Assessment of intraoral mucosal pain induced by the application of capsaicin.

OBJECTIVE To develop an objective method for assessing nociceptive behaviour in an animal model of capsaicin-induced intraoral pain. Changes in nociceptive responses were also examined after injury to the inferior alveolar nerve (IAN). DESIGN Nociceptive responses evoked by the intraoral application of various doses of capsaicin were analyzed in lightly anaesthetized rats. The number of c-Fos protein-like immunoreactive (Fos-LI) neurons in the medullary dorsal horn (MDH) induced by the intraoral application of capsaicin was measured. Behavioural and c-Fos responses were also examined 14 days after injury to the IAN. RESULTS Larger doses of intraoral capsaicin (1, 10 and 100μg) induced vigorous licking behaviour and c-Fos response in the MDH in a reproducible manner. The magnitudes of both behavioural activity and the c-Fos response from the 10 and 100μg doses of capsaicin were significantly greater than that by the 1μg dose. Injury to the IAN exaggerated the behavioural and c-Fos responses evoked by intraoral capsaicin. CONCLUSIONS The intraoral application of capsaicin is a valid and reliable method for studying intraoral pain and hyperalgesia following nerve injury.