Marijuana 'munchies' caused by heightened sense of smell

Why does smoking pot often lead to compulsive snacking? A new French study appears to have sniffed out the answer.

It appears cannabinoids such as tetrahydrocannabinol (THC), the active ingredient in marijuana, sharpens the sense of smell, making food more attractive, reports the study led by Giovanni Marsico, a research director the Institut national de la santé et de la recherche médicale in Bordeaux, France.

In the study on mice published this week in Nature Neuroscience, the researchers found key receptors for cannabinoids control a brain circuit that connects the olfactory bulb —  the part of the brain that collects smell signals from the nose —  to the part of the brain that processes smells.

When normal mice who are hungry are given THC, their appetite increases.

But when the researchers manipulated genetically modified mice so that they had very few cannabinoid receptors in their olfactory bulb, THC didn't affect their appetite.

The researchers performed additional experiments showing that activation of the cannabinoid receptor in the olfactory bulb of mice boosted their sense of smell and increased their food intake "proportionally."

Possible treatment for obesity

Prior to the study, researchers had already known that when mammals such as mice and humans haven't eaten in a while, there is an increase in levels of "endogenous cannabinoids" — chemicals found naturally in their brains that are similar to THC.

And cannabinoids are already known to alter both hunger and smell in humans, the researchers noted.

However, "to the best of our knowledge," they wrote, "a direct causal relationship between those two effects of cannabinoids has not yet been shown in humans, and specific experiments should be performed to test that hypothesis."

Nevertheless, the authors suggested in a news release the brain smell circuit they studied is likely "altered" in people who suffer from obesity or anorexia. Those people  may therefore have a weaker or better sense of smell than normal, they suggest.

A news release from Nature Communications suggested that because the cannabinoid receptors in the olfactory bulb are activated by hunger, it may be possible to develop drugs targeting those receptors in order to control people's food intake.