Differential Contribution of Spinal Mitogen-Activated Protein Kinases to the Phase of Long-Lasting Allodynia Evoked by Intrathecal Administration of ATP in Rats

Abstract

Several lines of evidence suggest that activation of spinal mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase (ERK) and p38 MAPK, contributes to the induction and maintenance of chronic pain. We recently reported that an intrathecal (i.t.) administration of ATP evoked tactile allodynia, which lasted more than 1 week in rats. The long-lasting allodynia was induced by activation of spinal P2X 2/3-receptors, and the induction and early phase of maintenance, but not the late phase, was mediated, at least in part, by the activation of spinal glial cells. In this study, we examined the involvement of spinal ERK and p38 MAPK in each phase of i.t. ATP-evoked long-lasting allodynia. I.t. administration of ATP (100 nmol) markedly increased phosphorylated ERK, which peaked at 1-8 h before gradually decreasing to a level that was sustained until 7 d after administration. In contrast, only a slight increase in phosphorylated p38 MAPK was observed. Consistent with the increased phosphorylation of MAPKs, the ERK kinase MEK inhibitor, U0126 (3 nmol), attenuated the induction phase (co-administration with ATP) and early maintenance phase (1-d post-ATP administration) of the i.t. ATP-evoked allodynia, but not the late maintenance phase (7-d post-ATP administration), while the p38 MAPK inhibitor, SB203580 (10 microg), had little effect. These results suggest that the induction phase and early maintenance phase, but not the late maintenance phase of long-lasting allodynia is mediated by the activation of ERK, rather than by the activation of p38 MAPK, in the spinal cord. These findings are informative for elucidating the mechanisms underlying the pathogenesis of chronic pain.