Differential synaptic distribution of AMPA receptor subunits in the ventral posterior and reticular thalamic nuclei of the rat

Although the mechanisms by which the cerebral cortex controls its ascending input are still poorly understood, it is known that cortical control at th e thalamic level is via direct glutamatergic projections to relay nuclei an d to the reticular nucleus. Here we confirm previous light microscopic repo rts of a high expression of the alpha -amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subunit, GluR4, in reticular and ventral pos terior thalamic nuclei of the rat, and moderate staining using an antibody recognizing both GluR2 and GluR3. In contrast only low levels of staining f or GluR2, and barely detectable levels of GluR1 immunoreactivity were obser ved. After injections of biotinylated dextran, electron microscopy revealed that anterogradely-labeled cortical synapses in both thalamic nuclei were small with fewer mitochondria and more densely-packed vesicles than termina ls likely to arise from intrinsic and ascending pathways. We performed post -embedding immunogold to provide quantitative data on the density of AMPA r eceptor subunits at morphologically-defined groups of synapses. We found th at corticothalamic synapses in the reticular thalamic nucleus contain twice as much GluR2/3, and at least three times more GluR4 protein than do intra thalamic synapses. In the ventral posterior nucleus, corticothalamic synaps es contain similar amounts of GluR2/3, but four times more GluR4 than do th ose from ascending afferents. Corticothalamic synapses in reticular nucleus contain slightly more GluR2/3, and three times more GluR4, than those in v entral posterior nucleus. We conclude that enrichment of GluR4 at morphologically-defined cortical sy napses is a feature common to both thalamic nuclei, and those in the reticu lar nucleus express higher levels of AMPA receptors. The rapid kinetics of GluR4-rich AMPA receptors we suggest indicate that cortical descending cont rol may be more temporally precise than previously recognized. (C) 2000 IBR O. Published by Elsevier Science Ltd. All rights reserved.