Receptive Field Properties of Starburst Cholinergic Amacrine Cells in the Rabbit Retina

Patch-clamp recordings were made from ON starburst cholinergic amacrine cells with somas located in the ganglion cell layer of an isolated, dark-adapted rabbit retina preparation. Light responses were analysed and cell identity was confirmed anatomically. The centre light response had a linear current-voltage relation with a reversal potential close to 0 mV. The receptive field size was similar to the dendritic field size. Cholinergic amacrine cells displayed significant surround inhibition. The receptive field profile consisted of a central excitatory region flanked by an inhibitory surround. The surround attenuated the central response to 36% of the maximum. The surround was probably mediated by a combination of presynaptic and postsynaptic inhibition. Starburst amacrine cells did not display action potentials and the presence of a large, voltage-dependent outward current limited depolarizing responses to a maximum potential of about -40 mV. Light responses were completely suppressed during application of 100 μM D, L-2-amino-4-phosphonobutyric acid (APB), consistent with activation exclusively through rod bipolar cells (on) and ON-cone bipolar cells. In darkness the cells displayed a tonic inward current that could be blocked by 100 μM APB and 2 μM CNQX.     

Direct excitatory and lateral inhibitory synaptic inputs to amacrine cells in the tiger salamander retina

Two distinct synaptic currents in amacrine cells were measured using whole cell patch clamp in retinal slices. Synaptic activity was elicited with transretinal current passed from photoreceptors to ganglion cells to depolarize bipolar cell terminals. Recordings made in line with the stimulating electrodes revealed inward synaptic currents that reversed near O mV, positive to spike threshold. Recording 400 micron lateral to the stimulating electrodes revealed synaptic currents that reversed near-60 mV, negative to spike threshold. The lateral signal was blocked by tetrodotoxin and strychnine. A current with reversal potential similar to the lateral input was elicited by direct application of glycine to the amacrine cells. The results suggest that bipolar, but not amacrine input can elicit spikes in neighboring amacrine cells, that amacrine cells are mutually inhibitory and the inhibitory lateral transmission is limited to the extent of processes of the excited, spiking cell.     

A differential effect of APB on ON- and OFF-center ganglion cells in the dark adapted rabbit retina

The glutamate analog, 2-amino-4-phosphonobutyric acid (APB) is a proven tool in exploring the retinal circuit; it has been shown to interfere specifically with the transmission from photoreceptor to depolarizing bipolar cell. Consequently, in photopic retinae, the application of APB disrupts the ON-channel leaving the OFF-channel undisturbed; on the other hand, in the scotopic state, APB application blocks all ganglion cell responses. In this paper, we will show that the ON- and OFF-channels have a differential sensitivity to application of APB. That is to say, APB blocks center responses in ON-ganglion cells at mean concentration of 22 ± 5.1 μM (mean ± standard error of the mean;n = 15) and in OFF-ganglion cells at mean concentration of 91 ± 15.5 μM (n = 16. Since considerable data rule out direct effects of APB on ganglion cells, we hypothesize that this effect is due to a difference in the synaptic gain of ON and OFF pathways in the inner retina.     

Synaptic inputs to physiologically identified retinal X-cells in the cat.

The cat retina contains a number of different classes of ganglion cells, each of which has a unique set of receptive field properties. The mechanisms that underlie the functional differences among classes, however, are not well understood. All of the afferent input to retinal ganglion cells are from bipolar and amacrine cell terminals in the inner plexiform layer, suggesting that the physiological differences among cat retinal ganglion cells might be due to differences in the proportion of input that they receive from these cell types. In this study, we have combined in vivo intracellular recording and labeling with subsequent ultrastructural analysis to determine directly the patterns of synaptic input to physiologically identified X-cells in the cat retina. Our primary aim in these analyses was to determine whether retinal X-cells receive a characteristic pattern of bipolar and amacrine cell input, and further, whether the functional properties of this cell type can be related to identifiable patterns of synaptic input in the inner plexiform layer. We reconstructed the entire dendritic arbor of an OFF-center X-cell and greater than 75% of the dendritic tree of an ON-center X-cell and found that 1) both ON- and OFF-center X-cells are contacted with approximately the same frequency by bipolar and amacrine cell terminals, 2) each of these input types is distributed widely over their dendritic fields, and 3) there is no significant difference in the pattern of distribution of bipolar and amacrine cell synapses onto the dendrites of either cell type. Comparisons of the inputs to the ON- and the OFF-center cell, however, did reveal differences in the complexity of the synaptic arrangements found in association with the two neurons; a number of complex synaptic arrangements, including serial amacrine cell synapses, were found exclusively in association with the dendrites of the OFF-center X-cell. Most models of retinal X-cell receptive fields, because their visual responses share a number of features with those of bipolar cells, have attributed X-cell receptive field properties to their bipolar cell inputs. The data presented here, the first obtained from analyzing the inputs to the entire dendritic arbors of retinal X-cells, demonstrate that these retinal ganglion cells receive nearly one-half of their input from amacrine cells. These results clearly indicate that further data concerning the functional consequences of amacrine cell input are needed to understand more fully visual processing in the X-cell pathway.     

The effects of strychnine and bicuculline on the responses of X- and Y-cells of the isolated eye-cup preparation of the cat

The effects of strychnine and bicuculline, the respective antagonists of glycine and GABA, on the inhibitory responses of X- and Y-type retinal ganglion cells of the cat were investigated using an isolated eye-cup preparation. The surround inhibition of the on-center X-cell was blocked by strychnine, whereas that of the on-center Y-cell was blocked by bicuculline. In the case of the off-center cells, bicuculline indifferently blocked the center and the surround responses of either the X-cell or the Y-cell, but strychnine did not.     

The inner plexiform layer of the vertebrate retina: a quantitative and comparative electron microscopic analysis

The inner plexiform layer of human, monkey, cat, rat, rabbit, ground squirrel, frog and pigeon retinas was studied by electron microscopy. All showed the same qualitative synaptic arrangements: bipolar cells made dyad ribbon synapses onto amacrine and ganglion cells; amacrine cells made conventional synaptic contacts onto bipolar, ganglion cells; amacrine cells montage of electron micrographs through the full thickness of the inner plexiform layer were made for each species and were scored for synaptic contacts. Both absolute and relative quantitative differences were found between species. The ratio of amacrine cell (conventional) synapses to bipolar cell (ribbon) synapses, the absolute number of amacrine cell synapses and the number of inter-amacrine cell synapses were all found to be higher in those animals which are known to have relatively complex retinal ganglion cell receptive field properties. It is suggested that the amacrine cell is involved in mediating complex visual transformations in certain vertebrate retinas.     

Synaptology of physiologically identified ganglion cells in the cat retina: A comparison of retinal X- and Y-cells

It has long been known that a number of functionally different types of ganglion cells exist in the cat retina, and that each responds differently to visual stimulation. To determine whether the characteristic response properties of different retinal ganglion cell types might reflect differences in the number and distribution of their bipolar and amacrine cell inputs, we compared the percentages and distributions of the synaptic inputs from bipolar and amacrine cells to the entire dendritic arbors of physiologically characterized retinal X- and Y- cells. Sixty-two percent of the synaptic input to the Y-cell was from amacrine and bipolar cells. We found no significant difference in the distributions of bipolar or amacrine cell inputs to X- and Y- cells, or ON-center and OFF-center cells, either as a function of dendritic branch order or distance from the origin of the dendritic arbor. While, on the basis of these data, we cannot exclude the possibility that the difference in the proportion of bipolar and amacrine cell input contributes to the functional differences between X- and Y- cells, the magnitude of this difference, and the similarity in the distributions of the input from the two afferent cell types, suggest that mechanisms other than a simple predominance of input from amacrine or bipolar cells underlie the differences in their response properties. More likely, perhaps, is that the specific response features of X- and Y- cells originated in differences in the visual responses of the bipolar and amacrine cells that provides their input, or in the complex synaptic arrangements found among amacrine and bipolar cell terminals and dendrites of specific types of retinal ganglion cells. © 1994 Wiley-Liss, Inc.     

Connectivity of glycine immunoreactive amacrine cells in the cat retina

The synaptic relationships of glycine immunoreactive amacrine cells in the cat retina were studied through the use of postembedding immunogold techniques. Glycine immunoreactive amacrine cells were found to synapse extensively with other amacrines and ganglion cells, particularly in strata 1-3 of the inner plexiform layer. This contrasts with GABA immunoreactive amacrine cells which provide major input to bipolar cells in strata 3-5. Glycine containing amacrine terminals exhibited diversity with respect to the morphology of their synaptic vesicles. The three types of terminals which could be distinguished were characterized by small pleomorphic (32-35 nm), medium-sized flattened (38-45 nm), or larger rounded (48-55 nm) vesicles. Comparison of retinal sections processed for glycine immunoreactivity with adjacent sections stained for GABA reactivity revealed a colocalization of glycine and GABA in 3% of the cells in the amacrine layer and approximately 40% of the cells in the ganglion cell layer. The amacrine terminals in which glycine and GABA were colocalized typically contained the small pleomorphic type of vesicles.     

Glycinergic input to carp retinal ganglion cells may be mediated by glycine receptors with homologous kinetics

Current responses of carp retinal ganglion cells (RGCs) retrogradely labeled and freshly dissociated to rapid application of glycine were recorded by whole-cell patch clamp techniques and effects of glycine antagonists on these responses were analyzed. The current response to maintained application of glycine at a concentration higher than 30 microM exhibited desensitization, which was well fitted to a monoexponential function. Strychnine (1 microM), a glycine receptor antagonist, completely blocked the response to 100 microM glycine. Strychnine at a concentration range between 10 and 200 nM suppressed the response to 100 microM glycine in a dose-dependent manner, and only a slow-activated and sustained current eventually remained in the presence of 200 nM strychnine. Power spectral density (PSD) analysis revealed no changes in the density-frequency dependence caused by strychnine. It was further shown that dissociation of strychnine from glycine receptors was rather slow. Moreover, Zn(2+) exerted similar dual action on this sustained response and the response in Ringer's: potentiating and reducing them at low and high concentrations of Zn(2+), respectively. 5,7-Dichlorokynurenic acid (DCKA, 500 microM), a selective blocker of the glycine recognition site at the NMDA receptor, partially reduced the glycine response, but without changing its kinetics. These results suggest that glycinergic input to carp ganglion cells may be mediated by strychnine-sensitive glycine receptors with homologous kinetics, and slow dissociation of strychnine from glycine receptors may partially account for the changes in glycine response kinetics occurring in the presence of strychnine.     

Agonist-stimulated cobalt uptake provides selective visualization of neurons expressing AMPA- or kainate-type glutamate receptors in the retina

Fast-acting excitatory neurotransmission in the retina is mediated primarily by glutamate, acting at alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) -selective and kainate-selective receptors. To localize these sites of action, cat retinas were stimulated with either AMPA or kainate and processed for histochemical visualization of cobalt uptake through calcium-permeable channels. Treatment with both agonists resulted in staining of A- and B-type horizontal cells and several types of OFF cone bipolar cells; there was no evidence for staining of ON cone bipolar cells or rod bipolar cells. The subpopulations of OFF cone bipolar cells differed in their responses with two distinct types that stained heavily with cobalt after exposure to AMPA and three different types that were preferentially labeled after exposure to kainate. Although many amacrine and ganglion cells appeared to respond to both agonists, AII amacrine cells were stained after stimulation by AMPA but not by kainate. The OFF cone bipolar cells that exhibit AMPA-stimulated cobalt uptake were found to have a high level of correspondence with cells that show immunocytochemical staining for the AMPA-selective glutamate receptor subunits GluR1 and GluR2/3. Similarly, the cone bipolar cells exhibiting kainate-stimulated cobalt uptake resemble those that are immunoreactive for the kainate subunit GluR5. The results indicate that, whereas many retinal neurons express both AMPA and kainate receptors, AII amacrine cells and subpopulations of OFF cone bipolar cells are limited to the expression of either AMPA or kainate receptors. This differential expression may contribute to the unique character of transmission by these cell types.     

GABA-like immunoreactive neurons in the retina of Bufo marinus: evidence for the presence of GABA-containing ganglion cells.

gamma-Aminobutyric acid (GABA)-like immunoreactive (IR) neurons in the retina of the cane toad Bufo marinus were revealed using immunohistochemistry on retinal wholemount preparation and sectioned material. GABA-IR neurons included horizontal, bipolar and amacrine cells in the inner nuclear layer and small to medium sized cells in the ganglion cell layer. A few IR axons were seen in the optic fiber layer of the retina. Following the injection of the carbocyanine dye, DiI into the optic tectum ganglion cells were retrogradely filled. A small population of DiI-filled ganglion cells (2.8%) was found to be GABA-IR. GABA-IR neurons in the ganglion cell layer without DiI label were considered to be displaced amacrine cells of which 45.3% were GABA positive. It is proposed that GABA-containing ganglion cells may form an inhibitory projection to visual centers of the anuran brain.     

Electrophysiological analysis of taurine and glycine action on neurons of the midpuppy retina. I. Intracellular recording

Intracellular recordings were carried out in the prefused retine-eyecup preparation of the mudpuppy. Taurine and glycine were added to the bathing medium to study their effects on different retinal neurons. In a few cases, gamma-aminobutyric acid was exogenously applied to compare GABA vs taurine/glycine action. Receptors and horizontal cells were relatively insensitive to taurine/glycine, while amacrines and ganglion cells were comparatively more sensitive to these agents. Bipolar cells proved to be differentially effected by inhibitory amino acids: hyperpolarizing (OFF) bipolars were depressed by taurine/glycine and proved less sensitive to GABA; depolarizing (ON) bipolars were suppressed by GABA and were comparatively less sensitive to glycine/taurine. Taurine and glycine had identical actions on neurons and both were about equally effective at the same concentration. Strychnine blocked the action of taurine and glycine. The patterns of glycine/taurine sensitivity and their effects on second order neurons eliminate taurine as a photoreceptor transmitter; one or both of these agents may be utilized by a subclass of amacrine cells which interact with hyperpolarizing bipolars, other amacrine cells and ganglion cells. It appears that taurine or glycine or both may be selectively involved in OFF channel activity, while GABA may subserve an equivalent role for the ON channel.     

Electrophysiological analysis of taurine and glycine action on neurons of the mudpuppy retina. I. Intracellular recording

Intracellular recordings were carried out in the perfused retina-eyecup preparation of the mudpuppy. Taurine and glycine were added to the bathing medium to study their effects on different retinal neurons. In a few cases, γ-aminobutyric acid was exogenously applied to compare GABA vs taurine/glycine action. Receptors and horizontal cells were relatively insensitive to taurine/glycine, while amacrines and ganglion cells were comparatively more sensitive to these agents. Bipolar cells proved to be differentially effected by inhibitory amino acids: hyperpolarizing (OFF) bipolars were depressed by taurine/glycine and proved less sensitive to GABA; depolarizing (ON) bipolars were suppressed by GABA and were comparatively less sensitive to glycine/taurine. Taurine and glycine had identical actions on neurons and both were about equally effective at the same concentration. Strychnine blocked the action of taurine and glycine. The patterns of glycine/taurine sensitivity and their effects on second order neurons eliminate taurine as a photoreceptor transmitter; one or both of these agents may be utilized by a subclass of amacrine cells which interact with hyperpolarizing bipolars, other amacrine cells and ganglion cells. It appears that taurine or glycine or both may be selectively involved in OFF channel activity, while GABA may subserve an equivalent role for the ON channel.     

Off-alpha and OFF-beta ganglion cells in cat retina: II. Neural circuitry as revealed by electron microscopy of HRP stains

An OFF-center alpha and an OFF-center beta ganglion cell in cat retina, which had been recorded from and intracellularly stained with horseradish peroxidase (HRP) were examined by serial section electron microscopy. We counted synapses and identified presynaptic neurons to the HRP-stained cells in 20 μm radial slices through the centers of their dendritic trees. Presynaptic amacrine and bipolar cells were identified on cytological criteria known from previous studies. The OFF-beta cell with a 62 μm dendritic arbor, restricted to S1 and S2 (sublamina a) of the inner plexiform layer (IPL), received 38% bipolar and 62% amacrine cell synapses. The bipolar input was from both cb1 and cb2 cone bipolar types. Input from three distinct amacrine cell types occurred upon the dendrites, namely from: (1) AII amacrine lobular appendages, (2) large pale amacrine profiles (possibly A2 or A3 cells), and (3) small, dark amacrine types (possibly A8 cells). Large pale amacrine profiles (possibly A13) were found on the cell body and apical dendrite in sublamina b of the IPL. In addition, several amacrine profiles synapsed directly on the sides and base of the cell body in the ganglion cell layer. We estimate that the complete dendritic tree of this beta cell received about 1,000 synapses contributed by 12–14 bipolar cells, 7–10 AII amacrines and 28–41 other amacrine cells. The OFF-alpha cell had a dendritic tree size of 680 × 920 μm. A 250 μm length of two major dendrites stratifying narrowly in S2 of the IPL was reconstructed. Amacrine cells provided most of the synaptic input (80%). This input came from: (1) AII amacrine lobular appendages, (2) amacrines exhibiting large, pale synaptic profiles (possibly A2 or A3 cells), (3) pale amacrines with large mitochondria and a few neurotubules (unknown type), and (4) densely neurotubule-filled amacrine profiles (possibly A19 cells). A large pale amacrine cell type (possibly A13) provided synaptic input to the cell body as a serial synaptic intermediary with rod bipolar cells. Cone bipolar synapses were from only one type of cone bipolar, the cb2 type and formed 20% of the total synaptic input. We estimate that a minimum of 142 bipolar cells, 256 AII amacrine cells and 1,011 other amacrine cells, altogether providing 6,000–10,000 synapses, converged on the dendritic tree of this OFF-alpha cell. © 1993 Wiley-Liss, Inc.     

Calbindin-D28k and calretinin as markers of retinal neurons in the anuran amphibian Rana perezi

In the present study we have analyzed the distribution of the calcium binding proteins calbindin-D28k (CB) and calretinin (CR) immunoreactive cells in the retina of the anuran Rana perezi using poly- and mono-clonal antibodies that were proven to be specific in the amphibian brain, without cross-reactivity. Double immunohistofluorescence techniques were used to demonstrate colocalization of both proteins in the same retinal cells. In addition, retrograde tracing experiments from the optic nerve were conducted to labeled ganglion cells and these were observed in combination with CB and/or CR immunohistochemistry. Cells containing CB were identified as all cones, scattered bipolar and amacrine cells together with cells in the ganglion cell layer. The pattern of CR immunoreactivity was strikingly different. Abundant cells contained CR in the inner retinal layers including horizontal, bipolar and amacrine cells, and cells in the ganglion cell layer. By means of double immunohistochemistry it was found that only subpopulations of amacrine cells and cells in the ganglion cell layer contained both CB and CR. Tracing from the optic nerve revealed retrogradely labeled ganglion cells with different morphologies and most of them contained CB and/or CR. All these data taken together suggest that in amphibians CB and CR are distinctly and precisely distributed in retinal neurons showing, however, peculiar features not observed previously in other vertebrates.     

Postnatal development of GABA- and glycine-mediated inhibition of feline retinal ganglion cells in the area centralis

Intraretinal iontophoresis in the optically intact eye of adult cats (18-22 weeks of age) and kittens (7-9 weeks of age) under pentobarbitone anaesthesia was performed. Studies were concentrated on retinal ganglion cells of the sustained (X) type in the area centralis under photopic conditions. In both the adult and kitten, gamma-aminobutyric acid (GABA) and muscimol inhibited the visually induced excitation, and bicuculline blocked the visually induced inhibition of on-cells. On the other hand, glycine inhibited the excitation and strychnine blocked the inhibition of off-cells. However, a greater current of GABA (muscimol) and glycine was required to produce total inhibition in kitten's on- and off-cells respectively when compared with the adult's. Furthermore, a smaller current of bicuculline and strychnine was needed to abolish the visually induced inhibition of kitten on- and off-cells respectively when compared with the adult's. In the adult, GABA and glycine did not affect the responses of off- and on-cells respectively, but in the kitten GABA inhibited off-cells and glycine inhibited on-cells to some extent. In neither the adult nor the kitten did bicuculline have any effect upon off-cells or strychnine any effect upon on-cells. Thus, the sustained on- and off-cells in the kitten area centralis exhibit: a reduced selectivity to inhibitory transmitters; a reduced sensitivity to exogenously applied inhibitory transmitter agonists; but a greater sensitivity to inhibitory transmitter antagonists, in comparison with the sustained on- and off-cells in the adult area centralis. The observed differences between the kitten and adult cat in transmitter actions on retinal ganglion cells appear to be analogous to those found in the postnatal development of functional synapses at the neuromuscular junction and sympathetic ganglia.     

Do N-methyl-D-aspartate receptors mediate synaptic responses in the mudpuppy retina?

Whole-cell recordings of amacrine and ganglion cells in the superfused retina-eyecup preparation of the mudpuppy were obtained in order to determine which excitatory amino acid receptor (EAAR) subtype mediates the synaptic responses of these neurons. All third-order retinal neurons tested were depolarized by kainic acid (KA, N-methyl-D-aspartate (NMDA), and quisqualate (QQ). The responses evoked by NMDA were blocked by the addition of D-2-amino-5-phosphonovaleric acid (D-AP5) and D-2-amino-7 phosphonoheptonoic acid (D-AP7) to the perfusate. When the actions of exogenously applied NMDA were completely blocked by D-AP5 and D-AP7, the light-evoked responses of inner retinal neurons persisted without any apparent reduction or, alternatively, a slight enhancement of the response was observed. Light-evoked responses of bipolar, amacrine, and ganglion cells associated with the On pathway were attenuated by L-AP5 in a manner similar to its lower-order homolog L-2-amino-4-phosphonobutyrate (AP4); nevertheless, L-AP5 was not an effective NMDA antagonist. Although synaptic transmission between retinal second- and third-order neurons appears to be mediated by EAARs, the NMDA receptor does not appear to play a prominent role under our experimental conditions. Nevertheless, our results suggest that the racemic mixture of AP5 should not be used as an NMDA antagonist in retinal research, due to the AP4-like actions of its L-enantiomer.     

Differential regulation of ciliary neurotrophic factor receptor-α expression in all major neuronal cell classes during development of the chick retina

Ciliary neurotrophic factor (CNTF) exerts a multiplicity of effects on a broad spectrum of target cells, including retinal neurons. To investigate how this functional complexity relates to the regulation of CNTF receptor α (CNTFRα) expression, we have studied the developmental expression of the receptor protein in chick retina by using immunocytochemistry. During the course of development, the receptor is expressed in all retinal layers, but three levels of specificity can be observed. First, the expression is regulated temporally with immunoreactivity observed in ganglion cells (embryonic day 8 [E8] to adult), photoreceptor precursors (E8–E12), amacrine cells (E10 to adult), bipolar cells (E12–E18), differentiated rods (E18 to adult), and horizontal cells (adult). Second, expression is restricted to distinct subpopulations of principal retinal neurons: preferentially, large ganglion cells; subpopulations of amacrine cells, including a particular type of cholinergic neuron; a distinctly located type of bipolar cell; and rod photoreceptors. Third, expression exhibits subcellular restriction: it is confined largely to dendrites in mature amacrine cells and is restricted entirely to outer segments in mature rods. These data correlate with CNTF effects on the survival of ganglion cells and mature photoreceptors, the in vitro differentiation of photoreceptor precursors and cholinergic amacrine cells, and the number of bipolar cells in culture described here or in previous studies. Thus, our results demonstrate an exceptional degree of complexity with respect to the regulation of neuronal CNTFRα expression in a defined model system. This suggests that the same signaling pathway is used to mediate a variety of regulatory influences, depending on the developmental stage and cell type. J. Comp. Neurol. 400:244–254, 1998. © 1998 Wiley-Liss, Inc.     

Distribution of muscarinic acetylcholine receptors on processes of isolated retinal cells

Binding of propylbenzilylcholine mustard, a muscarinic acetylcholine receptor antagonist, to isolated retinal cells was examined with light microscopic autoradiography. Dissociation of the adult tiger salamander retina yielded identifiable rod, cone, horizontal, bipolar, amacrine/ganglion, and Müller cells. Preservation of fine structure was assessed with conventional electron microscopy. For all cell types, the plasmalemma was intact and free of adhering debris; in addition, presynaptic ribbon complexes were present in photoreceptor and bipolar axon terminals indicating that synaptic structures were retained. Specific binding to cell bodies and processes was analyzed separately by using morphometric and statistical techniques. The highest grain densities occurred on processes of amacrine/ganglion cells and axons and 2 degrees and 3 degrees dendrites of bipolar neurons. Bipolar cells, however, seemed to be a heterogeneous population because there was great variation in the density of binding sites on both their axons and distal dendrites. Intermediate levels of binding were found on bipolar 1 degree dendrites and horizontal cells. No specific binding was detected on Müller cells and most parts of photoreceptors. Comparisons between cells showed that grain densities were similar for bipolar axons and amacrine/ganglion cell processes but bipolar dendrites were richer in binding sites than horizontal cell dendrites. Thus, muscarinic receptors in the salamander retina are located on amacrine/ganglion, bipolar, and horizontal cells and primarily confined to the processes which compose the two synaptic layers. In the inner plexiform layer, muscarinic receptors reside on processes from all three inner retinal neurons: in the outer synaptic layer, receptors are only on second-order cells and are more numerous along bipolar than horizontal cell dendrites.     

Ultrastructural demonstration of L-glutamate decarboxylase and cysteinesulfinic acid decarboxylase in rat retina by immunocytochemistry

The gamma-aminobutyric acid (GABA) synthesizing enzyme, L-glutamate decarboxylase (GAD), and the taurine synthesizing enzyme, cysteinesulfinic acid decarboxylase (CSAD) have been localized in rat retina at the ultrastructural level by indirect immunoelectron microscopy. GAD immunoreactivity (GAD-IR) was seen only in some amacrine cells and their terminals. CSAD immunoreactivity (CSAD-IR) was found in most retinal neuronal types and their processes including photoreceptor cells (rod and cone cells), bipolar cells, amacrine cells and ganglion cells. The GAD-IR positive amacrine terminals have been found to make synaptic contact with other GAD-IR negative bipolar and amacrine terminals, and ganglion cell dendrites. Most of the GAD-IR positive terminals are presynaptic. Occasionally, GAD-IR positive amacrine terminals are postsynaptic to another amacrine terminal or ganglion cell body. In the inner plexiform layer, CSAD-IR positive amacrine terminals also make synaptic contacts with other nerve terminals, similar to that of GAD-IR positive amacrine terminals. In addition, CSAD-IR positive bipolar terminals make synaptic contact with some CSAD-IR positive as well as negative amacrine terminals. Both CSAD-IR positive amacrine and bipolar terminals are mostly presynaptic to other CSAD-IR negative terminals. In the outer plexiform layer, CSAD-IR was found to be associated with synaptic vesicles and the synaptic membrane in certain cone pedicles and rod spherules. It is concluded that only a fraction of amacrine cells in rat retina may use GABA as a neurotransmitter. The presence of CSAD-IR in some amacrine, bipolar, photoreceptor and ganglion cells in rat retina is compatible with the notion that taurine may play some important roles, such as those of neurotransmitter or neuromodulator in mammalian retina.