The brain reward circuitry in mood disorders

A review of white matter microstructure alterations of pathways of the reward circuit in depression

BACKGROUND

Depressed mood, anhedonia, psychomotor retardation and alterations of circadian rhythm are core features of the depressive syndrome. Its neural correlates can be located within a frontal-striatal-tegmental neural network, commonly referred to as the reward circuit. It is the aim of this article to review literature on white matter microstructure alterations of the reward system in depression.

METHOD

We searched for diffusion tensor imaging (DTI)-studies that have explored neural deficits within the cingulum bundle, the uncinate fasciculus and the supero-lateral medial forebrain bundle/anterior thalamic radiation - in adolescent and adult depression (acute and remitted), melancholic depression, treatment-resistant depression and those at familial risk of depression. The relevant diffusion MRI literature was identified using PUBMED.

RESULTS

Thirty-five studies were included. In people at familial risk for depression the main finding was reduced fractional anisotropy (FA) in the cingulum bundle. Both increases and decreases of FA have been reported in the uncinate fasciculus in adolescents. Reductions of FA in the uncinate fasciculus and the anterior thalamic radiation/supero-lateral medial forebrain bundle during acute depressive episodes in adults were most consistently reported.

LIMITATIONS

Non-quantitative approach.

CONCLUSIONS

Altered cingulum bundle microstructure in unaffected relatives may either indicate resilience or vulnerability to depression. Uncinate fasciculus and supero-lateral medial forebrain bundle microstructure may be altered during depressive episodes in adult MDD. Future studies call for a careful clinical stratification of clinically meaningful subgroups.

Anhedonia and the brain reward circuitry in depression

Anhedonia, or the loss of pleasure in previously rewarding stimuli, is a core symptom of major depressive disorder that may reflect an underlying dysregulation in reward processing. The mesolimbic dopamine circuit, also known as the brain's reward circuit, is integral to processing the rewarding salience of stimuli to guide actions. Manifestation of anhedonia and associated depression symptoms like feelings of sadness, changes in appetite, and psychomotor effects, may reflect changes in the brain reward circuitry as a common underlying disease process. This review will synthesize the recent literature from human and rodent studies providing a circuit-level framework for understanding anhedonia in depression, with emphasis on the nucleus accumbens.

Vicarious social defeat stress: Bridging the gap between physical and emotional stress

BACKGROUND

Animal models capable of differentiating the neurobiological intricacies between physical and emotional stress are scarce. Current models rely primarily on physical stressors (e.g., chronic unpredictable or mild stress, social defeat, learned helplessness), and neglect the impact of psychological stress alone. This is surprising given extensive evidence that a traumatic event needs not be directly experienced to produce enduring perturbations on an individual's health and psychological well-being. Post-traumatic stress disorder (PTSD), a highly debilitating neuropsychiatric disorder characterized by intense fear of trauma-related stimuli, often occurs in individuals that have only witnessed a traumatic event.

NEW METHOD

By modifying the chronic social defeat stress (CSDS) paradigm to include a witness component (witnessing the social defeat of another mouse), we demonstrate a novel behavioral paradigm capable of inducing a robust behavioral syndrome reminiscent of PTSD in emotionally stressed adult mice.

RESULTS

We describe the vicarious social defeat stress (VSDS) model that is capable of inducing a host of behavioral deficits that include social avoidance and other depressive- and anxiety-like phenotypes in adult male mice. VSDS exposure induces weight loss and spike in serum corticosterone (CORT) levels. A month after stress, these mice retain the social avoidant phenotype and have an increased CORT response when exposed to subsequent stress.

COMPARISON WITH EXISTING METHOD(S)

The VSDS is a novel paradigm capable of inducing emotional stress by isolating physical stress/confrontation in mice.

CONCLUSIONS

The VSDS model can be used to study the short- and long-term neurobiological consequences of exposure to emotional stress in mice.

The Female Sexual Response: Current Models, Neurobiological Underpinnings and Agents Currently Approved or Under Investigation for the Treatment of Hypoactive Sexual Desire Disorder

How a woman responds to sexual cues is highly dependent on a number of distinct, yet related, factors. Researchers have attempted to explain the female sexual response for decades, but no single model reigns supreme. Proper female sexual function relies on the interplay of somatic, psychosocial and neurobiological factors; misregulation of any of these components could result in sexual dysfunction. The most common sexual dysfunction disorder is hypoactive sexual desire disorder (HSDD). HSDD is a disorder affecting women across the world; a recent in-person diagnostic interview study conducted in the USA found that an estimated 7.4% of US women suffer from HSDD. Despite the disorder's prevalence, it is often overlooked as a formal diagnosis. In a survey of primary care physicians and obstetrics/gynaecology specialists, the number one reason for not assigning an HSDD diagnosis was the lack of a safe and effective therapy approved by the US Food and Drug Administration (FDA). This changed with the recent FDA approval of flibanserin (Addyi™) for the treatment of premenopausal women with acquired, generalized HSDD; there are still, however, no treatments approved outside the USA. HSDD is characterized by a marked decrease in sexual desire, an absence of motivation (also known as avolition) to engage in sexual activity, and the condition's hallmark symptom, marked patient distress. Research suggests that HSDD may arise from an imbalance of the excitatory and inhibitory neurobiological pathways that regulate the mammalian sexual response; top-down inhibition from the prefrontal cortex may be hyperactive, and/or bottom-up excitation to the limbic system may be hypoactive. Key neuromodulators for the excitatory pathways include norepinephrine, oxytocin, dopamine and melanocortins. Serotonin, opioids and endocannabinoids serve as key neuromodulators for the inhibitory pathways. Evolving treatment strategies have relied heavily on these crucial research findings, as many of the agents currently being investigated as treatment options for HSDD target and influence key players within these excitatory and inhibitory pathways, including various hormone therapies and centrally acting drugs, such as buspirone, bupropion and bremelanotide.

Differential roles of dopamine D1 and D2 receptor-containing neurons of the nucleus accumbens shell in behavioral sensitization

The nucleus accumbens (Nac) mediates the reinforcing and motor stimulating properties of psychostimulants. It receives dopaminergic afferents from the ventral midbrain and is divided into two distinct subregions: shell and core. Each of these contains two subtypes of medium spiny neurons, which express either dopamine D1 (D1R) or D2 (D2R) receptors. However, functional dissociation between the two subtypes in psychostimulant response remains to be elucidated. We performed selective ablation of each subtype in the Nac shell in mice, using immunotoxin-mediated cell targeting, and examined the behavioral sensitization evoked by repeated administration of methamphetamine. The D1R cell-ablated mice exhibited delayed induction of sensitized locomotion compared to control mice, whereas the D2R cell-ablated mice showed a mildly enhanced rate of induction of sensitization. In vivo microdialysis revealed a marked blockade of the increase in extracellular dopamine in the Nac of the D1R cell-ablated animals in response to methamphetamine, indicating that the observed delay in behavioral sensitization in these mice involves an impairment in accumbal dopamine release. Our results reveal differential roles of D1R- and D2R-containing accumbal shell neurons in the development of behavioral sensitization to psychostimulants. Behavioral sensitization, enhanced motility by repetitive psychostimulant administration, is a model of drug addiction. Here, we show that the nucleus accumbens (Nac) shell neurons containing dopamine D1 receptor (D1R) or D2 receptor (D2R) play distinct roles in behavioral sensitization triggered by methamphetamine, and that D1R-containing neurons enhance the induction of behavioral sensitization at the early phase, whereas D2R-containing neurons act to suppress the rate of development of the behavior.

Nociception, Pain, Negative Moods, and Behavior Selection

Recent neuroimaging studies suggest that the brain adapts with pain, as well as imparts risk for developing chronic pain. Within this context, we revisit the concepts for nociception, acute and chronic pain, and negative moods relative to behavior selection. We redefine nociception as the mechanism protecting the organism from injury, while acute pain as failure of avoidant behavior, and a mesolimbic threshold process that gates the transformation of nociceptive activity to conscious pain. Adaptations in this threshold process are envisioned to be critical for development of chronic pain. We deconstruct chronic pain into four distinct phases, each with specific mechanisms, and outline current state of knowledge regarding these mechanisms: the limbic brain imparting risk, and the mesolimbic learning processes reorganizing the neocortex into a chronic pain state. Moreover, pain and negative moods are envisioned as a continuum of aversive behavioral learning, which enhance survival by protecting against threats.

Mefloquine in the nucleus accumbens promotes social avoidance and anxiety-like behavior in mice

Mefloquine continues to be a key drug used for malaria chemoprophylaxis and treatment, despite reports of adverse events like depression and anxiety. It is unknown how mefloquine acts within the central nervous system to cause depression and anxiety or why some individuals are more vulnerable. We show that intraperitoneal injection of mefloquine in mice, when coupled to subthreshold social defeat stress, is sufficient to produce depression-like social avoidance behavior. Direct infusion of mefloquine into the nucleus accumbens (NAc), a key brain reward region, increased stress-induced social avoidance and anxiety behavior. In contrast, infusion into the ventral hippocampus had no effect. Whole cell recordings from NAc medium spiny neurons indicated that mefloquine application increases the frequency of spontaneous excitatory postsynaptic currents, a synaptic adaptation that we have previously shown to be associated with increased susceptibility to social defeat stress. Together, these data demonstrate a role for the NAc in mefloquine-induced depression and anxiety-like behaviors.

Genetic influences on nicotinic α5 receptor (CHRNA5) CpG methylation and mRNA expression in brain and adipose tissue

INTRODUCTION

The nicotinic α5 receptor subunit, encoded by CHRNA5, harbors multiple functional single nucleotide polymorphisms (SNPs) that affect mRNA expression and alter the encoded protein. These polymorphisms are most notably associated with drug-taking behaviors and cognition. We previously identified common SNPs in a distant regulatory element (DRE) that increase CHRNA5 mRNA expression in the human prefrontal cortex (PFC) and confer risk for nicotine dependence. Genome-wide epigenetic studies in PFC and adipose tissue find strong effects of the DRE SNPs on CpG methylation. However, it is unclear whether DRE SNPs influence CpG methylation en route to modulating CHRNA5 mRNA expression. It is also unclear whether these polymorphisms affect expression in other brain regions, especially those mediating drug-taking behaviors.

RESULTS

By measuring total and allelic CHRNA5 mRNA expression in human habenula and putamen autopsy tissues, we found that CHRNA5 DRE variants considerably increase mRNA expression by up to 3.5-fold in both brain regions. Our epigenetic analysis finds no association between CpG methylation and CHRNA5 mRNA expression in the PFC or adipose tissues.

CONCLUSIONS

These finding suggests the mechanisms responsible for the genetic modulation of CpG methylation and mRNA expression are independent despite the DRE SNPs being highly associated with both measures. Our findings support a strong association between the DRE SNPs and mRNA expression or CpG methylation in the brain and periphery, but the independence of the two measures leads us to conclude that environmental factors affecting CpG methylation do not appear to directly modulate gene expression.

ACF chromatin-remodeling complex mediates stress-induced depressive-like behavior

Improved treatment for major depressive disorder (MDD) remains elusive because of the limited understanding of its underlying biological mechanisms. It is likely that stress-induced maladaptive transcriptional regulation in limbic neural circuits contributes to the development of MDD, possibly through epigenetic factors that regulate chromatin structure. We establish that persistent upregulation of the ACF (ATP-utilizing chromatin assembly and remodeling factor) ATP-dependent chromatin-remodeling complex, occurring in the nucleus accumbens of stress-susceptible mice and depressed humans, is necessary for stress-induced depressive-like behaviors. We found that altered ACF binding after chronic stress was correlated with altered nucleosome positioning, particularly around the transcription start sites of affected genes. These alterations in ACF binding and nucleosome positioning were associated with repressed expression of genes implicated in susceptibility to stress. Together, our findings identify the ACF chromatin-remodeling complex as a critical component in the development of susceptibility to depression and in regulating stress-related behaviors.

Elevated striatal reactivity across monetary and social rewards in bipolar I disorder

Bipolar disorder (BD) is associated with increased reactivity to rewards and heightened positive affectivity. It is less clear to what extent this heightened reward sensitivity is evident across contexts and what the associated neural mechanisms might be. The present investigation used both a monetary and social incentive delay task among adults with remitted BD Type I (n = 24) and a healthy nonpsychiatric control group (HC; n = 25) using fMRI. Both whole-brain and region-of-interest analyses revealed elevated reactivity to reward receipt in the striatum, a region implicated in incentive sensitivity, in the BD group. Post hoc analyses revealed that greater striatal reactivity to reward receipt, across monetary and social reward tasks, predicted decreased self-reported positive affect when anticipating subsequent rewards in the HC but not in the BD group. Results point toward elevated striatal reactivity to reward receipt as a potential neural mechanism of persistent reward pursuit in BD.

Probing the stress and depression circuits with a disease gene

Selectively deleting a gene that has been linked to depression from specific neurons in mice sheds new light on a neural circuit that controls stress-induced depressive behaviors.

Layer 2/3 pyramidal cells in the medial prefrontal cortex moderate stress induced depressive behaviors

Major depressive disorder (MDD) is a prevalent illness that can be precipitated by acute or chronic stress. Studies of patients with Wolfram syndrome and carriers have identified Wfs1 mutations as causative for MDD. The medial prefrontal cortex (mPFC) is known to be involved in depression and behavioral resilience, although the cell types and circuits in the mPFC that moderate depressive behaviors in response to stress have not been determined. Here, we report that deletion of Wfs1 from layer 2/3 pyramidal cells impairs the ability of the mPFC to suppress stress-induced depressive behaviors, and results in hyperactivation of the hypothalamic–pituitary–adrenal axis and altered accumulation of important growth and neurotrophic factors. Our data identify superficial layer 2/3 pyramidal cells as critical for moderation of stress in the context of depressive behaviors and suggest that dysfunction in these cells may contribute to the clinical relationship between stress and depression.

DOI:http://dx.doi.org/10.7554/eLife.08752.001

Around 16% of people will experience an episode of major depression at some point in their lives, with symptoms including a loss of motivation, a reduced enjoyment of previously pleasurable activities, and disturbances in sleep and appetite. Multiple genes and environmental factors have been implicated in depression, and one of the strongest risk factors for developing the disorder is exposure to stress.

Stress and depression affect many of the same brain regions, most notably the prefrontal cortex—an area that is involved in decision making, problem solving and regulating emotions. Shrestha et al. therefore reasoned that a good way of obtaining insights into the relationship between stress and depression would be to study prefrontal cortex cells that express genes that have been linked to depression.

One such gene is Wfs1. Mutations in this gene cause a rare disorder called Wolfram syndrome, in which affected individuals experience a wide range of symptoms that often include severe depression. Shrestha et al. identified a specific population of cells in the prefrontal cortex that express Wfs1. When subjected to a stressful event, such as being restrained, mice that had been genetically modified to lack this gene in their prefrontal cortex were more likely to exhibit depression-like behaviors than non-modified mice. The genetically modified mice also released more stress hormones when restrained and produced different amounts of a number of proteins that regulate the growth and signaling of neurons.

Shrestha et al. propose that these proteins act on neural circuits that control how the mice respond to stress. Furthermore, changes in the levels or the distribution of these proteins may increase the likelihood that a stressful event will trigger behaviors associated with depression. Further experiments are required to investigate the possibility that using drugs to manipulate cells that express Wfs1 could protect against the harmful effects of stress, or even treat existing episodes of depression.

DOI:http://dx.doi.org/10.7554/eLife.08752.002

Neural reactivity to rewards and losses in offspring of mothers and fathers with histories of depressive and anxiety disorders

Depression appears to be characterized by reduced neural reactivity to receipt of reward. Despite evidence of shared etiologies and high rates of comorbidity between depression and anxiety, this abnormality may be relatively specific to depression. However, it is unclear whether children at risk for depression also exhibit abnormal reward responding, and if so, whether risk for anxiety moderates this association. The feedback negativity (FN) is an event-related potential component sensitive to receipt of rewards versus losses that is reduced in depression. Using a large community sample (N = 407) of 9-year-old children who had never experienced a depressive episode, we examined whether histories of depression and anxiety in their parents were associated with the FN following monetary rewards and losses. Results indicated that maternal history of depression was associated with a blunted FN in offspring, but only when there was no maternal history of anxiety. In addition, greater severity of maternal depression was associated with greater blunting of the FN in children. No effects of paternal psychopathology were observed. Results suggest that blunted reactivity to rewards versus losses may be a vulnerability marker that is specific to pure depression, but is not evident when there is also familial risk for anxiety. In addition, these findings suggest that abnormal reward responding is evident as early as middle childhood, several years prior to the sharp increase in the prevalence of depression and rapid changes in neural reward circuitry in adolescence.

Reward function: a promising but (still) underexamined dimension in developmental psychopathology

The goal of this Special Section is to explore the ways that investigation of reward function can shed light on the development and pathophysiology of psychopathology. Reward function provides a promising starting point for clinical affective neuroscience research because, thanks to the extensive literature on the neural mechanisms of addiction, the functional neuroanatomy, cellular mechanisms, and genetic contributions to reward circuitry have been well delineated (see Russo & Nestler, 2013, for details).

Doping the Mind: Dopaminergic Modulation of Prefrontal Cortical Cognition

The prefrontal cortex is the center of cognitive control. Processing in prefrontal cortical circuits enables us to direct attention to behaviorally relevant events; to memorize, structure, and categorize information; and to learn new concepts. The prefrontal cortex receives strong projections from midbrain neurons that use dopamine as a transmitter. In this article, we review the crucial role dopamine plays as a modulator of prefrontal cognitive functions, in the primate brain in particular. Following a summary of the anatomy and physiology of the midbrain dopamine system, we focus on recent studies that investigated dopaminergic effects in prefrontal cortex at the cellular level. We then discuss how unregulated prefrontal dopamine signaling could contribute to major disorders of cognition. The studies highlighted in this review demonstrate the powerful influence dopamine exerts on the mind.

Neural correlates of successful psychotherapy of depression in adolescents

BACKGROUND

While major effort has been put in investigating neural correlates of depression and its treatment in adults, less is known about the effects of psychotherapy in adolescents. Given the concordance of the ventral striatum, amygdala, hippocampus and the subgenual anterior cingulate cortex (sgACC) as correlates of depression and their involvement in reward processing, we used functional magnetic resonance imaging (fMRI) during performance of a monetary reward task in an intervention versus waitlist-control design to investigate the clinical and neural effects of cognitive behavioral group therapy (CBT-G).

METHODS

22 medication naïve adolescents with major depressive disorder were scanned before and after five sessions of CBT-G (PAT-I), or before and after five weeks of waiting (PAT-W). Changes in symptom scales were analyzed along with neural activation changes within the amygdala, hippocampus, sgACC and ventral striatum regions of interest (ROI).

RESULTS

Psychometric assessments and ROI activation remained unchanged in PAT-W. In PAT-I, significant reduction in clinical symptoms accompanied significant changes in brain activation within the left amygdala, left hippocampus and bilateral sgACC. In line with previous findings in adults, pre-to-post-activation changes in the bilateral sgACC correlated with pre-to-post and pre-to-follow-up symptom improvement, and individual expressions of sgACC activation before treatment were related to pre-to-follow-up therapeutic success.

LIMITATIONS

Future studies should include larger sample sizes.

CONCLUSIONS

Successful group psychotherapy of depression in adolescents was related to signal changes in brain regions previously demonstrated to be reliably linked with successful, particularly pharmacological treatment in adults.

Role of the Brain's Reward Circuitry in Depression: Transcriptional Mechanisms

Increasing evidence supports an important role for the brain's reward circuitry in controlling mood under normal conditions and contributing importantly to the pathophysiology and symptomatology of a range of mood disorders, such as depression. Here we focus on the nucleus accumbens (NAc), a critical component of the brain's reward circuitry, in depression and other stress-related disorders. The prominence of anhedonia, reduced motivation, and decreased energy level in most individuals with depression supports the involvement of the NAc in these conditions. We concentrate on several transcription factors (CREB, ΔFosB, SRF, NFκB, and β-catenin), which are altered in the NAc in rodent depression models--and in some cases in the NAc of depressed humans, and which produce robust depression- or antidepressant-like effects when manipulated in the NAc in animal models. These studies of the NAc have established novel approaches toward modeling key symptoms of depression in animals and could enable the development of antidepressant medications with fundamentally new mechanisms of action.

Microglia disrupt mesolimbic reward circuitry in chronic pain

Chronic pain attenuates midbrain dopamine (DA) transmission, as evidenced by a decrease in opioid-evoked DA release in the ventral striatum, suggesting that the occurrence of chronic pain impairs reward-related behaviors. However, mechanisms by which pain modifies DA transmission remain elusive. Using in vivo microdialysis and microinjection of drugs into the mesolimbic DA system, we demonstrate in mice and rats that microglial activation in the VTA compromises not only opioid-evoked release of DA, but also other DA-stimulating drugs, such as cocaine. Our data show that loss of stimulated extracellular DA is due to impaired chloride homeostasis in midbrain GABAergic interneurons. Treatment with minocycline or interfering with BDNF signaling restored chloride transport within these neurons and recovered DA-dependent reward behavior. Our findings demonstrate that a peripheral nerve injury causes activated microglia within reward circuitry that result in disruption of dopaminergic signaling and reward behavior. These results have broad implications that are not restricted to the problem of pain, but are also relevant to affective disorders associated with disruption of reward circuitry. Because chronic pain causes glial activation in areas of the CNS important for mood and affect, our findings may translate to other disorders, including anxiety and depression, that demonstrate high comorbidity with chronic pain.

Social defeat-induced anhedonia: effects on operant sucrose-seeking behavior

Reduced capacity to experience pleasure, also known as anhedonia, is a key feature of the depressive state and is associated with poor disease prognosis and treatment outcome. Various behavioral readouts (e.g., reduced sucrose intake) have been employed in animal models of depression as a measure of anhedonia. However, several aspects of anhedonia are poorly represented within the repertoire of current preclinical assessments. We recently adopted the social defeat-induced persistent stress (SDPS) paradigm that models a maintained depressive-like state in the rat, including social withdrawal and deficits in short-term spatial memory. Here we investigated whether SDPS elicited persistent deficits in natural reward evaluation, as part of anhedonia. We examined cue-paired operant sucrose self-administration, enabling us to study acquisition, motivation, extinction, and relapse to sucrose seeking following SDPS. Furthermore, we addressed whether guanfacine, an α2-adrenergic agonist that reduces stress-triggered maladaptive behavioral responses to drugs of abuse, could relief from SDPS-induced anhedonia. SDPS, consisting of five social defeat episodes followed by prolonged (≥8 weeks) social isolation, did not affect sucrose consumption during acquisition of self-administration. However, it strongly enhanced the motivational drive to acquire a sucrose reward in progressive ratio training. Moreover, SDPS induced initial resilience to extinction and rendered animals more sensitive to cue-induced reinstatement of sucrose-seeking. Guanfacine treatment attenuated SDPS-induced motivational overdrive and limited reinstatement of sucrose seeking, normalizing behavior to control levels. Together, our data indicate that long after the termination of stress exposure, SDPS induces guanfacine-reversible deficits in evaluation of a natural reward. Importantly, the SDPS-triggered anhedonia reflects many aspects of the human phenotype, including impaired motivation and goal-directed conduct.

Chronic deep brain stimulation of the medial forebrain bundle reverses depressive-like behavior in a hemiparkinsonian rodent model

Preclinical and clinical evidence suggests that depression might be associated with a dysfunction in the reward/motivation circuitry. Deep brain stimulation (DBS) of the superolateral branch of the medial forebrain bundle (MFB) has been shown in a recent clinical trial to provide a prompt and consistent improvement of depressive symptoms in treatment-resistant patients. In order to better understand the underlying mechanisms of neuromodulation in the context of depression, the effects of chronic bilateral MFB-DBS were assessed in a combined rodent model of depression and Parkinson’s disease. Female Sprague-Dawley rats received unilateral 6-OHDA injection in the right MFB and were divided into three groups: CMS-STIM, CMS-noSTIM and control group. The CMS groups were submitted to chronic unpredictable mild stress (CMS) protocol for 6 weeks. MFB-DBS was applied only to the CMS-STIM group for 1 week. All groups were repeatedly probed on a series of behavioral tasks following each intervention, and to a postmortem histological analysis. CMS led to an increase in immobility in the forced swim test, to a decrease in sucrose solution consumption in the sucrose preference test, as well as to an increased production of ultrasonic vocalizations in the 22 kHz range, indicating increased negative affect. MFB-DBS reversed the anhedonic-like and despair-like behaviors. The results suggest that unilateral dopamine depletion did not preclude MFB-DBS in reversing depressive-like and anhedonic-like behavior in the rodent. Further understanding of the importance of hemispheric dominance in neuropsychiatric disorders is essential in order to optimize stimulation as a therapeutic strategy in these diseases.

Corticosterone mediates the synaptic and behavioral effects of chronic stress at rat hippocampal temporoammonic synapses

Chronic stress is thought to impart risk for depression via alterations in brain structure and function, but contributions of specific mediators in generating these changes remain unclear. We test the hypothesis that stress-induced increases in corticosterone (CORT), the primary rodent glucocorticoid, are the key mediator of stress-induced depressive-like behavioral changes and synaptic dysfunction in the rat hippocampus. In rats, we correlated changes in cognitive and affective behavioral tasks (spatial memory consolidation, anhedonia, and neohypophagia) with impaired excitatory strength at temporoammonic-CA1 (TA-CA1) synapses, an archetypical stress-sensitive excitatory synapse. We tested whether elevated CORT was sufficient and necessary to generate a depressive-like behavioral phenotype and decreased excitatory signaling observed at TA-CA1 after chronic unpredictable stress (CUS). Chronic CORT administration induced an anhedonia-like behavioral state and neohypophagic behavior. Like CUS, chronic, but not acute, CORT generated an impaired synaptic phenotype characterized by reduced α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-preferring glutamate receptor-mediated excitation at TA-CA1 synapses, decreased AMPA-type glutamate receptor subunit 1 protein expression, and altered serotonin-1B receptor-mediated potentiation. Repeatedly blunting stress-induced increases of CORT during CUS with the CORT synthesis inhibitor metyrapone (MET) prevented these stress-induced neurobehavioral changes. MET also prevented the CUS-induced impairment of spatial memory consolidation. We conclude that corticosterone is sufficient and necessary to mediate glutamatergic dysfunction underlying stress-induced synaptic and behavioral phenotypes. Our results indicate that chronic excessive glucocorticoids cause specific synaptic deficits in the hippocampus, a major center for cognitive and emotional processing, that accompany stress-induced behavioral dysfunction. Maintaining excitatory strength at stress-sensitive synapses at key loci throughout corticomesolimbic reward circuitry appears critical for maintaining normal cognitive and emotional behavior.

Activating positive memory engrams suppresses depression-like behaviour

Stress is considered a potent environmental risk factor for many behavioural abnormalities, including anxiety and mood disorders. Animal models can exhibit limited but quantifiable behavioural impairments resulting from chronic stress, including deficits in motivation, abnormal responses to behavioural challenges, and anhedonia. The hippocampus is thought to negatively regulate the stress response and to mediate various cognitive and mnemonic aspects of stress-induced impairments, although the neuronal underpinnings sufficient to support behavioural improvements are largely unknown. Here we acutely rescue stress-induced depression-related behaviours in mice by optogenetically reactivating dentate gyrus cells that were previously active during a positive experience. A brain-wide histological investigation, coupled with pharmacological and projection-specific optogenetic blockade experiments, identified glutamatergic activity in the hippocampus-amygdala-nucleus-accumbens pathway as a candidate circuit supporting the acute rescue. Finally, chronically reactivating hippocampal cells associated with a positive memory resulted in the rescue of stress-induced behavioural impairments and neurogenesis at time points beyond the light stimulation. Together, our data suggest that activating positive memories artificially is sufficient to suppress depression-like behaviours and point to dentate gyrus engram cells as potential therapeutic nodes for intervening with maladaptive behavioural states.

Anhedonia and the brain reward circuitry in depression

Anhedonia, or the loss of pleasure in previously rewarding stimuli, is a core symptom of major depressive disorder that may reflect an underlying dysregulation in reward processing. The mesolimbic dopamine circuit, also known as the brain's reward circuit, is integral to processing the rewarding salience of stimuli to guide actions. Manifestation of anhedonia and associated depression symptoms like feelings of sadness, changes in appetite, and psychomotor effects, may reflect changes in the brain reward circuitry as a common underlying disease process. This review will synthesize the recent literature from human and rodent studies providing a circuit-level framework for understanding anhedonia in depression, with emphasis on the nucleus accumbens.

Histamine H3 receptor activation inhibits dopamine synthesis but not release or uptake in rat nucleus accumbens

We studied the effect of activating histamine H3 receptors (H3Rs) on rat nucleus accumbens (rNAcc) dopaminergic transmission by analyzing [(3)H]-dopamine uptake by synaptosomes, and dopamine synthesis and depolarization-evoked [(3)H]-dopamine release in slices. The uptake of [(3)H]-dopamine by rNAcc synaptosomes was not affected by the H3R agonist RAMH (10(-10)-10(-6) M). In rNAcc slices perfusion with RAMH (1 μM) had no significant effect on [(3)H]-dopamine release evoked by depolarization with 30 mM K(+) (91.4 ± 4.5% of controls). The blockade of dopamine D2 autoreceptors with sulpiride (1 μM) enhanced K(+)-evoked [(3)H]-dopamine release (168.8 ± 15.5% of controls), but under this condition RAMH (1 μM) also failed to affect [(3)H]-dopamine release. Dopamine synthesis was evaluated in rNAcc slices incubated with the l-dihydroxyphenylalanine (DOPA) decarboxylase inhibitor NSD-1015 (1 mM). Forskolin-induced DOPA accumulation (220.1 ± 10.4% of controls) was significantly reduced by RAMH (41.1 ± 6.5% and 43.5 ± 9.1% inhibition at 100 nM and 1 μM, respectively), and this effect was prevented by the H3R antagonist ciproxifan (10 μM). DOPA accumulation induced by preventing cAMP degradation with IBMX (iso-butyl-methylxantine, 1 mM) or by activating receptors for the vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase-activating peptide (PACAP) with PACAP-27 (1 μM) was reduced (IBMX) or prevented (PACAP-27) by RAMH (100 nM). In contrast, DOPA accumulation induced by 8-Bromo-cAMP (1 mM) was not affected by RAMH (100 nM). These results indicate that in rNAcc H3Rs do not modulate dopamine uptake or release, but regulate dopamine synthesis by inhibiting cAMP formation and thus PKA activation. This article is part of the Special Issue entitled 'Histamine Receptors'.

Excitatory transmission at thalamo-striatal synapses mediates susceptibility to social stress

Postsynaptic remodeling of glutamatergic synapses on ventral striatum (vSTR) medium spiny neurons (MSNs) is critical for shaping stress responses. However, it is unclear which presynaptic inputs are involved. Susceptible mice exhibited increased synaptic strength at intralaminar thalamus (ILT), but not prefrontal cortex (PFC), inputs to vSTR MSNs following chronic social stress. Modulation of ILT-vSTR versus PFC-vSTR neuronal activity differentially regulated dendritic spine plasticity and social avoidance.

Stress and the social brain: behavioural effects and neurobiological mechanisms

Stress often affects our social lives. When undergoing high-level or persistent stress, individuals frequently retract from social interactions and become irritable and hostile. Predisposition to antisocial behaviours - including social detachment and violence - is also modulated by early life adversity; however, the effects of early life stress depend on the timing of exposure and genetic factors. Research in animals and humans has revealed some of the structural, functional and molecular changes in the brain that underlie the effects of stress on social behaviour. Findings in this emerging field will have implications both for the clinic and for society.

Activation of corticostriatal circuitry relieves chronic neuropathic pain

Neural circuits that determine the perception and modulation of pain remain poorly understood. The prefrontal cortex (PFC) provides top-down control of sensory and affective processes. While animal and human imaging studies have shown that the PFC is involved in pain regulation, its exact role in pain states remains incompletely understood. A key output target for the PFC is the nucleus accumbens (NAc), an important component of the reward circuitry. Interestingly, recent human imaging studies suggest that the projection from the PFC to the NAc is altered in chronic pain. The function of this corticostriatal projection in pain states, however, is not known. Here we show that optogenetic activation of the PFC produces strong antinociceptive effects in a rat model (spared nerve injury model) of persistent neuropathic pain. PFC activation also reduces the affective symptoms of pain. Furthermore, we show that this pain-relieving function of the PFC is likely mediated by projections to the NAc. Thus, our results support a novel role for corticostriatal circuitry in pain regulation.

[Depression and addiction comorbidity: towards a common molecular target?]

The comorbidity of depression and cocaine addiction suggests shared mechanisms and anatomical pathways. Specifically, the limbic structures, such as the nucleus accumbens (NAc), play a crucial role in both disorders. P11 (S100A10) is a promising target for manipulating depression and addiction in mice. We summarized the recent genetic and viral strategies used to determine how the titration of p11 levels within the NAc affects hedonic behavior and cocaine reward learning in mice. In particular, p11 in the ChAT+ cells or DRD1+ MSN of the NAc, controls depressive-like behavior or cocaine reward, respectively. Treatments to counter maladaptation of p11 levels in the NAc could provide novel therapeutic opportunities for depression and cocaine addiction in humans.

Optogenetic stimulation of infralimbic PFC reproduces ketamine's rapid and sustained antidepressant actions

Ketamine produces rapid and sustained antidepressant actions in depressed patients, but the precise cellular mechanisms underlying these effects have not been identified. Here we determined if modulation of neuronal activity in the infralimbic prefrontal cortex (IL-PFC) underlies the antidepressant and anxiolytic actions of ketamine. We found that neuronal inactivation of the IL-PFC completely blocked the antidepressant and anxiolytic effects of systemic ketamine in rodent models and that ketamine microinfusion into IL-PFC reproduced these behavioral actions of systemic ketamine. We also found that optogenetic stimulation of the IL-PFC produced rapid and long-lasting antidepressant and anxiolytic effects and that these effects are associated with increased number and function of spine synapses of layer V pyramidal neurons. The results demonstrate that ketamine infusions or optogenetic stimulation of IL-PFC are sufficient to produce long-lasting antidepressant behavioral and synaptic responses similar to the effects of systemic ketamine administration.

Gene network analysis shows immune-signaling and ERK1/2 as novel genetic markers for multiple addiction phenotypes: alcohol, smoking and opioid addiction

Background

Addictions to alcohol and tobacco, known risk factors for cancer, are complex heritable disorders. Addictive behaviors have a bidirectional relationship with pain. We hypothesize that the associations between alcohol, smoking, and opioid addiction observed in cancer patients have a genetic basis. Therefore, using bioinformatics tools, we explored the underlying genetic basis and identified new candidate genes and common biological pathways for smoking, alcohol, and opioid addiction.

Results

Literature search showed 56 genes associated with alcohol, smoking and opioid addiction. Using Core Analysis function in Ingenuity Pathway Analysis software, we found that ERK1/2 was strongly interconnected across all three addiction networks. Genes involved in immune signaling pathways were shown across all three networks. Connect function from IPA My Pathway toolbox showed that DRD2 is the gene common to both the list of genetic variations associated with all three addiction phenotypes and the components of the brain neuronal signaling network involved in substance addiction. The top canonical pathways associated with the 56 genes were: 1) calcium signaling, 2) GPCR signaling, 3) cAMP-mediated signaling, 4) GABA receptor signaling, and 5) G-alpha i signaling.

Conlusions

Cancer patients are often prescribed opioids for cancer pain thus increasing their risk for opioid abuse and addiction. Our findings provide candidate genes and biological pathways underlying addiction phenotypes, which may be future targets for treatment of addiction. Further study of the variations of the candidate genes could allow physicians to make more informed decisions when treating cancer pain with opioid analgesics.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-015-0167-x) contains supplementary material, which is available to authorized users.

Retinal dysfunction of contrast processing in major depression also apparent in cortical activity

Depressive disorder is often associated with the subjective experience of altered visual perception. Recent research has produced growing evidence for involvement of the visual system in the pathophysiology of depressive disorder. Using the pattern electroretinogram (PERG), we found reduced retinal contrast response in patients with major depression. Based on this observation, the question arises whether this change has a cortical correlate. To evaluate this, we analyzed the visual evoked potential (VEP) of the occipital cortex in 40 patients with depressive disorder and 28 healthy controls. As visual stimuli, checkerboard stimuli of 0.51° check size, 12.5 reversals per second and a contrast of 3-80% was used. In addition to the PERG, we recorded the VEP with an Oz versus FPz derivation. The amplitude versus contrast transfer function was compared across the two groups and correlated with the severity of depression, as measured by the Hamilton Depression Rating Scale and the Beck Depression Inventory. Patients with major depression displayed significantly reduced VEP amplitudes at all contrast levels compared to control subjects (p = 0.029). The VEP amplitude correlated with psychometric measures for severity of depression. The degree of depression reduced the contrast transfer function in the VEP to a lesser extent than in the PERG: While the PERG is reduced to ≈50%, the VEP is reduced to 75%. Our results suggest that depression affects the cortical response in major depression, but less so than the retinal responses. Modified contrast adaptation in the lateral geniculate nucleus or cortex possibly moderates the increased losses in the retina.

Pathogenesis of depression: Insights from human and rodent studies

Major depressive disorder (MDD) will affect one out of every five people in their lifetime and is the leading cause of disability worldwide. Nevertheless, mechanisms associated with the pathogenesis of MDD have yet to be completely understood and current treatments remain ineffective in a large subset of patients. In this review, we summarize the most recent discoveries and insights for which parallel findings have been obtained in human depressed subjects and rodent models of mood disorders in order to examine the potential etiology of depression. These mechanisms range from synaptic plasticity mechanisms to epigenetics and the immune system where there is strong evidence to support a functional role in the development of specific depression symptomology. Ultimately we conclude by discussing how novel therapeutic strategies targeting central and peripheral processes might ultimately aid in the development of effective new treatments for MDD and related stress disorders.

The Female Encounter Test: A Novel Method for Evaluating Reward-Seeking Behavior or Motivation in Mice

BACKGROUND

Reduced motivation is an important marker of psychiatric disorders, including depression. We describe the female encounter test, a novel method of evaluating reward-seeking behavior in mice.

METHODS

The test apparatus consists of three open chambers, formed with partitions that allow the animal to move freely from one chamber to another. A test male mouse is habituated in the apparatus, and subsequently a female and male mouse are introduced into a wire-mesh box in the left and right chamber, respectively. The time the test male mouse spends in the female or male area is measured for 10 min.

RESULTS

All six strains of mice tested showed a significant preference for female encounters. The preference was observed in 7-30-week-old mice. The preference was blocked by castration of the resident male test mouse, and was not affected by the phase of the menstrual cycle of the female intruder. The preference was impaired in mouse models of depression, including social isolation-reared, corticosterone-treated, and lipopolysaccharide-treated mice. The impairment was alleviated by fluvoxamine in isolation-reared and lipopolysaccharide-treated mice, and it was improved by the metabotropic glutamate 2/3 receptor antagonist LY341495 in corticosterone-treated mice. Encounter with a female, but not male, mouse increased c-Fos expression in the nucleus accumbens shell of test male mice. Furthermore, both the preference and encounter-induced increases in c-Fos expression were blocked by dopamine D1 and D2 receptor antagonists.

CONCLUSIONS

These findings indicate that motivation in adult male mice can be easily evaluated by quantitating female encounters.

Visible rodent brain-wide networks at single-neuron resolution

There are some unsolvable fundamental questions, such as cell type classification, neural circuit tracing and neurovascular coupling, though great progresses are being made in neuroscience. Because of the structural features of neurons and neural circuits, the solution of these questions needs us to break through the current technology of neuroanatomy for acquiring the exactly fine morphology of neuron and vessels and tracing long-distant circuit at axonal resolution in the whole brain of mammals. Combined with fast-developing labeling techniques, efficient whole-brain optical imaging technology emerging at the right moment presents a huge potential in the structure and function research of specific-function neuron and neural circuit. In this review, we summarize brain-wide optical tomography techniques, review the progress on visible brain neuronal/vascular networks benefit from these novel techniques, and prospect the future technical development.

Long-range projection neurons of the mouse ventral tegmental area: a single-cell axon tracing analysis

Pathways arising from the ventral tegmental area (VTA) release dopamine and other neurotransmitters during the expectation and achievement of reward, and are regarded as central links of the brain networks that create drive, pleasure, and addiction. While the global pattern of VTA projections is well-known, the actual axonal wiring of individual VTA neurons had never been investigated. Here, we labeled and analyzed the axons of 30 VTA single neurons by means of single-cell transfection with the Sindbis-pal-eGFP vector in mice. These observations were complemented with those obtained by labeling the axons of small populations of VTA cells with iontophoretic microdeposits of biotinylated dextran amine. In the single-cell labeling experiments, each entire axonal tree was reconstructed from serial sections, the length of terminal axonal arbors was estimated by stereology, and the dopaminergic phenotype was tested by double-labeling for tyrosine hydroxylase immunofluorescence. We observed two main, markedly different VTA cell morphologies: neurons with a single main axon targeting only forebrain structures (FPN cells), and neurons with multibranched axons targeting both the forebrain and the brainstem (F + BSPN cells). Dopaminergic phenotype was observed in FPN cells. Moreover, four “subtypes” could be distinguished among the FPN cells based on their projection targets: (1) “Mesocorticolimbic” FPN projecting to both neocortex and basal forebrain; (2) “Mesocortical” FPN innervating the neocortex almost exclusively; (3) “Mesolimbic” FPN projecting to the basal forebrain, accumbens and caudateputamen; and (4) “Mesostriatal” FPN targeting only the caudateputamen. While the F + BSPN cells were scattered within VTA, the mesolimbic neurons were abundant in the paranigral nucleus. The observed diversity in wiring architectures is consistent with the notion that different VTA cell subpopulations modulate the activity of specific sets of prosencephalic and brainstem structures.

Pre-stress performance in an instrumental training predicts post-stress behavioral alterations in chronically stressed rats

Stress is a major factor in the development of major depressive disorder (MDD), but few studies have assessed individual risk based on pre-stress behavioral and cognitive traits. To address this issue, we employed appetitive instrumental lever pressing with a progressive ratio (PR) schedule to assess these traits in experimentally naïve Sprague-Dawley rats. Based on four distinct traits that were identified by hierarchical cluster analysis, the animals were classified into the corresponding four subgroups (Low Motivation, Quick Learner, Slow Learner, and Hypermotivation), and exposed to chronic unpredictable stress (CUS) before monitoring their post-stress responses for 4 weeks. The four subgroups represented the following distinct behavioral phenotypes after CUS: the Low Motivation subgroup demonstrated weight loss and a late-developing paradoxical enhancement in PR performance that may be related to inappropriate decision-making in human MDD. The Quick Learner subgroup exhibited a transient loss of motivation and the habituation of serum corticosterone (CORT) response to repeated stress. The Slow Learner subgroup displayed resistance to demotivation and a suppressed CORT response to acute stress. Finally, the Hypermotivation subgroup exhibited resistance to weight loss, habituated CORT response to an acute stress, and a long-lasting amotivation. Overall, we identified causal relationships between pre-stress traits in the performance of the instrumental training and post-stress phenotypes in each subgroup. In addition, many of the CUS-induced phenotypes in rats corresponded to or had putative relationships with representative symptoms in human MDD. We concluded that the consequences of stress may be predictable before stress exposure by determining the pre-stress behavioral or cognitive traits of each individual in rats.

Reinforcement learning in depression: A review of computational research

Despite being considered primarily a mood disorder, major depressive disorder (MDD) is characterized by cognitive and decision making deficits. Recent research has employed computational models of reinforcement learning (RL) to address these deficits. The computational approach has the advantage in making explicit predictions about learning and behavior, specifying the process parameters of RL, differentiating between model-free and model-based RL, and the computational model-based functional magnetic resonance imaging and electroencephalography. With these merits there has been an emerging field of computational psychiatry and here we review specific studies that focused on MDD. Considerable evidence suggests that MDD is associated with impaired brain signals of reward prediction error and expected value ('wanting'), decreased reward sensitivity ('liking') and/or learning (be it model-free or model-based), etc., although the causality remains unclear. These parameters may serve as valuable intermediate phenotypes of MDD, linking general clinical symptoms to underlying molecular dysfunctions. We believe future computational research at clinical, systems, and cellular/molecular/genetic levels will propel us toward a better understanding of the disease.

Antidepressant drug development: Focus on triple monoamine reuptake inhibition

Many patients with major depressive disorder (MDD) only partially respond, and some have no clinically meaningful response, to current widely used antidepressant drugs. Due to the purported role of dopamine in the pathophysiology of depression, triple-reuptake inhibitors (TRIs) that simultaneously inhibit serotonin (5-HT), norepinephrine (NE) and dopamine reuptake could be a useful addition to the armamentarium of treatments for MDD. A TRI should more effectively activate mesolimbic dopamine-related reward-networks, restore positive mood and reduce potent 5-HT reuptake blockade associated "hypodopaminergic" adverse effects of decreased libido, weight gain and "blunting" of emotions. On the other hand, dopaminergic effects raise concern over abuse liability and TRIs may have many of the cardiovascular effects associated with NET inhibition. Several clinical development programs for potential TRI antidepressants have failed to demonstrate significantly greater efficacy than placebo or standard of care. Successful late-stage clinical development of a TRI is more likely if experimental research studies in the target population of depressed patients have demonstrated target engagement that differentially and dose-dependently improves assessments of reward-network dysfunction relative to existing antidepressants. TRI treatment could be individualized on the basis of predictive markers such as the burden of decreased positive mood symptoms and/or neuroimaging evidence of reward network dysfunction. This review focuses on how the next generation of monoamine-based treatments could be efficiently developed to address unmet medical need in MDD.

Comparing engage with PST in late-life major depression: a preliminary report

OBJECTIVE

The complexity of psychotherapies has been a barrier to community implementation. We used the Research Domain Criteria consensus as a guide to develop Engage, a streamlined, neurobiology-based psychotherapy for late-life depression that may match the skill set of practicing clinicians. This proof of concept study tested the hypotheses that Engage is bioequivalent to Problem Solving Therapy (PST) in reducing depressive symptoms, inducing remission, and ameliorating disability.

METHODS

Engage assumes that abnormal function of the positive valence systems fuels depression and uses "reward exposure" (engagement in meaningful, rewarding activities) as its principal intervention. Negativity bias, apathy, and emotional dysregulation are expressions of abnormalities in the negative valence, arousal and regulatory, and cognitive control systems, respectively. Engage targets each of them with simple interventions only if they interfere with reward exposure. We treated openly, with 9 weekly sessions of Engage, 39 older adults with unipolar major depression. We compared their course of depression (HAM-D), remission rate (HAM-D<10), and disability (WHODAS) with those of a historical comparison group (N = 97) treated with 9 weekly sessions of PST.

RESULTS

Community social workers and research therapists required one third as much training time in Engage as in PST. Engage was non-inferior to PST in reducing HAM-D and WHODAS. Remission rates for Engage at 6 and 9 weeks were 18.2% and 41.1%, respectively. The corresponding figures for PST were 13.7% and 35.0%, respectively.

CONCLUSION

These initial observations suggest that Engage has comparable efficacy with PST in reducing depressive symptoms and disability and warrants a randomized controlled trial.