Electron microscopy of the indoleamine-accumulating neurons in the retina of the rabbit

Summary The recently discovered indoleamine-accumulating retinal neurons were studied electron microscopically after destruction of the dopaminergic retinal neurons and subsequent labeling with 5,6-dihydroxytryptamine. These observations confirm earlier fluorescence microscopical studies on the distribution of the indoleamine-accumulating neurons in the rabbit retina. Their perikarya are known to be located in the inner nuclear layer (INL) among the amacrine cell bodies. Their processes are found only in the inner plexiform layer (IPL), most of them in the innermost third part of that layer. The indoleamine-accumulating terminals are pre- and postsynaptic to bipolar neurons in the innermost sublayer of the IPL. Reciprocal synapses are probably the rule. The synaptic vesicles of indoleamine-accumulating synapses onto bipolar cells are arranged in globular clusters around a central electron dense, round body. A number of synapses formed by unlabeled amacrine neurons with postsynaptic indoleamine-accumulating elements were also detected. These synapses were mainly found in the outermost third of the IPL. Synaptic contacts between presynaptic indoleamine-accumulating neurons and postsynaptic unlabeled processes of amacrine cells are very rare.     

Synaptic organization of the indoleamine-accumulating neurons in the cyprinid retina

The distribution and synaptic connections of the indoleamine-accumulating neurons in the retinae of the goldfish and carp were studied by means of fluorescence and electron microscopy. The indoleamine-accumulating neurons were visualized after intravitreal injection and uptake of the indoleamine 5,6-dihydroxytryptamine. This labeling procedure produced a characteristic yellow fluorescence of the indoleamine-accumulating neurons and also characteristic ultrastructural changes in these cells. To avoid interference from the dopaminergic neurons of the retina, their processes were either removed by prior treatment with 5-hydroxydopamine or prevented from taking up 5,6-dihydroxytryptamine by the simultaneous injection of the catecholamine alpha-methyl-noradrenaline. Fluorescence-microscopic studies confirmed earlier reports that the indoleamine-accumulating perikarya and processes are distributed similar to those of amacrine cells. The indoleamine-accumulating processes ramify in three bands in the inner plexiform layer, the outermost one being the densest. Electron-microscopic investigations showed the indoleamine-accumulating neurons to have synapses of the conventional type, similar to amacrine cells. Their main synaptic contacts are with other amacrine cells, but synapses with bipolar cell terminals are also present. Both the distribution of the indoleamine-accumulating processes and their synaptic arrangement in the cyprinid retina differ from those found in mammalian retinae investigated previously.     

Immunocytochemical and autoradiographic localization of GABA system in the vertebrate retina

Summary The localization of -aminobutyric acid (GABA) neurons in the goldfish and the rabbit retina has been studied by immunocytochemical localization of the GABA-synthesizing enzyme L-glutamate decarboxylase (GAD, L-glutamate 1-carboxy-lase, EC 4.1.1.15) and by [³H] GABA uptake autoradiography. In the goldfish retina, GAD is localized in some horizontal cells (H1 type), a few amacrine cells and sublamina b of the inner plexiform layer. Results from immunocytochemical studies of GAD-containing neurons and autoradiographic studies of GABA uptake reveals a marked similarity in the labeling pattern suggesting that in goldfish retina, the neurons which possess a high-affinity system for GABA uptake also contain significant levels of GAD. In the rabbit retina, when Triton X-100 was included in immunocytochemical incubations with a modified protein A-peroxidase-antiperoxidase method, reaction product was found in four broad, evenly spaced laminae within the inner plexiform layer. In the absence of the detergent, these laminae were seen to be composed of small, punctate deposits. When colchicine was injected intravitreally before glutamate decarboxylase staining, cell bodies with the characteristic shape and location of amacrine cells were found to be immunochemically labeled. Electron microscopic examination showed that these processes were presynaptic to ganglion cell dendrites (infrequently), amacrine cell telodendrons, and bipolar cell terminals. Often, bipolar cell terminals were found which were densely innervated by several GAD-positive processes. No definite synapses were observed in which a GAD-positive process represented the postsynaptic element. In autoradiographic studies by intravitreal injection of [³H] GABA a diffuse labeling of the inner plexiform layer and a dense labeling of certain amacrine cell bodies in the inner nuclear layer was observed. Both immunocytochemical and autoradiographic results support the notion that certain, if not all, amacrine cells use GABA as their neurotransmitter.     

Distribution of calcitonin gene-related peptide immunoreactivity in the central nervous system of the forg,Rana esculenta

Summary Tyrosine hydroxylase (TH) immunocytochemistry was utilized to quantify dopaminergic synapses in the inner plexiform layer of the retina of Bufo marinus. Since dopaminergic cells have bistratified dendritic arborisation in the inner plexiform layer, attention was given to the segregation of synapses between the scleral and the vitreal sublaminae. Light-microscopically, a more elaborate dendritic branching was observed in the scleral than in the vitreal sublamina. In contrast, about 55% of synapses occurred in the vitreal one fifth of the inner plexiform layer, 30% in the scleral fifth, and 15% in the intermediate laminae. Input sources and output targets showed only minor quantitative differences between sublaminae 1 and 5. TH-immunoreactive processes were found in presynaptic (62.8%) and postsynaptic (37.2%) positions. Synapses to the stained dendrites derived from bipolar (40.4%) and amacrine (59.6%) cells, whereas outputs from the TH-positive processes were directed to amacrine cells (56.8%) and to small and medium-sized dendrites (35.4%); at least some of these can be considered as ganglion cell dendrites. TH-positive profiles neither formed synapses with each other nor were presynaptic to bipolar cell terminals. Junctional appositions of the immunoreactive profiles were occasionally seen on non-stained amacrine and ganglion cell dendrites in the scleral sublamina of the inner plexiform layer and on optic axons in the optic fibre layer. Although dopaminergic cells are mainly involved in amacrine-amacrine interactions, inputs from bipolar terminals and outputs to ganglion cell dendrites were also substantial, suggestive of a role also in vertical information processing.     

THE DISTRIBUTION OF CHOLINE ACETYLTRANSFERASE ACTIVITY IN VERTEBRATE RETINA1

Abstract— Choline acetyltransferase (ChAc) activity was determined in retinal layers from 10 vertebrates. In all animals, the highest activity was in the inner plexiform layer, intermediate activity in the inner nuclear and ganglion cell layers, and very low activity in the photoreceptor and outer plexiform layers and optic nerve. The pattern of distribution of enzyme activity within the inner nuclear layer corresponds quantitatively to the distribution of amacrine cells within that layer. A species difference of almost 90-fold was found between the lowest and highest values for ChAc activity in inner plexiform layer. The variation in enzyme activity found among homeotherms in inner nuclear and inner plexiform layers is related to the number of amacrine cell synapses in the inner plexiform layer. But the differences in enzyme activity are generally greater than those which have been found in numbers of amacrine cell synapses between species. The data suggest that cholinergic neurons in retina are to be found predominantly among the amacrine cell types and that not all amacrine cells will be found to be cholinergic.     

Light- and electron-microscopic study of substance P-immunoreactive neurons in the guinea pig retina

Substance P (SP) immunoreactivity in the guinea pig retina was studied by light and electron microscopy. The morphology and distribution of SP-immunoreactive neurons was defined by light microscopy. The SP-immunoreactive neurons formed one population of amacrine cells whose cell bodies were located in the proximal row of the inner nuclear layer. A single dendrite emerged from each soma and descended through the inner plexiform layer toward the ganglion cell layer. SP-immunoreactive processes ramified mainly in strata 4 and 5 of the inner plexiform layer. SP-immunoreactive amacrine cells were present at a higher density in the central region around the optic nerve head and at a lower density in the peripheral region of the retina. The synaptic connectivity of SP-immunoreactive amacrine cells was identified by electron microscopy. SP-labeled amacrine cell processes received synaptic inputs from other amacrine cell processes in all strata of the inner plexiform layer and from bipolar cell axon terminals in sublamina b of the same layer. The most frequent postsynaptic targets of SP-immunoreactive amacrine cells were the somata of ganglion cells and their dendrites in sublamina b of the inner plexiform layer. Amacrine cell processes were also postsynaptic to SP-immunoreactive neurons in this sublamina. No synaptic outputs onto the bipolar cells were observed.     

Laminar Distributions of Choline Acetyltransferase and Acetylcholinesterase Activities in the Inner Plexiform Layer of Rat Retina

Choline acetyltransferase and acetylcholinesterase activities were measured in samples taken at 7-micron increments through the inner plexiform layer of rat retina. These enzyme activities were not uniformly distributed through the depth of the inner plexiform layer. Peaks of choline acetyltransferase activity occurred at about one-third and peaks of acetylcholinesterase activity at about one-fifth of the depth into the inner plexiform layer from either side. The positions of the two peaks of choline acetyltransferase activity most likely correspond to the locations of processes from cholinergic amacrine somata in the inner nuclear layer, which spread in sublamina a, and processes from cholinergic amacrine somata "displaced" in the ganglion cell layer which spread in sublamina b of the inner plexiform layer. The peaks of acetylcholinesterase activity may in addition correspond to the processes of cholinoceptive amacrine and ganglion cells. The magnitudes of choline acetyltransferase and acetylcholinesterase activities are as high as found anywhere in rat brain, emphasizing the important role of cholinergic mechanisms in visual processing through the rat inner plexiform layer.     

Comparative study of cholinergic cells in retinas of various mouse strains

We examined cholinergic cells in the retinas of BALB/C albino, C57BL/6J black, and 129/SvJ light chinchilla mice by using immunocytochemistry with specific antisera against choline acetyltransferase (ChAT). Two types of ChAT-immunoreactive amacrine cell bodies were found in the inner nuclear layer (INL) and ganglion cell layer in the retinas of all three mouse strains. They were distributed with mirror-image symmetry and their processes ramified in strata 2 and 4 of the inner plexiform layer. A distinct type of ChAT-immunoreactive cell was found only in C57BL/6J mouse retina. The somata of this third type of ChAT-immunoreactive cell were located in the outermost part of the INL, with their processes extending toward the outer plexiform layer. Double-labeling experiments demonstrated that these were not horizontal cells and that they were GABA-immunoreactive. The results suggested that these cells were probably misplaced cholinergic amacrine cells showing GABA immunoreactivity. This feature of the C57BL/6J mouse retina should be taken into account in studies of mutant mice having a mixed genetic background with a C57BL/6J contribution.Tae-Hoon Kang, Young-Han Ryu and In-Beom Kim contributed equally to this study.This work was supported by Neurobiology Support Grant (M1-0108-00-0059) of the Ministry of Science and Technology, Korea     

Immunocytochemical localization of glycine and glycine receptors in the retina of the frog <Emphasis Type="Italic">Rana ridibunda</Emphasis>

Glycine and glycine receptors (GlyRs) were analyzed immunocytochemically in the retina of the frog Rana ridibunda. Glycine was localized to somata of glycinergic amacrine and interplexiform cells. Approximately 50% of the cells in the amacrine cell layer were found to be glycinergic. GlyRs of the inner plexiform layer (IPL) were localized to brightly fluorescent puncta, probably representing postsynaptic clusters of GlyRs. GlyR clusters were not evenly distributed across the IPL but showed patterns of stratification specific for the various GlyR subunits. Clusters containing the 1 subunit formed four narrow strata within the IPL. Clusters containing the 3 subunit were more abundant and covered the whole IPL, with a band of higher density in stratum 3. Clusters of GlyRs were also observed in the outer plexiform layer. Thus, several isoforms of synaptic GlyRs involved with different synapses and inhibitory circuits are present in the frog retina.This work was supported by the Deutsche Forschungsgemeinschaft SFB269/B4     

Synaptic relationships in the plexiform layers of carp retina

Summary The synaptic contacts made by carp retinal neurons were studied with electron microscopic techniques. Three kinds of contacts are described: (1) a conventional synapse in which an accumulation of agranular vesicles is found on the presynaptic side along with membrane densification of both pre- and postsynaptic elements; (2) a ribbon synapse in which a presynaptic ribbon surrounded by a halo of agranular vesicles faces two postsynaptic elements; and (3) close apposition of plasma membranes without any vesicle accumulation or membrane densification.In the external plexiform layer, conventional synapses between horizontal cells are described. Horizontal cells possess dense-core vesicles about 1,000 Å in diameter. Membranes of adjacent horizontal cells of the same type (external, intermediate or internal) are found closely apposed over broad regions.In the inner plexiform layer ribbon synapses occur only in bipolar cell terminals. The postsynaptic elements opposite the ribbon may be two amacrine processes or one amacrine process and one ganglion cell dendrite. Amacrine processes make conventional synaptic contacts onto bipolar terminals, other amacrine processes, amacrine cell bodies, ganglion cell dendrites and bodies. Amacrine cells possess dense-core vesicles. Ganglion cells are never presynaptic elements. Serial synapses between amacrine processes and reciprocal synapses between amacrine processes and bipolar terminals are described. The inner plexiform layer contains a large number of myelinated fibers which terminate near the layer of amacrine cells.This work was supported by an N.I.H. grant NB 05404-05 and a Fight for Sight grant G-396 to P.W. and N.I.H. grant NB 05336 to J.E.D. The authors wish to thank Mrs. P. Sheppard and Miss B. Hecker for able technical assistance. P.W. is grateful to Dr. G. K. Smelser, Department of Ophthalmology, Columbia University, for the use of his electron microscope facilities.     

Electron microscopic observation of pituitary adenylate cyclase-activating polypeptide (PACAP)-containing neurons in the rat retina

The distribution and localization of pituitary adenylate cyclase-activating polypeptide (PACAP) in the rat retina were studied by immunocytochemistry with both light and electron microscopy. PACAP-like immunoreactivity (PACAP-LI) was detected in the amacrine and horizontal cells as well as in the inner plexiform layer, the ganglion cell layer and the nerve fiber layer. PACAP-LI seemed to be concentrated predominantly in the neuronal perikarya and their processes, but not in other cells in the retina. At the ultrastructural level, PACAP-LI was visible in the plasma membranes, rough endoplasmic reticulum, and cytoplasmic matrix in the PACAP-positive neurons in the inner nuclear layer. In the inner plexiform layer, PACAP-positive amacrine cell processes made synaptic contact with immunonegative amacrine cell processes, bipolar cell processes, and ganglion cell terminals. These findings suggest that PACAP may function as a neurotransmitter and/or neuromodulator.     

Morphological and pharmacological analysis of putative serotonergic bipolar and amacrine cells in the retina of a turtle,Pseudemys scripta elegans

Using immunocytochemical methods, we have been able to demonstrate serotonin-like immunoreactivity (SLI) in amacrine and bipolar cells of the turtle retina. Inhibition of monoamine oxidase with pargyline drastically increases the amount of 5-hydroxytryptamine within both cell types. The indoleamine 6-hydroxytryptamine is taken up by both cell types and both types are destroyed within 10 days following intraocular injection of 5,7-dihydroxytryptamine. Increasing the external potassium concentration induces release of serotonin in both cell types. Our data support the idea that these neurons use serotonin during neuronal processing. Morphologically, amacrine and bipolar cells with SLI can be subdivided into two and three subclasses, respectively, based on their ramification pattern within the inner plexiform layer. A comparison of the morphological data with those of intracellularly stained amacrine and bipolar cells suggests that all bipolar cells with SLI are center-hyperpolarizing cells and all amacrine cells center-depolarizing cells.     

Morphology and synaptic connectivity of nitric oxide synthase-immunoreactive neurons in the guinea pig retina

Immunocytochemical methods with an antiserum against neuronal nitric oxide synthase (NOS) were applied to identify the morphology and synaptic connectivity of NOS-like immunoreactive neurons in the guinea pig retina. In the present study, two types of amacrine cells were labeled with anti-NOS antisera. Type 1 cells had large somata located in the inner nuclear layer (INL) with long, sparsely branched processes ramifying mainly in stratum 3 of the inner plexiform layer (IPL). The somata of type 2 cells (smaller diameters) were located in the INL. Some displaced amacrine cells in the ganglion cell layer were labeled. The soma size of the displaced amacrine cells was similar to that of the type 2 amacrine cells. However, processes originating from type 2 amacrine cells and displaced amacrine cells stratified mainly in strata 1 and 5, respectively. Some cone bipolar cells were weakly NOS-immunoreactive. The synaptic connectivity of NOS-like immunoreactive amacrine cells was identified in the IPL by electron microscopy. NOS-labeled amacrine cell processes received synaptic input from other amacrine cell processes and bipolar cell axon terminals in all strata of the IPL. The most frequent postsynaptic targets of NOS-immunoreactive amacrine cells were other amacrine cell processes. Cone bipolar cells were postsynaptic to NOS-labeled amacrine cells in all strata of the IPL. Labeled amacrine cells synapsing onto ganglion cells were found only in sublamina b. A few synaptic contacts were observed between labeled cell processes. In the outer plexiform layer, dendrites of labeled bipolar cells made basal contact with cone pedicles or formed a synaptic triad opposed to a synaptic ribbon of cone pedicles.     

Localization of GABAA receptors in the rabbit retina

The distribution of gamma-aminobutyric acid_A (GABA_A) receptors in the rabbit retina is investigated and compared with the distribution of GABAergic neurons using immunocytochemical methods. Antibodies against the 1, 2/3, and 2 subunits of the GABA_A receptor label subpopulations of bipolar, amacrine and ganglion cells. Double labeling experiments show that the 2 subunit is colocalized with the 1 and the 2/3 subunits in bipolar, amacrine and ganglion cells. Electron microscopy reveals that in the outer plexiform layer, GABA_A receptor immunoreactivity is present on dendrites of cone bipolar cells adjacent to the cone pedicles. Bipolar cell dendrites are also receptor-positive at synapses from interplexiform cells. Some receptor immunoreactivity is found intracellularly in processes of horizontal cells. In the inner plexiform layer, GABA_A receptor immunoreactivity is present on both rod bipolar and cone bipolar axon terminals at putative GABAergic input sites. Amacrine and ganglion cell processes in sublamina a and b are also labeled.     

Putative neurotransmitters in the retinae of three urodele species (Triturus alpestris,Salamandra salamandra,Pleurodeles waltli)

Summary The immunocytochemical localization of several substances with putative neurotransmitter or modulator properties was investigated in the retinae of three urodele species. Gamma-aminobutyric acid-like immunoreactive labelling appeared in different types of amacrine and horizontal cells. In addition, labelled fibres in the optic nerve were detected. It was not possible to determine whether these fibres were ganglion-cell axons or part of an efferent projection. Endogenous serotonin was found in several populations of amacrine cells including stratified and diffuse types. Glucagon-like immunoreactivity appeared in one bistratified amacrine cell type, and neurotensin-like immunoreactivity was detected in a single monostratified amacrine cell type. Metenkephalin-like-immunoreactive labelling was rare but found in several sublaminae of the inner plexiform layer. Thus each peptide-like-immunoreactive cell type makes up a distinct and unique population of cells and probably has a special functional role in retinal processing. There are striking similarities in the peptide-like immunoreactive patterns of Triturus alpestris and Necturus maculosus whereas in Ambystomatidae the peptide-like-immunoreactive systems appear to be differently organized. This supports the hypothesis that Salamandridae and Proteidae are more closely related to each other than to the Ambystomatidae.Abbreviations GABA gamma-aminobutyric acid - GCL ganglion cell layer - Glu glucagon - HRP horseradish peroxidase - INL inner nuclear layer - IPL inner plexiform layer - IR immunoreactive or immunoreactivity - M-enk metenkephalin - Neu neurotensin - OFL optic fibre layer - ONL outer nuclear layer - OPL outer plexiform layer - Ser serotonin This work forms part of the doctoral thesis of Gaby Gläsener, Faculty of Biology, Technical University of Darmstadt, Federal Republic of Germany. Supported by a research grant from the Deutsche Forschungsgemeinschaft (Hi 306/1-1)     

Microscopic investigation of the comparative morphology of the retina

Morphological differences in the architectonics (the relations and composition of the layers and sublayers) of the retina are described in various vertebrates: pike, frog, and cat. These differences apply to both cellular and plexiform layers. The differences are particularly marked in the composition of the sublayers of the inner nuclear layer. In the frog the greatest degree of subdivision into layers of processes of the ganglion and amacrine cells is observed to correspond to the particularly complex differentiation of the inner plexiform layer of the retina (about 10 sublayers). In all the animals studied the ganglion cells can be divided into two principal types: symmetrical and asymmetrical, with many varieties. Asymmetrical amacrine cells are found in the pike and frog retina. The presence of vertical processes branching in the outer plexiform layer is confirmed for amacrine cells in the cat retina. The structural features of the retina are discussed in connection with physiological findings.     

Light-and electron-microscopic studies of the somatostatin-immunoreactive plexus in the cat retina

Summary Two monoclonal antibodies directed against somatostatin 14 were used to study immunoreactive neurons, their processes and their synapses in the cat retina. In retinal whole-mounts, a sparse population of wide-field displaced amacrine cells was observed predominantly in the ventral retina and near the retinal margin. Processes of these cells ramified mainly in two distinct strata within the inner plexiform layer: one near the inner nuclear layer (INL), and the other near the ganglion cell layer (GCL). The length of immunoreactive fibres within each plexus was measured: 232±32 mm/mm² near the INL and 230±74 mm/mm² near the GCL in all retinal regions. The immunoreactive processes were studied using electron-microscopic techniques; conventional and some ribbon-containing synapses (dyads) were found. Immunolabelled processes received input synapses from other amacrine cell processes. These investigations provide further evidence that this cell population has a diffuse, regulatory or modulatory role for visual-information processing in the inner plexiform layer.     

Immunohistochemical localization of GABAA receptors in the scotopic pathway of the cat retina

The distribution of GABA_A receptors in the inner plexiform layer of cat retina was studied using monoclonal antibodies against the 2/3 subunits. A dense band of receptor labeling was found in the inner region of the inner plexiform layer where the rod bipolar axons terminate. Three forms of evidence indicate that the GABA_A receptor labeling is on the indoleamine-accumulating, GABAergic amacrine cell that is synaptically interconnected with the rod bipolar cell terminal. (1) Electron microscopy showed that the anti-GABA_A receptor antibody (62-3G1) labeled profiles that were postsynaptic to rod bipolar axons and made reciprocal synapses. (2) Indoleamine uptake (and the subsequent autofluorescence) combined with GABA_A receptor immunohistochemistry showed co-localization of the two markers in half of the receptor-positive amacrine cells. (3) Double labeling demonstrated that half of the receptor-positive somata also contained GABA. These results indicate that a GABAergic amacrine cell interconnected with the rod bipolar cell, most likely the so-called A17 amacrine cell, itself bears GABA_A receptors.     

Morphology of calretinin and tyrosine hydroxylase-immunoreactive neurons in the pig retina

The morphology of calretinin- and tyrosine hydroxylase-immunoreactive (IR) neurons in adult pig retina was studied. These neurons were identified using antibody immunocytochemistry. Calretinin immunoreactivity was found in numerous cell bodies in the ganglion cell layer. Large ganglion cells, however, were not labeled. In the inner nuclear layer, the regular distribution of calretinin-IR neurons, the inner marginal location of their cell bodies in the inner nuclear layer, and the distinctive bilaminar morphologies of their dendritic arbors in the inner plexiform layer suggested that these calretinin-IR cells were AII amacrine cells. Calretinin immunoreactivity was observed in both A-and B-type horizontal cells. Neurons in the photoreceptor cell layer were not labeled by this antibody. The great majority of tyrosine hydroxylase-IR neurons were located at the innermost border of the inner nuclear layer (conventional amacrines). The processes were monostratified and ran laterally within layer 1 of the inner plexiform layer. Some of the tyrosine hydroxylase-IR neurons were located in the ganglion cell layer (displaced amacrines). The processes of displaced tyrosine hydroxylase-IR amacrine cells were also located within layer 1 of the inner plexiform layer. Some processes of a few neurons were located in the outer plexiform layer. A very low density of neurons had additional bands of tyrosine hydroxylase-IR processes in the middle and deep layers of the inner plexiform layer. The processes of tyrosine hydroxylase-IR neurons extended radially over a wide area and formed large, moderately branched dendritic fields. These processes occasionally had varicosities and formed "dendritic rings". These results indicate that calretinin- and tyrosine hydroxylase-IR neurons represent specific neuronal cell types in the pig retina.     

Differential expression of syntaxin-1 and synaptophysin in the developing and adult human retina

Synaptophysin and syntaxin-1 are membrane proteins that associate with synaptic vesicles and presynaptic active zones at nerve endings, respectively. The former is known to be a good marker of synaptogenesis; this aspect, however, is not clear with syntaxin-1. In this study, the expression of both proteins was examined in the developing human retina and compared with their distribution in postnatal to adult retinas, by immunohistochemistry. In the inner plexiform layer, both were expressed simultaneously at 11–12 weeks of gestation, when synaptogenesis reportedly begins in the central retina. In the outer plexiform layer, however, the immunoreactivities were prominent by 16 weeks of gestation. Their expression in both plexiform layers followed a centre-to-periphery gradient. The immunoreactivities for both proteins were found in the immature photoreceptor, amacrine and ganglion cells; however, synaptophysin was differentially localized in bipolar cells and their axons, and syntaxin was present in some horizontal cells. In postnatal-to-adult retinas, synaptophysin immunoreactivity was prominent in photoreceptor terminals lying in the outer plexiform layer; on the contrary, syntaxin-1 was present in a thin immunoreactive band in this layer. In the inner plexiform layer, however, both were homogeneously distributed. Our study suggests that (i) syntaxin-1 appears in parallel with synapse formation; (ii) synaptogenesis in the human retina might follow a centre-to-periphery gradient; (iii) syntaxin-1 is likely to be absent from ribbon synapses of the outer plexiform layer, but may occur at presynaptic terminals of photoreceptor and horizontal cells, as is apparent from its localization in these cells, which is hitherto unreported for any vertebrate retina.