A new study with rats is the first to demonstrate that a group of hormone receptors in the brain plays a key role in female sexual appetite. The research may one day lead to drugs for treating sexual dysfunction in women.
While men can turn to Viagra and other pills or potions to overcome impotence, there is nothing on the pharmacy shelf for women, although an estimated 30% of North American and European women suffer from various sexual dysfunctions, including a lack of sexual appetite. One reason for this inequality, says James Pfaus, a behavioral neuroendocrinologist at Concordia University in Montreal, Canada, is that it has only recently become clear that the kinds of chemicals that increase sexual arousal in men are ineffective in women. To uncover the crucial differences in sexual chemistry, Pfaus's team studied the effects on the behavior of female rats of small synthetic proteins that mimic a pituitary gland hormone called α-melanocyte stimulating hormone (α-MSH). The hormone's target, α-melanocyte receptors on the surface of cells in the brain, plays a central although poorly understood role in sexual arousal.
In a paper published online this week in the Proceedings of the National Academy of Sciences, the team reports that female rats injected with one α-MSH mimic, called PT-141, were significantly friskier in the presence of males, soliciting sex by hopping and darting around males about three times more often than control rats. Pfaus is surprised that PT-141 appears to increase sexual appetite in female rats but not their "downstream" sexual physiology, such as the reflexive bending of the female's back necessary for intercourse. This suggests that the drug is acting on a central regulator of sexual behavior, says Pfaus, which not only makes it the best candidate going for treating sexual dysfunction in women, but should also help us better understand the "chemical code" of sexual desire in the brain.
This is the first study to show that two components of female sexuality, the appetite for sex and the body's receptivity to intercourse, are under "separable control" in female mammals, says Joseph Herbert, a neurobiologist at the University of Cambridge, U.K. The next step toward understanding how sexual appetite is wired in the brain, says Herbert, is to find exactly which of the brain's α-MSH receptors are being activated.