The distribution of tyrosine hydroxylase-immunoreactive fibers in primate neocortex is widespread but regionally specific.
An antiserum directed against tyrosine hydroxylase (TH), an enzyme involved in dopamine and norepinephrine synthesis, was used to visualize axons immunohistochemically in monkey neocortex. Labeled fibers were distributed throughout the entire neocortex, but they had striking patterns of regional and laminar specialization. For example, primary motor cortex contained the greatest density of TH-labeled fibers, whereas primary sensory regions were sparsely innervated. Marked heterogeneity of fiber density was also present among the association regions of the frontal, parietal, and temporal lobes. In addition, the laminar pattern of innervation in a given region was correlated with its fiber density. Sparsely innervated regions had labeled fibers only in layer I and sometimes layer VI. In regions of intermediate density, labeled fibers tended to be located in layers I-superficial III and layers V-VI, whereas in densely innervated motor cortex TH-immunoreactive fibers were present in all cortical layers. Comparison of these distribution patterns with those produced by an antiserum directed against dopamine-beta-hydroxylase (DBH), a specific marker of neocortical noradrenergic axons, revealed marked differences. DBH-immunoreactive fibers were observed in some cortical locations where few or no TH-labeled fibers were present. In other regions, the density of TH-immunoreactive processes far exceeded that of DBH-labeled fibers. These findings indicate that nearly all of the immunoreactive fibers revealed by this anti-TH antiserum are dopaminergic. This interpretation was further supported by lesions of the ascending noradrenergic fibers in the brain stem, which reduced DBH immunoreactivity, but not TH immunoreactivity, in neocortex. The distinctive innervation patterns of TH-immunoreactive fibers suggest a functional specialization of the dopaminergic projections to primate neocortex.