Effect of Electrical Stimulation of the Preoptic Area on Plasma LH Concentrations in Proestrous Rats

Abstract

The present studies have measured, by radioimmunoassay, the changes which occur in plasma LH concentration in individual unanesthetized rats during various hours on the day of proestrus. These patterns have been compared with those which occur in proestrous rats treated with an ovulation blocking dosage of Nembutal (sodium pentobarbital) and in Nembutal- blocked rats in which “ovulation inducing” electrical stimuli were delivered either unilaterally or bilaterally to the medial preoptic area (POA). The “critical period” for hypothalamic activation of the adenohypophysis was established as occurring between 1300 and 1500 in these animals. In the normal proestrous rat, while the time of initial increase in plasma LH concentration varied from rat to rat, the majority of animals studied exhibited peak concentration in plasma LH between 1500 and 1600. Thereafter, plasma LH declined rapidly, and it reached approximately basal levels by 1900. Injection of Nembutal at 1245 prevented any increase in plasma LH during the afternoon of proestrus. Furthermore, simple stereotaxic placement of an electrode into the POA did not alter plasma LH concentrations beyond those basal levels observed after Nembutal treatment alone. When the POA was stimulated electrically with biphasic rectangular pulses (140 μA-100/sec-15 sec on/off for 40-60 min), a significant increase in plasma LH occurred at 100 or 40 min after cessation of stimulation. Thereafter, LH rapidly declined to prestimulation values at 120 min. Similar patterns of change occurred in plasma LH concentrations after bilateral POA stimulation. Peak plasma LH concentrations occurred 100 min after initiation of the stimulus, but, unlike the unilateral stimulated group, it was still elevated at 120 min. Thus, the patterns of change in plasma LH concentration induced by POA electrical stimulation are comparable to those observed in the normal proestrous rat. The temporal interrelationships which exist between the time of neurogenic activation of the hypothalamus and the release of pituitary LH are discussed. (Endocrinology88: 1175, 1971)