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

Flaxseed Oil Alleviates Trimethyltin-Induced Cell Injury and Inhibits the Pro-Inflammatory Activation of Astrocytes in the Hippocampus of Female Rats

Exposure to the neurotoxin trimethyltin (TMT) selectively induces hippocampal neuronal injury and astrocyte activation accompanied with resultant neuroinflammation, which causes severe behavioral, cognitive, and memory impairment. A large body of evidence suggests that flaxseed oil (FSO), as one of the richest sources of essential omega-3 fatty acids, i.e., α-linolenic acids (ALA), displays neuroprotective properties. Here, we report the preventive effects of dietary FSO treatment in a rat model of TMT intoxication. The administration of FSO (1 mL/kg, orally) before and over the course of TMT intoxication (a single dose, 8 mg/kg, i.p.) reduced hippocampal cell death, prevented the activation of astrocytes, and inhibited their polarization toward a pro-inflammatory/neurotoxic phenotype. The underlying protective mechanism was delineated through the selective upregulation of BDNF and PI3K/Akt and the suppression of ERK activation in the hippocampus. Pretreatment with FSO reduced cell death and efficiently suppressed the expression of inflammatory molecules. These beneficial effects were accompanied by an increased intrahippocampal content of n-3 fatty acids. In vitro, ALA pretreatment prevented the TMT-induced polarization of cultured astrocytes towards the pro-inflammatory spectrum. Together, these findings support the beneficial neuroprotective properties of FSO/ALA against TMT-induced neurodegeneration and accompanied inflammation and hint at a promising preventive use of FSO in hippocampal degeneration and dysfunction.

The potential neuroprotective effects of stingless bee honey

Tropical Meliponini bees produce stingless bee honey (SBH). Studies have shown beneficial properties, including antibacterial, bacteriostatic, anti-inflammatory, neurotherapeutic, neuroprotective, wound, and sunburn healing capabilities. High phenolic acid and flavonoid concentrations offer SBH its benefits. SBH can include flavonoids, phenolic acids, ascorbic acid, tocopherol, organic acids, amino acids, and protein, depending on its botanical and geographic origins. Ursolic acid, p-coumaric acid, and gallic acid may diminish apoptotic signals in neuronal cells, such as nuclear morphological alterations and DNA fragmentation. Antioxidant activity minimizes reactive oxygen species (ROS) formation and lowers oxidative stress, inhibiting inflammation by decreasing enzymes generated during inflammation. Flavonoids in honey reduce neuroinflammation by decreasing proinflammatory cytokine and free radical production. Phytochemical components in honey, such as luteolin and phenylalanine, may aid neurological problems. A dietary amino acid, phenylalanine, may improve memory by functioning on brain-derived neurotrophic factor (BDNF) pathways. Neurotrophin BDNF binds to its major receptor, TrkB, and stimulates downstream signaling cascades, which are crucial for neurogenesis and synaptic plasticity. Through BDNF, SBH can stimulate synaptic plasticity and synaptogenesis, promoting learning and memory. Moreover, BDNF contributes to the adult brain's lasting structural and functional changes during limbic epileptogenesis by acting through the cognate receptor tyrosine receptor kinase B (TrkB). Given the higher antioxidants activity of SBH than the Apis sp. honey, it may be more therapeutically helpful. There is minimal research on SBH's neuroprotective effects, and the related pathways contribute to it is unclear. More research is needed to elucidate the underlying molecular process of SBH on BDNF/TrkB pathways in producing neuroprotective effects.

Induction of Bdnf from promoter I following electroconvulsive seizures contributes to structural plasticity in neurons of the piriform cortex

BACKGROUND

Electroconvulsive therapy (ECT) efficacy is hypothesized to depend on induction of molecular and cellular events that trigger neuronal plasticity. Investigating how electroconvulsive seizures (ECS) impact plasticity in animal models can help inform our understanding of basic mechanisms by which ECT relieves symptoms of depression. ECS-induced plasticity is associated with differential expression of unique isoforms encoding the neurotrophin, brain-derived neurotrophic factor (BDNF).

HYPOTHESIS

We hypothesized that cells expressing the Bdnf exon 1-containing isoform are important for ECS-induced structural plasticity in the piriform cortex, a highly epileptogenic region that is responsive to ECS.

METHODS

We selectively labeled Bdnf exon 1-expressing neurons in mouse piriform cortex using Cre recombinase dependent on GFP technology (CRE-DOG). We then quantified changes in dendrite morphology and density of Bdnf exon 1-expressing neurons.

RESULTS

Loss of promoter I-derived BDNF caused changes in spine density and morphology in Bdnf exon 1-expressing neurons following ECS.

CONCLUSIONS

Promoter I-derived Bdnf is required for ECS-induced dendritic structural plasticity in Bdnf exon 1-expressing neurons.

Effects of early-life zinc deficiency on learning and memory in offspring and the changes in DNA methylation patterns

OBJECTIVE

To investigate the effect of maternal zinc deficiency on learning and memory in offspring and the changes in DNA methylation patterns.

METHODS

Pregnant rats were divided into zinc adequate (ZA), zinc deficient (ZD), and paired fed (PF) groups. Serum zinc contents and AKP activity in mother rats and offspring at P21 (end of lactation) and P60 (weaned, adult) were detected. Cognitive ability of offspring at P21 and P60 were determined by Morris water maze. The expression of proteins including DNMT3a, DNMT1, GADD45β, MeCP2 and BDNF in the offspring hippocampus were detected by Western-blot. The methylation status of BDNF promoter region in hippocampus of offspring rats was detected by MS-qPCR.

RESULTS

Compared with the ZA and PF groups, pups in the ZD group had lower zinc levels and AKP activity in the serum, spent more time finding the platform and spent less time going through the platform area. Protein expression of DNMT1 and GADD45b were downregulated in the ZD group during P0 and P21 but not P60 compared with the ZA and PF group, these results were consistent with a reduction in BDNF protein at P0 (neonate), P21. However, when pups of rats in the ZD group were supplemented with zinc ion from P21 to P60, MeCP2 and GADD45b expression were significantly downregulated compared with the ZA and PF group.

CONCLUSION

Post-weaning zinc supplementation may improve cognitive impairment induced by early life zinc deficiency, whereas it may not completely reverse the abnormal expression of particular genes that are involved in DNA methylation, binding to methylated DNA and neurogenesis.

BDNF, endurance activity, and mechanisms underlying the evolution of hominin brains

OBJECTIVES

As a complex, polygenic trait, brain size has likely been influenced by a range of direct and indirect selection pressures for both cognitive and non-cognitive functions and capabilities. It has been hypothesized that hominin brain expansion was, in part, a correlated response to selection acting on aerobic capacity (Raichlen & Polk, 2013). According to this hypothesis, selection for aerobic capacity increased the activity of various signaling molecules, including those involved in brain growth. One key molecule is brain-derived neurotrophic factor (BDNF), a protein that regulates neuronal development, survival, and plasticity in mammals. This review updates, partially tests, and expands Raichlen and Polk's (2013) hypothesis by evaluating evidence for BDNF as a mediator of brain size.

DISCUSSION

We contend that selection for endurance capabilities in a hot climate favored changes to muscle composition, mitochondrial dynamics and increased energy budget through pathways involving regulation of PGC-1α and MEF2 genes, both of which promote BDNF activity. In addition, the evolution of hairlessness and the skin's thermoregulatory response provide other molecular pathways that promote both BDNF activity and neurotransmitter synthesis. We discuss how these pathways contributed to the evolution of brain size and function in human evolution and propose avenues for future research. Our results support Raichlen and Polk's contention that selection for non-cognitive functions has direct mechanistic linkages to the evolution of brain size in hominins.

Prefrontal cortical trkB, glucocorticoids, and their interactions in stress and developmental contexts

The tropomyosin/tyrosine receptor kinase B (trkB) and glucocorticoid receptor (GR) regulate neuron structure and function and the hormonal stress response. Meanwhile, disruption of trkB and GR activity (e.g., by chronic stress) can perturb neuronal morphology in cortico-limbic regions implicated in stressor-related illnesses like depression. Further, several of the short- and long-term neurobehavioral consequences of stress depend on the developmental timing and context of stressor exposure. We review how the levels and activities of trkB and GR in the prefrontal cortex (PFC) change during development, interact, are modulated by stress, and are implicated in depression. We review evidence that trkB- and GR-mediated signaling events impact the density and morphology of dendritic spines, the primary sites of excitatory synapses in the brain, highlighting effects in adolescents when possible. Finally, we review the role of neurotrophin and glucocorticoid systems in stress-related metaplasticity. We argue that better understanding the long-term effects of developmental stressors on PFC trkB, GR, and related factors may yield insights into risk for chronic, remitting depression and related neuropsychiatric illnesses.

BDNF-Live-Exon-Visualization (BLEV) Allows Differential Detection of BDNF Transcripts in vitro and in vivo

Bdnf exon-IV and exon-VI transcripts are driven by neuronal activity and are involved in pathologies related to sleep, fear or memory disorders. However, how their differential transcription translates activity changes into long-lasting network changes is elusive. Aiming to trace specifically the network controlled by exon-IV and -VI derived BDNF during activity-dependent plasticity changes, we generated a transgenic reporter mouse for B DNF- l ive- e xon- v isualization (BLEV), in which expression of Bdnf exon-IV and -VI can be visualized by co-expression of CFP and YFP. CFP and YFP expression was differentially activated and targeted in cell lines, primary cultures and BLEV reporter mice without interfering with BDNF protein synthesis. CFP and YFP expression, moreover, overlapped with BDNF protein expression in defined hippocampal neuronal, glial and vascular locations in vivo. So far, activity-dependent BDNF cannot be explicitly monitored independent of basal BDNF levels. The BLEV reporter mouse therefore provides a new model, which can be used to test whether stimulus-induced activity-dependent changes in BDNF expression are instrumental for long-lasting plasticity modifications.

The effects of DNA methylation on human psychology

DNA methylation is a fundamental epigenetic modification in the human genome; pivotal in development, genomic imprinting, X inactivation, chromosome stability, gene expression and methylation aberrations are involved in an array of human diseases. Methylation at promoters is associated with transcriptional repression, whereas gene body methylation is generally associated with gene expression. Extrinsic factors such as age, diets and lifestyle affect DNA methylation which consequently alters gene expression. Stress, anxiety, depression, life satisfaction, emotion among numerous other psychological factors also modify DNA methylation patterns. This correlation is frequently investigated in four candidate genes; NR3C1, SLC6A4, BDNF and OXTR, since regulation of these genes directly impact responses to social situations, stress, threats, behaviour and neural functions. Such studies underpin the hypothesis that DNA methylation is involved in deviant human behaviour, psychological and psychiatric conditions. These candidate genes may be targeted in future to assess the correlation between methylation, social experiences and long-term behavioural phenotypes in humans; and may potentially serve as biomarkers for therapeutic intervention.

BDNF at the synapse: why location matters

Neurotrophic factors, a family of secreted proteins that support the growth, survival and differentiation of neurons, have been intensively studied for decades due to the powerful and diverse effects on neuronal physiology, as well as their therapeutic potential. Such efforts have led to a detailed understanding on the molecular mechanisms of neurotrophic factor signaling. One member, brain-derived neurotrophic factor (BDNF) has drawn much attention due to its pleiotropic roles in the central nervous system and implications in various brain disorders. In addition, recent advances linking the rapid-acting antidepressant, ketamine, to BDNF translation and BDNF-dependent signaling, has re-emphasized the importance of understanding the precise details of BDNF biology at the synapse. Although substantial knowledge related to the genetic, epigenetic, cell biological and biochemical aspects of BDNF biology has now been established, certain aspects related to the precise localization and release of BDNF at the synapse have remained obscure. A recent series of genetic and cell biological studies have shed light on the question-the site of BDNF release at the synapse. In this Perspectives article, these new insights will be placed in the context of previously unresolved issues related to BDNF biology, as well as how BDNF may function as a downstream mediator of newer pharmacological agents currently under investigation for treating psychiatric disorders.

Neuronal activity-regulated alternative mRNA splicing

Activity-regulated gene transcription underlies plasticity-dependent changes in the molecular composition and structure of neurons. Numerous genes whose expression is induced by different neuronal plasticity inducing pathways have been identified, but the alteration of gene expression levels represents only part of the complexity of the activity-regulated transcriptional program. Alternative splicing of precursor mRNA is an additional mechanism that modulates the activity-dependent transcriptional signature. Recently developed splicing sensitive transcriptome wide analyses improve our understanding of the underlying mechanisms and demonstrate to what extend the activity regulated transcriptome is alternatively spliced. So far, only for a small group of differentially spliced mRNAs of synaptic proteins, the functional implications have been studied in detail. These include examples in which differential exon usage can result in the expression of alternative proteins which interfere with or alter the function of preexisting proteins and cause a dominant negative functional block of constitutively expressed variants. Such altered proteins contribute to the structural and functional reorganization of pre- and postsynaptic terminals and to the maintenance and formation of synapses. In addition, activity-induced alternative splicing can affect the untranslated regions (UTRs) and generates mRNAs harboring different cis-regulatory elements. Such differential UTRs can influence mRNA stability, translation, and can change the targeting of mRNAs to subcellular compartments. Here, we summarize different categories of alternative splicing which are thought to contribute to synaptic remodeling, give an overview of activity-regulated alternatively spliced mRNAs of synaptic proteins that impact synaptic functions, and discuss splicing factors and epigenetic modifications as regulatory determinants.

Bdnf mRNA splice variants differentially impact CA1 and CA3 dendrite complexity and spine morphology in the hippocampus

Brain-derived neurotrophic factor (BDNF) is an activity-dependent neurotrophin critical for neuronal plasticity in the hippocampus. BDNF is encoded by multiple transcripts with alternative 5' untranslated regions (5'UTRS) that display activity-induced targeting to distinct subcellular compartments. While individual Bdnf 5'UTR transcripts influence dendrite morphology in cultured hippocampal neurons, it is unknown whether Bdnf splice variants impact dendrite arborization in functional classes of neurons in the intact hippocampus. Moreover, the contribution of Bdnf 5'UTR splice variants to dendritic spine density and shape has not been explored. We analyzed the structure of CA1 and CA3 dendrite arbors in transgenic mice lacking BDNF production from exon (Ex) 1, 2, 4, or 6 splice variants (Bdnf-e1, -e2, -e4, and -e6^-/- mice) and found that loss of BDNF from individual Bdnf mRNA variants differentially impacts the complexity of apical and basal arbors in vivo. Consistent with the subcellular localization studies, Bdnf Ex2 and Ex6 transcripts significantly contributed to dendrite morphology in both CA1 and CA3 neurons. While Bdnf-e2^-/- mice showed increased branching proximal to the soma in CA1 and CA3 apical arbors, Bdnf-e6^-/- mice showed decreased apical and basal dendrite complexity. Analysis of spine morphology on Bdnf-e6^-/- CA1 dendrites revealed changes in the percentage of differently sized spines on apical, but not basal, branches. These results provide further evidence that Bdnf splice variants generate a spatial code that mediates the local actions of BDNF in distinct dendritic compartments on structural and functional plasticity.

S 38093, a histamine H3 antagonist/inverse agonist, promotes hippocampal neurogenesis and improves context discrimination task in aged mice

Strategies designed to increase adult hippocampal neurogenesis (AHN) may have therapeutic potential for reversing memory impairments. H₃ receptor antagonists/inverse agonists also may be useful for treating cognitive deficits. However, it remains unclear whether these ligands have effects on AHN. The present study aimed to investigate the effects of a 28-day treatment with S 38093, a novel brain-penetrant antagonist/inverse agonist of H₃ receptors, on AHN (proliferation, maturation and survival) in 3-month-old and in aged 16-month-old mice. In addition, the effects of S 38093 treatment on 7-month-old APPSWE Tg2576 transgenic mice, a model of Alzheimer's disease, were also assessed. In all tested models, chronic treatment with S 38093 stimulated all steps of AHN. In aged animals, S 38093 induced a reversal of age-dependent effects on hippocampal brain-derived neurotrophic factor (BDNF) BDNF-IX, BDNF-IV and BDNF-I transcripts and increased vascular endothelial growth factor (VEGF) expression. Finally, the effects of chronic administration of S 38093 were assessed on a neurogenesis-dependent "context discrimination (CS) test" in aged mice. While ageing altered mouse CS, chronic S 38093 treatment significantly improved CS. Taken together, these results provide evidence that chronic S 38093 treatment increases adult hippocampal neurogenesis and may provide an innovative strategy to improve age-associated cognitive deficits.

Brain-derived neurotropic factor and GABAergic transmission in neurodegeneration and neuroregeneration

Neurotoxicity induced by stress, radiation, chemicals, or metabolic diseases, is commonly associated with excitotoxicity, oxidative stress, and neuroinflammation. The pathological process of neurotoxicity induces neuronal death, interrupts synaptic plasticity in the brain, and is similar to that of diverse neurodegenerative diseases. Animal models of neurotoxicity have revealed that clinical symptoms and brain lesions can recover over time via neuroregenerative processes. Specifically, brain-derived neurotropic factor (BDNF) and gamma-aminobutyric acid (GABA)-ergic transmission are related to both neurodegeneration and neuroregeneration. This review summarizes the accumulating evidences that suggest a pathogenic role of BDNF and GABAergic transmission, their underlying mechanisms, and the relationship between BDNF and GABA in neurodegeneration and neuroregeneration. This review will provide a comprehensive overview of the underlying mechanisms of neuroregeneration that may help in developing potential strategies for pharmacotherapeutic approaches to treat neurotoxicity and neurodegenerative disease.

Functional Role of BDNF Production from Unique Promoters in Aggression and Serotonin Signaling

Brain-derived neurotrophic factor (BDNF) regulates diverse biological functions ranging from neuronal survival and differentiation during development to synaptic plasticity and cognitive behavior in the adult. BDNF disruption in both rodents and humans is associated with neurobehavioral alterations and psychiatric disorders. A unique feature of Bdnf transcription is regulation by nine individual promoters, which drive expression of variants that encode an identical protein. It is hypothesized that this unique genomic structure may provide flexibility that allows different factors to regulate BDNF signaling in distinct cell types and circuits. This has led to the suggestion that isoforms may regulate specific BDNF-dependent functions; however, little scientific support for this idea exists. We generated four novel mutant mouse lines in which BDNF production from one of the four major promoters (I, II, IV, or VI) is selectively disrupted (Bdnf-e1, -e2, -e4, and -e6 mice) and used a comprehensive comparator approach to determine whether different Bdnf transcripts are associated with specific BDNF-dependent molecular, cellular, and behavioral phenotypes. Bdnf-e1 and -e2 mutant males displayed heightened aggression accompanied by convergent expression changes in specific genes associated with serotonin signaling. In contrast, BDNF-e4 and -e6 mutants were not aggressive but displayed impairments associated with GABAergic gene expression. Moreover, quantifications of BDNF protein in the hypothalamus, prefrontal cortex, and hippocampus revealed that individual Bdnf transcripts make differential, region-specific contributions to total BDNF levels. The results highlight the biological significance of alternative Bdnf transcripts and provide evidence that individual isoforms serve distinct molecular and behavioral functions.

Involvement of BDNF/ERK signaling in spontaneous recovery from trimethyltin-induced hippocampal neurotoxicity in mice

Trimethyltin (TMT) toxicity causes histopathological damage in the hippocampus and induces seizure behaviors in mice. The lesions and symptoms recover spontaneously over time; however, little is known about the precise mechanisms underlying this recovery from TMT toxicity. We investigated changes in the brain-derived neurotrophic factor/extracellular signal-regulated kinases (BDNF/ERK) signaling pathways in the mouse hippocampus following TMT toxicity. Mice (7 weeks old, C57BL/6) administered TMT (2.6 mg/kg intraperitoneally) showed acute and severe neurodegeneration with increased TUNEL-positive cells in the dentate gyrus (DG) of the hippocampus. The mRNA and protein levels of BDNF in the hippocampus were elevated by TMT treatment. Immunohistochemical analysis showed that TMT treatment markedly increased phosphorylated ERK1/2 expression in the mouse hippocampus 1-4 days after TMT treatment, although the intensity of ERK immunoreactivity in mossy fiber decreased at 1-8 days post-treatment. In addition, ERK-immunopositive cells were localized predominantly in doublecortin-positive immature progenitor neurons in the DG. In primary cultured immature hippocampal neurons (4 days in vitro), BDNF treatment alleviated TMT-induced neurotoxicity, via activation of the ERK signaling pathway. Thus, we suggest that BDNF/ERK signaling pathways may be associated with cell differentiation and survival of immature progenitor neurons, and will eventually lead to spontaneous recovery in TMT-induced hippocampal neurodegeneration.

Effects of developmental alcohol exposure vs. intubation stress on BDNF and TrkB expression in the hippocampus and frontal cortex of neonatal rats

Third trimester-equivalent alcohol exposure causes significant deficits in hippocampal and cortical neuroplasticity, resulting in alterations to dendritic arborization, hippocampal adult neurogenesis, and performance on learning tasks. The current study investigated the impact of neonatal alcohol exposure (postnatal days 4-9, 5.25 g/kg/day) on expression of brain-derived neurotrophic factor (BDNF) and the tropomyosin-related kinase B (TrkB) receptor in the hippocampal and frontal cortex of infant Long-Evans rats. Levels of BDNF protein were increased in the hippocampus, but not frontal cortex, of alcohol-exposed rats 24h after the last dose, when compared with undisturbed (but not sham-intubated) control animals. BDNF protein levels showed a trend toward increase in hippocampus of sham-intubated animals as well, suggesting an effect of the intubation procedure. TrkB protein was increased in the hippocampus of alcohol-exposed animals compared to sham-intubated pups, indicating an alcohol-specific effect on receptor expression. In addition, expression of bdnf total mRNA in alcohol-exposed and sham-intubated pups was enhanced in the hippocampus; however, there was a differential effect of alcohol and intubation stress on exon I- and IV-specific mRNA transcripts. Further, plasma corticosterone was found to be increased in both alcohol-exposed and sham-intubated pups compared to undisturbed animals. Upregulation of BDNF could potentially represent a neuroprotective mechanism activated following alcohol exposure or stress. The results suggest that alcohol exposure and stress have both overlapping and unique effects on BDNF, and highlight the need for the stress of intubation to be taken into consideration in studies that implement this route of drug delivery.

Involvement of Nitric Oxide, Neurotrophins and HPA Axis in Neurobehavioural Alterations Induced by Prenatal Stress

Several studies suggest that negative emotions during pregnancy generate adverse effects on the cognitive, behavioural and emotional development of the descendants. The psychoneuroendocrine pathways involve the transplacentary passage of maternal glucocorticoids in order to influence directly on fetal growth and brain development.Nitric oxide is a gaseous neurotransmitter that plays an important role in the control of neural activity by diffusing into neurons and participates in learning and memory processes. It has been demonstrated that nitric oxide is involved in the regulation of corticosterone secretion. Thus, it has been found that the neuronal isoform of nitric oxide synthase (nNOS) is an endogenous inhibitor of glucocorticoid receptor (GR) in the hippocampus and that nNOS in the hippocampus may participate in the modulation of hypothalamic-pituitary-adrenal axis activity via GR.Neurotrophins are a family of secreted growth factors consisting of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT3) and NT4. Although initially described in the nervous system, they regulate processes such as cell survival, proliferation and differentiation in several other compartments. It has been demonstrated that the NO-citrulline cycle acts together with BDNF in maintaining the progress of neural differentiation.In the present chapter, we explore the interrelation between nitric oxide, glucocorticoids and neurotrophins in brain areas that are key structures in learning and memory processes. The participation of this interrelation in the behavioural and cognitive alterations induced in the offspring by maternal stress is also addressed.

Regulation of GABAARs by phosphorylation

γ-Aminobutyric acid type A receptors (GABAARs) are the principal mediators of fast synaptic inhibition in the brain as well as the low persistent extrasynaptic inhibition, both of which are fundamental to proper brain function. Thus unsurprisingly, deficits in GABAARs are implicated in a number of neurological disorders and diseases. The complexity of GABAAR regulation is determined not only by the heterogeneity of these receptors but also by its posttranslational modifications, the foremost, and best characterized of which is phosphorylation. This review will explore the details of this dynamic process, our understanding of which has barely scratched the surface. GABAARs are regulated by a number of kinases and phosphatases, and its phosphorylation plays an important role in governing its trafficking, expression, and interaction partners. Here, we summarize the progress in understanding the role phosphorylation plays in the regulation of GABAARs. This includes how phosphorylation can affect the allosteric modulation of GABAARs, as well as signaling pathways that affect GABAAR phosphorylation. Finally, we discuss the dysregulation of GABAAR phosphorylation and its implication in disease processes.

Developmental and degenerative modulation of brain-derived neurotrophic factor transcript variants in the mouse hippocampus

Brain-derived neurotrophic factor (BDNF) is regarded as an important factor for neurogenesis, synaptic plasticity, and neuronal network organization in brain circuits. However, little is known about the regulation of BDNF transcript variants in the hippocampus during postnatal development and following chemically induced neurotoxicity. In the present study, we examined the expression of individual BDNF transcript variants in the mouse hippocampus on postnatal day (PD) 3, 7, 14, 21, and 56, as well as in the adult hippocampus 1, 2, 4, and 8 days after trimethyltin (TMT) treatment. During postnatal development, the expression levels of common BDNF-coding transcripts and BDNF transcript variants increased gradually in the hippocampus, but the temporal patterns of each exon transcript showed significant differences. In the TMT-treated hippocampus, the levels of common BDNF-coding transcripts and exon I, IIC, III, VII, VIII, and IXA transcripts were significantly increased 1 day post-treatment. These observations suggest that the differential regulation of BDNF exon transcripts may be associated with neuronal and synaptic maturation during postnatal development, and neuronal survival and synaptic plasticity in chemically induced neurodegeneration.

Incensole acetate reduces depressive-like behavior and modulates hippocampal BDNF and CRF expression of submissive animals

Incensole acetate (IA), a constituent of Boswellia resin ('frankincense'), was previously demonstrated to exhibit an antidepressive-like effect in the Forced Swim Test (FST) in mice following single dose administration (50 mg/kg). Here, we show that acute administration of considerably lower dose (10 mg/kg) IA to selectively bred mice, showing prominent submissive behavior, exerted significant antidepressant-like effects in the FST. Furthermore, chronic administration of 1 or 5 mg/kg per day of IA for three consecutive weeks dose- and time-dependently reduced the submissiveness of the mice in the Dominant-Submissive Relationship test, developed to screen the chronic effect of antidepressants. This behavioral effect was concomitant to reduced serum corticosterone levels, dose-dependent down-regulation of corticotropin releasing factor and up-regulation of brain derived neurotrophic factor transcripts IV and VI expression in the hippocampus. These data suggest that IA modulates the hypothalamic-pituitary-adrenal (HPA) axis and influences hippocampal gene expression, leading to beneficial behavioral effects supporting its potential as a novel treatment of depressive-like disorders.

Current perspectives on the neurobiology of drug addiction: a focus on genetics and factors regulating gene expression

Drug addiction is a chronic, relapsing disorder defined by cyclic patterns of compulsive drug seeking and taking interspersed with episodes of abstinence. While genetic variability may increase the risk of addictive behaviours in an individual, exposure to a drug results in neuroadaptations in interconnected brain circuits which, in susceptible individuals, are believed to underlie the transition to, and maintenance of, an addicted state. These adaptations can occur at the cellular, molecular, or (epi)genetic level and are associated with synaptic plasticity and altered gene expression, the latter being mediated via both factors affecting translation (epigenetics) and transcription (non coding microRNAs) of the DNA or RNA itself. New advances using techniques such as optogenetics have the potential to increase our understanding of the microcircuitry mediating addictive behaviours. However, the processes leading to addiction are complex and multifactorial and thus we face a major contemporary challenge to elucidate the factors implicated in the development and maintenance of an addicted state.

Opposite effects of acute ethanol exposure on GAP-43 and BDNF expression in the hippocampus versus the cerebellum of juvenile rats

The adolescent brain is particularly vulnerable to the effects of alcohol, with intoxications at this developmental age often producing long-lasting effects. The present study addresses the effects of a single acute ethanol exposure on growth-associated protein-43 (GAP-43) and brain-derived neurotrophic factor (BDNF) gene expression in neurons in the cerebellum and hippocampus of adolescent rats. Male postnatal day 23 (P23) Sprague-Dawley rats were exposed to ethanol vapors for 2h and after a recovery period of 2h, the cerebellum and hippocampus were harvested and samples were taken for blood alcohol concentration (BAC) determinations. We found that this exposure resulted in a mean BAC of 174 mg/dL, which resembles levels in human adolescents after binge drinking. Analyses of total RNA and protein by quantitative reverse transcription PCR and western blotting, respectively, revealed that this single ethanol exposure significantly decreased the levels of GAP-43 mRNA and protein in the cerebellum but increased the levels of mRNA and protein in the hippocampus. BDNF mRNA and protein levels were also increased in the hippocampus but not in the cerebellum of these animals. In situ hybridizations revealed that GAP-43 and BDNF mRNA levels were primarily increased by alcohol exposure in hippocampal dentate granule cells and CA3 neurons. Overall, the reported alterations in the expression of the plasticity-associated genes GAP-43 and BDNF in juvenile rats are consistent with the known deleterious effects of binge drinking on motor coordination and cognitive function.

Lasting epigenetic influence of early-life adversity on the BDNF gene

BACKGROUND

Childhood maltreatment and early trauma leave lasting imprints on neural mechanisms of cognition and emotion. With a rat model of infant maltreatment by a caregiver, we investigated whether early-life adversity leaves lasting epigenetic marks at the brain-derived neurotrophic factor (BDNF) gene in the central nervous system.

METHODS

During the first postnatal week, we exposed infant rats to stressed caretakers that predominately displayed abusive behaviors. We then assessed DNA methylation patterns and gene expression throughout the life span as well as DNA methylation patterns in the next generation of infants.

RESULTS

Early maltreatment produced persisting changes in methylation of BDNF DNA that caused altered BDNF gene expression in the adult prefrontal cortex. Furthermore, we observed altered BDNF DNA methylation in offspring of females that had previously experienced the maltreatment regimen.

CONCLUSIONS

These results highlight an epigenetic molecular mechanism potentially underlying lifelong and transgenerational perpetuation of changes in gene expression and behavior incited by early abuse and neglect.

Changes in alternative brain-derived neurotrophic factor transcript expression in the developing human prefrontal cortex

In this study, we determined when and through which promoter brain-derived neurotrophic factor (BDNF) transcription is regulated during the protracted period of human frontal cortex development. Using quantitative real-time polymerase chain reaction, we examined the expression of the four most abundant alternative 5' exons of the BDNF gene (exons I, II, IV, and VI) in RNA extracted from the prefrontal cortex. We found that expression of transcripts I-IX and VI-IX was highest during infancy, whereas that of transcript II-IX was lowest just after birth, slowly increasing to reach a peak in toddlers. Transcript IV-IX was significantly upregulated within the first year of life, and was maintained at this level until school age. Quantification of BDNF protein revealed that levels followed a similar developmental pattern as transcript IV-IX. In situ hybridization of mRNA in cortical sections showed the highest expression in layers V and VI for all four BDNF transcripts, whereas moderate expression was observed in layers II and III. Interestingly, although low expression of BDNF was observed in cortical layer IV, this BDNF mRNA low-zone decreased in prominence with age and showed an increase in neuronal mRNA localization. In summary, our findings show that dynamic regulation of BDNF expression occurs through differential use of alternative promoters during the development of the human prefrontal cortex, particularly in the younger age groups, when the prefrontal cortex is more plastic.

BHLHB2 controls Bdnf promoter 4 activity and neuronal excitability

Brain-derived neurotrophic factor (BDNF), via activation of TrkB receptors, mediates vital physiological functions in the brain, ranging from neuronal survival to synaptic plasticity, and has been implicated in the pathophysiology of neurodegenerative disorders. Although transcriptional regulation of the BDNF gene (Bdnf) has been extensively studied, much remains to be understood. We discovered a sequence within Bdnf promoter 4 that binds the basic helix-loop-helix protein BHLHB2 and is a target for BHLHB2-mediated transcriptional repression. NMDA receptor activation de-repressed promoter 4-mediated transcription and correlated with reduced occupancy of the promoter by BHLHB2 in cultured hippocampal neurons. Bhlhb2 gene -/- mice showed increased hippocampal exon 4-specific Bdnf mRNA levels compared with +/+ littermates under basal and activity-dependent conditions. Bhlhb2 knock-out mice also showed increased status epilepticus susceptibility, suggesting that BHLHB2 alters neuronal excitability. Together, these results support a role for BHLHB2 as a new modulator of Bdnf transcription and neuronal excitability.

Seizures in the developing brain: cellular and molecular mechanisms of neuronal damage, neurogenesis and cellular reorganization

Epilepsy is a common neurological disorder that occurs more frequently in children than in adults. The extent that prolonged seizure activity, i.e. status epilepticus (SE), and repeated, brief seizures affect neuronal structure and function in both the immature and mature brain has been the subject of increasing clinical and experimental research. Earlier studies suggest that seizure-induced effects in the immature brain compared with the adult brain are different. This is manifested as differences in neuronal vulnerability, cellular and synaptic reorganization and regenerative processes. The focus of this review is first to give a short overview of currently used experimental models of epilepsy in immature rats, and then discuss more thoroughly seizure-induced acute and sub-acute cellular and molecular alterations, highlight the contribution of inflammatory-like reactions and intracellular cytoskeleton to the insult, and reveal changes in the structure and function of inhibitory GABA(A) and excitatory glutamate receptors. The role of seizure-activated reparative, plastic processes, synaptic remodelling, neurogenesis as well as the long-term consequences of seizures are briefly outlined. The main emphasis is put on studies carried out in experimental animals, and the focus of interest is the hippocampus, the brain area of great vulnerability in epilepsy. In vitro studies are discussed only to limited extent. Collectively, recent studies suggest that the deleterious effects of seizures may not solely be a consequence of neuronal damage and loss per se, but could be due to the fact that seizures interfere with the highly regulated developmental processes in the immature brain.

Stressor-specific regulation of distinct brain-derived neurotrophic factor transcripts and cyclic AMP response element-binding protein expression in the postnatal and adult rat hippocampus

Stress regulation of brain-derived neurotrophic factor (BDNF) is implicated in the hippocampal damage observed in depression. BDNF has a complex gene structure with four 5' untranslated exons (I-IV) with unique promoters, and a common 3' coding exon (V). To better understand the stress regulation of BDNF, we addressed whether distinct stressors differentially regulate exon-specific BDNF transcripts in the postnatal and adult hippocampus. The early life stress of maternal separation (MS) resulted in a time point-dependent differential upregulation of BDNF transcripts restricted to early postnatal life (P14-BDNF II, P21-BDNF IV, V). In adulthood, distinct stressors regulated BDNF transcripts in a signature manner. Immobilization stress, administered once, decreased all BDNF splice variants but had differing effects on BDNF I/II (increase) and III/IV (decrease) when administered chronically. Although immobilization stress reduced BDNF (V) mRNA, chronic unpredictable stress did not influence total BDNF despite altering specific BDNF transcripts. Furthermore, a prior history of MS altered the signature pattern in which adult-onset stress regulated specific BDNF transcripts. We also examined the expression of cyclic AMP response element-binding protein (CREB), an upstream transcriptional activator of BDNF, and observed a CREB induction in the postnatal hippocampus following MS. As a possible consequence of enhanced CREB and BDNF expression following MS, we examined hippocampal progenitor proliferation and observed a significant increase restricted to early life. These results suggest that alterations in CREB/BDNF may contribute to the generation of individual differences in stress neurocircuitry, providing a substrate for altered vulnerability to depressive disorders.

Mouse and rat BDNF gene structure and expression revisited

Brain-derived neurotrophic factor (BDNF) has important functions in the development of the nervous system and in brain plasticity-related processes such as memory, learning, and drug addiction. Despite the fact that the function and regulation of rodent BDNF gene expression have received close attention during the last decade, knowledge of the structural organization of mouse and rat BDNF gene has remained incomplete. We have identified and characterized several mouse and rat BDNF transcripts containing novel 5' untranslated exons and introduced a new numbering system for mouse and rat BDNF exons. According to our results both mouse and rat BDNF gene consist of eight 5' untranslated exons and one protein coding 3' exon. Transcription of the gene results in BDNF transcripts containing one of the eight 5' exons spliced to the protein coding exon and in a transcript containing only 5' extended protein coding exon. We also report the distinct tissue-specific expression profiles of each of the mouse and rat 5' exon-specific transcripts in different brain regions and nonneural tissues. In addition, we show that kainic acid-induced seizures that lead to changes in cellular Ca(2+) levels as well as inhibition of DNA methylation and histone deacetylation contribute to the differential regulation of the expression of BDNF transcripts. Finally, we confirm that mouse and rat BDNF gene loci do not encode antisense mRNA transcripts, suggesting that mechanisms of regulation for rodent and human BDNF genes differ substantially.

Abnormalities of neurogenesis in the R6/2 mouse model of Huntington's disease are attributable to the in vivo microenvironment

Huntington's disease (HD) is an autosomal dominant neurodegenerative condition characterized by movement disorders, psychiatric disturbance, and cognitive decline. There are no treatments to halt or reverse the disease. Mammalian neurogenesis persists into adulthood in the subventricular zone (SVZ) and dentate gyrus (DG) of the hippocampus. In 2001, our laboratory published the hypothesis that neurogenesis is impaired in neurodegenerative diseases and that this may contribute to disease progression. Since then, it has been shown that neurogenesis is reduced in the DG of transgenic HD mice but increased in the SVZ of HD patients. We sought to characterize neurogenesis further. We found that, in the DG of the transgenic R6/2 mouse model of HD, newborn cell proliferation and morphology, but not differentiation or survival, was compromised. In R6/2 mice, neurogenesis failed to upregulate in the DG in response to seizures. Basal SVZ neurogenesis was similar between R6/2 mice and their wild-type littermates. There was no difference in the in vitro growth of adult neural precursor cells (NPCs) between genotypes. These results suggest that abnormal neurogenesis in the R6/2 mouse is not attributable to an intrinsic impairment of the NPC itself but is attributable to the environment in which the cell is located.

BDNF and trkB mRNA expression in the hippocampus and temporal cortex during the human lifespan

Brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase B (trkB) influence neuronal survival, differentiation, synaptogenesis, and maintenance. Using in situ hybridization we examined the spatial and temporal expression of mRNAs encoding these proteins during diverse stages of life in the human hippocampus and inferior temporal cortex. We examined six postnatal time points: neonatal (1-3 months), infant (4-12 months), adolescent (14-18 years), young adult (20-24 years), adult (34-43 years), and aged (68-86 years). Within the hippocampus, levels of BDNF mRNA did not change significantly with age. However, levels of both the full-length form of trkB (trkB TK+) mRNA and the truncated form of trkB (trkB TK-) decreased over the life span (p < 0.05). In the temporal cortex, BDNF and trkB TK+ mRNA levels were highest in neonates and decreased with age (r = -0.4 and r = -0.7, respectively, both p < 0.05). In contrast, TrkB TK- mRNA levels remained constant across the life span in the temporal cortex. The peak in both BDNF and trkB TK+ mRNA expression in the neonate temporal cortex differs from that previously described for the frontal cortex where both mRNAs peak in expression during young adulthood. The increase in BDNF and trkB TK+ mRNA in the temporal cortex of the neonate suggests that neurotrophin signaling is important in the early development of the temporal cortex. In addition, since BDNF and both forms of its high affinity receptor are expressed throughout the development, maturation, and aging of the human hippocampus and surrounding neocortex they are likely to play roles not only in early growth but also in maintenance of neurons throughout life.