Diurnal Variation in Luteinizing Hormone-Releasing Hormone and β-Endorphin Release in Pituitary Portal Plasma during the Rat Estrous Cycle1

Abstract

In this study, we investigated the pattern of both LHRH and beta-endorphin (beta-EP) secretion from the median eminence of cyclic female rats during the estrous cycle by collecting pituitary portal blood for hormone measurements by RIA. On diestrus, proestrus, or estrus (0900 h or 1300 h), rats were hypophysectomized (in order to prevent contamination of portal beta-EP with pituitary beta-EP) and were given a 50-60-min resting period of postsurgical recovery prior to blood sampling. Between 1000 h and 1800 h of the same day, pituitary portal blood samples were collected at 10-min intervals from these animals under Saffan anesthesia. Hormone RIA data revealed that LHRH and beta-EP levels in the portal plasma ranged from 25 to 600 pg/ml and from 0.6 to 4.6 ng/ml, respectively, whereas no discernible amounts of these hormones were detected in the peripheral plasma. Analysis further revealed that secretion of both LHRH and beta-EP showed a diurnal rhythmicity and that both neuropeptides were secreted in a pulsatile manner on each day of the cycle, with most dramatic changes seen in female rats on proestrus. In proestrus rats, portal plasma levels of beta-EP were highest during 1000-1500 h and lower around 1600-1800 h. In contrast, morning and early afternoon values of LHRH were obviously higher in rats on proestrus than in animals on other days of the cycle, and an even greater rise in portal plasma levels of LHRH occurred in late afternoon on proestrus, when beta-EP values were lowest.(ABSTRACT TRUNCATED AT 250 WORDS)