Psychotomimetic opiate receptors labeled and visualized with (+)-[3H]3-(3-hydroxyphenyl)-N-(1-propyl)piperidine

"Selective" serotonin 5-HT2A receptor antagonists

Blockade of the serotonin 5-HT2A G protein-coupled receptor (5-HT2AR) is a fundamental pharmacological characteristic of numerous antipsychotic medications, which are FDA-approved to treat schizophrenia, bipolar disorder, and as adjunctive therapies in major depressive disorder. Meanwhile, activation of the 5-HT2AR by serotonergic psychedelics may be useful in treating neuropsychiatric indications, including major depressive and substance use disorders. Serotonergic psychedelics and other 5-HT2AR agonists, however, often bind other receptors, and standard 5-HT2AR antagonists lack sufficient selectivity to make well-founded mechanistic conclusions about the 5-HT2AR-dependent effects of these compounds and the general neurobiological function of 5-HT2ARs. This review discusses the limitations and strengths of currently available "selective" 5-HT2AR antagonists, the molecular determinants of antagonist selectivity at 5-HT2ARs, and the utility of molecular pharmacology and computational methods in guiding the discovery of novel unambiguously selective 5-HT2AR antagonists.

Repurposing Sigma-1 Receptor Ligands for COVID-19 Therapy?

Outbreaks of emerging infections, such as COVID-19 pandemic especially, confront health professionals with the unique challenge of treating patients. With no time to discover new drugs, repurposing of approved drugs or in clinical development is likely the only solution. Replication of coronaviruses (CoVs) occurs in a modified membranous compartment derived from the endoplasmic reticulum (ER), causes host cell ER stress and activates pathways to facilitate adaptation of the host cell machinery to viral needs. Accordingly, modulation of ER remodeling and ER stress response might be pivotal in elucidating CoV-host interactions and provide a rationale for new therapeutic, host-based antiviral approaches. The sigma-1 receptor (Sig-1R) is a ligand-operated, ER membrane-bound chaperone that acts as an upstream modulator of ER stress and thus a candidate host protein for host-based repurposing approaches to treat COVID-19 patients. Sig-1R ligands are frequently identified in in vitro drug repurposing screens aiming to identify antiviral compounds against CoVs, including severe acute respiratory syndrome CoV-2 (SARS-CoV-2). Sig-1R regulates key mechanisms of the adaptive host cell stress response and takes part in early steps of viral replication. It is enriched in lipid rafts and detergent-resistant ER membranes, where it colocalizes with viral replicase proteins. Indeed, the non-structural SARS-CoV-2 protein Nsp6 interacts with Sig-1R. The activity of Sig-1R ligands against COVID-19 remains to be specifically assessed in clinical trials. This review provides a rationale for targeting Sig-1R as a host-based drug repurposing approach to treat COVID-19 patients. Evidence gained using Sig-1R ligands in unbiased in vitro antiviral drug screens and the potential mechanisms underlying the modulatory effect of Sig-1R on the host cell response are discussed. Targeting Sig-1R is not expected to reduce dramatically established viral replication, but it might interfere with early steps of virus-induced host cell reprogramming, aid to slow down the course of infection, prevent the aggravation of the disease and/or allow a time window to mature a protective immune response. Sig-1R-based medicines could provide benefit not only as early intervention, preventive but also as adjuvant therapy.

Sigma 1 receptor: A novel therapeutic target in retinal disease

Retinal degenerative diseases are major causes of untreatable blindness worldwide and efficacious treatments for these diseases are sorely needed. A novel target for treatment of retinal disease is the transmembrane protein Sigma 1 Receptor (Sig1R). This enigmatic protein is an evolutionary isolate with no known homology to any other protein. Sig1R was originally thought to be an opioid receptor. That notion has been dispelled and more recent pharmacological and molecular studies suggest that it is a pluripotent modulator with a number of biological functions, many of which are relevant to retinal disease. This review provides an overview of the discovery of Sig1R and early pharmacologic studies that led to the cloning of the Sig1R gene and eventual elucidation of its crystal structure. Studies of Sig1R in the eye were not reported until the late 1990s, but since that time there has been increasing interest in the potential role of Sig1R as a target for retinal disease. Studies have focused on elucidating the mechanism(s) of Sig1R function in retina including calcium regulation, modulation of oxidative stress, ion channel regulation and molecular chaperone activity. Mechanistic studies have been performed in isolated retinal cells, such as Müller glial cells, microglial cells, optic nerve head astrocytes and retinal ganglion cells as well as in the intact retina. Several compelling studies have provided evidence of powerful in vivo neuroprotective effects against ganglion cell loss as well as photoreceptor cell loss. Also described are studies that have examined retinal structure/function in various models of retinal disease in which Sig1R is absent and reveal that these phenotypes are accelerated compared to retinas of animals that express Sig1R. The collective evidence from analysis of studies over the past 20 years is that Sig1R plays a key role in modulating retinal cellular stress and that it holds great promise as a target in retinal neurodegenerative disease.

Tensor algebra-based geometric methodology to codify central chirality on organic molecules

A novel mathematical procedure to codify chiral features of organic molecules in the QuBiLS-MIDAS framework is introduced. This procedure constitutes a generalization to that commonly used to date, where the values 1 and -1 (correction factor) are employed to weight the molecular vectors when each atom is labelled as R (rectus) or S (sinister) according to the Cahn-Ingold-Prelog rules. Therefore, values in the range [Formula: see text] with steps equal to 0.25 may be accounted for. The atoms labelled R or S can have negative and positive values assigned (e.g. -3 for an R atom and 1 for an S atom, or vice versa), opposed values (e.g. -3 for an R atom and 3 for an S atom, or vice versa), positive values (e.g. 3 for an R atom and 1 for an S atom) or negative values (e.g. -3 for an R atom and -1 for an S atom). These proposed Chiral QuBiLS-MIDAS 3D-MDs are real numbers, non-symmetric and reduced to 'classical' (non-chiral) QuBiLS-MIDAS 3D-MDs when symmetry is not codified (correction factor equal to zero). In this report, only the factors with opposed values were considered with the purpose of demonstrating the feasibility of this proposal. From QSAR modelling carried out on four chemical datasets (Cramer's steroids, fenoterol stereoisomer derivatives, N-alkylated 3-(3-hydroxyphenyl)-piperidines, and perindoprilat stereoisomers), it was demonstrated that the use of several correction factors contributes to the building of models with greater robustness and predictive ability than those reported in the literature, as well as with respect to the models exclusively developed with QuBiLS-MIDAS 3D-MDs based on the factor 1 | -1. In conclusion, it can be stated that this novel strategy constitutes a suitable alternative to computed chirality-based descriptors, contributing to the development of good models to predict properties depending on symmetry.

The effect of the sigma-1 receptor selective compound LS-1-137 on the DOI-induced head twitch response in mice

Several receptor mediated pathways have been shown to modulate the murine head twitch response (HTR). However, the role of sigma receptors in the murine (±)-2,5-dimethoxy-4-iodoamphetamine (DOI)-induced HTR has not been previously investigated. We examined the ability of LS-1-137, a novel sigma-1 vs. sigma-2 receptor selective phenylacetamide, to modulate the DOI-induced HTR in DBA/2J mice. We also assessed the in vivo efficacy of reference sigma-1 receptor antagonists and agonists PRE-084 and PPCC. The effect of the sigma-2 receptor selective antagonist RHM-1-86 was also examined. Rotarod analysis was performed to monitor motor coordination after LS-1-137 administration. Radioligand binding techniques were used to determine the affinity of LS-1-137 at 5-HT2A and 5-HT2C receptors. LS-1-137 and the sigma-1 receptor antagonists haloperidol and BD 1047 were able to attenuate a DOI-induced HTR, indicating that LS-1-137 was acting in vivo as a sigma-1 receptor antagonist. LS-1-137 did not compromise rotarod performance within a dose range capable of attenuating the effects of DOI. Radioligand binding studies indicate that LS-1-137 exhibits low affinity binding at both 5-HT2A and 5-HT2C receptors. Based upon the results from these and our previous studies, LS-1-137 is a neuroprotective agent that attenuates the murine DOI-induced HTR independent of activity at 5-HT2 receptor subtypes, D2-like dopamine receptors, sigma-2 receptors and NMDA receptors. LS-1-137 appears to act as a sigma-1 receptor antagonist to inhibit the DOI-induced HTR. Therefore, the DOI-induced HTR can be used to assess the in vivo efficacy of sigma-1 receptor selective compounds.

The non-apoptotic action of Bcl-xL: regulating Ca(2+) signaling and bioenergetics at the ER-mitochondrion interface

Bcl-2 family proteins are known to competitively regulate Ca(2+); however, the specific inter-organelle signaling pathways and related cellular functions are not fully elucidated. In this study, a portion of Bcl-xL was detected at the ER-mitochondrion interface or MAM (mitochondria-associated ER membrane) in association with type 3 inositol 1,4,5-trisphosphate receptors (IP3R3); an association facilitated by the BH4 and transmembrane domains of Bcl-xL. Moreover, increasing Bcl-xL expression enhanced transient mitochondrial Ca(2+) levels upon ER Ca(2+) depletion induced by short-term, non-apoptotic incubation with thapsigargin (Tg), while concomitantly reducing cytosolic Ca(2+) release. These mitochondrial changes appear to be IP3R3-dependent and resulted in decreased NAD/NADH ratios and higher electron transport chain oxidase activity. Interestingly, extended Tg exposure stimulated ER stress, but not apoptosis, and further enhanced TCA cycling. Indeed, confocal analysis indicated that Bcl-xL translocated to the MAM and increased its interaction with IP3R3 following extended Tg treatment. Thus, the MAM is a critical cell-signaling junction whereby Bcl-xL dynamically interacts with IP3R3 to coordinate mitochondrial Ca(2+) transfer and alters cellular metabolism in order to increase the cells' bioenergetic capacity, particularly during periods of stress.

Biochemical Pharmacology of the Sigma-1 Receptor

The sigma-1 receptor (S1R) is a 223 amino acid two transmembrane (TM) pass protein. It is a non-ATP-binding nonglycosylated ligand-regulated molecular chaperone of unknown three-dimensional structure. The S1R is resident to eukaryotic mitochondrial-associated endoplasmic reticulum and plasma membranes with broad functions that regulate cellular calcium homeostasis and reduce oxidative stress. Several multitasking functions of the S1R are underwritten by chaperone-mediated direct (and indirect) interactions with ion channels, G-protein coupled receptors and cell-signaling molecules involved in the regulation of cell growth. The S1R is a promising drug target for the treatment of several neurodegenerative diseases related to cellular stress. In vitro and in vivo functional and molecular characteristics of the S1R and its interactions with endogenous and synthetic small molecules have been discovered by the use of pharmacologic, biochemical, biophysical, and molecular biology approaches. The S1R exists in monomer, dimer, tetramer, hexamer/octamer, and higher oligomeric forms that may be important determinants in defining the pharmacology and mechanism(s) of action of the S1R. A canonical GXXXG in putative TM2 is important for S1R oligomerization. The ligand-binding regions of S1R have been identified and include portions of TM2 and the TM proximal regions of the C terminus. Some client protein chaperone functions and interactions with the cochaperone 78-kDa glucose-regulated protein (binding immunoglobulin protein) involve the C terminus. Based on its biochemical features and mechanisms of chaperone action the possibility that the S1R is a member of the small heat shock protein family is discussed.

Synthesis and pharmacological characterization of a novel sigma receptor ligand with improved metabolic stability and antagonistic effects against methamphetamine

Methamphetamine interacts with sigma receptors at physiologically relevant concentrations suggesting a potential site for pharmacologic intervention. In the present study, a previous sigma receptor ligand, CM156, was optimized for metabolic stability, and the lead analog was evaluated against the behavioral effects of methamphetamine. Radioligand binding studies demonstrated that the lead analog, AZ66, displayed high nanomolar affinity for both sigma-1 and sigma-2 receptors (2.4 ± 0.63 and 0.51 ± 0.15, respectively). In addition, AZ66 had preferential affinity for sigma receptors compared to seven other sites and a significantly longer half-life than its predecessor, CM156, in vitro and in vivo. Pretreatment of male, Swiss Webster mice with intraperitoneal (10-20 mg/kg) or oral (20-30 mg/kg) dosing of AZ66 significantly attenuated the acute locomotor stimulatory effects of methamphetamine. Additionally, AZ66 (10-20 mg/kg, i.p.) significantly reduced the expression and development of behavioral sensitization induced by repeated methamphetamine administration. Taken together, these data indicate that sigma receptors can be targeted to mitigate the acute and subchronic behavioral effects of methamphetamine and AZ66 represents a viable lead compound in the development of novel therapeutics against methamphetamine-induced behaviors.

Partial agonists in schizophrenia--why some work and others do not: insights from preclinical animal models

While dopamine D2 receptor partial agonists (PAs) have been long considered for treating schizophrenia, only one, aripiprazole, is clinically available for therapeutic use. This raises critically important questions as to what is unique about aripiprazole and to what extent animal models can predict therapeutic success. A number of PAs whose clinical fate is known: aripiprazole, preclamol, terguride, OPC-4392 and bifeprunox were compared to haloperidol (a reference antipsychotic) in several convergent preclinical animal models; i.e. amphetamine-induced locomotion (AIL) and conditioned avoidance response (CAR), predictive of antipsychotic effects; unilateral nigrostriatal lesioned rats, a model of hypo-dopaminergia; striatal Fos induction, a molecular marker for antipsychotic activity; and side-effects common to this class of drugs: catalepsy (motor side-effects) and prolactaemia. The results were compared across drugs with reference to their measured striatal D2 receptor occupancy. All the PAs occupied striatal D2 receptors in a dose dependent manner, inhibited AIL and CAR, and lacked motor side-effects or prolactinaemia despite D2 receptor occupancy exceeding 80%. At comparative doses, aripiprazole distinguished itself from the other PAs by causing the least rotation in the hypo-dopaminergic model (indicating the least intrinsic activity) and showed the highest Fos expression in the nucleus accumbens (indicating functional D2 antagonism). Although a number of PAs are active in antipsychotic animal models, not all of them succeed. Given that only aripiprazole is clinically available, it can be inferred that low functional intrinsic activity coupled with sufficient functional antagonism as reflected in the animal models may be a marker of success.

Psychedelics and the human receptorome

We currently understand the mental effects of psychedelics to be caused by agonism or partial agonism of 5-HT(2A) (and possibly 5-HT(2C)) receptors, and we understand that psychedelic drugs, especially phenylalkylamines, are fairly selective for these two receptors. This manuscript is a reference work on the receptor affinity pharmacology of psychedelic drugs. New data is presented on the affinity of twenty-five psychedelic drugs at fifty-one receptors, transporters, and ion channels, assayed by the National Institute of Mental Health-Psychoactive Drug Screening Program (NIMH-PDSP). In addition, comparable data gathered from the literature on ten additional drugs is also presented (mostly assayed by the NIMH-PDSP). A new method is introduced for normalizing affinity (K(i)) data that factors out potency so that the multi-receptor affinity profiles of different drugs can be directly compared and contrasted. The method is then used to compare the thirty-five drugs in graphical and tabular form. It is shown that psychedelic drugs, especially phenylalkylamines, are not as selective as generally believed, interacting with forty-two of forty-nine broadly assayed sites. The thirty-five drugs of the study have very diverse patterns of interaction with different classes of receptors, emphasizing eighteen different receptors. This diversity of receptor interaction may underlie the qualitative diversity of these drugs. It should be possible to use this diverse set of drugs as probes into the roles played by the various receptor systems in the human mind.

The C-terminal amidated analogue of the substance P (SP) fragment SP(1-7) attenuates the expression of naloxone-precipitated withdrawal in morphine dependent rats

We previously demonstrated that intracerebroventricular (i.c.v.) administration of the substance P (SP) aminoterminal fragment SP(1-7) attenuates the expression of morphine withdrawal in the male rat. In this study we have used a synthetic analogue of this peptide, i.e. the SP(1-7) amide showing higher binding potency than the native heptapeptide, in a similar experimental set-up. Thus, Wistar male rats were made tolerant to morphine by daily injections of the opiate during 8 days. Following peptide administration (i.c.v.) and a subsequent naloxone challenge a variety of physical syndromes of withdrawal were recorded. We observed that the SP(1-7) amide potently and dose-dependently reduced several signs of reaction to morphine withdrawal. Interestingly, the effect of the peptide amide was significantly attenuated by the addition of the sigma agonist (+)-SKF-10047. We conclude that the SP(1-7) amide mimics the effect of the native SP fragment and that the mechanisms for its action involve a sigma receptor site.

An unambiguous assay for the cloned human sigma1 receptor reveals high affinity interactions with dopamine D4 receptor selective compounds and a distinct structure-affinity relationship for butyrophenones

The ability of the sigma(1) receptor to interact with a huge range of drug structural classes coupled with its wide distribution in the body has contributed to it being implicated as a possible therapeutic target for a broad array of disorders ranging from substance abuse to depression to Alzheimer's disease. Surprisingly, the reported affinity values for some sigma(1) receptor ligands vary more than 50-fold. The potential of the sigma(1) receptor as a pharmacotherapeutic target prompted us to develop an unambiguous assay system for measuring the affinity of ligands to the cloned human sigma(1) receptor. In the course of characterizing this system and determining the true affinity values for almost three dozen compounds, it was discovered that some dopamine D(4) receptor selective compounds bind sigma(1) receptors with high affinity. A systematic analysis of haloperidol-like compounds revealed a clear structure-affinity relationship amongst clinically relevant butyrophenones. The antidepressant fluvoxamine, the drug of abuse methamphetamine, and the neurosteroid progesterone were amongst the many ligands whose interactions with the sigma(1) receptor were confirmed with our screening assay.

Inhibition of hyperpolarization-activated cation currents by phencyclidine and some sigma ligands in rat hippocampal CA1 pyramidal neurons in vitro

Using whole-cell voltage-clamp recordings, hyperpolarization-activated cation currents (Ih) were elicited with hyperpolarizing voltage jumps in CA1 pyramidal neurons of rat hippocampal slices, and the effects of phencyclidine (PCP) and some sigma ligands on Ih were studied. PCP concentration-dependently (0.1-100 microM) suppressed Ih and shifted the activation curve of Ih to the negative direction. D-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP, 20 microM) and MK-801 (30 microM), competitive and non-competitive NMDA blockers, respectively, failed to mimic the inhibitory effect of PCP on Ih, and suppression of Ih by PCP was unaffected in the presence of these blockers. To explore the involvement of sigma1 receptors in the reduction of Ih, the effects of representative sigma1 ligands were studied. SKF10047 (100 microM), a sigma1 agonist, attenuated the maximal Ih and shifted the half-activation potential of Ih to the hyperpolarized direction. In the presence of the sigma1 antagonist NE-100 (1 microM), which alone did not affect Ih, the effect of SKF10047 on Ih was unaltered. By contrast, a higher concentration of NE-100 (10 microM) mimicked the effect of SKF10047. Again, no antagonism of Ih suppression by SKF10047 was obtained with rimcazole (100 microM), a sigma1 receptor antagonist that is structurally distinct from NE-100. This concentration of rimcazole alone resulted in a slight but significant reduction of Ih. Thus these major sigma1 ligands appear to suppress Ih independently of their agonistic or antagonistic properties. The results of this study suggest that PCP and some sigma ligands could modulate cell excitability partly through their action on Ih.

Animal models of focal dystonia

Animal models indicate that the abnormal movements of focal dystonia result from disordered sensorimotor integration. Sensorimotor integration involves a comparison of sensory information resulting from a movement with the sensory information expected from the movement. Unanticipated sensory signals identified by sensorimotor processing serve as signals to modify the ongoing movement or the planning for subsequent movements. Normally, this process is an effective mechanism to modify neural commands for ongoing movement or for movement planning. Animal models of the focal dystonias spasmodic torticollis, writer's cramp, and benign essential blepharospasm reveal different dysfunctions of sensorimotor integration through which dystonia can arise. Animal models of spasmodic torticollis demonstrate that modifications in a variety of regions are capable of creating abnormal head postures. These data indicate that disruption of neural signals in one structure may mutate the activity pattern of other elements of the neural circuits for movement. The animal model of writer's cramp demonstrates the importance of abnormal sensory processing in generating dystonic movements. Animal models of blepharospasm illustrate how disrupting motor adaptation can produce dystonia. Together, these models show mechanisms by which disruptions in sensorimotor integration can create dystonic movements.

Radiosynthesis, ex vivo and in vivo evaluation of [11C]preclamol as a partial dopamine D2 agonist radioligand for positron emission tomography

Dopamine D2 partial agonists have been successfully used as schizophrenia therapeutics. Radiolabeled D2 partial agonists may have application in elucidating dopaminergic transmission. It was the goal of this work to radiolabel (S)-(-)propyl-3-(3-hydroxyphenyl)piperidine (preclamol; (-)3-PPP), a partial dopamine D2 agonist with carbon-11 (half-life=20.4 min) and to evaluate this novel radiopharmaceutical for dopaminergic imaging in rodent models. [11C]Preclamol was synthesized by acylation of (S)-3-(3-hydroxyphenyl)piperidine hydrochloride with [11C]propionyl chloride, followed by LiAlH4 reduction, and HPLC purification. Male Sprague-Dawley rats were injected in the tail vein with a saline solution of [11C]preclamol (1.1 mug/kg) and sacrificed at 5, 15, 30 and 60 min postinjection. Brain regions were excised, weighed, and measured for radioactivity. In vivo binding kinetics of [11C]preclamol were determined with beta-sensitive microprobes implanted into the striatum and cerebellum of an anesthetized rat. A full production of [11C]preclamol resulted in 34 mCi ready for injection (corresponding to 4% uncorrected radiochemical yield, based on starting [11C]CO2) with specific activity of 535 mCi/micromol. The total synthesis time was 45 min and resulted in chemically and radiochemically pure [11C]preclamol (>99%; n=3). High levels of radioactivity were observed in rat brain indicating good blood-brain barrier penetration of [11C]preclamol, with 0.5 to 0.7% injected dose per gram of wet tissue present in all brain regions at 5 minutes postinjection. Unfortunately, [11C]preclamol displayed minimal preferential uptake in dopaminergic brain regions. A low striatal specific binding (SB) ratio of 0.32 was determined ex vivo at 60 min postinjection and was in close agreement with the microprobe study over 60 min (peaked at 27 min postinjection; SB ratio=0.6). The binding potential value was only 0.34 over a 1 hour time course, suggesting that [11C]preclamol is not suitable for cerebral PET studies.

Regulating ankyrin dynamics: Roles of sigma-1 receptors

Ankyrin is a cytoskeletal adaptor protein that controls important cellular functions, including Ca(2+) efflux at inositol 1,4,5-trisphosphate receptors (IP(3)R) on the endoplasmic reticulum. The present study found that sigma-1 receptors (Sig-1R), unique endoplasmic reticulum proteins that bind certain steroids, neuroleptics, and psychotropic drugs, form a trimeric complex with ankyrin B and IP(3)R type 3 (IP(3)R-3) in NG-108 cells. The trimeric complex could be coimmunoprecipitated by antibodies against any of the three proteins. Sig-1R agonists such as pregnenolone sulfate and cocaine caused the dissociation of an ankyrin B isoform (ANK 220) from IP(3)R-3. This effect caused by Sig-1R agonists was blocked by a Sig-1R antagonist. The degree of dissociation of ANK 220 from IP(3)R-3 caused by Sig-1R ligands correlates excellently with the ligands' efficacies in potentiating the bradykinin-induced increase in cytosolic free Ca(2+) concentration. Immunocytohistochemistry showed that Sig-1R, ankyrin B, and IP(3)R-3 are colocalized in NG-108 cells in perinuclear areas and in regions of cell-to-cell communication. These results suggest that Sig-1R and associated ligands may play important roles in cells by controlling the function of cytoskeletal proteins and that the Sig-1R/ANK220/IP(3)R-3 complex regulating Ca(2+) signaling may represent a site of action for neurosteroids and cocaine.

Regulating ankyrin dynamics: Roles of sigma-1 receptors

Ankyrin is a cytoskeletal adaptor protein that controls important cellular functions, including Ca(2+) efflux at inositol 1,4,5-trisphosphate receptors (IP(3)R) on the endoplasmic reticulum. The present study found that sigma-1 receptors (Sig-1R), unique endoplasmic reticulum proteins that bind certain steroids, neuroleptics, and psychotropic drugs, form a trimeric complex with ankyrin B and IP(3)R type 3 (IP(3)R-3) in NG-108 cells. The trimeric complex could be coimmunoprecipitated by antibodies against any of the three proteins. Sig-1R agonists such as pregnenolone sulfate and cocaine caused the dissociation of an ankyrin B isoform (ANK 220) from IP(3)R-3. This effect caused by Sig-1R agonists was blocked by a Sig-1R antagonist. The degree of dissociation of ANK 220 from IP(3)R-3 caused by Sig-1R ligands correlates excellently with the ligands' efficacies in potentiating the bradykinin-induced increase in cytosolic free Ca(2+) concentration. Immunocytohistochemistry showed that Sig-1R, ankyrin B, and IP(3)R-3 are colocalized in NG-108 cells in perinuclear areas and in regions of cell-to-cell communication. These results suggest that Sig-1R and associated ligands may play important roles in cells by controlling the function of cytoskeletal proteins and that the Sig-1R/ANK220/IP(3)R-3 complex regulating Ca(2+) signaling may represent a site of action for neurosteroids and cocaine.

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.

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

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

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.

Fluorine-18 radiolabelling, biodistribution studies and preliminary PET evaluation of a new memantine derivative for imaging the NMDA receptor

A synthetic method has been established for preparing [18F]1-amino-3-fluoromethyl-5-methyl-adamantane ([18F]AFA). Biodistribution of the radiotracer in mice showed high brain uptake. The peak uptake (3.7% I.D/g organ) for the brain occurred at 30 min after injection. Accumulation of radioactivity in mouse brain was consistent with the known distribution of the NMDA receptors. The binding of [18F]AFA to the phencyclidine (PCP) binding sites of the NMDA receptor complex and the sigma recognition sites in a Rhesus monkey was also examined using positron emission tomography (PET). The regional brain distribution of [18F]AFA was changed by memantine and by (+)-MK-801, indicating competition for the same binding sites. Treatment with haloperidol caused a marked reduction of radioactivity uptake in all the brain regions examined. (-)-Butaclamol, which has pharmacological specificity for sigma sites, did not have any significant effects.

Effects of cyclazocine on cocaine self-administration in rats

Cyclazocine is a kappa-opioid receptor agonist and mu-opioid receptor antagonist that was studied in the 1960s as a potential treatment for heroin addicts. Based on the evidence that opioid mechanisms modulate the reinforcing effects of cocaine, it has been suggested that cyclazocine be reconsidered for use in treating cocaine dependence. In the present study, the effects of orally administered (+/-)-cyclazocine, (+)-cyclazocine and (-)-cyclazocine on intravenous cocaine self-administration were assessed in rats. (+/-)-Cyclazocine produced a dose-related (2-8 mg/kg) decrease in cocaine intake without affecting bar-press responding for water. Neither enantiomer significantly altered responding for either cocaine or water. The efficacy of orally administered (+/-)-cyclazocine on cocaine self-administration was comparable to that previously observed using the intraperitoneal route. Distinct actions of the enantiomers of cyclazocine that might contribute to the unique efficacy of the racemate are discussed. Although the mechanistic basis for the results are not entirely understood, the data suggest that (+/-)-cyclazocine should be considered as a potential treatment for cocaine dependence.

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.

Uncompetitive inhibition of [3H]1,3-di-o-tolyl-guanidine-defined sigma binding sites by desipramine, propranolol and alprenolol in rat brain

Desipramine, imipramine, clomipramine, (-)-propranolol, (-)-alprenolol, (+/-)-pentazocine and risperidone caused a concentration-dependent inhibition of 6 nM [3H]DTG (1,3-di-o-tolylguanidine)-defined sigma (sigma) binding with Ki values of about 0.5-2.5 microM in well-washed homogenates obtained from rat cerebral cortex. The saturation studies revealed that the inhibition by desipramine (1-4 microM), (-)-propranolol (1 microM) and (-)-alprenolol (3 microM) resulted from a reduction of the Bmax value without alteration of the Kd of [3H]DTG binding to the cortex or hippocampus. In contrast, imipramine, (+/-)-pentazocine, clomipramine and risperidone competitively attenuated the cortical or hippocampal [3H]DTG binding. These findings demonstrate the uncompetitive inhibition of [3H]DTG binding by neuroactive drugs, thereby providing further support for the possible multiple regulation of cerebral sigma receptors.

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

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

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

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

Risperidone compared with new and reference antipsychotic drugs: in vitro and in vivo receptor binding

Risperidone and its active metabolite 9-OH-risperidone were compared to reference antipsychotic drugs (haloperidol, pipamperone, fluspirilene, clozapine, zotepine) and compounds under development (olanzapine, seroquel, sertindole, ORG-5222, ziprasidone) for in vitro binding to neurotransmitter receptors in brain tissue and on membranes of recombinant cells expressing cloned human receptors and for in vivo occupancy of neurotransmitter receptors in rat and guinea-pig brain following acute treatment (2 h., s.c.). An ex vivo autoradiography technique was applied to determine the receptor occupancy by the drugs administered in vivo. Of particular interest are the central 5HT2A receptors and D2-type receptors. Predominant 5HT2A receptor antagonism is supposed to add to an atypical profile of the antipsychotics (treatment of the negative symptoms, low incidence of extrapyramidal side effects). D2 antagonism is required the treatment of positive symptoms. A contribution of the new dopamine receptor subtypes D3 and in particular D4 receptors has been proposed. In vitro, all compounds, except the 'typical' antipsychotics haloperidol and fluspirilene, showed higher affinity for 5HT2A than for D2 receptors. Subnanomolar affinity for human 5HT2A receptors was observed for ORG-5222, sertindole, risperidone, 9-OH-risperidone and ziprasidone. Fluspirilene, ORG-5222, haloperidol, ziprasidone, risperidone, 9-OH-risperidone and zotepine displayed nanomolar affinity for human D2 receptors. Sertindole and olanzapine were slightly less potent. Pipamperone, clozapine and seroquel showed 2 orders of magnitude lower D2 affinity in vitro. Clozapine, but even more so pipamperone, displayed higher affinity for D4 than for D2 receptors. For most other compounds, D4 affinity was only slightly lower than their D2 affinity. Seroquel was totally devoid of D4 affinity. None of the compounds had nanomolar affinity for D1 receptors; their affinity for D3 receptors was usually slightly lower than for D2 receptors. In vivo, ORG-5222, risperidone, pipamperone, 9-OH-risperidone, sertindole, olanzapine, zotepine and clozapine maintained a higher potency for occupying 5HT2A than D2 receptors. Risperidone and ORG-5222 had 5HT2A versus D2 potency ratio of about 20. Highest potency for 5HT2A receptor occupancy was observed for ORG-5222 followed by risperidone and olanzapine. Ziprasidone exclusively occupied 5HT2A receptors. ORG-5222, haloperidol, fluspirilene and olanzapine showed the highest potency for occupying D2 receptors. No regional selectivity for D2 receptor occupancy in mesolimbic versus nigrostriatal areas was detected for any of the test compounds. Risperidone was conspicuous because of its more gradual occupancy of D2 receptors; none of the other compounds showed this property. The various compounds also displayed high to moderate occupancy of adrenergic alpha 1 receptors, except fluspirilene and ziprasidone. Clozapine, zotepine, ORG-5222 and sertindole occupied even more alpha 1 than D2 receptors. Clozapine showed predominant occupancy of H1 receptors and occupied cholinergic receptors with equivalent potency to D2 receptors. A stronger predominance of 5HT2A versus D2 receptor occupancy combined with a more gradual occupancy of D2 receptors differentiates risperidone and its 9-OH-metabolite from the other antipsychotic compounds in this study. The predominant 5HT2A receptor occupancy probably plays a role in the beneficial action of risperidone on the negative symptoms of schizophrenia, whereas maintenance of a moderate occupancy of D2 receptors seems adequate for treating the positive symptoms of schizophrenia. A combined 5HT2A and D2 occupancy and the avoidance of D2 receptor overblockade are believed to reduce the risk for extrapyra

In vitro receptor binding and in vivo receptor occupancy in rat and guinea pig brain: risperidone compared with antipsychotics hitherto used

Risperidone was compared with antipsychotics hitherto used for in vitro receptor binding using animal brain or cloned (human) receptors and in vivo receptor occupancy in rat and guinea pig brain following acute treatment. Both in vitro and in vivo, risperidone, 9-OH-risperidone, SM-9018, clozapine and clocapramine showed higher affinity for 5-HT2A- than for D2-receptors, whereas mosapramine, haloperidol, bromperidol and nemonapride had a slight to strong preference for D2- compared to 5-HT2A-receptors. In vivo, risperidone showed the highest potency for 5-HT2A-receptor occupancy; To obtain the same extent of D2-receptor occupancy, a 19-times higher dosage was required. 9-OH-Risperidone, the principal active metabolite of risperidone, showed a receptor occupancy profile comparable to that of risperidone. No regional selectivity for D2-receptor occupancy in mesolimbic vs nigrostriatal areas was detected for any of the compounds. Risperidone differed from the other compounds by the remarkably shallow slope of its D2-receptor dose-occupancy curve. A greater predominance of 5-HT2A-receptor vs D2-receptor occupancy and a more gradual occupancy of D2-receptors differentiate risperidone from the other compounds. Both properties probably assist in preventing an extensive blockade of D2-receptors, the cause for extrapyramidal symptoms (EPS). The predominant 5-HT2A-receptor occupancy most likely underlies risperidone's beneficial effects on the negative symptoms of schizophrenia and an adequately low D2-receptor occupancy adds to the treatment of positive symptoms with a low liability of EPS.

Effect of haloperidol and its metabolites on dopamine and noradrenaline uptake in rat brain slices

The effects of haloperidol and its metabolites on dopamine (DA) and noradrenaline (NA) uptake were investigated. Both direct uptake of [3H]DA and [3H]NA into the rat striatal and hippocampus slices and binding of a specific DA uptake inhibitor [3H]GBR-12935 were employed in the present study. Haloperidol pyridinium (HP+), haloperidol 1,2,3,6-tetrahydropyridine (HTP), 4-(4-chlorophenyl)-1,2,3,6-tetrahydropyridine (CPTP) and reduced haloperidol (RHAL) are potent inhibitors of DA uptake. HTP N-oxide (HTPNO) exhibits a relatively weak effect on DA uptake. Other metabolites of haloperidol, i.e. 4-(4-chlorophenyl)-4-hydroxypyridine (CPHP) and haloperidol N-oxide (HNO), as well as haloperidol itself possess negligible inhibitory effect on DA uptake. HP+ has been shown to be an amine releaser. It is possible that HP+ may induce amphetamine-like neurotoxicity. The effects of the metabolites of haloperidol on [3H]NA uptake are similar to those on [3H]DA uptake. HP+ appears to be different from MPP+, which is a more potent [3H]NA uptake blocker than on [3H]DA uptake. Although haloperidol exhibits no DA uptake inhibitory effect, it has a high affinity for the [3H]GBR-12935 binding site. The possible pharmacological implications such inhibitory effects on amine uptake are discussed.

Characteristics of binding of [3H]NE-100, a novel sigma-receptor ligand, to guinea-pig brain membranes

We examined the characteristics of binding of radiolabeled N,N-dipropyl-2-[4-methoxy-3-(2-phenyl-ethoxy)phenyl]- ethylamine monohydrochloride ([3H] NE-100), a highly potent and selective sigma-receptor ligand, to guinea-pig brain membranes. [3H]NE-100 showed saturable and reversible binding to sigma binding sites. A dissociation constant (Kd) and maximal numbers of binding sites (Bmax) obtained from Scatchard plot analysis were 1.2 +/- 0.1 nM and 1049.3 +/- 115.1 fmol/mg protein (n = 3), respectively. NE-100 was the most potent inhibitor of [3H]NE-100 binding among several structurally dissimilar sigma-receptor ligands, including haloperidol and (+)-pentazocine. (+)-Benzomorphanes had more than a 10-fold potent inhibitory activity over (-)-benzomorphanes, with regard to [3H]NE-100 binding. The binding of [3H]NE-100 was not influenced by histaminergic, dopaminergic, adrenergic, serotonergic cholinergic or glutaminergic agents at 10(-7) M. GTP-gamma-S and phenytoin also did not affect the binding of [3H]NE-100. A higher [3H]NE-100 binding was observed in the cerebellum and medulla oblongata. Except for the nuclear fraction, the highest level of [3H]NE-100 binding to subcellular fractions was observed in microsomal fractions. These results suggest that NE-100 selectively binds, with a high affinity, to sigma-1 binding sites in guinea-pig brain membranes, as an "antagonist".

Biphasic effects of sigma ligands on the neuronal response to N-methyl-D-aspartate

Previous studies from our laboratory have demonstrated that low doses of selective sigma (sigma) ligands potentiate the neuronal response to N-methyl-D-aspartate (NMDA) in the CA3 region of the rat dorsal hippocampus. Sertraline and clorgyline, two antidepressant drugs with a high affinity for receptors, also potentiate, at low doses, the NMDA response; however, when administered at higher sigma doses, the degree of potentiation induced by these two ligands progressively decreases (Bergeron et al. 1993). In the present experiments, the selective sigma ligands DTG, (+)pentazocine, BD-737, JO-1784 and L-687,384 were studied to determine if they would also generate bell-shaped dose-response curves. These ligands were administered intravenously at doses ranging from 1 micrograms/kg to 1 mg/kg or applied by microiontophoresis. They potentiated selectively, with bell-shaped dose-response curves, the NMDA-induced activation of pyramidal neurons in the CA3 region of the rat dorsal hippocampus. The potentiation of the NMDA response following the intravenous administration of a low dose of a sigma ligand persisted for at least 60 min, after which point in time a second injection of the same dose induced the same degree of potentiation. Moreover, a sustained potentiation was obtained during prolonged microiontophoretic applications of a sigma ligand. These two latter series of observations suggest that the lack of effect of the high doses of sigma ligands is not related to a rapid desensitisation of sigma receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute effects of sigma ligands on the extracellular DOPAC level in rat frontal cortex and striatum

Acute administration of (+)-N-allylnormetazocine ((+)-SKF-10,047) and (+/-)-pentazocine, was found to increase the extracellular level of 3,4-dihydroxyphenylacetic acid (DOPAC), a major dopamine (DA) metabolite, in the rat frontal cortex. By contrast, these benzomorphan sigma ligands did not change the extracellular DOPAC level in the rat striatum. On the other hand, 1,3-di(2-tolyl)guanidine (DTG) increased the extracellular DOPAC level in the frontal cortex, while it decreased that level in the striatum. Another non-benzomorphan sigma ligand, (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3-PPP) decreased the extracellular DOPAC level in both frontal cortex and striatum. Moreover, the increase of the extracellular DOPAC level elicited by (+)-SKF-10,047 was significantly inhibited by rimcazole, a putative sigma antagonist, while the DTG-induced increment was not reversed by rimcazole. These findings indicated that the effects of sigma ligands on the mesocortical DA neurons differed from those on the nigrostriatal DA neurons. In addition, the effects of benzomorphan sigma ligands on the central DA neurons were different from those of non-benzomorphan sigma ligands.

Further characterization of the effects of BMY 14802 on dopamine neuronal activity

Further evaluation of the effects of BMY 14802 on dopamine (DA) neuronal activity in the rat substantia nigra pars compacta (A9) was conducted with single-unit recording and microiontophoresis in anesthetized rats. Microiontophoretic administration of BMY 14802 (sigma, serotonin (5-HT)-1A and alpha-1 adrenoceptor ligand) had no effect on DA neurons. Microiontophoretic administration of (+)-3-PPP (weak D2 agonist with high affinity for sigma receptors) and quinpirole (D2/D3 agonist) inhibited A9 DA neuronal activity. Co-iontophoresis or i.v. pretreatment with BMY 14802 had no effect on the current-response curves for the effects of microiontophoretic (+)-3-PPP or quinpirole on A9 DA neurons. Co-iontophoretic administration of (-)-sulpiride, a selective D2 antagonist, blocked the inhibitory effects of microiontophoretic (+)-3-PPP. The effects of BMY 14802 (0.25-8 mg/kg, i.v.) on DA neurons (increased firing rate, increased burst-firing, reduced regularity of firing pattern) were not altered by acute brain hemitransection, but were blocked by pretreatment with NAN-190, an antagonist of 5-HT-1A and alpha-1 receptors. The alpha-1 receptor antagonist, prazosin, did not block these effects of BMY 14802. In conclusion, the effects of BMY 14802 on DA neuronal firing rate and firing pattern are indirect, perhaps due in part to the occupation of 5-HT-1A receptors.

Selective reduction of N-methyl-D-aspartate-evoked responses by 1,3-di(2-tolyl)guanidine in mouse and rat cultured hippocampal pyramidal neurones

1. The effects of 1,3-di(2-tolyl)guanidine (DTG) were examined on the responses of cultured hippocampal neurones to the excitatory amino acid analogues N-methyl-D-aspartate (NMDA), kainate, quisqualate and (RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA). 2. In rat hippocampal neurones loaded with the Ca(2+)-sensitive dye Fura-2, DTG (10-100 microM) produced a concentration-dependent depression of the NMDA-evoked rises in intracellular free calcium ([Ca2+]i), an effect that was not modified by changes in the extracellular glycine concentration. DTG (at 50 and 100 microM) also attenuated, although to a lesser extent, the rises in [Ca2+]i evoked by naturally-derived quisqualate. In contrast, 50 and 100 microM DTG did not depress responses evoked by kainate, AMPA and synthetic, glutamate-free (+)-quisqualate although on occasions DTG enhanced kainate- and AMPA-evoked rises in [Ca2+]i. 3. DTG attenuated NMDA-evoked currents recorded from mouse hippocampal neurones under whole-cell voltage-clamp with an IC50 (mean +/- s.e. mean) of 37 +/- 5 microM at a holding potential of -60 mV. The DTG block of NMDA-evoked responses was not competitive in nature and was not dependent on the extracellular glycine or spermine concentration. The block did, however, exhibit both voltage-, and use-, dependency. The steady-state current evoked by naturally-derived quisqualate was also attenuated by DTG whereas those evoked by kainate and AMPA were not. 4. We conclude that DTG, applied at micromolar concentrations, is a selective NMDA antagonist in cultured hippocampal neurones, the block exhibiting both Mg(2+)- and phencyclidine-like characteristics. Given the nanomolar affinity of DTG for sigma binding sites it is unlikely that the antagonism observed here is mediated by sigma-receptors, but the data emphasize the potential danger of ascribing the functional consequences of DTG administration solely to sigma receptor-mediated events.

Current hypotheses on sigma receptors and their physiological role: possible implications in psychiatry

During the last years, due to the availability of selective ligands, numerous investigations have been dedicated to sigma receptors. The existence of different subtypes of these receptors is now accepted; their endogenous ligand has not yet been identified, but some candidates have been proposed. Evidence suggests that one of their major roles might be to regulate the activity of the glutamatergic system via the N-methyl-D-aspartate receptor. The potential involvement of sigma receptors in psychiatry was suggested by the psychotomimetic effects of their earliest ligands and the fact that several neuroleptics have a high affinity for them. Recently, new arguments have strengthened this hypothesis: some molecules with high sigma affinity but low dopaminergic affinity display a "neuroleptic-like" pharmacological profile; post-mortem studies have shown a reduction of sigma binding sites in the brain of patients with schizophrenia; cocaine, which can induce psychotic episodes, has high affinity for sigma receptors. Hence, by modulating the glutamatergic inputs, by regulating directly the firing activity of dopaminergic neurons, or by both mechanisms, sigma receptors could be involved in the pathophysiology and/or in the treatment of schizophrenia.

The glycine/NMDA receptor antagonist, R-(+)-HA-966, blocks activation of the mesolimbic dopaminergic system induced by phencyclidine and dizocilpine (MK-801) in rodents

1. The effects of the glycine/N-methyl-D-aspartate (NMDA) receptor antagonist, R-(+)-HA-966 on the neurochemical and behavioural responses to phencyclidine (PCP) and dizocilpine (MK-801) have been determined in rodents. 2. In rats, pretreatment with PCP (5 and 10 mg kg-1) or MK-801 (0.25 and 0.5 mg kg-1) dose-dependently stimulated dopamine turnover in nucleus accumbens, amygdala and medial prefrontal cortex, but had no effect in striatum. In contrast, pretreatment with (+)-HA-966 (10 and 30 mg kg-1) did not affect dopamine turnover in any brain region investigated. 3. Pretreatment with (+)-HA-966 (10 and 30 mg kg-1) significantly antagonized the stimulation of dopamine turnover induced by both PCP (10 mg kg-1) and MK-801 (0.5 mg kg-1) in rat nucleus accumbens, amygdala and medial prefrontal cortex. 4. Intracerebral dialysis studies in conscious rats demonstrated that systemic injection of PCP (10 mg kg-1) markedly stimulated dopamine release from the nucleus accumbens, an effect that was abolished by pretreatment with (+)-HA-966 (30 mg kg-1). 5. Pretreatment with PCP (3-30 mg kg-1) or MK-801 (0.1-1.6 mg kg-1) significantly increased locomotor activity in mice. In contrast, subcutaneous injection of (+)-HA-966 (10-100 mg kg-1) failed to stimulate activity. 6. Pretreatment with (+)-HA-966 (10 and 30 mg kg-1) dose-dependently antagonized both PCP (10 mg kg-1) and MK-801 (0.4 mg kg-1) induced hyperactivity in mice. 7. Blockade of PCP-induced hyperactivity by (+)-HA-966 is unlikely to be explained by the induction or potentiation of sedation/ataxia since PCP-induced rotarod deficits were not significantly different in mice pretreated with (+)-HA-966 (30 mg kg-1) or saline.8. The results demonstrate that (+ )-HA-966 antagonizes both the neurochemical and behavioural effects of PCP and MK-801, possibly through interactions at the glycine/NMDA receptor.

Repeated administration of Sigma ligands alters the population activity of rat midbrain dopaminergic neurons

Acute and repeated administration of antipsychotic drugs produce distinctive profiles of electrophysiological effects on the population activity of midbrain dopaminergic (DA) neurons which correlate with their clinical effects. Sigma receptors have been hypothesized to be involved in psychosis and in the efficacy of antipsychotic drugs, but little is known about the effects of repeated treatment with sigma ligands on the activity of midbrain DA neuronal populations. In the present study, the cells-per-track cell-sampling method was used to evaluate the effects of 3 sigma ligands on the numbers of spontaneously active A9 and A10 DA neurons in chloral hydrate-anesthetized rats. One-hour pretreatment with either (+)-pentazocine (10 mg/kg, i.p.), DTG (2 mg/kg, i.p.), or JO 1784 (1 or 10 mg/kg, s.c.) did not alter the number of spontaneously active DA neurons encountered per electrode track. Repeated treatment (21 daily injections) with (+)-pentazocine (1 or 10 mg/kg) or DTG (0.2 or 2 mg/kg) increased the number of A10 DA cells per track; JO 1784 (10 mg/kg but not 1 mg/kg) moderately decreased the number of active A9 DA cells and increased the firing rate of A10 DA neurons. The effect of JO 1784 on A9 DA neurons was not due to depolarization inactivation. The effects of all 3 sigma ligands differ from those of antipsychotic drugs, all of which inactivate A10 DA neurons after repeated treatment. Clinical studies are necessary to determine if selective sigma ligands will provide a novel alternative to DA antagonists in the treatment of psychosis.

Similar ameliorating effects of benzomorphans and 5-HT2 antagonists on drug-induced impairment of passive avoidance response in mice: comparison with acetylcholinesterase inhibitors

Mice were trained to avoid electric shocks by means of step-down type passive avoidance learning tasks, and memory retention was measured 24 h after the training session. Memory impairment (amnesia) was produced by administering either p-chloroamphetamine (PCA), a serotonin (5-HT) releaser or scopolamine (SCOP), a muscarinic cholinoceptor antagonist, 30 min prior to the training session. Benzomorphans, 5-HT2 antagonists and acetylcholinesterase (AChE) inhibitors were administered immediately after the training session. PCA- but not SCOP-induced amnesia was attenuated by the post-training administration of two benzomorphans, (+)N-allylnormetazocine ((+)SKF-10,047) and (+/- )pentazocine ((+/- )PTZ). Similarly, PCA-induced amnesia was reversed by the post-training administration of 5-HT2 antagonists, ritanserin (RIT) and mianserin (MIA), but SCOP-induced amnesia was not. However, the AChE inhibitors, tetrahydroaminoacridine (THA) and physostigmine (PHY) attenuated both PCA- and SCOP-induced amnesia when administered immediately after the training session. These results indicated that benzomorphans and 5-HT2 antagonists have antiamnestic effects in mice, as do AChE inhibitors. In addition, it is interesting that the patterns of ameliorating effect of benzomorphans were similar to those of 5-HT2 antagonists, which differ from those of AChE inhibitors.

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.

BMY-14802 reverses the reduction of striatal dopamine release induced by (+)-3-[3-hydroxyphenyl]-N-(1-propyl)piperidine

Intraperitoneal injection of (+)-3-[3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3PPP), a sigma receptor agonist, significantly reduced the striatal levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) measured by in vivo microdialysis. These reductions were significantly greater at (+)-3PPP doses of 12 and 24 mg/kg than at 1 mg/kg. The levels of 5-hydroxyindoleacetic acid (5HIAA) were increased by the injection of (+)-3PPP in dose of 24 mg/kg, but were not affected at lower doses. BMY-14802, a sigma antagonist, alone at doses of 15 mg/kg and 30 mg/kg did not affect the levels of DA, DOPAC, HVA and 5HIAA. Pretreatment with 30 mg/kg BMY-14802 reversed the reduction of the levels of DA induced by 12 mg/kg (+)-3PPP. Although neither 30 mg/kg BMY-14802 nor 12 mg/kg (+)-3PPP affected the levels of striatal 5HIAA, combined treatment with both produced a significant elevation. These findings clearly demonstrate that sigma receptors may regulate DA release from the striatal presynapse.

Regional distribution and kinetics of haloperidol binding in human brain: a PET study with [18F]haloperidol

The regional distribution and the kinetics of haloperidol uptake in human brain were examined using [18F]haloperidol and PET in 9 controls and 5 schizophrenics while on haloperidol medication and after haloperidol washout. The regional distribution of [18F]N-methylspiroperidol, a tracer for D2 receptors, was measured in 1 normal subject for comparison. The uptake of [18F]haloperidol in the whole brain in normals was high (6.6% of the injected dose at 2 hr), and regional distribution was much more extensive than could be accounted for by the distribution of dopamine D2 receptors. In normals, the cerebellum, basal ganglia, and thalamus showed a greater concentration than the cortex, and there was minimal clearance of 18F from the brain during the 10-hr period of the study. Medicated schizophrenics showed a total brain uptake of 4.0% and had a significant clearance of [18F]haloperidol from brain and a higher concentration of [18F]haloperidol in plasma. After withdrawal from medication, [18F]haloperidol clearance from brain became slower than while on medication. These results are discussed in terms of the pharmacokinetics of haloperidol in the human brain and its binding to dopamine D2 receptors and to sigma receptors.

Sigma receptors. Putative links between nervous, endocrine and immune systems

The sigma receptor is a neuronal substrate that binds several psychoactive compounds. These include cocaine, some steroids, dextromethorphan, phencyclidine (PCP), and benzomorphans such as pentazocine and N-allyl-normatezocine (SKF-10047). Many newer atypical antipsychotic drugs also bind to the sigma receptor. The sigma receptor, however, is not the PCP receptor. The sigma receptor exists in the central nervous system, endocrine, immune and certain peripheral tissues. Progesterone and certain steroids have been shown to represent endogenous ligands for the sigma receptor. The sigma receptor resides likely in the nonsynaptic region of the plasma membrane. The sigma receptor exists in two forms: high-affinity and low-affinity. The solubilized sigma receptor retains all of the pharmacological characteristics of a membrane-bound receptor. A major physiological role of the sigma receptor may involve the modulation of a tonic potassium channel.

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

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

Characteristics of [3H]GBR 12935 binding in the human and rat frontal cortex

Binding characteristics of the selective dopamine uptake inhibitor [3H]GBR 12935 have been described for the striatum but not for the frontal cortex. We have developed assay conditions for quantifying [3H]GBR 12935 binding in the frontal cortex. In both the rat and human frontal cortex, the assay required four times more tissue (8 mg/ml) than in the striatum (2 mg/ml). [3H]GBR 12935 binding in the frontal is complex, as it involves multiple binding sites. The high-affinity binding site is sodium dependent and is inhibited by sodium. In human but not in rat frontal cortex, addition of K+ reversed the sodium inhibition. The pharmacological profile of the high-affinity [3H]GBR 12935 binding site is consistent with that of the dopamine transporter, because drugs with the most selective dopamine reuptake blocking activities are the most potent displacers of [3H]GBR 12935 binding. There is a positive correlation between the rat and human inhibitory constants, a finding indicating that there are similar pharmacological profiles across at least these two species. Rats with a 6-hydroxydopamine lesion had a 47% decrease in number of [3H]GBR 12935 binding sites, a result indicating that at least a portion of these sites had been on presynaptic dopamine terminals.

Mechanisms of action of atypical antipsychotic drugs. Implications for novel therapeutic strategies for schizophrenia

The mechanisms which contribute to the actions of atypical antipsychotic drugs, such as clozapine and the putative atypical agents remoxipride and raclopride, are reviewed. Examination of available preclinical and clinical data leads to two hypotheses concerning the mode of action of atypical antipsychotic drugs. The first hypothesis is that antagonism of the dopamine D2 receptor is both necessary and sufficient for the atypical profile, but that interaction with subtypes of the D2 receptor differentiates typical from atypical antipsychotic drugs. The second hypothesis has been previously advanced, and suggests that a relatively high ratio of serotonin 5-HT2:dopamine D2 receptor antagonism may subserve the atypical profile. It seems likely that the atypical antipsychotic drug profile may be achieved in more than one way.