Fluvoxamine as a sigma-1 receptor agonist improved cognitive impairments in a patient with schizophrenia

In vitro and in vivo sigma 1 receptor imaging studies in different disease states

The sigma receptor system has been classified into two distinct subtypes, sigma 1 (σ1R) and sigma 2 (σ2R). Sigma 1 receptors (σ1Rs) are involved in many neurodegenerative diseases and different central nervous system disorders such as Alzheimer's disease, Parkinson's disease, schizophrenia, and drug addiction, and pain. This makes them attractive targets for developing radioligands as tools to gain a better understanding of disease pathophysiology and clinical diagnosis. Over the years, several σ1R radioligands have been developed to image the changes in σ1R distribution and density providing insights into their role in disease development. Moreover, the involvement of both σ1Rs and σ2Rs with cancer make these ligands, especially those that are σ2R selective, great tools for imaging different types of tumors. This review will discuss the principles of molecular imaging using PET and SPECT, known σ1R radioligands and their applications for labelling σ1Rs under different disease conditions. Furthermore, this review will highlight σ1R radioligands that have demonstrated considerable potential as biomarkers, and an opportunity to fulfill the ultimate goal of better healthcare outcomes and improving human health.

Repurposing of CNS drugs to treat COVID-19 infection: targeting the sigma-1 receptor

The novel coronavirus disease 2019 (COVID-19) is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The escalating number of SARS-CoV-2-infected individuals has conferred the viral spread with the status of global pandemic. However, there are no prophylactic or therapeutic drugs available on the market to treat COVID-19, although several drugs have been approved. Recently, two articles using the comparative viral-human protein-protein interaction map revealed that the sigma-1 receptor in the endoplasmic reticulum plays an important role in SARS-CoV-2 replication in cells. Knockout and knockdown of SIGMAR1 (sigma-1 receptor, encoded by SIGMAR1) caused robust reductions in SARS-CoV-2 replication, which indicates that the sigma-1 receptor is a key therapeutic target for SARS-CoV-2 replication. Interestingly, a recent clinical trial demonstrated that treatment with the antidepressant fluvoxamine, which has a high affinity at the sigma-1 receptor, could prevent clinical deterioration in adult outpatients infected with SARS-CoV-2. In this review, we discuss the brief history of the sigma-1 receptor and its role in SARS-CoV-2 replication in cells. Here, we propose repurposing of traditional central nervous system (CNS) drugs that have a high affinity at the sigma-1 receptor (i.e., fluvoxamine, donepezil, ifenprodil) for the treatment of SARS-CoV-2-infected patients. Finally, we discussed the potential of other CNS candidates such as cutamesine and arketamine.

Imaging sigma receptors in the brain: New opportunities for diagnosis of Alzheimer's disease and therapeutic development

The sigma-1 (σ₁) receptor is a chaperone protein located on the mitochondria-associated membrane of the endoplasmic reticulum, while the sigma-2 receptor (σ₂) is an endoplasmic reticulum-resident membrane protein. Recent evidence indicates that both of these receptors figure prominently in the pathophysiology of Alzheimer's disease (AD) and thus are targets for the development of novel, disease-modifying therapeutic strategies. Radioligand-based molecular imaging technique such as positron emission tomography (PET) imaging is a powerful tool for the investigation of protein target expression and function in living subjects. In this review, we survey the development of PET radioligands for the σ₁ or σ₂ receptors and assess their potential for human imaging applications. The availability of PET imaging with σ₁ or σ₂ receptor-specific radioligands in humans will allow the investigation of these receptors in vivo and lead to further understanding of their respective roles in AD pathogenesis and progression. Moreover, PET imaging can be used in target occupancy studies to assess target engagement and correlate receptor occupancy and therapeutic response of σ₁ receptor agonists and σ₂ receptor antagonists currently in clinical trials. It is expected that neuroimaging of σ₁ and σ₂ receptors in the brain will shed new light on AD pathophysiology and may provide us with new biomarkers for diagnosis of AD and efficacy monitoring of emerging AD therapeutic strategies.

Evaluation of the Effect of Fluvoxamine in Patients With Schizophrenia Under Risperidone Treatment: A Clinical Trial

PURPOSE/BACKGROUND

The effectiveness of selective-serotonin reuptake inhibitors in the improvement of schizophrenia is a controversial issue. The aim of this study was to evaluate the effect of fluvoxamine on the symptoms of schizophrenia including positive and negative symptoms, cognitive impairment, and quality of life.

METHODS/PROCEDURES

This clinical trial was performed on 68 patients with chronic schizophrenia, treated with risperidone at 22 Bahman Hospital of Qazvin, Iran during 2015 to 2016. The patients were randomly divided into control and intervention groups (34 patients per group). The control group was treated with risperidone and biperiden, whereas the intervention group received fluvoxamine, besides risperidone, and biperiden. The participants completed the Wechsler Memory Scale, Scale for the Assessment of Positive Symptoms, Scale for the Assessment of Negative Symptoms (SANS), and the World Health Organization Quality of Life Scale, and the findings were statistically analyzed at baseline and postintervention (8 and 10 weeks).

FINDINGS/RESULTS

The mean ± SD Wechsler Memory Scale scores in the evaluated intervals (baseline, week 8, and week 10), respectively, were 70.58 ± 11.51, 70.76 ± 11.36, and 70.88 ± 11.40 in the control group and 74.76 ± 10.56, 77.76 ± 10.56, and 77.76 ± 10.73 in the intervention group (F = 126.73, P ≤ 0.001). The difference between the groups in terms of SANS and quality of life scores was significant in the specified intervals, SANS (F = 6.36, P = 0.004), and quality of life (F = 15.13, P ≤ 0.001). Nevertheless, no difference was observed in terms of Scale for the Assessment of Positive Symptoms scores (P > 0.05).

IMPLICATIONS/CONCLUSIONS

The results indicated that risperidone augmentation with fluvoxamine could significantly improve cognitive impairments and negative symptoms among patients with schizophrenia. Moreover, this augmentation led to higher quality of life among these patients.

Role of σ1 Receptors in Learning and Memory and Alzheimer's Disease-Type Dementia

The present chapter will review the role of σ1 receptor in learning and memory and neuroprotection , against Alzheimer's type dementia. σ1 Receptor agonists have been tested in a variety of pharmacological and pathological models of learning impairments in rodents these last past 20 years. Their anti-amnesic effects have been explained by the wide-range modulatory role of σ1 receptors on Ca2+ mobilizations, neurotransmitter responses, and particularly glutamate and acetylcholine systems, and neurotrophic factors. Recent observations from genetic and pharmacological studies have shown that σ1 receptor can also be targeted in neurodegenerative diseases, and particularly Alzheimer's disease . Several compounds, acting partly through the σ1 receptor, have showed effective neuroprotection in transgenic mouse models of Alzheimer's disease . We will review the data and discuss the possible mechanisms of action, particularly focusing on oxidative stress and mitochondrial integrity, trophic factors and a novel hypothesis suggesting a functional interaction between the σ1 receptor and α7 nicotinic acetylcholine receptor. Finally, we will discuss the pharmacological peculiarities of non-selective σ1 receptor ligands, now developed as neuroprotectants in Alzheimer's disease , and positive modulators, recently described and that showed efficacy against learning and memory deficits.

Sigma-1 Receptor Agonists and Their Clinical Implications in Neuropsychiatric Disorders

Accumulating evidence suggests that sigma-1 receptors play a role in the pathophysiology of neuropsychiatric diseases, as well as in the mechanisms of some selective serotonin reuptake inhibitors (SSRIs). Among the SSRIs, the order of affinity for sigma-1 receptors is as follows: fluvoxamine > sertraline > fluoxetine > escitalopram > citalopram >> paroxetine. Some SSRIs (e.g., fluvoxamine, fluoxetine and escitalopram) and other drugs (donepezil , ifenprodil , dehydroepiandeterone (DHEA)) potentiate nerve-growth factor (NGF)-induced neurite outgrowth in PC12 cells, and these effects could be antagonized by the selective sigma-1 receptor antagonist NE-100. Furthermore, fluvoxamine, donepezil, and DHEA, but not paroxetine or sertraline, improved phencyclidine-induced cognitive deficits in mice, and these effects could be antagonized by NE-100. Several clinical studies showed that sigma-1 receptor agonists such as fluvoxamine and ifenprodil could have beneficial effects in patients with neuropsychiatric disorders. In this chapter, the authors will discuss the role of sigma-1 receptors in the mechanistic action of some SSRIs, donepezil, neurosteroids, and ifenprodil, and the clinical implications for sigma-1 receptor agonists .

Sigma-1 receptor chaperones in neurodegenerative and psychiatric disorders

INTRODUCTION

Sigma-1 receptors (Sig-1Rs) are molecular chaperones that reside mainly in the endoplasmic reticulum (ER) but exist also in the proximity of the plasma membrane. Sig-1Rs are highly expressed in the CNS and are involved in many cellular processes including cell differentiation, neuritogenesis, microglia activation, protein quality control, calcium-mediated ER stress and ion channel modulation. Disturbance in any of the above cellular processes can accelerate the progression of many neurological disorders; therefore, the Sig-1R has been implicated in several neurological diseases.

AREAS COVERED

This review broadly covers the functions of Sig-1Rs including several neurodegenerative disorders in humans and drug addiction-associated neurological disturbance in the case of HIV infection. We discuss how several Sig-1R ligands could be utilized in therapeutic approaches to treat those disorders.

EXPERT OPINION

Emerging understanding of the cellular functions of this unique transmembrane chaperone may lead to the use of new agents or broaden the use of certain available ligands as therapeutic targets in those neurological disorders.

Effects of BD1047, a σ1 receptor antagonist, on the expression of mTOR, Camk2γ and GSK-3β in fluvoxamine-treated N2a cells

Fluvoxamine, a common antidepressant agent, is designed to exert its pharmacological effect by inhibiting synaptic serotonin reuptake. However, increasing evidence has demonstrated that σ₁ receptors are likely to be involved in the mechanism of action of fluvoxamine. The present study aimed to observe the effects of fluvoxamine on the expression levels of mammalian target of rapamycin (mTOR), Ca²⁺/calmodulin-dependent protein kinase 2γ (Camk2γ) and glycogen synthase kinase-3β (GSK-3β) in fluvoxamine-treated N2a cells and attempted to elucidate whether σ₁ receptors mediate the pharmacological effects of fluvoxamine. The N2a cells were randomly divided into three groups (each n=6): DMEM group (D group), 0.5 μmol/l fluvoxamine group (F group) and 0.2 μmol/l BD1047 (a σ₁ receptor antagonist) + 0.5 μmol/l fluvoxamine group (BF group). Western blotting was used to determine the expression levels of mTOR, Camk2γ and GSK-3β in the cultured N2a cells after two days of incubation. The F group exhibited significant increases in the expression levels of mTOR and Camk2γ and a significant reduction in the expression levels of GSK-3β compared with those in the D group (P<0.01). By contrast, the BF group demonstrated significant reductions in the expression levels of mTOR and Camk2γ and a significant increase in the expression levels of GSK-3β, compared with those in the F group (P<0.01). These results suggest that σ₁ receptors mediate fluvoxamine-elicited changes in the expression levels of mTOR, Camk2γ and GSK-3β in N2a cells, which indicates that σ₁ receptors are likely to be involved in the pharmacological effects of fluvoxamine.

Add-on fluvoxamine treatment for schizophrenia: an updated meta-analysis of randomized controlled trials

We performed an updated meta-analysis of fluvoxamine add-on therapy in patients with schizophrenia treated with antipsychotics based on two previous meta-analyses (Sepehry et al., in J Clin Psychiatry 68:604-610, 2007 and Singh et al., in Br J Psychiatry J Mental Sci 197:174-179, 2010). We searched PubMed, the Cochrane Library database, and PsycINFO up to January 2013. We conducted a systematic review and meta-analysis of individual patient data from randomized controlled trials comparing fluvoxamine add-on therapy with placebo. The risk ratio (RR), 95 % confidence intervals (CI), and standardized mean difference (SMD) were calculated. Seven studies (total n = 272) were identified. These included two clozapine studies, one olanzapine study, one second-generation antipsychotic (SGA) monotherapy study, and three first-generation antipsychotics (FGAs) monotherapy studies. There were significant effect of fluvoxamine add-on therapy on overall (SMD = -0.46, CI = -0.75 to -0.16, p = 0.003, I (2) = 0 %, 5 studies, n = 180) and negative symptoms (SMD = -0.44, CI = -0.74 to -0.14, p = 0.004, I (2) = 0 %, 5 studies, n = 180). However, fluvoxamine add-on therapy showed no significant effects on positive symptoms, depressive symptoms, and discontinuations from any cause or adverse events. Fluvoxamine add-on therapy in patients primarily treated with SGAs improved overall (p = 0.02) but not negative symptoms (p = 0.31). On the other hand, fluvoxamine add-on therapy in patients primarily treated with FGAs improved both overall (p = 0.04) and negative symptoms (p = 0.004) compared with control groups. Our results suggest that fluvoxamine add-on therapy is more beneficial on the psychopathology (especially negative symptoms) than controls in patients with schizophrenia who are primarily treated with FGAs. Given that a small number of studies were included in this meta-analysis, the results should be treated with caution.

The sigma-1 receptor: roles in neuronal plasticity and disease

Sigma-1 receptors (Sig-1Rs) have been implicated in many neurological and psychiatric conditions. Sig-1Rs are intracellular chaperones that reside specifically at the endoplasmic reticulum (ER)-mitochondrion interface, referred to as the mitochondrion-associated ER membrane (MAM). Here, Sig-1Rs regulate ER-mitochondrion Ca(2+) signaling. In this review, we discuss the current understanding of Sig-1R functions. Based on this, we suggest that the key cellular mechanisms linking Sig-1Rs to neurological disorders involve the translocation of Sig-1Rs from the MAM to other parts of the cell, whereby Sig-1Rs bind and modulate the activities of various ion channels, receptors, or kinases. Thus, Sig-1Rs and their associated ligands may represent new avenues for treating aspects of neurological and psychiatric diseases.

Roles of σ1 receptors in the mechanisms of action of CNS drugs

Accumulating evidence suggests that σ1 receptors play a role in the mechanisms of action of some therapeutic drugs, such as the selective serotonin reuptake inhibitors (SSRIs), donepezil, and ifenprodil. Among the SSRIs, fluvoxamine, a potent σ1 receptor agonist, has the highest affinity for σ1 receptors, while donepezil and ifenprodil also show high affinity for σ1 receptors. These drugs affect neuronal plasticity indicated by potentiation of nerve-growth factor (NGF)-induced neurite outgrowth in PC12 cells. Furthermore, phencyclidine (PCP)-induced cognitive impairment, associated with animal models of schizophrenia, is significantly improved by sub-chronic administration of fluvoxamine and donepezil. These pharmacological actions are antagonised by treatment with the selective σ1 receptor antagonist NE-100. Positron emission tomography (PET) with the σ1 specific ligand carbon-11-labelled 1-[2-(3,4-dimethoxyphenyl)ethyl]-4-(3-phenylpropyl)piperazine ([11C]SA4503) indicated that fluvoxamine and donepezil can bind to σ1 receptors in the healthy human brain in a dose-dependent manner. These findings suggest that σ1 receptors may be involved in the mechanisms of action of some therapeutic drugs.

Insights into the Sigma-1 receptor chaperone's cellular functions: a microarray report

We previously demonstrated that Sig-1Rs are critical regulators in neuronal morphogenesis and development via the regulation of oxidative stress and mitochondrial functions. In the present study, we sought to identify pathways and genes that are affected by Sig-1R. Gene expression profiles were examined in rat hippocampal neurons that had been cultured for 18 days in vitro (DIV). The cells were transduced with AAV siRNA targeting Sig-1R on DIV 10 for 7 days, followed by gene expression analysis using a rat genome cDNA array. The gene array results indicated that Sig-1R knockdown hampered cellular functions including steroid biogenesis, protein ubiquitination, actin cytoskeleton network, and Nrf-2 mediated oxidative stress. Many of the cellular components important for actin polymerization and synapse plasticity, including F-actin capping protein and neurofilaments, were significantly changed in AAV-siSig-1R neurons. Further, cytochrome c was reduced in AAV-Sig-1R neurons whereas free-radical generating enzymes including cytochrome p450 and cytochrome b-245 were increased. The microarray results also suggest that Sig-1Rs may regulate genes that are involved in the pathogenesis of many CNS diseases including Alzheimer's disease and Parkinson's disease. These data further confirmed that Sig-1Rs play critical roles in the CNS and thus these findings may aid in future development of therapeutic treatments targeting neurodegenerative disorders.

The cholinergic system, sigma-1 receptors and cognition

This article provides an overview of present knowledge regarding the relationship between the cholinergic system and sigma-1 receptors, and discusses potential applications of sigma-1 receptor agonists in the treatment of memory deficits and cognitive disorders. Sigma-1 receptors, initially considered as a subtype of the opioid family, are unique ligand-regulated molecular chaperones in the endoplasmatic reticulum playing a modulatory role in intracellular calcium signaling and in the activity of several neurotransmitter systems, particularly the cholinergic and glutamatergic pathways. Several central nervous system (CNS) drugs show high to moderate affinities for sigma-1 receptors, including acetylcholinesterase inhibitors (donepezil), antipsychotics (haloperidol, rimcazole), selective serotonin reuptake inhibitors (fluvoxamine, sertraline) and monoamine oxidase inhibitors (clorgyline). These compounds can influence cognitive functions both via their primary targets and by activating sigma-1 receptors in the CNS. Sigma-1 agonists show powerful anti-amnesic and neuroprotective effects in a large variety of animal models of cognitive dysfunction involving, among others (i) pharmacologic target blockade (with muscarinic or NMDA receptor antagonists or p-chloroamphetamine); (ii) selective lesioning of cholinergic neurons; (iii) CNS administration of β-amyloid peptides; (iv) aging-induced memory loss, both in normal and senescent-accelerated rodents; (v) neurodegeneration induced by toxic compounds (CO, trimethyltin, cocaine), and (vi) prenatal restraint stress.

The SIGMAR1 gene is associated with a risk of schizophrenia and activation of the prefrontal cortex

Several studies have identified the possible involvement of sigma non-opioid intracellular receptor 1 (SIGMAR1) in the pathogenesis of schizophrenia. The Gln2Pro polymorphism in the SIGMAR1 gene has been extensively examined for an association with schizophrenia. However, findings across multiple studies have been inconsistent. We performed a meta-analysis of the association between the functional Gln2Pro polymorphism and schizophrenia using combined samples (1254 patients with schizophrenia and 1574 healthy controls) from previously published studies and our own additional samples (478 patients and 631 controls). We then used near-infrared spectroscopy to analyze the effects of the Gln2Pro genotype, a schizophrenia diagnosis and the interaction between genotype and diagnosis on activation of the prefrontal cortex (PFC) during a verbal fluency task (127 patients and 216 controls). The meta-analysis provided evidence of an association between Gln2Pro and schizophrenia without heterogeneity across studies (odds ratio=1.12, p=0.047). Consistent with previous studies, patients with schizophrenia showed lower bilateral activation of the PFC when compared to controls (p<0.05). We provide evidence that Pro carriers, who are more common among patients with schizophrenia, have significantly lower activation of the right PFC compared to subjects with the Gln/Gln genotype (p=0.013). These data suggest that the SIGMAR1 polymorphism is associated with an increased risk of schizophrenia and differential activation of the PFC.

Behavioural phenotyping of knockout mice for the sigma-1 (σ₁) chaperone protein revealed gender-related anxiety, depressive-like and memory alterations

The sigma-1 (σ₁) protein regulates calcium homeostasis and acts as an endoplasmic reticulum chaperone. It can be activated by ligands which impact memory, depression, anxiety or addiction processes. We here characterized the behavioural phenotype of knockout (KO) mice for the σ₁ protein. Two-month old male σ₁⁻/⁻ mice showed signs of anxiety in the open-field, passive avoidance or elevated plus-maze test, but other activity or memory responses were unchanged. Female σ₁⁻/⁻ mice showed deficits in spontaneous alternation or water-maze learning. Twelve-month old σ₁⁺/⁻ female mice showed deficits in alternation and σ₁⁻/⁻ mice in avoidance escape latency. Two- and 14-month old female σ₁⁻/⁻ mice showed decreased plasma 17β-estradiol levels. Treatment with 17β-estradiol (0.1, 0.2 mg/kg i.p.) reversed the spatial memory deficits in young and aged mice. Male σ₁ KO mice showed enhanced response in the forced swimming test. Igmesine, a σ₁ agonist, failed to decrease immobility in σ₁ KO mice. Fluoxetine and sertraline were more efficient in σ₁ KO mice, an effect likely related to their σ₁ antagonist activity. Imipramine, desipramine and amitriptyline were equally active. σ₁ protein invalidation therefore affected stress or anxiety response but not memory in males. Changes in steroid tonus in female animals led, however, to memory impairments that increased with age.

Targeting ligand-operated chaperone sigma-1 receptors in the treatment of neuropsychiatric disorders

INTRODUCTION

Current drugs for the treatment of psychiatric or neurodegenerative disorders have limitations. Psychotherapeutic drugs such as typical and atypical antipsychotics, tricyclic antidepressants and selective monoamine reuptake inhibitors, aim to normalize the hyper- or hypo-neurotransmission of monoaminergic systems. Despite their contribution to the outcomes of psychiatric patients, these agents often exert severe side effects and require chronic treatments to promote amelioration of symptoms. Drugs available for the treatment of neurodegenerative disorders are severely limited.

AREAS COVERED

Recent evidence that has shed light on sigma-1 receptor ligands, which may serve as a new class of antidepressants or neuroprotective agents. Sigma-1 receptors are novel ligand-operated molecular chaperones regulating signal transduction, ER stress, cellular redox, cellular survival and synaptogenesis. Selective sigma-1 receptor ligands exert rapid antidepressant-like, anxiolytic, antinociceptive and robust neuroprotective actions in preclinical studies. Recent studies that suggest that reactive oxygen species might play a role as signal integrators downstream of Sig-1Rs are also covered.

EXPERT OPINION

The advances in sigma receptor research in the last decade have begun to elucidate the intracellular signal cascades upstream and downstream of sigma-1 receptors. The novel ligand-operated properties of the sigma-1 receptor chaperone may enable interventions by which stress-related cellular systems can be pharmacologically controlled.

Cognition and depression: the effects of fluvoxamine, a sigma-1 receptor agonist, reconsidered

Cognitive impairment is a primary feature of patients with major depressive disorder (MDD) and is characterised by stress-induced neural atrophy. Via alpha-adrenergic, anti-cholinergic and anti-histaminic activities, several antidepressants can cause significant counter-therapeutic cognitive impairment. Evidence is emerging of the involvement of sigma-1 receptor agonism in the mechanism of action of some antidepressants, notably fluvoxamine. Sigma-1 receptors are abundant in areas affected by depression/stress-induced cerebral atrophy and their ligands have a unique pharmacological profile; they may promote neurogenesis and initiate adaptive neural plasticity as a protection/reaction to stress. Fluvoxamine, as a potent sigma-1 receptor agonist, has shown ameliorating effects in animal models of psychosis, depression, stress, anxiety, obsessive-compulsive disorder (OCD) and aggression and has been shown to improve cognitive impairments. In humans, fluvoxamine may repair central nervous system (CNS) atrophy and restore cognitive function. The current review explores the mechanisms through which sigma-1 receptors can modulate cognitive function and examines how antidepressant therapy with fluvoxamine may help improve cognitive outcomes in patients with depression.

Sigma-1 receptors regulate hippocampal dendritic spine formation via a free radical-sensitive mechanism involving Rac1xGTP pathway

Sigma-1 receptors (Sig-1Rs) are endoplasmic reticulum (ER)-resident proteins known to be involved in learning and memory. Dendritic spines in hippocampal neurons play important roles in neuroplasticity and learning and memory. This study tested the hypothesis that Sig-1Rs might regulate denritic spine formation in hippocampal neurons and examined potential mechanisms therein. In rat hippocampal primary neurons, the knockdown of Sig-1Rs by siRNAs causes a deficit in the formation of dendritic spines that is unrelated to ER Ca(2+) signaling or apoptosis, but correlates with the mitochondrial permeability transition and cytochrome c release, followed by caspase-3 activation, Tiam1 cleavage, and a reduction in Rac1.GTP. Sig-1R-knockdown neurons contain higher levels of free radicals when compared to control neurons. The activation of superoxide dismutase or the application of the hydroxyl-free radical scavenger N-acetyl cysteine (NAC) to the Sig-1R-knockdown neurons rescues dendritic spines and mitochondria from the deficits caused by Sig-1R siRNA. Further, the caspase-3-resistant TIAM1 construct C1199DN, a stable guanine exchange factor able to constitutively activate Rac1 in the form of Rac1.GTP, also reverses the siRNA-induced dendritic spine deficits. In addition, constitutively active Rac1.GTP reverses this deficit. These results implicate Sig-1Rs as endogenous regulators of hippopcampal dendritic spine formation and suggest a free radical-sensitive ER-mitochondrion-Rac1.GTP pathway in the regulation of dendritic spine formation in the hippocampus.

The pharmacology of sigma-1 receptors

Originally considered an enigmatic protein, the sigma-1 receptor has recently been identified as a unique ligand-regulated molecular chaperone in the endoplasmic reticulum of cells. This discovery causes us to look back at the many proposed roles of this receptor, even before its molecular function was identified, in many diseases such as methamphetamine or cocaine addiction, amnesia, pain, depression, Alzheimer's disease, stroke, retinal neuroprotection, HIV infection, and cancer. In this review, we examine the reports that have clearly shown an agonist-antagonist relationship regarding sigma-1 receptors in models of those diseases and also review the relatively known mechanisms of action of sigma-1 receptors in an attempt to spur the speculation of readers on how the sigma-1 receptor at the endoplasmic reticulum might relate to so many diseases. We found that the most prominent action of sigma-1 receptors in biological systems including cell lines, primary cultures, and animals is the regulation and modulation of voltage-regulated and ligand-gated ion channels, including Ca(2+)-, K(+)-, Na(+), Cl(-), and SK channels, and NMDA and IP3 receptors. We found that the final output of the action of sigma-1 receptor agonists is to inhibit all above-mentioned voltage-gated ion channels, while they potentiate ligand-gated channels. The inhibition or potentiation induced by agonists is blocked by sigma-1 receptor antagonists. Other mechanisms of action of sigma-1 receptors, and to some extent those of sigma-2 receptors, were also considered. We conclude that the sigma-1 and sigma-2 receptors represent potential fruitful targets for therapeutic developments in combating many human diseases.

Phencyclidine-induced cognitive deficits in mice are ameliorated by subsequent subchronic administration of donepezil: role of sigma-1 receptors

This study was undertaken to examine the effects of two acetylcholinesterase inhibitors (donepezil and physostigmine) on cognitive deficits in mice after repeated administration of the NMDA receptor antagonist phencyclidine (PCP). In the novel object recognition test, PCP (10 mg/kg/day for 10 days)-induced cognitive deficits were significantly improved by subsequent subchronic (14 days) administration of donepezil (1.0 mg/kg/day), but not donepezil (0.1 mg/kg/day). Furthermore, the effect of donepezil (1.0 mg/kg/day) on PCP-induced cognitive deficits was significantly antagonized by co-administration of the selective sigma-1 receptor antagonist NE-100 (1.0 mg/kg/day), suggesting the role of sigma-1 receptors in the active mechanisms of donepezil. In contrast, PCP-induced cognitive deficits were not improved by subsequent subchronic (14 days) administration of physostigmine (0.25 mg/kg/day). Moreover, repeated administration of PCP significantly caused the reduction of sigma-1 receptors in the hippocampus. The present study suggests that agonistic activity of donepezil at sigma-1 receptors plays a role in the active mechanisms of donepezil on PCP-induced cognitive deficits in mice. Therefore, it is likely that donepezil would be potential therapeutic drugs for the treatment of the cognitive deficits in schizophrenia.