This study was performed to investigate the chemoreception of trigeminal afferents in the nose. Single unit activity was recorded from the anterior ethmoidal nerve in the anesthetized guinea pig breathing through a tracheostomy during nasal instillation of capsaicin (0.3 mM), nicotine (6 mM) and ammonia (1.5 M) solutions or with distilled water. Out of 36 fibers recorded, nineteen were stimulated by capsaicin, six by nicotine and seventeen by ammonia. Among those fibers, two were stimulated by both capsaicin and nicotine, six by both capsaicin and ammonia and one nicotine-responsive fiber was also stimulated by ammonia. A large proportion of capsaicin- and nicotine-responsive fibers exhibited long lasting discharges (170.4 +/- 17.7 sec and 120.7 +/- 29.3 sec, respectively), and were not stimulated by the second application of the same substance. However, fibers responding to ammonia discharged for a shorter time (31.5 +/- 6.5 sec), indicating a rapid adaptation. These results indicate that the ethmoidal nerve possesses a well-developed responsiveness to noxious stimuli. The nociceptive component of this nerve may be related to the various cardiorespiratory responses that can be elicited from the nasal cavity and also to local axonal reflexes (neurogenic inflammation) due to the release of chemical mediators from C-fiber endings.
Ammonia Animals