Progesterone increases levels of mu-opioid receptor mRNA in the preoptic area and arcuate nucleus of ovariectomized, estradiol-treated female rats

Estradiol (E-2) and progesterone (P) play different roles in generating the preovulatory surge release of luteinizing hormone-releasing hormone (LH-RH) and luteinizing hormone (LH). Results of our previous studies suggest that at least some of these steroid-specific effects may be mediated by beta-endorphinergic neurons. However, it is also possible that E-2 and P differentially regulate responsiveness to opioids by altering mu-opioid receptor gene expression. To test this hypothesis, we used quantitative in situ hybridization histochemistry (ISHH) to measure the effects of E-2 and P on mu-opioid receptor mRNA levels in cells of the preoptic area (POA) and arcuate nucleus (Arc). We examined several groups of animals in the morning and afternoon on the day of LH surge release: (1) 1-week ovariectomized (OVX) rats with or without E-2 treatment sacrificed between 09:00 and 09:30 h (48 h after E-2 capsules inserted); (2) OVX with or without E-2 treatment sacrificed between 15:30 and 16:00 h; and (3) OVX with both E-2 and P treatment sacrificed between 15:30 and 16:00 h (approximate to 54 h after E-2 and 6 h after P administration). We found that E-2 had no effect on morning or afternoon levels of mu-opioid receptor mRNA levels in either the POA or Arc. In contrast, P treatment increased afternoon levels of mu-opioid receptor mRNA in both regions. These findings indicate that differential effects of E-2 and P on LH-RH release may be mediated by steroid-specific effects on mu-opioid receptor gene expression in neurons of the POA and/or Arc. (C) 1997 Elsevier Science B.V.