To see whether a periodical central excitation related to the gonadotrophin release or production exists on the other stages than the proestrus in the 4-day cycling rat, the time course of the successful blockade of expected ovulation was investigated with pentobarbital anesthesia applied at different timing of the day throughout the estrous cycle.
Except the day of estrus, the anesthesia delayed the ovulation by one day, when it was given at a more or less limited period on the afternoon of each day of the cycle. Larger doses or more long-lasting anesthetic actions were required on the day of diestrus compared with the days of proestrus and metestrus. It was also noted that the effective period for blocking the ovulation by the anesthesia was much wider on the day of former than the days of latter of which period was almost corresponded with so called “critical period.”
A hypophyseal gonadotrophic activity, assayed by the induction of extra-ovulation in 4-day cyclic mice, showed a sudden change at around 14:00 on the day of metestrus in both rats and mice, though they showed somewhat different pattern.
Existence of some periodical, surging release of gonadotrophin, subjected to the activation of “cyclic release regulating” mechanism in the hypothalumus, is suggested on the day of metestrus in addition to the day of proestrus.