DIURNAL FLUCTUATIONS IN MATING-INDUCED OXYTOCINERGIC ACTIVITY WITHIN THE PARAVENTRICULAR AND SUPRAOPTIC NUCLEI DO NOT INFLUENCE PROLACTIN SECRETION

Previous studies have implicated oxytocin (OT) in the control of surge -type PRL secretion in the pregnant and pseudopregnant rat. The presen t studies examined the relationship between mating-induced activation of OT neurons in the paraventricular (PVN), supraoptic (SON), and ante rior commissural (ACN) nuclei and PRL secretion. Activity within OTerg ic neurons, as measured by increased c-fos expression, was examined im mediately and 5 days following mating in ovariectomized, estrogen-plus -progesterone-treated rats at the time when nocturnal PRL surges are e xpressed (0600 h) and at an intersurge time (2400 h). Females received fifteen intromissions (15I), 15 mounts-without-intromission (MO), or no stimulation (homecage, HC) from a sexually experienced male. Receip t of 15I at 0600 h induced significantly higher numbers of OT immunore active (OT-IR) cells and FOS/OT-IR double-labeled cells in the parvoce llular division of the PVN (PVN) and in the SON than did 15I. at 2400 h. Numbers of OT-IR and FOS/OT-IR cells in the ACN and in the magnocel lular compartment of the PVN (PVN) were not influenced by mating at ei ther time. In contrast, acute PRL secretion induced within 5-30 min by 15I was not influenced by whether mating occurred at 1800 h (diurnal surge), 2400 h, or 0600 h, nor were plasma OT levels elevated during t he 1 h following 15I or MO at these times. Examination of FOS-IR cells throughout the hypothalamus across the two times of day revealed prev iously unreported differences between 15I and control MO treatments in the PVN, SON, and the ventrolateral part of the arcuate nucleus (ARC( v1)). On day 5 post mating, numbers of OT-IR and FOS/OT-IR cells in th e PVN, SON, and ACN were very low and were similar between 0600 h and 2400 h and between females that showed (15I) or did not show (MO) mati ng-induced PRL surges characteristic of pregnancy. The results of thes e studies demonstrate that intromissive but not mounts-only stimulatio n from males induces a rapid increase in OT-IR staining and OT neuron activation in the PVN,, and the SON. These mating-induced responses in OT neurons occurred within Ih after mating only at 0600 h, suggesting a diurnal fluctuation in sensitivity to intromissive stimulation. Cha nges in OTergic function were not seen in response to mating at other times of day, nor at the time of the nocturnal PRL surge 5 days after mating. We conclude that OT activity induced by mating does not act to stimulate PRL secretion directly, but may be involved in the process( es) by which genitosensory stimulation initiates surge-type PRL secret ion.