NEUROFILAMENT PROTEIN IS DIFFERENTIALLY DISTRIBUTED IN SUBPOPULATIONSOF CORTICOCORTICAL PROJECTION NEURONS IN THE MACAQUE MONKEY VISUAL PATHWAYS

Previous studies of the primate cerebral cortex have shown that neurof ilament protein is present in pyramidal neuron subpopulations displayi ng specific regional and laminar distribution patterns. In order to ch aracterize further the neurochemical phenotype of the neurons furnishi ng feedforward and feedback pathways in the visual cortex of the macaq ue monkey, we performed an analysis of the distribution of neurofilame nt protein in corticocortical projection neurons in areas V1, V2, V3, V3A, V4, and MT. Injections of the retrogradely transported dyes Fast Blue and Diamidino Yellow were placed within areas V4 and MT, or in ar eas V1 and V2, in 14 adult rhesus monkeys, and the brains of these ani mals were processed for immunohistochemistry with an antibody to nonph osphorylated epitopes of the medium and heavy molecular weight subunit s of the neurofilament protein. Overall, there was a higher proportion of neurons projecting from areas V1, V2, V3, and V3A to area MT that were neurofilament protein-immunoreactive (57-100%), than to area V4 ( 25-36%). Tn contrast, feedback projections from areas MT, V4, and V3 e xhibited a more consistent proportion of neurofilament protein-contain ing neurons (70-80%), regardless of their target areas (V1 or V2). In addition, the vast majority of feedback neurons projecting to areas V1 and V2 were located in layers V and V1 in areas V4 and MT, while they were observed in both supragranular and infragranular layers in area V3. The laminar distribution of feedforward projecting neurons was het erogeneous. In area V1, Meynert and layer IVB cells were found to proj ect to area MT, while neurons projecting to area V4 were particularly dense in layer III within the foveal representation. In area V2, almos t all neurons projecting to areas MT or V4 were located in layer III, whereas they were found in both layers II-III and V-VI in areas V3 and V3A. These results suggest that neurofilament protein identifies part icular subpopulations of corticocortically projecting neurons with dis tinct regional and laminar distribution in the monkey visual system. I t is possible that the preferential distribution of neurofilament prot ein within feedforward connections to area MT and all feedback project ions is related to other distinctive properties of these corticocortic al projection neurons. (C) 1996 Wiley-Liss, Inc.