Dopaminergic innervation of the pallidum in the normal state, in MPTP-treated monkeys and in parkinsonian patients

The aim of the present study was to characterize the dopaminergic innervati on of the pallidum in primates (humans and Cercopithecus aethiops). Firstly , in monkeys, biotin dextran amine was injected into dopaminergic areas, an d the anterogradely labelled axons were reconstructed from serial sections and analysed in the pallidum. Secondly, in parkinsonian patients and MPTP-t reated monkeys, the dopaminergic innervation of the pallidum was studied us ing tyrosine hydroxylase-positive fibre quantification. Our study revealed that dopaminergic areas A8 and A9 innervated the two pallidal segments. Ind ividual axonal arborizations displayed a great heterogeneity. Some dopamine rgic axons crossed the pallidum without branching, other axons made small t erminal arborizations in a restricted region of one pallidal segment, where as others developed dense arborizations covering extended areas in the two pallidal segments. This heterogeneous organization suggests that dopamine c ould directly modulate the pallidum using either a point-to-point or a diff use projection pattern. A statistically significant loss of dopaminergic fi bres in the internal (-43%) and external pallidum (-39.6%) of humans, and i n the internal (-54.3%) and external pallidum (-59%) of monkeys was reveale d in parkinsonian states. The consequences of this alteration are still unk nown but it might participate in the triggering of motor symptoms observed in Parkinson's disease.