Abstract
In order to elucidate the facilitatory action of progesterone on the CNS mechanism (s) regulating ovulating hormone (OH) release, the steroid (5mg/rat) dissolved in sesame oil was injected subcutaneously to proestrous rats which had been treated with the minimal dose of sodium pentobarbital (PB, 25 or 22.5mg/kg, ip) to inhibit spontaneous OH release. Low (20-30%) incidence of ovulation (No.of rats ovulated against No.of rats treated) was obtained on the following day in the rats given PB alone. When progesterone was injected simultaneously with PB or until 21:00 proestrus, 4hr after PB injection, incidence of ovulation markedly increased to attain about 80 %. In these rats, ovulation was observed from 10:00 to 11:00 on the morning of estrus regardless of the timing of progesterone injection given between 17:00 and 21:00 proestrus. The rats given progesterone at 22:00 or later on proestrus, however, failed to show any increase in the incidence of ovulation. In the rats pretreated with 30mg/kg or more of PB, progesterone was ineffective to increase ovulation incidence. It is likely, therefore, that progesterone exerts its action on the CNS to prolong and possibly heighten the activity to induce OH release. This also suggests that endogenous progesterone, increased by the initial part of released OH on the afternoon of proestrus, acts, in turn, on the CNS ovulating mechanism (s) to accomplish normal OH release.
