Hypersensitivity of pulmonary chemosensitive neurons induced by activation of protease-activated receptor-2 in rats

This study was carried out to determine the effect of protease-activated receptor-2 (PAR₂) activation on the pulmonary chemoreflex responses and on the sensitivity of isolated rat vagal pulmonary chemosensitive neurons. In anaesthetized, spontaneously breathing rats, intratracheal instillation of trypsin (0.8 mg ml^-1, 0.1 ml), an endogenous agonist of PAR₂, significantly amplified the capsaicin-induced pulmonary chemoreflex responses. The enhanced responses were completely abolished by perineural capsaicin treatment of both cervical vagi, suggesting the involvement of pulmonary C-fibre afferents. In patch-clamp recording experiments, pretreatment with trypsin (0.1 μM, 2 min) potentiated the capsaicin-induced whole-cell inward current in isolated pulmonary sensory neurons. The potentiating effect of trypsin was mimicked by PAR₂-activating peptide (PAR₂-AP) in a concentration-dependent manner. PAR₂ -AP pretreatment (100 μM, 2 min) also markedly enhanced the acid-evoked inward currents in these sensory neurons. Furthermore, the sensitizing effect of PAR₂ was completely abolished by pretreatment with either U73122 (1 μM, 4 min), a phospholipase C inhibitor, or chelerythrine (10 μM, 4 min), a protein kinase C (PKC) inhibitor. In summary, our results have demonstrated that activation of PAR₂ upregulates the pulmonary chemoreflex sensitivity in vivo and the excitability of isolated pulmonary chemosensitive neurons in vitro, and this effect of PAR₂ activation was mediated through the PKC-dependent transduction pathway. These results further suggest that the hypersensitivity of these neurons may play a part in the development of airway hyper-responsiveness resulting from PAR₂ activation.