Differential effects of naloxone and diprenorphine on defensive behaviour in rats

Two opiate antagonists, naloxone and diprenorphine, were used to examine the proposed involvement of endogenous opioid mechanisms in the modulation of shock-induced defensive behaviour patterns in rats. Naloxone was found to exert a biphasic influence on defensive fighting, with small dose (0.1 mg/kg) facilitation and large dose (10 mg/kg) inhibition. This compound also induced a dose-dependent inhibitory effect on shock-elicited threat whilst facilitated escape/avoidance behaviour was observed with the largest doses only. None of these influences could be attributed to alterations in electric shock thresholds. In contrast to the effects of naloxone, diprenorphine (0.1-10 mg/kg) was largely ineffective in modifying shock-induced defensive behaviour. These data suggest that endogenous opioid mechanisms may not be involved in the effects of naloxone on defensive fighting and cast some doubt upon the general hypothesis of the involvement of endorphins in the regulation of innate defence patterns.