Effects of ethanol and naltrexone on aggressive display in the siamese fighting fish, Betta splendens

The present study used the aggressive display of Betta splendens in response to a mirror as an index of the effects of ethanol and the opiate antagonist, naltrexone. Naltrexone produces an opiate receptor blockade and thus provided a test of the hypothesis that ethanol effects on aggression are mediated by the opioid system. Eighty fish were randomly assigned to one of eight groups in a 4 X 2 factorial design with Ethanol (0, 0.25, 0.50, and 0.75 g%) and Naltrexone (0 and 5 mg/l) as the main factors. The 0.75 g% dose of ethanol reliably suppressed aggressive display as measured by number of gill show responses, but lower doses had no effect or tended to increase aggressive display. At the same time, all doses of ethanol increased arousal in the fish as measured by airgulping. Naltrexone alone did not have effects on aggression or arousal, and did not interact with ethanol on either measure. Thus the results did not support the ethanol-opioid common-link hypothesis.