Opioid and 17 beta-estradiol regulation of LH and FSH secretion during sexual maturation in heifers.

The objective of the present study was to examine the involvement of opioid neuropeptides and E2 in regulating circulating concentrations of gonadotropins during sexual maturation in the bovine female. Prepubertal (immature) and postpubertal (mature) bovine females were used. Mean concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in circulation before and after administration of naloxone were determined in ovariectomized heifers administered E2 and ovariectomized heifers not administered E2. A linear decline (P less than 0.01) in opioid suppression of LH and FSH occurred during the experimental period in immature heifers receiving E2. This decline in opioid suppression of LH and FSH occurred during the same period of time that intact control heifers were initiating estrous cycles at puberty. Little change of opioid suppression of LH and FSH occurred during the experimental period in immature heifers not receiving E2 and mature heifers receiving E2. Our research indicates that opioid neuropeptides and E2 act together to regulate LH and FSH secretion during sexual maturation in the bovine female.