Diet drug rimonabant (Acomplia / Zimulti), in addition to promoting weight loss by blocking the body's CB-1 receptors, may also block a receptor in the brain called TRPV1, possibly explaining why the diet pill produces depression, anxiety and suicidal thoughts in some patients, according to researchers.

Rimonabant is a CB-1 receptor antagonist that reduces appetite by blocking the body's cannebinoid receptors.

Rimonabant is widely available outside the United States, including in Eurolpe, but Sanofi Aventis pulled it back from consideration by the U.S. Food and Drug Administration last summer after an agency advisory panel unanimously expressed concern over the drug's psychiatric side-effects.

Now, in a study published in the March 13 issue of the journal Neuron, researcher Julie Kauer of Brown University and her colleagues report that the TRPV1 receptor -- a pain receptor that is central to learning and memory -- may also be blocked by large doses of rimonabant.

"It's possible that when the patient takes the drug, and the dose range is correct, it may not affect TRPV1 at all," Kauer said. "However, in some patients, Acomplia could be hitting TRPV1 and may well have an effect on activating this receptor."

In experiments with rat brains, Kauer and her fellow researchers found that TRPV1 controls a brain mechanism called long-term depression, which may relate to the psychiatric side effects seen with Acomplia.

"A large percentage of patients stop taking this drug as a result of psychiatric side effects, and our findings suggest the possibility that some of the central effects of [Acomplia] result from the antagonism of TRPV1 receptors ...," the researchers conclude.