Selective Oxytocin Receptor Activation in the Ventrolateral Portion of the Ventromedial Hypothalamus Is Required for Mating-Induced Pseudopregnancy in the Female Rat

Abstract

The ventrolateral region of the ventromedial hypothalamus (VMHvl) plays an essential role in female sexual behavior. Oxytocin (OT) is released from the paraventricular nucleus to downstream sites such as the VMHvl to facilitate female sexual behavior and shows characteristics of a prolactin (PRL)-releasing factor. During mating, vaginal cervical stimulation (VCS) received from a vasectomized male triggers twice-daily PRL surges that persist up to 12+ d, a period known as pseudopregnancy (PSP). To determine whether OT is involved in PSP by acting within the VMHvl, female rats were infused bilaterally with an oxytocin receptor antagonist (OTR-A), a vasopressin receptor-1a antagonist (V(1a)-A), or artificial cerebral spinal fluid 30 min before mating. All females received a sufficient amount of VCS, 15 intromissions, to induce PSP. Females infused with OTR-A (20 ng/0.4 microl) with implants targeting the VMHvl showed only a 22% induction of PSP, as measured using vaginal diestrus and serum PRL concentrations. In contrast, controls and V(1a)-A (80 ng/0.4 microl) infused females exhibited 100% induction of PSP. Females infused with OTR-A returned to estrus after 5 d, whereas females infused with either artificial cerebral spinal fluid or V(1a)-A remained in diestrus for 12-13 d in both the correct and missed placement groups. Although OT can act as a PRL releasing factor, the PRL surge does not begin until 18-24 h after mating. Together, our results suggest that OT release in the VMHvl mediates the effects of VCS on the induction of the PRL secretion needed to establish PSP.