Ovulatory Response to Brain Stimulation or Exogenous Luteinizing Hormone in Progesterone-Treated Rats

Injection of progesterone at certain early hours in proestrus (4-day cycles), followed at various intervals by atropine, caused release of an ovulatory quota of gonadotrophin only within a few hours immediately preceding the “critical period.” Late afternoon injection, in pentobarbital-blocked proestrous rats, failed to cause full ovulation. However, “delayed” release of gonadotrophin, normally occurring one day after pentobarbital in proestrus, was prevented by progesterone injection in pentobarbital-treated proestrous rats. “Delayed” release usually was not prevented if atropine was substituted as the blocker in proestrus. Spontaneous release of gonadotrophin was prevented by progesterone on diestrus day 2. A high dose (2 mg/100 g body wt) inhibited follicle growth and vaginal cornification. Rats so treated failed to ovulate after either “massive” bilateral preoptic brain stimulation or injection of LH in the next afternoon. After a low dose of progesterone (0.25 mg/100 g body wt), follicle growth continued and vaginal cornification occurred. Most such rats ovulated after either “moderate” unilateral preoptic stimulation or injection of LH in the next afternoon. Ovulation in the forthcoming cycle was prevented by daily afternoon injection of a low dose of progesterone. By contrast, daily morning injection of the same amount did not interfere. The experiments indicate that the ovulatory response to progesterone in proestrus (4-day cycles) is similar to that seen after progesterone in diestrus day 3 (5-day cycles). Also, failure of preoptic stimulation to cause ovulation in rats given a high dose of progesterone is probably due to the relative insensitivity of their ovaries to LH. Low-dose rats, which ovulate after preoptic stimulation, do not show a significant increase in their requirement for LH. With daily progesterone injections, suppression of gonadotrophin release in cycling rats is determined by the time of day when daily injections are given. (Endocrinology83: 1013, 1968)