Thrombin and trypsin directly activate vagal C-fibres in mouse lung via protease-activated receptor-1

The nature of protease-activated receptors (PARs) capable of activating respiratory vagal C-fibres in the mouse was investigated. Infusing thrombin or trypsin via the trachea strongly activated vagal lung C-fibres with action potential discharge, recorded with the extracellular electrode positioned in the vagal sensory ganglion. The intensity of activation was similar to that observed with the TRPV1 agonist, capsaicin. This was mimicked by the PAR1-activating peptide TFLLR-NH₂, whereas the PAR2-activating peptide SLIGRL-NH₂ was without effect. Patch clamp recording on cell bodies of capsaicin-sensitive neurons retrogradely labelled from the lungs revealed that TFLLR-NH₂ consistently evokes a large inward current. RT-PCR revealed all four PARs were expressed in the vagal ganglia. However, when RT-PCR was carried out on individual neurons retrogradely labelled from the lungs it was noted that TRPV1-positive neurons (presumed C-fibre neurons) expressed PAR1 and PAR3, whereas PAR2 and PAR4 were rarely expressed. The C-fibres in mouse lungs isolated from PAR1^-/- animals responded normally to capsaicin, but failed to respond to trypsin, thrombin, or TFLLR-NH₂. These data show that the PAR most relevant for evoking action potential discharge in vagal C-fibres in mouse lungs is PAR1, and that this is a direct neuronal effect.