Dopaminergic cell group A8 in the monkey: anatomical organization and projections to the striatum.

The first part of the study was a quantitative analysis of the distribution of A8 neurons compared with that of A9 and A10 neurons by means of tyrosine hydroxylase and calbindin-D(28K) immunohistochemistry and image analysis in monkeys. Then the striatal projection of A8 neurons was studied using retrograde and anterograde tracing methods. It was compared with that originating in cell groups A9 and A10 by performing injections of the retrograde tracer wheat germ agglutinin conjugated to horseradish peroxidase into different regions of the striatum. Ten percent of all mesencephalic dopaminergic neurons are located in cell group A8. This cell group, along with A10 and the dorsal part of A9, constitutes the dorsal tier, which accounts for 28% of mesencephalic dopaminergic neurons. Double-staining experiments showed that the neurons located in the dorsal tier were calbindin positive, whereas those from the ventral tier were not. In terms of anatomical projection, the dorsal tier mainly projects to the ventral part of the associative striatum, with preferential projections of A8 neurons to the ventrocaudal putamen, of A10 neurons to the nucleus accumbens, and of dorsal A9 neurons to both. Conversely, the main targets of the ventral tier of mesencephalic neurons (ventral part of A9) are the sensorimotor putamen and the associative caudate nucleus. In conclusion, each mesencephalic cell group projects primarily to one specific striatal region but also participates, albeit to a lesser extent, in the innervation of all the remaining striatal parts.