Differentiation Between the “Critical Period,” the “Activation Period” and the “Potential Activation Period” for Neurohumoral Stimulation of LH Release in Proestrous Rats1

Various ovulation blocking agents were administered to 4-day cycling rats just prior to the onset of the proestrous “critical period.” Blood was withdrawn at hourly intervals during the afternoon and evening through an atrial cannula implanted that morning. Radioimmunoassay of plasma LH revealed that ip injection of urethane (1.5 g/kg body wt), phenobarbital (75 mg/kg) or pentobarbital (32 mg/kg) at 1355 hr, deep anesthesia induced with ether from 1355–2000 hr, or slow infusion of 1.0 ml of 95% ethanol through the cannula from 1355–1500 hr, were all effective in inhibiting the spontaneous rise of LH. Plasma LH concentration remained low through 2300 hr, except for a small rise at 2100–2200 hr which was observed in all but phenobarbital-treated animals. None of these agents altered pituitary LH release in response to injection of 100 ng of synthetic LHRH shortly after 1400 hr, and saline and phenobarbital-treated rats also responded well to the releasing hormone at 2300 hr. Injection of urethane (0.8 g/kg ...