been described elsewhere, in this paper we describe the patterns of connectivity between identified second order neurons and identified photoreceptors or amongst second order neurons themselves. Basal telodendria emitted from double cone pedicles interconnect the two members of the double cone. Three morphologically different types of junction are made between bipolar cells and cone pedicles. H1 horizontal cells can be distinguished from H2 horizontal cells and synapses occur between them. Axon terminals of H1 cells are presynaptic to H1 cell bodies. Photoreceptors, H1 cell bodies and H1 axon terminals engage in electrical junctions while chemical synapses occur from both types of horizontal cell to bipolar cells. On rare occasions, bipolar cell dendrites were seen to be presynaptic to other bipolar cell dendrites. The significance of some of these contacts for the electrophysiological findings on the OPL of the turtle retina is discussed.     

Two classes of bipolar cell in the retina of the skateRaja erinacea

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

Dopaminergic transmission in the rat retina: evidence for volume transmission

The study was designed to determine whether dopaminergic neurotransmission in the retina can operate via volume transmission. In double immunolabelling experiments, a mismatch as well as a match was demonstrated in the rat retina between tyrosine hydroxylase (TH) and dopamine (DA) immunoreactive (ir) terminals and cell bodies and dopamine D2 receptor-like ir cell bodies and processes. The match regions were located in the inner nuclear and plexiform layers (D2 ir cell bodies plus processes). The mismatch regions were located in the ganglion cell layer, the outer plexiform layer, and the outer segment of the photoreceptor layer, where very few TH ir terminals can be found in relation to the D2 like ir processes. In similar experiments analyzing D1 receptor like ir processes versus TH ir nerve terminals, mainly a mismatch in their distribution could be demonstrated, with the D1 like ir processes present in the outer plexiform layer and the outer segment where a mismatch in D2 like receptors also exists. The demonstration of a mismatch between the localization of the TH terminal plexus and the dopamine D2 and D1 receptor subtypes in the outer plexiform layer, the outer segment and the ganglion cell layer (only D2 immunoreactivity (IR)) suggests that dopamine, mainly from the inner plexiform layer, may reach the D2 and D1 mismatch receptors via diffusion in the extracellular space. After injecting dopamine into the corpus vitreum, dopamine diffuses through the retina, and strong catecholamine (CA) fluorescence appears in the entire inner plexiform layer and the entire outer plexiform layer, representing the match and mismatch DA receptor areas, respectively. The DA is probably bound to D1 and D2 receptors in both plexiform layers, since the DA receptor antagonist chlorpromazine fully blocks the appearance of the DA fluorescence, while only a partial blockade is found after haloperidol treatment which mainly blocks D2 receptors. These results indicate that the amacrine and/or interplexiform DA cells, with sparse branches in the outer plexiform layer, can operate via volume transmission in the rat retina to influence the outer plexiform layer and the outer segment, as well as other layers of the rat retina such as the ganglion cell layer.     

Functional and structural modifications during retinal degeneration in the rd10 mouse

Mouse models of retinal degeneration are useful tools to study therapeutic approaches for patients affected by hereditary retinal dystrophies. We have studied degeneration in the rd10 mice both by immunocytochemistry and TUNEL-labeling of retinal cells, and through electrophysiological recordings. The cell degeneration in the retina of rd10 mice produced appreciable morphological changes in rod and cone cells by P20. Retinal cell death is clearly observed in the central retina and it peaked at P25 when there were 800 TUNEL-positive cells per mm(2). In the central retina, only one row of photoreceptors remained in the outer nuclear layer by P40 and there was a remarkable deterioration of bipolar cell dendrites postsynaptic to photoreceptors. The axon terminals of bipolar cells also underwent atrophy and the inner retina was subject to further changes, including a reduction and disorganization of AII amacrine cell population. Glutamate sensitivity was tested in rod bipolar cells with the single cell patch-clamp technique in slice preparations, although at P60 no significant differences were observed with age-matched controls. Thus, we conclude that rod and cone degeneration in the rd10 mouse model is followed by deterioration of their postsynaptic cells and the cells in the inner retina. However, the functional preservation of receptors for photoreceptor transmission in bipolar cells may open new therapeutic possibilities.     

Dopamine- and adenosine-3':5'-monophosphate (cAMP)-regulated phosphoprotein of Mr 32,000 (DARPP-32) in the retina of cat, monkey and human.

The cellular localization of a dopamine- and cAMP-regulated phosphoprotein of Mr 32,000 (DARPP-32) was investigated in cat, monkey and human retina by immunohistochemistry. In cat, DARPP-32-immunoreactive cell bodies identified as Müller cells were demonstrated in the inner nuclear layer (INL) with processes closely surrounding the cell soma of photoreceptors in the outer nuclear layer. Some DARPP-32-IR cells were also seen in the nerve fiber layer (NFL) sending processes to the inner plexiform layer. In monkey and human retina, DARPP-32-IR cell bodies were also demonstrated in the INL, with few cells located in the NFL.     

Immunocytochemical staining with antibodies against protein kinase C and its isozymes in the turtle retina

Summary An LM immunocytochemical study has investigated the patterns of staining in turtle retina with monoclonal antibodies to the , and isozymes of protein kinase C. The protein kinase C- antibody reveals cells in the ganglion cell layer, occasional amacrine cells and faint banding in strata 2 and 4 of the inner plexiform layer. The protein kinase C- antibody stains primarily amacrine cells that have dendrites running in strata 2, in 4 close to the 3/4 border and on the 4/5 border of the inner plexiform layer. Protein kinase C- immunoreactivity is seen in a population of bipolar cells. The latter are characterized by stained axon terminals in strata 3 and 4 of the inner plexiform layer. A type of amacrine cell, different from those seen with the other antibodies, is also immunoreactive to protein kinase C-. EM immunocytochemistry (using a polyclonal antibody) reveals protein kinase C immunoreactivity in photoreceptor cells, bipolar cells, amacrine cells and ganglion cells. In photoreceptors protein kinase C immunoreactivity occurs as patchy staining associated with vesicles and the plasmalemma in pedicles and telodendria. Some varieties of bipolar cell display protein kinase C reaction product throughout the entire cell. Their dendrites contact photoreceptor pedicles at wide-cleft basal junctions and ribbon and non-ribbon related narrow cleft junctions. A few lateral elements per cone or rod pedicle are always protein kinase C-immunoreactive. Amacrine and ganglion cells typically show small clumps of protein kinase C immunoreactivity around vesicles and close to the postsynaptic membranes. Synaptic boutons of some varieties of amacrine cell stain more uniformly. Protein kinase C-immunoreactive bipolar cells are most commonly presynaptic in stratum 4 of the inner plexiform layer, while protein kinase C-immunoreactive amacrine cells are both pre- and postsynaptic throughout strata 1, 2, 3 and 4. Stratum 5 appears to be almost devoid of protein kinase C-immunoreactive neural profiles.     

鲫鱼、牛蛙、鸡和大鼠视网膜锌离子分布的光镜观察

本文应用neo-Timm染色技术研究了鲫鱼、牛蛙、鸡和大鼠视网膜内锌离子的分布状况。结果发现上述动物视网膜内均存在锌离子。锌离子位于视网膜光感受器的内段、外网层、双极细胞、无长突细胞和神经节细胞等处。鲟鱼视网膜的部分光感受器胞体锌离子染色阳性。此外，牛蛙、鸡和大鼠等动物视网膜内同层锌离子亦呈弥漫性着色。提示在较高等动物作为神经调质的锌离子对视网膜神经无视觉信号的传导与调制可能具有更为广泛的意义。     

The light microscopic localization of substance P- and somatostatin-like immunoreactive cells in the larval tiger salamander retina

Summary Light microscopic immunocytochemistry was utilized to localize the populations of substance P (SP)- and somatostatin (SOM)-like immunoreactive cells in the larval tiger salamander retina. Of 104 SP-immunostained cells observed, 82% were Type 1 amacrine cells. Another 8% of the SP-cells were classified as Type 2 amacrine cells, while 10% of the SP-cells had their cell bodies located in the ganglion cell layer and were designated as displaced amacrine cells. Each type of SP-like immunoreactive cell was observed in the central and peripheral retina. SP-immunopositive processes were observed in the inner plexiform layer as a sparse plexus in sublamina 1 and as a denser network of fibers in sublamina 5. Seventy-eight percent of the 110 somatostatin-immunopositive cells observed were designated as Type 1 amacrine cells. Another 12% of SOM-cells were classified as displaced amacrine cells, while only two SOM-immunopositive Type 2 amacrine cells were observed. Nine percent of the SOM-cells were designated as interplexiform cells, based on their giving rise to processes distributing in the outer plexiform layer as well as processes ramifying in the inner plexiform layer. Each type of SOM-immunoreactive cell was observed in the central and peripheral retina, with the exception of the Type 2 amacrine cells, whose somas were only found in the central retina. Lastly, SOM-immunopositive processes in the inner plexiform layer appeared as a fine plexus in sublamina 1 and as a somewhat denser network of fibers in sublamina 5.     

Serotonin-like immunoreactivity in the retina of the clawed frogXenopus laevis

Summary Using an antiserum directed against serotonin, we have studied the morphology and distribution of serotonin-containing and serotonin-accumulating neurons in the retina ofXenopus laevis. Endogenous serotonin-like immunoreactivity was found in two classes of amacrine cell, one class of bipolar cell and a few centrifugal fibres. Kainic acid-induced depletion of serotonin, under various conditions, enabled us to determine the distribution of stained bipolars, amacrine cells and centrifugal fibres within the meshwork of serotonin-like immunoreactivity-labelled processes. Kainic acid-induced release of serotonin by bipolar cells is calcium dependent. Stimulation of release by kainic acid as well as the fact that all serotonin-like immunoreactive bipolar cells ramify in sublayer 1 of the inner plexiform layer suggest that serotonergic bipolars are OFF centre cells. Release of serotonin from amacrine cells is largely calcium independent. Serotonin-containing amacrines send primary dendrites into layer 1 of the inner plexiform layer; short off-shoots from the primary dendrites descend into sublayers 3–5 in which they ramify into a fine network. Serotonergic amacrines have an uneven distribution in theXenopus retina. Their highest density occurs in the posterolateral quadrant, whereas large portions of the anteromedial quadrant lack serotonin-like immunoreactivity altogether. The uneven distribution of serotonin-containing elements in theXenopus retina with its peak falling onto the retinal area which generates binocular vision, suggests its involvement in binocular perception.