Role of vagal afferents in the acute ventilatory responses to inhaled wood smoke in rats

Spontaneous inhalation of wood smoke (approximately 6 ml) via tracheostomy immediately evoked either a slowing of respiration (SR) or an augmented inspiration (AI) in each of the 76 anesthetized Sprague-Dawley rats studied. The SR was abolished after perineural capsaicin treatment of both cervical vagi (PCT; n = 20) but persisted during cooling of both vagi to 6.7 degrees C (VGC, n = 12); PCT selectively blocked the conduction of unmyelinated C fibers, whereas VGC differentially blocked the conduction of myelinated fibers. In contrast, the AI was eliminated during VGC (n = 10) but was unaffected by PCT (n = 5). Furthermore, after the SR was abolished in rats with PCT, an AI became the dominant immediate response to inhaled wood smoke. Conversely, after the AI was eliminated in rats receiving VGC, a SR was instantly evoked by inhaled wood smoke. Removal of smoke particulates did not affect the SR (n = 17) but did prevent the AI in 5 of the 12 rats studied. These results suggest the following: 1) the SR is a reflex resulting from stimulation of bronchopulmonary C-fiber nerve endings by the gas-phase smoke; 2) the AI results from excitation of lung irritant receptors by the gas-phase and/or smoke particulates; and 3) in intact animals, both types of vagal bronchopulmonary afferents are probably activated by inhaled wood smoke.