Marked species and age‐dependent differences in cell proliferation and neurogenesis in the hippocampus of wild‐living rodents

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Abstract
Variations in the extent of adult neurogenesis and natural and experimental factors controlling it have been described in laboratory animals. The wide range of variation seen even within a species, the mouse, raises the question as to which rates of neurogenesis can be expected in natural populations. Answering this question is important to evaluate the functional significance of adult neurogenesis under natural conditions and to define the factors controlling it. To address this issue, we investigated four species of wild‐living rodents and outbred laboratory mice using markers for proliferating cells, Ki‐67, and developing neurons, doublecortin and NeuroD. We found about four times as many Ki‐67‐positive cells per mm3 granule cell layer in two wood mouse species (Muridae; Apodemus spp.) than in bank and pine voles (Arvicolidae; Clethrionomys glareolus and Microtus subterraneus). Laboratory mice show proliferation rates between wood mice and voles. Markers for developing neurons, NeuroD and doublecortin, reflect the findings of proliferation activity. Hippocampal cell proliferation decreases dramatically with age in wild‐living species. The onset of the downregulation varies among species. It occurs late in the life span of the yellow‐necked wood mouse. In aged animals, the number of proliferating cells per mm3 granule cell layer is reduced to 19% of the adult value. Downregulation occurs early in pine voles, in which cell proliferation in adult animals is reduced to 33% of juvenile values. Proliferation and age‐dependent changes along the deep border of the alveus and angular bundle follow those of the dentate gyrus. We conclude that cell proliferation and neurogenesis in the dentate gyrus vary significantly among wild‐living rodents, and that they are downregulated with age, but at species‐specific time points.