Stimulatory effect of intracerebroventricular administration of orexin A on food intake in the zebrafish, Danio rerio

Orexin is a potent orexigenic neuropeptide implicated in feeding regulation of mammals. However, except for the case of goldfish, the involvement of orexin in the feeding behavior of teleost fish has not well been studied. Therefore, we investigated the role of orexin on food intake using a zebrafish (Danio rerio) model. We examined the effect of feeding status on orexin-like immunoreactivity and the expression level of orexin transcript in the brain. The number of neuronal cells showing orexin-like immunoreactivity in the hypothalamic region, including the posterior tuberal nucleus, was significantly increased in fish fasted for 7 days. Orexin precursor mRNA levels in the brain obtained from fish fasted for 7 days were higher than those in fish that had been fed normally. We then investigated the effect of intracerebroventricular (ICV) administration of orexin A on food intake. Cumulative food intake was significantly increased by ICV administration of orexin A (at 0.3 and 3 pmol/g body weight, BW) during a 60-min observation period after treatment. The orexin A-induced orexigenic action (at 0.3 pmol/g BW) was blocked by treatment with an orexin receptor antagonist, SB334867, at 10 pmol/g BW. These results indicate that orexin A acts as feeding regulator in the zebrafish.