, 2011), thus suggesting that these mice exhibit a modest overactivation of the HPA axis. On the other hand, it has been reported that ablation of adult hippocampal neurogenesis by X-ray irradiation does not impair basal HPA axis activity (Surget et al., 2011). Interestingly however, it has been reported that normalisation of HPA-axis overactivity by the antidepressant fluoxetine is dependent on intact adult hippocampal neurogenesis (Surget et al., 2011). Most studies report that ablating neurogenesis in rodents either with X-irradiation or with methylazoxymethanol (MAM) does

not increase their susceptibility to stress-induced changes in depression-related behaviour when compared with stressed neurogenesis-intact animals (Schloesser et al., 2010, Jayatissa et al., 2009, Lehmann et al., 2013 and Surget et al., 2011). For example, chemical ablation

of neurogenesis Ibrutinib chemical structure in rats with chronic injection of MAM did not see more induce anhedonia, a behaviour frequently observed following chronic stress, even though MAM reduced hippocampal cell proliferation to similar extent as exposure to a stress protocol (Jayatissa et al., 2009). Moreover, ablating neurogenesis prevented the ability of social defeat stress to induce social avoidance behaviour, thus suggesting that inhibiting neurogenesis may promote resilience rather than susceptibility to behavioural changes induced by this particular stressor (Lagace et al., 2010). Conversely, some studies have reported that neurogenesis ablated animals show a depressive-like

phenotype and increased susceptibility to stress-induced depression-like behaviour, including anhedonia and increased immobility in the forced swim test (Snyder et al., 2011 and Mateus-Pinheiro et al., 2013). Taken together, the precise role of adult hippocampal neurogenesis in stress susceptibility remains unclear as a lack of association as well as associations with both increased susceptibility and increased resilience being reported. On the other hand, it appears that adult hippocampal neurogenesis is required else for the ability of environmental factors such as environmental enrichment and physical exercise to promote stress resilience (Schloesser et al., 2010). Specifically, it was recently demonstrated that adult hippocampal neurogenesis is required for the beneficial effects of an enriched environment on antidepressant-like behaviours and recovery from stress-induced changes in behaviour (Schloesser et al., 2010). Stressed animals, when housed in an enriched environment, are rescued of the typical submissive behaviour induced by psychosocial stress, while animals housed in impoverished environment present a susceptible behaviour (Schloesser et al., 2010).