Direct blockade of inflammatory hypernociception by peripheral A1 adenosine receptors: involvement of the NO/cGMP/PKG/KATP signaling pathway.

Through activation of the A1 adenosine receptors (A1Rs) at both the central and peripheral level, adenosine produces antinociception in a wide range of tests. However, the mechanisms involved in the peripheral effect are still not fully understood. Therefore, the mechanisms by which peripheral activation of A1Rs reduces inflammatory hypernociception (a decrease in the nociceptive threshold) were addressed in the present study. Immunofluorescence of rat dorsal root ganglion revealed significant expression of A1Rs in primary sensory neurons associated with nociceptive pathways. Functionally, peripheral activation of A1Rs reduced inflammatory hypernociception because intraplantar (i.pl.) administration of an A1R antagonist (DPCPX) enhanced carrageenan-induced hypernociception. On the other hand, local (paw) administration of CPA (a selective A1R agonist) reversed mechanical hypernociception induced by carrageenan or by the directly acting hypernociceptive mediator prostaglandin E(2) (PGE(2)). Down-regulation of A1Rs expression in primary nociceptive neurons by intrathecal treatment with antisense oligodeoxinucleotides significantly reduced peripheral antinociceptive action of CPA. Direct blockade of PGE(2) inflammatory hypernociception by the activation of A1Rs depends on the nitric oxide/cGMP/Protein Kinase G/KATP signaling pathway because the peripheral antinociceptive effect of CPA was prevented by pretreatment with inhibitors of neuronal nitric oxide synthase (N-propyl-l-arginine), guanylyl cyclase (ODQ), and Protein Kinase G (KT5823) as well as with a KATP blocker (glibenclamide). However, this effect of CPA was not reduced by naloxone, excluding the participation of endogenous opioids. These results suggest that the peripheral activation of A1R plays a role in the regulation of inflammatory hypernociception by a mechanism that involves the NO/cGMP/PKG/KATP intracellular signaling pathway.