Increases in rat striatal extracellular dopamine and vacuous chewing produced by two sigma receptor ligands

Long-term Administration of 3-Di-O-Tolylguanidine Modulates Spatial Learning and Memory in Rats and Causes Transition in the Concentration of Neurotransmitters in the Hippocampus, Prefrontal Cortex and Striatum

The sigma-1 and sigma-2 (σ₁ and σ₂) receptors are found in high concentrations in the brain, and their altered expression leads to a variety of neuropsychiatric disorders. 3-di-tolylguanidine (DTG) stimulates the activity of both of these receptors. We assessed the effects of administering DTG to adult male Sprague Dawley rats on learning and memory consolidation processes and on the levels of neurotransmitters in selected brain structures. Spatial learning and memory were evaluated in the water maze test. The DTG was administered orally at daily doses of 3 mg/kg (DTG3), 10 mg/kg (DTG10) or 30 mg/kg (DTG30) for 10 weeks before and during the water-maze test. After completion of the experiment, the concentration of monoamines and their metabolites as well as amino acids in structures involved in cognitive performance - the hippocampus, prefrontal cortex, and striatum - were determined using high performance liquid chromatography (HPLC). The DTG10 group showed an improvement in memory processes related to the "new" platform location, whereas the DTG30 group was worse at finding the "old" platform location. Since the administration of DTG led to differences in dopaminergic transmission, it was assumed to influence memory processes in this way. Changes in histidine, serine, alanine, taurine, and glutamic acid levels in selected structures of the brains of rats with memory impairment were also observed. We conclude that long-term administration of DTG modulates spatial learning and memory in rats and changes the concentrations of neurotransmitters in the hippocampus, prefrontal cortex, and striatum..

Deferred Administration of Afobazole Induces Sigma1R-Dependent Restoration of Striatal Dopamine Content in a Mouse Model of Parkinson's Disease

Previously, we demonstrated that the immediate administration of multitarget anxiolytic afobazole slows down the progression of neuronal damage in a 6-hydroxidodamine (6-OHDA) model of Parkinson's disease due to the activation of chaperone Sigma1R. The aim of the present study is to evaluate the therapeutic potential of deferred afobazole administration in this model. Male ICR mice received a unilateral 6-OHDA lesion of the striatum. Fourteen days after the surgery, mice were treated with afobazole, selective Sigma1R agonist PRE-084, selective Sigma1R antagonist BD-1047, and a combination of BD-1047 with afobazole or PRE-084 for another 14 days. The deferred administration of afobazole restored the intrastriatal dopamine content in the 6-OHDA-lesioned striatum and facilitated motor behavior in rotarod tests. The action of afobazole accorded with the effect of Sigma1R selective agonist PRE-084 and was blocked by Sigma1R selective antagonist BD-1047. The present study illustrates the Sigma1R-dependent effects of afobazole in a 6-OHDA model of Parkinson's disease and reveals the therapeutic potential of Sigma1R agonists in treatment of the condition.

Hazards of Using Masking Protocols When Performing Ligand Binding Assays: Lessons From the Sigma-1 and Sigma-2 Receptors

Sigma-1 and sigma-2 receptors are emerging therapeutic targets. Although the molecular identity of the sigma-2 receptor has recently been determined, receptor quantitation has used, and continues to use, the sigma-1 selective agents (+) pentazocine or dextrallorphan to mask the sigma-1 receptor in radioligand binding assays. Here, we have assessed the suitability of currently established saturation and competition binding isotherm assays that are used to quantify parameters of the sigma-2 receptor. We show that whilst the sigma-1 receptor mask (+) pentazocine has low affinity for the sigma-2 receptor (K_i 406 nM), it can effectively compete at this site with [³H] di-O-tolyl guanidine (DTG) at the concentrations frequently used to mask the sigma-1 receptor (100 nM and 1 µM). This competition influences the apparent affinity of DTG and other ligands tested in this system. A more troublesome issue is that DTG can displace (+) pentazocine from the sigma-1 receptor, rendering it partly unmasked. Indeed, commonly used concentrations of (+) pentazocine, 100 nM and 1 µM, allowed 37 and 11% respectively of sigma-1 receptors to be bound by DTG (300 nM), which could result in an overestimation of sigma-2 receptor numbers in assays where sigma-1 receptors are also present. Similarly, modelled data for 1 µM dextrallorphan show that only 71–86% of sigma-1 receptors would be masked in the presence of 300 nM DTG. Therefore, the use of dextrallorphan as a masking agent would also lead to the overestimation of sigma-2 receptors in systems where sigma-1 receptors are present. These data highlight the dangers of using masking agents in radioligand binding studies and we strongly recommend that currently used masking protocols are not used in the study of sigma-2 receptors. In order to overcome these problems, we recommend the use of a cell line apparently devoid of sigma-1 receptors [e.g., MCF7 (ATCC HTB-22)] in the absence of any masking agent when determining the affinity of agents for the sigma-2 receptor. In addition, assessing the relative levels of sigma-1 and sigma-2 receptors can be achieved using [³H] DTG saturation binding followed by two-site analysis of (+) pentazocine competition binding with [³H] DTG.

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

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

Sigma-1 Agonist Binding in the Aging Rat Brain: a MicroPET Study with [(11)C]SA4503

PURPOSE

Sigma-1 receptor ligands modulate the release of several neurotransmitters and intracellular calcium signaling. We examined the binding of a radiolabeled sigma-1 agonist in the aging rat brain with positron emission tomography (PET).

PROCEDURES

Time-dependent uptake of [(11)C]SA4503 was measured in the brain of young (1.5 to 3 months) and aged (18 to 32 months) Wistar Hannover rats, and tracer-kinetic models were fitted to this data, using metabolite-corrected plasma radioactivity as input function.

RESULTS

In aged animals, the injected probe was less rapidly metabolized and cleared. Logan graphical analysis and a 2-tissue compartment model (2-TCM) fit indicated changes of total distribution volume (V T) and binding potential (BP ND) of the tracer. BP ND was reduced particularly in the (hypo)thalamus, pons, and medulla.

CONCLUSIONS

Some areas showed reductions of ligand binding with aging whereas binding in other areas (cortex) was not significantly affected.

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

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

Sigma 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.

Involvement of dopamine D2 receptor signal transduction in the discriminative stimulus effects of the κ-opioid receptor agonist U-50,488H in rats

We have reported previously that the inhibition of both dopaminergic and psychotomimetic/hallucinogenic components plays a role in the discriminative stimulus effects of U-50,488H. However, the mechanisms that underlie the discriminative stimulus effects of U-50,488H, and especially the component that plays a significant role, have not yet been clarified. The present study was designed to further investigate the mechanism(s) of the discriminative stimulus effects of the κ-opioid receptor agonist U-50,488H in rats that had been trained to discriminate between 3.0 mg/kg U-50,488H and saline. The dopamine D2 receptor antagonist sulpiride, but not the D1 receptor antagonist SCH23390, generalized to the discriminative stimulus effects of U-50,488H. The mood-stabilizing agents lithium chloride and valproic acid, which have attenuating effects on the Akt/GSK3 pathway, also partially generalized to the discriminative stimulus effects of U-50,488H. In contrast, the 5-HT-related compound racemic 3,4-methylenedioxymethamphetamine, the cannabinoid receptor agonist WIN55,212-2, and the μ-opioid receptor agonist morphine failed to generalize to the discriminative stimulus effects of U-50,488H. These results suggest that the inhibition of the dopaminergic activity mediated by the postsynaptic D2 receptor, followed by suppression of the Akt/GSK3 pathway may be critical for the induction of the discriminative stimulus effects induced by U-50,488H.

Self-administration of cocaine induces dopamine-independent self-administration of sigma agonists

Sigma(1) receptors (σ(1)Rs) are intracellularly mobile chaperone proteins implicated in several disease processes, as well as psychiatric disorders and substance abuse. Here we report that although selective σ(1)R agonists (PRE-084, (+)-pentazocine) lacked reinforcing effects in drug-naive rats, over the course of 28 experimental sessions, which was more than sufficient for acquisition of cocaine self-administration, responding was not maintained by either σ(1)R agonist. In contrast, after subjects self-administered cocaine σ(1)R agonists were readily self-administered. The induced reinforcing effects were long lasting; a response for which subjects had no history of reinforcement was newly conditioned with both σ(1)R agonists, extinguished when injections were discontinued, and reconditioned when σ(1)R agonists again followed responses. Experience with food reinforcement was ineffective as an inducer of σ(1)R agonist reinforcement. Although a variety of dopamine receptor antagonists blocked cocaine self-administration, consistent with its dopaminergic mechanism, PRE-084 self-administration was entirely insensitive to these drugs. Conversely, the σR antagonist, BD1063, blocked PRE-084 self-administration but was inactive against cocaine. In microdialysis studies i.v. PRE-084 did not significantly stimulate dopamine at doses that were self-administered in rats either with or without a cocaine self-administration experience. The results indicate that cocaine experience induces reinforcing effects of previously inactive σ(1)R agonists, and that the mechanism underlying these reinforcing effects is dopamine independent. It is further suggested that induced σ(1)R mechanisms may have an essential role in treatment-resistant stimulant abuse, suggesting new approaches for the development of effective medications for stimulant abuse.

Evidences for the involvement of sigma receptors in antidepressant like effect of quetiapine in mice

Although quetiapine is routinely used in the treatment of schizophrenia and bipolar disorders, the precise mechanism of its antidepressant activity is poorly understood. Since quetiapine binds with sigma receptor, the possibility exists that antidepressant action of quetiapine may be mediated through interaction with sigma receptors. In the present study, quetiapine [40-80 μg/mouse, intracerebroventricular (i.c.v.) and 40 mg/kg, intraperitoneal (i.p.)], sigma1 receptor agonist, (+)-pentazocine (120 μg/mouse, i.c.v.) and sigma2 receptor agonist, PB-28 [1-Cyclohexyl-4-[3-(1,2,3,4-tetrahydro-5-methoxy-1-naphthalenyl)propyl]piperazine] (20 μg/mouse, i.c.v.) significantly decreased immobility time in forced swim test. In combination studies, the antiimmobility effect of quetiapine (20 μg/mouse, i.c.v.) was significantly potentiated by pretreatment with (+)-pentazocine (30 and 60 μg/mouse, i.c.v.) or PB-28 (5 and 10 μg/mouse, i.c.v.). Conversely, prior administration of sigma1 receptor antagonist, BD-1063 [1-[2-(3,4-Dichlorophenyl)ethyl]-4-methylpiperazine] and sigma2 receptor antagonists, SM-21 [(±)-Tropanyl 2-(4-chlorophenoxy)butanoate] antagonized the antiimmobility effect induced by quetiapine and its synergistic combination with sigma receptor agonists. These results demonstrated the involvement of sigma receptors in the antidepressant like effect of quetiapine and suggest that sigma receptors can be explored as a potential therapeutic target for the treatment of depressive disorders.

The sigma receptor agonist SA4503 both attenuates and enhances the effects of methamphetamine

BACKGROUND

Methamphetamine's behavioral effects have been attributed to its interaction with monoamine transporters; however, methamphetamine also has affinity for sigma receptors.

METHOD

The present study investigated the effect of the sigma receptor agonist SA 4503 and the sigma receptor antagonists BD-1047 and BD-1063 on methamphetamine-evoked [(3)H]dopamine release from preloaded rat striatal slices. The effect of SA 4503 on methamphetamine-induced hyperactivity and on the discriminative stimulus properties of methamphetamine also was determined.

RESULTS

SA 4503 attenuated methamphetamine-evoked [(3)H]dopamine release in a concentration-dependent manner. BD-1047 and BD-1063 did not affect release. SA 4503 dose-dependently potentiated and attenuated methamphetamine-induced hyperactivity. SA 4503 pretreatment augmented the stimulus properties of methamphetamine.

CONCLUSIONS

Our findings indicate that SA 4503 both enhances and inhibits methamphetamine's effects and that sigma receptors are involved in the neurochemical, locomotor stimulatory and discriminative stimulus properties of methamphetamine.

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.

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

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

Synthesis, biological evaluation, and three-dimensional in silico pharmacophore model for sigma(1) receptor ligands based on a series of substituted benzo[d]oxazol-2(3H)-one derivatives

Novel benzo[d]oxazol-2(3H)-one derivatives were designed and synthesized, and their affinities against sigma receptors were evaluated. On the basis of 31 compounds, a three-dimensional pharmacophore model for the sigma(1) receptor binding site was developed using the Catalyst 4.9 software package. The best 3D pharmacophore hypothesis, consisting of one positive ionizable, one hydrogen bond acceptor, two hydrophobic aromatic, and one hydrophobic features provided a 3D-QSAR model with a correlation coefficient of 0.89. The best hypothesis was also validated by three independent methods, i.e., the Fisher randomization test included in the CatScramble functionality of Catalyst, the leave-one-out test, and activity prediction of an additional test set. The achieved results will allow researchers to use this 3D pharmacophore model for the design and synthesis of a second generation of high affinity sigma(1) ligands, as well as to discover other lead compounds for this class of receptors.

The sigma-1 antagonist BMY-14802 inhibits L-DOPA-induced abnormal involuntary movements by a WAY-100635-sensitive mechanism

RATIONALE

Levodopa (L-DOPA), the gold standard treatment for Parkinson's disease (PD), eventually causes L-DOPA-induced dyskinesia (LID) in up to 80% of patients. In the 6-hydroxydopamine (6-OHDA) rat model of PD, L-DOPA induces a similar phenomenon, which has been termed abnormal involuntary movement (AIM). We previously demonstrated that BMY-14802 suppresses AIM expression in this model.

OBJECTIVES

Although BMY-14802 is widely used as a sigma-1 antagonist, it is also an agonist at serotonin (5-HT) 1A and adrenergic alpha-1 receptors. The current study was conducted to determine which of these mechanisms underlies BMY-14802's AIM-suppressing effect. This characterization included testing the 5-HT1A agonist buspirone and multiple sigma agents. When these studies implicated a 5-HT1A mechanism, we subsequently undertook a pharmacological reversal study, evaluating whether the 5-HT1A antagonist WAY-100635 counteracted BMY-14802's AIM-suppressing effects.

RESULTS

Buspirone dose-dependently suppressed AIM, supporting past findings. However, no AIM-suppressing effects were produced by drugs with effects at sigma receptors, including BD-1047, finasteride, SM-21, DTG, trans-dehydroandrosterone (DHEA), carbetapentane, and opipramol. Finally, we show for the first time that the AIM-suppressing effect of BMY-14802 was dose-dependently prevented by WAY-100635 but not by the alpha-1 antagonist prazosin.

CONCLUSIONS

BMY-14802 exerts its AIM-suppressing effects via a 5-HT1A agonist mechanism, similar to buspirone. Other 5-HT1A agonists have failed clinical trials, possibly due to submicromolar affinity at other receptors, including D2, which may exacerbate PD symptoms. BMY-14802 is a promising candidate for clinical trials due to its extremely low affinity for the D2 receptor and lack of extrapyramidal effects during prior clinical trials for schizophrenia.

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.

Dehydroepiandrosterone alleviates copulatory disorder induced by social stress in male rats

INTRODUCTION

Social stress induces sexual dysfunction and reduces serum testosterone (T) level in rats. Stressful events exert an influence on a variety of behaviors and physiology through hormonal changes. The mechanism of stress-induced sexual dysfunction is unknown.

AIM

To investigate the role of dehydroepiandrosterone (DHEA) in copulatory behavior induced by social stress in rats.

METHODS

Stress-induced male rats were subjected to social stress in which the males lived in a wire-mesh siege located in a colony of male and female rats and were exposed daily to a brief defeat by the colony of males for five consecutive days. After the stress period, copulatory behavior and serum concentrations of DHEA and T were measured.

MAIN OUTCOME MEASURES

The effects of DHEA, T, and NE-100, a selective sigma 1 receptor antagonist, on copulatory behavior following social stress were examined.

RESULTS

The males exhibited a marked suppression of copulatory behavior (elongation of intromission and ejaculation latencies). Serum concentrations of DHEA and T were significantly lower than those in nonstressed control males. Another three groups of social stressed males were injected daily with DHEA, T, or DHEA + NE-100 during the stress period. Injections of DHEA attenuated the stress-induced suppression of copulatory behavior, whereas T had no effect. The combined treatment of NE-100 made DHEA ineffective at restoring copulatory behavior.

CONCLUSIONS

These results indicate that DHEA, but not its conversion to T, alleviates the suppressive effect of social stress on copulatory behavior via sigma 1 receptors. We suggest that the decreased endogenous DHEA is involved in copulatory disorders induced by social stress in rats.

Novel analogs of the σ receptor ligand BD1008 attenuate cocaine-induced toxicity in mice

Previous studies have shown that BD1008 (N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine) and related analogs attenuate the toxicity and stimulant effects of cocaine through antagonism of sigma receptors. In the present study, six analogs of BD1008 (UMB 98-103) were synthesized and evaluated in receptor binding and behavioral studies. Competition binding studies confirmed that all six compounds have high affinity for sigma1 receptors, moderate affinity for sigma2 receptors, and low to negligible affinity for monoamine transporters, opioid, N-methyl-D-aspartate, dopamine, and 5-HT receptors. In behavioral pharmacological studies, pretreatment of mice with UMB 100, UMB 101, or UMB 103 significantly attenuated cocaine-induced convulsions and lethality. Together with earlier studies, the data suggest that analogs of BD1008 are promising medication development leads for reducing the toxicity of cocaine.

Involvement of the sigma 1 receptor in the modulation of dopaminergic transmission by amantadine

Pharmacological effects of amantadine on dopaminergic transmission are proposed to result from an uncompetitive antagonism at glutamate N-methyl-D-aspartate (NMDA) receptors. However, our previous studies examining amantadine-mediated dopamine receptor regulation in the rat striatum revealed a discrepancy from a direct interference with glutamate transmission. Preliminary in vitro binding data from the literature suggested the interaction of amantadine with the sigma1 receptor. Therefore, we have now further characterized the pharmacological properties of amantadine and memantine at this receptor and investigated its involvement in the modulation of striatal dopaminergic transmission. Our binding studies using [3H]-(+)SKF-10,047 indicated that amantadine and memantine behave as ligands of the sigma(1) receptor in rat forebrain homogenates (Ki values of 7.44 +/- 0.82 and 2.60 +/- 0.62 microm, respectively). In NG108-15 neuroblastoma cells, both drugs (amantadine (100 microm) and memantine (10 microm)) potentiated the bradykinin-induced mobilization of intracellular Ca2+, mimicking the effect of the sigma1 receptor agonist PRE-084 (1 microm). Finally, we previously showed that in striatal membranes from amantadine-treated rats, the functional coupling of dopamine receptors with G-proteins was enhanced. Similarly, PRE-084 dose-dependently increased the [35S]GTPgammaS binding induced by dopamine (Emax 28 and 26% of basal, 0.3 and 1 mg/kg PRE-084, respectively). By contrast, BD1047, which is without effect on its own, antagonized the effects of amantadine and PRE-084. Together, these data demonstrate that aminoadamantanes behave as sigma1 receptor agonists, and confirm an involvement of this receptor in modulating dopamine receptors exerted by therapeutically relevant concentrations of amantadine.

Sigma receptors: potential medications development target for anti-cocaine agents

The ability of cocaine to interact with sigma receptors suggests a viable target for medications development. Recently, numerous novel compounds and antisense oligodeoxynucleotides targeting sigma receptors have been synthesized and shown to prevent the behavioral toxicity and psychomotor stimulant effects of cocaine in animals. Protective doses of sigma receptor antagonists have also been shown to prevent changes in gene expression that are induced by cocaine. Together, the studies provide insight and promising future directions for the development of potential medications for the treatment of cocaine addiction and overdose.

Age-related changes of the binding of [3h]SA4503 to sigma1 receptors in the rat brain

We have recently developed 1-([3-O-methyl-11C]3,4-dimethoxyphenethyl)-4-(3-phenylpropyl) piperazine ([11C]SA4503) as a selective radioligand for mapping sigma1 receptors in the brain by positron emission tomography (PET). In the present short communication we evaluated the age-related changes of the binding of this ligand to sigma1 receptors in Fisher-344 rats (1.5-, 6-, 12-, and 24-month-old) by the in vitro binding assay. We also measured the binding of [3H](+)-pentazocine to sigma1 receptors and the binding of [3H]1,3-di-O-tolylguanidine to sigma2 receptors, which are current standard methods. The specific binding of the three radioligands increased age-dependently. Both Kd and Bmax values of the 24-month-old rats for each radioligand were significantly higher than those of the young rats (1.5- and 6-month-old). The increased numbers of both sigma1 and sigma2 receptor subtypes in the aged rats compensate for the lowered affinity, and rather enhanced the radioligand-receptor binding. The results contrast strikingly with the age-dependent decrease in the dopaminergic, cholinergic and glutamatergic receptors that are reported to be correlated with the sigma receptors, and indicate that a PET study with [11C]SA4503 to evaluate the aging process in humans would be of great interest.

Sigma sites mediate DTG-evoked hypothermia in rats

1,3,-Di-o-tolylguanidine (DTG), a sigma agonist, produces hypothermia in rats, but the inability of purported sigma antagonists to block the hypothermia suggests that sites other than sigma may mediate the effect. Recently, N-[2-(3,4-dichlorophenyl) ethyl]-N-methyl-2-(dimethylamino) ethylamine (BD 1047) has been identified as a functional sigma antagonist in vivo because of its high selectivity for sigma sites and its ability to block DTG-induced dystonia and cocaine-evoked behaviors. Therefore, the present study investigated the effect of BD 1047 on DTG-evoked hypothermia. DTG (1, 10, 20 and 30 mg/kg sc) induced dose-dependent hypothermia. The onset of DTG-induced hypothermia was rapid, with a reduction in body temperature observed 15 min postinjection. To determine whether sigma sites mediated DTG-induced hypothermia, BD 1047 was injected 30 min prior to DTG. BD 1047 (1, 5, 7.5 and 10 mg/kg sc) attenuated the hypothermia in a dose-dependent fashion, thus revealing a sigma site mechanism. The injection of BD 1047 alone did not alter body temperature, suggesting that endogenous sigma systems do not play a tonic role in thermoregulation. The present experiments demonstrate for the first time that a selective sigma antagonist attenuates sigma agonist-induced hypothermia. Moreover, these data provide further evidence that BD 1047 is an effective antagonist for characterizing sigma-mediated effects in vivo.

Involvement of sigma receptors in the behavioral effects of cocaine: evidence from novel ligands and antisense oligodeoxynucleotides

Pharmacological and molecular biological tools were used to validate the involvement of sigma receptors in the actions of cocaine. Radioligand binding studies demonstrated significant levels of sigma receptors in the brain and heart, where cocaine interacts preferentially with the sigma(1) subtype. In behavioral pharmacological studies using mice, nine novel sigma receptor antagonists significantly attenuated cocaine-induced convulsions, while structural analogs with weak interactions with sigma receptors were ineffective. In contrast to the protection provided by the antagonists, a classical sigma receptor agonist exacerbated the convulsive effects of cocaine. The antagonists also attenuated cocaine-induced lethality, with the best compound protecting against death even when administered as a post-treatment. At doses where the antagonists had no effect on baseline locomotor activity, they significantly attenuated the locomotor stimulatory effects of cocaine, suggesting their ability to block the psychomotor as well as the toxic effects of cocaine. To further validate that the anti-cocaine effects were achieved by interfering with cocaine's access to sigma receptors, antisense oligodeoxynucleotides against sigma(1) receptors were shown to attenuate the convulsive and locomotor stimulatory effects of cocaine. Together, the studies support the involvement of sigma receptors, particularly the sigma(1) subtype, in the behavioral effects of cocaine.

Sigma(1) (sigma(1)) receptor antagonists represent a new strategy against cocaine addiction and toxicity

Cocaine is a highly addictive substance abused worldwide. Its mechanism of action involves initially inhibition of neuronal monoamine transporters in precise brain structures and primarily the dopamine reuptake system located on mesolimbic neurons. Cocaine rapidly increases the dopaminergic neurotransmission and triggers adaptive changes in numerous neuronal circuits underlying reinforcement, reward, sensitization and the high addictive potential of cocaine. Current therapeutic strategies focus on counteracting the cocaine effects directly on the dopamine transporter, through post-synaptic D(1), D(2) or D(3) receptors or through the glutamatergic, serotoninergic, opioid or corticotropin-releasing hormone systems. However, cocaine administration also results in the activation of numerous particular targets. Among them, the sigma(1) (sigma(1)) receptor is involved in several acute or chronic effects of cocaine. The present review will first bring concise overviews of the present strategies followed to alleviate cocaine addiction and animal models developed to analyze the pharmacology of cocaine addiction. Evidence involving activation of the sigma(1) receptor in the different aspects of cocaine abuse, will then be detailed, following acute, repeated, or overdose administration. The therapeutic potentials and neuropharmacological perspectives opened by the use of selective sigma(1) receptor antagonists in cocaine addiction will finally be discussed.

[11C]Raclopride binding was reduced in vivo by sigma(1) receptor ligand SA4503 in the mouse brain, while [11C]SA4503 binding was not by raclopride

[11C]Raclopride is widely used as a representative dopamine D(2)-like receptor ligand in positron emission tomography (PET) studies, and [11C]1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride ([11C]SA4503) is a recently developed selective ligand for mapping sigma(1) receptors in the brain. The striatal uptake of [11C]raclopride in mice was reduced by co-injection of an excess amount of SA4503, in spite of the fact that raclopride had no effect on the brain uptake of [11C]SA4503 as shown in a previous study. The blocking effect of SA4503 on the striatal uptake of [11C]raclopride was dose-dependent, but disappeared by 1 h or 6 h after intraperitoneal injection of SA4503. The brain uptake of [11C]SA4503 was not affected by a dopamine transporter inhibitor GBR 12909, nor was [11C]beta-CIT-FP inhibited by SA4503. The IC(50) values of raclopride for sigma(1) and sigma(2) receptor subtypes measured in vitro were 11800 nM and 4950 nM, respectively, suggesting that the affinity was too low for [11C]raclopride to bind in vivo to sigma receptors. On the other hand, the IC(50) value of SA4503 for dopamine D(2) receptors was 470 nM, that is approximate 1/25 of the affinity of raclopride for the dopamine D(2) receptors. Therefore, possible explanations for the partial blocking effects of SA4503 on the striatal uptake of [11C]raclopride are: (1) an excess amount of SA4503 may reduce the [11C]raclopride uptake due to its low affinity for dopamine D(2) receptors, or (2) SA4503 may enhance endogenous dopamine release, which results in the competitive inhibition of the [11C]raclopride uptake. These findings support that both [11C]raclopride and [11C]SA4503 are selective in vivo ligands for dopamine D(2)-like receptors and sigma(1) receptors, respectively, in spite of the partial blocking effect of SA4503 on the striatal uptake of [11C]raclopride.

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.

Correlation between neuroleptic binding to sigma(1) and sigma(2) receptors and acute dystonic reactions

Acute dystonic reactions are motor side effects that occur soon after the initiation of neuroleptic treatment. Although earlier studies indicate that these abnormal movements can be induced in animals and humans via activation of sigma receptors, the relative contribution of the different sigma receptor subtypes is unknown. Since sigma(1) and sigma(2) receptor are differentially represented in motor regions of the brain, the affinities of 17 neuroleptics for these sigma receptor subtypes were determined using competition binding studies. The results revealed that most neuroleptics do not exhibit selectivity for either of the sigma receptor subtypes, as reflected by a significant correlation between the affinities of the neuroleptics for sigma(1) vs. sigma(2) receptors. Moreover, when the sigma binding affinities of the neuroleptics were correlated with the tendency of the drugs to produce acute dystonic reactions in humans, there was a significant correlation for both subtypes. Together with earlier studies in animals, the data suggest that neuroleptic-induced motor side effects can be mediated through both sigma(1) and sigma(2) receptors.

The delayed effects of DTG and MK-801 on latent inhibition in a conditioned taste-aversion paradigm

The delayed effects of phencyclidine (PCP) have been shown to disrupt latent inhibition (LI) in a conditioned taste-aversion paradigm. In an attempt to understand the mechanism of this disruption, the delayed effects of the selective sigma receptor agonist 1,3-Di(2-tolyl)guanidine (DTG) and the selective NMDA receptor antagonist MK-801 on latent inhibition were assessed in the same paradigm. Water-deprived male rats were allowed access to either water (nonpreexposed; NPE) or 5% sucrose (preexposed; PE) for 30 min on 2 consecutive days. On the third day, animals were allowed access to sucrose and subsequently injected with lithium chloride. On the forth day, animals were allowed access to both sucrose and water. LI was assessed by comparing the percent sucrose consumed in PE and NPE groups on the fourth day. DTG (1.0, 5.0, or 10.0 mg/kg), MK-801 (0.5, 1.0, or 2.0 mg/kg), or vehicle was administered IP 20 h before preexposure (days 1 and 2) and conditioning (day 3). In vehicle-treated groups, PE animals consumed a significantly higher percent sucrose on the test day than NPE animals, indicating the presence of LI. DTG (10.0 mg/kg) and MK-801 (2.0 mg/kg) decreased the percent sucrose consumed by animals in the PE group to the level observed in the NPE group, indicating disrupted LI. However, this dose of MK-801 was found to produce a decrease in percent sucrose consumed in PE animals not treated with lithium chloride, indicating that the decrease observed in the LI paradigm could be due to MK-801-induced decrease in taste preference for sucrose rather than a disruption of LI. Lower doses of MK-801 that did not produce a decrease in taste preference for sucrose did not significantly disrupt LI. None of the doses of DTG tested altered taste preference for sucrose. These data suggest a role for sigma receptors in the previously observed PCP-induced disruption of LI. Published by Elsevier Science Inc., 2000

Sigma receptors: recent advances and new clinical potentials

Several recent advances are leading to a better understanding of sigma receptors. Here we focus on our recent findings regarding cellular functions of sigma-2 receptors and discuss their possible clinical implications. Agonists at sigma-2 receptors induced changes in cell morphology and apoptosis in various cell types. Sigma-2 receptor activation produced both transient and sustained increases in [Ca++]i, derived from different intracellular stores. These changes in [Ca++]i and cytotoxic effects are mediated by intracellular sigma-2 receptors. Sigma-2 agonists induced apoptosis in drug-resistant cancer cells, enhanced the potency of DNA damaging agents, and down-regulated expression of p-glycoprotein mRNA. Thus, sigma-2 receptor agonists may be useful in treatment of drug-resistant cancers. Sigma radioligands have been used in tumor imaging. We also discuss how sigma-2 antagonists might prevent the irreversible motor side effects of typical neuroleptics. Sigma-2 receptors may subserve a novel signalling pathway to apoptosis, involved in regulation of cell proliferation and/or viability.

Influence of 3-PPP, a sigma receptor ligand, on the anticonvulsive action of conventional antiepileptic drugs

(+)-3-(3-Hydroxyphenyl)-N-(1-propyl)-piperidine (3-PPP; a sigma receptor ligand), administered at 30 mg kg-1, 30 min before the test, significantly decreased the electroconvulsive threshold in mice, being ineffective in lower doses. 3-PPP (20 mg kg-1) diminished the protective activity of diphenylhydantoin, phenobarbital and valproate, but not that of carbamazepine against maximal electroshock. The effect of 3-PPP upon the electroconvulsive threshold and the 3-PPP-induced inhibition of the protective action of antiepileptics was reversed by haloperidol (0.5 mg kg-1). Moreover, 3-PPP did not alter the total and free plasma levels of antiepileptic drugs, so a pharmacokinetic interaction is not probable. The combined treatment of 3-PPP with antiepileptic drugs, providing a 50% protection against maximal electroshock, did not affect motor performance in mice, although resulted in significant long-term memory deficits. Our data indicate that sigma receptor-mediated events may play some role in seizure processes in the central nervous system and can modulate the protective activity of some conventional antiepileptic drugs.

Pain modulation by release of the endogenous cannabinoid anandamide

Synthetic cannabinoids produce behavioral analgesia and suppress pain neurotransmission, raising the possibility that endogenous cannabinoids serve naturally to modulate pain. Here, the development of a sensitive method for measuring cannabinoids by atmospheric pressure-chemical ionization mass spectrometry permitted measurement of the release of the endogenous cannabinoid anandamide in the periaqueductal gray (PAG) by in vivo microdialysis in the rat. Electrical stimulation of the dorsal and lateral PAG produced CB1 cannabinoid receptor-mediated analgesia accompanied by a marked increase in the release of anandamide in the PAG, suggesting that endogenous anandamide mediates the behavioral analgesia. Furthermore, pain triggered by subcutaneous injections of the chemical irritant formalin substantially increased the release of anandamide in the PAG. These findings indicate that the endogenous cannabinoid anandamide plays an important role in a cannabinergic pain-suppression system existing within the dorsal and lateral PAG. The existence of a cannabinergic pain-modulatory system may have relevance for the treatment of pain, particularly in instances where opiates are ineffective.

Involvement of sigma receptors in the modulation of the glutamatergic/NMDA neurotransmission in the dopaminergic systems

Extracellular single-unit recordings and iontophoresis were used to examine the effects of different selective sigma receptor ligands on dopaminergic and glutamatergic N-methyl-D-aspartate (NMDA) neurotransmissions both in origin (A10 and A9 areas) and terminal (nucleus accumbens and caudate nucleus) regions of the rat mesolimbic and nigrostriatal dopaminergic systems. The selective sigma1 receptor ligands 2-[4-(4-methoxy-benzyl)piperazin-1-yl-methyl]4-oxo[4H]-benzo-th iazolin-2-one (S-21377), systemically administered (1.2 mg/kg, i.v., cumulative dose), and 2[(4-benzyl piperazin-1-yl) mothyl] naphthalene, dichiorydrate (S-21378), iontophoretically applied, slightly increased the spontaneous firing rate and potentiated the NMDA-induced neuronal activation of dopaminergic neurons in the A9 and A10 regions. (+)N-cyclopropylmethyl-N-methyl-1,4-diphenyl-1-ethyl-butyl-2-N (JO-1784), another selective sigma1 receptor ligand produced no or little effect in these areas. The systemic administration of the selective sigma2 receptor ligand 1,4-bis-spiro[isobenzofuran-1(3H), 4'-piperidin-1'yl]butane (Lu 29-252) (2 mg/kg, i.v., cumulative dose) did not modify the firing activity of A9 and A10 dopaminergic neurons, but significantly potentiated the NMDA-induced increase in firing activity of A10 dopaminergic neurons. None of the sigma receptor ligands tested had any effects on the dopamine-induced suppression of firing. In the nucleus accumbens, the systemic administration of (JO-1784), (40 microg/kg, i.v.), (+)-pentazocine (30 microg/kg, i.v.), another selective sigma1 receptor ligand, and of the non selective sigma1 receptor ligand di-tolyl-guanidine (DTG) (20 microg/i.v.) produced a significant increase of NMDA-induced neuronal activation. Microiontophoretic applications of JO-1784 also potentiated the NMDA response. They also increased significantly the suppressant effect of dopamine on NMDA and kainate-induced activations of accumbens neurons. In the caudate nucleus, (+)-pentazocine, but not JO-1784, potentiated slightly the neuronal response to NMDA. None of the sigma receptor ligands tested did modify significantly the responses of caudate and accumbens neurons to kainate. These findings suggest that at least two subtypes of sigma1 receptors may affect differentially the glutamate NMDA neurotransmission in the terminal and origin regions of the mesolimbic and nigrostriatal dopaminergic systems. These results also demonstrate the existence of a functional interaction between sigma2 and NMDA receptors in the A10 region.

The functional sensitisation of sigma receptors following chronic selective serotonin reuptake inhibitor treatment

The purpose of the present study was to investigate the potential impairment of normal motor function following chronic selective serotonin reuptake inhibitor treatment that may result from sensitisation of sigma receptors. Rats were chronically treated with either sertraline, citalopram, paroxetine or fluvoxamine and a selective sigma receptor ligand, di-o-tolylguanidine (DTG), for 28 days. All animals then received an acute intra-rubral injection of either DTG or saline. Following the direct injection of DTG into the red nucleus, rats chronically treated with DTG exhibit a maximal behavioural response characterised as a pronounced dystonia. Animals chronically treated with sertraline and citalopram elicited a response similar to that of control animals following the acute DTG challenge, whereas chronic treatment with paroxetine and fluvoxamine significantly decreased and increased the dystonic response, respectively. Facial spasticity and vacuous chewing movements were associated with, and reflected the extent of, the DTG-induced dystonia. Changes in regional biogenic amine concentrations were also determined. The concentrations of serotonin and noradrenaline were determined in the brain stem and cerebellum following the intra-rubral injection of either saline or DTG in animals that had been chronically treated with a selective serotonin reuptake inhibitor or DTG. There was a significant increase in serotonin concentration in the brain stem as a result of chronic DTG and fluvoxamine treatments. The increase in serotonin correlated with the reported potentiation of dystonia in animals that received 28 days treatment with these drugs. The potentiation of dystonia following chronic DTG and fluvoxamine treatments suggests that these drugs sensitise the sigma2 receptors, an effect that does not appear to be shared by citalopram, sertraline or paroxetine.

Neuroleptic binding to sigma receptors: possible involvement in neuroleptic-induced acute dystonia

Several antipsychotic drugs, belonging to various chemical classes, were compared for their affinity for the sigma, dopamine-D2, and muscarinic receptors. Many neuroleptic drugs were found to bind with high affinity to sigma 2 receptors, and the binding affinity was clearly different from that observed for dopamine-D2 receptors. The dopaminergic and muscarinic theories for the physiopathology of acute dystonia are not completely satisfactory. Since the sigma receptors were reported to play a role in the control of movement, the high affinity of some neuroleptics for these sites suggests their possible involvement in some side effects, such as drug-induced dystonia. There was a correlation between the clinical incidence of neuroleptic-induced acute dystonia and binding affinity of drugs for the sigma receptor, except for some drugs, with a lower incidence, displaying significant affinity for the cholinergic muscarinic receptor. Therefore, we conclude that the affinity for the sigma receptor might be involved in neuroleptic-induced acute dystonia, but this might be partially corrected by the intrinsic anticholinergic properties of the drug.

The pharmacology of the novel and selective sigma ligand, PD 144418

The pharmacology of PD 144418 (1-propyl-5-(3-p-tolyl-isoxazol-5-yl)-1,2,3,6-tetrahydropyridine) was characterized using neurochemical, biochemical and behavioral techniques. For sigma (sigma 1 and sigma 2 respectively) sites, PD 144418 affinities were determined using whole guinea pig brain membranes with [3H](+)-pentazocine and neuroblastoma x glioma cell membranes using [3H]1,3,di-O-tolylguanidine (DTG) in the presence of 200 nM (+)-pentazocine. PD 144418 exhibited an affinity for sigma 1 of 0.08 nM (Ki) versus a K1 of 1377 nM for sigma 2 site. Additional receptor binding studies indicated that PD 144418 lacked affinity for dopaminergic, adrenergic, muscarinic and a variety of other receptors. In vitro studies indicated that PD 144418 reversed the N-methyl-D-aspartate (NMDA)-induced increase in cyclic GMP (cGMP) in rat cerebellar slices without affecting the basal levels, suggesting that sigma 1 sites may be important in the regulation of glutamine-induced actions. PD 144418 potentiated the decrease in 5-hydroxytryptophan caused by haloperidol in the mesolimbic region, but by itself had no effect in 5-hydroxytrypamine (5-HT) and dopamine (DA) synthesis. Behaviorally, similar to other sigma ligands, PD 144418 antagonized mescaline-induced scratching at doses that did not alter spontaneous motor activity. This action is suggestive of potential antipsychotic property. It exhibited no anxiolytic and antidepressant properties in the models used. These results show that PD 144418 is a very selective sigma 1 agent, devoid of any significant affinity for other receptors and that sigma 1 site may modulate actions in the CNS.

(+)-SKF-10,047 and dextromethorphan ameliorate conditioned fear stress via dopaminergic systems linked to phenytoin-regulated sigma 1 sites

Mice exhibited a marked suppression of motility when they were re-placed in the same environment in which they had previously received an electric footshock. (+)-SKF-10,047 ([2S-(2 alpha, 6 alpha, 11R*)]-1,2,3,4,5,6-hexahydro-6,11-dimethyl-3-(2-propenyl)-2,6-me tha no-3-benzazocin-8-ol hydrochloride; (+)-N-allylnormetazocine hydrochloride) and dextromethorphan, putative sigma receptor agonists, have been reported to reserve this psychological stress-induced motor suppression, defined as conditioned fear stress, through phenytoin-regulated type sigma 1, receptors. In the present study, we investigated the involvement of dopaminergic neurons in the ameliorating effects of (+)-SKF-10,047 and dextromethorphan on conditioned fear stress. (+)-SKF-10,047 and dextromethorphan attenuated conditioned fear stress at low doses (4 and 20 mg/kg, respectively) when they were co-administered with phenytoin (10 mg/kg), an anticonvulsant drug. The effects were antagonized by the sigma receptor antagonists. NE-100 (N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride) and BMY-14802 (a-(4-fluoro-phenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazine-butanol hydrochloride). Furthermore, the effects of (+)-SKF-10,047 or dextromethorphan in combination with phenytoin were blocked by the dopamine D1 receptor antagonist. SCH 23390 (R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3- benzazepine), and the dopamine D2 receptor antagonist, (-)-sulpiride, and they were also attenuated by 6-hydroxydopamine-induced lesions of dopaminergic neurons. The ameliorating effects of (+)-SKF-10,047 and dextromethorphan on conditioned fear stress at high doses (5 and 30 mg/kg, respectively) were also blocked by both the dopamine receptor antagonists. These results suggest that the stress-induced motor suppression is restored by the activation of dopaminergic neuronal systems as a result of the stimulation of phenytoin-regulated type sigma 1 receptors.

Induction of c-fos gene expression by the selective sigma receptor ligand EMD 57445 in rat brain

Based on animal studies it has been reasoned that ligands to sigma binding sites might be effective in the treatment of schizophrenic disorders and may also be used to investigate this largely elusive disorder on a molecular level. Expression patterns of c-fos in rat brain were studied following treatment with single doses of the sigma ligand EMD 57445 (0.3, 1, 3, 30 mg/kg s.c.). Specific c-fos gene expression was detected at all concentrations tested in various cortical areas. The signals observed were dose-dependent with the highest intensities in the piriform cortex. Strong signals were also detected in hippocampal areas CA 1,2,3 and the gyrus dentatus, as well as in the medial habenula nuclei. In the caudate putamen, nucleus accumbens and lateral septal nucleus signals were detectable after administration of doses > or = 1 mg/kg. Furthermore, c-fos hybridization was visible in the amygdala, in the mammillary bodies, the islands of Calleja and in the olfactory tubercle. In the hypothalamus, c-fos expression was seen in the median eminence area after 30 mg/kg EMD 57445. No hybridization signals were obtained in brainstem or cerebellum. Since c-fos expression induced by EMD 57445 resembled the pattern obtained with atypical neuroleptics and studies on animal behavior point to antipsychotic activity, it is concluded that the drug might be suitable in the treatment of schizophrenia.

Regional differences of the effect of sigma receptor ligands on the acetylcholine release in the rat brain

We found that sigma receptor ligands differentially regulated the acetylcholine (ACh) neurotransmission in the rat brain. Acute administration of (+)-N-allylnormetazocine [(+)-SKF-10,047], a prototype sigma 1 receptor ligand, and 1,3-di(2-tolyl)guanidine (DTG), a non-specific sigma receptor ligand, increased the extracellular ACh level in the rat hippocampus. This increase of hippocampal extracellular ACh level elicited by (+)-SKF-10,047 was more potent than that elicited by DTG. On the other hand, the striatal extracellular ACh level was slightly affected by (+)-SKF-10,047. In addition, DTG did not affect the striatal extracellular ACh level. Our previous studies have shown that both (+)-SKF-10,047 and DTG increased the extracellular ACh level in the rat frontal cortex. Taking all these data into consideration, the regulation of ACh neurotransmission by sigma receptor ligands are different depending upon the brain region.

Effects of sigma receptor ligands on the extracellular concentration of dopamine in the striatum and prefrontal cortex of the rat

The extracellular concentration of dopamine in the striatum and medial prefrontal cortex of the rat was determined following the systemic administration of sigma receptor ligands. The (+)-benzomorphan, (+)-pentazocine, significantly increased the extracellular concentration of dopamine in the striatum also was produced by the (+)-, but not the (-)-, enantiomer of N-allylnormetazocine, as well as by the non-benzomorphans 1-(cyclopropylmethyl)-4-(2'-(4"-fluorophenyl)-2'-oxoothyl-piper idi ne (DUP 734) and (-)-butaclamol. In contrast, the dopamine concentration was unaffected by di-o-tolylguanidine and markedly suppressed by (+)-3-[3-hydroxyphenyl]-N-(1-propyl)piperidine (3-PPP). Finally, the (+)-pentazocine-induced elevation of the extracellular concentration of dopamine was not suppressed by an inhibitor of the dopamine transporter, 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-[3-phenylpropyl]piperazine (GBR 12909). Thus, benzomorphan, e.g., (+)-pentazocine and (+)-N-allylnormetazocine, and non-benzomorphan, e.g., DUP 734 and (-)-butaclamol, sigma receptor ligands appear to facilitate dopamine release from nigrostriatal, and presumably mesocorticolimbic, neurons through a non-transporter-mediated mechanism.

Inotropic action of sigma receptor ligands in isolated cardiac myocytes from adult rats

High affinity binding sites for sigma receptor ligands were found in membranes of cardiac myocytes from adult rats. The sigma receptor ligand (+)-3-hydroxyphenyl-N-(1-propyl)piperidine ((+)-3-PPP) binds with a Kd of 17.9 +/- 4.0 nM and a Bmax of 275 +/- 32.1 fmol/mg protein. Competition experiments of (+)-pentazocine with [3H]1,3-di-O-tolylguanidine ([3H]DTG) binding yielded a Ki of 6.1 +/- 1.3 nM. The majority of the sites (> 80%) were of the sigma 1 subtype. Exposure of isolated cardiomyocytes from adult rats to (+)-3-PPP (10 nM-1.0 microM) caused a marked concentration-dependent increase in the amplitude of systolic cell contraction, reaching 149% of control level, with an apparent ED50 value of 4.5 nM. The increase in the contraction amplitude was markedly inhibited by pretreatment with verapamil or thapsigargin. An increase in the amplitude of [Ca2+]i transients, similar to that in the amplitude of cell contraction, was observed in indo-1-loaded cardiomyocytes exposed to 0.1 microM (+)-3-PPP. Exposure to 10 nM of haloperidol or (+)-pentazocine induced an increase in the amplitude of contraction, reaching 188% and 138% (respectively) of control level. A lower concentration of haloperidol or (+)-pentazocine (1 nM) did not induce an increase in the contraction amplitude but rather reduced the amplitude to 70-80% of control.

DTG-induced circling behaviour in rats may involve the interaction between sigma sites and nigro-striatal dopaminergic pathways

The distribution of sigma sites in brain areas enriched in dopamine, and the finding that circling behaviour can be elicited by specific sigma ligands such as DTG (di-o-tolylguanidine) suggest a modulatory role of these sites in the dopaminergic system. The present study was carried out to investigate further this hypothesis. Circling behaviour induced in rats by unilateral intranigral injection of DTG (10 nmol/rat) was decreased by haloperidol (0.1 mg/kg, i.p.), clebopride (0.25 mg/kg, i.p.) and SCH 23390 (0.03 mg/kg, s.c.) indicating that an interaction between sigma sites and the midbrain dopaminergic system may be involved in this rotational behaviour. Microdialysis experiments in freely moving rats showed that unilateral intranigral injection of DTG (5, 10, 20 nmol/rat) produced increases in extracellular levels of dopamine metabolites (DOPAC, HVA) in the ipsilateral striatum which correlated with the number of rotations. In addition intranigral injection of DTG (10 nmol/rat) produced increases in tissular dopamine metabolite levels in the ipsilateral striatum without affecting dopamine metabolite levels in limbic structures. These results indicate that sigma sites may be involved in the modulation of the dopaminergic motor system.

Stimulation of rat striatal tyrosine hydroxylase activity following intranigral administration of sigma receptor ligands

The effects of sigma ligands on turning behavior and striatal tyrosine hydroxylase activity were determined following microinjection of two chemically dissimilar sigma ligands into the rat substantia nigra. Striatal tyrosine hydroxylase activity was monitored by measuring the amount of 3,4-dihydroxyphenylalanine (DOPA) formed following inhibition of DOPA decarboxylase activity with m-hydroxybenzylhydrazine (NSD-1015). The sigma ligands, 1,3-di-o-tolylguanidine (DTG) and (-)-deoxy-N-benzylnormetazocine, produced a significant increase both in contralateral turning and in tyrosine hydroxylase activity. The DTG-induced increase in tyrosine hydroxylase activity was not antagonized by intranigral injection of the NMDA receptor antagonist, 3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP). CPP alone produced significant contralateral turning that was not accompanied by an increase in striatal tyrosine hydroxylase activity, indicating that turning per se is not sufficient to activate striatal tyrosine hydroxylase. The DTG-induced increase in tyrosine hydroxylase activity was antagonized by general anesthetics such as halothane and chloral hydrate. These results indicate that occupancy of sigma receptors in the substantia nigra is associated with an activation of dopamine formation in dopaminergic terminals in the striatum and support the concept that sigma activity in the substantia nigra produces an activation of dopamine-mediated responses in the striatum.

Acute effects of sigma ligands on the extracellular DOPAC level in rat frontal cortex and striatum

Acute administration of (+)-N-allylnormetazocine ((+)-SKF-10,047) and (+/-)-pentazocine, was found to increase the extracellular level of 3,4-dihydroxyphenylacetic acid (DOPAC), a major dopamine (DA) metabolite, in the rat frontal cortex. By contrast, these benzomorphan sigma ligands did not change the extracellular DOPAC level in the rat striatum. On the other hand, 1,3-di(2-tolyl)guanidine (DTG) increased the extracellular DOPAC level in the frontal cortex, while it decreased that level in the striatum. Another non-benzomorphan sigma ligand, (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3-PPP) decreased the extracellular DOPAC level in both frontal cortex and striatum. Moreover, the increase of the extracellular DOPAC level elicited by (+)-SKF-10,047 was significantly inhibited by rimcazole, a putative sigma antagonist, while the DTG-induced increment was not reversed by rimcazole. These findings indicated that the effects of sigma ligands on the mesocortical DA neurons differed from those on the nigrostriatal DA neurons. In addition, the effects of benzomorphan sigma ligands on the central DA neurons were different from those of non-benzomorphan sigma ligands.

Pharmacological evidence for the involvement of sigma sites in DTG-induced contralateral circling in rats

The central distribution of sigma sites labelled by di-o-tolylguanidine (DTG), a compound which has specific affinity for sigma sites, and its ability to produce postural movements, are consistent with the hypothesis that sigma sites may play a functional role in the regulation of movement. The aim of the present study was to evaluate the specificity of the circling behaviour induced by unilateral intranigral injection of DTG in rats. As previously described, DTG produced dose-dependent unilateral rotations (2.5-20 nmol/rat). A similar dose-dependent circling behaviour was observed with DMTG and (+) NANM (3-40 nmol/rat), compounds which bind to both sigma and PCP sites, and with haloperidol (3-20 nmol/rat) whereas raclopride and D,L-sulpiride did not elicit any circling (10 nmol/rat). DTG-induced circling after intranigral injection (10 nmol/rat) was decreased in a dose-dependent manner by rimcazole (20-40 mg/kg, i.p.), a selective ligand for sigma sites, and by BMY 14802 (3, 10, 30 mg/kg, i.p.), ifenprodil and eliprodil (1, 3, 10 mg/kg, i.p.), non-selective sigma ligands. In contrast, naloxone (1 mg/kg, s.c.) and CGS 19755 (1, 3, 10 mg/kg, i.p.) did not change the DTG-induced circling. Eliprodil failed to inhibit circling produced by compounds devoid of any affinity for sigma sites such as APV, dizocilpine or muscimol, indicating the specificity of the inhibition observed with eliprodil on the DTG-induced circling.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological comparison of antipsychotic drugs and sigma-antagonists in rodents

We compared antipsychotic drugs (haloperidol, chlorpromazine and clozapine) and sigma antagonists (remoxipride, cinuperone, alpha-(4-fluorophenyl)-4-(-fluoro-2-pyrimidinyl)-1-piperazine butanol (BMY 14802) and rimcazole) in the radio-ligand binding and behavioural experiments in rodents. A good correlation was established between the affinity of compounds at dopamine2-receptors in the striatum and their ability to block apomorphine-, amphetamine- and quipazine-induced behavioural effects in rodents. By contrast, no correlation was found between the behavioural effects of these drugs and their affinity at dopamine1-5-HT2- and sigma receptors. The rank order of potency among the studied antipsychotic drugs in the behavioural tests and at dopamine2-receptors was following: haloperidol >> chlorpromazine > or = clozapine. The effectiveness of chlorpromazine and clozapine was nearly similar against apomorphine-induced aggressiveness and yawning, whereas at 5-HT2-receptors clozapine was more active than chlorpromazine. The weak activity of sigma antagonists at dopamine2 receptors could be a possible reason why these compounds were less effective in the behavioural studies compared to antipsychotic drugs. However, the antagonism of remoxipride against apomorphine-induced stereotypy and aggressiveness is not related to its activity at sigma receptors, because the other sigma antagonists did not block these effects of apomorphine. It is probable that remoxipride exerts its action through blocking of dopamine2 receptors. In conclusion, the present study revealed only weak activity of sigma antagonists in the behavioural models widely used to study the antipsychotic drugs. Therefore, the antipsychotic activity of sigma antagonists is doubtful.