Direct projections from the centre median‐parafascicular complex to the subthalamic nucleus in the cat and rat

Single injections of a mixture of [ 3 H] leucine and L‐[ 3 H] made into the centre median‐parafascicular complex (CM‐Pf) of the cat. The ipsilateral subthalamic nucleus (STN) was most heavily labeled at its ros‐tral pole and moderately in the ventral and ventromedial portions of its rostral third, At middle subthalamic levels, label was more sparse and disappeared over the caudal third of the nucleus. Labeled fibers appeared to outline the borders of the STN throughout its rostral half. Control in‐jections of isotope into several other thalamic andmesodiencephalic regions produced no terminal labeling in the cat STN. After horseradish peroxidase (HRP) injections into the cat STN, a small number of the CM‐Pf neurons were labeled retrogradely. The labeled neurons were scattered diffusely in the CM‐Pf and were not obviously distinguishable from other unlabeled neurons in the nucleus. Single injections of L‐[ 3 H] leucine were made into the CM‐Pf of the rat. Anterograde labeling was seen in the rostral half of the ipsilateral STN. Heavy labeling was present throughout the most rostral STN, became less prominent more caudally, and was absent from the caudal half of the STN. HRP injections in the rat CM‐Pf produced a distribution of anterograde labeling in the STN similar to the isotope injections. It was concluded that a small number of scattered CM‐Pf neurons project primarily to rostral region of the STN. Thus, the CM‐Pf is in a unique position to regulate the basal ganglia by way of a newly established thalamosubthal‐amic pathway as well as a widespread, conspicuous thalamostriate pathway in those two mammalian species.