OXYTOCIN, VASOACTIVE-INTESTINAL PEPTIDE, AND SEROTONIN REGULATE THE MATING-INDUCED SURGES OF PROLACTIN SECRETION IN THE RAT

In female rats the mating stimulus induces a bimodal pattern of PRL secretion. A surge of PRL occurs at approximately 0300 h, called the nocturnal surge (N). Another surge occurs at 1700 h on the same day, called the diurnal surge (D). By lowering dopaminergic tone pharmacologically, we have recently demonstrated the existence of an endogenous rhythm stimulatory to PRL secretion in female rats. The periods of this stimulatory influence coincide with the periods of the N and D surges of PRL that occur in mated rats. In addition, we have shown that the 0300 h component of this endogenous rhythm is regulated by oxytocin (OT) and vasoactive intestinal peptide (VIP), and the 1700 h component is regulated by OT and serotonin (5-HT). In this study, we investigated the roles of OT, VIP, and 5-HT in controlling the N and D surges of PRL in ovariectomized (OVX) rats receiving a physiological dopamine-lowering stimulus, copulomimetic stimulation. Blood samples were obtained the day before the experiments between 1700 and 1900 h to verify that the rats used were having surges of PRL in response to cervical stimulation (CS). The role of OT was studied by infusing the OT antagonist l-deamino-2-D-Trp4-Val8-Orn-OT (OT-A; 0.5 μg/kg-min) beginning at either 0100 or 1500 h and continuing for 5 h on day 2 after the last CS. Serial blood samples were obtained immediately before infusion and 60, 90, 120, 150, 180, 240, and 300 min after the start of infusion. The samples overlapped either the N or D surge of PRL. All rats used in these studies demonstrated D surges of PRL the day before the experiment. Saline infusion had no effect on either the N or D surge of PRL in OVX-CS rats. However, infusion of OT-A completely blocked both the N and D surges of PRL. The role of VIP was studied by infusing the VIP antagonist [4-D-C1- Phe6-Leu17]VIP (VIP-A; 01 μg/kg-min) beginning at either 0100 or 1500 h and continuing for 5 h. VIP-A completely blocked both the N and D surges of PRL. To study the role of 5-HT, rats received an acute treatment with p-chlorophenylalanine (PCPA; 250 mg/kg, sc) at either 0100 or 1500 h, and blood samples were taken as before. PCPA had no effect on the N surge of PRL. Rats given PCPA at 0100 h produced N surges similar in timing and magnitude to those in control animals. However, acute PCPA treatment completely blocked the D surge of PRL. These data suggest that, similar to the endogenous rhythm, OT is involved in regulation of the N (0300 h) and D (1700 h) surges of PRL in OVX-CS rats, and 5-HT is involved in regulation of only the D (1700 h) surge. However, unlike the endogenous rhythm regulating PRL secretion, VIP is involved in regulation of both the N (0300 h) and D (1700 h) surges. Therefore, the mating-induced surges of PRL may be due to expression of the endogenous rhythm regulating PRL secretion superimposed upon the PRL regulatory effects of CS. © 1990 by The Endocrine Society.