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

The afferent activities that arise from sensory terminals located in the airways and lungs are conducted almost exclusively by vagus nerves and their branches (Paintal, 1973; Widdicombe, 1982). Cell bodies of these sensory nerves reside in two adjacent but distinct anatomical structures, the nodose and intracranial jugular ganglia. It has been shown that >75% of vagal bronchopulmonary afferents are non-myelinated C-fibres that exhibit exquisite chemosensitivity (Agostoni et al. 1957). The importance of these C-fibre afferents in regulating the cardiorespiratory functions under both normal and abnormal physiological conditions has been well documented (Paintal, 1973; Coleridge & Coleridge, 1984; Lee & Pisarri, 2001; Lee & Undem, 2005). Protease-activated receptors (PARs) are a family of G-protein-coupled, seven-transmembrane-domain receptors that are activated by proteolysis (Dery et al. 1998; Macfarlane et al. 2001; Ossovskaya & Bunnett, 2004). Certain proteases are known to cleave PARs within the extracellular N-terminus to expose tethered ligand domains that bind and activate the cleaved receptors. Four subtypes of PARs (PAR1 to PAR4) have been identified. PAR1 and PAR3 are both preferentially cleaved by thrombin, PAR2 by trypsin and tryptase, whereas PAR4 is activated by trypsin and thrombin. All PARs, except PAR3, can be selectively activated by the short synthetic peptide that mimics the tethered region of the receptor (Macfarlane et al. 2001; Vergnolle et al. 2001b; Ossovskaya & Bunnett, 2004). PAR2 is expressed in a variety of cells in the airways and lungs including epithelial cells, airway smooth muscle cells, endothelial cells and fibroblasts, as well as inflammatory cells such as mast cells, neutrophils and macrophages (Howells et al. 1997; D'Andrea et al. 1998; Cocks et al. 1999; Akers et al. 2000; Reed & Kita, 2004). It has been recently suggested that activation of PAR2 by endogenous or exogenous agonists such as mast cell tryptase, trypsin-like proteases and some house mite allergens contributes to airway inflammation and airway hyper-responsiveness, the hallmarks of certain airway inflammatory diseases such as asthma (Ricciardolo et al. 2000; Chambers et al. 2001; Schmidlin et al. 2002; Barrios et al. 2003; Ebeling et al. 2005), and an involvement of a neurogenic mechanism caused by the activation of pulmonary C-fibre afferents has been postulated (Ricciardolo et al. 2000; Cocks & Moffatt, 2001; Barrios et al. 2003). However, the effect of PAR2 activation on these pulmonary chemosensitive- (C-) neurons remains unclear. The present study was therefore carried out: (1) to determine whether activation of PAR2 enhances the sensitivity of the pulmonary chemoreflex, which is known to be elicited by stimulation of pulmonary C-fibre afferents (Lee & Pisarri, 2001), in anaesthetized and spontaneously breathing rats; (2) to determine whether this enhancing effect of PAR2 activation is caused by an increase in sensitivity of the vagal pulmonary sensory neurons; and (3) if so, to investigate the intracellular signalling pathway involved in the sensitizing effect caused by the PAR2 activation.