al-8810 and Pain

It was suggested that endocannabinoids are metabolized by cyclooxygenase (COX)-2 in the spinal cord of rats with kaolin/λ-carrageenan-induced knee inflammation, and that this mechanism contributes to the analgesic effects of COX-2 inhibitors in this experimental model. We report the development of a specific method for the identification of endocannabinoid COX-2 metabolites, its application to measure the levels of these compounds in tissues, and the finding of prostamide F(2α) (PMF(2α)) in mice with knee inflammation. Whereas the levels of spinal endocannabinoids were not significantly altered by kaolin/λ-carrageenan-induced knee inflammation, those of the COX-2 metabolite of AEA, PMF(2α), were strongly elevated. The formation of PMF(2α) was reduced by indomethacin (a non-selective COX inhibitor), NS-398 (a selective COX-2 inhibitor) and SC-560 (a selective COX-1 inhibitor). In healthy mice, spinal application of PMF(2α) increased the firing of nociceptive (NS) neurons, and correspondingly reduced the threshold of paw withdrawal latency (PWL). These effects were attenuated by the PMF(2α) receptor antagonist AGN211336, but not by the FP receptor antagonist AL8810. Also prostaglandin F(2α) increased NS neuron firing and reduced the threshold of PWL in healthy mice, and these effects were antagonized by AL8810, and not by AGN211336. In mice with kaolin/λ-carrageenan-induced knee inflammation, AGN211336, but not AL8810, reduced the inflammation-induced NS neuron firing and reduction of PWL. These findings suggest that inflammation-induced, and prostanoid-mediated, enhancement of dorsal horn NS neuron firing stimulates the production of spinal PMF(2α), which in turn contributes to further NS neuron firing and pain transmission by activating specific receptors.

Topics: Action Potentials; Animals; Arachidonic Acids; Cannabinoid Receptor Modulators; Chromatography, Liquid; Cyclooxygenase 1; Cyclooxygenase 2; Dinoprost; Dinoprostone; Endocannabinoids; Evoked Potentials; Hindlimb; Inflammation; Mass Spectrometry; Membrane Proteins; Mice; Nociceptors; Pain; Polyunsaturated Alkamides; Posterior Horn Cells; Rats

Nociceptive primary afferents have the capacity to induce a state of increased excitability in the dorsal horn neurons of the spinal cord. It is well accepted that capsaicin-sensitive C-fibers transduce noxious stimulation and acute pain and that capsaicin-insensitive A beta-fibers are responsible for touch and innocuous sensation. It has been reported that the intrathecal (i.t.) administration of prostaglandin F(2 alpha) (PGF(2 alpha)) and ATP induces mechanical allodynia via the capsaicin-insensitive primary afferent pathway. In the present study, we investigated the interaction of purinoceptor P2X and the PGF(2 alpha) receptor (FP) in the induction of allodynia by use of mice lacking FP (FP(-/-)). Both PGF(2 alpha) and the P2X receptor agonist alphabeta-methylene ATP administered i.t. strongly induced allodynia for 50 min by tactile stimuli to the flank of mice. The allodynia induced by alphabeta-methylene ATP, but not that by PGF(2 alpha), was suppressed by simultaneous i.t. administration of P2X receptor antagonists pyridoxalphosphate-6-azophenyl-2,4-disulphonic acid and A-317491. In contrast, the allodynia induced by alphabeta-methylene ATP as well as that by PGF(2 alpha) was not observed in FP(-/-) mice. Immunostaining of beta-galactosidase, a reporter knocked into the endogenous FP locus in FP(-/-) mice, showed that the FP receptor was co-localized with P2X(2) and P2X(3) receptors in neurons of the spinal cord. alphabeta-Methylene ATP evoked a transient or sustained [Ca(2+)](i) increase in most of the PGF(2 alpha)-responsive cells in the deeper layer of the spinal cord, and the alphabeta-methylene ATP-evoked increase was blocked by the FP receptor antagonist AL-8810 in two-thirds of the cells. Neither PGF(2 alpha) nor alphabeta-methylene ATP induced the activation of spinal microglia. The present study demonstrates that the alphabeta-methylene ATP-evoked allodynia is mediated by the FP receptor, possibly via the functional coupling between the activation of P2X(2/3) receptors on the central terminal of capsaicin-insensitive fibers and FP receptors on spinal neurons.

Topics: Adenosine Triphosphate; Afferent Pathways; Animals; Calcium Signaling; Dinoprost; Galactosides; Genes, Reporter; Hyperalgesia; Indoles; Male; Mice; Mice, Inbred C57BL; Mice, Knockout; Nociceptors; Pain; Posterior Horn Cells; Presynaptic Terminals; Purinergic P2 Receptor Agonists; Purinergic P2 Receptor Antagonists; Receptors, Prostaglandin; Receptors, Purinergic P2; Receptors, Purinergic P2X2; Receptors, Purinergic P2X3; Spinal Nerve Roots