The role of the N-methyl-D-aspartate receptor NR1 subunit in peripheral nerve injury-induced mechanical allodynia, glial activation and chemokine expression in the mouse

Abstract

The N-methyl-d-aspartate receptor (NMDAR) has been strongly implicated in mechanisms of persistent pain states. The purpose of the present study was to determine whether the NMDAR NR-1, a key subunit in regulation of NMDAR channel complex is directly contributing to the onset and propagation of peripheral nerve injury-induced allodynia and whether N-methyl-d-aspartate (NMDA) signaling interacts with spinal chemokine (chemotactic cytokines) expression and glial activation. We used genetically engineered male mice that had their normal NR1 gene knocked out and expressed a modified NR1 gene at either normal level (NR1 +/+, wild type) or at a low level (NR1+/-, knock down). Each mouse underwent a peripheral nerve injury in which the lumbar 5 spinal segment (L5) nerve was transected. Mechanical allodynia was assessed using 0.008 and 0.015 g von Frey filaments on days 1, 3, 5, 7, 10, 14, 17 and 21 post-surgery. Mice were killed on day 21 and the harvested L5 spinal cord was analyzed for chemokine expression using RNAse protection assay. In a separate study, glial expression using immunohistochemistry was assessed in both groups 7 days following peripheral nerve injury. The NR1+/- mice displayed decreased mechanical allodynia in comparison to their wild type counterparts. However, even with dramatically impaired NMDA receptor signaling, there was still evidence of tactile hypersensitivity. Using the RPA analysis, we found decreases in mRNA chemokine expression in the NR1+/- mice as compared with NR1+/+ mice. There were no apparent differences in microglial or astrocytic expression between the wild type and knock down mice. These data provide important insights into the cascade of events involving the dynamic interaction between NMDAR function and spinal chemokine and glial production in neuropathic pain states. The results support the findings that chemokine signaling releases glutamate in the spinal cord.