An electron microscopic tracer study of the projections from entopeduncular nucleus to the ventrolateral nucleus of the rat.

The entopeduncular (EP) nucleus is considered to be the major outflow nucleus of the basal ganglia (BG). The anterograde tracer dextran biotin was injected into EP to investigate its connections with the thalamus. Terminals from EP were found in the ipsilateral ventroanterior-ventrolateral (VAL) and ventromedial thalamic nuclei (VM), lateral habenular and centromedian-parafascicular complex. Electron microscopy of the terminals in VAL/VM revealed densely labelled small synapses that had prominent post-synaptic densities and round vesicles. Synapses were observed onto dendrites and dendritic spines but not onto other synapses or somas. No symmetrical synapses with pleomorphic vesicles were labelled. It is concluded that the synaptic contacts between EP and thalamus are Gray type 1. These findings have implications regarding current theories of BG pathophysiology.