In vivo pathway of thermal hyperalgesia by intrathecal administration of α,β-methylene ATP in mouse spinal cord:: Involvement of the glutamate-NMDA receptor system

1 The aim of the present study is to characterize the role of the P2X receptor in spinal nociceptive processing in vivo. We investigated the mechanisms of the P2X receptor agonist alpha,beta-methylene ATP (a,alpha,beta meATP)-induced modulation of acute nociceptive signalling in mouse spinal cord. 2 Intrathecal administration of or,alpha,beta meATP produced a significant and dose-dependent thermal hyperalgesic response. This response was completely blocked by intrathecal pretreatment with the non-selective PZ receptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonate (PPADS) and the selective P2X(1), P2X(3) and P2X(2+3), receptor antagonist, 2',3'-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate (TNP-ATP). Pretreatment with alpha,beta meATP 15, 30 and 60 min prior to administration of a second dose of a,alpha,beta meATP diminished the alpha,beta meATP-induced thermal hyperalgesia. 3 A potent agonist for the P2X, receptor, beta,gamma-methylene-L-ATP, did not show the hyperalgesic response, indicating that the P2X, receptor is not involved in the spinal nociceptive pathway. 4 In fura-2 experiments using mouse dorsal root ganglion (DRG) neurons, alpha,beta meATP (100 mu M) increased intracellular Ca(2+) ([Ca(2+)](i)). This was not produced by a second application of alpha,beta meATP. The same DRG neurons also showed a marked [Ca(2+)]i increase in response to capsaicin (3 mu M). 5 Intrathecal pretreatment with the Ca(2+)-dependent exocytosis inhibitor, botulinum neurotoxin B(1), abolished the thermal hyperalgesia by alpha,beta meATP. Furthermore, thermal hyperalgesia was significantly inhibited by the N-methyl-D-aspartate (NMDA) receptor antagonists, 2-amino-5-phosphonopentanoate (APV), dizocilpine and ifenprodil. 6 These findings suggest that alpha,beta meATP-induced thermal hyperalgesia may be mediated by the spinal P2X(3) receptor subtype that causes unresponsiveness by repetitive agonist applications, and that alpha,beta meATP (perhaps through P2X(3) receptors) may evoke spinal glutamate release which, in turn, leads to the generation of thermal hyperalgesia via activation of NMDA receptors.