BRAIN-STEM DOPAMINERGIC, CHOLINERGIC AND SEROTONINERGIC AFFERENTS TO THE PALLIDUM IN THE SQUIRREL-MONKEY

The retrograde tracer cholera toxin B subunit (CTb) was used in combin ation with immunohistochemistry for tyrosine hydroxylase (TH), calbind in D-28k (CaBP), choline acetyltransferase (ChAT) and 5-hydroxytryptam ine (5-HT) to determine the distribution and relative proportion of br ainstem chemospecific neurons that project to the pallidum in the squi rrel monkey (Saimiri sciureus). Large injections of CTb involving both pallidal segments produce numerous retrogradely labeled neurons in th e substantia nigra (SN), the pedunculopontine tegmental nucleus (PPN) and the dorsal raphe nucleus (DR). Labeled neurons are distributed uni formly in SN with a slight numerical increase at the junction between the pars compacta (SNc) and the ventral tegmental area (VTA). Retrogra dely labeled neurons abound also in PPN, principally in its pars dissi pata, whereas other CTb-labeled cells are scattered throughout the ros trocaudal extent of DR. After CTb injection involving specifically the internal pallidal segment (GPi), the same pattern of cell distributio n is found in SN, PPN and DR, except that the number of retrogradely l abeled cells is lower than after large pallidal complex injections. Ap proximately 70% of all CTb-labeled neurons in SNc-VTA complex display TH immunoreactivity, whereas 20% are immunoreactive for CaBP. About 39 % of all retrogradely labeled neurons in PPN are immunoreactive for Ch AT, whereas approximately 38% of the labeled neurons in DR display 5-H T immunoreactivity. Following CTb injection in the external pallidal s egment (GPe), the number of labeled cells is much smaller than after G Pi injection. The majority of CTb-labeled cells in SNc-VTA complex are located in the lateral half of SNc and approximately 93% of these neu rons display TH immunoreactivity compared to 10% that are immunoreacti ve for CaBP; very few CTb-labeled cells occur in PPN. Retrogradely lab eled cells in DR are located more laterally than those that projects t o the GPi and about 25% of them are immunoreactive for 5-HT. These res ults suggest that, in addition to their action at striatal and/or nigr al levels, the brainstem dopaminergic, cholinergic and serotoninergic neurons influence the output of the primate basal ganglia by acting di rectly upon GPi neurons.