Differential laminar distribution of tyrosine hydroxylase-immunoreactive axons in infant and adult monkey prefrontal cortex.

Immunohistochemical techniques were used to evaluate the laminar distribution of tyrosine hydroxylase (TH)-immunoreactive axons in area 9 of infant and adult rhesus monkey prefrontal cortex. In neonatal animals, TH-positive axons had a bilaminar location in the superficial and deep cortical layers, whereas in the adults, labeled fibers were more evenly distributed across all layers. These differences reflected the fact that fiber density in the superficial layers was over 35% greater in neonates than in adults, but in the middle cortical layers, fiber density was over 100% greater in adults than in neonates. The most striking changes in fiber distribution appeared to occur during the first few months of life. These findings may reveal differences in the role of dopamine in the regulation of prefrontal cortical function in neonatal and adult monkeys.