Physiological Substrates of a Circadian Oscillator

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DESCRIPTION (applicant's abstract): Our objective is to understand mechanisms whereby interacting chemical messengers transduce light information from the eye via the retinohypothalamic tract (RHT) to the circadian clock in the suprachiasmatic nucleus (SCN). This process decodes photic information from external light in the context of internal state. Under the present award, we discovered that the chemical signal from the RHT is more complex than previously thought. Pituitary adenyl cyclase-activating peptide (PACAP) and glutamate (Glu) co-localize within terminals of retinal ganglion cells innervating the SCN. We found evidence for functional interaction both in vivo and in vitro: in early night, PACAP potentiated Glu-induced phase delay of SCN rhythms, while in late night it blocked Glu-induced phase advance. Thus, responses to these signals are state-dependent and clock-controlled. How does PACAP interact with Glu to encode light signals at the SCN? What cellular processes integrate combinatorial signaling events to modulate amplitude and direction of phase resetting differentially in early vs. late night? We will evaluate PACAP and Glu actions and interactions during photic signaling in vivo, release from the RHT, signal transduction(s) and consequent molecular events in early vs. late night. Hypotheses to be tested are that: 1) the light signal contains both Glu- and PACAP-ergic components that interact producing graded changes in clock phase, and 2) the clock's responses to PACAP and Glu change between early and late night due to differential effects of clock-gated cAMP/PKA signaling and the state of the molecular clockworks. Multiple indices of change will be measured: rhythms of behavior, oscillation of SCN neuronal activity, and levels/localizations of putative clock elements in rodent models. This multidisciplinary approach will provide insights into classical (Glu) and modulatory (PACAP) neurotransmission, cellular and molecular mechanisms of signal integration, and decision-making processes that alter neuronal state. These are fundamental issues in neuroscience. Signal transduction is a cellular process, and by identifying the relevant neurotransmitters, receptors, second messenger systems and targets, we will be able to understand the causal mechanisms the mediate differential state changes in the clock. This research is basic to understanding integrative brain function. It has applied relevance for strategies in drug chronotherapeutics and will facilitate developing rationally-based therapies for timing disorders, including internal desynchronizations manifested as disordered patterns of sleep, cognitive and autonomic function, neurological impairment in aging and depressive states.