DIFFERENTIAL PROOPIOMELANOCORTIN GENE-EXPRESSION IN THE MEDIAL BASAL HYPOTHALAMUS OF RATS DURING PREGNANCY AND LACTATION

Hypothalamic proopiomelanocortin (POMC) gene expression was determined using in situ hybridization histochemistry (ISHH) during pregnancy an d lactation in rats with and without prior reproduction experience. PO MC mRNA levels in the arcuate nucleus were compared between primigravi d (first pregnancy) and multigravid (second pregnancy) and primiparous and multiparous lactating rats, and between these groups and age-matc hed, regularly cycling, nulliparous females in diestrus. Hybridization s were performed using a digoxigenin-labeled riboprobe complementary t o 837 bp of the POMC gene. The number of cells expressing POMC mRNA in the arcuate nucleus decreased in primiparous rats on day 12 of lactat ion when compared with the number of POMC cells in the arcuate nucleus of nulliparous rats in diestrus. In addition, the number of cells exp ressing POMC mRNA in multigravid animals was significantly less than i n the primigravid group on days 7 and 21 of pregnancy, and on day 12 o f lactation in primiparous animals. Repeated reproductive experience a ffected the number of POMC mRNA positive cells; there were fewer cells expressing POMC mRNA in the multigravid females on day 7 of pregnancy and an increase in the number of POMC cells in the multiparous group on day 12 of lactation compared to the primiparous animals. Optical de nsity measurements revealed a significant increase in reaction product in the labeled cells on all days of pregnancy compared with virgin fe males in diestrus and a significant decrease in reaction product an da y 12 of lactation in the multiparous group. The results of the present study indicate that POMC gene expression changes across pregnancy and lactation and that repeated reproductive experience has long-term, po ssibly permanent, effects on the endogenous opioid system.