Regulation of [3H] dopamine release from mesolimbic and mesocortical areas of guinea pig brain by sigma receptors

Exploring the Association between Schizophrenia and Cardiovascular Diseases: Insights into the Role of Sigma 1 Receptor

Contemporary society is characterized by rapid changes. Various epidemiological, political and economic crises represent a burden to mental health of nowadays population, which may at least partially explain the increasing incidence of mental disorders, including schizophrenia. Schizophrenia is associated with premature mortality by at least 13-15 years. The leading cause of premature mortality in schizophrenia patients is high incidence of cardiovascular diseases. The specific-cause mortality risk for cardiovascular diseases in schizophrenia patients is more than twice higher as compared to the general population. Several factors are discussed as the factor of cardiovascular diseases development. Intensive efforts to identify possible link between schizophrenia and cardiovascular diseases are made. It seems that sigma 1 receptor may represent such link. By modulation of the activity of several neurotransmitter systems, including dopamine, glutamate, and GABA, sigma 1 receptor might play a role in pathophysiology of schizophrenia. Moreover, significant roles of sigma 1 receptor in cardiovascular system have been repeatedly reported. The detailed role of sigma 1 receptor in both schizophrenia and cardiovascular disorders development however remains unclear. The article presents an overview of current knowledge about the association between schizophrenia and cardiovascular diseases and proposes possible explanations with special emphasis on the role of the sigma 1 receptor.

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.

S2RSLDB: a comprehensive manually curated, internet-accessible database of the sigma-2 receptor selective ligands

Background

Sigma (σ) receptors are accepted as a particular receptor class consisting of two subtypes: sigma-1 (σ₁) and sigma-2 (σ₂). The two receptor subtypes have specific drug actions, pharmacological profiles and molecular characteristics. The σ₂ receptor is overexpressed in several tumor cell lines, and its ligands are currently under investigation for their role in tumor diagnosis and treatment. The σ₂ receptor structure has not been disclosed, and researchers rely on σ₂ receptor radioligand binding assay to understand the receptor’s pharmacological behavior and design new lead compounds.

Description

Here we present the sigma-2 Receptor Selective Ligands Database (S2RSLDB) a manually curated database of the σ₂ receptor selective ligands containing more than 650 compounds. The database is built with chemical structure information, radioligand binding affinity data, computed physicochemical properties, and experimental radioligand binding procedures. The S2RSLDB is freely available online without account login and having a powerful search engine the user may build complex queries, sort tabulated results, generate color coded 2D and 3D graphs and download the data for additional screening.

Conclusion

The collection here reported is extremely useful for the development of new ligands endowed of σ₂ receptor affinity, selectivity, and appropriate physicochemical properties. The database will be updated yearly and in the near future, an online submission form will be available to help with keeping the database widely spread in the research community and continually updated. The database is available at http://www.researchdsf.unict.it/S2RSLDB.

Graphical abstract

Ethanol-related behaviors in mice lacking the sigma-1 receptor

RATIONALE

The Sigma-1 receptor (Sig-1R) is a chaperone protein that has been implicated in drug abuse and addiction. Multiple studies have characterized the role the Sig-1R plays in psychostimulant addiction; however, fewer studies have specifically investigated its role in alcohol addiction. We have previously shown that antagonism of the Sig-1R reduces excessive drinking and motivation to drink, whereas agonism induces binge-like drinking in rodents.

OBJECTIVES

The objectives of these studies were to investigate the impact of Sig-1R gene deletion in C57Bl/6J mice on ethanol drinking and other ethanol-related behaviors.

METHODS

We used an extensive panel of behavioral tests to examine ethanol actions in male, adult mice lacking Oprs1, the gene encoding the Sig-1R. To compare ethanol drinking behavior, Sig-1 knockout (KO) and wild type (WT) mice were subject to a two-bottle choice, continuous access paradigm with different concentrations of ethanol (3-20% v/v) vs. water. Consumption of sweet and bitter solutions was also assessed in Sig-1R KO and WT mice. Finally, motor stimulant sensitivity, taste aversion and ataxic effects of ethanol were assessed.

RESULTS

Sig-1R KO mice displayed higher ethanol intake compared to WT mice; the two genotypes did not differ in their sweet or bitter taste perception. Sig-1R KO mice showed lower sensitivity to ethanol stimulant effects, but greater sensitivity to its taste aversive effects. Ethanol-induced sedation was instead unaltered in the mutants.

CONCLUSIONS

Our results prove that the deletion of the Sig-1R increases ethanol consumption, likely by decreasing its rewarding effects, and therefore indicating that the Sig-1R is involved in modulation of the reinforcing effects of alcohol.

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.

Sigma-1 receptors and animal studies centered on pain and analgesia

INTRODUCTION

Neuropathic pain is difficult to relieve with standard analgesics and tends to be resistant to opioid therapy. Sigma-1 receptors activated during neuropathic injury may sustain pain. Neuropathic injury activates sigma-1 receptors, which results in activation of various kinases, modulates the activity of multiple ion channels, ligand activated ion channels and voltage-gated ion channels; alters monoamine neurotransmission and dampens opioid receptors G-protein activation. Activation of sigma-1 receptors tonically inhibits opioid receptor G-protein activation and thus dampens analgesic responses. Therefore, sigma-1 receptor antagonists are potential analgesics for neuropathic and adjuvants to opioid therapy.

AREAS COVERED

This article reviews the importance of sigma-1 receptors as pain generators in multiple animal models in order to illustrate both the importance of these unique receptors in pathologic pain and the potential benefits to sigma-1 receptor antagonists as analgesics.

EXPERT OPINION

Sigma-1 receptor antagonists have a great potential as analgesics for acute neuropathic injury (herpes zoster, acute postoperative pain and chemotherapy induced neuropathy) and may, as an additional benefit, prevent the development of chronic neuropathic pain. Antagonists are potentially effective as adjuvants to opioid therapy when used early to prevent analgesic tolerance. Drug development is complicated by the complexity of sigma-1 receptor pharmacodynamics and its multiple targets, the lack of a specific sigma-1 receptor antagonist, and potential side effects due to on-target toxicities (cognitive impairment, depression).

Sigma-1 receptor mediates acquisition of alcohol drinking and seeking behavior in alcohol-preferring rats

Sigma-1 receptor (Sig-1R) has been proposed as a novel therapeutic target for drug and alcohol addiction. We have shown previously that Sig-1R agonists facilitate the reinforcing effects of ethanol and induce binge-like drinking, while Sig-1R antagonists on the other hand block excessive drinking in genetic and environmental models of alcoholism, without affecting intake in outbred non-dependent rats. Even though significant progress has been made in understanding the function of Sig-1R in alcohol reinforcement, its role in the early and late stage of alcohol addiction remains unclear. Administration of the selective Sig-1R antagonist BD-1063 dramatically reduced the acquisition of alcohol drinking behavior as well as the preference for alcohol in genetically selected TSRI Sardinian alcohol preferring (Scr:sP) rats; the treatment had instead no effect on total fluid intake, food intake or body weight gain, proving selectivity of action. Furthermore, BD-1063 dose-dependently decreased alcohol-seeking behavior in rats trained under a second-order schedule of reinforcement, in which responding is maintained by contingent presentation of a conditioned reinforcer. Finally, an innate elevation in Sig-1R protein levels was found in the nucleus accumbens of alcohol-preferring Scr:sP rats, compared to outbred Wistar rats, alteration which was normalized by chronic, voluntary alcohol drinking. Taken together these findings demonstrate that Sig-1R blockade reduces the propensity to both acquire alcohol drinking and to seek alcohol, and point to the nucleus accumbens as a potential key region for the effects observed. Our data suggest that Sig-1R antagonists may have therapeutic potential in multiple stages of alcohol addiction.

Recent Advances in the Development of Sigma-1 Receptor Ligands

The existence of two distinct sigma (σ) receptor subtypes was established in the early 1990s. Sigma-1 and sigma-2 receptors (S1Rs and S2Rs, respectively) were shown to possess distinct molecular size, anatomical distribution, and ligand discrimination. S2R is overexpressed in numerous human cancers, and has therapeutic potential for the imaging and treatment of certain tumours. In contrast, S1R is more broadly involved in a wide variety of central nervous system (CNS) diseases including motor disorders, memory deficits, depression, schizophrenia, anxiety, pain, drug addiction, and many more. Since the human S1R was cloned in 1996, numerous high affinity ligands with excellent selectivity for S1R have been developed. This review focuses on recent developments in the generation of structurally diverse S1R-selective ligands and novel therapeutic candidates targeting S1Rs.

Dextromethorphan/quinidine for pseudobulbar affect associated with subarachnoid hemorrhage

ABSTRACT: 

Dextromethorphan/quinidine, sold under the brand name Nuedexta® (Avanir Pharmaceutics, CA, USA), is currently the only US FDA-approved medication for the treatment of pseudobulbar affect (PBA) in the USA. However, the clinical trials for approval of this medication only involved patients diagnosed with amyotrophic lateral sclerosis or multiple sclerosis. This case report describes a 43-year-old woman who developed PBA following an acute subarachnoid hemorrhage with a basilar artery aneurysm. After failing to respond to valproate or haloperidol, she was treated with dextromethorphan/quinidine and showed a reduction in her PBA symptoms. The patient's Center for Neurologic Study-Lability Scale score improved in 1 week and the improvement was maintained at 3 weeks.

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

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

Activation of σ-receptors induces binge-like drinking in Sardinian alcohol-preferring rats

Sigma (σ) receptors have been implicated in the behavioral and motivational effects of alcohol and psychostimulants. Sigma receptor antagonists reduce the reinforcing effects of alcohol and excessive alcohol intake in both genetic (alcohol-preferring rats) and environmental (chronic alcohol-induced) models of alcoholism. The present study tested the hypothesis that pharmacological activation of σ-receptors facilitates ethanol reinforcement and induces excessive, binge-like ethanol intake. The effects of repeated subcutaneous treatment with the selective σ-receptor agonist 1,3-di-(2-tolyl)guanidine (DTG; 15 mg/kg, twice a day for 7 days) on operant ethanol (10%) self-administration were studied in Sardinian alcohol-preferring (sP) rats. To confirm that the effect of DTG was mediated by σ-receptors, the effects of pretreatment with the selective σ-receptor antagonist BD-1063 (7 mg/kg, subcutaneously) were determined. To assess the specificity of action, the effects of DTG on the self-administration of equally reinforcing solutions of saccharin or sucrose were also determined. Finally, gene expression of opioid receptors in brain areas implicated in ethanol reinforcement was analyzed in ethanol-naive sP rats treated acutely or repeatedly with DTG, because of the well-established role of the opioid system in alcohol reinforcement and addiction. Repeatedly administered DTG progressively and dramatically increased ethanol self-administration in sP rats and increased blood alcohol levels, which reached mean values close to 100 mg% in 1 h drinking sessions. Repeated DTG treatment also increased the rats' motivation to work for alcohol under a progressive-ratio schedule of reinforcement. BD-1063 prevented the effects of DTG, confirming that σ-receptors mediate the effects of DTG. Repeated DTG treatment also increased the self-administration of the non-drug reinforcers saccharin and sucrose. Naive sP rats repeatedly treated with DTG showed increased mRNA expression of μ- and δ-opioid receptors in the ventral tegmental area. These results suggest a key facilitatory role for σ-receptors in the reinforcing effects of alcohol and identify a potential mechanism that contributes to binge-like and excessive drinking.

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.

Association analysis of SIGMAR1 with major depressive disorder and SSRI response

BACKGROUND

Several investigations have suggested the possible involvement of sigma 1 non-opioid intracellular receptor 1 (sigma 1 receptor) in the pathophysiology of major depressive disorder (MDD). Sigma 1 receptors are also one of the major pharmacological therapeutic targets of selective serotonin reuptake inhibitors (SSRIs). To evaluate the association of sigma 1 receptor gene (SIGMAR1) and MDD and SSRIs therapeutic response in MDD, we conducted a case-control study of Japanese samples (466 MDD patients, 516 controls and 208 MDD patients treated by fluvoxamine or sertraline).

METHOD

We defined a clinical response as a decrease of more than 50% in baseline the Structured Interview Guide for Hamilton Rating Scale for Depression (SIGH-D) within 8 weeks, and clinical remission as an SIGH-D score of less than 7 at 8 weeks. Therefore, we selected rs1800866 in SIGMAR1 for the following association analysis.

RESULTS

In the logistic regression analysis, we detected an association of the phenotypes (MDD or controls) with rs1800866 genotype. However, we did not detect an association between rs1800866 and SSRI therapeutic response in Japanese MDD. In addition, remission with SSRI was not associated with rs1800866. Also, we did not detect a novel polymorphism in SIGMAR1 when we performed a mutation search using MDD treated by SSRIs samples.

CONCLUSION

Our results suggest that rs1800866 in SIGMAR1 may play a role in the pathophysiology of MDD in the Japanese population. Also, SIGMAR1 does not play a role in the therapeutic response to SSRI in Japanese MDD patients. However, because our sample was small, a replication study using another population and larger sample will be required for conclusive results.

Dopamine-1 receptor agonist, but not cocaine, modulates sigma(1) gene expression in SVG cells

It has been hypothesized that sigma(1) receptors (sigma(1)Rs) are involved in the effects of cocaine abuse. Many in vitro and in vivo studies have already indicated an influence of sigma(1)R ligands on dopaminergic transmission; however, the direct effect on the brain is poorly understood. Herein we describe the effects of cocaine and the selective dopamine-1 receptor (D(1)R) agonist, (+)-SKF38393, on gene expression of the sigma(1)R in a human fetal astrocyte cell line (SVG cells). This study provides the first evidence for the expression of sigma(1)RmRNAin these cells. Our results show that treatment of SVG cells with various cocaine concentrations for several time durations showed no significant alterations in sigma(1)R gene expression, as detected by real-time quantitative RT-PCR, whereas treating cells for 24 h with (+)-SKF38393 caused a significant down-regulation in sigma(1) transcripts. This (+)-SKF38393-induced effect was blocked by the D(1)R selective antagonist (+)-SCH23390. These results suggest that the effect of cocaine on sigma(1) gene expression in the brain might be indirect and mediated through D(1)R.

Functional polymorphisms in the sigma1 receptor gene associated with alcoholism

BACKGROUND

Sigma1 receptors are involved in the pathogenesis of drug abuse. Two polymorphisms (GC-241-240TT and Gln2Pro) in the sigma1 receptor gene (SIGMAR1) have been identified. To investigate the role of SIGMAR1 in conveying susceptibility to alcoholism, we performed a functional analysis of polymorphisms in the SIGMAR1 and a case-control study.

METHODS

We initially screened for polymorphisms in the 5'-upstream region. The effects of the polymorphisms on transcriptional activity were determined using a gene reporter assay. The distribution of SIGMAR1 polymorphisms was analyzed in 307 alcoholic and 302 control subjects.

RESULTS

A novel T-485A polymorphism was identified. The transcriptional activity of the A-485 allele and the TT-241-240 allele was significantly reduced compared with that of the T-485 allele and the GC-241-240 allele. The frequencies of the A-485 allele (chi2=5.575, df=1, p=.0205) and the TT-241-240/Pro2 haplotype (chi2=21.464, df=1, p<.0001) were significantly higher in control subjects compared with alcoholic subjects. The T-485A and the GC-241-240TT may be functional polymorphisms, and the A-485 allele and TT-241-240/Pro2 haplotype are possible protective factors for the development of alcoholism.

Sigma1 receptor agonist-mediated regulation of N-methyl-D-aspartate-stimulated [3H]dopamine release is dependent upon protein kinase C

We have previously shown that sigma1 receptor agonists inhibit N-methyl-D-aspartate (NMDA)-stimulated [3H]dopamine from slices of rat striatum in a concentration-related manner and that the inhibition is reversed by sigma1 receptor-selective and nonsubtype-selective sigma receptor antagonists. Based on previous evidence from our laboratory as well as other laboratories, we hypothesized that sigma1 receptors might use a protein kinase C (PKC) signaling pathway to modulate stimulated dopamine release. We tested several inhibitors of PKC isozymes, as well as a phospholipase C inhibitor for their effects on sigma1 receptor agonist-mediated regulation of [3H]dopamine release. Although none of the inhibitors tested affected the ability of NMDA to stimulate [3H]dopamine release, they all abolished regulation by the sigma1 receptor agonist (+)-pentazocine in a concentration-related manner. We also found that prior exposure to 1 microM phorbol 2-myristate 13-acetate for 30 min abolished regulation by (+)-pentazocine. We concluded that an intact PKC system was required for sigma1 agonist-mediated regulation of NMDA-stimulated [3H]dopamine release from rat striatal slices. Based on the pharmacological profile of the PKC inhibitors tested, as well as reports in the literature on PKC

Decrease of adenylyl cyclase activity and expression by a sigma1 receptor ligand and putative atypical antipsychotic

We examined whether changes in the adenylyl cyclase system could be induced by the administration of the sigma1 receptor ligand and putative atypical antipsychotic 4-[4-fluorophenyl]-1,2,3,6-tetrahydro-1-[4-[1,-2,4-triazol-1-il]butyl]pyridine citrate) (E-5842). Repeated (21 days) but not acute (2 h) treatment with E-5842 induced a significant decrease in adenylyl cyclase type I immunoreactivity and adenylyl cyclase activity in rat frontal cortex membranes, with less or no effect in other brain regions such as the hippocampus or the striatum. Changes in immunoreactivity were not observed in other adenylyl cyclases (type V/VI). The reported changes, observed only after a chronic treatment, could be related to the mechanism of action of sigma receptor ligands in general or to that of E-5842 in particular and should be taken into account, given the long duration of treatment in psychiatric patients.

Conformationally restricted analogs of BD1008 and an antisense oligodeoxynucleotide targeting sigma1 receptors produce anti-cocaine effects in mice

Cocaine's ability to interact with sigma receptors suggests that these proteins mediate some of its behavioral effects. Therefore, three novel sigma receptor ligands with antagonist activity were evaluated in Swiss Webster mice: BD1018 (3S-1-[2-(3,4-dichlorophenyl)ethyl]-1,4-diazabicyclo[4.3.0]nonane), BD1063 (1-[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine), and LR132 (1R,2S-(+)-cis-N-[2-(3,4-dichlorophenyl)ethyl]-2-(1-pyrrolidinyl)cyclohexylamine). Competition binding assays demonstrated that all three compounds have high affinities for sigma1 receptors. The three compounds vary in their affinities for sigma2 receptors and exhibit negligible affinities for dopamine, opioid, GABA(A) and NMDA receptors. In behavioral studies, pre-treatment of mice with BD1018, BD1063, or LR132 significantly attenuated cocaine-induced convulsions and lethality. Moreover, post-treatment with LR132 prevented cocaine-induced lethality in a significant proportion of animals. In contrast to the protection provided by the putative antagonists, the well-characterized sigma receptor agonist di-o-tolylguanidine (DTG) and the novel sigma receptor agonist BD1031 (3R-1-[2-(3,4-dichlorophenyl)ethyl]-1,4-diazabicyclo[4.3.0]nonane) each worsened the behavioral toxicity of cocaine. At doses where alone, they produced no significant effects on locomotion, BD1018, BD1063 and LR132 significantly attenuated the locomotor stimulatory effects of cocaine. To further validate the hypothesis that the anti-cocaine effects of the novel ligands involved antagonism of sigma receptors, an antisense oligodeoxynucleotide against sigma1 receptors was also shown to significantly attenuate the convulsive and locomotor stimulatory effects of cocaine. Together, the data suggests that functional antagonism of sigma receptors is capable of attenuating a number of cocaine-induced behaviors.

Neurochemical and electrophysiological evidence that 5-HT4 receptors exert a state-dependent facilitatory control in vivo on nigrostriatal, but not mesoaccumbal, dopaminergic function

In this study we investigated, using in vivo microdialysis and single unit recordings, the role of serotonin4 (5-HT4) receptors in the control of nigrostriatal and mesoaccumbal dopaminergic (DA) pathway activity. In freely moving rats, the 5-HT4 antagonist GR 125487 (1 mg/kg, i.p.), without effect on its own, significantly reduced the enhancement of striatal DA outflow induced by 0.01 (-35%) and 0.1 (-66%), but not 1 mg/kg, s.c. haloperidol (HAL). Intrastriatal infusion of GR 125487 (1 microM) had no influence on basal DA outflow, but attenuated (-49%) the effect of 0.01 mg/kg HAL. Systemic administration of GR 125487 modified neither basal nor 0.01 mg/kg HAL-stimulated accumbal DA outflow. In halothane-anaesthetized rats, 1 or 10 mg/kg GR 125487, without effect by itself, failed to modify the changes in accumbal and striatal DA outflow elicited by electrical stimulation (300 microA, 1 ms, 20 Hz, 15 min) of the dorsal raphe nucleus. Finally, GR 125487 (444 microg/kg, i.v.), whilst not affecting basal firing of DA neurons within either the substantia nigra or the ventral tegmental area, reduced HAL-stimulated (1--300 microg/kg, i.v.) impulse flow of nigrostriatal DA neurons only. These results indicate that 5-HT4 receptors exert a facilitatory control on both striatal DA release and nigral DA neuron impulse flow only when nigrostriatal DA transmission is under activated conditions. Furthermore, they indicate that the striatum constitutes a major site for the expression of the control exerted by 5-HT4 receptors on DA release. In contrast, 5-HT4 receptors have no influence on mesoaccumbal DA activity in either basal or activated conditions.

Changes in phosphoinositide signalling activity and levels of the alpha subunit of G(q/11) protein in rat brain induced by E-5842, a sigma(1) receptor ligand and potential atypical antipsychotic

Changes in the phosphoinositide (PPI) signal transduction system induced by E-5842, a new sigma(1) (sigma(1)) receptor ligand and potential atypical antipsychotic, were studied in the rat frontal cortex, hippocampus and striatum. Acute treatment with E-5842 increased phospholipase C (PLC) activity in the striatum and the hippocampus. Chronic treatment with E-5842 induced an increase in the activity of PLC in the frontal cortex and the striatum. Similar up-regulation of the activity of the enzyme was also observed in rat frontal cortex membranes in presence of GTPgammaS. After chronic treatment with E-5842, it was also observed a significant increase of the immunoreactivity levels of G(q/11)alpha in the frontal cortex. Our results suggest that part of the antipsychotic effects of E-5842 could be related to the regulation of the PPI signal transduction pathway, especially after a prolonged treatment.

Phencyclidine and dizocilpine modulate dopamine release from rat nucleus accumbens via sigma receptors

Phencyclidine (PCP) binds to many sites in brain, including PCP receptors located within the N-methyl-D-aspartate (NMDA) receptor-operated cation channel and sigma (sigma) receptors. In this study, we compare mechanisms by which PCP, dizocilpine (MK-801), the prototypical sigma receptor agonist (+)-pentazocine, and the proposed endogenous sigma receptor ligand neuropeptide Y regulate potassium (K(+))-stimulated [3H]dopamine release from slices of rat nucleus accumbens. (+)-Pentazocine inhibits K(+)-stimulated [3H]dopamine release, and neuropeptide Y enhances it. Both effects are blocked by sigma(1) and neuropeptide Y receptor antagonists, suggesting possible inverse agonism at a subpopulation of sigma/neuropeptide Y receptors. In contrast, PCP and MK-801 both enhance K(+)-stimulated [3H]dopamine release via sigma(1) and sigma(2) receptor subtypes, as demonstrated by antagonist sensitivity. Regulation of release by both (+)-pentazocine and neuropeptide Y persists in the presence of tetrodotoxin suggests that the sigma/neuropeptide Y receptors mediating the modulation are located presynaptically on dopaminergic nerve terminals, but tetrodotoxin eliminates regulation by PCP and MK-801, suggesting that receptors mediating their effects are located upstream from dopaminergic nerve terminals.

Neuropeptide Y-mediated enhancement of NMDA-stimulated [3H]dopamine release from rat prefrontal cortex is reversed by sigma1 receptor antagonists

Sigma (sigma) receptors are located in limbic areas, including the prefrontal cortex, where decreased dopamine levels have been linked to negative symptoms. Although the endogenous ligands for sigma receptors are unknown, neuropeptide Y (NPY) has been named as the potential endogenous agonist at these receptors. NPY enhanced NMDA-stimulated [3H]dopamine release in rat prefrontal cortex. This was in contrast to the inhibition produced by the sigma agonists (+)pentazocine and BD737. However, four sigma antagonists, including one which is sigma1 selective, that reverse (+)pentazocine- or BD737-mediated inhibition all reversed the NPY-mediated enhancement. In addition, PYX-1, a Y receptor antagonist, reversed both the (+)pentazocine- and BD737-mediated inhibition and the NPY-mediated enhancement of release. Peptide YY (PYY), [Leu31,Pro34]NPY and NPY(13-36) did not mimic the effect of NPY. Our findings are consistent with NPY acting as an endogenous ligand for a subtype of sigma receptor with characteristics different from Y1, Y2 and Y3 receptors but sensitive to PYX-1. These findings suggest a role for NPY, via sigma receptors, as a modulator of dopamine levels in the prefrontal cortex.

Modulation of amphetamine-stimulated (transporter mediated) dopamine release in vitro by sigma2 receptor agonists and antagonists

Some sigma receptor ligands have been shown to bind with low affinity to the dopamine transporter and to inhibit [3H]dopamine uptake. It has not previously been shown whether any of these compounds influence release of dopamine via facilitated exchange diffusion. To further examine the nature of the interaction between sigma receptor ligands and the dopamine transporter, the effects of sigma receptor ligands on amphetamine-stimulated [3H]dopamine release were examined in slices prepared from rat caudate putamen. In the absence of exogenous Ca2+, both (+)-pentazocine and (-)-pentazocine potentiated amphetamine-stimulated [3H]dopamine release at concentrations consistent with their affinities for sigma2 receptors. In contrast, BD737 (1S.2R-(-)-cis-N-¿2-(3,4-dichlorophenyl)ethyl¿-N-methyl-2-(1-pyrrolidiny l)cyclohexylamine), a sigma1 receptor agonist, had no effect on amphetamine-stimulated release. Neither isomer of pentazocine alone had any effect on basal [3H]dopamine release under these conditions. Three antagonists at sigma receptors, one of which is non-selective for subtypes, and two of which are sigma2-selective, all blocked the enhancement of stimulated release produced by (+)-pentazocine. Enhancement of stimulated release by (-)-pentazocine was similarly blocked by sigma2 receptor antagonists. Our data support the contention that it is possible to regulate transporter-mediated events with drugs that act at a subpopulation of sigma receptors pharmacologically identified as the sigma2 subtype.

Regulation of [3H]norepinephrine release from guinea pig hippocampus by sigma2 receptors

The binding profile of the sigma2 receptor ligand endo-N-(8-methyl-8-azabicyclo[3.2.1.]oct-3-yl)-2,3-dihydro-(1-methyl)eth yl-2-oxo-1H-benzimidazole-1-carboxamidehydrochloride (BIMU-8) had previously been determined, but its agonist/antagonist status at sigma2 receptors had not been identified. We therefore investigated the effects of BIMU-8 for its ability to regulate the stimulated release of [3H]norepinephrine from slices of guinea pig hippocampus. BIMU-8 alone, at a concentration chosen to occupy 50% of sigma2 receptors, had no significant effect on N-methyl-D-aspartate (NMDA)-stimulated release of [3H]norepinephrine. We have shown previously that the sigma receptor agonist (+)-pentazocine inhibits NMDA-stimulated release in a concentration-dependent manner, producing a biphasic inhibition curve. Similarly, the sigma receptor agonist 1S,2R-(-)-cis-N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl )cyclohexylamine (BD737) produced a broad inhibition curve. The inhibition by low concentrations of (+)-pentazocine or BD737 that selectively activated sigma1 receptors was reversed by the sigma1-selective receptor antagonist (1-(cyclopropylmethyl)-4-2'-oxoethyl)piperidine HBr (DuP 734). In the current study, when the sigma1 component of inhibition by (+)-pentazocine was blocked by DuP 734, the remaining component of inhibition mediated by sigma2 receptors was reversed by BIMU-8. Our results suggest that (1) BIMU-8 is an antagonist at sigma2 receptors and that (2) sigma2 receptors contribute to regulation of norepinephrine release in guinea pig hippocampus.