Running wild: neurogenesis in the hippocampus across the lifespan in wild and laboratory-bred Norway rats.

The dogmatic view that new neurons are not produced in the adult mammalian brain has been overturned in light of mounting evidence that neurogenesis continues to occur within two neurogenic niches, the subventricular zone and the hippocampus. In mammals, new neurons are incorporated into the hippocampus throughout life and are influenced by environmental and genetic factors. Most studies use captive-bred animals, and no previous studies have examined neurogenesis in free-living rats despite the common use of laboratory rats. In particular, exercise upregulates neurogenesis in the hippocampus and exercise levels would certainly differ between wild and captive populations. Therefore, it is unclear whether results from captive populations can be generalized to natural populations or reflect variations from an artificial and inappropriate "baseline" level. To address this, we compared levels of cell proliferation and the number of immature neurons (using the endogenous markers Ki67 and doublecortin, respectively) in captured wild juvenile and adult Norway rats to three captive strains (Sprague-Dawley, Long-Evans, and Brown Norway) of the same species. Here, we show that the level of cell proliferation and young immature neuron survival in the dentate gyrus of juvenile wild rats is significantly higher than in Sprague-Dawley rats, but not Long-Evans or Brown Norway rats. However, cell proliferation and the number of immature neurons in the hippocampus of adult wild rats are within the normal range of captive-bred rats at all adult ages examined. This finding is surprising given the dissimilar environments, including stressors and opportunities for exercise, encountered by each population. Copyright 2008 Wiley-Liss, Inc.