The serotonin and norepinephrine innervation of primary visual cortex in the cynomolgus monkey (Macaca fascicularis)

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Abstract
The morphology and laminar distribution of norepinephrine (NE) and serotonin (5‐hydroxytryptamine, 5‐HT) axons in the primary visual cortex of cynomolgus monkeys (Macaca fascicularis) have been analyzed by immunocytochemistry with antibodies directed against dopamine‐beta‐hydroxylase (DBH) and against 5‐HT. The NE and 5‐HT innervation of primary visual cortex (area 17) in the monkey exhibit highly differentiated laminar patterns. Both of these monoamine axonal systems form tangentially continuous laminar bands that differ in density, morphology, and predominant orientation of constituent fibers. Serotonin axons, present in all cortical layers, form two especially prominent, dense bands of arborizing fibers; one extending from midlayer III through IVC‐alpha and the other from VA through VI. NE axons within cynomolgus visual cortex are markedly less dense than 5‐HT axons, and laminar differences in NE density are less prominent. NE axons form two broad bands of moderate density extending through deep and superficial cortical layers, separated by layer IVC, which is conspicuously poor in NE fibers. The laminar complementarity of 5‐HT and NE innervation that is a notable feature of the squirrel monkey visual cortex is not present in cynomolgus monkey; in this Old World monkey NE and 5‐HT axons show considerable overlap such that the same cortical layers appear to be innervated by both neurotransmitters. By virtue of their laminar distribution, 5‐HT and NE axons may each have a selective influence on the intrinsic circuitry of primate cerebral cortex, and, for 5‐HT in particular, the specific cellular targets of this influence are likely to differ in New and Old World monkeys.