(3H) glycine-accumulating neurons of the human retina

Specific cell types in cat retina express different forms of glutamic acid decarboxylase

We studied the expression of glutamate decarboxylase (GAD), GAD65 and GAD67, in cat retina by immunocytochemistry. About 10% of GABAergic amacrine cells express only GAD65 and 30% express only GAD67. Roughly 60% contain both forms of the enzyme, but GAD67 is present only at low levels in the majority of these double-labeled amacrine cells. The staining pattern in the inner plexiform layer (IPL) for the two GAD forms was also different. GAD65 was restricted to strata 1-4, and GAD67 was apparent throughout the IPL but was strongest in strata 1 and 5. This indicates that somas, as well as their processes, are differentially stained for the two forms of GAD. Cell types expressing only GAD65 include interplexiform cells, one type of cone bipolar cell, and at least one type of serotonin-accumulating amacrine cell. Cell types expressing only GAD67 include amacrine cells synthesizing dopamine, amacrine cells synthesizing nitric oxide (NO), and amacrine cells accumulating serotonin. Cholinergic amacrine cells express a low level of both GAD forms. Our findings in the retina are consistent with previous observations in the brain that GAD65 expression is greater in terminals than in somas. In addition, in retina most neurons expressing GAD67 also contain a second neurotransmitter as well as GABA, and they tend to be larger than neurons expressing GAD65. We propose that large cells have a greater demand for GABA than small cells, and thus require the constant, relatively unmodulated level of GABA that is provided by GAD67.

Early appearance and transient expression of putative amino acid neurotransmitters and related molecules in the developing rabbit retina: an immunocytochemical study

We have studied, by immunocytochemistry, the ontogeny of GABA, glycine, glutamate, glutamine, and taurine-containing cells in the rabbit retina. Amacrine cells show GABA immunoreactivity by embryonic day 25 (E25) and throughout postnatal life. By contrast, ganglion cells and horizontal cells are only transiently GABA-immunoreactive (-IR); few appear GABA-IR by the third postnatal week. At maturity, glycine is present in amacrine cells and in some bipolar cells. During development, putative ganglion cells transiently contained glycine between E25 and postnatal day 3 (P3), whereas immunolabelling in presumed amacrine cells and bipolar cells persists after birth. Ganglion cells, bipolar cells, photoreceptors, and some amacrine cells are glutamate-IR in the adult retina. Glutamate immunoreactivity first appears in the somata and processes of cytoblastic cells by E20 and is prominent by E25. Surprisingly, ganglion cells are not strongly glutamate-IR until just before eye-opening, at postnatal day 10 (P10), coincident with the appearance of glutamine in their somata and in Müller glial cells. Bipolar cells are glutamate-IR before they or Müller cells contain high levels of glutamine (at P10). Glutamate immunoreactivity in photoreceptors is progressively restricted to the inner segments by eye-opening. At no stage are presumed horizontal cells glutamate-IR or glutamine-IR, but some amacrine cells show glutamate- and glutamine-IR by P10. Taurine is localized to photoreceptors and Müller glial in the adult retina. Some cytoblasts are taurine-IR at E20; with ensuing development, taurine labelling becomes restricted primarily to Müller cells and photoreceptors; some putative bipolar cells may also be labelled. However, for a few days around birth, cells resembling horizontal cells, also show taurine immunoreactivity. The early appearance and often transient expression of these amino acids in retinal cells suggests that these neuroactive molecules may be involved in the structural and functional development of the retina.

Differential expression of glycine receptor subunits in the retina of the rat: a study using immunohistochemistry and in situ hybridization

Immunohistochemistry and in situ hybridization were used to study the distribution of glycine receptor (GlyR) subunits and the GlyR-associated protein gephyrin in the rat retina. Monoclonal antibodies against the alpha and beta subunits of the GlyR and gephyrin showed a strong punctate labeling pattern in the inner plexiform layer. Glycine receptor mRNAs were found in the inner nuclear layer and the ganglion cell layer. The alpha 1 subunit mRNA is predominantly present in the outer half of the INL and on some but not all ganglion cells. GlyR alpha 2 subunit mRNA is predominantly present in the inner half of the INL and on nearly all cells in the ganglion cell layer. GlyR alpha 3-, GlyR beta-, and gephyrin-mRNAs are present in the entire INL and in cells in the ganglion cell layer. The differential expression of glycine receptor subunits indicates a functional diversity of glycine receptors in the retina.

Diversity of neuronal phenotypes expressed in monolayer cultures from immature rabbit retina

We have used monolayer cultures prepared from early postnatal rabbit retinae (days 2-5) by the sandwich technique to study the capacity of immature neurons to express specific neuronal phenotypes in a homogeneous in vitro environment. Applying morphological, immunocytochemical, and autoradiographic criteria, we demonstrate that a variety of phenotypes could be distinguished after 7-14 days in vitro, and correlated with known retinal cell types. Bipolar cell-like neurons (approximately 4% of total cell number) were identified by cell type-specific monoclonal antibodies (115A10) and their characteristic bipolar morphology. Small subpopulations (about 1%) of GABA-immunoreactive neurons acquired elaborate morphologies strikingly similar to those of A- and B-type horizontal cells. Amongst putative amacrine cells several different subpopulations could be classified. GABA-immunoreactive amacrine-like neurons (6.5%), which also showed high affinity [3H]-GABA uptake, comprised cells of varying size and shape and could be subdivided into subpopulations with respect to their response to different glutamate receptor agonists (NMDA, kainic acid, quisqualic acid). In addition, a small percentage of [3H]-GABA accumulating cells with large dendritic fields showed tyrosine-hydroxylase immunoreactivity. Presumptive glycinergic amacrine cells (18.5%) were rather uniform in shape and had small dendritic fields. Release of [3H]-glycine from these neurons was evoked by kainic and quisqualic acid but not by NMDA. Small [3H]-glutamate accumulating neurons with few short processes were the most frequent cell type (73%). This cell type also exhibited opsin immunoreactivity and probably represented incompletely differentiated photoreceptor cells. Summing the numbers of characterized cells indicated that we were able to attribute a defined retinal phenotype to most, if not all of the cultured neurons. Thus, we have demonstrated that immature neuronal cells growing in monolayer cultures, in the absence of a structured environment, are capable of maintaining or producing specific morphological and functional properties corresponding to those expressed in vivo. These results stress the importance of intrinsic factors for the regulation of neuronal differentiation. On the other hand, morphological differentiation was far from perfect indicating the requirement for regulatory factors.

Immunocytochemical localization of the amino acid neurotransmitters in the chicken retina

Postembedding immunocytochemistry was used to determine the cellular localization of the amino acid neurotransmitters glutamate, aspartate, gamma-aminobutyric acid (GABA), and glycine in the avian retina. The through retinal pathway was glutamatergic, with all photoreceptors, bipolar cells, and ganglion cells being immunoreactive for glutamate. Bipolar cells displayed the highest level of glutamate immunoreactivity, with the cell bodies terminating just below the middle of the inner nuclear layer. All lateral elements, horizontal cells, amacrine cells, and interplexiform cells were immunoreactive for glycine or GABA. The GABAergic neurons consisted of two classes of horizontal cells and amacrine cells located in the lower part of the inner nuclear layer. GABA was also localized in displaced amacrine cells in the ganglion cell layer, and a population of ganglion cells that co-localize glutamate and GABA. Both the horizontal cells and GABAergic amacrine cells had high levels of glutamate immunoreactivity, which probably reflects a metabolic pool. At least two types of horizontal cells in the avian retina could be discriminated on the basis of the presence of aspartate immunoreactivity in the H2 horizontal cells. Glycine was contained in a subclass of amacrine cells, with their cell bodies located between the bipolar cells and GABAergic amacrine cells, two subclasses of bipolar cells, displaced amacrine cells in the ganglion cell layer, and ganglion cells that colocalize glutamate and glycine. Glycinergic amacrine cells had low levels of glutamate. We have also identified a new class of glycinergic interplexiform cell, with its stellate cell body located in the middle of the inner nuclear layer among the cell bodies of bipolar cells. Neurochemical signatures obtained by analyzing data from serial sections allowed the classification of subclasses of horizontal cells, bipolar cells, amacrine cells, and ganglion cells.

Immunocytochemical localization of glycine receptors in the mammalian retina

The distribution of glycinergic synapses in the mammalian retina was studied with monoclonal antibodies against glycine receptors and a glycine receptor-related protein (gephyrin). Monoclonal antibody 2b is specific for the alpha 1 subunit of the glycine receptor; monoclonal antibody 4a is specific for all known alpha subunits and the beta subunit, and monoclonal antibody 7a is specific for gephyrin. The three antibodies were applied to the retina of cat, macaque monkey, rat, and rabbit. The general staining pattern is comparable in all these species and it is similar but distinct with all of the three antibodies. Labeling is characterized by a punctate appearance indicating that it occurs at synapses. In the inner plexiform layer, labeling is concentrated in two bands. One band is located close to the inner nuclear layer; the other band is located in the middle of the inner plexiform layer. In the outer plexiform layer, sparse punctate labeling is seen. The distribution of gephyrin was also studied at the ultrastructural level in cat and monkey retina. Gephyrin is present on the postsynaptic membrane of amacrine cells and ganglion cells. The presynaptic profile to gephyrin immunoreactivity is always of an amacrine cell. The AII amacrine cell, the crucial glycinergic interneuron of the rod pathway, is presynaptic to gephyrin immunoreactivity in the OFF-sublamina and is itself gephyrin-positive at an input synapse from another (possibly GABAergic) amacrine cell in the ON-sublamina.

Comparison of immunolocalization patterns for the synaptic vesicle proteins p65 and synapsin I in macaque monkey retina

The distributions of the two synaptic vesicle proteins p65 [Matthew et al. (1981) J. Cell Biol., 91:257-269] and synapsin I [De Camilli et al. (1983) J. Cell Biol., 96:1337-1354] were compared in macaque monkey retina using pre-embedding immunocytochemistry for both light and electron microscopy. The monoclonal antibody AB-48 against p65 labeled ribbon-containing synaptic terminals of cone, rod, and bipolar cells as well as many conventional synapses of amacrine cells. In contrast, a polyclonal antiserum against synapsin I (SYN I) labeled many amacrine conventional synapses but no photoreceptor or bipolar ribbon synaptic terminals. Horizontal cell pre- and post-synaptic profiles in the outer plexiform layer were not labeled by either antibody. At the light microscopic level, the banding patterns in the inner plexiform layer also differed for the two antibodies, with four bands of AB-48 immunoreactivity in sublayers S1, S2, S4, and S5 but only three bands of SYN I immunoreactivity in S1, S3, and S5. SYN I also labeled varicose fibers in both the inner nuclear layer and the outer plexiform layer that are probably processes of dopaminergic and GABAergic interplexiform cells. Varicose fibers in the ganglion cell layer were labeled by both antibodies. These results provide the first electron microscopic immunocytochemical labeling for AB-48 and SYN I in intact retina and confirm that AB-48 labels both ribbon and conventional synaptic terminals, whereas SYN I labels only conventional synapses.

Differential staining of neurons in the human retina with antibodies to protein kinase C isozymes

Monoclonal antibodies to the three isozymes of protein kinase C (PKC) (alpha, beta, and gamma) were applied to postmortem human retina. Immunostaining was done on wholemount, or cryostat-sectioned retina, and visualized after ABC/DAB procedures by light (LM) and electron (EM) microscopy. The PCK-alpha antibody stained rod bipolar cells throughout the retina. EM analysis confirmed they were PKC-alpha-immunoreactive (IR) on their characteristic dendritic and axonal synaptology. Putative blue cone bipolar cells with wide-field axon terminals, stratifying in s5 of the inner plexiform layer (IPL), were also PKC-alpha-IR, and EM showed them to engage in narrow-cleft ribbon junctions in blue cone pedicles. The PKC-beta antibody stained cone bipolar cells, many amacrine cells, and most ganglion cells. Cone bipolar cells were stained all the way into the foveal center: both midget and diffuse varieties were included. The IPL was densely PKC-IR and individual neurons could not be identified on stratification patterns. EM of the outer plexiform layer (OPL) revealed that both flat and invaginating cone bipolar types were IR and that IR axon terminals were presynaptic in all strata of the IPL. The occurrence of PKC-beta-IR bipolar axons in stratum 2 of the IPL suggests that OFF-center as well as ON-center types were included. The PKC-gamma antibody gave inferior staining compared with results from the other two antibodies; however, two varieties of wide-field monostratified amacrine cell and a large-bodied ganglion cell type were discernible. PKC in one form or another appears to be a second messenger used in neurotransmission by both rod and cone systems and ON- and OFF-center systems in the human retina.

Spatial density and immunoreactivity of bipolar cells in the macaque monkey retina

The anatomical substrates of spatial and color vision in the primate retina are investigated by measuring the immunoreactivity and spatial density of bipolar, amacrine and horizontal cells in the inner nuclear layer of the macaque monkey retina. Bipolar cells can be distinguished from amacrine and horizontal cells by their differential immunoreactivity to antisera against glutamate, glycine, GABA, parvalbumin, calbindin (CaBP D-28K), and the L7 protein from mouse cerebellum. The spatial density of bipolar cells is compared to the densities of photoreceptors and ganglion cells at different retinal eccentricities. In the centralmost 2 mm, cone bipolar cells outnumber ganglion cells by about 1.4:1. The density of cone bipolar cells is thus high enough to allow for input to different (parasol and midget) ganglion cell classes by different (diffuse and midget) bipolar cell classes. The density gradient of cone bipolar cells follows closely that of ganglion cells in central retina but falls less steeply in peripheral retina. This suggests that the convergence of cone signals to the receptive fields of ganglion cells in the peripheral retina occurs in the inner plexiform layer. The density of cone bipolar cells is 2.5-4 times that of cones at all eccentricities studied, implying that cone connectivity to bipolar cells remains constant throughout the retina. Different subgroups of bipolar cells are distinguished by their relative immunoreactivity to the different antisera. All rod and cone bipolar cells show moderate to strong glutamate-like immunoreactivity. The bipolar cells that show weak to moderate GABA-like immunoreactivity are also labeled with the antiserum to the L7 protein and are thus identified as rod bipolar cells. Nearly half of all cone bipolar cells showed glycine-like immunoreactivity. The results suggest that the inhibitory neurotransmitter candidates GABA and glycine are segregated respectively in rod and cone bipolar cell pathways. A diffuse, cone bipolar cell type can be identified by the anti-parvalbumin and the anti-calbindin antisera. All horizontal cells show parvalbumin-like immunoreactivity. Nearly all amacrine cells show GABA-like or glycine-like immunoreactivity; a variety of subpopulations also show immunoreactivity to one or more of the other markers used.

Neurons of the human retina: a Golgi study

Golgi techniques have been applied to post mortem specimens of human retina. Analysis was possible on 150 human retinas processed and viewed by light microscopy as wholemounts. Camera lucida drawings and photography were used to classify the impregnated neurons into 3 types of horizontal cell, 9 types of bipolar cell, 24 basic types of amacrine cell, a single type of interplexiform cell, and 18 types of ganglion cell. We have distinguished two types of midget bipolar cell: fmB (flat) and imB (invaginating). In central retina, both types are typically single-headed, each clearly contacting a single cone. Peripherally, they may be two- or even three-headed, obviously contacting more than one cone. Two types of small-field diffuse cone bipolars occurring as flat and invaginating varieties are found across the entire retina from fovea to far periphery. The single rod bipolar type appears about 1 mm from the fovea and increases in dendritic tree diameter from there into the far periphery. The putative "ON-center" blue cone bipolar and the giant bistratified bipolar first described by Mariani are also present in human retina and we add two previously undescribed bipolar cell types: a putative giant diffuse invaginating and a candidate "OFF-center" blue cone bipolar. Taking into account the variation of cell size with eccentricity at all points on the retina, we observed three distinct varieties of horizontal cell. The HI is the well known, long-axon-bearing cell of Polyak. HII is the more recently described multibranched, wavy-axoned horizontal cell. The third variety, HIII, introduced here, has been separated from the HI type on morphological criteria of having a larger, more asymmetrical dendritic field and in contacting 30% more cones than the HI at any point on the retina. Amacrine cells proved to be most diverse in morphology. Many of the amacrine cell types that have been described in cat retina (Kolb et al., '81: Vision Res. 21; 1081-1114) were seen in this study. Where there are no equivalent cells in cat, we have adopted the descriptive terminology used by Mariani in monkey retina. Thus eight varieties of small-field amacrines (under 100 microns dendritic trees), eight varieties of medium-field cells (100-500 microns dendritic span), and eight large-field varieties (over 500 microns dendritic trees) have been classified. Often a broadly described variety of amacrine cell can be subdivided into as many as three subtypes dependent on stratification levels of their dendrites in the inner plexiform layer.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of GABA, glycine, glutamate and tyrosine hydroxylase in the human retina

A light microscope study using postembedding immunocytochemistry techniques to demonstrate the common neurotransmitter candidates gamma-aminobutyric acid (GABA), glycine, glutamate, and tyrosine hydroxylase for dopamine has been done on human retina. By using an antiserum to GABA, we found GABA-immunoreactivity (GABA-IR) to be primarily in amacrine cells lying in the inner nuclear layer (INL) or displaced to the ganglion cell layer (GCL). A few stained cells in the INL, which are probably interplexiform cells, were observed to project thin processes towards the outer plexiform layer (OPL). There were heavily stained bands of immunoreactivity in strata 1, 3 and 5 of the inner plexiform layer (IPL). An occasional ganglion cell was also GABA-IR. By using an antiserum to glycine, stained cells were observed at all levels of the INL. Most of these were amacrines, but a few bipolar cells were also glycine-IR. Displaced amacrine cells and large-bodied cells, which are probably ganglion cells, stained in the GCL. The bipolar cells that stained appeared to include both diffuse and midget varieties. The AII amacrine cell of the rod pathway was clearly stained in our material but at a lower intensity than two other amacrine cell types tentatively identified as A8 and A3 or A4. Again, there was stratified staining in the IPL, with strata 2 and 4 being most immunoreactive. An antiserum to glutamate revealed that most of the neurons of the vertical pathways in the human retina were glutamate-IR. Rod and cone photoreceptor synaptic endings labeled as did the majority of bipolar and ganglion cells. The rod photoreceptor stained more heavily than the cone photoreceptor in our material. While both midget and diffuse cone bipolar cell types were clearly glutamate-IR, rod bipolars were not noticeably stained. The most strongly staining glutamate-IR processes of the IPL lay in the outer half, in sublamina a. The antiserum to tyrosine hydroxylase (TOH) revealed two different amacrine cell types. Strongly immunoreactive cells (TOH1) had their cell bodies in the INL and their dendrites ramified in a dense plexus in stratum 1 of the IPL. Fine processes arising from their cell bodies or from the stratum 1 plexus passed through the INL to reach the OPL but did not produce long-ranging ramifications therein. The less immunoreactive amacrines (TOH2) lay in the INL, the center of the IPL or the GCL and emitted thick dendrites that were monostratified in stratum 3 of the IPL.

Interrelationship between retinal ischaemic damage and turnover and metabolism of putative amino acid neurotransmitters, glutamate and GABA

Conditions causing a reduction of oxygen availability (anoxia), such as stroke or diabetes, result in drastic changes in ion movements, levels of neurotransmitters and metabolites and subsequent neural death. Currently, there is no clinically available treatment for anoxia induced neural cell death resulting in drastic and permanent central nervous system dysfunction. However, there have been some exciting developments in experimentally induced anoxic conditions where several classes of drugs appear to significantly reduce neural cell death. This report aims to provide the foundations for understanding both the basic mechanisms involved in retinal ischaemic damage and experimental treatments used to prevent such damage. We discuss the normal release, actions and uptake of the fast retinal neurotransmitters, glutamate and GABA, in the vertebrate retina. Immunocytochemistry is used to demonstrate that both glutamate and GABA are found in the macaque retina. Following this is a discussion on how ischaemia may enhance neurotransmitter release or disrupt its uptake, thus causing an increase in extracellular concentration of these neurotransmitters and subsequent neuronal damage. The mechanisms involved in glutamate neurotoxicity are reviewed, because excess glutamate is the likely cause of retinal ischaemic damage. Finally, the mechanisms behind four possible modes of treatment of neurotransmitter toxicity and their advantages and disadvantages are discussed. Hopefully, further research in this area will lead to the development of a rational therapy for retinal, as well as cerebral ischaemia.

Synaptic inputs to physiologically defined turtle retinal ganglion cells

Two physiologically distinct, HRP-marked turtle retinal ganglion cells were examined for their morphology, GABAergic, glycinergic, and bipolar cell synaptic inputs, using electron-microscopic autoradiography and postembedding immunocytochemistry. One cell was a color-opponent, transient ON/OFF ganglion cell. Its center response to red was a sustained hyperpolarization, and its center response to green was a depolarization with increased spiking at onset. The HRP-injected cell most resembled G6, from previous Golgi-impregnation studies (Kolb, 1982; Kolb et al., 1988). It was a narrow-field bistratified cell, whose two broad dendritic strata peaked at approximately levels L20-25 (sublamina a) and L60 (sublamina b) of the inner plexiform layer. Bipolar cell synapses onto G6 were found evenly distributed between its distal and proximal dendritic strata, spanning L20-75. These inputs probably originated from several different bipolar cells, reflecting the complexity of the center response. GABAergic inputs were found onto both the distal and proximal strata, from near L20-L85. Only a few glycinergic inputs, confined to dendrites at L50-70, were observed. A second ganglion cell type that we physiologically characterized and HRP-injected had sustained ON-center, sustained OFF-surround responses. Two examples were studied; both were bistratified in sublamina b, near L60-70 and L85-100, with branches up to near L40. They resembled G10, from previous Golgi-impregnation studies (Kolb, 1982; Kolb et al., 1988). One cell was partially reconstructed to look at the distributions of GABAergic and glycinergic amacrine cell, and bipolar cell inputs. Although synapses from bipolar cells were equally divided between the two major dendritic strata of G10, the inputs to the distal stratum were close to the soma, and the inputs to the more proximal stratum were on the peripheral dendrites. This arrangement may reflect input from two distinct types of ON-bipolar cell. GABAergic and glycinergic inputs to G10 costratified to both strata and to the distal branches; but where glycinergic inputs were found distributed throughout the arbor, GABAergic inputs appeared to be confined to peripheral dendrites. We hypothesize on the neural elements involved and the circuitry that may underlie the physiologically recorded receptive fields of these two very different ganglion cell types in the turtle retina.

Glutamate, GABA, and glycine in the human retina: an immunocytochemical investigation

The distribution of the neuroactive amino acids glutamate, GABA, and glycine in the human retina was examined in consecutive semithin sections treated with antisera specific for fixed glutamate, GABA, and glycine, respectively. Glutamate immunoreactivity was conspicuous in all photoreceptor cells (rods more strongly labelled than cones), and in a majority (85-89%) of the cells in the inner nuclear layer (INL). Rod spherules and cone pedicles showed a greater enrichment of glutamate immunoreactivity than the parent cell bodies and inner segments. Also, structures of the inner plexiform layer (IPL) were labelled. A large majority (83-91%) of cells in the ganglion cell layer (GCL) was strongly stained, as were most axons in the nerve fibre layer. Müller cell processes appeared unstained. GABA immunoreactivity was present in presumed amacrine but not in bipolar-like cells. The stained cells were restricted to the inner 1/3 of the INL and were more frequent in central than in peripheral retina (40% and 26% of all cells in the inner 1/2 of INL, respectively). GABA positive cell processes, probably originating from interplexiform cells, appeared to traverse the INL and end in the outer plexiform layer. Dense immunolabeling was found in the IPL. GABA immunoreactive cells (some also stained for glutamate) comprised 23% of all GCL cells in the peripheral retina, but only 5% in the central retina. Most of them were localized adjacent to the IPL. A few GABA positive (possibly ganglion) cells extended a single fibre toward the nerve fibre layer. Solitary GABA positive fibres were seen in this layer and in the optic nerve. Glycine immunoreactivity was observed in cells with the location typical of amacrine and bipolar (peripheral retina) cells, as well as in punctate structures of the IPL. In contrast to the GABA positive cells, the glycine positive cells were more frequent in the peripheral than in the central retina (42% and 23% of all cells in inner 1/3 of INL, respectively). A few cells in the GCL (0.5-1.5%) were glycine positive. Glutamate colocalized with GABA or glycine in a majority of the cells stained for either of these inhibitory transmitters (90-95% of the GABA positive cells, and 80-86% of the glycine positive cells, in the INL). Some bipolar cells were stained for both glutamate and glycine. Colocalization of GABA and glycine occurred in a subpopulation (3-4%) of presumed amacrine cells, about half of which was also glutamate positive.

Involvement of glycinergic neurons in the diminished surround activity of ganglion cells in the dark-adapted rabbit retina

Previous studies have reported that the surround responses of retinal ganglion cells weaken or disappear upon dark adaptation. The mechanism(s) by which this occurs is largely unknown, although changes in activity of retinal dopaminergic neurons have been implicated. In the light-adapted rabbit retina, the surround ON responses of OFF-center ganglion cells have been shown to be markedly reduced or abolished by a dopamine antagonist. This effect of a dopamine antagonist was recently shown to be reversed by the glycine antagonist strychnine and by compounds that elevate intracellular cAMP levels. The present study was conducted to determine whether strychnine and cAMP-elevating compounds could bring out the surround ON responses in OFF-center ganglion cells that are diminished upon dark adaptation. Extracellular recordings of OFF-center brisk ganglion cells were made from isolated, superfused retinal preparations. During the course of dark adaptation, the surround ON responses of many cells decreased markedly. Application of low micromolar concentrations of strychnine to the bathing solution brought out the surround ON responses in both brisk-transient and brisk-sustained OFF-center ganglion cells. The center OFF responses of these cells, on the other hand, were not enhanced by strychnine. Of the cAMP-elevating compounds tested, 8-(4-chlorophenylthio) cyclic AMP was the most effective in bringing out the surround ON responses in dark-adapted OFF-center ganglion cells. The effects of bath application of this cAMP analog were very similar to those of strychnine. The findings from this study suggest that under dark-adapted conditions glycinergic neurons inhibit the surround component of OFF-center ganglion cells. The release of glycine from these neurons is suggested to be regulated by a cAMP-dependent mechanism.

Amacrine cells of the rhesus monkey retina

Amacrine cells of the rhesus monkey, Macaca mulatta, were studied in 38 retinas Golgi-impregnated as whole, flat preparations. By using criteria of dendritic morphology, span of arborization, and level of arborization in the inner plexiform layer, 26 types of amacrine cell ranging in size of dendritic span from 30 microns to 2 mm were identified and listed in increasing size of dendritic span. In some instances, different cell types could be grouped together due to similar morphological features. For example, 1 group, "knotty amacrine cells," has small cell bodies and a profusion of small, varicose, intertwined processes that span up to 30 microns and are essentially monostratified, but each of the 3 types ends in different strata. Another group is 2 types with about 20 fine radiating processes spanning 1 mm that possess some prominent varicosities. One of these has all of its processes terminating in the innermost stratum of the inner plexiform layer ("spidery"-type 2 amacrine cells). The other with predominantly similarly ending processes has some that also terminate in the outermost stratum ("spidery"-type 1 amacrines). These 2 cell types likely correspond to the type 1 and type 2 indolamine-accumulating amacrine cells in rabbit retina. Other types are individuals which cannot be grouped together but resemble familiar types in cat retina (AII and A13). Other types can be correlated with their putative neurotransmitter (type 1 CA-dopamine) or transmitter/drug receptor ("spiny"-benzodiazepine receptor) phenotype. Many types as yet have no known correlate from other Golgi studies or clues as to transmitter or receptor phenotype. This study provides evidence for an unprecedented number of amacrine cell types in the primate retina. The similar morphologies of different types of amacrine cell types within a group suggest other common features within these groups such as neurotransmitter phenotype.

Localization of neuropeptide-immunoreactive neurons in the human retina

Light microscopic immunocytochemistry was utilized to localize populations of neurons in the human retina immunoreactive for the following neuroactive peptides: substance P (SP), vasoactive intestinal polypeptide (VIP), somatostatin (SOM) and LANT-6-(H-Lys-Asn-Pro-Tyr-Ile-Leu-OH), a hexapeptide which is identical to the C-terminal half of neurotensin except for the amino acid substitutions Lys/Arg and Asn/Arg. The majority of SP immunoreactive cells were amacrine cells whose pear-shaped or oval cell bodies (about 8 microns in diameter) were situated in the proximal parts of the inner nuclear layer. A small number of SP-stained somas (about 10-15 microns in diameter) were located in the ganglion cell layer and were designated as those of displaced amacrine cells. The SP-immunoreactive processes were distributed in sublamina 1, 3 and 5 with the most dense plexus being found in sublamina 3 of the inner layer. VIP-positive cell bodies (8-9 microns) were oval or pear-shaped and were situated in the innermost cell rows of inner nuclear layer. The majority of fine VIP-immunoreactive processes extended to sublamina 3 with only a few branches distributing in sublamina 1 of the inner plexiform layer. The SOM-stained cell bodies (10-11 microns) were round and were situated in the innermost cell rows of inner nuclear layer. SOM-positive processes were observed in sublamina 1 and 2 of the inner plexiform layer. The LANT-6 immunoreactive cell bodies (12-22 microns) were either oval-, round- or pyriform-shaped and were situated in ganglion cell layer.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of 2-amino-4-phosphonobutyric acid (APB) and glycine on the oscillatory potentials of the rat electroretinogram

Oscillatory potentials of the electroretinogram were monitored in dark-adapted rats following intravitreal injection of 2-amino-4-phosphonobutyric acid (APB), a glutamate analog that preferentially blocks the light response of depolarizing bipolar cells, or glycine, a known endogenous inhibitory neurotransmitter that suppresses the light response of cells in the inner retina postsynaptic to glycinergic neurons. Oscillatory potentials were abolished in conjunction with the b-wave with APB and selectively reduced or eliminated by glycine: neither agent attenuated the a-wave. The results are compatible with the idea that light-induced depolarizing bipolar cell, hyperpolarizing bipolar cell, and glycinergic amacrine cell responses are all necessary for the generation of oscillatory potentials in the rat. The results also suggest that hyperpolarizing bipolar cells do not contribute to b-wave generation.

Immunocytochemical localization of glycine in the retina of the turtle (Pseudemys scripta)

We have localized glycine-like immunoreactivity to provide new anatomical detail about glycinergic neurons in the turtle retina. A rabbit antiserum directed against a glycine/albumin conjugate was used with standard fluorescent and avidin-biotin labeling techniques. Some processes in the outer plexiform layer and many processes in the inner plexiform layer, numerous somata in the inner nuclear layer, and isolated somata in the ganglion cell layer were immunoreactive. The vast majority of labeled neurons were amacrine cells. One class of amacrine cells had well-labeled somata near the inner nuclear/inner plexiform layer border, which gave rise to thick primary processes that entered the inner plexiform layer and arborized near the border of strata 1 and 2 and in stratum 3. A second class of glycinergic neurons, consisting of putative interplexiform cells, was unique in that it gave rise to dendritic arborizations in both the outer plexiform layer and the inner plexiform layer. Some of the immunoreactive neurons in the ganglion cell layer were apparently displaced amacrine cells, while others were probably true ganglion cells because they gave rise to labeled axons, and many labeled axons were visible in the ganglion cell axon layer. These results suggested that glycine played an extensive role in the turtle retina, and that it was involved in many diverse synaptic interactions in both the outer plexiform layer and the inner plexiform layer.

Localization of glycine-containing neurons in the Macaca monkey retina

Autoradiography following 3H-glycine (Gly) uptake and immunocytochemistry with a Gly-specific antiserum were used to identify neurons in Macaca monkey retina that contain a high level of this neurotransmitter. High-affinity uptake of Gly was shown to be sodium dependent whereas release of both endogenous and accumulated Gly was calcium dependent. Neurons labeling for Gly included 40-46% of the amacrine cells and nearly 40% of the bipolars. Synaptic labeling was seen throughout the inner plexiform layer (IPL) but with a preferential distribution in the inner half. Bands of labeled puncta occurred in S2, S4, and S5. Both light and postembedding electron microscopic (EM) immunocytochemistry identified different types of amacrine and bipolar cell bodies and their synaptic terminals. The most heavily labeled Gly+ cell bodies typically were amacrine cells having a single, thick, basal dendrite extending deep into the IPL and, at the EM level, electron-dense cytoplasm and prominent nuclear infoldings. This cell type may be homologous with the Gly2 cell in human retina (Marc and Liu: J. Comp. Neurol. 232:241-260, '85) and the AII/Gly2 of cat retina (Famiglietti and Kolb: Brain Res. 84:293-300, '75; Pourcho and Goebel: J. Comp. Neurol. 233:473-480, '85a). Gly+ amacrines synapse most frequently onto Gly- amacrines and both Gly- and Gly+ bipolars. Gly+ bipolar cells appeared to be cone bipolars because their labeled dendrites could be traced only to cone pedicles. The pattern of these labeled dendritic trees indicated that both diffuse and midget types of biopolars were Gly+. The EM distribution of labeled synapses showed Gly+ amacrine synapses throughout the IPL, but these composed only 11-23% of the amacrine population. Most of the Gly+ bipolar terminals were in the inner IPL, where 70% of all bipolar terminals were labeled. These findings are consistent with previous data from cats and humans and suggest that both amacrine and bipolar cells contribute to glycine-mediated neurotransmission in the monkey retina.

Stratified distribution of synapses in the inner plexiform layer of primate retina

Distributions of bipolar (B) and amacrine (A) synapses and postsynaptic ganglion cell (G) dendritic profiles in the inner plexiform layer (IPL) were analyzed in EM montages of monkey central and human foveal and peripheral retinae. Synapses and profiles were counted and plotted for each 5% interval of IPL, with 0% at the inner edge of the inner nuclear layer and 100% at the outer edge of the ganglion cell layer. In monkey and human retinae, both A and B synapses occur throughout the IPL, but the ratio of A to B synapses varies from 2:1 to more than 6:1. In the monkey central retina, four bands of A conventional synapses are concentrated at 15, 35, 60, and 80% depth. In the human foveal slope, there are two main A bands at 45 and 85%, whereas in the human periphery, there are five bands at 15, 35, 60, 75, and 90%. In both species, A processes containing large dense-core vesicles are concentrated in three bands at 10-20, 50, and 80-90% depth, corresponding to previously described levels of peptides, dopamine, and GABA. B ribbon synapses are distributed fairly evenly throughout the IPL, with a suggestion of four broadly overlapping bands. Most B ribbons are presynaptic to one A and one G (B----A/G). In the human, there are significantly more B dyads with postsynaptic G's (B----A/G, B----G/G) in the fovea (91%) than in the periphery (66%), implying greater A cell processing peripherally. Also in the human, B terminals containing glycogenlike granules are concentrated in the outer half of the IPL, with agranular terminals in the inner half. Our results demonstrate multiple strata containing different types of synaptic contacts in primate IPL.

Neurocircuitry of two types of neurotensin-containing amacrine cells in the turtle retina

The ultrastructural features and synaptic contacts of two types of neurotensin-containing amacrine cells in turtle retina were examined by electron immunocytochemistry, and the retinal peptides were characterized using radioimmunoassay and high-pressure liquid chromatography. The two types of cell were distinguished on the basis of their sizes, dendritic arborizations, synaptic connections and cytoplasmic staining characteristics. Type A cells had lightly labeled cytoplasm and large vertically elongated cell bodies which gave rise to a single primary process which in turn branched and ramified as smooth tapering processes in stratum 3 of the inner plexiform layer. Type A cells received approximately equal synaptic input from amacrine and bipolar cells. Type A amacrines had much more overall synaptic input than synaptic output, and they made conventional synaptic contacts onto bipolar, amacrine, and ganglion cells. Type B cells had a much darker-staining cytoplasm and a smaller cell body which gave rise to numerous delicate beaded dendrites which arborized in strata 3, 4 and 5 of the inner plexiform layer. Type B cells received primarily amacrine and some bipolar cell input. Type B cells had equal amounts of synaptic input and output and they made conventional synaptic contacts onto amacrine, bipolar, and ganglion cells. Whereas there were numerous large vesicles (120 nm diameter) that stained for neurotensin in both types of cells, conventional synaptic vesicles (60 nm diameter) were not labeled. In several cases these large labeled vesicles appeared to fuse with the cell membrane in non-synaptic regions and release their contents into extracellular space, which suggested a non-synaptic release of the neurotensin from type A neurons. Immunochemical and chromatographic studies demonstrated that the neurotensin-related material in retina was indistinguishable from neurotensin found in brain. These results are consistent with a neuroactive role for the neurotensin present within the large vesicles. The differences in the synaptic contacts and dendritic arborizations of the two amacrine cell types suggest they play distinctive functions in visual processing.

The presence of three neuroactive peptides in putative glycinergic amacrine cells of an avian retina

Amacrine cells are retinal interneurons which serve to mediate transmission between bipolar and ganglion cells. To date, in addition to several classical neurotransmitters, a number of neuroactive peptides have been localized to these cells. We have previously demonstrated in the chicken that both peptide-transmitter and peptide-peptide colocalization exists in some amacrine cells. In this report, we show that 3 neuroactive peptides (enkephalin, neurotensin and somatostatin) are present in subpopulations of amacrine cells which also possess a high affinity uptake system for glycine. These observations suggest that the simultaneous visualization by autoradiography of [3H]glycine-uptake and immunocytochemistry of peptides may be useful for distinguishing between different types of putative glycinergic amacrine cells.

The signature hypothesis: co-localizations of neuroactive substances as anatomical probes for circuitry analyses

The recent discoveries that a neuron in the vertebrate retina may contain more than one neuroactive substance (transmitter or neuropeptide) raise the possibility that within each class of neurons, every morphologically and physiologically distinct cell type may be uniquely identified and categorized by the neuroactive substances that it contains. This article examines the evidence to-date for such a conjecture and discusses some of its potential applications and implications.