Food restriction during lactation results in prolonged hyposensitivity to the positive-feedback effects of oestradiol.

Food restriction prolongs lactational infertility in rats. Here, we investigated whether an attenuated response to the positive-feedback effects of oestrogen on luteinizing hormone release contributed to this effect. The ability of oestrogen to induce surges in luteinizing hormone in ad libitum fed and food-restricted dams at different times of lactation was compared. The results showed that, on day 20 postpartum, ad libitum fed dams showed luteinizing hormone surges after oestrogen treatment, but food-restricted dams did not. Ovariectomy or RU486 treatment restored the ability of oestrogen to induce luteinizing hormone surges in food-restricted dams, and chronic progesterone exposure reduced oestrogen-induced surges of luteinizing hormone in ad libitum fed ovariectomized dams. Food restriction also resulted in a reduced ability of oestrogen to induce progesterone receptor immunoreactivity, but did not reduce the number of oestrogen receptors (ERalpha) in the anteroventralperiventricular area. As with the surge in luteinizing hormone, the effects of food restriction on oestrogen induction of progesterone receptors were mediated by progesterone. Together, these results suggest that the ability of food restriction to extend the length of lactational diestrus is mediated, in part, by a decrease in sensitivity to the positive-feedback effects of oestrogen. This results from high circulating concentrations of progesterone which apparently reduce the ability of oestrogen to induce progesterone receptor expression.