Melatonin, a key molecule of the vertebrate circadian system, can act as a modulator of stress-induced responses in avian and mammalian species. However, the possible role of melatonin as an anti-stress hormone has not been yet studied in fish. It is known that this hormone exerts an anorectic effect and reduces locomotor activity in some teleosts, including the goldfish (Carassius auratus). The objective of the present study was to analyze the possible role of melatonin as a modulator of the stress responses in a model of acute stress in goldfish. Fish were exposed to the air during 60 seconds and then, were intraperitoneally injected with melatonin (2 and 20 mg/g body weight) at 30 and 120 min postinjection. The plasma cortisol levels, food intake and locomotor activity were determined at 30 and 120 min post-treatment.

The protocol of acute stress used in our study significantly reduced food intake and increased plasma cortisol levels 30 min after stress exposition, indicating that it is an effective acute stress model in goldfish. Melatonin administration counteracted such both effects. The acute stress model used did not modify locomotor activity, but melatonin reduced significantly food anticipatory activity and diurnal and nocturnal locomotor activity in both fish, intact and stress-exposed ones. This melatonin dependent reduction of motor activity was observed in two housing conditions, isolated and grouped goldfish (n=6/aquarium). Altogether these data suggest that melatonin treatment can exert a possible anti-stress role in teleosts fish.