Capsaicin‐desensitization to the human nasal mucosa selectively reduces pain evoked by citric acid.

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Abstract
1. Kallidin (5‐500 nmol), hypertonic saline (0.9‐20% NaCl) or low pH medium (citric acid: pH 2.5‐1) applied (50 microliters) to the human nasal mucosa produced a pain response (evaluated by a visual analogue scale) that was related to the concentration of the peptide, NaCl or hydrogen ions, respectively. 2. Application (50 microliters) of capsaicin (50 nmol) to the human nasal mucosa produced overt pain. After repeated administrations (once a day for 5‐7 days) to one nostril this effect underwent almost complete desensitization, while in the contralateral nostril, treated with the vehicle, the response to capsaicin was unaffected. 3. The pain response produced in the human nasal mucosa by topical application (50 microliters) or kallidin (50‐ 500 nmol), NaCl (10‐20%) or citric acid (pH 1.5‐1) solutions was then studied before and after local capsaicin desensitization. 4. The pain response to pH 1.5 or 1 citric acid was markedly reduced (by 60% and 75%, respectively) in the capsaicin‐treated nostril. However, the pain response to 10% or 20% NaCl or the mild pain response to 50 or 500 nmol kallidin were unaffected by capsaicin pre‐treatment. 5. The present results suggest that prolonged topical capsaicin treatment to the human nasal mucosa may lead to selective desensitization to certain algesic stimuli such as capsaicin itself and hydrogen ions.