This is one of the most well-known effects of taking marijuana - the powerful increase in appetite felt by many users after smoking or taking marijuana extract, commonly referred to as "gastrophasia." For patients who have trouble eating due to chemotherapy, this can be one of the biggest benefits of taking marijuana.
For years, scientists have been trying to understand how marijuana's active ingredient, tetrahydrocannabinol (THC), stimulates appetite.
A report created by Nature Neuroscience brings us a little closer to solving the mystery. A team of European neuroscientists led by Giovanni Marsicana of the University of Bordeaux discovered that in mice, THC fits into receptors in the brain responsible for smell, significantly increasing the animals' ability to smell food. So the main reason you want to eat after using marijuana is simply because you can smell more, making everything tastier.
The researchers started by exposing mice (increasingly used in neuroscience research because of the surprising amount of cognitive similarities they share with humans) to banana and almond oils as a test of smell sensitivity. When they did so, the mice initially sniffed the oils intensely and then stopped showing interest in them - a well-known phenomenon called olfactory habituation. The mice that received THC continued to sniff, showing increased sensitivity to odors, and also ate much more food.
The researchers also genetically engineered some of the mice so that they did not have the cannabinoid receptor responsible for smell and subjected them to the same experiment. It turned out that even when the mice were given THC, it had no effect: they continued to habituate to the smell and showed no increased appetite after the drug was administered, showing that the "gastrophase" effect is dependent on the activity of the olfactory lobe.
How does this affect humans?
This effect of THC has to do with the underlying reason why this chemical affects the human brain so strongly. THC is likely produced by the marijuana plant as a self-defense against herbivores, who may feel confused after eating cannabis, causing them to avoid it in the future. THC fits into receptors that are part of the brain's natural endocannabinoid system, which helps control
- emotions, memory, pain sensitivity, and appetite.
Our brains typically produce their own chemicals (called cannabinoids) that match those same receptors, so by mimicking their activity, THC can artificially alter those same factors in dramatic ways.
This new discovery is likely part of the puzzle of THC and increased appetite. Previous studies have shown that the drug also acts on receptors in a region of the brain called the nucleus accumbens, increasing the release of the neurotransmitter dopamine and the feeling of pleasure that comes from eating after taking marijuana. Other work has shown that THC additionally interacts with the same types of receptors in the hypothalamus, leading to the release of the hormone ghrelin, which stimulates hunger.
The one aspect that these different mechanisms have in common is that they all involve the brain's natural endocannabinoid systems. THC does most of its work by following the same pathways the brain uses to regulate the senses normally.
THC manipulates this natural system by mimicking the sensation felt when we are deprived of food. As a final test, researchers forced some mice to fast for 24 hours and found that this led to an increase in natural cannabinoid levels in the olfactory lobe. Not surprisingly, these hungry mice showed greater sensitivity to smell and ate much more.
Most intriguing is that the genetically engineered mice that lacked cannabinoid receptors did not show increased odor sensitivity, even when they were starved. This suggests that both THC and the natural cannabinoids that arise from starvation act on the same neuronal pathway to allow us to smell and taste with greater sensitivity, and thus eat more. In other words, THC seems to create a "gastrophase" by convincing our brains that we are hungry.