Horizontal cells are non-spiking local circuit interneurons that mediate lateral interactions in the outer plexiform layer of the vertebrate retina. Despite numerous investigations of horizontal cell physiology there has been no systematic study of all the input-output relations of the horizontal cell in a single species. The goal of the proposed research is to characterize the mechanisms responsible for the generation of the horizontal cell response in Xenopus retina. Special emphasis will be placed on the interactions between rod and cone signals on the same horizontal cell, a problem for which Xenopus horizontal cells are particularly well suited because they receive mixed input from rods and cones. Results may be relevant to understanding rod-cone interactions in the human retina. By designing a suitable 'cocktail' that pharmacologically isolates the cell from some of its inputs it will be possible to monitor conductance changes associated with synaptic transmission and drug application and to identify the role of synaptic and intrinsic membrane channels in shaping the horizontal cell's response to light. The following methods will be used to dissect out the various factors contributing to the cell's response: 1) Rod and cone inputs will be isolated by appropriate choice of adaptational conditions. 2) Synaptic feedback onto cones and synaptic input from interplexiform cells will be manipulated by superfusing the retina with agonists and antagonists of GABA and glycine, respectively. 3) Electrical coupling between horizontal cells will be attenuated by the application of an uncoupling agent. 4) Voltage and time-dependent conductances intrinsic to the horizontal cell membrane will be blocked by the application of specific ion-channel blocking agents. The data obtained will provide insight into several basic and as yet unresolved problems in retinal neurophysiology including: 1) What is the basis for the differences between rod and cone mediated horizontal cell responses? 2) How does the adaptational state of the retina influence the horizontal cell response? 3) How does synaptic feedback onto cones, synaptic input from interplexiform cells and electrical coupling between horizontal cells contribute to the cell's response? 4) Which neurotransmitters contribute to the horizontal cell's response and what is their mechanism of action? 5) How do voltage-dependent conductances influence the horizontal cell response in the intact retina?