Gonadal steroid hormones upregulate medial preoptic mu-opioid receptors in the rat

The density of mu-opioid receptors is dependent upon the levels of gonadal steroid hormones in the medial preoptic area of ovariectomized female rats. Because the region containing these cyclical mu-opioid receptors is relatively small, autoradiographic analysis of the binding of a saturating concentration of mu-selective radioligand was used to determine whether hormone treatment affects maximal binding capacity of these receptors. The results reveal that gonadal steroid hormones upregulate 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 medial preoptic area mu-opioid receptors which could serve to regulate reproductive activities.