Motor effects of two sigma ligands mediated by nigrostriatal dopamine neurons

Loss of Sigma-2 Receptor/TMEM97 Is Associated with Neuropathic Injury-Induced Depression-Like Behaviors in Female Mice

Previous studies have shown that ligands that bind to sigma-2 receptor/TMEM97 (s2R/TMEM97), a transmembrane protein, have anxiolytic/antidepressant-like properties and relieve neuropathic pain-like effects in rodents. Despite medical interest in s2R/TMEM97, little affective and pain behavioral characterization has been done using transgenic mice, which limits the development of s2R/TMEM97 as a viable therapeutic target. Using wild-type (WT) and global Tmem97 knock-out (KO) mice, we sought to identify the contribution of Tmem97 in modulating affective and pain-like behaviors using a battery of affective and pain assays, including open field, light/dark preference, elevated plus maze, forced swim test, tail suspension test, and the mechanical sensitivity tests. Our results demonstrate that female Tmem97 KO mice show less anxiety-like and depressive-like behaviors in light/dark preference and tail suspension tests but not in an open field, elevated plus maze, and forced swim tests at baseline. We next performed spared nerve injury in WT and Tmem97 KO mice to assess the role of Tmem97 in neuropathic pain-induced anxiety and depression. WT mice, but not Tmem97 KO mice, developed a prolonged neuropathic pain-induced depressive-like phenotype when tested 10 weeks after nerve injury in females. Our results show that Tmem97 plays a role in modulating anxiety-like and depressive-like behaviors in naive animals with a significant change in the presence of nerve injury in female mice. Overall, these data demonstrate that Tmem97 could be a target to alleviate affective comorbidities of pain disorders.

Using sigma-ligands as part of a multi-receptor approach to target diseases of the brain

INTRODUCTION

The sigma receptors are found abundantly in the central nervous system and are targets for the treatment of various diseases, including Alzheimer's (AD), Parkinson's (PD), Huntington's disease (HD), depression, amyotrophic lateral sclerosis (ALS), and multiple sclerosis (MS). However, for many of these diseases, other receptors and targets have been the focus of the most, such as acetylcholine esterase inhibitors in Alzheimer's and dopamine replacement in Parkinson's. The currently available drugs for these diseases have limited success resulting in the requirement of an alternative approach to their treatment.

AREAS COVERED

In this review, we discuss the potential role of the sigma receptors and their ligands as part of a multi receptor approach in the treatment of the diseases mentioned above. The literature reviewed was obtained through searches in databases, including PubMed, Web of Science, Google Scholar, and Scopus.

EXPERT OPINION

Given sigma receptor agonists provide neuroprotection along with other benefits such as potentiating the effects of other receptors, further development of multi-receptor targeting ligands, and or the development of multi-drug combinations to target multiple receptors may prove beneficial in the future treatment of degenerative diseases of the CNS, especially when coupled with better diagnostic techniques.

Sigma Receptors and Substance Use Disorders

Thanks to advances in neuroscience, addiction is now recognized as a chronic brain disease with genetic, developmental, and cultural components. Drugs of abuse, including alcohol, are able to produce significant neuroplastic changes responsible for the profound disturbances shown by drug addicted individuals. The current lack of efficacious pharmacological treatments for substance use disorders has encouraged the search for novel and more effective pharmacotherapies. Growing evidence strongly suggests that Sigma Receptors are involved in the addictive and neurotoxic properties of abused drugs, including cocaine , methamphetamine , and alcohol. The present chapter will review the current scientific knowledge on the role of the Sigma Receptor system in the effects of drugs and alcohol, and proposes that this receptor system may represent a novel therapeutic target for the treatment of substance use disorders and associated neurotoxicity.

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

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

Sigma (σ) receptors as potential therapeutic targets to mitigate psychostimulant effects

Many psychostimulants, including cocaine and methamphetamine, interact with sigma (σ) receptors at physiologically relevant concentrations. The potential therapeutic relevance of this interaction is underscored by the ability to selectively target σ receptors to mitigate many behavioral and physiological effects of psychostimulants in animal and cell-based model systems. This chapter begins with an overview of these enigmatic proteins. Provocative preclinical data showing that σ ligands modulate an array of cocaine and methamphetamine effects are summarized, along with emerging areas of research. Together, the literature suggests targeting of σ receptors as an innovative option for combating undesired actions of psychostimulants through both neuronal and glial mechanisms.

Effects of the selective sigma receptor ligand, 1-(2-phenethyl)piperidine oxalate (AC927), on the behavioral and toxic effects of cocaine

BACKGROUND

Sigma receptors represent a unique structural class of proteins and they have become increasingly studied as viable medication development targets for neurological and psychiatric disorders, including drug abuse. Earlier studies have shown that cocaine and many other abused substances interact with sigma receptors and that antagonism of these proteins can mitigate their actions.

METHODS

In the present study, AC927 (1-(2-phenethyl)piperidine oxalate), a selective sigma receptor ligand, was tested against the behavioral and toxic effects of cocaine in laboratory animals.

RESULTS

Acute administration of AC927 in male, Swiss Webster mice significantly attenuated cocaine-induced convulsions, lethality, and locomotor activity, at doses that alone had no significant effects on behavior. Subchronic administration of AC927 also attenuated cocaine-induced conditioned place preference in mice, at doses that alone had no effects on place conditioning. In drug discrimination studies in male, Sprague-Dawley rats, AC927 partially substituted for the discriminative stimulus effects of cocaine. When it was administered with cocaine, AC927 shifted the cocaine dose-response curve to the left, suggesting an enhancement of the discriminative stimulus effects of cocaine. In non-human primates, AC927 was self-administered, maintaining responding that was intermediate between contingent saline and a maintenance dose of cocaine.

CONCLUSION

The ability of AC927 to elicit some cocaine-like appetitive properties and to also reduce many cocaine-induced behaviors suggests that it is a promising lead for the development of a medication to treat cocaine abuse.

Targeting ligand-operated chaperone sigma-1 receptors in the treatment of neuropsychiatric disorders

INTRODUCTION

Current drugs for the treatment of psychiatric or neurodegenerative disorders have limitations. Psychotherapeutic drugs such as typical and atypical antipsychotics, tricyclic antidepressants and selective monoamine reuptake inhibitors, aim to normalize the hyper- or hypo-neurotransmission of monoaminergic systems. Despite their contribution to the outcomes of psychiatric patients, these agents often exert severe side effects and require chronic treatments to promote amelioration of symptoms. Drugs available for the treatment of neurodegenerative disorders are severely limited.

AREAS COVERED

Recent evidence that has shed light on sigma-1 receptor ligands, which may serve as a new class of antidepressants or neuroprotective agents. Sigma-1 receptors are novel ligand-operated molecular chaperones regulating signal transduction, ER stress, cellular redox, cellular survival and synaptogenesis. Selective sigma-1 receptor ligands exert rapid antidepressant-like, anxiolytic, antinociceptive and robust neuroprotective actions in preclinical studies. Recent studies that suggest that reactive oxygen species might play a role as signal integrators downstream of Sig-1Rs are also covered.

EXPERT OPINION

The advances in sigma receptor research in the last decade have begun to elucidate the intracellular signal cascades upstream and downstream of sigma-1 receptors. The novel ligand-operated properties of the sigma-1 receptor chaperone may enable interventions by which stress-related cellular systems can be pharmacologically controlled.

Sigma1 receptor antagonists determine the behavioral pattern of the methamphetamine-induced stereotypy in mice

OBJECTIVE

The effects of sigma receptor antagonists on methamphetamine (METH)-induced stereotypy have not been examined. We examined the effects of sigma antagonists on METH-induced stereotypy in mice.

RESULTS

The administration of METH (10 mg/kg) to male ddY mice induced stereotyped behavior consisting of biting (90.1%), sniffing (4.2%), head bobbing (4.1%), and circling (1.7%) during an observation period of 1 h. Pretreatment of the mice with BMY 14802 (alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazinebutanol; 1, 5, and 10 mg/kg), a non-specific sigma receptor antagonist, significantly increased METH-induced sniffing (19.2%, 30.5%, and 43.8% of total stereotypical behavior) but decreased biting (76.6%, 66.9%, and 49.3% of total stereotypical behavior) in a dose-dependent manner. This response was completely abolished by (+)-SKF 10,047 ([2S-(2alpha,6alpha,11R)]-1,2,3,4,5,6-hexahydro-6,11-dimethyl-3-(2-propenyl)-2,6-methano-3-benzazocin-8-ol; 4 and 10 mg/kg), a putative sigma(1) receptor agonist, and partially by PB 28 (1-cyclohexyl-4-[3-(1,2,3,4-tetrahydro-5-methoxy-1-naphthalen-1-yl)-n-propyl]piperazine; 1 and 10 mg/kg), a putative sigma(2) receptor agonist. The BMY 14802 action on METH-induced stereotypy was mimicked by BD 1047 (N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine; 10 mg/kg), a putative sigma(1) receptor antagonist, but not by SM-21 ((+/-)-tropanyl 2-(4-chlorophenoxy)butanoate; 1 mg/kg), a putative sigma(2) receptor antagonist. The BD 1047 effect on METH-induced stereotypy was also abolished completely by (+)-SKF 10,047 and partially by PB 28. The overall frequency of METH-induced stereotypical behavior was unchanged with these sigma receptor ligands, despite the alteration in particular behavioral patterns. The BMY 14802 action on METH-induced stereotypy was unaffected by pretreatment with centrally acting histamine H(1) receptor antagonists (pyrilamine or ketotifen, 10 mg/kg), suggesting that these effects are independent of histamine H(1) receptor signaling systems.

CONCLUSION

In summary, modulation of central sigma(1) receptors alters the pattern of METH-induced stereotypy, producing a shift from stereotypical biting to stereotypical sniffing, without affecting the overall frequency of stereotypical behavior.

Commentary: Dopaminergic dysfunction in DYT1 dystonia

A three-base-pair deletion in the torsinA gene leads to generalized torsion dystonia (DYT1) in humans, an often devastating movement disorder in which voluntary movements are disrupted by sustained muscle spasms and abnormal limb posturing. In a recent issue of Experimental Neurology, Zhao et al. (2008) have provided a thorough behavioral, anatomic, and biochemical characterization of a mouse line that over-expresses human mutant torsinA, with particular emphasis on the possible role of dopaminergic dysfunction in these animals. This commentary provides an overview of the clinical and genetic features of the human disease and of the available transgenic mouse models for DYT1 dystonia, and discusses the evidence favoring the role of dopamine in the clinical manifestations of the disease.

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

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

Reproductive and developmental toxicity screening test of basic rubber accelerator, 1,3-di-o-tolylguanidine, in rats

Twelve male and female rats per group were exposed to the rubber accelerator 1,3-di-o-tolylguanidine (DTG) by gavage at 0, 8, 20 or 50 mg/kg bw/day. Males were dosed for a total of 49 days beginning 14 days before mating. Females were dosed for a total of 40-49 days beginning 14 days before mating to day 3 of lactation throughout the mating and gestation period. At 50 mg/kg bw/day, deaths were observed in two males and three females. Lowered body weight gain and food consumption were noted in males at 50 mg/kg bw/day and females at 20 and 50 mg/kg bw/day. Mydriasis, decreased locomotor activity, bradypnea, prone position, tremor and/or salivation were observed in males and females at 20 and 50 mg/kg bw/day. No effects of DTG were found on the estrous cyclicity, precoital interval, copulation, fertility and gestational indices, numbers of corpora lutea and implantations, or gestation length. A significant decrease in the number, body weight and viability of offspring and increase in the incidence of fetuses with external malformations were found at 50 mg/kg bw/day. Oligodactyly, anal atresia and tail anomalies were observed. These data suggest that DTG may be teratogenic. The NOAELs of DTG for general and developmental toxicity in rats are 8 and 20 mg/kg bw/day, respectively.

Prenatal developmental toxicity study of the basic rubber accelerator, 1,3-di-o-tolylguanidine, in rats

Pregnant rats were given 1,3-di-o-tolylguanidine (DTG) by gavage at 0, 10, 20 or 40 mg/kg bw/day on days 6-19 of pregnancy and the pregnancy outcome was determined on day 20 of pregnancy. At 40 mg/kg bw/day, deaths were observed in four out of 24 females. The incidences of females showing mydriasis at 20 and 40 mg/kg bw/day and showing decreased locomotor activity at 40 mg/kg bw/day were significantly increased. Alopecia, bradypnea, prone position and tremor were also observed at mg/kg bw/day. The maternal body weight gain at 20 and 40 mg/kg bw/day and food consumption at 40 mg/kg bw/day were significantly reduced. A significantly decreased weight of the gravid uterus, increased incidence of postimplantation loss, decreased number of live fetuses, and lowered weights of fetuses and placentae were found at 40 mg/kg bw/day. The incidences of the total number of fetuses with external malformations at 40 mg/kg bw/day and with skeletal malformations at 20 and 40 mg/kg bw/day were significantly increased. Significantly higher incidences of fetuses with brachydactyly and short tail and defects of caudal vertebrae, phalanges and metacarpals were observed at 40 mg/kg bw/day. Delayed ossification was also noted at 40 mg/kg bw/day. The data indicate that DTG is teratogenic at maternal toxic doses and the NOAELs of DTG for maternal and developmental toxicity are 10 mg/kg bw/day in rats.

Nicergoline reverts haloperidol-induced loss of detoxifying-enzyme activity

We evaluated the effects of nicergoline on antioxidant defense enzymes (detoxifying enzymes), during chronic treatment with haloperidol in rats. Chronic use of haloperidol (10 weeks, 1.5 mg/kg/day) induces a significant decrease in glutathione reductase, glutathione peroxidase and superoxide dismutase activity, in selected areas of the brain. Co-administration of nicergoline (20 days, 10 mg/kg/day) significantly restored the activity of these enzymes to levels comparable to those observed in control rats. These observations suggest beneficial effects of nicergoline in the prevention and in the treatment of haloperidol-induced side effects.

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

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

An increase of sigma receptors in the aged monkey brain

We evaluated in vivo the effect of aging on the sigma(1) receptors in the monkey brain by the quantitative analysis of the binding of [11C]SA4503 to sigma(1) receptors with positron emission tomography. Based on a three-compartment model, the influx rate constant K(1) of [11C]SA4503 from plasma to brain across the blood-brain barrier in all 10 regions investigated became smaller in the aged monkeys (20-28 years old, n=5) than in the young adult monkeys (4-8 years old, n=5), but the reduction was not significant due to the individual differences. On the other hand, the binding potential, which was calculated as the ratio of the association rate constant k(3) to the dissociation rate constant k(4) for the binding of [11C]SA4503 to sigma(1) receptors in the brain, significantly increased in nine of the brain regions of the aged monkeys to the 160-210% levels of the young monkeys. We concluded that the sigma(1) receptor binding sites increased in the aging process of the monkey brain.

A potential antidepressant activity of SA4503, a selective sigma 1 receptor agonist

The aim of the present study was to examine SA4503 [1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride], a novel selective receptor agonist, in respect of its potential antidepressant action. To this end we used a forced swimming test in rats to study SA4503 alone, as well as its interaction with imipramine, a classic tricyclic antidepressant. SA4503 decreased the immobility time in the forced swimming test in rats (although only at one of the three doses used); at the same time it did not change the locomotor activity recorded under the same experimental conditions. Moreover, SA4503 showed a synergistic effect with imipramine in the forced swimming test (both those compounds given jointly decreased the immobility time, but were ineffective when administered separately). It had previously been shown that repeated administration of antidepressants with different pharmacological profiles enhanced the action of D-amphetamine, quinpirole and other dopamine stimulants. SA4503 administered repeatedly increased the locomotor hyperactivity induced by D-amphetamine and quinpirole (a dopamine D /D receptor agonist), but not by (+/-)-7-hydroxy-dipropyloamino-tetralin hydrobromide [(+/-)-7-OH-DPAT; a dopamine D receptor agonist]. The results presented in this paper support the suggestion that SA4503 may have potential antidepressive properties.

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

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

Trishomocubanes: novel sigma-receptor ligands modulate amphetamine-stimulated [3H]dopamine release

Several trishomocubane analogues of the type 4-azahexacyclo [5.4.1.0(2,6).0(3,10).0(5,9).0(8,11)]dodecane exhibited moderate to high affinity at sigma-receptor subtypes and low or negligible affinity at dopamine and serotonin transporters (SERT). Selected compounds were examined for their effects on amphetamine-stimulated [3H]dopamine release from striatal slices in vitro. Compounds 1, 2, 3 and 4 significantly enhanced amphetamine-stimulated release in a concentration-dependent manner. Compound 4, with the highest affinity and selectivity for the sigma(2)-receptor subtype, displayed the greatest potency. The enhancement produced by 1 and 2 was fully reversed by the selective sigma(2) antagonists 1'-[4-[1-(4-fluorophenyl)-1-H-indol-3-yl]-1-butyl]spiro[iso-benzofuran-1(3H), 4'piperidine] (Lu28-179), endo-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2,3-dihydro-(1-methyl)ethyl-2-oxo-1-H-benzimidazole-1-carboxyamidehydrochloride (BIMU-8) and the non-subtype selective antagonist N-[2-(3,4-dichlorophenyl)-ethyl]-N-methyl-2-pyrrolidinyl)ethylamine (BD1008). These data suggested a potential role for compounds 1 through 4 as sigma(2)-receptor agonists in functional studies. In addition, a D(3)-trishomocubane compound 5 displayed low affinity at sigma receptors (K(i)=3 microM) and moderate affinity at dopamine transporters (K(i)=623 nM). Compound 5 significantly inhibited the potentiation mediated by compound 2, presumably through sigma(2)-receptor antagonism, or a direct action on dopamine transporters.

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

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

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

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

Haloperidol-induced changes in glutathione and energy metabolism: effect of nicergoline

The aim of this study was to evaluate the possible effects of nicergoline, a semisynthetic ergot derivative, on the biochemical changes observed during chronic treatment with haloperidol in male Sprague-Dawley rats. Chronic treatment with haloperidol induced a significant decrease in the cellular glutathione (GSH) content in selected areas of the brain (cerebellum, striatum and cortex) and in the liver. Prolonged nicergoline administration was able to antagonize the haloperidol-induced GSH decrease, maintaining the GSH concentration at levels comparable to those observed in the control group. Analysis of the energy charge revealed changes similar to those observed for GSH: haloperidol induced a significant decrease in ATP and energy charge that was completely reversed by repeated nicergoline administration. In conclusion, chronic treatment with the classical antipsychotic haloperidol induces profound biochemical changes in the brain and in the liver. Nicergoline treatment is able to counteract the haloperidol-induced decrease in GSH levels and energy charge, suggesting a potential role of the drug in the treatment of neuroleptic-induced side effects.

Characterization of novel N,N'-disubstituted piperazines as sigma receptor ligands

sigma Receptors have been the focus of extensive studies because of their potential functional role in several important physiological and biochemical processes. To further evaluate the properties of sigma receptors, especially sigma-1 and sigma-2 subtypes, we have synthesized a series of N,N'-disubstituted piperazine compounds (1-32). The design of these compounds was based upon the early structure-activity relationship (SAR) studies of the minimum structural requirements of a molecule necessary to elicit sigma receptor binding activity. In the N-(3-phenylpropyl)piperazine series, compounds with the ethylenediamine moiety (8-11, 15-17) showed 6-20-fold higher affinity for sigma-1 and 2-40-fold higher affinity for sigma-2 relative to their corresponding amides (1-7). The (m-nitrophenethyl)piperazine 10 exhibits a subnanomolar affinity for the sigma-1 site, whereas the corresponding o-nitro compound 9 shows the highest affinity for the sigma-2 site (Ki = 4.9 nM). Compounds with a free amino terminus were designed as precursors for use as bioconjugated affinity compounds. Some of these compounds displayed high affinity for sigma-1 and moderate affinity for sigma-2 sites and are currently used for the purification and characterization of the receptor subtypes.

Synthesis and quantitative structure-activity relationships of N-(1-benzylpiperidin-4-yl)phenylacetamides and related analogues as potent and selective sigma1 receptor ligands

A series of N-(1-benzylpiperidin-4-yl)phenylacetamide derivatives was synthesized and evaluated for affinity at sigma1 and sigma2 receptors. Most of these compounds showed a high affinity for sigma1 receptors and a low to moderate affinity for sigma2 receptors. The unsubstituted compound N-(1-benzylpiperidin-4-yl)phenylacetamide, 1, displayed a high affinity and selectivity for sigma1 receptors (Ki values of 3.90 nM for sigma1 receptors and 240 nM for sigma2 receptors). The influence of substitutions on the phenylacetamide aromatic ring on binding at both the sigma1 and sigma2 receptor has been examined through Hansch-type quantitative structure-activity relationship (QSAR) studies. In general, all 3-substituted compounds, except for the OH group, had a higher affinity for both sigma1 and sigma2 receptors when compared with the corresponding 2- and 4-substituted analogues. The selectivity for sigma1 receptors displayed a trend of 3 > 2 approximately 4 for Cl, Br, F, NO2, and OMe substituted analogues. Halogen substitution on the aromatic ring generally increased the affinity for sigma2 receptors while maintaining a similar affinity for sigma1 receptors. Substitution with electron-donating groups, such as OH, OMe, or NH2, resulted in weak or negligible affinity for sigma2 receptors and a moderate affinity for sigma1 receptors. The 2-fluoro-substituted analogue, 11, exhibited the highest selectivity for sigma1 receptors among all compounds tested, with a Ki value of 3.56 nM for sigma1 receptors and 667 nM for sigma2 receptors. Compounds 1, 5, 9, 11, and 20 had no affinity for dopamine D2 (IC50 > 10 000 nM) and D3 (IC50 > 10 000 nM) receptors. The nanomolar binding affinity and high selectivity for sigma1 receptors suggest that these compounds may be developed as potential radiotracers for positron emission tomography or single photon emission computerized tomography imaging studies.

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

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

A trial of dextromethorphan in parkinsonian patients with motor response complications

The effects of the NMDA antagonist dextromethorphan (DM) on levodopa-associated dyskinesias and motor fluctuations were studied in patients with advanced Parkinson's disease. During initial open-label dose escalation, 6 of 18 patients reported a beneficial effect at their individually determined optimal DM dose (range, 60-120 mg/day). The 12 remaining patients either experienced reversible side effects, particularly mild drowsiness, or decreased levodopa efficacy, and were therefore excluded from the study. The six responders entered the double-blind, placebo-controlled, crossover study with two 2-week arms separated by 1 week wash-out. On the last day of each arm, motor ratings were performed every 20 minutes for 8 consecutive hours. In addition, motor complications and Activities of Daily Living (ADL) were assessed using the Unified Parkinson's Disease Rating Scale (UPDRS) and patient diaries. With DM, dyskinesias improved by 25% according to physician's ratings and by 40% according to UPDRS interviews, without compromising the anti-Parkinson effect of levodopa. Motor fluctuations and ADL scores also improved significantly. Although the narrow therapeutic index of DM limits its clinical usefulness, these findings support the view that drugs acting to inhibit glutamatergic transmission at the NMDA receptor can ameliorate levodopa-associated motor complications.

Differential modulation of NMDA-stimulated [3H]dopamine release from rat striatum by neuropeptide Y and sigma receptor ligands

Although the identity of the endogenous ligands for sigma (sigma) receptors is unknown, neuropeptide Y (NPY) has been named as a possible candidate for a natural transmitter at these receptors. Using a superfusion system, we compared the effect of NPY on NMDA-stimulated [3H]dopamine release in rat striatum to that of the sigma agonists (+)-pentazocine and BD737. In contrast to (+)-pentazocine- or BD737-mediated inhibition of release, NPY enhanced release. However, the same sigma antagonists (BD1008, DuP734, haloperidol and DTG) that reverse (+)-pentazocine- or BD737-mediated inhibition, as well as a Y receptor antagonist, PYX-1, all reversed the enhancement. PYX-1 also reversed the (+)-pentazocine- and BD737-mediated inhibition of release. Peptide YY (PYY) and [Leu31,Pro34]NPY did not mimic the effect of NPY. NPY13-36 enhanced release to the same extent as NPY but the effect was not reversed by sigma antagonists. Our findings are consistent with the potential role of NPY as an endogenous ligand for a subtype of sigma receptor with characteristics different from Y1, Y2 and Y3 receptors but sensitive to PYX-1.

Subchronic administration of N-[2-(3,4-dichlorophenyl) ethyl]-N-methyl-2-(dimethylamino) ethylamine (BD1047) alters sigma 1 receptor binding

BD1047 (N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine) is known to bind with high affinity and selectivity to sigma sites in vitro. In prior in vivo studies, it has been shown to attenuate the dystonic postures and orofacial dyskinesias that are produced by sigma receptor ligands, including the neuroleptic haloperidol. Since abnormal movements, such as dystonic postures and orofacial dyskinesias, are side effects that are associated with many sigma-active neuroleptics, compounds such as BD1047 may have therapeutic potential for preventing and treating these unwanted movements. A possible limitation to the therapeutic potential of BD1047, however, is that at least in cell culture and albeit weak, it can be cytotoxic. Therefore, the present study analyzed the possible neurotoxic effects of in vivo subchronic intracerebroventricular infusion of BD1047 (10 nmol/h) or artificial cerebrospinal fluid (CSF) into rat brains using osmotic minipumps for 7 or 14 days. Following a 24 h wash-out period, the animals were killed, the brains removed, and P2 membranes prepared. Membranes from rats treated for 7 or 14 days with BD1047 showed a marked decrease in [3H](+)-pentazocine binding as compared to membranes from CSF-treated animals, suggesting a loss of sigma 1 receptor binding. Histological examination of brain sections processed for Nissl stains and glial fibrillary acidic protein (GFAP) immunohistochemistry excluded the possibility of a cytotoxically induced down-regulation, suggesting possible receptor internalization or desensitization mediated via sigma 1 sites. Under the conditions used in our study, BD1047 does not appear to be neurotoxic, and the data, when taken together with other studies, suggest that BD1047 acts as a partial agonist at sigma sites.

Behavioural effects of selective serotonin reuptake inhibitors following direct micro injection into the left red nucleus of the rat

The behavioural effects of selective serotonin reuptake inhibitors (paroxetine, sertraline, citalopram, fluvoxamine, fluoxetine) and reference compounds (N,N'-di(o-tolyl)guanidine, haloperidol, 3-(3-hydroxyphenyl)-N-(l-propyl)piperidine and chlorpromazine) were studied for their ability to produce dystonia and torticollis following direct micro injection into the left red nucleus of the rat, an area of the brain containing a high density of sigma2 receptors but relatively devoid of biogenic amine receptors. Each animal was monitored for abnormalities in posture and movement for a period of 30 min and then sacrificed 40 min following drug administation. Only fluvoxamine (100 nmol) and fluoxetine (100 nmol) elicited acute dystonic behaviour (1-5 min). The onset of dystonia was accompanied by facial spasticity, vacuous chewing movements and grooming behaviour which reflected the extent of dystonia. The dystonic behaviour following the direct intrarubal injection of fluvoxamine and fluoxetine suggest the possible activation of sigma2 receptors while citalopram, sertraline and paroxetine were without effect. The results of this study support the role of sigma2 receptors in the regulation and control of movement and coordination and provides preliminary evidence to suggest the in vivo activity of sigma receptors by fluoxetine and fluvoxamine.

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

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

Synthesis and evaluation of aryl-substituted N-(arylethyl)-N-methyl-2-(1-pyrrolidinyl)ethylamines and corresponding arylacetamides for sigma receptor affinity

A series of aryl-monosubstituted arylacetamides (4-9) and arylethylenediamine (10-18) compounds were synthesized based on the structure of the high-affinity sigma ligand N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine (2). These compounds were prepared to evaluate the effect of aromatic substitution patterns on sigma-1 and sigma-2 receptor binding affinity and selectivity. The data indicate that 10-18 possessed higher affinity than 4-9 for both sigma sites, especially when substituted with an electron-withdrawing group. The diamine compounds 10-18 were selective for sigma-1 binding sites, whereas the arylacetamide compounds 4-9 generally exhibited an increased selectivity for sigma-2 sites compared to sigma-1. No clear pattern between the orientation of aromatic substituents and the sigma binding activity was observed.

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

The role of sigma (sigma) receptors in brain function is poorly defined. They are located in limbic areas, including nucleus accumbens (NAC) and prefrontal cortex (PFC), both of which are thought to be involved in schizophrenia. Many antipsychotics (APs), including haloperidol, bind with high affinity to sigma receptors. Dopaminergic hyperactivity in NAC is thought to underlie positive symptoms of schizophrenia, while dopaminergic hypoactivity in PFC is thought to underlie negative symptoms. Sigma receptors regulate N-methyl-D-aspartate (NMDA)-stimulated [3H] dopamine ([3H]DA) release in caudate-putamen (CP), the neuroanatomical substrate for extrapyramidal side effects resulting from chronic AP treatment. In the current study, we investigated whether sigma receptors could similarly regulate DA release in mesolimbic and mesocortical tissue, and the relative participation of different sigma receptor subtypes in this process. We found that, in NAC, regulation of DA release by the prototypical sigma agonist (+)pentazocine was mediated predominantly by the sigma 1 receptor, whereas in the PFC a portion of the (+)pentazocine effect was likely mediated by the sigma 2 receptor. We also observed, in both the NAC and PFC, that regulation of DA release by the sigma agonist BD737 was mediated primarily by the sigma 1 receptor. In addition, we determined that (+)pentazocine or BD737 effects on DA release were not mediated via opioid receptors, nor the phencyclidine (PCP) binding site within the NMDA receptor-operated cation channel, nor by sigma receptor effects upon [3H]DA accumulated by noradrenergic terminals in PFC.

Dissociation of the motor effects of (+)-pentazocine from binding to sigma 1 sites

Radioligand binding and behavioral studies were conducted to determine whether a relationship existed between the motor effects produced by (+)-pentazocine and its binding to sigma sites. Scatchard analyses revealed decreased [3H](+)-pentazocine binding in middle aged rats (5-6 months old) compared to young adult rats (2-3 months old). However, there was no difference between the extent of circling behavior or dystonia produced by microinjection of (+)-pentazocine into the substantia nigra or red nucleus in the older animals compared to the young adult rats. There was also a significant decrease in [3H](+)-pentazocine binding in rats chronically treated with haloperidol. Again, however, despite the reduction in [3H](+)-pentazocine binding, there was no difference between the extent of dystonia produced by unilateral intrarubral microinjection of (+)-pentazocine into animals chronically treated with haloperidol vs. saline. The postural changes produced by (+)-pentazocine could not be attenuated with coadministration of the putative sigma receptor antagonist BD1047 (N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino) ethylamine), or the opiate receptor antagonist naloxone. However, the (+)-opiate, (+)-nordihydrocodeinone, partially attenuated the postural effects of (+)-pentazocine, despite its very low affinity for sigma 1, sigma 2, or opiate receptors. Taken together with previous studies, the results suggest that [3H](+)-pentazocine is a potent and selective probe for sigma 1 binding sites, but the in vivo effects of (+)-pentazocine cannot be fully attributed to actions through these sites. Some of the in vivo effects of (+)-pentazocine appear to involve other binding sites that are not detected under the conditions normally used in in vitro assays.

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

The effect of ibogaine on Sigma- and NMDA-receptor-mediated release of [3H]dopamine

The indole alkaloid ibogaine has been suggested to have potential for inhibiting dependency on stimulant drugs. Radioligand binding studies have suggested possible multisite actions of ibogaine: affinity at the kappa-opioid, NMDA, and sigma receptors, with effects on dopamine (DA) release. To further investigate the multiplicity of sites of action of ibogaine and the presynaptic regulation of the DA release, the effect of ibogaine on NMDA- and sigma-receptor-mediated efflux of [3H]DA was measured in striatal tissue from C57BL/6By mice. Striatal tissue was incubated in vitro with [3H]DA and the effect on DA release was measured. Both NMDA (25 microM) alone increased the efflux of DA. (+/)-Pentazocine (100 nM) did not inhibit the NMDA-evoked release. MK-801 (5 microM) completely inhibited the NMDA-evoked release and inhibited the (+/-)-pentazocine-evoked release by 49%. Ibogaine (10 microM) itself increased the efflux of DA; at 1 microM it was without effect. Ibogaine (1 microM) inhibited the NMDA-evoked release of DA by 31% and inhibited the (+/-)-pentazocine-evoked release by 48%. In addition, the level of basal release of DA obtained after the NMDA- or (+/)-pentazocine-evoked-release remained higher in the tissue exposed to ibogaine throughout. The results suggest that sigma receptors can regulate the release of DA, along with an action at the NMDA receptor. We previously reported action of ibogaine at the kappa-opioid site. The elevated basal release of DA in the presence of ibogaine after NMDA- or (+/-)-pentazocine-evoked release may reflect the ibogaine-induced removal of the tonically active kappa-opioid system that acts presynaptically to reduce dopamine release. The kappa-opioid system also appears to be inhibitory on both the NMDA and sigma receptors.

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

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

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

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

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

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

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

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

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

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

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.

Affinity of 1-aminoadamantanes for the sigma binding site in post-mortem human frontal cortex

The 1-aminoadamantanes memantine (1-amino-3,5-dimethyl-adamantane) and amantadine (1-amino-adamantane) are clinically used as anti-parkinsonian, anti-spasticity, anti-dementia and antiviral drugs. In the present investigation we have tested a series of 1-aminoadamantane derivatives including memantine and amantadine for their ability to compete with [3H](+)-pentazocine in homogenates of post-mortem human frontal cortex. The Ki values ranged from 0.237 +/- 0.019 microM for 1-N-dimethyl-amino-3,5-dimethyl-adamantane to 20.25 +/- 16.48 microM for amantadine. The Ki value of memantine was 19.98 +/- 3.08 microM and was thus very similar to that of amantadine. Memantine, at therapeutic concentrations, probably does not interact with the sigma binding site. Amantadine, at therapeutic concentrations, probably binds both to the sigma site and to the phencyclidine (PCP) binding site of the N-methyl-D-aspartate (NMDA) receptor.

Oculogyric crisis possibly related to pentazocine

OBJECTIVE

To report and describe the apparent first case of acute oculogyric crisis following administration of pentazocine, and to discuss the possible mechanism for this reaction.

DATA SOURCES/CASE SUMMARY

Patient case and relevant review of literature. The patient, a 39-year-old woman, developed acute oculogyric crisis following administration of Talacen (pentazocine and acetaminophen) for pain relief. The crisis resolved after discontinuation of the medication and administration of intravenous diphenhydramine 50 mg.

CONCLUSIONS

Based on the temporal relationship of drug administration to occurrence of the event, pentazocine is implicated as the cause of this acute oculogyric crisis. A plausible mechanism for precipitation of this crisis is the agonism of pentazocine on sigma opiate receptors, with postulated subsequent modulation of dopamine receptors.

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

The present studies were carried out to analyze the neurochemical and behavioral effects of peripheral sigma ligand administration in the rat. Based upon previous studies which showed an increase in turning behavior following unilateral intranigral administration of sigma ligands, we determined the effects of two sigma ligands, 1,3-di-o-tolylguanidine (DTG) and (+)-pentazocine, on extracellular dopamine levels in the rat striatum. Dopamine levels were monitored via microdialysis in awake freely moving animals following i.p. injection of the ligands. Both DTG (1 and 3 mg/kg) and (+)-pentazocine (10 mg/kg) produced a significant (30-50%) increase in extracellular dopamine. Given the relatively high concentration of sigma receptors in brain nuclei involved in facial and mouth movements, we have also determined the effects of the two sigma ligands on facial movements. Both ligands produced a significant increase in vacuous chewing movements, suggesting that studies on the consequences of sigma receptor activation may have relevance to animal models of human dystonia and/or dyskinesia.

A comparison of (-)-deoxybenzomorphans devoid of opiate activity with their dextrorotatory phenolic counterparts suggests role of sigma 2 receptors in motor function

Three novel benzomorphans, (+)-N-benzylnormetazocine, (-)-deoxy-N-benzylnormetazocine, and (-)-deoxypentazocine were tested for their ability to produce circling behavior in rats following intranigral microinjections. Dose studies revealed the following rank order of potency: (-)-deoxypentazocine > (-)-deoxy-N-benzylnormetazocine > (+)-N-benzylnormetazocine. This rank order approximates that for affinities for sigma 2 receptors but not sigma 1 receptors. It is very unlikely that the effects of the (-)-deoxybenzomorphans were mediated by opiate receptors for the following reasons: (1) consistent with the known requirement for the phenolic hydroxyl group for opiate activity, both (-)-deoxy compounds showed very low affinity for opiate receptors; (2) naloxone (4 micrograms) co-administered with (-)-deoxy-N-benzylnormetazocine failed to reduce its efficacy; (3) both (-)-deoxy compounds failed to produce marked analgesic effects in the tail flick test following systemic injections of 20 mg/kg s.c. These finding suggest that sigma 2 receptors mediate the motor effects of sigma ligands in rats.

Effects of 1,3-di-o-tolylguanidine (DTG), a sigma ligand, on local cerebral glucose utilization in rat brain

The 2-deoxy-D-[1-14C]glucose ([14C]DG) method was used to examine the effects of the relatively selective sigma ligand 1,3-di-o-tolylguanidine (DTG) on cerebral metabolism in freely moving rats. Each animal received an i.p. injection of DTG (0.2, 1, or 5 mg/kg) or normal saline 20 min prior to the infusion of [14C]DG. DTG induced dose-dependent changes in local cerebral glucose utilization (LCGU) in several motor and limbic structures. Most structures showed increases in LCGU, with a maximum effect at 1 mg/kg. The most profound increases in LCGU were observed in brain regions that are rich in sigma receptors. These included cerebellar and related nuclei (interpositus, lateral and medial cerebellar n., vestibular n., olivary n.), ambiguus n., superior colliculus (superior layers), hippocampus (CA2, CA3, DG), n. basalis of Meynert interpeduncular n., and the substantia nigra pars compacta and pars reticulata. No significant decreases in glucose utilization were observed at any dose. Although the areas affected by DTG are similar to those previously reported for other sigma ligands, future studies employing a range of doses for additional selective sigma ligands must be carried out in order to confirm whether these changes in LCGU were sigma-mediated.

Acute effects of sigma ligands on the electrophysiological activity of rat nigrostriatal and mesoaccumbal dopaminergic neurons

The effects of acute i.v. administration of several sigma ligands on the single-unit activity of nigrostriatal and mesoaccumbal dopaminergic (DA) neurons were evaluated in chloral hydrate-anesthetized rats. DTG (1,3-di(o-tolyl)guanidine) did not alter DA neuronal activity at nontoxic doses and JO 1784 [(+)-N-cyclopropylmethyl-N-methyl-1,4-diphenyl-1-ethylbut-3-en-1-+ ++ylamine] was inactive. (+)-Pentazocine was more effective in increasing mesoaccumbal vs. nigrostriatal DA cell firing rates. BMY 14802(alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazine-but anol) dose-dependently increased DA cell firing rate in both populations. The inhibition of nigrostriatal DA cell firing rate by (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine [(+)-3-PPP] was reversed by (-)-eticlopride and (+)-but not (-)-butaclamol, which supports previous evidence that (+)-3-PPP-induced inhibition is due to the DA agonist properties of the drug. From what is known of the pharmacological properties of these compounds, it is concluded that acute sigma receptor occupation does not markedly alter the firing rate of DA neurons. The dose-response curve for inhibition of nigrostriatal DA neuronal activity by the D2 DA agonist, quinpirole, was shifted to the right tenfold by BMY 14802 pretreatment (8 mg/kg, i.v.) and twofold by (+)-pentazocine (8 mg/kg, i.v.), but was not changed by DTG (2 mg/kg, i.v.). It is concluded that the marked effects of certain sigma ligands on DA cell electrophysiology are likely due to their non-sigma properties.

Freezing action of a metabolite of haloperidol in frogs

In vivo metabolism studies led to the identification of a previously proposed metabolite of haloperidol, 4-(4'-chlorophenyl)-4-piperidinol (CPPO), in the liver of a haloperidol-treated rat. However, the secondary metabolites of CPPO that we have proposed were not observed in this study. Neurotoxicity studies in frogs, which have been used to detect N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) action, showed that CPPO did not mimic the neurotoxicity of MPTP but caused a delayed and persistent freezing action in Rana pipiens frogs. It is proposed that this action may contribute to some of the delayed side-effects associated with haloperidol therapy.

Autoradiographic distribution of [3H](+)-pentazocine and [3H]1,3-di-o-tolylguanidine (DTG) binding sites in guinea pig brain: a comparative study

Binding studies suggested the selectivity of (+)-pentazocine for sigma receptors, and subsequent synthesis and testing of [3H](+)-pentazocine confirmed its high potency and selectivity for sigma sites. Newer data have demonstrated the selectivity of (+)-pentazocine for a subtype of the sigma receptor called sigma-1. Based on these findings, the distribution of [3H](+)-pentazocine binding sites in the guinea pig brain was examined using in vitro autoradiography. [3H](+)-Pentazocine binding was high in the cingulate cortex, dorsal diagonal band, periaqueductal gray, cerebellum and cranial nerve nuclei. It was relatively low in the nucleus accumbens, neocortical areas and caudate nucleus. A significant correlation was found between the binding of [3H](+)-pentazocine and [3H]1,3-di-o-tolylguanidine, a selective sigma ligand across brain regions. However, certain nuclei exhibited markedly different ratios of binding of the two ligands. Since DTG is not selective for the sigma subtypes, while (+)-pentazocine is selective for the sigma-1 type, the data are suggestive of relative differences in the distributions of sigma-1 and sigma-2 sites.