Development and Function of Sleep Regulatory Circuits in Zebrafish

Sleep is widely accepted as an essential behavior for optimum mental and physical health, yet the genetic and neural circuits that govern sleep remain poorly understood. In this thesis, I briefly introduce the behavioral criteria that define sleep, currently known sleep regulatory mechanisms, and the distinct advantages of the zebrafish, Danio rerio, as a simple animal model for studying sleep. I then investigate two factors previously implicated in sleep behavior: epidermal growth factor receptor and hypocretin. First, I show that epidermal growth factor receptor signaling is both necessary and sufficient for normal sleep behavior in zebrafish, just as it is in invertebrates. This demonstrates that sleep regulatory mechanisms can be conserved over large evolutionary distances, and is the first genetic study showing that the epidermal growth factor receptor signaling is necessary for normal sleep behavior in a vertebrate. Second, I capitalize upon the rapid external development of zebrafish embryos to screen for developmental factors that specify hypocretin neurons, which are known to promote arousal and consolidate sleep/wake bouts. I identify the LIM homeobox 9 transcription factor as necessary for hypocretin neuronal development in zebrafish and sufficient to specify additional hypocretin neurons in both zebrafish and mice. This is the first time any factor has been shown to induce hypocretin neurons in vivo and may be an important step towards curing narcolepsy, a debilitating sleep disorder caused by the selective loss of hypocretin neurons. These studies deepen our understanding of how sleep is regulated at a genetic and cellular level and underscore the potential for zebrafish to make future contributions to sleep research.