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This study presents an analysis of the distribution and organization pattern of axons originating from the substantia nigra pars reticulata and projecting to the thalamus in monkeys. Biotin dextran amine was iontophoretically injected into different parts of the substantia nigra pars reticulata of monkeys (African green monkeys and macaques). In whatever part of the substantia nigra the injection was made, numerous axonal endings were found to be distributed within different thalamic regions: the ventral anterior nucleus and mainly its magnocellular part, the most ventromedial part of the ventral lateral nucleus, and the mediodorsal and parafascicular nuclei. Moreover, the nigrothalamic projection appeared to be topographically organized. Ten anterogradely labeled axons were reconstructed from serial sections. The axons terminated in three to six terminal fields per axon located in a relatively small portion of only one thalamic region. These terminal fields were variable in size and comprised 4-43 very thin, varicose branches. They consisted either of different axonal branches of the same axon or of different axons and covered 10-31 thalamic cell bodies. These findings demonstrate that the overall morphological organization of individual nigral axons is complex and allows single axons to influence thalamic neurons via a combination of divergent, convergent, and amplification processes., (Copyright 2002 Wiley-Liss, Inc.)

The existence of a dopaminergic innervation of the subthalamic nucleus (STN) has been demonstrated in rats but has remained controversial in primates. The aim of the present study was first to demonstrate the existence of a dopaminergic innervation of the STN in monkeys using tracing methods and then to quantify the loss of dopaminergic fibers in the parkinsonian state in monkeys and humans. Following injection of Fluoro-Gold into the STN of a vervet monkey (Cercopithecus aethiops), retrogradely labeled neurons were found to be scattered in all dopaminergic areas of the mesencephalon. Injection of biotin dextran amine into dopaminergic areas A8 and A9 of two monkeys resulted in anterogradely labeled axons located throughout the whole extent of the STN. Labeled axons that also expressed tyrosine hydroxylase (TH) were reconstructed from serial sections. Some terminal axonal arborizations had profuse branching and occupied much of the STN, and others were restricted to small portions of the nucleus. In TH-immunoreactive sections, numerous sparse, fine, and varicose TH-positive fibers were observed in the STN of normal monkeys and humans. Quantification of these TH-positive fibers revealed a 51% loss of TH-positive fibers in MPTP-intoxicated monkeys and a 65% loss in Parkinson's disease patients compared with their respective controls. These findings demonstrate the existence of a dopaminergic innervation of the STN in primates. The loss of dopaminergic innervation in MPTP-intoxicated monkeys and in Parkinson's disease patients may directly affect the activity of STN neurons and could participate in the hyperactivity of the structure., (Copyright 2000 Wiley-Liss, Inc.)