Nitrooleic Acid, an Endogenous Product of Nitrative Stress, Activates Nociceptive Sensory Nerves via the Direct Activation of TRPA1

Transient Receptor Potential A1 (TRPA1) is a nonselective cation channel, preferentially expressed on a subset of nociceptive sensory neurons, that is activated by a variety of reactive irritants via the covalent modification of cysteine residues. Excessive nitric oxide during inflammation (nitrative stress), leads to the nitration of phospholipids, resulting in the formation of highly reactive cysteine modifying agents, such as nitrooleic acid (9-OA-NO2). Using calcium imaging and electrophysiology, we have shown that 9-OA-NO2 activates human TRPA1 channels (EC50, 1 mu M), whereas oleic acid had no effect on TRPA1. 9-OA-NO2 failed to activate TRPA1 in which the cysteines at positions 619, 639, and 663 and the lysine at 708 had been mutated. TRPA1 activation by 9-OA-NO2 was not inhibited by the NO scavenger carboxy-PTIO. 9-OA-NO2 had no effect on another nociceptive-specific ion channel, TRPV1. 9-OA-NO2 activated a subset of mouse vagal and trigeminal sensory neurons, which also responded to the TRPA1 agonist allyl isothiocyanate and the TRPV1 agonist capsaicin. 9-OA-NO2 failed to activate neurons derived from TRPA1(-/-) mice. The action of 9-OA-NO2 at nociceptive nerve terminals was investigated using an ex vivo extracellular recording preparation of individual bronchopulmonary C fibers in the mouse. 9-OA-NO2 evoked robust action potential discharge from capsaicin-sensitive fibers with slow conduction velocities (0.4-0.7 m/s), which was inhibited by the TRPA1 antagonist AP-18. These data demonstrate that nitrooleic acid, a product of nitrative stress, can induce substantial nociceptive nerve activation through the selective and direct activation of TRPA1 channels.