Cox-dependent fatty acid metabolites cause pain through activation of the irritant receptor TRPA1

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Abstract
Prostaglandins (PG) are known to induce pain perception indirectly by sensitizing nociceptors. Accordingly, the analgesic action of nonsteroidal anti-inflammatory drugs (NSAIDs) results from inhibition of cyclooxygenases and blockade of PG biosynthesis. Cyclopentenone PGs, 15-d-PGJ2, PGA2, and PGA1, formed by dehydration of their respective parent PGs, PGD2, PGE2, and PGE1, possess a highly reactive α,β-unsaturated carbonyl group that has been proposed to gate the irritant transient receptor potential A1 (TRPA1) channel. Here, by using TRPA1 wild-type (TRPA1+/+) or deficient (TRPA1−/−) mice, we show that cyclopentenone PGs produce pain by direct stimulation of nociceptors via TRPA1 activation. Cyclopentenone PGs caused a robust calcium response in dorsal root ganglion (DRG) neurons of TRPA1+/+, but not of TRPA1−/−mice, and a calcium-dependent release of sensory neuropeptides from the rat dorsal spinal cord. Intraplantar injection of cyclopentenone PGs stimulated c-fos expression in spinal neurons of the dorsal horn and evoked an instantaneous, robust, and transient nociceptive response in TRPA1+/+but not in TRPA1−/−mice. The classical proalgesic PG, PGE2, caused a slight calcium response in DRG neurons, increased c-fos expression in spinal neurons, and induced a delayed and sustained nociceptive response in both TRPA1+/+and TRPA1−/−mice. These results expand the mechanism of NSAID analgesia from blockade of indirect nociceptor sensitization by classical PGs to inhibition of direct TRPA1-dependent nociceptor activation by cyclopentenone PGs. Thus, TRPA1 antagonism may contribute to suppress pain evoked by PG metabolites without the adverse effects of inhibiting cyclooxygenases.