Birdwatchers have long known that a little chili pepper added to the birdfeeder keeps the squirrels away. Now biochemists can tell them why. A new study reveals the cellular difference that makes mammals uniquely sensitive to capsaicin--the fiery molecule that makes chili peppers the bane of gringos and the delight of culinary masochists. The finding may lead to a new class of pain medications.
In 1999, researchers showed that desert rodents scorn chilies, whereas birds wolf them down (ScienceNOW, 13 August 1999). That preference works out nicely for the chili plants--because the seeds break down in the rodent gut, but not in the avian gut, birds are better at dispersing the seeds. Cell biologists later found that in mice the same pain receptor is responsible for sensitivity to both heat and capsaicin (ScienceNOW, < AHREF="http://sciencenow.sciencemag.org/cgi/content/full/2000/413/1">13 April 2000). This led them to suspect that the receptor, called VR1, might differ between birds and mammals.
David Julius and Sven-Eric Jordt of the University of California, San Francisco, have confirmed this hunch. In the 8 February issue of Cell, they report the discovery of a segment of the VR1 protein that is unique to mammals and enables the receptor to bind capsaicin. To find the segment, the team tried inserting various snippets of the rat DNA that codes for VR1 into corresponding positions of chicken DNA. These hybrid stretches of DNA were then incorporated into easily-cultured kidney cells. A particular excerpt of rat DNA made the chicken receptor sensitive to capsaicin. This DNA encodes the region of the VR1 protein that binds capsaicin.
"The most important ramification is that the VR1 receptor is a very interesting target for pain," says David Clapham, a neurobiologist and pediatrician at Harvard Medial School in Boston. The receptor transmits other types of painful stimuli in addition to the chili's sting, and knowing where capsaicin binds narrows down the range of molecules that could prevent the receptor from sending pain impulses. But much more work is needed to fully understand the receptor structure, and novel painkillers that block capsaicin are at least 5 years off, Clapham says.