Overlapping corticostriatal projections from the supplementary motor area and the primary motor cortex in the macaque monkey: an anterograde double labeling study.

The purpose of the present study was to determine if the cortical efferents from homologous body regions of the supplementary motor area (SMA) and the primary motor cortex (MI) project to separate or to overlapping regions in the striatum. In order to investigate the dual corticostriatal projections, we employed an anterograde double labeling paradigm in which two tracers could be simultaneously detected in the same histological section. Prior to the injections, the forelimb representation in the two cortical motor areas was identified by using intracortical microstimulation in four Japanese monkeys (Macaca fuscata). Multiple injections of biotinylated dextran amine (BDA) were made into the forelimb regions of MI and wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) was injected into the arm region of the SMA. In additional animals, the tracers were reversed such that BDA was injected into the SMA and WGA-HRP was injected into the MI. The tissue was processed sequentially using different chromogens in order to visualize both tracers in a single section. We analyzed the distribution of the ipsilateral anterograde label. The striatal labeling from each cortical area basically consisted of a wide band of patchy dense labeling interrupted by lighter labeling. The SMA striatal projections were located mainly within the putamen, distributing from the level of the anterior commissure to the most posterior extent of the putamen. At an intermediate level, the label spread obliquely from the ventrolateral edge of the putamen dorsomedially as far as the lateral edge of the caudate nucleus. The label from the MI was observed in comparable portions of the putamen, although the SMA projections were shifted more anterior and dorsomedial to the MI projections and the heaviest projections from the SMA and the MI were separately located. On the basis of the double anterograde labeling technique, we found considerable overlap mainly in the central portion of the putamen from the SMA and MI forelimb representation. These results suggest that the homologous body regions of the SMA and MI send widespread, and substantially overlapping projections, to portions of the striatum.