Brain insults in rats induce increased expression of the BDNF gene through differential use of multiple promoters

BDNF signaling in context: From synaptic regulation to psychiatric disorders

Brain-derived neurotrophic factor (BDNF) is a neuropeptide that plays numerous important roles in synaptic development and plasticity. While its importance in fundamental physiology is well established, studies of BDNF often produce conflicting and unclear results, and the scope of existing research makes the prospect of setting future directions daunting. In this review, we examine the importance of spatial and temporal factors on BDNF activity, particularly in processes such as synaptogenesis, Hebbian plasticity, homeostatic plasticity, and the treatment of psychiatric disorders. Understanding the fundamental physiology of when, where, and how BDNF acts and new approaches to control BDNF signaling in time and space can contribute to improved therapeutics and patient outcomes.

Brain-Derived Neurotrophic Factor in Amygdala-Dependent Learning

The neurotrophin brain-derived neurotrophic factor (BDNF) has recently emerged as a possible molecular mediator of activity-dependent synaptic plasticity underlying learning and memory. Long-term potentiation (LTP) within the hippocampus and hippocampally dependent behaviors has been the primary model for examining the role of BDNF in learning and memory. However, these studies are limited by an incomplete understanding of the complex behavioral function of hippocampal circuitry, making it difficult to unravel the molecular machinery responsible for the formation and storage of these memories. In contrast, the amygdala and its role in Pavlovian fear conditioning promise to provide us with new insights into the mechanisms of BDNF-mediated synaptic plasticity during the learning and memory process. This article reviews the different levels of research on BDNF in learning and memory. The focus is primarily on the use of Pavlovian fear conditioning as a learning model that allows for the examination of the role of BDNF in the amygdala, following a single learning session and within a well-understood neural circuit.

The Antisense Transcriptome and the Human Brain

The transcriptome of a cell is made up of a varied array of RNA species, including protein-coding RNAs, long non-coding RNAs, short non-coding RNAs, and circular RNAs. The cellular transcriptome is dynamic and can change depending on environmental factors, disease state and cellular context. The human brain has perhaps the most diverse transcriptome profile that is enriched for many species of RNA, including antisense transcripts. Antisense transcripts are produced when both the plus and minus strand of the DNA helix are transcribed at a particular locus. This results in an RNA transcript that has a partial or complete overlap with an intronic or exonic region of the sense transcript. While antisense transcription is known to occur at some level in most organisms, this review focuses specifically on antisense transcription in the brain and how regulation of genes by antisense transcripts can contribute to functional aspects of the healthy and diseased brain. First, we discuss different techniques that can be used in the identification and quantification of antisense transcripts. This is followed by examples of antisense transcription and modes of regulatory function that have been identified in the brain.

An RNA binding protein promotes axonal integrity in peripheral neurons by destabilizing REST

The RE1 Silencing Transcription Factor (REST) acts as a governor of the mature neuronal phenotype by repressing a large consortium of neuronal genes in non-neuronal cells. In the developing nervous system, REST is present in progenitors and downregulated at terminal differentiation to promote acquisition of mature neuronal phenotypes. Paradoxically, REST is still detected in some regions of the adult nervous system, but how REST levels are regulated, and whether REST can still repress neuronal genes, is not known. Here, we report that homeostatic levels of REST are maintained in mature peripheral neurons by a constitutive post-transcriptional mechanism. Specifically, using a three-hybrid genetic screen, we identify the RNA binding protein, ZFP36L2, associated previously only with female fertility and hematopoiesis, and show that it regulates REST mRNA stability. Dorsal root ganglia in Zfp36l2 knock-out mice, or wild-type ganglia expressing ZFP36L2 shRNA, show higher steady-state levels of Rest mRNA and protein, and extend thin and disintegrating axons. This phenotype is due, at least in part, to abnormally elevated REST levels in the ganglia because the axonal phenotype is attenuated by acute knockdown of REST in Zfp36l2 KO DRG explants. The higher REST levels result in lower levels of target genes, indicating that REST can still fine-tune gene expression through repression. Thus, REST levels are titrated in mature peripheral neurons, in part through a ZFP36L2-mediated post-transcriptional mechanism, with consequences for axonal integrity.

Immune dysregulation and cognitive vulnerability in the aging brain: Interactions of microglia, IL-1β, BDNF and synaptic plasticity

Older individuals often experience declines in cognitive function after events (e.g. infection, or injury) that trigger activation of the immune system. This occurs at least in part because aging sensitizes the response of microglia (the brain's resident immune cells) to signals triggered by an immune challenge. In the aging brain, microglia respond to these signals by producing more pro-inflammatory cytokines (e.g. interleukin-1beta or IL-1β) and producing them for longer than microglia in younger brains. This exaggerated inflammatory response can compromise processes critical for optimal cognitive functioning. Interleukin-1β is central to the inflammatory response and is a key mediator and modulator of an array of associated biological functions; thus its production and release is usually very tightly regulated. This review will focus on the impact of dysregulated production of IL-1β on hippocampus dependent-memory systems and associated synaptic plasticity processes. The neurotrophin brain-derived neurotrophic factor (BNDF) helps to protect neurons from damage caused by infection or injury, and it plays a critical role in many of the same memory and hippocampal plasticity processes compromised by dysregulated production of IL-1β. This suggests that an exaggerated brain inflammatory response, arising from aging and a secondary immune challenge, may erode the capacity to provide the BDNF needed for memory-related plasticity processes at hippocampal synapses. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

Neurotrophins: transcription and translation

Neurotrophins are powerful molecules. Small quantities of these secreted proteins exert robust effects on neuronal survival, synapse stabilization, and synaptic function. Key functions of the neurotrophins rely on these proteins being expressed at the right time and in the right place. This is especially true for BDNF, stimulus-inducible expression of which serves as an essential step in the transduction of a broad variety of extracellular stimuli into neuronal plasticity of physiologically relevant brain regions. Here we review the transcriptional and translational mechanisms that control neurotrophin expression with a particular focus on the activity-dependent regulation of BDNF.

BDNF transcripts, proBDNF and proNGF, in the cortex and hippocampus throughout the life span of the rat

Neurotrophins are established molecular mediators of neuronal plasticity in the adult brain. We analyzed the impact of aging on brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) protein isoforms, their receptors, and on the expression patterns of multiple 5' exon-specific BDNF transcripts in the rat cortex and hippocampus throughout the life span of the rat (6, 12, 18, and 24 months of age). ProNGF was increased during aging in both structures. Mature NGF gradually decreased in the cortex, and, in 24-month-old animals, it was 30% lower than that in adult 6-month-old rats. The BDNF expression did not change during aging, while proBDNF accumulated in the hippocampus of aged rats. Hippocampal total BDNF mRNA was lower in 12-month-old animals, mostly as a result of a decrease of BDNF transcripts 1 and 2. In contrast to the region-specific regulation of specific exon-containing BDNF mRNAs in adult animals, the same BDNF RNA isoforms (containing exons III, IV, or VI) were present in both brain structures of aged animals. Deficits in neurotrophin signaling were supported by the observed decrease in Trk receptor expression which was accompanied by lower levels of the two main downstream effector kinases, pAkt and protein kinase C. The proteolytic processing of p75NTR observed in 12-month-old rats points to an additional regulatory mechanism in early aging. The changes described herein could contribute to reduced brain plasticity underlying the age-dependent decline in cognitive function.

The potent effects of ginseng root extract and memantine on cognitive dysfunction in male albino rats

The study determined the maximum intraperitoneal (ip) scopolamine dose inducing memory impairment in rats (2 mg/kg) compared to 0.5 or 1 mg/kg dose. The effect reflected by significant increase from normal in the latency time required for rats to find the hidden platform in water maze task and acetylcholinesterase (AChE) activities in cortex, hippocampus and striatum. The dose-related histopathological effect via the hemorrhage, vacuolation and gliosis in cortex and hippocampus is assessed. Then the study investigated the potency of Panax ginseng root extract on scopolamine cognitive dysfunction rat model compared to memantine hydrochloride as reference Food and Drug Administration approved. Ginseng extract was administered at dose 100 or 200 mg/kg/day and memantine at 20 mg/kg/day orally for 2 weeks. All treatments showed improvement in the water maze task, however, ginseng (200 mg/kg) group acquired the advantage without statistical difference control. Scopolamine (2 mg/kg ip) group showed significant increase in AChE reactivity and glutamate level and reduced monoamines (norepinephrine, dopamine and serotonin) and γ-aminobutyric acid contents in cortex, hippocampus and striatum. Ginseng extract in a dose-dependent manner appears effective as memantine and can improve memory impairment through the retrieved homeostasis via neurotransmitter levels and AChE activities in rat brain areas with partial effect on the histological feature of the brain tissue.

Hepatocyte growth factor overexpression in the nervous system enhances learning and memory performance in mice

Hepatocyte growth factor (HGF) and its receptor, c-Met, play pivotal roles in the nervous system during development and in disease states. However, the physiological roles of HGF in the adult brain are not well understood. In the present study, to assess its role in learning and memory function, we used transgenic mice that overexpress HGF in a neuron-specific manner (HGF-Tg) to deliver HGF into the brain without injury. HGF-Tg mice displayed increased alternation rates in the Y-maze test compared with age-matched wild-type (WT) controls. In the Morris water maze (MWM) test, HGF-Tg mice took less time to find the platform on the first day, whereas the latency to escape to the hidden platform was decreased over training days compared with WT mice. A transfer test revealed that the incidence of arrival at the exact location of the platform was higher for HGF-Tg mice compared with WT mice. These results demonstrate that overexpression of HGF leads to an enhancement of both short- and long-term memory. Western blot analyses revealed that the levels of N-methyl-D-aspartate (NMDA) receptor subunits NR2A and NR2B, but not NR1, were increased in the hippocampus of HGF-Tg mice compared with WT controls, suggesting that an upregulation of NR2A and NR2B could represent one mechanism by which HGF enhances learning and memory performance. These results demonstrate that modulation of learning and memory performance is an important physiological function of HGF that contributes to normal CNS plasticity, and we propose HGF as a novel regulator of higher brain functions.

Apoptotic neurodegeneration and spatial memory are not affected by sedative and anaesthetics doses of ketamine/medetomidine combinations in adult mice

BACKGROUND

Ketamine is increasingly popular in clinical practice and its combination with α(2)-agonists can provide good anaesthetic stability. Little is known about the effects of this combination in the brain. Therefore, we investigated the effects of different concentrations of ketamine combined with medetomidine on cognition and its potential apoptotic neurodegenerative effect in adult mice.

METHODS

Seventy-eight C57BL/6 adult mice were divided into six different groups (saline solution, 1 mg kg(-1) medetomidine, 25 mg kg(-1) ketamine+1 mg kg(-1) medetomidine, 75 mg kg(-1) ketamine+1 mg kg(-1) medetomidine, 25 mg kg(-1) ketamine, and 75 mg kg(-1) ketamine). Eight animals per group were tested in the T-maze, vertical pole, and open-field test. Five animals per group were used for histopathological [haematoxylin and eosin (HE) staining] and immunohistochemical analyses [caspase-3 activation and expression of neurotrophin brain-derived neurotrophic factor (BDNF)]. Cells showing clear HE staining and positive immunoreactions for caspase-3 and BDNF in the retrosplenial cortex, visual cortex, pyramidal cell layer of the cornu Ammonis 1 and cornu Ammonis 3 areas of the hippocampus, and in the granular layer of the dentate gyrus were counted.

RESULTS

There were no differences between groups regarding the number of dead cells and cells showing positive immunoreactions in the different areas of the brain studied. Similarly, no differences were detected in the number of trials to complete the T-maze task. Nevertheless, α(2)-agonist decreased hyperlocomotion caused by ketamine in the open field.

CONCLUSIONS

Neither apoptotic neurodegeneration nor alterations in spatial memory were observed with different concentrations of ketamine combined with medetomidine in adult mice.

Protective role of Ashwagandha leaf extract and its component withanone on scopolamine-induced changes in the brain and brain-derived cells

Background

Scopolamine is a well-known cholinergic antagonist that causes amnesia in human and animal models. Scopolamine-induced amnesia in rodent models has been widely used to understand the molecular, biochemical, behavioral changes, and to delineate therapeutic targets of memory impairment. Although this has been linked to the decrease in central cholinergic neuronal activity following the blockade of muscarinic receptors, the underlying molecular and cellular mechanism(s) particularly the effect on neuroplasticity remains elusive. In the present study, we have investigated (i) the effects of scopolamine on the molecules involved in neuronal and glial plasticity both in vivo and in vitro and (ii) their recovery by alcoholic extract of Ashwagandha leaves (i-Extract).

Methodology/Principal Findings

As a drug model, scopolamine hydrobromide was administered intraperitoneally to mice and its effect on the brain function was determined by molecular analyses. The results showed that the scopolamine caused downregulation of the expression of BDNF and GFAP in dose and time dependent manner, and these effects were markedly attenuated in response to i-Extract treatment. Similar to our observations in animal model system, we found that the scopolamine induced cytotoxicity in IMR32 neuronal and C6 glioma cells. It was associated with downregulation of neuronal cell markers NF-H, MAP2, PSD-95, GAP-43 and glial cell marker GFAP and with upregulation of DNA damage- γH2AX and oxidative stress- ROS markers. Furthermore, these molecules showed recovery when cells were treated with i-Extract or its purified component, withanone.

Conclusion

Our study suggested that besides cholinergic blockade, scopolamine-induced memory loss may be associated with oxidative stress and Ashwagandha i-Extract, and withanone may serve as potential preventive and therapeutic agents for neurodegenerative disorders and hence warrant further molecular analyses.

Aging and infection reduce expression of specific brain-derived neurotrophic factor mRNAs in hippocampus

Aging increases the likelihood of cognitive decline after negative life events such as infection or injury. We have modeled this increased vulnerability in aged (24-month-old), but otherwise unimpaired F344xBN rats. In these animals, but not in younger (3-month-old) counterparts, a single intraperitoneal injection of E. coli leads to specific deficits in long-term memory and long-lasting synaptic plasticity in hippocampal area CA1-processes strongly dependent on brain-derived neurotrophic factor (BDNF). Here we have investigated the effects of age and infection on basal and fear-conditioning-stimulated expression of Bdnf in hippocampus. We performed in situ hybridization with 6 probes recognizing: total (pan-)BDNF mRNA, the 4 predominant 5' exon-specific transcripts (I, II, IV, and VI), and BDNF mRNAs with a long 3' untranslated region (3' UTR). In CA1, aging reduced basal levels and fear-conditioning-induced expression of total BDNF mRNA, exon IV-specific transcripts, and transcripts with long 3' UTRs; effects of infection were similar and sometimes compounded the effects of aging. In CA3, aging reduced all of the transcripts to some degree; infection had no effect. Effects in dentate were minimal. Northern blot analysis confirmed an aging-associated loss of total BDNF mRNA in areas CA1 and CA3, and revealed a parallel, preferential loss of BDNF mRNA transcripts with long 3' UTRs.

A critical threshold of rehabilitation involving brain-derived neurotrophic factor is required for poststroke recovery

BACKGROUND

Enriched rehabilitation (ER; environmental enrichment plus skilled reaching) improves recovery after middle cerebral artery occlusion (MCAo) in rats. Fundamental issues such as whether ER is effective in other models, optimal rehabilitation intensity, and underlying recovery mechanisms have not been fully assessed.

OBJECTIVE

The authors tested whether the efficacy of ER varies with ischemia model and assessed the importance of rehabilitation intensity and brain-derived neurotrophic factor (BDNF) in recovery.

METHODS

Rats in experiment 1 received 8 weeks of ER or remained in standard housing. Functional outcome was assessed with the staircase and cylinder tasks. Surprisingly, ER provided no functional benefit in any model. In this experiment, ER was delivered during the light phase, whereas other studies delivered ER in the dark phase of the light cycle. It was hypothesized that in the light, rats engaged in less rehabilitation or alternatively that BDNF was lower. Experiment 2 tested these hypotheses. Following MCAo, rats received ER in either the light or dark phase of the light cycle. Functional outcome was assessed and BDNF levels were measured in the motor cortex and hippocampus.

RESULTS

Recovery was accompanied by increased BDNF. This occurred only in rats that received ER in the dark and these animals reached more than those in the light condition.

CONCLUSIONS

Data suggest that there is a critical threshold of rehabilitation, below which recovery will not occur, and that BDNF mediates functional recovery. The use of intensive rehabilitation therapies for stroke patients is strongly supported.

Enhanced hippocampal BDNF/TrkB signaling in response to fear conditioning in an animal model of posttraumatic stress disorder

Because the majority of patients with posttraumatic stress disorder (PTSD) exhibit long-lasting traumatic fear memory, we hypothesize that enhanced fear memory consolidation is closely involved in the pathophysiology of PTSD. Brain-derived neurotrophic factor (BDNF) and its receptor, tyrosine kinase receptor B (TrkB), are crucial for hippocampal-dependent learning and memory. In particular, differential induction of BDNF gene transcripts mediated by histone acetylation plays a role in the consolidation of fear memory. In the present study, total and exon-specific mRNA and protein levels of BDNF and TrkB in the hippocampus after contextual fear conditioning (FC) were compared between rats subjected to single prolonged stress (SPS) and sham treatment. In addition, we examined the degree of histone acetylation at the promoter of each exon of the BDNF gene by chromatin immunoprecipitation (ChIP). We previously demonstrated a significant increase in contextual freezing in SPS rats. In the present study, SPS rats also showed increased total BDNF mRNA (including exons I, IV) and BDNF protein levels in the hippocampus after FC, accompanied by increased acetylation of histone H3 and H4 at the promoter of exon I and IV relative to sham-treated rats. Furthermore, the TrkB protein levels in the hippocampus of SPS rats were significantly higher than those in sham rats. These findings suggest that the enhanced levels of BDNF as well as TrkB along with epigenetic regulation of the BDNF gene during fear memory consolidation is, at least in part, associated with long-lasting fear memory in patients with PTSD.

Analysis of gene expression changes associated with long-lasting synaptic enhancement in hippocampal slice cultures after repetitive exposures to glutamate

We have previously shown that repetitive exposures to glutamate (100 muM, 3 min, three times at 24-hr intervals) induced a long-lasting synaptic enhancement accompanied by synaptogenesis in rat hippocampal slice cultures, a phenomenon termed RISE (for repetitive LTP-induced synaptic enhancement). To investigate the molecular mechanisms underlying RISE, we first analyzed the time course of gene expression changes between 4 hr and 12 days after repetitive stimulation using an original oligonucleotide microarray: "synaptoarray." The results demonstrated that changes in the expression of synapse-related genes were induced in two time phases, an early phase of 24-96 hr and a late phase of 6-12 days after the third stimulation. Comprehensive screening at 48 hr after the third stimulation using commercially available high-density microarrays provided candidate genes responsible for RISE. From real-time PCR analysis of these and related genes, two categories of genes were identified, 1) genes previously reported to be induced by physiological as well as epileptic activity (bdnf, grm5, rgs2, syt4, ania4/carp/dclk) and 2) genes involved in cofilin-based regulation of actin filament dynamics (ywhaz, ssh1l, pak4, limk1, cfl). In the first category, synaptotagmin 4 showed a third stimulation-specific up-regulation also at the protein level. Five genes in the second category were coordinately up-regulated by the second stimulation, resulting in a decrease in cofilin phosphorylation and an enhancement of actin filament dynamics. In contrast, after the third stimulation, they were differentially regulated to increase cofilin phosphorylation and enhance actin polymerization, which may be a key step leading to the establishment of RISE.

Epigenetic regulation of BDNF gene in response to stress

Neuronal plasticity induced by changes in synaptic morphology and function is well known to play a pivotal role in leaning and memory as well as adaptation to stress. It is suggested that these plastic changes are due to orchestration of alterations in gene expression in the brain. Recent advances in molecular biology have provided evidence that epigenetic mechanisms, such as DNA methylation and histone modification, are crucial to gene transcription in the mammalian brain. Our research group has recently investigated the involvement of histone actylation at the promoter of the brain-derived neurotrophic factor (BDNF) gene in stress-induced reduction in BDNF, as well as in fear conditioning-induced enhancement of BDNF, in the rat hippocampus. The results of the stress study demonstrated that single-immobilization stress significantly reduced the levels of total, exon I, and exon IV BDNF mRNA, and also significantly reduced acetylation levels of histone H3, but not H4, at the promoter of exons I, IV, and VI. The results of the fear conditioning study showed that footshock stress significantly increased the levels of total, exon I, and exon IV BDNF mRNA, with significantly increased acetylation levels of both histone H3 and H4, at the promoter of exons I and IV, followed by enhanced freezing to fear-context exposure. These findings suggest that changes in BDNF transcription in the rat hippocampus in response to stressful stimuli are, at least in part, regulated by histone acetylation status.

An epigenetic hypothesis of aging-related cognitive dysfunction

This brief review will focus on a new hypothesis for the role of epigenetic mechanisms in aging-related disruptions of synaptic plasticity and memory. Epigenetics refers to a set of potentially self-perpetuating, covalent modifications of DNA and post-translational modifications of nuclear proteins that produce lasting alterations in chromatin structure. These mechanisms, in turn, result in alterations in specific patterns of gene expression. Aging-related memory decline is manifest prominently in declarative/episodic memory and working memory, memory modalities anatomically based largely in the hippocampus and prefrontal cortex, respectively. The neurobiological underpinnings of age-related memory deficits include aberrant changes in gene transcription that ultimately affect the ability of the aged brain to be "plastic". The molecular mechanisms underlying these changes in gene transcription are not currently known, but recent work points toward a potential novel mechanism, dysregulation of epigenetic mechanisms. This has led us to hypothesize that dysregulation of epigenetic control mechanisms and aberrant epigenetic "marks" drive aging-related cognitive dysfunction. Here we focus on this theme, reviewing current knowledge concerning epigenetic molecular mechanisms, as well as recent results suggesting disruption of plasticity and memory formation during aging. Finally, several open questions will be discussed that we believe will fuel experimental discovery.

Glutamate and neurotrophic factors in neuronal plasticity and disease

Glutamate's role as a neurotransmitter at synapses has been known for 40 years, but glutamate has since been shown to regulate neurogenesis, neurite outgrowth, synaptogenesis, and neuron survival in the developing and adult mammalian nervous system. Cell-surface glutamate receptors are coupled to Ca(2+) influx and release from endoplasmic reticulum stores, which causes rapid (kinase- and protease-mediated) and delayed (transcription-dependent) responses that change the structure and function of neurons. Neurotrophic factors and glutamate interact to regulate developmental and adult neuroplasticity. For example, glutamate stimulates the production of brain-derived neurotrophic factor (BDNF), which, in turn, modifies neuronal glutamate sensitivity, Ca(2+) homeostasis, and plasticity. Neurotrophic factors may modify glutamate signaling directly, by changing the expression of glutamate receptor subunits and Ca(2+)-regulating proteins, and also indirectly by inducing the production of antioxidant enzymes, energy-regulating proteins, and antiapoptotic Bcl-2 family members. Excessive activation of glutamate receptors, under conditions of oxidative and metabolic stress, may contribute to neuronal dysfunction and degeneration in diseases ranging from stroke and Alzheimer's disease to psychiatric disorders. By enhancing neurotrophic factor signaling, environmental factors such as exercise and dietary energy restriction, and chemicals such as antidepressants may optimize glutamatergic signaling and protect against neurological disorders.

Single immobilization stress differentially alters the expression profile of transcripts of the brain-derived neurotrophic factor (BDNF) gene and histone acetylation at its promoters in the rat hippocampus

Decreased levels of brain-derived neurotrophic factor (BDNF) in the hippocampus are implicated in the pathophysiology of major depression, although the mechanism has yet to be characterized. Epigenetic studies revealed that DNA methylation and histone modifications at the promoter of exons of the BDNF gene are the pivotal factors in the regulation of BDNF transcription. Histone acetylation regulates gene transcription through chromatin remodelling. We examined the influence of a single immobilization stress (SIS) at 2 h and 24 h afterwards on the levels of total BDNF mRNA with each exon mRNA by quantitative real-time PCR, acetylated histone at the promoters of the BDNF gene by chromatin immunoprecipitation followed by real-time PCR, and BDNF protein by ELISA in the rat hippocampus. SIS significantly decreased the levels of total BDNF mRNA with significantly reduced levels of exons I and IV mRNA followed by a significant reduction in BDNF protein 4 h after SIS. Significant decreases in the levels of acetylated histone H3, but not H4, were found at the promoters of exons I, IV, and VI. In contrast, no marked changes in the levels of either acetylated histone or BDNF mRNA and protein were found 24 h after SIS. This study demonstrated the involvement of histone acetylation in the regulation of BDNF transcription by SIS, and the plastic change in histone acetylation after SIS. These findings suggest that stress affects BDNF gene transcription via epigenetic regulation, and glucocorticoid may be involved in this regulation.

New insights into brain BDNF function in normal aging and Alzheimer disease

The decline observed during aging involves multiple factors that influence several systems. It is the case for learning and memory processes which are severely reduced with aging. It is admitted that these cognitive effects result from impaired neuronal plasticity, which is altered in normal aging but mainly in Alzheimer disease. Neurotrophins and their receptors, notably BDNF, are expressed in brain areas exhibiting a high degree of plasticity (i.e. the hippocampus, cerebral cortex) and are considered as genuine molecular mediators of functional and morphological synaptic plasticity. Modification of BDNF and/or the expression of its receptors (TrkB.FL, TrkB.T1 and TrkB.T2) have been described during normal aging and Alzheimer disease. Interestingly, recent findings show that some physiologic or pathologic age-associated changes in the central nervous system could be offset by administration of exogenous BDNF and/or by stimulating its receptor expression. These molecules may thus represent a physiological reserve which could determine physiological or pathological aging. These data suggest that boosting the expression or activity of these endogenous protective systems may be a promising therapeutic alternative to enhance healthy aging.

Time window for voluntary exercise-induced increases in hippocampal neuroplasticity molecules after traumatic brain injury is severity dependent

We recently found that an exercise-induced increase in hippocampal brain-derived neurotrophic factor (BDNF) is dependent when exercise is initiated after traumatic brain injury (TBI). When voluntary exercise was delayed by 2 weeks after a mild fluid-percussion injury (FPI) in rats, an increase in BDNF and an improvement in behavioral outcome were observed. This suggests that following FPI there is a therapeutic window for the implementation of voluntary exercise. To determine if more severely injured animals require more time after TBI before voluntary exercise can increase neuroplasticity, adult male rats with a moderate lateral FPI or sham injury were housed with or without access to a running wheel from post-injury-day (PID) 0-6, 14-20 or 30-36. Rats with a mild injury only had access to the running wheel from PID 0-6 or 14-20. Rats were sacrificed at PID 7, 21, or 37. BDNF, synapsin I, and cyclic AMP response element binding protein (CREB) were analyzed within the ipsilateral hippocampus. Whereas BDNF levels significantly increased with exercise in the mild FPI rats that were exercised from PID 14 to 20, the moderate FPI rats only showed significant increases in BDNF when exercised from PID 30 to 36. In addition, moderate FPI rats that were allowed to exercise from PID 30 to 36 also exhibited significant increases in synapsin I and CREB. These results indicate that the time window for exercise-induced increases in BDNF, synapsin I, and CREB is dependent on injury severity.

Effect of a neuroprotective exercise protocol on oxidative state and BDNF levels in the rat hippocampus

Daily moderate intensity exercise (2 weeks of 20 min/day of treadmill training), which reduces damage to hippocampal slices from rats submitted to in vitro ischemia, did not modify oxidative stress parameters in the hippocampus nor the brain-derived neurotrophic factor (BDNF) levels in different brain regions. The aim was to investigate whether the modulation of hippocampal oxidative status and/or brain BDNF content is involved in exercise-induced neuroprotection. Wistar rats were submitted to daily exercise in the treadmill and were sacrificed approximately 16 h after the last treadmill running. Some several oxidative stress parameters were determined, specifically the free radical levels, the macromolecule damage, the total reactive antioxidant potential and reactivity levels, which represent the total antioxidant capacity, in the hippocampus. In addition, BDNF levels in different rat cerebral regions (hippocampus, cortex, striatum, and the cerebellum) were measured by ELISA. The used exercise protocol did not affect any oxidative stress parameters studied in the hippocampus, suggesting that it does not cause a significant oxidative stress nor induce adaptations of the cellular antioxidant system. Treadmill training also did not change the BDNF content in brain areas studied. Considering the fact that this exercise protocol have been shown to be neuroprotective, we might speculate that BDNF levels and oxidative status may not be directly involved with the mechanisms of exercise-induced neuroprotection after ischemia.

Aspects of the general biology of adenosine A2A signaling

Many of our current hopes of finding better ways to treat Parkinson's disease or to stop its progression rely on studies of adenosine A2A receptors in the brain. Yet any drug targeting central receptors will also potentially affect receptors in other sites. Furthermore, several fundamental aspects of adenosine receptor biology must be taken into account. For these reasons the "Targeting adenosine A2A receptors in Parkinson's disease and other CNS disorders" meeting in Boston included selected aspects of the general biology of adenosine A2A receptor signaling. Some of the presentations from this part of the meeting are summarized in this first chapter. As will be apparent to the reader, these different parts do not form an integrated whole, but they do indicate areas the organizers felt might illuminate remaining questions regarding the roles of adenosine A2A receptors. The contributors to this part of the meeting have summarized some of the key questions below.

Oligomeric amyloid decreases basal levels of brain-derived neurotrophic factor (BDNF) mRNA via specific downregulation of BDNF transcripts IV and V in differentiated human neuroblastoma cells

Alzheimer's disease (AD) is a senile dementia characterized by amyloid plaques, neurofibrillary tangles, and synaptic and cell loss. The "amyloid cascade" hypothesis suggests that amyloid-beta (Abeta), the peptide deposited as amyloid plaques, is the primary insult in AD. However, debate continues over the mechanism of Abeta toxicity and whether fibrillar or oligomeric Abeta is the active species of the peptide that ultimately causes the synaptic loss and dementia associated with AD. Brain-derived neurotrophic factor (BDNF) is required for survival and function of cells compromised in AD. Decreased BDNF causes defects in long-term potentiation and memory and correlates with cognitive decline. We previously demonstrated that BDNF reduction occurs early in the course of AD, suggesting that decreased BDNF may promote neuronal dysfunction in AD. We also demonstrated that three of seven human BDNF transcripts are specifically downregulated in AD. What pathological feature(s) of AD leads to the decreased BDNF is unknown. In this study, we administered both fibrillar and oligomeric conformations of Abeta(1-42) to differentiated SH-SY5Y, a human neuroblastoma cell line, and measured both phosphorylated cAMP response element-binding protein (CREB), a regulator of BDNF transcription, and BDNF total mRNA. We found that oligomeric but not fibrillar preparations of Abeta(1-42) significantly decrease both phosphorylated CREB and total BDNF mRNA. Furthermore, oligomeric Abeta(1-42) decreases BDNF transcripts IV and V in these cells, demonstrating that Abeta(1-42) downregulates the major BDNF transcript decreased in vivo in the AD brain. Thus, oligomeric Abeta(1-42) could compromise neuronal function, causing memory loss and cognitive dysfunction by downregulation of BDNF in AD.

Biphasic change in BDNF gene expression following antidepressant drug treatment explained by differential transcript regulation

Brain-derived neurotrophic factor (BDNF) has been suggested as a possible target for the treatment of depression. The effect by antidepressant drugs on BDNF mRNA expression is, however, strictly dependent on both treatment duration and time after the last administration. The rat BDNF gene itself is complex and expresses four different mRNA isoforms which can be regulated by different signaling cascades. The aim of the present study was to test the hypothesis that the previously shown biphasic action by the antidepressant drugs on total BDNF expression is explained by differential BDNF transcript regulation. For this purpose, we used in situ hybridization with exon-specific oligo nucleotides for exon V (total BDNF mRNA), exon I (protein synthesis-dependent transcripts), exon III and exon IV (immediate early-gene like-transcripts). Following an acute injection, all three drugs tested: fluoxetine, desipramine and TCP decreased total BDNF mRNA (exon V) as well as exon IV mRNA, while no significant effect was recorded for exons I and III mRNAs. In contrast chronic administration of all three drugs resulted in increased expression of exon V- and exon I-containing transcripts (fluoxetine and TCP only) but no significant changes were recorded for exon III and IV mRNAs. Electroconvulsive shock administration showed up-regulation of all four BDNF mRNAs following a single shock, but after repeated administration increases were restricted to exons I- and V-containing transcripts. In summary, this study shows clear evidence of differential BDNF transcript regulation following acute and chronic antidepressant drug treatment.

Differential expression of the four untranslated BDNF exons in the adult mouse brain

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family of secreted proteins and has important functions in the peripheral and central nervous systems (CNS). Its gene organization is complex, comprising four untranslated exons, each with an associated promoter. We have examined the expression of the four untranslated exons in direct comparison to one another and to the fifth BDNF coding exon by in situ hybridization. Other studies have examined untranslated exon expression in several brain regions in response to specific stimuli; however, this is the first detailed study to describe endogenous exon 1-4 expression throughout the adult mouse brain at baseline conditions. The differential expression of the four untranslated exons seen throughout the brain indicates a complex regulation of BDNF.

What is the biological significance of BDNF mRNA targeting in the dendrites? Clues from epilepsy and cortical development

The neurotrophin brain-derived neurotrophic factor (BDNF) is a regulatory factor of several, partially contrasting, aspects of the biology of neural cells, including survival, growth, differentiation, and cell death. Regulation of the local availability of BDNF at distinct subcellular domains such as the cell soma, dendrites, axons, and spines appears to be the key to conferring spatial and temporal specificity of the different effects elicited by this neurotrophin. This article reviews recent findings in the context of epileptogenesis and visual cortex maturation that showed that different BDNF messenger RNA (mRNA) transcripts are localized at different subcellular locations in hippocampal and cortical neurons. It also reviews findings demonstrating that strong depolarizing stimuli, both in vitro and in vivo, elicit accumulation of BDNF mRNA and protein in the distal dendrites through a signaling pathway involving the activation of the N-methyl-D-aspartate and tyrosine kinase B receptors and an intracellular increase in Ca2+ concentration. Finally, this article proposes that the regulation of the delivery of BDNF mRNA and protein to the different subcellular domains--particularly the dendritic compartment--may represent a fundamental aspect of the processes of cellular and synaptic morphological rearrangements underlying epileptogenesis and postnatal development of the visual cortex.

Specific regulation of rat glial cell line-derived neurotrophic factor gene expression by riluzole in C6 glioma cells

Contrasting with its robust expression during embryogenesis, the glial cell line-derived neurotrophic factor (GDNF) is repressed in the adult organism. However, rapid induction of this neuronal growth factor is observed following diverse neuronal insults and it is now widely accepted that the control of its expression could constitute a powerful target in neuropharmacology. We investigated the effects of the neuroprotective drug, riluzole, on the GDNF gene expression in glial cells. Exposure of C6 glioma cells to riluzole (1 microM) significantly increased GDNF protein and mRNA levels. Using luciferase reporter gene constructs encoding fragments of the 5' untranslated region of the rat GDNF gene, we demonstrated that riluzole mediates its effect at the transcription level. Furthermore, luciferase assays revealed the presence of a negative regulatory region within the +343/+587 region of exon 1. This region was shown to contribute to the high sensitivity and specificity of the induction mediated by riluzole in the C6 glioma cell line at pharmacologically relevant concentrations. The effects of riluzole were inhibited by the mitogen-activated protein kinase extracellular signal-related kinase (MEK) inhibitor PD 98059. Together, these results indicated that the induction of GDNF release by riluzole in the C6 glioma cells results from the activation of its corresponding gene promoter through a signalling pathway involving MEK activity. This study suggests that the regulation of GDNF gene transcription in glial cells could contribute to the pharmacological properties of riluzole and possibly other neuroprotective drugs.

Differential effects of low glucose concentrations on seizures and epileptiform activityin vivoandin vitro

In vivo, severe hypoglycemia is frequently associated with seizures. The hippocampus is a structure prone to develop seizures and seizure-induced damage. Patients with repeated hypoglycemic episodes have frequent memory problems, suggesting impaired hippocampal function. Here we studied the effects of moderate hypoglycemia on primarily generalized flurothyl-induced seizures in vivo and, using EEG recordings, we determined involvement of the hippocampus in hypoglycemic seizures. Moderate systemic hypoglycemia had proconvulsant effects on flurothyl-induced clonic (forebrain) seizures. During hypoglycemic seizures, seizure discharges were recorded in the hippocampus. Thus, we continued the studies in combined entorhinal cortex-hippocampus slices in vitro. However, in vitro, decreases in extracellular glucose from baseline 10 mM to 2 or 1 mM did not induce any epileptiform discharges. In fact, low glucose (2 and 1 mM) attenuated preexisting low-Mg2+-induced epileptiform activity in the entorhinal cortex and hippocampal CA1 region. Osmolarity compensation in low-glucose solution using mannitol impaired slice recovery. Additionally, using paired-pulse stimuli we determined that there was no impairment of GABAA inhibition in the dentate gyrus during glucopenia. The data strongly indicate that, although forebrain susceptibility to seizures is increased during moderate in vivo hypoglycemia and the hippocampus is involved during hypoglycemic seizures, glucose depletion in vitro contributes to an arrest of epileptiform activity in the system of the entorhinal cortex-hippocampus network and there is no impairment of net GABAA inhibition during glucopenia.

Endothelin increases expression of exon III- and exon IV-containing brain-derived neurotrophic factor transcripts in cultured astrocytes and rat brain

The effects of endothelins (ETs) on brain-derived neurotrophic factor (BDNF) production in astrocytes were investigated. ET-1 (100 nM) increased the mRNA level and extracellular release of BDNF in cultured astrocytes. RT-PCR analyses using primer pairs that amplified exon-specific BDNF transcripts revealed that exon III- and exon IV-containing BDNF transcripts existed in cultured astrocytes, whereas exon I- and exon II-containing BDNF transcripts did not. ET-1 and Ala(1,3,11,15)-ET-1, an ET(B) receptor agonist, increased the expressions of the exon III and exon IV transcripts in cultured astrocytes. Intracerebroventricular administration of 500 pmol/day of Ala(1,3,11,15)-ET-1 increased exon III and exon IV BDNF transcripts in the rat striatum. In cultured astrocytes, Ca(2+)-chelation, W-7 (a calmodulin inhibitor), and KN93 (a Ca(2+)/calmodulin kinase inhibitor) inhibited the increases in exon IV BDNF mRNA and CCAAT enhancer-binding protein beta (C/EBPbeta) levels induced by ET-1. The ET-induced increases in exon III BDNF mRNA expression and phosphorylation of cAMP response element binding protein (CREB) were reduced by Ca(2+) chelation, W-7, KN93, PD98059 (a MEK inhibitor), and wortmannin (a phosphatidylinositol 3-kinase inhibitor). These results suggest that ETs stimulate the expressions of exon III and exon IV BDNF transcripts in astrocytes through CREB and C/EBPbeta-mediated mechanisms, respectively.

Human brain derived neurotrophic factor (BDNF) genes, splicing patterns, and assessments of associations with substance abuse and Parkinson's Disease

Potential roles for variants in the human BDNF gene in human brain disorders are supported by findings that include: (a) influences that this trophic factor can exert on important neurons, brain regions, and neurotransmitter systems, (b) changes in BDNF expression that follow altered neuronal activity and drug treatments, and (c) linkages or associations between genetic markers in or near BDNF and human traits and disorders that include depression, schizophrenia, addictions, and Parkinson's disease. We now report assembly of more than 70 kb of BDNF genomic sequence, delineation of 7 noncoding and 1 coding human BDNF exons, elucidation of BDNF transcripts that are initiated at several alternative promoters, identification of BDNF mRNA splicing patterns, elucidation of novel sequences that could contribute to activity-dependent BDNF mRNA transcription, targeting and/or translation, elucidation of tissue-specific and brain-region-specific use of the alternative human BDNF promoters and splicing patterns, identification of single nucleotide polymorphism (SNP), and simple sequence length polymorphism (SSLP) BDNF genomic variants and identification of patterns of restricted haplotype diversity at the BDNF locus. We also identified type 2 BDNF-locus transcripts that are coded by a novel gene that is overlapped with type 1 BDNF gene and transcribed in reverse orientation with several alternative splicing isoforms. Association studies of BDNF variants reveal no associations with Parkinson's disease. Comparisons between substance abusers and controls reveal modest associations. These findings increase interest in this diverse human gene.

Differential regulation of multiple brain-derived neurotrophic factor transcripts in the postnatal and adult rat hippocampus during development, and in response to kainate administration

Brain-derived neurotrophic factor (BDNF) is expressed at high levels in the hippocampus, where it has been implicated in physiological functions such as the modulation of synaptic strength as well as in the pathophysiology of epileptic seizures. BDNF expression is highly regulated and the BDNF gene can generate multiple transcript isoforms by alternate splicing of four 5' exons (exons I-IV) to one 3' exon (exon V). To gain insight into the regulation of different BDNF transcripts in specific hippocampal subfields during postnatal development, exon-specific riboprobes were used. Our data shows that BDNF exon I and exon II mRNAs are regulated in hippocampal subfields during postnatal development, in contrast to BDNF exon III and exon IV mRNA, which remain relatively stable through this period. Further, exons I and II show distinct temporal patterns of expression in the hippocampus: BDNF I mRNA peaks in adulthood in contrast to BDNF II mRNA which peaks at postnatal day 14 (P14). Finally, we have addressed whether kainate treatment in postnatal pups and adults regulates BDNF through the recruitment of the same, or distinct, BDNF promoters. Our data indicates that kainate-induced seizures induce strikingly different expression of distinct BDNF transcripts, both in magnitude as well as spatial patterns in the hippocampal subfields, of pups as compared to adults. These results suggest that kainate-mediated seizures differentially recruit BDNF promoters in the developing postnatal hippocampus in contrast to the adult hippocampus to achieve a hippocampal subfield specific regulation of exon-specific BDNF mRNAs.

Neuronal activity regulates the developmental expression and subcellular localization of cortical BDNF mRNA isoforms in vivo

Activity-dependent changes in BDNF expression have been implicated in developmental plasticity. Although its expression is widespread in visual cortex, developmental regulation of its different transcripts by visual experience has not been investigated. Here, we investigated the cellular expression of different BDNF transcripts in rat visual cortex during postnatal development. We found that transcripts I and II are expressed only in adults but III and IV are expressed from early postnatal stage. Total BDNF mRNA is expressed throughout the age groups. Transcripts III and IV show a differential intracellular localization, while former was detected only in cell bodies, latter is present both in cell bodies and dendritic processes. Inhibition of visual activity decreases the levels of exons, with exon IV transcript almost disappearing from dendrites. In vitro experiments also confirmed the above results, indicating activity-dependent regulation of different BDNF promoters with specific temporal and cellular patterns of expression in developing visual cortex.

Delayed hypothermia preferentially increases expression of brain-derived neurotrophic factor exon III in rat hippocampus after asphyxial cardiac arrest

Brain-derived neurotrophic factor (BDNF) protein levels increase in rats treated with a regimen of delayed, mild hypothermia that improve neurological recovery after asphyxial cardiac arrest. BDNF transcription in rat brain involves at least five different BDNF exons (exons I-V) that produce four different varieties of mRNA, each containing exon V paired with one of exons I-IV. This study examined whether these different BDNF transcripts are differentially affected by cardiac arrest and by therapeutic hypothermia in rat hippocampus using a reverse transcription and PCR-based method. At 24 h after asphyxial cardiac arrest, transcripts containing exons I and III increased. In rats treated with hypothermia after cardiac arrest, transcripts containing exon III were further increased. No significant alterations in transcripts from exons II or IV were observed, though there was a trend for hypothermia to decrease message from these exons. These data suggest that hypothermia after cardiac arrest produces exon-specific changes in BDNF transcription.

Functional interactions between somatodendritic dopamine release, glutamate receptors and brain-derived neurotrophic factor expression in mesencephalic structures of the brain

Dopaminergic nigrostriatal neurons may be considered as bipolar functional entities since they are endowed with the ability to synthesize, store and release the transmitter dopamine (DA) at the somatodendritic level in the substantia nigra (SN). Such dendritic DA release seems to be distinct from the transmitter release occurring at the axon terminal and seems to rely preferentially on volume transmission to exert its physiological effects. An increased glutamatergic (Gluergic) transmission into the SN facilitates such dendritic DA release via activation of NMDA-receptors (NMDA-Rs) and to a lesser extent through group II metabotropic glutamate receptors (mGluRs). In addition, nigral mGluRs functionally interact with NMDA-Rs in the SN, further modulating the NMDA-R-mediated increase of DA release from dendrites in the SN. In turn, dendritically released DA may exert, via D1 receptors, a tonic inhibitory control upon nigral glutamate (Glu). Furthermore, released DA, via D2/D3 autoreceptors, produces an autoinhibitory effect upon DA cell firing and its own release process. An increased Gluergic transmission into the SN may also induce, via activation of NMDA-Rs, an augmented expression of different brain-derived neurotrophic factor (BDNF) gene transcripts in this brain area. Pharmacological evidence suggests that non-NMDA-Rs could also participate in the regulation of BDNF gene expression in the SN. Glu-mediated changes of nigral BDNF expression could regulate, in turn, the expression of important transmitter-related proteins in the SN, such as different NMDA-R subunits, mGluRs and DA-D3 receptors. In conclusion, Glu-DA-BDNF interactions in the SN may play an important role in modulating the flow of neuronal information in this brain structure under normal conditions, as well as during adaptive and plastic responses associated with various neurological and psychiatric disorders.

Differential regulation of brain-derived neurotrophic factor transcripts during the consolidation of fear learning

Brain-derived neurotrophic factor (BDNF) has been implicated as a molecular mediator of learning and memory. The BDNF gene contains four differentially regulated promoters that generate four distinct mRNA transcripts, each containing a unique noncoding 5'-exon and a common 3'-coding exon. This study describes novel evidence for the differential usage of alternative BDNF promoters and 5'-exons during the consolidation of learning. We found a selective increase in BDNF transcripts containing exons I and III in the amygdala 2 h following fear conditioning, while mRNA levels of BDNF exons II and IV remained unchanged. These results provide the first evidence of differential splicing and/or differential BDNF promoter usage in response to a behaviorally relevant learning paradigm.

Early maternal deprivation induces alterations in brain-derived neurotrophic factor expression in the developing rat hippocampus

The effects of maternal deprivation (MD) during early postnatal life on the brain-derived neurotrophic factor (BDNF) level were investigated in the present study. Wistar rats were assigned to either maternal deprivation or mother-reared control (MRC) groups. MD manipulation was achieved by separating rat pups from their mothers for 3h a day during postnatal days (PND) 10-15. At 16, 20, 30, and 60 days of age, the level of BDNF mRNA in the hippocampal formation of each group was determined using real-time PCR analysis. Early postnatal maternal deprivation of rat pups resulted in a significant increase in body weight at 60 days of age. The expression of BDNF mRNA in the hippocampus was significantly decreased at 16 days of age, and increased at 30 and 60 days of age. These data indicate that even a brief period of maternal deprivation during early postnatal life can affect hippocampal BDNF expression.

Persistent regional increases in brain-derived neurotrophic factor in the flurothyl model of epileptogenesis are dependent upon the kindling status of the animal

Brain-derived neurotrophic factor (BDNF) appears to be both regulated by and a regulator of epileptogenesis. In the flurothyl (HFE) model of kindling mice exposed to successive flurothyl trials over 8 days express a rapid, long-lasting reduction in generalized seizure threshold and a more slowly evolving change in seizure phenotype in response to subsequent flurothyl exposure. The BDNF genotype of particular mouse strains appears to influence the epileptogenic progression in this model. Thus, we hypothesized that BDNF signaling pathways are altered by flurothyl-induced seizures. Following HFE kindling, fully kindled (eight seizures) adult male C57BI/6J mice had significantly elevated whole brain BDNF levels through at least 28 days after their final seizure. Mice that received only four HFE seizures (not kindled) had elevated BDNF levels, but only at 1 day post-seizure (DPSz), while BDNF levels were not significantly altered in mice receiving just one HFE seizure at any time point studied. Regional expression patterns of BDNF in the hippocampus, hypothalamus, and frontal cortex were also elevated by one DPSz and returned to control values by 14 DPSz in mice that received four HFE seizures. No changes were seen in the cerebellum, striatum, or piriform cortex. In contrast, fully kindled mice had significantly elevated BDNF levels within the hippocampus, hypothalamus, neocortex, and striatum that remained elevated through at least 14 DPSz, while levels were unchanged in the cerebellum and piriform cortex. Regional results were confirmed using anti-BDNF immunohistochemistry (IHC). Despite changes in BDNF levels following HFE kindling, we were unable to demonstrate alterations either in full-length tyrosine kinase receptor B (TrkB) expression (Western blot and IHC) or in truncated TrkB (IHC) expression levels. Together, these data suggest a model of a positive feedback loop involving seizure activity and seizure number and persistently modified BDNF signaling pathways that influences seizure phenotypes within the HFE kindling paradigm. Thus, long-term elevations in BDNF may be responsible in part for epileptogenic processes and the development of human refractory epilepsies.

Differential expression of brain-derived neurotrophic factor transcripts after pilocarpine-induced seizure-like activity is related to mode of Ca2+ entry

Activity-dependent brain-derived neurotrophic factor (BDNF) expression is Ca2+-dependent, yet little is known about the Ca2+ channel contributions that might direct selective expression of the multiple BDNF transcripts. Here, effects of pilocarpine-induced seizure activity on total BDNF expression and on the individual sensitivity of BDNF transcripts to glutamate receptor and Ca2+ channel blockers were evaluated using hippocampal slice cultures and in situ hybridization of transcript-specific cRNA probes directed against mRNAs for the four 5' exons (I-IV) of the BDNF gene. mRNAs for nerve growth factor (NGF) and tyrosine kinase B (trkB) also were studied. Pilocarpine (5 mM) induced a dose- and time-dependent increase in total BDNF (exon V) mRNA expression in the dentate granule cells and CA3-CA1 pyramidal cells with maximal effects at 6 and 24 h, respectively. Increases were blocked by co-treatment with the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX: 25 microM) and the N-methyl-d-aspartic acid receptor antagonist 2-amino-5-phosphonovaleric acid (APV; 25 microM), whereas the L-type voltage sensitive Ca2+ channel blocker nifedipine (20 microM) was without detectable effect. Maximal NGF and trkB mRNA expression was induced by pilocarpine at 4 and 12 h, respectively. For the individual BDNF transcripts, APV blocked pilocarpine-induced increases in transcript II, whereas nifedipine blocked increases in transcripts I and III. Transcript IV levels were not altered by treatment. These results indicate that transcript II makes the greatest contribution to pilocarpine effects on total BDNF mRNA content in this model and provides evidence for regional and Ca2+ channel-specific differences in activity-dependent regulation of the different BDNF transcripts in hippocampus.

Brain-derived neurotrophic factor activation of NFAT (nuclear factor of activated T-cells)-dependent transcription: a role for the transcription factor NFATc4 in neurotrophin-mediated gene expression

A member of the neurotrophin family, brain-derived neurotrophic factor (BDNF) regulates neuronal survival and differentiation during development. Within the adult brain, BDNF is also important in neuronal adaptive processes, such as the activity-dependent plasticity that underlies learning and memory. These long-term changes in synaptic strength are mediated through alterations in gene expression. However, many of the mechanisms by which BDNF is linked to transcriptional and translational regulation remain unknown. Recently, the transcription factor NFATc4 (nuclear factor of activated T-cells isoform 4) was discovered in neurons, where it is believed to play an important role in long-term changes in neuronal function. Interestingly, NFATc4 is particularly sensitive to the second messenger systems activated by BDNF. Thus, we hypothesized that NFAT-dependent transcription may be an important mediator of BDNF-induced plasticity. In cultured rat CA3-CA1 hippocampal neurons, BDNF activated NFAT-dependent transcription via TrkB receptors. Inhibition of calcineurin blocked BDNF-induced nuclear translocation of NFATc4, thus preventing transcription. Further, phospholipase C was a critical signaling intermediate between BDNF activation of TrkB and the initiation of NFAT-dependent transcription. Both inositol 1,4,5-triphosphate (IP3)-mediated release of calcium from intracellular stores and activation of protein kinase C were required for BDNF-induced NFAT-dependent transcription. Finally, increased expression of IP3 receptor 1 and BDNF after neuronal exposure to BDNF was linked to NFAT-dependent transcription. These results suggest that NFATc4 plays a crucial role in neurotrophin-mediated synaptic plasticity.

The influence of specific noradrenergic and serotonergic lesions on the expression of hippocampal brain-derived neurotrophic factor transcripts following voluntary physical activity

Previous studies have shown that hippocampal brain-derived neurotrophic factor (BDNF) mRNA levels are significantly increased in rats allowed free access to exercise wheels and/or administered antidepressant medications. Enhancement of BDNF may be crucial for the clinical effect of antidepressant interventions. Since increased function of the noradrenergic and/or serotonergic systems is thought to be an important initial mechanism of antidepressant medications, we sought to test the hypothesis that noradrenergic or serotonergic function is essential for the increased BDNF transcription occurring with exercise. In addition, individual transcript variants of BDNF were examined, as evidence exists they are differentially regulated by discrete interventions, and are expressed in distinct sub-regions of the hippocampus. The neurotransmitter system-specific neurotoxins p-chloroamphetamine (serotonergic) and N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (noradrenergic) were administered to rats prior to commencing voluntary wheel-running activity. In situ hybridization experiments revealed an absence of exercise-induced full-length BDNF mRNA elevations in the hippocampi of noradrenergic-lesioned rats. In addition, the striking elevation of the exon I transcript in the dentate gyrus was removed with this noradrenergic lesion. In contrast, other transcript variants (exons II and III) were elevated in several hippocampal regions as a result of this lesion. In serotonin-lesioned rats, the significant increases in full-length BDNF, exon I and exon II mRNA levels were sustained without alteration (with the exception of exon IV in the cornus ammonis subregion 4, CA4). Overall, these results indicate that an intact noradrenergic system may be crucial for the observed ability of exercise to enhance full-length and exon I hippocampal BDNF mRNA expression. In addition, these results suggest that the promoter linked to exon I may provide a major regulatory point for BDNF mRNA expression in the dentate gyrus. Elevations of other exons, such as II and III, may require the activation of separate neurotransmitter systems and intracellular pathways.

Generalization of rapidly recurring seizures is suppressed in mice lacking glial cell line-derived neurotrophic factor family receptor alpha2

Recent experimental evidence indicates that neurotrophic factors play a role in the pathophysiology of epilepsy. The objective of this study was to explore whether signaling through one of the glial cell line-derived neurotrophic factor family receptors, GFRalpha2, influences the severity of kindling-evoked, rapidly recurring seizures and the subsequent development of permanent hyperexcitability. We applied the rapid kindling model to adult mice, using 40 threshold stimulations delivered with 5-min interval in the ventral hippocampus. Generalized seizures were fewer and developed later in response to kindling stimulations in mice lacking GFRalpha2. However, GFRalpha2 gene deletion did not influence the acquisition of the permanent abnormal excitability as assessed 4 weeks later. In situ hybridization revealed marked and dynamic changes of GFRalpha2 mRNA levels in several forebrain areas following the stimulus-evoked seizures. Our findings provide evidence that signaling through the GFRalpha2 receptor contributes to seizure generalization in rapid kindling.

Reduced brain-derived neurotrophic factor in prefrontal cortex of patients with schizophrenia

Anatomical and molecular abnormalities of excitatory neurons in the dorsolateral prefrontal cortex (DLPFC) are found in schizophrenia. We hypothesized that brain-derived neurotrophic factor (BDNF), a protein capable of increasing pyramidal neuron spine density and augmenting synaptic efficacy of glutamate, may be abnormally expressed in the DLPFC of patients with schizophrenia. Using an RNase protection assay and Western blotting, we detected a significant reduction in BDNF mRNA (mean=23%) and protein (mean=40%) in the DLPFC of patients with schizophrenia compared to normal individuals. At the cellular level, BDNF mRNA was expressed at varying intensities in pyramidal neurons throughout layers II, III, V, and VI of DLPFC. In patients with schizophrenia; neuronal BDNF expression was decreased in layers III, V and VI. Our study demonstrates a reduction in BDNF production and availability in the DLPFC of schizophrenics, and suggests that intrinsic cortical neurons, afferent neurons, and target neurons may receive less trophic support in this disorder.

Immunohistochemical identification of multipotent adult progenitor cells from human bone marrow after transplantation into the rat brain

In the immunohistochemical analysis of the brain, tissue preparation and fixation are critical steps. During our studies of transplanting human bone marrow multipotent adult progenitor cells (hMAPCs) into the rat brain, we noticed that various methods of brain tissue preparation and fixation differentially influenced antigenic preservation in the transplants and in the host brain. Here, we report a simple, effective and reproducible method of tissue preparation and fixation that results in the immunohistochemical labeling of transplanted and host cells.

BDNF and activity-dependent synaptic modulation

It is widely accepted that neuronal activity plays a pivotal role in synaptic plasticity. Neurotrophins have emerged recently as potent factors for synaptic modulation. The relationship between the activity and neurotrophic regulation of synapse development and plasticity, however, remains unclear. A prevailing hypothesis is that activity-dependent synaptic modulation is mediated by neurotrophins. An important but unresolved issue is how diffusible molecules such as neurotrophins achieve local and synapse-specific modulation. In this review, I discuss several potential mechanisms with which neuronal activity could control the synapse-specificity of neurotrophin regulation, with particular emphasis on BDNF. Data accumulated in recent years suggest that neuronal activity regulates the transcription of BDNF gene, the transport of BDNF mRNA and protein into dendrites, and the secretion of BDNF protein. There is also evidence for activity-dependent regulation of the trafficking of the BDNF receptor, TrkB, including its cell surface expression and ligand-induced endocytosis. Further study of these mechanisms will help us better understand how neurotrophins could mediate activity-dependent plasticity in a local and synapse-specific manner.

Pilocarpine-induced seizure-like activity with increased BNDF and neuropeptide Y expression in organotypic hippocampal slice cultures

Organotypic hippocampal slice cultures were treated with the muscarinic agonist pilocarpine to study induced seizure-like activity and changes in neurotrophin and neuropeptide expression. For establishment of a seizure-inducing protocol, 2-week-old cultures derived from 6-8-day-old rats were exposed to 0.1 mM to 5 mM of pilocarpine for 4 h to 7 days. Other cultures were treated with pilocarpine for 7 days and left for 7-14 days in normal medium. Age-matched, non-treated cultures served as controls. Intracellular recordings from CA1 pyramidal cells revealed increased spontaneous activity in 31 of 35 cultures superfused with 0.1 or 5 mM pilocarpine. Epileptiform discharges were recorded in 17 of the 31 cultures, and 19 displayed frequencies specifically in the 6-12-Hz (Theta rhythm) range when superfused with pilocarpine. The pilocarpine effect was blocked by simultaneous superfusion with the muscarinic receptor antagonist atropine (100 microM). Regardless of dose and exposure time, the pilocarpine treatment induced very limited neuronal cell death, recorded as cellular propidium iodide uptake. Cultures exposed to 5 mM pilocarpine for up to 7 days displayed increased BDNF expression when analyzed by Western blot and ELISA. This BDNF increase correlated with increased neuropeptide Y immunoreactivity, known to accompany seizure activity. Addition of BDNF (200 ng/ml) to otherwise untreated cultures also upregulated NPY expression. The pilocarpine-induced seizure-like activity in hippocampal slice cultures, with concomitant increase in BDNF and NPY expression, is compared with in vivo observations and discussed in terms of the potential use of the easily accessible slice cultures in experimental seizure research.