Quantitative EM study of projection terminal in the rats AV thalamic nucleus. Autoradiographic and degeneration techniques compared.

An attempt was made to determine whether the labeled amino acid tracing technique combined with electron microscope (EM) autoradiography can be used to identify reliably the terminals of different to a cell group in the brain, using the rat's antero-ventral thalamic (AVTh) nucleus as a model. For this purpose in one group of rats [3H]leucine was injected in cingulate cortex, hippocampus and mammilary bodies, respectively, and in another group lesions were made in these structures. With one month autoradiographic exposure time it could be demonstrated that after cortical and hippocampal injections the radioactivity was transported to small terminals on distal dendrites, while after mammillary body injections it was transported to large terminals on proximal dendrites. After 4 months exposure time altogether 12% of the small terminals and 80% of the large ones had been labelled; A comparison with the degeneration findings showed that the EM autoradiography is in many respects more efficient in identifying the various terminals than the EM degeneration technique.