Role of the Brain's Reward Circuitry in Depression

Brain-derived neurotrophic factor-mediated projection-specific regulation of depressive-like and nociceptive behaviors in the mesolimbic reward circuitry

Increasing evidence suggests that the mesolimbic reward system plays critical roles in the regulation of depression and nociception; however, its circuitry and cellular mechanisms remain unclear. In this study, we investigated the output-specific regulatory roles of dopaminergic (DA) neurons within the ventral tegmental area (VTA) in depressive-like and nociceptive behaviors in mice subjected to unpredictable chronic mild stress (CMS), using the projection-specific electrophysiological recording, pharmacological manipulation, behavioral test, and molecular biology technologies. We demonstrated that CMS decreased the firing activity in VTA projecting to medial prefrontal cortex (VTA → mPFC), but not in VTA to nucleus accumbens (VTA → NAc), DA neurons. However, both VTA → mPFC and VTA → NAc DA neurons showed increased firing activity in response to morphine perfusion in CMS mice. Behavioral results showed that intra-VTA microinjection of morphine (25.5 ng/0.15 μL) relieved depressive-like behaviors, intriguingly, accompanied by a thermal hyperalgesia. Furthermore, the relief of depressive-like behaviors induced by intra-VTA injection of morphine in CMS mice could be prevented by blocking brain-derived neurotrophic factor (BDNF) signaling and mimicked by the administration of exogenous BDNF in mPFC rather than in NAc shell. Nociceptive responses induced by the activation of VTA DA neurons with morphine in CMS mice could be prevented by blocking BDNF signaling or mimicked by administration of exogenous BDNF in NAc shell, but not in mPFC. These results reveal projection-specific regulatory mechanisms of depression and nociception in the mesolimbic reward circuitry and provide new insights into the neural circuits involved in the processing of depressive and nociceptive information.

Identification of key genes, pathways, and miRNA/mRNA regulatory networks of CUMS-induced depression in nucleus accumbens by integrated bioinformatics analysis

INTRODUCTION

Major depressive disorder (MDD) is a recurrent, devastating mental disorder, which affects >350 million people worldwide, and exerts substantial public health and financial costs to society. Thus, there is a significant need to discover innovative therapeutics to treat depression efficiently. Stress-induced dysfunction in the subtype of neuronal cells and the change of synaptic plasticity and structural plasticity of nucleus accumbens (NAc) are implicated in depression symptomology. However, the molecular and epigenetic mechanisms and stresses to the NAc pathological changes in depression remain elusive.

MATERIALS AND METHODS

In this study, treatment group mice were treated continually with the chronic unpredictable mild stress (CUMS) until expression of depression-like behaviors were found. Depression was confirmed with sucrose preference, novelty-suppressed feeding, forced swimming, and tail suspension tests. We applied high-throughput RNA sequencing to assess microRNA expression and transcriptional profiles in the NAc tissue from depression-like behaviors mice and control mice. The regulatory network of miRNAs/mRNAs was constructed based on the high-throughput RNA sequence and bioinformatics software predictions.

RESULTS

A total of 17 miRNAs and 10 mRNAs were significantly upregulated in the NAc of CUMS-induced mice with depression-like behaviors, and 12 miRNAs and 29 mRNAs were downregulated. A series of bioinformatics analyses showed that these altered miRNAs predicted target mRNA and differentially expressed mRNAs were significantly enriched in the MAPK signaling pathway, GABAergic synapse, dopaminergic synapse, cytokine-cytokine receptor interaction, axon guidance, regulation of autophagy, and so on. Furthermore, dual luciferase report assay and qRT-PCR results validated the miRNA/mRNA regulatory network.

CONCLUSION

The deteriorations of GABAergic synapses, dopaminergic synapses, neurotransmitter synthesis, as well as autophagy-associated apoptotic pathway are associated with the molecular pathological mechanism of CUMS-induced depression.

Early life stress leads to sex differences in development of depressive-like outcomes in a mouse model

Childhood trauma and neglect influence emotional development and increase the risk for and severity of mental illness. Women have a heightened susceptibility to the effects of early life stress (ELS) and are twice as likely as men to develop debilitating, stress-associated disorders later in life, such as major depressive disorder (MDD). Until now, mouse models of depression have been largely unsuccessful at replicating the diverse symptomatology of this disease and the sex bias in vulnerability. From P4 to P11, a limited bedding model that leads to fragmented maternal care, was used to induce ELS. Early adolescent and young adult mice were tested on an array of assays to test for depressive-like behavior. This included our newly developed automated home cage behavioral recognition system, where the home cage behavior of ELS and control mice could be monitored over a continuous 5-10 day span. ELS females, but not males, exhibited depressive-like behaviors on traditional assays. These effects emerged during adolescence and became more severe in adulthood. Using the novel home cage video monitoring method, we identified robust and continuous markers of depressive-like pathology in ELS females that phenocopy many of the behavioral characteristics of depression in humans. ELS effects on home cage behavior were rapidly rescued by ketamine, a fast-acting antidepressant. Together, these findings highlight that limited bedding ELS (1) produces an early emerging, female-specific depressive phenotype that responds to a fast-acting antidepressant and (2) this model has the potential to inform sex-selective risk for the development of stress-induced mental illness.

Chronic brain stimulation rewarding experience ameliorates depression-induced cognitive deficits and restores aberrant plasticity in the prefrontal cortex

BACKGROUND

Major depressive disorder (MDD) is a multifactorial disease which often coexists with cognitive deficits. Depression-induced cognitive deficits are known to be associated with aberrant reward processing, neurochemical and structural alterations. Recent studies have shown that chronic electrical stimulation of brain reward areas induces a robust antidepressant effect. However, the effects of repeated electrical self-stimulation of lateral hypothalamus - medial forebrain bundle (LH-MFB) on depression-induced cognitive deficits and associated neurochemical and structural alterations in the prefrontal cortex (PFC) are unknown.

OBJECTIVES

We investigated the effect of chronic rewarding self-stimulation of LH-MFB in neonatal clomipramine (CLI) model of depression. During adulthood, neonatal CLI and saline administered rats were implanted with bilateral electrodes stereotaxically in the LH-MFB and trained to receive intracranial self-stimulation (ICSS) for 14 days. The rats were tested for depressive-like behaviors, learning and memory followed by estimation of PFC volumes, levels of monoamines and its metabolites in the PFC.

RESULTS

We found that chronic ICSS of LH-MFB reverses CLI-induced behavioral despair and anhedonia. Interestingly, self-stimulation normalizes the impaired novel object and location recognition memory in CLI rats. The amelioration of learning impairments in CLI rats was associated with the reversal of volume loss and restoration of monoamine metabolism in the PFC.

CONCLUSION

We demonstrated that repeated intracranial self-stimulation of LH-MFB ameliorates CLI-induced learning deficits, reverses altered monoamine metabolism and the atrophy of PFC. Our results support the hypothesis that chronic brain stimulation rewarding experience might be evolved as a potential treatment strategy for reversal of learning deficits in depression and associated disorders.

Depression as a Neuroendocrine Disorder: Emerging Neuropsychopharmacological Approaches beyond Monoamines

Depression is currently recognized as a crucial problem in everyday clinical practice, in light of ever-increasing rates of prevalence, as well as disability, morbidity, and mortality related to this disorder. Currently available antidepressant drugs are notoriously problematic, with suboptimal remission rates and troubling side-effect profiles. Their mechanisms of action focus on the monoamine hypothesis for depression, which centers on the disruption of serotonergic, noradrenergic, and dopaminergic neurotransmission in the brain. Nevertheless, views on the pathophysiology of depression have evolved notably, and the comprehension of depression as a complex neuroendocrine disorder with important systemic implications has sparked interest in a myriad of novel neuropsychopharmacological approaches. Innovative pharmacological targets beyond monoamines include glutamatergic and GABAergic neurotransmission, brain-derived neurotrophic factor, various endocrine axes, as well as several neurosteroids, neuropeptides, opioids, endocannabinoids and endovanilloids. This review summarizes current knowledge on these pharmacological targets and their potential utility in the clinical management of depression.

Neuropeptide and Small Transmitter Coexistence: Fundamental Studies and Relevance to Mental Illness

Neuropeptides are auxiliary messenger molecules that always co-exist in nerve cells with one or more small molecule (classic) neurotransmitters. Neuropeptides act both as transmitters and trophic factors, and play a role particularly when the nervous system is challenged, as by injury, pain or stress. Here neuropeptides and coexistence in mammals are reviewed, but with special focus on the 29/30 amino acid galanin and its three receptors GalR1, -R2 and -R3. In particular, galanin's role as a co-transmitter in both rodent and human noradrenergic locus coeruleus (LC) neurons is addressed. Extensive experimental animal data strongly suggest a role for the galanin system in depression-like behavior. The translational potential of these results was tested by studying the galanin system in postmortem human brains, first in normal brains, and then in a comparison of five regions of brains obtained from depressed people who committed suicide, and from matched controls. The distribution of galanin and the four galanin system transcripts in the normal human brain was determined, and selective and parallel changes in levels of transcripts and DNA methylation for galanin and its three receptors were assessed in depressed patients who committed suicide: upregulation of transcripts, e.g., for galanin and GalR3 in LC, paralleled by a decrease in DNA methylation, suggesting involvement of epigenetic mechanisms. It is hypothesized that, when exposed to severe stress, the noradrenergic LC neurons fire in bursts and release galanin from their soma/dendrites. Galanin then acts on somato-dendritic, inhibitory galanin autoreceptors, opening potassium channels and inhibiting firing. The purpose of these autoreceptors is to act as a 'brake' to prevent overexcitation, a brake that is also part of resilience to stress that protects against depression. Depression then arises when the inhibition is too strong and long lasting - a maladaption, allostatic load, leading to depletion of NA levels in the forebrain. It is suggested that disinhibition by a galanin antagonist may have antidepressant activity by restoring forebrain NA levels. A role of galanin in depression is also supported by a recent candidate gene study, showing that variants in genes for galanin and its three receptors confer increased risk of depression and anxiety in people who experienced childhood adversity or recent negative life events. In summary, galanin, a neuropeptide coexisting in LC neurons, may participate in the mechanism underlying resilience against a serious and common disorder, MDD. Existing and further results may lead to an increased understanding of how this illness develops, which in turn could provide a basis for its treatment.

Anti-cytokine agents for anhedonia: targeting inflammation and the immune system to treat dimensional disturbances in depression

The etiology of mood disorders is mechanistically heterogeneous, underscoring the need for a dimensional approach to identify and develop targeted treatments in psychiatry. Accumulating evidence implicates inflammation as an important contributor to the pathophysiology of depression and presents the immune system as a viable therapeutic target that may be more proximate to the pathogenic nexus of brain-based disorders in specific subpopulations. Anhedonia is a transdiagnostic (e.g. Parkinson's disease, diabetes mellitus, rheumatic diseases), yet specific, and clinically relevant symptom dimension subserved by well-characterized neurobiological and neurophysiological substrates of the positive valence systems (PVS). Brain circuits, nodes, and networks, as well as cellular and molecular pathways (e.g. dopaminergic transmission; excitotoxicity; synaptic plasticity), subserving anhedonia are preferentially affected by inflammatory processes. To our knowledge, no published randomized, controlled clinical trial in populations with mood disorders has, to date, primarily sought to determine the effects of an anti-inflammatory agent on PVS functions or pathophysiology. Three ongoing clinical trials aim to investigate the effects of anti-TNF-alpha biologic infliximab on measures of anhedonia [ClinicalTrials.gov identifier: NCT02363738], motivational behavior and circuitry [ClinicalTrials.gov identifier: NCT03006393], and glutamatergic changes in the basal ganglia [ClinicalTrials.gov identifier: NCT03004443] in clinical populations with unipolar or bipolar depression. Positive results would further instantiate the relevance of inflammatory processes and the immune system in the pathophysiology of mood disorders and provide the impetus to develop scalable treatments targeting inflammation and the immune system to mitigate transdiagnostic, dimensional disturbances in brain-based disorders.

Reward and aversion processing in patients with post-traumatic stress disorder: functional neuroimaging with visual and thermal stimuli

In patients with post-traumatic stress disorder (PTSD), a decrease in the brain reward function was reported in behavioral- and in neuroimaging studies. While pathophysiological mechanisms underlying this response are unclear, there are several lines of evidence suggesting over-recruitment of the brain reward regions by aversive stimuli rendering them unavailable to respond to reward-related content. The purpose of this study was to juxtapose brain responses to functional neuroimaging probes that reliably produce rewarding and aversive experiences in PTSD subjects and in healthy controls. The stimuli used were pleasant, aversive and neutral images selected from the International Affective Picture System (IAPS) along with pain-inducing heat applied to the dorsum of the left hand; all were administered during 3 T functional magnetic resonance imaging. Analyses of IAPS responses for the pleasant images revealed significantly decreased subjective ratings and brain activations in PTSD subjects that included striatum and medial prefrontal-, parietal- and temporal cortices. For the aversive images, decreased activations were observed in the amygdala and in the thalamus. PTSD and healthy subjects provided similar subjective ratings of thermal sensory thresholds and each of the temperatures. When 46 °C (hot) and 42 °C (neutral) temperatures were contrasted, voxelwise between-group comparison revealed greater activations in the striatum, amygdala, hippocampus and medial prefrontal cortex in the PTSD subjects. These latter findings were for the most part mirrored by the 44 vs. 42 °C contrast. Our data suggest different brain alterations patterns in PTSD, namely relatively diminished corticolimbic response to pleasant and aversive psychosocial stimuli in the face of exaggerated response to heat-related pain. The present findings support the hypothesis that brain sensitization to pain in PTSD may interfere with the processing of psychosocial stimuli whether they are of rewarding or aversive valence.

Activity Dependent Mammalian Target of Rapamycin Pathway and Brain Derived Neurotrophic Factor Release Is Required for the Rapid Antidepressant Effects of Puerarin

Puerarin is a traditional Chinese medicine with beneficial effects of reduced depression-like behaviors in mice with stress. Previous studies also show that puerarin can produce neuroprotective effect via activating the Akt or increased brain-derived neurotrophic factor (BDNF) expression. Interestingly, BDNF and Akt downstream target, mammalian target of rapamycin (mTOR) mediate the fast-acting antidepressant properties of ketamine. Until now, the involvement of the mTOR signaling pathway or BDNF on puerarin-induced antidepressant effect remains unknown. We aimed to investigate whether the antidepressant-like effect induced by puerarin would associate mTOR signaling pathway and BDNF release. The antidepressant-like effects of puerarin were evaluated using the forced swim test. The activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionaic acid receptor (AMPAR)-mTOR signaling pathway and release of BDNF in the prefrontal cortex were determined. We also investigated the effect of puerarin on AMPAR trafficking through measuring the PKA phosphorylation of AMPAR subunit GluR1. Our present results show that puerarin exerted antidepressant-like responses that was mediated by AMPAR-induced mTOR signaling pathway and associated with increased BDNF release. Moreover, a significant increase in the GluR1 phosphorylation at its PKA site was noted following puerarin treatment. Our findings are the first to demonstrate that the antidepressant-like actions of puerarin require AMPAR-mTOR signaling pathway activation, are associated with an increased BDNF level and facilitate AMPAR membrane insertion. These findings provide preclinical evidence that puerarin may possess antidepressant property which is mediated by the glutamatergic system.

Circuit Mechanisms of Reward, Anhedonia, and Depression

Pleasure and motivation are important factors for goal-directed behavior and well-being in both animals and humans. Intact hedonic capacity requires an undisturbed interplay between a number of different brain regions and transmitter systems. Concordantly, dysfunction of networks encoding for reward have been shown in depression and other psychiatric disorders. The development of technological possibilities to investigate connectivity on a functional level in humans and to directly influence networks in animals using optogenetics among other techniques has provided new important insights in this field of research.In this review, we aim to provide an overview on the neurobiological substrates of anhedonia on a network level. For this purpose, definition of anhedonia and the involved reward components are described first, then current data on reward networks in healthy individuals and in depressed patients are summarized, and the roles of different neurotransmitter systems involved in reward processing are specified. Based on this information, the impact of different therapeutic approaches on reward processing is described with a particular focus on deep brain stimulation (DBS) as a possibility for a direct modulation of human brain structures in vivo.Overall, results of current studies emphasize the importance of anhedonia in psychiatric disorders and the relevance of targeting this phenotype for a successful psychiatric treatment. However, more data incorporating these results for the refinement of methodological approaches are needed to be able to develop individually tailored therapeutic concepts based on both clinical and neurobiological profiles of patients.

Testing Animal Anxiety in Rats: Effects of Open Arm Ledges and Closed Arm Wall Transparency in Elevated Plus Maze Test

The elevated plus maze test is a behavioral test for assessing animal anxiety in rodents. Although this test is widely applied in the field of behavioral science, conflicting outcomes are often provided from different laboratories. To identify reasons for the different outcomes, we previously focused on arm features, which differ between laboratories, most notably the presence/absence of ledges at the sides of open arms and the transparency/opaqueness of closed arm walls. In a previous report, we used a custom designed container to compare rat behavior on different combinations of open and closed arm designs under otherwise identical experimental conditions, and showed that differences in arm features interfere with experimental outcomes. In brief, open arm ledges significantly increased anxiety-like behavior in rats, while transparent arms may also have decreased this behavior. Furthermore, we verified a higher detection sensitivity of the effect of an anxiolytic drug in a combination of no-ledged open arm + opaque walled closed arm compared with a combination of ledged open arm + transparent walled closed arm. In this report, we introduce our protocol for the elevated plus maze test, together with discussion of the key results from the previous report and our experimental experience. We believe this report will provide useful information for researchers who have employed or who plan to use the elevated plus maze in their studies.

Persistent Stress-Induced Neuroplastic Changes in the Locus Coeruleus/Norepinephrine System

Neural plasticity plays a critical role in mediating short- and long-term brain responses to environmental stimuli. A major effector of plasticity throughout many regions of the brain is stress. Activation of the locus coeruleus (LC) is a critical step in mediating the neuroendocrine and behavioral limbs of the stress response. During stressor exposure, activation of the hypothalamic-pituitary-adrenal axis promotes release of corticotropin-releasing factor in LC, where its signaling promotes a number of physiological and cellular changes. While the acute effects of stress on LC physiology have been described, its long-term effects are less clear. This review will describe how stress changes LC neuronal physiology, function, and morphology from a genetic, cellular, and neuronal circuitry/transmission perspective. Specifically, we describe morphological changes of LC neurons in response to stressful stimuli and signal transduction pathways underlying them. Also, we will review changes in excitatory glutamatergic synaptic transmission in LC neurons and possible stress-induced modifications of AMPA receptors. This review will also address stress-related behavioral adaptations and specific noradrenergic receptors responsible for them. Finally, we summarize the results of several human studies which suggest a link between stress, altered LC function, and pathogenesis of posttraumatic stress disorder.

Anhedonia Following Early-Life Adversity Involves Aberrant Interaction of Reward and Anxiety Circuits and Is Reversed by Partial Silencing of Amygdala Corticotropin-Releasing Hormone Gene

BACKGROUND

Anhedonia, the diminished ability to experience pleasure, is an important dimensional entity linked to depression, schizophrenia, and other emotional disorders, but its origins and mechanisms are poorly understood. We have previously identified anhedonia, manifest as decreased sucrose preference and social play, in adolescent male rats that experienced chronic early-life adversity/stress (CES). Here we probed the molecular, cellular, and circuit processes underlying CES-induced anhedonia and tested them mechanistically.

METHODS

We examined functional brain circuits and neuronal populations activated by social play in adolescent CES and control rats. Structural connectivity between stress- and reward-related networks was probed using high-resolution diffusion tensor imaging, and cellular/regional activation was probed using c-Fos. We employed viral-genetic approaches to reduce corticotropin-releasing hormone (Crh) expression in the central nucleus of the amygdala in anhedonic rats, and tested for anhedonia reversal in the same animals.

RESULTS

Sucrose preference was reduced in adolescent CES rats. Social play, generally considered an independent measure of pleasure, activated brain regions involved in reward circuitry in both control and CES groups. In CES rats, social play activated Crh-expressing neurons in the central nucleus of the amygdala, typically involved in anxiety/fear, indicating aberrant functional connectivity of pleasure/reward and fear circuits. Diffusion tensor imaging tractography revealed increased structural connectivity of the amygdala to the medial prefrontal cortex in CES rats. Crh-short hairpin RNA, but not control short hairpin RNA, given into the central nucleus of the amygdala reversed CES-induced anhedonia without influencing other emotional measures.

CONCLUSIONS

These findings robustly demonstrate aberrant interactions of stress and reward networks after early-life adversity and suggest mechanistic roles for Crh-expressing amygdala neurons in emotional deficits portending major neuropsychiatric disorders.

BDNF and BMI effects on brain structures of bipolar offspring: results from the global mood and brain science initiative

OBJECTIVE

To compare brain-derived neurotrophic factor (BDNF) levels between offspring of individuals with bipolar disorders (BD) and healthy controls (HCs) and investigate the effects of BDNF levels and body mass index (BMI) on brain structures.

METHOD

Sixty-seven bipolar offspring and 45 HCs were included (ages 8-28). Structural images were acquired using 3.0 Tesla magnetic resonance imaging. Serum BDNF levels were measured using enzyme-linked immunosorbent assay. Multivariate and univariate analyses of covariance were conducted.

RESULTS

Significantly higher BDNF levels were observed among bipolar offspring, relative to HCs (P > 0.025). Offspring status moderated the association between BDNF and BMI (F₁ =4.636, P = 0.034). After adjustment for relevant covariates, there was a trend for a significant interaction of group and BDNF on neuroimaging parameters (Wilks'λ F_56,94 =1.463, P = 0.052), with significant effects on cerebellar white matter and superior and middle frontal regions. Brain volume and BDNF were positively correlated among HCs and negatively correlated among bipolar offspring. Interactions between BDNF and BMI on brain volumes were non-significant among HCs (Wilks'λ F_28,2 =2.229, P = 0.357), but significant among bipolar offspring (Wilks'λ F_28,12 =2.899, P = 0.028).

CONCLUSION

Offspring status and BMI moderate the association between BDNF levels and brain structures among bipolar offspring, underscoring BDNF regulation and overweight/obesity as key moderators of BD pathogenesis.

The Nucleus Accumbens and Ketamine Treatment in Major Depressive Disorder

Animal models of depression repeatedly showed stress-induced nucleus accumbens (NAc) hypertrophy. Recently, ketamine was found to normalize this stress-induced NAc structural growth. Here, we investigated NAc structural abnormalities in major depressive disorder (MDD) in two cohorts. Cohort A included a cross-sectional sample of 34 MDD and 26 healthy control (HC) subjects, with high-resolution magnetic resonance imaging (MRI) to estimate NAc volumes. Proton MR spectroscopy (¹H MRS) was used to divide MDD subjects into two subgroups: glutamate-based depression (GBD) and non-GBD. A separate longitudinal sample (cohort B) included 16 MDD patients who underwent MRI at baseline then 24 h following intravenous infusion of ketamine (0.5 mg/kg). In cohort A, we found larger left NAc volume in MDD compared to controls (Cohen's d=1.05), but no significant enlargement in the right NAc (d=0.44). Follow-up analyses revealed significant subgrouping effects on the left (d⩾1.48) and right NAc (d⩾0.95) with larger bilateral NAc in non-GBD compared to GBD and HC. NAc volumes were not different between GBD and HC. In cohort B, ketamine treatment reduced left NAc, but increased left hippocampal, volumes in patients achieving remission. The cross-sectional data provided the first evidence of enlarged NAc in patients with MDD. These NAc abnormalities were limited to patients with non-GBD. The pilot longitudinal data revealed a pattern of normalization of left NAc and hippocampal volumes particularly in patients who achieved remission following ketamine treatment, an intriguing preliminary finding that awaits replication.

From Gene to Behavior: L-Type Calcium Channel Mechanisms Underlying Neuropsychiatric Symptoms

The L-type calcium channels (LTCCs) Cav1.2 and Cav1.3, encoded by the CACNA1C and CACNA1D genes, respectively, are important regulators of calcium influx into cells and are critical for normal brain development and plasticity. In humans, CACNA1C has emerged as one of the most widely reproduced and prominent candidate risk genes for a range of neuropsychiatric disorders, including bipolar disorder (BD), schizophrenia (SCZ), major depressive disorder, autism spectrum disorder, and attention deficit hyperactivity disorder. Separately, CACNA1D has been found to be associated with BD and autism spectrum disorder, as well as cocaine dependence, a comorbid feature associated with psychiatric disorders. Despite growing evidence of a significant link between CACNA1C and CACNA1D and psychiatric disorders, our understanding of the biological mechanisms by which these LTCCs mediate neuropsychiatric-associated endophenotypes, many of which are shared across the different disorders, remains rudimentary. Clinical studies with LTCC blockers testing their efficacy to alleviate symptoms associated with BD, SCZ, and drug dependence have provided mixed results, underscoring the importance of further exploring the neurobiological consequences of dysregulated Cav1.2 and Cav1.3. Here, we provide a review of clinical studies that have evaluated LTCC blockers for BD, SCZ, and drug dependence-associated symptoms, as well as rodent studies that have identified Cav1.2- and Cav1.3-specific molecular and cellular cascades that underlie mood (anxiety, depression), social behavior, cognition, and addiction.

Reward Network Immediate Early Gene Expression in Mood Disorders

Over the past three decades, it has become clear that aberrant function of the network of interconnected brain regions responsible for reward processing and motivated behavior underlies a variety of mood disorders, including depression and anxiety. It is also clear that stress-induced changes in reward network activity underlying both normal and pathological behavior also cause changes in gene expression. Here, we attempt to define the reward circuitry and explore the known and potential contributions of activity-dependent changes in gene expression within this circuitry to stress-induced changes in behavior related to mood disorders, and contrast some of these effects with those induced by exposure to drugs of abuse. We focus on a series of immediate early genes regulated by stress within this circuitry and their connections, both well-explored and relatively novel, to circuit function and subsequent reward-related behaviors. We conclude that IEGs play a crucial role in stress-dependent remodeling of reward circuitry, and that they may serve as inroads to the molecular, cellular, and circuit-level mechanisms of mood disorder etiology and treatment.

Association between COMT Val158Met and DAT1 polymorphisms and depressive symptoms in the obese population

OBJECTIVES

Depressive symptoms are common among patients with obesity. Abnormalities in dopamine signaling involved in the reward circuit may ensue excessive consumption of food, resulting in obesity and leading to neuropsychiatric disorders such as depression. This study sought to investigate the association of polymorphisms in the genes encoding DAT1/SLC6A3 and COMT with the intensity of depressive symptoms in obese subjects.

PARTICIPANTS AND METHODS

Prevalence and severity of depressive symptoms were assessed in a group of 364 obese patients using the Beck Depression Inventory (BDI) and the Hamilton Depression Rating Scale (HDRS). Genetic polymorphisms in DAT1 and COMT were evaluated in peripheral blood samples.

RESULTS

The results indicated an association between DAT1 alleles and depressive symptoms, as well as severity of obesity. Subjects homozygous for the nine-repeat allele scored higher in BDI (P=0.022) and HDRS (P=0.00001), suggesting higher intensity of depression in both sexes. This allele was also associated with the highest body mass index (BMI; P=0.001). Carriers of the Val158Met allele of COMT scored higher on both depression scales (BDI, P=0.0005; HRDS, P=0.002) and had the highest BMI values.

CONCLUSION

Polymorphisms in the DAT1 and COMT genes are associated with a greater intensity of depressive symptoms in the obese population.

Disruption of the Glutamate-Glutamine Cycle Involving Astrocytes in an Animal Model of Depression for Males and Females

Background: Women are twice as likely as men to develop major depression. The brain mechanisms underlying this sex disparity are not clear. Disruption of the glutamate–glutamine cycle has been implicated in psychiatric disturbances. This study identifies sex-based impairments in the glutamate–glutamine cycle involving astrocytes using an animal model of depression.

Methods: Male and female adult Long-Evans rats were exposed to chronic social defeat stress (CSDS) for 21 days, using a modified resident-intruder paradigm. Territorial aggression was used for males and maternal aggression was used for females to induce depressive-like deficits for intruders. The depressive-like phenotype was assessed with intake for saccharin solution, weight gain, estrous cycle, and corticosterone (CORT). Behaviors displayed by the intruders during daily encounters with residents were characterized. Rats with daily handling were used as controls for each sex. Ten days after the last encounter, both the intruders and controls were subjected to a no-net-flux in vivo microdialysis to assess glutamate accumulation and extracellular glutamine in the nucleus accumbens (NAc). The contralateral hemispheres were used for determining changes in astrocytic markers, including glial fibrillary acidic protein (GFAP) and glutamate transporter-1 (GLT-1).

Results: Both male and female intruders reduced saccharin intake over the course of CSDS, compared to their pre-stress period and to their respective controls. Male intruders exhibited submissive/defensive behaviors to territorial aggression by receiving sideways threats and bites. These males showed reductions in striatal GLT-1 and spontaneous glutamine in the NAc, compared to controls. Female intruders exhibited isolated behaviors to maternal aggression, including immobility, rearing, and selfgrooming. Their non-reproductive days were extended. Also, they showed reductions in prefrontal and accumbal GFAP⁺ cells and prefrontal GLT-1, compared to controls. When 10 μM of glutamate was infused, these females showed a significant accumulation of glutamate compared to controls. Infusions of glutamate reduced extracellular glutamine for both male and female intruders compared to their respective controls.

Conclusion: Twenty-one days of territorial or maternal aggression produced a depressive-like phenotype and impaired astrocytes in both male and female intruders. Disruption of the glutamate–glutamine cycle in the PFC-striatal network may be linked to depressive-like deficits more in females than in males.

Antidepressant-like effects of long-term sarcosine treatment in rats with or without chronic unpredictable stress

Sarcosine, an N-methyl-d-aspartate receptor enhancer, can improve depression-like behavior in rodent models and depression in humans. We found that a single dose of sarcosine exerted antidepressant-like effects with rapid concomitant increases in the mammalian target of rapamycin (mTOR) signaling pathway activation and enhancement of α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor (AMPAR) membrane insertion. Sarcosine may play a crucial role in developing novel therapy for depression. For a detailed understanding of sarcosine, this study examined the effects of long-term sarcosine treatment on the forced swim test (FST), mTOR signaling, and AMPAR membrane insertion in rats. The effects of long-term sarcosine treatment were examined in naive rats and rats exposed to chronic unpredictable stress (CUS). Long-term sarcosine treatment (560mg/kg/d for 21 d) significantly ameliorated the increased immobility induced by CUS in the FST, reaffirming the potential role of sarcosine as an antidepressant for depressed patients. The same long-term treatment exhibited no such effect in naive rats despite increased mTOR activation and AMPAR membrane insertion in both groups. Our findings clearly show CUS-exposed rats are sensitive to long-term sarcosine treatment in FST and the response at the same dose is absent in naïve rats. Nevertheless, the distinct sensitivity to long-term sarcosine treatment in rats with or without CUS is not associated with the activated mTOR signaling pathway or increased AMPAR membrane insertion. Additionally, understanding the behavioral and molecular basis of distinct responses is vital important for developing personalized treatment programs to increase the probability of success when treating depression.

Interleukin-1β overproduction is a common cause for neuropathic pain, memory deficit, and depression following peripheral nerve injury in rodents

BACKGROUND

Chronic pain is often accompanied by short-term memory deficit and depression. Currently, it is believed that short-term memory deficit and depression are consequences of chronic pain. Here, we test the hypothesis that the symptoms might be caused by overproduction of interleukin-1beta (IL-1β) in the injured nerve independent of neuropathic pain following spared nerve injury in rats and mice.

RESULTS

Mechanical allodynia, a behavioral sign of neuropathic pain, was not correlated with short-term memory deficit and depressive behavior in spared nerve injury rats. Spared nerve injury upregulated IL-1β in the injured sciatic nerve, plasma, and the regions in central nervous system closely associated with pain, memory and emotion, including spinal dorsal horn, hippocampus, prefrontal cortex, nucleus accumbens, and amygdala. Importantly, the spared nerve injury-induced memory deficits, depressive, and pain behaviors were substantially prevented by peri-sciatic administration of IL-1β neutralizing antibody in rats or deletion of IL-1 receptor type 1 in mice. Furthermore, the behavioral abnormalities induced by spared nerve injury were mimicked in naïve rats by repetitive intravenous injection of re combinant rat IL-1β (rrIL-1β) at a pathological concentration as determined from spared nerve injury rats. In addition, microglia were activated by both spared nerve injury and intravenous injection of rrIL-1β and the effect of spared nerve injury was substantially reversed by peri-sciatic administration of anti-IL-1β.

CONCLUSIONS

Neuropathic pain was not necessary for the development of cognitive and emotional disorders, while the overproduction of IL-1β in the injured sciatic nerve following peripheral nerve injury may be a common mechanism underlying the generation of neuropathic pain, memory deficit, and depression.

Role of hippocampal p11 in the sustained antidepressant effect of ketamine in the chronic unpredictable mild stress model

Although ketamine shows a rapid and sustained antidepressant effect, the precise mechanisms underlying its effect are unknown. Recent studies indicate a key role of p11 (also known as S100A10) in depression-like behavior in rodents. The present study aimed to investigate the role of p11 in the antidepressant-like action of ketamine in chronic unpredictable mild stress (CUMS) rat model. The open-field test, forced swimming test and sucrose preference test were performed after administration of ketamine (10 mg kg(-1)) or a combination of ketamine and ANA-12 (a tropomyosin-related kinase B (TrkB) antagonist; 0.5 mg kg(-1)). The lentivirus vector for p11 was constructed to knock down the hippocampal expression of p11. In the CUMS rats, ketamine showed a rapid (0.5 h) and sustained (72 h) antidepressant effect, and its effect was significantly blocked by co-administration of ANA-12. Furthermore, ketamine significantly increased the reduced expression of brain-derived neurotrophic factor (BDNF) in the hippocampus of CUMS rats, whereas ketamine did not affect the expression of p11 in CUMS rats 0.5 h after administration. In addition, ketamine significantly increased the reduced ratio of p-TrkB/TrkB in the hippocampus by CUMS rats, and its effect was also blocked by ANA-12. Moreover, the reduced expression of BDNF and p11 in the hippocampus of CUMS rats was significantly recovered to control levels 72 h after ketamine administration. Interestingly, knockdown of hippocampal p11 caused increased immobility time and decreased sucrose preference, which were not improved by ketamine administration. These results suggest that p11 in the hippocampus may have a key role in the sustained antidepressant effect of ketamine in the CUMS model of depression.