GluR2 AMPA receptor subunit expression in motoneurons at low and high risk for degeneration in amyotrophic lateral sclerosis.

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disorder that results in selective degeneration of most, but not all, groups of motoneurons. The greater susceptibility of vulnerable motoneurons to glutamate excitotoxicity and neurodegeneration has been hypothesized to result from their lower expression of the GluR2 AMPA receptor subunit under control conditions, which renders these receptors permeable to calcium. To address the question of whether there is differential expression of the GluR2 subunit in motoneurons, we compared in normal adult rats expression of GluR2 mRNA and protein within two cranial motor nuclei that are either resistant (III; oculomotor nucleus) or vulnerable (XII; hypoglossal nucleus) to degeneration in ALS. RT-PCR analysis of tissue punched from III and XII motor nuclei detected mRNA for all AMPA subunits (GluR1-R4). In situ hybridization demonstrated no significant difference in GluR2 mRNA expression between III and XII nuclei. Immunohistochemical examination of GluR2 (and GluR4) protein levels demonstrated a similar pattern of the subunit expression in both motor nuclei. This equivalent expression of GluR2 mRNA and protein in motoneurons that differ in their vulnerability to degeneration in ALS suggests that reduced expression of GluR2 is not a factor predisposing motoneurons to degeneration.