Comparison of mechanisms of allodynia induced by acromelic acid A between early and late phases

We previously showed that intrathecal administration of acromelic acid A (ACRO-A) provoked tactile allodynia in mice. As well, recent studies have demonstrated that the activation of NMDA glutamate receptor-neuronal nitric oxide synthase (nNOS) pathway and glia play crucial roles in the development and maintenance of neuropathic pain. In order to clarify their involvement in ACRO-A-induced allodynia, we investigated the effects of various agents on two mouse models at early and late-phase allodynia. The agents employed were Ca2+ channel α2δ ligands, NMDA and AMPA receptor antagonists, nNOS, and Ca2+/calmodulin kinase II inhibitors. When injected simultaneously with ACRO-A, all of these agents blocked allodynia in the early-phase group; however, they did not block allodynia when injected 7 days after the administration of ACRO-A in the late-phase group. In order to block glial activation, astrocytic inhibitor L-α-aminoadipate (LAA) or microglial inhibitor minocycline was administrated, and allodynia was examined on day 7. Activations of nNOS and glia in the spinal cord were histochemically examined at 1h or 1 week after injection of ACRO-A. We found that nNOS activity increased 1h after ACRO-A injection; however, it did not increase 1 week after ACRO-A injection. Conversely, microglial activation was observed 1 week after ACRO-A injection and was significantly inhibited with minocycline treatment. Moreover, only LAA was found to inhibit late-phase allodynia. In this study, we demonstrate that NMDA receptor activation is involved only in ACRO-A-induced tactile allodynia in the early phase, and that spinal astrocytic activation contributes to allodynia in the late phase.