The 'marginal division': a new subdivision in the neostriatum of the rat.

Using a combination of anterograde and retrograde (Phaseolus vulgaris leucoagglutinin; PHA-L and wheat germ agglutinin conjugated horseradish peroxidase; WGA-HRP) tract-tracing methods and histochemical techniques, a new subdivision of the neostriatum, the marginal division, has been found in the rat brain. The marginal division is approximately 120 microns wide and is located at the caudal extent of the neostriatum and surrounds the rostral edge of the globus pallidus. The neuronal somata of the marginal division are mostly fusiform in shape, with their long axes running parallel to the border between the striatum and the globus pallidus. Histochemically, the marginal division is lighter in AChE staining, is more densely filled with Met-enkephalin-immunoreactive terminals, and has fewer choline acetyltransferase (ChAT)-immunoreactive neurons than does the rest of the neostriatum. Injections of PHA-L or WGA-HRP demonstrated that the projections of the marginal division differ from those of the main body of the striatum. The striatopallidal projection from the marginal division terminates in the caudal-most part of the globus pallidus which is rich in cholinergic neurons. In contrast, the projection from the main region of the neostriatum terminates in two bands in the globus pallidus, both of which are rostral to the area of termination of the fibres from the marginal division. The striatonigral fibres from the marginal division terminate in the caudal part of the substantia nigra pars reticulata whereas the rest of neostriatum projects to a more rostral region. Based on its cellular morphology, immunohistochemistry and projection pattern, we conclude that the marginal division of the striatum is a distinct subdivision of the neostriatum.