DISSERTATION RESEARCH: Melatonin Regulation of Vocal Behavior

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Circadian rhythms that approximate the 24 hr day-night cycle, synchronize animal behavior and physiology to cyclical changes in environmental cues, such as light, temperature, and the availability of mates. How biological rhythms are generated by hormonal, genetic, and neural mechanisms is of great scientific interest. The nocturnal hormone melatonin plays a central role in entraining daily activity to the day-night cycle. In many mammals, activity and mating occur at night so there is potential for melatonin to regulate important reproductive behaviors such as vocalizations used in courtship. This project will investigate circadian and melatonin regulation of daily rhythms in the vocal behavior of a nocturnally active and highly vocal fish, the plainfin midshipman (Porichthys notatus) by answering three questions: (1) Is the daily vocal rhythm under internal, circadian control? (2) Does melatonin stimulate nocturnal vocal behavior? (3) Does melatonin regulate gene expression underlying neural excitability in vocal brain regions? These studies will contribute to a comparative framework for predicting melatonin regulation of vocal behavior in other species, including birds and mammals that exhibit divergent daily activity patterns. More broadly, melatonin has been implicated in sleep, jetlag, autism and epilepsy, making basic research on how it affects social communication and neural excitability important and widely applicable. This project will test the overall hypothesis that the midshipman's nocturnal courtship vocalization is under circadian control and stimulated by melatonin. Proposed studies will (1) record vocalizations from naturally behaving fish under constant external light conditions to test for an internally generated circadian vocal rhythm, (2) implant fish with melatonin to test for melatonin regulation of vocal behavior, and (3) seek a deeper understanding of how melatonin influences daily rhythms in vocal behavior by examining its regulation of neural excitability-related gene expression in a well-characterized vocal network. If the midshipman daily vocal rhythm is under endogenous circadian control, it is predicted that rhythms recorded under a normal light regime will persist under constant darkness with a period of ~24 hr. Additionally, if midshipman vocal behavior is dependent on melatonin action, it is predicted that vocalizations will be reduced or abolished under constant light, shown to abolish melatonin production, while exogenous melatonin replacement under light will rescue the occurrence of vocalizations. Finally, the researchers predict that melatonin increases the expression of previously identified candidate genes known to affect neural excitability. These genes encode three types of ion channels and exhibit increased expression in a hindbrain vocal nucleus collected during the summer night, a time of maximal natural vocal behavior. Given conserved functions of the candidate genes explored in this proposal, such mechanisms could provide the molecular basis for translating the nocturnal melatonin signal to increase or decrease neural excitability in nocturnal versus diurnal animals, respectively. This project will train current and new undergraduates of both genders from diverse backgrounds, including underrepresented minorities. The results will be disseminated in open-access peer-reviewed journals, popular articles via online platforms, at national and international conferences, and at local science outreach events.