Evidence for Substance P-immunoreactive Spinal Afferents That Mediate Bronchoconstriction

The origin and functional role of capsaicin-sensitive substance P-(SP-) immunoreactive (IR) nerve fibres in the lower airways were studied in the guinea-pig. Stellatectomy caused a significant reduction of SP-IR in the lung and pulmonary artery. Immunohistochemical analysis, however, did not reveal any clear-cut change in the number and distribution of SP-IR fibres in the lung of these animals. After combined stellatectomy plus local capsaicin treatment of the vagal nerves, most SP-IR nerves disappeared in the lower airways. The bronchoconstriction induced by capsaicin was significantly reduced after stellatectomy and abolished after stellatectomy plus capsaicin pretreatment of the vagal nerves. Ether inhalation caused bronchoconstriction, which was not influenced by stellatectomy but markedly reduced by combined capsaicin treatment of vagal nerves and stellatectomy. Stellate ganglion stimulation in animals that had been chemically sympathectomized by 6-OH-dopamine caused bronchoconstriction, which was resistant to cholinergic or adrenergic receptor blockade. This response was absent after systemic capsaicin pretreatment, suggesting that it was due to antidromic stimulation of afferent fibres traversing the stellate ganglion. In conclusion, the present data suggest that the lower airways receive SP-IR capsaicin-sensitive C-fibre afferents of both vagal and spinal origin. These sensory fibres seem to have branches both within the bronchial smooth muscle and around blood vessels.