Careful observers sometimes note increased mood swings during the two weeks after female orgasm in humans. We wondered if this were due to surges of prolactin, as prolactin can act as a stress hormone, and has been associated with hostility in women in other contexts.
In this experiment, research shows that vigorous intercourse causes prolactin surges in female rats for two weeks after mating, even where the males are sterilized. Are the females experiencing intense sexual arousal...akin to orgasm in women? Is it possible that in our mammalian evolutionary history females who pulled away from their mates after intense sexual stimulation protected their pregnancies somehow and that humans have inherited some vestige of this mechanism? Could intense sexual stimulation be linked to prolactin surges that affect moods?
Selective Oxytocin Receptor Activation in the Ventrolateral Portion of the Ventromedial Hypothalamus Is Required for Mating-induced Pseudopregnancy in the Female Rat
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-releasing factor. During mating, vaginal cervical stimulation (VCS) received from a vasectomized male triggers twice daily prolactin (PRL) surges which persist up to 12+ days, 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 either an oxytocin receptor antagonist (OTR-A), a vasopressin receptor antagonist (V1a-A) or artificial cerebral spinal fluid (aCSF) 30 min prior to 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 V1a-A (80 ng/0.4 microl) infused females exhibited 100% induction of PSP. Females infused with OTR-A returned to estrus after 5 days, whereas females infused with either aCSF or V1a-A remained in diestrus for 12-13 days in both the correct and missed placement groups. Although OT can act as a prolactin releasing factor, the PRL surge does not begin until 18-24 h after mating. Taken 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.
Northrop LE, Erskine MS. Department of Biology, Boston University, Boston, Massachusetts 02215.
Endocrinology 2007 Nov 15, 2007