Rapid Broad-Spectrum Analgesia through Activation of Peroxisome Proliferator-Activated Receptor-α

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Abstract
Severe pain remains a major area of unmet medical need. Here we report that agonists of the nuclear receptor PPAR-α (peroxisome proliferator-activated receptor-α) suppress pain behaviors induced in mice by chemical tissue injury, nerve damage, or inflammation. The PPAR-α agonists GW7647 [2-(4-(2-(1-cyclohexanebutyl)-3-cyclohexylureido)ethyl)phenylthio)-2-methylpropionic acid], Wy-14643 [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthioacetic acid], and palmitoylethanolamide (PEA) reduced nocifensive behaviors elicited in mice by intraplantar (i.pl.) injection of formalin or i.p. injection of magnesium sulfate. These effects were absent in PPAR-α-null mice yet occurred within minutes of agonist administration in wild-type mice, suggesting that they were mediated through a transcription-independent mechanism. Consistent with this hypothesis, blockade of calcium-operated IKca (KCa3.1) and BKca (KCa1.1) potassium channels prevented the effects of GW7647 and PEA in the formalin test. Three observations suggest that PPAR-α agonists may inhibit nocifensive responses by acting on peripheral PPAR-α. (i) PEA reduced formalin-induced pain at i.pl. doses that produced no increase in systemic PEA levels; (ii) PPAR-α was expressed in dorsal root ganglia neurons of wild-type but not PPAR-α-null mice; and (ii) GW7647 and PEA prevented formalin-induced firing of spinal cord nociceptive neurons in rats. In addition to modulating nociception, GW7647 and PEA reduced hyperalgesic responses in the chronic constriction injury model of neuropathic pain; these effects were also contingent on PPAR-α expression and were observed following either acute or subchronic PPAR-α agonist administration. Finally, acute administration of GW7647 and PEA reduced hyperalgesic responses in the complete Freund9s adjuvant and carrageenan models of inflammatory pain. Our results suggest that PPAR-α agonists may represent a novel class of analgesics.