Pharmacological properties of glycine transport in the frog retina

Glycine transport accounts for the differential role of glycine vs. D-serine at NMDA receptor coagonist sites in the salamander retina

In this study, we demonstrate that d-serine interacts with N-methyl-d-aspartate receptor (NMDAR) coagonist sites of retinal ganglion cells of the tiger salamander retina by showing that exogenous d-serine overcomes the competitive antagonism of 7-chlorokynurenic acid for this site. Additionally, we show that exogenous d-serine was more than 30 times as effective at potentiating NMDAR currents compared with glycine. We thus examined the importance of glycine transport through the application of selective antagonists of the GlyT1 (NFPS) and GlyT2 (ALX-5670) transport systems, while simultaneously evaluating the degree of occupancy of the NMDAR coagonist binding sites. Analysis was carried out with electrophysiological recordings from the inner retina, including whole-cell recordings from retinal ganglion cells and extracellular recordings of the proximal negative field potential. Blocking the GlyT2 transport system had no effect on the light-evoked NMDAR currents or on the sensitivity of these currents to exogenous d-serine. In contrast, when the GlyT1 system was blocked, the coagonist sites of NMDARs showed full occupancy. These findings clearly establish the importance of the GlyT1 transporter as an essential component for maintaining the coagonist sites of NMDARs in a non-saturated state. The normal, unsaturated state of the NMDAR coagonist binding sites allows modulation of the NMDAR currents, by release of either d-serine or glycine. These results are discussed in light of contemporary findings which favor d-serine over glycine as the major coagonist of the NMDARs found in ganglion cells of the tiger salamander retina.

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 transporters (glycine transporter 1 and glycine transporter 2) are expressed in retina

The amino acid glycine is an inhibitory neurotransmitter in the spinal cord, brain stem, and vertebrate retina. The effective synaptic concentrations of glycine are regulated by at least two transporters: glycine transporter 1 and glycine transporter 2. Here, we show retinal expression of glycine transporter 1 by in-situ hybridization and of glycine transporter 2 by reverse transcriptase-PCR and in-situ hybridization. In-situ hybridization signals were observed in the ganglionar and inner nuclear layer as well as in the outer nuclear layer of the frog and rat retinas. In addition, accumulation of H-glycine was observed in isolated photoreceptor cells. The expression of these transporters in nonglycinergic cells suggests that they may also modulate electrical signals.

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.

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.

Pharmacological properties of glycine uptake in the developing rat retina

A pharmacological characterization of glycine transport was performed in the rat retina at different postnatal ages. The uptake of 3H-glycine increased during the first 2 weeks of postnatal age, reaching maximum values at 12 days; then it decreased sharply to the adult values. We found a Na+ -dependent and high-affinity transport system with a Km of 100 microM. The Na+ Hill coefficient for glycine uptake was 1.76 +/- 0.07. Although glycine uptake was insensitive to staurosporine and phorbol ester, it was reduced 40-50% by sarcosine and ALX5407. Besides, amoxapine inhibited glycine uptake by 40 and 70% in adult and immature retina, respectively. These results suggest that the Glyt1 transporter was concentrated in the nerve terminals. In addition to the presence of Glyt1 in the retina, our results provided evidence of the occurrence of Glyt2 and/or another isoform of glycine transporter, which might have had a role in the retina development.

Expression of glycine receptor and transporter on bullfrog retinal Müller cells

The expression of the glycine receptor (GlyR) alpha1, alpha2 and beta subunits and glycine transporter (GlyT) on Müller cells was studied in bullfrog retina using double immunofluorescence labeling and confocal scanning microscopy. Double labeling of glial fibrillary acidic protein (GFAP), a specific marker for Müller cells, and the GlyR subunits showed that almost all Müller cells moderately expressed GlyR alpha1 and weakly GlyR beta, whereas no immunoreactivity for GlyR alpha2 was observed. The labeling for GlyR alpha1 and GlyR beta appeared in somata, major processes, endfeet and branchlets of the Müller cells. Müller cells were also GlyT1-labeled. Consistent with previous electrophysiological results, these findings suggest that Müller cells may be involved in modulation of glycinergic transmission by reciprocal interactions with retinal neurons through GlyR and GlyT.