NEW YORK, Feb 10 (Reuters Health) -- Although heart-shaped Valentines suggest otherwise, the brain is the true wellspring of amorous feelings. And now researchers say that a little brain protein by the unromantic name of DARPP-32 is key to sexual arousal -- at least in female rodents.

The discovery may one day lead to new treatments for reduced sex drives in humans.

Scientists have found that in female rats and mice, DARPP-32 couples two body chemicals known to affect sexual behavior: the "feel-good" nerve transmitter dopamine, and the female hormone progesterone. DARPP-32 exists in the brain pathways that dopamine and progesterone travel and is required for these two pathways to merge, according to Dr. Shaila Mani, a researcher in molecular and cellular biology at Baylor College of Medicine in Houston, Texas. Mani and her colleagues report their findings in Friday's issue of the journal Science.

"DARPP was known to be involved in dopamine-related pathways," Mani told Reuters Health. "But now we know it's also involved in progesterone-related ones."

This protein-controlled interaction of hormones and nerve signals means that sexual stimulation, far from being just a hormone rush, "is more complex than we've believed," Mani said.

To flesh out DARPP-32's role in sexual arousal, Mani and her colleagues studied genetically engineered mice that lacked the protein and rats in which it was inactivated. After they stopped hormone production in the animals by removing their ovaries, the investigators found that the animals showed no interest in sex and even fought off males' romantic advances. But subsequent injections of female hormones did not bring back loving feelings.

That's because progesterone cannot send out sexual signals without the help of DARPP-32, Mani explained. Whether males also need the protein is not yet certain.

The complex interplay of hormones, nerve signals, and proteins seen in this study is probably also acted out during other behavioral responses, including less-sexy processes like learning, according to Mani.

How brain pathways interact should be taken into account when people take certain drugs, she noted. For example, prescription drugs that act on dopamine pathways are used to treat a variety of conditions. Dopamine suppression, said Mani, may spur depressive symptoms, including the loss of sexual desire. Similarly, she added, cocaine use has been shown to act on dopamine pathways, so it may also alter hormone pathways.

On the other hand, finding drugs that stimulate these pathways might be a way to regulate sexual behavior, Mani noted.

"This research provides a model to study complex behavior," noted senior investigator Dr. Bert O'Malley, chairman of molecular and cellular biology at Baylor, in a statement.