[3H]DTG and [3H](+)-3-PPP label pharmacologically distinct sigma binding sites in guinea pig brain membranes

Allosteric Modulators of Sigma-1 Receptor: A Review

Allosteric modulators of sigma-1 receptor (Sig1R) are described as compounds that can increase the activity of some Sig1R ligands that compete with (+)-pentazocine, one of the classic prototypical ligands that binds to the orthosteric Sig1R binding site. Sig1R is an endoplasmic reticulum membrane protein that, in addition to its promiscuous high-affinity ligand binding, has been shown to have chaperone activity. Different experimental approaches have been used to describe and validate the activity of allosteric modulators of Sig1R. Sig1R-modulatory activity was first found for phenytoin, an anticonvulsant drug that primarily acts by blocking the voltage-gated sodium channels. Accumulating evidence suggests that allosteric Sig1R modulators affect processes involved in the pathophysiology of depression, memory and cognition disorders as well as convulsions. This review will focus on the description of selective and non-selective allosteric modulators of Sig1R, including molecular structure properties and pharmacological activity both in vitro and in vivo, with the aim of providing the latest overview from compound discovery approaches to eventual clinical applications. In this review, the possible mechanisms of action will be discussed, and future challenges in the development of novel compounds will be addressed.

Differences in the allosteric modulation by phenytoin of the binding properties of the σ1 ligands [3H](+)-pentazocine and [3H]NE-100

The present study evaluated the effects of phenytoin (DPH) on the binding to synaptosomal fraction membranes from guinea pig brain of the prototypic sigma1 (sigma1) receptor agonist [3H](+)-pentazocine and the putative sigma1 antagonist [3H]NE-100. Equilibrium and binding kinetics studies were done. The order of affinity of 12 sigma1 ligands for binding sites labeled with [3H](+)-pentazocine correlated well with their order of affinity for sites labeled with [3H]NE-100, suggesting that both radioligands label the same receptor. Phenytoin increased the binding of [3H](+)-pentazocine, enhancing its affinity (K(D) value) for sigma1 receptors and decreasing its dissociation rate from these receptors. The maximal number of receptors (B(max) value) labeled with [3H](+)-pentazocine was not changed. In contrast, phenytoin decreased the specific binding and maximal number of receptors labeled with [3H]NE-100, and increased its dissociation rate from sigma1 receptors. The affinity of this radioligand for sigma1 receptors was not modified. In conclusion, phenytoin behaved as a positive allosteric modulator on the binding of [3H](+)-pentazocine, whereas it negatively modulated the binding of [3H]NE-100. These results add evidence in favor of the use of phenytoin in vitro to distinguish between agonists and antagonists of sigma1 receptors.

Molecular modeling of sigma 1 receptor ligands: a model of binding conformational and electrostatic considerations

We have performed molecular modeling studies on several sigma 1 specific ligands, including PD144418, spipethiane, haloperidol, pentazocine, and others to develop a pharmacophore for sigma 1 receptor-ligand binding, under the assumption that all the compounds interact at the same receptor binding site. The modeling studies have investigated the conformational and electrostatic properties of the ligands. Superposition of active molecules gave the coordinates of the hypothetical 5-point sigma 1 pharmacophore, as follows: R1 (0.85, 7.26, 0.30); R2 (5.47, 2.40, -1.51); R3 (-2.57, 4.82, -7.10); N (-0.71, 3.29, -6.40); carbon centroid (3.16, 4.83, -0.60), where R1, R2 were constructed onto the aromatic ring of each compound to represent hydrophobic interactions with the receptor; and R3 represents a hydrogen bond between the nitrogen atom and the receptor. Additional analyses were used to describe secondary binding sites to electronegative groups such as oxygen or sulfur atom. Those coordinates are (2.34, 5.08, -4.18). The model was verified by fitting other sigma 1 receptor ligands. This model may be used to search conformational databases for other possibly active ligands. In conjunction with rational drug design techniques the model may be useful in design and synthesis of novel sigma 1 ligands of high selectivity and potency. Calculations were performed using Sybyl 6.5.

Binding of [3H]desglycinyl remacemide to rat brain membranes: association with the benzomorphan attachment site of the N-methyl-D-aspartic acid receptor channel

Desglycinyl remacemide (DGR), a biologically active metabolite of remacemide, was radiolabeled in an attempt to develop a ligand binding assay to identify its site of action. Incubation of the radioligand with membranes obtained from P2 fractions of whole rat brain revealed a single population of specific [3H]-DGR binding sites having a Kd of 290 nM and a Bmax of 1.3 pmole/mg protein. The specific binding of [3H]-DGR is most enriched in the P2 subcellular fraction and is heterogeneously distributed throughout the brain. The binding of [3H]-DGR to rat brain membranes was inhibited most potently by MK-801 and SKF-10,047. In contrast, haloperidol, and other sigma receptor-active agents, were relatively inactive at this site. These data suggest that DGR interacts with a channel blocking site on the NMDA receptor.

Relationship between modulation of the cerebellorubrospinal system in the in vitro turtle brain and changes in motor behavior in rats: effects of novel sigma ligands

Saturation and competition binding studies showed that the turtle brain contains sigma sites labeled by both [3H]di-o-tolylguanidine (DTG) and [3H](+)-pentazocine. There was a significant correlation between the IC50 values of sigma ligands for [3H]DTG sites in the turtle vs. rat brain, suggesting that the sites are comparable in the two species. In contrast, [3H](+)-pentazocine, which primarily labels sigma1 sites in the rodent brain, labels a heterogeneity of sites in the turtle brain. In extracellular recordings from the in vitro turtle brainstem, some sigma ligands enhanced the burst responses of red nucleus (RN) neurons (DTG, haloperidol, BD1031, BD1052, BD1069) while other sigma ligands decreased the burst responses (BD1047, BD1063). Control compounds (turtle Ringer vehicle control, opiate antagonist naloxone, atypical neuroleptic sulpiride) had no significant effects on the RN burst responses recorded from the in vitro turtle brain. The ED50s of the ligands for altering the burst responses in RN neurons from the turtle brain were correlated with their IC50s for turtle brain sites labeled with [3H]DTG, but not [3H](+)-pentazocine; this pattern is identical to that previously reported in rats, where there is a correlation between the potencies of sigma ligands for producing dystonic postures after microinjection into the rat RN and their binding to rat brain sites labeled with [3H]DTG, but not [3H](+)-pentazocine. When the novel sigma ligands were microinjected into the rat RN, dystonic postures were produced by ligands that increased the burst duration of RN neurons in the turtle brain. Novel sigma ligands that reduced the burst responses in the in vitro turtle brain have previously been reported to have no effects on their own when microinjected into the rat RN, but to block the dystonic postures produced by other sigma ligands. Taken together, the data suggest that the opposite effects of the novel ligands in the turtle electrophysiological studies represent the actions of agonists vs. antagonists, and that the directionality of the effects has predictive value for the expected motor effects of the drugs.

Purification and characterization of the human SR 31747A-binding protein. A nuclear membrane protein related to yeast sterol isomerase

SR 31747A, defined as a sigma ligand, is a novel immunosuppressive agent that blocks proliferation of human and mouse lymphocytes. Using a radiolabeled chemical probe, we here purified a target of SR 31747A and called it SR 31747A-binding protein (SR-BP). Purified SR-BP retained its binding properties and migrated on SDS-polyacrylamide gel as a Mr 28,000 protein. Cloning of the cDNA encoding human SR-BP shows an open reading frame for a 223-amino acid protein, which is homologous to the recently cloned sigma 1 receptor. Interestingly, the deduced amino acid sequence was found to be related to fungal C8-C7 sterol isomerase, encoded by the ERG2 gene. The ERG2 gene product has been identified recently as the molecular target of SR 31747A that mediates antiproliferative effects of the drug in yeast. Northern blot analysis of SR-BP gene expression revealed a single transcript of 2 kilobases which was widely expressed among organs, with the highest abundance in liver and the lowest abundance in brain. Subcellular localization analysis in various cells, using a specific monoclonal antibody raised against SR-BP, demonstrated that this protein was associated with the nuclear envelope. When studying the binding of SR 31747A on membranes from yeast expressing SR-BP, we found a pharmacological profile of sigma 1 receptors; binding was displaced by (+)-pentazocine, haloperidol, and (+)-SKF 10,047, with (+)-SKF 10, 047 being a more potent competitor than (-)-SKF 10,047. Scatchard plot analysis revealed Kd values of 7.1 nM and 0.15 nM for (+)-pentazocine and SR 31747A, respectively, indicating an affinity of SR-BP 50-fold higher for SR 31747A than for pentazocine. Additionally, we showed that pentazocine, a competitive inhibitor of SR 31747A binding, also prevents the immunosuppressive effect of SR 31747A. Taken together, these findings strongly suggest that SR-BP represents the molecular target for SR 31747A in mammalian tissues, which could be critical for T cell proliferation.

N-(N-benzylpiperidin-4-yl)-2-[18F]fluorobenzamide: a potential ligand for PET imaging of sigma receptors

Four nitro- and fluorobenzamides (1-4) have been synthesized in good yields from nitro- and fluoro-substituted benzoyl chloride with 4-amino-1-benzylpiperidine. In vitro studies showed that these compounds have high affinities to sigma receptors. N-(N-Benzylpiperidin-4-yl)-2-fluorobenzamide (3), in particular, bound to sigma receptors with high affinity (Ki = 3.4 nM, guinea pig brain membranes) and high selectivity (sigma-2/sigma-1 = 120). It was, therefore, labeled with 18F and evaluated as a sigma receptor radioligand. N-(N-Benzylpiperidin-4-yl)-2-[18F]fluorobenzamide (3a) was synthesized in one step by nucleophile substitution of the 2-nitro precursor (1) with [18F]fluoride in DMSO at 140 degrees C for 20 min followed by purification with HPLC in 4-10% yield (decay corrected). The synthesis time was 90 min and the specific activity was 0.4-1.0 Ci/mumol. Tissue distribution in mice revealed that the uptakes of 3a in the brain, heart, liver, lungs, spleen, kidneys and small intestine were high, and the radioactivity in these organs remained constant from 60 to 120 min post-injection. The radioactivity in the bone did not significantly increase, suggesting in vivo defluorination may not be the major route of metabolism of 3a in mice. Blocking studies with haloperidol in rats indicated that the uptake of compound 3a in the rat brain was selective to haloperidol-sensitive sigma sites. These results suggest that compound 3a is a potent sigma receptor radioligand and may be a potential ligand for PET imaging of sigma receptors in humans.

2(3H)-benzoxazolone and 2(3H)-benzothiazolone derivatives: novel, potent and selective sigma1 receptor ligands

A series of original 2(3H)-benzoxazolone and 2(3H)-benzothiazolone derivatives were evaluated for their affinity at sigma1 and sigma2 receptor subtypes in competition binding experiments, using [3H](+)-pentazocine or [3H]1,3-di-o-tolyl-guanidine (DTG) in the presence of 100 nM (+)-N-allylnormetazocine (NANM) in guinea-pig brain membranes. Several of these derivatives showed preferential selectivity for sigma1 binding sites. Compound 1 [3-(1-piperidinoethyl)-6-propylbenzothiazolin-2-one] emerged as a potent sigma1 receptor ligand (Ki = 0.6 nM) and displayed a moderate selectivity over the sigma2 receptor subtype (Ki for sigma2/Ki for sigma1 = 29). Compounds 2 [3-(1-piperidinopropyl)-6-propanoylbenzothiazolin-2-one] and 3 [3-(1-piperidinopropyl)-6-propanoylbenzoxazolin-2-one] still showed rather high affinities for sigma1 binding sites with Ki values of 2.3 and 8.5 nM, respectively. On the contrary, they had 87- and 58-fold less affinity at sigma2 receptors, respectively. Unlike their potent affinity for sigma binding sites, these compounds had negligible affinity for mu-, delta- and kappa-opioid receptors, 5-HT2, dopamine D2, and muscarinic M2 receptors. Sigma receptor ligands may affect neuronal transmission and display, in animal models, antipsychotic, cognitive, motor, neuroprotective and anticonvulsant activity. Therefore, on the basis of these findings, these novel sigma receptor ligands were assayed, in mice, in three tests: maximal electroshock, subcutaneous pentylenetetrazol and rotarod neurotoxicity. Compound 1, administered intraperitoneally, was the most effective against maximal electroshock-induced seizures and was devoid of significant neurotoxic effects.

Effect of short-term and long-term treatments with sigma ligands on the N-methyl-D-aspartate response in the CA3 region of the rat dorsal hippocampus

1. Long-term treatments with the sigma ligand haloperidol decrease the density of sigma receptors in mammalian CNS. We have shown that sigma ligands, such as di(2-tolyl)guanidin (DTG), potentiate dose-dependently, with bell-shaped dose-response curves, the neuronal response of pyramidal neurones to N-methyl-D-aspartate (NMDA) in the CA3 region of the rat dorsal hippocampus. sigma Ligands producing such a potentiation were denoted 'agonists'. This potentiation was suppressed by low doses of other sigma ligands denoted 'antagonists'. High doses of DTG and JO-1784 did not modify the NMDA response but acted as 'antagonists' by suppressing the potentiation induced by sigma 'agonists'. 2. Following a 21-day treatment with haloperidol as well as with high doses of DTG or JO-1784, after a 48 h washout, the acute administration of sigma 'agonists' failed to induce any potentiation of the NMDA response. Following a 21 day treatment with a low dose of DTG or JO-1784, after a 48 h washout, the neuronal response to microiontophoretic applications of NMDA was markedly increased. A 21 day treatment with low or high doses of (+)-pentazocine, after a 48 h washout, did not produce any change. 3. Following a two day treatment with a high dose of haloperidol, DTG, JO-1784 and (+)-pentazocine, after a 24 h washout, the potentiation of the NMDA response induced by the acute administration of the sigma 'agonists' was unchanged. 4. With the minipumps on board, with DTG and JO-1784, a dose-dependent enhancement of the NMDA response was seen but no effect was observed in the groups of rats treated at the same doses with haloperidol or (+)-pentazocine. 5. The present data suggest that long-term treatments with sigma 'antagonists' induce a desensitization of the th receptors, whereas long-term treatments with th 'agonists' induce a supersensitivity of the th receptors.

Differential localization of three distinct binding sites for sigma receptor ligands in rat spleen

In vitro receptor autoradiography was used to localize sigma 1 receptors, sigma 2 receptors, and novel haloperidol/DTG-inaccessible sites for sigma and opiate ligands in rat spleen. Sigma-1 receptors were present throughout the spleen, but were most concentrated in the T cell zones. Binding under "sigma 2 receptor-selective' conditions was 70% nonspecific, and sigma 2 receptors could not be detected. Haloperidol/DTG-inaccessible sites had a coarse, punctate distribution in the red pulp and marginal zones of the white pulp. This anatomical localization suggests types of cells and functions that should be examined for modulation by sigma receptors.

Basis of the antiseizure action of phenytoin

1. Phenytoin has been used with much clinical success against all types of epileptiform seizures, except petit mal epilepsy, for over 50 years. Its mechanism of action, however, is still open to interpretation. 2. Several potential targets for phenytoin action have been identified within the central nervous system. These include the Na-K-ATPase, the GABAA receptor complex, ionotropic glutamate receptors, calcium channels and sigma binding sites. 3. To date, though, the best evidence hinges on the inhibition of voltage-sensitive Na+ channels in the plasma membrane of neurons undergoing seizure activity. Quieter nerve cells are far less affected. Moreover, the fact that phenytoin also has important cardiac antiarrhythymic effects and can inhibit Na+ influx into cardiac cells supports the idea that the primary target of phenytoin is, indeed, the Na+ channel.

[3H]1,3-di(2-tolyl) guanidine binds to a sigma 2 receptor on Jurkat cell membranes, but sigma compounds fail to influence immunomodulatory events in human peripheral blood lymphocytes

The binding characteristics of the sigma ligand [3H]1.3-di(2-tolyl)guanidine (DTG) were investigated in membranes prepared from the Jurkat T cell line. Binding was saturable with a KD of 56 +/- 3 nM and a Bmax of 11706 +/- 3173 fmol/mg protein (n = 3). The rank order of potency for sigma reference compounds to inhibit binding in the Jurkat cell line was ifenprodil > 1,3-di(2-tolyl)guanidine > haloperidol > carbetapentane > (+)3-(3-hydroxyphenyl)-N-propylpiperidine ((+)3-PPP) > (-)pentazocine > caramiphen > (+)pentazocine, and significantly correlated with potency at sigma 2 binding sites in guinea pig brain (r = 0.90, p < 0.01). The immunomodulatory activities of the sigma ligands 1,3-di(2-tolyl)guanidine, haloperidol. (-)pentazocine and (+)pentazocine on CD3-induced proliferation, IL-2 production and Ca2+ flux in human lymphocytes did not reveal any consistent pharmacology that could be ascribed to potency of these compounds at sigma binding sites. Collectively the data demonstrate that the [3H]1,3-di(2-tolyl)guanidine binding site on Jurkat cell membranes has a pharmacology consistent with sigma receptors, but no modulation of functional activity or intracellular events in human peripheral blood lymphocytes correlating with sigma receptors was found.

RU 3117 a steroidal compound with high affinity for sigma sites in rat testis membranes

RU 3117 belongs to a new series of steroids which exhibited a high relative binding affinity (RBA) for (+)[3H]PPP sites in rat testis membranes; its RBA was about 40 times higher than that of progesterone. Furthermore, it is devoid of any binding to classical steroid receptors; therefore in order to study its binding parameters on rat testis membranes it was tritiated. [3H]RU 3117 bound at least two distinct sites with Ka values of 0.4 +/- 0.06 x 10(9) M(-1) and 1.3 +/- 0.2 x 10(7) M(-1). Using this marker, competition studies with cold haloperidol showed that a part of this binding was haloperidol-sensitive, whereas another part was haloperidol-resistant. Interestingly, progesterone described as a sigma ligand competes with [3H]RU 3117 binding, with an RBA of 1.6%. When haloperidol was preincubated (250 nM) with rat testis membranes, in order to mask the sigma sites, we observed that DTG (1,3-di-O-tolylguanidine) and haloperidol displayed a very low RBA (< 0.1%) and were not able totally to displace the [3H]RU 3117 binding up to 50 microM. Furthermore, benztropine exhibited a significant RBA of 19% but its displacement curve showed a plateau (500-50,000 nM). These results showed that part of the haloperidol-resistant sites was benztropine sensitive but another part was displaced neither by haloperidol nor by benztropine. The presence of these remaining binding sites was confirmed by preincubating a mixture of haloperidol and benztropine with testis membranes. Under these conditions, [3H]RU 3117 displayed a Ka of 1.0 +/- 0.01 x 10(7) M(-1), and we observed that these sites were recognized, up to now, only by the steroids RU 1968 and RU 54173 which are also devoid of any binding to classical nuclear steroid receptors.

Solubilization and characterization of binding sites for [3H]NE-100, a novel and potent sigma 1 ligand, from guinea pig brain

The binding sites for [3H]NE-100, a newly defined sigma 1 ligand, was solubilized from guinea pig brain, using zwitterionic detergent 3-[(3-c holamidopropyl) dimethylamino]-1-propanesulfonate (CHAPS), and the properties of the solubilized binding sites were compared to those for [3H](+)-pentazocine, a selective sigma 1 ligand. The pharmacological selectivity of solubilized sites for both [3H]NE-100 and [3H](+)-pentazocine was identical to that obtained from membrane preparations. Stereoselectivity of benzomorphan such as pentazocine and SKF10,047 was preserved in displacing [3H]NE-100 binding in solubilized preparations as observed in membrane preparations. The inhibitory potencies of several sigma ligands on [3H]NE-100 binding were similar to those on [3H](+)-pentazocine binding, indicating that the pharmacological characteristics of the binding sites for [3H]NE-100 are retained after solubilization. Phenytoin augmented the binding of [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl) piperidine hydrochloride (3-PPP) to solubilized sigma binding sites while it had no effect on the binding of [3H]NE-100. Furthermore, the inhibitory effect of putative sigma receptor agonists such as (+)-3-PPP and dextromethorphan were enhanced by phenytoin; the effects of haloperidol, a putative sigma receptor antagonist, were unaltered. Molecular weight of [3H]NE-100 binding protein was estimated to be 440KDa by Sepharose CL-6B gel filtration chromatography, and the value was identical to that of [3H](+)-pentazocine binding protein, a putative sigma 1 binding protein. These findings indicate that [3H]NE-100 binding sites are putative sigma 1 binding sites, and that NE-100 may act as an antagonist at sigma 1 binding sites.

Kappa opioid receptor agonists inhibit sigma-1 (sigma 1) receptor binding in guinea-pig brain, liver and spleen: autoradiographical evidence

The present study examined whether the kappa-opioid agonists U50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N[-2-(1-pyrrolidinyl)- cyclohexyl]-benzeacetamide methane sulphonate), bremazocine, spiradoline and ICI 197067 bind to sigma sites in guinea-pig tissues using in vitro, semi-quantitative receptor autoradiography and receptor binding, and compared the binding profile so obtained with those for several selective sigma ligands. Guinea-pigs were killed and their brians, livers and spleens were removed, tissue sections cut and processed for sigma binding site autoradiography using (+)-[3H]-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-[3H]-3-PPP), or tissue was wiped and determined by liquid scintillation. Serial slide-mounted sections were incubated with 9-10 concentrations (1 nM-10 microM) of kappa opioids and their potency to inhibit (+)-[3H]-3-PPP binding compared with that of the sigma ligands haloperidol, DTG (1,3 di(o)-tolylguanidine), (+)-3-PPP, (+) and (-)pentazocine, SR 31742A and rimcazole (n = 3, duplicate determinations). Binding of (+)-[3H]-3-PPP to untreated, matched serial tissue sections was used as control. Kd values were estimated in brain, liver and spleen using quantitative, saturation binding analysis, IC50 values were determined from the binding data obtained by slide wiping experiments for each drug, and Ki values were calculated using the Cheng-Prussoff equation. All four kappa opioids inhibited (+)-[3H]-3-PPP binding to sigma 1-receptors with order of potency: brain: U50,488H = spiradoline > bremazocine > ICI 197067; liver: spiradoline > U50,488H > ICI 197067 > bremazocine; spleen: U50,488H > spiradoline > ICI 197067 > bremazocine. By comparison, the sigma ligands inhibited (+)-[3H]-3-PPP binding to matched, serial slide-mounted brain tissue sections (similar results in liver and spleen) with order of potency: SR 31742A > haloperidol > (+)pentazocine > (+)-3-PPP > DTG > (-)pentazocine > rimcazole. (+)-[3H]-3-PPP autoradiography confirmed these binding data. It is concluded that the kappa opioids tested moderately inhibit (+)-[3H]-3-PPP binding to sigma 1-receptors in guinea-pig brain, liver and spleen tissue with Ki values comparable to some selective sigma ligands and therefore are not opioid selective.

Initial characterization and autoradiographic localization of a novel sigma/opioid binding site in immune tissues

High concentrations of novel, haloperidol- and DTG-inaccessible (+)-[3H]-3-PPP binding sites were found in human peripheral blood leukocytes rat spleen and splenocytes, but not in rat brain. Splenic sites were localized in a course punctate pattern in the marginal zones and red pulp. The pharmacology of the splenic sites was: (-)-SKF 10,047 > or = naltrexone = (-)-pentazocine > (+)-pentazocine = (-)-3-PPP = (+)-SKF 10,047 > or = (+)-3-PPP > or = dextrorphan > dextromethorphan > PCP > clorgyline. DTG, haloperidol, TCP, (-)-deprenyl and SKF 525-A did not complete. Binding activity was destroyed by heating and phospholipase C, but not by proteases or glycosidases. These sites may be involved in immunomodulation by opiate and sigma receptor agonists.

Lack of binding of acetaminophen to 5-HT receptor or uptake sites (or eleven other binding/uptake assays)

The mechanism of analgesic action of acetaminophen (paracetamol) remains unknown. However, a central component distinct from that of the NSAIDs (non-steroidal antiinflammatory drugs) seems likely. A recent report (NeuroReport 6:1546-1548, 1995) suggests the involvement of 5-HT3 receptors. In the present study, we measured the affinity of acetaminophen at 5-HT3, as well as 5-HT1A, 5-HT1B, 5-HT1D, 5-HT2, 5-HT2C, 5-HT4, 5-HT6, 5-HT7 and eleven other receptor sites and at serotonin and norepinephrine reuptake sites. At 10 microM, acetaminophen inhibited less than 10% specific radioligand binding at any site. These findings: (i) suggest that acetaminophen's effect on serotonergic pathways is indirect, and (ii) circumscribe acetaminophen's possible central analgesic mechanism(s).

(+)-SKF-10,047 and dextromethorphan ameliorate conditioned fear stress through the activation of phenytoin-regulated sigma 1 sites

Mice exhibited a marked suppression of motility when they were replaced in the same environment in which they had previously received an electric footshock. This psychological stress-induced motor suppression, known as conditioned fear stress, was dose dependently attenuated by (+)-N-allylnormetazocine ((+)-SKF-10,047) and by dextromethorphan, putative sigma receptor agonists, but not by other sigma receptor ligands, (+)-pentazocine and 1,3-di-(2-tolyl)guanidine (DTG). Unlike (+)-SKF-10,047 and dextromethorphan, the non-competitive NMDA receptor antagonists, phencyclidine and dizocilpine, attenuated the conditioned fear stress only at high doses that induced marked hypermotility in non-stressed mice. The effects of (+)-SKF-10,047 and dextromethorphan, but not phencyclidine and dizocilpine, on the conditioned fear stress were antagonized by the sigma receptor antagonists, NE-100 (N,N-dipropyl-2-[4-methoxy-3-(2- phenylethoxy)phenyl]-ethylamine monohydrochloride) and BMY-14802 (alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazine- butanol hydrochloride). Interestingly, the effects of (+)-SKF-10,047 and dextromethorphan on the stress response were enhanced by combination with phenytoin, an anticonvulsant drug, whereas those of (+)- pentazocine, DTG, phencyclidine, and dizocilpine were not. These results suggest that activation of phenytoin-regulated type sigma 1 receptors, but not of phencyclidine receptors, is involved in the ameliorating effects of (+)-SKF-10,047 and dextromethorphan on stress-induced motor suppression.

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.

Modulation of NMDA and dopaminergic neurotransmissions by sigma ligands: possible implications for the treatment of psychiatric disorders

Sigma (sigma) receptors, improperly classified as belonging to the opiate receptor family when discovered in 1976, were subsequently confused with phencyclidine binding sites for several years. It's only recently, with the emergence of new selective ligands that their functional significance could be meaningfully addressed. Several subtypes of sigma receptors are present in high densities in the limbic structures as well as in motor-related areas of the CNS. Different lines of evidence suggest that a major role for sigma receptors might be to regulate the activity of the glutamatergic system via the modulation one of its subtype of receptor, the NMDA receptor. This modulation of the glutamatergic system could in turn interfere with the dopaminergic neurotransmission with which, however, sigma ligands could also interact directly. The potential involvement of sigma receptors in schizophrenia has been considered ever since their discovery. The initial suggestion to this respect emerged from the observation that several of the earliest sigma ligands induced psychotomimetic symptoms such as delusions, hallucinations and depersonalization. This link was later reinforced with the demonstration that several neuroleptics, such as haloperidol, have a high affinity for sigma receptors, whereas, some new molecules with a high affinity for sigma receptors, but a low affinity for dopaminergic receptors demonstrated a "neuroleptic-like" pharmacological profile. However, the therapeutic efficacy of selective sigma ligands in schizophrenia has not yet been established and it has even been suggested that sigma receptors might be responsible for some side effects of the classical neuroleptics. The possible implication of sigma receptors in affective disorders has also been suggested by reports showing that some antidepressant drugs have a high affinity for sigma receptors and that long-term treatments with anti- depressant drugs, even with those devoid of affinity for sigma receptors, modify their binding characteristics. In conclusion, indirect evidence suggests possible etiological and/or therapeutic roles for sigma receptors in some psychiatric disorders. However, despite several attempts, no clear indications of a therapeutic efficacy of sigma ligands has yet emerged. More selective ligands and fundamental studies on the respective role of the different subtypes of sigma receptors are needed before clear concepts can be formulated. p3

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.

Discrepancies in characterization of sigma sites in the mouse central nervous system

The characteristics of [3H](+)-pentazocine and [3H]1,3-di(2-tolyl) guanidine (DTG) binding to mouse whole brain, cortex, cerebellum and spinal cord membranes were investigated in radioreceptor assays. [3H](+)-Pentazocine bound to a single, high affinity site (Kd = 1.2-1.6 nM) with increasing density along the neuraxis from the cortex (Bmax = 543 fmol/mg protein) to the spinal cord (Bmax = 886 fmol/mg protein). Hot saturation studies resolved the presence of one binding site for [3H]DTG showing no tissue variations in terms of density (Bmax = 1075-1264 fmol/mg protein) or affinity (Kd = 16.6-22.3 nM). Incubation with 100 nM (+)-pentazocine revealed two classes of high affinity [3H]DTG labeled binding sites corresponding to sigma 1 and sigma 2 subtypes. A preponderance of sigma 2 sites was revealed in all investigated tissues. Different pharmacological profiles were demonstrated for the sigma 2 sites in mouse whole bain compared to mouse spinal cord. However, competition studies indicated that the whole brain and spinal [3H](+)-pentazocine labeled sigma 1 binding sites exhibited similar pharmacological properties. The density of [3H](+)-pentazocine labeled sigma 1 population was found not to match that of [3H]DTG labeled sigma 1 site throughout the mouse central nervous system. The presence of low affinity [3H]DTG labeled sites was demonstrated in cold saturation experiments. Equilibrium binding data for the low affinity [3H]DTG binding site resulted in an increasing density (Bmax = 1973-11,369 fmol/mg protein) with a decreasing affinity (Kd = 242-943 nM) in mouse cortex through the spinal cord.

Haloperidol and reduced haloperidol-induced exacerbation of the dystonia produced by the kappa opioid U50,488H in guinea-pigs is associated with inhibition of sigma binding sites: behavioural and autoradiographical studies

A single dose of haloperidol and reduced haloperidol has been found to exacerbate the dystonic response produced by U50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) -cyclohexyl]-benzeacetamide methane sulphonate) in guinea-pigs [8]. The present study sought to correlate the behavioural effect of haloperidol and reduced haloperidol with their effect on inhibition of sigma binding sites in guinea-pig brain using receptor binding and semi-quantitative autoradiography. In the first experiments, groups of guinea-pigs were injected with saline (control, n = 12), haloperidol (0.1 and 1 mg/kg i.p., n = 5) or reduced haloperidol (0.1 and 1 mg/kg i.p., n = 5) 1, 3 and 10 days before, followed by U50,488H (10 mg/kg s.c.) and the effect on the dystonic response rated using a behavioural rating scale [8]. In the second experiments, animals (n = 5) were injected with saline, haloperidol and reduced haloperidol as above and killed 1, 3 and 10 days later, their brains removed, dissected and tissue sections processed for sigma binding site autoradiography using [3H]3-(3-hydroxyphenyl)-N-(n-propyl)piperidine ([3H]-3-PPP). Triplicate tissue sections were wiped using GF/C filters and radioactivity counted. Injection of haloperidol and reduced haloperidol 1, 3 and 10 days earlier exacerbated the dystonic response by decreasing the latency to maximal dystonia and increasing the duration of the response at each dose tested compared with saline-treated animals. These effects of haloperidol and reduced haloperidol on latency and duration were time-related since the effect at 1 > 3 > 10 days. In addition, [3H]-3-PPP binding was inhibited by haloperidol and reduced haloperidol in a dose-and time-related manner. For example, % inhibition of [3H]-3-PPP binding for haloperidol (1 mg/kg) > haloperidol (0.1 mg/kg) and % inhibition of binding (mean +/- SEM) produced by haloperidol (0.1 mg/kg) at 1 (96.1 +/- 2.4) > 3 (74.8 +/- 4.8) > 10 days (36.2 +/- 1.6). Similar results were obtained for haloperidol (1 mg/kg) and reduced haloperidol (0.1 and 1 mg/kg). [3H]-3-PPP autoradiography confirmed these binding data. The results indicate that the exacerbation by sigma ligands of the dystonia produced by U50,488H was associated with the degree of inhibition of [3H]-3-PPP binding.

N-substituted aminohydroxypyridines as potential non-opioid analgesic agents

A series of new N-substituted aminohydroxypyridines have been synthesized, pharmacologically evaluated and compared with their N-substituted oxazolopyridone analogs. The compound with the maximal combination of safety and analgesic efficacy was 3-[2-[4-(4-fluorophenyl)-1-piperazinyl]ethyl]amino-2-hydroxypyridine (compound 10a), with ED50 values 0.4 mg kg-1 po (mouse: phenylquinone writhing test) and 0.5 mg kg-1 po (rat: acetic acid writhing test). Compound 10a possesses a potent non-opioid antinociceptive activity with moderate anti-inflammatory properties.

Sigma binding site ligands inhibit cell proliferation in mammary and colon carcinoma cell lines and melanoma cells in culture

Recent evidence suggests a role for sigma (sigma) binding sites in maintenance of cell growth and/or proliferation. The present study examines, for the first time, the effect of sigma binding site ligands on in vitro growth of tumour cells derived from human mammary adenocarcinoma (MCF-7, MDA) and colon carcinoma (LIM 1215, WIDr), and melanoma (Chinnery). Addition of the sigma ligands haloperidol, reduced haloperidol, 1,3-di(2-tolyl)guanidine (DTG), (+)- and (-)-N-allylnormetazocine (SKF 10,047), (+)- and (-)-pentazocine and rimcazole at 6.25, 12.5, 25, 50, 100 microM at the beginning of culture or 24 h later, inhibited cell proliferation in a dose-dependent manner. Light microscopy revealed cell detachment, rounding and cell death. The potency of sigma ligands on melanoma cells was rimcazole > reduced haloperidol > haloperidol = (+)-pentazocine, whereas DTG and (+)- and (-)-SKF 10,047 and (-)-pentazocine had no effect even at 100 microM. In contrast, in MCF-7 cells, rimcazole > reduced haloperidol > haloperidol > (-)-pentazocine > DTG > (+)-pentazocine > (+)-SKF 10,047 > (-)-SKF 10,047. For colon cancer cells, reduced haloperidol > DTG > haloperidol = (-)-pentazocine = (+)-pentazocine = (+)-SKF 10,047. Of all the ligands tested, rimcazole and reduced haloperidol were the most potent inhibitors of cell proliferation. With the exception of one slow-growing colon cancer cell line (LIM 1215), the order of sensitivity of various cell lines to reduced haloperidol, SFK 10,047, DTG, haloperidol and (+)- and (-)-pentazocine was colon carcinoma > mammary adenocarcinoma > melanoma, whereas to rimcazole, the sensitivities of mammary adenocarcinoma and melanoma cells were comparable. The effect of sigma ligands in MCF-7 and melanoma cells was not due to blockade of dopamine D1 and D2 receptors, serotonin (5-HT2) receptors, N-methyl-D-aspartate (NMDA)/phencyclidine receptors, beta-adrenoceptors or opioid receptors, since 100 microM SCH 23390, raclopride, mianserin, (+)-MK-801, propranolol and 1 microM naloxone respectively, were ineffective. However, mianserin and raclopride were inhibitory to melanoma cells and one colon carcinoma cell line, respectively. Taken together, the results are consistent with the recent observation that sigma binding sites may play a role in cell growth and/or cell proliferation.

Substance P analogs displace sigma binding differentially in the brain and spinal cord of the adult mouse

We have previously observed similarities in the behavioral effects produced by the NH2-terminus of the undecapeptide substance P (SP) and by 1,3-di(2-tolyl)-guanidine (DTG) in the adult mouse. The present series of experiments indicate differences in the rank-order of potency of sigma ligands [DTG; haloperidol (HAL)], SP analogs [SP; SP(1-7); SP(5-11); [D-Pro2, D-Phe7]-SP(1-7) (D-SP(1-7))] and miscellaneous compounds [morphine (MOR), naloxone (NAL)] at competing for [3H]-DTG binding sites in the mouse brain and spinal cord in vitro: Brain; DTG = HAL >> SP = MOR = NAL >> SP(1-7) >> D-SP(1-7) >> SP(5-11): Spinal cord; DTG = HAL >> SP(1-7) = MOR = NAL >> SP >> D-SP(1-7) = SP(5-11). The observed difference in the rank-order potencies of the displacing ligands at these same binding sites supports the notion of two distinct populations of sigma binding sites in these tissues in the adult mouse. Given the low (micromolar) potency of SP analogs at displacing [3H]-DTG binding in the present series of experiments, it is unlikely that the similar behavioral effects we have previously observed elicited by SP(1-7) and DTG in the adult mouse are a result of a direct action of SP(1-7) at the sigma binding site.

Allosteric modulation of peripheral sigma binding sites by a new selective ligand: SR 31747

The interactions of a new compound SR 31747 with sigma sites were examined in rat spleen membranes and in human peripheral blood leukocytes (PBL). Nanomolar concentrations of SR 31747 selectively inhibited in a non-competitive manner the binding of the prototypic sigma ligands [3H](+)-pentazocine, [3H](+)-3PPP and [3H]DTG on rat spleen membranes. Characterization of SR 31747 binding sites using [3H]SR 31747 as a ligand showed that this compound binds reversibly, with high affinity to one class of sites on rat spleen membranes (Kd 0.66 nM, Bmax 5646 fmol/mg protein). The pharmacological profile of [3H]SR 31747 binding sites was consistent with the presence of specific sites distinct from classical sigma 1 and sigma 2 receptor subtypes strongly suggesting an allosteric modulation of sigma sites by SR 31747. Similarly, [3H]SR 31747 binding sites were demonstrated on human PBL and also on purified subpopulations of human mononuclear cells (granulocytes, NK cells, T4, T8 and B lymphocytes). Administered to mice by i.p. or oral route 30 min before sacrifice, SR 31747 strongly inhibited the binding of [3H](+)-3PPP to mice spleen membranes with ED50 values of 0.18 and 1.43 mg/kg, respectively. Taken together these results could suggest a potential immunological activity of SR 31747 either directly or through allosteric modulation of peripheral sigma sites.

Sigma-1 and sigma-2 sites in rat brain: comparison of regional, ontogenetic, and subcellular patterns

Radioligand binding assay conditions were established for the selective labeling of sigma-1 and sigma-2 sites in membrane homogenates of rat brain. Selective sigma-1 assays were conducted using 5 nM(+)[3H]SKF-10,047 in the presence of 300 nM dizocilpine (MK-801). Selective sigma-2 assays were conducted using 5 nM [3H]DTG in the presence of 1 microM (+)SKF-10,047. Distributions of sigma-1 and sigma-2 binding among brain regions were found to differ. While the brain stem yields the highest level of sigma-1 binding, it yields among the lowest levels of sigma-2 binding. The reverse is true in hippocampal membranes. Different ontogenetic patterns were also observed. Sigma-2 binding decreases substantially during brain development, whereas sigma-1 binding does not vary significantly. Patterns of distribution among subcellular fractions of rat brain homogenates were found to be similar. Both sigma-1 and sigma-2 sites are most enriched in microsomal fractions, and neither is enriched in synaptosomal or mitochondrial fractions. The present results suggest that sigma-1 and sigma-2 sites are distinct entities; they do not appear to be located on a common macromolecule, and they do not represent two different affinity states of a single type of binding site. While the precise subcellular locations of sigma-1 and sigma-2 sites remain to be determined, we conclude that localization of either type of binding site to synaptic regions of plasma membrane or to mitochondria is highly unlikely.

Rat liver and kidney contain high densities of sigma 1 and sigma 2 receptors: characterization by ligand binding and photoaffinity labeling

Rat liver and kidney were investigated for the presence of sigma (sigma) receptor subtypes by radioligand binding with three highly selective sigma probes and by photoaffinity labeling using [3H]azido-di-o-tolylguanidine ([3H]azido-DTG). [3H](+)-Pentazocine, a highly selective sigma 1 probe, bound to sites in liver membranes with Kd = 7.5 nM and Bmax3 = 2929 fmol/mg protein. [3H](+)-Pentazocine binding sites in kidney had Kd = 23.3 nM and Bmax = 229 fmol/mg protein. [3H]1,3-Di-o-tolylguanidine ([3H]DTG) and [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ([3H](+)-3-PPP) label both sigma 1 and sigma 2 receptors. Parameters for [3H]DTG in the liver were Kd = 17.9 nM and Bmax = 11,895 fmol/mg protein. Similar parameters were observed for [3H](+)-3-PPP, Kd = 51.9 nM and Bmax = 11,070 fmol/mg protein. [3H]DTG bound to rat kidney with Kd = 45.8 nM and Bmax = 1190 fmol/mg protein. The observation that either [3H]DTG or [3H](+)-3-PPP and [3H](+)-3-PPP labeled a higher number of sites relative to [3H](+)-pentazocine suggested that liver and kidney contain both subtypes of sigma receptor. This was confirmed by competition studies vs. [3H](+)-pentazocine and [3H]DTG (in the presence of dextrallorphan to mask sigma 1 sites). In both tissues, [3H](+)-pentazocine labeled sites with high affinity for haloperidol and enantioselectivity for (+)-benzomorphans over (-)-benzomorphans. [3H]DTG + dextrallorphan labeled sites in both tissues which also had high affinity for haloperidol, but which had the characteristic sigma 2 property of low affinity for (+)-benzomorphans and enantioselectivity for (-)-benzomorphans over the corresponding (+)-isomer. Similar results were obtained with [3H](+)-3-PPP + dextrallorphan. Several novel aryl diamines, such as 1S,2R-cis-N-[2-(3,4-dichlorophenylethyl]-N-methyl-2- (1-pyrrolidinyl)cyclohexylamine (BD737) and N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine (BD1008), bound to both sites with high affinity. Photoaffinity labeling with 10 nM [3H]azido-DTG resulted in specific labeling of polypeptides of 25 kDa and 21.5 kDa. Dextrallorphan (100 nM or 500 nM) completely blocked labeling of the 25 kDa polypeptide, but had no effect on labeling of the lower molecular weight protein. (+)-10,11-Dihydro-5-methyl-5H-dibenzo[a,d]cyclohepten-5,10- imine((+)-MK-801) had no effect on labeling of either polypeptide. These data are consistent with the notion that the 25 kDa and 21.5 kDa proteins represent sigma 1 and sigma 2 receptors, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple [3H]DTG binding sites in guinea pig cerebellum: evidence for the presence of non-specific binding

The characteristics of the low affinity component of 1,3-di(2-[5-3H]tolyl)guanidine binding to the guinea pig cerebellum were investigated. Saturation binding assays where sigma 1 receptors were masked with dextrallorphan indicated that 1,3-di(2-[5-3H]tolyl)guanidine bound to cerebellar membranes in a fashion best described by a 1 site+non-specific binding model with a low density of specific binding sites (Bmax approximately 200 fmol/mg protein). Boiling the cerebellar membranes before addition to the saturation assay had no effect on the density of 1,3-di(2-[5-3H]tolyl)guanidine binding. In contrast, both the Kd and Bmax for 1,3-di(2-[5-3H]tolyl)guanidine binding to liver membranes was significantly reduced by boiling, as was the density of [3H](+)-pentazocine binding to cerebellum and liver. Thus, a substantial component of 1,3-di(2-[5-3H]tolyl)guanidine binding in the guinea pig cerebellum is to non-specific, proteinaceous binding sites with some of the pharmacological characteristics of the sigma 2 binding site.

Differential effects of repeated dl-pentazocine treatment on sigma binding sites in discrete brain areas of the rat

Repeated treatment with dl-pentazocine (5-10 mg/kg i.p.), but not d-methamphetamine (4.8 mg/kg i.p.), twice daily for 8 days diminished the specific [3H]1,3-di-o-tolylguanidine (DTG) binding to the haloperidol-sensitive sigma sites in rat striatum and hippocampus. Either drug failed to cause any changes in the binding to the hypothalamus and prefrontal cortex. The pentazocine-induced diminution in the striatum was accompanied by a decrease in the density with an increase in the affinity of the [3H]DTG binding. These data suggest that repetitive use of dl-pentazocine may influence the regulation systems of the sigma site in specific brain regions.

[125I](S)-trans-7-OH-PIPAT: a potential spect imaging agent for sigma binding sites

[125I](S)-trans-7-hydroxy-2-[(N-n-propyl-N-(3'-iodo-2'-propenyl)] aminotetralin ([125I](S)-trans-7-OH-PIPAT) has been prepared as an iodinated radioligand for studying the sigma binding site. [125I](S)-trans-7-OH-PIPAT binds to rat cerebellar membranes with a Kd = 1.67 +/- 0.07 nM and Bmax = 240 +/- 72 fmol/mg of protein (determined in the presence of 15 nM spiperone). This new ligand appears to bind to only one site with Hill coefficients close to unity. Inhibition constants for competing ligands determined in the cerebellar tissue homogenates (in the presence of 15 nM spiperone) are closely comparable to inhibition constants determined in the whole brain tissue homogenates (in the absence of spiperone). Furthermore, these inhibition constants are consistent with the values reported for typical sigma ligands. In vivo uptake of [125I](S)-trans-7-OH-PIPAT in the rat brain is initially high (2.52% dose/organ at 2 min post i.v. injection) and displays a rapid washout from the brain (0.8% dose/organ at 30 min post i.v. injection). Uptake of [125I](S)-trans-7-OH-PIPAT shows moderate target to non-target ratios at 30 minutes (1.54, 1.66 and 1.92 for cerebellar, hypothalamic and hindbrain uptake over striatal uptake, respectively). Pre-injection with haloperidol reduced these ratios to unity suggesting that the ligand binds specifically to haloperidol-sensitive sites in vivo. The selectivity and affinity of [125I](S)-trans-7-OH-PIPAT suggest that this new iodinated ligand may be useful for in vitro studies of the sigma sites and can be used in vivo as a potential SPECT imaging agent.

Effects of sigma ligands on rat cerebellar Purkinje neuron firing: an iontophoretic study

The electrophysiological responses of rat cerebellar Purkinje neurons to selective sigma ligands applied iontophoretically was examined in urethane anesthetized male Sprague-Dawley rats. 1,3-Di-o-tolylguanidine (DTG), dextrallorphan (DEX), (+)-pentazocine((+)-PENT), (+)-3-(3-Hydroxyphenyl)-N-propylpiperidine ((+)-3-PPP), and the novel diamine BD1008, were ejected from multibarrel pipettes onto individual Purkinje cells. In some neurons, cell firing was inhibited following ejections of all compounds. These inhibitory effects were dose dependent and occurred without changes in spike amplitude or duration, thus ruling out local anesthetic effects as a mechanism. (+)-3-PPP and DEX increased firing rate in 27% and 14% (n = 15, n = 14, respectively) of cells studied. The results of this study indicate that sigma ligands significantly alter the spontaneous firing of Purkinje neurons, consistent with previous work suggesting motor effects of sigma ligands via the rubro-cerebellar circuitry.

Discrimination of (+)-3-PPP sites from DTG sites by FH-510, a novel potent sigma ligand, in rat brain

The effect of 5,8-dimethyl-4-(2-di-n-propylaminoethyl)carbazol monohydrochloride (FH-510) on the binding of sigma ligands such as [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ([3H](+)-3-PPP) and [3H]1,3-di-o-tolylguanidine ([3H]DTG) to rat brain membranes was studied. The inhibitory effect of FH-510 on [3H](+)-3-PPP binding to membranes of rat brain was 260 times more potent than that on [3H]DTG binding. Scatchard plot analysis showed that FH-510 inhibited [3H](+)-3-PPP binding in a competitive manner, while the inhibitory effect of FH-510 on [3H]DTG binding was noncompetitive. These results suggest that (+)-3-PPP sites could be discriminated from DTG sites, and that FH-510 binds preferentially to (+)-3-PPP recognition sites in rat brain.

Haloperidol treatment differentially regulates [3H]DTG and [3H](+)-3-PPP labeled sigma binding sites

The effect of repeated haloperidol administration on sigma binding sites in brain membranes was assessed using [3H](+)-3-(3-hydroxyphenyl)-N-(1- propyl)piperidine ((+)-3-PPP) and [3H]1,3-di-o-tolylguanidine (DTG). Administration of haloperidol (1 mg/kg, i.p.) to guinea pigs for 14 consecutive days followed by a 4 day drug-free period prior to sacrifice resulted in 75% and 6% decreases in the specific binding of [3H](+)-3-(3-hydroxyphenyl)-N-(1- propyl)piperidine and [3H]1,3-di-o-tolylguanidine, respectively, when measured using a single concentration (2 nM) of radioligand. Scatchard analysis revealed a reduction in both the maximum number of [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine binding sites and the affinity of these sites for the radioligand; the potency of 1,3-di-o-tolylguanidine to inhibit [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine binding was also reduced. In parallel studies, the potency of 1,3-di-o-tolylguanidine to inhibit [3H]1,3-di-o-tolylguanidine binding was unaffected by haloperidol treatment, but the potency of (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine against [3H]1,3-di-o-tolylguanidine was reduced 3-fold. Phenytoin, which increased (10-fold) the potency of dextromethorphan to inhibit [3H]1,3-di-o-tolylguanidine binding in control membranes, had no effect in membranes obtained from haloperidol-treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

The kappa-opioid receptor agonist, U50,488H, exerts antidystonic activity in a mutant hamster model of generalized dystonia

The kappa-opioid receptor agonist, U50,488H (trans-(+-)-3,4-dichloro-N-methyl-[2-(1-pyrrolidinyl)-cyclohexyl]- benzeneacetamide), has recently been reported to induce dystonia after s.c. administration of 5-10 mg/kg in guinea pigs. The dystonic movements observed in response to U50,488H resembled those previously reported to occur spontaneously or in response to mild environmental stimuli in a mutant hamster model of paroxysmal generalized dystonia. This prompted us to study the effects of opioid receptor antagonists and of U50,488H in the mutant hamster model. Naloxone and naltrexone, 1 and 10 mg/kg i.p., were either ineffective or tended to induce prodystonic effects in the mutant hamsters. In contrast, U50,488H markedly reduced the severity of dystonic movements at doses of 1-10 mg/kg s.c. In non-dystonic hamsters, U50,488H reduced locomotor activity but did not produce dystonic-like symptoms. The data from mutant hamsters demonstrate that stimulation of kappa-opioid receptors is a powerful means of attenuating dystonic movements in a genetic animal model of idiopathic dystonia. The antidystonic effects of U50,488H might relate to interactions between kappa-opioid receptors and dopaminergic and glutamatergic neurotransmission.

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.

Multiple sigma binding sites in guinea-pig and rat brain membranes: G-protein interactions

1. Evidence is accumulating for multiple sigma (sigma) sites in the mammalian CNS. 2. We have addressed this problem and have examined sigma site - G-protein coupling in guinea-pig and rat brain membranes. 3. Ditolylorthoguanidine (DTG), (+)-3-(3-hydroxyphenyl)-N-1-(propyl)piperidine (3PPP) and dextromethorphan displaced [3H]-DTG (3.4 nM) with low Hill slopes of 0.5, 0.6 and 0.6, respectively in guinea-pig brain membranes. 4. In the presence of 5'-guanylylimidodiphosphate (Gpp(NH)p; 100 microM), the specific binding of [3H]-DTG was reduced by 36.7%, the Hill slope of 3PPP was increased to near unity, the ability of dextromethorphan to displace DTG was virtually abolished and the Hill slope for DTG remained low (0.7), indicating the presence of at least two binding sites. These data indicate that although Gpp(NH)p removes a dextromethorphan high affinity site, two DTG selective sites remain in the presence of Gpp(NH)p. 5. The present study suggests that DTG binds to at least three sites in guinea-pig brain membranes, at least one of which is G-protein linked. 6. In rat brain membranes, DTG displaced itself (3.4 nM) with a Hill slope near 1. 3PPP displacement of [3H]-DTG was comparable with the guinea-pig (Hill slope 0.5) and displaced from more than 1 site. Dextromethorphan did not displace [3H]-DTG at concentrations below 10 microM. 7. The heterogeneity of sigma sites appears to be less in rat than in guinea-pig brain membranes.

Allosteric modulation of ligand binding to [3H](+)pentazocine-defined sigma recognition sites by phenytoin

The allosteric modulation of sigma recognition sites by phenytoin (diphenylhydantoin) has been demonstrated by the ability of phenytoin to stimulate binding of various [3H] sigma ligands, as well as to slow dissociation from sigma sites and to shift sigma sites from a low- to a high-affinity state. Phenytoin stimulated the binding of the sigma 1- selective ligand [3H](+)pentazocine in a dose-dependent manner. Stimulation of binding at a final concentration of 250 microM phenytoin was associated with a decrease in the KD. The affinities of the sigma reference compounds caramiphen, dextromethorphan, dextrophan, (+)3-PPP and (+)SKF-10,047 were three- to eight-fold higher, while the affinities of benzetimide, BMY-14802, carbetapentane, DTG and haloperidol were unchanged in the presence of 250 microM phenytoin. The relative sensitivity of sigma compounds to allosteric modulation by phenytoin is not a property of all sigma ligands, and may provide an in vitro basis for distinguishing actions of sigma compounds and predicting sigma effects in vivo.

Characterization of the binding of [3H](+)-pentazocine to σ recognition sites in guinea pig brain

The selective sigma compound (+)-pentazocine was radiolabeled and its binding characteristics in guinea pig brain membranes were investigated. [3H](+)-Pentazocine bound to a single high-affinity site with a KD of 2.9 nM and a Bmax of 1998 fmol/mg protein. Saturation was achieved at a ligand concentration of 15 nM. Maximal specific binding was observed at 37 degrees C and was greater than 90% of total binding. Equilibrium was reached by 120 min and dissociation was complete by 420 min, with a t1/2 of 121 min. Li+, Ca2+ and Mg2+ inhibited binding at high concentrations, and binding was insensitive to adenyl and guanyl nucleotides. Stereoselectivity was observed for the inhibition of binding by benzomorphans, 3-(3-hydroxyphenyl)-N-propylpiperidine and butaclamol, and the (+) enantiomers and alpha diastereomers of pentazocine and cyclazocine were more potent than their corresponding (-) enantiomers and beta diastereomers. The rank order of potency for the sigma reference agents to displace [3H](+)-pentazocine binding was similar to that reported using the [3H]sigma ligands dextromethorphan, 1,3-di(2-tolyl)guanidine and (+)-3-(3-hydroxyphenyl)-N-propylpiperidine. Haloperidol, (+)-pentazocine, (+)-3-(3-hydroxyphenyl)-N-propylpiperidine and rimcazole were competitive inhibitors of binding to the [3H](+)-pentazocine-defined sigma recognition site, suggesting that these different structural classes of compounds all bind to a single molecular entity.

Lack of neuroprotective effect of some sigma ligands in a model of focal cerebral ischemia in the mouse

The potential neuroprotective effect of seven sigma ligands has been evaluated in a mouse model of focal cerebral ischemia (induced by coagulation of the left middle cerebral artery) and compared to that of known N-methyl-D-aspartate (NMDA) antagonists. When given after the induction of cerebral ischemia, the NMDA receptor antagonists dizocilpine and CGS 19755 and the mixed NMDA antagonist/sigma ligand eliprodil (SL 82.0715) afforded very substantial protection against cortical infarction (92, 44 and 72%, at the doses of 1, 10 and 10 mg/kg, i.p., for dizocilpine, CGS 19755 and eliprodil, respectively). In contrast, none of the sigma ligands investigated--DTG, DMTG, GBR 12909, (+)- and (-)-3-PPP (up to 10 mg/kg), BMY 14802 (up to 30 mg/kg), except haloperidol at a high dose (3 mg/kg)--had neuroprotective effects.

Iontophoretic effects of sigma ligands on rubral neurons in the rat

Iontophoretic application of the sigma ligands, 1,3-di-o-tolylguanidine (DTG), dextrallorphan, and (+)-pentazocine reliably inhibited the firing rate of rubral neurons. Dextrallorphan inhibited 87% of the neurons tested, DTG inhibited 76%, and (+)-pentazocine inhibited 50%. These inhibitions were current dependent and occurred without significant changes in spike amplitude or duration, suggesting that local anesthetic effects were not involved. In contrast to the other sigma ligands, iontophoretic application of (+)-3-PPP in the rat red nucleus resulted in very few inhibitions and tended to elicit weak excitations instead. Only 14% of rubral neurons were inhibited by (+)-3PPP, while 36% were excited. Although unusual, (+)-3-PPP has atypical effects when compared to other sigma ligands in numerous functional assays for sigma receptor activity. (+)-3-PPP, therefore, appears to have complex effects and may act through nonsigma mechanisms or through a different type of sigma binding site than the other compounds. The inhibition of firing rate produced by the more typical sigma ligands may contribute to the postural changes produced by microinjection of sigma ligands into the rat red nucleus.

Sigma receptors are associated with cortical limbic areas in the primate brain

Putative sigma receptors are a current target for antipsychotic drug development. Novel antipsychotic agents which possess selective and high affinity for sigma binding sites may serve as an alternative to the principal neuroleptic drugs currently in clinical use which mediate extrapyramidal side effects and dyskinesias through their blockade of dopamine receptors. We have used in vitro autoradiography to localize putative sigma receptors labelled with (+)-[3H]-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine [(+)-[3H]-3-PPP] in the brain of the rhesus macaque. The binding characteristics of (+)-[3H]-3-PPP in the primate brain were comparable to those previously described in the rodent. Saturation analysis demonstrated a single class of sites in cerebellar and hippocampal membranes with a Kd value of 28 nM. Sigma receptors labeled with (+)-[3H]-3-PPP in the primate brain displayed the appropriate rank order of potency and stereoselectivity in competition binding assays. Haloperidol displaced (+)-[3H]-3-PPP binding in the low nanomolar range, and the (+) isomer of pentazocine was 50-fold more potent than (-) pentazocine. Computerized densitometric analysis of the autoradiograms demonstrated a striking enrichment of sigma binding sites over the paralimbic belt cortices, including the orbitofrontal, cingulate, insular, parahippocampal, and temporopolar gyri. Peak densities of sigma receptors were seen over the medial and central nuclei of the amygdala and were widely distributed within the hippocampal formation. Sigma binding sites densities were elevated over the suprachiasmatic and supraoptic nuclei of the hypothalamus. Moderate sigma receptor densities were observed over the ventromedial sectors of the caudate and the putamen. Sigma receptors were also elevated over autonomic relay nuclei of the brainstem, including the nucleus of the solitary tract and the dorsal motor nucleus of the vagus. The distribution of sigma receptors in the primate brain suggests that the paralimbic belt cortices, amygdala, hippocampus, hypothalamus, and autonomic relay nuclei of the brainstem may be interrelated by a topographic chemical linkage. The autoradiographic visualization of sigma receptor distributions in the primate brain provides further support for a role of sigma receptor mechanisms in the functions of the limbic system.

1,3-Di(2-[5-3H]tolyl)guanidine labels more than one site in rat forebrain

Studies of 1,3-di-(2-[5-3H]tolyl)guanidine ([3H]DTG) binding to rat brain membranes revealed that [3H]DTG binds to a high and a low affinity site with Kd values of 19.8 nM and 1.31 microM (corresponding Bmax values 291 fmol/mg protein and 8.68 pmol/mg protein). The order of potency of competitors for [3H]DTG binding revealed a binding profile typical of sigma site ligands. Several sigma ligands such as the enantiomers of 3-PPP (3-(3-hydroxyphenyl)-N- (n-propyl)piperidine) and (+/-)-pentazocine exhibited biphasic competition profiles for [3H]DTG binding, whereas other sigma ligands such as haloperidol displayed monotonic competition curves. Neither phenytoin nor carbamazepine were observed to enhance [3H]DTG binding. These data support the hypothesis that multiple sigma binding sites exist. The lack of phenytoin and carbamazepine modulation of [3H]DTG binding are in agreement with the proposed greater density of sigma site 2 in the rat, since allosteric modulation has been ascribed to the DM1/sigma 1 site.