Glycinergic contacts in the outer plexiform layer of the Xenopus laevis retina characterized by antibodies to glycine, GABA and glycine receptors

Role of neurotransmitter receptors in mediating light-evoked responses in retinal interplexiform cells

Interplexiform (IP) cells are a long-neglected group of retinal neurons the function of which is yet to be determined. Anatomical study indicates that IP cells are located in the inner nuclear layer, juxtaposed with the third-order neurons. However, the synaptic transmission of IP cells in the inner retina is poorly understood. Using whole cell patch-clamp and pharmacological techniques, we extensively studied synaptic receptors in IP cells. The IP cells in amphibian retinal slices were identified by electrical and morphological properties with voltage-clamp recording and Lucifer yellow dialysis. We find that light-evoked excitatory postsynaptic currents (L-EPSCs) are mediated by AMPA and N-methyl-d-aspartate receptors in IP cells. Although both receptors contributed to the amplitude and kinetics of L-EPSCs, AMPA receptor desensitization substantially shaped L-EPSCs in the neurons, similar to those found in the third-order neurons. The light-evoked inhibitory postsynaptic currents (L-IPSCs) in IP cells were primarily mediated by strychnine-sensitive glycine receptors with a small component of GABA(C) receptors. GABA(C) receptor rho2 subunits were detected in IP cells with single-cell RT-PCR assays. Expression of GABA(C) receptors is one of the special features for IP cells, distinct from most of the third-order neurons that depend on GABA(A) and glycine receptors to relay the inhibitory signals. However, GABA(A) receptors in IP cells acted like nonsynaptic receptors that were activated by exogenous GABA application. Furthermore, L-IPSCs in IP cells were inhibited by the serial inhibitions between amacrine cells in the inner retina. In addition, application of neurotransmitters on the axon terminals of IP cells had no significant current generated in the cells, indicating that the synaptic inputs of IP cells are mainly from the inner retina. This study demonstrates the important role that light signals are encoded by both experiment of inhibitory receptors in IP cells.

Caffeine inhibition of ionotropic glycine receptors

We found that caffeine is a structural analogue of strychnine and a competitive antagonist at ionotropic glycine receptors (GlyRs). Docking simulations indicate that caffeine and strychnine may bind to similar sites at the GlyR. The R131A GlyR mutation, which reduces strychnine antagonism without suppressing activation by glycine, also reduces caffeine antagonism. GlyR subtypes have differing caffeine sensitivity. Tested against the EC(50) of each GlyR subtype, the order of caffeine potency (IC(50)) is: alpha2beta (248 +/- 32 microm) alpha3beta (255 +/- 16 microm) > alpha4beta (517 +/- 50 microm) > alpha1beta(837 +/- 132 microm). However, because the alpha3beta GlyR is more than 3-fold less sensitive to glycine than any of the other GlyR subtypes, this receptor is most effectively blocked by caffeine. The glycine dose-response curves and the effects of caffeine indicate that amphibian retinal ganglion cells do not express a plethora of GlyR subtypes and are dominated by the alpha1beta GlyR. Comparing the effects of caffeine on glycinergic spontaneous and evoked IPSCs indicates that evoked release elevates the glycine concentration at some synapses whereas summation elicits evoked IPSCs at other synapses. Caffeine serves to identify the pharmacophore of strychnine and produces near-complete inhibition of glycine receptors at concentrations commonly employed to stimulate ryanodine receptors.

Functional expression of the glycine transporter 1 on bullfrog retinal cones

Using patch clamp techniques, we characterized glycine-induced currents from cones in bullfrog retinal slices. Application of glycine to cone terminals induced an inward current, which was in part suppressed by strychnine. The remaining strychnine-resistant current component, which did not show polarity reversion in a range of -120 mV to +40 mV, was blocked by N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl] sarcosine, an antagonist of glycine transporter 1 (GlyT1), but not affected by amoxapine, an inhibitor of glycine transporter 2. Application of sarcosine, an agonist of GlyT1, to cone terminals induced an inward current that was completely suppressed by N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl] sarcosine or when external Na in Ringer's was replaced by choline. All these results show for the first time the functional expression of GlyT1 on bullfrog cones.

Glycine input induces the synaptic facilitation in salamander rod photoreceptors

Glycinergic synapses in photoreceptors are made by centrifugal feedback neurons in the network, but the function of the synapses is largely unknown. Here we report that glycinergic input enhances photoreceptor synapses in amphibian retinas. Using specific antibodies against a glycine transporter (GlyT2) and glycine receptor beta subunit, we identified the morphology of glycinergic input in photoreceptor terminals. Electrophysiological recordings indicated that 10 muM glycine depolarized rods and activated voltage-gated Ca(2+) channels in the neurons. The effects facilitated glutamate vesicle release in photoreceptors, meanwhile increased the spontaneous excitatory postsynaptic currents in Off-bipolar cells. Endogenous glycine feedback also enhanced glutamate transmission in photoreceptors. Additionally, inhibition of a Cl(-) uptake transporter NKCC1 with bumetanid effectively eliminated glycine-evoked a weak depolarization in rods, suggesting that NKCC1 maintains a high Cl(-) level in rods, which causes to depolarize in responding to glycine input. This study reveals a new function of glycine in retinal synaptic transmission.

Differential distribution of glycine transporters in Müller cells and neurons in amphibian retinas

Amphibian retinas are commonly used for electrophysiological studies on neural function and transduction because they share the same general properties as higher vertebrate retinas. Glycinergic synapses have been well described in amphibian retinas. However, the role of glycine transporters in the synapses is largely unknown. We studied the distribution and function of glycine transporters in the retinas from tiger salamanders, mudpuppies, and leopard frogs by immunofluorescence labeling and whole-cell recording methods. Our results indicated that GlyT1- and GlyT2-like transporters were present in Müller cells and neurons, respectively. GlyT1 labeling was present in Müller glial cells and co-localized with Glial fibrillary acidic protein (GFAP), a Müller cell marker, whereas the GlyT2 immunoreactivity was present in the somas of amacrine cells (ACs) and processes in the inner plexiform layer (IPL) and the outer plexiform layer (OPL). Because the axon processes of glycinergic interplexiform cells (IPCs) are the only source of glycine input in the OPL, GlyT2 staining revealed a spatial pattern of the axon processes of IPCs in the OPL. The function of GlyT2 in the IPCs was studied in tiger salamander retinal horizontal cells (HCs) by whole-cell gramicidin perforated recording. The results demonstrated that inhibition of GlyT2 by a specific inhibitor, amoxapine, increased a tonic glycine input to HCs. Thus, the GlyT2 transporter is responsible for uptake of synaptic glycine in the outer retina. We also compared the distribution of glycine transporters in other amphibian species: salamander, mudpuppy, and frog. The results are consistent with the general pattern that GlyT1-like transporters are present in Müller cells and GlyT2-like transporters in neurons in amphibian retinas.

Glycine receptors are functionally expressed on bullfrog retinal cone photoreceptors

Using immunocytochemical and whole cell recording techniques, we examined expression of glycine receptors on bullfrog retinal cone photoreceptors. Immunofluorescence double labeling experiments conducted on retinal sections and isolated cell preparations showed that terminals and inner segments of cones were immunoreactive to both alpha1 and beta subunits of glycine receptors. Moreover, application of glycine induced a sustained inward current from isolated cones, which increased in amplitude in a dose-dependent manner, with an EC50 (concentration of glycine producing half-maximal response) of 67.3+/-4.9 microM, and the current was blocked by the glycine receptor antagonist strychnine, but not 5,7-dichlorokynurenic acid (DCKA) of 200 microM, a blocker of the glycine recognition site at the N-methyl-D-aspartate (NMDA) receptor. The glycine-induced current reversed in polarity at a potential close to the calculated chloride equilibrium potential, and the reversal potential was changed as a function of the extracellular chloride concentration. These results suggest that strychnine-sensitive glycine receptors are functionally expressed in bullfrog cones, which may mediate signal feedback from glycinergic interplexiform cells to cones in the outer retina.

Characterization of glycinergic synapses in vertebrate retinas

Glycine is one of the essential neurotransmitters modulating visual signals in retina. Glycine activates Cl(-) permeable receptors that conduct either inhibitory or excitatory actions, depending on the Cl(-) electrical-chemical gradient (E (Cl)) positive or negative to the resting potential in the cells. Interestingly, both glycine-induced inhibitory and excitatory responses are present in adult retinas, and the effects are confined in the inner and outer retinal neurons. Glycine inhibits glutamate synapses in the inner plexiform layer (IPL), resulting in shaping light responses in ganglion cells. In contrast, glycine excites horizontal cells and On-bipolar dendrites in the outer plexiform layer (OPL). The function of glycinergic synapse in the outer retina represents the effect of network feedback from a group of centrifugal neurons, glycinergic interplexiform cells. Moreover, immunocytochemical studies identify glycine receptor subunits (alpha1, alpha2, alpha3 and beta) in retinas, forming picrotoxin-sensitive alpha-homomeric and picrotoxin-insensitive alpha/beta-heteromeric receptors. Glycine receptors are modulated by intracellular Ca(2+) and protein kinas C and A pathways. Extracellular Zn(2+) regulates glycine receptors in a concentration-dependent manner, nanomolar Zn(2+) enhancing glycine responses, and micromolar Zn(2+) suppressing glycine responses in retinal neurons. These studies describe the function and mechanism of glycinergic synapses in retinas.

Characterization of the glycinergic input to bipolar cells of the mouse retina

Glycine and gamma-aminobutyric acid (GABA) are the major inhibitory transmitters of the mammalian retina, and bipolar cells receive GABAergic and glycinergic inhibition from multiple amacrine cell types. Here we evaluated the functional properties and subunit composition of glycine receptors (GlyRs) in bipolar cells. Patch-clamp recordings were performed from retinal slices of wild-type, GlyRalpha1-deficient (Glra1(spd-ot)) and GlyRalpha3-deficient (Glra3(-/-)) mice. Whole-cell currents following glycine application and spontaneous inhibitory postsynaptic currents (IPSCs) were analysed. During the recordings the cells were filled with Alexa 488 and, thus, unequivocally identified. Glycine-induced currents of bipolar cells were picrotoxinin-insensitive and thus represent heteromeric channels composed of alpha and beta subunits. Glycine-induced currents and IPSCs were absent from all bipolar cells of Glra1(spd-ot) mice, indicating that GlyRalpha1 is an essential subunit of bipolar cell GlyRs. By comparing IPSCs of bipolar cells in wild-type and Glra3(-/-) mice, no statistically significant differences were found. OFF-cone bipolar (CB) cells receive a strong glycinergic input from AII amacrine cells, that is preferentially based on the fast alpha1beta-containing channels (mean decay time constant tau = 5.9 +/- 1.4 ms). We did not observe glycinergic IPSCs in ON-CB cells and could elicit only small, if any, glycinergic currents. Rod bipolar cells receive a prominent glycinergic input that is mainly mediated by alpha1beta-containing channels (tau = 5.5 +/- 1.6 ms). Slow IPSCs, the characteristic of GlyRs containing the alpha2 subunit, were not observed in bipolar cells. Thus, different bipolar cell types receive kinetically fast glycinergic inputs, preferentially mediated by GlyRs composed of alpha1 and beta subunits.

Immunocytochemical study of glycine receptors in the retina of the frog Xenopus laevis

The expression of glycine receptors in the retina of clawed frog, Xenopus laevis was studied immunocytochemically. Glycine receptors (GlyRs), as revealed by means of several different antibodies, were mainly distributed in the inner (IPL) and the outer plexiform layers. Their composition was determined to include alpha2 and alpha3 subunits. Typical punctate appearance and specific lamination in the IPL were seen with each of the antibodies directed against the different GlyRs' subunits. A notion for diversity of the glycine receptors was put forward, according to which the alpha2 and alpha3 subunits are located in different subtypes of glycine synapses.

Glycine receptors in a population of adult mammalian cones

Glycinergic interplexiform cells provide a feedback signal from the inner retina to the outer retina. To determine if cones receive such a signal, glycine was applied on cultured porcine cone photoreceptors recorded with the patch clamp technique. A minor population of cone photoreceptors was found to generate large currents in response to puff application of glycine. These currents reversed close to the calculated equilibrium potential for chloride ions. These glycine-elicited currents were sensitive to strychnine but not to picrotoxin consistent with the expression of alpha-beta-heteromeric glycine receptors. Glycine receptors were also activated by taurine and beta-alanine. The glycine receptor antibody mAb4a labelled a minority of the cone photoreceptors identified by an antibody specific for cone arrestin. Finally, expression of the beta subunit of the glycine receptor was demonstrated by single cell RT-PCR in a similar proportion (approximately 13%) of cone photoreceptors freshly isolated by lectin-panning. The identity of cone photoreceptors was assessed by their specific expression of the cone arrestin mRNA. The population of cone photoreceptors expressing the glycine receptor was not correlated to a specific colour-sensitive subtype as demonstrated by single cell RT-PCR experiments using primers for S opsin, cone arrestin and glycine receptor beta subunit. This glycine receptor expression in a minority of cones defines a new cone population suggesting an unexpected role for glycine in the visual information processing in the outer retina.

Glycine modulates the center response of ON type rod-dominant bipolar cells in carp retina

Effects of glycine on ON type rod-dominant bipolar cells (RBCs) were studied in isolated, superfused carp retina by intracellular recording technique and in carp retinal slice preparation by whole cell recording. Glycine of 4mM hyperpolarized RBCs and potentiated their light responses to large light spots, which was reversed by co-application of 10 microM strychnine. It was further found that illumination of the receptive field surround did not affect the depolarizing center response of RBCs. The above result therefore suggests that glycine modulates the center response of RBCs. Focal application of glycine to either dendrites or axon terminals of RBCs failed to induce any currents in both isolated cell and retinal slice preparations. On the other hand, glycine of 4mM increased the amplitude of the scotopic electroretinographic PIII component, which reflects the activity of rod photoreceptors. It seems likely that modulation by glycine of the RBC center response may be in part ascribed to a consequence of the potentiation of rod responses by glycine.

Repetitive light stimulation inducing glycine receptor plasticity in the retinal neurons

Neurotransmitter receptor plasticity is a mechanism that can regulate the temporal and intensity encoding of a synapse. While this has been extensively studied as a mechanism of learning, less is known about such processes in sensory systems. This study examines modulation of glycine receptor function at the first synapse in the retina. It was found that horizontal cells, which are postsynaptic to photoreceptors, have glycine receptor currents that are enhanced when internal calcium is elevated. This can be achieved by glutamatergic synaptic input or by activation of voltage-gated calcium channels. When the retina was maintained in a dark-adapted state, the calcium levels in horizontal cells were relatively low. After a series of brief light stimuli, the internal calcium concentration in horizontal cells was elevated, and the glycine currents were faster and greater in amplitude. The increase of internal calcium levels was caused by increased transmitter release from photoreceptors. Thus glycine receptor function is state dependent and can be rapidly altered by synaptic input from photoreceptors. Light stimulation drives glycine receptor plasticity in the retinal neural network.

Characterization of strychnine-sensitive glycine receptor in the intact frog retina: modulation by protein kinases

We studied 3H-glycine and 3H-strychnine specific binding to glycine receptor (GlyR) in intact isolated frog retinas. To avoid glycine binding to glycine uptake sites, experiments were performed at low ligand concentrations in a sodium-free medium. The binding of both radiolabeled ligands was saturated. Scatchard analysis of bound glycine and strychnine revealed a KD of 2.5 and 2.0 microM, respectively. Specific binding of glycine was displaced by beta-alanine, sarcosine, and strychnine. Strychnine binding was displaced 50% by glycine, and sarcosine. Properties of the strychnine-binding site in the GlyR were modified by sarcosine. Binding of both radioligands was considerably reduced by compounds that inhibit or activate adenylate cyclase and increased cAMP levels. A phorbol ester activator of PKC remarkably decreased glycine and strychnine binding. These results suggest modulation of GlyR in response to endogenous activation of protein kinases A and C, as well as protein phosphorylation modulating GlyR function in retina.

Diversity of glycine receptors in the mouse retina: localization of the alpha2 subunit

Gamma-aminobutyric acid (GABA) and glycine are the major inhibitory neurotransmitters in the retina, glycine being produced in approximately half of all amacrine cells. Whereas retinal cell types expressing the glycine receptor (GlyR) alpha1 and alpha3 subunits have been mapped, the role of the alpha2 subunit in retinal circuitry remains unclear. By using immunocytochemistry, we localized the alpha2 subunit in the inner plexiform layer (IPL) in brightly fluorescent puncta, which represent postsynaptically clustered GlyRs. This was shown by doubly labeling sections for GlyR alpha2 and bassoon (a presynaptic marker) or gephyrin (a postsynaptic marker). Synapses containing GlyR alpha2 were rarely found on ganglion cell dendrites but were observed on bipolar cell axon terminals and on amacrine cell processes. Recently, an amacrine cell type has been described that is immunopositive for glycine and for the vesicular glutamate transporter vGluT3. The processes of this cell type were presynaptic to GlyR alpha2 puncta, suggesting that vGluT3 amacrine cells release glycine. Double labeling of sections for GlyR alpha1 and GlyR alpha2 subunits showed that they are clustered at different synapses. In sections doubly labeled for GlyR alpha2 and GlyR alpha3, approximately one-third of the puncta were colocalized. The most abundant GlyR subtype in retina contains alpha3 subunits, followed by those containing GlyR alpha2 and GlyR alpha1 subunits.

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

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

Bullfrog retinal bipolar cells may express heterogeneous glycine receptors at dendrites and axon terminals

Subcellular localization and properties of glycine receptors on bipolar cells (BCs) were studied using whole-cell recordings and non-stationary noise analysis (NSNA) in bullfrog retinal slices. The currents elicited by focally applied glycine were of comparable amplitudes at the dendrites and axon terminals of both OFF and ON BCs. Moreover, glycine receptors were also expressed at the axons of some BCs. NSNA revealed that the weighted mean single-channel conductance of the glycine receptors at the dendrites (18.2 pS) was significantly larger than that of those at the axon terminals (8.1 pS), thus implying that the glycine receptors on bullfrog retinal BCs may be heterogeneous at these two sites.

Regional distribution of glycine receptor messenger RNA in the central nervous system of zebrafish

We report the cloning of the zebrafish beta subunit of the glycine receptor and compare the anatomical distribution of three glycine receptor subunit constituents in adult zebrafish brain (alphaZ1, alphaZ2 and betaZ) to the expression pattern of homologous receptor subunits (alpha1, alpha2 and beta) in the mammalian adult CNS. Non-radioactive hybridization was used to map the distribution of the alphaZ1, alphaZ2 and betaZ glycine receptor subunit messenger RNAs in the adult zebrafish brain. The anterior-posterior expression gradient found in adult zebrafish brain was similar to that reported in mammalian CNS. However, the glycine receptor transcripts, notably the alphaZ1 subunit, were more widely distributed in the anterior regions of the zebrafish than in the adult mammalian brain. The isoform-specific distribution pattern was less regionalized in zebrafish than in the rat mammalian CNS. Nevertheless, there was some regionalization of alphaZ1, alphaZ2 and betaZ transcripts in the diencephalic and mesencephalic nuclei where different sensory and motor centers express either alphaZ1/betaZ or alphaZ2 subunits. In contrast to the widespread distribution of the beta subunit in adult mammalian brain, alphaZ2 messenger RNA presented the widest expression territory of all three glycine receptor subunits tested. alphaZ2 messenger RNA was expressed in the absence of alphaZ1 and betaZ messenger RNA in the outer nuclear layer of the retina, the inferior olive and the raphe of the medulla oblongata, as well as in the nucleus of Cajal of the medulla spinalis. In contrast, an identified central neuron of the reticular formation, the Mauthner cell, expresses all three glycine receptor subunits (alphaZ1, alphaZ2 and betaZ). This report extends the already described glycine receptor expression in the vertebrate CNS and confirms the importance of glycine-mediated inhibition in spinal cord and brainstem.

Photoreceptor classes and transmission at the photoreceptor synapse in the retina of the clawed frog, Xenopus laevis

The photoreceptor population in Xenopus consists of a green-sensitive rod (lambda(max) = 523 nm), a blue-sensitive rod (lambda(max) = 445 nm) and three classes of cone. The largest cone is red-sensitive (lambda(max) = 611 nm). The intermediate cone is presumed to be blue-sensitive based on physiological criteria, whereas the miniature cone may be UV-sensitive. Horizontal cells (HC) are of two sorts: axon-bearing and axonless. The axon-bearing HC is of the luminosity type and probably contacts all types of photoreceptor. The axonless HC is of the chromaticity type and contacts only intermediate (blue) cones and at least one type of rod. During development dendrites of HCs and bipolar neurons penetrate photoreceptor bases. A progressive maturation of HC and bipolar synapses with rods and cones occurs between tadpoles stages 37/8 and 46. Neighboring rods and cones are joined by gap junctions. During this same period, the outer segments are laid down and photopigments synthesized. A linear relation was found between the quantum capturing ability of the rod and its absolute threshold. Mature rods of the Xenopus retina release glutamate in a calcium-dependent manner. Glutamate release was found to be a linear function of calcium influx through L-type calcium channels. Both types of HC possess ionotropic glutamate receptors of the AMPA subtype.

Mini-review: gephyrin, a major postsynaptic protein of GABAergic synapses

gamma-aminobutyric acid type A (GABAA) receptors are located at the majority of inhibitory synapses in the mammalian brain. However, the mechanisms by which GABAA receptor subunits are targeted to, and clustered in, the postsynaptic membrane are poorly understood. Recent studies have demonstrated that gephyrin, a protein first identified as a component of the glycine receptor (GlyR) complex, is colocalized with several subtypes of GABAA receptors and is involved in the stabilization of postsynaptic GABAA receptor clusters. Thus, gephyrin functions as a clustering protein for major subtypes of inhibitory ion channel receptors.

The embryogenesis of rod photoreceptors in the teleost fish retina, Haplochromis burtoni

Development of the retina, like that of other tissues, occurs via an orderly sequence of cell division and differentiation, producing the functional retina. In teleost fish, however, cell division and differentiation in the retina continue throughout the life of the animal in two distinct ways. Stem cells in a circumferential germinal zone at the periphery of the retina give rise to all retinal cell types and progenitor cells located throughout the retina in the outer nuclear layer (ONL) produce new rod photoreceptors. These processes in adult retina recapitulate in space the embryonic events responsible for forming the retina. Analysis of these events in an African cichlid fish, Haplochromis burtoni, confirmed that cone photoreceptors differentiate first, followed by rod photoreceptors. Correspondingly, at the margin of the eye, cone photoreceptors differentiate nearer to the margin than do rods. Control of photoreceptor production is not understood. Here we present the time of appearance and distribution pattern of GABA and vimentin which are candidates for the control of retinal cell division and differentiation. Antibody staining reveals that both GABA and vimentin exhibit unique patterns of expression during embryonic retinal development. Vimentin immunoreactivity is evident throughout the retina in a spoke-like pattern between developmental Days 4 and 7, as both cone and rod photoreceptors are being formed. GABA is expressed in horizontal cells between Days 5 and 7, corresponding to the onset of rod differentiation in time and in position within the retina. Moreover, the wave of GABAergic staining in the horizontal cells parallels the wave of rod differentiation across the embryonic retina of H. burtoni. Thus, GABA may play a role in the development of rod photoreceptors.

Glycinergic synaptic inputs to bipolar cells in the salamander retina

1. Glycine activated strychnine-sensitive chloride conductances at both the dendrites and the axonal telodendria of most bipolar cells in the salamander retina. 2. The chloride equilibrium potential of bipolar cells was found to be negative to -50 mV, indicating that glycinergic synapses on bipolar cells are inhibitory. 3. Some bipolar cells exhibited discrete, strychnine-sensitive, chloride-mediated inhibitory postsynaptic currents (IPSCs). These were elicited by focal application of glutamate at the inner plexiform layer (IPL). Glycinergic synapses were localized using simultaneous focal application of calcium to retinal slices bathed in calcium-free media. Both dendritic and telodendritic glycinergic IPSCs were observed. 4. The decay of the telodendritic IPSCs was well fitted by a single exponential with a time constant of 17.7 +/- 8.7 ms. Similar kinetics were observed for dendritic IPSCs in some cells, but in one class of on-centre bipolar cell the decay of the dendritic IPSCs was better fitted by a sum of two exponentials with time constants 9.9 +/- 4.3 and 51.3 +/- 24.3 ms. 5. The dendritic IPSCs were best driven by application of glutamate at the distal IPL (the off sublamina), while the telodendritic IPSCs were driven best by application near the telodendria. These results suggest that bipolar cell dendrites receive inhibitory glycinergic inputs from interplexiform cells that are excited by off-centre bipolar cells, whereas bipolar cell telodendria receive glycinergic amacrine cell inputs that are antagonistic to the photoreceptor inputs. 6. Both inputs could be elicited in the presence of tetrodotoxin (TTX), but the dendritic IPSCs were sometimes abolished by TTX, suggesting that sodium-dependent spikes play an important role in the transmission of interplexiform cell signals to the outer plexiform layer.

Synaptic clustering of GABA(C) receptor rho-subunits in the rat retina

Polyclonal antibodies which recognize the rho-subunits of the GABA(C) receptor were applied to sections of the rat retina. Strong punctate immunoreactivity was found in the inner plexiform layer (IPL), which was shown by electron microscopy to represent a clustering of the GABA(C) receptors at synaptic sites. During postnatal development diffuse rho-immunoreactivity was first observed at postnatal day P3. Distinct labelling of bipolar cells appeared at P7 and punctate, synaptic labelling was observed at P10. In order to show that the rho-immunoreactive puncta coincide with the axons of bipolar cells, double immunostainings of retinal sections with an antiserum against syntaxin 3 and with the rho-antiserum were performed. The experiments showed that rho-immunoreactive puncta are preferentially located on the axon terminals of rod and cone bipolar cells. In order to determine whether GABA(C) receptor rho-subunits coassemble with GABA(A) receptor subunits, double-labelling experiments were performed with subunit specific antisera. Punctate, putative synaptic clustering was observed with all antisera applied, however, GABA(C) receptor expressing puncta did not coincide with GABA(A) receptor containing puncta. This suggests that there are no synaptic GABA receptors in the retina in which GABA(A) and GABA(C) receptor subunits are coassembled. Similar double-labelling experiments were also performed to find out whether GABA(C) receptors and glycine receptors are colocalized. They were clustered at different synapses. This suggests that synaptic GABA(C) receptors consist of rho-subunits and are not coassembled with GABA(A)- or glycine-receptor subunits.

Both high- and low voltage-activated calcium currents contribute to the light-evoked responses of luminosity horizontal cells in the Xenopus retina

We examined the contribution of two intrinsic voltage-dependent calcium channels to the light-evoked responses of a non-spiking retinal neuron, the horizontal cell (HC). HC's isolated from the Xenopus retina were studied by the whole cell version of the patch clamp. In a mixture of agents which suppressed Na- and K-dependent currents, we identified a transient, low voltage-activated Ca current suppressed by Ba2+ and blocked by Ni2+ (T-type) and a sustained, high voltage-activated, dihydropyridine-sensitive Ca current that was enhanced by Ba2+ (L-type). We made simultaneous intracellular recordings from rods and HC's in the intact, dark-adapted Xenopus retina. Under certain stimulus conditions, transient oscillations appeared in HC responses but were absent in rod light-evoked waveforms. One type of transient was seen at relatively hyperpolarized potentials (< -45 mV), was enhanced by Sr2+ and inhibited by Ni2+. It thus appears to depend on a T-type Ca-current. A second type of oscillation was seen to be superimposed on a prolonged depolarizing wave following light off in the HC and as spike-like depolarizations in rods. These oscillations were enhanced by Ba2+ and Sr2+, but blocked by the dihydropyridine, nifedipine, indicating their dependence on an L-type calcium conductance. All calcium-dependent oscillations were suppressed by 0.05-0.5 mM Co2+. Suppression of glutamate neurotransmission with CNQX or kynurenate, or glycine neurotransmission with strychnine, enhanced the HC oscillations.

Synaptic inputs mediating bipolar cell responses in the tiger salamander retina

Postsynaptic receptors in bipolar cells were studied by focal application of glutamate and GABA to the outer and inner plexiform layers (OPL and IPL) under visual guidance in living retinal slices of the tiger salamander. Two different types of conductance change could be elicited in bipolar cells by applying glutamate to the OPL. In off-center cells, which had axon telodendria ramifying in the distal 55% of the IPL, glutamate elicited a conductance increase associated with a reversal potential near -5 mV. In on-center cells, which had telodendria stratified in the proximal 45% of the IPL, glutamate caused a conductance decrease associated with a reversal potential near -11 mV. These observations suggest that glutamate gates relatively nonspecific cation channels at synapses between photoreceptors and bipolar cell dendrites. Application of glutamate to the IPL elicited no conductance change in Co2+ Ringer's solution, but in normal Ringer's it generated a conductance increase associated with a reversal potential near the chloride equilibrium potential (ECl). These findings are consistent with the notion that glutamate receptors exist in GABAergic and/or glycinergic amacrine cells, and that glutamate in the IPL depolarizes these cells, causing GABA and/or glycine release and the opening of chloride channels in bipolar cell axon terminals. In Co2+ Ringer's, application of GABA at the OPL elicited no conductance changes in bipolar cells, suggesting that GABA receptors do not exist on bipolar cell dendrites. Applied at the IPL, GABA elicited large conductance increases associated with a reversal potential near ECl. Implications of these results for the functional circuitry of the tiger salamander retina are discussed.

The number and distribution of bipolar to ganglion cell synapses in the inner plexiform layer of the anuran retina

The main route of information flow through the vertebrate retina is from the photoreceptors towards the ganglion cells whose axons form the optic nerve. Bipolar cells of the frog have been so far reported to contact mostly amacrine cells and the majority of input to ganglion cells comes from the amacrines. In this study, ganglion cells of frogs from two species (Bufo marinus, Xenopus laevis) were filled retrogradely with horseradish peroxidase. After visualization of the tracer, light-microscopic cross sections showed massive labeling of the somata in the ganglion cell layer as well as their dendrites in the inner plexiform layer. In cross sections, bipolar output and ganglion cell input synapses were counted in the electron microscope. Each synapse was assigned to one of the five equal sublayers (SLs) of the inner plexiform layer. In both species, bipolar cells were most often seen to form their characteristic synaptic dyads with two amacrine cells. In some cases, however, the dyads were directed to one amacrine and one ganglion cell dendrite. This type of synapse was unevenly distributed within the inner plexiform layer with the highest occurrence in SL2 both in Bufo and Xenopus. In addition, SL4 contained also a high number of this type of synapse in Xenopus. In both species, we found no or few bipolar to ganglion cell synapses in the marginal sublayers (SLs 1 and 5). In Xenopus, 22% of the bipolar cell output synapses went onto ganglion cells, whereas in Bufo this was only 10%. We conclude that direct bipolar to ganglion cell information transfer exists also in frogs although its occurrence is not as obvious and regular as in mammals. The characteristic distribution of these synapses, however, suggests that specific type of the bipolar and ganglion cells participate in this process. These contacts may play a role in the formation of simple ganglion cell receptive fields.

Different specific binding sites of [3H]glycine and [3H]strychnine in synaptosomal membranes isolated from frog retina

Synaptosomal fractions were isolated from frog retina: a fraction enriched in photoreceptor terminals (P1) and a second one (P2) containing interneurons terminals. We compared the binding of [3H]glycine and [3H]strychnine to membranes of these synaptosomes. The binding of both radioactive ligands was saturable and Na(+)-independent. [3H]Glycine bound to a single site in P1 and P2 synaptosomal fractions, with KD = 12 and 82 nM and BMax = 3.1 and 3.06 pmol/mg protein respectively. [3H]Strychnine bound to two sites in each one of the synaptosomal fractions. For P1 KD values were 3.9 and 18.7 nM, and BMax values were 1.1 and 7.1 pmol/mg protein, respectively. Membranes from the P2 synaptosomal fraction showed KD's of 0.6 and 48 nM and BMax's of 0.4 and 4.5 pmol/mg. Specific [3H]glycine binding was displaced by beta-alanine, 1-serine, d-serine and HA966, but not by strychnine, 7-chlorokynurenic or 5,7-dichloro-kynurenic acids. Specific [3H]strychnine binding was partially displaced by glycine and related amino acids and totally displaced only by 2-NH2-strychnine. Our results indicate the presence of high affinity binding sites for glycine and strychnine in frog retinal synaptosomal membranes. The pharmacological binding pattern indicates the presence of the strychnine sensitive glycine receptor as well as other sites. These might not include the NMDA receptor-associated glycine site.

Colocalization of gephyrin and GABAA-receptor subunits in the rat retina

Gephyrin is a protein that copurifies with the glycine receptor (GlyR) and is required for the clustering of GlyRs at postsynaptic sites. Previously, it was thought that antibody mAb 7a, directed against gephyrin, was a specific marker for GlyR. However, there is evidence that gephyrin can also be found at nonglycinergic synapses. Here, immunocytochemistry was applied to show this directly for the rat retina. Both gephyrin and different subunits of the gamma-aminobutyric acid (GABA)A receptor were localized to discrete puncta in the inner plexiform layer, and these puncta were shown by electron microscopy to represent synaptic sites. Double immunocytochemistry revealed that GABAA receptors and GlyRs are not colocalized. However, gephyrin and different subunits of GABAA receptors were found to occur at the same synapses. The amount of colocalization varied with the GABAA receptor subunit composition and was most extensive for the alpha 2 subunit, less for the alpha 3 subunit, and minimal for the alpha 1 subunit. The gephyrin present at GABAergic synapses of the retina might also be involved with clustering of receptors at the postsynaptic sites. Hence, localization of gephyrin can no longer be considered as a unique marker of glycinergic synapses.

Postsynaptic gephyrin immunoreactivity exhibits a nearly one-to-one correspondence with gamma-aminobutyric acid-like immunogold-labeled synaptic inputs to sympathetic preganglionic neurons

Peripheral regulation of cardiovascular function is fundamentally influenced by central excitation and inhibition of sympathetic preganglionic neurons in thoracic spinal cord. This electron microscopy study investigated whether the gamma-aminobutyric acid (GABA)-ergic and glycinergic inhibitory innervation of sympathetic preganglionic neurons arises from mutually exclusive afferent populations. Sympathetic preganglionic neurons were retrogradely labeled with cholera beta subunit. GABAergic terminals were identified using strict quantitative statistical analyses as those boutons containing significantly elevated levels of GABA-like immunogold labeling (GABA+). Glycinergic terminals were classified as those boutons opposite postsynaptic gephyrin immunostaining containing background levels of GABA-like immunogold labeling (gephyrin+/GABA- association). Approximately 43% of the synaptic terminals that contacted sympathetic preganglionic somata and proximal dendrites and that were opposite gephyrin were GABA-; the remaining 57% were GABA+. Only two GABA+ boutons (4%) that synapsed on identified sympathetic preganglionic neuron (SPN) processes were not opposite gephyrin immunostaining (GABA+/gephyrin- association). GABA-/gephyrin+ associations were anticipated given prior anatomical, physiological, and pharmacological data. The observed nearly one-to-one correspondence between postsynaptic gephyrin immunoreactivity and GABA+ boutons was unexpected. Prior physiological and pharmacological experiments suggest that the postsynaptic effects of GABAergic inputs to sympathetic preganglionic neurons are mediated by activation of GABAA receptors. Those data, the present results, and other molecular, biochemical, and anatomical studies of gephyrin in the central nervous system (CNS) are consistent with two hypotheses: 1) Postsynaptic gephyrin is associated with GABAA receptors in the membranes of sympathetic preganglionic neurons, and 2) GABA+/gephyrin+ associations do not necessarily predict colocalization of GABA and glycine within single boutons synapsing on sympathetic preganglionic somata and dendrites.

Regulation of endogenous dopamine release in amphibian retina by gamma-aminobutyric acid and glycine

Endogenous dopamine release in the retina of the African clawed frog (Xenopus laevis) increases in light and decreases in darkness. The roles of the inhibitory amino acid transmitters gamma-aminobutyric acid (GABA) and glycine in regulating this light/dark difference in dopamine release were explored in the present study. Exogenous GABA, the GABA-A receptor agonist muscimol, the GABA-B receptor agonist baclofen, and the GABA-C receptor agonist cis-aminocrotonic acid (CACA) suppressed light-evoked dopamine overflow from eyecups. The effects of GABA-A and -B receptor agonists were selectively reversed by their respective receptor-specific antagonists, whereas the effect of CACA was reversed by the competitive GABA-A receptor antagonist bicuculline. The benzodiazepine diazepam enhanced the effect of muscimol on light-evoked dopamine release. Both GABA-A and -B receptor antagonists stimulated dopamine release in light or darkness. Bicuculline was more potent in light than in darkness. These data suggest that retinal dopaminergic neurons are inhibited by GABA-A and -B receptor activation in both light and darkness but that GABA-mediated inhibitory tone may be greater in darkness than in light. Exogenous glycine inhibited light-stimulated dopamine release in a concentration-dependent and strychnine-sensitive manner. However, strychnine alone did not increase dopamine release in light or darkness, nor did it augment bicuculline-stimulated release in darkness. Additionally, both strychnine and 7-chlorokynurenate, an antagonist of the strychnine-insensitive glycine-binding site of the N-methyl-D-aspartate subtype of glutamate receptor, suppressed light-evoked dopamine release. Thus, the role of endogenous glycine in the regulation of dopamine release remains unclear.

D1 dopamine receptor immunoreactivity in human and monkey cerebral cortex: predominant and extrasynaptic localization in dendritic spines

Antibodies to the D1 dopamine receptor were used to localize this protein in several areas of human and monkey cerebral cortex with light and electron microscopy. In addition to cell body labeling in monkeys, all areas of humans and monkeys had a neuropil label with a laminar distribution predicted by previous D1 receptor autoradiography studies. Using electron microscopy, this neuropil label was seen in numerous dendritic spines, in dendritic shafts, and in occasional axon terminals. While labeled spines were common, they represented only a subset of all cortical spines. Serial sectioning through labeled spines showed that the diaminobenzidine reaction product was usually not at postsynaptic densities but instead was displaced to the side of the large asymmetric (presumed glutamatergic) synapse. Furthermore, most labeled spines did not receive synapses with dopaminergic features, suggesting that many D1 receptors are at extrasynaptic sites, possibly receiving dopamine via diffusion in the neuropil. Similarly, double labeling failed to reveal D1 labeling at synapses of tyrosine hydroxylase immunoreactive axons. Localization to numerous dendritic spines suggests that a primary role of D1 receptors is modulation of glutamatergic input to cortical pyramidal 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.

Cone synapses of a flat diffuse cone bipolar cell in the primate retina

A Golgi-stained flat diffuse cone bipolar cell from a vervet monkey's retina (Cercopithecus aethiops), contacting six cones, was serially sectioned for electron microscopy (EM) to determine the types of synapses it made with the cone pedicles. All the synapses were basal (flat) contacts. Their distribution and ultrastructural type were similar at each pedicle. Approximately half the synapses were definable as triad-associated and the rest were elsewhere on the cone pedicle base. Their ultrastructure is the same regardless of those positions. About 25 synapses were made with each cone. Thus this type (DB2 of Boycott & Wässle, 1991) of flat diffuse cone bipolar cell is in contact with six cones through about 150 synapses. At the eccentricity studied each cone pedicle probably makes 90-100 basal synapses with between three and four DB2 bipolar cells. This is between two and three times the number that are made with all the types of invaginating bipolar cells. A brief review of cone photoreceptor synapses with bipolar cells shows that, for those so far examined in the primate retina, the dichotomy into two types of bipolar cell invaginating (ribbon-related), with axons ending in the b-layer of the inner plexiform layer (IPL) (hence presumptive On-bipolars) and flat (basal synapses), with axons ending in the a-layer of the inner plexiform layer (hence presumptive Off-bipolars) is the rule. But other vertebrate retinae, including that of the cat, also have bipolar cells which vary from this pattern.

Synaptic connections involving immunoreactive glycine receptors in the turtle retina

The distribution of glycine receptors in the turtle retina was studied with the aid of a monoclonal antibody that detects the 93-kD protein associated with the strychnine-sensitive glycine receptor. Light microscopically, receptors were found in the inner plexiform layer and, more sparsely, in the innermost parts of the inner nuclear layer. No receptors were seen to be associated with photoreceptor cells, horizontal cells, or any other structures in the distal inner nuclear layer or outer plexiform layer. Ultrastructurally, glycine receptors were found on the inner face of postsynaptic membranes of processes from amacrine and presumed ganglion cells and always involved amacrine cell processes as the presynaptic element. Such glycine receptor immunoreactive synapses onto amacrine cell processes were distributed throughout the inner plexiform layer with a peak density near the middle. On the other hand, output synapses onto ganglion cell processes displaying immunoreactive glycine receptor sites showed a bimodal distribution in the inner plexiform layer. Glycine receptor immunoreactivity was not detected on bipolar cells, but presumed glycine-utilizing processes (i.e. those presynaptic to immunoreactive glycine receptors) were occasionally found to be postsynaptic in bipolar cell dyads. The majority of the synaptic input to the presumed glycine-utilizing amacrine cell processes was from other amacrine processes, some of which were themselves glycine utilizing. The observations suggest that glycinergic synapses in the turtle retina are, to a large extent, engaged in processing interamacrine signals.

Association of m1 and m2 muscarinic receptor proteins with asymmetric synapses in the primate cerebral cortex: morphological evidence for cholinergic modulation of excitatory neurotransmission

Muscarinic m1 receptors traditionally are considered to be postsynaptic to cholinergic fibers, while m2 receptors are largely presynaptic receptors associated with axons. We have examined the distribution of these receptor proteins in the monkey cerebral cortex and obtained results that are at odds with this expectation. Using immunohistochemistry with specific antibodies to recombinant m1 and m2 muscarinic receptor proteins, we have demonstrated that both m1 and m2 receptors are prominently associated with noncholinergic asymmetric synapses as well as with the symmetric synapses that characterize the cholinergic pathways in the neocortex. At asymmetric synapses, both m1 and m2 receptor immunoreactivity is observed postsynaptically within spines and dendrites; the m2 receptor is also found in presynaptic axon terminals which, in the visual cortex, resemble the parvicellular geniculocortical pathway. In addition, m2 labeling was also found in a subset of nonpyramidal neurons. These findings establish that the m2 receptor is located postsynaptically as well as presynaptically. The association of m1 and m2 receptors with asymmetric synapses in central pathways, which use excitatory amino acids as neurotransmitters, provides a morphological basis for cholinergic modulation of excitatory neurotransmission.

Physiological and morphological properties of off- and on-center bipolar cells in the Xenopus retina: effects of glycine and GABA

We studied the morphology and center-surround organization of Lucifer Yellow injected OFF- and ON-center bipolar cells in the light-adapted Xenopus retina and the effects of glycine and GABA on their cone-mediated light responses. In both classes of cell, prominent antagonistic surround responses up to 20 mV in amplitude could be evoked without first suppressing the center responses with steady illumination. An additional feature of the light-evoked bipolar cell response was a pronounced (up to -24 mV) delayed hyperpolarizing after potential (DHAP) which followed the depolarizing responses of both classes of bipolar cell. The morphological features of dye-injected bipolar cells conformed to the general idea of segregation of ON and OFF pathways in the inner and outer interplexiform layer, however, the morphology of axonal arborizations was different for both classes. OFF-center cells ramified symmetrically around the primary branchpoint, whereas ON-center cells had a strongly asymmetrical arrangement of their axonal tree. The center and surround responses were differentially sensitive to glycine and GABA. Glycine eliminated the antagonistic surround responses in both OFF and ON cells; the center responses were reduced to some extent but were not eliminated. In contrast, GABA affected the hyperpolarizing responses much more strongly than the depolarizing response components. That is, the amplitude of the center response in the OFF cell and the surround response in the ON cell was reduced 80-90% during exposure to GABA, whereas the surround and center depolarizations of OFF and ON cells, respectively, were reduced only 0-10%. Our findings implicate a role for GABAergic and glycinergic pathways in the center-surround organization of bipolar cells in Xenopus retina. In addition, the results suggest that the pathways mediating center-surround antagonism may be different in OFF-bipolar cells vs. ON-bipolar cells.

Tyrosine hydroxylase-immunoreactive elements in the distal retina of Bufo marinus: a light and electron microscopic study

Tyrosine hydroxylase-immunoreactive elements in the distal retina of Bufo marinus were investigated using light and electron microscopic immunocytochemistry. At the light microscopic level, immunoreactive somas were seen in the proximal part of the inner nuclear layer, and immunoreactive processes projected both to the inner and outer plexiform layers. In some instances stained axon-like processes traveled from the inner plexiform layer, across the inner nuclear layer to the distal retina. Immunolabeled elements formed basket-like structures around the photoreceptor inner segments. At the ultrastructural level immunostained fibers were observed in close contact with the necks, lateral walls, bases and the outer surfaces of rod outer segments. Synaptic specializations were neither observed at rod contacts nor at other possible contact sites such as bipolar dendrites and horizontal cell somata and processes in the outer plexiform layer. In contrast, synaptic specializations between immunolabeled profiles and amacrine, bipolar and ganglion cells were regularly present in the inner plexiform layer. These findings suggest that a population of dopaminergic interplexiform cells is present in the Bufo retina and sends axon-like processes towards the distal retina. It is assumed that dopamine is probably released non-synaptically from the immunolabeled terminals in the distal retina influencing rods directly, by which the quality of photopic vision is enhanced in the anuran retina.

Glycine-receptor immunoreactivity in retinal bipolar cells is postsynaptic to glycinergic and GABAergic amacrine cell synapses

Glycinergic innervation of the synaptic terminals of mixed rod-cone bipolar cells in the goldfish retina was investigated by electron microscopical immunocytochemistry with presynaptic and postsynaptic markers for glycinergic neurons: a monoclonal antibody (mAb 7A) against the 93 kDa subunit of the strychnine-sensitive glycine receptor and polyclonal antisera against a glycine/BSA conjugate. Conventional "glycinergic" synaptic contacts, made by amacrine cell processes, accounted for 7-10% of the input to the bipolar cell terminals, whether determined by glycine receptor immunoreactivity (GlyR-IR) or glycine-IR. In addition to the conventional synapses, the large bipolar cell terminals in the proximal inner plexiform layer (type Mb) gave rise to spinules (spine-like protrusions) that invaginated into presynaptic amacrine cell processes. Although 85% of the spinules were GlyR-IR, no spinules were postsynaptic to glycine-IR processes; yet 86% of the spinules were postsynaptic to GAD-IR processes, suggesting that the GlyR-IR spinules were postsynaptic to GABAergic terminals. Furthermore, a single amacrine cell process could make two synapses with an Mb terminal: a GlyR-IR contact onto a spinule and a conventional synapse that was not GlyR-IR. We suggest that glycinergic innervation of bipolar cell terminals involves conventional glycinergic synapses as well as an unconventional situation in which GABA and glycine may interact in as yet undetermined manner, perhaps by potentiation.

Glycinergic interplexiform cells make synaptic contact with amacrine cell bodies in goldfish retina

Recent works utilizing glycine-immunoreactivity (IR) and combined Golgi impregnation and 3H-glycine uptake autoradiography indicate that glycinergic interplexiform cells (IPC) may synapse upon cell bodies in the inner nuclear and ganglion cell layers in fish retina. This possibility was investigated with immunocytochemical techniques using presynaptic and postsynaptic markers for glycinergic neurons: a monoclonal antibody (mAb 7A) against the 93 kDa subunit of the strychnine-sensitive glycine receptor and a polyclonal antiserum against a glycine/BSA conjugate. Synaptic contacts onto the lateral and proximal surfaces of amacrine cell bodies and onto the distal surface of cells in the ganglion cell layer were identified with both probes. The contacts were rare with one contacted amacrine cell/section of 500 linear micron. Serial 1-micron sections were processed alternately for glycine and GABA antisera using postembedding techniques at the light microscopic level. Glycine-IR processes + boutons were apposed to GABA-IR cell bodies in 16 of 17 examples, indicating that the dendro-somatic contacts were onto GABA-immunoreactive amacrine cell bodies. In context of other published morphological data, we suggest that the dendro-somatic synapses were derived from glycinergic IPCs. Glycinergic IPCs receive input from GABAergic horizontal cells and, via a shunt conductance produced by the dendro-somatic contacts, may be involved in controlling the sensitivity, temporal, or spatial properties of amacrine cell responses to large field illumination.

The organization of dopaminergic neurons in vertebrate retinas

A survey of the shapes of dopaminergic (DA) neurons in the retinas of representative vertebrates reveals that they are divisible into three groups. In teleosts and Cebus monkey, DA cells are interplexiform (IPC) neurons with an ascending process that ramifies to create an extensive arbor in the outer plexiform layer (OPL). All other vertebrates studied, including several primate species, have either DA amacrine cells or IPCs with an ascending process that either does not branch within the OPL or does so to a very limited degree. DA neurons of non-teleosts exhibit a dense plexus of fine caliber fibers which extends in the distal most sublamina of the inner plexiform layer (IPL). Teleosts lack this plexus. In all vertebrates, DA cells are distributed more or less evenly and at a low density (10-60 cells/mm2) over the retinal surface. Dendritic fields of adjacent DA neurons overlap. Most of the membrane area of the DA cell is contained within the plexus of fine fibers, which we postulate to be the major source of dopamine release. Thus, dopamine release can be modeled as occurring uniformly from a thin sheet located either in the OPL (teleosts) or in the distal IPL (most other vertebrates) or both (Cebus monkey). Assuming that net lateral spread of dopamine is zero, the fall of dopamine concentration with distance at right angles to the sheet (i.e. in the scleral-vitreal axis) will be exponential. The factors that influence the rate of fall-diffusion in extracellular space, uptake, and transport--are not yet quantified for dopamine, hence the dopamine concentration around its target cells cannot yet be assessed. This point is important in relation to the thresholds for activation of D1 and D2 dopamine receptors that are found on a variety of retinal cells.