Further evidence for an antidepressant potential of the selective sigma1 agonist SA 4503: electrophysiological, morphological and behavioural studies

Sigma-1 receptor activation produces faster antidepressant-like effect through enhancement of hippocampal neuroplasticity: Focus on sigma-1-5-HT1A heteroreceptor complex

The sigma-1 receptor (S1R) has garnered significant attention as a potential target for rapid-onset antidepressant-like effects, particularly owing to its ability to swiftly stimulate serotonergic neurons in the dorsal raphe nucleus (DRN). However, the precise mechanisms underlying its regulatory effects remain unclear. Therefore, this study aims to examine the interaction between SA-4503 (a selective S1R agonist) and 8-OH-DPAT (a serotonin1A (5-HT1A) receptor agonist) in mice with depressive-like behavior induced by chronic restraint stress (CRS). Preliminary studies were conducted to explore the potential mechanisms underlying the accelerated antidepressant-like effects resulting from the combined activation of S1R and 5-HT1A receptors. The results showed that the coadministration of SA4503 (1.0 mg/kg, orally) and 8-OH-DPAT (0.3 mg/kg, i. g.) produced antidepressant-like effects. However, the doses of 8-OH-DPAT used in this study did not exhibit intrinsic antidepressant-like activity in this model. Moreover, using an in-situ proximity ligation assay provided the first evidence of S1R-5-HT1A heteroreceptor complexes in the midbrain DRN and dentate gyrus (DG) of the forebrain in mice. The formation of these heterocomplexes was influenced by pharmacological agents and was closely associated with depressive-like behavior development in mice. Mechanistic analysis revealed that the combined activation of S1R and 5-HT1A receptors synergistically enhanced neurogenesis and plasticity in the dorsal DG region of the hippocampus in mice subjected to CRS. These findings significantly advance our understanding of S1R-mediated neuroplasticity, suggesting potential therapeutic strategies for developing rapid-acting antidepressants.

The role of 5-HTergic neuron activation in the rapid antidepressant-like effects of hypidone hydrochloride (YL-0919) in mice

Introduction

Major depressive disorder (MDD) is a common and disabling mental health condition; the currently available treatments for MDD are insufficient to meet clinical needs due to their limited efficacy and slow onset of action. Hypidone hydrochloride (YL-0919) is a sigma-1 receptor agonist and a novel fast-acting antidepressant that is currently under clinical development.

Methods

To further understand the fast-acting antidepressant activity of YL-0919, this study focused on the role of 5-HTergic neurons in the dorsal raphe nucleus (DRN) in mice. Using fiber photometry to assess neural activity in vivo and two behavioral assays (tail suspension test and forced swimming test) to evaluate antidepressant-like activity.

Results

It was found that 3 or 7 days of YL-0919 treatment significantly activated serotonin (5-HT) neurons in the DRN and had significant antidepressant-like effects on mouse behaviors. Chemogenetic inhibition of 5-HTergic neurons in the DRN significantly blocked the antidepressant-like effect of YL-0919. In addition, YL-0919 treatment significantly increased the 5-HT levels in the prefrontal cortex (PFC). These changes were drastically different from those of the selective serotonin reuptake inhibitor (SSRI) fluoxetine, which suggested that the antidepressant-like effects of the two compounds were mechanistically different.

Conclusion

Together, these results reveal a novel role of 5-HTergic neurons in the DRN in mediating the fast-acting antidepressant-like effects of YL-0919, revealing that these neurons are potential novel targets for the development of fast-acting antidepressants for the clinical management of MDD.

[18F]Fluspidine-A PET Tracer for Imaging of σ1 Receptors in the Central Nervous System

σ1 receptors play a crucial role in various neurological and neurodegenerative diseases including pain, psychosis, Alzheimer's disease, and depression. Spirocyclic piperidines represent a promising class of potent σ1 receptor ligands. The relationship between structural modifications and σ1 receptor affinity and selectivity over σ2 receptors led to the 2-fluoroethyl derivative fluspidine (2, Ki = 0.59 nM). Enantiomerically pure (S)-configured fluspidine ((S)-2) was prepared by the enantioselective reduction of the α,β-unsaturated ester 23 with NaBH4 and the enantiomerically pure co-catalyst (S,S)-24. The pharmacokinetic properties of both fluspidine enantiomers (R)-2 and (S)-2 were analyzed in vitro. Molecular dynamics simulations revealed very similar interactions of both fluspidine enantiomers with the σ1 receptor protein, with a strong ionic interaction between the protonated amino moiety of the piperidine ring and the COO- moiety of glutamate 172. The 18F-labeled radiotracers (S)-[18F]2 and (R)-[18F]2 were synthesized in automated syntheses using a TRACERlab FX FN synthesis module. High radiochemical yields and radiochemical purity were achieved. Radiometabolites were not found in the brains of mice, piglets, and rhesus monkeys. While both enantiomers revealed similar initial brain uptake, the slow washout of (R)-[18F]2 indicated a kind of irreversible binding. In the first clinical trial, (S)-[18F]2 was used to visualize σ1 receptors in the brains of patients with major depressive disorder (MDD). This study revealed an increased density of σ1 receptors in cortico-striato-(para)limbic brain regions of MDD patients. The increased density of σ1 receptors correlated with the severity of the depressive symptoms. In an occupancy study with the PET tracer (S)-[18F]2, the selective binding of pridopidine at σ1 receptors in the brain of healthy volunteers and HD patients was shown.

Sigma-1 receptor agonist properties that mediate the fast-onset antidepressant effect of hypidone hydrochloride (YL-0919)

The most intriguing characteristic of the sigma-1 receptor is its ability to regulate multiple functional proteins directly via protein-protein interactions, giving the sigma-1 receptor the powerful ability to regulate several survival and metabolic functions in cells, fine tune neuronal excitability, and regulate the transmission of information within brain circuits. This characteristic makes sigma-1 receptors attractive candidates for the development of new drugs. Hypidone hydrochloride (YL-0919), a novel structured antidepressant candidate developed in our laboratory, possess a selective sigma-1 receptor agonist profile, as evidenced by molecular docking, radioligand receptor binding assays, and receptor functional experiments. In vivo studies have revealed that YL-0919 elicits a fast-onset antidepressant activity (within one week) that can be attenuated with pretreatment of the selective sigma-1 receptor antagonist, BD-1047. Taken together, the findings of the current study suggest that YL-0919 activates the sigma-1 receptor to partially mediate the rapid onset antidepressant effects of YL-0919. Thus, YL-0919 is a promising candidate as a fast-onset antidepressant that targets the sigma-1 receptor.

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.

Fenfluramine modulates the anti-amnesic effects induced by sigma-1 receptor agonists and neuro(active)steroids in vivo

Fenfluramine (N-ethyl-α-methl-3-(trifluoromethyl)phenethylamine) is an anti-seizure medication (ASM) particularly effective in patients with Dravet syndrome, a severe treatment-resistant epileptic encephalopathy. Fenfluramine acts not only as neuronal serotonin (5-HT) releaser but also as a positive modulator of the sigma-1 receptor (S1R). We here examined the modulatory activity of Fenfluramine on the S1R-mediated anti-amnesic response in mice using combination analyses. Fenfluramine and Norfenfluramine, racemate and isomers, were combined with either the S1R agonist (PRE-084) or the S1R-acting neuro(active)steroids, pregnenolone sulfate (PREGS), Dehydroepiandrosterone sulfate (DHEAS), or progesterone. We report that Fenfluramine racemate or (+)-Fenfluramine, in the 0.1-1 mg/kg dose range, attenuated the dizocilpine-induced learning deficits in spontaneous alternation and passive avoidance, and showed low-dose synergies in combination with PRE-084. These effects were blocked by the S1R antagonist NE-100. Dehydroepiandrosterone sulfate or PREGS attenuated dizocilpine-induced learning deficits in the 5-20 mg/kg dose range. Co-treatments at low dose between steroids and Fenfluramine or (+)-Fenfluramine were synergistic. Progesterone blocked Fenfluramine effect. Finally, Fenfluramine and (+)-Fenfluramine effects were prevented by the 5-HT1A receptor antagonist WAY-100635 or 5-HT2A antagonist RS-127445, but not by the 5-HT1B/1D antagonist GR 127935 or the 5-HT2C antagonist SB 242084, confirming a 5-HT1A and 5-HT2A receptor involvement in the drug effect on memory. We therefore confirmed the positive modulation of Fenfluramine racemate or dextroisomer on S1R and showed that, in physiological conditions, the drug potentiated the low dose effects of neuro(active)steroids, endogenous S1R modulators. The latter are potent modulators of the excitatory/inhibitory balance in the brain, and their levels must be considered in the antiepileptic action of Fenfluramine.

Pridopidine reduces mutant huntingtin-induced endoplasmic reticulum stress by modulation of the Sigma-1 receptor

The endoplasmic reticulum (ER)-localized Sigma-1 receptor (S1R) is neuroprotective in models of neurodegenerative diseases, among them Huntington disease (HD). Recent clinical trials in HD patients and preclinical studies in cellular and mouse HD models suggest a therapeutic potential for the high-affinity S1R agonist pridopidine. However, the molecular mechanisms of the cytoprotective effect are unclear. We have previously reported strong induction of ER stress by toxic mutant huntingtin (mHtt) oligomers, which is reduced upon sequestration of these mHtt oligomers into large aggregates. Here, we show that pridopidine significantly ameliorates mHtt-induced ER stress in cellular HD models, starting at low nanomolar concentrations. Pridopidine reduced the levels of markers of the three branches of the unfolded protein response (UPR), showing the strongest effects on the PKR-like endoplasmic reticulum kinase (PERK) branch. The effect is S1R-dependent, as it is abolished in cells expressing mHtt in which the S1R was deleted using CRISPR/Cas9 technology. mHtt increased the level of the detergent-insoluble fraction of S1R, suggesting a compensatory cellular mechanism that responds to increased ER stress. Pridopidine further enhanced the levels of insoluble S1R, suggesting the stabilization of activated S1R oligomers. These S1R oligomeric species appeared in ER-localized patches, and not in the mitochondria-associated membranes nor the ER-derived quality control compartment. The colocalization of S1R with the chaperone BiP was significantly reduced by mHtt, and pridopidine restored this colocalization to normal, unstressed levels. Pridopidine increased toxic oligomeric mHtt recruitment into less toxic large sodium dodecyl sulfate-insoluble aggregates, suggesting that this in turn reduces ER stress and cytotoxicity.

Bi-phasic dose response in the preclinical and clinical developments of sigma-1 receptor ligands for the treatment of neurodegenerative disorders

Introduction: The sigma-1 receptor (S1R) is attracting much attention for disease-modifying therapies in neurodegenerative diseases. It is a conserved protein, present in plasma and endoplasmic reticulum (ER) membranes and enriched in mitochondria-associated ER membranes (MAMs). It modulates ER-mitochondria Ca2+ transfer and ER stress pathways. Mitochondrial and MAM dysfunctions contribute to neurodegenerative processes in diseases such as Alzheimer, Parkinson, Huntington or Amyotrophic Lateral Sclerosis. Interestingly, the S1R can be activated by small druggable molecules and accumulating preclinical data suggest that S1R agonists are effective protectants in these neurodegenerative diseases.Area covered: In this review, we will present the data showing the high therapeutic potential of S1R drugs for the treatment of neurodegenerative diseases, focusing on pridopidine as a potent and selective S1R agonist under clinical development. Of particular interest is the bi-phasic (bell-shaped) dose-response effect, representing a common feature of all S1R agonists and described in numerous preclinical models in vitro, in vivo and in clinical trials.Expert opinion: S1R agonists modulate inter-organelles communication altered in neurodegenerative diseases and activate intracellular survival pathways. Research will continue growing in the future. The particular cellular nature of this chaperone protein must be better understood to facilitate the clinical developement of promising molecules.

The Sigma-1 Receptor Mediates Pridopidine Rescue of Mitochondrial Function in Huntington Disease Models

Pridopidine is a selective Sigma-1 receptor (S1R) agonist in clinical development for Huntington disease (HD) and amyotrophic lateral sclerosis. S1R is a chaperone protein localized in mitochondria-associated endoplasmic reticulum (ER) membranes, a signaling platform that regulates Ca2+ signaling, reactive oxygen species (ROS) and mitochondrial fission. Here, we investigate the protective effects of pridopidine on various mitochondrial functions in human and mouse HD models. Pridopidine effects on mitochondrial dynamics were assessed in primary neurons from YAC128 HD mice expressing the mutant human HTT gene. We observe that pridopidine prevents the disruption of mitochondria-ER contact sites and improves the co-localization of inositol 1,4,5-trisphosphate receptor (IP3R) and its chaperone S1R with mitochondria in YAC128 neurons, leading to increased mitochondrial activity, elongation, and motility. Increased mitochondrial respiration is also observed in YAC128 neurons and in pridopidine-treated HD human neural stem cells (hNSCs). ROS levels were assessed after oxidative insult or S1R knockdown in pridopidine-treated YAC128 neurons, HD hNSCs, and human HD lymphoblasts. All HD models show increased ROS levels and deficient antioxidant response, which are efficiently rescued with pridopidine. Importantly, pridopidine treatment before H2O2-induced mitochondrial dysfunction and S1R presence are required for HD cytoprotection. YAC128 mice treated at early/pre-symptomatic age with pridopidine show significant improvement in motor coordination, indicating a delay in symptom onset. Additionally, in vivo pridopidine treatment reduces mitochondrial ROS levels by normalizing mitochondrial complex activity. In conclusion, S1R-mediated enhancement of mitochondrial function contributes to the neuroprotective effects of pridopidine, providing insight into its mechanism of action and therapeutic potential.

Efficacy of dextromethorphan for the treatment of depression: a systematic review of preclinical and clinical trials

INTRODUCTION

The large percentage of adults with major depressive disorder (MDD) insufficiently responding and/or tolerating conventional monoamine-based antidepressants invites the need for mechanistically novel treatments. Convergent evidence implicates glutamatergic signaling as a potential therapeutic target in MDD.

AREAS COVERED

The synthesis herein of preclinical and clinical studies indicates that dextromethorphan (DXM) is well tolerated and exhibits clinically significant antidepressant effects; DXM combined with bupropion has demonstrated replicated and relatively rapid onset efficacy in adults with MDD. DXM efficacy has been preliminarily reported in adults with bipolar depression. The combination of DXM and bupropion represents a pharmacokinetic and pharmacodynamic synergy which may account for the rapidity of action in MDD.

EXPERT OPINION

The combination of DXM and bupropion is a safe, well tolerated and efficacious treatment option in adults with MDD. Priority questions are whether DXM/bupropion is uniquely effective across discrete domains of psychopathology (e.g. anhedonia, reward processing, general cognitive systems) and/or whether it is able to significantly improve patient-reported outcomes (e.g. quality of life, psychosocial functioning). The availability of ketamine/esketamine and DXM/bupropion instantiates the relevance of glutamate as a treatment target in MDD. Studies in bipolar depression with DXM/bupropion are warranted as well as in MDD with suicidality.

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.

Sigma-1 (σ1) receptor activity is necessary for physiological brain plasticity in mice

The sigma-1 receptor (S1R) is a membrane-associated protein expressed in neurons and glia at mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs). S1R interacts with different partners to regulate cellular responses, including ER stress, mitochondrial physiology and Ca2+ fluxes. S1R shapes cellular plasticity by directly modulating signaling pathways involved in inflammatory responses, cell survival and death. We here analyzed its impact on brain plasticity in vivo, in mice trained in a complex maze, the Hamlet test. The device, providing strong enriched environment (EE) conditions, mimics a small village. It has a central agora and streets expanding from it, leading to functionalized houses where animals can Drink, Eat, Hide, Run, or Interact. Animals were trained in groups, 4 h/day for two weeks, and their maze exploration and topographic memory could be analyzed. Several groups of mice were considered: non-trained vs. trained; repeatedly administered with saline vs. NE-100, a selective S1R antagonist; and wildtype vs. S1R KO mice. S1R inactivation altered maze exploration and prevented topographic learning. EE induced a strong plasticity measured through resilience to behavioral despair or to the amnesic effects of scopolamine, and increases in S1R expression and bdnf mRNA levels in the hippocampus; increases in neurogenesis (proliferation and maturation); and increases of histone acetylation in the hippocampus and cortex. S1R inactivation altered all these parameters significantly, showing that S1R activity plays a major role in physiological brain plasticity. As S1R is a major resident protein in MAMs, modulating ER responses and mitochondrial homeostasy, MAM physiology appeared impacted by enriched environment.

Effects of Pridopidine on Functional Capacity in Early-Stage Participants from the PRIDE-HD Study

BACKGROUND

No pharmacological treatment has been demonstrated to provide a functional benefit for persons with Huntington's disease (HD). Pridopidine is a sigma-1-receptor agonist shown to have beneficial effects in preclinical models of HD.

OBJECTIVE

To further explore the effect of pridopidine on Total Functional Capacity (TFC) in the recent double-blind, placebo-controlled PRIDE-HD study.

METHODS

We performed post-hoc analyses to evaluate the effect of pridopidine on TFC at 26 and 52 weeks. Participants were stratified according to baseline TFC score and analyzed using repeated measures (MMRM) and multiple imputation assuming missing not-at-random (MNAR) and worst-case scenarios.

RESULTS

The pridopidine 45 mg bid dosage demonstrated a beneficial effect on TFC for the entire population at week 52 of 0.87 (nominal p = 0.0032). The effect was more pronounced for early HD participants (HD1/HD2, TFC = 7-13), with a change from placebo of 1.16 (nominal p = 0.0003). This effect remained nominally significant using multiple imputation with missing not at random assumption as a sensitivity analysis. Responder analyses showed pridopidine 45 mg bid reduced the probability of TFC decline in early HD patients at Week 52 (nominal p = 0.02).

CONCLUSION

Pridopidine 45 mg bid results in a nominally significant reduction in TFC decline at 52 weeks compared to placebo, particularly in patients with early-stage HD.

Blockade of the Sigma-1 Receptor Relieves Cognitive and Emotional Impairments Associated to Chronic Osteoarthritis Pain

Osteoarthritis is the most common musculoskeletal disease worldwide, often characterized by degradation of the articular cartilage, chronic joint pain and disability. Cognitive dysfunction, anxiety and depression are common comorbidities that impact the quality of life of these patients. In this study, we evaluated the involvement of sigma-1 receptor (σ1R) on the nociceptive, cognitive and emotional alterations associated with chronic osteoarthritis pain. Monosodium iodoacetate (MIA) was injected into the knee of Swiss-albino CD1 mice to induce osteoarthritis pain, which then received a repeated treatment with the σ1R antagonist E-52862 or its vehicle. Nociceptive responses and motor performance were assessed with the von Frey and the Catwalk gait tests. Cognitive alterations were evaluated using the novel object recognition task, anxiety-like behavior with the elevated plus maze and the zero-maze tests, whereas depressive-like responses were determined using the forced swimming test. We also studied the local effect of the σ1R antagonist on cartilage degradation, and its central effects on microglial reactivity in the medial prefrontal cortex. MIA induced mechanical allodynia and gait abnormalities that were prevented by the chronic treatment with the σ1R antagonist. E-52862 also reduced the memory impairment and the depressive-like behavior associated to osteoarthritis pain. Interestingly, the effect of E-52862 on depressive-like behavior was not accompanied by a modification of anxiety-like behavior. The pain-relieving effects of the σ1R antagonist were not due to a local effect on the articular cartilage, since E-52862 treatment did not modify the histological alterations of the knee joints. However, E-52862 induced central effects revealed by a reduction of the cortical microgliosis observed in mice with osteoarthritis pain. These findings show that σ1R antagonism inhibits mechanical hypersensitivity, cognitive deficits and depressive-like states associated with osteoarthritis pain in mice. These effects are associated with central modulation of glial activity but are unrelated to changes in cartilage degradation. Therefore, targeting the σ1R with E-52862 represents a promising pharmacological approach with effects on multiple aspects of chronic osteoarthritis pain that may go beyond the strict inhibition of nociception.

Ketamine and Ketamine Metabolite Pharmacology: Insights into Therapeutic Mechanisms

Ketamine, a racemic mixture consisting of (S)- and (R)-ketamine, has been in clinical use since 1970. Although best characterized for its dissociative anesthetic properties, ketamine also exerts analgesic, anti-inflammatory, and antidepressant actions. We provide a comprehensive review of these therapeutic uses, emphasizing drug dose, route of administration, and the time course of these effects. Dissociative, psychotomimetic, cognitive, and peripheral side effects associated with short-term or prolonged exposure, as well as recreational ketamine use, are also discussed. We further describe ketamine's pharmacokinetics, including its rapid and extensive metabolism to norketamine, dehydronorketamine, hydroxyketamine, and hydroxynorketamine (HNK) metabolites. Whereas the anesthetic and analgesic properties of ketamine are generally attributed to direct ketamine-induced inhibition of N-methyl-D-aspartate receptors, other putative lower-affinity pharmacological targets of ketamine include, but are not limited to, γ-amynobutyric acid (GABA), dopamine, serotonin, sigma, opioid, and cholinergic receptors, as well as voltage-gated sodium and hyperpolarization-activated cyclic nucleotide-gated channels. We examine the evidence supporting the relevance of these targets of ketamine and its metabolites to the clinical effects of the drug. Ketamine metabolites may have broader clinical relevance than was previously considered, given that HNK metabolites have antidepressant efficacy in preclinical studies. Overall, pharmacological target deconvolution of ketamine and its metabolites will provide insight critical to the development of new pharmacotherapies that possess the desirable clinical effects of ketamine, but limit undesirable side effects.

Inducing a long-term potentiation in the dentate gyrus is sufficient to produce rapid antidepressant-like effects

Recent hypotheses propose that one prerequisite to obtain a rapid antidepressant (AD) effect would reside in processes of synaptic reinforcement occurring within the dentate gyrus (DG) of the hippocampus independently from neurogenesis. However, to date no relationship has been established between an increased DG synaptic plasticity, and rapid AD-like action. To the best of our knowledge, this study shows for the first time that inducing a long-term potentiation (LTP) within the DG by stimulating the perforant pathway (PP) is sufficient to induce such effects. Thus, Sprague-Dawley rats having undergone a successful LTP displayed a significant reduction of immobility when passed acutely 3 days thereafter in the forced swimming test (FST). Further, in a longitudinal paradigm using the pseudo-depressed Wistar-Kyoto rat strain, LTP elicited a decrease of FST immobility after only 2 days, whereas the AD desipramine was not effective before 16 days. In both models, the influence of LTP was transient, as it was no more observed after 8-9 days. No effects were observed on the locomotor activity or on anxiety-related behavior. Theta-burst stimulation of a brain region anatomically adjacent to the PP remained ineffective in the FST. Immunoreactivity of DG cells for phosphorylated histone H3 and doublecortin were not modified three days after LTP, indicating a lack of effect on both cell proliferation and neurogenesis. Finally, depleting brain serotonin contents reduced the success rate of LTP but did not affect its subsequent AD-like effects. These results confirm the 'plastic DG' theory of rapid AD efficacy. Beyond, they point out stimulations of the entorhinal cortex, from which the PP originates, as putative new approaches in AD research.

Drug repurposing may generate novel approaches to treating depression

Objectives

The breakthrough advancements in scientific medical research have greatly improved our understanding of the pathogenesis of depression, encouraging drug discoverers to take a shorter path than ever through drug repurposing to generate new antidepressant medications. In addition to reduced noradrenergic and serotonergic neurotransmission in the brain, other coincidence features such as glutamate neurotoxicity, inflammation and/or cerebrovascular insufficiency are implicated in the pathogenesis of major depressive disorder and late-life depression. This short review discusses the progress made in repurposing drugs for antidepressant actions.

Key findings

Drugs being repurposed as antidepressants act on novel drug targets, thereby treating resistant depression and improving remission rate. Drugs such as ketamine, dextromethorphan/quinidine and scopolamine are rapidly acting antidepressants targeting glutamate receptors. Nimodipine and quetiapine are efficient add-on therapy for late-life depression. Anti-inflammatory drugs, statins, insulin sensitizers, minocycline could remarkably contribute to treating refractory depression.

Summary

Drug repurposing represents an alternative approach to cope with major obstacles, including financial insufficiency and unavoidable long lag evaluation time, undermining the classical pathway of developing new hit compounds into clinically approved antidepressants.

Synthesis and pharmacological evaluation of benzamide derivatives as potent and selective sigma-1 protein ligands

A series of novel benzamide-derived compounds was designed, synthesized and pharmacologically evaluated. Among all 37 synthesized compounds, two series were developed with the modulation of the nature, the position of atoms or groups on the benzamide scaffold, but also the nature of the amine group separated from the benzamide with 2, 3 or 4 methylene groups. In vitro competition binding assays against sigma proteins (sigma-1 S1R and sigma-2 S2R) revealed that most of them conferred S2R/S1R selectivity toward without cytotoxic effects on SY5Y cells, especially with the first series with compounds 7a-z. Some selected compounds were also evaluated for their agonist and antagonist activities on a panel of 40 receptors. Results showed the importance of the nature and the position with halogeno atom on the benzamide scaffold, the length chain but also the contribution of the hydrophobic part on the amine group. Among them, compounds 7i, w, y with Cl, CN or NO₂ groups at the 4-position of the benzamide scaffold showed excellent affinity for S1R (Ki = 1.2-3.6 nM), selectivity for S2R (Ki up to 1400 nM) and high selectivity index (IC_50(SY5Y)/Ki_(S1R) ratio from 28 000 to 83 000). Futhermore, these compounds presented an excellent safety profile over 40 other receptors. These derivatives will be selected for further biological investigations.

Anti-anhedonic effect of selective serotonin reuptake inhibitors with affinity for sigma-1 receptors in picrotoxin-treated mice

BACKGROUND AND PURPOSE

Prefrontal dopamine release by the combined activation of 5-HT1A and sigma-1 (σ1 ) receptors is enhanced by the GABAA receptor antagonist picrotoxin in mice. Here, we examined whether this neurochemical event was accompanied by behavioural changes.

EXPERIMENTAL APPROACH

Male mice were treated with picrotoxin to decrease GABAA receptor function. Their anhedonic behaviour was measured using the female encounter test. The expression of c-Fos was determined immunohistochemically.

KEY RESULTS

Picrotoxin caused an anxiogenic effect on three behavioural tests, but it did not affect the immobility time in the forced swim test. Picrotoxin decreased female preference in the female encounter test and attenuated the female encounter-induced increase in c-Fos expression in the nucleus accumbens. Picrotoxin-induced anhedonia was ameliorated by fluvoxamine and S-(+)-fluoxetine, selective serotonin reuptake inhibitors with high affinity for the σ1 receptor. The effect of fluvoxamine was blocked by a 5-HT1A or a σ1 receptor antagonist, and co-administration of the σ1 receptor agonist (+)-SKF-10047 and the 5-HT1A receptor agonist osemozotan mimicked the effect of fluvoxamine. By contrast, desipramine, duloxetine and paroxetine, which have little affinity for the σ1 receptor, did not affect picrotoxin-induced anhedonia. The effect of fluvoxamine was blocked by a dopamine D2/3 receptor antagonist. Methylphenidate, an activator of the prefrontal dopamine system, ameliorated picrotoxin-induced anhedonia.

CONCLUSION AND IMPLICATIONS

Picrotoxin-treated mice show anhedonic behaviour that is ameliorated by simultaneous activation of 5-HT1A and σ1 receptors. These findings suggest that the increased prefrontal dopamine release is associated with the anti-anhedonic effect observed in picrotoxin-treated mice.

Sigma-1 Receptors and Neurodegenerative Diseases: Towards a Hypothesis of Sigma-1 Receptors as Amplifiers of Neurodegeneration and Neuroprotection

Sigma-1 receptors are molecular chaperones that may act as pathological mediators and targets for novel therapeutic applications in neurodegenerative diseases. Accumulating evidence indicates that sigma-1 ligands can either directly or indirectly modulate multiple neurodegenerative processes, including excitotoxicity, calcium dysregulation, mitochondrial and endoplasmic reticulum dysfunction, inflammation, and astrogliosis. In addition, sigma-1 ligands may act as disease-modifying agents in the treatment for central nervous system (CNS) diseases by promoting the activity of neurotrophic factors and neural plasticity. Here, we summarize their neuroprotective and neurorestorative effects in different animal models of acute brain injury and chronic neurodegenerative diseases, and highlight their potential role in mitigating disease. Notably, current data suggest that sigma-1 receptor dysfunction worsens disease progression, whereas enhancement amplifies pre-existing functional mechanisms of neuroprotection and/or restoration to slow disease progression. Collectively, the data support a model of the sigma-1 receptor as an amplifier of intracellular signaling, and suggest future clinical applications of sigma-1 ligands as part of multi-therapy approaches to treat neurodegenerative diseases.

Fluorinated PET Tracers for Molecular Imaging of σ1 Receptors in the Central Nervous System

At first the role of σ1 receptors in various neurological, psychiatric and neurodegenerative disorders is discussed. In the second part, the principle of positron emission tomography (PET ) is described and the known fluorinated PET tracers for labeling of σ1 receptors are presented. The third part focuses on fluoroalkyl substituted spirocyclic PET tracers, which represent the most promising class of fluorinated PET tracers reported so far. The homologous fluoroalkyl derivatives 12-15 show high σ1 affinity (K i = 0.59-1.4 nM) and high selectivity over the σ2 subtype (408-1331-fold). The enantiomers of the fluoroethyl derivative fluspidine 13 were prepared and pharmacologically characterized. Whereas the (S)-configured enantiomer (S)-13 (K i = 2.3 nM) is 4-fold less active than the (R)-enantiomer (R)-13 (K i = 0.57 nM), (S)-13 is metabolically more stable. The interactions of (S)-13 and (R)-13 with the σ1 receptor were analyzed at the molecular level using the 3D homology model. In an automated radiosynthesis [18F](S)-13 and [18F](R)-13 were prepared by nucleophilic substitution of the tosylates (S)-17 and (R)-17 with K[18F]F in high radiochemical yield, high radiochemical purity and short reaction time. Application of both enantiomers [18F](S)-13 and [18F](R)-13 to mice and piglets led to fast uptake into the brain, but [18F](R)-13 did not show washout from the brain indicating a quasi-irreversible binding. Both radiotracers [18F](S)-13 and [18F](R)-13 were able to label regions in the mouse and piglet brain with high σ1 receptor density. The specific binding of the enantiomeric tracers [18F](S)-13 and [18F](R)-13 could be replaced by the selective σ1 ligand SA4503.

Sigma-1 Receptor and Neuronal Excitability

The sigma-1 receptor (Sig-1R), via interaction with various proteins, including voltage-gated and ligand-gated ion channels (VGICs and LGICs), is involved in a plethora of neuronal functions. This capability to regulate a variety of ion channel targets endows the Sig-1R with a powerful capability to fine tune neuronal excitability, and thereby the transmission of information within brain circuits. This versatility may also explain why the Sig-1R is associated to numerous diseases at both peripheral and central levels. To date, how the Sig-1R chooses its targets and how the combinations of target modulations alter overall neuronal excitability is one of the challenges in the field of Sig-1R-dependent regulation of neuronal activity. Here, we will describe and discuss the latest findings on Sig-1R-dependent modulation of VGICs and LGICs, and provide hypotheses that may explain the diverse excitability outcomes that have been reported so far.

Effects of dextromethorphan on MDMA-induced serotonergic aberration in the brains of non-human primates using [123I]-ADAM/SPECT

3,4-Methylenedioxymethamphetamine (MDMA), a common recreational drug, is known to cause serotonergic neurotoxicity in the brain. Dextromethorphan (DM) is a widely used antitussive reported to exert anti-inflammatory effect in vivo. In this study, we examined the long-term effect of MDMA on the primate serotonergic system and the protective property of DM against MDMA-induced serotonergic abnormality using single photon emission computed tomography (SPECT). Nine monkeys (Macaca cyclopis) were divided into three groups, namely control, MDMA and co-treatment (MDMA/DM). [¹²³I]-ADAM was used as the radioligand for serotonin transporters (SERT) in SPECT scans. SERT levels of the brain were evaluated and presented as the uptake ratios (URs) of [¹²³I]-ADAM in several regions of interest of the brain including midbrain, thalamus and striatum. We found that the URs of [¹²³I]-ADAM were significantly lower in the brains of MDMA than control group, indicating lower brain SERT levels in the MDMA-treated monkeys. This MDMA-induced decrease in brain SERT levels could persist for over four years. However, the loss of brain SERT levels was not observed in co-treatment group. These results suggest that DM may exert a protective effect against MDMA-induced serotonergic toxicity in the brains of the non-human primate.

Dextromethorphan: An update on its utility for neurological and neuropsychiatric disorders

Dextromethorphan (DM) is a commonly used antitussive and is currently the only FDA-approved pharmaceutical treatment for pseudobulbar affect. Its safety profile and diverse pharmacologic actions in the central nervous system have stimulated new interest for repurposing it. Numerous preclinical investigations and many open-label or blinded clinical studies have demonstrated its beneficial effects across a variety of neurological and psychiatric disorders. However, the optimal dose and safety of chronic dosing are not fully known. This review summarizes the preclinical and clinical effects of DM and its putative mechanisms of action, focusing on depression, stroke, traumatic brain injury, seizure, pain, methotrexate neurotoxicity, Parkinson's disease and autism. Moreover, we offer suggestions for future research with DM to advance the treatment for these and other neurological and psychiatric disorders.

Sigma-1 receptor deficiency reduces MPTP-induced parkinsonism and death of dopaminergic neurons

Sigma-1 receptor (σ1R) has been reported to be decreased in nigrostriatal motor system of Parkinson's disease patients. Using heterozygous and homozygous σ1R knockout (σ1R+/- and σ1R-/-) mice, we investigated the influence of σ1R deficiency on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-impaired nigrostriatal motor system. The injection of MPTP for 5 weeks in wild-type mice (MPTP-WT mice), but not in σ1R+/- or σ1R-/- mice (MPTP-σ1R+/- or MPTP-σ1R-/- mice), caused motor deficits and ~40% death of dopaminergic neurons in substantia nigra pars compacta with an elevation of N-methyl-d-aspartate receptor (NMDAr) NR2B phosphorylation. The σ1R antagonist NE100 or the NR2B inhibitor Ro25-6981 could alleviate the motor deficits and the death of dopaminergic neurons in MPTP-WT mice. By contrast, MPTP-σ1R+/- mice treated with the σ1R agonist PRE084 or MPTP-σ1R-/- mice treated with the NMDAr agonist NMDA appeared to have similar motor deficits and loss of dopaminergic neurons as MPTP-WT mice. The pharmacological or genetic inactivation of σ1R suppressed the expression of dopamine transporter (DAT) in substantia nigra, which was corrected by NMDA. The activation of σ1R by PRE084 enhanced the DAT expression in WT mice or σ1R+/- mice. By contrast, the level of vesicular monoamine transporter 2 (VMAT2) in σ1R+/- mice or σ1R-/- mice had no difference from WT mice. Interestingly, MPTP-WT mice showed the reduction in the levels of DAT and VMAT2, but MPTP-σ1R-/- mice did not. The inactivation of σ1R by NE100 could prevent the reduction of VMAT2 in MPTP-WT mice. In addition, the activation of microglia cells in substantia nigra was equally enhanced in MPTP-WT mice and MPTP-σ1R-/- mice. The number of activated astrocytes in MPTP-σ1R-/- mice was less than that in MPTP-WT mice. The findings indicate that the σ1R deficiency through suppressing NMDAr function and DAT expression can reduce MPTP-induced death of dopaminergic neurons and parkinsonism.

Neuroprotective effects of cutamesine, a ligand of the sigma-1 receptor chaperone, against noise-induced hearing loss

The sigma-1 receptor, which is expressed throughout the brain, provides physiological benefits that include higher brain function. The sigma-1 receptor functions as a chaperone in the endoplasmic reticulum and may control cell death and regeneration within the central nervous system. Cutamesine (1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl) piperazine dihydrochloride) is a ligand selective for this receptor and may mediate neuroprotective effects in the context of neurodegenerative disease. We therefore assessed whether cutamesine protects the inner ear from noise-induced or aging-associated hearing loss. Immunohistochemistry and Western blotting revealed that the sigma-1 receptor is present in adult cochlea. We treated mice with 0, 3, or 30 mg/kg cutamesine from 10 days before noise exposure until the end of the study. All subjects were exposed to a 120-dB, 4-kHz octave-band noise for 2 hr. We assessed auditory thresholds by measuring the auditory-evoked brainstem responses at 4, 8, and 16 kHz, prior to and 1 week, 1 month, or 3 months following noise exposure. For the aging study, measurements were made before treatment was initiated and after 3 or 9 months of cutamesine treatment. Damage to fibrocytes within the cochlear spiral limbus was assessed by quantitative histology. Cutamesine significantly reduced threshold shifts and cell death within the spiral limbus in response to intense noise. These effects were not dose or time dependent. Conversely, cutamesine did not prevent aging-associated hearing loss. These results suggest that cutamesine reduces noise-induced hearing loss and cochlear damage during the acute phase that follows exposure to an intense noise.

The interaction of escitalopram and R-citalopram at the human serotonin transporter investigated in the mouse

RATIONALE

Escitalopram appears to be a superior antidepressant to racemic citalopram. It has been hypothesized that binding of R-citalopram to the serotonin transporter (SERT) antagonizes escitalopram binding to and inhibition of the SERT, there by curtailing the elevation of extracellular 5-hydroxytryptamine (5-HTExt), and hence anti-depressant efficacy. Further, it has been suggested that a putative allosteric binding site is important for binding of escitalopram to the primary, orthosteric, site, and for R-citalopram's inhibition here of.

OBJECTIVES

Primary: Investigate at the human (h)SERT, at clinical relevant doses, whether R-citalopram antagonizes escitalopram-induced 5-HTExt elevation. Secondary: Investigate whether abolishing the putative allosteric site affects escitalopram-induced 5-HTExt elevation and/or modulates the effect of R-citalopram.

METHODS

Recombinant generation of hSERT transgenic mice; in vivo microdialysis; SERT binding; pharmacokinetics; 5-HT sensitive behaviors (tail suspension, marble burying).

RESULTS

We generated mice expressing either the wild-type human SERT (hSERT(WT)) or hSERT carrying amino acid substitutions (A505V, L506F, I507L, S574T and I575T) collectively abolishing the putative allosteric site (hSERT(ALI/VFL+SI/TT)). One mg/kg escitalopram yielded clinical relevant plasma levels and brain levels consistent with therapeutic SERT occupancy. The hSERT mice showed normal basal 5-HTExt levels. Escitalopram-induced 5-HTExt elevation was not decreased by R-citalopram co-treatment and was unaffected by loss of the allosteric site. The behavioral effects of the clinically relevant escitalopram dose were small and tended to be enhanced by R-citalopram co-administration.

CONCLUSIONS

We find no evidence that R-citalopram directly antagonizes escitalopram or that the putative allosteric site is important for hSERT inhibition by escitalopram.

The development of radiotracers for imaging sigma (σ) receptors in the central nervous system (CNS) using positron emission tomography (PET)

Sigma (σ) receptors are unique mammalian proteins, distributed in the central nervous system and elsewhere, which are increasingly implicated in the pathophysiology of virtually all major central nervous system disorders. The heterogeneous but wide distribution of σ1 in the brain has prompted the development of selective radiotracers for imaging these sites using positron emission tomography (PET). To date, some 50 carbon-11-labelled and fluorine-18-labelled candidate PET radioligands targeting σ receptors have been reported. The historical development of selective σ1 receptor ligands as potential PET imaging agents, as well as the radiochemistry and application of the most recently developed examples, is described herein.

Involvement of sigma-1 receptors in the antidepressant-like effects of dextromethorphan

Dextromethorphan is an antitussive with a high margin of safety that has been hypothesized to display rapid-acting antidepressant activity based on pharmacodynamic similarities to the N-methyl-D-aspartate (NMDA) receptor antagonist ketamine. In addition to binding to NMDA receptors, dextromethorphan binds to sigma-1 (σ1) receptors, which are believed to be protein targets for a potential new class of antidepressant medications. The purpose of this study was to determine whether dextromethorphan elicits antidepressant-like effects and the involvement of σ1 receptors in mediating its antidepressant-like actions. The antidepressant-like effects of dextromethorphan were assessed in male, Swiss Webster mice using the forced swim test. Next, σ1 receptor antagonists (BD1063 and BD1047) were evaluated in conjunction with dextromethorphan to determine the involvement of σ receptors in its antidepressant-like effects. Quinidine, a cytochrome P450 (CYP) 2D6 inhibitor, was also evaluated in conjunction with dextromethorphan to increase the bioavailability of dextromethorphan and reduce exposure to additional metabolites. Finally, saturation binding assays were performed to assess the manner in which dextromethorphan interacts at the σ1 receptor. Our results revealed dextromethorphan displays antidepressant-like effects in the forced swim test that can be attenuated by pretreatment with σ1 receptor antagonists, with BD1063 causing a shift to the right in the dextromethorphan dose response curve. Concomitant administration of quinidine potentiated the antidepressant-like effects of dextromethorphan. Saturation binding assays revealed that a Ki concentration of dextromethorphan reduces both the Kd and the Bmax of [(3)H](+)-pentazocine binding to σ1 receptors. Taken together, these data suggest that dextromethorphan exerts some of its antidepressant actions through σ1 receptors.

Sigma (σ) receptors as potential therapeutic targets to mitigate psychostimulant effects

Many psychostimulants, including cocaine and methamphetamine, interact with sigma (σ) receptors at physiologically relevant concentrations. The potential therapeutic relevance of this interaction is underscored by the ability to selectively target σ receptors to mitigate many behavioral and physiological effects of psychostimulants in animal and cell-based model systems. This chapter begins with an overview of these enigmatic proteins. Provocative preclinical data showing that σ ligands modulate an array of cocaine and methamphetamine effects are summarized, along with emerging areas of research. Together, the literature suggests targeting of σ receptors as an innovative option for combating undesired actions of psychostimulants through both neuronal and glial mechanisms.

Structure activity relationship study of benzo[d]thiazol-2(3H)one based σ receptor ligands

Herein we report the SAR study which involved structural modifications to the linker length, aryl substitution and alkylamine ring size of the benzo[d]thiazol-2(3H)one based sigma receptor (σ) ligands. Many compounds in this series displayed low nanomolar affinity for the σ receptor subtypes. In particular, 8a showed high affinity (σ-1 Ki = 4.5 nM) for σ-1 receptors and moderately high selectivity (483-fold) over σ-2 receptors.

Involvement of the sigma1 receptor in the antidepressant-like effects of fluvoxamine in the forced swimming test in comparison with the effects elicited by paroxetine

We studied the involvement of the sigma(1) receptor in the antidepressant-like effects of the selective serotonin reuptake inhibitor (SSRI) fluvoxamine in DBA/2 mice using the forced swimming test. The effects of the selective sigma(1) receptor antagonist N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino) ethylamine (BD1047) at 1mg/kg significantly antagonized the anti-immobility elicited by fluvoxamine (10mg/kg). However, the anti-immobility effects elicited by another SSRI, paroxetine (5m/kg), were not altered by BD1047. The selective sigma(1) receptor agonist 2S-(2α,6α,11R(*))-1,2,3,4,5,6-hexahydro-6,11-dimethyl-3-(2-propenyl)-2,6-methano-3-benzazocin-8-ol ((+)SKF-10047) elicited dose-dependent anti-immobility effects in DBA/2 mice. BD1047 significantly blocked the anti-immobility effects induced by (+)SKF-10047 at 10mg/kg. These results suggested that the sigma(1) receptor was associated with fluvoxamine-induced antidepressant-like effects but not with paroxetine-induced antidepressant-like effects.

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.

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.

N-Arylalkyl-2-azaadamantanes as cage-expanded polycarbocyclic sigma (σ) receptor ligands

A series of racemic N-arylalkyl-2-azaadamantan-1-ols (9-15) and the corresponding deoxygenated, achiral N-arylalkyl-2-azaadamantanes (23-29) were synthesized and screened in competition binding assays against a panel of CNS targets. Adamantyl hemiaminals 9-15 displayed generally low affinity for both σ(1) (K(i) values= 294-1950 nM) and σ(2) receptors (K(i) values=201-1020 nM), and negligible affinity for 42 other CNS proteins. Deoxygenation of 9-15 to give the corresponding achiral azaadamantanes 23-29 greatly improved affinity for σ(1) (K(i) values=8.3-239 nM) and σ(2) receptors (K(i) values=34-312 nM).

Sigma receptors: potential targets for a new class of antidepressant drug

Despite the widespread and devastating impact of depression on society, our current understanding of its pathogenesis is limited. Likewise, existing treatments are inadequate, providing relief to only a subset of people suffering from depression. The search for more effective antidepressant drugs includes the investigation of new molecular targets. Among them, current data suggests that sigma receptors are involved in multiple processes effecting antidepressant-like actions in vivo and in vitro. This review summarizes accumulated evidence supporting a role for sigma receptors in antidepressant effects and provides a conceptual framework for delineating their potential roles over the course of antidepressant treatment.

Selective serotonin reuptake inhibitors potentiate the rapid antidepressant-like effects of serotonin4 receptor agonists in the rat

BACKGROUND

We have recently reported that serotonin(4) (5-HT(4)) receptor agonists have a promising potential as fast-acting antidepressants. Here, we assess the extent to which this property may be optimized by the concomitant use of conventional antidepressants.

METHODOLOGY/PRINCIPAL FINDINGS

We found that, in acute conditions, the 5-HT(4) agonist prucalopride was able to counteract the inhibitory effect of the selective serotonin reuptake inhibitors (SSRI) fluvoxamine and citalopram on 5-HT neuron impulse flow, in Dorsal Raphé Nucleus (DRN) cells selected for their high (>1.8 Hz) basal discharge. The co-administration of both prucalopride and RS 67333 with citalopram for 3 days elicited an enhancement of DRN 5-HT neuron average firing rate, very similar to what was observed with either 5-HT(4) agonist alone. At the postsynaptic level, this translated into the manifestation of a tonus on hippocampal postsynaptic 5-HT(1A) receptors, that was two to three times stronger when the 5-HT(4) agonist was combined with citalopram. Similarly, co-administration of citalopram synergistically potentiated the enhancing effect of RS 67333 on CREB protein phosphorylation within the hippocampus. Finally, in the Forced Swimming Test, the combination of RS 67333 with various SSRIs (fluvoxamine, citalopram and fluoxetine) was more effective to reduce time of immobility than the separate administration of each compound.

CONCLUSIONS/SIGNIFICANCE

These findings strongly suggest that the adjunction of an SSRI to a 5-HT(4) agonist may help to optimize the fast-acting antidepressant efficacy of the latter.

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.