As any stoner will tell you, smoking a blunt invariably gives you the munchies. But how getting high leads to your reaching for that extra brownie or bag of chips isn't entirely clear.
Last year, a group of researchers found that marijuana might heighten the sense of smell, leading to increased hunger. And thanks to a new study published this week in the journal Nature, we may have a better grasp on what might be going on inside a pot-smoker's brain after the first puff.
But first some basics: when you smoke or eat cannabis, you get high thanks to marijuana’s best-known cannabinoid, tetrahydrocannabinol—THC, for short. This molecule latches on to a protein called the cannabinoid 1 (CB1) receptor and activates it. The CB1 receptor, in certain brain areas, is what controls the activity of neurons that makes us feel hungry or less hungry.
For the Nature study, Yale University researchers injected mice with ACEA, a molecule known to activate the CB1 receptor. (We’ll spare you the long, long scientific name.) According to the study, rodents injected with ACEA ended up eating more food than mice injected with a placebo.
So, no, the researchers didn’t actually give the mice marijuana, or even pure THC. But the results of the study hint at what happens when marijuana does start to work on our brains.
For instance, the researchers took slivers of mouse brains and puffed some ACEA on them in a dish in order to look at the molecule's effects on hypothalamic pro-opiomelanocortin neurons, the brain cells that regulate feeding. When researchers recorded the activity of these cells with a tiny needle, they found that ACEA made them fire almost three times as often. This was surprising, because usually hypothalamic pro-opiomelanocortin neurons suppress hunger, not promote it.
So what was going on? Well, it turns out that when CB1 receptors are active—as happens right after you smoke pot—the amount of beta-endorphin in your body goes up. (If you're an athlete, you may know beta-endorphin: it's what gives you a runner's high.) Beta-endorphin binds to opioid receptors, and when that happens, the neurons that typically slow down your hypothalamic pro-opiomelanocortin neurons (the ones that regulate how hungry you are) become less active. The result is that your brain's hunger-inducing neurons actually fire faster, making you feel hungrier.
"It's like pressing a car's brakes and accelerating instead," said Tamas Horvath, the lead author of the study, 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."
And so, we munch.
Daniela Hernandez is a senior writer at Fusion. She likes science, robots, pugs, and coffee.