PEDUNCULOPONTINE NUCLEUS IN THE SQUIRREL-MONKEY - CHOLINERGIC AND GLUTAMATERGIC PROJECTIONS TO THE SUBSTANTIA-NIGRA

The distribution and chemospecificity of the pedunculonigral neurons h ave been studied in squirrel monkeys (Saimiri sciureus) with cholera t oxin subunit B (CTb) and fluorogold (FG) as retrograde tracers combine d with immunohistochemistry for choline acetyltransferase (ChAT), glut amate, and the calcium binding protein calbindin D-28k. The injection of either CTb or FG into the substantia nigra produces prominent retro grade cell labeling in the mesopontine tegmentum. Labeled neurons are particularly numerous at the level of the decussation of the superior cerebellar peduncle, where they abound principally in the pars dissipa ta of the pedunculopontine nucleus (PPN). A significant proportion of retrogradely labeled neurons in the PPN display ChAT immunoreactivity. Within the entire PPN, approximately 25% of the retrogradely labeled neurons express ChAT immunoreactivity, but proportions of doubly label ed neurons are about 35%, 25%, and 15% in the rostral, middle, and cau dal thirds of the PPN, respectively. These doubly labeled neurons are scattered among numerous retrogradely labeled neurons that are ChAT-ne gative and whose number increases along the rostrocaudal extent of the PPN. Several retrogradely labeled neurons in the PPN display glutamat e immunoreactivity, but very few express calbindin. This study provide s the first direct evidence for the involvement of cholinergic and glu tamatergic neurons in the pedunculonigral projection in primates. Furt hermore, the fact that some neurons of the PPN display both ChAT and g lutamate immunoreactivity indicates that single neurons in the mesopon tine tegmentum may exert a two-fold effect upon dopaminergic neurons o f the substantia nigra. This dual cholinergic and glutamatergic pedunc ulonigral projection may play a crucial role in the functional organiz ation of primate basal ganglia. (C) 1994 Wiley-Liss, Inc.