Neonatal capsaicin treatment chronically ablates unmyelinated C fibers, yet also destroys a small amount of myelinated fibers. Inhalation of wood smoke evokes respiratory reflexes resulting from stimulation of both lung C-fiber nerve endings (unmyelinated afferents) and irritant receptors (myelinated afferents). This study investigated the influences of neonatal capsaicin treatment on the immediate ventilatory responses to inhaled wood smoke in adult rats. Inhalation of wood smoke (~6 ml) via a tracheostomy immediately triggered only an augmented inspiration in 16 rats neonatally treated with capsaicin (50 mg/kg, subcutaneous injection). In contrast, inhaled wood smoke evoked a slowing of respiration in 11 neonatal vehicle-treated rats and an augmented inspiration in another five. The inability to exhibit the slowing of respiration and the persistence of the augmented inspiration in capsaicin-treated rats are consistent with our hypothesis that these two reflex responses originate from stimulation of lung vagal C-fiber afferents and irritant receptors, respectively. Since all capsaicin-treated rats responded to smoke with an augmented inspiration, it is further suggested that neonatal capsaicin treatment selectively impairs the reflex functions of C-fiber afferents and well preserves the reflex functions of lung irritant receptors. Copyright (C) 1999 Elsevier Science B.V.
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