Role of the muscarinic M₂ receptor in human nasal mucosa

Introduction: Neurally mediated secretory hyperresponsiveness is a feature of allergic rhinitis (AR). Muscarinic M₂ receptors are inhibitory autoreceptors which limit acetylcholine release at postganglionic parasympathetic nerve terminals, dysfunction of which has been implicated as a cause of bronchial hyperresponsiveness in asthma. The purpose of this study was to investigate the presence and function of M₂ receptors in the human nose. Methods: In the first part of the study, nasal biopsies from subjects with AR (n=12) and normal controls (n=10) were examined for the presence of M₂ receptor mRNA using polymerase chain reaction (PCR). In the second part, subjects with symptomatic AR (n=17), with established contralateral secretory reflexes to bradykinin, underwent unilateral bradykinin challenge after premedication with pilocarpine, a M₂ receptor agonist. The effect of pilocarpine on contralateral secretory reflexes was studied. Results: M₂ receptor mRNA was detected in 12 subjects (54%). Pilocarpine premedication led to a significant reduction of the contralateral secretory reflex to bradykinin. Conclusion: M₂ receptors are present in the human nasal mucosa, though not universally. The function of M₂ receptors in subjects with AR can be demonstrated using pilocarpine, however, more work is required to define their importance and potential role in nasal hyperresponsiveness.