A Longitudinal Study of the Changes in Responsiveness of the Hypothalamo-Pituitary Unit to Medial Preoptic Area Stimulation and to Luteinizing Hormone-Releasing Hormone Administration During Diestrus-2 and Proestrus in Rats

Abstract

Whether certain components of the preoptico-tuberal-pituitary unit show differential changes in responsiveness to activating stimuli during various hours on diestrus day-2 (D2) and proestrus (P) was studied in rats. In experiment I, the medial preoptic area (MPOA) of Nembutal-treated 4-day cyclic rats was electrochemically stimulated (ECS), blood was sequentially collected at 30 min intervals to 210 min on various hours during D2 and P, and plasma was radioimmunoassayed for LH/FSH [luteinizing hormone/follicle stimulating hormone]. While MPOA-ECS provoked a significant release of LH in all groups, the peak plasma concentrations varied considerably between 1000 h D2 and 1300 h P. By 2300 h D2, plasma LH rose significantly above baseline but FSH values remained unchanged. At 0300 h P preoptic stimulation induced further significant rises in LH and at this hour plasma FSH also had increased by 200 min poststimulation. While the greatest peak LH concentrations occurred in rats stimulated at 1100-1300 h P, plasma FSH values did not exceed those observed between 0300-0900 h P. In experiment II, pituitary responsiveness to pulse injections of synthetic LHRH [luteinizing hormone releasing hormone] was tested. Injection of 250 ng LHRH between 1000 h D2 and 0700 h P elevated plasma LH from basal 5 .+-. 2 ng/ml to only 36-50 ng/ml within 10-20 min after treatment. Peak plasma concentrations of LH in rats injected at 1100 and 1300 h P were 85 .+-. 2.7 and 151 .+-. 24 ng/ml, respectively. The injection of 1000 ng LHRH induced a near linear increase in plasma LH in all groups studied between 0300-1300 h P. Peak LH concentrations were reached more rapidly in the 1300 h P animals than in the other groups. Plasma FSH levels were increased above baseline at 120 and 180 min following LHRH injections but only in the 0700-1300 h groups. Seemingly, the preoptico-tuberal system becomes competent to respond to MPOA-ECS by the release of LH/FSH at a time (0300 h P) when pituitary responsiveness to LHRH is minimal.