Dynamic changes in N-methyl-D-aspartate receptors after closed head injury in mice: Implications for treatment of neurological and cognitive deficits

Abstract

Traumatic brain injury is a leading cause of mortality and morbidity among young people. For the last couple of decades, it was believed that excess stimulation of brain receptors for the excitatory neurotransmitter glutamate was a major cause of delayed neuronal death after head injury, and several major clinical trials in severely head injured patients used blockers of the glutamate N-methyl-D-aspartate (NMDA) receptor. All of these trials failed to show efficacy. Using a mouse model of traumatic brain injury and quantitative autoradiography of the activity-dependent NMDA receptor antagonist MK801, we show that hyperactivation of glutamate NMDA receptors after injury is short-lived (<1 h) and is followed by a profound and long-lasting (> or =7 days) loss of function. Furthermore, stimulation of NMDA receptors by NMDA 24 and 48 h postinjury produced a significant attenuation of neurological deficits (blocked by coadministration of MK801) and restored cognitive performance 14 days postinjury. These results provide the underlying mechanism for the well known but heretofore unexplained short therapeutic window of glutamate antagonists after brain injury and support a pharmacological intervention with a relatively long (> or =24 h) time window easily attainable for treatment of human accidental head injury.

Fig. 1.

Dynamic changes in NMDAR open channel availability after CHI. Brain sections at the level of dorsal hippocampus from a noninjured animal (Top), a CHI mouse killed 15 min postinjury (Middle), and an injured animal killed 8 h postinjury (Bottom) illustrate the dynamic nature of changes in NMDAR after injury. The original autoradiograms were pseudocolored by using the extended rainbow color scale shown above the top section (red and white representing the highest values). Note the bilateral increase in MK801 binding throughout the hippocampus, cortex, and thalamus of the animal killed 15 min postinjury and the profound decline in the same regions in an animal killed 8 h after CHI.

Fig. 2.

Activation of NMDAR through systemic agonist administration improves neurological recovery after CHI. Mice subjected to CHI were treated with NMDA, NMDA+MK801, or vehicle and dNSS calculated as described in Materials and Methods. dNSS was significantly higher in the NMDA-treated animals (top curve, open symbols) compared to vehicle-treated CHI mice (middle curve) at 7 days (P = 0.05) and 14 days (P = 0.016) by the Mann-Whitney nonparametric t test. Administration of MK801 1 mg/kg in addition to NMDA obliterated the benefi-cial effect of the agonist (bottom curve). NMDA+MK801 animals had signifi-cantly lower dNSS values (P < 0.001) at 7 (P = 0.001) and 14 (P < 0.0001) days compared to NMDA alone. Recovery was also significantly worse when compared to vehicle at 7 (P = 0.017) and 14 (P = 0.05) days.

Fig. 3.

Activation of NMDAR through systemic agonist administration improves cognitive performance in the object recognition test after CHI. Results are means and standard errors of eight to nine animals per treatment group. Mice were subjected to the object recognition test 14 days after CHI. Injured, vehicle-treated animals (filled bars) lost the ability to recognize the new object, whereas NMDA-treated CHI animals (gray bars) spent a significantly higher percentage of their exploration time near the novel object (^*, P < 0.0001; Student's t test) and were indistinguishable from intact untreated animals (open bars). NMDA at the dose administered in this study had no effect on the performance of intact animals (hatched bars).