Changes in Pituitary LH Content During the Rat Estrous Cycle1

Abstract

Evidence in the literature suggests that a facilitation of neural centers concerned with LH release occurs daily between 2 and 4 PM in the cyclic female rat. LH release itself has been shown to occur during this time on the day of proestrus or, under special circumstances, the day before or after. The possibility has never been eliminated that under normal conditions daily release of LH occurs between 2 and 4 PM, but that ovulation occurs only when prepared follicles are present at proestrus. To investigate this possibility LH must be measured by exogenous bio-assay, not by means of endogenous ovulation as done previously. Measurements of pituitary LH content (by means of the ovarian ascorbic acid depletion assay) have been carried out in every stage of the cycle in rats running either 4-day or 5-day cycles, at 10–11 AM and 4–5 PM. A drop in potency occurred only between proestrus and estrus, indicating that the neural facilitation which takes place daily between 2 and 4 PM is inadequate by itself to cause LH release and that another facilitating event (gonadal steroid feedback?) occurs at the time of proestrus. Pituitary LH content is similar in 4-day and 5-day cyclers, and is minimal at estrus and metestrus and maximal (2 times estrous values) at diestrus or on the morning of proestrus. Male pituitaries do not show a change in LH content between 10 AM and 4 PM. The potency of male pituitaries resembles that of female diestrous or proestrous morning pituitaries.