Telencephalic projections of the A8 dopamine cell group.

The telencephalic projections of the A8 dopamine cell group of the rat were assessed using both anterograde and combined retrograde-immunohistochemical methods. The projections of the A8 neurons onto the forebrain were more extensive than hitherto realized, and encompassed striatal, limbic, and allocortical regions. The A8 neurons were shown to contribute to the dopaminergic innervation of the striatum, nucleus accumbens, olfactory tubercles, amygdala, and bed nucleus of the stria terminalis, and also innervate the pyriform and entorhinal cortices. In addition, projections within the midbrain were observed, and suggested that there may be direct interconnections between the dopaminergic neurons of the A8, A9, and A10 cell groups. These data therefore suggest that the A8 dopamine cell group is uniquely situated to modulate functional activity within both nigrostriatal and mesocorticolimbic regions, and further suggests that heterogeneities of the midbrain dopamine neurons are embedded within a larger homogeneous mesotelencephalic dopamine system.