Intact spatial learning and memory in transgenic mice with reduced BDNF

Reduced Levels of Brain-Derived Neurotrophic Factor Affect Body Weight, Brain Weight and Behavior

Neurotrophins, which belong to the family of growth factors, not only play crucial roles during development but are also involved in many processes in the postnatal brain. One representative of neurotrophins is brain-derived neurotrophic factor (BDNF). BDNF plays a role in the regulation of body weight and neuronal plasticity and is, therefore, also involved in processes associated with learning and memory formation. Many of the studies on BDNF have been carried out using BDNF-deficient mice. Unfortunately, homozygous deletion of BDNF is lethal in the early postnatal stage, so heterozygous BDNF-deficient mice are often studied. Another possibility is the use of conditional BDNF-deficient mice in which the expression of BDNF is strongly downregulated in some brain cells, for example, in the neurons of the central nervous system, but the expression of BDNF in other cells in the brain is unchanged. To further reduce BDNF expression, we crossed heterozygous BDNF-deficient mice with mice carrying a deletion of BDNF in neurofilament L-positive neurons. These offspring are viable, and the animals with a strong reduction in BDNF in the brain show a strongly increased body weight, which is accompanied by a reduction in brain weight. In addition, these animals show behavioral abnormalities, particularly with regard to locomotion.

Long-Term Paracetamol Treatment Impairs Cognitive Function and Brain-Derived Neurotrophic Factor in Adult Rat Brain

Paracetamol (acetaminophen, APAP) is known as a safe pain reliever; however, its negative effects on the central nervous system have gradually been reported. We examined alterations in learning and memory, and brain-derived neurotrophic factor (BDNF) expression in the frontal cortex and hippocampus at different durations of APAP treatment in rats. Novel object recognition (NOR) and Morris water maze (MWM) paradigms were used to assess learning and memory in rats fed with 200 mg/kg APAP at single-dose, 15-day or 30-day treatments. BDNF expression was evaluated through immunohistochemistry and Western blotting. The single-dose APAP treatment did not alter the NOR performance. However, deficits in the NOR and MWM capacities were detected in the rats with longer durations of APAP treatment. An analysis of BDNF expression revealed no significant change in BDNF expression in the single-dose APAP treatment, while rats given APAP for extended periods as treatment showed a significant decrement in this protein in the frontal cortex and hippocampus. Short-term APAP treatment has no effect on learning and memory, or BDNF expression; however, long-term APAP exposure causes cognitive impairment. The diminishment of the BDNF level in the frontal cortex and hippocampus due to the long period of treatment with APAP may at least in part be involved in altered learning and memory in rats.

The molecular memory code and synaptic plasticity: A synthesis

The most widely accepted view of memory in the brain holds that synapses are the storage sites of memory, and that memories are formed through associative modification of synapses. This view has been challenged on conceptual and empirical grounds. As an alternative, it has been proposed that molecules within the cell body are the storage sites of memory, and that memories are formed through biochemical operations on these molecules. This paper proposes a synthesis of these two views, grounded in a computational model of memory. Synapses are conceived as storage sites for the parameters of an approximate posterior probability distribution over latent causes. Intracellular molecules are conceived as storage sites for the parameters of a generative model. The model stipulates how these two components work together as part of an integrated algorithm for learning and inference.

Propolis Promotes Memantine-Dependent Rescue of Cognitive Deficits in APP-KI Mice

Propolis is a complex resinous substance that is relevant as a therapeutic target for Alzheimer's disease (AD) and other neurodegenerative diseases. In this study, we confirmed that propolis (Brazilian green propolis) further enhances the rescue of cognitive deficits by the novel AD drug memantine in APP-KI mice. In memory-related behavior tasks, administration of a single dose of propolis at 1-100 mg/kg p.o. significantly enhanced the rescue of cognitive deficits by memantine at 1 mg/kg p.o. in APP-KI mice. In in vitro studies, propolis significantly increased intracellular Ca²⁺ concentration and calcium/calmodulin-dependent protein kinase II (CaMKII) autophosphorylation in Kir6.2-overexpressed N2A cells treated with memantine. Propolis also significantly increased adenosine 5'-triphosphate (ATP) contents and CaMKII autophosphorylation, which was impaired in Aβ-treated Kir6.2-overexpressed N2A cells. Similarly, repeated administration of propolis at 100 mg/kg p.o. for 8 weeks further enhanced the rescue of cognitive deficits by memantine in APP-KI mice. Consistent with the rescued cognitive deficits in APP-KI mice, repeated administration of propolis markedly ameliorated memantine-dependent rescue of injured long-term potentiation (LTP) in APP-KI mice, concomitant with increased CaMKII autophosphorylation and calcium/calmodulin-dependent protein kinase IV (CaMKIV) phosphorylation in the hippocampal CA1 region. Furthermore, repeated administration of both memantine and propolis significantly restored the decreased ATP contents in the CA1 region of APP-KI mice. Finally, we confirmed that repeated administration of memantine at 1 mg/kg p.o. and propolis at 100 mg/kg p.o. for 8 weeks failed to restore the cognitive deficits in Kir6.2-/- mice. Our study demonstrates that propolis increases ATP contents and promotes the amelioration of cognitive deficits rescued by memantine via Kir6.2 channel inhibition in the CA1 region.

Learning Ability and Hippocampal Transcriptome Responses to Early and Later Life Environmental Complexities in Dual-Purpose Chicks

In this study, we hypothesized that complex early-life environments enhance the learning ability and the hippocampal plasticity when the individual is faced with future life challenges. Chicks were divided into a barren environment group (BG), a litter materials group (LG), and a perches and litter materials group (PLG) until 31 days of age, and then their learning abilities were tested following further rearing in barren environments for 22 days. In response to the future life challenge, the learning ability showed no differences among the three groups. In the hippocampal KEGG pathways, the LG chicks showed the downregulation of neural-related genes neuronal growth regulator 1 (NEGR1) and neurexins (NRXN1) in the cell adhesion molecules pathway compared to the BG (p < 0.05). Immune-related genes TLR2 in Malaria and Legionellosis and IL-18 and IL18R1 in the TNF signaling pathway were upregulated in the LG compared to in the BG (p < 0.05). Compared to the BG, the PLG displayed upregulated TLR2A in Malaria (p < 0.05). The PLG showed upregulated neural-related gene, i.e., neuronal acetylcholine receptor subunit alpha-7-like (CHRNA8) in the nicotine addiction pathway and secretagogin (SCGN) gene expression, as compared to the LG (p < 0.05). In conclusion, early-life environmental complexities had limited effects on the learning ability in response to a future life challenge. Early-life perches and litter materials can improve neural- and immune-related gene expression and functional pathways in the hippocampus of chicks.

BDNF haploinsufficiency induces behavioral endophenotypes of schizophrenia in male mice that are rescued by enriched environment

Brain-derived neurotrophic factor (BDNF) is implicated in a number of processes that are crucial for healthy functioning of the brain. Schizophrenia is associated with low BDNF levels in the brain and blood, however, not much is known about BDNF's role in the different symptoms of schizophrenia. Here, we used BDNF-haploinsufficient (BDNF^+/-) mice to investigate the role of BDNF in different mouse behavioral endophenotypes of schizophrenia. Furthermore, we assessed if an enriched environment can prevent the observed changes. In this study, male mature adult wild-type and BDNF^+/- mice were tested in mouse paradigms for cognitive flexibility (attentional set shifting), sensorimotor gating (prepulse inhibition), and associative emotional learning (safety and fear conditioning). Before these tests, half of the mice had a 2-month exposure to an enriched environment, including running wheels. After the tests, BDNF brain levels were quantified. BDNF^+/- mice had general deficits in the attentional set-shifting task, increased startle magnitudes, and prepulse inhibition deficits. Contextual fear learning was not affected but safety learning was absent. Enriched environment housing completely prevented the observed behavioral deficits in BDNF^+/- mice. Notably, the behavioral performance of the mice was negatively correlated with BDNF protein levels. These novel findings strongly suggest that decreased BDNF levels are associated with several behavioral endophenotypes of schizophrenia. Furthermore, an enriched environment increases BDNF protein to wild-type levels and is thereby able to rescue these behavioral endophenotypes.

Cholinergic Basal Forebrain Lesion Decreases Neurotrophin Signaling without Affecting Tau Hyperphosphorylation in Genetically Susceptible Mice

Alzheimer's disease (AD) is a progressive, irreversible neurodegenerative disease that destroys memory and cognitive function. Aggregates of hyperphosphorylated tau protein are a prominent feature in the brain of patients with AD, and are a major contributor to neuronal toxicity and disease progression. However, the factors that initiate the toxic cascade that results in tau hyperphosphorylation in sporadic AD are unknown. Here we investigated whether degeneration of basal forebrain cholinergic neurons (BFCNs) and/or a resultant decrease in neurotrophin signaling cause aberrant tau hyperphosphorylation. Our results reveal that the loss of BFCNs in pre-symptomatic pR5 (P301L) tau transgenic mice results in a decrease in hippocampal brain-derived neurotrophic factor levels and reduced TrkB receptor activation. However, there was no exacerbation of the levels of phosphorylated tau or its aggregation in the hippocampus of susceptible mice. Furthermore the animals' performance in a hippocampal-dependent learning and memory task was unaltered, and no changes in hippocampal synaptic markers were observed. This suggests that tau pathology is likely to be regulated independently of BFCN degeneration and the corresponding decrease in hippocampal neurotrophin levels, although these features may still contribute to disease etiology.

Reduced CaM Kinase II and CaM Kinase IV Activities Underlie Cognitive Deficits in NCKX2 Heterozygous Mice

Among five members of the K⁺-dependent Na⁺/Ca²⁺ exchanger (NCKX) family (NCKX1-5), only NCKX2 is highly expressed in mouse brain. NCKX2 in plasma membranes mediates cytosolic calcium excretion through electrogenic exchange of 4 Na⁺ for 1 Ca²⁺ and 1 K⁺. Here, we observed significantly decreased levels of NCKX2 protein and mRNA in the CA1 region of APP23 mice, a model of Alzheimer's disease. We also found that, like APP23 mice, heterozygous NCKX2-mutant mice exhibit mildly impaired hippocampal LTP and memory acquisition, the latter based on novel object recognition and passive avoidance tasks. When we addressed underlying mechanisms, we found that both CaMKII autophosphorylation and CaMKIV phosphorylation significantly decreased in CA1 regions of NCKX2+/- relative to control mice. Likewise, phosphorylation of GluA1 (Ser-831) and CREB (Ser-133), respective downstream targets of CaMKII and CaMKIV, also significantly decreased in the CA1 region. BDNF protein and mRNA levels significantly decreased in CA1 of NCKX2+/- relative to control mice. Finally, CaN activity increased in CA1 of NCKX2+/- mice. Our findings suggest that like APP23 mice, NCKX2+/- mice may exhibit impaired learning and hippocampal LTP due to decreased CaM kinase II and CaM kinase IV activities.

The Medial Orbitofrontal Cortex Regulates Sensitivity to Outcome Value

An essential component of goal-directed decision-making is the ability to maintain flexible responding based on the value of a given reward, or "reinforcer." The medial orbitofrontal cortex (mOFC), a subregion of the ventromedial prefrontal cortex, is uniquely positioned to regulate this process. We trained mice to nose poke for food reinforcers and then stimulated this region using CaMKII-driven Gs-coupled designer receptors exclusively activated by designer drugs (DREADDs). In other mice, we silenced the neuroplasticity-associated neurotrophin brain-derived neurotrophic factor (BDNF). Activation of Gs-DREADDs increased behavioral sensitivity to reinforcer devaluation, whereas Bdnf knockdown blocked sensitivity. These changes were accompanied by modifications in breakpoint ratios in a progressive ratio task, and they were recapitulated in Bdnf(+/-)mice. Replacement of BDNF selectively in the mOFC in Bdnf(+/-)mice rescued behavioral deficiencies, as well as phosphorylation of extracellular-signal regulated kinase 1/2 (ERK1/2). Thus, BDNF expression in the mOFC is both necessary and sufficient for the expression of typical effort allocation relative to an anticipated reinforcer. Additional experiments indicated that expression of the immediate-early gene c-fos was aberrantly elevated in the Bdnf(+/-)dorsal striatum, and BDNF replacement in the mOFC normalized expression. Also, systemic administration of an MAP kinase kinase inhibitor increased breakpoint ratios, whereas the addition of discrete cues bridging the response-outcome contingency rescued breakpoints in Bdnf(+/-)mice. We argue that BDNF-ERK1/2 in the mOFC is a key regulator of "online" goal-directed action selection.

SIGNIFICANCE STATEMENT

Goal-directed response selection often involves predicting the consequences of one's actions and the value of potential payoffs. Lesions or chemogenetic inactivation of the medial orbitofrontal cortex (mOFC) in rats induces failures in retrieving outcome identity memories (Bradfield et al., 2015), suggesting that the healthy mOFC serves to access outcome value information when it is not immediately observable and thereby guide goal-directed decision-making. Our findings suggest that the mOFC also bidirectionally regulates effort allocation for a given reward and that expression of the neurotrophin BDNF in the mOFC is both necessary and sufficient for mice to sustain stable representations of reinforcer value.

The requirement of BDNF for hippocampal synaptic plasticity is experience-dependent

Brain‐derived neurotrophic factor (BDNF) supports neuronal survival, growth, and differentiation and has been implicated in forms of hippocampus‐dependent learning. In vitro, a specific role in hippocampal synaptic plasticity has been described, although not all experience‐dependent forms of synaptic plasticity critically depend on BDNF. Synaptic plasticity is likely to enable long‐term synaptic information storage and memory, and the induction of persistent (>24 h) forms, such as long‐term potentiation (LTP) and long‐term depression (LTD) is tightly associated with learning specific aspects of a spatial representation. Whether BDNF is required for persistent (>24 h) forms of LTP and LTD, and how it contributes to synaptic plasticity in the freely behaving rodent has never been explored. We examined LTP, LTD, and related forms of learning in the CA1 region of freely dependent mice that have a partial knockdown of BDNF (BDNF^+/−). We show that whereas early‐LTD (<90min) requires BDNF, short‐term depression (<45 min) does not. Furthermore, BDNF is required for LTP that is induced by mild, but not strong short afferent stimulation protocols. Object‐place learning triggers LTD in the CA1 region of mice. We observed that object‐place memory was impaired and the object‐place exploration failed to induce LTD in BDNF^+/− mice. Furthermore, spatial reference memory, that is believed to be enabled by LTP, was also impaired. Taken together, these data indicate that BDNF is required for specific, but not all, forms of hippocampal‐dependent information storage and memory. Thus, very robust forms of synaptic plasticity may circumvent the need for BDNF, rather it may play a specific role in the optimization of weaker forms of plasticity. The finding that both learning‐facilitated LTD and spatial reference memory are both impaired in BDNF^+/− mice, suggests moreover, that it is critically required for the physiological encoding of hippocampus‐dependent memory. © 2015 The Authors Hippocampus Published by Wiley Periodicals, Inc.

The role of brain-derived neurotrophic factor in learned fear processing: an awake rat fMRI study

Brain‐derived neurotrophic factor (BDNF) signaling is implicated in the etiology of many psychiatric disorders associated with altered emotional processing. Altered peripheral (plasma) BDNF levels have been proposed as a biomarker for neuropsychiatric disease risk in humans. However, the relationship between peripheral and central BDNF levels and emotional brain activation is unknown. We used heterozygous BDNF knockdown rats (BDNF^+/−) to examine the effects of genetic variation in the BDNF gene on peripheral and central BDNF levels and emotional brain activation as assessed by awake functional magnetic resonance imaging (fMRI). BDNF^+/− and control rats were trained to associate a flashing light (conditioned stimulus; CS) with foot‐shock, and brain activation in response to the CS was measured 24 h later in awake rats using fMRI. Central and peripheral BDNF levels were decreased in BDNF^+/− rats compared with control rats. Activation of fear circuitry (amygdala, periaqueductal gray, granular insular) was seen in control animals; however, activation of this circuitry was absent in BDNF^+/− animals. Behavioral experiments confirmed impaired conditioned fear responses in BDNF^+/− rats, despite intact innate fear responses. These data confirm a positive correlation [r = 0.86, 95% confidence interval (0.55, 0.96); P = 0.0004] between peripheral and central BDNF levels and indicate a functional relationship between BDNF levels and emotional brain activation as assessed by fMRI. The results demonstrate the use of rodent fMRI as a sensitive tool for measuring brain function in preclinical translational studies using genetically modified rats and support the use of peripheral BDNF as a biomarker of central affective processing.

Interaction does Count: A Cross-Fostering Study on Transgenerational Effects of Pre-reproductive Maternal Enrichment

Pre-reproductive environmental enrichment of female rats influences sensorimotor development and spatial behavior of the offspring, possibly through the changed maternal nurturing. Nevertheless, maternal care could be not the solely responsible for changing offspring developmental trajectories. To disentangle the specific contribution to the transgenerational inheritance of pre- and post-natal factors, a cross-fostering study was performed. Female rats were reared in an enriched environment from weaning to sexual maturity, while control female rats were reared under standard conditions. Following mating with standard-reared males, all females were housed individually. Immediately after delivery, in- or cross-fostering manipulations were performed so that any foster dams received pups born to another dam of the same (in-fostering) or the opposite (cross-fostering) pre-reproductive rearing condition. In lactating dams maternal care and nesting activities were assessed, while in their male pups spatial abilities were assessed through Morris Water Maze (MWM) test at post-natal day 45. Moreover, the expression of Brain-Derived-Neurotrophic-Factor (BDNF) was evaluated in the hippocampus and frontal cortex of dams and pups at weaning. Pre-reproductive maternal environmental enrichment, followed by adoption procedures, loosened its potential in modifying maternal care and offspring developmental trajectories, as indicated by the lack of differences between in-fostered groups of dams and pups. In addition, enriched dams rearing standard pups showed the least complex maternal repertoire (the highest sniffing duration and the lowest nest quality), and their pups showed a reduced spatial learning in the MWM. Nevertheless, pre-reproductive maternal enrichment kept influencing neurotrophic pattern, with enriched dams expressing increased frontal BDNF levels (regardless of the kind of fostered pups), and their offspring expressing increased hippocampal BDNF levels. The present findings enlighten the crucial importance of the early mother-pups interactions in influencing maternal care and offspring phenotype, with the enriched dam-standard pups couple resulting in the most maladaptive encounter. Our study thus sustains that the bidirectional interactions between mother and pups are able to deeply shape offspring phenotype.

Chronic BDNF deficiency leads to an age-dependent impairment in spatial learning

Brain-derived neurotrophic factor (BDNF) is a crucial mediator of neural plasticity and, consequently, of memory formation. In hippocampus-dependent learning tasks BDNF also seems to play an essential role. However, there are conflicting results concerning the spatial learning ability of aging BDNF(+/-) mice in the Morris water maze paradigm. To evaluate the effect of chronic BDNF deficiency in the hippocampus on spatial learning throughout life, we conducted a comprehensive study to test differently aged BDNF(+/-) mice and their wild type littermates in the Morris water maze and to subsequently quantify their hippocampal BDNF protein levels as well as expression levels of TrkB receptors. We observed an age-dependent learning deficit in BDNF(+/-) animals, starting at seven months of age, despite stable hippocampal BDNF protein expression and continual decline of TrkB receptor expression throughout aging. Furthermore, we detected a positive correlation between hippocampal BDNF protein levels and learning performance during the probe trial in animals that showed a good learning performance during the long-term memory test.

DREAM controls the on/off switch of specific activity-dependent transcription pathways

Changes in nuclear Ca(2+) homeostasis activate specific gene expression programs and are central to the acquisition and storage of information in the brain. DREAM (downstream regulatory element antagonist modulator), also known as calsenilin/KChIP-3 (K(+) channel interacting protein 3), is a Ca(2+)-binding protein that binds DNA and represses transcription in a Ca(2+)-dependent manner. To study the function of DREAM in the brain, we used transgenic mice expressing a Ca(2+)-insensitive/CREB-independent dominant active mutant DREAM (daDREAM). Using genome-wide analysis, we show that DREAM regulates the expression of specific activity-dependent transcription factors in the hippocampus, including Npas4, Nr4a1, Mef2c, JunB, and c-Fos. Furthermore, DREAM regulates its own expression, establishing an autoinhibitory feedback loop to terminate activity-dependent transcription. Ablation of DREAM does not modify activity-dependent transcription because of gene compensation by the other KChIP family members. The expression of daDREAM in the forebrain resulted in a complex phenotype characterized by loss of recurrent inhibition and enhanced long-term potentiation (LTP) in the dentate gyrus and impaired learning and memory. Our results indicate that DREAM is a major master switch transcription factor that regulates the on/off status of specific activity-dependent gene expression programs that control synaptic plasticity, learning, and memory.

An investigation into "two hit" effects of BDNF deficiency and young-adult cannabinoid receptor stimulation on prepulse inhibition regulation and memory in mice

Reduced brain-derived neurotrophic factor (BDNF) signaling has been shown in the frontal cortex and hippocampus in schizophrenia. The aim of the present study was to investigate whether a BDNF deficit would modulate effects of chronic cannabis intake, a well-described risk factor for schizophrenia development. BDNF heterozygous mice (HET) and wild-type controls were chronically treated during weeks 6, 7, and 8 of life with the cannabinoid receptor agonist, CP55,940 (CP). After a 2-week delay, there were no CP-induced deficits in any of the groups in short-term spatial memory in a Y-maze task or novel object recognition memory. Baseline prepulse inhibition (PPI) was lower but average startle was increased in BDNF HET compared to wild-type controls. Acute CP administration before the PPI session caused a marked increase in PPI in male HET mice pre-treated with CP but not in any of the other male groups. In females, there were small increases of PPI in all groups upon acute CP administration. Acute CP administration furthermore reduced startle and this effect was greater in HET mice irrespective of chronic CP pre-treatment. Analysis of the levels of [³H]CP55,940 binding by autoradiography revealed a significant increase in the nucleus accumbens of male BDNF HET mice previously treated with CP but not in any of the other groups or in the caudate nucleus. These results show that BDNF deficiency and chronic young-adult cannabinoid receptor stimulation do not interact in this model on learning and memory later in life. In contrast, male “two hit” mice, but not females, were hypersensitive to the effect of acute CP on sensorimotor gating. These effects may be related to a selective increase of [³H]CP55,940 binding in the nucleus accumbens, reflecting up-regulation of CB1 receptor density in this region. These data could be of relevance to our understanding of differential “two hit” neurodevelopmental mechanisms in schizophrenia.

Loss of BDNF or its receptors in three mouse models has unpredictable consequences for anxiety and fear acquisition

BDNF-induced signaling is essential for the development of the central nervous system and critical for plasticity in adults. Mature BDNF signals through TrkB, while its precursor proBDNF employs p75(NTR), resulting in activation of signaling cascades with opposite effects on neuronal survival, growth cone decisions, and synaptic plasticity. Accordingly, variations in the genes encoding BDNF and its receptors sometimes have opposing influences in psychiatric disorders, and despite the vast literature, consensus is lacking about the behavioral consequences of disrupting the activity of the BDNF system in mice. To dissect the behavioral traits affected by dysfunctional BDNF/TrkB vs. proBDNF/p75(NTR) activity, we studied Bdnf(+/-), Ntrk2(+/-), and Ngfr(-/-) mice in parallel with respect to exploratory behavior, anxiety, startle, and fear acquisition. Our data reveal that the effect of proBDNF/BDNF and its receptors on behavior is more complex than expected. Strikingly, receptor-deficient mice displayed increased risk-taking behavior in the open field and elevated plus maze, whereas lack of proBDNF/BDNF had the opposite effect on mouse behavior. On the other hand, although TrkB signaling is instrumental for acquisition of fear memory in an inhibitory avoidance experiment, lack of p75(NTR) or proBDNF/BDNF conferred increased memory in this task. Importantly, none of the genotypes displayed any deficits in startle reflex, indicating unimpaired response to shock. The combined data illustrate an apparent paradox in the role of the BDNF system in controlling complex behavior and suggest that the individual components may also engage independently in separate signaling pathways.

A Tale of Two Maladies? Pathogenesis of Depression with and without the Huntington's Disease Gene Mutation

Huntington’s disease (HD) is an autosomal dominant disorder caused by a tandem repeat expansion encoding an expanded tract of glutamines in the huntingtin protein. HD is progressive and manifests as psychiatric symptoms (including depression), cognitive deficits (culminating in dementia), and motor abnormalities (including chorea). Having reached the twentieth anniversary of the discovery of the “genetic stutter” which causes HD, we still lack sophisticated insight into why so many HD patients exhibit affective disorders such as depression at very early stages, prior to overt appearance of motor deficits. In this review, we will focus on depression as the major psychiatric manifestation of HD, discuss potential mechanisms of pathogenesis identified from animal models, and compare depression in HD patients with that of the wider gene-negative population. The discovery of depressive-like behaviors as well as cellular and molecular correlates of depression in transgenic HD mice has added strong support to the hypothesis that the HD mutation adds significantly to the genetic load for depression. A key question is whether HD-associated depression differs from that in the general population. Whilst preclinical studies, clinical data, and treatment responses suggest striking similarities, there are also some apparent differences. We discuss various molecular and cellular mechanisms which may contribute to depression in HD, and whether they may generalize to other depressive disorders. The autosomal dominant nature of HD and the existence of models with excellent construct validity provide a unique opportunity to understand the pathogenesis of depression and associated gene-environment interactions. Thus, understanding the pathogenesis of depression in HD may not only facilitate tailored therapeutic approaches for HD sufferers, but may also translate to the clinical depression which devastates the lives of so many people.

Rescue of cAMP response element-binding protein signaling reversed spatial memory retention impairments induced by subanesthetic dose of propofol

AIMS

The intravenous anesthetic propofol caused episodic memory impairments in human. We hypothesized propofol caused episodic-like spatial memory retention but not acquisition impairments in rats and rescuing cAMP response element-binding protein (CREB) signaling using selective type IV phosphodiesterase (PDEIV) inhibitor rolipram reversed these effects.

METHODS

Male Sprague-Dawley rats were randomized into four groups: control; propofol (25 mg/kg, intraperitoneal); rolipram; and rolipram + propofol (pretreatment of rolipram 25 min before propofol, 0.3 mg/kg, intraperitoneal). Sedation and motor coordination were evaluated 5, 15, and 25 min after propofol injection. Invisible Morris water maze (MWM) acquisition and probe test (memory retention) were performed 5 min and 24 h after propofol injection. Visible MWM training was simultaneously performed to resist nonspatial effects. Hippocampal CREB signaling was detected 5 min, 50 min, and 24 h after propofol administration.

RESULTS

Rolipram did not change propofol-induced anesthetic/sedative states or impair motor skills. No difference was found on the latency to the platform during the visible MWM. Propofol impaired spatial memory retention but not acquisition. Rolipram reversed propofol-induced spatial memory impairments and suppression on cAMP levels, CaMKIIα and CREB phosphorylation, brain-derived neurotropic factor (BDNF) and Arc protein expression.

CONCLUSIONS

Propofol caused spatial memory retention impairments but not acquisition inability possibly by inhibiting CREB signaling.

Age-dependent decline of motor neocortex but not hippocampal performance in heterozygous BDNF mice correlates with a decrease of cortical PSD-95 but an increase of hippocampal TrkB levels

Brain-derived neurotrophic factor (BDNF) is a key player in learning and memory processes. However, little is known about brain area-specific functions of this neurotrophin. Here we investigated whether BDNF could differently affect motor neocortical and hippocampal-related cognitive and plastic morphologic changes in young (12-week-old) and middle-aged (30-week-old) BDNF heterozygous (BDNF⁺/⁻) and wild type (wt) mice. We found that at 30 weeks of age, BDNF⁺/⁻ mice showed impaired performance in accelerating rotarod and grasping tests while preserved spatial learning in a T-maze and recognition memory in an object recognition task compared with wt mice suggesting a specific neocortical dysfunction. Accordingly, a significant reduction of synaptic markers (PSD-95 and GluR1) and corresponding puncta was observed in motor neocortex but not in hippocampus of BDNF⁺/⁻ mice. Interestingly, 30-week-old BDNF⁺/⁻ mice displayed increased TrkB levels in the hippocampus but not in the motor neocortex, which suggests specific hippocampal compensatory mechanisms as a consequence of BDNF decrease. In conclusion, our data indicates that BDNF could differentially regulate the neuronal micro-structures and cognition in a region-specific and in an age-dependent manner.

Sustained expression of brain-derived neurotrophic factor is required for maintenance of dendritic spines and normal behavior

Brain-derived neurotrophic factor (BDNF) plays important roles in the development, maintenance, and plasticity of the mammalian forebrain. These functions include regulation of neuronal maturation and survival, axonal and dendritic arborization, synaptic efficacy, and modulation of complex behaviors including depression and spatial learning. Although analysis of mutant mice has helped establish essential developmental functions for BDNF, its requirement in the adult is less well documented. We have studied late-onset forebrain-specific BDNF knockout (CaMK-BDNF(KO)) mice, in which BDNF is lost primarily from the cortex and hippocampus in early adulthood, well after BDNF expression has begun in these structures. We found that although CaMK-BDNF(KO) mice grew at a normal rate and can survive more than a year, they had smaller brains than wild-type siblings. The CaMK-BDNF(KO) mice had generally normal behavior in tests for ataxia and anxiety, but displayed reduced spatial learning ability in the Morris water task and increased depression in the Porsolt swim test. These behavioral deficits were very similar to those we previously described in an early-onset forebrain-specific BDNF knockout. To identify an anatomical correlate of the abnormal behavior, we quantified dendritic spines in cortical neurons. The spine density of CaMK-BDNF(KO) mice was normal at P35, but by P84, there was a 30% reduction in spine density. The strong similarities we find between early- and late-onset BDNF knockouts suggest that BDNF signaling is required continuously in the CNS for the maintenance of some forebrain circuitry also affected by developmental BDNF depletion.

Reduction of BDNF expression in Fmr1 knockout mice worsens cognitive deficits but improves hyperactivity and sensorimotor deficits

Fragile X syndrome (FXS) is a common cause of inherited intellectual disability and a well-characterized form of autism spectrum disorder. As brain-derived neurotrophic factor (BDNF) is implicated in the pathophysiology of FXS we examined the effects of reduced BDNF expression on the behavioral phenotype of an animal model of FXS, Fmr1 knockout (KO) mice, crossed with mice carrying a deletion of one copy of the Bdnf gene (Bdnf(+/-)). Fmr1 KO mice showed age-dependent alterations in hippocampal BDNF expression that declined after the age of 4 months compared to wild-type controls. Mild deficits in water maze learning in Bdnf(+/-) and Fmr1 KO mice were exaggerated and contextual fear learning significantly impaired in double transgenics. Reduced BDNF expression did not alter basal nociceptive responses or central hypersensitivity in Fmr1 KO mice. Paradoxically, the locomotor hyperactivity and deficits in sensorimotor learning and startle responses characteristic of Fmr1 KO mice were ameliorated by reducing BNDF, suggesting changes in simultaneously and in parallel working hippocampus-dependent and striatum-dependent systems. Furthermore, the obesity normally seen in Bdnf(+/-) mice was eliminated by the absence of fragile X mental retardation protein 1 (FMRP). Reduced BDNF decreased the survival of newborn cells in the ventral part of the hippocampus both in the presence and absence of FMRP. Since a short neurite phenotype characteristic of newborn cells lacking FMRP was not found in cells derived from double mutant mice, changes in neuronal maturation likely contributed to the behavioral phenotype. Our results show that the absence of FMRP modifies the diverse effects of BDNF on the FXS phenotype.

Long-term behavioral and NMDA receptor effects of young-adult corticosterone treatment in BDNF heterozygous mice

Psychiatric illnesses, such as schizophrenia, are most likely caused by an interaction between genetic predisposition and environmental factors, including stress during development. The neurotrophin, brain-derived neurotrophic factor (BDNF) has been implicated in this illness as BDNF levels are decreased in the brain of patients with schizophrenia. The aim of the present study was to assess the combined effect of reduced BDNF levels and postnatal stress, simulated by chronic young-adult treatment with the stress hormone, corticosterone. From 6 weeks of age, female and male BDNF heterozygous mice and their wild-type controls were chronically treated with corticosterone in their drinking water for 3 weeks. At 11 weeks of age, male, but not female BDNF heterozygous mice treated with corticosterone exhibited a profound memory deficit in the Y-maze. There were no differences between the groups in baseline prepulse inhibition (PPI), a measure of sensorimotor gating, or its disruption by treatment with MK-801. However, an increase in startle caused by MK-801 treatment was absent in male, but not female BDNF heterozygous mice, irrespective of corticosterone treatment. Analysis of protein levels of the NMDA receptor subunits NR1, NR2A, NR2B and NR2C, showed a marked increase of NR2B levels in the dorsal hippocampus of male BDNF heterozygous mice treated with corticosterone. In the ventral hippocampus, significantly reduced levels of NR2A, NR2B and NR2C were observed in male BDNF heterozygous mice. The NMDA receptor effects in hippocampal sub-regions could be related to the spatial memory deficits and the loss of the effect of MK-801 on startle in these mice, respectively. No significant changes in NMDA receptor subunit levels were observed in any of the female groups. Similarly, no significant changes in levels of BDNF or its receptor, TrkB, were found other than the expected reduced levels of BDNF in heterozygous mice. In conclusion, the data show differential interactive effects of reduced levels of BDNF expression and corticosterone treatment on spatial memory and startle in male and female mice, accompanied by significant, but region-specific changes in NMDA receptor subunit levels in the dorsal and ventral hippocampus. These results could be important for our understanding of the interaction of neurodevelopmental stress and BDNF deficiency in cognitive and anxiety-related symptoms of psychiatric illnesses, such as schizophrenia.

Mouse models of genetic effects on cognition: relevance to schizophrenia

Cognitive dysfunction is a core feature of schizophrenia. Growing evidence indicates that a wide variety of genetic mutations and polymorphisms impact cognition and may thus be implicated in various aspects of this mental disorder. Despite differences between human and rodent brain structure and function, genetic mouse models have contributed critical information about brain mechanisms involved in cognitive processes. Here, we summarize discoveries of genetic modifications in mice that impact cognition. Based on functional hypotheses, gene modifications within five model systems are described: 1) dopamine (D1, D2, D3, D4, D5, DAT, COMT, MAO); 2) glutamate (GluR-A, NR1, NR2A, NR2B, GRM2, GRM3, GLAST); 3) GABA (α(5), γ(2), α(4), δGABA(A), GABA(B(1)), GAT1); 4) acetylcholine (nAChRβ2, α7, CHRM1); and 5) calcium (CaMKII-α, neurogranin, CaMKKβ, CaMKIV). We also consider other risk-associated genes for schizophrenia such as dysbindin (DTNBP1), neuregulin (NRG1), disrupted-in-schizophrenia1 (DISC1), reelin and proline dehydrogenase (PRODH). Because of the presumed importance of environmental factors, we further consider genetic modifications within the stress-sensitive systems of corticotropin-releasing factor (CRF), brain-derived neurotrophic factor (BDNF) and the endocannabinoid systems. We highlight the missing information and limitations of cognitive assays in genetically modified mice models relevant to schizophrenia pathology.

Voluntary exercise does not enhance long-term potentiation in the adolescent female dentate gyrus

The hippocampus is a dynamic brain structure involved with learning and memory. Long-term potentiation (LTP) is a neuronal model of learning and memory and, in adult rodents, is enhanced by voluntary exercise (VEx). The current study sought to elucidate whether synaptic plasticity in the male and female adolescent hippocampus is augmented by VEx. Consistent with previous studies, VEx significantly enhanced LTP in adolescent males following weak and strong theta-burst stimulation. Despite running the same amount as males, however, VEx did not enhance LTP in females above non-runner females. Surprisingly, the exercise-induced enhancement to LTP in males was seen in the absence of a change in brain derived neurotrophic factor in the dentate gyrus (DG). These findings indicate that adolescent males and females are differentially sensitive to the potentiating effect of exercise on hippocampal synaptic plasticity.

Deficit in BDNF does not increase vulnerability to stress but dampens antidepressant-like effects in the unpredictable chronic mild stress

Despite growing evidences of an association between brain-derived neurotrophic factor (BDNF) and antidepressant effects, the neurotrophic hypothesis of depression is challenged by the paucity of direct links between BDNF deficit and depressive-like behaviors. The unpredictable chronic mild stress (UCMS) paradigm might take our understanding a step further by examining whether a decrease in bdnf expression can lead to enhanced vulnerability to stress and prevent antidepressant efficacy in all or specific UCMS-induced alterations. Wild-type bdnf(+/+) and heterozygous bdnf(+/-) mice were exposed to an 8-week UCMS regimen and, from the third week onward, treated with either vehicle or imipramine (20mg/kg/day, ip). Physical, behavioral and biological (plasma corticosterone levels, bdnf expression in the dentate gyrus) measures were further analyzed regarding to the genotype and the treatment. Heterozygous bdnf(+/-) mice displayed hyperactivity and increase of body weight but no enhancement of the sensitivity to stress exposure in all the measures investigated here. In contrast, while imipramine treatment reduced anxiety-like behaviors in the novelty-suppressed feeding test in both genotypes, it decreased aggressiveness in the resident/intruder test and immobility in the tail suspension test in wild-type but not in heterozygous mice. Furthermore, imipramine induced a twofold increase of bdnf expression in the dentate gyrus in both genotypes, while bdnf(+/-) mice displayed roughly half-reduced level for the same treatment. In summary, we demonstrate here that depletion in BDNF dampened the antidepressant effects in several behaviors but failed to increase vulnerability to chronic stress exposure.

Brain-derived neurotrophic factor modulates the severity of cognitive alterations induced by mutant huntingtin: involvement of phospholipaseCgamma activity and glutamate receptor expression

The involvement of brain-derived neurotrophic factor (BDNF) in cognitive processes and the decrease in its expression in Huntington's disease suggest that this neurotrophin may play a role in learning impairment during the disease progression. We therefore analyzed the onset and severity of cognitive deficits in two different mouse models with the same mutant huntingtin but with different levels of BDNF (R6/1 and R6/1:BDNF+/- mice). We observed that BDNF modulates cognitive function in different learning tasks, even before the onset of motor symptoms. R6/1:BDNF+/- mice showed earlier and more accentuated cognitive impairment than R6/1 mice at 5 weeks of age in discrimination learning; at 5 weeks of age in procedural learning; and at 9 weeks of age in alternation learning. At the earliest age at which cognitive impairment was detected, electrophysiological analysis was performed in the hippocampus. All mutant genotypes showed reduced hippocampal long term potentiation (LTP) with respect to wild type but did not show differences between them. Thus, we evaluated the involvement of BDNF-trkB signaling and glutamate receptor expression in the hippocampus of these mice. We observed a decrease in phospholipaseCgamma activity, but not ERK, in R61, BDNF+/- and R6/1:BDNF+/- hippocampus at the age when LTP was altered. However, a specific decrease in the expression of glutamate receptors NR1, NR2A and GluR1 was detected only in R6/1:BDNF+/- hippocampus. Therefore, these results show that BDNF modulates the learning and memory alterations induced by mutant huntingtin. This interaction leads to intracellular changes, such as specific changes in glutamate receptors and in BDNF-trkB signaling through phospholipaseCgamma.

The neurotrophin-inducible gene Vgf regulates hippocampal function and behavior through a brain-derived neurotrophic factor-dependent mechanism

VGF is a neurotrophin-inducible, activity-regulated gene product that is expressed in CNS and PNS neurons, in which it is processed into peptides and secreted. VGF synthesis is stimulated by BDNF, a critical regulator of hippocampal development and function, and two VGF C-terminal peptides increase synaptic activity in cultured hippocampal neurons. To assess VGF function in the hippocampus, we tested heterozygous and homozygous VGF knock-out mice in two different learning tasks, assessed long-term potentiation (LTP) and depression (LTD) in hippocampal slices from VGF mutant mice, and investigated how VGF C-terminal peptides modulate synaptic plasticity. Treatment of rat hippocampal slices with the VGF-derived peptide TLQP62 resulted in transient potentiation through a mechanism that was selectively blocked by the BDNF scavenger TrkB-Fc, the Trk tyrosine kinase inhibitor K252a (100 nm), and tPA STOP, an inhibitor of tissue plasminogen activator (tPA), an enzyme involved in pro-BDNF cleavage to BDNF, but was not blocked by the NMDA receptor antagonist APV, anti-p75(NTR) function-blocking antiserum, or previous tetanic stimulation. Although LTP was normal in slices from VGF knock-out mice, LTD could not be induced, and VGF mutant mice were impaired in hippocampal-dependent spatial learning and contextual fear conditioning tasks. Our studies indicate that the VGF C-terminal peptide TLQP62 modulates hippocampal synaptic transmission through a BDNF-dependent mechanism and that VGF deficiency in mice impacts synaptic plasticity and memory in addition to depressive behavior.

Adenosine A(2A) receptors are required for normal BDNF levels and BDNF-induced potentiation of synaptic transmission in the mouse hippocampus

Brain-derived neurotrophic factor (BDNF), a member of neurotrophin family, enhances synaptic transmission and regulates neuronal proliferation and survival. Both BDNF and its tyrosine kinase receptors (TrkB) are highly expressed in the hippocampus, where an interaction with adenosine A(2A) receptors (A(2A)Rs) has been recently reported. In the present paper, we evaluated the role of A(2A)Rs in mediating functional effects of BDNF in hippocampus using A(2A)R knock-out (KO) mice. In hippocampal slices from WT mice, application of BDNF (10 ng/mL) increased the slope of excitatory post-synaptic field potentials (fEPSPs), an index of synaptic facilitation. This increase of fEPSP slope was abolished by the selective A(2A) antagonist ZM 241385. Similarly, genetic deletion of the A(2A)Rs abolished BDNF-induced increase of the fEPSP slope in slices from A(2A)R KO mice The reduced functional ability of BDNF in A(2A)R KO mice was correlated with the reduction in hippocampal BDNF levels. In agreement, the pharmacological blockade of A(2)Rs by systemic ZM 241385 significantly reduced BDNF levels in the hippocampus of normal mice. These results indicate that the tonic activation of A(2A)Rs is required for BDNF-induced potentiation of synaptic transmission and for sustaining a normal BDNF tone in the hippocampus.

Bidirectional changes in water-maze learning following recombinant adenovirus-associated viral vector (rAAV)-mediated brain-derived neurotrophic factor expression in the rat hippocampus

Alterations in hippocampal brain-derived neurotrophic factor (BDNF) expression have been implicated in the pathogenesis of emotional and cognitive dysfunction. Here, we induced BDNF overexpression in the rat hippocampus using recombinant adenovirus-associated viral (rAAV) vectors, and studied its long-term (2 months postinduction) effects on anxiety-related behaviour, exploration in the open field, and spatial learning in the water maze. Although the treatment successfully led to substantial elevation of hippocampal BDNF levels, its effect on spatial learning was bidirectional: a subset of rAAV-induced BDNF-overexpressing rats performed well above control level, whereas the rest were clearly impaired. This behavioural distinction corresponded to two markedly different levels of BDNF overexpression. The increase in dorsal hippocampal BDNF content achieved in the 'water-maze-impaired' subgroup was twice that attained in the 'water-maze-improved' rats. Although neither subgroup of rAAV-induced BDNF-overexpressing rats differed from controls in the open field, the 'water-maze-impaired' subgroup also showed a significant anxiolytic effect. Our results suggest that hippocampal BDNF elevation significantly affects cognitive and emotional behaviours, but the direction and magnitude of the effects critically depend on the precise levels of overexpression. This factor must be taken into account in future studies examining the functional consequences of hippocampal BDNF overexpression.

Neurobiology of memory and anxiety: from genes to behavior

Interaction of anxiety and memory represents an essential feature of CNS functioning. This paper reviews experimental data coming from neurogenetics, neurochemistry, and behavioral pharmacology (as well as parallel clinical findings) reflecting different mechanisms of memory-anxiety interplay, including brain neurochemistry, circuitry, pharmacology, neuroplasticity, genes, and gene-environment interactions. It emphasizes the complexity and nonlinearity of such interplay, illustrated by a survey of anxiety and learning/memory phenotypes in various genetically modified mouse models that exhibit either synergistic or reciprocal effects of the mutation on anxiety levels and memory performance. The paper also assesses the putative role of different neurotransmitter systems and neuropeptides in the regulation of memory processes and anxiety, and discusses the role of neural plasticity in these mechanisms.

Central and medial amygdaloid brain-derived neurotrophic factor signaling plays a critical role in alcohol-drinking and anxiety-like behaviors

Brain-derived neurotrophic factor (BDNF) is a member of the nerve growth factor family of neurotrophins and plays a vital role in synaptic plasticity. This study investigated the involvement of the amygdaloid BDNF system in molecular mechanisms underlying anxiety and alcohol-drinking behaviors. Male Sprague Dawley rats were cannulated targeting central amygdala (CeA), medial amygdala (MeA), or basolateral amygdala (BLA), and BDNF expression was manipulated using an antisense oligodeoxynucleotide (ODN) strategy. Anxiety-like and alcohol-drinking behaviors were measured after infusion of BDNF sense and antisense ODNs with or without BDNF coinfusion, using the elevated plus-maze test and two-bottle free-choice paradigm, respectively. Here we report that BDNF antisense ODN infusions into the CeA and MeA, but not BLA, provoked anxiety-like behaviors in rats, which were rescued by BDNF coinfusion. The levels of BDNF, p-ERK1/2 (phosphorylated extracellular signal-regulated kinases 1/2), and p-CREB (phosphorylated cAMP responsive-element binding protein) were decreased by BDNF antisense, but not by sense, ODN infusions, which were restored to normal after BDNF coinfusions. Furthermore, BDNF antisense ODN infusions into the CeA or MeA, but not into BLA, increased alcohol intake, which was attenuated by BDNF coinfusions. These novel results suggest that decreased BDNF levels in the CeA and MeA, but not in the BLA, are crucial in regulating alcohol-drinking and anxiety-like behaviors in rats.

Structural and functional modifications in glutamateric synapses following prolonged ethanol exposure

This article summarizes the proceedings of a symposium presented at the 2005 annual meeting of the Research Society on Alcoholism in Santa Barbara, California, USA. The organizer and chair was L. Judson Chandler. The presentations were (1) Chronic Ethanol Exposure, N-Methyl-D-Aspartate (NMDA) Receptor Dynamics, and Withdrawal Hyperexcitability, by Adam Hendricson, Regina Maldve, and Richard Morrisett; (2) Ethanol-Induced Synaptic Targeting of NMDA Receptors Is Associated With Enhanced Postsynaptic Density-95 Clustering and Spine Size, by Judson Chandler and Ezekiel Carpenter-Hyland; (3) Presynaptic and Postsynaptic Alterations in the Nucleus Accumbens Following Chronic Alcohol Exposure, by Feng Zhou, Youssef Sari, and Richard Bell; and (4) An Active Role for Accumbens Homer2 Expression in Alcohol-Induced Neural Plasticity, by Karen Szumlinski.

Early social enrichment augments adult hippocampal BDNF levels and survival of BrdU-positive cells while increasing anxiety- and “depression”-like behavior

Early experiences affect brain function and behavior at adulthood. Being reared in a communal nest (CN), consisting of a single nest where three mothers keep their pups together and share care-giving behavior from birth to weaning (postnatal day [PND] 25), provides an highly socially stimulating environment to the developing pup. Communal nest characterizes the natural ecologic niche of many rodent species including the mouse. At adulthood, CN reared mice, compared to mice reared in standard nesting laboratory condition (SN), show an increase in BDNF protein levels and longer survival of BrdU-positive cells in the hippocampus. Open field and elevated plus maze results indicate that CN mice, although showing levels of exploratory and locomotor activity similar to those of SN mice, displayed increased anxiety-like behavior, performing more thigmotaxis in the open field and spending less time in the open arms of the plus maze. Furthermore, CN mice displayed higher levels of immobility behavior in the forced swim test. Overall, these findings show that CN, an highly stimulating early social environment, increases adult neuronal plasticity, as suggested by high BDNF levels and augmented number of newly generated cells in the hippocampus, which is associated to an increased anxiety- and "depression"-like behavior. These findings are discussed in the framework of the neurotrophin hypothesis of depression.

AIT-082, a novel purine derivative with neuroregenerative properties

Preclinical and clinical evidence support the effectiveness of neurotrophins in Alzheimer's disease (AD) and neurodegenerative diseases. Delivery of neurotrophins to target sites in the brain remains a major obstacle for their use in humans. Development of orally active agents that mimic the effects of nerve growth factor and other neurotrophins provides a promising alternative therapeutic strategy. AIT-082, a purine analogue, has been shown to reverse age-induced memory deficits in mice and is a growth factor-mimetic agent. It is orally active, rapidly penetrates the blood-brain barrier and induces the production of multiple growth factors at the appropriate target site in the central nervous system (CNS).

Sexual dimorphisms in the effect of low-level p25 expression on synaptic plasticity and memory

p25, a degradation product of p35, has been reported to accumulate in the forebrain of patients with Alzheimer's disease. p25 as well as p35 are activators of cyclin-dependent kinase 5 (Cdk5) although p25/Cdk5 and p35/Cdk5 complexes have distinct properties. Several mouse models with high levels of p25 expression exhibit signs of neurodegeneration. On the contrary, we have shown that low levels of p25 expression do not cause neurodegeneration and are even beneficial for particular types of learning and memory [Angelo et al., (2003) Eur J. Neurosci., 18, 423-431]. Here, we have studied the influence of low-level p25 expression in hippocampal synaptic plasticity and in learning and memory for each sex separately in two different genetic backgrounds (129B6F1 and C57BL/6). Surprisingly, we found that low-level p25 expression had different consequences in male and female mutants. In the two genetic backgrounds LTP induced by a strong stimulation of the Schaffer's collaterals (four trains, 1-s duration, 5-min interval) was severely impaired in male, but not in female, p25 mutants. Furthermore, in the two genetic backgrounds spatial learning in the Morris water maze was faster in female p25 mutants than in male transgenic mice. These results suggest that, in women, the production of p25 in Alzheimer's disease could be a compensation for some early learning and memory deficits.

Brain-derived neurotrophic factor plays a critical role in contextual fear conditioning

In this study, brain-derived neurotrophic factor (BDNF) heterozygous knock-outs were tested on fear conditioning, and their wild-type littermates were used as controls. Results showed that BDNF(+/-) mice are impaired in contextual learning, whereas tone learning remains intact. Because BDNF is involved in synaptic transmission and contextual learning is hippocampal dependent, we hypothesized that this deficit is attributable to abnormal BDNF-modulated synaptic plasticity in the hippocampus. A "gain-of-function" experiment was performed next by infusing recombinant BDNF protein into the hippocampal formation to investigate whether this deficit can be rescued. Infusion of BDNF protein into the hippocampus appeared to partially restore contextual fear learning of BDNF(+/-) mice. In conclusion, the present study suggests that BDNF plays a critical role in fear conditioning. Loss of one copy of the BDNF gene leads to impairment of contextual fear learning in BDNF(+/-). This deficit can be partially rescued by infusing BDNF protein into the hippocampus. Other brain regions interacting with the hippocampus in the context conditioned stimulus pathway, for example, the amygdala, may also require normal BDNF expression levels to fully rescue this impairment.

Intrahippocampal administration of BDNF in adult rats affects short-term behavioral plasticity in the Morris water maze and performance in the elevated plus-maze

The present study evaluated the effects of a single intrahippocampal administration of brain-derived neurotrophic factor (BDNF) on memory retention in a water maze. Adult rats were trained in a water maze (acquisition phase, day 1). Immediately after the last training trial subjects were injected in the right hippocampus with either BDNF (24 microg) or phosphate-buffered saline (1 microl). On day 2, all subjects were tested for memory retention in a probe trial and were subsequently tested for reversal learning. While no differences emerged in the probe trial, BDNF-treated subjects showed a shorter latency and a shorter path length to reach the platform during the reversal phase. A significant difference in their "turn angle" and in their swim paths suggests that they might have used a different search strategy compared with controls. Moreover, all subjects also underwent an elevated-plus maze test. BDNF-treated-animals showed a clear tendency to spend a greater amount of time in the open arms and a significantly higher frequency of grooming behavior and of the stretched-attend posture in this maze area, but no differences in locomotion. Overall, these results indicate that administration of BDNF improves performance in a spatial memory task and has enduring effects on emotional behavior.

Transgenic mice overexpressing the full-length neurotrophin receptor trkB exhibit increased activation of the trkB-PLCgamma pathway, reduced anxiety, and facilitated learning

We have investigated the biochemical, physiological, and behavioral properties of transgenic mice overexpressing the full-length neurotrophin receptor trkB (trkB.TK+). The highest trkB.TK+ mRNA overexpression was achieved in the cerebral cortex and hippocampal subfields, both areas also showing strongly increased trkB.TK+ receptor protein expression and phosphorylation. Furthermore, as a result of trkB.TK+ overexpression, partial activation of trkB downstream signaling was observed. Phosphorylation of phospholipaseCgamma-1 was increased but unexpectedly, the expression and phosphorylation levels of signaling molecules Shc and mitogen-activated protein kinase (MAPK) were unaltered. Behavioral studies revealed improved learning and memory in the water maze, contextual fear conditioning, and conditioned taste aversion tests, and reduced anxiety in the elevated plus maze (EPM) and light-dark exploration tests in trkB.TK+ transgenic mice. Electrophysiological studies revealed a reduced long-term potentiation (LTP) at the Schaffer collateral-CA1 synapse in trkB.TK+ mice. Altogether, overexpression of the trkB.TK+ receptor postnatally leads to selective activation of trkB signaling pathways and enhanced learning and memory.

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

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.

Spatial learning is delayed and brain-derived neurotrophic factor mRNA expression inhibited by administration of MK-801 in rats

Brain-derived neurotrophic factor (BDNF) is involved in activity-dependent plasticity and interacts with the neurotransmitter glutamate. Glutamate N-methl-D-aspartate (NMDA) receptor activation increases BDNF expression, while BDNF facilitates NMDA activity, with both involved in spatial learning. Administration of the NMDA receptor antagonist MK-801 can impair this leaning. The interaction between NMDA and BDNF in learning is examined in this study. Adult male Sprague-Dawley rats received either i.p. MK-801 or saline and were trained to locate a submerged water maze platform. Sedentary and activity yoked groups were included for biochemical comparisons. Control rats quickly learned the platform location while MK-801-treated rats learned at a significantly slower rate (P < 0.0001). In situ hybridization for hippocampal BDNF mRNA indicated significant increases in the yoked and learning groups. However, MK-801 attenuated the BDNF mRNA increase in the learning and activity-yoked conditions (P < 0.05). Administration of MK-801 to the sedentary group did not alter baseline mRNA levels. These data suggest that BDNF expression is important for NMDA-dependent learning and memory. Interestingly, learning still occurs across trials independent of the NMDA and BDNF interaction. Increases in BDNF and NMDA activity may be significant components in learning and memory, and modulation of these systems may be beneficial for developing strategies to improve cognitive function.

Increased neurogenesis and brain-derived neurotrophic factor in neurokinin-1 receptor gene knockout mice

It has previously been shown that chronic treatment with antidepressant drugs increases neurogenesis and levels of brain-derived neurotrophic factor in the hippocampus. These changes have been correlated with changes in learning and long-term potentiation and may contribute to the therapeutic efficacy of antidepressant drug treatment. Recently, antagonists at the neurokinin-1 receptor, the preferred receptor for the neuropeptide substance P, have been shown to have antidepressant activity. Mice with disruption of the neurokinin-1 receptor gene are remarkably similar both behaviourally and neurochemically to mice maintained chronically on antidepressant drugs. We demonstrate here that there is a significant elevation of neurogenesis but not cell survival in the hippocampus of neurokinin-1 receptor knockout mice. Neurogenesis can be increased in wild-type but not neurokinin-1 receptor knockout mice by chronic treatment with antidepressant drugs which preferentially target noradrenergic and serotonergic pathways. Hippocampal levels of brain-derived neurotrophic factor are also two-fold higher in neurokinin-1 receptor knockout mice, whereas cortical levels are similar. Finally, we examined hippocampus-dependent learning and memory but found no clear enhancement in neurokinin-1 receptor knockout mice. These data argue against a simple correlation between increased levels of neurogenesis or brain-derived neurotrophic factor and mnemonic processes in the absence of increased cell survival. They support the hypothesis that increased neurogenesis, perhaps accompanied by higher levels of brain-derived neurotrophic factor, may contribute to the efficacy of antidepressant drug therapy.

Learning deficits in forebrain-restricted brain-derived neurotrophic factor mutant mice

Brain-derived neurotrophic factor (BDNF) participates in synaptic plasticity and the adaptive changes in the strength of communication between neurons thought to underlie aspects of behavioral adaptation. By selectively deleting BDNF from the forebrain of mice using the Cre site-specific DNA recombinase, we were able to study the requirements for BDNF in behaviors such as learning and anxiety. Early-onset forebrain-restricted BDNF mutant mice (Emx-BDNF(KO)) that develop in the absence of BDNF in the dorsal cortex, hippocampus, and parts of the ventral cortex and amygdala failed to learn the Morris Water Maze task, a hippocampal-dependent visuo-spatial learning task. Freezing during all phases of cued-contextual fear conditioning, a behavioral task designed to study hippocampal-dependent associative learning, was enhanced. These mice learned a brightness discrimination task well but were impaired in a more difficult pattern discrimination task. Emx-BDNF(KO) mice did not exhibit altered sensory processing and gating, as measured by the acoustic startle response or prepulse inhibition of the startle response. Although they were less active in an open-field arena, they did not show alterations in anxiety, as measured in the elevated-plus maze, black-white chamber or mirrored chamber tasks. Combined, these data indicate that although an absence of forebrain BDNF does not disrupt acoustic sensory processing or alter baseline anxiety, specific forms of learning are severely impaired.

Anxiety and alcohol abuse disorders: a common role for CREB and its target, the neuropeptide Y gene

It has been hypothesized that anxiety disorders play an important role in the initiation and maintenance of alcohol drinking behaviors. However, the molecular mechanisms for the association between anxiety and alcohol abuse are not well understood. Structures of the extended amygdala, particularly the central nucleus of amygdala, are involved in anxiety and in motivational aspects of alcohol drinking behaviors. Here, I propose that cAMP response element-binding protein (CREB) has a role in anxiety and alcohol drinking behaviors. The CREB gene transcription factor regulates the expression of the gene encoding neuropeptide Y (NPY), and decreased concentrations of NPY are implicated in anxiety and alcohol drinking behaviors. Therefore, decreased function of CREB in the central nucleus of the amygdala might regulate anxiety and alcohol intake via decreased expression of NPY, and might provide a common link between anxiety and alcohol abuse disorders. I also suggest that, via CREB, NPY might interact with other CREB target genes, such as the gene encoding brain-derived neurotrophic factor, and that this CREB-mediated interaction might be important in the regulation of anxiety and alcohol drinking behaviors.

Reduced behavioral effects of cocaine in heterozygous brain-derived neurotrophic factor (BDNF) knockout mice

Brain-derived neurotrophic factor (BDNF) affects the development of brain neurotransmitter systems, including dopamine and serotonin systems that are important for cocaine's rewarding and locomotor stimulatory properties. Human genomic markers within or near the BDNF locus have been linked to or associated with substance abuse. Post-mortem human brain specimens reveal individual differences in the levels of BDNF mRNA and in mRNA splicing patterns. To assess the effects of lifelong alterations in the levels of BDNF expression on a measure of psychostimulant reward, we have compared locomotor stimulant and rewarding effects of cocaine in heterozygous BDNF knockout mice with effects in their wild-type littermates. Heterozygous BDNF knockout mice displayed less locomotion during habituation and less locomotion after cocaine injections. Cocaine-conditioned place preferences were reduced in the BDNF heterozygotes. These mice displayed no significant difference from saline control values at a dose of 10 mg/kg s.c. cocaine, although they exhibited cocaine-induced preference at a 20 mg/kg dose. These data confirm important roles for BDNF in psychostimulant actions, presumably via neurotrophic effects on dopamine and serotonin systems. Furthermore, these data support suggestions that differences in human BDNF expression may underlie associations between markers near the human BDNF gene locus and drug addiction.

Heterozygous knock-out mice for brain-derived neurotrophic factor show a pathway-specific impairment of long-term potentiation but normal critical period for monocular deprivation

Genetic deletion of a single allele of the BDNF gene affects hippocampal LTP and causes several behavioral phenotypes, including deficits in spatial learning. In the developing visual cortex, overexpression of BDNF accelerates the time course of the critical period for monocular deprivation (MD), and exogenous administration of BDNF alters the outcome of MD. We asked whether reduced levels of BDNF could affect visual cortex plasticity by studying long-term potentiation (LTP) induction and the effects of MD in heterozygous BDNF knock-out mice. We found that theta burst stimulation that induced LTP in the layer IV-III pathway of wild-type (wt) mice caused only a transient potentiation in BDNF+/- mice, and that this potentiation vanished in 25 min. In contrast, LTP elicited by stimulation of the white matter (WM), a form of LTP that can be induced only during the critical period, occurred normally in wt and BDNF+/- mice. The effects of MD during the critical period were similar in wt and BDNF+/- mice, indicating that layer IV-evoked, layer III LTP is not required for ocular dominance plasticity. We then asked whether reduction of cortical BDNF levels could prolong the critical period for MD and for the WM-evoked, layer III LTP induction. We found that in adult BDNF+/- mice, WM-evoked, layer III LTP was not inducible, and that the critical period for MD terminated normally. We conclude that deletion of one copy of the BDNF gene selectively impairs LTP of the layer IV-III pathway but does not alter ocular dominance plasticity.