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We have examined rat basal forebrain projections to the mediodorsal thalamic nucleus (MD) by making injections of retrogradely transported fluorescent tracers into the MD. Additionally, in some animals, we also stained sections for glutamate decarboxylase (GAD) by the indirect fluorescent antibody technique. Our results demonstrate that the following basal forebrain areas project to the MD: lateral orbital cortex, agranular insular cortex superficial to claustrum, primary olfactory cortex, diagonal band nuclei, ventral pallidum, and amygdala. A large number of labeled cells are present in the olfactory tubercle, and these cells are almost without exception located in dense GAD-positive ventral pallidal areas rather than in striatal regions of the tubercle. This ventral pallidal projection to the MD strengthens the concept of a ventral striatal-pallidal system in parallel to the classic striatal-pallidal system which projects to the ventral thalamus. These results are also discussed in relationship to the olfactory system.

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