Chronic stress prior to pregnancy potentiated long-lasting postpartum depressive-like behavior, regulated by Akt-mTOR signaling in the hippocampus

Abstract

Postpartum depression (PPD) affects over 10% of new mothers and adversely impacts the health of offspring. One of the greatest risk factors for PPD is prepregnancy stress but the underlying biological mechanism is unknown. Here we constructed an animal model which recapitulated prepregnancy stress induced PPD and tested the role of Akt-mTOR signaling in the hippocampus. Female virgin Balb/c mice received chronic restraint stress, followed by co-housing with a normal male mouse. We found that the chronic stress led to a transient depressive-like condition that disappeared within two weeks. However, prepregnantly stressed females developed long-term postpartum depressive-like (PPD-like) symptoms as indicated by deficient performance in tests of sucrose preference, forced swim, and novelty-suppressed feeding. Chronic stress induced transient decrease in Akt-mTOR signaling and altered expressions of glutamate receptor subunits NR1 and GluR1, in contrast to long-term deficits in Akt-mTOR signaling, GluR1/NR1 ratio, and hippocampal neurogenesis in PPD-like mice. Acute ketamine improved the molecular signaling abnormality, and reversed the behavioral deficits in PPD-like mice in a rapid and persistent manner, in contrast to ineffectiveness by chronic fluoxetine treatment. Taken together, we find that chronic prepregnancy stress potentiates a long-term PPD, in which Akt-mTOR signaling may play a crucial role.

Figure 1. Changes in depressive-like behaviors and Akt/mTOR signaling after chronic stress.

(A) Sucrose preference test (SPT) and (B) Immobility time in the forced swim test (FST) in stressed (S) and control (C) groups. (C) The effect of stress on the body weight. (D,E)Hippocampal protein expressions of NR1, Akt, mTOR. p70s6k, 4EBP1, GluR1 1 day (D) and 15 days (E) after chronic stress. Data represent mean ± SEM. *p < 0.05, **p < 0.01, ***p < 0.001, n = 6–8/group.

Figure 2. Influences of prepregnancy stress on PPD at 3 weeks postpartum.

(A) Schematic timeline for experiments. (B)SPT from 1 to 12 postpartum weeks in stress-only (S), parturition-only (P), stressed + parturition (SP), and control (C) females. *difference in comparison with C group, ^#difference in comparison with S group, Δdifference in comparison with P group, *p < 0.01, ^#p < 0.01, Δp < 0.001,n = 5–7/group. (C) Immobility time in FST at 3 weeks postpartum. n = 5–7/group. (D,E) Latency to take a food pellet (D) and food consumption (E) in Novelty-suppressed-feeding, NSF test at 3 weeks postpartum. (F,G,H) Effects of prepregnancy stress on the number of pups at birth (F) and infant mortality (G) and body weights of pups in P and SP groups at 3 weeks postpartum (H). n = 6/group. The results are the mean ± SEM. (I) Hippocampal protein expressions of NR1, Akt, mTOR, p70s6k, 4EBP1 and GluR1 at 3 weeks postpartum. n = 5/group. Data represent mean ± SEM. *p < 0.05, **p < 0.01, ***p < 0.001, compared to control group; ^#p < 0.05, ^##p < 0.01, ^###p < 0.001, compared to stress only group; Δp < 0.05, ΔΔp < 0.01, ΔΔΔp < 0.001, compared to parturition group.

Figure 3. Behavioral effects by acute ketamine or chronic fluoxetine treatment in SP mice and alternations of Akt/mTOR signaling.

(A,B) Effects on the SPT (A) or immobility time in FST (B) 24 hours after an acute ketamine or 3-week fluoxetine treatment in SP mice. n = 5–7/group.***p < 0.001, compared to control, ^###p < 0.001, compared to vehicle.(C,D) Latency to eat (C) and food consumption (D) in NSF test at 24 hours after an acute administration of ketamine (ket, 30 mg/kg, i.p.) and the last chronic fluoxetine (flx, 18 mg/kg/day x21d, s.c.) treatment in SP mice. n = 5–7/group. *p < 0.05, ***p < 0.001, compared to control, ^#p < 0.05, ^###p < 0.001, compared to vehicle. (E,F) Effects of pharmacological inhibition of mTOR using rapamysin (rapa) on the antidepressant effects of ketamine (ket) on SPT (E) and FST (F) in SP group mice.n = 8/group.***p < 0.001, compared to non-drug control, ^###p < 0.001, compared to group of ket + veh. (G) Hippocampal protein expressions of NR1, Akt, mTOR, p70s6k, 4EBP1 and GluR1 at 24 hours after an acute administration of ketamine in SP mice, n = 5/group. Data represent mean ± SEM.*p < 0.05, ***p < 0.001, compared to control; ^#p < 0.05, ^###p < 0.001, compared to vehicle.

Figure 4. Photomicrographs and quantification of cell proliferation in the hippocampus of PPD-like mice at 3 weeks postpartum and the effects of chronic fluoxetine treatment.

BrdU immunoreactivity was detected in the dentate gyrus of hippocampus in control (con, C), stress-only (str, S), parturition-only (par, P) and stress +parturition (SP) groups in the saline (Sal) or fluoxetine (Flx) treatment condition. Left panels (A–D) represent photomicrographs of saline-treated groups, and right panels (E–H) represent those of fluoxetine-treated groups. The bottom panel shows the quantification of cell proliferation in each group (I), n = 6/group. Data represent mean ± SEM. **p < 0.01, ***p < 0.001, compared to control group; ^##p < 0.01, ^###p < 0.001, compared to stress only group; Δp < 0.05, ΔΔΔp < 0.001, compared to parturition group.

Figure 5. A model of Akt-mTOR signaling pathways in developing PPD potentiated by prepregnancy stress.

Chronic mild stress results in up-regulation of NMDA receptor subunit NR1, which suppresses the activation of Akt, in turn down-regulates mTOR signaling in the hippocampus. This abnormal signaling underscores the depressive-like behavior, although the deficit can be gradually restored, responsible for behavioral recovery. With the occurrence of parturition, the chronic stress, however, facilitates a prolonged NR1/Akt/mTOR signaling deficiency and neurogenesis postpartum, subsequently induces a long-term postpartum depression. The defect Akt-mTOR signaling can be corrected by a single dose of ketamine and thus a quick and enduring antidepressant effect is achieved.