How the nose cools the brain during copulation in the male rat

Copulation in the male rat is accompanied by a progressive increase in both body temperature and hypothalamic temperature and, soon after ejaculation, by a rapid and selective decrease in hypothalamic temperature. We hypothesized that two changes occur in tandem within the vasculature of the nasal mucosa that contribute, respectively, to hypothalamic heating and hypothalamic cooling. The first takes place prior to ejaculation and involves mucosal vasoconstriction and warm venous blood flowing from the nose to the base of the brain. We thought of such warm blood as retarding heat loss from the hypothalamus. The second takes place immediately following ejaculation and involves the same venous blood, but now cool owing to an abrupt dilation of nasal blood vessels. We hypothesized that such cool venous blood is largely responsible for the observed postejaculatory reduction in hypothalamic temperature. To test our hypothesis, we measured temperature at the surface of the nasal mucosa and in the hypothalamus during successive copulatory bouts. In accord with prediction, we found a reduction in mucosal-surface temperature prior to ejaculation (reflecting vasoconstriction and heat retention) and a substantial rise in mucosal-surface temperature following ejaculation (reflecting vasodilation and heat dissipation). Accompanying these changes in nasal vasomotor tone was a progressive preejaculatory rise in hypothalamic temperature and a rapid postejaculatory decrease. We conclude that nasal venous blood modulates the temperature of the ventral brain through conductive heat exchange and that such heat exchange is conspicuous during sexual behavior.