The midbrain dopaminergic cell groups in the baboon Papio ursinus

The present study evaluates the cytoarchitecture of midbrain dopaminergic r egions in baboons using similar methodology to that recently applied to com pare humans and rats. This information is relevant for the interpretation o f nonhuman primate models of Parkinson's disease (PD). The midbrains of fou r alpha male baboons were serially sectioned into 10 evenly spaced series o f 50 mu m sections. Series were stained with either cresyl violet or immuno histochemically reacted for tyrosine hydroxylase, substance P, calbindin-D- 28k, or parvalbumin. The organization of dopaminergic cell groups and the d istribution of proteins within these groups were found to be very similar t o that previously described in humans [McRitchie et al,, J. Comp. Neurol, 3 64:121-150; 1996], Dorsal and ventral tiers of the A9 substantia nigra (SN) pars compacta and all divisions of the A8 and A10 cell groups were identif ied revealing a high degree of homology in the arrangement of chemically di stinct midbrain neurons between primates. The major difference between the organization of human and baboon midbrain dopaminergic neurons is the anter oposterior extent of the dense cell clusters within the SN pars compacta, I n baboons the dorsomedial cell cluster is absent at posterior levels. The v entral tier cell clusters, which are targeted by PD in humans, are restrict ed to the posterior and ventral regions of the SN pars compacta of the babo on. In humans these cell clusters are found throughout the rostrocaudal ext ent of the SN, These ventral cell clusters have been previously shown to ha ve reciprocal connections with sensorimotor regions of the putamen. (C) 199 9 Elsevier Science Inc.