BDNF and activity-dependent synaptic modulation

Cochlear implants stimulate activity-dependent CREB pathway in the deaf auditory cortex: implications for molecular plasticity induced by neural prosthetic devices

Neural activity modulates the maturation of synapses and their organization into functional circuits by regulating activity-dependent signaling pathways. Phosphorylation of cyclic AMP/Ca(2+)-responsive element-binding protein (CREB) is widely accepted as a stimulus-inducible event driven by calcium influx into depolarized neurons. In turn, phosphorylated CREB (pCREB) activates the transcription of brain-derived neurotrophic factor (BDNF), which is needed for synaptic transmission and long-term potentiation. We examined how these molecular events are influenced by sensorineural hearing loss and long-term reactivation via cochlear implants. Sensorineural hearing loss reduced the expression of pCREB and BDNF. In contrast, deafened animals subject to long-term, unilateral intracochlear electrical stimulation exhibited an increased expression of pCREB and BDNF in the contralateral auditory cortical neurons, relative to ipsilateral ones. These changes induced by cochlear implants are further accompanied by the activation of the mitogen-activated protein kinase (MAPK) signaling pathway, which has been implicated in long-lasting forms of synaptic plasticity. Because CREB and BDNF are critical modulators of synaptic plasticity, our data describe for the first time possible molecular candidate genes, which are altered in the auditory cortex, following cochlear implantation. These findings provide insights into adaptive, molecular mechanisms recruited by the brain upon functional electrical stimulation by neural prosthetic devices.

Neurotrophins and electrical stimulation for protection and repair of spiral ganglion neurons following sensorineural hearing loss

Exogenous neurotrophins (NTs) have been shown to rescue spiral ganglion neurons (SGNs) from degeneration following a sensorineural hearing loss (SNHL). Furthermore, chronic electrical stimulation (ES) has been shown to retard SGN degeneration in some studies but not others. Since there is evidence of even greater SGN rescue when NT administration is combined with ES, we examined whether chronic ES can maintain SGN survival long after cessation of NT delivery. Young adult guinea pigs were profoundly deafened using ototoxic drugs; five days later they were unilaterally implanted with an electrode array and drug delivery system. Brain derived neurotrophic factor (BDNF) was continuously delivered to the scala tympani over a four week period while the animal simultaneously received ES via bipolar electrodes in the basal turn (i.e., turn 1) scala tympani. One cohort (n=5) received ES for six weeks (i.e., including a two week period after the cessation of BDNF delivery; ES(6)); a second cohort (n=5) received ES for 10 weeks (i.e., a six week period following cessation of BDNF delivery; ES(10)). The cochleae were harvested for histology and SGN density determined for each cochlear turn for comparison with normal hearing controls (n=4). The withdrawal of BDNF resulted in a rapid loss of SGNs in turns 2-4 of the deafened/BDNF-treated cochleae; this was significant as early as two weeks following removal of the NT when compared with normal controls (p<0.05). Importantly, there was not a significant reduction in SGNs in turn 1 (i.e., adjacent to the electrode array) two and six weeks after NT removal, as compared with normal controls. This result suggests that chronic ES can prevent the rapid loss of SGNs that occurs after the withdrawal of exogenous NTs. Implications for the clinical delivery of NTs are discussed.

Distinct role of long 3' UTR BDNF mRNA in spine morphology and synaptic plasticity in hippocampal neurons

The brain produces two brain-derived neurotrophic factor (BDNF) transcripts, with either short or long 3' untranslated regions (3' UTRs). The physiological significance of the two forms of mRNAs encoding the same protein is unknown. Here, we show that the short and long 3' UTR BDNF mRNAs are involved in different cellular functions. The short 3' UTR mRNAs are restricted to somata, whereas the long 3' UTR mRNAs are also localized in dendrites. In a mouse mutant where the long 3' UTR is truncated, dendritic targeting of BDNF mRNAs is impaired. There is little BDNF in hippocampal dendrites despite normal levels of total BDNF protein. This mutant exhibits deficits in pruning and enlargement of dendritic spines, as well as selective impairment in long-term potentiation in dendrites, but not somata, of hippocampal neurons. These results provide insights into local and dendritic actions of BDNF and reveal a mechanism for differential regulation of subcellular functions of proteins.

Postsynaptic action of brain-derived neurotrophic factor attenuates alpha7 nicotinic acetylcholine receptor-mediated responses in hippocampal interneurons

Nicotinic mechanisms acting on the hippocampus influence attention, learning, and memory and constitute a significant therapeutic target for many neurodegenerative, neurological, and psychiatric disorders. Here, we report that brain-derived neurotrophic factor (BDNF) (1-100 ng/ml), a member of the neurotrophin gene family, rapidly decreases alpha7 nicotinic acetylcholine receptor responses in interneurons of the hippocampal CA1 stratum radiatum. Such effect is dependent on the activation of the TrkB receptor and involves the actin cytoskeleton; noteworthy, it is compromised when the extracellular levels of the endogenous neuromodulator adenosine are reduced with adenosine deaminase (1 U/ml) or when adenosine A(2A) receptors are blocked with SCH 58261 (2-(2-furanyl)-7-(2-phenylethyl)-7H-pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-amine) (100 nm). The intracellular application of U73122 (1-[6[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione) (5 mum), a broad-spectrum inhibitor of phospholipase C, or GF 109203X (bisindolylmaleimide I) (2 mum), a general inhibitor of protein kinase C isoforms, blocks BDNF-induced inhibition of alpha7 nicotinic acetylcholine receptor function. Moreover, in conditions of simultaneous intracellular dialysis of the fast Ca(2+) chelator BAPTA (10 mm) and removal of extracellular Ca(2+) ions, the inhibitory action of BDNF is further prevented. The present findings disclose a novel target for rapid actions of BDNF that might play important roles on synaptic transmission and plasticity in the brain.

Full-length human mutant huntingtin with a stable polyglutamine repeat can elicit progressive and selective neuropathogenesis in BACHD mice

To elucidate the pathogenic mechanisms in Huntington's disease (HD) elicited by expression of full-length human mutant huntingtin (fl-mhtt), a bacterial artificial chromosome (BAC)-mediated transgenic mouse model (BACHD) was developed expressing fl-mhtt with 97 glutamine repeats under the control of endogenous htt regulatory machinery on the BAC. BACHD mice exhibit progressive motor deficits, neuronal synaptic dysfunction, and late-onset selective neuropathology, which includes significant cortical and striatal atrophy and striatal dark neuron degeneration. Power analyses reveal the robustness of the behavioral and neuropathological phenotypes, suggesting BACHD as a suitable fl-mhtt mouse model for preclinical studies. Additional analyses of BACHD mice provide novel insights into how mhtt may elicit neuropathogenesis. First, unlike previous fl-mhtt mouse models, BACHD mice reveal that the slowly progressive and selective pathogenic process in HD mouse brains can occur without early and diffuse nuclear accumulation of aggregated mhtt (i.e., as detected by immunostaining with the EM48 antibody). Instead, a relatively steady-state level of predominantly full-length mhtt and a small amount of mhtt N-terminal fragments are sufficient to elicit the disease process. Second, the polyglutamine repeat within fl-mhtt in BACHD mice is encoded by a mixed CAA-CAG repeat, which is stable in both the germline and somatic tissues including the cortex and striatum at the onset of neuropathology. Therefore, our results suggest that somatic repeat instability does not play a necessary role in selective neuropathogenesis in BACHD mice. In summary, the BACHD model constitutes a novel and robust in vivo paradigm for the investigation of HD pathogenesis and treatment.

A bi-directional carboxypeptidase E-driven transport mechanism controls BDNF vesicle homeostasis in hippocampal neurons

Anterograde transport of brain-derived neurotrophic factor (BDNF) vesicles from the soma to neurite terminals is necessary for activity-dependent secretion of BDNF to mediate synaptic plasticity, memory and learning, and retrograde BDNF transport back to the soma for recycling. In our study, overexpression of the cytoplasmic tail of the carboxypeptidase E (CPE) found in BDNF vesicles significantly reduced localization of BDNF in neurites of hippocampal neurons. Live-cell imaging showed that the velocity and distance of movement of fluorescent protein-tagged CPE- or BDNF-containing vesicles were reduced in both directions. In pulldown assays, the CPE tail interacted with dynactin along with kinesin-2 and kinesin-3, and cytoplasmic dynein. Competition assays using a CPE tail peptide verified specific interaction between the CPE tail and dynactin. Thus, the CPE cytoplasmic tail binds dynactin that recruits kinesins or dynein for driving bi-directional transport of BDNF vesicle to maintain vesicle homeostasis and secretion in hippocampal neurons.

Lack of effect of antipsychotics on BNDF and NGF levels in hippocampus of Wistar rats

Schizophrenia is a common and serious mental disorder, in which the majority of patients require long-term antipsychotic treatment. Several studies have suggested that schizophrenia is associated with decreased neurotrophins such as brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF). Investigation of the mechanisms of pharmacological agents that are used in the treatment of schizophrenia has been used to better understand the basis of the pathology associated with this mental illness. The present study aims to investigate the effect of chronic treatment with antipsychotics, named haloperidol (HAL), clozapine (CLO), olanzapine (OLZ) or aripiprazole (ARI) on BDNF and NGF levels in rat hippocampus. Adult male Wistar rats received daily injections of HAL (1.5 mg/kg), CLO (25 mg/kg), OLZ (2.5, 5 or 10 mg/kg) or ARI (2, 10 or 20 mg/kg), whereas control animals were given vehicle. BDNF and NGF levels were measured in rat hippocampus by sandwich-ELISA. The results showed that chronic administration of antipsychotics did not modify BDNF and NGF levels in rat hippocampus, suggesting that their therapeutic properties are not mediated by stimulation of these neurotrophins.

Impact of genetic variant BDNF (Val66Met) on brain structure and function

A common single-nucleotide polymorphism in the human brain-derived neurotrophic factor (BDNF) gene, a methionine (Met) substitution for valine (Val) at codon 66 (Val66Met), is associated with alterations in brain anatomy and memory, but its relevance to clinical disorders is unclear. We generated a variant BDNF mouse (BDNF(MET/Met)) that reproduces the phenotypic hallmarks in humans with the variant allele. Variant BDNF(Met) was expressed in brain at normal levels, but its secretion from neurons was defective. In this context, the BDNF(Met/Met) mouse represents a unique model that directly links altered activity-dependent release of BDNF to a defined set of in vivo consequences. Our subsequent analyses of these mice elucidated a phenotype that had not been established in human carriers: increased anxiety. When placed in conflict settings, BDNF(Met/Met) mice display increased anxiety-related behaviours that were not normalized by the antidepressant, fluoxetine. A genetic variant BDNF may thus play a key role in genetic predispositions to anxiety and depressive disorders.

Cell biology of BDNF and its relevance to schizophrenia

BDNF is a key regulator of synaptic plasticity and hence is thought to be uniquely important for various cognitive functions. While correlations of schizophrenia with polymorphisms in the BDNF gene and changes in BDNF mRNA levels have been reported, specific links remain to be established. Cell biology studies may provide clues as to how BDNF signalling impacts schizophrenia aetiology and pathogenesis: (1) the Val-Met polymorphism in the pro-domain affects activity-dependent BDNF secretion and short-term, hippocampus-mediated episodic memory. (2) pro-BDNF and mBDNF, by interacting with their respective p75(NTR) and TrkB receptors, facilitate long-term depression (LTD) and long-term potentiation (LTP), two common forms of synaptic plasticity working in opposing directions. (3) BDNF transcription is controlled by four promoters, which drive expression of four BDNF-encoding transcripts in different brain regions, cell types and subcellular compartments (dendrites, cell body, etc.), and each is regulated by different genetic and environmental factors. A role for BDNF in early- and late-phase LTP and short- and long-term, hippocampal-dependent memory has been firmly established. Extending these studies to synaptic plasticity in other areas of the brain may help us to better understand how altered BDNF signalling could contribute to intermediate phenotypes associated with schizophrenia.

A causal role for brain-derived neurotrophic factor in the homeostatic regulation of sleep

Slow-wave activity (SWA), the EEG power between 0.5 and 4 Hz during non-rapid eye movement (NREM) sleep, is one of the best characterized markers of sleep need, because it increases as a function of preceding waking duration and decreases during sleep, but the underlying mechanisms remain unknown. We hypothesized that SWA is high at sleep onset because it reflects the occurrence, during the previous waking period, of widespread synaptic potentiation in cortical and subcortical areas. Consistent with this hypothesis, we recently showed that the more rats explore, the stronger is the cortical expression of BDNF during wakefulness, and the larger is the increase in SWA during the subsequent sleep period. There is compelling evidence that BDNF plays a causal role in synaptic potentiation, and exogenous application of BDNF in vivo is sufficient to induce long-term increases in synaptic strength. We therefore performed cortical unilateral microinjections of BDNF in awake rats and measured SWA during the subsequent sleep period. SWA during NREM sleep was higher in the injected hemisphere relative to the contralateral one. The effect was reversible within 2 h, and did not occur during wakefulness or rapid eye movement sleep. Asymmetries in NREM SWA did not occur after vehicle injections. Furthermore, microinjections, during wakefulness, of a polyclonal anti-BDNF antibody or K252a, an inhibitor of BDNF TrkB receptors, led to a local SWA decrease during the following sleep period. These effects were also reversible and specific for NREM sleep. These results show a causal link between BDNF expression during wakefulness and subsequent sleep regulation.

Activity-dependent expression of brain-derived neurotrophic factor in dendrites: facts and open questions

Long-lasting synaptic changes in transmission and morphology at the basis of memory storage, require delivery of newly synthesized proteins to affected synapses. Although many of these proteins are generated in the cell body, several key molecules for plasticity can be delivered in the form of silent mRNAs at synapses in extra somatic compartments where they are locally translated. One of such mRNAs encodes brain-derived neurotrophic factor (BDNF), a key molecule in neuronal development, learning and memory. A single BDNF protein is produced from several splice variants having a different 5' untranslated region. These mRNA variants have a different subcellular localization (soma, proximal or distal dendritic compartment) and may represent a spatial code for a local control of BDNF availability. This review will highlight current knowledge on the mechanisms of spatial and temporal regulation of activity-dependent BDNF mRNA localization in dendrites in relation with synaptic plasticity.

Relapse to cocaine seeking increases activity-regulated gene expression differentially in the prefrontal cortex of abstinent rats

RATIONALE

Alterations in the activity of the prefrontal and orbitofrontal cortices of cocaine addicts have been linked with re-exposure to cocaine-associated stimuli.

OBJECTIVES

Using an animal model of relapse to cocaine seeking, the present study investigated the expression patterns of four different activity-regulated genes within prefrontal cortical brain regions after 22 h or 15 days of abstinence during context-induced relapse.

MATERIALS AND METHODS

Rats self-administered cocaine or received yoked-saline for 2 h/day for 10 days followed by 22 h or 2 weeks of abstinence when they were re-exposed to the self-administration chamber with or without levers available to press for 1 h. Brains were harvested and sections through the prefrontal cortex were processed for in situ hybridization using radioactive oligonucleotide probes encoding c-fos, zif/268, arc, and bdnf.

RESULTS

Re-exposure to the chamber in which rats previously self-administered cocaine but not saline, regardless of lever availability, increased the expression of all genes in the medial prefrontal and orbitofrontal cortices at both time points with one exception: bdnf mRNA was significantly increased in the medial prefrontal cortex at 22 h only if levers previously associated with cocaine delivery were available to press. Furthermore, re-exposure of rats to the chambers in which they received yoked saline enhanced both zif/268 and arc expression selectively in the orbitofrontal cortex after 15 days of abstinence.

CONCLUSIONS

These results support convergent evidence that cocaine-induced changes in the prefrontal cortex are important in regulating drug seeking following abstinence and may provide additional insight into the molecular mechanisms involved in these processes.

Identifying molecular substrates in a mouse model of the serotonin transporter x environment risk factor for anxiety and depression

BACKGROUND

A polymorphism in the serotonin transporter (5-HTT) gene modulates the association between adverse early experiences and risk for major depression in adulthood. Although human imaging studies have begun to elucidate the neural circuits involved in the 5-HTT x environment risk factor, a molecular understanding of this phenomenon is lacking. Such an understanding might help to identify novel targets for the diagnosis and therapy of mood disorders. To address this need, we developed a gene-environment screening paradigm in the mouse.

METHODS

We established a mouse model in which a heterozygous null mutation in 5-HTT moderates the effects of poor maternal care on adult anxiety and depression-related behavior. Biochemical analysis of brains from these animals was performed to identify molecular substrates of the gene, environment, and gene x environment effects.

RESULTS

Mice experiencing low maternal care showed deficient gamma-aminobutyric acid-A receptor binding in the amygdala and 5-HTT heterozygous null mice showed decreased serotonin turnover in hippocampus and striatum. Strikingly, levels of brain-derived neurotrophic factor (BDNF) messenger RNA in hippocampus were elevated exclusively in 5-HTT heterozygous null mice experiencing poor maternal care, suggesting that developmental programming of hippocampal circuits might underlie the 5-HTT x environment risk factor.

CONCLUSIONS

These findings demonstrate that serotonin plays a similar role in modifying the long-term behavioral effects of rearing environment in diverse mammalian species and identifies BDNF as a molecular substrate of this risk factor.

Exercise and brain health--implications for multiple sclerosis: Part 1--neuronal growth factors

The benefits of regular exercise to promote general health and reduce the risk of hypokinetic diseases associated with sedentary lifestyles are well recognized. Recent studies suggest that exercise may enhance neurobiological processes that promote brain health in aging and disease. A current frontier in the neurodegenerative disorder multiple sclerosis (MS) concerns the role of physical activity for promoting brain health through protective, regenerative and adaptive neural processes. Research on neuromodulation, raises the possibility that regular physical activity may mediate favourable changes in disease factors and symptoms associated with MS, in part through changes in neuroactive proteins. Insulin-like growth factor-I appears to act as a neuroprotective agent and studies indicate that exercise could promote this factor in MS. Neurotrophins, brain-derived neurotrophic factor (BDNF) and nerve growth factor likely play roles in neuronal survival and activity-dependent plasticity. Physical activity has also been shown to up-regulate hippocampal BDNF, which may play a role in mood states, learning and memory to lessen the decline in cognitive function associated with MS. In addition, exercise may promote anti-oxidant defences and neurotrophic support that could attenuate CNS vulnerability to neuronal degeneration. Exercise exposure (preconditioning) may serve as a mechanism to enhance stress resistance and thereby may support neuronal survival under heightened stress conditions. Considering that axonal loss and cerebral atrophy occur early in the disease, exercise prescription in the acute stage could promote neuroprotection, neuroregeneration and neuroplasticity and reduce long-term disability. This review concludes with a proposed conceptual model to connect these promising links between exercise and brain health.

Zinc-mediated transactivation of TrkB potentiates the hippocampal mossy fiber-CA3 pyramid synapse

The receptor tyrosine kinase, TrkB, is critical to diverse functions of the mammalian nervous system in health and disease. Evidence of TrkB activation during epileptogenesis in vivo despite genetic deletion of its prototypic neurotrophin ligands led us to hypothesize that a non-neurotrophin, the divalent cation zinc, can transactivate TrkB. We found that zinc activates TrkB through increasing Src family kinase activity by an activity-regulated mechanism independent of neurotrophins. One subcellular locale at which zinc activates TrkB is the postsynaptic density of excitatory synapses. Exogenous zinc potentiates the efficacy of the hippocampal mossy fiber (mf)-CA3 pyramid synapse by a TrkB-requiring mechanism. Long-term potentiation of this synapse is impaired by deletion of TrkB, inhibition of TrkB kinase activity, and by CaEDTA, a selective chelator of zinc. The activity-dependent activation of synaptic TrkB in a neurotrophin-independent manner provides a mechanism by which this receptor can regulate synaptic plasticity.

Rapid activity-dependent modifications in synaptic structure and function require bidirectional Wnt signaling

Activity-dependent modifications in synapse structure play a key role in synaptic development and plasticity, but the signaling mechanisms involved are poorly understood. We demonstrate that glutamatergic Drosophila neuromuscular junctions undergo rapid changes in synaptic structure and function in response to patterned stimulation. These changes, which depend on transcription and translation, include formation of motile presynaptic filopodia, elaboration of undifferentiated varicosities, and potentiation of spontaneous release frequency. Experiments indicate that a bidirectional Wnt/Wg signaling pathway underlies these changes. Evoked activity induces Wnt1/Wg release from synaptic boutons, which stimulates both a postsynaptic DFz2 nuclear import pathway as well as a presynaptic pathway involving GSK-3beta/Shaggy. Our findings suggest that bidirectional Wg signaling operates downstream of synaptic activity to induce modifications in synaptic structure and function. We propose that activation of the postsynaptic Wg pathway is required for the assembly of the postsynaptic apparatus, while activation of the presynaptic Wg pathway regulates cytoskeletal dynamics.

Emergence of brain-derived neurotrophic factor-induced postsynaptic potentiation of NMDA currents during the postnatal maturation of the Kolliker-Fuse nucleus of rat

The Kölliker-Fuse nucleus (KF) contributes essentially to respiratory pattern formation and adaptation of breathing to afferent information. Systems physiology suggests that these KF functions depend on NMDA receptors (NMDA-R). Recent investigations revealed postnatal changes in the modulation of glutamatergic neurotransmission by brain-derived neurotrophic factor (BDNF) in the KF. Therefore, we investigated postnatal changes in NMDA-R subunit composition and postsynaptic modulation of NMDA-R-mediated currents by BDNF in KF slice preparations derived from three age groups (neonatal: postnatal day (P) 1-5; intermediate: P6-13; juvenile: P14-21). Immunohistochemistry showed a developmental up-regulation of the NR2D subunit. This correlated with a developmental increase in decay time of NMDA currents and a decline of desensitization in response to repetitive exogenous NMDA applications. Thus, developmental up-regulation of the NR2D subunit, which reduces the Mg(2+) block of NMDA-R, causes these specific changes in NMDA current characteristics. This may determine the NMDA-R-dependent function of the mature KF in the control of respiratory phase transition. Subsequent experiments revealed that bath-application of BDNF progressively potentiated these repetitively evoked NMDA currents only in intermediate and juvenile age groups. Pharmacological inhibition of protein kinase C (PKC), as a downstream component of the BDNF-tyrosine kinase B receptor (trkB) signalling, prevented BDNF-induced potentiation of NMDA currents. BDNF-induced potentiation of NMDA currents in later developmental stages might be essential for synaptic plasticity during the adaptation of the breathing pattern in response to peripheral/central commands. The lack of plasticity in neonatal neurones strengthens the hypothesis that the respiratory network becomes permissive for activity-dependent plasticity with ongoing postnatal development.

Pain facilitation and activity-dependent plasticity in pain modulatory circuitry: role of BDNF-TrkB signaling and NMDA receptors

Pain modulatory circuitry in the brainstem exhibits considerable synaptic plasticity. The increased peripheral neuronal barrage after injury activates spinal projection neurons that then activate multiple chemical mediators including glutamatergic neurons at the brainstem level, leading to an increased synaptic strength and facilitatory output. It is not surprising that a well-established regulator of synaptic plasticity, brain-derived neurotrophic factor (BDNF), contributes to the mechanisms of descending pain facilitation. After tissue injury, BDNF and TrkB signaling in the brainstem circuitry is rapidly activated. Through the intracellular signaling cascade that involves phospholipase C, inositol trisphosphate, protein kinase C, and nonreceptor protein tyrosine kinases; N-methyl-D-aspartate (NMDA) receptors are phosphorylated, descending facilitatory drive is initiated, and behavioral hyperalgesia follows. The synaptic plasticity observed in the pain pathways shares much similarity with more extensively studied forms of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD), which typically express NMDA receptor dependency and regulation by trophic factors. However, LTP and LTD are experimental phenomena whose relationship to functional states of learning and memory has been difficult to prove. Although mechanisms of synaptic plasticity in pain pathways have typically not been related to LTP and LTD, pain pathways have an advantage as a model system for synaptic modifications as there are many well-established models of persistent pain with clear measures of the behavioral phenotype. Further studies will elucidate cellular and molecular mechanisms of pain sensitization and further our understanding of principles of central nervous system plasticity and responsiveness to environmental challenge.

TrkB kinase is required for recovery, but not loss, of cortical responses following monocular deprivation

Changes in visual cortical responses that are induced by monocular visual deprivation are a widely studied example of competitive, experience-dependent neural plasticity. It has been thought that the deprived-eye pathway will fail to compete against the open-eye pathway for limited amounts of brain-derived neurotrophic factor, which acts on TrkB and is needed to sustain effective synaptic connections. We tested this model by using a chemical-genetic approach in mice to inhibit TrkB kinase activity rapidly and specifically during the induction of cortical plasticity in vivo. Contrary to the model, TrkB kinase activity was not required for any of the effects of monocular deprivation. When the deprived eye was re-opened during the critical period, cortical responses to it recovered. This recovery was blocked by TrkB inhibition. These findings suggest a more conventional trophic role for TrkB signaling in the enhancement of responses or growth of new connections, rather than a role in competition.

Major depression after breast cancer: a review of epidemiology and treatment

OBJECTIVE

While many breast cancer patients experience "normal" distress, there is a subset who experience clinically significant depression. We examined the current knowledge about the prevalence, impact and treatment of major depression in women with breast cancer.

METHOD

We reviewed the evidence for the prevalence of depression in women with breast cancer from the last 20 years and summarized the medical literature on the pharmacology and psychotherapy of depression in this population.

RESULTS

Despite evidence that depression significantly impacts quality of life in breast cancer patients, few studies focus on the epidemiology and treatment of major depression. Treatment studies have focused on distress and mixed depressive states, with resulting lack of replicable studies showing treatment efficacy. Potential biological and psychosocial determinants of major depression following breast cancer are discussed in a proposed model. The need for further research on the epidemiology and treatment of major depression in this population is proposed.

CONCLUSION

Major depression is a frequent but underrecognized and undertreated condition among breast cancer patients, which causes amplification of physical symptoms, increased functional impairment and poor treatment adherence. More research on the epidemiology and treatment of major depression in this population is needed.

Persistent neural activity in the prefrontal cortex: a mechanism by which BDNF regulates working memory?

Working memory is the ability to maintain representations of task-relevant information for short periods of time to guide subsequent actions or make decisions. Neurons of the prefrontal cortex exhibit persistent firing during the delay period of working memory tasks. Despite extensive studies, the mechanisms underlying this persistent neural activity remain largely obscure. The neurotransmitter systems of dopamine, NMDA, and GABA have been implicated, but further investigations are necessary to establish their precise roles and relationships. Recent research has suggested a new component: brain-derived neurotrophic factor (BDNF) and its high-affinity receptor, TrkB. We review the research on persistent activity and suggest that BDNF/TrkB signaling in a distinct class of interneurons plays an important role in organizing persistent neural activity at the single-neuron and network levels.

Interaction between BDNF and serotonin: role in mood disorders

Brain-derived neurotrophic factor (BDNF) and serotonin (5-hydroxytryptamine, 5-HT) are two seemingly distinct signaling systems that play regulatory roles in many neuronal functions including survival, neurogenesis, and synaptic plasticity. A common feature of the two systems is their ability to regulate the development and plasticity of neural circuits involved in mood disorders such as depression and anxiety. BDNF promotes the survival and differentiation of 5-HT neurons. Conversely, administration of antidepressant selective serotonin reuptake inhibitors (SSRIs) enhances BDNF gene expression. There is also evidence for synergism between the two systems in affective behaviors and genetic epitasis between BDNF and the serotonin transporter genes.

Hormesis: a promising strategy to sustain endogenous neuronal survival pathways against neurodegenerative disorders

The brain developed adaptive mechanisms in the face of changing environments and stresses imposed on the nervous system. The addition of glutamate as the major excitatory amino acid neurotransmitter to the brain's complement of amino acids and peptides dictated a coordinated transcriptional and translational program to meet the demands of excitatory neurotransmission. One such program is the ability of neurons to sustain and maintain their survival given the nature of glutamate-mediated receptor activation. The unique development of endogenous neuronal pathways activated by glutamate receptors transformed neurons and allowed them to survive under conditions of high energy demands. These same endogenous survival pathways also mediate plastic responses to meet another demand of the brain, adaptation. An endogenous protein that plays a central role in glutamate receptor-mediated survival pathways is brain-derived neurotrophic factor (BDNF). Intermittent but frequent synaptic ionotropic glutamate receptor activation ensures neuronal survival through a BDNF autocrine loop. In sharp contrast, overactivation of ionotropic glutamate receptors leads to neuronal cell death. Thus, innovative strategies that induce endogenous neuronal survival pathways through low-level activation of ionotropic glutamate receptors or those that bypass receptor activation but upregulate endogenous survival pathways may not only prevent neurodegenerative disorders that involve glutamate as a final common pathway that kills neurons, but may also provide treatment alternatives critical for neurons to survive stressful conditions such as stroke, status epilepticus and hypoglycemic-induced neuronal cell death.

Permissive and repulsive cues and signalling pathways of axonal outgrowth and regeneration

Successful axonal outgrowth in the adult central nervous system (CNS) is central to the process of nerve regeneration and brain repair. To date, much of the knowledge on axonal guidance and outgrowth comes from studies on neuritogenesis and patterning during development where distal growth cones constantly sample the local environment and respond to specific physical and trophic influences. Opposing permissive (e.g., growth factors) and hostile signals (e.g., repulsive cues) are processed, leading to growth cone remodelling, and a concomitant restructuring of the cytoskeleton, thereby permitting pioneering extension and a potential for establishing synaptic connections. Repulsive cues, such as semaphorins, ephrins and myelin-secreted inhibitory glycoproteins, act through their respective receptors to affect the collapsing or turning of growth cones via several pathways, such as the Rho GTPases signalling which precipitates the cytoskeletal changes. One of the direct modulators of microtubules is the family of brain-specific proteins, collapsin response mediator protein (CRMP). Exciting evidence emerged recently that cleavage of CRMPs in response to injury-activated proteases, such as calpain, signals axonal retraction and neuronal death in adult post-mitotic neurons, while blocking this signal transduction prevents axonal retraction and death following excitotoxic insult and cerebral ischemia. Regeneration is minimal in injured postnatal CNS, albeit the occurrence of some limited remodelling in areas where synaptic plasticity is prevalent. Frequently in the absence of axonal regeneration, there is not only an inevitable loss of functional connections, but also a loss of neurons, such as through the actions of dependence receptors. Deciphering the cues and signalling pathways of axonal guidance and outgrowth may hold the key to fully understanding nerve regeneration and brain repair, thereby opening the way for developing potential therapeutics.

The missing piece in the 'use it or lose it' puzzle: is inhibition regulated by activity or does it act on its own accord?

We have gained enormous insight into the mechanisms underlying both activity-dependent and (to a lesser degree) -independent plasticity of excitatory synapses. Recently, cortical inhibition has been shown to play a vital role in the formation of critical periods for sensory plasticity. As such, sculpting of neuronal circuits by inhibition may be a common mechanism by which activity organizes or reorganizes brain circuits. Disturbances in the balance of excitation and inhibition in the neocortex provoke abnormal activities, such as epileptic seizures and abnormal cortical development. However, both the process of experience-dependent postnatal maturation of neocortical inhibitory networks and its underlying mechanisms remain elusive. Mechanisms that match excitation and inhibition are central to achieving balanced function at the level of individual circuits. The goal of this review is to reinforce our understanding of the mechanisms by which developing inhibitory networks are able to adapt to sensory inputs, and to maintain their balance with developing excitatory networks. Discussion is centered on the following questions related to experience-dependent plasticity of neocortical inhibitory networks: 1) What are the roles of GABAergic inhibition in the postnatal maturation of neocortical circuits? 2) Does the maturation of neocortical inhibitory circuits proceed in an activity-dependent manner or do they develop independently of sensory inputs? 3) Does activity regulate inhibitory networks in the same way it regulates excitatory networks? 4) What are the molecular and cellular mechanisms that underlie the activity-dependent maturation of inhibitory networks? 5) What are the functional advantages of experience-dependent plasticity of inhibitory networks to network processing in sensory cortices?

Developmental changes in the BDNF-induced modulation of inhibitory synaptic transmission in the Kölliker-Fuse nucleus of rat

The Kölliker-Fuse nucleus (KF), part of the pontine respiratory group, is involved in the control of respiratory phase duration, and receives both excitatory and inhibitory afferent input from various other brain regions. There is evidence for developmental changes in the modulation of excitatory inputs to the KF by the neurotrophin brain-derived neurotrophic factor (BDNF). In the present study we investigated if BDNF exerts developmental effects on inhibitory synaptic transmission in the KF. Recordings of inhibitory postsynaptic currents (IPSCs) in KF neurons in a pontine slice preparation revealed general developmental changes. Recording of spontaneous and evoked IPSCs (sIPSCs, eIPSCS) revealed that neonatally the gamma-aminobutyric acid (GABA)ergic fraction of IPSCs was predominant, while in later developmental stages glycinergic neurotransmission significantly increased. Bath-application of BDNF significantly reduced sIPSC frequency in all developmental stages, while BDNF-mediated modulation on eIPSCs showed developmental differences. The eIPSCs mean amplitude was uniformly and significantly reduced following BDNF application only in neurons from rats younger than postnatal day 10. At later postnatal stages the response pattern became heterogeneous, and both augmentations and reductions of eIPSC amplitudes occurred. All BDNF effects on eIPSCs and sIPSCs were reversed with the tyrosine kinase receptor-B inhibitor K252a. We conclude that developmental changes in inhibitory neurotransmission, including the BDNF-mediated modulation of eIPSCs, relate to the postnatal maturation of the KF. The changes in BDNF-mediated modulation of IPSCs in the KF may have strong implications for developmental changes in synaptic plasticity and the adaptation of the breathing pattern to afferent inputs.

Association between brain-derived neurotrophic factor Val66Met gene polymorphism and progressive brain volume changes in schizophrenia

OBJECTIVE

Factors underlying progressive brain volume changes in schizophrenia remain poorly understood. The authors investigated whether a gene polymorphism influencing neuroplasticity may contribute to longitudinal brain volume alterations.

METHOD

High-resolution magnetic resonance (MR) images of the whole brain were obtained for 119 patients with recent-onset schizophrenia spectrum disorders. Changes in brain volumes over an average of 3 years were compared between brain-derived neurotrophic factor (BDNF) val66met genotype groupings. Exploratory analyses were conducted to examine relationships between antipsychotic treatment and brain volume changes as well as the effects of BDNF genotype on changes in cognition and symptoms.

RESULTS

Significant genotype effects were observed on within-subject changes in volumes of frontal lobe gray matter, lateral ventricles, and sulcal CSF. Met allele carriers had significantly greater reductions in frontal gray matter volume, with reciprocal volume increases in the lateral ventricles and sulcal (especially frontal and temporal) CSF than Val homozygous patients. Independent of BDNF genotype, more antipsychotic exposure between MRI scans correlated with greater volume reductions in frontal gray matter, particularly among patients who were initially treatment naive. There were no statistically significant genotype effects on within-subject changes in cognition or symptoms.

CONCLUSIONS

BDNF(Met) variant may be one of several factors affecting progressive brain volume changes in schizophrenia.

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.

Expression profiling of Huntington's disease models suggests that brain-derived neurotrophic factor depletion plays a major role in striatal degeneration

Many pathways have been proposed as contributing to Huntington's disease (HD) pathogenesis, but generally the in vivo effects of their perturbation have not been compared with reference data from human patients. Here we examine how accurately mechanistically motivated and genetic HD models recapitulate the striatal gene expression phenotype of human HD. The representative genetic model was the R6/2 transgenic mouse, which expresses a fragment of the huntingtin protein containing a long CAG repeat. Pathogenic mechanisms examined include mitochondrial dysfunction; profiled in 3-nitropropionic acid-treated rats, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mice, and PGC-1alpha knock-out mice; and depletion of brain-derived neurotrophic factor (BDNF) using heterozygous and forebrain-specific BDNF-knock-out mice (BDNF(HET), Emx-BDNF(KO)). Based on striatal gene expression, we find the BDNF models, both heterozygous and homozygous knock-outs, to be more like human HD than the other HD models. This implicates reduced trophic support as a major pathway contributing to striatal degeneration in HD. Because the majority of striatal BDNF is synthesized by cortical neurons, the data also imply that cortical dysfunction contributes to HD's hallmark effects on the basal ganglia. Finally, the results suggest that striatal lesions caused by mitochondrial toxins may arise via pathways different from those that drive neurodegeneration in HD. Based on these findings, we present a testable model of HD pathogenesis that, unlike most models, begins to account for regional specificity in human HD and the absence of such specificity in genetic mouse models of HD.

Brain-derived neurotrophic factor activation of extracellular signal-regulated kinase is autonomous from the dominant extrasynaptic NMDA receptor extracellular signal-regulated kinase shutoff pathway

NMDA receptors bidirectionally modulate extracellular signal-regulated kinase (ERK) through the coupling of synaptic NMDA receptors to an ERK activation pathway that is opposed by a dominant ERK shutoff pathway thought to be coupled to extrasynaptic NMDA receptors. In the present study, synaptic NMDA receptor activation of ERK in rat cortical cultures was partially inhibited by the highly selective NR2B antagonist Ro25-6981 (Ro) and the less selective NR2A antagonist NVP-AAM077 (NVP). When Ro and NVP were added together, inhibition appeared additive and equal to that observed with the NMDA open-channel blocker MK-801. Consistent with a selective coupling of extrasynaptic NMDA receptors to the dominant ERK shutoff pathway, pre-block of synaptic NMDA receptors with MK-801 did not alter the inhibitory effect of bath-applied NMDA on ERK activity. Lastly, in contrast to a complete block of synaptic NMDA receptor activation of ERK by extrasynaptic NMDA receptors, activation of extrasynaptic NMDA receptors had no effect upon ERK activation by brain-derived neurotrophic factor. These results suggest that the synaptic NMDA receptor ERK activation pathway is coupled to both NR2A and NR2B containing receptors, and that the extrasynaptic NMDA receptor ERK inhibitory pathway is not a non-selective global ERK shutoff.

Consensus paper of the WFSBP Task Force on Biological Markers: biological markers in depression

Biological markers for depression are of great interest to aid in elucidating the causes of major depression. We assess currently available biological markers to query their validity for aiding in the diagnosis of major depression. We specifically focus on neurotrophic factors, serotonergic markers, biochemical markers, immunological markers, neuroimaging, neurophysiological findings, and neuropsychological markers. We delineate the most robust biological markers of major depression. These include decreased platelet imipramine binding, decreased 5-HT1A receptor expression, increase of soluble interleukin-2 receptor and interleukin-6 in serum, decreased brain-derived neurotrophic factor in serum, hypocholesterolemia, low blood folate levels, and impaired suppression of the dexamethasone suppression test. To date, however, none of these markers are sufficiently specific to contribute to the diagnosis of major depression. Thus, with regard to new diagnostic manuals such as DSM-V and ICD-11 which are currently assessing whether biological markers may be included in diagnostic criteria, no biological markers for major depression are currently available for inclusion in the diagnostic criteria.

BDNF: a key regulator for protein synthesis-dependent LTP and long-term memory?

It is generally believed that late-phase long-term potentiation (L-LTP) and long-term memory (LTM) require new protein synthesis. Although the full complement of proteins mediating the long-lasting changes in synaptic efficacy have yet to be identified, several lines of evidence point to a crucial role for activity-induced brain-derived neurotrophic factor (BDNF) expression in generating sustained structural and functional changes at hippocampal synapses thought to underlie some forms of LTM. In particular, BDNF is sufficient to induce the transformation of early to late-phase LTP in the presence of protein synthesis inhibitors, and inhibition of BDNF signaling impairs LTM. Despite solid evidence for a critical role of BDNF in L-LTP and LTM, many issues are not resolved. Given that BDNF needs to be processed in Golgi outposts localized at the branch point of one or few dendrites, a conceptually challenging problem is how locally synthesized BDNF in dendrites could ensure synapse-specific modulation of L-LTP. An interesting alternative is that BDNF-TrkB signaling is involved in synaptic tagging, a prominent hypothesis that explains how soma-derived protein could selectively modulate the tetanized (tagged) synapse. Finally, specific roles of BDNF in the acquisition, retention or extinction of LTM remain to be established.

BDNF induces calcium elevations associated with IBDNF, a nonselective cationic current mediated by TRPC channels

Brain-derived neurotrophic factor (BDNF) has potent actions on hippocampal neurons, but the mechanisms that initiate its effects are poorly understood. We report here that localized BDNF application to apical dendrites of CA1 pyramidal neurons evoked transient elevations in intracellular Ca(2+) concentration, which are independent of membrane depolarization and activation of N-methyl-d-aspartate receptors (NMDAR). These Ca(2+) signals were always associated with I(BDNF), a slow and sustained nonselective cationic current mediated by transient receptor potential canonical (TRPC3) channels. BDNF-induced Ca(2+) elevations required functional Trk and inositol-tris-phosphate (IP(3)) receptors, full intracellular Ca(2+) stores as well as extracellular Ca(2+), suggesting the involvement of TRPC channels. Indeed, the TRPC channel inhibitor SKF-96365 prevented BDNF-induced Ca(2+) elevations and the associated I(BDNF). Thus TRPC channels emerge as novel mediators of BDNF-induced intracellular Ca(2+) elevations associated with sustained cationic membrane currents in hippocampal pyramidal neurons.

The Met Allele of the BDNF Val66Met Polymorphism Predicts Poor Outcome Among Survivors of Aneurysmal Subarachnoid Hemorrhage

BACKGROUND AND PURPOSE

Brain-derived neurotrophic factor (BDNF) plays a role in neuronal survival, plasticity and neurogenesis. The BDNF gene contains a common Val66Met polymorphism; the Met allele is associated with lower depolarization-induced BDNF release and differences in memory functions and brain morphology. We hypothesized that the Met allele is associated with poor recovery from subarachnoid hemorrhage.

METHODS

A sample of 105 survivors was assessed at 3 months after subarachnoid hemorrhage using Glascow Outcome Scale. Poor outcome was defined as severe disability or worse. DNA samples were genotyped for the Val66Met polymorphism.

RESULTS

Higher percentage of the Met carriers had a poor outcome (29%) as compared with the Val/Val group (10%; P=0.011). In multiple logistic regression, this association between the Met allele and poor outcome was independent of several other prognostic factors such as patient age, clinical condition, and radiological severity of the bleeding (odds ratio 8.40; 95% CI, 1.60 to 44.00; P=0.012).

CONCLUSIONS

Genetically influenced variation in BDNF function plays a role in recovery from subarachnoid hemorrhage. These data indicate that augmentation of BDNF signaling may be beneficial to recovery from brain injury.

Pharmacologically diverse antidepressants rapidly activate brain-derived neurotrophic factor receptor TrkB and induce phospholipase-Cgamma signaling pathways in mouse brain

Previous studies suggest that brain-derived neurotrophic factor and its receptor TrkB are critically involved in the therapeutic actions of antidepressant drugs. We have previously shown that the antidepressants imipramine and fluoxetine produce a rapid autophosphorylation of TrkB in the rodent brain. In the present study, we have further examined the biochemical and functional characteristics of antidepressant-induced TrkB activation in vivo. We show that all the antidepressants examined, including inhibitors of monoamine transporters and metabolism, activate TrkB rapidly in the rodent anterior cingulate cortex and hippocampus. Furthermore, the results indicate that acute and long-term antidepressant treatments induce TrkB-mediated activation of phospholipase-Cgamma1 (PLCgamma1) and increase the phosphorylation of cAMP-related element binding protein, a major transcription factor mediating neuronal plasticity. In contrast, we have not observed any modulation of the phosphorylation of TrkB Shc binding site, phosphorylation of mitogen-activated protein kinase or AKT by antidepressants. We also show that in the forced swim test, the behavioral effects of specific serotonergic antidepressant citalopram, but not those of the specific noradrenergic antidepressant reboxetine, are crucially dependent on TrkB signaling. Finally, brain monoamines seem to be critical mediators of antidepressant-induced TrkB activation, as antidepressants reboxetine and citalopram do not produce TrkB activation in the brains of serotonin- or norepinephrine-depleted mice. In conclusion, our data suggest that rapid activation of the TrkB neurotrophin receptor and PLCgamma1 signaling is a common mechanism for all antidepressant drugs.

Adenosine A(2A) receptors modulate BDNF both in normal conditions and in experimental models of Huntington's disease

Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, enhances synaptic transmission and regulates neuronal proliferation and survival. Functional interactions between adenosine A_2A receptors (A_2ARs) and BDNF have been recently reported. In this article, we report some recent findings from our group showing that A_2ARs regulate both BDNF functions and levels in the brain. Whereas BDNF (10 ng/ml) increased the slope of excitatory postsynaptic field potentials (fEPSPs) in hippocampal slices from wild-type (WT) mice, it was completely ineffective in slices taken from A_2AR knock-out (KO) mice. Furthermore, enzyme immunoassay studies showed a significant reduction in hippocampal BDNF levels in A_2AR KO vs. WT mice. Having found an even marked reduction in the striatum of A_2AR KO mice, and as both BDNF and A_2ARs have been implicated in the pathogenesis of Huntington’s disease (HD), an inherited striatal neurodegenerative disease, we then evaluated whether the pharmacological blockade of A_2ARs could influence striatal levels of BDNF in an experimental model of HD-like striatal degeneration (quinolinic acid-lesioned rats) and in a transgenic mice model of HD (R6/2 mice). In both QA-lesioned rats and early symptomatic R6/2 mice (8 weeks), the systemic administration of the A_2AR antagonist SCH58261 significantly reduced striatal BDNF levels. These results indicate that the presence and the tonic activation of A_2ARs are necessary to allow BDNF-induced potentiation of synaptic transmission and to sustain a normal BDNF tone. The possible functional consequences of reducing striatal BDNF levels in HD models need further investigation.

Locomotor ability in spinal rats is dependent on the amount of activity imposed on the hindlimbs during treadmill training

Studies have shown that treadmill training with body weight support is effective for enhancing locomotor recovery following a complete spinal cord transection (ST) in animals. However, there have been no studies that have investigated the extent that functional recovery in ST animals is dependent on the amount of activity imposed on the hindlimbs during training. In rats transected as neonates (P5), we used a robotic device to impose either a high or a low amount of hindlimb activity during treadmill training starting 23 days after transection. The rats were trained 5 days per week for 4 weeks. One group (n = 13) received 1000 steps/training session and a second group (n = 13) received 100 steps/training session. During training, the robotic device imposed the maximum amount of weight that each rat could bear on the hindlimbs, and counted the number of stepping movements during each session. After 4 weeks of training, the number of steps performed during treadmill testing was not significantly different between the two groups. However, the quality of stepping in the group that received 1000 steps/training session improved over a range of levels of weight bearing on the hindlimbs and at different treadmill speeds. In contrast, little improvement in the quality of stepping was observed in the group that received only 100 steps/training session. These findings indicate that the ability of the lumbar spinal cord to adjust to load- and speed-related sensory stimuli associated with stepping is dependent on the number of repetitions of the same activity that is imposed on the spinal circuits during treadmill training.

Brain-derived neurotrophic factor and TrkB modulate visual experience-dependent refinement of neuronal pathways in retina

Sensory experience refines neuronal structure and functionality. The visual system has proved to be a productive model system to study this plasticity. In the neonatal retina, the dendritic arbors of a large proportion of ganglion cells are diffuse in the inner plexiform layer. With maturation, many of these arbors become monolaminated. Visual deprivation suppresses this remodeling. Little is known of the molecular mechanisms controlling maturational and experience-dependent refinement. Here, we tested the hypothesis that brain-derived neurotrophic factor (BDNF), which is known to regulate dendritic branching and synaptic function in the brain, modulates the developmental and visual experience-dependent refinement of retinal ganglion cells. We used a transgenic mouse line, in which a small number of ganglion cells were labeled with yellow fluorescence protein, to delineate their dendritic structure in vivo. We found that transgenic overexpression of BDNF accelerated the laminar refinement of ganglion cell dendrites, whereas decreased TrkB expression or retina-specific deletion of TrkB, the cognate receptor for BDNF, retarded it. BDNF-TrkB signaling regulated the maturational formation of new branches in ON but not the bilaminated ON-OFF ganglion cells. Furthermore, BDNF overexpression overrides the requirement for visual inputs to stimulate laminar refinement and dendritic branching of ganglion cells. These experiments reveal a previously unrecognized action of BDNF and TrkB in controlling cell-specific, experience-dependent remodeling of neuronal structures in the visual system.

A local reduction in cortical GABAergic synapses after a loss of endogenous brain-derived neurotrophic factor, as revealed by single-cell gene knock-out method

To address questions of whether brain-derived neurotrophic factor (BDNF) released from active excitatory neurons acts locally only on GABAergic presynaptic terminals contacting these neurons or generally also on GABAergic terminals contacting other inactive neurons, we developed a single-cell gene knock-out method in organotypic slice culture of visual cortex of floxed BDNF transgenic mice. A biolistic transfection of Cre recombinase with green fluorescence protein (GFP) plasmids to layer II/III of the cortex resulted in loss of BDNF in a single neuron or a small number of neurons, which expressed GFP at 13-14 d in vitro. Analysis with in situ hybridization and immunohistochemistry confirmed that neurons expressing GFP lacked BDNF mRNA and protein, respectively. Analysis with immunohistochemistry using antibody against GABA synthesizing enzyme showed that the number of GABAergic terminals on the soma of BDNF knock-out neurons was smaller than that of neighboring control neurons. Morphological analysis indicated that there was no significant difference in the soma size and branch points and length of dendrites between the BDNF knock-out and control neurons. Recordings of miniature IPSCs (mIPSCs) showed that the frequency of mIPSCs of BDNF knock-out neurons was lower than that of control neurons, although the amplitude was not significantly different, suggesting the smaller number of functional GABAergic synapses on whole the BDNF knock-out neuron. The present results suggest that BDNF released from postsynaptic target neurons promotes the formation or proliferation of GABAergic synapses through its local actions in layer II/III of visual cortex.

Neurotrophic factors promote and enhance locomotor recovery in untrained spinalized cats

In spinal cats, locomotor recovery without rehabilitation is limited, but weight-bearing stepping returns with treadmill training. We studied whether neurotrophins administered to the injury site also restores locomotion in untrained spinal cats and whether combining both neurotrophins and training further improves recovery. Ordinary rat fibroblasts or a mixture of fibroblasts secreting brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) (Fb-NTF) were grafted into T12 spinal transection sites. Cats with each type of transplant were divided into two groups: one receiving daily training and the other receiving no training. As expected, trained cats with/without neurotrophin-producing transplants could step on the treadmill. Untrained cats without neurotrophin-producing transplants could not locomote. However, untrained cats with neurotrophin-secreting transplants performed plantar weight-bearing stepping at speeds up to 0.8 m/s as early as 2 wk after transection. Locomotor capability and stance lengths in these animals were similar to those in animals receiving training alone, suggesting that administration of BDNF/NT-3 was equivalent to treadmill training in restoring locomotion in chronically spinalized cats. Cats receiving both interventions showed the greatest improvement in step length. Anatomical evaluation indicated that all transections were complete and that axons did not enter the cord caudal to the graft. Thus BDNF/NT-3 secreting fibroblasts were equivalent to training in their ability to engage the locomotor circuitry in chronic spinal cats. Furthermore, the rapid time-course of recovery and the absence of axonal growth through the transplants indicate that the restorative mechanisms were not related to supraspinal axonal growth. Finally, the results show that transplants beneficial in rodents are applicable to larger mammals.

Dissecting the human BDNF locus: bidirectional transcription, complex splicing, and multiple promoters

Brain-derived neurotrophic factor (BDNF), a member of the nerve growth factor family of neurotrophins, has central roles in the development, physiology, and pathology of the nervous system. We have elucidated the structure of the human BDNF gene, identified alternative transcripts, and studied their expression in adult human tissues and brain regions. In addition, the transcription initiation sites for human BDNF transcripts were determined and the activities of BDNF promoters were analyzed in transient overexpression assays. Our results show that the human BDNF gene has 11 exons and nine functional promoters that are used tissue and brain-region specifically. Furthermore, noncoding natural antisense RNAs that display complex splicing and expression patterns are transcribed in the BDNF gene locus from the antiBDNF gene (approved gene symbol BDNFOS). We show that BDNF and antiBDNF transcripts form dsRNA duplexes in the brain in vivo, suggesting an important role for antiBDNF in regulating BDNF expression in human.

TRPC3 channels are necessary for brain-derived neurotrophic factor to activate a nonselective cationic current and to induce dendritic spine formation

Brain-derived neurotrophic factor (BDNF) exerts prominent effects on hippocampal neurons, but the mechanisms that initiate its actions are poorly understood. We report here that BDNF evokes a slowly developing and sustained nonselective cationic current (I(BDNF)) in CA1 pyramidal neurons. These responses require phospholipase C, IP3 receptors, Ca2+ stores, and Ca2+ influx, suggesting the involvement of transient receptor potential canonical subfamily (TRPC) channels. Indeed, I(BDNF) is absent after small interfering RNA-mediated TRPC3 knockdown. The sustained kinetics of I(BDNF) appears to depend on phosphatidylinositol 3-kinase-mediated TRPC3 membrane insertion, as shown by surface biotinylation assays. Slowly emerging membrane currents after theta burst stimulation are sensitive to the scavenger TrkB-IgG and TRPC inhibitors, suggesting I(BDNF) activation by evoked released of endogenous, native BDNF. Last, TRPC3 channels are necessary for BDNF to increase dendritic spine density. Thus, TRPC channels emerge as novel mediators of BDNF-mediated dendritic remodeling through the activation of a slowly developing and sustained membrane depolarization.

Developmental changes in brain-derived neurotrophic factor-mediated modulations of synaptic activities in the pontine Kölliker-Fuse nucleus of the rat

The Kölliker-Fuse nucleus (KF), part of the respiratory network, is involved in the modulation of respiratory phase durations in response to peripheral and central afferent inputs. The KF is immature at birth. Developmental changes in its physiological and anatomical properties have yet to be investigated. Since brain-derived neurotrophic factor (BDNF) is of major importance for the maturation of neuronal networks, we investigated its effects on developmental changes in the KF on different postnatal days (neonatal, P1-5; intermediate, P6-13; juvenile, P14-21) by analysing single neurones in the in vitro slice preparation and network activities in the perfused brainstem preparation in situ. The BDNF had only weak effects on the frequency of mixed excitatory and inhibitory spontaneous postsynaptic currents (sPSCs) in neonatal slice preparations. Postnatally, in the intermediate and juvenile age groups, a significant augmentation of the sPSC frequency was observed in the presence of 100 pm BDNF (+23.5+/-12.6 and +76.7+/-28.4%, respectively). Subsequent analyses of BDNF effects on evoked excitatory postsynaptic currents (eEPSCs) revealed significant enhancement of eEPSC amplitude of +20.8+/-7.0% only in juvenile stages (intermediates, -13.2+/-4.8%). On the network level, significant modulation of phrenic nerve activity following BDNF microinjection into the KF was also observed only in juveniles. The data suggest that KF neurones are subject to BDNF-mediated fast synaptic modulation after completion of postnatal maturation. After maturation, BDNF contributes to modulation of fast excitatory neurotransmission in respiratory-related KF neurones. This may be important for network plasticity associated with the processing of afferent information.

Transducer of regulated CREB and late phase long-term synaptic potentiation

In the central nervous system, long-term adaptive responses to changes in the environment, such as the processes involved in learning and memory, require the conversion of extracellular stimuli into intracellular signals. Many of these signals involve the induction of gene expression. The late, transcription- and translation-dependent phase of long-term synaptic potentiation (L-LTP) is an attractive cellular model for long-lasting memory formation. The transcription factor cAMP response element-binding protein (CREB) plays an essential role in the maintenance of L-LTP. However, how synaptic signals propagate to the nucleus to initiate CREB-target gene expression is unclear. Recent studies indicate that the CREB transducer of regulated CREB activity 1 coactivator undergoes neuronal activity-dependent translocation from the cytoplasm to the nucleus, a process required for CRE-dependent gene expression and the maintenance of L-LTP in the hippocampus.

Acute role of the brain-derived neurotrophic factor (BDNF) on the respiratory neural network activity in mice in vitro

In humans, several pathologies are associated with disturbances of the respiratory control, some of them including alteration in the brain-derived neurotrophic factor (BDNF) signalling pathway. BDNF has long been known as a neurotrophic factor involved in survival, differentiation and maintenance of neuronal populations in the peripheral and central nervous system. More recently BDNF has also been discovered to be a potent neuromodulator with acute effects on neuronal excitability and synaptic plasticity. Animals deleted for the gene encoding BDNF exhibit respiratory alteration suggesting an important but yet undefined role of the neurotrophin in respiratory rhythmogenesis either by a trophic and/or an acute action. The possibility that BDNF might exert an acute regulatory role on the rhythmic activity of the respiratory generator of the pre-Bötzinger complex has been recently examined in newborn mice in vitro. Results obtained, reviewed in the present paper, will help getting insights in respiratory rhythm regulatory mechanisms that involve BDNF signalling.