The phenomenon of the "munchies," the intense increase in appetite experienced by many users of marijuana, has long been a subject of scientific curiosity. The active ingredient in marijuana, tetrahydrocannabinol (THC), has been shown to stimulate the appetite by affecting the brain's natural endocannabinoid system, which controls emotions, memory, pain sensitivity, and appetite. The new study published in Nature Neuroscience reveals that THC fits into receptors in the brain's olfactory bulb, increasing sensitivity to scents and flavors, and thereby convincing the brain that it is hungry.

The research team, led by Giovanni Marsicano of the University of Bordeaux, conducted experiments on mice by exposing them to scents of banana and almond oils. They observed that while mice without THC exposure showed a well-known phenomenon of losing interest in the scents over time (olfactory habituation), mice with THC exposure continued to sniff the oils, demonstrating an enhanced sensitivity to the scents. The THC-dosed mice also ate more food when given the chance, demonstrating an increased appetite.

The researchers believe that THC's effect on the brain is likely a self-defense mechanism developed by the marijuana plant to prevent herbivores from avoiding it after eating it and feeling disorientated. THC mimics the activity of chemicals produced by the brain called cannabinoids, which fit into the same receptors and alter the same factors in dramatic ways. This scientific explanation sheds light on how marijuana causes the munchies and highlights the complex relationship between THC and the brain.

The researchers also conducted an experiment on mice that were genetically altered to not have a specific cannabinoid receptor in their olfactory bulbs. The results showed that even when these mice were given THC, there was no effect on their sensitivity to scent or appetite. This showed that the drug's ability to enhance scent and increase appetite is dependent on the activity in the olfactory lobe of the brain.

This new discovery sheds light on one of the ways THC stimulates appetite - by making individuals more sensitive to the smell of food, and therefore, the taste as well. However, this is just one piece of the puzzle as previous studies have found that THC also influences the release of dopamine in the nucleus accumbens, leading to an increase in pleasure associated with eating. It also interacts with receptors in the hypothalamus to release the hormone ghrelin, which causes hunger.

All of these mechanisms are related as they involve the brain's natural endocannabinoid system and THC manipulates these pathways to alter the senses.

Interestingly, the study also provides insight into the way THC imitates sensations felt when food is deprived. In a final test, the researchers forced some mice to fast for 24 hours and found that this increased levels of natural cannabinoids in the olfactory lobe, leading to greater sensitivity to scent and an increase in appetite. However, the genetically altered mice without cannabinoid receptors in the olfactory lobe did not show a change in sensitivity or appetite, even when they were deprived of food. This indicates that both THC and the natural cannabinoids produced during starvation target the same neural pathway to enhance the sense of smell and taste, and thus, increase food consumption. In other words, THC tricks the brain into thinking it is in a state of starvation, causing the munchies.