Glycine receptors (GlyRs) and their role in
retinal circuitry were analyzed immunocytochemically in the rat retina. Specific
antibodies against the alpha 1 subunit of the GlyR and against the GlyR-associated
protein gephyrin, respectively, were used. In the inner plexiform layer (IPL), both
antibodies produced a punctate label that was shown by electron microscopy to occur at synapses.
Gephyrin-like immunoreactivity (-LI) was more widely distributed, indicating that
gephyrin might also occur at nonglycinergic synapses. At the ultrastructural level,
gephyrin-LI was found at the cytoplasmic face of postsynaptic membranes of amacrine and
ganglion cells, but was never detected in bipolar cell axons. Immunoreactivity for the alpha 1 subunit was concentrated in the cleft of conventional synapses made by amacrine cell processes onto
ganglion cell dendrites and cone bipolar axons. The latter synapses differ from other glycinergic synapses since they are not labeled by the antibody against
gephyrin used in this study. In order to identify the type of bipolar cell involved in these synapses, the distribution of the alpha 1 subunit was compared with that of
recoverin-immunoreactive cone bipolar cells and with that of
parvalbumin-immunoreactive All-amacrine cells. Double-label immunofluorescence showed that, in the outer part of the IPL, 75% of the alpha 1-immunoreactive puncta were colocalized with
recoverin-positive bipolar cell axons and 71% of the alpha 1-immunoreactive puncta were colocalized with
parvalbumin-positive All-amacrine processes. Hence, the alpha 1 subunit of the GlyR is present at the chemical synapses established by All-amacrine cells with OFF-cone bipolar cells and OFF-
ganglion cells. These synapses play a key role in the transmission of scotopic signals through the OFF-channel of the rod pathway.