People can reject food, abstain from sex and control their thirst, but cannot keep from falling asleep. What is the genetic and neural basis of this most insistent bodily need? Why is it essential in nearly all multicellular animals, and how is it regulated?
Answers to these questions remain elusive despite years of study, and we are taking a new approach using zebrafish. Zebrafish are amenable to high-throughput genetic and small molecule screens that are powerful approaches to identify genetic pathways that regulate sleep. The optical transparency of zebrafish larvae make them an ideal system to monitor and manipulate neural circuits that regulate sleep, and zebrafish and mammalian brains are structurally similar. Zebrafish are therefore uniquely well suited to unravel the mysteries of sleep.
We are interested in two general questions: What are the genetic circuits that regulate sleep, and what are the neural circuits that regulate sleep?
Genetic Regulation of Sleep
To test whether similar genetic pathways regulate zebrafish and mammalian sleep, we are studying Hypocretin/Orexin, whose loss causes the mammalian sleep disorder narcolepsy. We found that overexpression of zebrafish Hypocretin impairs the initiation and maintenance of sleep, consolidates wakefulness and induces a state of hyperarousal, as it does in mammals. We are now performing screens to identify novel genes and small molecules that regulate sleep, and are characterizing several candidate sleep regulators.
Neural Regulation of Sleep
Hypocretin is expressed in ~20 hypothalamic neurons in zebrafish larvae compared to 3,000 neurons in mice and 70,000 neurons in humans, making it much simpler to characterize the zebrafish Hypocretin circuit in detail. We found that zebrafish Hypocretin neurons project to wake-promoting centers of the brain and are active during periods of consolidated wakefulness, as in mammals.
We are now taking advantage of the optical transparency of zebrafish larvae, the relative simplicity of zebrafish neural circuits and recently developed optical tools to study the development and function of Hypocretin neurons, as well as other hypothalamic neural circuits that may regulate sleep. We are characterizing these neural circuits down to the level of individual neurons, testing the effects of activating and inhibiting specific neurons on behavior, and monitoring the activity of genetically defined neurons in freely behaving zebrafish larvae.