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 (alpha 1, alpha 2, alpha 3 and beta) in retinas, forming picrotoxin-sensitive alpha-homomeric and picrotoxin-insensitive alpha/beta-heteromeric receptors. Glycine receptors are modulated by intracellular Ca2+ and protein kinas C and A pathways. Extracellular Zn2+ regulates glycine receptors in a concentration-dependent manner, nanomolar Zn2+ enhancing glycine responses, and micromolar Zn2+ suppressing glycine responses in retinal neurons. These studies describe the function and mechanism of glycinergic synapses in retinas.