Light-evoked changes of glycine-like immunoreactivity in the rat retina

Glycine is an important inhibitory neurotransmitter in the vertebrate retina. Although many reports of glycine localization by the immunocytochemical method have been published, none has distinguished the neurotransmitter function of glycine from metabolic change. If glycine is the neurotransmitter, it seems that it must be affected or released by stimulation. We evaluated the possibility that glycine in retinal synapses changes with light stimulation. Glycine-like immunoreactivity was found by light microscopy to be present mainly in the inner plexiform layer and amacrine cells. In the inner plexiform layer these immunoreactive products decreased after illumination. The distribution of glycine-like immunoreactivity in the rat retina was also investigated after embedding anti-rabbit IgG-gold colloidal particles (GCP). In the dark-adapted eyes, glycine-like immunoreactivity was marked in the inner plexiform layer (27.8 +/- 5.7 GCP/microns2) and amacrine cells (33.5 +/- 5.5 GCP/microns2). This immunoreactive product significantly decreased only in the inner plexiform layer after illumination (15.5 +/- 4.5 GCP/microns2). These changes in immunoreactivity may reflect the release of endogenous glycine. Our results provide evidence for the role of glycine as a retinal neurotransmitter.