Oxytocin in the Medial Preoptic Area Facilitates Male Sexual Behavior in the Rat

Oxytocin is a versatile neuropeptide that is involved in a variety of mammalian behaviors, and its role in reproductive behavior has been well established. Oxytocin stimulates male sexual behavior, in part, by promoting erectile function and seminal emission. Previous research has focused on the paraventricular nucleus (PVN) of the hypothalamus, which sends oxytocinergic projections to other neural sites and releases oxytocin into the circulation via the posterior pituitary. This dissertation explores the role of the medial preoptic area (MPOA) in oxytocin-induced facilitation of sexual behavior. Oxytocin infused into the MPOA facilitates copulation in both sexually naïve and experienced males, whereas an oxytocin antagonist (OTA) administered into the MPOA delays the onset of copulation, inhibits anogenital investigation in a dose-dependent manner, and inhibits sexual efficiency following the first ejaculation in experienced rats. The inhibitory effects of OTA treatment are not due to a decrease in locomotor activity, as OTA treatment has no effect on locomotor activity in an open field. There is an inverse relationship between sexual efficiency and oxytocin receptor binding in the rostral MPOA; these findings may suggest that efficient males have higher levels of oxytocin in this hypothalamic area, which may be causing downregulation of oxytocin receptors. Sexual experience significantly increases oxytocin receptor protein levels in the MPOA, while oxytocin receptor mRNA levels are significantly elevated immediately following the first sexual experience, suggesting that the oxytocin receptor may play a role in the MPOA in experienced-induced facilitation of copulation. Oxytocin in the MPOA attenuates NAcc dopamine release during copulation and also appears to maintain NAcc dopamine levels within a normal range. The data reported here provide strong evidence that oxytocin facilitates male sexual behavior, in part, by acting in the MPOA of the hypothalamus, a major integrative site for male sexual behavior in all vertebrate species studied thus far.