Luteinizing hormone-releasing hormone in peripheral plasma and hypothalamus of normal and ovariectomized rats.

Plasma and hypothalamic LHRH was measured by specific radioimmuno-assay in intact and ovariectomized rats, and the values were correlated with peripheral plasma (PP) FSH and LH titers. At most stages of the estrous cycle, plasma LHRH was either undetectable or present at very minimal values. An increase in PP LHRH concentration was observed in some animals between 13.00 and 20.00 h on proestrus. The mean elevation in LHRH was greatest in rats when blood samples were taken by decapitation; elevation was somewhat less when samples were taken from etherized rats and minimal when taken from rats bearing indwelling jugular cannulae. LHRH was elevated in approximately half the ovariectomized animals; repeated samples were drawn at 15-min intervals from intrajugular cannulae. In animals with LHRH elevations, LHRH was highly variable, which indicates that it is released in pulsatile fashion. Plasma LHRH and LH titers were correlated in ovariectomized animals. The relatively low correlation between LHRH and LH may be explained by the fact that a pulse of LHRH can elicit LH release over a considerable time span; also, LHRH is cleared much more rapidly from the circulation than is LH, as revealed by the time course of disappearance of exogenous LHRH given by bolus injection. In intact rats, hypothalamic LHRH content was slightly lower at 10.00 h on diestrus day 1 than at other sample times. LHRH was significantly lower 4 weeks following ovariectomy compared to levels in intact rats at any sample time. It would appear that LHRH's resynthesis does not keep pace with its release in ovariectomized rats, resulting in a decline in hypothalamic stores.