Hypothalamic factors involved in the endogenous stimulatory rhythm regulating prolactin secretion

We recently reported that acute pharmacologic depression of dopaminergic tone at different times of day unmasks a sex-specific endogenous stimulatory rhythm regulating PRL secretion. The PRL secretory responses of ovariectomized rats to the dopamine antagonist domperidone (DOM) were higher at 0300 and 1700 h than at 1200 h. These are the times during which surges of PRL appear in mated rats. This experimental paradigm was used to investigate the roles of the putative PRL-releasing factors (PRFs) oxytocin (OT), vasoactive intestinal peptide (VIP), and serotonin (5-HT) in this rhythm. The role of OT was studied by infusion of the OT antagonist 1-deamino-2-D-Trp-4-Val-8-Orn-Oxytocin (OT-A, 0.5 microgram/kg min) for 6 h. Two hours after beginning the OT-A infusion DOM was administered, as a single injection of 200 micrograms/kg iv at either 0300, 1200, or 1700 h. Serial blood samples were collected immediately before and 5, 10, 20, 30, 60, 120, 180, and 240 min after DOM administration. Infusion of OT-A attenuated the heightened PRL secretory responses to DOM given at both 0300 and 1700 h but did not affect the response at 1200 h. The role of VIP was studied by infusing the VIP antagonist [D, 4-Cl-Phe6,Leu17] VIP (VIP-A, 0.1 microgram/kg.min) as described above. VIP-A infusion had no effect on the PRL secretory responses to DOM given at 1200 or 1700 h but attenuated the heightened response at 0300 h. In order to study the role of 5-HT in the rhythm, rats were pretreated with p-chlorophenylalanine (250 mg/kg sc) 48 and again 24 h before the experiment. Pretreatment with p-chlorophenylalanine had no effect on the PRL secretory responses to DOM given at 0300 or 1200 h, but it attenuated the augmented PRL secretory response at 1700 h. These data suggest that both VIP and OT act as endogenous PRFs at 0300 h and 5-HT and OT act as PRFs at 1700 h. We propose that VIP and 5-HT are continuously active oscillatory neurotransmitters regulating OT release into pituitary portal blood and that these daily events only eventuate in PRL release when the mating stimulus has release the lactotroph from the inhibitory effects of dopamine.