Antisera directed against human dopamine-ß-hydroxylase and against serotonin were used to characterize the noradrenergic (NA) and serotoninergic (5-HT) innervation of several cortical and subcortical visual areas in squirrel monkey (Saimiri sciureus) and cynomolgus monkey (Macaca fascicularis). Few species differences were observed for either monoamine. Cortical areas 17 and 18, as well as visual areas in the temporal and parietal lobe were found to exhibit regional specialization of both 5-HT and NA innervation. Precisely at the border between areas 17 and 18, the laminar innervation patterns and density characteristic of NA fibers in area 17 (Morrison et al., '82a; Kosofsky et al., '84) shift so that layer IV of area 18 contains more fibers than layer IV of area 17, and the overall density of fibers in area 18 is higher. For 5-HT, the highly laminated patterns characteristic of area 17 (Morrison et al., '82a; Kosofsky et al., '84) also observe this cytoarchitectonic boundary. Fibers in area 18 are more evenly distributed across laminae, and the overall density of fibers decreases. The visual region of the inferotemporal cortex was found to be very lightly innervated by NA fibers and very densely innervated by 5-HT fibers. Area 7 of the parietal lobule was more densely innervated by NA fibers, and less densely innervated by 5-HT fibers, than any other visual cortical region examined.
The visual thalamic nuclei exhibited even greater regional differences in the density of NA innervation. The lateral geniculate nucleus was found to be virtually devoid of NA fibers, while the pulvinar-lateral posterior complex was densely innervated. The density of 5-HT fibers was more uniform across thalamic visual nuclei. The lateral geniculate, pulvinar, and lateral posterior nuclei all exhibit a moderate to high density of immunoreactive fibers. In the mesencephalon, the superficial layers of the superior colliculus were found to be densely innervated by NA fibers, whereas 5-HT fibers were most dense in the intermediate layers.
These patterns of innervation indicate that, in these primate species, functionally related visual regions share common and distinguishable densities of NA innervation. Specifically, tecto-pulvinar-juxtastriate structures are more densely innervated than geniculo-striate and inferotemporal structures. These relationships suggest that, within the visual system, NA fibers preferentially innervate the regions involved in spatial analysis and visuomotor response rather than those involved in feature extraction and pattern analysis. Although there are some examples of complementary innervation of cortical visual areas by 5-HT and NA fibers, the 5-HT innervation of visual areas at the mesencephalic, diencephalic, and telencephalic levels was of high density and exhibited less specialization than was evident for NA innervation.