Specific effects of beta-endorphin infused into the amygdala on sexual behaviour in the male rat.

Bilateral intra-amygdaloid infusions of 0, 20, 60, 120 and 200 pmol beta-endorphin were administered to sexually experienced male rats in a Latin-square design. Sixty, 120 and 200 pmol beta-endorphin significantly decreased preintromission investigation rate and increased intromission latency with an oestrous female. No other effects on the male rats' sexual behaviour were produced. beta-Endorphin-induced effects on preintromission investigation and intromission latency were naloxone reversible (5 mg/kg, i.p.). Similar doses of intra-amygdaloid beta-endorphin had no effect on aggressive interaction with another, strange, male behaviour and similar doses of intra-amygdaloid corticotropin-releasing factor had no effect on sexual behaviour. Thus, there was both behavioural and chemical specificity of intra-amygdaloid beta-endorphin-induced effects on male sexual behaviour. Bilateral 60 pmol beta-endorphin infusions into the caudate-putamen had no effects on male sexual behaviour, demonstrating anatomical specificity of the intra-amygdaloid-induced effects. Basolateral excitotoxic lesions did not prevent the behavioural effects of intra-amygdaloid beta-endorphin infusions. These results demonstrate that intra-amygdaloid beta-endorphin specifically suppresses the precopulatory phase of male rat sexual behaviour but leaves the execution of the copulatory series intact once it has been initiated. This effect appears to be mediated via the corticomedial amygdaloid region. The change in precopulatory behaviour suggests that beta-endorphin may interfere with the processing of sensory information from the female, thus delaying appropriate initiation of the copulatory series.