μ-Opiate receptor binding in the medial preoptic area is cyclical and sexually dimorphic

The density and distribution of μ- and κ-opiate receptors in the medial preoptic area (MPOA) of male and female rats across the estrous cycle was examined using quantitative in vitro autoradiography of [³H]D-Ala²,MePhe⁴,Gly-ol⁵-enkephalin (DAGO), [³H]naloxone and [³H]bremazocine binding. While no difference in κ-receptor labeling was observed across sex or estrous stage, selective μ-receptor labeling with [³H]DAGO revealed a significant variation of density and distribution in the MPOA across the estrous cycle and between sexes. A dense concentration of μ-receptors located in the central, sexually dimorphic portion of the MPOA was observed during metestrus and diestrus in females, but not during proestrus nor in males. This region appeared to be the same as that labeled similarly using [³H]naloxone. These results suggest that a regional substrate for functional activation by endogenous opiod peptides (e.g. β-endorphin) is cyclically regulated in females, which may explain the gonadal streoid-dependent effects of MPOA β-endorphin on lordosis and luteinizing hormone secretion in females.