Effects of melatonin, serotonin, and naloxone on aggression in isolated cichlid fish (Aequidens pulcher).

Intracranial injections of 10 micrograms melatonin, with or without 4 micrograms naloxone, result in reduced aggressive responsiveness by the cichlid fish Aequidens pulcher to a mirror presented 20 min later. Naloxone, when administered on its own, had no behavioral effect. Intracranial injections of 25 micrograms serotonin-creatinine sulphate complex (5HT-CS) also reduced aggression. 5HT-CS's action was abolished if the injection included 10 micrograms S-adenosyl homocysteine (SAH), which inhibits the conversion of serotonin to melatonin. This suggests that the behavioral action of 5HT-CS may be mediated by its conversion to melatonin. There is inconclusive evidence to suggest that 37.5 micrograms 5HT-CS, administered in conjunction with 10 micrograms SAH, may reduce aggressiveness; injection of 50 micrograms 5HT-CS, with or without 10 micrograms SAH, induced a pronounced escape behavior which precluded any measurement of aggressiveness in the fish.