The cortico-pallidal projection in the rat: an anterograde tracing study with biotinylated dextran amine.

The projections from the frontal cortex to the pallidum (e.g. the globus pallidus and the entopeduncular nucleus) were studied in the rat using the biotinylated dextran amine (BDA) anterograde tracing method. Injections of BDA into the precentral medial and precentral lateral cortices consistently yielded labeling of thin fibers with small boutons in the ipsilateral pallidum although not in the contralateral pallidum. Electron microscopic observation revealed that BDA-labeled boutons form asymmetric synapses mainly with small dendrites and dendritic spines. When the injection sites included the insular, orbital, and prelimbic cortices, the labeled fibers and boutons were also seen in the ventral pallidum, the basal nucleus of Meynert, and the substantia innominata. Injections of BDA in the parietal and occipital cortices resulted in either very few labeling or no labeled boutons in the pallidum. The cortico-pallidal projections were topographically organized. The density of labeled boutons in the globus pallidus was found to be approximately 10% of the density of cortical terminals in the neostriatum.