Upregulation of NMDARI mRNA induced by MK-801 is associated with massive death of axotomized motor neurones in adult rats

Studies on the pathogenesis of human motor neurone disease have suffered from the absence of models of motor neurone degeneration in adult animals. Normally in adult rodents, transection of motor neurone axons results in only a modest degree of neuronal death. We reasoned that axotomy-induced motor neurone death might be enhanced by modulating glutamatergic transmission. By axotomizing the facial nerve in adult rats and then administering MK-801 for the first week of a 4-week or 8-week post-lesion survival period, we induced a 67% motor neurone loss by 8 weeks as compared with a 19% loss in controls. A possible explanation for the increased motor neurone loss after MK-801 treatment is that transient blockade of NMDA receptors may upregulate synthesis of NMDA receptor components. In order to test this idea, we employed quantitativein situhybridization to determine the response of NMDAR1 mRNA to axotomy and axotomy + MK-801 treatment. Quantification of the percentage of area occupied by NMDAR1 silver grains per motor neurone somata indicated that axotomy alone did not provoke a change in NMDAR1 mRNA. However, axotomy and MK-801 combined treatment resulted in a highly significant upregulation of NMDAR1 mRNA when compared with controls or animals treated solely with axotomy.Our results suggest that motor neurone death in adult animals can be enhanced after axotomy in association with the upregulation of NMDA receptor mRNA. Thus, abnormalities in glutamate receptor signalling may lead to subacute motor neurone deathin vivo. Furthermore these results indicate that transient treatment with MK-801 is a convenient method for enhancing the degree of motor neurone death after axotomy in adult animals.