From acquisition to consolidation: on the role of brain-derived neurotrophic factor signaling in hippocampal-dependent learning

Escalating ethanol intake is associated with altered corticostriatal BDNF expression

Alcoholism is a chronically relapsing condition, indicative of long-term neuronal adaptations maintaining the disease even after prolonged abstinence. Previously, we identified brain-derived neurotrophic factor (BDNF) in the dorsal striatum as the central mediator of a homeostatic mechanism which is activated by acute alcohol (ethanol) exposure and functions to decrease the sensitivity of rodents to ethanol-related behaviors. We hypothesized that extensive exposure to ethanol would result in dysregulation of this BDNF-mediated protective mechanism, accompanied by heightened ethanol intake. In this study, we demonstrate that while a single bout of ethanol intake increases BDNF mRNA expression in the dorsal striatum, this effect is no longer observed after 6 weeks of daily ethanol access. Additionally, 6 weeks of ethanol consumption decreases BDNF in the cortex, a main source of BDNF for the striatum. Importantly, these ethanol-induced changes in BDNF levels are not ameliorated by 2 weeks' abstinence. Together, these data suggest that the BDNF pathway, which is activated following a single bout of ethanol drinking, breaks down by the end of 6 weeks of access and does not recover its protective function after a 2-week deprivation period. These results suggest that the persistence of altered BDNF signaling may contribute to the inflexibility of addictive behaviors.

Effects of dietary Na+ deprivation on epithelial Na+ channel (ENaC), BDNF, and TrkB mRNA expression in the rat tongue

BACKGROUND

In rodents, dietary Na+ deprivation reduces gustatory responses of primary taste fibers and central taste neurons to lingual Na+ stimulation. However, in the rat taste bud cells Na+ deprivation increases the number of amiloride sensitive epithelial Na+ channels (ENaC), which are considered as the "receptor" of the Na+ component of salt taste. To explore the mechanisms, the expression of the three ENaC subunits (alpha, beta and gamma) in taste buds were observed from rats fed with diets containing either 0.03% (Na+ deprivation) or 1% (control) NaCl for 15 days, by using in situ hybridization and real-time quantitative RT-PCR (qRT-PCR). Since BDNF/TrkB signaling is involved in the neural innervation of taste buds, the effects of Na+ deprivation on BDNF and its receptor TrkB expression in the rat taste buds were also examined.

RESULTS

In situ hybridization analysis showed that all three ENaC subunit mRNAs were found in the rat fungiform taste buds and lingual epithelia, but in the vallate and foliate taste buds, only alpha ENaC mRNA was easily detected, while beta and gamma ENaC mRNAs were much less than those in the fungiform taste buds. Between control and low Na+ fed animals, the numbers of taste bud cells expressing alpha, beta and gamma ENaC subunits were not significantly different in the fungiform, vallate and foliate taste buds, respectively. Similarly, qRT-PCR also indicated that Na+ deprivation had no effect on any ENaC subunit expression in the three types of taste buds. However, Na+ deprivation reduced BDNF mRNA expression by 50% in the fungiform taste buds, but not in the vallate and foliate taste buds. The expression of TrkB was not different between control and Na+ deprived rats, irrespective of the taste papillae type.

CONCLUSION

The findings demonstrate that dietary Na+ deprivation does not change ENaC mRNA expression in rat taste buds, but reduces BDNF mRNA expression in the fungiform taste buds. Given the roles of BDNF in survival of cells and target innervation, our results suggest that dietary Na+ deprivation might lead to a loss of gustatory innervation in the mouse fungiform taste buds.

Preliminary evidence of cannabinoid effects on brain-derived neurotrophic factor (BDNF) levels in humans

BACKGROUND

Acute and chronic exposure to cannabinoids has been associated with cognitive deficits, a higher risk for schizophrenia and other drug abuse. However, the precise mechanism underlying such effects is not known. Preclinical studies suggest that cannabinoids modulate brain-derived neurotrophic factor (BDNF). Accordingly, we hypothesized that Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the principal active component of cannabis, would alter BDNF levels in humans.

MATERIALS AND METHODS

Healthy control subjects (n = 14) and light users of cannabis (n = 9) received intravenous administration of (0.0286 mg/kg) Delta(9)-THC in a double-blind, fixed order, placebo-controlled, laboratory study. Serum sampled at baseline, after placebo administration, and after Delta(9)-THC administration was assayed for BDNF using ELISA.

RESULTS

Delta(9)-THC increased serum BDNF levels in healthy controls but not light users of cannabis. Further, light users of cannabis had lower basal BDNF levels. Delta(9)-THC produced psychotomimetic effects, perceptual alterations, and "high" and spatial memory impairments.

IMPLICATIONS

The effects of socially relevant doses of cannabinoids on BDNF suggest a possible mechanism underlying the consequences of exposure to cannabis. This may be of particular importance for the developing brain and also in disorders believed to involve altered neurodevelopment such as schizophrenia. Larger studies to investigate the effects of cannabinoids on BDNF and other neurotrophins are warranted.

Impaired long term memory consolidation in transgenic mice overexpressing the human soluble form of IL-1ra in the brain

Interleukin-1 (IL-1) is expressed following LTP induction and is required for long-term memory consolidation. We demonstrate that the long-term, but not short-term memory is impaired in a transgenic mouse strain overexpressing the human soluble interleukin-1 receptor antagonist (hsIL-1ra) in the brain. Overexpression of IL-1ra was found to reduce the basal as well as the novelty-induced upregulation of activity-regulated cytoskeleton-associated protein (Arc) in the dentate gyrus and in the retrosplenial cortex. Together with the finding that blocking IL-1 receptors interferes with the BDNF-ERK1/2 pathway, our data suggest an essential role played by physiological levels of IL-1 in long-term memory consolidation.

Object-place recognition learning triggers rapid induction of plasticity-related immediate early genes and synaptic proteins in the rat dentate gyrus

Long-term recognition memory requires protein synthesis, but little is known about the coordinate regulation of specific genes. Here, we examined expression of the plasticity-associated immediate early genes (Arc, Zif268, and Narp) in the dentate gyrus following long-term object-place recognition learning in rats. RT-PCR analysis from dentate gyrus tissue collected shortly after training did not reveal learning-specific changes in Arc mRNA expression. In situ hybridization and immunohistochemistry were therefore used to assess possible sparse effects on gene expression. Learning about objects increased the density of granule cells expressing Arc, and to a lesser extent Narp, specifically in the dorsal blade of the dentate gyrus, while Zif268 expression was elevated across both blades. Thus, object-place recognition triggers rapid, blade-specific upregulation of plasticity-associated immediate early genes. Furthermore, Western blot analysis of dentate gyrus homogenates demonstrated concomitant upregulation of three postsynaptic density proteins (Arc, PSD-95, and α-CaMKII) with key roles in long-term synaptic plasticity and long-term memory.

Estrogen-growth factor interactions and their contributions to neurological disorders

Estrogen has diverse and powerful effects in the brain, including actions on neurons, glia, and the vasculature. It is not surprising, therefore, that there are many changes in the female brain as serum estradiol levels rise and fall during the normal ovarian cycle. At times of life when estradiol levels change dramatically, such as puberty, postpartum, or menopause, there also are dramatic changes in the central nervous system. Changes that occur because of fluctuations in serum estrogen levels are potentially relevant to neurological disorders because symptoms often vary with the time of the ovarian cycle. Moreover, neurological disorders (eg, seizures and migraine) often increase in frequency in women when estradiol levels change. In this review, the contribution of 2 growth factors targeted by estrogen, the neurotrophin brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF), will be discussed. Estrogen-sensitive response elements are present on the genes for both BDNF and VEGF, and they are potent modulators of neuronal, glial, and vascular function, making them logical candidates to mediate the multitude of effects of estrogen. In addition, BDNF induces neuropeptide Y, which has diverse actions that are relevant to estrogen action and to the same neurological disorders.

Variations in the transcriptome of Alzheimer's disease reveal molecular networks involved in cardiovascular diseases

Analysis of microarray data reveals extensive links between Alzheimer’s disease and cardiovascular diseases.

Background

Because of its polygenic nature, Alzheimer's disease is believed to be caused not by defects in single genes, but rather by variations in a large number of genes and their complex interactions. A systems biology approach, such as the generation of a network of co-expressed genes and the identification of functional modules and cis-regulatory elements, to extract insights and knowledge from microarray data will lead to a better understanding of complex diseases such as Alzheimer's disease. In this study, we perform a series of analyses using co-expression networks, cis-regulatory elements, and functions of co-expressed gene modules to analyze single-cell gene expression data from normal and Alzheimer's disease-affected subjects.

Results

We identified six co-expressed gene modules, each of which represented a biological process perturbed in Alzheimer's disease. Alzheimer's disease-related genes, such as APOE, A2M, PON2 and MAP4, and cardiovascular disease-associated genes, including COMT, CBS and WNK1, all congregated in a single module. Some of the disease-related genes were hub genes while many of them were directly connected to one or more hub genes. Further investigation of this disease-associated module revealed cis-regulatory elements that match to the binding sites of transcription factors involved in Alzheimer's disease and cardiovascular disease.

Conclusion

Our results show the extensive links between Alzheimer's disease and cardiovascular disease at the co-expression and co-regulation levels, providing further evidence for the hypothesis that cardiovascular disease and Alzheimer's disease are linked. Our results support the notion that diseases in which the same set of biochemical pathways are affected may tend to co-occur with each other.

BDNF modulation of NMDA receptors is activity dependent

Brain-derived neurotrophic factor (BDNF), a potent modulator of synaptic transmission, is known to influence associative synaptic plasticity and refinement of neural connectivity. We now show that BDNF modulation of glutamate currents in hippocampal neurons exhibits the additional property of use dependence, a postsynaptic mechanism resulting in selective modulation of active channels. We demonstrate selectivity by varying the repetition rate of iontophoretically applied glutamate pulses during BDNF exposure. During relatively high-frequency glutamate pulses (0.1 Hz), BDNF application elicited a doubling of the glutamate current. During low-frequency pulses (0.0033 Hz), however, BDNF evoked a dramatically diminished response. This effect was apparently mediated by calcium because manipulations that prevented elevation of intracellular calcium largely eliminated the action of BDNF on glutamate currents. To confirm N-methyl-D-aspartate (NMDA) receptor involvement and assess spatial requirements, we made cell-attached single-channel recordings from somatic NMDA receptors. Inclusion of calcium in the pipette was sufficient to produce enhancement of channel activity by BDNF. Substitution of EGTA for calcium prevented BDNF effects. We conclude that BDNF modulation of postsynaptic NMDA receptors requires concurrent neuronal activity potentially conferring synaptic specificity on the neurotrophin's actions.

The influence of forepaw palmar sensorimotor deprivation on learning and memory in young rats

This study examined the effects of early palmar forepaw sensorimotor deprivation on learning and memory in rats. Sensorimotor deprivation was performed on 18-day-old male rats. Controls were sham operated. Studies were performed on rats aged 18, 25, 35, 45 and 60 days. Morris water maze testing was used to assess learning and memory. Long-term potentiation (LTP) was assessed by electrophysiological means in slices obtained from the hippocampal Schaffer collateral pathway. Nissl staining was performed to assess pyramidal cell number in hippocampal CA1 and CA3 regions. Hippocampal N-methyl-d-aspartate receptor 1 (NMDAR1) mRNA and protein levels were assessed. Learning and short-term memory were significantly depressed in 25 and 35 day old sensorimotor deprived rats (P<0.01). LTP was also significantly depressed in sensorimotor deprived rats at these ages, while hippocampal CA1 pyramidal cell counts were significantly decreased (P<0.05). CA3 cell numbers were significantly lower in 25-day-old sensorimotor deprived rats (P<0.05). Both NMDAR1 mRNA and protein levels were significantly lower in sensorimotor deprived rats aged 25 and 35 days (P<0.05). These findings indicate that palmar surface forepaw sensorimotor deprivation impairs subsequent learning and memory in young rats. Decreased hippocampal pyramidal cell numbers and altered NMDAR1 expression may underlie this impairment.

Disturbances in selective information processing associated with the BDNF Val66Met polymorphism: evidence from cognition, the P300 and fronto-hippocampal systems

In this study, we examined whether the Met allele of the BDNF Val66Met polymorphism is associated with selective disruptions to task-relevant information processing. In 475 non-clinical participants for whom BDNF genotype status was determined we used the 'IntegNeuro' computerized battery of neuropsychological tests to assess cognitive performance, an auditory oddball task to elicit the P300 event-related potential (ERP) and, in smaller subsets of these subjects, high resolution structural MRI imaging to quantify fronto-hippocampal grey matter (n=161), and functional magnetic resonance imaging to assess fronto-hippocampal BOLD activation (n=37). Met/Met (MM) homozygotes had higher verbal recall errors, in the absence of differences in attention, executive function, verbal ability or sensori-motor function. Further, MM homozygotes demonstrated a slowed P300 ERP during the oddball task, with corresponding alterations in hippocampal and lateral prefrontal activation, and a localized reduction in hippocampal grey matter. These results are consistent with a subtle impact of the Met allele on fronto-hippocampal systems involved in selective information processing of stimulus context and memory updating within the normal population. The findings also indicate that heritable endophenotypes such as the P300 have value in elucidating genotype-phenotype relationships.

Backpropagating action potentials trigger dendritic release of BDNF during spontaneous network activity

Brain-derived neurotrophic factor (BDNF) is a major regulator of activity-dependent synapse development and plasticity. Because BDNF is a secreted protein, it has been proposed that BDNF is released from target neurons in an activity-dependent manner. However, direct evidence for postsynaptic release of BDNF triggered by ongoing network activity is still lacking. Here we transfected cultures of dissociated hippocampal neurons with green fluorescent protein (GFP)-tagged BDNF and combined whole-cell recording, time-lapse fluorescent imaging, and immunostaining to monitor activity-dependent dendritic release of BDNF. We found that spontaneous backpropagating action potentials, but not synaptic activity alone, led to a Ca2+-dependent dendritic release of BDNF-GFP. Moreover, we provide evidence that endogenous BDNF released from a single neuron can phosphorylate CREB (cAMP response element-binding protein) in neighboring neurons, an important step of immediate early gene activation. Therefore, together, our results support the hypothesis that BDNF might act as a target-derived messenger of activity-dependent synaptic plasticity and development.

Persistent downregulation of hippocampal CREB mRNA parallels a Y-maze deficit in adolescent rats following semi-chronic amphetamine administration

BACKGROUND AND PURPOSE

We investigated possible differences in the impact of chronic amphetamine administration during adolescence and adulthood on aspects of behaviour and brain chemistry.

EXPERIMENTAL APPROACH

Adult (n=32) and adolescent (n=32) male Sprague-Dawley rats were given either D-amphetamine sulphate (10 mg kg(-1) daily, i.p.) or saline (1 mL kg(-1), i.p.) for 10 days. Rats were subsequently tested for anxiety-like behaviour, learning and memory, and sensorimotor gating. Nine weeks later, rats received saline (1 mL kg(-1)) or acute amphetamine challenge (1.5 mg kg(-1)) and the expression levels of mRNA for tyrosine kinase B (TrkB) or cAMP response element-binding protein (CREB) were measured in the hippocampus.

KEY RESULTS

The adolescent amphetamine pretreated group revealed a deficit in exploration on the Y-maze during a 6 h retention test. The frequency of visits to the novel arm was 35% lower for the amphetamine group compared with controls. In parallel, a 43% decrease in hippocampal CREB mRNA, but not TrkB mRNA, was observed in periadolescent rats treated chronically with amphetamine 9 weeks earlier. None of the effects were detected in the adult treated cohort.

CONCLUSIONS AND IMPLICATIONS

Chronic amphetamine treatment during periadolescence resulted in altered behaviour on the Y-maze and persistent downregulation of hippocampal CREB mRNA expression. Given that this group had intact spatial learning and reference memory, it would appear that the deficits observed on the Y-maze reflect a dysfunction in response to novelty. Because no effects of amphetamine treatment were observed in the adult cohort, these data suggest idiosyncratic sensitivity of periadolescence to the long-term effects of psychostimulants.

Enhancement of active shuttle avoidance response by the NO-cGMP-PKG activator YC-1

Although much has been learned about the role of the amygdala in Pavlovian fear conditioning, relatively little is known about the signaling pathway involved in the acquisition of an active avoidance reaction. The aim of this study is to investigate the potentiating effects of the NO-guanylate cyclase activator YC-1 on learning and memory of shuttle avoidance test in rats. YC-1 enhanced the induction of long-term potentiation (LTP) in amygdala through NO-cGMP-PKG-ERK pathway and the increase of BDNF expression. The Western blot and PCR methods were used to examine the signaling pathways involved in fear memory. It was found that YC-1 increased the avoidance responses during learning period and the memory retention lasted longer than one week. The enhancement of learning behavior by YC-1 was antagonized by intracerebroventricular injection of NOS inhibitor l-NAME, PKG inhibitor Rp-8-Br-PET-cGMPS and MEK inhibitor PD98059, indicating that NO-cGMP-PKG and ERK pathways are involved in the learning potentiating action of YC-1. In addition, YC-1 increased the activation of ERK and Akt 30 min after Day-1 training in amygdala. YC-1 also potentiated the expression of BDNF and CREB in response to fear memory test. Taken together, these findings suggest that NO-cGMP-PKG-ERK signaling pathway is involved in the action of YC-1 in enhancing the fear memory.

Effects of multiparity on recognition memory, monoaminergic neurotransmitters, and brain-derived neurotrophic factor (BDNF)

Recognition memory and anxiety were examined in nulliparous (NP: 0 litters) and multiparous (MP: 5-6 litters) middle-aged female rats (12 months old) to assess possible enduring effects of multiparity at least 3 months after the last litter was weaned. MP females performed significantly better than NP females on the non-spatial memory task, object recognition, and the spatial memory task, object placement. Anxiety as measured on the elevated plus maze did not differ between groups. Monoaminergic activity and levels were measured in prefrontal cortex, CA1 hippocampus, CA3 hippocampus, and olfactory bulb (OB). NP and MP females differed in monoamine concentrations in the OB only, with MP females having significantly greater concentrations of dopamine and metabolite DOPAC, norepinephrine and metabolite MHPG, and the serotonin metabolite 5-HIAA, as compared to NP females. These results indicate a long-term change in OB neurochemistry as a result of multiparity. Brain-derived neurotrophic factor (BDNF) was also measured in hippocampus (CA1, CA3, dentate gyrus) and septum. MP females had higher BDNF levels in both CA1 and septum; as these regions are implicated in memory performance, elevated BDNF may underlie the observed memory task differences. Thus, MP females (experiencing multiple bouts of pregnancy, birth, and pup rearing during the first year of life) displayed enhanced memory task performance but equal anxiety responses, as compared to NP females. These results are consistent with previous studies showing long-term changes in behavioral function in MP, as compared to NP, rats and suggest that alterations in monoamines and a neurotrophin, BDNF, may contribute to the observed behavioral changes.

Neuroplasticity in the mesolimbic dopamine system and cocaine addiction

The main characteristics of cocaine addiction are compulsive drug use despite adverse consequences and high rates of relapse during periods of abstinence. A current popular hypothesis is that compulsive cocaine use and cocaine relapse is due to drug-induced neuroadaptations in reward-related learning and memory processes, which cause hypersensitivity to cocaine-associated cues, impulsive decision making and abnormal habit-like learned behaviours that are insensitive to adverse consequences. Here, we review results from studies on the effect of cocaine exposure on selected signalling cascades, growth factors and physiological processes previously implicated in neuroplasticity underlying normal learning and memory. These include the extracellular signal-regulated kinase (ERK) signalling pathway, brain-derived neurotrophic factor (BDNF), glutamate transmission, and synaptic plasticity (primarily in the form of long-term potentiation and depression, LTP and LTD). We also discuss the degree to which these cocaine-induced neuroplasticity changes in the mesolimbic dopamine system mediate cocaine psychomotor sensitization and cocaine-seeking behaviours, as assessed in animal models of drug addiction. Finally, we speculate on how these factors may interact to initiate and sustain cocaine psychomotor sensitization and cocaine seeking.

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

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

Viral vector-mediated blockade of the endocrine stress-response modulates non-spatial memory

Stress results in the release of glucocorticoids (GCs) which at high levels, impair performance on hippocampus-dependent tasks. Estrogen is neurotrophic and can rescue stress-induced memory impairments. Here we report the use of a viral vector to overexpress a chimeric gene (ER/GR) that converts the deleterious effects of glucocorticoids into beneficial estrogenic effects. A short immobilization stress regimen was sufficient to impair non-spatial memory. In contrast, viral vector-mediated overexpression of ER/GR in the dentate gyrus of the hippocampus protected against stress-induced impairments of non-spatial memory. These data add to the growing evidence that increasing estrogenic signaling can protect against the impairing effects of stress on non-spatial memory.

BDNF is essential to promote persistence of long-term memory storage

Persistence is a characteristic attribute of long-term memories (LTMs). However, little is known about the molecular mechanisms that mediate this process. We recently showed that persistence of LTM requires a late protein synthesis- and BDNF-dependent phase in the hippocampus. Here, we show that intrahippocampal delivery of BDNF reverses the deficit in memory persistence caused by inhibition of hippocampal protein synthesis. Importantly, we demonstrate that BDNF induces memory persistence by itself, transforming a nonlasting LTM trace into a persistent one in an ERK-dependent manner. Thus, BDNF is not only necessary, but sufficient to induce a late postacquisition phase in the hippocampus essential for persistence of LTM storage.

Brain-derived neurotropic factor/TrkB signaling in the pathogenesis and novel pharmacotherapy of schizophrenia

The role of neurotropins, predominantly brain-derived neurotropic factor (BDNF), has been implicated in the pathophysiology as well as treatment outcome of schizophrenia. Both human and rodent studies indicate that the beneficial effects of antipsychotic drugs are mediated, at least in part, through BDNF and its receptor, TrkB. This review will discuss the available data on the levels of BDNF and TrkB in subjects with schizophrenia and in animals with and without conventional antipsychotics. The data concerning the impact of the antipsychotic drugs on BDNF/TrkB signaling will also be discussed. More importantly, this review will provide future perspective on BDNF/TrkB signaling as a novel molecular target to correct the pathogenesis and improve the long-term clinical outcome by treatments with conventional and adjunctive drugs.

Interaction between BDNF and serotonin: role in mood disorders

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

The actions of BDNF on dendritic spine density and morphology in organotypic slice cultures depend on the presence of serum in culture media

We have previously shown that brain-derived neurotrophin factor (BDNF) increases dendritic spine density and the proportion of stubby spines in apical dendrites of CA1 pyramidal neurons of hippocampal slice cultures maintained in serum-free media. We show here that serum withdrawal causes an increase in the proportion of thin spines and a decrease in the fraction of stubby spines, without changing the overall density of dendritic spines. When slices are maintained in serum-containing media, BDNF also increased spine density but had the opposite effect on spine morphology: it increased the proportion of mushroom and thin spines and decreased the proportion of stubby spines. Intriguingly, slices maintained in serum media showed a lower p75NTR-to-TrkB expression level than serum-free slices, even after BDNF exposure. The differential actions of BDNF on spine morphology depending on the presence of serum in culture media, together with the difference in neurotrophin receptor expression are reminiscent of opposing functional signaling by p75NTR and Trk receptors, and reveal a complex modulation of dendritic morphology by BDNF signaling.

Differential regional expression of brain-derived neurotrophic factor following olfactory fear learning

We examined brain-derived neurotrophic factor (BDNF) mRNA expression across the olfactory system following fear conditioning. Mice received 10 pairings of odor with footshock or equivalent unpaired odors and shocks. We found increased BDNF mRNA in animals receiving paired footshocks in the multiple regions examined including the posterior piriform cortex (PPC) and basolateral amygdala (BLA). This was in contrast to the unpaired and odor-alone treatments, where BDNF mRNA was increased in the olfactory bulb (OB) and anterior piriform cortex (APC) only, but not the higher olfactory areas. We propose that odor exposure increases expression of BDNF in the OB and APC while the PPC and BLA increase BDNF mRNA only when associative learning occurs.

Interaction between BDNF Val66Met and dopamine transporter gene variation influences anxiety-related traits

The involvement in neural plasticity and the mediation of effects of repeated stress exposure and long-term antidepressant treatment on hippocampal neurogenesis supports a critical role of brain-derived neurotrophic factor (BDNF) in the pathophysiology of affective and other stress-related disorders. A previously reported valine to methionine substitution at amino-acid position 66 (BDNF Val66Met) seems to account for memory disturbance and hippocampal dysfunction. In the present study, we evaluated the impact of the BDNF Val66Met polymorphism on individual differences in personality traits in a sample of healthy volunteers in relation to other common gene variants thought to be involved in the pathophysiology of affective disorders, such as the serotonin transporter promoter polymorphism (5-HTTLPR) and a variable number of tandem repeat polymorphism of the dopamine transporter gene (DAT VNTR). Personality traits were assessed using the NEO personality inventory (NEO-PI-R) and Tridimensional Personality Questionnaire (TPQ). There was a significant DAT VNTR-dependent association between NEO-PI-R Neuroticism and the BDNF Val66Met polymorphism. Among individuals with at least one copy of the DAT 9-repeat allele, carriers of the BDNF Met allele exhibited significantly lower Neuroticism scores than noncarriers. This interaction was also observed for TPQ Harm Avoidance, a personality dimension related to Neuroticism. Our results support the notion that allelic variation at the BDNF locus--in interaction with other gene variants--influences anxiety- and depression-related personality traits.

Reviews: BDNF and Memory Formation and Storage

During the past decade, a large body of evidence has implicated BDNF in synaptic plasticity. In this review, we focus on the newer experiments that involve BDNF in different aspects of learning and memory processing—in particular, in memory persistence and storage. NEUROSCIENTIST 14(2):147—156, 2008. DOI: 10.1177/1073858407305850

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

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

Changes in hippocampal function of ovariectomized rats after sequential low doses of estradiol to simulate the preovulatory estrogen surge

In adult female rats, robust hippocampal changes occur when estradiol rises on the morning of proestrus. Whether estradiol mediates these changes, however, remains unknown. To address this issue, we used sequential injections of estradiol to simulate two key components of the preovulatory surge: the rapid rise in estradiol on proestrous morning, and the slower rise during the preceding day, diestrus 2. Animals were examined mid-morning of simulated proestrus, and compared to vehicle-treated or intact rats. In both simulated and intact rats, CA1-evoked responses were potentiated in hippocampal slices, and presynaptic mechanisms appeared to contribute. In CA3, multiple population spikes were evoked in response to mossy fiber stimuli, and expression of brain-derived neurotrophic factor was increased. Simulation of proestrous morning also improved performance on object and place recognition tests, in comparison to vehicle treatment. Surprisingly, effects on CA1-evoked responses showed a dependence on estradiol during simulated diestrus 2, as well as a dependence on proestrous morning. Increasing estradiol above the physiological range on proestrous morning paradoxically decreased evoked responses in CA1. However, CA3 pyramidal cell activity increased further, and became synchronized. Together, the results confirm that physiological estradiol levels are sufficient to profoundly affect hippocampal function. In addition: (i) changes on proestrous morning appear to depend on slow increases in estradiol during the preceding day; (ii) effects are extremely sensitive to the peak serum level on proestrous morning; and (iii) there are striking subfield differences within the hippocampus.

Investigating the neurobiology of music: brain-derived neurotrophic factor modulation in the hippocampus of young adult mice

It has been shown that music might be able to improve mood state in people affected by psychiatric disorders, ameliorate cognitive deficits in people with dementia and increase motor coordination in Parkinson patients. Robust experimental evidence explaining the central effects of music, however, is missing. This study was designed to investigate the effect of music on brain neurotrophin production and behavior in the mouse. We exposed young adult mice to music with a slow rhythm (6 h/day; mild sound pressure levels, between 50 and 60 db) for 21 consecutive days. At the end of the treatment, mice were tested for passive avoidance learning and then killed for analysis of brain-derived neurotrophic factor (BDNF) and nerve growth factor with enzyme-linked immunosorbent assay (ELISA) in selected brain regions. We found that music-exposed mice showed increased BDNF, but not nerve growth factor in the hippocampus. Furthermore, we observed that music exposure significantly enhanced learning performance, as measured by the passive avoidance test. Our results demonstrate that exposure to music can modulate the activity of the hippocampus by influencing BDNF production. Our findings also suggest that music exposure might be of help in several central nervous system pathologies.

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

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

Neurotrophins and their receptors: roles in plasticity, neurodegeneration and neuroprotection

It is beyond doubt that the neurotrophin family of proteins plays key roles in determining the fate of the neuron, not only during embryonic development, but also in the adult brain. Neurotrophins such as NGF (nerve growth factor) and BDNF (brain-derived neurotrophic factor) can play dual roles: first, in neuronal survival and death, and, secondly, in activity-dependent plasticity. The neurotrophins manifest their effects by binding to two discrete receptor subtypes: the Trk (tropomyosin receptor kinase) family of RTKs (receptor tyrosine kinases) and the p75NTR (p75 neurotrophin receptor). The differential activation of these receptors by the mature neurotrophins and their precursors, the proneurotrophins, renders analysis of the biological functions of these receptors in the adult brain highly complex. Here, we briefly give a broad review of current knowledge of the roles of neurotrophins in the adult brain, including expression of hippocampal plasticity, neurodegeneration and exercise-induced neuroprotection.

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

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

Functional genomics and schizophrenia: endophenotypes and mutant models

This article summarizes the rationale, methods, and results of gene discovery programs in schizophrenia research and describes functional methods of investigating potential candidate genes. It focuses next on the most prominent current candidate genes and describes (1) evidence for their association with schizophrenia and research into the function of each gene; (2) investigation of the clinical phenotypes and endophenotypes associated with each gene, at the levels of psychopathologic, neurocognitive, electrophysiologic, neuroimaging, and neuropathologic findings; and (3) research into the ethologic, cognitive, social, and psychopharmacologic phenotype of mutants with targeted deletion of each gene. It examines gene-gene and gene-environment interactions. Finally, it looks at future directions for research.

Brain-derived neurotrophic factor (BDNF) gene not associated with antidepressant-induced mania

BACKGROUND

Brain-derived neurotrophic factor (BDNF) plays an important role in the regulation of synaptic plasticity and neurotransmitter release across multiple neurotransmitter systems. Recent studies have suggested that BDNF plays a role in the pathogenesis of bipolar disorder (BPD). Moreover, increasing BDNF production might be one of the mechanisms involved in the alleviation of depression and aggravation of mania in antidepressant treatment.

OBJECTIVES

Thus, we hypothesized that a genetic variant within the BDNF gene might influence susceptibility to antidepressant-induced mania, as has been suggested previously.

METHODS

We performed a case-control study to test for allelic frequency and genotype distribution differences across six BDNF polymorphisms between 27 patients with antidepressant-induced mania (IM+) and 29 patients without antidepressant-induced mania (IM-).

RESULTS

We did not observe any significant difference in either allelic or genotype frequencies between the two groups.

CONCLUSIONS

Our results did not support the BDNF link to mania hypothesis proposed previously. However, a larger sample would allow for greater power to determine smaller effects of the BDNF gene in antidepressant-induced mania.

Correlated network activity enhances synaptic efficacy via BDNF and the ERK pathway at immature CA3 CA1 connections in the hippocampus

At early developmental stages, correlated neuronal activity is thought to exert a critical control on functional and structural refinement of synaptic connections. In the hippocampus, between postnatal day 2 (P2) and P6, network-driven giant depolarizing potentials (GDPs) are generated by the synergistic action of glutamate and GABA, which is depolarizing and excitatory. Here the rising phase of GDPs was used to trigger Schaffer collateral stimulation in such a way that synchronized network activity was coincident with presynaptic activation of afferent input. This procedure produced a persistent increase in spontaneous and evoked alpha-amino-3-hydroxy-5-methyl-4-isoxadepropionic acid-mediated glutamatergic currents, an effect that required calcium influx through postsynaptic L-type calcium channels. No potentiation was observed when a delay of 3 sec was introduced between GDPs and afferent stimulation. Pairing-induced potentiation was prevented by scavengers of endogenous BDNF or tropomyosin-related kinase receptor B (TrkB) receptor antagonists. Blocking TrkB receptors in the postsynaptic cell did not prevent the effects of pairing, suggesting that BDNF, possibly secreted from the postsynaptic cell during GDPs, acts on TrkB receptors localized on presynaptic neurons. Application of exogenous BDNF mimicked the effects of pairing on synaptic transmission. In addition, pairing-induced synaptic potentiation was blocked by ERK inhibitors, suggesting that BDNF activates the MAPK/ERK cascade, which may lead to transcriptional regulation and new protein synthesis in the postsynaptic neuron. These results support the hypothesis that, during a critical period of postnatal development, GABAA-mediated GDPs are instrumental in tuning excitatory synaptic connections and provide insights into the molecular mechanisms involved in this process.

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

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

Brain-derived neurotrophic factor: linking fear learning to memory consolidation

Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, plays an important role in synaptic plasticity. In this issue of Molecular Pharmacology, Ou and Gean (p. 350) thoroughly describe the molecular cascade by which fear learning leads to an increase in BDNF expression in the lateral amygdala (LA). Calcium influx through N-methyl-D-aspartate receptors and L-type voltage-dependent calcium channels, which occurs in the LA during fear conditioning, activates protein kinase A and Ca2+/calmodulin-dependent protein kinase IV. Each induces phosphorylation of cAMP response element-binding protein, which binds to the BDNF promoter, leading to BDNF expression in the LA, and contributes to fear memory consolidation.

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.

Leptin promotes rapid dynamic changes in hippocampal dendritic morphology

Recent studies have implicated the hormone leptin in synaptic plasticity associated with neuronal development and learning and memory. Indeed, leptin facilitates hippocampal long-term potentiation and leptin-insensitive rodents display impaired hippocampal synaptic plasticity suggesting a role for endogenous leptin. Structural changes are also thought to underlie activity-dependent synaptic plasticity and this may be regulated by specific growth factors. As leptin is reported to have neurotrophic actions, we have examined the effects of leptin on the morphology and filopodial outgrowth in hippocampal neurons. Here, we demonstrate that leptin rapidly enhances the motility and density of dendritic filopodia and subsequently increases the density of hippocampal synapses. This process is dependent on the synaptic activation of NR2A-containing NMDA receptors and is mediated by the MAPK (ERK) signaling pathway. As dendritic morphogenesis is associated with activity-dependent changes in synaptic strength, the rapid structural remodeling of dendrites by leptin has important implications for its role in regulating hippocampal synaptic plasticity and neuronal development.

Maternal isolation alters the expression of neural proteins during development: 'Stroking' stimulation reverses these effects

Rat pups reared apart from their siblings, mother, and nest environment in the 'pup-in-a-cup' regime show many alterations in behavior reminiscent of the Institutional Inattention/Overactivity Syndrome that characterizes children whose first few months are spent in institutions. In this report, we compare mother-reared (MR) and artificially reared (AR) male rats in concentrations and distributions of brain proteins that are involved in normal brain development. When assessed during the juvenile period and in adulthood, AR animals showed elevations in Neu-N (a neuronal marker) and in S-100 (an astrocyte marker) but reductions in synaptophysin (synapse protein), N-CAM (cell-adhesion molecule), GAP-43 (axon elongation protein), and BDNF (brain derived neurotrophic factor) in comparison to MR controls in many brain sites involved in attention, impulsivity, activity, and social behavior. Daily 'licking-like' stimulation provided to AR animals (AR-MAX) throughout early development that reverses many of the behavioral deficits, also reverses many of the isolation effects on brain proteins. Study 2 showed that elevations in the number of neurons in combination with decreases in functionality are associated with a reduction in neuronal pruning and apoptosis during the very early post-partum period in AR animals and their reversal through daily 'licking-like' stimulation.

beta-Estradiol induces synaptogenesis in the hippocampus by enhancing brain-derived neurotrophic factor release from dentate gyrus granule cells

We investigated the effect of beta-estradiol (E2) on synaptogenesis in the hippocampus using organotypic hippocampal slice cultures and subregional hippocampal neuron cultures. E2 increased the expression of PSD95, a postsynaptic marker, specifically in stratum lucidum of Cornu Ammonis 3 (CA3SL) in cultured hippocampal slices. E2 also increased the spine density at the proximal site of CA3 apical dendrites in CA3SL and PSD95 was clustered on these spine heads. The effects of E2 on the expression of PSD95 and the spine density disappeared when the dentate gyrus (DG) had been excised at 1 day in vitro (DIV). FM1-43 analysis of subregional hippocampal neuron cultures which were comprised of Ammon's horn neurons, DG neurons, or a mixture of these neurons, revealed that E2 increased the number of presynaptic sites in the cultures that contained DG neurons. K252a, a potent inhibitor of the high affinity receptor of brain-derived neurotrophic factor (BDNF), and function-blocking antibody to BDNF (BDNFAB) completely inhibited the effects of E2 in hippocampal slice cultures and subregional neuron cultures, whereas ICI182,780 (ICI), a strong antagonist of nuclear estrogen receptors (nERs), did not. Expression of BDNF in DG neurons was markedly higher than that in Ammon's horn neurons and E2 did not affect these expression levels. E2 significantly increased the BDNF release from DG neurons. KT5720, a specific inhibitor of 3'-5'-cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA), and Rp-adenosine 3', 5'-cyclic monophosphorothioate triethylammonium salt (Rp-cAMP), a non-hydrolyzable diastereoisomer and a potent inhibitor of PKA, completely suppressed the E2-induced increase in BDNF release, whereas ICI and U0126, a potent inhibitor of MAP kinase kinase (MEK), did not. These results suggest that E2 induces synaptogenesis between mossy fibers and CA3 neurons by enhancing BDNF release from DG granule cells in a nER-independent and PKA-dependent manner.