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

Attachment and emotional regulation: examining the role of prefrontal cortex functions, executive functions, and mindfulness in their relationship

Attachment is a prominent area of psychological research, with its relevance linked to executive functions, mindfulness, and emotional regulation. The purpose of this study is to examine this relationship among these aforementioned four constructs and propose a model to be tested in the future. Based on the current trends using the Interpersonal Neurobiology approach, which assumes prefrontal cortex functions to include other socioemotional resources such as empathy, morality, insight, behavior, and body regulation. Our study included prefrontal cortical functions alongside executive functions. The assessment instruments used were Attachment-Based Cognitive Representations Scale, Prefrontal Cortex Functions Scale, Webexec, Five Facet Mindfulness Scale, and Difficulties in Emotion Regulation Scale. We hypothesized that attachment would be the strongest predictor of emotional regulation. The participants in the study were 539 college students (mean = 20.21; sd = 1.57); (68% female and 32%, male). Our a priori research hypothesis was supported, with an additional finding that trait mindfulness was also a significant predictor. The strongest correlations with attachment styles were with trait mindfulness and emotional regulation. We conducted path analyses of two different models for secure and insecure attachment. The path analyses showed that secure attachment scores were negatively related, and insecure attachment scores were positively related to difficulties in emotional regulation scores. Furthermore, trait mindfulness and prefrontal cortex functions also mediated this relationship. However, there was no significant relationship between executive functions and difficulties in emotional regulation scores, even though it was significantly related to attachment. Results and implications are discussed.

Association of polymorphisms rs4680 of the Catechol-O-Methyltransferase gene and rs6265 of the Brain Derived Neurotrophic Factor gene with the Behavioral Inhibition and Behavioral Activation Systems

Background

The Behavioral Inhibition System (BIS) comprises limbic circuitry implicated in avoidance behaviors. Its increased activation has been identified as a risk factor for anxiety and depressive disorders. In addition, both Catechol-O-Methyltransferase (COMT) and Brain Derived Neurotrophic Factor (BDNF) have been postulated as candidate genes that constitute a vulnerability for the onset of anxiety and depressive disorders. The aim of this study was to evaluate the possible association between the rs4680 polymorphism of the COMT gene and the rs6265 polymorphism of the BDNF gene with the BIS and the Behavioral Activation System (BAS) in a population sample from Colombia.

Methods

Genetic information was obtained by extracting DNA from blood samples of 80 participants and using Taqman probes designed for each polymorphism. In addition, participants completed a BIS/BAS scale in order to establish a neuropsychological classification.

Results

The frequency of the Met allele of the BDNF gene was greater in the group with BIS sensitivity compared to the group with BAS sensitivity. On the contrary, the frequency of the Met allele of the COMT gen did not show a significant association with the BIS.

Conclusions

The rs6265 polymorphism of BDNF gene is associated with the BIS which in turn constitutes a risk factor for anxiety and depression.

Ketamine treatment upon memory retrieval reduces fear memory in marmoset monkeys

Emotionally arousing experiences are retained very well as seen in posttraumatic stress disorder (PTSD). Various lines of evidence indicate that reactivation of these memories renders them labile which offers a potential time-window for intervention. We tested in non-human primates whether ketamine, administered during fear memory reactivation, affected passive (inhibitory) avoidance learning. For the consolidation of contextual emotional memory, the unescapable foot-shock paradigm in a passive avoidance task with two compartments (dark vs illuminated) was used. After entering the dark compartment, marmoset monkeys received four random foot-shocks (1 mA, 4 s) within 15-min. This stressful exposure increased the saliva cortisol and heart rate and impaired REM-sleep (p<0.05). One week later the monkeys were re-exposed to the stressful situation for the reconsolidation of the fearful experience. During the re-exposure the monkeys were treated with ketamine (0.5 mg/kg) or saline. In week 3, the monkeys were placed in the experimental setting to test their memory for the fearful experience. In contrast to the vehicle-treated monkeys, who avoided the dark compartment, the ketamine-treated monkeys entered the dark compartment that was previously associated with the fearful experience (p<0.05). Post-mortem analysis of the hippocampus showed that ketamine-treated animals exhibited less doublecortin positive neurons and BrdU-labeled cells in the dentate gyrus. This study reveals that a single low dose of ketamine, administered upon fear retrieval in monkeys, reduce contextual fear memory and attenuate neurogenesis in the hippocampus. These are important findings for considering ketamine as a potential candidate to target traumatic memories in PTSD.

Folic Acid Supplementation in the Gestational Phase of Female Rats Improves Age-Related Memory Impairment and Neuroinflammation in Their Adult and Aged Offspring

Folic acid (FA) supplementation is important during pregnancy to avoid malformations in the offspring. However, it is unknown if it can affect the offspring throughout their lives. To evaluate the offspring, female mother rats (dams) were separated into 5 groups: Four groups received the AIN-93 diet, divided into control and FA (5, 10, and 50 mg/kg), and an additional group received a FA-deficient diet, and the diet was performed during pregnancy and lactation. We evaluated the female offspring of these dams (at 2 and 18 months old). The aged offspring fed with FA-deficient diet presented habituation, spatial and aversive memory impairment and the FA maternal supplementation prevented this. The natural aging caused an increase in the TNF-α and IL-1β levels in the hippocampus from 18-month-old offspring. FA maternal supplementation was able to prevent the increase of these cytokines. IL-4 levels decreased in the prefrontal cortex from aged control rats and FA prevented it. FA deficiency decreased the levels of IL-4 in the hippocampus of the young offspring. In addition, natural aging and FA deficiency decreased brain-derived neurotrophic factor levels in the hippocampus and nerve growth factor levels in the prefrontal cortex and FA supplementation prevented it. Thus, the present study shows for the first time the effect of FA maternal supplementation on memory, cytokines, and neurotrophins in the aged offspring.

Understanding stress: Insights from rodent models

Through incorporating both physical and psychological forms of stressors, a variety of rodent models have provided important insights into the understanding of stress physiology. Rodent models also have provided significant information with regards to the mechanistic basis of the pathophysiology of stress-related disorders such as anxiety disorders, depressive illnesses, cognitive impairment and post-traumatic stress disorder. Additionally, rodent models of stress have served as valuable tools in the area of drug screening and drug development for treatment of stress-induced conditions. Although rodent models do not accurately reproduce the biochemical or physiological parameters of stress response and cannot fully mimic the natural progression of human disorders, yet, animal research has provided answers to many important scientific questions. In this review article, important studies utilizing a variety of stress models are described in terms of their design and apparatus, with specific focus on their capabilities to generate reliable behavioral and biochemical read-out. The review focusses on the utility of rodent models by discussing examples in the literature that offer important mechanistic insights into physiologically relevant questions. The review highlights the utility of rodent models of stress as important tools for advancing the mission of scientific research and inquiry.

•

Stressful life events may lead to the onset of severe psychopathologies in humans.

•

Rodents may model many features of stress exposure in human populations.

•

Induction of stress via pharmacological and psychological manipulations alter rodent behavior.

•

Mechanistic rodent studies reveal key molecular targets critical for new therapeutic targets.

Effects of Exercise on Memory Interference in Neuropsychiatric Disorders

There are several mechanisms that cause memory impairment, including motivated forgetting, active forgetting, natural decay, and memory interference. Interference occurs when one is attempting to recall something specific, but there is conflicting information making it more difficult to recall the target stimuli. In laboratory settings, it is common to measure memory interference with paired associate tasks-usually utilizing the AB-CD, AB-AC, AB-ABr, or AB-DE AC-FG method. Memory impairments are frequent among those with neuropsychiatric disorders such as depression, schizophrenia, and multiple sclerosis. The memory effects of each condition differ, but are all related to alterations in brain physiology and general memory deterioration. Exercise, or physical activity, has been demonstrated to attenuate memory interference in some cases, but the mechanisms are still being determined. Further research is needed on memory interference, in regard to exercise and neuropsychiatric disorders.

Human BDNF rs6265 polymorphism as a mediator for the generalization of contextual anxiety

The Met allele of the human brain-derived neurotrophic factor (BDNF) gene might be a risk factor for anxiety disorders and is associated with reduced hippocampal volume. Notably, hippocampus plays a crucial role in contextual learning and generalization. The role of the BDNF gene variation in human context-conditioning and generalization is still unknown. We investigated 33 carriers of the Met allele (18 females) and 32 homozygous carriers of the Val allele (15 females) with a virtual-reality context-conditioning paradigm. Electric stimulations (unconditioned stimulus, US) were unpredictably delivered in one virtual office (CTX+), but never in another virtual office (CTX-). During generalization, participants revisited CTX+ and CTX- and a generalization office (G-CTX), which was a mix of the other two. Rating data indicated successful conditioning (more negative valence, higher arousal, anxiety and contingency ratings for CTX+ than CTX-), and generalization of conditioned anxiety by comparable ratings for G-CTX and CTX+. The startle data indicated discriminative learning for Met allele carriers, but not for Val homozygotes. Moreover, a trend effect suggests that startle responses of only the Met carriers were slightly potentiated in G-CTX versus CTX-. In sum, the BDNF polymorphism did not affect contextual learning and its generalization on a verbal level. However, the physiological data suggest that Met carriers are characterized by fast discriminative contextual learning and a tendency to generalize anxiety responses to ambiguous contexts. We propose that such learning may be related to reduced hippocampal functionality and the basis for the risk of Met carriers to develop anxiety disorders.

Effects of stressor controllability on transcriptional levels of c-fos, Arc, and brain-derived neurotrophic factor in mouse amygdala and medial prefrontal cortex

Controllability is an important factor in determining stress outcomes. Uncontrollable stress is associated with the development of psychopathology such as post-traumatic stress disorder, whereas controllable stress is associated with adaptive stress responses and positive outcomes. In this study, we investigated how controllability affects poststress neurobiology by assessing transcriptional levels of activity-dependent genes in medial prefrontal cortex (mPFC) and amygdala, regions important in mediating stress outcomes. Mice were subjected to either escapable shock (ES) or yoked inescapable shock (IS) as models of controllable and uncontrollable stress, respectively. Immediately (0 h) or at 2 h after shock training (20 trials; 0.5 mA, 5.0 s maximum duration; 1.0 min interstimulus interval), mice were killed, and we interrogated expression levels of the immediate-early genes, c-fos and Arc, and a delayed primary response gene, brain-derived neurotrophic factor, in mPFC, amygdala, and somatosensory cortex (a control region), using real-time reverse transcription quantitative PCR (RT qPCR). We found ES-associated up-regulation of brain-derived neurotrophic factor in amygdala as well as in mPFC. IS suppressed c-fos in mPFC (0 h) but induced more Arc in amygdala (2 h) in comparison with ES. Freezing, an index of fear memory, and serum level corticosterone, an index of the stress response, did not differ between mice trained with ES or IS. The data are discussed with respect to the potential functional involvements of the amygdala and mPFC in mediating differential outcomes of controllable and uncontrollable stress.

Changes in Brain-Derived Neurotrophic Factor Expression Influence Sleep-Wake Activity and Homeostatic Regulation of Rapid Eye Movement Sleep

Study Objectives

Brain-derived neurotrophic factor (BDNF) expression and homeostatic regulation of rapid eye movement (REM) sleep are critical for neurogenesis and behavioral plasticity. Accumulating clinical and experimental evidence suggests that decreased BDNF expression is causally linked with the development of REM sleep-associated neuropsychiatric disorders. Therefore, we hypothesize that BDNF plays a role in sleep–wake (S–W) activity and homeostatic regulation of REM sleep.

Methods

Male and female wild-type (WT; BDNF +/+) and heterozygous BDNF (KD; BDNF +/−) rats were chronically implanted with S–W recording electrodes to quantify baseline S–W activity and REM sleep homeostatic regulatory processes during the light phase.

Results

Molecular analyses revealed that KD BDNF rats had a 50% decrease in BDNF protein levels. During baseline S–W activity, KD rats exhibited fewer REM sleep episodes that were shorter in duration and took longer to initiate. Also, the baseline S–W activity did not reveal any sex difference. During the 3-hour selective REM sleep deprivation, KD rats failed to exhibit a homeostatic drive for REM sleep and did not exhibit rebound REM sleep during the recovery S–W period.

Conclusion

Interestingly, both genotypes did not reveal any sex difference in the quality and/or quantity of REM sleep. Collectively, these results, for the first time, unequivocally demonstrate that an intact BDNF system in both sexes is a critical modulator for baseline and homeostatic regulation of REM sleep. This study further suggests that heterozygous BDNF knockdown rats are a useful animal model for the study of the cellular and molecular mechanisms of sleep regulation and cognitive functions of sleep.

Regulatory role of NGFs in neurocognitive functions

Nerve growth factors (NGFs), especially the prototype NGF and brain-derived neurotrophic factor (BDNF), have a diverse array of functions in the central nervous system through their peculiar set of receptors and intricate signaling. They are implicated not only in the development of the nervous system but also in regulation of neurocognitive functions like learning, memory, synaptic transmission, and plasticity. Evidence even suggests their role in continued neurogenesis and experience-dependent neural network remodeling in adult brain. They have also been associated extensively with brain disorders characterized by neurocognitive dysfunction. In the present article, we aimed to make an exhaustive review of literature to get a comprehensive view on the role of NGFs in neurocognitive functions in health and disease. Starting with historical perspective, distribution in adult brain, implied molecular mechanisms, and developmental basis, this article further provides a detailed account of NGFs’ role in specified neurocognitive functions. Furthermore, it discusses plausible NGF-based homeostatic and adaptation mechanisms operating in the pathogenesis of neurocognitive disorders and has presents a survey of such disorders. Finally, it elaborates on current evidence and future possibilities in therapeutic applications of NGFs with an emphasis on recent research updates in drug delivery mechanisms. Conclusive remarks of the article make a strong case for plausible role of NGFs in comprehensive regulation of the neurocognitive functions and pathogenesis of related disorders and advocate that future research should be directed to explore use of NGF-based mechanisms in the prevention of implicated diseases as well as to target these molecules pharmacologically.

Treatment-resistant panic disorder: clinical significance, concept and management

Panic disorder is commonly prevalent in the population, but the treatment response for panic disorder in clinical practice is much less effective than that in our imagination. Increasing evidence suggested existence of a chronic or remitting-relapsing clinical course in panic disorder. In this systematic review, we re-examine the definition of treatment-resistant panic disorder, and present the potential risk factors related to the treatment resistance, including the characteristics of panic disorder, other psychiatric and physical comorbidities, and psychosocial stresses. Furthermore, we summarize the potential pathophysiologies, such as genetic susceptibility, altered brain functioning, brain-derived neurotrophic factor, and long-term inflammation, to explain the treatment resistance. Finally, we conclude the current therapeutic strategies for treating treatment-resistant panic disorder from pharmacological and non-pharmacological views.

Preventing the Return of Fear Using Reconsolidation Update Mechanisms Depends on the Met-Allele of the Brain Derived Neurotrophic Factor Val66Met Polymorphism

BACKGROUND

Memory reconsolidation is the direct effect of memory reactivation followed by stabilization of newly synthesized proteins. It has been well proven that neural encoding of both newly and reactivated memories requires synaptic plasticity. Brain derived neurotrophic factor (BDNF) has been extensively investigated regarding its role in the formation of synaptic plasticity and in the alteration of fear memories. However, its role in fear reconsolidation is still unclear; hence, the current study has been designed to investigate the role of the BDNF val66met polymorphism (rs6265) in fear memory reconsolidation in humans.

METHODS

An auditory fear-conditioning paradigm was conducted, which comprised of three stages (acquisition, reactivation, and spontaneous recovery). One day after fear acquisition, the experimental group underwent reactivation of fear memory followed by the extinction training (reminder group), whereas the control group (non-reminder group) underwent only extinction training. On day 3, both groups were subjected to spontaneous recovery of earlier learned fearful memories. The treat-elicited defensive response due to conditioned threat was measured by assessing the skin conductance response to the conditioned stimulus. All participants were genotyped for rs6265.

RESULTS

The results indicate a diminishing effect of reminder on the persistence of fear memory only in the Met-allele carriers, suggesting a moderating effect of the BDNF polymorphism in fear memory reconsolidation.

CONCLUSIONS

Our findings suggest a new role for BDNF gene variation in fear memory reconsolidation in humans.

The effects of environmental enrichment and age-related differences on inhibitory avoidance in zebrafish (Danio rerio Hamilton)

The inhibitory avoidance paradigm allows the study of mechanisms underlying learning and memory formation in zebrafish (Danio rerio Hamilton). For zebrafish, the physiology and behavior associated with this paradigm are as yet poorly understood. We therefore assessed the effects of environmental enrichment and fish age on inhibitory avoidance learning. Fish raised in an environmentally enriched tank showed decreased anxiety-like behavior and increased exploration. Enrichment greatly reduced inhibitory avoidance in 6-month (6M)- and 12-month (12 M)-old fish. Following inhibitory avoidance, telencephalic mRNA levels of proliferating cell nuclear antigen (pcna), neurogenic differentiation (neurod), cocaine- and amphetamine-regulated transcript 4 (cart4), and cannabinoid receptor 1 (cnr1) were lower in enriched-housed fish, while the ratios of mineralocorticoid receptor (nr3c2)/glucocorticoid receptor α [nr3c1(α)] and glucocorticoid receptor β [nr3c1(β)]/glucocorticoid receptor α [nr3c1(α)] were higher. This was observed for 6M-old fish only, not for 24-month (24 M) old fish. Instead, 24 M-old fish showed delayed inhibitory avoidance, no effects of enrichment, and reduced expression of neuroplasticity genes. Overall, our data show strong differences in inhibitory avoidance behavior between zebrafish of different ages and a clear reduction in avoidance behavior following housing under environmental enrichment.

Estrogenic mediation of serotonergic and neurotrophic systems: implications for female mood disorders

Clinical research has demonstrated a significant sex difference in the occurrence of depressive disorders. Beginning at pubertal onset, women report a higher incidence of depression than men. Women are also vulnerable to the development of depressive disorders such as premenstrual dysphoric disorder, postpartum depression, and perimenopausal depression. These disorders are associated with reproductive stages involving changes in gonadal hormone levels. Specifically, female depression and female affective behaviors are influenced by estradiol levels. This review argues two major mechanisms by which estrogens influence depression and depressive-like behavior: through interactions with neurotrophic factors and through an influence on the serotonergic system. In particular, estradiol increases brain derived neurotrophic factor (BDNF) levels within the brain, and alters serotonergic expression in a receptor subtype-specific manner. We will take a regional approach, examining these effects of estrogens in the major brain areas implicated in depression. Finally, we will discuss the gaps in our current knowledge of the effects of estrogens on female depression, and the potential utility for estrogen receptor modulators in treatment for this disorder.

Inhibitory avoidance learning in zebrafish (Danio rerio): effects of shock intensity and unraveling differences in task performance

The zebrafish (Danio rerio) is increasingly used as a model in neurobehavioral and neuroendocrine studies. The inhibitory avoidance paradigm has been proposed as tool to study mechanisms underlying learning and memory in zebrafish. In this paradigm subjects receive a shock after entering the black compartment of a black-white box. On the next day, latency to enter the black compartment is assessed; higher latencies are indicative of increased avoidance learning. Here, we aimed to understand the effects of different shock intensities (0, 1, 3, and 9 V) and to unravel variation in inhibitory avoidance learning in an in-house reared Tuebingen Long-Fin zebrafish (D. rerio) strain. While median latencies had increased in the 1, 3, and 9 V groups, no increase in median latency was found in the 0 V group. In addition, higher shock intensities resulted in a higher number of avoiders (latency ≥180 s) over nonavoiders (latency <60 s). Both changes are indicative of increased avoidance learning. We assessed whole-body cortisol content and the expression levels of genes relevant to stress, anxiety, fear, and learning 2 h after testing. Shock intensity was associated with whole-body cortisol content and the expression of glucocorticoid receptor alpha [nr3c1(alpha)], cocaine- and amphetamine-regulated transcript (cart4), and mineralocorticoid receptor (nr3c2), while avoidance behavior was associated with whole-body cortisol content only. The inhibitory avoidance paradigm in combination with measuring whole-body cortisol content and gene expression is suitable to unravel (genetic) mechanisms of fear avoidance learning. Our data further show differences in brain-behavior relationships underlying fear avoidance learning and memory in zebrafish. These findings serve as starting point for further unraveling differences in brain-behavior relationships underlying (fear avoidance) learning and memory in zebrafish.

Translational findings on brain-derived neurotrophic factor and anxiety: contributions from basic research to clinical practice

BACKGROUND/AIMS

Anxious responses are evolutionarily adaptive, but excessive fear can become disabling and lead to anxiety disorders. Translational models of anxiety might be useful sources for understanding the neurobiology of fear and anxiety and can contribute to future proposals of therapeutic intervention for the disorders studied. Brain-derived neurotrophic factor (BDNF), which is known for its importance on neuroplasticity and contextual memory, has emerged as a relevant element for emotional memory. Recent studies show that the Val(66)Met BDNF polymorphism correlates with various psychiatric disorders, including anxiety, but there are several differences between experimental and clinical studies.

METHODS

In this work, we review the literature focused on the BDNF Val(66)Met polymorphism and anxiety, and discuss biological findings from animal models to clinical studies.

RESULTS

As occurs with other psychiatric disorders, anxiety correlates with anatomical, behavioral and physiological changes related to the BDNF polymorphism. In animal studies, it has been shown that a significant decrease in regulated secretion from both BDNFVal/Met and BDNFMet/Met neurons represented a significant decrease in available BDNF.

CONCLUSION

These studies suggest that developing pharmacological strategies facilitating the release of BDNF from synapses or prolongation of the half-life of secreted BDNF may improve the therapeutic responses of humans expressing the BDNF polymorphism.

Neuropeptide S and BDNF gene expression in the amygdala are influenced by social decision-making under stress

In a newly developed conceptual model of stressful social decision-making, the Stress-Alternatives Model (SAM; used for the 1st time in mice) elicits two types of response: escape or remain submissively. Daily (4d) aggressive social interaction in a neutral arena between a C57BL6/N test mouse and a larger, novel aggressive CD1 mouse, begin after an audible tone (conditioned stimulus; CS). Although escape holes (only large enough for smaller test animals) are available, and the aggressor is unremittingly antagonistic, only half of the mice tested utilize the possibility of escape. During training, for mice that choose to leave the arena and social interaction, latency to escape dramatically decreases over time; this is also true for control C57BL6/N mice which experienced no aggression. Therefore, the open field of the SAM apparatus is intrinsically anxiogenic. It also means that submission to the aggressor is chosen despite this anxiety and the high intensity of the aggressive attacks and defeat. While both groups that received aggression displayed stress responsiveness, corticosterone levels were significantly higher in animals that chose submissive coexistence. Although both escaping and non-escaping groups of animals experienced aggression and defeat, submissive animals also exhibited classic fear conditioning, freezing in response to the CS alone, while escaping animals did not. In the basolateral amygdala (BLA), gene expression of brain-derived neurotrophic factor (BDNF) was diminished, at the same time neuropeptide S (NPS) expression was significantly elevated, but only in submissive animals. This increase in submission-evoked NPS mRNA expression was greatest in the central amygdala (CeA), which coincided with decreased BDNF expression. Reduced expression of BDNF was only found in submissive animals that also exhibit elevated NPS expression, despite elevated corticosterone in all socially interacting animals. The results suggest an interwoven relationship, linked by social context, between amygdalar BDNF, NPS and plasma corticosterone.

CACNA1C genotype explains interindividual differences in amygdala volume among patients with schizophrenia

Affective deficits are one common denominator of schizophrenia (SZ), bipolar disorder (BD) and obsessive compulsive disorder (OCD) with the amygdala indicated as one of the major structures involved in emotion regulation. Previous findings of differences in amygdala volume between healthy controls and patients with SZ, BD or OCD diverge with respect to the affected hemisphere, size and direction of the effect. Variability in the CACNA1C gene has been linked to BD, SZ as well as structural and functional variation in the amygdala in healthy people and patients with BD. We were interested to investigate whether amygdala volumes differ between hemispheres, diagnostic or genotype groups, and whether any interactive effects exist. We combined genotyping of SNP rs1006737 in CACNA1C with structural MRI measurements of relative gray matter (GM) amygdala volume in patients with SZ, BD or OCD as well as healthy controls (N Total = 72). The CACNA1C genotype showed a significant effect on relative GM amygdala volume in patients with SZ. There was a significant left versus right relative GM amygdala volume decrease in patients with SZ or BD. The effects of hemisphere and diagnosis (controls vs. patients with SZ) on relative GM amygdala volume were genotype specific. Our data suggest that the CACNA1C genotype may account for some heterogeneity in the effects of hemisphere and diagnosis on amygdala volume when comparing patients with SZ and controls and point to disturbed Ca(2+)-signaling as a plausible mechanism contributing to the pathology in patients with SZ.

Deep brain stimulation of the ventral striatum increases BDNF in the fear extinction circuit

Deep brain stimulation (DBS) of the ventral capsule/ventral striatum (VC/VS) reduces the symptoms of treatment-resistant obsessive compulsive disorder (OCD), and improves response to extinction-based therapies. We recently reported that DBS-like stimulation of a rat homologue of VC/VS, the dorsal-VS, reduced conditioned fear and enhanced extinction memory (Rodriguez-Romaguera et al., 2012). In contrast, DBS of the ventral-VS had the opposite effects. To examine possible mechanisms of these effects, we assessed the effects of VS DBS on the expression of the neural activity marker Fos and brain-derived neurotrophic factor (BDNF), a key mediator of extinction plasticity in prefrontal-amygdala circuits. Consistent with decreased fear expression, DBS of dorsal-VS increased Fos expression in prelimbic and infralimbic prefrontal cortices and in the lateral division of the central nucleus of amygdala, an area that inhibits amygdala output. Consistent with improved extinction memory, we found that DBS of dorsal-VS, but not ventral-VS, increased neuronal BDNF expression in prelimbic and infralimbic prefrontal cortices. These rodent findings are consistent with the idea that clinical DBS of VC/VS may augment fear extinction through an increase in BDNF expression.

Melanocortin-4 receptor regulates hippocampal synaptic plasticity through a protein kinase A-dependent mechanism

Learning and memory require orchestrated regulation of both structural and functional synaptic plasticity in the hippocampus. While a neuropeptide alpha-melanocyte-stimulating hormone, α-MSH, has been implicated in memory acquisition and retention, the functional role of its cognate receptor, melanocortin-4 receptor (MC4R), in hippocampal-dependent synaptic plasticity has not been explored. In this study, we report that activation of MC4R enhances synaptic plasticity through the regulation of dendritic spine morphology and abundance of AMPA receptors. We show that activation of postsynaptic MC4R increases the number of mature dendritic spines and enhances surface expression of AMPA receptor subunit GluA1, resulting in synaptic accumulation of GluA1-containing AMPA receptors. Moreover, MC4R stimulates surface GluA1 trafficking through phosphorylation of GluA1 at Ser845 in a Gα(s)-cAMP/PKA-dependent manner. Blockade of protein kinase A (PKA) signaling abolishes the MC4R-mediated enhancement of neurotransmission and hippocampal long-term potentiation. Importantly, in vivo application of MC4R agonists increases LTP in the mouse hippocampal CA1 region. These findings reveal that MC4R in the hippocampus plays a critical role in the regulation of structural and functional plasticity.

Neural and cellular mechanisms of fear and extinction memory formation

Over the course of natural history, countless animal species have evolved adaptive behavioral systems to cope with dangerous situations and promote survival. Emotional memories are central to these defense systems because they are rapidly acquired and prepare organisms for future threat. Unfortunately, the persistence and intrusion of memories of fearful experiences are quite common and can lead to pathogenic conditions, such as anxiety and phobias. Over the course of the last 30 years, neuroscientists and psychologists alike have attempted to understand the mechanisms by which the brain encodes and maintains these aversive memories. Of equal interest, though, is the neurobiology of extinction memory formation as this may shape current therapeutic techniques. Here we review the extant literature on the neurobiology of fear and extinction memory formation, with a strong focus on the cellular and molecular mechanisms underlying these processes.

Epigenetic mechanisms in experience-driven memory formation and behavior

Epigenetic mechanisms have long been associated with the regulation of gene-expression changes accompanying normal neuronal development and cellular differentiation; however, until recently these mechanisms were believed to be statically quiet in the adult brain. Behavioral neuroscientists have now begun to investigate these epigenetic mechanisms as potential regulators of gene-transcription changes in the CNS subserving synaptic plasticity and long-term memory (LTM) formation. Experimental evidence from learning and memory animal models has demonstrated that active chromatin remodeling occurs in terminally differentiated postmitotic neurons, suggesting that these molecular processes are indeed intimately involved in several stages of LTM formation, including consolidation, reconsolidation and extinction. Such chromatin modifications include the phosphorylation, acetylation and methylation of histone proteins and the methylation of associated DNA to subsequently affect transcriptional gene readout triggered by learning. The present article examines how such learning-induced epigenetic changes contribute to LTM formation and influence behavior. In particular, this article is a survey of the specific epigenetic mechanisms that have been demonstrated to regulate gene expression for both transcription factors and growth factors in the CNS, which are critical for LTM formation and storage, as well as how aberrant epigenetic processing can contribute to psychological states such as schizophrenia and drug addiction. Together, the findings highlighted in this article support a novel role for epigenetic mechanisms in the adult CNS serving as potential key molecular regulators of gene-transcription changes necessary for LTM formation and adult behavior.

Epigenetic mechanisms in memory and synaptic function

Although the term 'epigenetics' was coined nearly seventy years ago, its critical function in memory processing by the adult CNS has only recently been appreciated. The hypothesis that epigenetic mechanisms regulate memory and behavior was motivated by the need for stable molecular processes that evade turnover of the neuronal proteome. In this article, we discuss evidence that supports a role for neural epigenetic modifications in the formation, consolidation and storage of memory. In addition, we will review the evidence that epigenetic mechanisms regulate synaptic plasticity, a cellular correlate of memory. We will also examine how the concerted action of multiple epigenetic mechanisms with varying spatiotemporal profiles influence selective gene expression in response to behavioral experience. Finally, we will suggest key areas for future research that will help elucidate the complex, vital and still mysterious, role of epigenetic mechanisms in neural function and behavior.

Contrasting hippocampal and amygdalar expression of genes related to neural plasticity during escape from social aggression

Social subjugation has widespread consequences affecting behavior and underlying neural systems. We hypothesized that individual differences in stress responsiveness were associated with differential expression of neurotrophin associated genes within the hippocampus and amygdala. To do this we examined the brains of hamsters placed in resident/intruder interactions, modified by the opportunity to escape from aggression. In the amygdala, aggressive social interaction stimulated increased BDNF receptor TrK(B) mRNA levels regardless of the ability to escape the aggressor. In contrast, the availability of escape limited the elevation of GluR(1) AMPA subunit mRNA. In the hippocampal CA(1), the glucocorticoid stress hormone, cortisol, was negatively correlated with BDNF and TrK(B) gene expression, but showed a positive correlation with BDNF expression in the DG. Latency to escape the aggressor was also negatively correlated with CA(1) BDNF expression. In contrast, the relationship between amygdalar TrK(B) and GluR(1) was positive with respect to escape latency. These results suggest that an interplay of stress and neurotrophic systems influences learned escape behavior. Animals which escape faster seem to have a more robust neurotrophic profile in the hippocampus, with the opposite of this pattern in the amygdala. We propose that changes in the equilibrium of hippocampal and amygdalar learning result in differing behavioral stress coping choices.

Synergistic activation of dopamine D1 and TrkB receptors mediate gain control of synaptic plasticity in the basolateral amygdala

Fear memory formation is thought to require dopamine, brain-derived neurotrophic factor (BDNF) and zinc release in the basolateral amygdala (BLA), as well as the induction of long term potentiation (LTP) in BLA principal neurons. However, no study to date has shown any relationship between these processes in the BLA. Here, we have used in vitro whole-cell patch clamp recording from BLA principal neurons to investigate how dopamine, BDNF, and zinc release may interact to modulate the LTP induction in the BLA. LTP was induced by either theta burst stimulation (TBS) protocol or spaced 5 times high frequency stimulation (5xHFS). Significantly, both TBS and 5xHFS induced LTP was fully blocked by the dopamine D1 receptor antagonist, SCH23390. LTP induction was also blocked by the BDNF scavenger, TrkB-FC, the zinc chelator, DETC, as well as by an inhibitor of matrix metalloproteinases (MMPs), gallardin. Conversely, prior application of the dopamine reuptake inhibitor, GBR12783, or the D1 receptor agonist, SKF39393, induced robust and stable LTP in response to a sub-threshold HFS protocol (2xHFS), which does not normally induce LTP. Similarly, prior activation of TrkB receptors with either a TrkB receptor agonist, or BDNF, also reduced the threshold for LTP-induction, an effect that was blocked by the MEK inhibitor, but not by zinc chelation. Intriguingly, the TrkB receptor agonist-induced reduction of LTP threshold was fully blocked by prior application of SCH23390, and the reduction of LTP threshold induced by GBR12783 was blocked by prior application of TrkB-FC. Together, our results suggest a cellular mechanism whereby the threshold for LTP induction in BLA principal neurons is critically dependent on the level of dopamine in the extracellular milieu and the synergistic activation of postsynaptic D1 and TrkB receptors. Moreover, activation of TrkB receptors appears to be dependent on concurrent release of zinc and activation of MMPs.

Effects of the BDNF Val66Met polymorphism on neural responses to facial emotion

The brain derived neurotrophic factor (BDNF) Val66Met polymorphism has been associated with affective disorders, but its role in emotion processing has not been fully established. Due to the clinically heterogeneous nature of these disorders, studying the effect of genetic variation in the BDNF gene on a common attribute such as fear processing may elucidate how the BDNF Val66Met polymorphism impacts brain function. Here we use functional magnetic resonance imaging examine the effect of the BDNF Val66Met genotype on neural activity for fear processing. Forty healthy participants performed an implicit fear task during scanning, where subjects made gender judgments from facial images with neutral or fearful emotion. Subjects were tested for facial emotion recognition post-scan. Functional connectivity was investigated using psycho-physiological interactions. Subjects were genotyped for the BDNF Val66Met polymorphism and the measures compared between genotype groups. Met carriers showed overactivation in the anterior cingulate cortex (ACC), brainstem and insula bilaterally for fear processing, along with reduced functional connectivity from the ACC to the left hippocampus, and impaired fear recognition ability. The results show that during fear processing, Met allele carriers show an increased neural response in regions previously implicated in mediating autonomic arousal. Further, the Met carriers show decreased functional connectivity with the hippocampus, which may reflect differential retrieval of emotional associations. Together, these effects show significant differences in the neural substrate for fear processing with genetic variation in BDNF.

BDNF signaling during learning is regionally differentiated within hippocampus

Learning-induced neurotrophic signaling at synapses is widely held to be critical for neuronal viability in adult brain. A previous study provided evidence that unsupervised learning of a novel environment is accompanied by activation of the TrkB receptor for brain-derived neurotrophic factor (BDNF) in hippocampal field CA1b of adult rats. Here we report that this effect is regionally differentiated, in accord with "engram" type memory encoding. A 30 min exposure to a novel, complex environment caused a marked, NMDA receptor-dependent increase in postsynaptic densities associated with activated (phosphorylated) Trk receptors in rostral hippocampus. Increases were pronounced in field CA3a, moderate in the dentate gyrus, and absent in field CA1a. Synapses with Trk activation were significantly larger than their neighbors. Surprisingly, unsupervised learning had no effect on Trk phosphorylation in more temporal sections of hippocampus. It thus appears that commonplace forms of learning interact with regional predispositions to produce spatially differentiated effects on BDNF signaling.

Learning strategies during fear conditioning

This paper describes a model of fear learning, in which subjects have an option of behavioral responses to impending social defeat. The model generates two types of learning: social avoidance and classical conditioning, dependent upon (1) escape from or (2) social subordination to an aggressor. We hypothesized that social stress provides the impetus as well as the necessary information to stimulate dichotomous goal-oriented learning. Specialized tanks were constructed to subject rainbow trout to a conditioning paradigm where the conditioned stimulus (CS) is cessation of tank water flow (water off) and the unconditioned stimulus (US) is social aggression from a larger conspecific. Following seven daily CS/US pairings, approximately half of the test fish learned to consistently escape the aggression to a neutral chamber through a small escape hole available only during the interaction. The learning curve for escaping fish was dramatic, with an 1100% improvement in escape time over 7 days. Fish that did not escape exhibited a 400% increase in plasma cortisol and altered brain monoamine response to presentation of the CS alone. Elevated plasma cortisol levels represent classical fear conditioning in non-escaping fish, while a lack of fear conditioning and a decreased latency to escape over the training period in escapers indicates learned escape.

Genetic variation in brain-derived neurotrophic factor and human fear conditioning

Brain-derived neurotrophic factor (BDNF) has been implicated in hippocampal-dependent learning processes, and carriers of the Met allele of the Val66Met BDNF genotype are characterized by reduced hippocampal structure and function. Recent nonhuman animal work suggests that BDNF is also crucial for amygdala-dependent associative learning. The present study sought to examine fear conditioning as a function of the BDNF polymorphism. Fifty-seven participants were genotyped for the BDNF polymorphism and took part in a differential-conditioning paradigm. Participants were shocked following a particular conditioned stimulus (CS+) and were also presented with stimuli that ranged in perceptual similarity to the CS+ (20, 40 or 60% smaller or larger than the CS+). The eye blink component of the startle response was measured to quantify fear conditioning; post-task shock likelihood ratings for each stimulus were also obtained. All participants reported that shock likelihood varied with perceptual similarity to the CS+ and showed potentiated startle in response to CS +/- 20% stimuli. However, only the Val/Val group had potentiated startle responses to the CS+. Met allele carrying individuals were characterized by deficient fear conditioning--evidenced by an attenuated startle response to CS+ stimuli. Variation in the BDNF genotype appears related to abnormal fear conditioning, consistent with nonhuman animal work on the importance of BDNF in amygdala-dependent associative learning. The relation between genetic variation in BDNF and amygdala-dependent associative learning deficits is discussed in terms of potential mechanisms of risk for psychopathology.

The BDNF Val66Met polymorphism affects amygdala activity in response to emotional stimuli: evidence from a genetic imaging study

Mounting evidence shows that the brain derived neurotrophic factor (BDNF) plays a crucial role in synaptic plasticity. Due to its potential involvement in psychiatric diseases like depression and anxiety disorders BDNF lately became a major target in research. A functional variant of the BDNF gene--the BDNF Val66Met polymorphism--is of particular interest, because it influences the BDNF secretion which is followed by signaling at the TrkB receptor leading to dendritic growth of neurons. Findings from genetic association studies in humans yield heterogenous results with respect to the question of which allele represents a potential risk factor for an affective disorder. Although structural MRT studies revealed that the 66Met variant is associated with smaller hippocampi and could therefore present the risk allele, fMRI studies investigating the processing of emotion with respect to the BDNF Val66Met polymorphism are lacking. N=37 healthy female subjects participated in an fMRI experiment with an affective startle reflex paradigm. Carriers of the 66Met variant showed stronger amygdala activation in the right hemisphere in response to emotional stimuli compared to neutral stimuli. The results of this study add to growing literature, showing that it is the 66Met, which is associated with higher trait anxiety.

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

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