Functional down-regulation of axotomized rat facial motoneurons.

Functional alterations in injured motoneurons were quantitatively analyzed in axotomized rat facial nuclei. Choline acetyltransferase (ChAT), vesicular acetylcholine transporter (VAchT) and m2 muscarinic acetylcholine receptor (m2MAchR) were chosen as indicators of motoneuron function. Immunoblotting showed that the amounts of ChAT in the ipsilateral facial nucleus significantly decreased to below 20% from 3 to 14 days after transection. The decreased level of ChAT in injured motoneurons was ascertained by immunohistochemical study. However, at 4-5 weeks after transection the level of ChAT was restored to that of control side. The amounts of VAchT in the transected nucleus were observed to decrease to below 20% in the first 14 days after transection. The down-regulated levels of VAchT in injured motoneurons were confirmed by immunohistochemical results. The reduced VAchT levels returned to the control levels at 4-5 weeks following insult. The level of m2MAchR in the ipsilateral nucleus was recognized to decrease to below 10% starting on the 5th day after insult, and the low levels were sustained for 5 weeks. Nissl staining at 5 days and 12 days after insult revealed that facial motoneurons in the transected nucleus were almost all alive. Altogether, these results indicate that transected adult rat facial motoneurons are functionally depressed with down-regulated levels of ChAT, VAchT and m2MAchR during the first 14 days after insult, and during weeks 4-5 ChAT and VAchT levels are restored while the levels of m2MAchR remain low.