Despite the various experiences that activate the HPA axis and produce a suppressive effect on adult neurogenesis (Table 1), there are some behaviors that activate the HPA axis but are associated with increased rates of adult neurogenesis. For example, physical exercise activates the HPA axis and increases glucocorticoid levels in the blood (Droste et al., 2003; Makatsori et al., 2003; Stranahan et al., 2006). Physical exercise also enhances cell proliferation, neuronal differentiation and survival of new neurons in the dentate gyrus of the adult mouse (Klaus et al., 2009; van Praag et al., 1999; Snyder et al., 2009b) and rat (Stranahan et al., 2006; Yi et al., 2009). Running also rescues cell proliferation in the dentate gyrus from alcohol-induced inhibition (Crews et al., 2004). This suggests that running engages mechanisms that protect progenitor cells or new neurons from the detrimental effects of elevated glucocorticoids.
