Which receptor, also triggered by physical heat, does capsaicin activate?

TRPV1, an ion channel in sensory neurons that opens above about 42 degrees Celsius - genuine tissue-damaging heat. Capsaicin opens the same channel chemically, so the nervous system reports a burn because as far as it is concerned there is one. The pain is not a metaphor; it is the identical signal.

The related channel TRPM8 does the opposite: it opens in the cold, and menthol opens it too, which is why mint feels cold and why cold water on a mouthful of mint feels colder still. David Julius, who identified both, shared the 2021 Nobel for it - work that started by screening a library of genes for one that made cells respond to chilli extract.

Birds are unaffected, because their version of the channel does not bind capsaicin, and this appears to be the point: mammals chew seeds and destroy them, birds swallow them whole and disperse them, so the plant evolved a deterrent aimed at exactly half its consumers. Capsaicin is also used medically - repeated exposure exhausts the neurons that carry pain signals, so a high-dose patch can relieve nerve pain for months, by burning out the messenger.