Autoradiographic identification and characterization of sigma receptors in guinea pig brain using [3H]1(cyclopropylmethyl)-4-(2'-(4''-fluorophenyl)-2'-oxoethyl) piperidine ([3H]DuP 734), a novel sigma receptor ligand

Antinociception by Sigma-1 Receptor Antagonists: Central and Peripheral Effects

There is plenty of evidence supporting the modulatory role of sigma-1 receptors (σ1Rs) in nociception, mainly based on the pain-attenuated phenotype of σ1R knockout mice and on the antinociceptive effect exerted by σ1R antagonists, particularly in nonacute sensitizing conditions involving sustained afferent drive, activity-dependent plasticity/sensitization, and ultimately pain hypersensitivity, as it is the case in chronic pains of different etiology. Antinociceptive effects of σ1R antagonists both when acting alone and in combination with opioids (to enhance opioid analgesia) have been reported at both central and peripheral sites. At the central level, findings at the behavioral (animal pain models), electrophysiological (spinal wind-up recordings), neurochemical (spinal release of neurotransmitters) and molecular (NMDAR function) level supports a role for σ1R antagonists in inhibiting augmented excitability secondary to sustained afferent input. Attenuation of activity-induced plastic changes (central sensitization) following tissue injury/inflammation or nerve damage could thus underlie the central inhibitory effect of σ1R antagonists. Moreover, recent pieces of information confirm the involvement of σ1R in mechanisms regulating pain at the periphery, where σ1Rs are highly expressed, particularly in dorsal root ganglia. Indeed, local peripheral administration of σ1R antagonists reduces inflammatory hyperalgesia. Potentiation of opioid analgesia is also supported, particularly at supraspinal sites and at the periphery, where locally administered σ1R antagonists unmask opioid analgesia. Altogether, whereas σ1R activation is coupled to pain facilitation and inhibition of opioid antinociception, σ1R antagonism inhibits pain hypersensitivity and "releases the brake" enabling opioids to exert enhanced antinociceptive effects, both at the central nervous system and at the periphery.

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.

Possible involvement of sigma-1 receptors in the anti-immobility action of bupropion, a dopamine reuptake inhibitor

Sigma receptors particularly, sigma-1 subtype is known to modulate the release of catecholamines in the brain and may participate in the mechanism of action of various antidepressants. The present study investigated the possible involvement of sigma receptors in modulating the anti-immobility-like effect of bupropion (a dopamine reuptake inhibitor) using the forced swim test (FST) in mice. Bupropion produced dose-dependent (10-40 mg/kg, i.p.) reduction in immobility period and the ED(50) value was found to be 18.5 (7.34-46.6) mg/kg, i.p. (+)-Pentazocine (2.5 mg/kg, i.p.), a high-affinity sigma-1 receptor agonist, produced synergistic response when it was co-administered with a subeffective dose of bupropion (10 mg/kg, i.p.). On the contrary, pretreatment with progesterone (10 mg/kg, s.c.), a sigma-1 receptor antagonist neurosteroid, rimcazole (5 mg/kg, i.p.), another sigma-1 receptor antagonist, or BD 1047 (1 mg/kg, i.p.), a novel sigma-1 receptor antagonist, reversed the anti-immobility effects of bupropion (20 mg/kg, i.p.). The various modulators used in the study did not show any effect per se on locomotor activity except bupropion which at a higher dose (15-40 mg/kg, i.p.) significantly increased the locomotor activity. The results for the first time demonstrated the involvement of sigma-1 receptors in the anti-immobility effects of bupropion.

Involvement of sigma-1 receptor modulation in the antidepressant action of venlafaxine

Multiple lines of investigation have explored the role of sigma receptors in mental depression. Sigma receptors particularly, sigma-1 subtype is known to modulate the release of various catecholamines in the brain and may play, in some way, a role in the mechanism of action of various antidepressants. The present study investigated the possible involvement of sigma receptors in modulating the antidepressant-like effect of venlafaxine (dual serotonin and norepinephrine reuptake inhibitor) in the mouse forced swim test (FST). Immobility period in the forced swim test was registered for a total period of 6 min. Venlafaxine produced dose-dependent (4-16 mg/kg, i.p.) reduction in immobility period. Pretreatment of mice with (+)-pentazocine (2.5 mg/kg, i.p.), a high-affinity sigma-1 receptor agonist, produced synergism with subeffective dose of venlafaxine (2 mg/kg, i.p.). On the contrary, pretreatment with progesterone (10 mg/kg, s.c.), a sigma-1 receptor antagonist neurosteroid, rimcazole (5 mg/kg, i.p.), another sigma-1 receptor antagonist, or BD 1047 (1 mg/kg, i.p.), a novel sigma-1 receptor antagonist, reversed the anti-immobility effects of venlafaxine (8 mg/kg i.p.). The various modulators used in the study did not produce any changes in locomotor activity per se except venlafaxine which at higher dose (16 mg/kg, i.p.) significantly increased the locomotor activity in mice. The results for the first time demonstrated that the anti-immobility effects of venlafaxine in the FST possibly involve an interaction with sigma-1 receptors.

Non-MAO A binding of clorgyline in white matter in human brain

Clorgyline is an irreversible inhibitor of monoamine oxidase (MAO A) which has been labeled with carbon-11 (C-11) and used to measure human brain MAO A with positron emission tomography (PET). In this study we compared [11C]clorgyline and deuterium-substituted [11C]clorgyline ([11C]clorgyline-D2) to better understand the molecular link between [11C]clorgyline binding and MAO A. In PET studies of five normal healthy volunteers scanned with [11C]clorgyline and [11C]clorgyline-D2 2 h apart, deuterium substitution generally produced the expected reductions in the brain uptake of [11C]clorgyline. However, the reduction was not uniform with the C-11 binding in white matter being significantly less sensitive to deuterium substitution than other brain regions. The percentages of the total binding attributable to MAO A is largest for the thalamus and smallest for the white matter and this is clearly seen in PET images with [11C]clorgyline-D2. Thus deuterium-substituted [11C]clorgyline selectively reduces the MAO A binding component of clorgyline in the human brain revealing non-MAO A binding which is most apparent in the white matter. The characterization of the non-MAO A binding component of this widely used MAO A inhibitor merits further investigation.

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

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

Phencyclidine (PCP) acts at sigma sites to induce c-fos gene expression

Phencyclidine (PCP) is a compound that results in abnormal human behavior and has been proposed as a chemical model for schizophrenia. It was hypothesized that PCP induction of the immediate-early gene, c-fos, should be seen in areas associated with emotional behavior, such as the cortex and limbic system. It was also proposed that PCP may induce c-fos via the sigma receptor. PCP and two sigma ligands, 1,3-di(2-tolyl)guanidine (DTG) and pentazocine, were shown to induce c-fos in similar patterns. The three compounds abundantly induced c-fos in the cingulate, parietal, and piriform cortices and the midline structures of the thalamus and hypothalamus. Neither PCP nor the sigma ligands induced c-fos in the hippocampus. This suggests that PCP binding at NMDA receptors does not result in significant c-fos induction. Rimcazole, a putative sigma2 receptor antagonist, and other sigma ligands have been shown to ameliorate PCP stereotypic behavior. Rimcazole inhibited PCP c-fos induction in the cingulate and parietal cortices and DTG c-fos induction in the cingulate cortex. DTG shows both sigma1 and sigma2 binding affinity. Rimcazole failed to inhibit pentazocine c-fos induction. Pentazocine binds only to sigma1 receptors. This suggests that PCP may produce a significant fraction of its c-fos induction via sigma2 receptors.

Effects of sigma ligands on the ability of rimcazole to inhibit PCP hsp70 induction

Phencyclidine (PCP) can result in schizophrenia-like behavior. It binds at the PCP site on the NMDA-receptor calcium channel and at the sigma receptor. PCP also induces the heat shock gene hsp7O in retrosplenial cortex neurons. An antipsychotic drug, rimcazole, inhibits PCP hsp7O induction. Rimcazole binds predominantly to sigma-2 sites. It is hypothesized that sigma ligands without antipsychotic properties and with some sigma-2 affinity should partially reverse the effects of rimcazole. (+)-3-PPP, (+)-cyclazocine, and (+)-pentazocine bind predominantly to sigma-I sites. (+)-3-PPP is also a modest sigma-2 ligand. Female Sprague-Dawley rats (200-260 g) were injected intraperitoneally (IP) with (+)-3-PPP (50 mg/kg), rimcazole (60 mg/kg) and, after 5 min, with PCP (40 mg/kg). Brains were sectioned (100 mu m) and presence of the hsp7O gene protein product, HSP7O, was determined immunocytochemically. (+)-3-PPP significantly (0 <0.05) diminished the ability of rimcazole to inhibit PCP hsp7O induction in the retrosplenial cortex. (+)-Cyclazocine (15mg/kg, IP) and (+)-pentazocine (8Omg/kg, IP) given in an analogous manner did not diminish the ability of rimcazole to inhibit PCP hsp7O induction.

sigma1 Receptors in rat striatum regulate NMDA-stimulated [3H]dopamine release via a presynaptic mechanism

The role of the sigma1 receptor in the regulation of N-methyl-D-aspartate (NMDA)-stimulated [3H]dopamine release from rat striatal slices was examined. The sigma receptor agonist 1S,2R-(--)-N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)cy clohexylamine (BD737) inhibited stimulated release in a concentration-dependent manner. The sigma1 receptor antagonist, 1-(cyclopropylmethyl)-4-(2'-(4"-fluorophenyl)-2'-oxoethyl)piperidi ne HBr (DuP 734), reversed inhibition of release by BD737. Haloperidol, di-o-tolylguanidine (DTG) and N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine (BD1008) reversed the BD737-mediated inhibition of release. Haloperidol and DTG also antagonized inhibition of stimulated release by (+)-pentazocine. Furthermore, BD737 and (+)-pentazocine inhibited stimulated release in the presence of tetrodotoxin, suggesting that sigma1 receptors regulating dopamine release are located on dopaminergic nerve terminals. These data suggest that sigma1 receptors may be important in the regulation of glutamate-stimulated dopamine release.

Inhibition of the ATP-sensitive potassium channel in the guinea pig substantia nigra by BMS 181100 is not mediated by a sigma-binding site

Quantitative autoradiography of brain tissue has revealed a high density of binding sites for the K-ATP channel antagonists, the sulphonylureas, and for sigma-ligands in the substantia nigra (SN). In view of the high density of the two binding sites in the SN the possibility has been investigated that the K-ATP channel and the sigma-binding site are functionally linked. The K-ATP channel-mediated membrane hyperpolarisation and decrease in input resistance produced by hypoxia and by the metabolic inhibitor, cyanide, in rostral substantia nigra pars compacta neurons are antagonised by the sigma-ligand BMS 181100. In addition, BMS 181100 antagonises activation of the K-ATP channel by diazoxide; cromakalim is found to be without effect in these neurons. Antagonism of the cyanide-induced hyperpolarisation is dose dependent and is observed at concentrations of the drug which have no observable effect on the resting membrane properties of the neurons. By contrast, the nonselective sigma ligands 1,3-di-O-tolylguanidine (10 microM) and (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine (100 microM), and the selective sigma 1-ligand (+)-pentazocine (5-10 microM) have no effect on the cyanide-induced hyperpolarisation. 5-HT (50-100 microM) and the selective 5-HT1A receptor agonist 8-OH-DPAT (50 microM) also fail to antagonise the cyanide-induced hyperpolarisation. The antagonism of the cyanide-induced hyperpolarisation by BMS 181100 persists in the presence of tetrodotoxin (1 microM) and in the presence of high concentrations of (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine, but not under conditions of reduced calcium (0.1-0.2 mM) and raised magnesium (6 mM) concentrations, which block synaptic transmission. It is concluded that in substantia nigra phasic neurons the sigma-binding site does not regulate activation of the ATP-sensitive channel. However, BMS 181100 antagonises K-ATP channel activation in these neurons independently of sigma-binding sites and 5-HT receptors. This action of BMS 181100 is TTX insensitive and Ca2+ dependent.

DNQX inhibits phencyclidine (PCP) and ketamine induction of the hsp70 heat shock gene in the rat cingulate and retrosplenial cortex

Phencyclidine (PCP) and ketamine are known to block NMDA receptor mediated excitotoxicity by non-competitively blocking the NMDA receptor calcium channel. PCP and ketamine have the paradoxical effect of also inducing the heat shock gene, hsp70, in the cingulate and retrosplenial cortex of the rat. The present study shows that DNQX, a specific AMPA receptor antagonist, given as either a 5 mg/kg or 10 mg/kg intraperitoneal dose or into the lateral cerebral ventricle (5 microliters of 0.5 mg/ml) significantly diminished PCP (40 mg/kg) and ketamine (80, 100, 120 mg/kg) hsp70 induction in the posterior cingulate and retrosplenial cortex. The most dramatic decrease of hsp70 induction was seen with the intraventricular dose of DNQX. Present findings show that the AMPA receptor has a role in PCP/ketamine induction of hsp70 in the cortex. DNQX inhibition of PCP/ketamine hsp70 induction was likely related to AMPA receptor antagonism which prevented excess calcium influx via voltage-gated calcium channels.

Determination of a novel sigma receptor antagonist, DuP 734, in rat plasma by reversed-phase high-performance liquid chromatography with ultraviolet detection

A selective high-performance liquid chromatographic (HPLC) assay for a sigma receptor antagonist, DuP 734 (I), in rat plasma has been developed. Compound I and internal standard, XC031 (I.S.), were first extracted from plasma into an ethyl acetate-toluene mixture (3:7, v/v) and then back-extracted into freshly prepared phosphoric acid (0.03 M). Separation of I and I.S. with no interference from endogenous substances was achieved on a reversed-phase octyl column and detection was by UV at 229 nm. The mobile phase consisted of acetonitrile-glacial acetic acid-triethylamine-0.05 M ammonium acetate (670:4:2:2000, v/v). Using 0.5 ml of rat plasma for extraction, the limit of quantitation was 43 ng/ml and the assay was linear from 43 to 8536 ng/ml. The intra- and inter-day coefficients of variation ranged from 0.7 to 3.0%, and from 1.4 to 14.5%, respectively, over the entire concentration range. The accuracy was within 16.1% of the spiked concentrations. I was stable in frozen plasma at -20 degrees C for at least 68 days.

FH-510, a potent and selective ligand for rat brain sigma recognition sites

FH-510 (5,8-dimethyl-4-(2-di-n-propylaminoethyl)carbazol) is a potent ligand for sigma ligand binding sites in rat brain membranes and has an IC50 value of 4.8 +/- 1.0 nM, which is comparable to that of haloperidol (2.2 +/- 0.2 nM). The high selectivity of FH-510 for sigma binding sites was evaluated by its lack of significant affinity for other binding sites, including those for dopamine D2 and phencyclidine. When administered to mice orally, FH-510 suppressed (+)-N-allylnormetazocine ((+)-SKF10,047)-induced stereotyped behavior with an ED25 value of 0.54 mg/kg, which is 14-fold more potent than that for (+)-alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazine butanol ((+)-BMY14802) (7.6 mg/kg). These results suggest that FH-510 may be a potent and selective sigma ligand.