DIURNAL-VARIATION IN LUTEINIZING-HORMONE-RELEASING HORMONE AND BETA-ENDORPHIN RELEASE IN PITUITARY PORTAL PLASMA DURING THE RAT ESTROUS-CYCLE

In this study, we investigated the pattern of both LHRH and beta-endor phin (beta-EP) secretion from the median eminence of cyclic female rat s during the estrous cycle by collecting pituitary portal blood for ho rmone measurements by RIA, On diestrus, proestrus, or estrus (0900 h o r 1300 h), rats were hypophysectomized (in order to prevent contaminat ion of portal beta-EP with pituitary beta-EP) and were given a 50-60-m in resting period of postsurgical recovery prior to blood sampling. Be tween 1000 h and 1800 h of the same day, pituitary portal blood sample s were collected at 10-min intervals from these animals under Saffan a nesthesia, Hormone RIA data revealed that LHRH and beta-EP levels in t he 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 secr etion of both LHRH and beta-EP showed a diurnal rhythmicity and that b oth neuropeptides were secreted in a pulsatile manner on each day of t he cycle, with most dramatic changes seen in female rats on proestrus. In proestrous rats, portal plasma levels of beta-EP were highest duri ng 1000-1500 h and lowest around 1600-1800 h. In contrast, morning and early afternoon values of LHRH were obviously higher in rats on proes trus than in animals on other days of the cycle, and an even greater r ise in portal plasma levels of LHRH occurred in late afternoon on proe strus, when beta-EP values were lowest. The latter result seems to sug gest an inverse relationship between LHRH and beta-EP secretion. These results reveal pulsatile patterns of LHRH and beta-EP secretion in cy clic female rats and suggest a possible interaction between these neur opeptide systems in the hypothalamus.