An examination of the putative sigma-receptor in the mouse isolated vas deferens

Genetic inactivation of the sigma-1 chaperone protein results in decreased expression of the R2 subunit of the GABA-B receptor and increased susceptibility to seizures

There is a growing body of evidence demonstrating the significant involvement of the sigma-1 chaperone protein in the modulation of seizures. Several sigma-1 receptor (Sig1R) ligands have been demonstrated to regulate the seizure threshold in acute and chronic seizure models. However, the mechanism by which Sig1R modulates the excitatory and inhibitory pathways in the brain has not been elucidated. The aim of this study was to compare the susceptibility to seizures of wild type (WT) and Sig1R knockout (Sig1R-/-) mice in intravenous pentylenetetrazol (PTZ) and (+)-bicuculline (BIC) infusion-induced acute seizure and Sig1R antagonist NE-100-induced seizure models. To determine possible molecular mechanisms, we used quantitative PCR, Western blotting and immunohistochemistry to assess the possible involvement of several seizure-related genes and proteins. Peripheral tissue contractile response of WT and Sig1R-/- mice was studied in an isolated vasa deferentia model. The most important finding was the significantly decreased expression of the R2 subunit of the GABA-B receptor in the hippocampus and habenula of Sig1R-/- mice. Our results demonstrated that Sig1R-/- mice have decreased thresholds for PTZ- and BIC-induced tonic seizures. In the NE-100-induced seizure model, Sig1R-/- animals demonstrated lower seizure scores, shorter durations and increased latency times of seizures compared to WT mice. Sig1R-independent activities of NE-100 included downregulation of the gene expression of iNOS and GABA-A γ2 and inhibition of KCl-induced depolarization in both WT and Sig1R-/- animals. In conclusion, the results of this study indicate that the lack of Sig1R resulted in decreased expression of the R2 subunit of the GABA-B receptor and increased susceptibility to seizures. Our results confirm that Sig1R is a significant molecular target for seizure modulation and warrants further investigation for the development of novel anti-seizure drugs.

Fenfluramine acts as a positive modulator of sigma-1 receptors

OBJECTIVE

Adjunctive fenfluramine hydrochloride, classically described as acting pharmacologically through a serotonergic mechanism, has demonstrated a unique and robust clinical response profile with regard to its magnitude, consistency, and durability of effect on seizure activity in patients with pharmacoresistant Dravet syndrome. Recent findings also support long-term improvements in executive functions (behavior, emotion, cognition) in these patients. The observed clinical profile is inconsistent with serotonergic activity alone, as other serotonergic medications have not been demonstrated to have these clinical effects. This study investigated a potential role for σ1 receptor activity in complementing fenfluramine's serotonergic pharmacology.

METHODS

Radioligand binding assays tested the affinity of fenfluramine for 47 receptors associated with seizures in the literature, including σ receptors. Cellular function assays tested fenfluramine and norfenfluramine (its major metabolite) activity at various receptors, including adrenergic, muscarinic, and serotonergic receptors. The σ1 receptor activity was assessed by the mouse vas deferens isometric twitch and by an assay of dissociation of the σ1 receptor from the endoplasmic reticulum stress protein binding immunoglobulin protein (BiP). In vivo mouse models assessed fenfluramine activity at σ1 receptors in ameliorating dizocilpine-induced learning deficits in spatial and nonspatial memory tasks, alone or in combination with the reference σ1 receptor agonist PRE-084.

RESULTS

Fenfluramine and norfenfluramine bound ≥30% to β2-adrenergic, muscarinic M1, serotonergic 5-HT1A, and σ receptors, as well as sodium channels, with a Ki between 266 nM (σ receptors) and 17.5 μM (β-adrenergic receptors). However, only σ1 receptor isometric twitch assays showed a positive functional response, with weak stimulation by fenfluramine and inhibition by norfenfluramine. Fenfluramine, but not the 5-HT2C agonist lorcaserin, showed a positive modulation of the PRE-084-induced dissociation of σ1 protein from BiP. Fenfluramine also showed dose-dependent antiamnesic effects against dizocilpine-induced learning deficits in spontaneous alternation and passive avoidance responses, which are models of σ1 activation. Moreover, low doses of fenfluramine synergistically potentiated the low-dose effect of PRE-084, confirming a positive modulatory effect at the σ1 receptor. Finally, all in vivo effects were blocked by the σ1 receptor antagonist NE-100.

SIGNIFICANCE

Fenfluramine demonstrated modulatory activity at σ1 receptors in vitro and in vivo in addition to its known serotonergic activity. These studies identify a possible new σ1 receptor mechanism underpinning fenfluramine's central nervous system effects, which may contribute to its antiseizure activity in Dravet syndrome and positive effects observed on executive functions in clinical studies.

Effects of sigma ligands on the cloned mu-, delta- and kappa-opioid receptors co-expressed with G-protein-activated K+ (GIRK) channel in Xenopus oocytes

1. Taking advantage of the functional coupling of the opioid receptors with the G-protein-activated K+ (GIRK) channel, we investigated the effects of sigma (sigma) ligands of various structural and pharmacological classes, (+)-N-allylnormetazocine ((+)-SKF10047) and (+)-cyclazocine, (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3PPP), 1,3-di-(2-tolyl)guanidine (DTG), carbetapentane and haloperidol, on the inward K+ current responses in Xenopus oocytes co-injected with each of the cloned mu-, delta- and kappa-opioid receptor mRNAs and the GIRK1 mRNA. 2. (+)-SKF10047 acted as a delta- and kappa-agonist (EC50 values (microM) = 0.618 and 0.652, respectively) and mu-antagonist (IC50 value (microM) = 8.51). (+)-Cyclazocine acted as a kappa-agonist and mu-antagonist (IC50 = 33.2). (+)-3PPP acted as a kappa-agonist (EC50 = 18.08 and a mu-antagonist. DTG acted as a mu- and kappa-agonist (EC50 = more than 30 and 14.88, respectively). Carbetapentane acted as a kappa-agonist and mu-antagonist (IC50 = 11.2). Haloperidol acted as a mu- and delta-agonist (EC50 = 5.683 and 7.389, respectively). 3. All currents induced by sigma ligands were reduced by 1 microM naloxone, an opioid receptor antagonist, and blocked by 300 microM Ba2+, a GIRK channel blocker. It was also indicated that the antagonism by naloxone at the delta-- and kappa-opioid receptors was weaker than that of naloxone at the mu-opioid receptor. The sigma ligands tested had no effect on the current responses in the oocytes injected with each of the opioid receptor mRNAs alone or with the GIRK1 mRNA alone. 4. We conclude that various sigma ligands directly interact with the cloned mu-, delta- and kappa-opioid receptors in Xenopus oocytes. Our results suggest that the effects of the sigma ligands may be partly mediated by the opioid receptors.

Apparent desensitization of a sigma receptor sub-population in neonatal rat cardiac myocytes by pre-treatment with sigma receptor ligands

sigma Receptor ligands induce marked effects on contractility in cardiac myocytes from neonatal and adult rats (Ela et al., 1994, J. Pharmacol. Exp. Ther. 269, 1300-1309; Novakova et al., 1995, Eur. J. Pharmacol. 286, 19-30). Augmentation or attenuation of the contractile amplitude was observed under different experimental conditions. Preincubation of neonatal cardiomyocytes with a sigma receptor ligand ((+)-(3-hydroxyphenyl)-N-(1-propyl)-piperidine ((+)-3PPP), (+)-pentazocine, or haloperidol) changed the response to re-application of the ligand after cell wash. The inhibitory effect was abolished, while the stimulatory effect became much more pronounced. We suggest that the effects of sigma receptor ligands are mediated via two receptor subtypes, one stimulatory and the other inhibitory, and only the inhibitory subtype is subject to desensitization.

Involvement of sigma-receptors in the increase in contraction of mouse vas deferens induced by exogenous ATP

The effects of sigma-receptor ligands on the twitch contraction elicited by the exogenous application of adenosine 5'-triphosphate (ATP) in the unstimulated mouse vas deferens were studied. (-)-Pentazocine, 1,3-di(2-tolyl)guanidine(DTG) and two pairs of optical isomers of 3-(3-hydroxyphenyl)-N-(1-propyl)piperidine(3-PPP) and N-allylnormetazocine (SKF-10,047) potentiated the exogenous application of ATP-induced twitch-type contraction in a concentration-dependent manner, while (+)-pentazocine did not affect it. The order of potentiating ability was: (+)-3-PPP > (-)pentazocine > (-)-SKF-10,047> DTG > (-)-3-PPP > (+)-SKF-10,047. On the other hand, haloperidol and rimcazole, putataive sigma-receptor antagonists, suppressed this twitch contraction. In addition, these antagonists significantly blocked the (+)-3-PPP- and (-)-pentazocine-induced potentiation at concentrations which did not affect contractions per se. These findings indicate that the exogenous application of ATP-induced twitch contraction in the mouse vas deferens is regulated by sigma-receptors. In addition, the present ranking order suggests that the sigma-receptor potentiating the ATP-induced twitch contraction at post-junctional sites may differ from the sigma 1- and/or sigma 2-receptor subtypes.

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.

Characterization of the currents induced by sigma ligands in NCB20 neuroblastoma cells

Electrical and pharmacological properties of currents induced by compounds having affinities for putative sigma receptors were investigated with NCB20 cells by use of the whole-cell patch-clamp technique. Antipsychotics and naloxone induced inward currents with a decrease in membrane conductance at a holding potential of -60 mV. The rank order of potency for compounds inducing these currents was bromperidol > haloperidol > mosapramine = clocapramine > carpipramine > chlorpromazine > remoxipride > naloxone. Sulpiride, which does not have affinity for sigma receptors, induced inward currents only slightly. Haloperidol-induced currents were not affected by the pretreatments with 10 microM of sulpiride, dopamine, atropine, N-methyl-D-aspartate, 2-amino-7-phosphonoheptanoic acid, morphine or A23187, 100 nM of ICS 205-930, 100 microM of forskolin, 1 microM of phorbol-12,13-dibutyrate, or 100 ng/ml of cholera or pertussis toxins. The reversal potential of the currents induced by haloperidol, naloxone or remoxipride was dependent on the concentration of external or internal potassium. These results indicate that the currents induced by the tested compounds are due to blockade of tonic, outward potassium currents and suggest that these agents act on putative sigma receptors and that the second messenger systems within the cell are not essential for the coupling between the receptors and the channels.

Is the sigma 2 receptor in rat brain related to the K+ channel of class III antiarrhythmic drugs?

The sigma 2 receptor subtype was studied in rat cerebral cortex and in C6 glioma cells homogenates using various compounds including class III antiarrhythmic drugs. The characteristics of (+)-[3H]-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-[3H]-3-PPP) binding were assessed in competition experiments with different displacers which revealed the presence of sigma 2 receptors. Various class III antiarrhythmic drugs inhibited (+)-[3H]-3-PPP binding with high affinity and their binding affinity was found to correlate with the potency of these compounds to increase the duration of action potentials measured in Purkinje fibers in electrophysiological studies. Since class III antiarrhythmic drugs are known to interact with voltage-dependent K+ channels, the present results provide evidence that the (+)-[3H]-3-PPP binding sites in rat brain possess the characteristics of K+ channels of class III antiarrhythmic drugs.

The sigma receptor ligand 1,3-di(2-tolyl)guanidine is anticonvulsant in the rat prepiriform cortex

Unilateral focal injection of 1,3-di(2-tolyl)guanidine (DTG) caused a dose-dependent and potent (ED50 = 5.25 nmol, 95% confidence limits 1.1 to 25.0 nmol) suppression of generalized motor seizures induced by (-)-bicuculline methiodide in the rat prepiriform cortex. These findings indicate that DTG is equipotent to the noncompetitive NMDA receptor antagonist MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate) as an anticonvulsant. This potent pharmacological effect of DTG distinguishes it from two other prototypic sigma ligands, haloperidol and (+)-pentazocine, which are ineffective as anticonvulsants. Pretreatment of animals with haloperidol failed to block the anticonvulsant effects of DTG. These data therefore document a novel anticonvulsant action of DTG in vivo by a mechanism that does not involve sigma receptors.

Correlation between potentiation of neurogenic twitch contraction and benzomorphan sigma receptor binding potency in the mouse vas deferens

The effects of sigma receptor ligands on the neurogenic twitch contraction in the ddY mouse vas deferens were studied. In functional studies, (+)-N-allylnormetazocine ((+)-SKF-10,047) and (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3-PPP) potentiated neurogenic twitch contractions in a concentration-dependent manner. The potentiation by each (+) enantiomer was significantly more potent than that by the respective (-) enantiomer. In addition, haloperidol and (+/-)-pentazocine also potentiated neurogenic twitch contractions. The order of potentiating ability was: haloperidol > (+/-)-pentazocine > (+)-3-PPP > (-)-3-PPP > (+)-SKF-10,047 > (-)-SKF-10,047. In contrast, other sigma receptor ligands, 1,3-di(2-tolyl)guanidine (DTG) and rimcazole, suppressed this twitch contraction. In addition, rimcazole significantly antagonized the (+)-SKF-10,047-induced potentiation at concentrations which did not affect contractions per se. Furthermore, binding studies showed that the kinetic parameters and the inhibitory potencies of sigma receptor ligands for the binding of [3H](+)-SKF-10,047 in the mouse vas deferens were similar to those in the guinea pig brain. The order of potency of sigma receptor ligands to potentiate the neurogenic twitch contraction in the mouse vas deferens was significantly correlated with the potency to inhibit [3H](+)-SKF-10,047 binding in both mouse vas deferens and guinea pig brain. These results indicate that sigma receptor ligands regulate the neurogenic twitch contraction, which is mediated by rimcazole-sensitive benzomorphan-type sigma receptors.

The sigma ligand 1,3-di-o-tolylguanidine depresses amino acid-induced excitation non-selectively in rat brain

The sigma ligand 1,3-di-o-tolylguanidine (DTG) has been applied by microiontophoresis to neurones in the rat hippocampal slice and to neurones in the neocortex and hippocampus of rats anaesthetised with urethane. DTG depressed the excitatory responses of cells to both N-methyl-D-aspartate (NMDA) and quisqualate on a majority of the units tested, in no case causing an enhancement. Haloperidol had no consistent effect of its own and did not prevent the depressant effects of DTG. It is concluded that in the preparations used, DTG did not selectively modify neuronal sensitivity to NMDA.

Lack of correlation between sigma binding potency and inhibition of contractions in the mouse vas deferens preparation

The existence of sigma receptors in the mouse, rat and guinea pig vasa deferentia has previously been proposed, although drug effects are inconsistent and generally occur only at high concentrations. The purpose of the present study was to evaluate lower, physiologically relevant concentrations of ligands for possible sigma effects on electrically stimulated twitch contractions in the mouse vas deferens (MVD). Putative sigma agonists and antagonists all inhibited 0.1 Hz electrically stimulated twitch contractions in nM concentrations. Inhibitory activity plateaued between 20 and 60% for all compounds except 1,3-di(2-tolyl)guanidine (DTG), which had a shallow concentration-effect curve. Subsequent to the plateau, higher concentrations (30 microM) of rimcazole and haloperidol fully inhibited electrically stimulated twitch contractions. There was no correlation between inhibitory potency or maximal effect in the MVD and binding potency at sigma sites in either MVD or guinea pig brain. The inhibitory effects of R(+)-3-(3-hydroxyphenyl)-N-1-propylpiperidine ((+)3-PPP) or DTG on electrically stimulated twitch contractions were not antagonized by the putative sigma antagonists DTG, haloperidol, rimcazole or BMY-14802, nor by alpha 2-adrenoceptor, dopamine D1, dopamine D2 or opiate antagonists. Although the mechanism of sigma ligand effects in the MVD has not been established, the data caution against a presumption that effects of sigma ligands on electrically stimulated twitch contractions in this preparation are mediated by sigma receptors.

Focal stimulation of specific pathways in the rat hippocampus causes a reduction in radioligand binding to the haloperidol-sensitive sigma receptor

Focal electrical stimulation of selected excitatory pathways in the hippocampal slice caused a decrease in the binding of [3H]-1,3-di(2-tolyl)guanidine (DTG) or [3H]-(+)-3-[hydroxyphenyl]-N-(1-propyl)piperidine [( 3H )-(+)3-PPP) to haloperidol-sensitive sigma binding sites in the slice. Activation of the mossy fibers or perforant path by high frequency electrical stimulation caused the reduction in [3H]-DTG binding; whereas activation of fibers in the strata radiatum, lacunosum-moleculare, alveus, or oriens did not affect [3H]-DTG binding. The decrease in binding observed was calcium-dependent and tetrodotoxin sensitive and varied with the frequency, intensity, and duration of stimulation. Although haloperidol-sensitive [3H]-DTG binding sites are distributed throughout the hippocampus, stimulation of the perforant path or mossy fibers resulted in a significant reduction in binding only in the dentate region of the slice. The decrease in binding following perforant path stimulation was blocked by the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX); whereas the decrease in binding caused by mossy fiber stimulation was not affected by CNQX or DL-APV. The results obtained support the hypothesis that activation of the granule cells in the hippocampal slice caused the release of an endogenous ligand which acts at the haloperidol-sensitive sigma binding site in the dentate gyrus.

The lack of utility of the rat vas deferens as a functional bioassay for sigma ligands

The present study examined the utility of the rat vas deferens preparation as a bioassay for sigma site ligands. sigma Ligands such as (+/-)-pentazocine, phencyclidine (PCP) and (+)-SK&F 10047 potentiated neurogenic twitch contractions. However, neither the order of potency nor the absolute potency of (+/-)-pentazocine and (+)-SK&F 10047 correlated with their affinity at central sigma sites. Furthermore, another potent sigma ligand, ditolyl-ortho guanidine (DTG) neither affected neurogenic twitch contractions nor inhibited twitch potentiation by PCP or (+)-SK&F 10047 at concentrations up to 30 mumol/l. These data indicate that the rat vas deferens is not a useful bioassay for the evaluation of sigma ligands. PCP, (+)-SK&F 10047 and (+/-)-pentazocine probably enhance neurogenic contractions in rat vas deferens primarily by inhibition of the neuronal uptake of noradrenaline.

Interactions of N-cyclopropylmethyl(-)-6 beta-acetylthionormorphine with mu-, kappa-, delta- and sigma-opioid receptors

1. Affinities of N-cyclopropylmethyl(-)-6 beta-acetylthionormorphine (KT-90) to mu-, kappa-, delta- and sigma-receptors were tested in rat brain membrane fractions using radioligand-receptor assays. 2. Though KT-90 had nonselectively high affinities to mu-, kappa- and delta-receptors, affinity of KT-90 to sigma-receptors was lower than those to the other three receptors. 3. Affinity of KT-90 to sigma-receptors was 1000 times lower than that of buprenorphine.

The sigma [corrected] ligand rimcazole antagonises (+)SKF 10,047, but not (+)3-PPP, in the mouse isolated vas deferens

We have characterized the actions of several sigma receptor ligands on the electrically evoked, neurogenic contractions of the mouse isolated vas deferens. (-)SKF 10,047 was significantly more potent than (+)SKF 10,047 in potentiating twitch contractions and was equipotent with (+)3-PPP. Rimcazole (1 and 3 microM) antagonised the potentiation induced by 100 microM (+)SKF 10,047 and, to a lesser extent, that induced by 30 microM (-)SKF 10,047 but increased that elicited by (+)3-PPP (30 microM). This apparent contradiction may arise from sigma agonists acting in this tissue at both sigma and non-sigma sites.

Inhibition of potassium currents by the sigma receptor ligand (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine in sympathetic neurons of the mouse isolated hypogastric ganglion

The actions of (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine [(+)3-PPP] on sympathetic neurons of the mouse isolated hypogastric ganglion were studied using the current clamp and single electrode voltage clamp techniques. In neurons studied under current clamp (+)3-PPP (10(-5) to 3 x 10(-4) M) evoked a concentration-dependent depolarization, which was fully reversible on washout of the drug. The depolarization was associated with an increase in membrane input resistance. At membrane potentials between -43 and -65 mV the amplitude of the depolarization was inversely related to the membrane potential. (+)3-PPP had no effect on membrane potential at potentials negative to -65 mV. The effect of (+)3-PPP on the M-current was studied in cells voltage clamped at -40 mV and stepped to -60 mV for 300-500 ms. The slow current relaxations seen during and after the voltage step are largely due to the M-current. (+)3-PPP (3 x 10(-5) to 3 x 10(-4) M) inhibited the M-current and produced an inward current in a concentration-dependent manner. (-)3-PPP (3 x 10(-5) M) had similar effects, but was less potent than (+)3-PPP. (+)3-PPP (3 x 10(-5) M) also inhibited the A-current and a calcium-dependent potassium current, but to a lesser degree than the M-current.(ABSTRACT TRUNCATED AT 250 WORDS)