Circadian rhythms regulate the function of living systems at virtually every level of organization - from molecular to organismal. A fundamental question in the field concerns the molecular mechanism of circadian oscillators. How are circadian oscillations generated, and what components at the tissue, cellular or subcellular level are required for circadian properties? Our long-term objectives are to understand the cellular and biochemical events that underlie circadian rhythms among the vertebrates. The mechanisms that generate circadian rhythms will be studied in vitro using cultured vertebrate pineal tissue as a model system. The isolated pineal gland of birds and lizards contains circadian oscillators, with photoreceptive input, which regulate the rhythmic synthesis of melatonin. Immunocytochemical methods will be used to identify cell types that compose the gland. Photoreceptor-and melatonin-specific determinants will be used to identify cell types and to correlate structural information with functional properties. The biochemical events mediating the effects of light on melatonin biosynthesis will be defined with emphasis on the role of cyclic nucleotides. Light and pharmacological agents will be used to determine whether an oscillation of cyclic AMP controls the melatonin rhythm. To test whether cyclic AMP is a molecular component of the circadian oscillator itself, perturbations of cyclic AMO levels will be used to perturb parameters of the oscillator. If a molecular component is part of the timing mechanism, then a minimum of two conditions must be fulfilled: the level of the component must oscillate and perturbation of its level must perturb the circadian oscillation in a determinate fashion. Finally, the mechanism generating the circadian oscillation of intracellular cyclic AMP will be studied by measuring the enzymes that synthesize and degrade cyclic AMP. In two model systems, the vertebrate pineal and the molluscan eye, there exists compelling evidence for important regulatory roles of cyclic nucleotides in circadian function. The elucidation of the role of cyclic nucleotides in these systems should aid ultimately in the search for the molecular components of the circadian clock. An understanding of the biological basis of circadian rhythms in vertebrates may lead to procedures useful in the diagnosis and treatment of pathophysiologic conditions associated with circadian rhythm dysfunctions such as sleep disorders, mental health and endocrine abnormalities.