If recent laws legalizing marijuana in more U.S. states also boost sales of potato chips and brownies, scientists will know why: In an unexpected finding, a new study has revealed 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’, researchers reported in the journal Nature.
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, called cannabinoids, affect the olfactory center 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.
In this latest study, scientists led by Dr. 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.
To their surprise, Dr. 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 traveled to the brain’s appetite-control region, the hypothalamus, stimulating the mice’s desire to eat.
“It’s like pressing a car’s brakes and accelerating instead,” said Dr. Horvath. “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.”
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.