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 present studies examined the relationship between mating-induced activation of OT neurons in the paraventricular (PVN), supraoptic (SON), and anterior commissural (ACN) nuclei and PRL secretion. Activity within OTergic neurons, as measured by increased c-fos expression, was examined immediately and 5 days following mating in ovariectomized, estrogen-plus-progesterone-treated rats at the time when nocturnal PRL surges are expressed (0600 h) and at an intersurge time (2400 h). Females received fifteen intromissions (151), 15 mounts-without-intromission (MO), or no stimulation (homecage, HC) from a sexually experienced male. Receipt of 151 at 0600 h induced significantly higher numbers of OT immunoreactive (OT-IR) cells and FOS/OT-IR double- labeled cells in the parvocellular division of the PVN (PVN(parv)) 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 magnocellular compartment of the PVN (PVN(mag)) were not influenced by mating at either time. In contrast, acute PRL secretion induced within 5-30 min by 151 was not influenced by whether mating occurred at 1800 h (diurnal surge), 2400 h, or 0600 h, nor were plasma OT levels elevated during the 1 h following 151 or MO at these times. Examination of FOS-IR cells throughout the hypothalamus across the two times of day revealed previously unreported differences between 15I and control MO treatments in the PVN, SON, and the ventrolateral part of the arcuate nucleus (ARC(vl)). On day 5 post mating, numbers of OT-IR and FOS/OT-IR cells in the 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) mating-induced PRL surges characteristic of pregnancy. The results of these studies demonstrate that intromissive but not mounts-only stimulation from males induces a rapid increase in OT-IR staining and OT neuron activation in the PVN(parv) and the SON. These mating-induced responses in OT neurons occurred within 1 h after mating only at 0600 h, suggesting a diurnal fluctuation in sensitivity to intromissive stimulation. Changes 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 secretion.