Estrogen-Dependent Twenty-four-Hour Periodicity in Pituitary LH Release in the Female Hamster1

Ovariectomized hamsters were used to study the sites and mechanisms of action of estrogen (E) and progesterone (P) in controlling the synthesis and release of LH by the anterior pituitary. Progesterone alone or in combination with E had no effect on the pituitary content or concentration of LH. Correlated with a lower concentration of pituitary LH in E-treated animals was an increased release of LH in the afternoon. Estrogen alone depressed morning levels of serum LH to 40% of that observed in controls; in combination with P, E further depressed serum LH to values comparable to those found on diestrus. No effect on serum LH by P alone was observed. Pituitary responsiveness to purified exogenous LH-RH was diminished by the combination of E and P, but not by either steroid alone. Serum LH was elevated at 1500 hr for 3 consecutive days in ovariectomized (ovx) hamsters primed with E; this elevation, which was both quantitatively and temporally comparable to the preovulatory LH surge at proestrus was not observed in ovx controls or in ovx + P and ovx + EP groups. This elevation in serum LH was blocked by phenobarbital, which inhibits LH release by action on the central nervous system. Furthermore, when administered during the LH surge on the first day after E treatment, P inhibited the increase in LH on the second day. These observations indicate that in the hamster there is a 24-hr periodicity in the central nervous mechanism responsible for the preovulatory release of LH. This LH release mechanism is enhanced by estrogen and is inhibited by progesterone. Part of the inhibitory effect of progesterone appears to be at the pituitary. (Endocrinology93: 965, 1973)