Brain-derived neurotrophic factor and antidepressant drugs have different but coordinated effects on neuronal turnover, proliferation, and survival in the adult dentate gyrus

Abstract

Antidepressants increase proliferation of neuronal progenitor cells and expression of brain-derived neurotrophic factor (BDNF) in the hippocampus. We investigated the role of BDNF signaling in antidepressant-induced neurogenesis by using transgenic mice with either reduced BDNF levels (BDNF+/-) or impaired trkB activation (trkB.T1-overexpressing mice). In both transgenic strains, chronic (21 d) imipramine treatment increased the number of bromodeoxyuridine (BrdU)-positive cells to degree similar to that seen in wild-type mice 24 h after BrdU administration, although the basal proliferation rate was increased in both transgenic strains. Three weeks after BrdU administration and the last antidepressant injection, the amount of newborn (BrdU- or TUC-4-positive) cells was significantly reduced in both BDNF+/- and trkB.T1-overexpressing mice, which suggests that normal BDNF signaling is required for the long-term survival of newborn hippocampal neurons. Moreover, the antidepressant-induced increase in the surviving BrdU-positive neurons seen in wild-type mice 3 weeks after treatment was essentially lost in mice with reduced BDNF signaling. Furthermore, we observed that chronic treatment with imipramine or fluoxetine produced a temporally similar increase in both BrdU-positive and terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end-labeled neurons in the dentate gyrus, indicating that these drugs simultaneously increase both neurogenesis and neuronal elimination. These data suggest that antidepressants increase turnover of hippocampal neurons rather than neurogenesis per se and that BDNF signaling is required for the long-term survival of newborn neurons in mouse hippocampus.

Figure 1.

Antidepressants increase neuronal turnover in the dentate gyrus of the adult mouse hippocampus. Imipramine or saline was injected daily for 21 d; saline was replaced by fluoxetine during the last 20, 10, or 5 d of treatment. All mice were treated with BrdU 1 d before perfusion. A, Both BrdU-positive (left arrow) and BrdU-TUNEL double-positive cells (right arrow) are present in the dentate gyrus (DG). H, Hilus. B, Chronic imipramine treatment (30 mg/kg) induces a similar increase in both cell proliferation (BrdU⁺) and apoptosis (TUNEL) compared with saline-treated mice (sal). C, Fluoxetine (10 mg/kg) significantly increases both TUNEL-positive cells (hatched bars) and BrdU-positive cells (black bars) after 10 and 20 d of treatment (Fluox-10 and Fluox-20, respectively) but not after 5 d of treatment (Fluox-5). The apparently high response to fluoxetine (C) reflects a low basal-proliferation rate in the NMRI mice used in the fluoxetine experiment when compared with that in CD2F₁ mice used in the imipramine experiment. Error bars indicate means ± SEM. ^*p < 0.01 compared with the wild-type saline group. AD, Antidepressant; Fluox-20d, fluoxetine treatment for 20 d; Fluox-10d, fluoxetine treatment for 10 d; Fluox-5d, fluoxetine treatment for 5 d; imi, imipramine; S, staining; SAL/Sal/sal, saline.

Figure 2.

Chronic antidepressant treatment increases the number of BrdU-positive cells in the adult hippocampus. A, Mice treated with imipramine [antidepressant (AD); 30 mg/kg] daily for 21 d received BrdU injections 24 h after the last imipramine injection and were prepared for staining (S) 24 h after the last BrdU injection. Immunohistochemical staining shows BrdU-labeled cells in the granule cell layer of the dentate gyrus in saline-treated wild-type mice (B), saline-treated trkB.T1 transgenic mice (C), imipramine-treated wild-type mice (D), and imipramine-treated trkB.T1 transgenic mice (E). imi, Imipramine; sal, saline; wt, wild type. Magnification in B-E is 200×. F, G, Quantitative analysis of the BrdU-labeled cells in the granule cell layer of the dentate gyrus 24 h after BrdU injections. Imipramine produced a significant and similar increase in cell proliferation in all genotypes: wild-type mice (WT; F, G), BDNF^+/- mice (F), and trkB.T1 transgenic mice (G). The basal-proliferation rate was increased in both BDNF^+/- and trkB.T1 mice compared with controls (F, G). Error bars indicate means ± SEM. ^*p < 0.01, saline versus imipramine; ^#p < 0.05, wild-type versus transgenic mice.

Figure 3.

Normal BDNF signaling is required for the survival of newborn neurons after chronic antidepressant treatment. A, The number of BrdU-positive cells was determined 21 and 22 d after BrdU administration and the last imipramine injection [antidepressant (AD)], respectively. S, Staining. B, C, E, The number of surviving cells was significantly reduced in the hippocampus of both BDNF^+/- mice (BrdU-positive cells) and trkB.T1 transgenic mice (C, BrdU-positive/GFAP-negative cells; E, TUC-4-positive neurons) (also see supplementary Fig. 4, available at www.jneurosci.org as supplemental material). The positive effect of imipramine on cell proliferation is maintained for 3 weeks in wild-type mice (B, C, E) and BDNF^+/- transgenic mice (B). D, TUC-4 staining was used as a marker to identify newborn neurons. DG, Dentate gyrus; H, hilus. E, Bar height indicates TUC-4-positive cells, the black portion of each bar denotes BrdU/TUC-4 double-positive cells. Error bars indicate means ± SEM. ^*p < 0.01 compared with the wild-type saline group; ^#p < 0.01 compared with the saline-treated group in the same genotype. imi, Imipramine; sal, saline; WT, wild type.