Food intake inhibition by melatonin in goldfish (Carassius auratus)

Feeding regulation by monoamines, neuropeptides and certain hormones has be en studied in fish, but a possible role of melatonin is unknown. The purpos e of the present study was to investigate the effects of melatonin on food intake in goldfish. Fishes were housed in 12L:12D and injected with differe nt doses of either melatonin or 2-iodomelatonin. Two routes of administrati on, intracerebroventricular and intraperitoneal injections, and two times o f the daily photocycle, midday and midnight, were tested. Food intake was m easured at 2, 5 and 8 h postinjection. Melatonin and its analog, 2-iodomela tonin intracerebroventricularly injected had no effect on food intake at an y time. However, intraperitoneal injections of both indoleamines significan tly reduced food intake at different postinjection times. The inhibitory ef fect of melatonin was blocked by intraperitoneal administration of its anta gonist, luzindole. These results demonstrate the in vivo efficiency of luzi ndole as melatonin antagonist, and thus provide a useful experimental tool to investigate melatonin functions. In conclusion, both melatonin and its a gonist 2-iodomelatonin administered peripherally, inhibit food intake in go ldfish, and this inhibitory effect appears to be mediated via luzindole-sen sitive melatonin receptors. Our results strongly suggest that melatonin is involved in the peripheral satiety mechanisms in goldfish. (C) 2001 Elsevie r Science Inc. All rights reserved.