Title: Efferent Control of Photoreceptor Function: Voltage-clamp Study The ventral photoreceptor of Limulus continues to be the favored preparation for detailed electrophysiological studies of phototransduction and light-adaptation: it is large, isolated, and readily voltage-clamped. In this proposal, the PI reveals new evidence that: (i) ventral photoreceptors (in situ) undergo circadian changes in responsivity to light; and (ii) pharmacological agents, shown to mimic the effects of endogenous efferent neurotransmission in the lateral eye, change function in ventral photoreceptors as well. These findings open up the possibility for uncovering the mechanisms mediating efferent control of photoreceptor gain (change in membrane conductance per photon), shape or duration of quantal bumps, and other cellular mechanisms associated with phototransduction or light-adaptation that are under efferent control. The specific aims of this study are to address the following questions: - What is the specific nature of the changes in photoreceptor gain and bump duration? - Are these changes the result of specific voltage-sensitive or light-activated conductances, and how are these related to changes in photoreceptor gain and bump duration? The methods will employ standard procedures for the voltage- clamping of ventral photoreceptors while recording (a) light- intensity functions and (b) i-v (current-voltage) relationships before, during, and after superfusion of specific pharmacological agents (those that mimic various components of endogenous efferent neurotransmission in lateral eye photoreceptors). The PI fully expects these studies to provide the pilot data for future investigations aimed at a deeper understanding of cellular mechanisms involved with phototransduction, light-adaptation, and the biochemistry underlying these processes.