Activation of bronchopulmonary vagal afferent nerves with bradykinin, acid and vanilloid receptor agonists in wild‐type and TRPV1–/– mice

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Abstract
The vanilloid receptor TRPV1 (formerly VR1) has been implicated in the activation of nociceptive sensory nerves by capsaicin, noxious heat, protons, bradykinin, cannabinoids such as anandamide, and certain metabolites of arachidonic acid. Using TRPV1 knockout mouse (TRPV1–/–) we address the question of whether TRPV1 is obligatory for action potential discharge in vagal C‐fibre terminals evoked by capsaicin, anandamide, acid and bradykinin. The response of a defined subtype of the vagal afferent bronchopulmonary C‐fibres (conduction velocity < 0.7 ms−1) to the putative TRPV1 activators was studied in vitro in the mouse isolated/perfused lung–nerve preparation. Capsaicin (1 μm) evoked action potential discharge of ∼90% (28/31) of C‐fibres in the TRPV1+/+ mice, but failed to activate bronchopulmonary C‐fibres in TRPV1–/– animals (n= 10). Anandamide (3–100 μm) induced concentration‐dependent activation of capsaicin‐sensitive TRPV1+/+ C‐fibres with a threshold of 3–10 μm, but failed to evoke substantive discharge in TRPV1–/– C‐fibres. In the TRPV1+/+ mice, the B2 receptor‐mediated activation by bradykinin (1 μm) was restricted to the capsaicin‐sensitive C‐fibres. Bradykinin was effective in evoking B2 receptor‐mediated action potential discharge in TRPV1–/– C‐fibres, but the response was significantly (P < 0.05) less persistent than in TRPV1+/+ C‐fibres. Exposing the tissue to acid (pH = 5) excited both TRPV1+/+ and TRPV1–/– C‐fibres. We conclude that TRPV1 is obligatory for vagal C‐fibre activation by capsaicin and anandamide. By contrast, whereas TRPV1 may have a modulatory role in bradykinin and acid‐induced activation of bronchopulmonary C‐fibres, it is not required for action potential discharge evoked by these stimuli.