Expression patterns of immature neuronal markers PSA‐NCAM, CRMP‐4 and NeuroD in the hippocampus of young adult and aged rodents

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Abstract
Neurogenesis is known to continue in the adult hippocampus of mammals, including humans. The present experiments were undertaken to examine the nature of developing neurons generated in the dentate gyrus of young and older rodents using immature neuronal markers such as highly polysialylated neural cell adhesion molecules (PSA‐NCAM), collapsin response‐mediated protein‐4 (CRMP‐4) and NeuroD. Most PSA‐expressing cells are simultaneously positive for CRMP‐4 and NeuroD in young rats. More than half of the PSA‐positive cells were also positive for mature neuronal markers such as NeuN and MAP2, although the intensity of the immunoreactivities was relatively weak. BrdU analysis revealed that CRMP‐4 is expressed for a longer period than PSA in BrdU‐labeled neurons. The number of immature neurons expressing PSA, NeuroD or CRMP‐4 decreased in older rodents, but no qualitative difference was found in the expression patterns of these molecular markers between young and older rodents. These results suggest not only that immunohistochemistry, using a combination of these immature and mature neuronal markers, is helpful for clarifying the developmental state of newly generated neurons, but also that newly generated neurons in young adult and older rodents have similar properties.