Artificial copulomimetic cervical stimulation (CS) induces an immediate release of oxytocin (OT) and prolactin (PRL) followed by a daily PRL rhythm characterized by nocturnal and diurnal surges. Although we have shown that the initial release of PRL is induced by the immediate release of OT, we tested whether the PRL that is released in response to CS is responsible for the initiation and maintenance of the subsequent PRL surges. Thus, we injected OVX rats centrally or peripherally with ovine PRL (oPRL) at 2200 h. Central oPRL induced PRL surges in OVX rats that were similar in size and timing to those of CS rats, whereas peripheral oPRL induced surges that were of smaller amplitude and delayed. We then infused a PRL antagonist (S179D, 0.1 ng/h) centrally into OVX and OVX-CS rats and measured the release of endogenous PRL and the activity of neuroendocrine dopaminergic neurons. Central infusion of S179D did not influence basal PRL secretion in OVX rats but prevented the expression of the CS-induced PRL surges and the accompanying noontime increase of CS-induced dopaminergic activity when continued for 3 d. However, central infusion of S179D only on the day of CS did not prevent the daily rhythm of PRL surges. These results demonstrate that PRL acts centrally to induce the PRL rhythm and that PRL in the brain is essential for the maintenance but not for the initiation of the CS-induced rhythmic PRL surges.
