The eye gathers light from the environment and transmits the neural signals to the brain that result in vision. In most animals, the brain sends fibers back to the eye. The purpose of these returning efferent fibers is not well known in most species, especially mammals, because of the complexity of the visual system as a whole and the difficulty of direct experimental examination of efferent effects. Experiments on the horseshoe crab, Limulus polyphemus, have revealed a wealth of information about how the signals returning from the brain via the efferent fibers affect visual processing in the eye itself. In Limulus a.circadian clock in the brain drives the efferent fibers at night and is silent during the day. Efferent inputs modulate the structure and function of the retina in phase with the circadian clock, and prime or enable processes such as the daily turnover of photosensitive membrane. Indeed most of the major retinal processes appear to be modulated or controlled in some way by efferent inputs from the brain. This project investigates how the effects of efferent activity are realized at the cellular level in the retina. Specific aims are: 1) Determine the source of the efferent fibers which innervate the retina from a circadian clock. 2) Identify the efferent neurotransmitters. 3) Investigate the cellular mechanisms underlying the structural changes in the retina that result from circadian efferent activity. 4) Investigate further the efferent control and light initiation of rhabdom turnover. 5) Analyze the functional effects of efferent activity. 6) Develop the ventral eye as a companion model system for our studies in the lateral eye. Our methodology emphasizes anatomical procedures including high-resolution light-microscope autoradiography, immunocytochemistry at the light and electron microscope levels, computer-assisted, 3-dimensional reconstruction, and morphometric studies. Our primary tenet is that comprehensive study of retinal cellular processes and their efferent control will yield additional fundamental understanding about the mechanisms that maintain visual sensitivity in the visual systems of animals including man.