Ionotropic glutamate receptor subunit distribution on hypoglossal motoneuronal pools in the rat

The expression of ionotropic glutamate receptor subunits in the motoneurona l pools of the hypoglossal nucleus was studied using specific antibodies ag ainst subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate and N-methyl-D-aspartate (NMDA) subtypes. The highest numbe rs of intensely immunolabelled motoneurons were found in the dorsal tier an d caudoventromedial part of the hypoglossal nucleus with all antibodies exc ept that against the GluR1 AMPA subunit. Labelling for the GluR1 subunit wa s weak except for caudally located groups of motoneurons which innervate to ngue muscles related to respiratory activity. By contrast, most motoneurons were intensely immunostained with antibodies against GluR2/3 and GluR4 sub units of the AMPA subtype. The low staining observed using an antibody spec ific for the GluR2 subunit (which prevents Ca2+-entry through AMPA channels ) strongly suggests that AMPA receptors in hypoglossal motoneurons are Ca2-permeable. Immunolabelling for the GluR5/6/7 kainate receptor subunits was found in many motoneuronal somata as well as in thin axon-like profiles an d puncta that resembled synaptic boutons. Most motoneurons were intensely i mmunostained for the NMDA receptor subunit NR1. These results show that the hypoglossal nucleus contains five heterogeneous pools of motoneurons which innervate functionally defined groups of tongue muscles. The uneven expres sion of the different receptor subunits analysed here could reflect diverse phenotypic properties of hypoglossal motoneurons which might be expected t o generate different patterns of motor responses under different physiologi cal or pathological conditions.