Chronological changes of mechanical allodynia and spinal microglia activation by an intrathecal injection of MK-801.

The neuropathic pain that occurs after peripheral nerve injury may be related to abnormal central activity. The present experiments investigated the effects of MK-801 [N-methyl-D-aspartate (NMDA) receptor antagonist] on neuropathic pain behaviors and microglial activity in rats. Neuropathic pain was produced by L5 spinal nerve ligation of rats. MK-801 was injected to determine whether spinal microglial activation after nerve injury plays a crucial role in the development and/or maintenance of neuropathy through the NMDA receptor. Mechanical allodynia of the hind paw was examined with von Frey filaments postoperatively. Microglial activity was measured by observing changes in immunoreactivity with a microglia marker, OX-42. The MK-801, at a dose of 3 or 30 µg/5 µl, injection group showed higher neuropathic pain threshold and reduction of microglial activity. These results suggest that neuropathic pain behaviors following L5 spinal nerve ligation may be related to altered activity of the microglia involving the NMDA receptor, and chronological changes of microglial activation by MK-801 are related to maintenance of mechanical allodynia.