Immunocytochemical localization of the AMPA receptor subunits in the mesencephalic trigeminal nucleus of the rat

The mesencephalic trigeminal nucleus is composed of large (35-50 mu m) pseu do-unipolar neurons. Closely associated with them are small (< 20 mu m) mul tipolar neurons. An unique peculiarity of the pseudo-unipolar perikarya is that they receive synaptic input from various sources, which sets them apar t from the dorsal root and cranial nerves sensory ganglia neurons. Whereas glutamate is the best neurotransmitter candidate in pseudo-unipolar neurons , glutamatergic input into them has not yet been reported. AMPA glutamate r eceptors are implicated in fast excitatory glutamatergic synaptic transmiss ion. They have been localized ultrastructurally at postsynaptic sites. This study demonstrates that the pseudo-unipolar neurons of the mesencephalic t rigeminal nucleus express AMPA glutamate receptor subunits, which indicates that these neurons receive glutamatergic input. Serial sections from the r ostral pens and midbrain of Sprague-Dawley rats were immunostained with ant ibodies against C-terminus of AMPA receptor subunits: GluR1, GluR2/3, and G luR4. The immunoreaction was visualized with avidin-biotin-peroxidase/DAB f or light and electron microscopy. With GluR1 antibody only the smallest mul tipolar neurons were recognized as immunopositive within the mesencephalic trigeminal nucleus. GluR2/3 stained the pseudo-unipolar neurons intensely w ithin the entire rostro-caudal extent of the nucleus. In addition the forme r antibody stained small multipolar neurons within the mesencephalic trigem inal nucleus, though with somewhat larger dimensions than those immunoreact ive for GluR1. Whereas the overall staining with GluR4 antibody was scant, those pseudo-unipolar neurons that were stained, were strongly stained. Fur thermore, a considerable number of microglial cells within and surrounding the mesencephalic trigeminal nucleus displayed very intense immunoreactivit y for GluR4. These results are discussed in the light of the glutamate rece ptor subunit composition.