topographical organization of efferent projection from rat substantia nigra to the mediodorsal thalamic, by hrp tracer study

introduction: nigrothalamocortical tract is one of the main outputs of the basal nuclei, but its role in motor disturbances is vauge, because there is little informations on its connection structure. some electrophysiological and pharmacological surveys report that none-dopaminergic outputs originate from reticular part of substantia nigra and end at thalamus. in this study the topographical connection of substantia nigra with thalamus was studied using hrp tracer. materials and methods: a tracer, stereotaxically was injected into the md nucleus of thalamus of 25 rats. the animals were perfused transcardially 48-72 hours later, and the brain tissue was fixed. 40-50 micrometer sections were prepared from diencephalon and midbrain. following enzymatic reactions, the sections were stained by tetramethyl-benzidin method. results: the light microscopic study showed that there is a high concentration of neurons which were projected to the md nucleus in the rostro lateral parts of pars reticulata (snr) and the number of labeled cells decrease in the caudal parts. other labeled neurons are located at the border of snc, snr and vta, specially close to the passage of iii cranial nerve. in general, the size of neurons was small, medium; and multipolar in shape labeled. we didn't see any labeled cell in the substantia nigra opposite to injection site. conclusion: our findings shows that none-dopaminergic efferents which originate from substantia nigra nuclei and terminate in md nucleus. it seems that these fibers not only affect motor mechanisms, but also influence the limbic system and sensory functions.