THE EFFECTS OF PROGESTERONE AND OF PENTOBARBITONE ADMINISTERED AT THE DIOESTROUS STAGE ON THE OVARIAN CYCLE OF THE RAT

Abstract

SUMMARY Changes in the uterus:plasma and oviduct:plasma concentration ratios for radioactive iodide after the administration of progesterone at the dioestrous stage of the oestrous cycle have confirmed that significant oestrogen secretion begins on the afternoon of the day of dioestrus in a 4-day cycle. Progesterone injected at dioestrus delays ovulation and the change to a facilitatory effect on luteinizing hormone (LH) release does not occur until early on the day of pro-oestrus, about 12 hr. after the estimated time at which oestrogen secretion begins. Pentobarbitone (Nembutal) injected at 13.30 hr. on the day of pro-oestrus blocks ovulation, but does not do so when injected 3 hr. later. When injected at either 13.30 or 16.30 hr. in dioestrus, it delays ovulation and may prevent it completely in that cycle. The injection of Nembutal at dioestrus does not appear to prevent the release of gonadotrophin responsible for oestrogen secretion and the delay of ovulation may be due to interference with some of the normal effects of oestrogen secreted at this stage of the cycle on the hypothalamo-hypophysial system. The delay of ovulation is not prevented by the injection of oestrogen with Nembutal at dioestrus but is prevented by the subsequent injection of progesterone at pro-oestrus. The results are discussed in relation to other studies on the time of onset of oestrogen secretion in the oestrous cycle and to the change in the response to progesterone injection from inhibition to facilitation of LH release following the period of oestrogen secretion.