During the last four years, our laboratory has focused its attention on the encoding of hormone signals with pulsatile patterns. As a result of our efforts, we have been able to establish that PRL-mediated trophic inputs to the corpus luteum of the estrous cycle are encoded in a special subtype of PRL episodes which we have denominated as big mass (BM) PRL pulses. This particular type of PRL secretory event, which presents the highest incidence during the afternoon of estrus, appears to be exquisitely regulated, since BM pulses are the consequence of the instauration of a period of sporadic interruptions in the tonic inhibitory influence exhibited by hypothalamic dopamine. Due to the particular endocrine characteristics of the day of estrus in the rat, we evaluated whether estradiol and/or progesterone imprinting during the days prior to estrus could be responsible for establishing the interrupted dopaminergic tone required to observe BM PRL pulses. Experiments were conducted in long-term ovariectomized (OVX) rats which were injected with estradiol, progesterone or both three days before the bleeding. Under this pharmacological paradigm, we were able to demonstrate that estradiol is needed for obtaining a pattern of PRL release consisting of BM and small mass PRL pulses. Progesterone, in contrast, appears to reduce the incidence of BM pulses when administered with estradiol. In order to confirm our observations and to extend them to a more physiological paradigm, we designed a series of experiments aimed at analyzing the time in which the ovary is required for establishing the hormonal environment leading to BM PRL pulses. Timed ovariectomies were performed either in the morning of diestrus or the afternoon of proestrus after the preovulatory surge of gonadotropins and PRL has occurred. Animals were bled on the expected afternoon of estrus. Under these circumstances, the hypothalamus either receives (rats after the preovulatory surges) or is deprived (diestrous OVX rats) of the endogenous influences of the ovary during proestrus. Ovariectomy during diestrus abolished BM PRL pulse-incidence, indicating that the ovary of proestrus participates in the mechanisms leading to BM pulse appearance. In contrast, when the animals were OVX in the evening of proestrus, PRL secretory patterns were identical to those observed in sham-operated rats. These data clearly show that the ovary, during the periovulatory period, participates in the physiological mechanisms leading to the genesis of BM PRL pulses.
