Sigma receptors: from molecule to man

Role of the Sigma-1 receptor in Amyotrophic Lateral Sclerosis (ALS)

Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative disease affecting spinal cord motoneurons (MN) with an associative connection to Frontotemporal Lobar Dementia (FTLD). The endoplasmic reticulum (ER) bound Sigma-1 Receptor (S1R) chaperone protein localizes to specialized ER cisternae within 10 nm of the plasma membrane in spinal cord ventral horn cholinergic post synaptic C-terminals. Removal of the S1R gene in the Superoxide Dismutase-1 (SOD-1) mouse model of ALS exacerbated the neurodegenerative condition and resulted in a significantly reduced longevity when compared to the SOD-1/S1R wild type (WT) mouse. The proposed amelioration of the ALS phenotype by the S1R is likely due to a "brake" on excitation of the MN as evidenced by a reduction in action potential generation in the MN of the WT when compared to the S1R KO mouse MN. Although the precise signal transduction pathway(s) regulated by the S1R in the MN has/have not been elucidated at present, it is likely that direct or indirect functional interactions occur between the S1R in the ER cisternae with voltage gated potassium channels and/or with muscarinic M2 receptor signaling in the post synaptic plasma membrane. Possible mechanisms for regulation of MN excitability by S1R are discussed.

Recent Advances in the Development of Sigma-1 Receptor Ligands

The existence of two distinct sigma (σ) receptor subtypes was established in the early 1990s. Sigma-1 and sigma-2 receptors (S1Rs and S2Rs, respectively) were shown to possess distinct molecular size, anatomical distribution, and ligand discrimination. S2R is overexpressed in numerous human cancers, and has therapeutic potential for the imaging and treatment of certain tumours. In contrast, S1R is more broadly involved in a wide variety of central nervous system (CNS) diseases including motor disorders, memory deficits, depression, schizophrenia, anxiety, pain, drug addiction, and many more. Since the human S1R was cloned in 1996, numerous high affinity ligands with excellent selectivity for S1R have been developed. This review focuses on recent developments in the generation of structurally diverse S1R-selective ligands and novel therapeutic candidates targeting S1Rs.

Biocatalytic synthesis of chiral alcohols and amino acids for development of pharmaceuticals

Chirality is a key factor in the safety and efficacy of many drug products and thus the production of single enantiomers of drug intermediates and drugs has become increasingly important in the pharmaceutical industry. There has been an increasing awareness of the enormous potential of microorganisms and enzymes derived there from for the transformation of synthetic chemicals with high chemo-, regio- and enatioselectivities. In this article, biocatalytic processes are described for the synthesis of chiral alcohols and unntural aminoacids for pharmaceuticals.

Early development of sigma-receptor ligands

Sigma receptors (σ-1 and σ-2) are non-opioid proteins implicated in the pathophysiology of various neurological disorders and cancer. The σ-1 subtype is a chaperon protein widely distributed in the CNS and peripheral tissues. These receptors are involved in the modulation of K+- and Ca2+-dependent signaling cascades at the endoplasmic reticulum and modulation of neurotransmitter release. σ-1 receptors are emerging targets for the treatment of neurophychiatric diseases (schizophrenia and depression) and cocaine addiction. σ-2 receptors are lipid raft proteins. They are highly expressed on many tumor cells and hence considered potential targets for anticancer drugs. σ receptors bind to a diverse class of pharmacological compounds like cocaine, methamphetamine, benzomorphans like (±)-pentazocine, (±)-SKF-10,047 and endogenous neurosteroids and sphingolipids. In this review we focus on the early development of σ receptor-specific ligands and radiolabeling agents.

Small molecule receptors as imaging targets

The aberrant expression and function of certain receptors in tumours and other diseased tissues make them preferable targets for molecular imaging. PET and SPECT radionuclides can be used to label specific ligands with high affinity for the target receptors. The functional information obtained from imaging these receptors can be used to better understand the systems under investigation and for diagnostic and therapeutic applications. This review discusses some of the aspects of receptor imaging with small molecule tracers by PET and SPECT and reviews some of the tracers for the receptor imaging of tumours and brain, heart and lung disorders.

Divergent enantioselective synthesis of (-)-galanthamine and (-)-morphine

An efficient divergent synthetic strategy for the synthesis of the opiate and amaryllidaceae alkaloids emerges by employing a Pd-catalyzed asymmetric allylic alkylation (AAA) to set the stereochemistry. Three generations of syntheses of galanthamine are discussed in detail with particular focus on the scope of the palladium-catalyzed AAA reactions and intramolecular Heck reactions. The pivotal tricyclic intermediate is available in six steps from 2-bromovanillin and the monoester of methyl 6-hydroxycyclohexene-1-carboxylate. This intermediate requires only two steps to convert to (-)-galanthamine. Using a Heck vinylation, we found that the fourth ring of codeine/morphine could be formed. The final ring formation involves a novel visible light-promoted hydroamination. Thus, six steps are required to convert the pivotal tricyclic intermediate into codeine, which has been demethylated in high yield to morphine.

Sigma-1 receptors at galactosylceramide-enriched lipid microdomains regulate oligodendrocyte differentiation

In the brain, myelin is important in regulating nerve conduction and neurotransmitter release by providing insulation at axons. Myelin is a specialized yet continuous sheet structure of differentiated oligodendrocytes (OLs) that is enriched in lipids, specifically galactosylceramides (GalCer) originated at the endoplasmic reticulum (ER). GalCer are known to affect OL differentiation. However, the mechanism whereby GalCer affect OL differentiation is not well understood. Sigma-1 receptors (Sig-1Rs), shown by us to exist in detergent-insoluble lipid microdomains at lipid-enriched loci of ER in NG108 cells, are important in the compartmentalization/transport of ER-synthesized lipids and in cellular differentiation. In this study, we used rat primary hippocampal cultures and found that Sig-1Rs form GalCer-enriched lipid rafts at ER lipid droplet-like structures in the entire myelin sheet of mature OLs. In rat OL progenitors (CG-4 cells), levels of lipid raft-residing Sig-1Rs and GalCer increase as cells differentiate. Sig-1Rs also increase in OLs and myelin of developing rat brains. Sig-1R, GalCer, and cholesterol are colocalized and are resistant to the Triton X-100 solubilization. Treating cells with a Sig-1R agonist or targeting Sig-1Rs at lipid rafts by overexpression of Sig-1Rs in CG-4 cells enhances differentiation, whereas reducing Sig-1Rs at lipid rafts by transfection of functionally dominant-negative Sig-1Rs attenuates differentiation. Furthermore, Sig-1R siRNA inhibits differentiation. Our findings indicate that, in the brain, Sig-1Rs targeting GalCer-containing lipid microdomains are important for OL differentiation and that Sig-1Rs may play an important role in the pathogenesis of certain demyelinating diseases.

The sigma receptor ligand (+)-pentazocine prevents apoptotic retinal ganglion cell death induced in vitro by homocysteine and glutamate

Recent studies demonstrated that the excitotoxic amino acid homocysteine induces apoptotic death of retinal ganglion cells in vivo. In the present study, an in vitro rat retinal ganglion cell (RGC-5), culture system was used to analyze the toxicity of acute exposure to high levels of homocysteine, the mechanism of homocysteine-induced toxicity, and the usefulness of type 1 sigma receptor (sigmaR1) ligands as neuroprotectants. When cultured RGC-5 cells were subjected to treatment with 1 mM D,L-homocysteine, a significant increase in cell death was detected by terminal dUTP nick end labeling (TUNEL) analysis and analysis of activated caspase. When cells were treated with homocysteine- or glutamate in the presence of MK-801, an antagonist of the N-methyl-D-aspartate (NMDA) receptor, the cell death was inhibited significantly. In contrast, NBQX, an antagonist of the AMPA/Kainate receptor, and nifedipine, a calcium channel blocker, did not prevent the homocysteine- or glutamate-induced cell death. Semiquantitative RT-PCR and immunocytochemical analysis demonstrated that RGC-5 cells were exposed to homocysteine or glutamate express type 1 sigma receptor at levels similar to control cells. Treatment of RGC-5 cells with 3 or 10 microM concentrations of the sigmaR1-specific ligand (+)-pentazocine inhibited significantly the apoptotic cell death induced by homocysteine or glutamate. The results suggest that homocysteine is toxic to ganglion cells in vitro, that the toxicity is mediated via NMDA receptor activation, and that the sigmaR1-specific ligand (+)-pentazocine can block the RGC-5 cell death induced by homocysteine and glutamate.

Amisulpride: limbic specificity and the mechanism of antipsychotic atypicality

Amisulpride clearly has the clinical profile of an atypical antipsychotic, characterised in particular by its lower propensity to induce extrapyramidal side effects as well as its greater efficacy in treating negative symptoms compared with classical neuroleptics. In addition to the clinical advantages over classical neuroleptics, it has also been demonstrated that the clinical profile of amisulpride is comparable to that of other modern atypical neuroleptics. Animal data also allow the conclusion to be drawn that amisulpride has an atypical profile. For example, amisulpride does not provoke catalepsy which is characteristic of postsynaptic D2 blockade in the rat. The induction of catalepsy in animal models is usually seen as an indicator of the propensity to induce extrapyramidal side effects in patients. In relation to the widely accepted hypothesis that the inclusion of 5-HT2A antagonism in addition to D2 antagonism is of great relevance for the atypicality of an antipsychotic, and given the fact that amisulpride lacks 5-HT2A antagonism, the pharmacological explanation of the clinically well-proven atypicality of amisulpride is of great interest. Based on basic research and in vivo imaging studies, two mechanisms in particular seem to explain the atypicality of amisulpride: preferential action on limbic D2/D3 receptors and preferential blockade of presynaptic D2/D3 receptors. In addition, the fast dissociation hypothesis can contribute to the explanation of the atypical clinical profile of amisulpride. The relevance of the D3 blockade in the context of atypicality is not yet completely clear.

Antagonism of NMDA receptors by sigma receptor ligands attenuates chemical ischemia-induced neuronal death in vitro

We investigated the effects of sigma receptor ligands on neuronal death induced by chemical ischemia using primary cultures of rat cerebral cortical neurons. The induction of chemical ischemia by sodium azide and 2-deoxy-D-glucose led to delayed neuronal death in a time- and concentration-dependent manner, as determined by trypan blue exclusion. The neurotoxicity was inhibited by N-methyl-D-aspartate (NMDA) receptor antagonists, indicating the involvement of glutamate. The sigma receptor ligands (+)-N-allylnormetazocine ((+)-SKF10,047) and haloperidol, but not carbetapentane and R(+)-3-(3-hydroxyphenyl)-N-propylpiperidine ((+)-3-PPP), prevented chemical ischemia-induced neurotoxicity in a concentration-dependent manner. The protective effects of (+)-SKF10,047 and haloperidol were not affected by the sigma receptor antagonists. (+)-SKF10,047 and haloperidol, but not carbetapentane and (+)-3PPP, inhibited the glutamate-induced increase in intracellular Ca(2+), and the inhibitory effects were not attenuated by sigma receptor antagonists. These results suggest that direct interaction with NMDA receptors but not sigma receptors is crucial to the neuroprotective effects of sigma receptor ligands with affinity for NMDA receptors.

Analysis of sigma receptor (sigmaR1) expression in retinal ganglion cells cultured under hyperglycemic conditions and in diabetic mice

The type 1 sigma receptor (sigmaR1) is a nonopiate and nonphencyclidine binding site that has numerous pharmacological and physiological functions. In some studies, agonists for sigmaR1 have been shown to afford neuroprotection against overstimulation of the NMDA receptor. sigmaR1 expression has been demonstrated recently in retinal ganglion cells (RGC). RGCs undergo apoptosis early in diabetic retinopathy via NMDA receptor overstimulation. In the present study we asked whether RGCs cultured under hyperglycemic conditions and RGCs of diabetic mice continue to express sigmaR1. RGCs were cultured 48 h in RPMI medium containing either 45 mM glucose or 11 mM glucose plus 34 mM mannitol (osmolar control). C57BL/6 mice were made diabetic using streptozotocin. The retina was dissected from normal and streptozotocin-induced diabetic mice 3, 6 and 12 weeks post-onset of diabetes. sigmaR1 was analyzed in cells using semiquantitative RT-PCR and in tissues by semiquantitative RT-PCR, in situ hybridization, Western blot analysis and immunolocalization. The RT-PCR analysis of cultured RGCs showed that sigmaR1 mRNA is expressed under hyperglycemic conditions at levels similar to control cells. Similarly, analysis of retinas of diabetic mice showed no difference in levels of mRNA encoding sigmaR1 compared to retinas of control mice. In situ hybridization analysis showed that expression patterns of sigmaR1 mRNA in the ganglion cell layer were similar between diabetic and control mice. Western blot analysis suggested that levels of sigmaR1 in retina were similar between diabetic and control retinas. Immunohistochemical analysis of sigmaR1 showed a similar pattern of sigmaR1 protein expression between control and diabetic retina. These studies demonstrate that sigmaR1 is expressed under hyperglycemic conditions in vitro and in vivo.

Expression pattern of sigma receptor 1 mRNA and protein in mammalian retina

Sigma receptors are nonopiate and nonphencyclidine binding sites that are thought to be neuroprotective due to modulation of N-methyl-D-aspartate (NMDA) receptors. Sigma receptor 1 expression has been demonstrated in numerous tissues including brain. Recently, studies using binding assays have demonstrated sigma receptor 1 in neural retina, however these studies did not demonstrate in which retinal cell type(s) sigma receptor 1 was present nor did they establish unequivocally the molecular identity of the receptor. The present study was designed to address these issues. Reverse transcription-polymerase chain reaction (RT-PCR) analysis amplified sigma receptor 1 in neural retina, RPE-choroid complex, and lens isolated from mice. A similar RT-PCR product was amplified also in three cultured cell lines, rat Müller cells, rat ganglion cells and human ARPE-19 cells. In situ hybridization analysis revealed abundant sigma receptor 1 expression in ganglion cells, cells of the inner nuclear layer, inner segments of photoreceptor cells and retinal pigment epithelial (RPE) cells. Immunohistochemical studies detected the sigma receptor 1 protein in retinal ganglion, photoreceptor, RPE cells and surrounding the soma of cells in the inner nuclear layer. These data provide the first cellular localization of sigma receptor 1 in neural retina and establish the molecular identity of sigma receptor 1 in retinal cells. The demonstration that sigma receptor 1 is present in ganglion cells is particularly noteworthy given the well-documented susceptibility of these cells to glutamate toxicity. Our findings suggest that retinal ganglion cells may be amenable to the neuroprotective effects of sigma ligands under conditions of neurotoxicity such as occurs in diabetes.

Expression pattern of the type 1 sigma receptor in the brain and identity of critical anionic amino acid residues in the ligand-binding domain of the receptor

The type 1 sigma receptor (sigmaR1) has been shown to participate in a variety of functions in the central nervous system. To identify the specific regions of the brain that are involved in sigmaR1 function, we analyzed the expression pattern of the receptor mRNA in the mouse brain by in situ hybridization. SigmaR1 mRNA was detectable primarily in the cerebral cortex, hippocampus, and Purkinje cells of cerebellum. To identify the critical anionic amino acid residues in the ligand-binding domain of sigmaR1, we employed two different approaches: chemical modification of anionic amino acid residues and site-directed mutagenesis. Chemical modification of anionic amino acids in sigmaR1 with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide reduced the ligand-binding activity markedly. Since it is known that a splice variant of this receptor which lacks exon 3 does not have the ability to bind sigma ligands, the ligand-binding domain with its critical anionic amino acid residues is likely to be present in or around the region coded by exon 3. Therefore, each of the anionic amino acids in this region was mutated individually and the influence of each mutation on ligand binding was assessed. These studies have identified two anionic amino acids, D126 and E172, that are obligatory for ligand binding. Even though the ligand-binding function was abolished by these two mutations, the expression of these mutants was normal at the protein level. These results show that sigmaR1 is expressed at high levels in specific areas of the brain that are involved in memory, emotion and motor functions. The results also provide important information on the chemical nature of the ligand-binding site of sigmaR1 that may be of use in the design of sigmaR1-specific ligands with potential for modulation of sigmaR1-related brain functions.

Different effects of lectins on the ligand binding of the NMDA receptors and sigma sites in rat brain hippocampus synaptic membranes

The effects of the lectins concanavalin A, WGA, ricin, abrin, and the mistletoe lectins from Viscum album MLI, MLII, and MLIII on the binding of ligands of the NMDA and sigma receptors in rat hippocampus synaptic plasma membranes were investigated. Binding of [3H]MK-801, [3H]glutamate, [3H]5,7-DCKA, and [3H]glycine to the membranes was decreased by 40-60% after addition of galactose-specific lectins (mistletoe lectins MLI, MLII, ricin, abrin) at concentrations of 0.01 mg/ml, but was not affected by the glucose- and mannose-specific lectin Con A, an acetylglucosamine-specific lectin WGA, or an acetylgalactosamine-specific lectin MLIII. The binding of [3H]SKF 10047 was decreased only in the presence of MLIII and did not change after addition of the other lectins. It is suggested that lectin-sensitive ligand binding sites of sigma- and NMDA receptors are located separately, and that the carbohydrate side chains of the sigma receptor do not participate in the modulation of the NMDA-receptor.

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.

Involvement of direct inhibition of NMDA receptors in the effects of sigma-receptor ligands on glutamate neurotoxicity in vitro

This study was performed to examine the roles of the N-methyl-D-aspartate (NMDA) receptor/phencyclidine (PCP) channel complex in the protective effects of sigma-receptor ligands against glutamate neurotoxicity in cultured cortical neurons derived from fetal rats. A 1-h exposure of cultures to glutamate caused a marked loss of viability, as determined by Trypan blue exclusion. This acute neurotoxicity of glutamate was prevented by NMDA receptor antagonists. Expression of sigma(1) receptor mRNA in cortical cultures was confirmed by reverse transcription polymerase chain reaction (RT-PCR). sigma Receptor ligands with affinity for NMDA receptor channels including the PCP site, such as (+)-N-allylnormetazocine ((+)-SKF10,047), haloperidol, and R(-)-N-(3-phenyl-1-propyl)-1-phenyl-2-aminopropane ((-)-PPAP), prevented glutamate neurotoxicity in a concentration-dependent manner. In contrast, other sigma-receptor ligands without affinity for NMDA receptors, such as carbetapentane and R(+)-3-(3-hydroxyphenyl)-N-propylpiperidine ((+)-3-PPP), did not show neuroprotective effects. Putative endogenous sigma receptor ligands such as pregnenolone, progesterone, and dehydroepiandrosterone did not affect glutamate neurotoxicity. The protective effects of (+)-SKF10,047, haloperidol, and (-)-PPAP were not affected by the sigma(1) receptor antagonist rimcazole. These results suggested that a direct interaction with NMDA receptors but not with sigma receptors plays a crucial role in the neuroprotective effects of sigma receptor ligands with affinity for NMDA receptors.

Repeated haloperidol treatment decreases sigma(1) receptor binding but does not affect its mRNA levels in the guinea pig or rat brain

The effects of chronic treatment with haloperidol on sigma (sigma) receptors were investigated across brain regions and species. The regional distribution of [3H](+)-pentazocine binding to sigma(1) receptor was similar between the guinea pig and rat brains. The highest level of binding was detected in the brain stem and lowest in the striatum and hippocampus. The regional distribution of [3H]1, 3-di (2-tolyl) guanidine ([3H]DTG) binding in the presence of 100 nM (+)-pentazocine to sigma(2) receptor was similar to that of the [3H](+)-pentazocine binding in the guinea pig brain, while in the rat brain high levels of [3H]DTG binding were detected in the cortex, frontal cortex and cerebellum. The intraperitoneal administration of 2 mg/kg of haloperidol to guinea pig and rats once a day for 21 days produced inhibition of [3H](+)-pentazocine binding but did not affect [3H]DTG binding to sigma(2) receptors in any brain region examined. The effects of haloperidol on [3H](+)-penazocine binding in the rat were much weaker than those in the guinea pig. The regional distribution of the level of sigma(1) receptor mRNA determined by the ribonuclease protection assay was similar to that of the [3H](+)-pentazocine binding activity, except in the cortex and cerebellum where the levels of sigma(1) receptor mRNA were low in guinea pig and rat. Treatment with haloperidol did not affect the levels of sigma(1) receptor mRNA in any brain region in either species. These findings suggested that the sigma receptors differentially distributed in brain regions are down regulated by treatment with haloperidol across sigma receptor subtypes and animal species without changing the transcriptional activity of the sigma(1) receptor. The mechanisms by which sigma receptors could be differently regulated in vivo by chronic treatment with haloperidol in different species may contribute to the therapeutic efficacy of haloperidol.

Steroidal sigma receptor ligands affect signaling pathways in human spermatozoa

In human spermatozoa, Ca(2+) entry is stimulated by progesterone or prostaglandin E(1) (PGE(1)). The regulation of cation currents by progestins involves sigma receptors, and sigma binding sites are abundant in testis. We examined the effects of sigma ligands on human spermatozoa. Ca(2+) entry induced by progesterone or PGE(1) was not altered by the sigma ligands haloperidol and ditolylguanidine. However, the steroidal sigma ligands RU 3117 and RU 1968 had distinct effects. Stimulation by RU 3117 resulted in activation and homologous desensitization of the sperm progesterone receptor but not of the PGE(1) receptor. Because haloperidol and ditolylguanidine did not affect RU 3117 and progesterone actions in spermatozoa, we conclude that sigma receptors are not involved. However, RU 1968 potently inhibited both the progesterone- and PGE(1)-induced Ca(2+) entry and acrosome reaction. At higher concentrations, RU 1968 also inhibited hormonal Ca(2+) signaling in fibroblasts. Despite suppression of Ca(2+) mobilization, inhibition of phospholipase C by RU 1968 was not observed. Furthermore, RU 1968 did not impair the binding of inositol-1,4,5-trisphosphate to its endoplasmic reticulum receptor. Because RU 1968 preferentially inhibits signaling pathways in spermatozoa, the future development of more selective drugs structurally related to RU 1968 may be a novel approach for pharmacological contraception.

sigma Receptor ligand-induced up-regulation of the H(+)/peptide transporter PEPT1 in the human intestinal cell line Caco-2

We determined the effects of (+)pentazocine, a selective sigma(1) ligand, on the uptake of glycylsarcosine (Gly-Sar) in the human intestinal cell line Caco-2 which expresses the low affinity/high capacity peptide transporter PEPT1. Confluent Caco-2 cells were treated with various concentrations of (+)pentazocine for desired time (mostly 24 hr). The activity of PEPT1 was assessed by measuring the uptake of [(14)C]Gly-Sar in the presence of a H(+) gradient. (+)Pentazocine increased the uptake of [(14)C]Gly-Sar mediated by PEPT1 in a concentration- and time-dependent manner. Kinetic analyses have indicated that (+)pentazocine increased the maximal velocity (V(max)) for Gly-Sar uptake in Caco-2 cells without affecting the Michaelis-Menten constant (K(t)). In addition, semi-quantitative RT-PCR revealed that treatment of (+)pentazocine increased PEPT1 mRNA in Caco-2 cells in a concentration-dependent manner. These data suggest that sigma(1) receptor ligand (+)pentazocine up-regulates PEPT1 in Caco-2 cells at the level of increased mRNA, causing an increase in the density of the transporter protein in the cell membrane.

Drugs of abuse and placental transport

The placenta provides the only link between the mother and the developing fetus. The function of the placenta as a transport organ is obligatory for fetal development because this process, mediated by a variety of transport systems, is responsible for the delivery of nutrients from the mother to the fetus. Some of the transport systems in the placenta also play a role in the clearance of vasoactive compounds, thus maintaining optimal blood flow to this organ. There is strong supporting evidence to indicate that several of these placental transport systems are either direct or indirect targets for the abusable drugs cocaine, amphetamines, nicotine, and cannabinoids. These drugs of abuse compromise the placental transport function and consequently produce detrimental effects on the developing fetus.

Sigma1 receptor subtype does not interact with stereotyped behaviors in rats

In the present study, we clearly showed that the sigma1 receptor subtype did not interact with the induction of stereotyped behaviors in rats. Namely, (+)-N-allylnormetazocine [(+)-SKF-10,047] (5.0, 10.0, and 20.0 mg/kg, SC), a traditional sigma receptor ligand that has affinities for the sigma1 receptor subtype and the N-methyl-D-aspartate (NMDA)/phencyclidine (PCP) receptor channel complex, markedly produced PCP-like stereotyped behaviors, such as head weaving, turning, and backpedaling, in rats. On the contrary, 1-(3,4-dimethoxyphenyl)-4-(3-phenylpropyl)piperazine dihydrochloride (SA4503), a potent and selective sigma1 receptor agonist, did not produce these behaviors. Additionally, PCP-induced stereotyped behaviors were significantly augmented by (+)-SKF-10,047, but not by SA4503. We thus suggest that the induction of PCP-like stereotyped behaviors elicited by (+)-SKF-10,047 closely interacts with NMDA/PCP receptor channel complex but not with the sigma1 receptor subtype.

Association between polymorphisms in the type 1 sigma receptor gene and schizophrenia

Several antipsychotic agents such as haloperidol and rimcazole are known to bind to sigma receptors with high affinity, and evidence for a potential link between sigma receptors and the etiology of schizophrenia has been reported. The present study was conducted to systematically search for nucleotide variants of the type 1 sigma receptor gene in 48 schizophrenics. Two polymorphisms were found: GC-241-240TT in the 5' flanking region and Gln2Pro. These two polymorphisms were in nearly complete linkage disequilibrium with each other. The Pro2 variant of the Gln2Pro polymorphism changes the endoplasmic reticulum retention signal motif. These polymorphisms were examined in an extended sample of schizophrenics (n = 308) and controls (n = 433) and a significant association between the presence of the TT/Pro2 haplotype and schizophrenia was observed (odds ratio = 1.27, P = 0.04).

Regulation of NMDA-induced [3H]dopamine release from rat hippocampal slices through sigma-1 binding sites

To examine the interaction between ionotropic glutamate receptors and sigma binding sites, we made use of [3H]dopamine release from rat hippocampal slices. Agonists for ionotropic glutamate receptors such as N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and kainate evoked release of [3H]dopamine from rat hippocampal slices, in a dose-dependent manner. (+)-Pentazocine, a prototype sigma1 agonist, attenuated the NMDA-induced [3H]dopamine release dose-dependently and significantly as did non-competitive NMDA antagonists such as 5-methyl-10,11-dihydro-5H-dibenzo(a,b)cyclohepten-5,10-imine maleate (MK-801) and phencyclidine. In contrast, (+)-pentazocine had no effect on AMPA- or on kainate-induced [3H]dopamine release. Sigma-1 receptor antagonists including N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl] ethylamine monohydrochloride (NE-100), 1(cyclopropylmethyl)-4-(2'-(4"-fluorophenyl)-2'-oxoethylpiperidine (DuP734) and 1-(cyclopropylmethyl)-4-(2',4"-cianophenyl)-2'-oxoethyl)-pip eridine hydrobromide (XJ448) prevented significantly the inhibitory effect of (+)-pentazocine on NMDA-induced [3H]dopamine release, without affecting the release of [3H]dopamine evoked by NMDA. The inhibitory effect of (+)-pentazocine on [3H]dopamine release was preserved even in the presence of tetrodotoxin. These results suggest that sigma1 binding sites selectively interact with the NMDA receptor channel complex among ionotropic glutamate receptors, and that sigma1 binding sites may be involved in modulating the release of dopamine in the rat hippocampus by interacting with the NMDA receptor on dopaminergic nerve terminal.

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

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

N-(N-benzylpiperidin-4-yl)-2-[18F]fluorobenzamide: a potential ligand for PET imaging of sigma receptors

Four nitro- and fluorobenzamides (1-4) have been synthesized in good yields from nitro- and fluoro-substituted benzoyl chloride with 4-amino-1-benzylpiperidine. In vitro studies showed that these compounds have high affinities to sigma receptors. N-(N-Benzylpiperidin-4-yl)-2-fluorobenzamide (3), in particular, bound to sigma receptors with high affinity (Ki = 3.4 nM, guinea pig brain membranes) and high selectivity (sigma-2/sigma-1 = 120). It was, therefore, labeled with 18F and evaluated as a sigma receptor radioligand. N-(N-Benzylpiperidin-4-yl)-2-[18F]fluorobenzamide (3a) was synthesized in one step by nucleophile substitution of the 2-nitro precursor (1) with [18F]fluoride in DMSO at 140 degrees C for 20 min followed by purification with HPLC in 4-10% yield (decay corrected). The synthesis time was 90 min and the specific activity was 0.4-1.0 Ci/mumol. Tissue distribution in mice revealed that the uptakes of 3a in the brain, heart, liver, lungs, spleen, kidneys and small intestine were high, and the radioactivity in these organs remained constant from 60 to 120 min post-injection. The radioactivity in the bone did not significantly increase, suggesting in vivo defluorination may not be the major route of metabolism of 3a in mice. Blocking studies with haloperidol in rats indicated that the uptake of compound 3a in the rat brain was selective to haloperidol-sensitive sigma sites. These results suggest that compound 3a is a potent sigma receptor radioligand and may be a potential ligand for PET imaging of sigma receptors in humans.

SA4503, a novel cognitive enhancer, with sigma 1 receptor agonistic properties

We found a potent and selective sigma 1 (sigma 1) receptor ligand, SA4503 (1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride). This compound had a high affinity for sigma 1 receptor subtype (IC50 = 17 +/- 1.9 nM), but a low affinity for sigma 2 receptor subtype (IC50 = 1800 +/- 310 nM). The present study examines the effect of this compound on the central cholinergic functions, since sigma receptor has been reported to interact with the central cholinergic neurons. SA4503 elicited the increase in extracellular acetylcholine level in rat frontal cortex, while it did not affect the striatal acetylcholine level. On the other hand, tetrahydroaminoacridine (THA), an acetylcholinesterase (AChE) inhibitor, increased the extracellular acetylcholine level in both regions. Although both compounds had anti-amnesic effect against scopolamine-induced memory impairment, THA also induced catalepsy in rats. These results suggest that SA4503 may be a novel cognitive enhancer, with sigma 1 receptor agonistic properties. In addition, SA4503 does not cause striatal cholinomimetic side-effects, which is different from THA.

Reduction of the scopolamine-induced impairment of passive-avoidance performance by sigma receptor agonist in mice

We examined the ameliorating effects of several sigma receptor agonists on scopolamine-induced memory impairment in mice. Scopolamine was administered IP 30 min before the training session. Each sigma receptor agonist was administered 60 min before or immediately after the training session, or 60 min before the retention test in the passive-avoidance performance experiments. (+)-N-Allylnormetazocine ((+)-SKF-10,047), a prototype sigma 1 receptor agonist, showed an ameliorating effect on the scopolamine-induced memory impairment in these 3 administration schedules, and (-)-SKF-10,047, a stereoisomer with low affinity for the sigma 1 receptor subtype, failed to reduce this memory impairment in mice. In addition, 1,3-di(2-toly1)guanidine (DTG) and (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperizine ((+)-3-PPP), nonselective sigma receptor agonists, did not affect this memory impairment. Physostigmine, an acetylcholinesterase (AChE) inhibitor, alleviated the scopolamine-induced memory impairment in all these drug administration schedules. In addition, (+)-SKF-10,047-induced antiamnesic effect was antagonized by the concurrent administration of haloperidol, a sigma receptor antagonist, or N,N-dipropyl-2-(4-methoxy-3-(2-phenylethoxy) phenyl)ethylamine monohydrochloride (NE-100), a selective sigma 1 receptor antagonist. These findings indicate that the sigma 1 receptor agonist has ameliorating effects on all phases of learning and memory processes. This profile of sigma 1 receptor agonist is similar to that of an AChE inhibitor.

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

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

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

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

Ameliorating effect of SA4503, a novel sigma 1 receptor agonist, on memory impairments induced by cholinergic dysfunction in rats

We found a potent and selective sigma 1 receptor agonist, SA4503 (1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydro-chloride), with high affinity for the sigma 1 receptor subtype (IC50 = 17 nM), but low affinity for the sigma 2 receptor subtype (IC50 = 1800 nM). The binding activity and selectivity of SA4503 resembled those of (+)-pentazocine, a prototype sigma 1 receptor agonist. We have previously shown that the sigma 1 receptor agonist activated central cholinergic functions. Therefore, we examined the effects of SA4503 on the cholinergic dysfunction-induced memory impairments in a passive avoidance task. Scopolamine, a muscarinic acetylcholine receptor antagonist, produced memory impairment, when it was administered 30 min before the training session of the passive avoidance task in rats. Single administration of SA4503 significantly reduced the scopolamine-induced memory impairment. In addition, the lesioning by injection of alpha-amino-3-hydroxy-5-isoxazole acetic acid (ibotenic acid) into the basal forebrain area produced memory impairment in rats. Repeated administration of SA4503 after lesioning of the basal forebrain area ameliorated the basal forebrain lesion-induced memory impairment. Moreover, the ameliorating effect of SA4503 against the scopolamine-induced memory impairment was antagonized by both 4-[4-(4-chlorophenyl)-4-hydroxy-1-piperidinyl]-1-(4-fluorophenyl)-1-buta none (haloperidol), a sigma receptor antagonist, and N,N-dipropyl-2- [4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE-100), a putative sigma 1 receptor antagonist. These results suggest that SA4503 has an anti-amnesic effect against cholinergic dysfunction-induced memory impairment, and that the effect of SA4503 is mediated by the sigma 1 receptor subtype.

Regional differences of the effect of sigma receptor ligands on the acetylcholine release in the rat brain

We found that sigma receptor ligands differentially regulated the acetylcholine (ACh) neurotransmission in the rat brain. Acute administration of (+)-N-allylnormetazocine [(+)-SKF-10,047], a prototype sigma 1 receptor ligand, and 1,3-di(2-tolyl)guanidine (DTG), a non-specific sigma receptor ligand, increased the extracellular ACh level in the rat hippocampus. This increase of hippocampal extracellular ACh level elicited by (+)-SKF-10,047 was more potent than that elicited by DTG. On the other hand, the striatal extracellular ACh level was slightly affected by (+)-SKF-10,047. In addition, DTG did not affect the striatal extracellular ACh level. Our previous studies have shown that both (+)-SKF-10,047 and DTG increased the extracellular ACh level in the rat frontal cortex. Taking all these data into consideration, the regulation of ACh neurotransmission by sigma receptor ligands are different depending upon the brain region.

Differential solubilization of rat liver sigma 1 and sigma 2 receptors: retention of sigma 2 sites in particulate fractions

Rat liver membranes (crude P2 membranes) were solubilized in 10 mM Tris-HCl, pH 7.4 containing 7 mM 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate (CHAPS). The soluble fraction was designated the Extract 1. The 105,000 x g pellet was washed once, and then extracted a second time (Extract 2). The various resulting fractions were assayed for sigma (sigma) binding characteristics, using [3H](+)-pentazocine to label sigma 1 sites and [3H]1,3-di-o-tolylguanidine (DTG) in the presence of 1 microM dextrallorphan to label sigma 2 sites. Both of the extracts and resultant pellets (Pellet 1 and Pellet 2) contained sigma 1 and sigma 2 receptors, as indicated by the pharmacological profiles upon competition studies. The Kd and Bmax values for sigma 1 activity in the original P2 membranes were 8.3 +/- 0.73 nM and 5333 +/- 572 fmol/mg protein; Kd and Bmax for sigma 2 activity was 19 +/- 0.17 nM and 9190 +/- 800 fmol/mg protein. There were no changes in the radioligand Kd values of the two sites in the subsequent soluble and particulate fractions. However, while the sigma 1 and sigma 2 Bmax values in extracts and pellets were generally on the same order as those of P2 membranes, the actual sigma 2 to sigma 1 Bmax ratio varied markedly across the fractions. The ratio of sigma 2/ sigma 1 binding in Extract 1 and Extract 2 was 0.86 and 0.68, respectively, compared to a ratio of 1.7 in the original P2. However, the ratio in Pellet 2 was 3.8, twice that of the original P2 membranes. Furthermore, the Bmax value for sigma 1 sites in Pellet 2 did not change, whereas the sigma 2 Bmax increased 1.8 fold relative to the original P2 membranes. The changes in sigma 2/ sigma 1 binding ratio in extracts were observed using two different assay methods for soluble receptors (retention on polyethyleneimine-coated filters and polyethylene glycol precipitation) and is therefore not an artifact of assay procedure. These data suggest that, relative to sigma 1 receptors, sigma 2 receptors are more resistant to solubilization and become somewhat enriched in the particulate fractions. This supports the notion that sigma 1 and sigma 2 receptors are distinct macromolecules and may indicate different modes of association with the cell membrane.

sigma Receptor antagonists block the development of sensitization to cocaine

The effects of putative sigma receptor antagonists, BMY-14802 (alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazine), rimcazole and SR-31742A (cis-3-(hexahydroazepin-1-yl)1-(3-chloro-4- cyclohexylphenyl)propene-1), on the development of behavioral sensitization induced by repeated administration of cocaine were investigated. Acute intraperitoneal injection of 15 mg/kg cocaine in rats induced moderate hyperactivity which mainly consisted of sniffing and rearing. These acute effects of cocaine were hardly affected by co-administration of the sigma receptor antagonists, except that BMY-14802 enhanced, but not significantly cocaine-induced locomotion. While repeated cocaine administration induced a progressive increase in stereotyped behaviors and resulted in sensitization, every sigma receptor antagonists tested attenuated the development of sensitization to cocaine. These prophylactic effects of sigma receptor antagonists against cocaine-induced sensitization were confirmed by the challenge test with cocaine alone after an abstinence. These results were consistent with results of our previous study which revealed that BMY-14802 blocked the sensitization to methamphetamine, another psychostimulant. Therefore, sigma receptors play a crucial role in the development of the psychostimulant-induced sensitization phenomenon, which is a pharmacological model of schizophrenia.

BMY-14802 reversed the sigma receptor agonist-induced neck dystonia in rats

To clarify clinical roles of sigma receptor binding affinity of neuroleptics, neck dystonia induced by microinjection of sigma receptor ligands and neuroleptics into rat red nucleus was investigated. DTG and (+)-3-PPP, putative sigma receptor agonists, induced neck dystonia in dose-dependent and reversible manner. Haloperidol and perphenazine induced dystonia in the same way as sigma receptor agonists, whereas zotepine and (-)-sulpiride did not. The rank order of potency in induction of dystonia and sigma receptor affinity of these compounds showed positive correlation. Although BMY-14802 has a high affinity for sigma receptors, it never produced dystonia by itself. On the other hand, combined injection of BMY-14802 with DTG attenuated DTG-induced dystonia. Therefore, it is suggested that typical neuroleptics such as haloperidol act agonistic and atypical neuroleptics such as BMY-14802 act antagonistic at rubral sigma receptors in the induction of neck dystonia.

Involvement of sigma-receptors in the increase in contraction of mouse vas deferens induced by exogenous ATP

The effects of sigma-receptor ligands on the twitch contraction elicited by the exogenous application of adenosine 5'-triphosphate (ATP) in the unstimulated mouse vas deferens were studied. (-)-Pentazocine, 1,3-di(2-tolyl)guanidine(DTG) and two pairs of optical isomers of 3-(3-hydroxyphenyl)-N-(1-propyl)piperidine(3-PPP) and N-allylnormetazocine (SKF-10,047) potentiated the exogenous application of ATP-induced twitch-type contraction in a concentration-dependent manner, while (+)-pentazocine did not affect it. The order of potentiating ability was: (+)-3-PPP > (-)pentazocine > (-)-SKF-10,047> DTG > (-)-3-PPP > (+)-SKF-10,047. On the other hand, haloperidol and rimcazole, putataive sigma-receptor antagonists, suppressed this twitch contraction. In addition, these antagonists significantly blocked the (+)-3-PPP- and (-)-pentazocine-induced potentiation at concentrations which did not affect contractions per se. These findings indicate that the exogenous application of ATP-induced twitch contraction in the mouse vas deferens is regulated by sigma-receptors. In addition, the present ranking order suggests that the sigma-receptor potentiating the ATP-induced twitch contraction at post-junctional sites may differ from the sigma 1- and/or sigma 2-receptor subtypes.

Involvement of sigma 1 receptor in (+)-N-allylnormetazocine-stimulated hippocampal cholinergic functions in rats

The effects of the stereoisomers of N-allylnormetazocine (SKF-10,047) on the hippocampal cholinergic functions were compared in rats. A putative sigma 1 receptor agonist, (+)-SKF-10,047, elicited an increase of hippocampal extracellular acetylcholine level and anti-amnesic effect against scopolamine-induced memory dysfunctions in rats. These phenomena were not produced by (-)-SKF-10,047, and were reversed by haloperidol, a putative sigma 1 receptor antagonist. Such stereoselectivity and antagonism imply an involvement of sigma 1 receptors in these (+)-SKF-10,047-stimulated hippocampal cholinergic functions.

In vivo labeling of sigma receptors in mouse brain with [3H]4-phenyl-1-(4-phenylbutyl)piperidine

4-Phenyl-1-(4-phenylbutyl)piperidine (4-PPBP) is a very potent ligand for sigma (Sigma) receptors. The present study was undertaken to evaluate [3H]4-PPBP as a radioligand for in vivo labeling of cerebral sigma receptors. After intravenous administration of [3H]4-PPBP to mice, there is high uptake of radioactivity in the brain. The regional distribution of radioactivity in the brain 2 h after intravenous injection of [3H]4-PPBP parallels the in vitro binding of the radioligand in rat brain (pons/medulla > cerebellum > or = prefrontal cortex > or = parietal cortex > hypothalamus > olfactory tubercle > or = thalamus > hippocampus > striatum). Pretreatment with haloperidol (2 mg/kg) significantly decreases the radioactivity measured in the brain 30-120 min after injection of [3H]4-PPBP. Pretreatment with unlabeled 4-PPBP or ifenprodil also significantly decreases radioactivity in the brain 2 h after injection of [3H]4-PPBP, in a dose-dependent manner. The in vivo binding of [3H]4-PPBP in the brain also is significantly inhibited by SL 82.0715, BMY 14802, 1,3-di-o-tolylguanidine (DTG), and (+)-enantiomers of pentazocine, SKF 10,047, and 3-PPP, but not by the corresponding (-)-enantiomers, consistent with stereoselectivity of inhibition obtained in in vitro binding studies. In contrast, pretreatment with dizocilpine and spiperone does not inhibit in vivo binding of [3H]4-PPBP. The results indicate that [3H]4-PPBP would be a suitable radioligand for in vivo labeling of sigma receptors in brain.

Interactions of erythro-ifenprodil, threo-ifenprodil, erythro-iodoifenprodil, and eliprodil with subtypes of sigma receptors

Observations of sigma (sigma) receptor heterogeneity have prompted interest in identifying ligands for sigma receptor subtypes. Selective ligands for the sigma-2 are unavailable, but [3H]ifenprodil labels sigma-2 sites. Therefore, isomers and analogues of ifenprodil were compared as potential sigma-2 ligands. Threo-ifenprodil and erythro-ifenprodil had high affinity (Ki congruent to 2 nM) for sigma-2 sites; erythro-iodoifenprodil had moderate affinity (Ki congruent to 46 nM); eliprodil had lowest affinity (Ki congruent to 630 nM). Threo-ifenprodil, which has less affinity for alpha 1-adrenoceptors than erythro-ifenprodil, was slightly more selective than erythro-ifenprodil for sigma-2 sites. These results identify threo-ifenprodil as potentially useful for studies of sigma-2 receptors.