In previous studies, we have demonstrated that pulsatile PRL secretion depicts a distinct pattern depending on the estrous cycle phase. This pattern is characterized by the presence of big mass PRL pulses which present the highest incidence during the afternoon of estrus. In this phase of the estrous cycle, there is an important steroid imprinting, therefore, it is reasonable to think that the highest incidence of BM PRL pulses may be due to this steroid environment. This possibility was evaluated by analyzing pulsatile PRL secretion OVX animals after treatment with either estradiol, progesterone or both combined. The results indicate that OVX dramatically reduces the incidence of big mass PRL pulses. In fact, PRL pulsatility in two-week OVX animals is characterized by an almost complete absence of large pulses although the pulsatile nature of PRL secretion was not abolished. Three-day estradiol replacement therapy induces a proestrus-like surge 48 h after estrogen administration and, therefore, an endocrine situation similar to that found in the morning of estrus. In these rats, the incidence of BM PRL pulses is restored to that observed in the morning of estrus. In contrast, progesterone (P4) administration the day prior to the bleeding, which stimulates the proestrus P4 surge, impaired the ability of estradiol in reverting the effects of OVX. These observations clearly suggest that estradiol imprinting may favor the establishment of a discontinuous dopaminergic tone, whereas P4, at least at the doses used in this study, blocks this ability. These studies indicate that the preovulatory estradiol imprinting may set a period of interrupted dopaminergic input leading to the establishment of the pulsatile PRL profile observed during estrus. In addition, the ability of progesterone to abolish estradiol effects suggests that progesterone during diestrus is responsible for the insaturation of a period of tonic dopaminergic inhibition, producing, thereby, a profile solely consisting of small mass PRL pulses.
