Ovariectomy at different stages of the estrous cycle modifies the pulsatile secretory pattern of prolactin in rat.

The role of estrogens in the synchronization of pulsatile prolactin secretion throughout the estrous cycle in the adult female rat is studied. Mean values of prolactin, in sham-operated animals increased from diestrus-2 to proestrus and then decreased to the values found at diestrus-1. Ovariectomy did not modify the pattern which follows mean serum levels of the hormone throughout the estrous cycle; although increased values of prolactin were found at both proestrus and estrus, when compared to control animals. No changes in mean values of the hormone were observed at any other phase of the estrous cycle. The number of peaks was fairly constant in sham-operated rats in any phase of the estrous cycle, but a decrease in its number, after ovariectomy, was observed at proestrus and estrus. The absolute amplitude of the peaks increased numerically but not statistically significant from diestrus-2 to proestrus, then decreasing to diestrus-1 in control animals whereas an increase in the absolute amplitude of the prolactin peaks was detected in proestrus and estrous after ovariectomy. The duration of the prolactin peaks was not changed either by the phase of the estrous cycle or by ovariectomy. The relative amplitude of the prolactin peaks was only changed in rats ovariectomized at diestrus-1, as compared to sham-operated animals. All these data indicated that changes in estrogen/secretion by ovariectomy throughout the estrous cycle of the rat induced a differential effect on the pulsatile pattern of prolactin, being more marked in proestrus and estrus, associated with the highest circulating estrogen levels.