Expression of glutamate receptor subunits in alpha-motoneurons

Whole-cell recordings from 6.5 day embryonic chick alpha-motoneurons indicated the presence of AMPA, kainate, and NMDA glutamate receptor subtypes in each motoneuron tested. AMPA consistently evoked a desensitizing response, while kainate could evoke either a desensitizing or non-desensitizing whole-cell response. In excised membrane patches, desensitizing AMPA responses appeared to be colocalized with non-desensitizing kainate responses. Desensitizing kainate responses were seen in some patches which were not responsive to AMPA, suggesting that kainate selective subunits and AMPA selective subunits localize separately on the motoneuron membrane. To determine which of the known glutamate receptor subunits might underlie these responses, we used RT-PCR amplification to detect subunits present in mRNA isolated from adult rat spinal cord and from a highly enriched motoneuron population from embryonic chick. Sequencing of the amplified cDNA was used to verify the identity of the products and of the alternative splice variants of GluR1-4. In rat spinal cord, all subunits that we attempted to detect, including AMPA selective subunits GluR1-4, kainate selective subunits GluR5-7 and KA1-2, and NMDA subunit NR1 were present. The isolated motoneurons also contained AMPA subunits GluR1, 2, and 4, and kainate subunits GluR6 and 7. The GluR2 and 4 subunits were specifically processed by splicing, present primarily as the flip splice form.