BioEd Online

Ever since Viagra worked for men - and didn't for women - researchers have been searching for a drug that turns women on. Now a hormone-like drug may hit the spot, by targeting not the genitals, but the brain.

The way Viagra works is simple; it dilates blood vessels, increasing blood flow in the genitals. This purely physical effect seems to be sufficient for most men to perform and enjoy sex.

But women are more complicated. "The difference between male and female orgasms is that brain effects are more important in women," says John Stevenson, an endocrinologist at the Royal Brompton and Harefield NHS Trust. In other words, physical arousal doesn't happen without desire. And that can be much harder to trigger.

"People may get the sensory input, but they don't think, 'ooh I'm horny'."

According to James Pfaus, a behavioural neurobiologist at Concordia University in Montreal, Canada, disorders of sexual desire affect 30% of women in North America and Europe. The loss of sex drive may be down to personal factors in many cases, but Pfaus says that the cause is sometimes physiological.

"People may get the sensory input, but they don't think, 'ooh I'm horny'," he says.

All in the brain

Pfaus believes that a drug called PT-141 could help women in this situation by stimulating their brains to increase libido. PT-141 mimics a hormone called alpha-melanocyte-stimulating hormone, which binds to certain receptors in the brain and is thought to stimulate sexual activity. The drug has already been shown to work on men who don't respond to Viagra.

To test whether females respond to it too, Pfaus and his team administered the drug to groups of 20 or 40 rats. The animals each received one of four doses of PT-141, and then each group was tested for a different sign of sexual desire, including showing interest in individual males and changing body position to aid sexual intercourse.

In all the groups, the higher the dose of PT-141, the more sexual behaviour the females displayed. The next step, says Pfaus, is to test the drug in women.

Stevenson agrees the approach looks promising. But he points out that women who lose their sex drive are often in menopause and suffer from other symptoms too, such as hot flushes and mood swings. The drug could be most useful in combination with other therapies, he suggests.