Abstract
Both vitamin A deficiency and high doses of retinoids can result in learning and memory impairments, depression as well as decreases in cell proliferation, neurogenesis and cell survival. Physical activity enhances hippocampal neurogenesis and can also exert an antidepressant effect. Here we elucidate a putative link between running, retinoid signaling, and neurogenesis in hippocampus. Adult transgenic reporter mice designed to detect ligand‐activated retinoic acid receptors (RAR) or retinoid X receptors (RXR) were used to localize the distribution of activated RAR or RXR at the single‐cell level in the brain. Two months of voluntary wheel‐running induced an increase in hippocampal neurogenesis as indicated by an almost two‐fold increase in doublecortin‐immunoreactive cells. Running activity was correlated with neurogenesis. Under basal conditions a distinct pattern of RAR‐activated cells was detected in the granule cell layer of the dentate gyrus (DG), thalamus, and cerebral cortex layers 3–4 and to a lesser extent in hippocampal pyramidal cell layers CA1–CA3. Running did not change the number of RAR‐activated cells in the DG. There was no correlation between running and RAR activation or between RAR activation and neurogenesis in the DG of hippocampus. Only a few scattered activated retinoid X receptors were found in the DG under basal conditions and after wheel‐running, but RXR was detected in other areas such as in the hilus region of hippocampus and in layer VI of cortex cerebri. RAR agonists affect mood in humans and reduce neurogenesis, learning and memory in animal models. In our study, long‐term running increased neurogenesis but did not alter RAR ligand activation in the DG in individually housed mice. Thus, our data suggest that the effects of exercise on neurogenesis and other plasticity changes in the hippocampal formation are mediated by mechanisms that do not involve retinoid receptor activation. © 2008 Wiley‐Liss, Inc.