Anxiety- rather than depression-like behavior is associated with adult neurogenesis in a female mouse model of higher trait anxiety- and comorbid depression-like behavior

Abstract

Adult neurogenesis has been implicated in affective disorders and the action of antidepressants (ADs) although the functional significance of this association is still unclear. The use of animal models closely mimicking human comorbid affective and anxiety disorders seen in the majority of patients should provide relevant novel information. Here, we used a unique genetic mouse model displaying higher trait anxiety (HAB) and comorbid depression-like behavior. We demonstrate that HABs have a lower rate of hippocampal neurogenesis and impaired functional integration of newly born neurons as compared with their normal anxiety/depression-like behavior (NAB) controls. In HABs, chronic treatment with the AD fluoxetine alleviated their higher depression-like behavior and protected them from relapse for 3 but not 7 weeks after discontinuation of the treatment without affecting neurogenesis. Similar to what has been observed in depressed patients, fluoxetine treatment induced anxiogenic-like effects during the early treatment phase in NABs along with a reduction in neurogenesis. On the other hand, treatment with AD drugs with a particularly strong anxiolytic component, namely the neurokinin-1-receptor-antagonist L-822 429 or tianeptine, increased the reduced rate of neurogenesis in HABs up to NAB levels. In addition, challenge-induced hypoactivation of dentate gyrus (DG) neurons in HABs was normalized by all three drugs. Overall, these data suggest that AD-like effects in a psychopathological mouse model are commonly associated with modulation of DG hypoactivity but not neurogenesis, suggesting normalization of hippocampal hypoactivity as a neurobiological marker indicating successful remission. Finally, rather than to higher depression-related behavior, neurogenesis seems to be linked to pathological anxiety.

Figure 1

HABs, in comparison with NABs, showed higher depression-like behavior, reduced neurogenesis/functional integration of newly born neurons and reduced DG activation on exposure to the FST. (a) Schematic representation of the experimental design. HABs display (b) higher immobility time, (c) a lower number of BrdU+ cells, (d) a lower number of DCX+ cells, (e) a lower proportion of BrdU+ cells in DCX+/NeuN+ cells, (f) a lower number of c-Fos+ cells and (g) a lower proportion of BrdU+ cells colabeled with c-Fos+ cells. **P<0.01, ***P<0.001 HAB vs NAB, n=6–9 mice per group. Data are represented as mean±s.e.m. BrdU, bromodeoxyuridine; DCX, doublecortin; FST, forced swim test; HAB, high anxiety/depression-like behavior; NAB, normal anxiety/depression-like behavior.

Figure 2

Representative image showing (a, b) BrdU+ cells in NABs and HABs as represented by brownish spots in the subgranular and granular layer of the DG indicated by black arrows. (c, d) DCX+ cells in NABs and HABs as represented by brownish spots indicating the cell body in the subgranular layer (indicated by black arrows) and the dendrites projecting into the GCL and ML (indicated by white arrows) of the DG. (e) A majority of BrdU+ cells (indicated by white arrows) are also DCX+ suggesting that these cells have become immature neurons (f) magnified image of an immature neuron indicated by white arrow. (g) A BrdU+ cell colabeled with both DCX and NeuN showing that it has become a fully mature neuron. This figure also shows a decline in DCX expression as the newly born cell starts expressing NeuN. Scale bar in (b, d): 50 μm; (e–g): 10 μm. BrdU, bromodeoxyuridine; DCX, doublecortin; DG, dentate gyrus; GCL, granular cell layer; HAB, high anxiety/depression-like behavior; ML, molecular layer; NAB, normal anxiety/depression-like behavior.

Figure 3

Effect of chronic fluoxetine treatment on depression-like behavior, neurogenesis and DG activity on exposure to FST in HABs and NABs. (a) Schematic representation of the experimental design. Chronic fluoxetine treatment (b) decreased the immobility time in HABs but not NABs (c) did not alter the number of BrdU+ cells in HABs but reduced it in NABs, (d) did not alter the proportion of BrdU+ cells in DCX+/NeuN+ in HABs, but reduced it in NABs, (e) increased the number of c-Fos+ cells in HABs but not in NABs. *P<0.05, **P<0.01, ***P<0.001 HAB-Ctl vs NAB-Ctl. ^$P<0.05, ^$$P<0.01 NAB-Ctl vs NAB-fluoxetine. ^#P<0.05 HAB-Ctl vs HAB-fluoxetine. n=6–9 mice per group. Data are represented as mean±s.e.m. BrdU, bromodeoxyuridine; Ctl, control; DCX, doublecortin; DG, dentate gyrus; FST, forced swim test; HAB, high anxiety/depression-like behavior; NAB, normal anxiety/depression-like behavior.

Figure 4

Effect of fluoxetine treatment for 3 weeks followed by discontinuation of the drug for either 3 or 7 weeks on depression-like behavior, neurogenesis and DG activity on exposure to FST in HABs and NABs. (a) Schematic representation of the experimental design for 3-week fluoxetine discontinuation period. A 3-week drug-free period (b) decreased the immobility time in HABs but not in NABs, (c) did not alter the number of BrdU+ cells in HABs but reduced it in NABs, (d) increased the number of c-Fos+ cells in HABs but not in NABs. (e) Depression-related behavior was negatively correlated with the number of c-Fos immunoreactive cells in the DG of HABs. (f) Schematic representation of the experimental design for the 7-week drug-free period. Chronic fluoxetine treatment for 3 weeks followed by a discontinuation for the next 7 weeks did not alter (g) the immobility time, (h) the number of BrdU+ cells or (i) or c-Fos+ cells. *P<0.05, **P<0.01, ***P<0.001 HAB-Ctl vs NAB-Ctl. ^$$P<0.01 NAB-Ctl vs NAB-fluoxetine. ^##P<0.01 HAB-Ctl vs HAB-fluoxetine. n=6–9 mice per group. Data are represented as mean±s.e.m. BrdU, bromodeoxyuridine; Ctl, control; DG, dentate gyrus; FST, forced swim test; HAB, high anxiety/depression-like behavior; NAB, normal anxiety/depression-like behavior.

Figure 5

Chronic fluoxetine treatment did not decrease neurogenesis in NABs when BrdU was administered 12–16 days after fluoxetine onset. (a) Schematic representation of the experimental design. Chronic fluoxetine treatment for 3 weeks (b) decreased the immobility time in HABs but not in NABs, (c) did not alter the number of BrdU+ cells in HABs or in NABs and (d) did not alter the number of DCX+ cells in HABs or NABs. **P<0.01, HAB-Ctl vs NAB-Ctl. ^##P<0.01 HAB-Ctl vs HAB-fluoxetine. n=6-9 mice per group. Data are represented as mean±s.e.m. BrdU, bromodeoxyuridine; Ctl, control; DCX, doublecortin; FST, forced swim test; HAB, high anxiety/depression-like behavior; NAB, normal anxiety/depression-like behavior.

Figure 6

Chronic treatment with L-822,429 or tianeptine had anxiolytic and antidepressant-like effects in HABs along with an increase in neurogenesis and normalization of challenge-induced DG hypoactivity. (a) Schematic representation of the experimental design. Chronic (b) L-822,429 or (c) tianeptine treatment increased the total time spent in the light compartment in the LD test in HABs, (d) reduced the immobility time in the FST in HABs without affecting NABs, (e) increased the proportion of BrdU+ cells in DCX+/NeuN+ cells in HABs without affecting NABs and (f) increased the number of c-Fos+ cells in the DG of HABs but not of NABs. *P<0.05, **P<0.01, HAB-Ctl vs NAB-Ctl. ^#P<0.05, ^##P<0.01, ^###P<0.001, HAB-Ctl vs HAB- L-822,429/ HAB-tia. n=6–9 mice per group. Data are represented as mean±s.e.m. BrdU, bromodeoxyuridine; Ctl, control; DCX, doublecortin; DG, dentate gyrus; FST, forced swim test; IHC, immunohistochemistry; LD, light–dark; HAB, high anxiety/depression-like behavior; NAB, normal anxiety/depression-like behavior; Tia, tianeptine.