Correlation between potentiation of neurogenic twitch contraction and benzomorphan sigma receptor binding potency in the mouse vas deferens

Fenfluramine acts as a positive modulator of sigma-1 receptors

OBJECTIVE

Adjunctive fenfluramine hydrochloride, classically described as acting pharmacologically through a serotonergic mechanism, has demonstrated a unique and robust clinical response profile with regard to its magnitude, consistency, and durability of effect on seizure activity in patients with pharmacoresistant Dravet syndrome. Recent findings also support long-term improvements in executive functions (behavior, emotion, cognition) in these patients. The observed clinical profile is inconsistent with serotonergic activity alone, as other serotonergic medications have not been demonstrated to have these clinical effects. This study investigated a potential role for σ1 receptor activity in complementing fenfluramine's serotonergic pharmacology.

METHODS

Radioligand binding assays tested the affinity of fenfluramine for 47 receptors associated with seizures in the literature, including σ receptors. Cellular function assays tested fenfluramine and norfenfluramine (its major metabolite) activity at various receptors, including adrenergic, muscarinic, and serotonergic receptors. The σ1 receptor activity was assessed by the mouse vas deferens isometric twitch and by an assay of dissociation of the σ1 receptor from the endoplasmic reticulum stress protein binding immunoglobulin protein (BiP). In vivo mouse models assessed fenfluramine activity at σ1 receptors in ameliorating dizocilpine-induced learning deficits in spatial and nonspatial memory tasks, alone or in combination with the reference σ1 receptor agonist PRE-084.

RESULTS

Fenfluramine and norfenfluramine bound ≥30% to β2-adrenergic, muscarinic M1, serotonergic 5-HT1A, and σ receptors, as well as sodium channels, with a Ki between 266 nM (σ receptors) and 17.5 μM (β-adrenergic receptors). However, only σ1 receptor isometric twitch assays showed a positive functional response, with weak stimulation by fenfluramine and inhibition by norfenfluramine. Fenfluramine, but not the 5-HT2C agonist lorcaserin, showed a positive modulation of the PRE-084-induced dissociation of σ1 protein from BiP. Fenfluramine also showed dose-dependent antiamnesic effects against dizocilpine-induced learning deficits in spontaneous alternation and passive avoidance responses, which are models of σ1 activation. Moreover, low doses of fenfluramine synergistically potentiated the low-dose effect of PRE-084, confirming a positive modulatory effect at the σ1 receptor. Finally, all in vivo effects were blocked by the σ1 receptor antagonist NE-100.

SIGNIFICANCE

Fenfluramine demonstrated modulatory activity at σ1 receptors in vitro and in vivo in addition to its known serotonergic activity. These studies identify a possible new σ1 receptor mechanism underpinning fenfluramine's central nervous system effects, which may contribute to its antiseizure activity in Dravet syndrome and positive effects observed on executive functions in clinical studies.

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.

Sigma1 Pharmacology in the Context of Cancer

Sigma1 (also known as sigma-1 receptor, Sig1R, σ1 receptor) is a unique pharmacologically regulated integral membrane chaperone or scaffolding protein. The majority of publications on the subject have focused on the neuropharmacology of Sigma1. However, a number of publications have also suggested a role for Sigma1 in cancer. Although there is currently no clinically used anti-cancer drug that targets Sigma1, a growing body of evidence supports the potential of Sigma1 ligands as therapeutic agents to treat cancer. In preclinical models, compounds with affinity for Sigma1 have been reported to inhibit cancer cell proliferation and survival, cell adhesion and migration, tumor growth, to alleviate cancer-associated pain, and to have immunomodulatory properties. This review will highlight that although the literature supports a role for Sigma1 in cancer, several fundamental questions regarding drug mechanism of action and the physiological relevance of aberrant SIGMAR1 transcript and Sigma1 protein expression in certain cancers remain unanswered or only partially answered. However, emerging lines of evidence suggest that Sigma1 is a component of the cancer cell support machinery, that it facilitates protein interaction networks, that it allosterically modulates the activity of its associated proteins, and that Sigma1 is a selectively multifunctional drug target.

Small molecule antagonists of the sigma-1 receptor cause selective release of the death program in tumor and self-reliant cells and inhibit tumor growth in vitro and in vivo

The acquisition of resistance to apoptosis, the cell's intrinsic suicide program, is essential for cancers to arise and progress and is a major reason behind treatment failures. We show in this article that small molecule antagonists of the sigma-1 receptor inhibit tumor cell survival to reveal caspase-dependent apoptosis. sigma antagonist-mediated caspase activation and cell death are substantially attenuated by the prototypic sigma-1 agonists (+)-SKF10,047 and (+)-pentazocine. Although several normal cell types such as fibroblasts, epithelial cells, and even sigma receptor-rich neurons are resistant to the apoptotic effects of sigma antagonists, cells that can promote autocrine survival such as lens epithelial and microvascular endothelial cells are as susceptible as tumor cells. Cellular susceptibility appears to correlate with differences in sigma receptor coupling rather than levels of expression. In susceptible cells only, sigma antagonists evoke a rapid rise in cytosolic calcium that is inhibited by sigma-1 agonists. In at least some tumor cells, sigma antagonists cause calcium-dependent activation of phospholipase C and concomitant calcium-independent inhibition of phosphatidylinositol 3'-kinase pathway signaling. Systemic administration of sigma antagonists significantly inhibits the growth of evolving and established hormone-sensitive and hormone-insensitive mammary carcinoma xenografts, orthotopic prostate tumors, and p53-null lung carcinoma xenografts in immunocompromised mice in the absence of side effects. Release of a sigma receptor-mediated brake on apoptosis may offer a new approach to cancer treatment.

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.

Preclinical evaluation of [11C]SA4503: radiation dosimetry, in vivo selectivity and PET imaging of sigma1 receptors in the cat brain

Our previous in vivo study with rats has demonstrated that 11C-labeled 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine ([11C]SA4503) is a potential radioligand for mapping CNS sigmal receptors by positron emission tomography (PET). In the present study, we further characterized this ligand. The radiation absorbed-dose of [11C]SA4503 in humans estimated with the tissue distribution in mice, was higher in the liver, kidney and pancreas than in other organs studied, but was low enough for clinical use. The brain uptake of [11C]SA4503 in mice was reduced to approximately 60-70% by co-injection of carrier SA4503 and haloperidol, but not by co-injection of any of six ligands for sigma2 or other receptors, for which SA4503 showed in vitro >100 times weaker affinity than for signal receptor. In the cat brain, the uptake in the cortex was higher than that in the cerebellum. The radioactivity in the cortex and cerebellum accumulated for the first 10 min and then gradually decreased until 81.5 min in the baseline measurement, but rapidly decreased in the carrier-loading condition. The receptor-mediated uptake was estimated to be approximately 60-65% of the total radioactivity in the cortex and cerebellum at 76 min after tracer injection. We have concluded that [11C]SA4503 has the potential for mapping sigma1 receptor by PET.

Effects of trazodone on neurotransmitter release from rat mossy fibre cerebellar synaptosomes

The effects of trazodone and putative sigma (sigma) receptor ligands were investigated on KCl-stimulated release of glutamate (Glu) and gamma-aminobutyric acid (GABA) from cerebellar mossy fibre synaptosomes. Both trazodone and serotonin (5-HT) inhibited the increase of Glu and GABA release evoked by 15 mM KCl. Trazodone increased the inhibition of Glu release caused by 0.01 microM 5-HT, while it antagonized the inhibition induced by higher 5-HT concentrations. Despite the low affinity of trazodone for both sigma(1) and sigma(2) binding sites, with a pK(i) of 5.9 and 6.0 respectively, two sigma receptor ligands, (+)-3-[3-hydroxypheny]-N-(1-propyl)piperidine ((+)-3-PPP) and N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine (BD 1047) antagonized the effects of trazodone. The putative sigma receptor ligand N-allylnormetazocine ((+)-SKF 10,047) mimicked the inhibitory effect of trazodone. As with trazodone, (+)-3-PPP and BD 1047 antagonized the activity of (+)-SKF 10,047 but not that of 5-HT. On the whole, these results suggest that trazodone shares a common molecular target with sigma compounds distinct from that of 5-HT and is involved in K(+)-stimulated Glu and GABA release from mossy fibre cerebellar synaptosomes.

In vivo evaluation of [(11)C]SA4503 as a PET ligand for mapping CNS sigma(1) receptors

The potential of the (11)C-labeled selective sigma(1) receptor ligand 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine ([(11)C]SA4503) was evaluated in vivo as a positron emission tomography (PET) ligand for mapping sigma(1) receptors in rats. SA4503 is known to have a high affinity (IC(50) = 17.4 nM) and a higher selectivity (sigma(1)/sigma(2) = 103) for the sigma(1) receptor. A high and increasing brain uptake of [(11)C]SA4503 was found. Pre-, co- and postinjection of cold SA4503 significantly decreased uptake of [(11)C]SA4503 in the brain, spleen, heart, lung, and kidney in which sigma receptors are present as well as in the skeletal muscle. In the blocking study with one of four sigma receptor ligands including haloperidol, (+)-pentazocine, SA4503, and (-)-pentazocine (in the order of their affinity for sigma(1) receptor subtype), SA4503 and haloperidol significantly reduced the brain uptake of [(11)C]SA4503 to approximately 30% of the control, but the other two benzomorphans did not. A high specific uptake of [(11)C]SA4503 by the brain was also confirmed by ex vivo autoradiography (ARG) and PET. Ex vivo ARG showed a higher uptake in the vestibular nucleus, temporal cortex, cingulate cortex, inferior colliculus, thalamus, and frontal cortex, and a moderate uptake in the parietal cortex and caudate putamen. Peripherally, the blocking effects of the four ligands depended on their affinity for sigma(1) receptors. No (11)C-labeled metabolite was detected in the brain 30 min postinjection, whereas approximately 20% of the radioactivity was found as (11)C-labeled metabolites in plasma. These results have demonstrated that the (11)C-labeled sigma(1) receptor ligand [(11)C]SA4503 has a potential for mapping sigma(1) receptors in the central nervous system and peripheral organs.

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.

Binding properties of SA4503, a novel and selective sigma 1 receptor agonist

The binding profiles of SA4503 (1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride), a novel sigma receptor ligand, to sigma 1 and sigma 2 receptor subtypes in guinea pig and rat brain membranes were evaluated. SA4503 showed a high affinity for the sigma 1 receptor subtype labeled by (+)-[3H]pentazocine (IC50 = 17.4 +/- 1.9 nM), while it had about 100-fold less affinity for the sigma 2 receptor subtype labeled by [3H]1,3-di(2-tolyl)guanidine ([3H]DTG) in the presence of 200 nM (+)-pentazocine. SA4503 showed little affinity for 36 other receptors, ion channels and second messenger systems. The inhibition curves of SA4503 for (+)-[3H]pentazocine binding were shifted to the right in the presence of guanosine 5'-o-(3-thiotriphosphate) (GTP gamma S), as similar to those of (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3-PPP) and (+)-pentazocine, sigma 1 receptor agonists. SA4503 significantly increased the KD value, but did not affect the Bmax value for specific (+)-[3H]pentazocine binding. These results indicated that SA4503 is a potent and selective agonist for the sigma 1 receptor subtype in the brain. In addition, SA4503 inhibited specific (+)-[3H]pentazocine binding in a competitive manner.

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.

Inotropic action of sigma receptor ligands in isolated cardiac myocytes from adult rats

High affinity binding sites for sigma receptor ligands were found in membranes of cardiac myocytes from adult rats. The sigma receptor ligand (+)-3-hydroxyphenyl-N-(1-propyl)piperidine ((+)-3-PPP) binds with a Kd of 17.9 +/- 4.0 nM and a Bmax of 275 +/- 32.1 fmol/mg protein. Competition experiments of (+)-pentazocine with [3H]1,3-di-O-tolylguanidine ([3H]DTG) binding yielded a Ki of 6.1 +/- 1.3 nM. The majority of the sites (> 80%) were of the sigma 1 subtype. Exposure of isolated cardiomyocytes from adult rats to (+)-3-PPP (10 nM-1.0 microM) caused a marked concentration-dependent increase in the amplitude of systolic cell contraction, reaching 149% of control level, with an apparent ED50 value of 4.5 nM. The increase in the contraction amplitude was markedly inhibited by pretreatment with verapamil or thapsigargin. An increase in the amplitude of [Ca2+]i transients, similar to that in the amplitude of cell contraction, was observed in indo-1-loaded cardiomyocytes exposed to 0.1 microM (+)-3-PPP. Exposure to 10 nM of haloperidol or (+)-pentazocine induced an increase in the amplitude of contraction, reaching 188% and 138% (respectively) of control level. A lower concentration of haloperidol or (+)-pentazocine (1 nM) did not induce an increase in the contraction amplitude but rather reduced the amplitude to 70-80% of control.