If recent laws legalizing marijuana in more U.S. states also boost sales of potato chips and brownies, scientists will know why: A study in mice published on Wednesday found, unexpectedly, that the active ingredients in pot essentially make appetite-curbing regions of the brain reverse functions.

When that happens, neurons that ordinarily transmit a signal that means, "you're full, stop eating," instead give the brain the munchies, neurobiologists reported in the journal Nature.

Marijuana fools the brain's feeding system. - Tamas Horvath, Yale University

The fact that smoking marijuana makes users crave salty, crunchy or sweet snacks has long been enshrined in popular lore and comedy. But how that happens has been a scientific enigma.

One idea had involved heightened sensory perception. A 2014 study by neuroscientists in Europe, for instance, found that the active ingredients in marijuana, cannabinoids, affect the olfactory centre in the brains of mice. As a result, the animals better smell food, which can stimulate appetite.

But that didn't explain the marijuana-fueled appeal of foods without much aroma.

Surprising results

In their study, scientists led by Tamas Horvath of Yale University focused on molecules called receptors that cannabinoids bind to and activate in the brains of both mice and men. They expected to find that when cannabinoids did so, the receptors sent out a signal quieting nearby neurons that suppress appetite. That could lead to the munchies.

The researchers expected to find that cannabinoids in marijuana would quiet nearby neurons that suppress appetite. Instead, they activated the appetite-suppressing neurons - but still caused an increase in appetite. (iStock)

To their surprise, Horvath said, they found that activating the cannabinoid receptors in mice's brains instead increased, not decreased, the activity of appetite-suppressing neurons.

The reason that did not suppress appetite was that the neurons, instead of emitting their usual appetite-killing neurochemicals, emitted completely different ones. Called endorphins, they travelled to the brain's appetite-control region, the hypothalamus, stimulating the mice's desire to eat.

"Neurons that normally shut down eating instead promoted it, even when the mice were full," Horvath said in an interview. "Marijuana fools the brain's feeding system."

It does not fool the brain into eating just anything, however. Smoking marijuana rarely leads to a craving for broccoli. Instead, he said, the brain mechanisms create a desire for calorie-dense foods like salty, fatty chips and rich sweets.

There are likely additional brain pathways by which marijuana causes the munchies, which could be tapped for one of the drug's medical uses: increasing appetite in cancer patients and others who have lost the desire to eat.