Ultrastructural study of rabbit lung tissue subsequent to capsaicin treatment over several days

The neurogenic basis of nonspecific airway hyperresponsiveness (NAH) is poorly understood. Under experimental conditions isocyanates can elicit bronchial hyperresponsiveness in animals. The purpose of our study was to determine whether reactions of neurosecretory granules in nonmedullated C fibers might play a role in NAH. Our experiments were based on the fact that capsaicin treatment causes depletion of neurosecretory granules in vicinity of C fibers. We gave rabbits repeated subcutaneous injections of capsaicin. The animals were then treated with toluene diisocyanate (TDI), inducing airway hyperresponsiveness upon acetylcholine (ACH) inhalation. In capsaicin-treated animals the neurosecretory granules were not evident ultrastructurally and airway hyperresponsiveness did not occur in response to TDI treatment. Controls that were not treated with capsaicin displayed both neurosecretory granules ultrastructurally as well as airway hyperresponsiveness to ACH. We conclude that in the rabbit, NAH is related to the presence of neurosecretory granules adjacent to nonmedullated C fibers.