Drug specificity of pharmacological dystonia

Physiology of Dystonia: Animal Studies

Dystonia is currently ranked as the third most prevalent motor disorder. It is typically characterized by involuntary muscle over- or co-contractions that can cause painful abnormal postures and jerky movements. Dystonia is a heterogenous disorder-across patients, dystonic symptoms vary in their severity, body distribution, temporal pattern, onset, and progression. There are also a growing number of genes that are associated with hereditary dystonia. In addition, multiple brain regions are associated with dystonic symptoms in both genetic and sporadic forms of the disease. The heterogeneity of dystonia has made it difficult to fully understand its underlying pathophysiology. However, the use of animal models has been used to uncover the complex circuit mechanisms that lead to dystonic behaviors. Here, we summarize findings from animal models harboring mutations in dystonia-associated genes and phenotypic animal models with overt dystonic motor signs resulting from spontaneous mutations, neural circuit perturbations, or pharmacological manipulations. Taken together, an emerging picture depicts dystonia as a result of brain-wide network dysfunction driven by basal ganglia and cerebellar dysfunction. In the basal ganglia, changes in dopaminergic, serotonergic, noradrenergic, and cholinergic signaling are found across different animal models. In the cerebellum, abnormal burst firing activity is observed in multiple dystonia models. We are now beginning to unveil the extent to which these structures mechanistically interact with each other. Such mechanisms inspire the use of pre-clinical animal models that will be used to design new therapies including drug treatments and brain stimulation.

In vitro and in vivo sigma 1 receptor imaging studies in different disease states

The sigma receptor system has been classified into two distinct subtypes, sigma 1 (σ1R) and sigma 2 (σ2R). Sigma 1 receptors (σ1Rs) are involved in many neurodegenerative diseases and different central nervous system disorders such as Alzheimer's disease, Parkinson's disease, schizophrenia, and drug addiction, and pain. This makes them attractive targets for developing radioligands as tools to gain a better understanding of disease pathophysiology and clinical diagnosis. Over the years, several σ1R radioligands have been developed to image the changes in σ1R distribution and density providing insights into their role in disease development. Moreover, the involvement of both σ1Rs and σ2Rs with cancer make these ligands, especially those that are σ2R selective, great tools for imaging different types of tumors. This review will discuss the principles of molecular imaging using PET and SPECT, known σ1R radioligands and their applications for labelling σ1Rs under different disease conditions. Furthermore, this review will highlight σ1R radioligands that have demonstrated considerable potential as biomarkers, and an opportunity to fulfill the ultimate goal of better healthcare outcomes and improving human health.

Drug-induced movement disorders

Movement disorders are frequently a result of prescription drugs or of illicit drug use. This article focuses on prescribed drugs but briefly mentions drugs of abuse. The main emphasis is on movement disorders caused by dopamine receptor-blocking agents. However, movement disorders caused by other drugs are also briefly discussed.

A step forward in the sigma enigma: a role for chirality in the sigma1 receptor–ligand interaction?

To investigate the role of chirality in the ligand–σ₁ receptor interaction, a series of enantiomeric arylalkylaminoalcohols and arylpyrrolidinols was evaluated by means of both in silico and in vitro studies.

Animal models for dystonia

Symptomatic animal models have clinical features consistent with human disorders and are often used to identify the anatomical and physiological processes involved in the expression of symptoms and to experimentally demonstrate causality where it would be infeasible in the patient population. Rodent and primate models of dystonia have identified basal ganglia abnormalities, including alterations in striatal GABAergic (ie, transmitting or secreting γ-aminobutyric acid) and dopaminergic transmission. Symptomatic animal models have also established the critical role of the cerebellum in dystonia, particularly abnormal glutamate signaling and aberrant Purkinje cell activity. Further, experiments suggest that the basal ganglia and cerebellum are nodes in an integrated network that is dysfunctional in dystonia. The knowledge gained from experiments in symptomatic animal models may serve as the foundation for the development of novel therapeutic interventions to treat dystonia. © 2013 Movement Disorder Society.

Acute cervical dystonia after the first dose of butamirate citrate

Butamirate citrate is a central-acting antitussive drug and is widely used in clinical practice in childhood. It is thought that to be centrally active antitussive drugs act through receptors in the brainstem to inhibit cough, and these findings were based on the evidence of animal models. Central nervous system adverse effects of cough suppressants are rare and include irritability, lethargy, hallucinations, and dystonic reactions. In this report, we present the first patient who developed cervical dystonia shortly after the first dose of butamirate citrate, and the patient's symptoms improved immediately after a single intramuscular dosage of biperiden.

Development of the sigma-1 receptor in C-terminals of motoneurons and colocalization with the N,N'-dimethyltryptamine forming enzyme, indole-N-methyl transferase

The function of the sigma-1 receptor (S1R) has been linked to modulating the activities of ion channels and G-protein-coupled receptors (GPCR). In the CNS, the S1R is expressed ubiquitously but is enriched in mouse motoneurons (MN), where it is localized to subsurface cisternae of cholinergic postsynaptic densities, also known as C-terminals. We found that S1R is enriched in mouse spinal MN at late stages of embryonic development when it is first visualized in the endoplasmic reticulum. S1Rs appear to concentrate at C-terminals of mouse MN only on the second week of postnatal development. We found that indole-N-methyl transferase (INMT), an enzyme that converts tryptamine into the sigma-1 ligand dimethyltryptamine (DMT), is also localized to postsynaptic sites of C-terminals in close proximity to the S1R. This close association of INMT and S1Rs suggest that DMT is synthesized locally to effectively activate S1R in MN.

Sigma receptor agonists: receptor binding and effects on mesolimbic dopamine neurotransmission assessed by microdialysis

BACKGROUND

Subtypes of sigma (σ) receptors, σ₁ and σ₂, can be pharmacologically distinguished, and each may be involved in substance-abuse disorders. σ-Receptor antagonists block cocaine place conditioning and σ-receptor agonists are self-administered in rats that previously self-administered cocaine. Self-administration of abused drugs has been related to increased dopamine (DA) neurotransmission, however, σ-receptor agonist effects on mesolimbic DA are not fully characterized.

METHODS

Receptor-binding studies assessed affinities of σ-receptor ligands for σ-receptor subtypes and the DA transporter; effects on DA transmission in the rat nucleus accumbens shell were assessed using in vivo microdialysis.

RESULTS

Cocaine (.1-1.0 mg/kg intravenous [IV]), the nonselective σ(½)-receptor agonist DTG (1.0-5.6 mg/kg IV), and the selective σ₁-receptor agonist PRE-084 (.32-10 mg/kg IV) dose-dependently increased DA to ∼275%, ∼150%, and ∼160% maxima, respectively. DTG-induced stimulation of DA was antagonized by the nonselective σ(½)-receptor antagonist BD 1008 (10 mg/kg intraperitoneal [IP]) and the preferential σ₂-receptor antagonist SN 79 (1-3 mg/kg IP), but not by the preferential σ₁-receptor antagonist, BD 1063 (10-30 mg/kg IP). Neither PRE-084 nor cocaine was antagonized by BD 1063 or BD 1008.

CONCLUSIONS

σ-Receptor agonists stimulated DA in a brain area critical for reinforcing effects of cocaine. DTG effects on DA appear to be mediated by σ₂-receptors rather than σ₁-receptors. However, DA stimulation by cocaine or PRE-084 does not likely involve σ-receptors. The relatively low potency on DA transmission of the selective σ₁-receptor agonist, PRE-084, and its previously reported potent reinforcing effects, suggest a dopamine-independent reinforcing pathway that may contribute to substance-abuse disorders.

Reinforcing effects of sigma-receptor agonists in rats trained to self-administer cocaine

sigma-Receptor (sigmaR) antagonists have been reported to block certain effects of psychostimulant drugs. The present study examined the effects of sigmaR ligands in rats trained to self-administer cocaine (0.032-1.0 mg/kg/inj i.v.) under fixed-ratio 5-response schedules of reinforcement. Maximal rates of responding were maintained by 0.32 mg/kg/inj cocaine, or by the sigmaR agonists, 1,3-di-(2-tolyl)guanidine (DTG; 1.0 mg/kg/inj) or 2-(4-morpholinethyl) 1-phenylcyclohexane-1-carboxylate hydrochloride (PRE-084; 0.32 mg/kg/inj), when substituted for cocaine. Lower response rates were maintained at higher and lower doses of the compounds. No dose of the sigmaR antagonists [N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine (BD 1008), N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine (BD 1047), N-[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine (BD 1063)] maintained responding appreciably above levels obtained when responding had no consequences. Presession treatment with sigmaR agonists dose-dependently shifted the cocaine self-administration dose-effect curve leftward. The dopamine-uptake inhibitor, (-)-2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane (WIN 35,428), dose-dependently shifted the DTG and PRE-084 self-administration dose-effect curves leftward. Treatment with the sigmaR antagonists dose-dependently decreased response rates maintained by DTG or PRE-084, but did not affect cocaine self-administration. Response rates maintained by maximally effective DTG or PRE-084 doses were decreased by sigmaR antagonists at lower doses than those that decreased response rates maintained by food reinforcement. Although sigmaR antagonists block some cocaine-induced effects, the lack of effect on cocaine self-administration suggests that the primary reinforcing effects of cocaine do not involve direct effects at sigmaRs. However, the self-administration of sigmaR agonists in cocaine-trained subjects, facilitation of cocaine self-administration by sigmaR-agonist pretreatment, and the facilitation of sigmaR-agonist self-administration by WIN 35,428, together suggest enhanced abuse-related effects resulting from concomitant dopaminergically mediated actions and sigmaR-mediated actions of the drugs.

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

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

Neuroprotective profile of dextromethorphan in an experimental model of penetrating ballistic-like brain injury

Dextromethorphan (DM) has been well-characterized as a neuroprotective agent in experimental models of CNS injury. The goal of this study was to determine the neuroprotective profile of DM in a military-relevant model of penetrating ballistic-like brain injury (PBBI). In an acute (3 day) dose-response study, anesthetized male Sprague-Dawley rats were exposed to a unilateral frontal PBBI with DM (0.156-10 mg/kg) or vehicle delivered as an i.v. bolus from 30 min to 48 h post-injury. In a follow-up (7 day) experiment, the 10-mg/kg bolus injections of DM were administered in conjunction with a 6-h infusion (5 mg/kg/h). DM bolus injections alone produced a dose-dependent improvement in motor recovery on a balance beam task at 3 days post-injury. However, more rapid recovery (24 h) was observed on this task when the bolus injections were combined with the 6-h infusion. Moreover, the DM bolus/infusion treatment regimen resulted in a significant (76%) improvement in cognitive performance in a novel object recognition (NOR) task at 7 days post-injury. Although post-injury administration of DM (all doses) failed to reduce core lesion size, the maximum dose of DM (10 mg/kg) was effective in reducing silver-stained axonal fiber degeneration in the cortical regions adjacent to the injury.

Sigma-1 receptor activation prevents intracellular calcium dysregulation in cortical neurons during in vitro ischemia

Sigma receptors are putative targets for neuroprotection following ischemia; however, little is known on their mechanism of action. One of the key components in the demise of neurons following ischemic injury is the disruption of intracellular calcium homeostasis. Fluorometric calcium imaging was used to examine the effects of sigma receptor activation on changes in intracellular calcium concentrations ([Ca(2+)](i)) evoked by in vitro ischemia in cultured cortical neurons from embryonic rats. The sigma receptor agonist, 1,3-di-o-tolyl-guanidine (DTG), was shown to depress [Ca(2+)](i) elevations observed in response to ischemia induced by sodium azide and glucose deprivation. Two sigma receptor antagonists, metaphit [1-(1-(3-isothiocyanatophenyl)-cyclohexyl)-piperidine] and BD-1047 (N-[2-3,4-dichlorophenyl)-ethyl]-N-methyl-2-(dimethylamino)ethylamine), were shown to blunt the ability of DTG to inhibit ischemia-evoked increases in [Ca(2+)](i), revealing that the effects are mediated by activation of sigma receptors and not via the actions of DTG on nonspecific targets such as N-methyl-d-aspartate receptors. DTG inhibition of ischemia-induced increases in [Ca(2+)](i) was mimicked by the sigma-1 receptor-selective agonists, carbetapentane, (+)-pentazocine and PRE-084 [2-(4-morpholinethyl) 1-phenylcyclohexanecarboxylate hydrochloride], but not by the sigma-2-selective agonist, ibogaine, showing that activation of sigma-1 receptors is responsible for the effects. In contrast, DTG, carbetapentane, and ibogaine blocked spontaneous, synchronous calcium transients observed in our preparation at concentrations consistent with sigma receptor-mediated effects, indicating that both sigma-1 and sigma-2 receptors regulate events that affect [Ca(2+)](i) in cortical neurons. Our studies show that activation of sigma receptors can ameliorate [Ca(2+)](i) dysregulation associated with ischemia in cortical neurons and, thus, identify one of the mechanisms by which these receptors may exert their neuroprotective properties.

Rodent models for dystonia research: Characteristics, evaluation, and utility

A large number of different genetic and acquired disorders of the nervous system may be associated with dystonia. To elucidate its pathogenesis and to facilitate the discovery of potential novel treatments, there has been a growing interest in the development of animal models and particularly rodent models. Multiple animal models for dystonia have now been developed and partially characterized. The results obtained from studies of these models often lead in very different directions, in part because the different models target different aspects of a very heterogeneous disorder. A recent workshop addressed four main issues affecting those who conduct dystonia research with animal models, including the different ways in which dystonic disorders can be modeled in rodents, key features that constitute a useful model, methods used in the evaluation of these models, and recommendations for future research. This review summarizes the main outcomes of this conference. 2005 Movement Disorder Society.

The role of σ-receptors in levodopa-induced dyskinesia in patients with advanced Parkinson disease: a positron emission tomography study

Object. Levodopa-induced dyskinesia (LID) in patients with Parkinson disease (PD) mimics acute dystonic reactions induced by antipsychotic agents, possibly mediated by σ-receptors; however, there are few reports in which the relationship between σ-receptors and LID in advanced PD is investigated. The binding potential of cerebellar σ-receptors before and after a pallidal surgery for dyskinesia in patients with advanced PD is assessed.

Methods. Six patients with advanced PD (male/female ratio 3:3, age 56.7 ± 9.8 years) underwent stereotactic pallidal surgery (two posteroventral pallidotomy procedures and four deep brain stimulation of the globus pallidus internus, including one bilateral case). Clinical features of patients with PD were assessed using Hoehn and Yahr (H & Y) stages, the Unified Parkinson's Disease Rating Scale (UPDRS), and the Schwab and England Activities of Daily Life Scale (S & E). The LID was evaluated by LID severity score. The binding potential of cerebellar σ-receptors was determined before and after the surgery by 11C-nemonapride positron emission tomoraphy, a specific radioligand for σ-receptors in the cerebellum. All clinical scores, especially the LID severity score, were dramatically improved after the surgery (p < 0.05). Preoperatively, contralateral cerebellar binding potential was significantly elevated (p < 0.01), and it was reduced after the surgery, but it was still higher than that of healthy volunteers (p < 0.05). The ipsilateral cerebellar binding potential remained unchanged after the surgery. The level of binding potential did not correlate with H & Y stage, UPDRS, or S & E score, but a strong positive correlation was seen between the binding potential and the preoperative LID severity score when the patients were receiving medication (r = 0.893, p < 0.05).

Conclusions. Cerebellar σ-receptors may potentially involve the genesis of LID in advanced PD.

Abnormal cerebellar signaling induces dystonia in mice

Dystonia is a relatively common neurological syndrome characterized by twisting movements or sustained abnormal postures. Although the basal ganglia have been implicated in the expression of dystonia, recent evidence suggests that abnormal cerebellar function is also involved. In these studies, a novel mouse model was developed to study the role of the cerebellum in dystonia. Microinjection of low doses of kainic acid into the cerebellar vermis of mice elicited reliable and reproducible dystonic postures of the trunk and limbs. The severity of the dystonia increased linearly with kainate dose. Kainate-induced dystonia was blocked by the glutamatergic antagonist 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide and reproduced by domoic acid microinjection, suggesting that the induction of dystonia is dependent on glutamatergic activation in this model. The abnormal movements were not associated with kainate-induced seizures, because EEG recordings showed no epileptiform activity during the dystonic events. Neuronal activation, as assessed by in situ hybridization for c-fos, revealed c-fos mRNA expression in the cerebellum, locus ceruleus, and red nucleus. In contrast, regions associated with epileptic seizures, such as the hippocampus, did not exhibit increased c-fos expression after cerebellar kainate injection. Furthermore, in transgenic mice lacking Purkinje cells, significantly less dystonia was induced after kainic acid injection, implicating Purkinje cells and the cerebellar cortex in this model of dystonia. Together, these data suggest that abnormal cerebellar signaling produces dystonia and that the cerebellum should be considered along with the basal ganglia in the pathophysiology of this movement disorder.

N-arylalkylpiperidines as high-affinity sigma-1 and sigma-2 receptor ligands: phenylpropylamines as potential leads for selective sigma-2 agents

To delineate the differences between the structural requirements necessary for recognition at sigma-1 and sigma-2 receptors, a range of phenethyl- and phenylpropylpiperidines were evaluated in binding assays. Phenethylpiperidines were found to favor sigma-1 receptors, whereas phenylpropylpiperidines tend to favor sigma-2 receptors. It appears that phenylpropylamine is a potential pharmacophore for selective sigma-2 ligands.

The interaction between neuroactive steroids and the sigma1 receptor function: behavioral consequences and therapeutic opportunities

Steroids, synthesized in peripheral glands or centrally in the brain--the latter being named neurosteroids--exert an important role as modulators of the neuronal activity by interacting with different receptors or ion channels. In addition to the modulation of GABA(A), NMDA or cholinergic receptors, neuroactive steroids interact with an atypical intracellular receptor, the sigma(1) protein. This receptor has been cloned in several species, and highly selective synthetic ligands are available. At the cellular level, sigma1 agonists modulate intracellular calcium mobilization and extracellular calcium influx, NMDA-mediated responses, acetylcholine release, and alter monoaminergic systems. At the behavioral level, the sigma1 receptor is involved in learning and memory processes, the response to stress, depression, neuroprotection and pharmacodependence. Pregnenolone, dehydroepiandrosterone, and their sulfate esters behave as sigma1 agonists, while progesterone is a potent antagonist. This review will detail the physiopathological consequences of these interactions, focusing on recent results on memory and depression. The therapeutical interest of selective sigma1 receptor agonists in alleviating aging-related cognitive deficits will be discussed.

Conformationally restricted analogs of BD1008 and an antisense oligodeoxynucleotide targeting sigma1 receptors produce anti-cocaine effects in mice

Cocaine's ability to interact with sigma receptors suggests that these proteins mediate some of its behavioral effects. Therefore, three novel sigma receptor ligands with antagonist activity were evaluated in Swiss Webster mice: BD1018 (3S-1-[2-(3,4-dichlorophenyl)ethyl]-1,4-diazabicyclo[4.3.0]nonane), BD1063 (1-[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine), and LR132 (1R,2S-(+)-cis-N-[2-(3,4-dichlorophenyl)ethyl]-2-(1-pyrrolidinyl)cyclohexylamine). Competition binding assays demonstrated that all three compounds have high affinities for sigma1 receptors. The three compounds vary in their affinities for sigma2 receptors and exhibit negligible affinities for dopamine, opioid, GABA(A) and NMDA receptors. In behavioral studies, pre-treatment of mice with BD1018, BD1063, or LR132 significantly attenuated cocaine-induced convulsions and lethality. Moreover, post-treatment with LR132 prevented cocaine-induced lethality in a significant proportion of animals. In contrast to the protection provided by the putative antagonists, the well-characterized sigma receptor agonist di-o-tolylguanidine (DTG) and the novel sigma receptor agonist BD1031 (3R-1-[2-(3,4-dichlorophenyl)ethyl]-1,4-diazabicyclo[4.3.0]nonane) each worsened the behavioral toxicity of cocaine. At doses where alone, they produced no significant effects on locomotion, BD1018, BD1063 and LR132 significantly attenuated the locomotor stimulatory effects of cocaine. To further validate the hypothesis that the anti-cocaine effects of the novel ligands involved antagonism of sigma receptors, an antisense oligodeoxynucleotide against sigma1 receptors was also shown to significantly attenuate the convulsive and locomotor stimulatory effects of cocaine. Together, the data suggests that functional antagonism of sigma receptors is capable of attenuating a number of cocaine-induced behaviors.

N-alkyl substituted analogs of the sigma receptor ligand BD1008 and traditional sigma receptor ligands affect cocaine-induced convulsions and lethality in mice

Cocaine binds to sigma receptors with comparable affinity to its well-established interaction with dopamine transporters. Previous studies have shown BD1008 (N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine) to have high affinity and selectivity for sigma receptors, and to additionally attenuate the locomotor stimulatory effects of cocaine. Therefore, in the present study, three N-alkyl substituted analogs of BD1008 were characterized in receptor binding and behavioral studies: BD1060 (N-[2-(3,4-dichlorophenyl)ethyl]-2-(1-pyrrolidinyl)ethylamine), BD1067 (N-[2-(3,4-dichlorophenyl)ethyl]-N-ethyl-2-(1-pyrrolidinyl)ethylamine), and BD1052 (N-[2-(3,4-dichlorophenyl)ethyl]-N-allyl-2-(1-pyrrolidinyl)ethylamine). Similarly to BD1008, all three analogs exhibited high affinity and selectivity for sigma receptors. In behavioral studies, BD1008, BD1060 or BD1067 attenuated cocaine-induced convulsions and lethality in Swiss Webster mice. The protective effects appear to be mediated through sigma receptor antagonism because traditional sigma receptor antagonists with high to moderate affinity for these receptors also attenuated the behavioral toxicity of cocaine. In contrast, traditional and novel sigma receptor agonists such as di-o-tolylguanidine and BD1052 worsened the behavioral toxicity of cocaine. To further characterize the actions of the N-alkyl substituted compounds, they were microinjected into the rat red nucleus, a functional assay of sigma receptor activity, where they produced agonist vs. antagonist actions that were consistent with their effects on cocaine-induced behaviors. Together, the data demonstrate that BD1008, BD1060 or BD1067 can attenuate the behavioral toxicity of cocaine, most likely through functional antagonism of sigma receptors.

Synthesis and binding affinity of cis-(-)- and cis-(+)-N-ethyleneamino-N-nordeoxymetazocine and cis-(-)-N-normetazocine analogues at sigma1, sigma2 and kappa opioid receptors

The synthesis of cis-(+)- and cis-(-)-N-ethyleneamino-N-nordeoxymetazocine and cis-(-)-N-normetazocine analogues is described and their affinities to sigma1, sigma2 and kappa opioid receptors are evaluated. The cis-(+)-deoxy compounds displayed high sigma/kappa selectivity with nanomolar K(i) values for sigma1 receptors, whereas in the cis-(-)-N-normetazocine series the compound (-)-7b was found to bind with nanomolar affinity to the kappa opioid receptor (K(i)=21.5 nM). Compound (-)-7b showed good selectivity for the kappa opioid receptor in comparison to the sigma1 and sigma2 sites and to the mu and delta opioid receptors. A correlation of the binding affinities between cis-(-)- and cis-(+)-N-deoxynormetazocine derivatives show that both isomers of the deoxy analogs have similar sigma1 and sigma2 binding profiles as the cis-(+)-N-normetazocine derivatives.

Pharmacological identification of SM-21, the novel sigma(2) antagonist

SM-21 is a tropane analogue with high affinity and selectivity for sigma(2) receptor subtype. In the absence of highly selective sigma(2) antagonists, the aim of the present study was to determine whether SM-21 is endowed with antagonistic activity. The experiments were conducted in rats by inducing neck dystonia, which is reported to be subsequent to activation of sigma(2) receptors. SM-21 (10 nmol/0.5 microl) was able to prevent torsion of the neck obtained by administration of the sigma(1)-sigma(2) agonist 1,3-di-(2-tolyl)guanidine (DTG, 5 nmol/0.5 microl) in the red nucleus. These data indicate that SM-21 is a potent and selective sigma(2) antagonist.

Biperiden hydrochlorate ameliorates dystonia of rats produced by microinjection of sigma ligands into the red nucleus

It has been reported that the imbalance of anticholinergic and antidopaminergic activity of each neuroleptic drug correlates with the capacity to produce neuroleptic-induced acute dystonia (NAD) and the major focus of NAD is thought to be the striatum. Anticholinergic drugs are highly effective on NAD, but they are partially effective on neuroleptic-induced tardive dystonia and their effect on idiopathic dystonia is disappointing. Recently, it has been reported that the unilateral microinjection of sigma (sigma) ligands into the red nucleus induces torticollis of rats. This animal model appears to be a model of dystonia, but it is not clear whether it is suitable for NAD in man. To clarify this issue, we investigated the effect of an anticholinergic drug, biperiden hydrochlorate (BH), on this animal model. This study revealed that BH dose-dependently ameliorated dystonia of rats induced by two sigma ligands, whether each sigma ligand had dopaminergic affinity or not. This animal model of dystonia appears to be a model of NAD in man from the viewpoint of treatment-response. The results also suggest that not only dopaminergic and cholinergic systems but also sigma system, and not only the striatum but also the red nucleus, may play an important role in the pathophysiology of NAD.

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.

Sigma receptors: recent advances and new clinical potentials

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

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

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

Asymmetric synthesis of 9-alkyl-2-benzyl-6,7-benzomorphans: characterization as novel sigma receptor ligands

A convenient enantioselective synthesis of (1R,5R,9R)- and (1S,5S, 9S)-9-alkyl-2-benzyl-6,7-benzomorphans (2a-c) which starts with naphthaldehyde is described. These compounds were designed to gain additional information on the structure-sigma binding relationship of the 6,7-benzomorphan class of sigma ligands. In contrast to pentazocine and most 6,7-benzomorphans, the (1R,5R,9R)-isomers of 2a-c showed greater affinity for the sigma(1) receptor than the (1S, 5S,9S)-isomers. Despite reversal of enantioselectivity at the sigma(1) sites, moderate affinity and enantioselectivity at the sigma(2) sites [greater affinity for (1R,5R,9R)-isomers than (1S,5S, 9S)-isomers] were maintained. A comparison of the binding affinities of 2a-c to the more conformationally flexible trans-2-alkyl-1-benzaminoethyl-1,2-dihydronaphthalenes (10a-c) suggested that the relatively rigid structure of 2a-c played an important part in their sigma(1) binding properties. These compounds, particularly (1R,5R,9R)-2-benzyl-9-methyl-6,7-benzomorphan [(-)-2a], which has a K(i) value of 0.96 nM, will be useful in further characterization of the sigma(1) receptor.

The attenuation of learning impairments induced after exposure to CO or trimethyltin in mice by sigma (sigma) receptor ligands involves both sigma1 and sigma2 sites

1. Sigma (sigma) receptor ligands were previously reported to alleviate learning and memory impairments on several pharmacological and pathological rodent models of amnesia. Such effect was demonstrated as involving the sigma1 subtype of sigma receptor. 2. In this study, we characterized the pharmacological effect mediated by sigma ligands on two lesional models of amnesia in mice: (1) the hypoxia-related learning and memory impairment model induced by repeated exposure to carbon monoxide (CO) gas; and (2) the intoxication with trimethyltin (1 mg kg(-1)). 3. The selective sigma1 ligand PRE-084 (1 mg kg(-1)) or the non-selective sigma1/sigma2 compounds DTG (0.1 mg kg(-1)), BD1008 (3 mg kg(-1)), and haloperidol (0.1 mg kg(-1)) reversed significantly the spontaneous alternation deficits observed 7 days after exposure to CO or 14 days after intoxication with trimethyltin. 4. The selective sigma1 receptor antagonist NE-100 (1 mg kg(-1)) was ineffective by itself, but blocked completely the PRE-084 effects, partially the DTG effects, and did not affect the effects induced by BD1008 or haloperidol. 5. A similar pharmacological profile was observed in the step-down type passive avoidance test performed 8 days after exposure to CO. 6. These results show that, in contrast to the previously reported amnesia models, the impairments induced after exposure to CO or intoxication with trimethyltin could be alleviated not only by sigma1 receptor agonists but also by sigma2 agonists. The particular pattern of neurodegeneration observed in these lesional models may explain these differences.

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.

Possible involvement of a sigma receptor subtype in the neck dystonia in rats

To clarify which subtype of sigma receptors is involved in the sigma receptor-mediated neck dystonia in rats, we examined whether 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (SA4503), a selective sigma1 receptor agonist, and 1,3-di-(2-tolyl)guanidine (DTG), a sigma1 and sigma2 receptor agonist, induce neck dystonia in rats. Microinjection of SA4503 into the red nucleus of rat brain scarcely produced neck dystonia at the concentration of 10 nmol/0.5 microl. On the contrary, DTG produced significant dystonia at a concentrations of more than 5 nmol/0.5 microl. These results indicate that the sigma2 receptor subtype, but not sigma1 receptor subtype, may play an important role in the sigma receptor-mediated neck dystonia in rats.

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.

(+)-cis-N-ethyleneamino-N-normetazocine derivatives. Novel and selective sigma ligands with antagonist properties

A series of (+)-cis-N-normetazocine derivatives has been described, and their affinities for sigma1, sigma2, and phencyclidine (PCP) sites and opioid, muscarinic (M2), dopamine (D2), and serotonin (5-HT2) receptors were evaluated. The effect of the N-substitution with a substituted ethylamino spacer was investigated. Compounds 8c-11c displayed high affinities for sigma1 sites and for opioid receptors. Substitution of the second basic nitrogen either with alkyl or cycloalkyl substituents give compounds (1a-6a) with high affinity and selectivity for sigma1 binding sites. Compounds 1a-5a were further characterized in vivo, and their agonist/antagonist activity was evaluated. In mouse, compound 1a and 2a as well as haloperidol suppressed in a dose-related manner the stereotyped behavior induced by (+)-SKF 10,047. Compounds 3a-5a and (+)-pentazocine do not affect the stereotyped behavior induced by ip injection of (+)-SKF 10,047. Therefore, from this series of compounds we identified potent and selective sigma1 ligands which might prove useful to unveil the functional role of sigma1 sites.

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

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

Changes in human motor cortex excitability induced by dopaminergic and anti-dopaminergic drugs

Transcranial magnetic stimulation was used to probe the acute effect of a single oral dose of various dopaminergic (levodopa, selegiline, bromocriptine) and antidopaminergic drugs (sulpiride, haloperidol) on motor cortex excitability in healthy volunteers. Motor threshold, intracortical inhibition and intracortical facilitation were tested in the abductor digiti minimi muscle. The latter two parameters were studied in a conditioning-test paired stimulus paradigm. The principal findings were an increase in intracortical inhibition by bromocriptine, and, conversely, a decrease in intracortical inhibition and an increase in intracortical facilitation by haloperidol. Effects peaked at delays consistent with the pharmacokinetics of the two drugs and were fully reversible. In conclusion, dopamine receptor agonists and antagonists can be considered inverse modulators of motor cortex excitability: the former enhance inhibition while the latter reduce it. The relation of the present findings to current models of motor excitability abnormalities in movement disorders will be discussed.

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.

The electrical activity is impaired in the red nucleus of dt(sz) mutant hamsters with paroxysmal dystonia: an EEG power spectrum analysis of depth electrode recordings

The genetically dystonic (dt(sz)) hamster is an animal model of paroxysmal dystonia that displays attacks of sustained abnormal movements and postures in response to mild stress. Dysfunctions within the basal ganglia may be critically involved in the pathophysiology of dystonia in mutant hamsters. Furthermore, previous observations from autoradiographic studies pointed to an altered neural activity in the red nucleus (RN). In the present study, computerized EEG spectral analysis of depth electrode recordings from the RN was performed before and after dystonic attacks in freely moving dt(sz) hamsters and compared to age-matched non-dystonic controls. No epileptic activity was seen in any of the recordings, substantiating previous notions that paroxysmal dystonia in these mutants has no epileptogenic basis. The predominant EEG changes in RN of dystonic hamsters were a decrease in total power over the range of 1.25-42.00 Hz, a decrease in maximum power and a shift of frequency at maximum power to lower frequencies. With regard to selected frequency bands, there was a decrease in the alpha, beta and gamma band. Although the observed changes of neural activity in the RN are probably based on a primary dysfunction in related structures, the present data demonstrate its importance in the expression of dystonic movements.

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.

[3H]1,3-di(2-tolyl)guanidine and [3H](+)pentazocine binding sites in the rat brain: autoradiographic visualization of the putative sigma1 and sigma2 receptor subtypes

Sigma (sigma) receptors have generated a great deal of interest on the basis of their possible role in psychosis and on locomotor behaviors. The effects of sigma drugs on these various functions are apparently mediated by different sigma receptor subtypes (sigma1 and sigma2). However, little information is currently available on the discrete anatomical distribution of these putative sigma receptor subtypes in the rat brain. The aim of the present study was to investigate, by quantitative autoradiography, the respective distribution of purported sigma1 and sigma2 receptor subtypes in the rat brain using [3H]1,3-di(2-tolyl)guanidine, a universal sigma ligand, and [3H](+)pentazocine, a selective sigma1 ligand. Putative sigma2 receptor sites were visualized using [3H]1,3-di(2-tolyl)guanidine in presence of a saturating concentration of (+)pentazocine. Specific [3H]1,3-di(tolyl)guanidine and [3H](+)pentazocine binding sites were found to be widely but discretely distributed in the rat brain. The highest densities of specific labeling were seen in various cranial nerve nuclei, followed by certain hippocampal sub-fields and laminae, the red nucleus, the interpeduncular nucleus and mid-layers of primary and secondary motor cortices. Lower amounts of specific binding were present in various other structures including most thalamic and hypothalamic nuclei, and the cerebellum. Interestingly, [3H]1,3-di(2-tolyl)guanidine binding in the motor cortex was found to be particularly resistant to a saturating concentration of (+)pentazocine suggesting an enrichment in the putative sigma2 receptor subtype. This also applies for a few other structures such as the nucleus accumbens, substantia nigra pars reticulata, central gray matter, occulomotor nucleus and cerebellum. On the other hand, the sigma1 subtype is more abundant in most other regions with the highest densities seen in the dentate gyrus of the hippocampal formation, facial nucleus, and various thalamic and hypothalamic nuclei. The comparative localization of the sigma1 and sigma2 receptor binding sites probably relates to the differential effects of sigma1 and sigma2 drugs in the rat brain.

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.

[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.

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.

The sigma ligand JO 1784 prevents trimethyltin-induced behavioural and sigma-receptor dysfunction in the rat

Recently much research interest has focused on the possible therapeutic uses of sigma-receptor ligands in psychiatric and neurodegenerative disorders. In the present study, the potential neuroprotective effects of chronic (52 days) administration of (+) cinnamyl-l-phenyl-l-N-methyl-N-cyclo propylene (JO 1784) (1 and 3 mg/kg subcutaneously), a potent and selective sigma receptor ligand, were assessed in the trimethyltin (8 mg/kg intraperitoneally) model of memory dysfunction. JO 1784 (3 mg/kg subcutaneously) prevented the trimethyltin-induced deficits in locomotor activity, passive avoidance and radial maze performance, while the lower dose of JO 1784 had little or no effect. Trimethyltin was also shown to produce a marked reduction in the binding of [3H] (+)-pentazocine to sigma-receptor sites in limbic brain structures, as detected by quantitative autoradiography, which was particularly evident in the hippocampal pyramidal cells. JO 1784 (3 mg/kg subcutaneously) successfully attenuated this loss of [3H] (+)-pentazocine binding sites in the hippocampus (CA1, CA3 and CA4 regions) and in the substantia innominata. This neuroprotective effect of JO 1784 in the trimethyltin model would seem to be related to the modulatory effects of this sigma ligand on trimethyltin-induced glutamate neurotoxicity.

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.

sigma2 Site-mediated inhibition of electrically evoked guinea pig ileum longitudinal muscle/myenteric plexus contractions

Functional and binding studies were performed in order to characterize the relative efficacy and affinity of a number of compounds that bind to sigma sites. The ability of sigma site ligands to inhibit electrically evoked contraction of the guinea pig ileum longitudinal muscle/myenteric plexus preparation was compared to the affinities of these compounds for sigma1 sites (assessed by displacement of [3H](+)-pentazocine) and sigma2 sites (assessed by displacement of [3H]1,3-di-o-tolylguanidine (DTG) in the presence of 5 microM dextromethorphan). It was shown that the rank order of potencies for suppression of electrically evoked contractions of guinea pig ileum perfectly matched the rank order of affinities of these compounds for the sigma2 binding site, while correlating poorly with the sigma1 binding site. In addition, no significant correlations were found between the efficacy of the tested compounds to inhibit contraction of the guinea pig ileum preparation and previously reported affinities for muscarinic, dopamine D2 or MK-801 binding sites. Thus, the present study represents the first functional bioassay selectively sensitive to agents interacting with the sigma2 receptor subtype binding site, and provides a means with which to further elucidate the functional role of sigma2 sites.

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.