by Angus Chen, The Salt at NPR Food (2/18/15)
Shortly after toking up, a lot of marijuana users find that there's one burning question on their minds: "Why am I so hungry?" Researchers have been probing different parts of the brain looking for the root cause of the marijuana munchies for years. Now, a team of neuroscientists report that they have stumbled onto a major clue buried in a cluster of neurons they thought was responsible for making you feel full.
This cluster, called the POMC neurons, is in the hypothalamus, a region of the brain that scientists typically associate with base instincts like sexual arousal, alertness and feeding. Tamas Horvath, a neuroscientist at the Yale School of Medicine and the team's leader, says that the POMC neurons normally work by sending out a chemical signal telling the brain, you're sated, stop eating.
In the past, when neuroscientists shut down POMC neurons in mice, all the mice became morbidly obese. Horvath figured that in order for the drug in marijuana — compounds called cannabinoids — to spawn that undeniable impulse to feed, it would have to bind the activity of these neurons and make them fire less. Paradoxically, Horvath says, "We found the exact opposite."
The team discovered that when they injected cannabinoids into mice, the drug was turning off adjacent cells that normally command the POMC neurons to slow down. As a result, the POMC neurons' activity leapt up. At the same time, the cannabinoids activate a receptor inside the POMC neuron that causes the cell to switch from making a chemical signal telling the brain you're full to making endorphins, a neurotransmitter that's known to increase appetite.