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

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

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

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.

Sigma receptors: potential medications development target for anti-cocaine agents

The ability of cocaine to interact with sigma receptors suggests a viable target for medications development. Recently, numerous novel compounds and antisense oligodeoxynucleotides targeting sigma receptors have been synthesized and shown to prevent the behavioral toxicity and psychomotor stimulant effects of cocaine in animals. Protective doses of sigma receptor antagonists have also been shown to prevent changes in gene expression that are induced by cocaine. Together, the studies provide insight and promising future directions for the development of potential medications for the treatment of cocaine addiction and overdose.

Involvement of sigma receptors in the behavioral effects of cocaine: evidence from novel ligands and antisense oligodeoxynucleotides

Pharmacological and molecular biological tools were used to validate the involvement of sigma receptors in the actions of cocaine. Radioligand binding studies demonstrated significant levels of sigma receptors in the brain and heart, where cocaine interacts preferentially with the sigma(1) subtype. In behavioral pharmacological studies using mice, nine novel sigma receptor antagonists significantly attenuated cocaine-induced convulsions, while structural analogs with weak interactions with sigma receptors were ineffective. In contrast to the protection provided by the antagonists, a classical sigma receptor agonist exacerbated the convulsive effects of cocaine. The antagonists also attenuated cocaine-induced lethality, with the best compound protecting against death even when administered as a post-treatment. At doses where the antagonists had no effect on baseline locomotor activity, they significantly attenuated the locomotor stimulatory effects of cocaine, suggesting their ability to block the psychomotor as well as the toxic effects of cocaine. To further validate that the anti-cocaine effects were achieved by interfering with cocaine's access to sigma receptors, antisense oligodeoxynucleotides against sigma(1) receptors were shown to attenuate the convulsive and locomotor stimulatory effects of cocaine. Together, the studies support the involvement of sigma receptors, particularly the sigma(1) subtype, in the behavioral effects of cocaine.

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.

Two novel sigma receptor ligands, BD1047 and LR172, attenuate cocaine-induced toxicity and locomotor activity

The ability of cocaine to interact with sigma receptors indicates that these sites may mediate the negative properties associated with cocaine use, such as toxicity and addiction. Previous studies have shown that the novel sigma receptor ligand, BD1008 (N-[2-(3,4-dicholophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylam ine), effectively protects against cocaine-induced convulsions and locomotor activity in mice. Therefore, BD1047 ([2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(diamino)ethylamine) and LR172 (N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-homopiperidinyl)eth ylamine), two analogs of BD1008, were tested to determine if they also have anti-cocaine properties. Receptor binding assays showed that BD1047 and LR172 both have high affinities for a receptors, but low to negligible affinities for dopamine, opioid, phencyclidine, and 5-HT2 sites. In behavioral studies, pretreatment of mice with BD1047 or LR172 reduced the convulsions, lethality, and locomotor activity produced by cocaine. The data indicates a possible role for sigma receptor ligands in the treatment of cocaine overdose and addiction.

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.

[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.

In vivo modulation of sigma receptor sites by calcitonin gene-related peptide in the mouse and rat hippocampal formation: radioligand binding and electrophysiological studies

Possible interactions between sigma (sigma) receptor sites and calcitonin gene-related peptides (CGRP) were investigated using receptor subtype-related analogues and fragment in in vivo [3H](+)SKF 10 047/sigma binding in the hippocampus, and electrophysiological recording of the N-methyl-D-aspartate (NMDA)-induced activation of CA3 pyramidal neurons, two well-established sigma assays. In both paradigms, CGRP and the agonist [Cys(ACM)2,7]hCGRPalpha modulated sigma systems. In vivo binding experiments demonstrated that CGRP and [Cys(ACM)2,7]hCGRPalpha inhibited 25-40% of specific [3H](+)SKF 10 047 labelling in the mouse hippocampal formation while the purported antagonist hCGRP8-37 was inactive. The specificity of this modulation was demonstrated further by the lack of effect of other vasoactive peptides, including the atrial natriuretic peptide, substance P, and its N-terminal fragment, substance P1-7. In the CA3 subfield of the rat dorsal hippocampus, hCGRP alpha decreased (up to 61%) the NMDA-induced activation of the pyramidal neurons. Conversely, the linear analogue [Cys(ACM)2,7]hCGRP alpha enhanced (by 85%) the NMDA-induced activation of CA3 pyramidal neurons, while the antagonistic fragment hCGRP8-37 had no effect. Haloperidol, a high-affinity sigma receptor ligand, inhibited by 90% the in vivo [3H](+)SKF 10 047 labelling, and prevented the modulation of the NMDA-induced activation by hCGRP alpha and [Cys(ACM)2,7]hCGRP alpha. It thus appears that CGRP can modulate sigma-related systems in the hippocampal formation.

Characterization of two novel sigma receptor ligands: antidystonic effects in rats suggest sigma receptor antagonism

The novel sigma receptor ligands, N(-)[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine (BD1047) and 1(-)[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine (BD1063), were characterized in rats using binding assays and behavioral studies. In radioligand binding studies, the novel ligands showed marked selectivity for sigma binding sites, generally having a 100-fold or better affinity for sigma sites compared to nine other tested receptors (opiate, phencyclidine, muscarinic, dopamine, alpha 1-, alpha 2-, beta-adrenoceptor, 5-HT1, 5-HT2); the only exception was the affinity of BD1047 for beta-adrenoceptors. Competition assays further revealed that the drugs interacted with both sigma 1 and sigma 2 binding sites. Although both drugs had preferential affinities for sigma 1 sites, BD1047 exhibited a higher affinity for sigma 2 sites than BD1063. In behavioral studies, BD1047 and BD1063 had no effects on their own when unilaterally microinjected into the red nucleus of rats, but both compounds attenuated the dystonia produced by the high affinity sigma ligands, di-o-tolylguanidine (DTG) and haloperidol. BD1047 and BD1063 dose-dependently attenuated the dystonia produced by DTG, suggesting a receptor-mediated mechanism, and the dose curve for DTG was shifted to the right in the presence of the novel ligands. BD1047 and BD1063 appear to act as antagonists at sigma sites and may represent promising new tools for probing other functional effects associated with sigma binding sites.

Endogenous opioid regulation of norepinephrine release in guinea pig hippocampus

Release of endogenous norepinephrine was detected in guinea pig hippocampal slices using a radioligand displacement assay. Focal electrical stimulation released endogenous norepinephrine and caused a calcium-dependent reduction in specific [3H]propranolol binding at beta-adrenergic receptors in the brain slice. The mu-opioid agonist PL017 decreased norepinephrine release, and the inhibition by PL017 could be blocked by the opioid antagonist naloxone. Endogenous opioid peptides concomitantly released by tissue stimulation also decreased norepinephrine release in a naloxone-sensitive manner. These results support the hypothesis that endogenous opioids can regulate excitability in the hippocampus by presynaptic modulation of norepinephrine release.