ABSTRACT
The role of acetylcholine as an excitatory neurotransmitter is well established, and cholinergic neurons appear to play an important role in the mammalian retinae. Though it has been reported that certain conventional and displaced amacrine cells are consistently labeled with anti-choline acetyltransferase antiserum in the mammalian retinae, little has been studied on the synaptic circuitry of cholinergic neurons to clarify mechanism of its action in the visual processing of the mammalian retinae. This study was conducted to localize cholinergic neurons and to define their synaptic circuitry in the rat retina by immunocytochemical method using anti-choline acetyltransferase antiserum. The results were as follows: 1. Cholinergic neurons of the rat retina were conventional amacrine cells located in the inner nuclear layer and displaced amacrine cells in the ganglion cell layer. 2. Cholinergic amacrine cells were branched in the middle of the sublamina a of the inner plexiform layer, and cholinergic displaced amacrine cells branched in the sublamina b, forming one prominent band, respectively. 3. Presynaptic processes to cholinergic amacrine cell processes were axon terminals of invaginating and flat cone bipolar cells, and unlabelled amacrine cell processes in the inner plexiform layer. Postsynaptic dyads at the ribbon synapses of axon terminals of cone bipolar cells were cholinergic amacrine cell process and dendrite of ganglion cell, cholinergic amacrine cell process and unlabelled amacrine cell process and cholinergic amacrine cell process and cholinergic amacrine cell process. In addition, cholinergic amacrine cell process formed postsynaptic monoad at the ribbon synapse. 4. Cholinergic amacrine cell processes made output conventional chemical synapses onto the dendrites of ganglion cells, unlabelled amacrine cell processes and cholinergic amacrine cell processes in the inner plexiform layer. These results demonstrate that (1) cholinergic neurons are conventional amacrine cells and displaced amacrine cells of which somata are located in the inner nuclear layer and ganglion cell layer, respectively, (2) cholinergic conventional amacrine cells are involved in OFF pathway, and cholinergic displaced amacrine cells play an important role in ON pathway in visual processing of lightness, and (3) acetylcholine released from cholinergic neurons by light excites directly ON and OFF ganglion cells or indirectly ON and OFF ganglion cells via non-cholinergic amacrine cells.