Synthesis and evaluation of optically pure [3H]-(+)-pentazocine, a highly potent and selective radioligand for sigma receptors

Evaluation of the role of Sigma 1 receptor and Cullin3 in retinal photoreceptor cells

Sigma 1 receptor (Sig1R), a pluripotent modulator of cell survival, is neuroprotective in models of retinal degeneration when activated by the high-affinity, high-specificity ligand (+)-pentazocine ((+)-PTZ). The molecular mechanisms of Sig1R-mediated retinal neuroprotection are under investigation. We previously reported that the antioxidant regulatory transcription factor Nrf2 may be involved in Sig1R-mediated retinal photoreceptor cell (PRC) rescue. Cullin 3 (Cul3) is a component of the Nrf2-Keap1 antioxidant pathway and facilitates Nrf2 ubiquitination. Our earlier transcriptome analysis revealed decreased Cul3 in retinas lacking Sig1R. Here, we asked whether Sig1R activation can modulate Cul3 expression in 661 W cone PRCs. Proximity ligation and co-immunoprecipitation (co-IP) showed that Cul3 resides closely to and co-IPs with Sig1R. Activation of Sig1R using (+)-PTZ significantly increased Cul3 at the gene/protein level; silencing Sig1R decreased Cul3 gene/protein levels. Experiments in which Cul3 was silenced in cells exposed to tBHP resulted in increased oxidative stress, which was not attenuated with Sig1R activation by (+)-PTZ, whereas cells transfected with scrambled siRNA (and incubated with tBHP) responded to (+)-PTZ treatment by decreasing levels of oxidative stress. Assessment of mitochondrial respiration and glycolysis revealed significantly improved maximal respiration, spare capacity and glycolytic capacity in oxidatively-stressed cells transfected with scrambled siRNA and treated with (+)-PTZ, but not in (+)-PTZ treated, oxidatively-stressed cells in which Cul3 had been silenced. The data provide the first evidence that Sig1R co-localizes/interacts with Cul3, a key player in the Nrf2-Keap1 antioxidant pathway. The data suggest that the preservation of mitochondrial respiration/glycolytic function and reduction of oxidative stress observed upon activation of Sig1R occur in part in a Cul3-dependent manner.

Defining the ligand-dependent proximatome of the sigma 1 receptor

Sigma 1 Receptor (S1R) is a therapeutic target for a wide spectrum of pathological conditions ranging from neurodegenerative diseases to cancer and COVID-19. S1R is ubiquitously expressed throughout the visceral organs, nervous, immune and cardiovascular systems. It is proposed to function as a ligand-dependent molecular chaperone that modulates multiple intracellular signaling pathways. The purpose of this study was to define the S1R proximatome under native conditions and upon binding to well-characterized ligands. This was accomplished by fusing the biotin ligase, Apex2, to the C terminus of S1R. Cells stably expressing S1R-Apex or a GFP-Apex control were used to map proximal proteins. Biotinylated proteins were labeled under native conditions and in a ligand dependent manner, then purified and identified using quantitative mass spectrometry. Under native conditions, S1R biotinylates over 200 novel proteins, many of which localize within the endomembrane system (endoplasmic reticulum, Golgi, secretory vesicles) and function within the secretory pathway. Under conditions of cellular exposure to either S1R agonist or antagonist, results show enrichment of proteins integral to secretion, extracellular matrix formation, and cholesterol biosynthesis. Notably, Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) displays increased binding to S1R under conditions of treatment with Haloperidol, a well-known S1R antagonist; whereas Low density lipoprotein receptor (LDLR) binds more efficiently to S1R upon treatment with (+)-Pentazocine ((+)-PTZ), a classical S1R agonist. Furthermore, we demonstrate that the ligand bound state of S1R correlates with specific changes to the cellular secretome. Our results are consistent with the postulated role of S1R as an intracellular chaperone and further suggest important and novel functionalities related to secretion and cholesterol metabolism.

Sphingoid Bases Regulate the Sigma-1 Receptor-Sphingosine and N,N'-Dimethylsphingosine Are Endogenous Agonists

Both bioactive sphingolipids and Sigma-1 receptor (S1R) chaperones occur ubiquitously in mammalian cell membranes. Endogenous compounds that regulate the S1R are important for controlling S1R responses to cellular stress. Herein, we interrogated the S1R in intact Retinal Pigment Epithelial cells (ARPE-19) with the bioactive sphingoid base, sphingosine (SPH), or the pain-provoking dimethylated SPH derivative, N,N'-dimethylsphingosine (DMS). As informed by a modified native gel approach, the basal and antagonist (BD-1047)-stabilized S1R oligomers dissociated to protomeric forms in the presence of SPH or DMS (PRE-084 as control). We, thus, posited that SPH and DMS are endogenous S1R agonists. Consistently, in silico docking of SPH and DMS to the S1R protomer showed strong associations with Asp126 and Glu172 in the cupin beta barrel and extensive van der Waals interactions of the C18 alkyl chains with the binding site including residues in helices 4 and 5. Mean docking free energies were 8.73-8.93 kcal/mol for SPH and 8.56-8.15 kcal/mol for DMS, and calculated binding constants were ~40 nM for SPH and ~120 nM for DMS. We hypothesize that SPH, DMS, and similar sphingoid bases access the S1R beta barrel via a membrane bilayer pathway. We further propose that the enzymatic control of ceramide concentrations in intracellular membranes as the primary sources of SPH dictates availability of endogenous SPH and DMS to the S1R and the subsequent control of S1R activity within the same cell and/or in cellular environments.

Novel High Affinity Sigma-1 Receptor Ligands from Minimal Ensemble Docking-Based Virtual Screening

Sigma-1 receptor (S1R) is an intracellular, multi-functional, ligand operated protein that also acts as a chaperone. It is considered as a pluripotent drug target in several pathologies. The publication of agonist and antagonist bound receptor structures has paved the way for receptor-based in silico drug design. However, recent studies on this subject payed no attention to the structural differences of agonist and antagonist binding. In this work, we have developed a new ensemble docking-based virtual screening protocol utilizing both agonist and antagonist bound S1R structures. This protocol was used to screen our in-house compound library. The S1R binding affinities of the 40 highest ranked compounds were measured in competitive radioligand binding assays and the sigma-2 receptor (S2R) affinities of the best S1R binders were also determined. This way three novel high affinity S1R ligands were identified and one of them exhibited a notable S1R/S2R selectivity.

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.

Comparison of Sigma 1 Receptor Ligands SA4503 and PRE084 to (+)-Pentazocine in the rd10 Mouse Model of RP

Purpose

Sigma 1 receptor is a novel therapeutic target for retinal disease. Its activation, using a high-affinity, high-specificity ligand (+)-pentazocine ((+)-PTZ), rescues photoreceptor cells in the rd10 mouse model of RP. Here, we asked whether the robust retinal neuroprotective properties of (+)-PTZ are generalizable to SA4503 and PRE084, two other high-affinity sigma 1 receptor ligands.

Methods

We treated 661W cells with SA4503 or PRE084. Cell viability, oxidative stress, and expression of Nrf2 and NRF2-regulated antioxidant genes (Nqo1, Cat, and Sod1) were assessed. Rd10 mice were administered SA4503 (1 mg/kg), PRE084 (0.5 mg/kg), or (+)-PTZ (0.5 mg/kg). Visual acuity, retinal architecture, and retinal electrophysiologic function were measured in vivo and retinal structure was assessed histologically.

Results

Similar to (+)-PTZ, SA4503 and PRE084 improved cell viability, attenuated oxidative stress, and increased Nrf2, Nqo1 and Cat expression. Although treatment of rd10 mice with (+)-PTZ improved visual acuity, increased outer retinal thickness, and improved photopic a- and b-wave responses compared with nontreated rd10 mice, treatment with SA4503 or PRE084 did not. The number of photoreceptor nuclei/100 µm retinal length in SA4503- and PRE084-treated rd10 mice (approximately 11/100) did not differ significantly from nontreated rd10 mice, whereas (+)-PTZ-treated mice had significantly more nuclei (approximately 22/100 µm).

Conclusions

Cell survival and gene regulation experiments yielded similar outcomes when SA4503, PRE084, or (+)-PTZ were conducted in vitro, however neither SA4503 or PRE084 afforded in vivo protection in the severe rd10 retinopathy model comparable to (+)-PTZ. Despite all three compounds demonstrating the potential to activate sigma 1 receptor, the retinal neuroprotective properties of the three ligands differ significantly.

Using sigma-ligands as part of a multi-receptor approach to target diseases of the brain

INTRODUCTION

The sigma receptors are found abundantly in the central nervous system and are targets for the treatment of various diseases, including Alzheimer's (AD), Parkinson's (PD), Huntington's disease (HD), depression, amyotrophic lateral sclerosis (ALS), and multiple sclerosis (MS). However, for many of these diseases, other receptors and targets have been the focus of the most, such as acetylcholine esterase inhibitors in Alzheimer's and dopamine replacement in Parkinson's. The currently available drugs for these diseases have limited success resulting in the requirement of an alternative approach to their treatment.

AREAS COVERED

In this review, we discuss the potential role of the sigma receptors and their ligands as part of a multi receptor approach in the treatment of the diseases mentioned above. The literature reviewed was obtained through searches in databases, including PubMed, Web of Science, Google Scholar, and Scopus.

EXPERT OPINION

Given sigma receptor agonists provide neuroprotection along with other benefits such as potentiating the effects of other receptors, further development of multi-receptor targeting ligands, and or the development of multi-drug combinations to target multiple receptors may prove beneficial in the future treatment of degenerative diseases of the CNS, especially when coupled with better diagnostic techniques.

The molecular chaperone sigma 1 receptor mediates rescue of retinal cone photoreceptor cells via modulation of NRF2

Sigma 1 receptor (Sig1R), a putative molecular chaperone, has emerged as a novel therapeutic target for retinal degenerative disease. Earlier studies showed that activation of Sig1R via the high-affinity ligand (+)-pentazocine ((+)-PTZ) induced profound rescue of cone photoreceptor cells in the rd10 mouse model of retinitis pigmentosa; however the mechanism of rescue is unknown. Improved cone function in (+)-PTZ-treated mice was accompanied by reduced oxidative stress and normalization of levels of NRF2, a transcription factor that activates antioxidant response elements (AREs) of hundreds of cytoprotective genes. Here, we tested the hypothesis that modulation of NRF2 is central to Sig1R-mediated cone rescue. Activation of Sig1R in 661W cone cells using (+)-PTZ induced dose-dependent increases in NRF2-ARE binding activity and NRF2 gene/protein expression, whereas silencing Sig1R significantly decreased NRF2 protein levels and increased oxidative stress, although (+)-PTZ did not disrupt NRF2-KEAP1 binding. In vivo studies were conducted to investigate whether, in the absence of NRF2, activation of Sig1R rescues cones. (+)-PTZ was administered systemically for several weeks to rd10/nrf2+/+ and rd10/nrf2-/- mice. Through post-natal day 42, cone function was significant in rd10/nrf2+/+, but minimal in rd10/nrf2-/- mice as indicated by electroretinographic recordings using natural noise stimuli, optical coherence tomography and retinal histological analyses. Immunodetection of cones was limited in (+)-PTZ-treated rd10/nrf2-/-, though considerable in (+)-PTZ-treated rd10/nrf2+/+mice. The data suggest that Sig1R-mediated cone rescue requires NRF2 and provide evidence for a previously-unrecognized relationship between these proteins.

Choline Is an Intracellular Messenger Linking Extracellular Stimuli to IP3-Evoked Ca2+ Signals through Sigma-1 Receptors

Sigma-1 receptors (Sig-1Rs) are integral ER membrane proteins. They bind diverse ligands, including psychoactive drugs, and regulate many signaling proteins, including the inositol 1,4,5-trisphosphate receptors (IP3Rs) that release Ca2+ from the ER. The endogenous ligands of Sig-1Rs are unknown. Phospholipase D (PLD) cleaves phosphatidylcholine to choline and phosphatidic acid (PA), with PA assumed to mediate all downstream signaling. We show that choline is also an intracellular messenger. Choline binds to Sig-1Rs, it mimics other Sig-1R agonists by potentiating Ca2+ signals evoked by IP3Rs, and it is deactivated by metabolism. Receptors, by stimulating PLC and PLD, deliver two signals to IP3Rs: IP3 activates IP3Rs, and choline potentiates their activity through Sig-1Rs. Choline is also produced at synapses by degradation of acetylcholine. Choline uptake by transporters activates Sig-1Rs and potentiates Ca2+ signals. We conclude that choline is an endogenous agonist of Sig-1Rs linking extracellular stimuli, and perhaps synaptic activity, to Ca2+ signals.

Sigma 1 receptor: A novel therapeutic target in retinal disease

Retinal degenerative diseases are major causes of untreatable blindness worldwide and efficacious treatments for these diseases are sorely needed. A novel target for treatment of retinal disease is the transmembrane protein Sigma 1 Receptor (Sig1R). This enigmatic protein is an evolutionary isolate with no known homology to any other protein. Sig1R was originally thought to be an opioid receptor. That notion has been dispelled and more recent pharmacological and molecular studies suggest that it is a pluripotent modulator with a number of biological functions, many of which are relevant to retinal disease. This review provides an overview of the discovery of Sig1R and early pharmacologic studies that led to the cloning of the Sig1R gene and eventual elucidation of its crystal structure. Studies of Sig1R in the eye were not reported until the late 1990s, but since that time there has been increasing interest in the potential role of Sig1R as a target for retinal disease. Studies have focused on elucidating the mechanism(s) of Sig1R function in retina including calcium regulation, modulation of oxidative stress, ion channel regulation and molecular chaperone activity. Mechanistic studies have been performed in isolated retinal cells, such as Müller glial cells, microglial cells, optic nerve head astrocytes and retinal ganglion cells as well as in the intact retina. Several compelling studies have provided evidence of powerful in vivo neuroprotective effects against ganglion cell loss as well as photoreceptor cell loss. Also described are studies that have examined retinal structure/function in various models of retinal disease in which Sig1R is absent and reveal that these phenotypes are accelerated compared to retinas of animals that express Sig1R. The collective evidence from analysis of studies over the past 20 years is that Sig1R plays a key role in modulating retinal cellular stress and that it holds great promise as a target in retinal neurodegenerative disease.

The sigma-1 receptor as a regulator of dopamine neurotransmission: A potential therapeutic target for methamphetamine addiction

Methamphetamine (METH) abuse is a major public health issue around the world, yet there are currently no effective pharmacotherapies for the treatment of METH addiction. METH is a potent psychostimulant that increases extracellular dopamine levels by targeting the dopamine transporter (DAT) and alters neuronal activity in the reward centers of the brain. One promising therapeutic target for the treatment of METH addiction is the sigma-1 receptor (σ1R). The σ1R is an endoplasmic reticulum-localized chaperone protein that is activated by cellular stress, and, unique to this chaperone, its function can also be induced or inhibited by different ligands. Upon activation of this unique "chaperone receptor", the σ1R regulates a variety of cellular functions and possesses neuroprotective activity in the brain. Interestingly, a variety of σ1R ligands modulate dopamine neurotransmission and reduce the behavioral effects of METH in animal models of addictive behavior, suggesting that the σ1R may be a viable therapeutic target for the treatment of METH addiction. In this review, we provide background on METH and the σ1R as well as a literature review regarding the role of σ1Rs in modulating both dopamine neurotransmission and the effects of METH. We aim to highlight the complexities of σ1R pharmacology and function as well as the therapeutic potential of the σ1R as a target for the treatment of METH addiction.

Sigma 1 receptor regulates ERK activation and promotes survival of optic nerve head astrocytes

The sigma 1 receptor (S1R) is a unique transmembrane protein that has been shown to regulate neuronal differentiation and cellular survival. It is expressed within several cell types throughout the nervous system and visceral organs, including neurons and glia within the eye. S1R ligands are therapeutic targets for diseases ranging from neurodegenerative conditions to neoplastic disorders. However, effects of S1R activation and inhibition within glia cells are not well characterized. Within the eye, the astrocytes at the optic nerve head are crucial to the health and survival of the neurons that send visual information to the brain. In this study, we used the S1R-specific agonist, (+)-pentazocine, to evaluate S1R activation within optic nerve head-derived astrocytes (ONHAs). Treatment of ONHAs with (+)-pentazocine attenuated the level and duration of stress-induced ERK phosphorylation following oxidative stress exposure and promoted survival of ONHAs. These effects were specific to S1R activation because they were not observed in ONHAs that were depleted of S1R using siRNA-mediated knockdown. Collectively, our results suggest that S1R activation suppresses ERK1/2 phosphorylation and protects ONHAs from oxidative stress-induced death.

Absence of Sigma 1 Receptor Accelerates Photoreceptor Cell Death in a Murine Model of Retinitis Pigmentosa

Purpose

Sigma 1 Receptor (Sig1R) is a novel therapeutic target in neurodegenerative diseases, including retinal disease. Sig1R-/- mice have late-onset retinal degeneration with ganglion cell loss that worsens under stress. Whether Sig1R plays a role in maintaining other retinal neurons is unknown, but was investigated here using rd10 mice, a model of severe photoreceptor degeneration.

Methods

Wild-type, rd10, and rd10/Sig1R-/- mice were subjected to ERG and spectral-domain optical coherence tomography (SD-OCT) to assess visual function/structure in situ. Retinas imaged microscopically were subjected to morphometric analysis, immunodetection of cones, and analysis of gliosis. Oxidative and endoplasmic reticulum (ER) stress was evaluated at mRNA/protein levels.

Results

Photopic ERG responses were reduced significantly in rd10/Sig1R-/- versus rd10 mice at P28 (31 ± 6 vs. 56 ± 7 μV), indicating accelerated cone loss when Sig1R was absent. At P28, SD-OCT revealed reduced retinal thickness in rd10/Sig1R-/- mice (60% of WT) versus rd10 (80% of WT). Morphometric analysis disclosed profound photoreceptor nuclei loss in rd10/Sig1R-/- versus rd10 mice. rd10/Sig1R-/- mice had 35% and 60% fewer photoreceptors, respectively, at P28 and P35, than rd10. Peanut agglutinin cone labeling decreased significantly; gliosis increased significantly in rd10/Sig1R-/- versus rd10 mice. At P21, NRF2 levels increased in rd10/Sig1R-/- mice versus rd10 and downstream antioxidants increased indicating oxidative stress. At P28, ER stress genes/proteins, especially XBP1, a potent transcriptional activator of the unfolded protein response and CHOP, a proapoptotic transcription factor, increased significantly in rd10/Sig1R-/- mice versus rd10.

Conclusions

Photoreceptor cell degeneration accelerates and cone function diminishes much earlier in rd10/Sig1R-/- than rd10 mice emphasizing the importance of Sig1R as a modulator of retinal cell survival.

σ Receptor Effects of N-Substituted Benztropine Analogs: Implications for Antagonism of Cocaine Self-Administration

Several N-substituted benztropine (BZT) analogs are atypical dopamine transport inhibitors as they have affinity for the dopamine transporter (DAT) but have minimal cocaine-like pharmacologic effects and can block numerous effects of cocaine, including its self-administration. Among these compounds, N-methyl (AHN1-055), N-allyl (AHN2-005), and N-butyl (JHW007) analogs of 3α-[bis(4'-fluorophenyl)methoxy]-tropane were more potent in antagonizing self-administration of cocaine and d-methamphetamine than in decreasing food-maintained responding. The antagonism of cocaine self-administration (0.03-1.0 mg/kg per injection) with the above BZT analogs was reproduced in the present study. Further, the stimulant-antagonist effects resembled previously reported effects of pretreatments with combinations of standard DAT inhibitors and σ1-receptor (σ1R) antagonists. Therefore, the present study examined binding of the BZT analogs to σRs, as well as their in vivo σR antagonist effects. Each of the BZT analogs displaced radiolabeled σR ligands with nanomolar affinity. Further, self-administration of the σR agonist DTG (0.1-3.2 mg/kg/injection) was dose dependently blocked by AHN2-005 and JHW007 but potentiated by AHN1-055. In contrast, none of the BZT analogs that were active against DTG self-administration was active against the self-administration of agonists at dopamine D1-like [R(+)-SKF 81297, (±)-SKF 82958 (0.00032-0.01 mg/kg per injection each)], D2-like [R(-)-NPA (0.0001-0.0032 mg/kg per injection), (-)-quinpirole (0.0032-0.1 mg/kg per injection)], or μ-opioid (remifentanil, 0.0001-0.0032 mg/kg per injection) receptors. The present results indicate that behavioral antagonist effects of the N-substituted BZT analogs are specific for abused drugs acting at the DAT and further suggest that σR antagonism contributes to those actions.

A role for sigma receptors in stimulant self-administration and addiction

Sigma-1 receptors (σ1Rs) are structurally unique intracellular proteins that function as chaperones. σ1Rs translocate from the mitochondria-associated membrane to other subcellular compartments, and can influence a host of targets, including ion channels, G-protein-coupled receptors, lipids, and other signaling proteins. Drugs binding to σRs can induce or block the actions of σRs. Studies indicate that stimulant self-administration induces the reinforcing effects of σR agonists, because of dopamine transporter actions. Once established, the reinforcing effects of σR agonists are independent of dopaminergic mechanisms traditionally thought to be critical to the reinforcing effects of stimulants. Self-administered doses of σR agonists do not increase dopamine concentrations in the nucleus accumbens shell, a transmitter and brain region considered important for the reinforcing effects of abused drugs. However, self-administration of σR agonists is blocked by σR antagonists. Several effects of stimulants have been blocked by σR antagonists, including the reinforcing effects, assessed by a place-conditioning procedure. However, the self-administration of stimulants is largely unaffected by σR antagonists, indicating fundamental differences in the mechanisms underlying these two procedures used to assess the reinforcing effects. When σR antagonists are administered in combination with dopamine uptake inhibitors, an effective and specific blockade of stimulant self-administration is obtained. Actions of stimulant drugs related to their abuse induce unique changes in σR activity and the changes induced potentially create redundant and, once established, independent reinforcement pathways. Concomitant targeting of both dopaminergic pathways and σR proteins produces a selective antagonism of stimulant self-administration, suggesting new avenues for combination chemotherapies to specifically combat stimulant abuse.

Effects of the psychotomimetic benzomorphan N-allylnormetazocine (SKF 10,047) on prepulse inhibition of startle in mice

N-allylnormetazocine (NANM; SKF 10,047) is a benzomorphan opioid that produces psychotomimetic effects. (+)-NANM is the prototypical agonist for the sigma-1 (σ1) receptor, and there is a widespread belief that the hallucinogenic effects of NANM and other benzomorphan derivatives are mediated by interactions with σ1 sites. However, NANM is also an agonist at the κ opioid receptor (KOR) and binds to the PCP site located within the channel pore of the NMDA receptor, interactions that could potentially contribute to the effects of NANM. NMDA receptor antagonists such as phencyclidine (PCP) and ketamine are known to disrupt prepulse inhibition (PPI) of acoustic startle, a measure of sensorimotor gating, in rodents. We recently found that racemic NANM disrupts PPI in rats, but it is not clear whether the effect is mediated by blockade of the NMDA receptor, or alternatively whether interactions with KOR and σ1 receptors are involved. The present studies examined whether NANM and its stereoisomers alter PPI in C57BL/6J mice, and tested whether the effects on PPI are mediated by KOR or σ1 receptors. Racemic NANM produced a dose-dependent disruption of PPI (3-30mg/kg SC). (+)-NANM also disrupted PPI, whereas (-)-NANM was ineffective. Pretreatment with the selective KOR antagonist nor-binaltorphimine (10mg/kg SC) or the selective σ1 antagonist NE-100 (1mg/kg IP) failed to attenuate the reduction in PPI produced by racemic NANM. We also found that the selective KOR agonist (-)-U-50,488H (10-40mg/kg SC) had no effect on PPI. These findings confirm that NANM reduces sensorimotor gating in rodents, and indicate that the effect is mediated by interactions with the PCP receptor and not by activation of KOR or σ1 receptors. This observation is consistent with evidence indicating that the σ1 receptor is not linked to hallucinogenic or psychotomimetic effects.

σ1 receptor ligands control a switch between passive and active threat responses

Humans and many animals show 'freezing' behavior in response to threatening stimuli. In humans, inappropriate threat responses are fundamental characteristics of several mental illnesses. To identify small molecules that modulate threat responses, we developed a high-throughput behavioral assay in zebrafish (Danio rerio) and evaluated 10,000 compounds for their effects on freezing behavior. We found three classes of compounds that switch the threat response from freezing to escape-like behavior. We then screened these for binding activity across 45 candidate targets. Using target profile clustering, we identified the sigma-1 (σ1) receptor as having a role in the mechanism of behavioral switching and confirmed that known σ1 ligands also disrupt freezing behavior. Furthermore, mutation of the gene encoding σ1 prevented the behavioral effect of escape-inducing compounds. One compound, which we call finazine, potently bound mammalian σ1 and altered threat-response behavior in mice. Thus, pharmacological and genetic interrogation of the freezing response revealed σ1 as a mediator of threat responses in vertebrates.

Role of Sigma 1 Receptor in Retinal Degeneration of the Ins2Akita/+ Murine Model of Diabetic Retinopathy

PURPOSE

Sigma receptor 1 (Sigma1R), a nonopioid putative molecular chaperone, has neuroprotective properties in retina. This study sought to determine whether delaying administration of (+)-pentazocine, a high-affinity Sigma1R ligand after onset of diabetes in Ins2Akita/+ diabetic mice would afford retinal neuroprotection and to determine consequences on retinal phenotype in Ins2Akita/+ diabetic mice in the absence of Sigma1R.

METHODS

Ins2Akita/+ diabetic and WT mice received intraperitoneal injections of (+)-pentazocine beginning 4 or 8 weeks after onset of diabetes; eyes were harvested at 25 weeks. Retinal histologic sections were analyzed to determine thicknesses of retinal layers, number of ganglion cells, and evidence of gliosis (increased glial fibrillary acidic protein levels). Ins2Akita/+/Sig1R-/-mice were generated and subjected to in vivo assessment of retinal architecture (optical coherence tomography [OCT]) and retinal vasculature using fluorescein angiography (FA) at 12 and 16 weeks compared with age-matched Ins2Akita/+ mice. Eyes were then harvested for retinal morphometric assessment and gliosis assessment.

RESULTS

Wild-type mice had 13 ± 0.06 cells/100 μm retinal length; cell bodies in Ins2Akita/+ mice injected 4 and 8 weeks after onset of diabetes with (+)-pentazocine retained significantly more ganglion cells compared with Ins2Akita/+ mice (9 ± 0.04) and demonstrated significant attenuation of gliosis. Ins2Akita/+/Sig1R-/-mouse retinas, analyzed to determine whether the Ins2Akita/+ phenotype was accelerated when lacking Sigma1R, revealed increased nerve fiber layer thickness (OCT), evidence of vitreal opacities, and vessel beading (FA) compared with Ins2Akita/+ mice. Morphometric analysis revealed significantly fewer ganglion cells in Ins2Akita/+/Sig1R-/-mice compared with Ins2Akita/+ mice.

CONCLUSIONS

Sigma1R may be a novel retinal stress modulator, and targeting it even after disease onset may afford retinal neuroprotection.

Antinociceptive effects of a new sigma-1 receptor antagonist (N-(2-morpholin-4-yl-ethyl)-2-(1-naphthyloxy)acetamide) in two types of nociception

Pain has become an active clinical challenge due its etiological heterogeneity, symptoms and mechanisms of action. In the search for new pharmacological therapeutic alternatives, sigma receptors have been proposed as drug targets. This family consists of sigma-1 and sigma-2 receptors. The sigma-1 system is involved in nociception through its chaperone activity. Additionally, it has been shown that agonist to these receptors promote related sensitisation and pain hypersensitisation, suggesting the possible use of antagonists for sigma-1 receptors as an alternative therapy. The aim of this study was to evaluate the antinociceptive effect of a new sigma-1 receptor antagonist N-(2-morpholin-4-yl-ethyl)-2-(1-naphthyloxy)acetamida (NMIN) in two types of pain (arthritic and neuropathic) and to compare its efficacy and potency with reference drugs. The antinociceptive effects of NMIN were quantitatively evaluated using the pain-induced functional impairment model in the rat and the acetone test in a rat model of neuropathic pain. NMIN (sigma-1 receptor affinity of 324nM) did not show any antinociceptive activity in the arthritic pain model but showed a dose-dependent anti-allodynic effect in neuropathic pain. NMIN showed a similar efficacy compared to the effects obtained with morphine and the sigma-1 antagonist BD-1063. However, these reference drugs showed increased potency compared with NMIN. Our results suggest that sigma-1 receptors may play an important direct role in neuropathic pain but not in arthritic pain, supporting the hypothesis that NMIN may be useful for the treatment of neuropathic pain.

Biochemical Pharmacology of the Sigma-1 Receptor

The sigma-1 receptor (S1R) is a 223 amino acid two transmembrane (TM) pass protein. It is a non-ATP-binding nonglycosylated ligand-regulated molecular chaperone of unknown three-dimensional structure. The S1R is resident to eukaryotic mitochondrial-associated endoplasmic reticulum and plasma membranes with broad functions that regulate cellular calcium homeostasis and reduce oxidative stress. Several multitasking functions of the S1R are underwritten by chaperone-mediated direct (and indirect) interactions with ion channels, G-protein coupled receptors and cell-signaling molecules involved in the regulation of cell growth. The S1R is a promising drug target for the treatment of several neurodegenerative diseases related to cellular stress. In vitro and in vivo functional and molecular characteristics of the S1R and its interactions with endogenous and synthetic small molecules have been discovered by the use of pharmacologic, biochemical, biophysical, and molecular biology approaches. The S1R exists in monomer, dimer, tetramer, hexamer/octamer, and higher oligomeric forms that may be important determinants in defining the pharmacology and mechanism(s) of action of the S1R. A canonical GXXXG in putative TM2 is important for S1R oligomerization. The ligand-binding regions of S1R have been identified and include portions of TM2 and the TM proximal regions of the C terminus. Some client protein chaperone functions and interactions with the cochaperone 78-kDa glucose-regulated protein (binding immunoglobulin protein) involve the C terminus. Based on its biochemical features and mechanisms of chaperone action the possibility that the S1R is a member of the small heat shock protein family is discussed.

Sigma Receptor Binding Assays

Sigma receptors, both Sigma-1(S1R) and Sigma-2 (S2R), are small molecule-regulated, primarily endoplasmic reticulum (ER) membrane-associated sites. A number of drugs bind to sigma receptors, including the antipsychotic haloperidol and (+)-pentazocine, an opioid analgesic. Sigma receptors are implicated in many central nervous system disorders, in particular Alzheimer's disease and conditions associated with motor control, such as Amyotrophic Lateral Sclerosis (ALS). Described in this unit are radioligand binding assays used for the pharmacological characterization of S1R and S2R. Methods detailed include a radioligand saturation binding assay for defining receptor densities and a competitive inhibition binding assay employing [³H]-(+)-pentazocine for identifying and characterizing novel ligands that interact with S1R. Procedures using [³H]-1,3-di(2-tolyl)guanidine ([³H]-DTG), a nonselective sigma receptor ligand, are described for conducting a saturation binding and competitive inhibition assays for the S2R site. These protocols are of value in drug discovery in identifying new sigma ligands and in the characterization of these receptors.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

Improving selectivity preserving affinity: new piperidine-4-carboxamide derivatives as effective sigma-1-ligands

We report the design, synthesis and binding evaluation against σ1 and σ2 receptors of a series of new piperidine-4-carboxamide derivatives variously substituted on the amide nitrogen atom. Specifically, we assessed the effects exerted on σ receptor affinity by substituting the N-benzylcarboxamide group present on a series of compounds previously synthesized in our laboratory with different cyclic or linear moieties. The synthesized compounds 2a-o were tested to estimate their affinity and selectivity toward σ1 and σ2 receptors. Very high σ1 affinity (Ki = 3.7 nM) and Kiσ2/Kiσ1 selectivity ratio (351) were found for the tetrahydroquinoline derivative 2k, featuring a 4-chlorobenzyl moiety linked to the piperidine nitrogen atom.

Targeting sigma receptors: novel medication development for drug abuse and addiction

Psychostimulant abuse is a serious health and societal problem in industrialized and developing countries. However, the identification of an effective pharmacotherapy to treat it has remained elusive. It has long been known that many psychostimulant drugs, including cocaine and methamphetamine, interact with sigma receptors in the brain and heart, offering a logical target for medication development efforts. However, selective pharmacological agents and molecular biological tools have only recently become available to rigorously evaluate these receptors as viable medication development targets. The current review will summarize provocative preclinical data, demonstrating the ability of sigma receptor antagonists and antisense oligonucleotides to ameliorate cocaine-induced convulsions, lethality, locomotor activity and sensitization, and conditioned place-preference in rodents. Recent studies suggest that the protective effects of sigma receptor antagonists also extend to actions produced by methamphetamine, 3,4-methylenedioxymethamphetamine, ethanol and other abused substances. Together, the data indicate that targeting sigma receptors, particularly the σ(1)-subtype, may offer an innovative approach for combating the effects of cocaine, and perhaps other abused substances.

Homology Model and Docking-Based Virtual Screening for Ligands of the σ1 Receptor

This study presents for the first time the 3D model of the σ1 receptor protein as obtained from homology modeling techniques, shows the applicability of this structure to docking-based virtual screening, defines a computational strategy to optimize the results based on a combination of 3D pharmacophore-based docking and MM/PBSA free energy of binding scoring, and provides evidence that these in silico models and recipes are powerful tools on which virtual screening of new σ1 ligands can be based. In particular, the validation of the applicability of docking-based virtual screening to homology models is of utmost importance, since no crystal structure is available to date for the σ1 receptor, and this missing information still constitutes a major hurdle for a rational ligand design for this important protein target.

A Role for Sigma Receptors in Stimulant Self Administration and Addiction

Sigma₁ receptors (σ₁Rs) represent a structurally unique class of intracellular proteins that function as chaperones. σ₁Rs translocate from the mitochondria-associated membrane to the cell nucleus or cell membrane, and through protein-protein interactions influence several targets, including ion channels, G-protein-coupled receptors, lipids, and other signaling proteins. Several studies have demonstrated that σR antagonists block stimulant-induced behavioral effects, including ambulatory activity, sensitization, and acute toxicities. Curiously, the effects of stimulants have been blocked by σR antagonists tested under place-conditioning but not self-administration procedures, indicating fundamental differences in the mechanisms underlying these two effects. The self administration of σR agonists has been found in subjects previously trained to self administer cocaine. The reinforcing effects of the σR agonists were blocked by σR antagonists. Additionally, σR agonists were found to increase dopamine concentrations in the nucleus accumbens shell, a brain region considered important for the reinforcing effects of abused drugs. Although the effects of the σR agonist, DTG, on dopamine were obtained at doses that approximated those that maintained self administration behavior those of another agonist, PRE-084 required higher doses. The effects of DTG were antagonized by non-selective or a preferential σ₂R antagonist but not by a preferential σ₁R antagonist. The effects of PRE-084 on dopamine were insensitive to σR antagonists. The data suggest that the self administration of σR agonists is independent of dopamine and the findings are discussed in light of a hypothesis that cocaine has both intracellular actions mediated by σRs, as well as extracellular actions mediated through conventionally studied mechanisms. The co-activation and potential interactions among these mechanisms, in particular those involving the intracellular chaperone σRs, may lead to the pernicious addictive effects of stimulant drugs.

Synthesis and characterization of [³H]-SN56, a novel radioligand for the σ₁ receptor

The study of the binding characteristics of σ ligands in vivo and in vitro requires radiolabeled probes with high affinity and selectivity. The radioligand presently used for in vitro studies of the σ₁ receptor, [³H](+)-pentazocine, has significant limitations; it is difficult to synthesize, has limited chemical stability, and can be problematic to obtain. Evaluation of a series of novel 2(3H)-benzothiazolone compounds revealed SN56 to have sub-nanomolar and preferential affinity for the σ₁ subtype, relative to σ₂ and non-sigma, binding sites. The goal of this study was to characterize the binding of [³H]-SN56 to σ₁ receptors isolated from rat brain. Standard in vitro binding techniques were utilized to 1) determine the specificity and affinity of binding to σ₁ receptors, 2) confirm that[³H]-SN56 labels sites previously identified as σ₁ by comparing binding to sites labeled by [³H](+)-pentazocine, and 3) characterize the kinetics of binding. The results indicate that [³H]-SN56 exhibits 1) specific, saturable, and reversible binding to the σ₁ receptor, with B(max)=340±10 fmol/mg and K(d)=0.069±0.0074 nM, 2) competitive displacement by classical sigma compounds, yielding σ₁ K(i) values consistent with those reported in the literature, and 3) binding kinetics compatible with a 90 min incubation, and filtration for separation of free and bound radioligand. The results of these studies suggest that [(3)H]-SN56 may serve as a viable alternative to [³H](+)-pentazocine in radioligand binding assays.

Early development of sigma-receptor ligands

Sigma receptors (σ-1 and σ-2) are non-opioid proteins implicated in the pathophysiology of various neurological disorders and cancer. The σ-1 subtype is a chaperon protein widely distributed in the CNS and peripheral tissues. These receptors are involved in the modulation of K+- and Ca2+-dependent signaling cascades at the endoplasmic reticulum and modulation of neurotransmitter release. σ-1 receptors are emerging targets for the treatment of neurophychiatric diseases (schizophrenia and depression) and cocaine addiction. σ-2 receptors are lipid raft proteins. They are highly expressed on many tumor cells and hence considered potential targets for anticancer drugs. σ receptors bind to a diverse class of pharmacological compounds like cocaine, methamphetamine, benzomorphans like (±)-pentazocine, (±)-SKF-10,047 and endogenous neurosteroids and sphingolipids. In this review we focus on the early development of σ receptor-specific ligands and radiolabeling agents.

The sigma-1 receptor chaperone as an inter-organelle signaling modulator

Inter-organelle signaling plays important roles in many physiological functions. Endoplasmic reticulum (ER)-mitochondrion signaling affects intramitochondrial calcium (Ca(2+)) homeostasis and cellular bioenergetics. ER-nucleus signaling attenuates ER stress. ER-plasma membrane signaling regulates cytosolic Ca(2+) homeostasis and ER-mitochondrion-plasma membrane signaling regulates hippocampal dendritic spine formation. Here, we propose that the sigma-1 receptor (Sig-1R), an ER chaperone protein, acts as an inter-organelle signaling modulator. Sig-1Rs normally reside at the ER-mitochondrion contact called the MAM (mitochondrion-associated ER membrane), where Sig-1Rs regulate ER-mitochondrion signaling and ER-nucleus crosstalk. When cells are stimulated by ligands or undergo prolonged stress, Sig-1Rs translocate from the MAM to the ER reticular network and plasmalemma/plasma membrane to regulate a variety of functional proteins, including ion channels, receptors and kinases. Thus, the Sig-1R serves as an inter-organelle signaling modulator locally at the MAM and remotely at the plasmalemma/plasma membrane. Many pharmacological/physiological effects of Sig-1Rs might relate to this unique action of Sig-1Rs.

The novel sigma-2 receptor ligand SW43 stabilizes pancreas cancer progression in combination with gemcitabine

Background

Sigma-2 receptors are over-expressed in proliferating cancer cells, making an attractive target for the targeted treatment of pancreatic cancer. In this study, we investigated the role of the novel sigma-2 receptor ligand SW43 to induce apoptosis and augment standard chemotherapy.

Results

The binding affinity for sigma-2 ligands is high in pancreas cancer, and they induce apoptosis with a rank order of SV119 < SW43 < SRM in vitro. Combining these compounds with gemcitabine further increased apoptosis and decreased viability. Our in vivo model showed that sigma-2 ligand treatment decreased tumor volume to the same extent as gemcitabine. However, SW43 combination treatment with gemcitabine was superior to the other compounds and resulted in stabilization of tumor volume during treatment, with minimal toxicities.

Conclusions

This study shows that the sigma-2 ligand SW43 has the greatest capacity to augment gemcitabine in a pre-clinical model of pancreas cancer and has provided us with the rationale to move this compound forward with clinical investigations for patients with pancreatic cancer.

In vivo protection against retinal neurodegeneration by sigma receptor 1 ligand (+)-pentazocine

PURPOSE

To evaluate the neuroprotective properties of the sigma receptor 1 (sigmaR1) ligand, (+)-pentazocine in an in vivo model of retinal neurodegeneration.

METHODS

Spontaneously diabetic Ins2(Akita/+) and wild-type mice received intraperitoneal injections of (+)-pentazocine for 22 weeks beginning at diabetes onset. Retinal mRNA and protein were analyzed by RT-PCR and Western blot analysis. Retinal histologic sections were measured to determine total retinal thickness, thicknesses of inner-outer nuclear and plexiform layers (INL, ONL, IPL, INL), and the number of cell bodies in the ganglion cell layer (GCL). Immunolabeling experiments were performed using antibodies specific for 4-hydroxynonenal and nitrotyrosine, markers of lipid peroxidation, and reactive nitrogen species, respectively, and an antibody specific for vimentin to view radial Müller fibers.

RESULTS

sigmaR1 mRNA and protein levels in the Ins2(Akita/+) retina were comparable to those in the wild-type, indicating that sigmaR1 is an available target during the disease process. Histologic evaluation of eyes of Ins2(Akita/+) mice showed disruption of retinal architecture. By 17 to 25 weeks after birth, Ins2(Akita/+) mice demonstrated approximately 30% and 25% decreases in IPL and INL thicknesses, respectively, and a 30% reduction in ganglion cells. In the (+)-pentazocine-treated group, retinas of Ins2(Akita/+) mice showed remarkable preservation of retinal architecture; IPL and INL thicknesses of (+)-pentazocine-treated Ins2(Akita/+) mouse retinas were within normal limits. The number of ganglion cells was 15.6 +/- 1.5 versus 10.4 +/- 1.2 cells/100 mum retinal length in (+)-pentazocine-treated versus nontreated mutant mice. Levels of nitrotyrosine and 4-hydroxynonenal increased in Ins2(Akita/+) retinas, but were reduced in (+)-pentazocine-treated mice. Retinas of Ins2(Akita/+) mice showed loss of the uniform organization of radial Müller fibers. Retinas of (+)-pentazocine-treated mice maintained the radial organization of glial processes.

CONCLUSION

Sustained (+)-pentazocine treatment in an in vivo model of retinal degeneration conferred significant neuroprotection, reduced evidence of oxidative stress, and preserved retinal architecture, suggesting that sigmaR1 ligands are promising therapeutic agents for intervention in neurodegenerative diseases of the retina.

Prevention of excitotoxicity in primary retinal ganglion cells by (+)-pentazocine, a sigma receptor-1 specific ligand

PURPOSE

Sigma receptors (sigmaRs) are nonopioid, nonphencyclidine binding sites with robust neuroprotective properties. Previously, the authors induced death in the RGC-5 cell line using very high concentrations (1 mM) of the excitatory amino acids glutamate (Glu) and homocysteine (Hcy) and demonstrated that the sigmaR1 ligand (+)-pentazocine ((+)-PTZ) could protect against cell death. The purpose of the present study was to establish a physiologically relevant paradigm for testing the neuroprotective effect of (+)-PTZ in retinal ganglion cells (RGCs).

METHODS

Primary ganglion cells (GCs) were isolated by immunopanning from retinas of 1-day-old mice, maintained in culture for 3 days, and exposed to 10, 20, 25, or 50 microM Glu or 10, 25, 50, or 100 microM Hcy for 6 or 18 hours in the presence or absence of (+)-PTZ (0.5, 1, 3 microM). Cell viability was measured using the viability and apoptosis detection fluorescein in situ assays. Expression of sigmaR1 was assessed by immunocytochemistry, RT-PCR, and Western blotting. Morphologic appearance of live ganglion cells and their processes was examined over time (0, 3, 6, 18 hours) by differential interference contrast (DIC) microscopy after exposure to excitotoxins in the presence or absence of (+)-PTZ.

RESULTS

Primary GCs showed robust sigmaR1 expression. The cells were exquisitely sensitive to Glu or Hcy toxicity (6-hour treatment with 25 or 50 microM Glu or 50 or 100 microM Hcy induced marked cell death). Primary GCs pretreated for 1 hour with (+)-PTZ followed by 18-hour cotreatment with 25 microM Glu and (+)-PTZ showed a marked decrease in cell death: 25 microM Glu alone, 50%; 25 microM Glu/0.5 microM (+)-PTZ, 38%; 25 microM Glu/1 microM (+)-PTZ, 20%; 25 microM Glu/3 microM (+)-PTZ, 18%. Similar results were obtained with Hcy. sigmaR1 mRNA and protein levels did not change in the presence of the excitotoxins. DIC examination of cells exposed to excitotoxins revealed substantial disruption of neuronal processes; cotreatment with (+)-PTZ revealed marked preservation of these processes. The stereoselective effect of (+)-PTZ for sigmaR1 was established in experiments in which (-)-PTZ, the levo-isomer form of pentazocine, had no neuroprotective effect on excitotoxin-induced ganglion cell death.

CONCLUSIONS

Primary GCs express sigmaR1; their marked sensitivity to Glu and Hcy toxicity mimics the sensitivity observed in vivo, making them a highly relevant model for testing neuroprotection. Pretreatment of cells with 1 to 3 microM (+)-PTZ, but not (-)-PTZ, affords significant protection against Glu- and Hcy-induced cell death. sigmaR1 ligands may be useful therapeutic agents in retinal diseases in which ganglion cells die.

dextro-Morphine attenuates the morphine-produced conditioned place preference via the sigma(1) receptor activation in the rat

An unbiased conditioned place preference paradigm was used to evaluate the effect of dextro-morphine on the morphine-produced reward in male CD rats. Morphine sulfate (1-10 mg/kg) given intraperitoneally dose-dependently produced the conditioned place preference. Pretreatment with dextro-morphine at a dose from 0.1 to 3 microg/kg given subcutaneously dose-dependently attenuated the morphine-produced conditioned place preference. However, dextro-morphine at a higher dose 100 microg/kg did not affect the morphine-produced conditioned place preference. Thus, dextro-morphine pretreatment induces a U-shaped dose-response curve for attenuating the morphine-produced conditioned place preference. The attenuation of the morphine-produced conditioned place preference was reversed by the pretreatment with the sigma(1) receptor antagonist BD1047 (N-[2-(3,4-Dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine dihydrobromide. dextro-Morphine or BD1047 given alone did not affect the baseline place conditioning. It is concluded that dextro-morphine attenuated the morphine-produced conditioned place preference via the sigma(1) receptor activation.

Purification and characterization of the guinea pig sigma-1 receptor functionally expressed in Escherichia coli

Sigma receptors once considered as a class of opioid receptors are now regarded as unique orphan receptors, distinguished by the ability to bind various pharmacological agents such as the progesterone (steroid), haloperidol (anti-psychotic), and drugs of abuse such as cocaine and methamphetamine. The sigma-1 receptor is a 223 amino acid protein, proposed to have two transmembrane segments. We have developed a scheme for the purification of the guinea pig sigma-1 receptor following overexpression in Escherichia coli as a maltose binding protein (MBP) fusion and extraction with Triton X-100. Affinity chromatography using an amylose column and Ni2+ affinity column was used to purify the sigma-1 receptor. The sigma-1 receptor purified by this method is a 26 kDa polypeptide as assessed by SDS-PAGE, binds sigma ligands with high affinity and can be specifically photoaffinity labeled with the sigma-1 receptor photoprobe, [125I]-iodoazidococaine. Ligand binding using [3H]-(+)-pentazocine indicated that approximately half of the purified protein in Triton X-100 bound to radioligand. The MBP-sigma-1 receptor and the sigma-1 receptor in 0.5% triton were maximally stable for approximately two weeks at -20 degrees C in buffer containing 30% glycerol.

Sigma-1 receptor ligands: potential in the treatment of neuropsychiatric disorders

The sigma receptor was originally proposed to be a subtype of the opioid receptor. However, it is now clear that sigma receptors are unique non-opioid, non-phencyclidine brain proteins. Two types of sigma receptor exist, the sigma-1 receptor and the sigma-2 receptor. sigma-1 receptors have been cloned and their distribution, physiological functions and roles in signal transduction were recently characterised. Certain sex hormones in the brain (neurosteroids) are known to interact with sigma-1 receptors. sigma-1 receptors regulate glutamate NMDA receptor function and the release of neurotransmitters such as dopamine. They are thus proposed to be involved in learning and memory as well as in certain neuropsychiatric disorders. Selective sigma-1 receptor ligands have been suggested to represent a new class of therapeutic agents for neuropsychiatric disorders, although none have yet been introduced into therapeutic use. Early studies showed that psychotomimetic benzomorphans, as well as several antipsychotics, can bind to sigma-1 receptors. As a result of these findings, sigma-1 receptor ligands have been proposed as being of potential use in the treatment of schizophrenia. Nevertheless, the relationship of sigma-1 receptors to the underlying pathogenesis of schizophrenia is still unclear. sigma-1 receptor ligands have failed to improve acute psychotic symptoms of schizophrenia in clinical trials, but, interestingly, a few studies have shown an improvement in negative symptoms in schizophrenic patients. A number of preclinical studies have shown that selective agonists of sigma-1 receptors affect higher-ordered brain functions such as learning and memory, cognition and mood. These studies indicate that sigma-1 receptor agonists may exert therapeutic effects in depression and senile dementia. Indeed, the sigma-1 receptor agonist igmesine, has been shown to improve depression in a clinical trial. The most distinctive feature of the action of sigma-1 receptor ligands is their "modulatory" role. In behavioural studies of depression and memory, they exert beneficial effects only when brain functions are perturbed. Given the recently accumulated preclinical and clinical data, it is time to reconstruct the concept of sigma-1 receptors and the associated pathophysiological conditions that ligands of these receptors target. This would allow clinical trials to be performed more efficiently, and the results may confirm a long-speculated possibility that sigma-1 receptor ligands represent a new class of therapeutic agents for neuropsychiatric disorders.

Different brain kinetics of two sigma 1 receptor ligands, [3H](+)-pentazocine and [11C]SA4503, by P-glycoprotein modulation

We compared the brain kinetics of radiolabeled (+)-pentazocine and SA4503, which have a high and selective affinity for sigma(1) receptors. Brain uptake of [(11)C]SA4503 was high after intravenous injection followed by a gradual decrease in mice, whereas that of [(3)H](+)-pentazocine rapidly decreased. The brain uptake of the two radioligands was dose-dependently reduced, but the reduction of [(3)H](+)-pentazocine was found at higher doses. Percentages of the saturable binding of [(3)H](+)-pentazocine was much lower than that of [(11)C]SA4503. The brain uptake of [(3)H](+)-pentazocine was greatly blocked by SA4503 at a dose of 2 micromol/kg, while that of [(11)C]SA4503 was blocked by (+)-pentazocine at a dose of 20 micromol/kg and over. When mice were treated with cyclosporin A, a P-glycoprotein modulator, the uptake of [(3)H](+)-pentazocine was enhanced, but that of [(11)C]SA4503 was not. Under control and P-glycoprotein-modulated conditions, the brain uptake of both radioligands was reduced by haloperidol, another representative sigma receptor ligand, to a different extent. We concluded that the P-glycoprotein modulation resulted in the different brain kinetics of the two radioligands. The radiolabeled SA4503 is suitable as an in vivo probe, but radiolabeled (+)-pentazocine is not.

Syntheses and opioid receptor binding affinities of 8-amino-2,6-methano-3-benzazocines

8-Amino-2,6-methano-3-benzazocine derivatives have been made using Pd-catalyzed amination procedures, and their affinities for opioid receptors were assessed. The 8-amino group was hypothesized to be a replacement for the prototypic 8-OH substituent for 2,6-methano-3-benzazocines and related opiates. This OH group is generally required for binding yet is implicated in unfavorable pharmacokinetic characteristics such as low oral bioavailability and rapid clearance via O-glucuronidation. The core structures in which the 8-OH group was replaced were cyclazocine and its enantiomers, ethylketocyclazocine and its enantiomers, ketocyclazocine, and Mr2034. Many new analogues had high affinity for opioid receptors with several in the subnanomolar range. Highest affinity was seen in analogues with secondary 8-(hetero)arylamino appendages. Binding to opioid receptors was enantioselective with the (2R,6R,11R)-configuration preferred and high selectivity for mu and kappa over delta opioid receptors was observed within the series. Several derivatives were shown to have intrinsic opioid-receptor-mediated activity in [(35)S]GTPgammaS assays.

Trishomocubanes: novel sigma-receptor ligands modulate amphetamine-stimulated [3H]dopamine release

Several trishomocubane analogues of the type 4-azahexacyclo [5.4.1.0(2,6).0(3,10).0(5,9).0(8,11)]dodecane exhibited moderate to high affinity at sigma-receptor subtypes and low or negligible affinity at dopamine and serotonin transporters (SERT). Selected compounds were examined for their effects on amphetamine-stimulated [3H]dopamine release from striatal slices in vitro. Compounds 1, 2, 3 and 4 significantly enhanced amphetamine-stimulated release in a concentration-dependent manner. Compound 4, with the highest affinity and selectivity for the sigma(2)-receptor subtype, displayed the greatest potency. The enhancement produced by 1 and 2 was fully reversed by the selective sigma(2) antagonists 1'-[4-[1-(4-fluorophenyl)-1-H-indol-3-yl]-1-butyl]spiro[iso-benzofuran-1(3H), 4'piperidine] (Lu28-179), endo-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2,3-dihydro-(1-methyl)ethyl-2-oxo-1-H-benzimidazole-1-carboxyamidehydrochloride (BIMU-8) and the non-subtype selective antagonist N-[2-(3,4-dichlorophenyl)-ethyl]-N-methyl-2-pyrrolidinyl)ethylamine (BD1008). These data suggested a potential role for compounds 1 through 4 as sigma(2)-receptor agonists in functional studies. In addition, a D(3)-trishomocubane compound 5 displayed low affinity at sigma receptors (K(i)=3 microM) and moderate affinity at dopamine transporters (K(i)=623 nM). Compound 5 significantly inhibited the potentiation mediated by compound 2, presumably through sigma(2)-receptor antagonism, or a direct action on dopamine transporters.

Repeated haloperidol treatment decreases sigma(1) receptor binding but does not affect its mRNA levels in the guinea pig or rat brain

The effects of chronic treatment with haloperidol on sigma (sigma) receptors were investigated across brain regions and species. The regional distribution of [3H](+)-pentazocine binding to sigma(1) receptor was similar between the guinea pig and rat brains. The highest level of binding was detected in the brain stem and lowest in the striatum and hippocampus. The regional distribution of [3H]1, 3-di (2-tolyl) guanidine ([3H]DTG) binding in the presence of 100 nM (+)-pentazocine to sigma(2) receptor was similar to that of the [3H](+)-pentazocine binding in the guinea pig brain, while in the rat brain high levels of [3H]DTG binding were detected in the cortex, frontal cortex and cerebellum. The intraperitoneal administration of 2 mg/kg of haloperidol to guinea pig and rats once a day for 21 days produced inhibition of [3H](+)-pentazocine binding but did not affect [3H]DTG binding to sigma(2) receptors in any brain region examined. The effects of haloperidol on [3H](+)-penazocine binding in the rat were much weaker than those in the guinea pig. The regional distribution of the level of sigma(1) receptor mRNA determined by the ribonuclease protection assay was similar to that of the [3H](+)-pentazocine binding activity, except in the cortex and cerebellum where the levels of sigma(1) receptor mRNA were low in guinea pig and rat. Treatment with haloperidol did not affect the levels of sigma(1) receptor mRNA in any brain region in either species. These findings suggested that the sigma receptors differentially distributed in brain regions are down regulated by treatment with haloperidol across sigma receptor subtypes and animal species without changing the transcriptional activity of the sigma(1) receptor. The mechanisms by which sigma receptors could be differently regulated in vivo by chronic treatment with haloperidol in different species may contribute to the therapeutic efficacy of haloperidol.

Binding properties of [3H]MS-377, a novel sigma receptor ligand, to rat brain membranes

MS-377 ((R)-(+)-1-(4-chlorophenyl)-3-[4-(2-methoxyethyl)piperazin-1-yl]++ +methy l-2-pyrrolidinone L-tartrate) is a novel selective sigma receptor ligand, currently being developed for the treatment of schizophrenia. MS-377 showed anti-phencyclidine (PCP), anti-dopaminergic and anti-serotonergic activities, and we anticipated that the anti-psychotic activities of MS-377 were associated with sigma(1) receptors. However, its pharmacological profile is partly distinct from those of selective sigma(1) receptor ligands. Thus, one of the possible speculations is that MS-377 has another site of action. In the present study, we examined the binding properties of radiolabeled MS-377 ([3H]MS-377) to rat brain membranes. [3H]MS-377 showed saturable and reversible binding to rat brain membranes. Scatchard plot and Hill plot from saturation studies were linear, with K(d) of 15.2+/-6.6 nM, B(max) of 599.4+/-58.6 fmol/mg protein and Hill coefficient of 1.01+/-0.01, indicating that [3H]MS-377 bound to a single high-affinity site in rat brain membranes. Displacement studies revealed that the other sigma reference compounds with different structures inhibited the specific binding of [3H]MS-377 in a competitive manner. Stereoselectivity was observed for the inhibition of [3H]MS-377 binding, (+)-isomers were more potent than (-)-isomers. Non-sigma receptor ligand PCP showed weak inhibition of [3H]MS-377 binding. The rank order of potency for the sigma reference compounds to displace [3H]MS-377 binding were as following: haloperidol>MS-377=(+)-pentazocine>DTG (1, 3-Ditolylguanidine)=(-)-pentazocine>BMY14802 (alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyramidinyl)-1-piperazine butanol)>(+)-SKF-10,047>(-)-SKF-10,047=PCP. These results suggested that the MS-377 selectively binds to sigma binding site with high affinity in rat brain membranes. Therefore, the anti-psychotic activities of MS-377 are attributable to association with sigma(1) receptors.

8-Aminocyclazocine analogues: synthesis and structure-activity relationships

Opioid binding affinities were assessed for a series of cyclazocine analogues where the prototypic 8-OH substituent of cyclazocine was replaced by amino and substituted-amino groups. For mu and kappa opioid receptors, secondary amine derivatives having the (2R,6R,11R)-configuration had the highest affinity. Most targets were efficiently synthesized from the triflate of cyclazocine or its enantiomers using Pd-catalyzed amination procedures.

Asymmetric synthesis of 9-alkyl-2-benzyl-6,7-benzomorphans: characterization as novel sigma receptor ligands

A convenient enantioselective synthesis of (1R,5R,9R)- and (1S,5S, 9S)-9-alkyl-2-benzyl-6,7-benzomorphans (2a-c) which starts with naphthaldehyde is described. These compounds were designed to gain additional information on the structure-sigma binding relationship of the 6,7-benzomorphan class of sigma ligands. In contrast to pentazocine and most 6,7-benzomorphans, the (1R,5R,9R)-isomers of 2a-c showed greater affinity for the sigma(1) receptor than the (1S, 5S,9S)-isomers. Despite reversal of enantioselectivity at the sigma(1) sites, moderate affinity and enantioselectivity at the sigma(2) sites [greater affinity for (1R,5R,9R)-isomers than (1S,5S, 9S)-isomers] were maintained. A comparison of the binding affinities of 2a-c to the more conformationally flexible trans-2-alkyl-1-benzaminoethyl-1,2-dihydronaphthalenes (10a-c) suggested that the relatively rigid structure of 2a-c played an important part in their sigma(1) binding properties. These compounds, particularly (1R,5R,9R)-2-benzyl-9-methyl-6,7-benzomorphan [(-)-2a], which has a K(i) value of 0.96 nM, will be useful in further characterization of the sigma(1) receptor.

Sodium channel activity and sigma binding of 2-aminopropanamide anticonvulsants

Sodium channel blocking, anticonvulsant activity, and sigma (sigma) binding of selected leads in a series of alpha-amino amide anticonvulsants were examined. While anticonvulsant compounds were always endowed with low micromolar sodium (Na+) channel site-2 binding, compounds with low site-2 Na+ channel affinity failed to control seizures. No correlation could be drawn with sigma1 binding. Both anticonvulsant and Na+ channel blocking activities were independent of stereochemistry, while sigma1 binding seems to be favoured by an S-configuration on the aminoamide moiety.

Two novel sigma receptor ligands, BD1047 and LR172, attenuate cocaine-induced toxicity and locomotor activity

The ability of cocaine to interact with sigma receptors indicates that these sites may mediate the negative properties associated with cocaine use, such as toxicity and addiction. Previous studies have shown that the novel sigma receptor ligand, BD1008 (N-[2-(3,4-dicholophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylam ine), effectively protects against cocaine-induced convulsions and locomotor activity in mice. Therefore, BD1047 ([2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(diamino)ethylamine) and LR172 (N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-homopiperidinyl)eth ylamine), two analogs of BD1008, were tested to determine if they also have anti-cocaine properties. Receptor binding assays showed that BD1047 and LR172 both have high affinities for a receptors, but low to negligible affinities for dopamine, opioid, phencyclidine, and 5-HT2 sites. In behavioral studies, pretreatment of mice with BD1047 or LR172 reduced the convulsions, lethality, and locomotor activity produced by cocaine. The data indicates a possible role for sigma receptor ligands in the treatment of cocaine overdose and addiction.

Relationship between modulation of the cerebellorubrospinal system in the in vitro turtle brain and changes in motor behavior in rats: effects of novel sigma ligands

Saturation and competition binding studies showed that the turtle brain contains sigma sites labeled by both [3H]di-o-tolylguanidine (DTG) and [3H](+)-pentazocine. There was a significant correlation between the IC50 values of sigma ligands for [3H]DTG sites in the turtle vs. rat brain, suggesting that the sites are comparable in the two species. In contrast, [3H](+)-pentazocine, which primarily labels sigma1 sites in the rodent brain, labels a heterogeneity of sites in the turtle brain. In extracellular recordings from the in vitro turtle brainstem, some sigma ligands enhanced the burst responses of red nucleus (RN) neurons (DTG, haloperidol, BD1031, BD1052, BD1069) while other sigma ligands decreased the burst responses (BD1047, BD1063). Control compounds (turtle Ringer vehicle control, opiate antagonist naloxone, atypical neuroleptic sulpiride) had no significant effects on the RN burst responses recorded from the in vitro turtle brain. The ED50s of the ligands for altering the burst responses in RN neurons from the turtle brain were correlated with their IC50s for turtle brain sites labeled with [3H]DTG, but not [3H](+)-pentazocine; this pattern is identical to that previously reported in rats, where there is a correlation between the potencies of sigma ligands for producing dystonic postures after microinjection into the rat RN and their binding to rat brain sites labeled with [3H]DTG, but not [3H](+)-pentazocine. When the novel sigma ligands were microinjected into the rat RN, dystonic postures were produced by ligands that increased the burst duration of RN neurons in the turtle brain. Novel sigma ligands that reduced the burst responses in the in vitro turtle brain have previously been reported to have no effects on their own when microinjected into the rat RN, but to block the dystonic postures produced by other sigma ligands. Taken together, the data suggest that the opposite effects of the novel ligands in the turtle electrophysiological studies represent the actions of agonists vs. antagonists, and that the directionality of the effects has predictive value for the expected motor effects of the drugs.

Novel sigma receptor ligands attenuate the locomotor stimulatory effects of cocaine

Cocaine interacts with sigma receptors, suggesting that these sites are important for many of its behavioral effects. Therefore, two novel sigma receptor ligands, BD1008 (N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine) and BD1063 (1-[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine), were evaluated for their ability to attenuate cocaine-induced locomotor activity. Receptor binding studies showed that BD1008 and BD1063 have nanomolar affinities for sigma1 and sigma2 sites, but a 250-fold or lower affinity for nine other receptors, making them among the most selective sigma receptor ligands identified. In behavioral studies, pretreatment of mice with BD1008 or BD1063 produced a two-fold increase in the ED50 for the locomotor stimulatory effects of cocaine. These results suggest that sigma receptors are involved in the behavioral effects of cocaine.

Substituted halogenated arylsulfonamides: a new class of sigma receptor binding tumor imaging agents

The discovery of a series of novel halogenated arylsulfonamides (HAS) as new sigma receptor binding tumor imaging agents is described. Several substituted halogenated sulfonamides have been prepared and characterized. Target compounds were examined for their affinity for sigma1 and sigma2 receptor subtypes using guinea pig brain membranes and rat liver membranes, respectively. A number of substituted halogenated sulfonamides displayed subnanomolar affinities for sigma1 sites and low nanomolar affinities for sigma2 subtype receptors. A limited structure-activity relationship study of this chemical series is discussed. The radioiodination (I-125) of one congener member (4-[125I]iodo-N-[2-(1'-piperidinyl)ethyl]benzenesulfonamide, 4-[125I]IPBS) was accomplished in high yields. The in vitro competition binding studies of 4-[125I]IPBS in guinea pig brain membranes with sigma receptor binding ligands confirmed its sigma pharmacology. The rank order of potency was BD1008 (N-[2-(3, 4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine) > 4-IPBS > haloperidol > (+)-pentazocine > DTG (1, 3-di-o-tolylguanidine) > (-)-pentazocine. The inhibition constants (IC50) were 0.70, 1.46, 6.28, 10.4, 87.2, and 152 nM, respectively, and are consistent with labeling of sigma1 receptors. The tumor imaging potential of 4-[125I]IPBS was studied in C57 black mice bearing B16 melanoma xenograft. A high tumor uptake of 4-[125I]IPBS was observed (7.40% ID/g) at 1 h postinjection. The wash out of activity from the tumor was slow at 6 h postinjection (7.22% ID/g). The tumor also had the highest amount of radioactivity (1.54% ID/g) at 24 h postinjection. These results demonstrate that radiohalogenated benzenesulfonamides could be a potentially useful class of compounds in nuclear oncologic scintigraphy.