Sigma-1 receptor deficiency reduces GABAergic inhibition in the basolateral amygdala leading to LTD impairment and depressive-like behaviors

The sigma-1 receptor: a mechanistically-informed therapeutic target for antidepressants

INTRODUCTION

The mechanism of action of antidepressants is not fully ascertained. In addition to monoamines, disparate other effectors are also implicated in the molecular and cellular effects of chronic stress including neurogenesis, neurodifferentiation, and neuroplasticity. Evidence suggests sigma-1 receptors (S1Rs) as a putative target and possible mediator of antidepressant activity.

AREAS COVERED

Data from preclinical and clinical trials was synthesized from inception to August 2024. Results showed that S1Rs regulate neurotransmitter availability and release (e.g. monoamines, glutamate), and influence intracellular Ca2+ levels as well as immune inflammatory responses. The introduction of the N-Methyl-D-aspartic Acid (NMDA) antagonist/S1R agonist dextromethorphan-bupropion in August of 2022 represented the first time the Food and Drug Administration (FDA) permitted language that the hypothesized mechanism of an antidepressant involved activity at S1Rs. We also describe the physiology, pathophysiology, and function of S1Rs.

EXPERT OPINION

Sigma-1 modulation is relevant to the mechanism of action of agents currently FDA-approved in major depressive disorder (MDD) (e.g. dextromethorphan-bupropion). Modulating sigma-1 systems is fit for purpose as it relates to future therapeutic discoveries and development in depressive and other mental disorders. Whether sigma-1 modulation is uniquely relevant to targeting dimensions of psychopathology that are more difficult to treat (i.e. anhedonia) awaits determination.

Selective activation of SIGMAR1 in anterior cingulate cortex glutamatergic neurons facilitates comorbid pain in depression in male mice

Depression and comorbid pain are frequently encountered clinically, and the comorbidity complicates the overall medical management. However, the mechanism whereby depression triggers development of pain needs to be further elucidated. Here, by using the chronic restraint stress (CRS) mouse model of depression and comorbid pain, we showed that CRS hyperactivated the glutamatergic neurons in the anterior cingulate cortex (ACC), as well as increasing the dendrite complexity and number. Chemogenetic activation of these neurons can induce depression and pain, while chemogenetic blockade can reverse such depression-induced pain. Moreover, we utilized translating ribosome affinity purification (TRAP) in combination with c-Fos-tTA strategy and pharmacological approaches and identified SIGMAR1 as a potential therapeutic molecular target. These results revealed a previously unknown neural mechanism for depression and pain comorbidity and provided new mechanistic insights into the antidepressive and analgesic effects of the disease.

Potential antidepressant-like effects of N-3 polyunsaturated fatty acids through inhibition of endoplasmic reticulum stress

RATIONALE

The growing evidence has demonstrated the importance of endoplasmic reticulum stress (ERS) in the pathophysiology of depression. ERS genes were considered to be potential novel therapeutic targets for depression.

OBJECTIVES

To clarify the mechanisms of the chronic unpredictable mild stress (CUMS)-induced ERS response and the potential contributing pathways in depression, and further investigate the potential link between N-3 polyunsaturated fatty acids (PUFAs) and stress-induced ERS disturbances.

METHODS

This study analyzed the expression of ERS-related genes including GRP78, ATF-4, ATF-6, XBP-1, and CHOP, and sigma-1R with real-time PCR in peripheral blood mononuclear cell (PBMC) RNA samples from participants. All of the rats except for those in the control groups were subjected to 5 consecutive weeks of CUMS to establish the depression model, and the antidepressant effects of N-3 PUFAs were observed by behavior tests. Moreover, the effect of diet and stress on the ERS pathways was also investigated using the western blot.

RESULTS

Blood CHOP, ATF-4, and XBP-1 levels were notably elevated in depressed patients relative to healthy individuals. Moreover, increased sigma-1R and decreased ATF-6 implied the protective role of sigma-1R through modulating ERS in patients with depression. Animal studies disclosed the novel findings that supplementary N-3 PUFAs in rats alleviated CUMS-induced disturbance of ERS through the ATF-4/XBP-1/CHOP pathway, implying its potential strategy for depression.

CONCLUSION

CUMS-induced depressive-like behaviors are related to the disturbance of ERS. Furthermore, supplementary N-3 PUFAs might be an effective way to alleviate ERS.

Pharmacological Analysis of GABAA Receptor and Sigma1R Chaperone Interaction: Research Report I-Investigation of the Anxiolytic, Anticonvulsant and Hypnotic Effects of Allosteric GABAA Receptors' Ligands

Two groups of facts have been established in previous drug development studies of the non-benzodiazepine anxiolytic fabomotizole. First, fabomotizole prevents stress-induced decrease in binding ability of the GABAA receptor's benzodiazepine site. Second, fabomotizole is a Sigma1R chaperone agonist, and exposure to Sigma1R antagonists blocks its anxiolytic effect. To prove our main hypothesis of Sigma1R involvement in GABAA receptor-dependent pharmacological effects, we performed a series of experiments on BALB/c and ICR mice using Sigma1R ligands to study anxiolytic effects of benzodiazepine tranquilizers diazepam (1 mg/kg i.p.) and phenazepam (0.1 mg/kg i.p.) in the elevated plus maze test, the anticonvulsant effects of diazepam (1 mg/kg i.p.) in the pentylenetetrazole-induced seizure model, and the hypnotic effects of pentobarbital (50 mg/kg i.p.). Sigma1R antagonists BD-1047 (1, 10, and 20 mg/kg i.p.), NE-100 (1 and 3 mg/kg i.p.), and Sigma1R agonist PRE-084 (1, 5, and 20 mg/kg i.p.) were used in the experiments. Sigma1R antagonists have been found to attenuate while Sigma1R agonists can enhance GABAARs-dependent pharmacological effects.

Early 7,8-Dihydroxyflavone Administration Ameliorates Synaptic and Behavioral Deficits in the Young FXS Animal Model by Acting on BDNF-TrkB Pathway

Fragile X syndrome (FXS) is the leading inherited form of intellectual disability and the most common cause of autism spectrum disorders. FXS patients exhibit severe syndromic features and behavioral alterations, including anxiety, hyperactivity, impulsivity, and aggression, in addition to cognitive impairment and seizures. At present, there are no effective treatments or cures for FXS. Previously, we have found the divergence of BDNF-TrkB signaling trajectories is associated with spine defects in early postnatal developmental stages of Fmr1 KO mice. Here, young fragile X mice were intraperitoneal injection with 7,8-Dihydroxyflavone (7,8-DHF), a high affinity tropomyosin receptor kinase B (TrkB) agonist. 7,8-DHF ameliorated morphological abnormities in dendritic spine and synaptic structure and rescued synaptic and hippocampus-dependent cognitive dysfunction. These observed improvements of 7,8-DHF involved decreased protein levels of BDNF, p-TrkB^Y816, p-PLCγ, and p-CaMKII in the hippocampus. In addition, 7,8-DHF intervention in primary hippocampal neurons increased p-TrkB^Y816 and activated the PLCγ1-CaMKII signaling pathway, leading to improvement of neuronal morphology. This study is the first to account for early life synaptic impairments, neuronal morphological, and cognitive delays in FXS in response to the abnormal BDNF-TrkB pathway. Present studies provide novel evidences about the effective early intervention in FXS mice at developmental stages and a strategy to produce powerful impacts on neural development, synaptic plasticity, and behaviors.

Sigma-1 receptor and seizures

Over the last decade, sigma-1 receptor (Sig1R) has been recognized as a valid target for the treatment of seizure disorders and seizure-related comorbidities. Clinical trials with Sig1R ligands are underway testing therapies for the treatment of drug-resistant seizures, developmental and epileptic encephalopathies, and photosensitive epilepsy. However, the direct molecular mechanism by which Sig1R modulates seizures and the balance between excitatory and inhibitory pathways has not been fully elucidated. This review article aims to summarize existing knowledge of Sig1R and its involvement in seizures by focusing on the evidence obtained from Sig1R knockout animals and the anti-seizure effects of Sig1R ligands. In addition, this review article includes a discussion of the advantages and disadvantages of the use of existing compounds and describes the challenges and future perspectives on the use of Sig1R as a target for the treatment of seizure disorders.

Design and Synthesis of Orally Active Quinolyl Pyrazinamides as Sigma 2 Receptor Ligands for the Treatment of Pancreatic Cancer

Sigma 2 receptor (σ2R) is overexpressed in select cancers and is regarded as a biomarker for tumor proliferation. σ2R ligands are emerging as promising theranostics for cancer and neurodegenerative diseases. Herein, we describe the design and synthesis of a series of novel quinolyl pyrazinamides as selective and potent σ2R ligands that show sub-micromolar potency in pancreatic cancer cell lines. Compounds 14 (JR1-157) and 17 (JR2-298) bind σ2R with K_i of 47 and 10 nM, respectively. Importantly, compound 14 has an oral bioavailability of 60% and shows significant in vivo efficacy without obvious toxicity in a syngeneic model of pancreatic cancer. The cytotoxicity of the quinolyl pyrazinamides significantly enhanced in the presence of copper and diminished in the presence of the copper-chelator tetrathiomolybdate. In conclusion, compound 14 is water-soluble, metabolically stable, orally active, and increases the expression of the autophagy marker LC3B and warrants further development for the treatment of pancreatic cancer.

Sigma-1 receptor: A potential target for the development of antidepressants

Though a great many of studies on the development of antidepressants for the therapy of major depression disorder (MDD) and the development of antidepressants have been carried out, there still lacks an efficient approach in clinical practice. The involvement of Sigma-1 receptor in the pathological process of MDD has been verified. In this review, recent research focusing on the role of Sigma-1 receptor in the etiology of MDD were summarized. Preclinical studies and clinical trials have found that stress induce the variation of Sigma-1 receptor in the blood, brain and heart. Dysfunction and absence of Sigma-1 receptor result in depressive-like behaviors in rodent animals. Agonists of Sigma-1 receptor show not only antidepressant-like activities but also therapeutical effects in complications of depression. The mechanisms underlying antidepressant-like effects of Sigma-1 receptor may include suppressing neuroinflammation, regulating neurotransmitters, ameliorating brain-derived neurotrophic factor and N-Methyl-D-Aspartate receptor, and alleviating the endoplasmic reticulum stress and mitochondria damage during stress. Therefore, Sigma-1 receptor represents a potential target for antidepressants development.

Repeated inhibition of sigma-1 receptor suppresses GABAA receptor expression and long-term depression in the nucleus accumbens leading to depressive-like behaviors

Sigma-1 receptor (σ₁R) downregulation in male mice is known to cause a depressive-like phenotype. The nucleus accumbens (NAc), a region associated with affective regulation, has high levels of σ₁R. Here, we investigated the effect of repeated inhibition of σ₁R in the NAc on depressive-like behaviors and synaptic plasticity by microinjecting σ₁R antagonist NE-100 into NAc nuclei in mice (NE-100 mice); this was followed by behavioral tests and field potentials recordings. We first examined the effect of NE-100 administration on σ₁R expression and found that cell surface levels of σ₁R were significantly reduced in the NAc of NE-100 mice. Compared to control mice, NE-100 mice exhibited significantly prolonged immobility in forced swim test (FST) and tail suspension test (TST), impaired long-term depression (LTD) as well as multi-spike waveform field excitatory postsynaptic potential (fEPSP) with an extended duration and an increased paired-pulse ratio (PPR). Reduced levels of GABA_A receptor (GABA_AR)-α1, -α2, -β2, and -β3 subunits, membrane D2R, and PKC phosphorylation in the NAc were observed in NE-100 mice. Activation of GABA_AR by muscimol corrected the extended fEPSP duration and increased PPR, restored LTD maintenance as well as alleviated depressive-like behaviors in NE-100 mice. The decline of PKC phosphorylation in the NAc of NE-100 mice was corrected by injecting NAc with quinpirole, a D2R agonist. Injections of quinpirole or PMA (a PKC activator) into NAc of NE-100 mice rescued the expression levels of GABA_AR, and alleviated the increase in PPR and impairment in LTD; these effects were sensitive to GF109203X, a PKC inhibitor. Furthermore, injecting NAc with quinpirole or PMA relieved depressive-like behaviors in NE-100 mice. Collectively, these results indicate that repeated inhibition of σ₁R in the NAc reduces D2R-mediated PKC phosphorylation and suppresses GABA_AR expression, thus impairing LTD maintenance and leading to depressive-like behaviors.

Sigma-1 Receptors in Depression: Mechanism and Therapeutic Development

Depression is the most common type of neuropsychiatric illness and has increasingly become a major cause of disability. Unfortunately, the recent global pandemic of COVID-19 has dramatically increased the incidence of depression and has significantly increased the burden of mental health care worldwide. Since full remission of the clinical symptoms of depression has not been achieved with current treatments, there is a constant need to discover new compounds that meet the major clinical needs. Recently, the roles of sigma receptors, especially the sigma-1 receptor subtype, have attracted increasing attention as potential new targets and target-specific drugs due to their translocation property that produces a broad spectrum of biological functions. Even clinical first-line antidepressants with or without affinity for sigma-1 receptors have different pharmacological profiles. Thus, the regulatory role of sigma-1 receptors might be useful in treating these central nervous system (CNS) diseases. In addition, long-term mental stress disrupts the homeostasis in the CNS. In this review, we discuss the topical literature concerning sigma-1 receptor antidepressant mechanism of action in the regulation of intracellular proteostasis, calcium homeostasis and especially the dynamic Excitatory/Inhibitory (E/I) balance in the brain. Furthermore, based on these discoveries, we discuss sigma-1 receptor ligands with respect to their promise as targets for fast-onset action drugs in treating depression.

Estradiol-dependent gene expression profile in the amygdala of young ovariectomized spontaneously hypertensive rats

Estrogen plays a role in cardiovascular functions, emotional health, and energy homeostasis via estrogen receptors expressed in the brain. The comorbid relationship between rising blood pressure, a decline in mood and motivation, and body weight gain after menopause, when estrogen levels drop, suggests that the same brain area(s) contributes to protection from all of these postmenopausal disorders. The amygdala, a major limbic system nucleus known to express high estrogen receptor levels, is involved in the regulation of such physiological and psychological responses. We hypothesized that elevated estrogen levels contribute to premenopausal characteristics by activating specific genes and pathways in the amygdala. We examined the effect of 1-month estradiol treatment on the gene expression profile in the amygdala of ovariectomized young adult female spontaneously hypertensive rats. Estradiol substitution significantly decreased blood pressure, prevented body weight gain, and enhanced the voluntary physical activity of ovariectomized rats. In the amygdala of ovariectomized rats, estradiol treatment downregulated the expression of genes associated with estrogen signaling, cholinergic synapse, dopaminergic synapse, and long-term depression pathways. These findings indicate that the transcriptomic characteristics of the amygdala may be involved in estrogen-dependent regulation of blood pressure, physical activity motivation, and body weight control in young adult female spontaneously hypertensive rats.

Neuropsychiatric Drugs Against COVID-19: What is the Clinical Evidence?

Since the beginning of the coronavirus disease (COVID)-19 pandemic, the need for effective treatments for COVID-19 led to the idea of “repurposing” drugs for antiviral treatment. Several antipsychotics and antidepressants have been tested for in vitro activity against the severe acute respiratory syndrome coronavirus 2. Chlorpromazine, other phenothiazine antipsychotics, and the antidepressant fluoxetine were found to be rather potent in these studies. However, whether effective plasma concentrations can be obtained with clinically accepted doses of these drugs is not clear. Data of COVID-19 patients are not yet available but several clinical studies are currently underway.

The specific serotonin reuptake inhibitor fluvoxamine is a potent Sigma-1 receptor agonist and reduces inflammation in animal models of cytokine-stress. Accordingly, fluvoxamine treatment was superior to placebo in reducing impaired respiratory function and other symptoms of inflammation in COVID-19 patients in a placebo-controlled clinical study and another open clinical trial. The beneficial effects of fluvoxamine on the course of COVID-19 were recently confirmed in a large placebo-controlled double-blind trial with several hundred patients.

Inflammation represents a major risk factor for many psychiatric disorders which explains the high susceptibilitiy of COVID-19 patients for psychiatric diseases. Many antidepressants and antipsychotics possess anti-inflammatory properties independent of sigma-1 activity which might be important to reduce psychiatric symptoms of COVID-19 patients and to improve respiratory dysfunction and other consequences of inflammation. This might explain the rather unspecific benefit which has been reported for several cohorts of COVID-19 patients treated with different psychotropic drugs.

Revisiting the sigma-1 receptor as a biological target to treat affective and cognitive disorders

Depression and cognitive disorders are diseases with complex and not-fully understood etiology. Unfortunately, the COVID-19 pandemic dramatically increased the prevalence of both conditions. Since the current treatments are inadequate in many patients, there is a constant need for discovering new compounds, which will be more effective in ameliorating depressive symptoms and treating cognitive decline. Proteins attracting much attention as potential targets for drugs treating these conditions are sigma-1 receptors. Sigma-1 receptors are multi-functional proteins localized in endoplasmic reticulum membranes, which play a crucial role in cellular signal transduction by interacting with receptors, ion channels, lipids, and kinases. Changes in their functions and expression may lead to various diseases, including depression or memory impairments. Thus, sigma-1 receptor modulation might be useful in treating these central nervous system diseases. Importantly, two sigma-1 receptor ligands entered clinical trials, showing that this compound group possesses therapeutic potential. Therefore, based on preclinical studies, this review discusses whether the sigma-1 receptor could be a promising target for drugs treating affective and cognitive disorders.

Emerging Benefits: Pathophysiological Functions and Target Drugs of the Sigma-1 Receptor in Neurodegenerative Diseases

The sigma-1 receptor (Sig-1R) is encoded by the SIGMAR1 gene and is a nonopioid transmembrane receptor located in the mitochondrial-associated endoplasmic reticulum membrane (MAM). It helps to locate endoplasmic reticulum calcium channels, regulates calcium homeostasis, and acts as a molecular chaperone to control cell fate and participate in signal transduction. It plays an important role in protecting neurons through a variety of signaling pathways and participates in the regulation of cognition and motor behavior closely related to neurodegenerative diseases. Based on its neuroprotective effects, Sig-1R has now become a breakthrough target for alleviating Alzheimer's disease and other neurodegenerative diseases. This article reviews the most cutting-edge research on the function of Sig-1R under normal or pathologic conditions and target drugs of the sigma-1 receptor in neurodegenerative diseases.

D-Serine produces antidepressant-like effects in mice through suppression of BDNF signaling pathway and regulation of synaptic adaptations in the nucleus accumbens

OBJECTIVE

D-Serine is a crucial endogenous co-agonist of N-methyl-D-aspartate receptors (NMDARs) in the central nervous system and can affect the function of the brain derived neurotrophic factor (BDNF) system, which plays an essential role in modulating synaptic plasticity. The current study aimed to systematically evaluate the role and mechanisms of D-serine in depressive behavior in nucleus accumbens (NAc).

METHODS

D-Serine concentration in the chronic social defeat stress (CSDS) model in NAc was measured using high-performance liquid chromatography (HPLC). The antidepressant-like effects of D-serine were identified using forced swim test (FST) and tail suspension test (TST) in control mice and then assessed in CSDS model. We applied social interaction and sucrose preference tests to identify the susceptibility of CSDS model. Western blotting was further performed to assess the changes of BDNF signaling cascade in NAc after CSDS and D-serine treatment. The BDNF signaling inhibitor (K252a) was also used to clarify the antidepressant-like mechanism of D-serine. Moreover, D-serine effects on synaptic plasticity in NAc were investigated using electrophysiological methods.

RESULTS

D-Serine concentration was decreased in depression susceptible mice in NAc. D-Serine injections into NAc exhibited antidepressant-like effects in FST and TST without affecting the locomotor activity of mice. D-Serine was also effective in CSDS model of depression. Moreover, D-serine down-regulated the BDNF signaling pathway in NAc during CSDS procedure. Furthermore, BDNF signaling inhibitor (K252a) enhanced the antidepressant effects of D-serine. We also found that D-serine was essential for NMDARs-dependent long-term depression (LTD).

CONCLUSION

D-Serine exerts antidepressant-like effects in mice mediated through restraining the BDNF signaling pathway and regulating synaptic plasticity in NAc.

Sigma-1 Receptor Is Critical for Mitochondrial Activity and Unfolded Protein Response in Larval Zebrafish

The sigma-1 receptor (S1R) is a highly conserved transmembrane protein highly enriched in mitochondria-associated endoplasmic reticulum (ER) membranes, where it interacts with several partners involved in ER-mitochondria Ca2+ transfer, activation of the ER stress pathways, and mitochondria function. We characterized a new S1R deficient zebrafish line and analyzed the impact of S1R deficiency on visual, auditory and locomotor functions. The s1r+25/+25 mutant line showed impairments in visual and locomotor functions compared to s1rWT. The locomotion of the s1r+25/+25 larvae, at 5 days post fertilization, was increased in the light and dark phases of the visual motor response. No deficit was observed in acoustic startle response. A critical role of S1R was shown in ER stress pathways and mitochondrial activity. Using qPCR to analyze the unfolded protein response genes, we observed that loss of S1R led to decreased levels of IRE1 and PERK-related effectors and increased over-expression of most of the effectors after a tunicamycin challenge. Finally, S1R deficiency led to alterations in mitochondria bioenergetics with decreased in basal, ATP-linked and non-mitochondrial respiration and following tunicamycin challenge. In conclusion, this new zebrafish model confirmed the importance of S1R activity on ER-mitochondria communication. It will be a useful tool to further analyze the physiopathological roles of S1R.

Sigma1 Receptor Inhibits TRPC1-Mediated Ca2+ Entry That Promotes Dopaminergic Cell Death

Regulation of Ca2+ homeostasis is essential for neuronal function and its survival. Recent data suggest that TRPC1 function as the endogenous store-mediated Ca2+ entry channel in dopaminergic cells, and loss of TRPC1 function leads to neurodegeneration; however, its regulation is not fully identified. Here we provide evidence that the sigma 1 receptor contributes to the loss of dopaminergic cells by blocking TRPC1-mediated Ca2+ entry. Importantly, downregulation of sigma 1 receptor expression significantly decreased neurotoxin-induced loss of dopaminergic cells as measured by MTT assays and caspase activity was also inhibited. Importantly, sigma 1 receptor inhibited TRPC1-mediated Ca2+ entry and silencing of sigma 1 receptor significantly restored store-dependent Ca2+ influx. Although co-immunoprecipitation failed to show an interaction between the TRPC1 and sigma 1 receptor, store depletion promoted a decrease in the sigma 1 receptor-STIM1 association. Neurotoxin-induced loss of Ca2+ entry was significantly restored in cells that had decreased sigma 1 receptor expression. Furthermore, TRPC1 or STIM1 silencing inhibited store-mediated Ca2+ entry, which was further increased upon the downregulation of the sigma 1 receptor expression. TRPC1 silencing prevented the increased neuroprotection and caspase activity observed upon the downregulation of sigma 1 receptor. Finally, sigma 1 receptor activation also significantly decreased TRPC1-mediated Ca2+ entry and lead to an increase in neurodegeneration. In contrast, addition of sigma 1 receptor antagonist prevented neurotoxin-induced neurodegeneration and facilitated TRPC1-mediated Ca2+ influx. Together these results suggest that the sigma 1 receptor is involved in the inhibition of TRPC1- mediated Ca2+ entry, which leads to the degeneration in the dopaminergic cells, and prevention of sigma 1 receptor function could protect neuronal cell death as observed in Parkinson's disease.

Sigmar1's Molecular, Cellular, and Biological Functions in Regulating Cellular Pathophysiology

The Sigma 1 receptor (Sigmar1) is a ubiquitously expressed multifunctional inter-organelle signaling chaperone protein playing a diverse role in cellular survival. Recessive mutation in Sigmar1 have been identified as a causative gene for neuronal and neuromuscular disorder. Since the discovery over 40 years ago, Sigmar1 has been shown to contribute to numerous cellular functions, including ion channel regulation, protein quality control, endoplasmic reticulum-mitochondrial communication, lipid metabolism, mitochondrial function, autophagy activation, and involved in cellular survival. Alterations in Sigmar1’s subcellular localization, expression, and signaling has been implicated in the progression of a wide range of diseases, such as neurodegenerative diseases, ischemic brain injury, cardiovascular diseases, diabetic retinopathy, cancer, and drug addiction. The goal of this review is to summarize the current knowledge of Sigmar1 biology focusing the recent discoveries on Sigmar1’s molecular, cellular, pathophysiological, and biological functions.

Genetic inactivation of the sigma-1 chaperone protein results in decreased expression of the R2 subunit of the GABA-B receptor and increased susceptibility to seizures

There is a growing body of evidence demonstrating the significant involvement of the sigma-1 chaperone protein in the modulation of seizures. Several sigma-1 receptor (Sig1R) ligands have been demonstrated to regulate the seizure threshold in acute and chronic seizure models. However, the mechanism by which Sig1R modulates the excitatory and inhibitory pathways in the brain has not been elucidated. The aim of this study was to compare the susceptibility to seizures of wild type (WT) and Sig1R knockout (Sig1R-/-) mice in intravenous pentylenetetrazol (PTZ) and (+)-bicuculline (BIC) infusion-induced acute seizure and Sig1R antagonist NE-100-induced seizure models. To determine possible molecular mechanisms, we used quantitative PCR, Western blotting and immunohistochemistry to assess the possible involvement of several seizure-related genes and proteins. Peripheral tissue contractile response of WT and Sig1R-/- mice was studied in an isolated vasa deferentia model. The most important finding was the significantly decreased expression of the R2 subunit of the GABA-B receptor in the hippocampus and habenula of Sig1R-/- mice. Our results demonstrated that Sig1R-/- mice have decreased thresholds for PTZ- and BIC-induced tonic seizures. In the NE-100-induced seizure model, Sig1R-/- animals demonstrated lower seizure scores, shorter durations and increased latency times of seizures compared to WT mice. Sig1R-independent activities of NE-100 included downregulation of the gene expression of iNOS and GABA-A γ2 and inhibition of KCl-induced depolarization in both WT and Sig1R-/- animals. In conclusion, the results of this study indicate that the lack of Sig1R resulted in decreased expression of the R2 subunit of the GABA-B receptor and increased susceptibility to seizures. Our results confirm that Sig1R is a significant molecular target for seizure modulation and warrants further investigation for the development of novel anti-seizure drugs.

Chronic sigma-1 receptor activation ameliorates ventricular remodeling and decreases susceptibility to ventricular arrhythmias after myocardial infarction in rats

The present study aimed to assess the effect of sigma-1 receptor (S1R) stimulation on ventricular remodeling and susceptibility to ventricular arrhythmias (VAs) after myocardial infarction (MI) in rats. Wild-type male rats were placed into one of the following four treatment groups. For four weeks, animals in the Sham group and MI group received intraperitoneal (i.p.) injections of 0.9% saline (1 ml/kg/day); those in the MI + F group received fluvoxamine (FLV) (0.3 mg/kg/day); and those in the MI + F + BD group received FLV plus BD1047 (0.3 mg/kg/day). After that, the ventricular electrophysiological parameters were measured via the langendorff system. Ventricular fibrosis quantification was determined with Masson staining. Cardiac function was evaluated by echocardiography. The protein levels of S1R, connexin (Cx)43, Cav1.2, Kv4.2, Kv4.3, tyrosine hydroxylase (TH), nerve growth factor (NGF), growth-associated protein 43 (GAP43) were detected by Western blot assays. Our results indicated that fluvoxamine significantly prolonged the ventricular effective refractory period (ERP), shortened action potential duration (APD), reduced susceptibility to VAs after MI. Masson staining showed a decrease in ventricular fibrosis in the MI + F group. Furthermore, the contents of Cx43, S1R, Cav1.2, Kv4.2, Kv4.3 were increased in the MI + F group compared with the MI group (all P < 0.05). The contents of TH, NGF, GAP43 were reduced in the MI + F group compared with the MI group. (all P < 0.05). However, BD1047 reduces all of these effects of FLV. The results suggest that S1R stimulation reduces susceptibility to VAs and improves cardiac function by improving myocardial fibrosis, lightning sympathetic remodeling, electrical remodeling, gap junction remodeling and upregulating S1R content.

Impaired LTD-like motor cortical plasticity in female patients with major depression disorder

BACKGROUNDS

Long-term depression (LTD) is a form of physiologic plasticity that is important for reversal learning and may be linked to major depression. Few studies have examined LTP-like plasticity in patients with depression. It is unclear if continuous theta burst stimulation (cTBS) induced LTD is altered in depression patients.

METHODS

Here we recruited 29 healthy control subjects and 31 female patients with depression. We performed cTBS protocol on left motor cortex and employed motor evoked potentials (MEPs) response to measure LTD-like plasticity. Peripheral molecules were measured for correlation analyses to cortical plasticity.

RESULTS

Our results revealed persistent LTD-like plasticity deficits in female patients with depression. LTD-like plasticity was impaired in patients with depression despite the fact that peripheral concentrations of BDNF were comparable to that of healthy subjects.

CONCLUSIONS

Our findings provide evidence for impaired LTD-like plasticity in patients with depression which may be an important mechanism linked to the pathophysiology and treatment of this disorder.

Chaperone Sigma1R and Antidepressant Effect

This review analyzes the current scientific literature on the role of the Sigma1R chaperone in the pathogenesis of depressive disorders and pharmacodynamics of antidepressants. As a result of ligand activation, Sigma1R is capable of intracellular translocation from the endoplasmic reticulum (ER) into the region of nuclear and cellular membranes, where it interacts with resident proteins. This unique property of Sigma1R provides regulation of various receptors, ion channels, enzymes, and transcriptional factors. The current review demonstrates the contribution of the Sigma1R chaperone to the regulation of molecular mechanisms involved in the antidepressant effect.

Chronic inhibition of the sigma-1 receptor exacerbates atrial fibrillation susceptibility in rats by promoting atrial remodeling

AIMS

This study aimed to evaluate the effects of the sigma-1 receptor (S1R) on atrial fibrillation (AF) susceptibility in rats.

MAIN METHODS

Rats were randomly assigned into three groups for intraperitoneal treatment with saline (CTL group), BD1047 (an antagonist of the S1R, BD group) or BD1047 plus fluvoxamine (an agonist of the S1R, BD + F group) for 4 weeks. The heart rate variability (HRV) and atrial electrophysiological parameters were measured via the PowerLab system and analyzed by LabChart 8.0 software. Atrial histology was determined with Masson staining. The protein levels of connexin (Cx) 40, Cav1.2, S1R, eNOS, p-eNOS, and p-AKT were detected by western blot assays.

KEY FINDINGS

Our results showed that BD1047 significantly shortened the atrial effective refractory period (ERP) and action potential duration (APD), increased AF inducibility and duration, augmented sympathetic activity, depressed parasympathetic activity, and reduced heart rate variability (HRV) compared with the CTL group. Masson staining also showed a significant increase in atrial fibrosis in the BD group. Furthermore, the expressions of S1R, Cx40, Cav1.2, p-eNOS, and p-AKT were dramatically reduced in the BD group compared with the CTL group (all P < 0.01). However, fluvoxamine administration mitigated most of the abovementioned alterations.

SIGNIFICANCE

Our findings indicated that S1R inhibition contributed to atrial electrical remodeling, cardiac autonomic remodeling and atrial fibrosis, which could be attenuated by fluvoxamine, thus providing new insights into the relationship between the S1R and AF.

Modulation of the sigma-1 receptor-IRE1 pathway is beneficial in preclinical models of inflammation and sepsis

Sepsis is an often deadly complication of infection in which systemic inflammation damages the vasculature, leading to tissue hypoperfusion and multiple organ failure. Currently, the standard of care for sepsis is predominantly supportive, with few therapeutic options available. Because of increased sepsis incidence worldwide, there is an urgent need for discovery of novel therapeutic targets and development of new treatments. The recently discovered function of the endoplasmic reticulum (ER) in regulation of inflammation offers a potential avenue for sepsis control. Here, we identify the ER-resident protein sigma-1 receptor (S1R) as an essential inhibitor of cytokine production in a preclinical model of septic shock. Mice lacking S1R succumb quickly to hypercytokinemia induced by a sublethal challenge in two models of acute inflammation. Mechanistically, we find that S1R restricts the endonuclease activity of the ER stress sensor IRE1 and cytokine expression but does not inhibit the classical inflammatory signaling pathways. These findings could have substantial clinical implications, as we further find that fluvoxamine, an antidepressant therapeutic with high affinity for S1R, protects mice from lethal septic shock and dampens the inflammatory response in human blood leukocytes. Our data reveal the contribution of S1R to the restraint of the inflammatory response and place S1R as a possible therapeutic target to treat bacterial-derived inflammatory pathology.

Hyperactivity of Hypothalamic-Pituitary-Adrenal Axis Due to Dysfunction of the Hypothalamic Glucocorticoid Receptor in Sigma-1 Receptor Knockout Mice

Sigma-1 receptor knockout (σ₁R-KO) mice exhibit a depressive-like phenotype. Because σ₁R is highly expressed in the neuronal cells of hypothalamic paraventricular nuclei (PVN), this study investigated the influence of σ₁R deficiency on the regulation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Here, we show that the levels of basal serum corticosterone (CORT), adrenocorticotropic hormone (ACTH) and corticotrophin releasing factor (CRF) as well as the level of CRF mRNA in PVN did not significantly differ between adult male σ₁R-KO mice and wild-type (WT) mice. Acute mild restraint stress (AMRS) induced a higher and more sustainable increase in activity of HPA axis and CRF expression in σ₁R-KO mice. Percentage of dexamethasone (Dex)-induced reduction in level of CORT was markedly attenuated in σ₁R^-/- mice. The levels of glucocorticoid receptor (GR) and protein kinase C (PKC) phosphorylation were reduced in the PVN of σ₁R-KO mice and σ₁R antagonist NE100-treated WT mice. The exposure to AMRS in σ₁R-KO mice induced a stronger phosphorylation of cAMP-response element binding protein (CREB) in PVN than that in WT mice. Intracerebroventricular (i.c.v.) injection of PKC activator PMA for 3 days in σ₁R-KO mice not only recovered the GR phosphorylation and the percentage of Dex-reduced CORT but also corrected the AMRS-induced hyperactivity of HPA axis and enhancement of CRF mRNA and CREB phosphorylation. Furthermore, the injection (i.c.v.) of PMA in σ₁R-KO mice corrected the prolongation of immobility time in forced swim test (FST) and tail suspension test (TST). These results indicate that σ₁R deficiency causes down-regulation of GR by reducing PKC phosphorylation, which attenuates GR-mediated feedback inhibition of HPA axis and facilitates the stress response of HPA axis leading to the production of depressive-like behaviors.

MPTP-Induced Dopamine Depletion in Basolateral Amygdala via Decrease of D2R Activation Suppresses GABAA Receptors Expression and LTD Induction Leading to Anxiety-Like Behaviors

Anxiety disorders commonly occur in Parkinson’s disease. Using field potential recording and patch-clamp recording, we evaluated influence of MPTP-reduced dopaminergic afferent in basolateral amygdala (BLA), a main region for affective regulation, on excitatory–inhibitory circuits and synaptic plasticity. Field excitatory post-synaptic potential (fEPSP) slopes at external capsule-BLA synapses were increased in MPTP-mice with decreases in paired-pulse facilitation and long-term potentiation amplitude, which were corrected by bath-application of D2R agonist quinpirole or cannabinoid type 1 receptors agonist WIN55,212-2, but not D1R agonist SKF38393. Compared to single waveform fEPSP in control mice, a multi-spike waveform fEPSP was observed in MPTP-mice with prolongation of duration and an increase in paired-pulse inhibition, which were recovered by BLA-injection of quinpirole for 2 days rather than bath-application. Density of GABA-evoked current (I_GABA) in BLA principal neurons and GABA_AR-α2 subunit expression were reduced in MPTP-mice, which were recovered by administration of quinpirole. Decline of PKC phosphorylation in BLA of MPTP-mice was corrected by bath-application of quinpirole, but not SKF38393. In MPTP-mice, BLA-injection of quinpirole or PKC activator PMA could recover GABA_AR expression, which was sensitive to PKC inhibitor GF109203X. The impairment of long-term depression (LTD) in MPTP-mice was rescued by bath-application of GABA_AR agonist muscimol or BLA-injection of quinpirole and PMA. Finally, BLA-injection of muscimol, quinpirole or PMA relieved anxiety-like behaviors in MPTP-mice. The results indicate that the MPTP-induced dopamine depletion in BLA principal neurons through reducing D2R-mediated PKC phosphorylation suppresses GABA_AR expression and activity, which impairs GABA_AR-mediated inhibition and LTD induction leading to anxiety-like behaviors.