Effects of Antagonists of Mu and Kappa Opioid Receptors on Aggressivity in Male Mice with Repeated Experience of Aggression

Abstract

Mu and kappa opioid receptors often show antagonism in the regulation of physiological responses and behavior, including aggressive behavior. We report here our studies of the effects of acute administration of the selective mu opioid receptor antagonist CTAP (1 and 2 mg/kg, s.c.) and the selective kappa opioid receptor antagonist nor-BNI (0.5 mg/kg, s.c.) on manifestations of aggressive behavior in male C57BL/6J mice with short (three days) and long (20 days) experience of victory in intermale confrontations. Animals with short experience of aggression were insensitive to blockade of both mu and kappa receptors. In males with long experience of aggression, administration of CTAP led to dose-dependent increases in the latent period of attacks but had no effect on attack duration. Administration of nor-BNI had no effect on the behavior of males with prolonged experience of aggression in antagonistic confrontations. Possible changes in the sensitivity of opioid receptors in male C57BL/6J mice following prolonged experience of aggression are discussed.