Tamas Horvath, the study's lead author and a Yale professor and neurobiologist, likened the reaction to hitting a car's brakes and accelerating instead.
"It fools the brain's central feeding system," Horvath said in an announcement accompanying the research, which was funded in part by the National Institutes of Health and the American Diabetes Association. "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."
In an interview Tuesday, Horvath said that while the link between cannabis and increased appetite itself is not surprising -- either to researchers or to pot smokers -- "what drives that, nobody has ever really known. We accidentally bumped into that."
Researchers have long known that activating receptors in the brain — known as cannabinoid receptor 1, or CB1R — can contribute to overeating. They also have documented that the active ingredient in marijuana, tetrahydrocannabinol, has the ability to trigger hunger. But in recent years, they have continued to investigate the mechanisms underlying the hunger effect that marijuana users know so well.
In a 2005 study, scientists founds that cannabis use rendered specific neurons in the brain "more excitable" and inhibited the appetite-suppressing hormone leptin. In 2009, research out of Japan found that marijuana might interact with taste receptors to enhance the sweet taste in foods, thus boosting cravings. In a study published last year in Nature Neuroscience, a group of European scientists found that exposing mice to tetrahydrocannabinol, or THC -- the primary active ingredient in marijuana -- enhanced their ability to smell certain foods and led them to eat more.
Horvath made clear Tuesday that more research is needed to determine whether the reaction he and colleagues observed in mice is happening in people. "Obviously, this is a very primitive mechanism that's likely to be the same in humans," he said. "Nevertheless, there needs to be confirmation of that."
Beyond merely figuring out the neurological mystery behind the munchies, Horvath and other scientists are hoping that a clearer understanding of the appetite triggers in the brain could lead to an array of practical uses. For instance, it could lead to new medications, perhaps even a pill, to jump-start hunger in cancer patients who often lose their appetite during chemotherapy. (Some patients already use marijuana to relieve pain, control nausea and vomiting and stimulate appetite.) At the same time, the ability to block cannabinoid receptors in the brain eventually could lead to new approaches to treating obesity.