Postovulatory sensitization to the negative oestrogen feedback in female rats is probably induced by the preceding decline of the oestrogen concentration in the medial preoptic area.

To examine the question if an endogenous oestrogen-independent rhythm is involved in the cyclic variation of sensitivity to the negative feedback of oestrogen recorded in a former study, adult female rats were ovariectomized on subsequent days of a 4-day ovarian cycle, injected with 3 micrograms oestradiol benzoate (OB)/100 g b.w. or oil three days after castration, and autopsied on the following day. Estimation of the serum LH concentration revealed a similar LH-inhibiting effect of OB in all experimental groups. Female rats were then implanted with OB or cholesterol in the medial preoptic area (MPOA) in metoestrus. In part of the rats, the implants were removed on the presumptive day of pro-oestrus to imitate the periovulatory decline of the circulating oestrogen level acting on the MPOA. Evaluation of the sensitivity to the negative oestrogen feedback during oestrus and metoestrus demonstrated that s.c. injected OB was highly effective in suppressing the LH secretion after removal of the OB implants in pro-oestrus, but not in rats with the implants left in place till autopsy. In a final experiment, the pro-oestrous progesterone surge was inactivated by the injection of specific antibodies. An influence of this treatment on the LH-inhibiting effect of OB examined during oestrus and metoestrus could not be found. Taken together the results suggest that the high sensitivity to the negative oestrogen feedback recorded during the postovulatory period in cyclic female rats is mainly induced by the periovulatory fall of the circulating oestrogen level leading to reduction of the medial preoptic oestrogen concentration.