Scientists have found out why smoking cannabis can cause over-eating, with marijuana working on the part of the brain that normally suppresses appetite.

People who empty the refrigerator after smoking cannabis can blame their cravings on brain cells that normally suppress appetite, a study says.

Researchers using genetically-modified mice pinpointed circuitry in the hypothalamus used by the active ingredient in cannabis to cause the "munchies".

"We were able to see what drives the hunger brought about by cannabis and how that same mechanism that normally turns off feeding becomes a driver of eating," said Tamas Horvath of Yale University, who led the unusual study.

The active compounds, cannabinoids, work on a select group of nerve cells called pro-opiomelanocortin (POMC) neurons, his team found.

POMC cells have until now been considered the big stoppers of over-eating, for they send out signals to say when satiety has been reached.

But to the scientists' surprise, the cells' activity flipped over when affected by cannabinoids.

Instead of a sending out a chemical signal that said in essence "I'm full", they sent out a neurotransmitter called endorphins, which are known appetite-boosters.

"It's like pressing a car's brakes and accelerating instead," Horvath said in a press release.

"We were surprised to find that the neurons we thought were responsible for shutting down eating were suddenly being activated and promoting hunger, even when you are full. It fools the brain's central feeding system."

Cannabinoids have also been found to affect other areas of the brain that touch on appetite, enhancing perceptions of smell and taste.

If replicated among humans, the findings could help to treat people with eating problems, such as cancer patients who lose their appetite during treatment.

The study appears in the journal Nature.


Copyright AAP 2015