Marijuana is reputed to set the mood for love, particularly in females. Now scientists have new insights into how the drug exerts its effects--at least in rats. A study published in the 23 January Proceedings of the National Academy of Sciences shows that the drug's active compound influences acts through two chemical messengers known to have a strong influence on reproductive behavior.
In rats as in people, sexual behavior requires certain hormones. If the ovaries are removed from a female rat, for example, she will no longer raise her rump when she's ready to mate. This behavior can be restored with injections of the sex hormones estrogen and progesterone--and also with shots of marijuana's active ingredient, 9-tetrahydrocannabinol (THC).
To get a better handle on this effect, molecular biologist Shaila Mani of Baylor College of Medicine in Houston injected rats with chemicals that block one of three receptors in the brain: cannabinoid receptor 1 (CB1), which is known to bind to THC and thought to trigger its mind-altering effects; a receptor for progesterone; and a receptor for dopamine, a neurotransmitter known to be involved in pleasurable activities like sex. With any of these receptors blocked, a shot of THC could no longer restore the rats' sexual behavior. Apparently, THC somehow employs both the dopamine and the progesterone pathways, Mani says.
That's a surprise. "Nobody anticipated the fact that cannabinoids would interact with dopamine and progesterone," says pharmacologist Nephi Stella of the University of Washington, Seattle. He thinks the study may spark new interest in THC's effect on sexual behavior. But Mani cautions that such behavior, while directly controlled by progesterone in rats, is much more complex in humans. In other words, smoking a joint may have a more subtle effect on women's behavior.