Increasing Adult Hippocampal Neurogenesis is Sufficient to Reduce Anxiety and Depression-Like Behaviors

Abstract

Adult hippocampal neurogenesis is increased by antidepressants, and is required for some of their behavioral effects. However, it remains unclear whether expanding the population of adult-born neurons is sufficient to affect anxiety and depression-related behavior. Here, we use an inducible transgenic mouse model in which the pro-apoptotic gene Bax is deleted from neural stem cells and their progeny in the adult brain, and thereby increases adult neurogenesis. We find no effects on baseline anxiety and depression-related behavior; however, we find that increasing adult neurogenesis is sufficient to reduce anxiety and depression-related behaviors in mice treated chronically with corticosterone (CORT), a mouse model of stress. Thus, neurogenesis differentially affects behavior under baseline conditions and in a model of chronic stress. Moreover, we find no effect of increased adult hippocampal neurogenesis on hypothalamic-pituitary-adrenal (HPA) axis regulation, either at baseline or following chronic CORT administration, suggesting that increasing adult hippocampal neurogenesis can affect anxiety and depression-related behavior through a mechanism independent of the HPA axis. The use of future techniques to specifically inhibit BAX in the hippocampus could be used to augment adult neurogenesis, and may therefore represent a novel strategy to promote antidepressant-like behavioral effects.

Figure 1

In the absence of stress, genetically increasing adult hippocampal neurogenesis in iBax mice increases adult hippocampal neurogenesis, but does not affect anxiety- and antidepressant-responsive behavior. (a) Schematic: vehicle-treated animals have normal levels of adult hippocampal neurogenesis where stem cells (red) can undergo symmetric or asymmetric division to produce mature granule cells (purple), however, the majority of produced cells undergo BAX-dependent cell death during maturation. Tamoxifen treatment in iBax mice prevents BAX-mediated cell death in recombined cells, leading to an increased number of adult-born granule cells. (b) Experimental timeline: iBax mice were injected with either vehicle (top line) or tamoxifen (TAM; bottom line) 6 weeks before behavioral testing. (c) TAM increases the total number of BrdU-positive cells in the dentate gyrus (P=0.007). Significant differences are seen both in the dorsal (P=0.004) and ventral (P=0.017) subregions. The majority of BrdU-labeled cells are colabeled with the mature neuronal marker NeuN, and this percentage does not differ between groups when assessed throughout the whole, dorsal, or ventral hippocampus (P>0.05). (d) TAM has no effect on the number of BrdU-positive cells observed in the arcuate nucleus of the hypothalamus (P>0.05). (e and f) Mice treated with vehicle or TAM show similar locomotion in the open field test, as assessed by total distance (P>0.05). Both groups also show anxiety-like behavior in the open field test, as assessed by percent center distance (P>0.05), and in the elevated plus maze, as assessed by time spent in the open arms (P>0.05) or open arm entries (P>0.05). (g) No differences were observed between mice treated with vehicle or TAM in depression-like behavior as observed by mobility in the tail suspension test (P>0.05). In the line graph, data is represented in 1-min bins for the duration of the test. All error bars represent SEM. *P<0.05, **P<0.01. BrdU, bromodeoxyuridine; NeuN, neuronal nuclei; TAM, Tamoxifen.

Figure 2

Genetically increasing adult hippocampal neurogenesis does not affect the HPA axis response to acute stress. (a) Experimental timeline. (b and c) There were no differences between vehicle and TAM groups in plasma CORT levels before or at various time points following 30 min of restraint stress or immediately following 15 min in a novel cage (P>0.05). n=4–11/group. All error bars represent SEM. CORT, corticosterone; HPA, hypothalamic–pituitary–adrenal; TAM, Tamoxifen.

Figure 3

Genetic ablation of Bax in neural stem cells and progenitors protects against chronic CORT-induced reduction in adult hippocampal neurogenesis. (a) Experimental timeline: iBax mice were injected with either vehicle or TAM. Six weeks later, mice received vehicle or CORT via drinking water for the remainder of the experiment. Three behavioral groups were tested, vehicle (top line), CORT alone (middle line), and TAM+CORT (bottom line). (b and c) Representative images of BrdU and DCX in the dentate gyrus (scale bars 100 um). (d) While no significant difference in the number of BrdU-positive cells is observed between vehicle- and CORT-treated mice (P>0.05), TAM+CORT treatment increases the number of BrdU-positive cells (P=0.046) (dorsal P=0.064; ventral P=0.027). The majority of BrdU-labeled cells are colabeled with the mature neuronal marker NeuN. The percent of BrdU-labeled cells colabeled with NeuN is the same between all groups in the dorsal dentate gyrus. In the ventral dentate gyrus, a lower percentage of BrdU-positive cells are colabeled with NeuN in CORT-treated mice, compared with those treated with TAM+CORT (P=0.022). (e) CORT treatment decreases the total number of DCX-positive neurons (P=0.005), and there is a trend for a decrease in the number of DCX-positive neurons with tertiary dendrites (P=0.07). TAM+CORT prevents these effects (P=0.002 for total DCX-positive neurons; P=0.002 for DCX-positive neurons with tertiary dendrites). In the dorsal and ventral subregions, the total number of DCX-positive neurons is decreased by CORT (dorsal P=0.07; ventral P=0.039), and rescued in TAM+CORT-treated mice (dorsal P=0.028; ventral P=0.002); the number of DCX-positive neurons with tertiary dendrites is increased in TAM+CORT-treated mice compared with mice treated with CORT alone (dorsal P=0.013; ventral P=0.001). n=4–7/group in all analyses. All error bars represent SEM. *P<0.05, **P<0.01. BrdU, bromodeoxyuridine; CORT, corticosterone; DCX, doublecortin; NeuN, neuronal nuclei; TAM, Tamoxifen.

Figure 4

Genetically increasing adult hippocampal neurogenesis in iBax mice prevents the effects of chronic CORT on mood-related behavior, but does not affect HPA axis regulation. (a) Experimental timeline. (b) No statistically significant differences were seen between groups in total distance or percent center distance in the open field test (P>0.05). n=8–10/group. (c) In the elevated plus maze, CORT-treated mice spent significantly less time in the open arms (P=0.015) and had fewer open arm entries (P=0.017) than controls. These effects were reversed in TAM+CORT-treated mice (P=0.018 for open arm time, P=0.049 for open arm entries). n=12–15/group. (d) In the tail suspension test, CORT-treated mice displayed decreased mobility (P=0.032), which was reversed in TAM+CORT-treated mice (P=0.005). In the line graph, data is represented in 1-min bins for the duration of the test. n=14–15/group. (e) Trunk blood was collected 5 min after a 1-min swim stress, from which plasma was isolated. There is a strong trend for CORT-treated mice to have lower plasma CORT levels than controls following forced swim stress (P=0.05), but no difference between CORT and TAM+CORT groups (P=0.91). n=5–6 group. All error bars represent SEM. *P<0.05, **P<0.01. CORT, corticosterone; HPA, hypothalamic–pituitary–adrenal; TAM, Tamoxifen.