Changes in NMDA receptor subunit expression in response to contusive spinal cord injury

Differential assembly of N-methyl-D-aspartate (NMDA) receptor subunits determines their functional characteristics. Using in situ hybridization, we found a selective increase of the subunits NR1 and NR2A mRNA at 24 h in ventral motor neurons (VMN) caudal to a standardized spinal cord contusion injury (SCI). Other neuronal cell populations and VMN rostral to the injury site appeared unaffected. Significant up-regulation of NR2A mRNA also was seen 1 month after SCI in thoracic and lumbar VMN. The selective effects on VMN caudal to the injury site suggest that the loss of descending innervation leads to increased NMDA receptor subunit expression in these cells after SCI, which may alter their responses to glutamate. In contrast, protein levels determined by western blot analysis show decreased levels of NR2A 1 month after SCI in whole thoracic segments of spinal cord that included the injury sites. No effects of injury were seen on subunit levels in cervical or lumbar segments. Taken together with our previous study showing alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor subunit down-regulation after injury, our data suggest that glutamate receptor composition is significantly altered after SCI. These changes need to be taken into account to properly understand the function of, and potential pharmacotherapy for, the chronically injured spinal cord.