The heat of four-alarm chili and the coolness of spearmint chewing gum aren't just metaphorical: To some cells, taste and temperature are the same. This year, researchers tunneled into ion channels that respond to such sensations. They're tuned to warmth, minty coolness, or, in mice at least, another steamy stimulus: pheromones.
So-called transient receptor potential (TRP) ion channels are proteins that snake in and out of the cell membrane. When they're tickled appropriately, they allow calcium or other ions to surge into a cell. In neurons, this can make the cell fire off a signal to its neighbors. Mammals harbor at least 21 flavors of TRP channels, but most of their functions are unknown.
The first report that certain TRP channels promiscuously respond to either a chemical or a thermal stimulus came in 1997, with the identification of a TRP channel that gets steamed up by either hot temperatures (above 43°C) or capsaicin, the active ingredient in chili peppers. This year, a similar multitasking channel was found in nerves of the mouth and skin. It reacts when exposed to either cool temperatures (15° to 25°C) or menthol, the chemical that makes mint minty.
A warmth-sensitive (~34°C) TRP channel debuted this year as well. It is concentrated in skin cells, suggesting that the skin itself senses heat and passes the message to neurons.
TRP channels are necessary for another type of body heat as well. Male mice lacking a certain TRP channel are particularly thickheaded when it comes to mating, seemingly unable to distinguish between females and fellow males. These TRP channels inhabit a part of the nose that sniffs out pheromones.
Mutations in other TRP channels are responsible for certain cancers and other diseases, including, as discovered this year, an inability to regulate magnesium levels. These and most other TRP functions are still poorly understood, but researchers are picking up more coherent signals from TRP channels that buzz to sensations of taste, heat, and pheromones.