Adult hippocampal neurogenesis buffers stress responses and depressive behaviour

Abstract

Glucocorticoids are released in response to stressful experiences and serve many beneficial homeostatic functions. However, dysregulation of glucocorticoids is associated with cognitive impairments and depressive illness. In the hippocampus, a brain region densely populated with receptors for stress hormones, stress and glucocorticoids strongly inhibit adult neurogenesis. Decreased neurogenesis has been implicated in the pathogenesis of anxiety and depression, but direct evidence for this role is lacking. Here we show that adult-born hippocampal neurons are required for normal expression of the endocrine and behavioural components of the stress response. Using either transgenic or radiation methods to inhibit adult neurogenesis specifically, we find that glucocorticoid levels are slower to recover after moderate stress and are less suppressed by dexamethasone in neurogenesis-deficient mice than intact mice, consistent with a role for the hippocampus in regulation of the hypothalamic-pituitary-adrenal (HPA) axis. Relative to controls, neurogenesis-deficient mice also showed increased food avoidance in a novel environment after acute stress, increased behavioural despair in the forced swim test, and decreased sucrose preference, a measure of anhedonia. These findings identify a small subset of neurons within the dentate gyrus that are critical for hippocampal negative control of the HPA axis and support a direct role for adult neurogenesis in depressive illness.

Figure 1

GFAP-TK mice show specific inhibition of adult neurogenesis. a) Confocal image of endogenous GFAP and transgenic TK expression in a radial precursor cell in the dentate gyrus (arrow). b) Confocal photographs of valganciclovir-treated mice show GFAP+ astrocytes in the hilus and molecular layer in both genotypes, despite strong TK expression in all GFAP-expressing cells in TK mice. c) Valganciclovir treatment did not affect numbers of GFAP+ astrocytes (genotype effect F_1,20=1.7, P=0.2; valganciclovir effect F_1,20=1.0, P=0.3; interaction F_1,20=1.5, P=0.2; n=6/group), confirming the expectation that valganciclovir does not kill astrocytes, which are postmitotic in the adult. d) Confocal photographs of dentate gyrus doublecortin (DCX) immunostaining in mice treated with valganciclovir (v-WT and v-TK) for 4 weeks. DCX+ young neurons are abundant in v-WT mice but absent in v-TK mice. e) The number of BrdU+/DCX+ young neurons in was unaltered in v-WT mice but reduced by 99% in v-TK mice (genotype effect F_1,20=20, P=0.0002; valganciclovir effect F_1,20=19, P=0.0003; interaction F_1,20=40, P<0.0001; *P<0.001 vs. untreated TK and v-WT; n=6/group). Inset shows example of 1-day-old BrdU+/DCX+ neuron (arrow). Error bars show s.e.m.; scale bars 10 μm in a, e and 100 μm in b, d. MOL, molecular layer; GCL, granule cell layer; HIL, hilus.

Figure 2

The glucocorticoid response to stress is increased in neurogenesis-deficient mice. a) Restraint, a moderate psychogenic stressor, resulted in higher corticosterone in neurogenesis-deficient v-TK mice than in v-WT mice 30 min after the end of stress. b) The effect of restraint was still observed after repeated exposure to stress (for both days: genotype effect F_1,65>5, P<0.05; time effect F_2,65>24, P<0.001; *P<0.05 post hoc vs. v-WT; n=6-17/group/timepoint). c) In untreated control mice, corticosterone levels 30 min after restraint stress were not different between WT and TK, indicating that altered glucocorticoid response to stress is not a non-specific effect of transgene expression (t₃₀=0.1, P=0.95; n=16/group). d) Valganciclovir-treated v-TK mice show impaired dexamethasone suppression of corticosterone in response to restraint (*t₁₃=2.5, P=0.03; n=7-8/group). Error bars show s.e.m.

Figure 3

Increased stress response is not due to reduced neurogenesis in the subventricular zone. a) Increased corticosterone response 30 min after restraint was confirmed in mice in which neurogenesis was reduced by irradiation (irradiation effect F_1,50=2.0, P=0.16; time effect F_2,50=5.1, P=0.01; irradiation × time interaction F_2,50=5.7, P=0.006; *P<0.01 post hoc; n=4-6/group/time point). Green squares at the 30 min time point indicate corticosterone levels in irradiated mice that showed unaffected neurogenesis in the subventricular zone (SVZ). b) Confocal images of BrdU+ and DCX+ cells in the dentate gyrus; neurogenesis was reduced in all irradiated mice. c) Confocal images of BrdU+ and DCX+ cells illustrating sparing of neurogenesis in the SVZ. Scale bars 100 μm. LV, lateral ventricle; MOL, molecular layer; GCL, granule cell layer; HIL, hilus.

Figure 4

Mice lacking neurogenesis show increased anxiety/depression-like behaviors. a) In the novelty-suppressed feeding (NSF) test, v-TK mice showed increased latency to feed in a novel environment following restraint stress but not under control conditions (genotype effect F_1,75=5.9, P=0.02; stress effect F_1,75=1.9, P=0.17; genotype × stress interaction F_1,75=4.8, P=0.03; *P<0.05 vs. v-TK control and *P<0.01 vs. v-WT stressed; n=13-25/group). b) Cumulative distribution of feeding latencies for the NSF test (log-rank test; *P<0.05 vs. all other groups). c) Neurogenesis-deficient v-TK mice became immobile faster in the forced swim test. Restraint stress reduced the latency to immobility in v-WT mice but did not affect v-TK mice (genotype effect F_1,88=1.1, P=0.3; stress effect F_1,88=2.2, P=0.14; genotype × stress interaction F_1,88=2.6, P=0.11; *T₄₆=2.1, P<0.05 vs. v-WT stressed; v-TK control vs. v-TK stressed T₄₂=0.1, P=0.9; n=22-26/group). d) Under control conditions, the total time spent immobile was greater in v-TK mice than in v-WT mice. Restraint stress significantly increased total immobility in v-WT mice but had no effect on v-TK mice (genotype effect F_1,88=0.3, P=0.6; stress effect F_1,88=4.2, P=0.04; genotype × stress interaction F_1,88=9.1, P=0.003; *P<0.05 vs. control v-TK, *P<0.001 vs. stressed v-WT, stressed v-WT vs. stressed v-TK P>0.05; n=22-26/group). e) Neurogenesis-deficient v-TK mice showed reduced preference for sucrose in an acute test, compared to v-WT mice, under both control and restraint conditions (genotype effect F_1,20=11.2, P<0.01; stress effect F_1,20=3.1, P=0.09; genotype × stress interaction F_1,20=0.2, P=0.7; n=4-8/group). f) Sucrose preference remained lower in v-TK mice compared to v-WT mice during the subsequent dark cycle (genotype effect F_1,25=6.8, P=0.01; stress effect F_1,25=0.8, P=0.4; genotype × stress interaction F_1,25=0.5, P=0.5; n=4-10/group). Error bars show s.e.m.