Immunocytochemical Study on Synaptic Circuitry of Glycinergic Neurons in the Rat Retina

Uhm, Seung Ryong; Kim, In Bum; Lee, Moon Yong; Chun, Myung Hoon

Abstract

The role of glycine as an inhibitory neurotransmitter is well established, and glycinergic neurons appear to play an important role in the mammalian retinae[Ikeda & Sheardown, 1983 ; Bolz et al., 1985]. Though it has been reported that certain conventional and displaced amacrine cells and a few of bipolar cells are consistently labeled with anti-glycine antiserum in the mammalian retinae so far[W ssle et al., 1986 ; Pourcho & Goebel, 1987 ; Davanger et al., 1991 ; Yoo & Chung, 1992], little has been studied on the synaptic circuitry of glycinergic neurons to clarify mechanism of its action in the visual processing of the mammalian retinae. This study was conducted to localize glycinergic neurons and to define their synaptic circuitry in the rat retina by immunocytochemical method using anti -glycine antiserum. The results were as follows : 1. Glycinergic neurons of the rat retina were conventional and displaced amacrine cells, interstitial cells and bipolar cells. 2. Glycinergic amacrine cells could be subdivided into two types, that is, A II amacrine cells and other amacrine cells, according to their ultrastructures. Glycinergic A II amacrine and other amacrine cell processes comprised postsynaptic dyad at the ribbon synapse of rod bipolar axon terminals in the sublamina b of the inner plexiform layer of the retina. Glycinprgic A II amacrine cell processes made gap junctions with axon terminals of unlabeled invaginating cone bipolar cells in the sublamina b, and made chemical synapses onto axon terminals of unlabeled flat cone bipolar cells and onto dendrites of ganglion cells in the sublamina a of the inner plexiform layer. In the sublamina b of the inner plexiform layer, g1ycinergic amacrine cell processes were postsynaptic to axon terminals of unlabeled invaginating cone bipolar cells, and made chemical output synapses onto axon terminals of unlabeled invaginating cone bipolar and rod bipolar cells and onto the dendrites of ganglion cells. Such cases that pre- and post-synaptic processes of glycinergic amacrine cell processes were non- glycinergic amacrine cell processes were frequently observed throughout the inner plexiform layer. In some cases, glycinergic amacrine cell processes receiving synaptic inputs from other glycinergic amacrine cell process made synaptic outputs onto the non-glycinergic or glycinergic amacrine cell processes. 3. Glycinergic bipolar cells could be subdivided into invaginating and flat cone bipolar cells. Postsynaptic dyads of cone bipolar cells at the ribbon synapses were non-glycinergic amacrine and amacrine cell processes, glycinergic amacrine and amacrine cell processes, glycinergic amacrine and non-glycinergic amacrine cell processes, and dendrite and dendrite of ganglion cells. These results demonstrate that [1] glycinergic A II amacrine cell receiving synaptic input from rod bipolar cells inhibit flat cone bipolar cells and OFF ganglion cells via chemical synapse, and excite ON cone bipolar cells via electrical synapse ; thereby visual information in the darkness can be transmitted to ON ganglion cells via ON cone bipolar cells, and [2] glycine released from glycinergic neurons inhibits directly ON and OFF ganglion cells or indirectly ON and OFF ganglion cells via non-glycinergic amacrine or bipolar cells.