GONADAL-STEROID HORMONES UP-REGULATE MEDIAL PREOPTIC MU-OPIOID RECEPTORS IN THE RAT

The density of mu-opioid receptors is dependent upon the levels of gon adal steroid hormones in the medial preoptic area of ovariectomized fe male rats. Because the region containing these cyclical mu-opioid rece ptors is relatively small, autoradiographic analysis of the binding of a saturating concentration of mu-selective radioligand was used to de termine whether hormone treatment affects maximal binding capacity of these receptors. The results reveal that gonadal steroid hormones upre gulate mu-opioid receptor binding capacity specifically in the medial preoptic area 27 h later, without affecting binding in adjacent brain regions. This suggests that hormones induce a functional increase of m edial preoptic area mu-opioid receptors which could serve to regulate reproductive activities.