Spatial relationship of the striatonigral and mesostriatal pathways: double-label immunocytochemistry for DARPP-32 and tyrosine hydroxylase.

Antibodies to tyrosine hydroxylase and DARPP-32 were used to examine the spatial arrangement between mesostriatal dopamine projections and the reciprocal pathway from DARPP-32-containing neurons in the basal forebrain. Use of a double-labeling immunocytochemical procedure demonstrated that the mesostriatal and striatonigral pathways run in close proximity throughout the rostral mesencephalon and basal forebrain. The majority of descending axons immunoreactive for DARPP-32 appear to originate in the striatum, including the nucleus accumbens, and run through the internal capsule to innervate the globus pallidus, entopeduncular nucleus, and all subdivisions of the substantia nigra. The ventral tegmental area is sparsely invested with DARPP-32-immunoreactive axons. At all levels, there are also fascicles of DARPP-32-containing fibers which run ventromedial to the internal capsule in the medial forebrain bundle, and which are coextensive with ascending axons of the mesencephalic dopamine the internal capsule in the medial forebrain bundle, and which are coextensive with ascending axons of the mesencephalic dopamine cell groups. Tyrosine hydroxylase-immunoreactive axons are coextensive with DARPP-32-immunoreactive axons in the internal capsule entopeduncular nucleus, and globus pallidus, as well as much of the remainder of the basal forebrain. Although the main source of descending DARPP-32 immunoreactive axons would appear to be the striatum, other possible sources are also discussed.