Distribution of cholinergic pallidal neurons in the squirrel monkey (Saimiri sciureus) based upon choline acetyltransferase.

The distribution of cells immunoreactive to choline acetyltransferase (ChAT-IR) in, and around the globus pallidus were studied in the squirrel monkey. Intrinsic pallidal ChAT-IR neurons in the globus pallidus were most numerous in ventrocaudal regions of the lateral pallidal segment (LPS) and in the oral pole of the medial pallidal segment (MPS). Smaller numbers of ChAT-positive cells were seen in portions of the medullary laminae of the pallidum. Computer measurements of somal areas of ChAT-IR cells in the globus pallidus, substantia innominata and putamen were made. Morphological features and somal areas of ChAT-IR cells in the globus pallidus and in the Ch4 group of the substantia innominata were strikingly similar. Cholinergic pallidal neurons appear to be part of the Ch4 cell group and have similar widespread cortical projections. The smaller cholinergic neurons in the striatum are considered to be intrinsic neurons which primarily act upon spiny striatal projection neurons. The possible local interaction of pallidal cholinergic neurons upon GABAergic neurons is unknown.