Effects of sigma ligands on the ability of rimcazole to inhibit PCP hsp70 induction

Targeting sigma receptors: novel medication development for drug abuse and addiction

Psychostimulant abuse is a serious health and societal problem in industrialized and developing countries. However, the identification of an effective pharmacotherapy to treat it has remained elusive. It has long been known that many psychostimulant drugs, including cocaine and methamphetamine, interact with sigma receptors in the brain and heart, offering a logical target for medication development efforts. However, selective pharmacological agents and molecular biological tools have only recently become available to rigorously evaluate these receptors as viable medication development targets. The current review will summarize provocative preclinical data, demonstrating the ability of sigma receptor antagonists and antisense oligonucleotides to ameliorate cocaine-induced convulsions, lethality, locomotor activity and sensitization, and conditioned place-preference in rodents. Recent studies suggest that the protective effects of sigma receptor antagonists also extend to actions produced by methamphetamine, 3,4-methylenedioxymethamphetamine, ethanol and other abused substances. Together, the data indicate that targeting sigma receptors, particularly the σ(1)-subtype, may offer an innovative approach for combating the effects of cocaine, and perhaps other abused substances.

Early brain development disruption from NMDA receptor hypofunction: relevance to schizophrenia

Disruption to brain development at an early stage can potentially alter chemically coded neural networks and can affect behavior in later life. During early brain development antagonism of glutamate NMDA receptors, which play an important role in neuronal outgrowth and survival, leads to neuronal damage in several brain regions and causes behavioral alterations in rodents that mimic schizophrenia symptoms and endophenotypes. There are several lines of evidence implicating involvement of a dysfunctional glutamate system in schizophrenia. In normal subjects, NMDA receptor antagonists produce behavioral and neurochemical changes that mimic schizophrenia symptoms better than any other psychotomimetic drug. Moreover, these drugs worsen symptoms in schizophrenia patients and can trigger a recrudescence of the acute psychotic state in patients with stable chronic schizophrenia. In addition, genes consistently reported as being altered in schizophrenia play roles in development, neuroplasticity and glutamate/GABAergic neurotransmission. Perinatal NMDA receptor antagonist treatment is a useful model for studying the neurodevelopmental and NMDA receptor hypofunction hypotheses of schizophrenia because neurochemical and behavioral changes, reminiscent of those seen in schizophrenia, are present long after cessation of drug administration, which suggests that a permanent change in brain structure and organization has occurred during brain development.

Review of the pharmacological and clinical profile of rimcazole

Rimcazole is a carbazole derivative that acts in part as a sigma receptor antagonist. Wellcome Research Laboratories introduced this compound during the 1980s when it was hypothesized to be a novel antipsychotic with an improved side effect profile. However, subsequent clinical trials demonstrated that rimcazole lacked efficacy in schizophrenic patients and it is now primarily used as an experimental tool. In addition to its actions as a sigma receptor antagonist, rimcazole also has high affinity for dopamine transporters, and in recent years it has served as a lead compound for the development of novel dopamine transporter ligands. Although rimcazole cannot be considered a selective ligand for sigma receptors, the recent development of other selective agonists and antagonists for sigma receptors have aided in clarifying the involvement of these receptors in the actions of rimcazole. Many of the physiological and behavioral effects of rimcazole can in fact be ascribed to its action as a sigma receptor antagonist, although there are exceptions. Rimcazole is likely to have a continued role in elucidating sigma receptor function in either in vitro or in vivo systems where sigma receptor-mediated effects can be studied independently of the influence of dopamine and serotonin transporters.

Psychosis: pathological activation of limbic thalamocortical circuits by psychomimetics and schizophrenia?

Non-competitive NMDA receptor antagonists, such as phencyclidine, ketamine and MK801, produce psychosis in humans. These drugs also produce injury to cingulate-retrosplenial cortex in adult rodents that can be prevented by GABA-receptor agonists and antipsychotics such as haloperidol and clozapine. MK801 injections into anterior thalamus reproduce limbic cortex injury, and GABA-receptor agonist injections into anterior thalamus prevent injury produced by systemic MK801. Inhibition of NMDA receptors on GABAergic thalamic reticular nucleus neurons might activate thalamocortical 'injury' circuits in animals. Pathological activation of thalamocortical circuits might also mediate the psychosis produced by NMDA-receptor antagonists in humans, and might contribute to psychosis in schizophrenia.

Phencyclidine (PCP) acts at sigma sites to induce c-fos gene expression

Phencyclidine (PCP) is a compound that results in abnormal human behavior and has been proposed as a chemical model for schizophrenia. It was hypothesized that PCP induction of the immediate-early gene, c-fos, should be seen in areas associated with emotional behavior, such as the cortex and limbic system. It was also proposed that PCP may induce c-fos via the sigma receptor. PCP and two sigma ligands, 1,3-di(2-tolyl)guanidine (DTG) and pentazocine, were shown to induce c-fos in similar patterns. The three compounds abundantly induced c-fos in the cingulate, parietal, and piriform cortices and the midline structures of the thalamus and hypothalamus. Neither PCP nor the sigma ligands induced c-fos in the hippocampus. This suggests that PCP binding at NMDA receptors does not result in significant c-fos induction. Rimcazole, a putative sigma2 receptor antagonist, and other sigma ligands have been shown to ameliorate PCP stereotypic behavior. Rimcazole inhibited PCP c-fos induction in the cingulate and parietal cortices and DTG c-fos induction in the cingulate cortex. DTG shows both sigma1 and sigma2 binding affinity. Rimcazole failed to inhibit pentazocine c-fos induction. Pentazocine binds only to sigma1 receptors. This suggests that PCP may produce a significant fraction of its c-fos induction via sigma2 receptors.