Essential role of brain-derived neurotrophic factor in adult hippocampal function

Abstract

Brain-derived neurotrophic factor (BDNF) regulates neuronal development and function. However, it has been difficult to discern its role in the adult brain in influencing complex behavior. Here, we use a recently developed inducible knockout system to show that deleting BDNF in broad forebrain regions of adult mice impairs hippocampal-dependent learning and long-term potentiation. We use the inducible nature of this system to show that the loss of BDNF during earlier stages of development causes hyperactivity and more pronounced hippocampal-dependent learning deficits. We also demonstrate that the loss of forebrain BDNF attenuates the actions of desipramine, an antidepressant, in the forced swim test, suggesting the involvement of BDNF in antidepressant efficacy. These results establish roles for BDNF in the adult, and demonstrate the strength of this inducible knockout system in studying gene function in the adult brain.

Figures

Fig. 1.

Inducible, cell-targeted KO system. (A and B) The NSE promoter drives expression of tTA. (A) In the presence of dox, tTA cannot activate the TetOp promoter. (B) In the absence of dox, tTA binds to TetOp and drives Cre expression. Cre then mediates recombination of a floxed (e.g., BDNF) gene locus. (C–E) Coronal sections from the brain of adult (6-month-old) male NSE–tTA × TetOp-Cre × Rosa26 trigenic mice stained with X-gal. (C) In mice maintained on dox, no recombination was observed. In contrast, comparable levels of recombination were observed in mice bred and maintained in the absence of dox (early KO; D) and in mice bred on dox and then removed at 12 weeks of age (adult KO; E). (F–I) Coronal hippocampal sections of adult trigenic mice subjected to immunofluorescence staining with antibodies against β-gal (F and H) and either NeuN (G) or GFAP (I). (J–L) In situ hybridization analysis of BDNF mRNA levels in the brains of NSE–tTA × TetOp-Cre × fBDNF trigenic mice. Images of coronal brain sections with a 35S-labeled BDNF cRNA probe in a wild-type animal (J); trigenic animal maintained on dox (adult CTL; K); and trigenic animal bred on dox and then removed at 12 weeks of age (adult KO; L).

Fig. 2.

Behavioral effects of BDNF KO from adult brain. (A) Locomotor activity in the BDNF adult KO mice (n = 26) was indistinguishable from genotype controls maintained on dox (adult CTL; n = 21). Locomotor activity was assessed over a 10-min period, and the number of photocell beam breaks was recorded. (B and C) Adult KO mice (n = 16) do not display altered levels of aggression in the resident intruder paradigm compared with adult CTLs (n = 13). (D) Context-dependent fear conditioning is decreased in adult KO mice (n = 26) 24 h after training compared with adult CTL mice (*, P < 0.05; n = 22). The deficit in context-dependent fear conditioning persists on repeat testing 7 days after training (*, P < 0.05). No significant difference was observed in baseline freezing behavior. Cue-dependent fear conditioning was indistinguishable in inducible BDNF adult KO and adult CTL mice.

Fig. 3.

BDNF modulates LTP of the CA3 to CA1 inputs in adult hippocampus. (A–F) Traces show fEPSPs recorded in the stratum radiatum before (Left) and after (Right) two independent CA3 to CA1 inputs are stimulated according to the protocol indicated (Center). Stimulus intensities of 0% (A and D), 40% (B and E), or 100% (C and F) of the theta burst stimulation (TBS) intensity to induce a maximal fEPSP were used. Baseline amplitude of the slope of the fEPSP evoked by input 2 was established over 10 min; then the paired stimuli were applied and the fEPSP slope was monitored for 130 min (G and H). With a 40% stimulus intensity, the average control fEPSP was enhanced over the recording period by >150% in control (n = 9), but the BDNF KO (n = 9) slices showed no enhancement (G). At 100% stimulus intensity, control showed >170% enhancement and the BDNF KO slices showed an enhancement of ≈140% (H).

Fig. 4.

Behavioral effects of BDNF KO from late embryonic brain. (A) Locomotor activity is increased in BDNF early KO mice compared with littermate controls (early CTLs), as well as with adult KO and adult CTL mice. Locomotor activity was assessed over a 2-h period, and the number of photocell beam breaks was recorded. (B) Selective deficit in context- and cue-dependent (*, P < 0.05) fear conditioning in BDNF early KO mice (n = 10) compared with littermate controls (n = 10) as assessed 24 h after testing. No significant difference was observed in baseline freezing behavior.

Fig. 5.

Behavioral response of CTL and BDNF KO mice in the forced swim test to subchronic antidepressant treatment. Animals were administered saline or desipramine 24, 4, and 1 h before the swim test (10 mg/kg i.p., 10 mg/kg i.p., and 20 mg/kg s.c., respectively). Saline-treated control (n = 10) and BDNF KO (n = 10) mice exhibited similar immobility times. Subchronic desipramine treatment significantly reduced immobility time in the control mice (n = 10) but not in the BDNF KO mice (n = 10). Results are presented as mean immobility (sec) ± SEM; ANOVA and post hoc Tukey's test revealed a significance of P < 0.05 (asterisks) for saline control versus desipramine control [F(1, 57) = 8.51; P < 0.01].