Ovulation blockade and receptivity in the cyclic rat

The onset and decline of sexual receptivity was determined around oestrus in intact 5-day cyclic rats and in animals in which spontaneous ovulation was inhibited with nembutal. It was observed that early receptivity (before the critical hours at 14--16 h) could be induced by prior exposure of pro-oestrous females to male rats during the morning at pro-oestrus. In most cases early receptivity was associated with a failure to block ovulation with nembutal administered at 14 h. In cyclic rats receptivity disappeared on the day of oestrus between 8 and 10 a.m. By contrast the period of receptivity was prolonged for 24 h in animals in which ovulation was inhibited by nembutal. Reflex ovulation could be induced by exposure of nembutal-blocked rats to intact as well as to pudendectomized male rats, which latter cannot achieve intromissions. This illustrates the limited role of female genital stimuli in the induction of reflex ovulation. Progesterone (1.25 or 5 mg) injected at pro-oestrus during nembutal sedation did not have an attenuating effect on receptivity 24 h later. It is concluded that LH-surge dependent progesterone secretion at pro-oestrus is not essential for the induction and termination of receptivity around oestrus and that hormones, presumably oestrogens, secreted by the persistent pre-ovulatory follicles, are responsible for the prolonged period of receptivity following nembutal blockade of ovulation.