Low-affinity nerve growth factor receptor (p75NGFR) was analyzed in the adult monkey and human cerebral cortex and hippocampus by light and electron microscopic immunohistochemistry using a mouse monoclonal antibody raised against human p75NGFR. In the monkey only, the morphology and laminar and areal distribution of the p75NGFR-immunoreactive fibers were further compared with distribution of the ACh-synthesizing enzyme ChAT. We found that the p75NGFR fiber distribution is remarkably dense throughout the cerebral neocortex and highly similar in homologous cortical areas of monkey and human. In both species, p75NGFR-immunoreactive fibers are contracted in layers I, II, and the superficial portions of layer III in all frontal, parietal, temporal, and occipital areas examined. In the primary visual and somatosensory cortices, dense p75NGFR fiber plexuses are additionally found in layer IV. In the hippocampal formation, this fiber system is most dense in the CA2 and CA3 subfields as well as in the molecular and polymorphic layers of the dentate gyrus. The distribution and characteristics of the p75NGFR-positive fibers in the primates correspond almost exactly to those of the cholinergic fiber system as revealed by ChAT immunohistochemistry in the present study as well as in the literature (Lewis, 1991; Mesulam et al., 1992; Voytko et al., 1992). Electron microscopy revealed that p75NGFR is associated mainly with axonal membranes and portions of the axoplasm in labeled fibers. Although the function of p75NGFR is still a matter of debate, its widespread distribution opens up new questions regarding its physiological role in the large and functionally differentiated cerebral cortex and hippocampus of monkey and human.

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