Differential postnatal development of catecholamine and serotonin inputs to identified neurons in prefrontal cortex of rhesus monkey

The monoaminergic innervation of cerebral cortex has long been implicated i n its development. Methods now exist to examine catecholamine and serotonin inputs to identified neurons in the cerebral cortex. We have quantified su ch inputs on pyramidal and nonpyramidal cells in prefrontal cortex of rhesu s monkeys ranging in age from 2 weeks to 10 years. Individual layer III neu rons were filled with Lucifer yellow and double-immunostained with axons co ntaining either tyrosine hydroxylase (TH) or 5-hydroxytryptamine (5-HT). Th e filled cells were reconstructed, and putative appositions between the axo ns and dendritic spines and shafts were quantified at high magnification us ing light microscopy. The density of catecholamine appositions on pyramidal neurons matures slowl y, reaching only half the adult level by 6 months of age and thereafter ris ing gradually to adult levels by 2 years of age. By contrast, the density o f serotonin appositions on pyramidal cells reaches the adult level before t he second week after birth. The average adult pyramidal neuron in layer III of area 9m receives three times stronger input from catecholaminergic than from serotoninergic axons. The overall density of both inputs to interneur ons does not appear to change during postnatal development. Selective chang es in the TH innervation of pyramidal cells against a backdrop of constant TH innervation of interneurons suggest that the balance between excitation and inhibition may change developmentally in the prefrontal cortex. By cont rast, 5-HT innervation of both types of neurons remains relatively constant over the age range studied.