During retinal development, the neuroepithelium of the early optic cup consists of colummar cells. These cells proliferate, cease dividing, elaborate processes, and become highly specialized as they mature. The neurons, in particular, make the commitment to use a certain neurotransmitter and proceed to acquire mechanisms for their uptake, storage, synthesis, and release. This proposal seeks to study histogenesis in the embryonic chick retina using neurotransmitter candidates as anatomical and physiological markers. In particular, the emergence and maturation of the retinal transmitter systems, GABA, glycine, dopamine, aspartate, glutamate, and enkephalin, will be examined over the time couse of embryonic development. Autoradiography and immunocytochemistry will be used to identify the time and order of appearance of retinal cell subpopulations. Once identified, synaptic pathways between continuous neurons will traced at the EM level by doubly labeling single sections using a 3H-labeled transmitter, and an antibody against either L-glutamic acid decaroxylase (GAD) or Met-enkephalin. The acquisition of intraretinal biosynthetic capabilities and 'release' mechanisms will be measured by HPLC with electrochemical detection, HPLC with fluorescence detection, and radioimmunoassay. How can transmitters help us to understand the events of retinal histogenesis? First, the different transmitter-specific properties can obviously be used as markers to determine the signals and mechanisms regulating retinal differentiation and maturation. Secondly, these same properties will allow us to examine the effects of extrinsic (e.g., drugs) and/or intrinsic parameters (e.g., genetic variations) on neurogenesis. These issues are especially important in view of the many known congenital and drug-induced disorders in the developing CNS, and the retina is likely to be an excellent model system for some of these studies.