If you’ve ever felt sick after eating a new type of food, you’ll probably never eat that food again. “Taste aversion learning is probably the most profound learning of any kind,” says Linda Parker, psychology professor and Canada Research Chair in Behavioural Neuroscience.
“Almost everyone has experienced a taste aversion to a relatively novel food that is followed by the experience of illness. Even if the food wasn’t responsible for causing the illness, we attribute sickness and nausea to the foods that we eat, especially if they are new.” People are more likely to blame the food they ate for making them sick rather than a stomach bug contracted from contaminated surfaces or unwashed hands.
Alcohol can also cause taste aversion, adds Parker, especially if it caused a severe hangover. If you overindulged in an unfamiliar alcoholic beverage and became sick, you probably won’t touch that drink again — even though it was the amount you consumed and not the drink itself that made you sick. “The taste has to be novel, something that’s not familiar to you, in order to develop taste aversion.”
Taste aversion causes a “yuck reaction” in both animals and humans, says Parker. Rats show a gape face when they taste a food that made them feel sick in the past. Researchers can recreate this reaction in the lab by feeding rats a novel food followed by an injection of a nausea-inducing drug. When rats feel ill after eating a novel food, they will avoid it in the future, even if the sickness occurs up to 12 hours later. Researchers previously believed that this type of learning could occur only if cause-and-effect happened within a few seconds, says Parker. Taste aversion is “very difficult to extinguish,” she adds. “The rat won’t go back to the food.”
Rats are especially picky eaters because they can’t vomit to rid their bodies of toxins. When they encounter a new food, they will nibble on it and wait several hours for any effects before returning to the food. “Rats will avoid tastes paired with pretty much any drug that changes their physiological state, even a rewarding drug that they learn to self-administer,” says Parker. “However, they only display the gape face to a taste paired with an aversive nauseating drug.”
Although everyone has experienced nausea at some point, the neurobiology of nausea is poorly understood due to a lack of animal models, she adds. “We know nothing about nausea. We know about vomiting. The vomiting reflex is very well characterized, but the experience of nausea is something that little is known about. How is it generated? Where is it generated?”
Using the gape face as a rat model of nausea, her research team has identified the visceral insular cortex as the part of the brain that causes nausea. Delivering serotonin to this region causes nausea; blocking serotonin reduces nausea. When researchers delivered ondansetron, a drug used to treat nausea in chemotherapy patients, directly into the visceral insular cortex, it blocked the serotonin receptors and eliminated the gape face reaction produced by a nauseating drug.
Medical marijuana also reduces nausea by stimulating cannabinoid receptors in the brain. Delivering cannabinoid compounds to the visceral insular cortex reduces nausea and the gape face reaction. “What we believe is that cannabinoids ultimately act to reduce serotonin in this region, thereby reducing nausea,” says Parker.
She is also working with clinical studies professor Paul Woods to study the efficacy of cannabidiol, a non-psychoactive compound found in marijuana, to treat nausea in dogs undergoing chemotherapy. Cannabidiol works in the same way as ondansetron but is less expensive. “It might be a new veterinary medicine for dogs,” says Parker. The dog study is funded by the Pet Trust Fund.