The active ingredient in cannabis is sometimes used to halt weight loss

"We know that smoking cannabis stimulates the appetite, but we were wondering whether the substances in the brain which work on the same receptors would have the same effect on appetite," explained Dr George Kunos.

Dr Kunos is scientific director of the National Institute on Alcohol Abuse and Alcoholism at the National Institutes of Health in the United States.

Together with colleagues from Italy, Japan and the US, he found that, just like cannabis, natural endocannabinoids did indeed stimulate the appetite.

Mice studies

He and his colleagues found that mice bred without cannabinoid receptors did not eat very much, even after being deprived of food.

Normal mice, on the other hand, would eat to compensate for periods of starvation.

The research also linked cannabinoid levels to leptin, a hormone known to suppress the appetite and produced by fatty tissue in the body.

When mice get fatter, leptin levels rise and cannabinoid levels fall, reducing the appetite, it seems.

But there were some surprises.

Complex system

The team found that the regulation of appetite depends on other mechanisms, too.

Some of the mice they tested should have stayed thin, but developed alternative ways of compensating for their missing receptors and regained their appetites.

"Perhaps appetite is such an important matter that the body does not entrust it to just one pathway," he told BBC News Online.

The implications for drugs to treat appetite problems are clear:

"If we want to control appetite therapeutically then we have to knock out more than one pathway.

"Leptin was disappointing in clinical trials," he said.

Natural alternative?

The active ingredient in cannabis is licensed for prescription in the United States, but as Dr Konos says, it has unwanted psychoactive effects.

"What would make sense would be to try and boost natural levels of cannabinoids, for instance by blocking the enzyme which degrades them," he said.

The research is published in the scientific journal Nature.