Naloxone-inaccessible sigma receptor in rat central nervous system

Structure-Affinity relationships of novel σ2R/TMEM97 ligands

The sigma 2 receptor (σ2R), which was recently identified as the transmembrane protein 97 (TMEM97), is increasingly attracting interest as a possible therapeutic target for indications in neuroscience. Toward identifying novel modulators of σ2R/TMEM97, we prepared a collection of benzoxazocine, benzomorphan, and methanobenzazepine ligands related to the known bioactive norbenzomorphans DKR-1677, FEM-1689, and EES-1686 and determined their Ki values for σ2R/TMEM97 and the sigma 1 receptor (σ1R). The σ2R/TMEM97 binding affinities and selectivities relative to σ1R of these new benzoxazocine, benzomorphan, and methanobenzazepine analogs are lower, often significantly lower, than their respective norbenzomorphan counterparts, suggesting the spatial orientation of pharmacophoric substituents is critical for binding to the two proteins. The benzoxazocine, benzomorphan, and methanobenzazepine congeners of DKR-1677 and FEM-1689 tend to be weakly selective for σ2R/TMEM97 versus σ1R, whereas EES-1686 derivatives exhibit the greatest selectivity, suggesting the size and/or nature of the substituent on the nitrogen atom of the scaffold may be important for selectivity. Computational docking studies were performed for the 1S,5R-and 1R,5S-enantiomers of DKR-1677, FEM-1689, and EES-1686 and their benzoxazocine, benzomorphan, and methanobenzazepine counterparts. These computations predict that the protonated amino group of each ligand forms a highly conserved salt bridge and a H-bonding interaction with Asp29 as well as a cation-π interaction with Tyr150 of σ2R/TMEM97. These electrostatic interactions are major driving forces for binding to σ2R/TMEM97 and are similar, though not identical, for each ligand. Other interactions within the well-defined binding pocket also tend to be comparable, but there are some major differences in how the hydrophobic aryl groups of various ligands interact with the protein surface external to the binding pocket. Overall, these studies show that the orientations of aryl and N-substituents on the norbenzomorphan and related scaffolds are important determinants of binding affinity of σ2R/TMEM97 ligands, and small changes can have significant effects upon binding profiles.

Recent advances in drug discovery efforts targeting the sigma 1 receptor system: Implications for novel medications designed to reduce excessive drug and food seeking

Psychiatric disorders characterized by uncontrolled reward seeking, such as substance use disorders (SUDs), alcohol use disorder (AUD) and some eating disorders, impose a significant burden on individuals and society. Despite their high prevalence and substantial morbidity and mortality rates, treatment options for these disorders remain limited. Over the past two decades, there has been a gradual accumulation of evidence pointing to the sigma-1 receptor (S1R) system as a promising target for therapeutic interventions designed to treat these disorders. S1R is a chaperone protein that resides in the endoplasmic reticulum, but under certain conditions translocates to the plasma membrane. In the brain, S1Rs are expressed in several regions important for reward, and following translocation, they physically associate with several reward-related GPCRs, including dopamine receptors 1 and 2 (D1R and D2R). Psychostimulants, alcohol, as well as palatable foods, all alter expression of S1R in regions important for motivated behavior, and S1R antagonists generally decrease behavioral responses to these rewards. Recent advances in structural modeling have permitted the development of highly-selective S1R antagonists with favorable pharmacokinetic profiles, thus providing a therapeutic avenue for S1R-based medications. Here, we provide an up-to-date overview of work linking S1R with motivated behavior for drugs of abuse and food, as well as evidence supporting the clinical utility of S1R antagonists to reduce their excessive consumption. We also highlight potential challenges associated with targeting the S1R system, including the need for a more comprehensive understanding of the underlying neurobiology and careful consideration of the pharmacological properties of S1R-based drugs.

Structure-affinity relationships of stereoisomers of norbenzomorphan-derived σ2R/TMEM97 modulators

The sigma 2 receptor (σ₂R), which is identical to transmembrane protein 97 (TMEM97), is attracting increasing interest as a possible therapeutic target for various indications in neuroscience. In continuation of a program to identify novel compounds that bind with high affinity and selectivity to σ₂R/TMEM97, we performed structure-affinity-relationship (SAfiR) studies of several sets of σ₂R/TMEM97 ligands having a B-norbenzomorphan ring core. Binding data for σ₂R/TMEM97 and σ₁R of several enantiomeric pairs of piperazine-substituted norbenzomorphans show the (1S,5R)-enantiomers have affinities (K_i = 9-75 nM) for σ₂R/TMEM97 that are 2-3-fold higher than their enantiomorphic (1R,5S)-analogs; however, there is no clear trend for selectivity for σ₂R/TMEM97 vs σ₁R. A series of N-alkyl piperazino (1S,5R)-norbenzomorphans was then evaluated, and with the exception of compounds having N-alkyl groups substituted with oxygen or amino groups at C (2) of an ethylene chain, K_i values for σ₂R/TMEM97 are less than 25 nM, and several compounds have good selectivities (ca 7-16-fold) for σ₂R/TMEM97 vs σ₁R. Mono-substituted carbobenzyloxy analogs have K_i values for σ₂R/TMEM97 comparable to the unsubstituted parent (K_i = ca 7-27 nM), but replacing the N-acyloxy group with N-acyl or N-arylsulfonyl groups provides analogs having lower affinity and selectivity. Some congeners with bioisosteric replacements of the piperazine group on the (1S,5R)-norbenzomorphan core have high affinity (K_i = <30 nM) for σ₂R/TMEM97, but selectivities are modest. Computational docking studies for racemic pairs of piperazino norbenzomorphans show that individual (1S,5R)- and (1R,5S)-enantiomers adopt distinct poses upon binding to σ₂R/TMEM97, whereas ligands belongingto the same enantiomeric series adopt closely similar binding poses. The protonated amino group in each of the enantiomorphic ligands engages in highly conserved salt bridges with Asp29 and cation-π interactions with Tyr150 that are the primary determinants of binding affinity. There is no correlation between any of the computational parameter outputs and K_i values, but this is unsurprising given the small energetic differences involved. Modeling also suggest sthat some compounds can extend deeper into σ₂R/TMEM97 binding pocket forming salt bridges with Glu73.

Discovery of a σ1 receptor antagonist by combination of unbiased cell painting and thermal proteome profiling

Phenotypic screening for bioactive small molecules is typically combined with affinity-based chemical proteomics to uncover the respective molecular targets. However, such assays and the explored bioactivity are biased toward the monitored phenotype, and target identification often requires chemical derivatization of the hit compound. In contrast, unbiased cellular profiling approaches record hundreds of parameters upon compound perturbation to map bioactivity in a broader biological context and may link a profile to the molecular target or mode of action. Herein we report the discovery of the diaminopyrimidine DP68 as a Sigma 1 (σ1) receptor antagonist by combining morphological profiling using the Cell Painting assay and thermal proteome profiling. Our results highlight that integration of complementary profiling approaches may enable both detection of bioactivity and target identification for small molecules.

Synthesis and σ receptor affinity of spiro[[2]benzopyran-1,1'-cyclohexanes] with an exocyclic amino moiety in the 3'-position

The main functions of σ ₁ receptors include the modulation of release and reuptake of neurotransmitters, the regulation of ion channels and the influence on intracellular signaling through modulation of calcium levels. Due to these properties, σ ₁ receptors are interesting drug targets for the treatment of various neurological disorders, pain and cancer. In order to modify the distance between the pharmacophoric elements (the benzene ring of 2-benzopyran and an amino moiety), a set of spiro[[2]benzopyran-1,1'-cyclohexan]-3'-amines was synthesized. The key step of the synthesis was a Parham cyclization of 1-bromo-2-(2-bromoethyl)benzene (6) with the mono ketal 7 of cyclohexane-1,3-dione, which led in a one-pot reaction to the spirocyclic framework 8. Reductive amination of ketone 9 stereoselectively provided secondary amines cis-4, which were methylated to afford tertiary amines cis-5. Whereas spirocyclic compounds cis-4a and cis-5a bearing a benzyl moiety at the exocyclic amino moiety showed rather low σ ₁ affinity, the corresponding cyclohexylmethyl derivatives cis-4b and cis-5b exhibited low nanomolar σ ₁ affinity. The secondary amine cis-4b displayed the highest σ ₁ receptor affinity (K _i = 5.4 nM) in this series. Methylation of the secondary amine cis-4b led to a slightly decreased σ ₁ receptor affinity of cis-5b (K _i = 15 nM).

Repurposing Sigma-1 Receptor Ligands for COVID-19 Therapy?

Outbreaks of emerging infections, such as COVID-19 pandemic especially, confront health professionals with the unique challenge of treating patients. With no time to discover new drugs, repurposing of approved drugs or in clinical development is likely the only solution. Replication of coronaviruses (CoVs) occurs in a modified membranous compartment derived from the endoplasmic reticulum (ER), causes host cell ER stress and activates pathways to facilitate adaptation of the host cell machinery to viral needs. Accordingly, modulation of ER remodeling and ER stress response might be pivotal in elucidating CoV-host interactions and provide a rationale for new therapeutic, host-based antiviral approaches. The sigma-1 receptor (Sig-1R) is a ligand-operated, ER membrane-bound chaperone that acts as an upstream modulator of ER stress and thus a candidate host protein for host-based repurposing approaches to treat COVID-19 patients. Sig-1R ligands are frequently identified in in vitro drug repurposing screens aiming to identify antiviral compounds against CoVs, including severe acute respiratory syndrome CoV-2 (SARS-CoV-2). Sig-1R regulates key mechanisms of the adaptive host cell stress response and takes part in early steps of viral replication. It is enriched in lipid rafts and detergent-resistant ER membranes, where it colocalizes with viral replicase proteins. Indeed, the non-structural SARS-CoV-2 protein Nsp6 interacts with Sig-1R. The activity of Sig-1R ligands against COVID-19 remains to be specifically assessed in clinical trials. This review provides a rationale for targeting Sig-1R as a host-based drug repurposing approach to treat COVID-19 patients. Evidence gained using Sig-1R ligands in unbiased in vitro antiviral drug screens and the potential mechanisms underlying the modulatory effect of Sig-1R on the host cell response are discussed. Targeting Sig-1R is not expected to reduce dramatically established viral replication, but it might interfere with early steps of virus-induced host cell reprogramming, aid to slow down the course of infection, prevent the aggravation of the disease and/or allow a time window to mature a protective immune response. Sig-1R-based medicines could provide benefit not only as early intervention, preventive but also as adjuvant therapy.

Computational screening of natural and natural-like compounds to identify novel ligands for sigma-2 receptor

Sigma-2 (σ₂) receptor is a transmembrane protein shown to be linked with neurodegenerative diseases and cancer development. Thus, it emerges as a potential biological target for the advancement of anticancer and anti-Alzheimer's agents. The current study was aimed to identify potential σ₂ receptor ligands using integrated computational approaches including homology modelling, combined pharmacophore- and docking-based virtual screening, and molecular dynamics (MD) simulation. Pharmacophore-based screening was conducted against a database composed of 20,523 small natural and natural-like products. In total, 1200 structures were found to satisfy the required pharmacophore features and were then exposed to docking-based screening against the generated homology model of σ₂ receptor. On the basis of the pharmacophore fit scores, docking scores, and mechanism of binding interaction, 20 potential hits were retained. Five promising candidates were selected (SR84, SR823, SR300, SR413, and SR530) on the basis of their binding score and interaction. Further, in silico ADMET profiling of these compounds showed that the selected compounds possess favourable ADME properties with low toxicity risk. The mechanism of interaction of these compounds with σ₂ receptor as well as their binding stability were characterized by MD simulation.

Pharmacological characterization of high-affinity σ1 receptor ligands with spirocyclic thienopyran and thienofuran scaffold

OBJECTIVES

In this study, the pharmacological properties of six spirocyclic piperidines 1-6 showing very high σ1 receptor affinity (Ki  = 0.2-16 nm) were investigated.

METHODS

In vitro receptor binding studies, retinal ganglion assay and in vivo capsaicin assay were used to determine the affinity, selectivity and activity. Influence on human tumour cell growth (cell lines A427, LCLC-103H, 5637 and DAN-G) was determined in different assays. The effect on the ergosterol and cholesterol biosynthesis was determined by GLC/MS analysis.

KEY FINDINGS

Receptor binding studies demonstrated high selectivity for the σ1 receptor. The increased Ca2+ influx mediated by 2 and the analgesic activity of 1, 4, 5 and 6 confirm σ1 receptor antagonistic activity. Inhibition of human tumour cell growth further supports the σ1 antagonistic effects. Treatment of A427 tumour cells with 2 led to cell detachment and cell degradation. Whereas the ergosterol biosynthesis was not affected, the sterol C14-reductase, a key enzyme in the cholesterol biosynthesis, was weakly inhibited.

CONCLUSIONS

Due to the high selectivity, off-target effects are not expected. The antiallodynic activity underlines the clinical potential of the spirocyclic piperidines for the treatment of neuropathic pain. Due to the antiproliferative activity, the spirocyclic σ1 antagonists represent promising antitumour agents.

The Molecular Function of σ Receptors: Past, Present, and Future

The σ1 and σ2 receptors are enigmatic proteins that have attracted attention for decades due to the chemical diversity and therapeutic potential of their ligands. However, despite ongoing clinical trials with σ receptor ligands for multiple conditions, relatively little is known regarding the molecular function of these receptors. In this review, we revisit past research on σ receptors and discuss the interpretation of these data in light of recent developments. We provide a synthesis of emerging structural and genetic data on the σ1 receptor and discuss the recent cloning of the σ2 receptor. Finally, we discuss the major questions that remain in the study of σ receptors.

Trends in Sigma-1 Receptor Research: A 25-Year Bibliometric Analysis

Purpose: There are previous reviews focused on Sigma-1 receptor but no bibliometric studies examining this field as a whole. This article aims to present a global view of Sigma-1 receptor research and its intellectual structure.

Methods: We used bibliometric indicators of a basic nature as well as techniques for the visualization and analysis of networks of scientific information extracted from Scopus database.

Results: In total, 1,102 articles from 1992 to 2017 were identified. The growth in the production of articles is not constant over time, with periods of stagnation of approximately 5 years. Only 247 authors have five or more publications. The authors appear grouped in relatively independent clusters, thus suggesting a low level of collaborations between those dedicated to the Sigma-1 receptor. The United States was the country with the highest production followed by Japan and Germany. Spain, Japan, and Italy showed the highest per million inhabitants ratio. The highest citation/article ratio was reached in France, United States, and Canada. The leading institutions were the University of Münster, the National Institutes of Health, ESTEVE, and INSERM. The top authors in number of publications were Wünsch-B, Schepmann-D, and Maurice-T. Hayashi-T, Su-TP and Bowen-WD showed the highest citations per article. The article by Hayashi-T and Su-TP in Cell (2007) describing the Sigma-1 receptor as a chaperone protein is the top cited reference. Cluster labeling from author co-citation analysis shows that research has been focused on specific diseases such as addiction, neuroprotection and neurodegenerative diseases, psychiatric disorders, and pain. High-frequency terms in author keywords suggest that the research efforts in some areas such as neuroimaging, cocaine addiction or psychiatric disorders have declined over time, while others such as neurodegenerative diseases or pain are currently most popular.

Perspective: A greater involvement of the scientific community, with an increase in the scientific production related to Sigma-1, is desirable. Additional boost needed to improve research performance is likely to come from combining data from different laboratories to overcome the limitations of individual approaches. The resulting maps are a useful and attractive tool for the Sigma-1 receptor research community, as they reveal the main lines of exploration at a glance.

Molecular Interplay Between the Sigma-1 Receptor, Steroids, and Ion Channels

Cell excitability is tightly regulated by the activity of ion channels that allow for the passage of ions across cell membranes. Ion channel activity is controlled by different mechanisms that change their gating properties, expression or abundance in the cell membrane. The latter can be achieved by forming complexes with a diversity of proteins like chaperones such as the Sigma-1 receptor (Sig-1R), which is one with unique features and exhibits a role as a ligand-operated chaperone. This molecule also displays high intracellular mobility according to its activation level since, depletion of internal Ca⁺² stores or the presence of specific ligands, produce Sig-1R’s mobilization from the endoplasmic reticulum toward the plasma membrane or nuclear envelope. The function of the Sig-1R as a chaperone is regulated by synthetic and endogenous ligands, with some of these compounds being a steroids and acting as key endogenous modifiers of the actions of the Sig-1R. There are cases in the literature that exemplify the close relationship between the actions of steroids on the Sig-1R and the resulting negative or positive effects on ion channel function/abundance. Such interactions have been shown to importantly influence the physiology of mammalian cells leading to changes in their excitability. The present review focuses on describing how the Sig-1R regulates the functional properties and the expression of some sodium, calcium, potassium, and TRP ion channels in the presence of steroids and the physiological consequences of these interplays at the cellular level are also discussed.

Bicycloheptylamine-Doxorubicin Conjugate: Synthesis and Anticancer Activities in σ2 Receptor-Expressing Cell Lines

BACKGROUND

Novel bicycloheptylamines were designed and synthesized. These compounds were found to be selective for sigma-2 receptors. These receptors have been found to be up to 10 fold over-expressed in certain cancer cell lines, leading to investigation of possible uses as a biomarker in diagnosis and/or treatment especially in cancers with poor prognosis.

OBJECTIVES

The aim was to conjugate a novel sigma-2 receptor ligand to doxorubicin to examine anticancer activities, with and without conjugation, and therefore possibilities in drug delivery.

METHODS

Conjugation was conducted using N-(3-Dimethylaminopropyl)-N'-ethylcarbodiimide HCl as a coupling agent. Affinity towards the sigma-2 receptor was tested using ligand-receptor binding studies. Anticancer activities against cancer cell lines were carried out using cell viability assays. Caspase dependency was tested using Z-VAD-FMK, a pan-caspase inhibitor, to begin to investigate mechanisms of action.

RESULTS

The target compound retained affinity towards the sigma-2 receptor and exhibited potent anticancer activities on cancer cell lines expressing the sigma-2 receptor. The potencies exceeded those of doxorubicin, the lead sigma-2 receptor ligand, as well as non-covalent combination of both drugs. The activity was also found to be caspase-dependent.

CONCLUSION

The conjugation of target bicycloheptylamines with cytotoxic moieties may yield potent and selective molecules for detection and/or treatment of certain cancers.

Cardiac sigma receptors - an update

More than four decades passed since sigma receptors were first mentioned. Since then, existence of at least two receptor subtypes and their tissue distributions have been proposed. Nowadays, it is clear, that sigma receptors are unique ubiquitous proteins with pluripotent function, which can interact with so many different classes of proteins. As the endoplasmic resident proteins, they work as molecular chaperones - accompany various proteins during their folding, ensure trafficking of the maturated proteins between cellular organelles and regulate their functions. In the heart, sigma receptor type 1 is more dominant. Cardiac sigma 1 receptors regulate response to endoplasmic reticulum stress, modulates calcium signaling in cardiomyocyte and can affect function of voltage-gated ion channels. They contributed in pathophysiology of cardiac hypertrophy, heart failure and many other cardiovascular disorders. Therefore, sigma receptors are potential novel targets for specific treatment of cardiovascular diseases.

Synthesis, Receptor Affinity, and Antiallodynic Activity of Spirocyclic σ Receptor Ligands with Exocyclic Amino Moiety

In order to detect novel σ receptor ligands, the rigid spiro[[2]benzopyran-1,1'-cyclohexan]-4'-one was connected with amino moieties derived from σ₂ receptor preferring lead compounds resulting in mixtures of trans- and cis-configured amines 6, 18, and 27. In a four step synthesis the methyl acetals 6 were converted into fluoroethyl derivatives 13 and 30. The most promising σ₂ receptor ligand is the methyl acetal 6a bearing a 2,4-dimethylbenzylamino moiety. The fluoroethyl derivatives 13c and 13d reveal high σ₁ affinity but moderate selectivity over the σ₂ subtype. In mice 13c and 13d showed antiallodynic activity that is stronger than that of the reference σ₁ antagonist BD-1063 (34). Since the antiallodynic activity of 13c could only be partially reversed by the σ₁ agonist PRE-084 (35), it is postulated that a second mechanism contributes to its overall antiallodynic effect. In contrast, the antiallodynic effect of its diastereomer 13d can be totally explained by a σ₁ antagonism.

Chiral resolution of serial potent and selective σ1 ligands and biological evaluation of (-)-[18F]TZ3108 in rodent and the nonhuman primate brain

Twelve optically pure enantiomers were obtained using either crystallization or chiral high performance liquid chromatography (HPLC) separation methodologies to resolve six racemic sigma-1 (σ₁) receptor ligands. The in vitro binding affinities of each enantiomer for σ₁, σ₂ receptors and vesicular acetylcholine transporter (VAChT) were determined. Out of the 12 optically pure enantiomers, five displayed very high affinities for σ₁ (K_i<2nM) and high selectivity for σ₁ versus σ₂ and VAChT (>100-fold). The minus enantiomer, (-)-14a ((-)-TZ3108) (K_i-σ1=1.8±0.4nM, K_i-σ2=6960±810nM, K_i-VAChT=980±87nM), was chosen for radiolabeling and further in vivo evaluation in rodents and nonhuman primates (NHPs). A biodistribution study in Sprague Dawley rats showed brain uptake (%ID/gram) of (-)-[¹⁸F]TZ3108 reached 1.285±0.062 at 5min and 0.802±0.129 at 120min. NHP microPET imaging studies revealed higher brain uptake of (-)-[¹⁸F]TZ3108 and more favorable pharmacokinetics compared to its racemic counterpart. Pretreatment of the animal using two structurally different σ₁ ligands significantly decreased accumulation of (-)-[¹⁸F]TZ3108 in the brain. Together, our in vivo evaluation results suggest that (-)-[¹⁸F]TZ3108 is a promising positron emission tomography (PET) tracer for quantifying σ₁ receptor in the brain.

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.

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 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.

Synthesis and σ receptor affinity of regioisomeric spirocyclic furopyridines

In order to investigate systematically the effect of the position of the pyridine N-atom on the σ1 receptor affinity four regioisomeric furopyridines 2a-d were synthesized and pharmacologically evaluated. The key steps of the synthesis comprise bromine/lithium exchange at regioisomeric bromopyridinecarbaldehyde acetals 7a-d, subsequent addition to 1-benzylpiperidin-4-one and cyclization. The regioisomeric acetals 7a-d were obtained either by o-metalation of bromopyridines 5b and 5c or by oxidation of bromopicolines 3a and 3d. In radioligand binding studies the regioisomeric furopyridines 2a-d showed 7- to 12-fold lower σ1 affinity than the benzofuran analog 1. The reduced σ1 affinity of the furopyridines 2a-d is explained with the reduced electron density of the pyridine ring. Since the four regioisomeric furopyridines show almost the same σ1 affinity (Ki = 4.9-10 nM), a directed interaction of the pyridine N-atom with the receptor protein can be excluded.

Preclinical efficacy of N-substituted benztropine analogs as antagonists of methamphetamine self-administration in rats

Atypical dopamine-uptake inhibitors have low abuse potential and may serve as leads for development of cocaine-abuse treatments. Among them, the benztropine (BZT) derivatives, N-butyl (JHW007), N-allyl (AHN2-005), and N-methyl (AHN1-055) analogs of 3α-[bis(4'-fluorophenyl)methoxy]-tropane dose-dependently decreased cocaine self-administration without effects on food-maintained responding. Our study examined selectivity by assessing their effects on self-administration of other drugs. As with cocaine, each BZT analog (1.0-10.0 mg/kg i.p.) dose-dependently decreased maximal self-administration of d-methamphetamine (0.01-0.32 mg/kg/infusion) but was inactive against heroin (1.0-32.0 µg/kg/infusion) and ketamine (0.032-1.0 mg/kg/infusion) self-administration. Further, standard dopamine indirect-agonists [WIN35,428 ((-)-3β-(4-fluorophenyl)-tropan-2-β-carboxylic acid methyl ester tartrate), d-amphetamine (0.1-1.0 mg/kg i.p., each)] dose-dependently left-shifted self-administration dose-effect curves for d-methamphetamine, heroin, and ketamine. Noncompetitive NMDA-glutamate receptor/channel antagonists [(+)-MK-801 (0.01-0.1 mg/kg i.p.), memantine (1.0-10.0 mg/kg i.p.)] also left-shifted dose-effect curves for d-methamphetamine and ketamine (but not heroin) self-administration. The µ-agonists [dl-methadone and morphine (1.0-10.0 mg/kg i.p., each)] dose-dependently decreased maximal self-administration of µ-agonists (heroin, remifentanil) but not d-methamphetamine or ketamine self-administration. The µ-agonist-induced decreases were similar to the effects of BZT analogs on stimulant self-administration and effects of food prefeeding on responding maintained by food reinforcement. Radioligand-binding and behavioral studies suggested that inhibition of dopamine transporters and σ receptors were critical for blocking stimulant self-administration by BZT-analogs. Thus, the present results suggest that the effects of BZT analogs on stimulant self-administration are similar to effects of µ-agonists on µ-agonist self-administration and food prefeeding on food-reinforced responding, which implicates behavioral mechanisms for these effects and further supports development of atypical dopamine uptake inhibitors as medications for stimulant abuse.

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 of the σ1 receptor and its ligand binding site

The exact 3D structure of the enigmatic σ1 receptor is unknown, as the crystal structure of this protein has not been solved so far. Many efforts have been devoted to unveiling the structure of the σ1 receptor and specifically its binding site, which include photoaffinity labeling, site directed mutagenesis, and homology modeling. The aim of the present miniperspective is to give a short overview of all results that contribute to the current knowledge of the σ1 receptor and its ligand binding site.

Stimulants as specific inducers of dopamine-independent σ agonist self-administration in rats

A previous study showed that cocaine self-administration induced dopamine-independent reinforcing effects of σ agonists mediated by their selective actions at σ1 receptors (σ1Rs), which are intracellularly mobile chaperone proteins implicated in abuse-related effects of stimulants. The present study assessed whether the induction was specific to self-administration of cocaine. Rats were trained to self-administer the dopamine releaser, d-methamphetamine (0.01-0.32 mg/kg per injection), the μ-opioid receptor agonist, heroin (0.001-0.032 mg/kg per injection), and the noncompetitive N-methyl-d-aspartate receptor/channel antagonist ketamine (0.032-1.0 mg/kg per injection). As with cocaine, self-administration of d-methamphetamine induced reinforcing effects of the selective σ1R agonists PRE-084 [2-(4-morpholinethyl)1-phenylcyclohexanecarboxylate hydrochloride] and (+)-pentazocine (0.032-1.0 mg/kg per injection, each). In contrast, neither self-administration of heroin nor ketamine induced PRE-084 or (+)-pentazocine (0.032-10 mg/kg per injection, each) self-administration. Although the σ1R agonists did not maintain responding in subjects with histories of heroin or ketamine self-administration, substitution for those drugs was obtained with appropriate agonists (e.g., remifentanil, 0.1-3.2 µg/kg per injection, for heroin and (5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine ((+)-MK 801; dizocilpine), 0.32-10.0 µg/kg per injection, for ketamine). The σR antagonist N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine dihydrobromide (BD 1008; 1.0-10 mg/kg) dose-dependently blocked PRE-084 self-administration but was inactive against d-methamphetamine, heroin, and ketamine. In contrast, PRE-084 self-administration was affected neither by the dopamine receptor antagonist (+)-butaclamol (10-100 μg/kg) nor by the opioid antagonist (-)-naltrexone (1.0-10 mg/kg), whereas these antagonists were active against d-methamphetamine and heroin self-administration, respectively. The results indicate that experience specifically with indirect-acting dopamine agonists induces reinforcing effects of previously inactive σ1R agonists. It is further suggested that induced σ1R reinforcing mechanisms may play an essential role in treatment-resistant stimulant abuse, suggesting new approaches for the development of effective medications for its treatment.

The σ2 receptor: a novel protein for the imaging and treatment of cancer

The σ2 receptor is an important target for the development of molecular probes in oncology because of its 10-fold higher density in proliferating tumor cells compared with that in quiescent tumor cells and because of the observation that σ2 receptor agonists are able to kill tumor cells via apoptotic and nonapoptotic mechanisms. Although recent evidence indicates that the σ2 receptor binding site is localized within the progesterone receptor membrane component 1 (PGRMC1), most information regarding this protein has been obtained using either radiolabeled or fluorescent receptor-based probes and from biochemical analysis of the effect of σ2 selective ligands on cells grown in culture. This article reviews the development of σ2 receptor ligands and presents an overview of how they have been used in vitro and in vivo to increase our understanding of the role of the σ2 receptor in cancer and proliferation.

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.

Synthesis and pharmacological characterization of a novel sigma receptor ligand with improved metabolic stability and antagonistic effects against methamphetamine

Methamphetamine interacts with sigma receptors at physiologically relevant concentrations suggesting a potential site for pharmacologic intervention. In the present study, a previous sigma receptor ligand, CM156, was optimized for metabolic stability, and the lead analog was evaluated against the behavioral effects of methamphetamine. Radioligand binding studies demonstrated that the lead analog, AZ66, displayed high nanomolar affinity for both sigma-1 and sigma-2 receptors (2.4 ± 0.63 and 0.51 ± 0.15, respectively). In addition, AZ66 had preferential affinity for sigma receptors compared to seven other sites and a significantly longer half-life than its predecessor, CM156, in vitro and in vivo. Pretreatment of male, Swiss Webster mice with intraperitoneal (10-20 mg/kg) or oral (20-30 mg/kg) dosing of AZ66 significantly attenuated the acute locomotor stimulatory effects of methamphetamine. Additionally, AZ66 (10-20 mg/kg, i.p.) significantly reduced the expression and development of behavioral sensitization induced by repeated methamphetamine administration. Taken together, these data indicate that sigma receptors can be targeted to mitigate the acute and subchronic behavioral effects of methamphetamine and AZ66 represents a viable lead compound in the development of novel therapeutics against methamphetamine-induced behaviors.

Decreases in cocaine self-administration with dual inhibition of the dopamine transporter and σ receptors

Sigma receptor (σR) antagonists attenuate many behavioral effects of cocaine but typically not its reinforcing effects in self-administration procedures. However, the σR antagonist rimcazole and its N-propylphenyl analogs, [3-(cis-3,5-dimethyl-4-[3-phenylpropyl]-1-piperazinyl)-propyl]diphenylamine hydrochloride (SH 3-24) and 9-[3-(cis-3,5-dimethyl-4-[3-phenylpropyl]-1-piperazinyl)-propyl]carbazole hydrobromide (SH 3-28), dose-dependently decreased the maximal rates of cocaine self-administration without affecting comparable responding maintained by food reinforcement. In contrast, a variety of σR antagonists [N-phenethylpiperidine oxalate (AC927), N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine dihydrobromide (BD 1008), N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino) ethylamine dihydrobromide (BD 1047), N-[2-(3,4-dichlorophenyl) ethyl]-4-methylpiperazine dihydrochloride (BD 1063), and N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE-100)] had no effect on cocaine self-administration across the range of doses that decreased rates of food-maintained responding. Rimcazole analogs differed from selective σR antagonists in their dual affinities for σRs and the dopamine transporter (DAT) assessed with radioligand binding. Selective DAT inhibitors and σR antagonists were studied alone and in combination on cocaine self-administration to determine whether actions at both σRs and the DAT were sufficient to reproduce the effects of rimcazole analogs. Typical DAT inhibitors [2β-carbomethoxy-3β-(4-fluorophenyl)tropane (WIN 35,428), methylphenidate, and nomifensine] dose-dependently shifted the cocaine dose-effect curve leftward. Combinations of DAT inhibitor and σR antagonist doses that were behaviorally inactive alone decreased cocaine self-administration without effects on food-maintained responding. In addition, whereas the DAT inhibitors were self-administered at rates similar to those of cocaine, neither rimcazole analogs nor typical σR antagonists (NE-100 and AC927) maintained responding above control levels across a wide range of doses. These findings suggest that the unique effects of rimcazole analogs are due to dual actions at the DAT and σRs and that a combined target approach may have utility in development of medical treatments for cocaine abuse.

Design, synthesis and pharmacological evaluation of spirocyclic σ(1) receptor ligands with exocyclic amino moiety (increased distance 1)

Various pharmacophore models for potent σ(1) ligands specify a basic amino group flanked by two different hydrophobic regions in defined distances to the basic amine (distance 1 and distance 2, respectively). According to these models distance 1 of the potent spirocyclic σ(1) ligand 1 is too short. In order to find a new class of more potent σ(1) ligands and to verify the distance hypothesis of the pharmacophore models spirocyclic compounds 2 with an exocyclic amino group were designed and synthesized. The secondary amines 8 and 9 with N-benzyl residues are >100-fold less potent than the spirocyclic piperidine 1. However, the tertiary methylamines trans-11 and cis-11 represent potent σ(1) ligands with K(i)-values of 43 and 24 nM, respectively. Whereas one large benzyl moiety is required for high σ(1) receptor binding, a second large N-substituent is not tolerated by the σ(1) receptor protein. As a rule, cis-configured diastereomers with a longer distance 1 (predominantly 7.16-7.23 Å) show higher σ(1) affinities than their trans-configured counterparts (distance 1 is predominantly 5.88-6.26 Å).

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.

Sigma receptor agonists: receptor binding and effects on mesolimbic dopamine neurotransmission assessed by microdialysis

BACKGROUND

Subtypes of sigma (σ) receptors, σ₁ and σ₂, can be pharmacologically distinguished, and each may be involved in substance-abuse disorders. σ-Receptor antagonists block cocaine place conditioning and σ-receptor agonists are self-administered in rats that previously self-administered cocaine. Self-administration of abused drugs has been related to increased dopamine (DA) neurotransmission, however, σ-receptor agonist effects on mesolimbic DA are not fully characterized.

METHODS

Receptor-binding studies assessed affinities of σ-receptor ligands for σ-receptor subtypes and the DA transporter; effects on DA transmission in the rat nucleus accumbens shell were assessed using in vivo microdialysis.

RESULTS

Cocaine (.1-1.0 mg/kg intravenous [IV]), the nonselective σ(½)-receptor agonist DTG (1.0-5.6 mg/kg IV), and the selective σ₁-receptor agonist PRE-084 (.32-10 mg/kg IV) dose-dependently increased DA to ∼275%, ∼150%, and ∼160% maxima, respectively. DTG-induced stimulation of DA was antagonized by the nonselective σ(½)-receptor antagonist BD 1008 (10 mg/kg intraperitoneal [IP]) and the preferential σ₂-receptor antagonist SN 79 (1-3 mg/kg IP), but not by the preferential σ₁-receptor antagonist, BD 1063 (10-30 mg/kg IP). Neither PRE-084 nor cocaine was antagonized by BD 1063 or BD 1008.

CONCLUSIONS

σ-Receptor agonists stimulated DA in a brain area critical for reinforcing effects of cocaine. DTG effects on DA appear to be mediated by σ₂-receptors rather than σ₁-receptors. However, DA stimulation by cocaine or PRE-084 does not likely involve σ-receptors. The relatively low potency on DA transmission of the selective σ₁-receptor agonist, PRE-084, and its previously reported potent reinforcing effects, suggest a dopamine-independent reinforcing pathway that may contribute to substance-abuse disorders.

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.

Pharmacological and metabolic characterisation of the potent σ1 receptor ligand 1′-benzyl-3-methoxy-3H-spiro[[2]benzofuran-1,4′-piperidine]

Objectives

The pharmacology and metabolism of the potent σ1 receptor ligand 1′-benzyl-3-methoxy-3H-spiro[[2]benzofuran-1,4′-piperidine] were evaluated.

Methods

The compound was tested against a wide range of receptors, ion channels and neurotransmitter transporters in radioligand binding assays. Analgesic activity was evaluated using the capsaicin pain model. Metabolism by rat and human liver microsomes was investigated, and the metabolites were identified by a variety of analytical techniques.

Key findings

1′-Benzyl-3-methoxy-3H-spiro[[2]benzofuran-1,4′-piperidine] (compound 1) is a potent σ1 receptor ligand (Ki 1.14 nM) with extraordinarily high σ1/σ2 selectivity (&gt;1100). It was selective for the σ1 receptor over more than 60 other receptors, ion channels and neurotransmitter transporters, and did not interact with the human ether-a-go-go-related gene (hERG) cardiac potassium channel. Compound 1 displayed analgesic activity against neuropathic pain in the capsaicin pain model (53% analgesia at 16 mg/kg), indicating that it is a σ1 receptor antagonist. It was rapidly metabolised by rat liver microsomes. Seven metabolites were unequivocally identified; an N-debenzylated metabolite and a hydroxylated metabolite were the major products. Pooled human liver microsomes formed the same metabolites. Studies with seven recombinant cytochrome P450 isoenzymes revealed that CYP3A4 produced all the metabolites identified. The isoenzyme CYP2D6 was inhibited by 1 (IC50 88 nM) but did not produce any metabolites.

Conclusions

1′-Benzyl-3-methoxy-3H-spiro[[2]benzofuran-1,4′-piperidine] is a potent and selective σ1 receptor antagonist, which is rapidly metabolised. Metabolically more stable σ1 ligands could be achieved by stabilising the N-benzyl substructure.

The C-terminal amidated analogue of the substance P (SP) fragment SP(1-7) attenuates the expression of naloxone-precipitated withdrawal in morphine dependent rats

We previously demonstrated that intracerebroventricular (i.c.v.) administration of the substance P (SP) aminoterminal fragment SP(1-7) attenuates the expression of morphine withdrawal in the male rat. In this study we have used a synthetic analogue of this peptide, i.e. the SP(1-7) amide showing higher binding potency than the native heptapeptide, in a similar experimental set-up. Thus, Wistar male rats were made tolerant to morphine by daily injections of the opiate during 8 days. Following peptide administration (i.c.v.) and a subsequent naloxone challenge a variety of physical syndromes of withdrawal were recorded. We observed that the SP(1-7) amide potently and dose-dependently reduced several signs of reaction to morphine withdrawal. Interestingly, the effect of the peptide amide was significantly attenuated by the addition of the sigma agonist (+)-SKF-10047. We conclude that the SP(1-7) amide mimics the effect of the native SP fragment and that the mechanisms for its action involve a sigma receptor site.

Juxtaposition of the steroid binding domain-like I and II regions constitutes a ligand binding site in the sigma-1 receptor

sigma-1 receptors represent unique binding sites that are capable of interacting with a wide range of compounds to mediate different cellular events. The composition of the ligand binding site of this receptor is unclear, since no NMR or crystal structures are available. Recent studies in our laboratory using radiolabeled photoreactive ligands suggested that the steroid binding domain-like I (SBDLI) (amino acids 91-109) and the steroid binding domain-like II (SBDLII) (amino acids 176-194) regions are involved in forming the ligand binding site(s) ( Chen, Y., Hajipour, A. R., Sievert, M. K., Arbabian, M., and Ruoho, A. E. (2007) Biochemistry 46, 3532-3542 ; Pal, A., Hajipour, A. R., Fontanilla, D., Ramachandran, S., Chu, U. B., Mavlyutov, T., and Ruoho, A. E. (2007) Mol. Pharmacol. 72, 921-933 ). In this report, we have further addressed this issue by utilizing our previously developed sulfhydryl-reactive, cleavable, radioiodinated photocross-linking reagent: methanesulfonothioic acid, S-((4-(4-amino-3-[125I]iodobenzoyl) phenyl)methyl) ester (Guo, L. W., Hajipour, A. R., Gavala, M. L., Arbabian, M., Martemyanov, K. A., Arshavsky, V. Y., and Ruoho, A. E. (2005) Bioconjugate Chem. 16, 685-693). This photoprobe was shown to derivatize the single cysteine residues as mixed disulfides at position 94 in the SBDLI region of the wild type guinea pig sigma-1 receptor (Cys94) and at position 190 in the SBDLII region of a mutant guinea pig sigma-1 receptor (C94A,V190C), both in a sigma-ligand (haloperidol or (+)-pentazocine)-sensitive manner. Significantly, photocross-linking followed by Endo Lys-C cleavage under reducing conditions and intramolecular radiolabel transfer from the SBDLI to the SBDLII region in the wild type receptor and, conversely, from the SBDLII to the SBDLI region in the mutant receptor were observed. These data support a model in which the SBDLI and SBDLII regions are juxtaposed to form, at least in part, a ligand binding site of the sigma-1 receptor.

Nitrile analogs of meperidine as high affinity and selective sigma-1 receptor ligands

A series of N-substituted-4-cyano-4-phenylpiperidine analogs were synthesized and evaluated for binding affinity at opioid receptors and showed no affinity. The series similarity to previously reported sigma ligands prompted analysis at sigma receptors to determine the SAR for affinity at sigma receptors. Within the N-substituent series the saturated analogs showed increased affinity at both sigma receptors. Optimal chain length in the N-arylalkyl series for sigma(1) and sigma(2) receptors proved to be N-propylphenyl; extension to a four carbon chain dramatically decreased affinity at both receptors. Substituents in the 4-position affect only sigma(1) affinity; no change in affinity at sigma(2) was shown. The N-isobutyl, N-phenylpropyl, and N-benzyl analogs are worth pursuing due to their good affinity and selectivity at the sigma(1) receptor, whereas the N-benzyl analog exhibits the greatest selectivity for sigma(1).

An unambiguous assay for the cloned human sigma1 receptor reveals high affinity interactions with dopamine D4 receptor selective compounds and a distinct structure-affinity relationship for butyrophenones

The ability of the sigma(1) receptor to interact with a huge range of drug structural classes coupled with its wide distribution in the body has contributed to it being implicated as a possible therapeutic target for a broad array of disorders ranging from substance abuse to depression to Alzheimer's disease. Surprisingly, the reported affinity values for some sigma(1) receptor ligands vary more than 50-fold. The potential of the sigma(1) receptor as a pharmacotherapeutic target prompted us to develop an unambiguous assay system for measuring the affinity of ligands to the cloned human sigma(1) receptor. In the course of characterizing this system and determining the true affinity values for almost three dozen compounds, it was discovered that some dopamine D(4) receptor selective compounds bind sigma(1) receptors with high affinity. A systematic analysis of haloperidol-like compounds revealed a clear structure-affinity relationship amongst clinically relevant butyrophenones. The antidepressant fluvoxamine, the drug of abuse methamphetamine, and the neurosteroid progesterone were amongst the many ligands whose interactions with the sigma(1) receptor were confirmed with our screening assay.

(+)-Morphine and (-)-morphine stereoselectively attenuate the (-)-morphine-produced tail-flick inhibition via the naloxone-sensitive sigma receptor in the ventral periaqueductal gray of the rat

We have previously demonstrated that (+)-morphine and (-)-morphine pretreated spinally for 45 min stereoselectively attenuates the tail-flick inhibition produced by (-)-morphine given spinally in the mouse. The present study is then undertaken to determine if the same phenomenon observed in the mouse spinal cord can also take place in the ventral periaqueductal gray of the rat. Pretreatment with (+)-morphine for 45 min at 0.3 to 3.3 fmol dose-dependently attenuated the tail-flick inhibition produced by (-)-morphine (9 nmol) given into the ventral periaqueductal gray. Likewise, pretreatment with (-)-morphine for 45 min at a higher dose (3-900 pmol), which given alone did not affect the baseline tail-flick latency, also dose-dependently attenuated the tail-flick inhibition produced by (-)-morphine. Thus, (+)-morphine is approximately 270,000-fold more potent than (-)-morphine in attenuating the (-)-morphine-produced tail-flick inhibition. The attenuation of the (-)-morphine-produced tail-flick inhibition induced by (+)-morphine or (-)-morphine was dose-dependently reversed by (+)-naloxone (27.5 to 110 pmol) pretreatment for 50 min given into the ventral periaqueductal gray. Pretreatment with the sigma receptor antagonist BD1047 (N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine dihydrobromide) (11-45 nmol) for 45 min given into the ventral periaqueductal gray also reversed dose-dependently the attenuation of the (-)-morphine-produced tail-flick inhibition induced by (+)-morphine or (-)-morphine, indicating that the effects are mediated by the activation of the sigma receptors. Since (+)-morphine, (-)-morphine and (+)-naloxone do not have any affinity for the naloxone-inaccessible sigma receptors, we therefore propose that (+)-morphine and (-)-morphine attenuate the (-)-morphine-produced tail-flick inhibition via the activation of the naloxone-sensitive sigma receptor originally proposed by Tsao and Su [Tsao, L.T., Su, T.P., 1997. Naloxone-sensitive, haloperidol-sensitive, [(3)H](+)-SKF-1047-binding protein partially purified from rat liver and rat brain membranes: an opioid/sigma receptor. Synapse 25, 117-124].