Immunoelectron microscopic study of glutamate inputs from the retrosplenial granular cortex to identified thalamocortical projection neurons in the anterior thalamus of the rat

We have carried out an ultrastructural study to determine the characteristi cs and distribution of glutamate-containing constituents of the anterodorsa l (AD) and anteroventral (AV) thalamic nuclei in adult rats. We used a poly clonal antibody to glutamate and a postembedding immunogold detection metho d in animals in which the neurons of AD/AV projecting to the cortex had bee n retrogradely labelled and the terminals of corticothalamic afferents ante rogradely labelled by injection of cholera toxin-horseradish peroxidase (HR P) into the retrosplenial granular cortex. The heaviest immunogold labellin g was over axon terminals 0.42 to 2.2 mu m in diameter containing round syn aptic vesicles and establishing Gray type 1 (asymmetric) synaptic contact ( type 1 terminals) on HRP-labelled or non-labelled dendrites. Mean gold part icle densities over such terminals were 3-4 times higher than the densities over the dendrites to which they were presynaptic and 5-6 times higher tha n over terminals establishing Gray type 2 (symmetric) synaptic contacts (ty pe 2 terminals). Gold particle densities over neuronal cell bodies and dend rites and over a subpopulation of myelinated axons were intermediate betwee n the densities over type 1 and type 2 terminals. In adjacent serial sectio ns immunoreacted for gamma aminobutyric acid, type 2 terminals were heavily immunolabelled whereas type 1 terminals and other profiles with moderate g old particle densities after glutamate immunoreaction displayed very low la belling. A subpopulation of small type 1 axon terminals (up to 1 mu m diame ter) contained HRP reaction product identifying them as cortical in origin; they contacted small dendritic profiles (most <1 mu m diameter) many of wh ich also contained HRP reaction product. We conclude that terminals of the corticothalamic projection from retrosplenial granular cortex to AD/AV are glutamatergic and innervate predominantly distal dendrites of thalamocortic al projection neurons. All rights reserved (C) 1999 Elsevier Science Inc.