The orbital and medial prefrontal circuit through the primate basal ganglia.

The ventral striatum is considered an interface between limbic and motor systems. We followed the orbital and medial prefrontal circuit through the monkey basal ganglia by analyzing the projection from this cortical area to the ventral striatum and the representation of orbitofrontal cortex via the striatum, in the globus pallidus and substantia nigra. Following injections of Lucifer yellow and horse radish peroxidase into the medial ventral striatum, there is a very densely labeled distribution of cells in areas 13a and 13b, primarily in layers V and VI, and in medial prefrontal areas 32 and 25. Injections into the shell of the nucleus accumbens labeled primarily areas 25 and 32. The reaction product in the globus pallidus and the substantia nigra supports previous studies demonstrating that efferent projections from the ventral striatum are represented topographically in the ventral pallidum and nontopographically in the substantia nigra, pars compacta. Tritiated amino acid or PHA-L tracer injections into orbitofrontal cortex produce dense patches of terminal labeling along the medial edge of the caudate nucleus and the dorsal part of the nucleus accumbens. These results demonstrate that the orbital prefrontal cortex projects primarily to the medial edge of the ventral striatum and to the core of the nucleus accumbens. The arrangement of terminals in the globus pallidus and substantia nigra show two different patterns. Thus, the orbital and medial prefrontal cortex is represented in a confined region of the globus pallidus but throughout an extensive area of the dorsal substantia nigra. Terminals are extensive throughout the region of the dopaminergic neurons, suggesting that this input may influence a wide area of both the striatum and frontal cortex.