Characterization of the AMPA-activated receptors present on motoneurons

Motoneurons have been shown to be particularly sensitive to Ca2+-dependent glutamate excitotoxicity, mediated via AMPA receptors (AMPARs), To determine the molecular basis for this susceptibility we have used immunocytochemistry, RT-PCR, and electrophysiology to profile AMPARs on embryonic day 14.5 rat motoneurons, Motoneurons show detectable AMPAR-mediated calcium permeabiiity in vitro and in vivo as determined by cobalt uptake and electrophysiology. Motoneurons express all four AMPAR subunit mRNAs, with glutamate receptor (GluR)2 being the most abundant (63.9 +/- 4.8%). GluR2 is present almost exclusively in the edited form, and electrophysiology confirms that most AMPARs present are calcium-impermeant. However, the kainate current in motoneurons was blocked an average of 32.0% by Joro spider toxin, indicating that a subset of the AMPARs is Ca2+-permeable. Therefore, heterogeneity of AMPARs, rather than the absence of GluR2 or the presence of unedited GluR2, explains AMPAR-mediated Ca2+ permeability. The relative levels of flip/flop isoforms of each subunit were also examined by semiquantitative PCR. Both isoforms were present, but the relative proportion varied for each subunit, and the flip isoform predominated. Thus, our data show that despite high levels of edited GluR2 mRNA, some AMPARs are Ca2+ permeable, and this subset of AMPARs can account for the AMPAR-mediated Ca2+ inflow inferred from cobalt uptake and electrophysiology studies.