Comparison of the effects of beta-endorphin and morphine on exploratory and socio-sexual behaviour in the male rat.

The socio-sexual and exploratory behaviour profiles were studied in male rats after intracerebral administration of beta-endorphin and morphine. Two social test situations were used, one with a sexually receptive female and one with a castrated male as the stimulus subject. Time (10-60 min)-response and dose (0.25-5.0 millimicron)-response relationships were investigated. A dose of 5 millimicron of beta-endorphin or morphine decreased exploratory activity and socio-sexual approaches. At lower doses (0.5-1.0 millimicron) beta-endorphin produced a more selective inhibitory effect on copulatory behaviour than morphine, which still caused a general depression of both social and sexual behaviours. In contrast to morphine, beta-endorphin increased amicable social contacts at a dose (1 millimicron) which decrease sexual responses. The dissimilarities in morphine and beta-endorphin effects suggest involvement of different brain areas or opiate receptors. The particular behaviour profiles observed after treatment with beta-endorphin indicate the possibility that beta-endorphin influences mechanisms implicated in perception or interpretation of environmental stimuli.