Most people are well aware of the effects of “enjoying” hot chili peppers and tend to approach them with caution or stay away altogether. Birds, on the other hand, will happily consume the hottest of peppers with no ill effects whatsoever.

A paper from Sven-Eric Jordt and David Julius published in this week's issue of Cell explains the cellular mechanism behind this long observed difference between mammals and birds.

Hot chili peppers contain varying amounts of capsaicin, a chemical compound similar to the active compound in vanilla. In mammals, it is the capsaicin from hot peppers that interacts with a specific receptor protein called VR1 (vannilloid receptor 1) on sensory nerve endings and evokes sensations of burning and pain.

VR1 also activates pain pathways when stimulated by heat. The researchers identified a receptor from chicken sensory neurons that is structurally and functionally similar to mammalian VR1.

Although chicken VR1 was activated by heat, it differs from mammalian VR1 in that it is insensitive to stimulation by capsaicin. The scientists were able to pinpoint an exact region of the VR1 receptor in mammals that binds capsaicin and found that this small region is not present in the avian form of the receptor.

This subtle difference in the structure of the VR1 receptors between the species makes mammals extremely sensitive to the spiciness of hot peppers while birds are relatively unaffected.

This species-specific difference is good for the pepper plant as well as the bird. Mammalian predators, whose digestive systems would damage the seeds of the plant, are repelled, while birds can consume the peppers with ease.

Wild birds are almost entirely responsible for dispersing the seeds of hot peppers, which pass through their gut without being digested. The pepper plant has therefore developed a unique chemical mechanism to improve its reproductive success.

We now know that the basis of this interesting evolutionary mechanism hinges on a very tiny region of a specific protein receptor in sensory nerves.


[Contact: David Julius]

08-Feb-2002