Hippocampal PPARα is a novel therapeutic target for depression and mediates the antidepressant actions of fluoxetine in mice

Apoptosis-induced decline in hippocampal microglia mediates the development of depression-like behaviors in adult mice triggered by unpredictable stress during adolescence

Depression triggered by harmful stress during adolescence is a common problem that can affect mental health. To date, the mechanisms underlying this type of depression remain unclear. One mechanism for the promotion of depression by chronic stress in adulthood is the loss of hippocampal microglia. Since deleterious stress in adolescence also activates microglia, we investigated the dynamic changes of microglia in the hippocampus in mice exposed to chronic unpredictable stress (CUS) in adolescence. Our results showed that 12 days of CUS stimulation in adolescence induced typical depression-like behaviors in adult mice, which were accompanied by a significant decrease and dystrophy of microglia in the dentate gyrus of the hippocampus. Further analysis showed that this decrease in microglia was mediated by the initial response of microglia to unpredictable stress in the dentate gyrus of the hippocampus and their subsequent apoptosis. Blocking the initial response of microglia to unpredictable stress by pretreatment with minocycline was able to prevent apoptosis and microglial decline as well as the development of depression-like behaviors in adult mice induced by adolescent CUS. Moreover, administration of lipopolysaccharide (LPS) or macrophage-colony stimulatory factor (M-CSF), two drugs that reversed microglia decline in the dentate gyrus, ameliorated the depression-like behaviors induced by CUS stimulation in adolescence. These findings reveal a novel mechanism for the development of depression-like behaviors in animals triggered by deleterious stress in adolescence and suggest that reversing microglial decline in the hippocampus may be a hopeful strategy for the treatment of depression triggered by deleterious stress in adolescence.

Integrative omics analysis reveals epigenomic and transcriptomic signatures underlying brain structural deficits in major depressive disorder

Several lines of evidence support the involvement of transcriptomic and epigenetic mechanisms in the brain structural deficits of major depressive disorder (MDD) separately. However, research in these two areas has remained isolated. In this study, we proposed an integrative strategy that combined neuroimaging, brain-wide gene expression, and peripheral DNA methylation data to investigate the genetic basis of gray matter abnormalities in MDD. The MRI T1-weighted images and Illumina 850 K DNA methylation microarrays were obtained from 269 patients and 416 healthy controls, and brain-wide transcriptomic data were collected from Allen Human Brain Atlas. The between-group differences in gray matter volume (GMV) and differentially methylated CpG positions (DMPs) were examined. The genes with their expression patterns spatially related to GMV changes and genes with DMPs were overlapped and selected. Using principal component regression, the associations between DMPs in overlapped genes and GMV across individual patients were investigated, and the region-specific correlations between methylation status and gene expression were examined. We found significant associations between the decreased GMV and DMPs methylation status in the anterior cingulate cortex, inferior frontal cortex, and fusiform face cortex regions. These DMPs genes were primarily enriched in the neurodevelopmental and synaptic transmission process. There was a significant negative correlation between DNA methylation and gene expression in genes associated with GMV changes of the frontal cortex in MDD. Our findings suggest that GMV abnormalities in MDD may have a transcriptomic and epigenetic basis. This imaging-transcriptomic-epigenetic integrative analysis provides spatial and biological links between cortical morphological deficits and peripheral epigenetic signatures in MDD.

Berberine exerts antidepressant effects in vivo and in vitro through the PI3K/AKT/CREB/BDNF signaling pathway

BACKGROUND

Depression, a global neuropsychiatric disorder, brings a serious burden to patients and society as its incidence continues to rise. Berberine is one of the main compounds of a variety of Chinese herbal medicines and has been shown to have multiple pharmacological effects. However, whether berberine can exert antidepressant effects in vivo and in vitro and its related mechanisms remain to be explored.

METHODS

The chronic restraint stress (CRS) method and corticosterone (CORT) were applied to simulate depression-like behavior in vivo and neuronal apoptosis in vitro, respectively. The antidepressant effects of berberine were evaluated by behavioral tests and changes in the content of monoamine neurotransmitters. Inflammatory cytokines were detected and immunofluorescence staining was used to observe the expression levels of apoptosis-related proteins. RT-qPCR and Western blot were used to examine the mRNA and protein expression (or phosphorylation) levels of biomarkers of the PI3K/AKT/CREB/BDNF signaling pathways.

RESULTS

Behavioral tests and levels of neurotransmitters proved that berberine could effectively ameliorate depression-like symptoms in CRS mice. Meanwhile, the results of ELISA and immunofluorescence staining showed that berberine could alleviate inflammatory status and reduce cell apoptosis in vivo and in vitro. Moreover, the changes of the PI3K/AKT/CREB/BDNF signaling pathway induced by CRS or CORT in mouse hippocampus or HT-22 cells were significantly reversed by berberine.

CONCLUSION

Our current study suggested that berberine could exert antidepressant effects in vitro and in vivo, which may be associated with the PI3K/AKT/CREB/BDNF signaling pathway.

A tau fragment links depressive-like behaviors and cognitive declines in Alzheimer's disease mouse models through attenuating mitochondrial function

Introduction

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease characterized by extracellular senile plaques including amyloid-β peptides and intracellular neurofibrillary tangles consisting of abnormal Tau. Depression is one of the most common neuropsychiatric symptoms in AD, and clinical evidence demonstrates that depressive symptoms accelerate the cognitive deficit of AD patients. However, the underlying molecular mechanisms of depressive symptoms present in the process of AD remain unclear.

Methods

Depressive-like behaviors and cognitive decline in hTau mice were induced by chronic restraint stress (CRS). Computational prediction and molecular experiments supported that an asparagine endopeptidase (AEP)-derived Tau fragment, Tau N368 interacts with peroxisome proliferator-activated receptor delta (PPAR-δ). Further behavioral studies investigated the role of Tau N368-PPAR-δ interaction in depressive-like behaviors and cognitive declines of AD models exposed to CRS.

Results

We found that mitochondrial dysfunction was positively associated with depressive-like behaviors and cognitive deficits in hTau mice. Chronic stress increased Tau N368 and promoted the interaction of Tau N368 with PPAR-δ, repressing PPAR-δ-mediated transactivation in the hippocampus of mice. Then we predicted and identified the binding sites of PPAR-δ. Finally, inhibition of AEP, clearance of Tau N368 and pharmacological activation of PPAR-δ effectively alleviated CRS-induced depressive-like behaviors and cognitive decline in mice.

Conclusion

These results demonstrate that Tau N368 in the hippocampus impairs mitochondrial function by suppressing PPAR-δ, facilitating the occurrence of depressive-like behaviors and cognitive decline. Therefore, our findings may provide new mechanistic insight in the pathophysiology of depression-like phenotype in mouse models of Alzheimer's disease.

Activation of mTORC1 Signaling Cascade in Hippocampus and Medial Prefrontal Cortex Is Required for Antidepressant Actions of Vortioxetine in Mice

BACKGROUND

Although thought of as a multimodal-acting antidepressant targeting the serotonin system, more molecules are being shown to participate in the antidepressant mechanism of vortioxetine. A previous report has shown that vortioxetine administration enhanced the expression of rapamycin complex 1 (mTORC1) in neurons. It has been well demonstrated that mTORC1 participates in not only the pathogenesis of depression but also the pharmacological mechanisms of many antidepressants. Therefore, we speculate that the antidepressant mechanism of vortioxetine may require mTORC1.

METHODS

Two mouse models of depression (chronic social defeat stress and chronic unpredictable mild stress) and western blotting were first used together to examine whether vortioxetine administration produced reversal effects against the chronic stress-induced downregulation in the whole mTORC1 signaling cascade in both the hippocampus and medial prefrontal cortex (mPFC). Then, LY294002, U0126, and rapamycin were used together to explore whether the antidepressant effects of vortioxetine in mouse models of depression were attenuated by pharmacological blockade of the mTORC1 system. Furthermore, lentiviral-mTORC1-short hairpin RNA-enhanced green fluorescence protein (LV-mTORC1-shRNA-EGFP) was adopted to examine if genetic blockade of mTORC1 also abolished the antidepressant actions of vortioxetine in mice.

RESULTS

Vortioxetine administration produced significant reversal effects against the chronic stress-induced downregulation in the whole mTORC1 signaling cascade in both the hippocampus and mPFC. Both pharmacological and genetic blockade of the mTORC1 system notably attenuated the antidepressant effects of vortioxetine in mice.

CONCLUSIONS

Activation of the mTORC1 system in the hippocampus and mPFC is required for the antidepressant actions of vortioxetine in mice.

The salt-inducible kinases inhibitor HG-9-91-01 exhibits antidepressant-like actions in mice exposed to chronic unpredictable mild stress

Major depressive disorder is a frequently occurring neuropsychiatric disorder throughout the world. However, the limited and delayed therapeutic efficacy of monoaminergic medications has led to intensive research efforts to develop novel antidepressants. We have previously demonstrated that hippocampal salt-inducible kinase 2 (SIK2) plays a role in the pathogenesis of depression via regulating the downstream CREB-regulated transcription coactivator 1 (CRTC1)-cAMP response element-binding protein (CREB)-brain derived neurotrophic factor (BDNF) pathway. HG-9-91-01 is a potent and selective inhibitor of salt-inducible kinases (SIKs). The present study aims to explore whether HG-9-91-01 has antidepressant-like actions in male C57BL/6J mice. The chronic unpredictable mild stress (CUMS) model of depression, various behavioral tests, western blotting, co-immunoprecipitation, immunofluorescence, stereotactic infusion, and viral-mediated genetic knockdown were used together. It was found that hippocampal infusion of HG-9-91-01 induced significant antidepressant-like effects in the CUMS model, accompanied with preventing the enhancement of CUMS on the hippocampal SIK2 expression and cytoplasmic translocation of CRTC1. HG-9-91-01 treatment also reversed the decreasing effects of CUMS on the BDNF signaling cascade and adult neurogenesis in the hippocampus. Moreover, the antidepressant-like actions of HG-9-91-01 in mice required the hippocampal CRTC1-CREB-BDNF pathway. In conclusion, HG-9-91-01 has potential of being a novel antidepressant candidate.

Resveratrol alleviates postpartum depression-like behavior by activating autophagy via SIRT1 and inhibiting AKT/mTOR pathway

BACKGROUND

Postpartum depression (PPD) causes maternal mortality, and has a high disability rate. In recent years, studies have suggested the Sirt1 gene to be involved in the pathogenesis of depression. Resveratrol (RSV), an activator of Sirt1, has been investigated in depressive behavior. However, its effect on PPD remains to be thoroughly elucidated.

METHODS

We employed a mice model with bilateral oophorectomy combined with hormone-simulated pregnancy to assess postpartum depression-like behavior. The behavioral tests were performed 2 days after the withdrawal of estradiol benzoate. RSV was administered subcutaneously to the PPD model mice. Several behavioral tests were executed, including the open field test, forced swimming test, and tail suspension test. Western blot analyses and immunofluorescence staining were used to evaluate protein expression levels of SIRT1, autophagy markers, and the AKT/mTOR.

RESULTS

Postpartum depressive-like behavior was triggered following the withdrawal of estradiol benzoate after hormone-stimulated-pregnancy. RSV improved postpartum depressive-like behavior of mice via its upregulation of the SIRT1 and autophagy markers, such as Beclin1, ATG5 and LC3B. Also, the downregulation of the p62 protein expression was observed. More importantly, we also detected the inhibition of phosphorylated AKT and mTOR in the hippocampus of postpartum depressive-like mice.

CONCLUSION

RSV could alleviate postpartum depression-like behavior in mice by stimulating the SIRT1, induce autophagy and inhibit the AKT/ mTOR signaling pathway.

The Strategy of Targeting Peroxisome Proliferator-Activated Receptor (PPAR) in the Treatment of Neuropsychiatric Disorders

Peroxisome proliferator-activated receptors (PPARs) are nonsteroid nuclear receptors and transcription factors that regulate several neuroinflammatory and metabolic processes, recently involved in several neuropsychiatric conditions, including Alzheimer's disease, Parkinson's disease, major depressive disorder, post-traumatic stress disorder (PTSD), schizophrenia spectrum disorders, and autism spectrum disorders. PPARs are ligand-activated receptors that, following stimulation, induce neuroprotective effects by decreasing neuroinflammatory processes through inhibition of the nuclear factor kappa-light-chain-enhancer of activated B cell (NF-κB) expression and consequent suppression of pro-inflammatory cytokine production. PPARs heterodimerize with the retinoid X-receptor (RXR) and bind to PPAR-responsive regulatory elements (PPRE) in the promoter region of target genes involved in lipid metabolism, synthesis of cholesterol, catabolism of amino acids, and inflammation. Interestingly, PPARs are considered functionally part of the extended endocannabinoid (eCB) system that includes the classic eCB, anandamide, which act at cannabinoid receptor types 1 (CB1) and 2 (CB2) and are implicated in the pathophysiology of stress-related neuropsychiatric disorders. In preclinical studies, PPAR stimulation improves anxiety and depression-like behaviors by enhancing neurosteroid biosynthesis. The peculiar functional role of PPARs by exerting anti-inflammatory and neuroprotective effects and their expression localization in neurons and glial cells of corticolimbic circuits make them particularly interesting as novel therapeutic targets for several neuropsychiatric disorders characterized by underlying neuroinflammatory/neurodegenerative mechanisms. Herein, we discuss the pathological hallmarks of neuropsychiatric conditions associated with neuroinflammation, as well as the pivotal role of PPARs with a special emphasis on the subtype alpha (PPAR-α) as a suitable molecular target for therapeutic interventions.

Adolescent microglia stimulation produces long-lasting protection against chronic stress-induced behavioral abnormalities in adult male mice

Our previous studies had reported that microglia activation one day before stress exposure prevented the behavioral abnormalities induced by chronic stress in adult mice, and a 10-day interval between microglia stimulation and stress exposure can abolish the prophylactic effect of LPS preinjection on the behavioral abnormalities induced by chronic stress, which, however, could be rescued by repeated LPS injection. This suggests that increased stimulation of microglia results in animals developing a strong ability to prevent deleterious stress stimuli. Because microglia in the adolescent brain exhibit flexible immunological plasticity, we hypothesize that a single low-dose LPS injection during adolescence may provide long-lasting protection against behavioral abnormalities induced by chronic stress in adult mice. As expected, our results showed that a single injection of LPS (100 μg/kg) at post-natal day 28 (PND 28) prevented the development of abnormal behaviors and shifted neuroinflammatory responses toward an anti-inflammatory phenotype in adult mice treated with CSDS at their different stages of the age (PND 56, 140, and 252). Moreover, pretreatment with minocycline or PLX3397 to inhibit microglial activation abolished the prophylactic effect of LPS preinjection after PND 28 on behavioral abnormalities and neuroinflammatory responses induced by CSDS in adult mice at their different stages of the age, PND 56, 140, and 252. These results indicate that stimulation of microglia in adolescence may confer long-lasting protection against neuroinflammatory responses and behavioral abnormalities induced by chronic stress in adult mice. This may offer the potential for the development of a "vaccine-like strategy" to prevent mental disorders.

PPARα Signaling: A Candidate Target in Psychiatric Disorder Management

Peroxisome proliferator-activator receptors (PPARs) regulate lipid and glucose metabolism, control inflammatory processes, and modulate several brain functions. Three PPAR isoforms have been identified, PPARα, PPARβ/δ, and PPARγ, which are expressed in different tissues and cell types. Hereinafter, we focus on PPARα involvement in the pathophysiology of neuropsychiatric and neurodegenerative disorders, which is underscored by PPARα localization in neuronal circuits involved in emotion modulation and stress response, and its role in neurodevelopment and neuroinflammation. A multiplicity of downstream pathways modulated by PPARα activation, including glutamatergic neurotransmission, upregulation of brain-derived neurotrophic factor, and neurosteroidogenic effects, encompass mechanisms underlying behavioral regulation. Modulation of dopamine neuronal firing in the ventral tegmental area likely contributes to PPARα effects in depression, anhedonia, and autism spectrum disorder (ASD). Based on robust preclinical evidence and the initial results of clinical studies, future clinical trials should assess the efficacy of PPARα agonists in the treatment of mood and neurodevelopmental disorders, such as depression, schizophrenia, and ASD.

Disruption of 5-hydroxytryptamine 1A receptor and orexin receptor 1 heterodimer formation affects novel G protein-dependent signaling pathways and has antidepressant effects in vivo

G protein-coupled receptor (GPCR) heterodimers are new targets for the treatment of depression. Increasing evidence supports the importance of serotonergic and orexin-producing neurons in numerous physiological processes, possibly via a crucial interaction between 5-hydroxytryptamine 1A receptor (5-HT1AR) and orexin receptor 1 (OX1R). However, little is known about the function of 5-HT1AR/OX1R heterodimers. It is unclear how the transmembrane domains (TMs) of the dimer affect its function and whether its modulation mediates antidepressant-like effects. Here, we examined the mechanism of 5-HT1AR/OX1R dimerization and downstream G protein-dependent signaling. We found that 5-HT1AR and OX1R form constitutive heterodimers that induce novel G protein-dependent signaling, and that this heterodimerization does not affect recruitment of β-arrestins to the complex. In addition, we found that the structural interface of the active 5-HT1AR/OX1R dimer transforms from TM4/TM5 in the basal state to TM6 in the active conformation. We also used mutation analyses to identify key residues at the interface (5-HT1AR R151^4.40, 5-HT1AR Y198^5.41, and OX1R L230^5.54). Injection of chronic unpredictable mild stress (CUMS) rats with TM4/TM5 peptides improved their depression-like emotional status and decreased the number of endogenous 5-HT1AR/OX1R heterodimers in the rat brain. These antidepressant effects may be mediated by upregulation of BDNF levels and enhanced phosphorylation and activation of CREB in the hippocampus and medial prefrontal cortex. This study provides evidence that 5-HT1AR/OX1R heterodimers are involved in the pathological process of depression. Peptides including TMs of the 5-HT1AR/OX1R heterodimer interface are candidates for the development of compounds with fast-acting antidepressant-like effects.

Central Adiponectin Signaling – A Metabolic Regulator in Support of Brain Plasticity

Brain plasticity and metabolism are tightly connected by a constant influx of peripheral glucose to the central nervous system in order to meet the high metabolic demands imposed by neuronal activity. Metabolic disturbances highly affect neuronal plasticity, which underlies the prevalent comorbidity between metabolic disorders, cognitive impairment, and mood dysfunction. Effective pro-cognitive and neuropsychiatric interventions, therefore, should consider the metabolic aspect of brain plasticity to achieve high effectiveness. The adipocyte-secreted hormone, adiponectin, is a metabolic regulator that crosses the blood-brain barrier and modulates neuronal activity in several brain regions, where it exerts neurotrophic and neuroprotective properties. Moreover, adiponectin has been shown to improve neuronal metabolism in different animal models, including obesity, diabetes, and Alzheimer’s disease. Here, we aim at linking the adiponectin’s neurotrophic and neuroprotective properties with its main role as a metabolic regulator and to summarize the possible mechanisms of action on improving brain plasticity via its role in regulating the intracellular energetic activity. Such properties suggest adiponectin signaling as a potential target to counteract the central metabolic disturbances and impaired neuronal plasticity underlying many neuropsychiatric disorders.

Indole-3-Carbinol Selectively Prevents Chronic Stress-Induced Depression-but not Anxiety-Like Behaviors via Suppressing Pro-Inflammatory Cytokine Production and Oxido-Nitrosative Stress in the Brain

Indole-3-carbinol (I3C), a phytochemical enriched in most cruciferous vegetables, has been shown to display various biological activities such as anti-oxidative stress, anti-inflammation, and anti-carcinogenesis. In this study, we investigated the regulatory effect of I3C on chronic stress-induced behavioral abnormalities in mice. Results showed that repeated I3C treatment at the dose of 10, 30, and 60 mg/kg prevented chronic social defeat stress (CSDS)-induced behavioral abnormalities in the tail suspension test, forced swimming test, sucrose preference test, and social interaction test in mice, and did not affect CSDS-induced behavioral abnormalities in the elevated plus maze, light-dark test, and open-field test, suggesting that the I3C treatment selectively prevents the onset of depression- but not anxiety-like behaviors in chronically stressed mice. Further analysis demonstrated that repeated I3C treatment (60 mg/kg, 10 days) prevented CSDS-induced increases in levels of interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α (TNF-α) mRNA and protein, but did not affect CSDS-induced decreases in levels of IL-4, IL-10, and Ym-1 mRNA and/or protein in the hippocampus and prefrontal cortex, suggesting that I3C can selectively prevent chronic stress-induced pro-inflammatory but not anti-inflammatory responses in the brain. Further analysis showed that repeated I3C treatment (60 mg/kg, 10 days) prevented CSDS-induced increases in levels of nitrite and malondialdehyde (MDA), decreases in contents of glutathione (GSH), and decreases in levels of brain derived neurotrophic factor (BDNF) protein in the hippocampus and prefrontal cortex. These results demonstrated that I3C selectively prevents chronic stress-induced depression-like behaviors in mice likely through suppressing neuroinflammation and oxido-nitrosative stress in the brain.

Hippocampal NR6A1 impairs CREB-BDNF signaling and leads to the development of depression-like behaviors in mice

Chronic stress exposure is a risk factor that can induce the development of depression-like behaviors by impairing the hippocampal cyclic adenosine monophosphate-response element binding protein (CREB)-brain-derived neurotrophic factor (BDNF) signaling, but its underlying mechanisms remain largely unknown. We identified an orphan receptor that can suppress the activity of CREB, nuclear receptor sub-family 6, group A, member 1 (NR6A1), in mouse brain neurons. Given the critical role of the impaired CREB-BDNF signaling in depression, we speculate that the neuronal NR6A1 may mediate the pathogenesis of depression. Results showed that chronic unpredictable stress (CUS) markedly increased the expression levels of hippocampal NR6A1 protein, which reduced hippocampal CREB phosphorylation and BDNF protein expression. Overexpression of hippocampal NR6A1 in stress-naïve mice simulated chronic stress, inducing depression-like behaviors in the tail suspension test, forced swimming test, and sucrose preference test, and impairing the hippocampal CREB-BDNF signaling cascade. Genetic knockdown of hippocampal NR6A1 did not affect mouse behaviors but prevented the CUS-induced depression-like behaviors in mice and impairment in hippocampal CREB-BDNF signaling. Furthermore, genetic knockdown of hippocampal CREB or BDNF abrogated the preventive effect of hippocampal NR6A1 down-regulation on CUS-induced depression-like behaviors in mice. Collectively, these results for the first time identified a nuclear expression of NR6A1 in mouse brain neurons, and showed that the abnormally increased NR6A1 protein in the hippocampus in mice treated with or without chronic stress can impair the CREB-BDNF signaling cascade and lead to the development of depression-like behaviors. Hippocampal NR6A1 could be a novel target for the development of antidepressants.

Promoting the hippocampal PPARα expression participates in the antidepressant mechanism of reboxetine, a selective norepinephrine reuptake inhibitor

Reboxetine, the first selective norepinephrine (NA) reuptake inhibitor used in the treatment of depression, mainly acts by binding to the NA transporter and blocking reuptake of extracellular NA. Recently, some other pharmacological targets beyond the NA transporter are being demonstrated for reboxetine. Peroxisome proliferator activated receptor α (PPARα) is a member of the nuclear hormone receptor family of ligand-dependent transcription factors. Previous reports have demonstrated the role of hippocampal PPARα in the pathophysiology of depression. Here we assume that hippocampal PPARα may participate in the antidepressant mechanism of reboxetine. Therefore, the chronic social defeat stress (CSDS) model of depression, various behavioral tests, the western blotting and adenovirus associated virus (AAV)-mediated genetic knockdown methods were used together in the present study. Our results showed that repeated reboxetine treatment markedly restored the decreasing effects of CSDS on the expression of hippocampal PPARα, brain-derived neurotrophic factor (BDNF) and phosphorylated cAMP response element binding protein (pCREB). Pharmacological blockade of PPARα notably prevented the antidepressant-like effects of reboxetine in the CSDS model. Furthermore, genetic knockdown of hippocampal PPARα also fully abolished the antidepressant-like effects of reboxetine in the CSDS model. Taken together, promoting the hippocampal PPARα expression participates in the antidepressant mechanism of reboxetine.

Virus-mediated decrease of LKB1 activity in the mPFC diminishes stress-induced depressive-like behaviors in mice

As a highly prevalent neuropsychiatric disorder worldwide, the pathophysiology of depression is not yet fully understood and based on multiple factors among which chronic stress is critical. Numerous previous studies have shown the role of central mammalian target of rapamycin complex 1 (mTORC1) signaling in depression. However, so far it remains elusive by which way chronic stress down-regulates the activity of central mTORC1. Liver kinase b1 (LKB1) has been demonstrated to regulate the activity of the mTORC1 signaling cascade by phosphorylating AMP activated protein kinase (AMPK). Here, this study aimed to explore whether LKB1 participates in depression by regulating the downstream AMPK-mTORC1 signaling, and various methods including mouse models of depression, western blotting and immunofluorescence were used together. Our results showed that chronic stress significantly enhanced the expression of both phosphorylated LKB1 and total LKB1 in the medial prefrontal cortex (mPFC) but not the hippocampus. Furthermore, genetic knockdown of LKB1 in the mPFC fully reversed not only the depressive-like behaviors induced by chronic stress in mice but also the effects of chronic stress on the activity of AMPK and the mTORC1 system. Taken together, this study preliminarily suggests that LKB1 in the mPFC could be a feasible target for antidepressants. This study also provides support for the potential use of LKB1 inhibition strategies against the chronic stress-related neuropsychiatric disorders.

Hippocampal miR-206-3p participates in the pathogenesis of depression via regulating the expression of BDNF

As a widely-known neuropsychiatric disorder, the exact pathogenesis of depression remains elusive. MiRNA-206 (miR-206) is conventionally known as one of the myomiRs and has two forms: miR-206-3p and miR-206-5p. Recently, miR-206 has been demonstrated to regulate the biosynthesis of brain-derived neurotrophic factor (BDNF), a very popular target involved in depression and antidepressant responses. Here we assumed that miR-206 may play a role in depression, and various methods including the chronic social defeat stress (CSDS) model of depression, quantitative real-time reverse transcription PCR, western blotting, immuofluorescence and virus-mediated gene transfer were used together. It was found that CSDS robustly increased the level of miR-206-3p but not miR-206-5p in the hippocampus. Both genetic overexpression of hippocampal miR-206-3p and intranasal administration of AgomiR-206-3p induced not only notable depressive-like behaviors but also significantly decreased hippocampal BDNF signaling cascade and neurogenesis in naïve C57BL/6J mice. In contrast, both genetic knockdown of hippocampal miR-206-3p and intranasal administration of AntagomiR-206-3p produced significant antidepressant-like effects in the CSDS model of depression. Furthermore, it was found that the antidepressant-like effects induced by miR-206-3p inhibition require the hippocampal BDNF-TrkB system. Taken together, hippocampal miR-206-3p participates in the pathogenesis of depression by regulating BDNF biosynthesis and is a feasible antidepressant target.

Repurposing Peroxisome Proliferator-Activated Receptor Agonists in Neurological and Psychiatric Disorders

Common pathophysiological mechanisms have emerged for different neurological and neuropsychiatric conditions. In particular, mechanisms of oxidative stress, immuno-inflammation, and altered metabolic pathways converge and cause neuronal and non-neuronal maladaptative phenomena, which underlie multifaceted brain disorders. The peroxisome proliferator-activated receptors (PPARs) are nuclear receptors modulating, among others, anti-inflammatory and neuroprotective genes in diverse tissues. Both endogenous and synthetic PPAR agonists are approved treatments for metabolic and systemic disorders, such as diabetes, fatty liver disease, and dyslipidemia(s), showing high tolerability and safety profiles. Considering that some PPAR-acting drugs permeate through the blood-brain barrier, the possibility to extend their scope from the periphery to central nervous system has gained interest in recent years. Here, we review preclinical and clinical evidence that PPARs possibly exert a neuroprotective role, thereby providing a rationale for repurposing PPAR-targeting drugs to counteract several diseases affecting the central nervous system.

Hippocampal alpha-synuclein mediates depressive-like behaviors

Alpha-synuclein (α-syn) which encoded by SNCA plays a critical role in the neurotransmission, vesicle dynamics, and neuroplasticity. Alteration to SNCA expression is associated with major depressive disorder. However, the pathogenic mechanism of SNCA in depression remains unknown. Herein, we reported that SNCA was up-regulated in the peripheral blood of major depressive disorder (MDD) patients and the depressive mice. Chronic restraint stress (CRS) also up-regulated the SNCA expression in the hippocampus. Moreover, over-expression of SNCA in the hippocampus triggered spontaneous depressive-like behaviors under the non-stressed conditions in mice, and knockout of SNCA could reverse CRS-induced depressive-like behaviors. SNCA led to synapse loss and neuronal cell death in the hippocampus possibly via complement-mediated microglial engulfment and inflammation, and thus contributed to the pathogenesis of depressive disorder. Overall, hippocampal SNCA and complement system are involved in the pathogenesis of depressive disorder and it provides a new perspective for the occurrence of depressive disorder.

Hippocampal PPARα Plays a Role in the Pharmacological Mechanism of Vortioxetine, a Multimodal-Acting Antidepressant

As a well-known multimodal-acting antidepressant, vortioxetine is thought to aim at several serotonin (5-HT) receptors and the 5-HT transporter. However, recently more and more proteins besides 5-HT are being reported to participate in the antidepressant mechanism of vortioxetine. As a widely known nuclear hormone receptor, peroxisome proliferator activated receptor α (PPARα) possesses transcriptional activity and is very important in the brain. Several reports have suggested that hippocampal PPARα is implicated in antidepressant responses. Here we speculate that hippocampal PPARα may participate in the antidepressant mechanism of vortioxetine. In this study, chronic unpredictable mild stress (CUMS), chronic social defeat stress (CSDS), behavioral tests, the western blotting and adenovirus associated virus (AAV)-mediated gene knockdown methods were used together. It was found that vortioxetine administration significantly reversed the inhibitory actions of both CUMS and CSDS on the hippocampal PPARα expression. Pharmacological blockade of PPARα notably prevented the antidepressant actions of vortioxetine in the CUMS and CSDS models. Moreover, genetic knockdown of PPARα in the hippocampus also significantly blocked the protecting effects of vortioxetine against both CUMS and CSDS. Therefore, the antidepressant effects of vortioxetine in mice require hippocampal PPARα.

A prophylactic effect of macrophage-colony stimulating factor on chronic stress-induced depression-like behaviors in mice

Innate immune activation has been shown to reduce the severity of nervous system disorders such as brain ischemia and traumatic brain damage. Macrophage-colony stimulating factor (M-CSF), a drug that is used to treat hematological system disease, is an enhancer of the innate immune response. In the present study, we evaluated the effect of M-CSF preconditioning on chronic social defeat stress (CSDS)-induced depression-like behaviors in mice. Results showed that a single M-CSF injection 1 day before stress exposure at the dose of 100 and 500 μg/kg, or a single M-CSF injection (100 μg/kg) 1 or 5 days but not 10 days before stress exposure prevented CSDS-induced depression-like behaviors in mice. Further analysis showed that a second M-CSF injection 10 days after the first M-CSF injection and a 2 × or 4 × M-CSF injections 10 days before stress exposure also prevented CSDS-induced depression-like behaviors. Molecular studies revealed that a single M-CSF injection prior to stress exposure skewed the neuroinflammatory responses in the brain in CSDS-exposed mice towards an anti-inflammatory phenotype. These behavioral and molecular actions of M-CSF were correlated with innate immune stimulation, as pre-inhibiting the innate immune activation by minocycline pretreatment (40 mg/kg) abrogated the preventive effect of M-CSF on CSDS-induced depression-like behaviors and neuroinflammatory responses. These results provide evidence to show that innate immune activation by M-CSF pretreatment may prevent chronic stress-induced depression-like behaviors via preventing the development of neuroinflammatory response in the brain, which may help to develop novel strategies for the prevention of depression.

Brain circuit dysfunction in specific symptoms of depression

Since the depressive disorder manifests complex and diverse symptoms clinically, its pathological mechanism and therapeutic options are difficult to determine. In recent years, the advent of optogenetics, chemogenetics and viral tracing techniques, along with the well-established rodent model of depression, has led to a shift in the focus of depression research from single molecules to neural circuits. In virtue of the powerful tools above, psychiatric disorder such as depression could be well related to the disfunction of brain's connection. Moreover, compelling studies also support that the diversity of depressive behaviour could be involved with the discrete changes in a distinct circuit of the brain. Therefore, summarising the differential changes of the neural circuits in mice with depression-like behaviour may provide a better understanding of the causal relationships between neural circuit and depressive behaviour. Here, we focus on the changes in the neural circuitry underlying various depression-like phenotypes, including motivation, despair, social avoidance and comorbid sequelae, which may provide an explanation to circuit-specific discrepancy in depression-like behaviour.

Involvement of dorsal raphe nucleus serotonergic systems in social approach-avoidance behaviour and in the response to fluoxetine treatment in peri-adolescent female BALB/c mice

Serotonergic systems are involved in the development and regulation of social behaviour, and drugs that target serotonin neurotransmission, such as selective serotonin reuptake inhibitors (SSRIs), also alter aspects of social approach-avoidance. The midbrain dorsal raphe nucleus (DR), which is a major serotonergic nucleus and main source of serotonergic innervation of the forebrain, has been proposed as an important target for SSRIs, although evidence in females is lacking. In this study, we examined the involvement of the DR serotonergic systems in social behaviour and in response to SSRI treatment, using peri-adolescent female BALB/c mice. Mice were exposed to the SSRI fluoxetine either chronically (18 mg/kg/day, in drinking water, for 12 days) or acutely (18 mg/kg, i.p.), or to vehicle control condition (0.9 % saline, i.p.), prior to being exposed to the three-chambered sociability test. Activation of serotonergic neurons across subregions of the DR were subsequently measured, using dual-label immunohistochemistry for TPH2 and c-Fos. Acute fluoxetine administration increased generalised and social avoidance, while mice exposed to chronic fluoxetine treatment showed levels of social approach behaviour that were comparable to controls. Serotonergic populations across the DR showed reduced activity following acute fluoxetine treatment. Further, activation of serotonergic neurons in the ventral DR correlated with social approach behaviour in vehicle-treated control mice. These data provide some support for the involvement of discrete populations of DR serotonergic neurons in the regulation of social approach-avoidance, although more research is needed to understand the effects and mechanisms of chronic SSRI treatment in females.

Differential and paradoxical roles of new-generation antidepressants in primary astrocytic inflammation

Background

Selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs) are commonly used new-generation drugs for depression. Depressive symptoms are thought to be closely related to neuroinflammation. In this study, we used up-to-date protocols of culture and stimulation and aimed to understand how astrocytes respond to the antidepressants.

Methods

Primary astrocytes were isolated and cultured using neurobasal-based serum-free medium. The cells were treated with a cytokine mixture comprising complement component 1q, tumor necrosis factor α, and interleukin 1α with or without pretreatments of antidepressants. Cell viability, phenotypes, inflammatory responses, and the underlying mechanisms were analyzed.

Results

All the SSRIs, including paroxetine, fluoxetine, sertraline, citalopram, and fluvoxamine, show a visible cytotoxicity within the range of applied doses, and a paradoxical effect on astrocytic inflammatory responses as manifested by the promotion of inducible nitric oxide synthase (iNOS) and/or nitric oxide (NO) and the inhibition of interleukin 6 (IL-6) and/or interleukin 1β (IL-1β). The SNRI venlafaxine was the least toxic to astrocytes and inhibited the production of IL-6 and IL-1β but with no impact on iNOS and NO. All the drugs had no regulation on the polarization of astrocytic A1 and A2 types. Mechanisms associated with the antidepressants in astrocytic inflammation route via inhibition of JNK1 activation and STAT3 basal activity.

Conclusions

The study demonstrated that the antidepressants possess differential cytotoxicity to astrocytes and function differently, also paradoxically for the SSRIs, to astrocytic inflammation. Our results provide novel pieces into understanding the differential efficacy and tolerability of the antidepressants in treating patients in the context of astrocytes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02097-z.

Identification of the antidepressive properties of C1, a specific inhibitor of Skp2, in mice

We have reported that SMIP004, an inhibitor of S-phase kinase-associated protein 2 (Skp2), displays antidepressant-like activities in stress-naïve and chronically stressed mice. Here, we investigated the antidepressant-like effect of C1, another inhibitor of Skp2, in mouse models following acute or chronic drug administration at different doses and treatment times by using the tail suspension test (TST), forced swimming test (FST), and social interaction test (SIT). The time- and dose-dependent results showed that the antidepressant-like effect of C1 occurred 8 days after the drug treatment, and C1 produced antidepressant-like activities at the dose of 5 and 10 but not 1 mg/kg in male or female mice. C1 administration (5 mg/kg) also induced antidepressant-like effects in stress-naïve mice in a three-times administration mode within 24 h (24, 5, and 1 h before the test) but not in an acute administration mode (1 h before the test). The C1 and fluoxetine co-administration produced additive effect on depression-like behaviors in stress-naïve mice. The antidepressant-like effect of C1 was not associated with the change in locomotor activity, as no increased locomotor activity was observed in different treatment modes. Furthermore, the long-term C1 treatment (5 mg/kg) was found to ameliorate the depression-like behaviors in chronic social defeat stress-exposed mice, suggesting that C1 can produce antidepressant-like actions in stress conditions. Since C1 is a specific inhibitor of Skp2, our results demonstrate that inhibition of Skp2 might be a potential strategy for the treatment of depression, and Skp2 may be potential target for the development of novel antidepressants.

The Selective SIK2 Inhibitor ARN-3236 Produces Strong Antidepressant-Like Efficacy in Mice via the Hippocampal CRTC1-CREB-BDNF Pathway

Depression is a widespread chronic medical illness affecting thoughts, mood, and physical health. However, the limited and delayed therapeutic efficacy of monoaminergic drugs has led to intensive research efforts to develop novel antidepressants. ARN-3236 is the first potent and selective inhibitor of salt-inducible kinase 2 (SIK2). In this study, a multidisciplinary approach was used to explore the antidepressant-like actions of ARN-3236 in mice. Chronic social defeat stress (CSDS) and chronic unpredictable mild stress (CUMS) models of depression, various behavioral tests, high performance liquid chromatography-tandem mass spectrometry, stereotactic infusion, viral-mediated gene transfer, western blotting, co-immunoprecipitation and immunofluorescence were used together. It was found that ARN-3236 could penetrate the blood-brain barrier. Repeated ARN-3236 administration induced significant antidepressant-like effects in both the CSDS and CUMS models of depression, accompanied with fully preventing the stress-enhanced SIK2 expression and cytoplasmic translocation of cyclic adenosine monophosphate response element binding protein (CREB)-regulated transcription coactivator 1 (CRTC1) in the hippocampus. ARN-3236 treatment also completely reversed the down-regulating effects of CSDS and CUMS on the hippocampal brain-derived neurotrophic factor (BDNF) system and neurogenesis. Moreover, we demonstrated that the hippocampal CRTC1-CREB-BDNF pathway mediated the antidepressant-like efficacy of ARN-3236. Collectively, ARN-3236 possesses strong protecting effects against chronic stress, and could be a novel antidepressant beyond monoaminergic drugs.

Endocannabinoid-Like Lipid Neuromodulators in the Regulation of Dopamine Signaling: Relevance for Drug Addiction

The family of lipid neuromodulators has been rapidly growing, as the use of different -omics techniques led to the discovery of a large number of naturally occurring N-acylethanolamines (NAEs) and N-acyl amino acids belonging to the complex lipid signaling system termed endocannabinoidome. These molecules exert a variety of biological activities in the central nervous system, as they modulate physiological processes in neurons and glial cells and are involved in the pathophysiology of neurological and psychiatric disorders. Their effects on dopamine cells have attracted attention, as dysfunctions of dopamine systems characterize a range of psychiatric disorders, i.e., schizophrenia and substance use disorders (SUD). While canonical endocannabinoids are known to regulate excitatory and inhibitory synaptic inputs impinging on dopamine cells and modulate several dopamine-mediated behaviors, such as reward and addiction, the effects of other lipid neuromodulators are far less clear. Here, we review the emerging role of endocannabinoid-like neuromodulators in dopamine signaling, with a focus on non-cannabinoid N-acylethanolamines and their receptors. Mounting evidence suggests that these neuromodulators contribute to modulate synaptic transmission in dopamine regions and might represent a target for novel medications in alcohol and nicotine use disorder.

Drug repositioning for treatment-resistant depression: Hypotheses from a pharmacogenomic study

About 20-30% of patients with major depressive disorder (MDD) develop treatment-resistant depression (TRD) and finding new effective treatments for TRD has been a challenge. This study aimed to identify new possible pharmacological options for TRD. Genes in pathways included in predictive models of TRD in a previous whole exome sequence study were compared with those coding for targets of drugs in any phase of development, nutraceuticals, proteins and peptides from Drug repurposing Hub, Drug-Gene Interaction database and DrugBank database. We tested if known gene targets were enriched in TRD-associated genes by a hypergeometric test. Compounds enriched in TRD-associated genes after false-discovery rate (FDR) correction were annotated and compared with those showing enrichment in genes associated with MDD in the last Psychiatric Genomics Consortium genome-wide association study. Among a total of 15,475 compounds, 542 were enriched in TRD-associated genes (FDR p < .05). Significant results included drugs which are currently used in TRD (e.g. lithium and ketamine), confirming the rationale of this approach. Interesting molecules included modulators of inflammation, renin-angiotensin system, proliferator-activated receptor agonists, glycogen synthase kinase 3 beta inhibitors and the rho associated kinase inhibitor fasudil. Nutraceuticals, mostly antioxidant polyphenols, were also identified. Drugs showing enrichment for TRD-associated genes had a higher probability of enrichment for MDD-associated genes compared to those having no TRD-genes enrichment (p = 6.21e-55). This study suggested new potential treatments for TRD using a in silico approach. These analyses are exploratory only but can contribute to the identification of drugs to study in future clinical trials.

Tolerance-inducing effect and properties of innate immune stimulation on chronic stress-induced behavioral abnormalities in mice

Over-activation of the innate immune system constitutes a risk factor for the development of nervous system disorders but may reduce the severity of these disorders by inducing tolerance effect. Here, we studied the tolerance-inducing effect and properties of innate immune stimulation on chronic social defeat stress (CSDS)-induced behavioral abnormalities in mice. A single injection of the innate immune enhancer lipopolysaccharide (LPS) one day before stress exposure prevented CSDS-induced impairment in social interaction and increased immobility time in the tail suspension test and forced swimming test. This effect was observed at varying doses (100, 500, and 1000 μg/kg) and peaked at 100 μg/kg. A single LPS injection (100 μg/kg) either one or five but not ten days before stress exposure prevented CSDS-induced behavioral abnormalities. A second LPS injection ten days after the first LPS injection, or a 2 × or 4 × LPS injections ten days before stress exposure also induced tolerance against stress-induced behavioral abnormalities. Our results furthermore showed that a single LPS injection one day before stress exposure skewed the neuroinflammatory response in the hippocampus and prefrontal cortex of CSDS-exposed mice toward an anti-inflammatory phenotype. Inhibiting the central innate immune response by pretreatment with minocycline or PLX3397 abrogated the tolerance-inducing effect of LPS preconditioning on CSDS-induced behavioral abnormalities and neuroinflammatory responses in the brain. These results provide evidence for a prophylactic effect of innate immune stimulation on stress-induced behavioral abnormalities via changes in microglial activation, which may help develop novel strategies for the prevention of stress-induced psychological disorders.

Antidepressant-like activity of L-701324 in mice: A behavioral and neurobiological characterization

Antidepressants currently used in clinical practice have limitations such as low efficacy, slow onset and various adverse reactions. It has become necessary to develop novel antidepressants beyond monoaminergic drugs. L-701,324 is a potent NMDA receptor antagonist, and the purpose of this study was to investigate the possible antidepressant effects of L-701,324 in mice. Here, various methods including the forced swim test (FST), tail suspension test (TST), chronic unpredictable mild stress (CUMS) model of depression, western blotting and immunofluorescence, were used together. A single injection of L-701,324 exhibited antidepressant-like potential in the FST and TST without affecting the locomotor activity of mice. Repeated injection of L-701,324 not only prevented CUMS-induced depressive-like behaviors in mice, but also ameliorated the downregulating effects of CUMS on the hippocampal BDNF signaling cascade and neurogenesis. Furthermore, K252a, a potent inhibitor of the BDNF system, fully blocked the antidepressant-like activity of L-701,324 in mice. K252a administration also abolished the activating actions of L-701,324 on the hippocampal BDNF signaling cascade and neurogenesis in CUMS-treated mice. Collectively, these data indicated that L-701,324 possesses antidepressant-like activity in mice, which was mediated, at least in part, by promoting the hippocampal BDNF system.

Venlafaxine protects against chronic stress-related behaviors in mice by activating the mTORC1 signaling cascade

BACKGROUND

Recent studies have suggested the role of mammalian target of rapamycin complex 1 (mTORC1) in the pathophysiology of depression. Although venlafaxine was thought to be a serotonin and norepinephrine reuptake inhibitor (SNRI), its pharmacological mechanism remain elusive. In this study, the effects of venlafaxine on the mTORC1 system were studied in both chronic unpredictable mild stress (CUMS) and chronic social defeat stress (CSDS) models.

METHOD

First, we examined whether repeated venlafaxine treatment reversed the effects of CUMS and CSDS on the mTORC1 signaling cascade in both the hippocampus and medial prefrontal cortex (mPFC). Second, several selective pharmacological inhibitors of the mTORC1 system, including rapamycin, LY294002 and U0126, were used together to determine whether the protective effects of venlafaxine against the CUMS and CSDS models were prevented by mTORC1 system blockade. Finally, genetic knockdown of mTORC1 by mTORC1-shRNA was further adopted to test whether mTORC1 was necessary for the anti-stress effects of venlafaxine in mice.

RESULT

Our results showed that the decreasing effects of CUMS and CSDS on the mTORC1 signaling cascade in the hippocampus and mPFC were restored by venlafaxine, and the use of rapamycin, LY294002, U0126 and mTORC1-shRNA fully abolished the anti-stress actions of venlafaxine in mice.

CONCLUSION

The mTORC1 system is involved in the pharmacological mechanism of venlafaxine.

Emotional Impairments and Neuroinflammation are Induced in Male Mice Invulnerable to Repeated Social Defeat Stress

Prolonged stress triggers neuroinflammation, which plays a significant role in the development of depression; however, stressed people do not always suffer from depression because of individual differences in stress vulnerability. Negative cognitive bias (NCB) toward pessimistic judgment often underlies depressive episodes. However, a relationship between stress vulnerability, neuroinflammation, and NCB remains elusive. In addition, an animal model with all the traits would be a powerful tool for studying the etiology of depression and its therapeutic approaches. Accordingly, this study evaluated the effect of stress vulnerability on neuroinflammation and depression-related behaviors, including NCB in males, using a modified version of repeated social defeat stress (mRSDS) paradigm, a validated animal model of psychosocial stress. Exposure to mRSDS, consisting of 5 min of social defeat by unfamiliar CD-1 aggressor mice for five consecutive days, caused NCB, which co-occurred with depressive- and anxiety-like behaviors, and neuroinflammation in male BALB/c mice. Treatment with minocycline, an antibiotic with anti-inflammatory property, blocked mRSDS-induced depressive-like behaviors and neuroinflammation, but not NCB, indicating the limited effect of an anti-inflammatory intervention. In addition, marked differences were found in neuroinflammatory profiles and hippocampal gene expression patterns between resilient and unstressed mice, as well as between susceptible and resilient mice. Therefore, mice resilient to mRSDS are indeed not intact. Our findings provide insights into the unique features of the mRSDS model in male BALB/c mice, which could be used to investigate the etiological mechanisms underlying depression as well as bridge the gap in the relationship between stress vulnerability, neuroinflammation, and NCB in males.

LIMK1/2 in the mPFC Plays a Role in Chronic Stress-Induced Depressive-Like Effects in Mice

BACKGROUND

Depression is one of the most common forms of mental illness and also a leading cause of disability worldwide. Developing novel antidepressant targets beyond the monoaminergic systems is now popular and necessary. LIM kinases, including LIM domain kinase 1 and 2 (LIMK1/2), play a key role in actin and microtubule dynamics through phosphorylating cofilin. Since depression is associated with atrophy of neurons and reduced connectivity, here we speculate that LIMK1/2 may play a role in the pathogenesis of depression.

METHODS

In this study, the chronic unpredictable mild stress (CUMS), chronic restraint stress (CRS), and chronic social defeat stress (CSDS) models of depression, various behavioral tests, stereotactic injection, western blotting, and immunofluorescence methods were adopted.

RESULTS

CUMS, CRS, and CSDS all significantly enhanced the phosphorylation levels of LIMK1 and LIMK2 in the medial prefrontal cortex (mPFC) but not the hippocampus of mice. Administration of fluoxetine, the most commonly used selective serotonin reuptake inhibitor in clinical practice, fully reversed the effects of CUMS, CRS, and CSDS on LIMK1 and LIMK2 in the mPFC. Moreover, pharmacological inhibition of LIMK1 and LIMK2 in the mPFC by LIMKi 3 infusions notably prevented the pro-depressant effects of CUMS, CRS, and CSDS in mice.

CONCLUSIONS

In summary, these results suggest that LIMK1/2 in the mPFC has a role in chronic stress-induced depressive-like effects in mice and could be a novel pharmacological target for developing antidepressants.

Is There a Future for PPARs in the Treatment of Neuropsychiatric Disorders?

Recently, peroxisome proliferator-activated receptor (PPAR)-α and γ isoforms have been gaining consistent interest in neuropathology and treatment of neuropsychiatric disorders. Several studies have provided evidence that either the receptor expression or the levels of their endogenously-produced modulators are downregulated in several neurological and psychiatric disorders and in their respective animal models. Remarkably, administration of these endogenous or synthetic ligands improves mood and cognition, suggesting that PPARs may offer a significant pharmacological target to improve several neuropathologies. Furthermore, various neurological and psychiatric disorders reflect sustained levels of systemic inflammation. Hence, the strategy of targeting PPARs for their anti-inflammatory role to improve these disorders is attracting attention. Traditionally, classical antidepressants fail to be effective, specifically in patients with inflammation. Non-steroidal anti-inflammatory drugs exert potent antidepressant effects by acting along with PPARs, thereby strongly substantiating the involvement of these receptors in the mechanisms that lead to development of several neuropathologies. We reviewed running findings in support of a role for PPARs in the treatment of neurological diseases, including Alzheimer's disease or psychiatric disorders, such as major depression. We discuss the opportunity of targeting PPARs as a future pharmacological approach to decrease neuropsychiatric symptoms at the same time that PPAR ligands resolve neuroinflammatory processes.

Hippocampal overexpression of chordin protects against the chronic social defeat stress-induced depressive-like effects in mice

Depression is a serious and worldwide neuropsychiatric disesase, and developing novel antidepressant targets beyond the monoaminergic systems is now popular and necessary. Bone morphogenetic protein (BMP) signals modulate numerous developmental, physiological, and homeostatic processes. The functions of BMPs are also regulated by secreted extracellular antagonists such as chordin and noggin. Chordin has abundant expression in adult brain, and may play critical role in the central nervous system. In this study, the chronic social defeat stress (CSDS) model of depression, various behavioral tests, western blotting, quantitative real-time reverse transcription PCR, immunohistochemistry, recombinant mouse chordin protein and AAV-Chordin-EGFP were together used to explore the role of chordin in the pathogenesis of depression. It was found that CSDS significantly decreased the expression of chordin in the hippocampus but not other related brain regions. Moreover, both pharmacological and genetic overexpression of hippocampal chordin fully protected against the CSDS-induced depressive-like effects in mice. Collectively, hippocampal chordin could be a novel antidepressant target, and this study further highlights the importance of the hippocampal BMP system in the pathophysiology of depression.

Stimulation of Peroxisome Proliferator-Activated Receptor-α by N-Palmitoylethanolamine Engages Allopregnanolone Biosynthesis to Modulate Emotional Behavior

BACKGROUND

The endocannabinoid and neurosteroid systems regulate emotions and stress responses. Activation of peroxisome proliferator-activated receptor (PPAR)-α by the endocannabinoid congener N-palmitoylethanolamine (PEA) regulates pathophysiological systems (e.g., inflammation, oxidative stress) and induces peripheral biosynthesis of allopregnanolone, a gamma-aminobutyric acidergic neurosteroid implicated in mood disorders. However, effects of PPAR-α on emotional behavior are poorly understood.

METHODS

We studied the impact of PPAR-α activation on emotional behavior in a mouse model of posttraumatic stress disorder. Neurosteroid levels before and after PEA treatment were measured by gas chromatography-mass spectrometry in relevant brain regions of socially isolated versus group-housed mice exposed to the contextual fear conditioning test, elevated plus maze test, forced swim test, and tail suspension test. Neurosteroidogenic enzyme levels were quantified in hippocampus by Western blot.

RESULTS

PEA administered in a model of conditioned contextual fear reconsolidation blockade facilitated fear extinction and fear extinction retention and induced marked antidepressive- and anxiolytic-like effects in socially isolated mice with reduced brain allopregnanolone levels. These effects were mimicked by the PPAR-α synthetic agonists, fenofibrate and GW7647, and were prevented by PPAR-α deletion, PPAR-α antagonists, and neurosteroid-enzyme inhibitors. Behavioral improvements correlated with PEA-induced upregulation of PPAR-α, neurosteroidogenic enzyme expression, and normalization of corticolimbic allopregnanolone levels.

CONCLUSIONS

This evidence supports a previously unknown role for PPAR-α in behavior regulation and suggests new strategies for the treatment of neuropsychopathologies characterized by deficient neurosteroidogenesis, including posttraumatic stress disorder and major depressive disorder.

Hippocampal Salt-Inducible Kinase 2 Plays a Role in Depression via the CREB-Regulated Transcription Coactivator 1-cAMP Response Element Binding-Brain-Derived Neurotrophic Factor Pathway

BACKGROUND

Developing novel pharmacological targets beyond monoaminergic systems is now a popular strategy for finding new ways to treat depression. Salt-inducible kinase (SIK) is a kinase that regulates the nuclear translocation of cyclic adenosine monophosphate response element binding protein (CREB)-regulated transcription coactivator (CRTC) by phosphorylation. Here, we hypothesize that dysfunction of the central SIK-CRTC system may contribute to the pathogenesis of depression.

METHODS

Chronic social defeat stress (CSDS) and chronic unpredictable mild stress (CUMS) models of depression, various behavioral tests, viral-mediated gene transfer, Western blotting, coimmunoprecipitation, quantitative real-time reverse transcription polymerase chain reaction, and immunohistochemistry were used in this study (for in vivo studies, n = 10; for in vitro studies, n = 5).

RESULTS

Both CSDS and CUMS markedly increased the expression of hippocampal SIK2, which reduced CRTC1 nuclear translocation and binding of CRTC1 and CREB in the hippocampus. Genetic overexpression of hippocampal SIK2 in naïve mice simulated chronic stress, inducing depressive-like behaviors in the forced swim test, tail suspension test, sucrose preference test, and social interaction test, as well as decreasing the brain-derived neurotrophic factor signaling cascade and neurogenesis in the hippocampus. In contrast, genetic knockdown and knockout of hippocampal SIK2 protected against CSDS and CUMS, exerting significant antidepressant-like effects that were mediated via the downstream CRTC1-CREB-brain-derived neurotrophic factor pathway. Moreover, fluoxetine, venlafaxine, and mirtazapine all significantly restored the effects of CSDS and CUMS on the hippocampal SIK2-CRTC1 pathway, which was necessary for their antidepressant actions.

CONCLUSIONS

The hippocampal SIK2-CRTC1 pathway is involved in the pathogenesis of depression, and hippocampal SIK2 could be a novel target for the development of antidepressants.

The antidepressant effects of hesperidin on chronic unpredictable mild stress-induced mice

Hesperidin, a kind of citrus bioflavonoid distributed in foods including grapefruits, oranges and lemons, has many pharmacological activities. This study was aimed to evaluate the anti-depressant-like effect of hesperidin on chronic unpredictable mild stress (CUMS)-induced mice. Depressive-like behavior was detected by the sucrose preference test (SPT), tail suspension test (TST) and forced swimming test (FST). A 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) assay was performed to assess the cell viability of corticosterone-induced PC12 cells. The serum, hippocampal and cell supernatant concentrations of interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α were determined using enzyme-linked immunosorbent assay (ELISA) commercial kits. Furthermore, the protein expression levels of high-mobility group box 1 protein (HMGB1), receptor for advanced glycation end-products (RAGE)/NF-κB and brain-derived neurotrophic factor (BDNF)/tropomyosin-related kinase B (TrkB) pathway in the hippocampus and corticosterone-induced PC12 cells were detected by Western blot. Our results showed that hesperidin (100, 200 mg/kg) significantly relieved depressive-like behaviors, including decreased sucrose consumption in sucrose preference test (SPT), immobility in the forced swimming test (FST), tail suspension test, and locomotor activity in the open field test (OFT). Hesperidin reduced inflammatory cytokine levels by attenuating the HMGB1/RAGE/NF-κB signaling pathway and BDNF/TrkB pathway both in vivo and in vitro. In conclusion, hesperidin possessed efficient neuroprotective effects on depression, which was associated with neuroinflammation mediated by the HMGB1/RAGE/NF-κB and BDNF/TrkB pathways.

Sulforaphane triggers a functional elongation of microglial process via the Akt signal

Microglia are a kind of innate immune cells in the nervous system. The amoeboid morphology in microglia indicates a pro-inflammatory status, while their ramified morphologies are associated with anti-neuroinflammation. Recently, we and others have reported that drugs that trigger microglial process elongation may be beneficial for neuroinflammation inhibition. In this study, we found that sulforaphane (SFN), a compound extracted from broccoli sprouts, promotes primary cultured microglial process elongation in both normal and pro-inflammatory conditions in a reversible manner. This pro-elongation effect of SFN was also observed in the prefrontal cortex in vivo and accompanied with an attenuation of pro-inflammatory response as well as an enhancement of anti-inflammatory response in primary cultured microglia. Mechanistic studies revealed that the SFN treatment increased Akt phosphorylation levels in primary cultured microglia and Akt inhibition blocked the effect of SFN on microglial process elongation, suggesting that the regulation of microglial process by SFN is mediated by Akt activation. Functional studies showed that Akt inhibition reversed the effect of SFN on both pro- and anti-inflammatory responses in lipopolysaccharide (LPS)-stimulated microglia. In an inflammation model in vivo, SFN pretreatment not only prevented LPS-induced retractions of microglial process in the prefrontal cortex, but improved LPS-induced behavioral abnormalities in mice, including the increase in immobility time in the tail suspension test and forced swim test as well as the decrease in sucrose preference. These results indicate that the SFN inhibits microglial activation and neuroinflammation-triggered behavioral abnormalities likely through triggering Akt-mediated microglial process elongation.

Hippocampal PPARα is a novel therapeutic target for depression and mediates the antidepressant actions of fluoxetine in mice

BACKGROUND AND PURPOSE

Developing novel pharmacological targets beyond the monoaminergic system is now a popular strategy for treating depression. PPARα is a nuclear receptor protein that functions as a transcription factor,-regulating gene expression. We have previously reported that both WY14643 and fenofibrate, two pharmacological agonists of PPARα, have antidepressant-like effects in mice, implying that PPARα is a potential antidepressant target.

EXPERIMENTAL APPROACH

We first used various biotechnological methods to evaluate the effects of chronic stress and fluoxetine on hippocampal PPARα. The viral-mediated genetic approach was then employed to explore whether hippocampal PPARα was an antidepressant target. PPARα inhibitors, PPARα-knockout (KO) mice and PPARα-knockdown (KD) mice were further used to determine the role of PPARα in the antidepressant effects of fluoxetine.

KEY RESULTS

Chronic stress significantly decreased mRNA and protein levels of PPARα in the hippocampus, but not other regions, and also fully reduced the recruitment of hippocampal PPARα to the cAMP response element-binding (CREB) promoter. Genetic overexpression of hippocampal PPARα induced significant antidepressant-like actions in mice by promoting CREB-mediated biosynthesis of brain-derived neurotrophic factor. Moreover, fluoxetine notably restored the stress-induced negative effects on hippocampal PPARα. Using PPARα antagonists fully blocked the antidepressant effects of fluoxetine in mice, and similarly, both PPARα-KO and PPARα-KD abolished the effects of fluoxetine. Besides, PPARα-KO and PPARα-KD aggravated depression in mice.

CONCLUSIONS AND IMPLICATIONS

Hippocampal PPARα is a potential novel antidepressant target that mediates the antidepressant actions of fluoxetine in mice.