Brain-derived neurotrophic factor in the ventral midbrain-nucleus accumbens pathway: a role in depression

Sex and litter effects on anxiety and DNA methylation levels of stress and neurotrophin genes in adolescent rats

Maternal care variations associate with DNA methylation of the glucocorticoid receptor gene, Nr3c1, in hippocampus at a nerve-growth factor-inducible protein 1 binding site. Epigenetic regulation of brain-derived neurotrophin factor is affected by early stress. These systems contribute to anxiety and fear. Early stress has sex-dependent effects perhaps reflecting sex differences in maternal care. Altering litter gender composition affects maternal behavior and DNA methylation levels of another gene in hippocampus and nucleus accumbens (NAc). We now test if DNA methylation levels of Nr3c1, Egr1, and Bdnf differ by litter composition or sex. Rats from mixed- or single-sex litters were tested for anxiety and fear on postnatal day 35. Brain tissues were collected and analyzed using direct sequencing methods. Females showed hypermethylation of Nr3c1 of hippocampal DNA and litter composition modified sex effects on methylation of Egr1 in NAc. Few differences were seen for Bdnf. LGC modified some sex differences in behavior.

Val66Met BDNF genotypes in melancholic depression: effects on brain structure and treatment outcome

BACKGROUND

A brain-derived neurotrophic factor (BDNF) prodomain single-nucleotide polymorphism resulting in a valine to methionine substitution (Val66Met) has been associated with depression-related phenotypes and brain alterations involving regions consistently associated with major depressive disorder (MDD). The aim of our study was to evaluate the association of regional gray matter (GM) volume within the hippocampus and other unpredicted regions at the whole-brain level with the BDNF Val66Met polymorphism in MDD patients with melancholic features and their impact on treatment outcome.

METHODS

A sample of 37 MDD inpatients was assessed with three-dimensional magnetic resonance imaging (1.5-T scanner). GM volume was analyzed with voxel-based morphometry (VBM) using Statistical Parametric Mapping (SPM5). The BDNF Val66Met variant was genotyped using SNPlex technology. MDD patients were classified according to genotype distribution under a dominant model of inheritance and thus comparing Val66 homozygotes (n = 22) versus Met66 carriers (n = 15).

RESULTS

A significant GM volume reduction in the left hippocampus was observed in Met66 carriers. Conversely, in the same group, a volume increase in the right orbitofrontal cortex was detected. Moreover, a significant negative correlation between left hippocampal volume and days to remission was found in Val66 homozygotes, whereas right orbitofrontal volume was inversely correlated to days to remission in Met66 carriers.

CONCLUSIONS

Our results suggest that the Val66Met BDNF variant may have a differential impact on the brain structure of melancholic patients with possible treatment outcome implications.

Treatment with tianeptine induces antidepressive-like effects and alters the neurotrophin levels, mitochondrial respiratory chain and cycle Krebs enzymes in the brain of maternally deprived adult rats

Maternally deprived rats were treated with tianeptine (15 mg/kg) once a day for 14 days during their adult phase. Their behavior was then assessed using the forced swimming and open field tests. The BDNF, NGF and energy metabolism were assessed in the rat brain. Deprived rats increased the immobility time, but tianeptine reversed this effect and increased the swimming time; the BDNF levels were decreased in the amygdala of the deprived rats treated with saline and the BDNF levels were decreased in the nucleus accumbens within all groups; the NGF was found to have decreased in the hippocampus, amygdala and nucleus accumbens of the deprived rats; citrate synthase was increased in the hippocampus of non-deprived rats treated with tianeptine and the creatine kinase was decreased in the hippocampus and amygdala of the deprived rats; the mitochondrial complex I and II-III were inhibited, and tianeptine increased the mitochondrial complex II and IV in the hippocampus of the non-deprived rats; the succinate dehydrogenase was increased in the hippocampus of non-deprived rats treated with tianeptine. So, tianeptine showed antidepressant effects conducted on maternally deprived rats, and this can be attributed to its action on the neurochemical pathways related to depression.

Interleukin-1β causes anxiety by interacting with the endocannabinoid system

Interleukin-1β (IL-1β) is involved in mood alterations associated with inflammatory illnesses and with stress. The synaptic basis of IL-1β-induced emotional disturbances is still unknown. To address the possible involvement of the endocannabinoid system in IL-1β-induced anxiety, we performed behavioral and neurophysiological studies in mice exposed to stress or to intracerebroventricular injections of this inflammatory cytokine or of its antagonist. We found that a single intracerebroventricular injection of IL-1β caused anxiety in mice, and abrogated the sensitivity of cannabinoid CB1 receptors (CB1Rs) controlling GABA synapses in the striatum. Identical behavioral and synaptic results were obtained following social defeat stress, and intracerebroventricular injection of IL-1 receptor antagonist reverted both effects. IL-1β-mediated inhibition of CB1R function was secondary to altered cholesterol composition within membrane lipid rafts, and required intact function of the transient receptor potential vanilloid 1 (TRPV1) channel, another element of the endocannabinoid system. Membrane lipid raft disruption and inhibition of cholesterol synthesis, in fact, abrogated IL-1β-CB1R coupling, and TRPV1-/- mice were indeed insensitive to the synaptic and behavioral effects of both IL-1β and stress. On the other hand, cholesterol enrichment of striatal slices mimicked the synaptic effects of IL-1β on CB1Rs only in control mice, while the same treatment was ineffective in slices prepared from TRPV1-/- mice. The present investigation identifies a previously unrecognized interaction between a major proinflammatory cytokine and the endocannabinoid system in the pathophysiology of anxiety.

Different roles of BDNF in nucleus accumbens core versus shell during the incubation of cue-induced cocaine craving and its long-term maintenance

Brain-derived neurotrophic factor (BDNF) contributes to diverse types of plasticity, including cocaine addiction. We investigated the role of BDNF in the rat nucleus accumbens (NAc) in the incubation of cocaine craving over 3 months of withdrawal from extended access cocaine self-administration. First, we confirmed by immunoblotting that BDNF levels are elevated after this cocaine regimen on withdrawal day 45 (WD45) and showed that BDNF mRNA levels are not altered. Next, we explored the time course of elevated BDNF expression using immunohistochemistry. Elevation of BDNF in the NAc core was detected on WD45 and further increased on WD90, whereas elevation in shell was not detected until WD90. Surface expression of activated tropomyosin receptor kinase B (TrkB) was also enhanced on WD90. Next, we used viral vectors to attenuate BDNF-TrkB signaling. Virus injection into the NAc core enhanced cue-induced cocaine seeking on WD1 compared with controls, whereas no effect was observed on WD30 or WD90. Attenuating BDNF-TrkB signaling in shell did not affect cocaine seeking on WD1 or WD45 but significantly decreased cocaine seeking on WD90. These results suggest that basal levels of BDNF transmission in the NAc core exert a suppressive effect on cocaine seeking in early withdrawal (WD1), whereas the late elevation of BDNF protein in NAc shell contributes to incubation in late withdrawal (WD90). Finally, BDNF protein levels in the NAc were significantly increased after ampakine treatment, supporting the novel hypothesis that the gradual increase of BDNF levels in NAc accompanying incubation could be caused by increased AMPAR transmission during withdrawal.

Neurobiological sequelae of witnessing stressful events in adult mice

BACKGROUND

It is well known that exposure to severe stress increases the risk for developing mood disorders. However, most chronic stress models in rodents involve at least some form of physically experiencing traumatic events.

METHODS

This study assessed the effects of a novel social stress paradigm that is insulated from the effects of physical stress. Specifically, adult male C57BL/6J mice were exposed to either emotional (ES) or physical stress (PS) for 10 minutes per day for 10 days. The ES mice were exposed to the social defeat of a PS mouse by a larger, more aggressive CD-1 mouse from the safety of an adjacent compartment.

RESULTS

Like PS mice, ES mice exhibited a range of depression- and anxiety-like behaviors both 24 hours and 1 month after the stress. Increased levels of serum corticosterone, part of the stress response, accompanied these behavioral deficits. Based on previous work that implicated gene expression changes in the ventral tegmental area (a key brain reward region) in the PS phenotype, we compared genome-wide mRNA expression patterns in this brain region of ES and PS mice using RNA-seq. We found significant overlap between these conditions, which suggests several potential gene targets for mediating the behavioral abnormalities observed.

CONCLUSIONS

These findings demonstrate that witnessing traumatic events is a potent stress in adult male mice capable of inducing long-lasting neurobiological perturbations.

The Role of Neurotrophins in Major Depressive Disorder

Neurotrophins and other growth factors have been advanced as critical modulators of depressive behavior. Support for this model is based on analyses of knockout and transgenic mouse models, human genetic studies, and screens for gene products that are regulated by depressive behavior and/or antidepressants. Even subtle alteration in the regulated secretion of brain-derived neurotrophic factor (BDNF), for example, due to a single nucleotide polymorphism (SNP)-encoded Val-Met substitution in proBDNF that affects processing and sorting, impacts behavior and cognition. Alterations in growth factor expression result in changes in neurogenesis as well as structural changes in neuronal cytoarchitecture, including effects on dendritic length and spine density, in the hippocampus, nucleus accumbens, and prefrontal cortex. These changes have the potential to impact the plasticity and stability of synapses in the CNS, and the complex brain circuitry that regulates behavior. Here we review the role that neurotrophins play in the modulation of depressive behavior, and the downstream signaling targets they regulate that potentially mediate these behavioral pro-depressant and antidepressant effects.

Novel insights into depression and antidepressants: a synergy between synaptogenesis and neurogenesis?

Major depressive disorder has been associated with manifold pathophysiological changes. These include metabolic abnormalities in discreet brain areas; modifications in the level of stress hormones, neurotransmitters, and neurotrophic factors; impaired spinogenesis and synaptogenesis in crucial brain areas, such as the prefrontal cortex and the hippocampus; and impaired neurogenesis in the hippocampus. Antidepressant therapy facilitates remission by reversing most of these disturbances, indicating that these dysfunctions may participate causally in depressive symptomatology. However, few attempts have been made to integrate these different pathophysiologies into one model. The present chapter endeavors (1) to review the extant literature in the field, with particular focus on the role of neurogenesis and synaptogenesis in depression; (2) and to suggest a possible interplay between these two processes, as well as, describe the ways by which improving both neurogenesis and synaptogenesis may enable effective recovery by acting on a larger neuronal network.

Sigma-1 receptor chaperone and brain-derived neurotrophic factor: emerging links between cardiovascular disease and depression

Epidemiological studies have demonstrated a close relationship between depression and cardiovascular disease (CVD). Although it is known that the central nervous system (CNS) contributes to this relationship, the detailed mechanisms involved in this process remain unclear. Recent studies suggest that the endoplasmic reticulum (ER) molecular chaperone sigma-1 receptor and brain-derived neurotrophic factor (BDNF) play a role in the pathophysiology of CVD and depression. Several meta-analysis studies have showed that levels of BDNF in the blood of patients with major depressive disorder (MDD) are lower than normal controls, indicating that blood BDNF might be a biomarker for depression. Furthermore, blood levels of BDNF in patients with CVD are also lower than normal controls. A recent study using conditional BDNF knock-out mice in animal models of myocardial infarction highlighted the role of CNS-mediated mechanisms in the cardioprotective effects of BDNF. In addition, a recent study shows that decreased levels of sigma-1 receptor in the mouse brain contribute to the association between heart failure and depression. Moreover, sigma-1 receptor agonists, including the endogenous neurosteroid dehydroepiandosterone (DHEA) and the selective serotonin reuptake inhibitor (SSRI) fluvoxamine, show potent cardioprotective and antidepressive effects in rodents, via sigma-1 receptor stimulation. Interestingly, agonist activation of sigma-1 receptors increased the secretion of mature BDNF from its precursor proBDNF via chaperone activity in the ER. Given the role of ER stress in the pathophysiology of CVD and MDD, the author will discuss the potential link between sigma-1 receptors and BDNF-TrkB pathway in the pathophysiology of these two diseases. Finally, the author will make a case for potent sigma-1 receptor agonists and TrkB agonists as new potential therapeutic drugs for depressive patients with CVD.

Targeted delivery of GDNF through the blood-brain barrier by MRI-guided focused ultrasound

Neurotrophic factors, such as glial cell line-derived neurotrophic factor (GDNF), are promising therapeutic agents for neurodegenerative diseases. However, the application of GDNF to treat these diseases effectively is limited because the blood–brain barrier (BBB) prevents the local delivery of macromolecular therapeutic agents from entering the central nervous system (CNS). Focused ultrasound combined with microbubbles (MBs) using appropriate parameters has been previously demonstrated to be able to open the BBB locally and noninvasively. This study investigated the targeted delivery of GDNF MBs through the BBB by magnetic resonance imaging (MRI)-guided focused ultrasound. Evans Blue extravasation and histological examination were used to determine the optimum focused ultrasound parameters. Enzyme-linked immunosorbent assay was performed to verify the effects of GDNF bound on MBs using a biotin–avidin bridging chemistry method to promote GDNF delivery into the brain. The results showed that GDNF can be delivered locally and noninvasively into the CNS through the BBB using MRI-guided focused ultrasound combined with MBs under optimum parameters. MBs that bind GDNF combined with MRI-guided focused ultrasound may be an effective way of delivering neurotrophic factors directly into the CNS. The method described herein provides a potential means of treating patients with CNS diseases.

The neurobiology of depression and antidepressant action

We present a comprehensive overview of the neurobiology of unipolar major depression and antidepressant drug action, integrating data from affective neuroscience, neuro- and psychopharmacology, neuroendocrinology, neuroanatomy, and molecular biology. We suggest that the problem of depression comprises three sub-problems: first episodes in people with low vulnerability ('simple' depressions), which are strongly stress-dependent; an increase in vulnerability and autonomy from stress that develops over episodes of depression (kindling); and factors that confer vulnerability to a first episode (a depressive diathesis). We describe key processes in the onset of a 'simple' depression and show that kindling and depressive diatheses reproduce many of the neurobiological features of depression. We also review the neurobiological mechanisms of antidepressant drug action, and show that resistance to antidepressant treatment is associated with genetic and other factors that are largely similar to those implicated in vulnerability to depression. We discuss the implications of these conclusions for the understanding and treatment of depression, and make some strategic recommendations for future research.

Somatic drugs for psychiatric diseases: aspirin or simvastatin for depression?

The evolution in the understanding of the neurobiology of most prevalent mental disorders such as major depressive disorder (MDD), bipolar disorder or schizophrenia has not gone hand in hand with the synthesis and clinical use of new drugs that would represent a therapeutic revolution such as that brought about by selective serotonin reuptake inhibitors (SSRIs) or atypical antipsychotics. Although scientists are still a long way from understanding its true aetiology, the neurobiological concept of depression has evolved from receptor regulation disorder, to a neurodegenerative disorder with a hippocampal volume decrease with the controversial reduction in neurotrophins such as BDNF, to current hypotheses that consider depression to be an inflammatory and neuroprogressive process. As regards antidepressants, although researchers are still far from knowing their true mechanism of action, they have gone from monoaminergic hypotheses, in which serotonin was the main protagonist, to emphasising the anti-inflammatory action of some of these drugs, or the participation of p11 protein in their mechanism of action.In the same way, according to the inflammatory hypothesis of depression, it has been proposed that some NSAIDS such as aspirin or drugs like simvastatin that have an anti-inflammatory action could be useful in some depressive patients. Despite the fact that there may be some data to support their clinical use, common sense and the evidence advise us to use already tested protocols and wait for the future to undertake new therapeutic strategies.

[The development of depression: the role of brain-derived neurotrophic factor]

An association between the presence of psychosocial stress, its pathological processing and the development of depression is well documented. This review reports and discusses studies suggesting a reduced release of brain-derived neurotrophic factor (BDNF) under stress as a possible mechanism. The studies show a reduction of BDNF secretion in stressful situations, a decreased blood concentration in depression and a normalization of BDNF by successful antidepressant therapy. As a possible mechanism of BDNF action, a reactivation of neuroplasticity is being discussed, especially in hippocampal and cortical networks. On the other hand, methodological limitations, such as the impossibility of determining the cerebral BDNF concentration in vivo and ruling out a variety of possible confounders, may restrict the significance of the studies. The question of whether the ascertained changes of BDNF levels are causally involved in the pathophysiology of depression or whether they are just an epiphenomenal result of depression-induced stress is still under debate.

The Neurobiology of Depression and Anxiety: How Do We Change from Models of Drug Efficacy to Understanding Mood and Anxiety Disorders?

Current treatments for depression and anxiety disorders are only effective in approximately half of the patient population. Effective treatments have negative side-effects including sexual dysfunction, weight gain and gastrointestinal problems. Furthermore, even when patients achieve remission, they often need to increase dosage or change treatment across their lifetime as efficacy weakens. The majority of treatments being used today are based on the monoamine hypothesis of depression, a theory of depression that was based on the effectiveness of drugs discovered by chance to alleviate the symptoms of depression. This chapter provides an overview of the neurobiology of depression and anxiety disorders within the context of drug discovery. The chapter starts with what we currently know about these disorders through the lens of the monoamine hypothesis of depression. We then provide a background into the animal models of depression and anxiety that are being used to understand the underlying biology of these disorders and test new treatments. Work conducted using these animal models has directed human imagining and has provided us with new information about both the molecular and cellular mechanism of depression and anxiety as well as the neural circuitry controlling these disorders. Finally, we will conclude with a discussion of new treatments being developed based on empirical evidence about the neurobiology of depression and anxiety and the need to develop more personalized treatments in the future. It is hoped that these new drugs will be able to provide effective treatment for more people, with fewer negative side-effects.

Neurotrophins role in depression neurobiology: a review of basic and clinical evidence

Depression is a neuropsychiatric disorder affecting a huge percentage of the active population especially in developed countries. Research has devoted much of its attention to this problematic and many drugs have been developed and are currently prescribed to treat this pathology. Yet, many patients are refractory to the available therapeutic drugs, which mainly act by increasing the levels of the monoamines serotonin and noradrenaline in the synaptic cleft. Even in the cases antidepressants are effective, it is usually observed a delay of a few weeks between the onset of treatment and remission of the clinical symptoms. Additionally, many of these patients who show remission with antidepressant therapy present a relapse of depression upon treatment cessation. Thus research has focused on other possible molecular targets, besides monoamines, underlying depression. Both basic and clinical evidence indicates that depression is associated with several structural and neurochemical changes where the levels of neurotrophins, particularly of brain-derived neurotrophic factor (BDNF), are altered. Antidepressants, as well as other therapeutic strategies, seem to restore these levels. Neuronal atrophy, mostly detected in limbic structures that regulate mood and cognition, like the hippocampus, is observed in depressed patients and in animal behavioural paradigms for depression. Moreover, chronic antidepressant treatment enhances adult hippocampal neurogenesis, supporting the notion that this event underlies antidepressants effects. Here we review some of the preclinical and clinical studies, aimed at disclosing the role of neurotrophins in the pathophysiological mechanisms of depression and the mode of action of antidepressants, which favour the neurotrophic/neurogenic hypothesis.

Taking the Perspective that a Depressive State Reflects Inflammation: Implications for the Use of Antidepressants

This paper reviews both the evidence that supports the characterization of depression as an inflammatory disorder and the different biochemical mechanisms that have been postulated for the connection between inflammation and depression. This association offers credible explanation for the short term efficacy of antidepressants, which have short term anti-inflammatory effects. Evidence for those anti-inflammatory effects is discussed. Evidence of the contrary long-term effects of antidepressants, which increase rather than decrease inflammation, is also reviewed. It is argued that this increase in inflammation would predict an increase in chronicity among depressed patients that have been treated with antidepressants drugs, which has been noted in the literature. A brief discussion of alternatives for decreasing inflammation, some of which have demonstrated efficacy in ameliorating depression, is presented.

TrkB inhibition as a therapeutic target for CNS-related disorders

The interaction of brain-derived neurotrophic factor (BDNF) with its tropomyosin-related kinase receptor B (TrkB) is involved in fundamental cellular processes including neuronal proliferation, differentiation and survival as well as neurotransmitter release and synaptic plasticity. TrkB signaling has been widely associated with beneficial, trophic effects and many commonly used psychotropic drugs aim to increase BDNF levels in the brain. However, it is likely that a prolonged increased TrkB activation is observed in many pathological conditions, which may underlie the development and course of clinical symptoms. Interestingly, genetic and pharmacological studies aiming at decreasing TrkB activation in rodent models mimicking human pathology have demonstrated a promising therapeutic landscape for TrkB inhibitors in the treatment of various diseases, e.g. central nervous system (CNS) disorders and several types of cancer. Up to date, only a few selective and potent TrkB inhibitors have been developed. As such, the use of crystallography and in silico approaches to model BDNF-TrkB interaction and to generate relevant pharmacophores represent powerful tools to develop novel compounds targeting the TrkB receptor.

Recurrent long-lasting tethering reduces BDNF protein levels in the dorsal hippocampus and frontal cortex in pigs

Brain-derived neurotrophic factor (BDNF) signaling has been implicated in the onset of depression and in antidepressant efficacy, although the exact role of this neurotrophin in the pathophysiology of depression remains to be elucidated. Also, the interaction between chronic stress, which may precede depression, corticosteroids and BDNF is not fully understood. The present study aimed at investigating whether long-lasting, recurrent tethering of sows during a period of 1.5 or 4.5 years leads to enduring effects on measures that may be indicative of chronic stress, compared with animals kept in a group housing system ('loose' sows). Immediately after slaughter, the frontal cortex, dorsal and ventral hippocampus were dissected and protein levels of BDNF and its receptors were analyzed and compared with plasma cortisol levels and adrenal weights. Results indicate that tethering stress reduced BDNF protein levels in the dorsal hippocampus and the frontal cortex, but not in the ventral hippocampus. In addition, levels of TrkB, the high affinity receptor for BDNF, were increased in the dorsal hippocampus. Plasma cortisol levels and adrenal weight were increased after tethering. These stress effects on BDNF levels were more pronounced after 4.5 years of recurrent tethering and negatively correlated in particular in the frontal cortex with cortisol levels and adrenal weight. This suggests that the stress effect of tethered housing on neurotrophin levels may be mediated via cortisol. Taken together, these data indicate that recurrent tethering stress in sows over 4.5 years results in a loss of neurotrophic support by BDNF, mediated by an overactive neuroendocrine system.

Depression and treatment response: dynamic interplay of signaling pathways and altered neural processes

Since the 1960s, when the first tricyclic and monoamine oxidase inhibitor antidepressant drugs were introduced, most of the ensuing agents were designed to target similar brain pathways that elevate serotonin and/or norepinephrine signaling. Fifty years later, the main goal of the current depression research is to develop faster-acting, more effective therapeutic agents with fewer side effects, as currently available antidepressants are plagued by delayed therapeutic onset and low response rates. Clinical and basic science research studies have made significant progress towards deciphering the pathophysiological events within the brain involved in development, maintenance, and treatment of major depressive disorder. Imaging and postmortem brain studies in depressed human subjects, in combination with animal behavioral models of depression, have identified a number of different cellular events, intracellular signaling pathways, proteins, and target genes that are modulated by stress and are potentially vital mediators of antidepressant action. In this review, we focus on several neural mechanisms, primarily within the hippocampus and prefrontal cortex, which have recently been implicated in depression and treatment response.

Differential BDNF responses of triple versus dual reuptake inhibition in neuronal and astrocytoma cells as well as in rat hippocampus and prefrontal cortex

Monoamine reuptake inhibitors increase brain-derived neurotrophic factor (BDNF) activity, and this growth factor is regarded as an interesting target for developing new antidepressant drugs. The aims of this study were to evaluate whether monoaminergic reuptake inhibition increases BDNF in vivo and in vitro as predicted by the neurotrophic hypothesis of depression, and whether triple reuptake inhibition has a superior BDNF response compared to dual reuptake inhibition. Twenty-one days of oral treatment (30 mg/kg) with the dual serotonin/noradrenaline reuptake inhibitor duloxetine or the triple serotonin/noradrenaline/dopamine reuptake inhibitor DOV 216,303 restored BDNF protein levels in the rat hippocampus, which were initially decreased due to injection stress. The prefrontal cortex contained increased BDNF levels only after DOV 216,303 treatment. In vitro, neither duloxetine nor DOV 216,303 altered intracellular BDNF levels in murine HT22 neuronal cells. In contrast, BDNF release was more effectively decreased following treatment with DOV 216,303 in these cells. In rat C62B astrocytomas, both antidepressants increased intracellular BDNF levels at their highest nontoxic concentration. C62B astrocytomas did not release BDNF, even after antidepressant treatment. Increased BDNF levels support the neurotrophic hypothesis of depression, but our findings do not clearly evidence that the BDNF response after triple reuptake inhibitors is more effective than after dual reuptake inhibitors. Moreover, the data suggest that the role of BDNF in neurons and astrocytes is complex and likely depends on factors including specificity of cell types in different brain regions, cell–cell interactions, and different mechanisms of action of antidepressants used.

Rescue from excitotoxicity and axonal degeneration accompanied by age-dependent behavioral and neuroanatomical alterations in caspase-6-deficient mice

Apoptosis, or programmed cell death, is a cellular pathway involved in normal cell turnover, developmental tissue remodeling, embryonic development, cellular homeostasis maintenance and chemical-induced cell death. Caspases are a family of intracellular proteases that play a key role in apoptosis. Aberrant activation of caspases has been implicated in human diseases. In particular, numerous findings implicate Caspase-6 (Casp6) in neurodegenerative diseases, including Alzheimer disease (AD) and Huntington disease (HD), highlighting the need for a deeper understanding of Casp6 biology and its role in brain development. The use of targeted caspase-deficient mice has been instrumental for studying the involvement of caspases in apoptosis. The goal of this study was to perform an in-depth neuroanatomical and behavioral characterization of constitutive Casp6-deficient (Casp6-/-) mice in order to understand the physiological function of Casp6 in brain development, structure and function. We demonstrate that Casp6-/- neurons are protected against excitotoxicity, nerve growth factor deprivation and myelin-induced axonal degeneration. Furthermore, Casp6-deficient mice show an age-dependent increase in cortical and striatal volume. In addition, these mice show a hypoactive phenotype and display learning deficits. The age-dependent behavioral and region-specific neuroanatomical changes observed in the Casp6-/- mice suggest that Casp6 deficiency has a more pronounced effect in brain regions that are involved in neurodegenerative diseases, such as the striatum in HD and the cortex in AD.

Sustained expression of brain-derived neurotrophic factor is required for maintenance of dendritic spines and normal behavior

Brain-derived neurotrophic factor (BDNF) plays important roles in the development, maintenance, and plasticity of the mammalian forebrain. These functions include regulation of neuronal maturation and survival, axonal and dendritic arborization, synaptic efficacy, and modulation of complex behaviors including depression and spatial learning. Although analysis of mutant mice has helped establish essential developmental functions for BDNF, its requirement in the adult is less well documented. We have studied late-onset forebrain-specific BDNF knockout (CaMK-BDNF(KO)) mice, in which BDNF is lost primarily from the cortex and hippocampus in early adulthood, well after BDNF expression has begun in these structures. We found that although CaMK-BDNF(KO) mice grew at a normal rate and can survive more than a year, they had smaller brains than wild-type siblings. The CaMK-BDNF(KO) mice had generally normal behavior in tests for ataxia and anxiety, but displayed reduced spatial learning ability in the Morris water task and increased depression in the Porsolt swim test. These behavioral deficits were very similar to those we previously described in an early-onset forebrain-specific BDNF knockout. To identify an anatomical correlate of the abnormal behavior, we quantified dendritic spines in cortical neurons. The spine density of CaMK-BDNF(KO) mice was normal at P35, but by P84, there was a 30% reduction in spine density. The strong similarities we find between early- and late-onset BDNF knockouts suggest that BDNF signaling is required continuously in the CNS for the maintenance of some forebrain circuitry also affected by developmental BDNF depletion.

Activation of signaling pathways following localized delivery of systemically administered neurotrophic factors across the blood-brain barrier using focused ultrasound and microbubbles

The brain-derived neurotrophic factor (BDNF) has been shown to have broad neuroprotective effects in addition to its therapeutic role in neurodegenerative disease. In this study, the efficacy of delivering exogenous BDNF to the left hippocampus is demonstrated in wild-type mice (n = 7) through the noninvasively disrupted blood-brain barrier (BBB) using focused ultrasound (FUS). The BDNF bioactivity was found to be preserved following delivery as assessed quantitatively by immunohistochemical detection of the pTrkB receptor and activated pAkt, pMAPK, and pCREB in the hippocampal neurons. It was therefore shown for the first time that systemically administered neurotrophic factors can cross the noninvasively disrupted BBB and trigger neuronal downstream signaling effects in a highly localized region in the brain. This is the first time that the administered molecule is tracked through the BBB and localized in the neuron triggering molecular effects. Additional preliminary findings are shown in wild-type mice with two additional neurotrophic factors such as the glia-derived neurotrophic factor (n = 12) and neurturin (n = 2). This further demonstrates the impact of FUS for the early treatment of CNS diseases at the cellular and molecular level and strengthens its premise for FUS-assisted drug delivery and efficacy.

Structural and synaptic plasticity in stress-related disorders

Abstract Stress can have a lasting impact on the structure and function of brain circuitry that results in long-lasting changes in the behavior of an organism. Synaptic plasticity is the mechanism by which information is stored and maintained within individual synapses, neurons, and neuronal circuits to guide the behavior of an organism. Although these mechanisms allow the organism to adapt to its constantly evolving environment, not all of these adaptations are beneficial. Under prolonged bouts of physical or psychological stress, these mechanisms become dysregulated, and the connectivity between brain regions becomes unbalanced, resulting in pathological behaviors. In this review, we highlight the effects of stress on the structure and function of neurons within the mesocorticolimbic brain systems known to regulate mood and motivation. We then discuss the implications of these spine adaptations on neuronal activity and pathological behaviors implicated in mood disorders. Finally, we end by discussing recent brain imaging studies in human depression within the context of these basic findings to provide insight into the underlying mechanisms leading to neural dysfunction in depression.

Serotonergic and BDNF genes and risk of depression after stroke

BACKGROUND

Polymorphisms of serotonin transporter (5-HTT) and brain-derived neurotrophic factor (BDNF) have been investigated as candidate genes for post-stroke depression (PSD). Serotonin 2a receptor (5-HTR2a) genes have not been yet investigated in PSD. This study aimed to investigate whether the 5-HTT, 5-HTR2a, and BDNF genes are associated with PSD independently and/or interactively in a Korean sample with high prevalence of risk alleles.

METHODS

In 276 stroke cases, depression was diagnosed using DSM-IV at 2 weeks after stroke, further classified to major PSD (N=29), all (major plus minor) PSD (N=77), and control (N=199) groups. Associations between PSD and 5-HTTLPR, STin2 VNTR, 5-HTR2a 1438A/G, 5-HTR2a 102T/C, and BDNF val66met genotypes were estimated using logistic regression models, and gene-gene interactions were investigated using the generalized multifactor dimensionality reduction method.

RESULTS

5-HTR2a 1438 A/A genotype was associated with major PSD, while 5-HTTLPR s/s and BDNF met/met genotypes were associated with all PSD. There was a significant interaction between 5-HTR2a 1438A/G and BDNF val66met polymorphisms for major PSD and a borderline significant interaction between 5-HTTLPR and BDNF val66met polymorphisms for all PSD.

CONCLUSIONS

In a large cohort, we found evidence for serotonin and BDNF polymorphisms as susceptibility factors and gene-gene interactions between these systems for depression at 2 weeks post-stroke.

Estrogen effects on the forced swim test differ in two outbred rat strains

Changes in reproductive hormones, such as estrogen, play a role in mood regulation. The present study examined strain differences (Long-Evans vs. Wistar-Hannover) in the behavioral and biochemical effects of estrogen manipulation. Adult ovariectomized female rats were treated with estradiol, vehicle, or withdrawn from estradiol. The two strains demonstrated differential behavioral responses to short-term estradiol administration in the forced swim test; estradiol induced an antidepressant-like effect in Long-Evans rats but not in Wistar rats. Conversely, withdrawal from estradiol resulted in a depressive-like state in the Wistar rats but not in the Long-Evans rats. Western blot analyses found no differences in estrogen receptors α and β within the hippocampus or the frontal cortex, two brain areas strongly implicated in affective disorders. These data demonstrate the importance of strain as a variable when interpreting behavioral effects of estrogen.

BDNF function as a potential mediator of bipolar disorder and post-traumatic stress disorder comorbidity

Bipolar disorder (BD) and post-traumatic stress disorder (PTSD) frequently co-occur among psychiatric patients, leading to increased morbidity and mortality. Brain-derived neurotrophic factor (BDNF) function is associated with core characteristics of both BD and PTSD. We propose a neurobiological model that underscores the role of reduced BDNF function resulting from several contributing sources, including the met variant of the BDNF val66met (rs6265) single-nucleotide polymorphism, trauma-induced epigenetic regulation and current stress, as a contributor to the onset of both illnesses within the same person. Further studies are needed to evaluate the genetic association between the val66met allele and the BD-PTSD population, along with central/peripheral BDNF levels and epigenetic patterns of BDNF gene regulation within these patients.

The hippocampus, neurotrophic factors and depression: possible implications for the pharmacotherapy of depression

Depression is a prevalent, highly debilitating mental disorder affecting up to 15% of the population at least once in their lifetime, with huge costs for society. Neurobiological mechanisms of depression are still not well known, although there is consensus about interplay between genetic and environmental factors. Antidepressant medications are frequently used in depression, but at least 50% of patients are poor responders, even to more recently discovered medications. Furthermore, clinical response only occurs following weeks to months of treatment and only chronic treatment is effective, suggesting that actions beyond the rapidly occurring effect of enhancing monoaminergic systems, such as adaptation of these systems, are responsible for the effects of antidepressants. Recent studies indicate that an impairment of synaptic plasticity (neurogenesis, axon branching, dendritogenesis and synaptogenesis) in specific areas of the CNS, particularly the hippocampus, may be a core factor in the pathophysiology of depression. The abnormal neural plasticity may be related to alterations in the levels of neurotrophic factors, namely brain-derived neurotrophic factor (BDNF), which play a central role in plasticity. As BDNF is repressed by stress, epigenetic regulation of the BDNF gene may play an important role in depression. The hippocampus is smaller in depressed patients, although it is unclear whether smaller size is a consequence of depression or a pre-existing, vulnerability marker for depression. Environmental stressors triggering activation of the hypothalamic-pituitary-adrenal axis cause the brain to be exposed to corticosteroids, affecting neurobehavioural functions with a strong downregulation of hippocampal neurogenesis, and are a major risk factor for depression. Antidepressant treatment increases BDNF levels, stimulates neurogenesis and reverses the inhibitory effects of stress, but this effect is evident only after 3-4 weeks of administration, the time course for maturation of new neurons. The ablation of hippocampal neurogenesis blocks the behavioural effects of antidepressants in animal models. The above findings suggest new possible targets for the pharmacotherapy of depression such as neurotrophic factors, their receptors and related intracellular signalling cascades; agents counteracting the effects of stress on hippocampal neurogenesis (including antagonists of corticosteroids, inflammatory cytokines and their receptors); and agents facilitating the activation of gene expression and increasing the transcription of neurotrophins in the brain.

Functional biomarkers of depression: diagnosis, treatment, and pathophysiology

Major depressive disorder (MDD) is a heterogeneous illness for which there are currently no effective methods to objectively assess severity, endophenotypes, or response to treatment. Increasing evidence suggests that circulating levels of peripheral/serum growth factors and cytokines are altered in patients with MDD, and that antidepressant treatments reverse or normalize these effects. Furthermore, there is a large body of literature demonstrating that MDD is associated with changes in endocrine and metabolic factors. Here we provide a brief overview of the evidence that peripheral growth factors, pro-inflammatory cytokines, endocrine factors, and metabolic markers contribute to the pathophysiology of MDD and antidepressant response. Recent preclinical studies demonstrating that peripheral growth factors and cytokines influence brain function and behavior are also discussed along with their implications for diagnosing and treating patients with MDD. Together, these studies highlight the need to develop a biomarker panel for depression that aims to profile diverse peripheral factors that together provide a biological signature of MDD subtypes as well as treatment response.

Antidepressant-like effect of artemin in mice: a mechanism for acetyl-L-carnitine activity on depression

RATIONALE

Depression may be associated with altered plasticity of the nervous system. The importance of neurotrophic factor levels is strongly suggested, and the neuronal-related family is extensively studied with respect to glial-derived one.

OBJECTIVES

Aimed to contribute to the study of nervous plasticity modulation as therapeutical target in mood disorders, the role of the glial-derived factor artemin (ARTN) in depression and in the pharmacodynamics of the antidepressant and trophic compound acetyl-L: -carnitine (ALCAR) was evaluated.

METHODS

Male mice were treated with 100 mg kg(-1) ALCAR daily for 7 days; 0.6 μg/mouse ARTN was acutely injected intracerebroventricularly. Gene knockdown of ARTN and GDNF family receptor alpha (GFRalpha3) was obtained by oligonucleotide antisense strategy. The forced swimming test was performed to evaluate antidepressant-like effects.

RESULTS

Repeated ALCAR administration increased ARTN levels in spinal cord, hippocampus, and prefrontal cortex. No modulatory effect was detected on BDNF and glial cell line-derived neutrotrophic factor (GDNF). ARTN, 30 min after administration, showed a dose-dependent antidepressant-like effect. ALCAR needed a 7-day treatment to reach a comparable effect; nevertheless, both substances were able to induce a phosphorylation of the GDNF family receptor Ret. A decrease of the free ARTN level by a specific ARTN antibody impaired the antidepressant-like effect of acute ARTN and repeated ALCAR. Gene knockdown of ARTN or, alternatively, of its receptor GFRalpha3 fully prevented ALCAR effectiveness.

CONCLUSIONS

A mechanism for the antidepressant property of ALCAR is proposed, and the novelty of the possible role of ARTN in depression is suggested.

A role for repressive histone methylation in cocaine-induced vulnerability to stress

Substance abuse increases an individual's vulnerability to stress-related illnesses, which is presumably mediated by drug-induced neural adaptations that alter subsequent responses to stress. Here, we identify repressive histone methylation in nucleus accumbens (NAc), an important brain reward region, as a key mechanism linking cocaine exposure to increased stress vulnerability. Repeated cocaine administration prior to subchronic social defeat stress potentiated depressive-like behaviors in mice through decreased levels of histone H3 lysine 9 dimethylation in NAc. Cre-mediated reduction of the histone methyltransferase, G9a, in NAc promoted increased susceptibility to social stress, similar to that observed with repeated cocaine. Conversely, G9a overexpression in NAc after repeated cocaine protected mice from the consequences of subsequent stress. This resilience was mediated, in part, through repression of BDNF-TrkB-CREB signaling, which was induced after repeated cocaine or stress. Identifying such common regulatory mechanisms may aid in the development of new therapies for addiction and depression.

Juvenile administration of concomitant methylphenidate and fluoxetine alters behavioral reactivity to reward- and mood-related stimuli and disrupts ventral tegmental area gene expression in adulthood

There is a rise in the concurrent use of methylphenidate (MPH) and fluoxetine (FLX) in pediatric populations. However, the long-term neurobiological consequences of combined MPH and FLX treatment (MPH + FLX) during juvenile periods are unknown. We administered saline (VEH), MPH, FLX, or MPH + FLX to juvenile Sprague Dawley male rats from postnatal day 20 to 34, and assessed their reactivity to reward- and mood-related stimuli 24 h or 2 months after drug exposure. We also assessed mRNA and protein levels within the ventral tegmental area (VTA) to determine the effect of MPH, FLX, or MPH + FLX on the extracellular signal-regulated protein kinase-1/2 (ERK) pathway--a signaling cascade implicated in motivation and mood regulation. MPH + FLX enhanced sensitivity to drug (i.e., cocaine) and sucrose rewards, as well as anxiety (i.e., elevated plus maze)- and stress (i.e., forced swimming)-eliciting situations when compared with VEH-treated rats. MPH + FLX exposure also increased mRNA of ERK2 and its downstream targets cAMP response element-binding protein (CREB), BDNF, c-Fos, early growth response protein-1 (Zif268), and mammalian target of rapamycin (mTOR), and also increased protein phosphorylation of ERK2, CREB, and mTOR 2 months after drug exposure when compared with VEH-treated rats. Using herpes simplex virus-mediated gene transfer to block ERK2 activity within the VTA, we rescued the MPH and FLX-induced behavioral deficits seen in the forced-swimming task 2 months after drug treatment. These results indicate that concurrent MPH + FLX exposure during preadolescence increases sensitivity to reward-related stimuli while simultaneously enhancing susceptibility to stressful situations, at least in part, due to long-lasting disruptions in ERK signaling within the VTA.

A food restriction protocol that increases drug reward decreases tropomyosin receptor kinase B in the ventral tegmental area, with no effect on brain-derived neurotrophic factor or tropomyosin receptor kinase B protein levels in dopaminergic forebrain regions

Food restriction (FR) decreases brain-derived neurotrophic factor (BDNF) expression in hypothalamic and hindbrain regions that regulate feeding and metabolic efficiency, while increasing expression in hippocampal and neocortical regions. Drugs of abuse alter BDNF expression within the mesocorticolimbic dopamine (DA) pathway, and modifications of BDNF expression within this pathway alter drug-directed behavior. Although FR produces a variety of striatal neuroadaptations and potentiates the rewarding effects of abused drugs, the effects of FR on BDNF expression and function within the DA pathway are unknown. The primary purpose of the present study was to examine the effect of FR on protein levels of BDNF and its tropomyosin receptor kinase B (TrkB) receptor in component structures of the mesocorticolimbic pathway. Three to four weeks of FR, with stabilization of rats at 80% of initial body weight, did not alter BDNF or TrkB levels in nucleus accumbens, caudate-putamen, or medial prefrontal cortex. However, FR decreased TrkB levels in the ventral tegmental area (VTA), without change in levels of BDNF protein or mRNA. The finding that FR also decreased TrkB levels in substantia nigra, with elevation of BDNF protein, suggests that decreased TrkB in VTA could be a residual effect of increased BDNF during an earlier phase of FR. Voltage-clamp recordings in VTA DA neurons indicated decreased glutamate receptor transmission. These data might predict lower average firing rates in FR relative to ad libitum fed subjects, which would be consistent with previous evidence of decreased striatal DA transmission and upregulation of postsynaptic DA receptor signaling. However, FR subjects also displayed elevated VTA levels of phospho-ERK1/2, which is an established mediator of synaptic plasticity. Because VTA neurons are heterogeneous with regard to neurochemistry, function, and target projections, the relationship(s) between the three changes observed in VTA, and their involvement in the augmented striatal and behavioral responsiveness of FR subjects to drugs of abuse, remains speculative.

A meta-analysis of circulating BDNF concentrations in anorexia nervosa

OBJECTIVES

Brain derived neurotrophic factor (BDNF) is involved in neuroplasticity, and in the homeostatic regulation of food intake and energy expenditure. It also has a role in stress responsivity and reward processing. On the basis of its involvement in these various processes, BDNF can be hypothesized to be an important factor in the development and maintenance of anorexia nervosa (AN). This study meta-analytically summarizes investigations of serum BDNF concentrations in people currently ill with AN, in comparison to healthy controls.

METHODS

Seven studies measuring BDNF in serum of individuals with AN (n=155) and healthy controls (n=174) were identified and included in the meta-analysis of the mean differences between case and control groups.

RESULTS

This study confirms that AN is associated with decreased serum BDNF concentrations, in comparison to healthy controls. The combined effect size (standardized mean difference, SMD) was large (SMD=-0.96; 95% CI -1.33 to -0.59; P<0.001). Significant heterogeneity of effect sizes was identified (I(2)=58.3%; P<0.001), which emerged as being primarily attributable to the first published study on the investigated association.

CONCLUSIONS

The present meta-analytical summary of studies measuring circulating BDNF concentrations in women with AN and healthy controls confirms that it is significantly reduced in this patient group. Difficulties associated with the measurement of BDNF have been identified and potential confounding factors have been discussed. Current data do not allow inferences to be made about causal links between levels of circulating BDNF and AN. However, possible explanations for the relationship between BDNF and AN have been presented.

Brain-derived neurotrophic factor (BDNF) infusion restored astrocytic plasticity in the hippocampus of a rat model of depression

Brain-derived neurotrophic factor (BDNF) is closely associated with hippocampal plasticity in psychiatric disorders. Glial cells (particularly astrocytes) are the most abundant cell type in the central nervous system. Previous studies have demonstrated that distinct alterations of astrocytes are associated with major depressive disorder, but there is a paucity of data describing whether such alterations of astrocytic plasticity are present in depressive-like rat hippocampus after BDNF administration. In this paper, we investigated the effects of chronic unpredictable mild stress (CUMS) and BDNF infusion on astrocyte immunoreactivity in rat hippocampus using sucrose preference test, open field test, and Western blot analysis. Results revealed that CUMS induced anhedonic-like behaviors in sucrose consumption and open field performances, which were partially reversed by BDNF infusion. Moreover, CUMS produced decreased glial fibrillary acidic protein (GFAP) expression and increased s100 calcium binding protein b (s100b) expression in rat hippocampus, which were partially rescued by BDNF administration. Therefore, BDNF might restore astrocyte immunoreactivity in depressive-like rat hippocampus, providing insights into the potential pharmacological role of BDNF in stress-related disorders.

Viral depletion of VTA BDNF in rats modulates social behavior, consequences of intermittent social defeat stress, and long-term weight regulation

Mesolimbic brain-derived neurotrophic factor (BDNF) is implicated in sustained behavioral changes following chronic social stress, and its depletion may reduce susceptibility to such behavioral alterations. Enhanced mesolimbic BDNF is proposed as pro-depressive and anhedonic, while depleting ventral tegmetal area (VTA) BDNF increases weight by enhancing hedonic eating. Here, we questioned whether depletion of VTA BDNF would alleviate social defeat stress-induced deficits in weight regulation, or affect social behavior in the presence or absence of social stress. Male Sprague-Dawley rats received bilateral intra-VTA infusions of adeno-associated virus (AAV) vectors containing shRNA against BDNF or a control virus. Three weeks later, rats underwent 4 episodes of social defeat stress involving exposure to an aggressive Long-Evans resident rat, or control handling every third day. Depleted VTA BDNF conferred resistance to the deficient weight regulation normally observed during intermittent social defeat stress, and enhanced long-term weight gain regardless of stress history. In addition, social approach and avoidance behavior towards a novel social target were measured 7 weeks after stress. Social defeat stress chronically reduced social behavior, whereas depletion of VTA BDNF chronically increased social behavior. Our results reveal that depletion of VTA BDNF alleviates some consequences of intermittent social defeat stress, enhances social behavior, and may contribute to weight gain. These data implicate VTA BDNF in protracted behavioral responses to stress, social stimuli, and weight regulation.

Cannabinoid receptor signalling in neurodegenerative diseases: a potential role for membrane fluidity disturbance

Type-1 cannabinoid receptor (CB(1)) is the most abundant G-protein-coupled receptor (GPCR) in the brain. CB(1) and its endogenous agonists, the so-called 'endocannabinoids (eCBs)', belong to an ancient neurosignalling system that plays important functions in neurodegenerative and neuroinflammatory disorders like Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and multiple sclerosis. For this reason, research on the therapeutic potential of drugs modulating the endogenous tone of eCBs is very intense. Several GPCRs reside within subdomains of the plasma membranes that contain high concentrations of cholesterol: the lipid rafts. Here, the hypothesis that changes in membrane fluidity alter function of the endocannabinoid system, as well as progression of particular neurodegenerative diseases, is described. To this end, the impact of membrane cholesterol on membrane properties and hence on neurodegenerative diseases, as well as on CB(1) signalling in vitro and on CB(1) -dependent neurotransmission within the striatum, is discussed. Overall, present evidence points to the membrane environment as a critical regulator of signal transduction triggered by CB(1) , and calls for further studies aimed at better clarifying the contribution of membrane lipids to eCBs signalling. The results of these investigations might be exploited also for the development of novel therapeutics able to combat disorders associated with abnormal activity of CB(1).

5-HT1B mRNA expression after chronic social stress

Chronic stress contributes to vulnerability for depression and drug addiction. The function of the serotonergic system has been found to be modified by chronic stress and these changes may play an important role in stress-related relapses to drug craving. The 5-HT(1B) receptor is expressed in nucleus accumbens (NAc) projection neurons and modulates drug reward mechanisms and there is evidence suggesting that stress alters the regulation and function of these receptors. To examine the role of these receptors in integrating the effects of stress on reward mechanisms, we examined whether chronic or acute social defeat stress (SDS) regulates 5-HT(1B) mRNA in dorsal and ventral striatum, regions that are critical for integrating the effects of environmental stressors on reward motivated behavior. In addition, 5-HT(1B) mRNA regulation in response to another acute stressor, inescapable tailshock, was measured. Our results indicate that intermittent and daily SDS procedures attenuated body weight gain, induced adrenal hypertrophy, and reduced the preference for saccharin, a sweet solution preferred by normal rats. There was a trend for daily, but not intermittent SDS to increase 5-HT(1B) receptor mRNA levels in nucleus accumbens. Therefore, in the next experiment, we examined daily SDS in greater detail and found that it increased 5-HT(1B) receptor mRNA expression in rostral nucleus accumbens shell, an area especially associated with reward functions. Neither acute SDS, nor acute tailshock stress had a significant impact on 5-HT(1B) mRNA expression in the striatum. Since increased 5-HT(1B) receptor expression in nucleus accumbens shell neurons can facilitate cocaine and alcohol reward mechanisms, this adaptation in endogenous 5-HT(1B) mRNA may be involved in the SDS-associated increase in vulnerability for developing addiction.

Enhanced cocaine-conditioned place preference and associated brain regional levels of BDNF, p-ERK1/2 and p-Ser845-GluA1 in food-restricted rats

Previously, a learning-free measure was used to demonstrate that chronic food restriction (FR) increases the reward magnitude of a wide range of abused drugs. Moreover, a variety of striatal neuroadaptations were detected in FR subjects, some of which are known to be involved in synaptic plasticity but have been ruled out as modulators of acute drug reward magnitude. Little is known about effects of FR on drug-conditioned place preference (CPP) and brain regional mechanisms that may enhance CPP in FR subjects. The purpose of the present study was to compare the expression and persistence of a conditioned place preference (CPP) induced by a relatively low dose of cocaine (7.0mg/kg, i.p.) in ad libitum fed (AL) and FR rats and take several brain regional biochemical measures following the first CPP conditioning session to probe candidate mechanisms that may underlie the more robust CPP observed in FR subjects. Behaviorally, AL subjects displayed a CPP upon initial testing which extinguished rapidly over the course of subsequent test sessions while CPP in FR subjects persisted. Despite previous reports of elevated BDNF protein in forebrain regions of FR rats, the FR protocol used in the present study did not alter BDNF levels in dorsal hippocampus, nucleus accumbens or medial prefrontal cortex. On the other hand, FR rats, whether injected with cocaine or vehicle, displayed elevated p-ERK1/2 and p-Ser845-GluA1 in dorsal hippocampus. FR rats also displayed elevated p-ERK1/2 in medial prefrontal cortex and elevated p-ERK1 in nucleus accumbens, with further increases produced by cocaine. The one effect observed exclusively in cocaine-treated FR rats was increased p-Ser845-GluA1 in nucleus accumbens. These findings suggest a number of avenues for continuing investigation with potential translational significance.

Identification of a low-molecular weight TrkB antagonist with anxiolytic and antidepressant activity in mice

The neurotrophin brain-derived neurotrophic factor (BDNF) and its receptor tropomyosin-related kinase B (TrkB) have emerged as key mediators in the pathophysiology of several mood disorders, including anxiety and depression. However, therapeutic compounds that interact with TrkB receptors have been difficult to develop. Using a combination of structure-based in silico screening and high-capacity functional assays in recombinant and neuronal cells, we identified a low-molecular weight TrkB ligand (ANA-12) that prevented activation of the receptor by BDNF with a high potency. ANA-12 showed direct and selective binding to TrkB and inhibited processes downstream of TrkB without altering TrkA and TrkC functions. KIRA-ELISA analysis demonstrated that systemic administration of ANA-12 to adult mice decreased TrkB activity in the brain without affecting neuronal survival. Mice administered ANA-12 demonstrated reduced anxiety- and depression-related behaviors on a variety of tests predictive of anxiolytic and antidepressant properties in humans. This study demonstrates that structure-based virtual screening strategy can be an efficient method for discovering potent TrkB-selective ligands that are active in vivo. We further propose that ANA-12 may be a valuable tool for studying BDNF/TrkB signaling and may constitute a lead compound for developing the next generation of therapeutic agents for the treatment of mood disorders.

Region-specific involvement of BDNF secretion and synthesis in conditioned taste aversion memory formation

Brain-derived neurotrophic factor (BDNF) and its receptor, tropomyosin-related kinase receptor B (TrkB), play a critical role in activity-dependent plasticity processes such as long-term potentiation, learning, and memory. It has been shown that BDNF exerts different or even opposite effects on behavior depending on the neural circuit. However, the detailed role of BDNF in memory process on the basis of its location has not been fully understood. Here, we aim to investigate the regional specific involvement of BDNF/TrkB in hippocampal-independent conditioned taste aversion (CTA) memory processes. We found region-specific changes in BDNF expression during CTA learning. CTA conditioning induced increased BDNF levels in the central nuclei of amygdala (CeA) and insular cortex, but not in the basolateral amygdala (BLA) and ventromedial prefrontal cortex. Interestingly, we found that the enhanced TrkB phosphorylation occurred at the time point before the increased BDNF expression, suggesting rapid induction of activity-dependent BDNF secretion by CTA learning. Moreover, targeted infusion of BDNF antibodies or BDNF antisense oligonucleotides revealed that activity-dependent BDNF secretion and synthesis in the CeA, but not the BLA, was respectively involved in the short- and long-term memory formation of CTA. Finally, we found that infusion of exogenous BDNF into the CeA could enhance CTA learning. These data suggest that region-specific BDNF release and synthesis temporally regulate different CTA memory phases through activation of TrkB receptors.

Hcrtr1 and 2 signaling differentially regulates depression-like behaviors

The orexin/hypocretin system has the potential to significantly modulate affect, based on both the neuroanatomical projection patterns of these neurons and on the sites of orexin receptor expression. However, there is little data supporting the role of specific orexin receptors in the modulation of depression-like behavior. Here we report behavioral profiling of mice after genetic or pharmacologic inhibition of hcrtr1 and 2 receptor signaling. Hcrtr1 null mice displayed a significant reduction in behavioral despair in the forced swim test and tail suspension test. Wild-type mice treated with the hcrtr1 antagonist SB-334867 also displayed a similar reduction in behavioral despair. No difference in anxiety-like behavior was noted following hcrtr1 deletion. In contrast, hcrtr2-null mice displayed an increase in behavioral despair with no effect on measures of anxiety. These studies suggest that the balance of orexin action at either the hcrtr1 or the hcrtr2 receptor produces an anti-depressant or pro-depressant like effect, depending on the receptor subtype activated.

The hydrophobic dipeptide Leu-Ile inhibits immobility induced by repeated forced swimming via the induction of BDNF

Depression has recently become a serious problem in society worldwide. However, we lack appropriate therapeutic tools, since the causes of depression remain unclear. Degeneration of neuronal cells and a decrease in neurogenesis have been suggested recently as two of the factors responsible for depression-like behavior. Furthermore, brain-derived neurotrophic factor (BDNF) is also suggested to be an important factor in recovering from such behavior. We have previously demonstrated that the hydrophobic dipeptide leucyl-isoleucine (Leu-Ile) induces BDNF in cultured neuronal cells. We therefore investigated possible antidepressant-like effects of Leu-Ile in an animal model using the repeated forced swim test (FST). Mice were forced to swim for 6 min once a day in a cylinder containing water. The mice were treated with Leu-Ile s.c. or p.o. immediately after each FST. Five-day repeated Leu-Ile treatment significantly increased BDNF mRNA levels and activated the BDNF/Akt/mTOR signaling pathway in the hippocampi of the mice. While 2-week repeated FST increased immobility time, Leu-Ile treatment for 2 weeks offset this increase. In C57BL/6J-BDNF heterozygous knockout (BDNF(+/-)) mice, Leu-Ile failed to reduce the immobility time increased by repeated FST. We next investigated the extent of cell proliferation in the hippocampus as 5-bromo-2'-deoxy-uridine (BrdU) uptake into hippocampal cells. Repeated FST significantly reduced the number of BrdU-positive cells in the hippocampal dentate gyrus, while this deficit was prevented by repeated Leu-Ile treatment. These results suggest that Leu-Ile has an antidepressant-like effect, at least in part by supporting cell proliferation through the BDNF signaling pathway.

Hippocampal-dependent antidepressant-like activity of histone deacetylase inhibition

Chronic social defeat stress in mice significantly decreases subsequent social interactions and induces other depression-like behaviors. Here we measured and manipulated levels of acetylated histone H3 (acH3), a chromatin mark of transcriptional activation, in the hippocampus and amygdala after ten continuous days of social defeat stress in male C57/Bl6J mice. This form of social stress causes a transient increase, followed by a persistent decrease, in the levels of acH3 in hippocampus. By comparison, increased acH3 in amygdala was more robust but also highly transient. The persistent decrease in acH3 in hippocampus may be pathological, since it is reversed by chronic fluoxetine administration. Consistent with this hypothesis, infusion of a histone deacetylase (HDAC) inhibitor MS-275 (100 μM) into hippocampus reverses a defeat-induced deficit in sucrose preference, although it does not restore social interaction behavior. Next, different forms of social enrichment were examined with or without hippocampal infusion of MS-275. After social stress, simple pair-housing with another male C57, or female C57, mouse does not reverse social avoidance. However, when HDAC inhibitors are infused into hippocampus during social housing with another male, social avoidance is attenuated. Interestingly, social avoidance is reversed when MS-275 is infused directly into amygdala. Together, these findings further support the antidepressant potential of HDAC inhibitors, and indicate that temporally overlapping environmental and molecular events are required to optimally reverse specific stress-induced behavioral symptoms.

p11 and gene therapy for severe psychiatric disorders: a practical goal?

In this issue of Science Translational Medicine, Alexander and colleagues describe coherent evidence drawn from humans and from modeled animals that supports a brain region-specific gene therapy for depression: adeno-associated virus (AAV)-mediated transfer of the gene encoding p11 to the nucleus accumbens (NAcc). The investigators found that focal NAcc knockdown of p11 expression in mice resulted in behavioral deficits related to depression and that AAV-mediated p11 gene transfer to the NAcc rescued the depression-related behavioral deficits of mice in which endogenous p11 had been genetically knocked out. They also found that p11 levels were lower in the NAcc of patients with depression than in the NAcc of matched controls. Taken together, the data suggest that gene therapies aimed at enhancing p11 in the NAcc may represent promising new approaches for treating depression; however, a large number of clinical and regulatory issues must be overcome before such therapies can be implemented.

BDNF-restricted knockout mice as an animal model for aggression

Mice with global deletion of one brain-derived neurotrophic factor (BDNF) allele or with forebrain-restricted deletion of both alleles show elevated aggression, but this phenotype is accompanied by other behavioral changes, including increases in anxiety and deficits in cognition. Here we performed behavioral characterization of conditional BDNF knockout mice generated using a Cre recombinase driver line, KA1-Cre, which expresses Cre in few areas of brain: highly at hippocampal area CA3 and moderately in dentate gyrus, cerebellum and facial nerve nucleus. The mutant animals exhibited elevated conspecific aggression and social dominance, but did not show changes in anxiety-like behaviors assessed using the elevated plus maze and open field test. There were no changes in depression-like behaviors tested in the forced swim test, but small increase in immobility in the tail suspension test. In cognitive tasks, mutants showed normal social recognition and normal spatial and fear memory, but exhibited a deficit in object recognition. Thus, this knockout can serve as a robust model for BDNF-dependent aggression and object recognition deficiency.