Hurts so good: Chili peppers, carbonated drinks, and nicotine all deliver painful sensations to the brain. And we learn to like it.
That’s according to UC Davis neurophysiologist Earl Carstens. "Humans have to learn to like these irritants, because they are all activating pain pathways," said Carstens.
In fact, pain may be an important part of food flavor, alongside taste and smell, according to food scientist Michael O’Mahony, who jointly heads the research team with Carstens. Most cultures have foods with this effect, such as strong malt vinegar on fish and chips, pretzels crusted with salt, black pepper, chilies, mustard oil or horseradish.
In a study published in the October issue of the Journal of Neurophysiology, the researchers compared the effects of capsaicin, which gives chilies their burn, with nicotine on nerve activity in rats. When dripped onto the tongue, both capsaicin and nicotine caused firing of trigeminal nerves, the first pain relay on the way to the brain.
"As you continue to ingest capsaicin, the response increases over time," said Carstens. But if capsaicin was stopped for a few minutes, the nerves became desensitized and the response decreased. The response returned when further doses of capsaicin were given, a phenomenon called "stimulus-induced response."
These results are comparable to those found in experiments on humans, said Carstens, and correspond to the experience of eating a spicy meal and finding that the food becomes less hot the more you eat.
Nicotine, in contrast, is very irritating on first exposure, but the nerves immediately become desensitized, so the response declines.
"If you walk into a smoky room, you immediately have an irritating sensation in the eyes, but you quickly become desensitized," said Carstens.
Carbonated drinks, like sodas, beer or sparkling wine, cause painful sensations on the tongue. "If you stick your tongue in carbonated water for a few seconds, that gets painful," said Carstens.
His laboratory has shown that the painful sensation is due to formation of carbonic acid from carbon dioxide in fizzy bubbles. Some draught beers such as Guinness, which mostly get their fizz from nitrogen, have a different, smoother feel in the mouth, because they do not produce as much acid.
The team is beginning to explore the interactions between these different irritants. Capsaicin seems to have the strongest general effect. If the mouth is desensitized to capsaicin, this affects the response to nicotine and carbonated drinks as well. In contrast, nicotine does reduce the response to capsaicin, but only at high doses.
O’Mahony’s laboratory studies how the brain interprets flavor. His team has developed very accurate methods for measuring differences in taste tests with human subjects.
"The way the brain scans information is different, depending on the circumstances," said O’Mahony. "The way you ask the question can change the effect."
Desensitization by capsaicin can also have medical applications, said Carstens. Capsaicin creams probably soothe muscular aches and pains because they are absorbed through the skin and desensitize pain receptors. Menthol rubs have a similar effect; menthol seems to activate "cold" receptors as well as pain receptors, giving a mixed cooling/heating sensation.