We previously demonstrated that intracerebroventricular (ICV) injection of orexin-A induces arousal and increased metabolic turnover rates of norepinephrine, dopamine, and serotonin in layer (egg-type) chicks. Because monoamine oxidase-A (MAO-A) is a potent degrading enzyme of these monoamines, we hypothesized that orexin-A may mediate its arousal-inducing effects through MAOA. Therefore, we simultaneously injected clorgyline, a specific inhibitor of MAO-A, with orexin-A and examined behavior of chicks. Behaviors associated with arousal were attenuated in the group of chicks that received clorgyline and orexin-A compared with those that received orexin-A alone. For the monoamine turnover rate, enhancement of the turnover rate of serotonin by orexin-A was attenuated by clorgyline. Therefore, we conclude that orexin-A-induced arousal is dependent upon monoamine neural activities stimulated by MAO-A in chicks.
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