Involvement of activated microglia in increased vulnerability of motoneurons after facial nerve avulsion in presymptomatic amyotrophic lateral sclerosis model rats

Activated microglia are observed in various neurodegenerative diseases and are thought to be involved in the processes of neuronal cell death. Motoneuron damage in the facial nuclei after facial nerve avulsion is accelerated in presymptomatic transgenic rats expressing human mutant Cu²⁺/Zn²⁺ superoxide dismutase 1 (SOD1), compared with that in wild-type rats. To reveal the functional role of microglia in motoneuronal death, we investigated the microglial response after facial nerve avulsion in presymptomatic mutant SOD1^H46R (mSOD1^H46R) rats. At 3 days after avulsion, microglial clusters were observed in the facial nuclei of both wild-type and mSOD1^H46R rats. The numbers of microglial clusters, proliferating microglia, and microglial attachments to motoneurons were significantly higher in mSOD1^H46R rats, compared with those in wild-type rats. Immunopositive signals for the phagocytic marker ED1 were significantly stronger in mSOD1^H46R rats, compared with that in wild-type rats, at 2 weeks after avulsion. Furthermore, primary microglia prepared from mSOD1^H46R rats showed enhanced phagocytic activity, compared with that in wild-type rats. The expression of P2Y₁₂ mRNA was higher in the facial nuclei of mSOD1^H46R rats, compared with that in wild-type rats. A laser microdissection system revealed that the expression of ATF3 mRNA was higher in the motoneurons of mSOD1^H46R rats, compared with that in wild-type rats, at 2 days after avulsion. These results indicate that microglial activation in response to early neuronal damage increased in mSOD1^H46R rats and suggest that the enhanced activation of microglia may lead to an increase in the vulnerability of motoneurons after avulsion in mSOD1^H46R rats.