Capsaicin pretreatment inhibits heat-induced oedema in the rat skin.

Exposure of rat paws to noxious heat (hot water, 48 degrees C) led to an increase in vascular permeability to macromolecules as indicated by extravasation of Evans blue dye. After pretreatment of neonatal rats with 50 mg X kg-1 capsaicin s.c., which is known to cause degeneration of unmyelinated afferent neurones, this protein leakage was reduced by 74% (P less than 0.01). It is concluded that the oedema induced by noxious heat is partially caused by excitation of peripheral endings of capsaicin-sensitive primary afferent neurones.