Orexin is a neuropeptide distributed widely among vertebrates. In mammals, orexin and its receptor system are involved in the regulation of food intake, locomotion, and psychomotor activities including the sleep/wakefulness cycle. With regard to nonmammalian vertebrates, there has also been intensive study aimed at the identification and functional characterization of orexin and its receptor, and recent investigations of the role of orexin have revealed that it exerts behavioral effects in teleost fish. Goldfish and zebrafish are excellent teleost fish models, and in these species it has been demonstrated that orexin increases food consumption as an orexigenic factor and enhances locomotor activity, as well as being involved in the regulation of active and rest status (circadian rhythmicity and the sleep/wakefulness cycle), as is the case in mammals. This chapter reviews current knowledge of orexin derived from studies of teleost fish, as representative nonmammals, focusing particularly on the role of the orexin system, and examines its significance from a comparative viewpoint.
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