This Is Why Weed Gives You the Munchies

This article originally appeared on MUNCHIES in February 2015.

If you've ever wondered why exactly smoking weed makes you want to tear through an entire bag of chips, a microwaved plate of frozen taquitos, and even that four-month-old box of peanut brittle that Grandma sent you for the holidays, a team of neuroscientists is one step closer to the answer.

We don't need science to tell us that getting stoned and getting the munchies tend to go hand in hand. But how exactly cannabis affects the brain's hunger circuitry hasn't been explored until now.

Tomas Horvath, a professor of neurobiology at Yale, and his team of researchers focus on how the brain controls feeding, appetite, and fullness. A few years ago, Horvath met Marco Koch of the University of Leipzig, who was interested in joining Horvath's lab in order to study how cannabinoids—the class of chemicals of which THC is the most famous—mess with normal brain function.

Together, they discovered that a set of neurons in the hypothalamus—known as POMCs—literally become scrambled when exposed to marijuana. Their findings were published last week in Nature.

"When you administer cannabinoids to animals, as in humans, they start to eat more even if they're full," Horvath tells MUNCHIES. "So [we said], let's look at those pathways that control fullness. Are they turned off by exposure to cannabinoids?" Horvath hypothesized that exposure to cannabinoids would simply shut the POMCs off—turning out the lights on neurons that tell you you're full.

But when they exposed mice to cannabinoids—via injection, not a gravity bong—they found that the exact opposite had happened. "Shockingly, what we found was not only were [the POMCs] not turned off, they were much more turned on," Horvath says.

The POMCs usually secrete a chemical called alpha-melanocyte-stimulating hormone (alpha-MSH), which is believed to play a role in feelings of satiety. When they exposed the mice to cannabinoids, the POMCs started releasing an entirely different chemical: beta-endorphins, which stimulate appetite and promote cravings.

"All of a sudden, these neurons that normally serve the purpose of stopping eating become the driver of food intake," Horvath says.

In subsequent tests, the researchers artificially shut off the POMCs in mice that were already full. ("We wanted to make the animals overeat when otherwise they would not," Horvath notes, "which is obviously the situation when somebody smokes marijuana after dinner.") When they exposed those mice to cannabinoids, they didn't eat very much. When they artificially cranked up the POMCs in another group of well-fed mice and exposed them to cannabinoids, the rodents gorged themselves like your sophomore roommate during a stoned Sopranos marathon.

This was beyond exciting for Horvath and his team, and it also posed a whole new set of questions about both brain function and marijuana's ability to act on it. Specifically, could cannabinoids flip the function of other neurons? Is marijuana actually rewiring our brain circuitry, even if only temporarily?

"If you think about it, when you smoke marijuana or eat hashish cookies or whatever, obviously your brain functions change. That's the whole idea behind them," Horvath says with a laugh. "So the question is, are those brain functions changed because neuronal functions flip in other parts of the brain too? I think it's an interesting question to ask."

Horvath and his team are now curious to explore the role that these neurons might play in the overall feeling of being stoned.

While this study isn't the first to explore the role of cannabinoids on the munchies, Horvath notes that his team is the first to explore this specific set of neurons in this capacity. Another recent study suggested that marijuana could affect the olfactory bulb in the brain, essentially making food smell much better than it usually does.

"I can't exclude the possibility that the olfactory system plays a role in the munchies as well," Horvath says. "We're simply saying that these neurons have a major impact on how animals respond to cannabinoids regarding feeding—and we don't say they are the only ones."

Indeed, the phenomenon of the munchies is most likely a complex physiological response that can't be reduced to a single set of neurons. But Horvath and his team are now curious to explore the role that these neurons might play in the overall feeling of being stoned.

"If it turns out that these neurons are important for the general high, then it's an interesting observation," Horvath says. "If it's only for feeding, then you could argue that if you interfere with these neurons, you could eliminate that 'side effect' of marijuana use. You know, if you want to." For example, drugs could potentially be developed that would alter the function of these neurons in, say, chemotherapy patients who have a reduced appetite. (Then again, who needs drugs when you can have a plate of chicken pot-cciatore?)

One of the study's other key findings might provide a clue as to why snacking stoners aren't all morbidly obese. "When you use cannabinoids, these neurons start to secrete [endorphins that promote feeding]—but they are still producing and storing the other guy, which is the anti-feeding chemical," Horvath says. Once your high wears off, the POMC neurons are able to release the alpha-MSH, which could cause a rebound phase in which you eat less. "I think it sort of balances itself out and may not contribute to elevated body weight—unless you eat and smoke nonstop."

Nature always strives for equilibrium, doesn't it?