Laryngeal C-fiber afferents are not involved in the apneic response to laryngeal wood smoke in anesthetized rats

• Published in: Life sciences (1973) Vol. 66; no. 18; pp. 1695 - 1704
• Main Authors: Shuei Lin, You, Ho, Ching-Yin, Chang, Shyue-Yih, Ru Kou, Yu
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier Inc 24.03.2000
• Subjects: Aerosols; Animals; Apnea - chemically induced; Apnea - physiopathology; Capsaicin - administration & dosage; Capsaicin - pharmacology; Female; Hemodynamics - drug effects; In Vitro Techniques; inhaled irritants; Laryngeal Nerves - cytology; Laryngeal Nerves - drug effects; Laryngeal Nerves - physiopathology; Larynx - drug effects; Larynx - physiopathology; Male; Nerve Fibers - physiology; Neurons, Afferent - physiology; perineural capsaicin treatment; Rats; Rats, Sprague-Dawley; Respiratory Mechanics - drug effects; ruthenium red; Ruthenium Red - pharmacology; smoke; Smoke - adverse effects; superior laryngeal nerves; Wood; perineural capsaicin treatment; smoke; superior laryngeal nerves; ruthenium red; inhaled irritants
• ISSN: 0024-3205; 1879-0631
• DOI: 10.1016/S0024-3205(00)00492-6

Laryngeal exposure to wood smoke in rats evokes a reflex apnea which is mediated through superior laryngeal afferents ( J. Appl. Physiol. 83: 723–730, 1997). To study the role of laryngeal C-fiber afferents in eliciting this response, capsaicin aerosol (0.05 – 0.2 μg ml ) and 5 ml of wood smoke were delivered separately into a functionally isolated larynx of anesthetized Sprague-Dawley rats at a constant flow rate of 1.4 ml s , while animals breathed spontaneously. Studies were repeated after either an intravenous injection of ruthenium red (2 mg kg ; n = 8), a perineural capsaicin treatment (200 μg ml for 5 min; n = 8) of the superior laryngeal nerves, or a perineural sham treatment (n = 8); Ruthenium red inhibits the stimulation of afferent C-fiber nerve endings by capsaicin, whereas perineural capsaicin treatment selective blocks the conduction of C-fiber afferents. Either ruthenium red or perineural capsaicin treatment abolished the apneic response to laryngeal capsaicin, but did not significantly affect the apneic response to laryngeal wood smoke. Furthermore, the apneic responses to both types of irritants were not significantly altered by perineural sham treatment, yet were completely eliminated by a subsequent denervation of superior laryngeal nerves. Our results suggest that superior laryngeal C-fiber afferents are not involved in eliciting the reflex apneic response to laryngeal wood smoke in anesthetized rats. It is speculated that this response may result mainly from the stimulation of myelinated afferents, possibly laryngeal irritant receptors.