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Bdnf val66met impairs fluoxetine-induced enhancement of adult hippocampus plasticity

Neuropsychopharmacology (2012), 1–8& 2012 American College of Neuropsychopharmacology. All rights reserved 0893-133X/12 BDNF Val66Met Impairs Fluoxetine-Induced Enhancementof Adult Hippocampus Plasticity Kevin G Bath*,1,2, Deqiang Q Jing1, Iva Dincheva1, Christine C Neeb1, Siobhan S Pattwell1, Moses V Chao3, Francis S Lee*,1,4 and Ipe Ninan*,5 Department of Psychiatry, Weill Medical College of Cornell, New York, NY, USA; 2Department of Neuroscience, Brown University, Providence, RI, USA; 3Department of Cell Biology, Physiology and Neuroscience, and Psychiatry, The Helen L. and Martin S. Kimmel Center for Biology and Medicine at the Skirball Institute for Biomolecular Medicine, New York University School of Medicine, New York, NY, USA; 4Department of Pharmacology, Weill Medical College of Cornell, New York, NY, USA; 5Department of Psychiatry, New York University School of Medicine, New York, NY, USA Recently, a single-nucleotide polymorphism (SNP) in the brain-derived neurotrophic factor (BDNF) gene (BDNF Val66Met) has been linked to the development of multiple forms of neuropsychiatric illness. This SNP, when genetically introduced into mice, recapitulates core phenotypes identified in human BDNF Val66Met carriers. In mice, this SNP also leads to elevated expression of anxiety-like behaviors that are not rescued with the prototypic selective serotonin reuptake inhibitor (SSRI), fluoxetine. A prominent hypothesis is that SSRI- induced augmentation of BDNF protein expression and the beneficial trophic effects of BDNF on neural plasticity are critical components for drug response. Thus, these mice represent a potential model to study the biological mechanism underlying treatment-resistant forms of affective disorders. To test whether the BDNF Val66Met SNP alters SSRI-induced changes in neural plasticity, we used wild-type (BDNFVal/Val) mice, and mice homozygous for the BDNF Val66Met SNP (BDNFMet/Met). We assessed hippocampal BDNF protein levels, survival rates of adult born cells, and synaptic plasticity (long-term potentiation, LTP) in the dentate gyrus either with or without chronic (28-day) fluoxetine treatment. BDNFMet/Met mice had decreased basal BDNF protein levels in the hippocampus that did not significantly increase following fluoxetine treatment. BDNFMet/Met mice had impaired survival of newly born cells and LTP in the dentate gyrus; the LTP effects remained blunted following fluoxetine treatment. The observed effects of the BDNF Val66Met SNP on hippocampal BDNF expression and synaptic plasticity provide a possible mechanistic basis by which this common BDNF SNP may impair efficacy of SSRI drug Neuropsychopharmacology advance online publication, 4 January 2012; Keywords: BDNF Val66Met SNP; hippocampus; neurogenesis; plasticity; fluoxetine; LTP In rodent models, nearly allantidepressant regimens tested to date increase BDNF Brain-derived neurotrophic factor (BDNF) is a critical within the adult hippocampus regulator of activity-dependent plasticity in the adult brain a brain region heavily implicated in the expression of anxiety and depressive-like symptoms Genetic or environmental factors that disrupt BDNF . In rodents, the genetic ablation of expression or signaling have been associated with the BDNF leads to a blockade in the response to standard development of a number of neuropsychiatric disorders, antidepressant regimens ; including major depression ( Furthermore, the infusion of BDNF proteininto the brains of rodents has antidepressant effects *Correspondence: Dr FS Lee, Department of Psychiatry, Weill Medical College of Cornell University, Weill Cornell Medical College, 1300 some to propose that augmentation of BDNF expression via York Avenue (LC-905), Box 244, New York, NY 10021, USA, Tel: + 1 chronic antidepressant treatment, and the subsequent 212 746 5403, Fax: + 1 212 746 8529, E-mail: trophic effects of BDNF on neural plasticity, may be one or Dr I Ninan, Department of Psychiatry, New York University School of the critical mechanisms underlying antidepressant of Medicine, 540 1Ave, SKI 5-3, New York NY 10016, USA, Tel: + 1 347 535 0710, Fax: + 1 212 263 0723, E-mail: The neurotrophic hypothesis of antidepressant response or Dr KG Bath, Department of Neuroscience, Brown University, 185 has become particularly relevant given the recent discovery Meeting Street, Providence, RI 02916, USA, Tel: + 1 401 863 1054,E-mail: of a uniquely human single-nucleotide polymorphism Received 5 August 2011; revised 28 October 2011; accepted 17 (SNP) in the BDNF gene (BDNF Val66Met; This SNP leads to a nucleotide change from G to A at BDNF Val66Met impairs SSRI-augmented plasticity position 196 and subsequent amino acid change at position MATERIALS AND METHODS 66 in the BDNF prodomain, from valine to methionine, andinterferes with the trafficking of BDNF mRNA ), as well as a selective impairment in the activity- All mice used in these studies were adult males (3–4 months dependent release of BDNF ( of age), housed in standard shoebox cages on a 12 : 12 Human carriers of this SNP have reverse light/dark schedule. BDNF knockout mice as impairments in hippocampus-dependent memories and described in and TrkB knockout mice the extinction of learned fear as described in (along with wild-type Male BDNF Val66Met carriers have been shown littermate controls were also used for cell survival studies.
to be at increased risk to develop depression, an effect that For the remainder of studies, BDNF Val66Met knock-in has recently been confirmed by the largest meta-analysis to mice (BDNFMet/Met) and wild-type (BDNFVal/Val) littermate date ). In addition, carriers of the controls as described in were used.
BDNF Val66Met SNP that develop depression often All mouse lines were fully backcrossed (minimum of 10 demonstrate more severe symptoms than individuals who generations) onto an inbred C57BL/6 background. Animal are non-carriers (; care was in accordance with Weill Medical College of Cornell University IACUC and FDA standards.
these findings, significant questions remain regarding themechanisms underlying BDNF's contribution to the devel-opment of affective disorders and the potential role of this Tissue Preparation and Immunohistochemistry SNP in treatment response Animalmodels have provided a fruitful testing ground to address Control and genetically modified lines of mice were deeply such questions.
anesthetized with pentobarbital and transcardially perfused We have generated a mouse model in which we knocked- with a solution of 0.9% saline and 0.1% sodium nitrite in the BDNF Val66Met SNP into the mouse bdnf gene, followed by 4% paraformaldehyde in phosphate buffer.
replacing the endogenous BDNF. Mice homozygous for the Brains were dissected out and postfixed in 4% paraforma- BDNF Val66Met SNP (BDNFMet/Met) phenocopy human dehyde for 1 h. Brains were then incubated in a 30% sucrose SNP carriers (and solution overnight at 41C. Coronal sections were serially cut have elevated expression of anxiety-like behaviors that are at 40 mm using a freezing microtome. Immunohistochem- unresponsive to chronic treatment with the selective istry was performed on floating sections. Briefly, sections serotonin reuptake inhibitor (SSRI) fluoxetine. Thus, this were first incubated in a blocking solution for 1 h and then mouse may provide a model system to gain insights into the transferred to the primary antibody solution for 48 h at 41C mechanisms by which BDNF Val66Met suppresses the before incubation with the corresponding fluorescent- response of some individuals to SSRI treatment.
labeled (Invitrogen, Carlsbad, CA) or biotinylated second- In animal models of depression, two of the potential ary antibody (Vector Laboratories, Burlingame, CA).
targets of increased trophic support are through augmenta- Antibodies and dilutions used included a rabbit polyclonal tion of hippocampal neurogenesis anti-pTrkB (1 : 500), anti-NeuN (Millipore; 1 : 1000), anti- polysialylated neural cell adhesion molecule (PSA-NCAM; Millipore; 1 : 400), anti-GFAP (Millipore; 1 : 1000), anti- both processes are BrdU (BD Biosciences; 1 : 200). BrdU-labeled sections were impacted by altered BDNF levels analyzed using a standard Nikon (Tokyo, Japan) upright microscope, digital camera mount, and the MetaMorph software package (Molecular Devices, Sunnyvale, CA). For BDNFMet/Met mice had decreased basal levels of BDNF in the co-labeling studies we performed confocal fluorescence hippocampus, and unlike BDNFVal/Val mice, BDNF protein microscopy using a Zeiss (Oberkochen, Germany) LSM510 levels did not increase following chronic fluoxetine treat- microscope fitted with a Zeiss  63 1.4 numerical aperture ment. We tested for alterations in the functional plasticity objective with standard filter sets and a standard (1 Airy disk) pinhole.
following chronic fluoxetine treatment in BDNFMet/Met andBDNFVal/Val mice. BDNFMet/Met mice also had fewer surviv- ing newly born cells in the adult dentate gyrus (DG) of thehippocampus relative to BDNFVal/Val controls. Following Fluoxetine was dissolved in water (160 mg/l, as described in fluoxetine treatment, neurogenesis in BDNFMet/Met mice (and administered in light-protected normalized to untreated wildtype levels. Finally, we found water bottles. Fluoxetine infused water was changed every impairments in long-term potentiation (LTP) in the DG of 72 h to insure fresh and active drug. Inclusion of fluoxetine BDNFMet/Met mice compared with BDNFVal/Val controls, an led to no change in water consumption, and based on the effect that was not rescued by chronic fluoxetine treatment.
average daily water intake of mice (3.1 ml per mouse, per These studies are the first to identify impairments in BDNF day with a range of 2.5–3.8 ml), this concentration led to an protein expression, the incorporation of newly born cells, average daily dose of 0.5 mg of fluoxetine per day (or and synaptic plasticity in the DG of adult BDNFMet/Met mice.
B16 mg/kg). This dosing regimen has been shown to lead to Furthermore, these studies provide a possible mechanistic therapeutic levels in blood and to be highly effective in framework through which the BDNF Val66Met SNP could reducing anxiety-like symptoms in standard inbred mouse impair SSRI response.
strains (Dulwala et al., 2004; Dulwala and Hen, 2005).

BDNF Val66Met impairs SSRI-augmented plasticityKG Bath et al Digital stimulator PG4000A (Cygnus Technology, PA) andstimulus isolator A365 (World Precision Instruments, To measure endogenous BDNF levels, a BDNF enzyme- Sarasota, FL). MPP-DG pathway was confirmed by assessing linked immunosorbent assay (ELISA) was used (BDNF paired-pulse depression (PPD) at 50 ms inter-pulse interval Emax Immunoassay System, Promega, Madison, WI) with recombinant BDNF as a standard. This methodology stable basal recording, input–output curves were generated.
demonstrates low cross-reactivity (o3%) with other A 15-minute baseline was recorded every 20 s at an intensity neurotrophic factors and is capable of detecting a minimum that evoked a response approximately 35% of the maximum of 15.6 pg/ml of BDNF. Briefly, control and fluoxetine evoked response. LTP was induced using 100 Hz stimulation treated BDNFVal/Val and BDNFMet/Met mice were killed by (4 trains, 100 Hz for 1 min, separated by 20 s). Responses cervical dislocation and then decapitation. Brains were were recorded for 1 h after tetanization and measured fEPSP collected on ice and total bilateral hippocampi were slope expressed as percentage of baseline.
dissected and then lysed in 700 ml TNE lysis buffer (0.1 MTris HCl, 0.15 M NaCl, 0.001 M EDTA, 1% NP-40). Lysateswere centrifuged for 10 min at 41C and the clarified supernatant was collected. Using the Bradford method, For comparisons in which greater than two groups were total levels of protein were quantified. Tissue and assay were included, a one-way analysis of variance (ANOVA) was prepared and run in accordance with the manufacturers used. In the event of a significant main effect, post hoc suggested protocol. BDNF levels were then corrected based group comparisons were made using a least significant on the total amount of protein loaded.
differences (LSD) approach, correcting for multiple tests.
In cases in which two-groups were compared, we used BrdU Labeling, Detection, and Quantification Student's t-test. For all statistics a-value was set at o0.05.
SPSS software (IBM) was used for all analyses.
BrdU labeling and immunohistochemcial detection wascarried out as described in To quantifythe density of BrdU positive cells in the sub-granular & granule cell layer of the DG, a profile counting method wasused (described below). To estimate the rate of survival of Chronic Fluoxetine Fails to Normalize Hippocampal newly born cells, mice were given a single injection of BrdU BDNF Levels in BDNFMet/Met Mice (160 mg/kg) and allowed to survive for 4-weeks prior to In rodent models, nearly all antidepressant regimens tested sacrifice. To avoid double counting, every third serially to date have been shown to increase BDNF protein levels in obtained section (80 mm interval) and was counterstained the hippocampus and other brain regions ( with Nissl to allow discrimination of individual cells that an effect implicated in antidepressant may be clustered. All BrdU-positive nuclei within the efficacy. To test if the BDNF Val66Met polymorphism granule cell layer of the DG were counted. The total number affects basal or SSRI-related augmentation of hippocampal of counted cells was then divided into the total volume BDNF protein levels in mice, we used a BDNF ELISA. We measured in order to obtain an estimate of the density of found a significant main effect of group (ANOVA, BrdU positive cells within this structure.
F(3,35) ¼ 6.058, po0.002). More specifically, we found thatBDNF protein levels in untreated BDNFMet/Met mice weresignificantly lower than that observed in untreated wild- LTP and Basal Transmission in DG type (BDNFVal/Val) mice (post hoc LSD, po0.01; Figure 1).
Control or fluoxetine treated mice were killed by decapita- As anticipated, following 28-days of fluoxetine treatment, tion after pentobarbital anesthesia. Brains were quicklyremoved and placed in ice-cold artificial cerebrospinal fluid(ACSF) consisting of (in mM): NaCl (118), KCl (4.4), CaCl2(2), MgCl2 (2), NaHCO3 (26), NaH2PO4 (1), D-glucose (10)aerated by 95% O2/ 5% CO2 (pH 7.4; ).
Hippocampi were quickly removed. Transverse hippocam-pal slices (400 mm) were cut and transferred to a slice pre-incubator (Scientific System Design, Canada) at roomtemperature for at least 1-h to allow for recovery. A singleslice was then transferred to an interface-recording chambermaintained at 32 1C (Scientific System Design, Canada). Thechamber was continuously perfused by ACSF at a constantrate of 2 ml/min. Recording electrodes were filled with 2 MNaCl solution and field excitatory postsynaptic potentials(fEPSPs) were recorded from the molecular layer of the DGwith IE-210 amplifier (Warner Instruments, USA) using Bar graphs depicting BDNF protein levels (pg/mg of protein) in Digidata 1440A and pClamp 10 software (Molecular hippocampal lysates from adult wild-type (control n ¼ 11, fluox n ¼ 11;BDNFVal/Val) and BDNF Val66Met homozygous (control n ¼ 9, fluox Devices, USA). The stimulating electrode (concentric n ¼ 11; BDNFMet/Met) mice maintained for 28 days on water (Control) or bipolar electrodes, FHC, Bowdoinham, ME) was placed in 160 mg/l fluoxetine (Fluox); * indicates significant difference, po0.05.
the medial perforant path (MPP) and stimulated using a N.S., not significant difference.

BDNF Val66Met impairs SSRI-augmented plasticity BDNFVal/Val mice showed a significant increase in BDNF BDNF Val66Met SNP was genetically knocked-in, we found protein levels compared with untreated BDNFVal/Val con- a main effect of genotype (ANOVA, F(2,9) ¼ 2.939, po0.05; trols (post hoc LSD, po0.01; Interestingly, see Supplementary Figure 3); BDNFMet/Met mice had a fluoxetine treatment did not increase BDNF protein significant reduction in the density of surviving BrdU- expression in BDNFMet/Met mice above levels observed in labeled cells compared with BDNFVal/Val mice (Post Hoc untreated BDNFMet/Met controls (post hoc LSD, p40.05) and remained significantly lower than that observed in un-treated BDNFVal/Val mice (post hoc LSD, po0.05).
Impact of Chronic Fluoxetine on Cell Survival in the DG Altered BDNF and TrkB Expression Decrease Basal of BDNFMet/Met Mice Rates of Cell Survival in the Adult DG Chronic antidepressant treatment has been shown to In previous reports, the genetic ablation of BDNF or TrkB in augment the survival of newly born cells in the adult DG mice leads to a selective loss of surviving newly born but not (To test whether the BDNF Val66Met proliferating cells of the DG ; polymorphism altered the antidepressant-induced increase in cell survival, we again used BrdU labeling to track cell Furthermore, during neurogenesis, the BDNF receptor, survival. We found a significant main effect of group TrkB, is selectively expressed by postmitotic neuroblasts (ANOVA, F(3,20) ¼ 6.257, po0.01; We noted a and neurons The loss of BDNF alters significant difference in basal rates of neurogenesis between the neuronal fate of these cells and leads to decreased BDNFVal/Val and BDNFMet/Met mice, with BDNFMet/Met mice dendritic complexity of differentiated DG neurons ( having fewer surviving BrdU-positive cells (post hoc LSD, Using immunohistochemical labeling for the po0.05). Chronic fluoxetine led to a significant increase in phosphorylated form of TrkB (pTrkB), we found pTrkB to the survival of newly born cells in both BDNFVal/Val (post be most highly expressed on immature (PSA-NCAM- hoc LSD, po0.05) and BDNFMet/Met mice (post hoc LSD, positive) neuroblasts (In the rostral po0.05) compared with untreated control groups.
migratory stream pTrkB expression was not detectable onGFAP-positive or NeuN-positive cells These effects were identical to what we observed in the DG(Supplementary Figure 1). On the basis on these observa-tions, we focused our investigation of BDNF-mediatedeffects following fluoxetine treatment upon measures of cellsurvival within the DG.
We used BrdU labeling (Supplementary Figure 2) to test if altered BDNF or TrkB expression impacts the survival ofnewly born cells of the adult DG. Consistent with previousreports the densityof BrdU-positive cells in the DG of heterozygous BDNFknock-out mice (BDNF + /) was significantly lower thanthat observed in wild-type littermate controls (BDNF + / + ;Student's t-test, t(6) ¼ 5.74, po0.01; Similarly,genetic ablation of one copy of the trkb gene (TrkB + /) Bar graphs depicting BrdU cell density (cells/mm3) in the DG of resulted in a significant reduction in the density of adult mice 28 days following BrdU administration in the DG adult surviving newly born granule cells in the DG (Student's t- BDNFVal/Val (n ¼ 6 per group) and BDNFMet/Met (n ¼ 6 per group) mice test, t(6) ¼ 2.44, po0.05; compared with wild- maintained for 28 days on water (Control) or 160 mg/l fluoxetine (Fluox); type littermate controls (TrkB + / + ). For mice in which the * indicates significant difference, po0.05. N.S., not significant difference.
Bar graphs depicting BrdU cell density (cells/mm3) in the dentate gyrus of adult mice 28 days following BrdU injection in (a) BDNF wild-type (n ¼ 4; BDNF + / + ) and BDNF heterozygous (n ¼ 4; BDNF + /) mice, (b) TrkB wild-type (n ¼ 4; TrkB + / + ) and TrkB heterozygous (n ¼ 4; TrkB + /) mice.
BrdU cell density (cells/mm3) (c) in BDNFVal/Val (n ¼ 5) and BDNFMet/Met (n ¼ 5) mice; * indicates significant difference, po0.05.

BDNF Val66Met impairs SSRI-augmented plasticityKG Bath et al BDNFMet/Met Impairs Synaptic Plasticity Within the might be independent of neurogenesis ().
However, the suppression of PPD in fluoxetine-treatedBDNFMet/Met did not reach statistical significance (ANOVA Earlier studies have demonstrated that chronic fluoxetine followed by post hoc comparison, p40.05). Input–output treatment can enhance synaptic neurotransmission and relationship analysis of fEPSP slope revealed no significant plasticity in the DG We tested whether differences in basal synaptic neurotransmission between the BDNF Val66Met polymorphism interfered with the untreated BDNFMet/Met and matched BDNFVal/Val mice fluoxetine-induced enhancement of synaptic transmission (Two-way ANOVA, p40.05, Although fluox- (fEPSP) and synaptic plasticity (LTP) in the DG. We etine treatment induced a robust enhancement of basal analyzed PPD to confirm the MPP-DG pathway and to synaptic neurotransmission in the BDNFVal/Val mice (Two- determine whether this short-term plasticity was modified way ANOVA, F(1,14) ¼ 14.5, po0.01, fluoxetine- by the BDNF Val66Met polymorphism or fluoxetine treated BDNFMet/Met mice revealed a slight but statistically treatment . Although nonsignificant enhancement of basal synaptic neurotrans- we did not observe a difference in PPD between mission compared with the untreated mice (p40.05, BDNFMet/Met and BDNFVal/Val mice, fluoxetine treatment As the long-lasting potentiation in the decreased PPD in BDNFVal/Val mice (ANOVA followed by MPP–DG pathway in the absence of GABA blockers is post hoc comparison, po0.05, Our results are sensitive to the incorporation of new cells into the consistent with the earlier report that chronic fluoxetine decreased PPD at the MPP-DG synapses and this effect we hypothesized that impaired survival of newly Effect of fluoxetine treatment on synaptic neurotransmission and plasticity in the DG. (a) PPD of fEPSPs in BDNFVal/Val (n ¼ 10), BDNFMet/Met (n ¼ 8), fluoxetine + BDNFVal/Val (n ¼ 10) and fluoxetine + BDNFMet/Met groups (n ¼ 8). Fluoxetine treatment suppressed the PPD in BDNFVal/Val mice butnot in BDNFMet/Met mice. (b) Input–output curves of fEPSPs in BDNFVal/Val (n ¼ 10), BDNFMet/Met (n ¼ 8), fluoxetine + BDNFVal/Val (n ¼ 10) and fluoxetine + BDNFMet/Met groups (n ¼ 8). Fluoxetine treatment induced a robust enhancement of basal synaptic neurotransmission in the BDNFVal/Val mice but not in BDNFMet/Met mice. (c) LTP in BDNFVal/Val (n ¼ 10), BDNFMet/Met (n ¼ 8), fluoxetine + BDNFVal/Val (n ¼ 10) and fluoxetine + BDNFMet/Met groups (n ¼ 8).
LTP was significantly impaired in the BDNFMet/Met mice. Fluoxetine selectively enhanced LTP in the BDNFVal/Val mice but not in the BDNFMet/Met mice.
BDNF Val66Met impairs SSRI-augmented plasticity born cells in the BDNFMet/Met mice could interfere with BDNFMet/Met mice had significantly fewer surviving newly fluoxetine-induced enhancement of long-lasting potentiation born cells in the DG compared with BDNFVal/Val mice.
in the MPP-DG pathway. Consistent with the earlier reports Disruptions in neurogenesis have been reliably linked with the development of anxiety-like behavior and antidepres- application of a 100 Hz stimulus (4 trains, 100 Hz for 1 min, sant response in animal models of anxiety and depressive- separated by 20 s) induced long-lasting potentiation in the like disorders (. Here we demonstrate untreated BDNFVal/Val mice (. However, the that cell survival is impaired in the DG of BDNFMet/Met mice, long-lasting potentiation in the BDNFMet/Met mice was an effect that is correlated with their development of significantly lower than that observed in BDNFVal/Val mice anxiety-like behaviors. We further show that SSRI's can (Two-way ANOVA, F(1,16) ¼ 24.4, po0.01, significantly potentiate the survival of cells in the DG of Although fluoxetine treatment significantly enhanced long- BDNFMet/Met mice. These data could suggest that SSRI- lasting potentiation in the BDNFVal/Val mice (Two-way induced augmentation in newly born cell survival may not ANOVA, F(1,18) ¼ 8.6, po0.01, ), the BDNFMet/Met be critical for the behavioral effects of antidepressants in mice did not show any significant improvement in long- BDNFMet/Met mice. Alternatively, animals that have had a lasting potentiation after fluoxetine treatment compared chronic blunting in the survival of adult born cells with untreated BDNFMet/Met controls (Two-way ANOVA, throughout life may simply not benefit from the modest yet significant increase in cell survival (eg the degree towhich cell survival increased may have been simplyinsufficient).
We have shown that the BDNF Val66Met SNP leads to decreased trafficking of BDNF into secretory granules and thus a reduction in the activity- In this report, we provide the first mechanistic links dependent release of BDNF ( between the BDNF Val66Met polymorphism and SSRI drug In BDNF Val66Met mice, we anticipate that the resistance. We found that BDNFMet/Met mice had decreased reduced activity-dependent release of BDNF onto immature basal levels of hippocampal BDNF and did not show cells and thus a decrease in critical trophic support during antidepressant-induced augmentation of BDNF protein the process of cellular differentiation is the critical under- levels. BDNFMet/Met mice had impairments in the survival lying factor leading to decreased survival of newly born of newly born cells in the DG, an effect that was normalized cells. Such findings are consistent with previously published to control wild-type levels by SSRI treatment. Finally, we reports demonstrating that the genetic or pharmacological observed a blunting of synaptic plasticity within the DG of ablation of BDNF results in decreased adult neurogenesis BDNFMet/Met mice, an effect that could not be rescued with within the hippocampus the SSRI fluoxetine. These studies identify several potential ; , with specific effects on the mechanisms underlying blunted treatment response in mice maturation and survival of newly born neurons and the that are homozygous for the BDNF Val66Met SNP, and may selective expression and activation of the BDNF receptor have implications for understanding mechanisms under- TrkB on postmitotic neuroblasts.
lying treatment non-responsiveness in human populations.
We uncovered significant impairments in synaptic Under baseline conditions, BDNFMet/Met mice had sig- plasticity in the dentate gyrus of BDNFMet/Met mice, under nificantly lower BDNF protein levels in the hippocampus both basal conditions and following chronic fluoxetine compared with control BDNFVal/Val mice. In previous treatment. We have recently described significant effects of reports, we have shown that the BDNF Val66Met SNP does this SNP on synaptic transmission and plasticity at the not lead to a reduction in total BDNF levels in the whole CA3–CA1 synapse ). To our knowledge, brain of BDNFMet/Met mice, cultured cortical neurons from these are the first studies to identify impairments in BDNFMet/Met mice ), or cortical tissues synaptic plasticity resulting from the BDNF Val66Met SNP (This SNP does lead to a decrease in the in the DG. On the basis of our observations, SSRI-induced trafficking of BDNF to secretory vesicles and the subsequent augmentations in LTP were completely blocked in activity-dependent release of BDNF ( BDNFMet/Met mice an effect that parallels the failure of these As the hippocampus is target of many mice to show behavioral benefits of antidepressant treat- cortical and subcortical projections, the decreased levels of ment. Despite the rescue of neurogenesis to control levels, BDNF could in part be due to decreased BDNF secretion from afferent inputs to the hippocampus. In recent reports, BDNFMet/Met mice. These results suggest that fluoxetine the BDNF Val66Met SNP has also been shown to impact affects plasticity by mechanisms other than neurogenesis.
trafficking of BDNF mRNA to dendritic processes where it The lack of effect of fluoxetine on BDNF levels or a potential can be locally transcribed ). Reduced alteration of synaptic function might be responsible for transport and local translation of BDNF mRNA in fluoxetine-resistant LTP impairment in BDNFMet/Met mice BDNFMet/Met mice may also contribute to the lower protein expression observed in the hippocampus. Such effects may reliably been shown to be a significant promoter of synaptic contribute to previously described alterations in dendritic excitability and LTP within the hippocampus ( complexity of hippocampal neurons of BDNFMet/Met mice ) and the loss of TrkB from newborn neurons in this (. However, further studies are required to region significantly impact LTP fully understand the basis of this marked decrease in basal We hypothesize that the observed effects of BDNFMet/Met hippocampal BDNF protein levels.
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Supplementary Information accompanies the paper on the Neuropsychopharmacology website ()

Source: https://www.sacklerinstitute.org/cornell/CBGB/publications/Bath.etal.2012.pdf