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The exercise-induced enhancement of learning and memory, and its ability to slow age-related cognitive decline in humans led us to investigate whether running stimulates periventricular (PVR) neural stem cells (NSCs) in aging mice, thereby augmenting the regenerative capacity of the brain. To establish a benchmark of normal aging on endogenous NSCs, we harvested the PVR from serial vibratome sections through the lateral ventricles of juvenile (6-8 weeks), 6-, 12-, 18-, and 24-month-old mice, culturing the cells in the neural colony-forming cell assay. A significant decline in NSC frequency was apparent by 6 months (∼40%), ultimately resulting in a ∼90% reduction by 24 months. Concurrent with this decline was a progressive loss in regenerative capacity, as reflected by an incomplete repopulation of neurosphere-forming cells following gamma cell irradiation-induced depletion of the PVR. However, voluntary exercise (i.e., 21 days of running) significantly increased NSC frequency in mice ≥ 18 months of age, augmenting the regeneration of irradiation-ablated periventricular cells and restoring NSC numbers to youthful levels. Importantly, and consistent with the demonstrated ability of growth hormone (GH) to increase NSC proliferation, and the elevated secretion of GH during exercise, exercise failed to stimulate NSCs in GH receptor-null mice. These findings now provide a novel basis for understanding the ability of exercise to delay the onset and rate of decline in neurodegenerative conditions not typically associated with the hippocampus and suggest that the GH-dependent activation of endogenous NSCs may be effective in reversing or preventing age-related neurodegeneration in humans. Disclosure of potential conflicts of interest is found at the end of this article.

The detection of growth hormone (GH) and its receptor in germinal regions of the mammalian brain prompted our investigation of GH and its role in the regulation of endogenous neural precursor cell activity. Here we report that the addition of exogenous GH significantly increased the expansion rate in long-term neurosphere cultures derived from wild-type mice, while neurospheres derived from GH null mice exhibited a reduced expansion rate. We also detected a doubling in the frequency of large (i.e. stem cell-derived) colonies for up to 120 days following a 7-day intracerebroventricular infusion of GH suggesting the activation of endogenous stem cells. Moreover, gamma irradiation induced the ablation of normally quiescent stem cells in GH-infused mice, resulting in a decline in olfactory bulb neurogenesis. These results suggest that GH activates populations of resident stem and progenitor cells, and therefore may represent a novel therapeutic target for age-related neurodegeneration and associated cognitive decline.

The neurosphere assay can detect and expand neural stem cells (NSCs) and progenitor cells, but it cannot discriminate between these two populations. Given two assays have purported to overcome this shortfall, we performed a comparative analysis of the distribution and frequency of NSCs and progenitor cells detected in 400 μm coronal segments along the ventricular neuraxis of the adult mouse brain using the neurosphere assay, the neural colony forming cell assay (N-CFCA), and label-retaining cell (LRC) approach. We observed a large variation in the number of progenitor/stem cells detected in serial sections along the neuraxis, with the number of neurosphere-forming cells detected in individual 400 μm sections varying from a minimum of eight to a maximum of 891 depending upon the rostral-caudal coordinate assayed. Moreover, the greatest variability occurred in the rostral portion of the lateral ventricles, thereby explaining the large variation in neurosphere frequency previously reported. Whereas the overall number of neurospheres (3730 ± 276) or colonies (4275 ± 124) we detected along the neuraxis did not differ significantly, LRC numbers were significantly reduced (1186 ± 188, 7 month chase) in comparison to both total colonies and neurospheres. Moreover, approximately two orders of magnitude fewer NSC-derived colonies (50 ± 10) were detected using the N-CFCA as compared to LRCs. Given only 5% of the LRCs are cycling (BrdU+/Ki-67+) or competent to divide (BrdU+/Mcm-2+), and proliferate upon transfer to culture, it is unclear whether this technique selectively detects endogenous NSCs. Overall, caution should be taken with the interpretation and employment of all these techniques. Disclosure of potential conflicts of interest is found at the end of this article.