Ionotropic glutamate receptor subunits are differentially regulated in themotoneuronal pools of the rat hypoglossal nucleus in response to axotomy

Unilateral hypoglossal nerve axotomy was used as a model to analyse immunoh istochemically the expression of the GluR1, GluR2, GluR3, and GluR4 glutama te receptor subunits of the alpha -amino-3-hydroxy-5-methyl-4-isoxazoleprop ionate (AMPA) subtype and the NR1 subunit of the N-methyl-D-aspartate (NMDA ) subtype in the different morphofunctional hypoglossal pools from 1 to 45 days postaxotomy. Following hypoglossal nerve axotomy, the percentage of mo toneurons that were GluR1-immunopositive and the labeling intensity for thi s subunit was increased in some hypoglossal pools. Immunolabeling for the G luR2 subunit was undetectable. These results contrast with the unchanged pa ttern for these two subunits after sciatic nerve axotomy previously describ ed. Image analysis showed a significant decrease in the intensity of immuno histochemical labeling for the GluR2/3 and GluR4 subunits in motoneurons, a lthough most motoneurons were still immunopositive for these 2 subunits aft er axotomy. The intensity of immunolabeling for the NR1 subunit was slightl y decreased postlesion, whereas the percentage of NR1-immunopositive motone urons increased. Immunoreactivity returned to basal levels 45 days postlesi on. These findings show that in axotomized hypoglossal motoneurons, i) AMPA and NMDA receptor subunits are still expressed, ii) the composition of the ionotropic glutamate receptor subunit pool is subjected to continuous chan ges during the regeneration process, iii) AMPA receptors, if functional, wo uld have physiological properties different to those in intact motoneurons, and iv) the various AMPA receptor subunits are differentially regulated. T he present results also suggest a faster recovery of basal levels of immuno reactivity for caudally localised groups of motoneurons which could reflect a caudo-rostral sequential functional revovery in the hypoglossal nucleus.