Estrous Cycle- and Pregnancy-Related Differences in Expression of the Proenkephalin and Proopiomelanocortin Genes in the Ovary and Uterus*

Expression of the genes for two opioid peptide precursors, proenkephalin and POMC, was examined within the female reproductive system of rodents as a function of the estrous cycle and during pregnancy. Proenkephalin RNA was found to change markedly during the estrous cycle in both the ovary and uterus (approximately 6- and 3-fold, respectively). The highest concentrations occurred at estrus in the rat ovary and at metestrus and diestrus in the rat uterus. In sharp contrast to proenkephalin RNA, the abundance of POMC RNA remained relatively constant throughout the estrous cycle in both tissues. Similar results were obtained in the cycling hamster ovary. During pregnancy, the concentrations of proenkephalin RNA in the rat ovary showed little variation, while in the uterus a 4-fold increase in this transcript was observed. The effects of pregnancy on POMC RNA were the reverse of this pattern; its abundance increased 2-fold in the ovary and did not vary substantially in the uterus. These differences in the expression of proenkephalin and POMC genes during the estrous cycle and pregnancy suggest that these two opioid peptide precursors are associated with distinct functional roles within the female reproductive system.