iodoresiniferatoxin and Neuralgia

Neuropathic pain elevates spinal anandamide (AEA) levels in a way further increased when URB597, an inhibitor of AEA hydrolysis by fatty acid amide hydrolase (FAAH), is injected intrathecally. Spinal AEA reduces neuropathic pain by acting at both cannabinoid CB1 receptors and transient receptor potential vanilloid-1 (TRPV1) channels. Yet, intrathecal URB597 is only partially effective at counteracting neuropathic pain. We investigated the effect of high doses of intrathecal URB597 on allodynia and hyperalgesia in rats with chronic constriction injury (CCI) of the sciatic nerve. Among those tested, the 200 µg/rat dose of URB597 was the only one that elevated the levels of the FAAH non-endocannabinoid and anti-inflammatory substrates, oleoylethanolamide (OEA) and palmitoylethanolamide (PEA), and of the endocannabinoid FAAH substrate, 2-arachidonoylglycerol, and fully inhibited thermal and tactile nociception, although in a manner blocked almost uniquely by TRPV1 antagonism. Surprisingly, this dose of URB597 decreased spinal AEA levels. RT-qPCR and western blot analyses demonstrated altered spinal expression of lipoxygenases (LOX), and baicalein, an inhibitor of 12/15-LOX, significantly reduced URB597 analgesic effects, suggesting the occurrence of alternative pathways of AEA metabolism. Using immunofluorescence techniques, FAAH, 15-LOX and TRPV1 were found to co-localize in dorsal spinal horn neurons of CCI rats. Finally, 15-hydroxy-AEA, a 15-LOX derivative of AEA, potently and efficaciously activated the rat recombinant TRPV1 channel. We suggest that intrathecally injected URB597 at full analgesic efficacy unmasks a secondary route of AEA metabolism via 15-LOX with possible formation of 15-hydroxy-AEA, which, together with OEA and PEA, may contribute at producing TRPV1-mediated analgesia in CCI rats.

Topics: Amides; Amidohydrolases; Analgesia; Animals; Arachidonate 15-Lipoxygenase; Arachidonic Acids; Benzamides; Calcium Signaling; Carbamates; Diterpenes; Endocannabinoids; Ethanolamines; Flavanones; Glycerides; HEK293 Cells; Humans; Hyperalgesia; Injections, Spinal; Lipoxygenase Inhibitors; Male; Neuralgia; Oleic Acids; Palmitic Acids; Polyunsaturated Alkamides; Posterior Horn Cells; Rats; Rats, Wistar; Sciatic Nerve; Spinal Cord; TRPV Cation Channels

The vanilloid TRPV1 receptor, present on primary afferent fibres, is activated by noxious heat, low pH and endogenous vanilloids. Changes in the function or distribution of TRPV1 receptors may play an important role in pain induced by inflammation or neuropathy. The aim of the present study was to evaluate the role of peripheral TRPV1 receptors in thermal nociception in rat models of inflammatory and neuropathic pain. Here, we have determined the effects of peripheral administration of the potent TRPV1 receptor antagonist iodoresiniferatoxin (IRTX) on noxious heat (45 degrees C)-evoked responses of spinal wide dynamic range (WDR) neurons in naïve, carrageenan-inflamed, sham-operated and L5/6 spinal nerve-ligated (SNL) anaesthetized rats in vivo. In addition, effects of peripheral administration of IRTX on mechanically evoked responses of WDR neurons were determined in sham-operated and SNL rats. Carrageenan inflammation significantly (P<0.05) increased the 45 degrees C-evoked responses of WDR neurons. Intraplantar injection of the lower dose of IRTX (0.004 microg) inhibited (P<0.05) 45 degrees C-evoked responses of WDR neurons in carrageenan-inflamed, but not in naïve, rats. The higher dose of IRTX (0.4 microg) significantly (P<0.05) inhibited 45 degrees C-evoked responses in both inflamed and naïve rats. In sham-operated and SNL rats, IRTX (0.004 and 0.4 microg) significantly (P<0.05) inhibited 45 degrees C-evoked, but had no effect on mechanically evoked responses of WDR neurons. These data support the role of peripheral TRPV1 receptors in noxious thermal transmission in naïve, inflamed and neuropathic rats, and suggest that there is an increased functional contribution of peripheral TRPV1 receptors following acute inflammation.

Topics: Animals; Carrageenan; Disease Models, Animal; Diterpenes; Dose-Response Relationship, Drug; Drug Interactions; Evoked Potentials; Hot Temperature; Hyperalgesia; Inflammation; Ion Channels; Ligation; Male; Neuralgia; Nociceptors; Posterior Horn Cells; Rats; Rats, Sprague-Dawley; Spinal Nerves; Statistics, Nonparametric; Time Factors; TRPV Cation Channels