NEUROTRANSMITTERS IN THE VISUAL SYSTEM

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The vertebrate retina offers unique advantages for studying the mechanisms of information processing in the central nervous system. Although the physiological responses and synaptic connections of the different cell types have been extensively studied, identities of neurotransmitters used by most retinal cells are unknown. For the past few years, we have studied the synthesis of transmitter candidates in dissociated, identified retinal cells (Lam, 1976). Such studies establish the feasibility of using isolated cells for physiological and chemical characterizations of retinal cells, and also suggest that some retinal neurons may not use any known candidates as transmitters. Based on these findings, the main objective of this research proposal is to continue the identification of neurotransmitters in the vertebrate retina. In particular, there are five specific aims: (1) The synthesis of acetylcholine, gamma-aminobutyric acid, catecholamines and serotonin by each identified retinal cell type will be studied using the method of Lam (1976); (2) Since amino acids such as glutamate, aspartate, taurine and glycine have also been implicated as transmitters, the endogenous levels of these amino acids in identified retinal cells will be measured by dansylation and amino-acid analyses; (3) The release of transmitter candidates from identified retinal cells will be studied using light or high external K ion as stimuli; (4) From our previous studies, some cell types may not use any known candidates as transmitters (Lam, 1976), substances such as peptides will therefore be extracted and purified from bovine retinas and the physiological effects of these substances on the electroretinogram and ganglion cell activities of isolated perfused rabbit retinas will be tested using the method of Ames and Pollen (1969); attempts will be also made to determine the structures of retinal substances which have specific and reversible effects on retinal cells; these studies may establish the existence of new transmitters in the retina; (5) The appearance of transmitter syntheses during the development of the tadpole retina will be examined and correlated with the electrophysiological and morphological studies of Nilsson and Crescitelli (1969).