Antagonists show GTP‐sensitive high‐affinity binding to the sigma‐1 receptor

Abstract

BACKGROUND AND PURPOSESigma-1 receptors are atypical receptors with potentially two transmembrane domains. Antagonists require doses significantly higher than their published affinities to have biological effects. We have reassessed the binding characteristics of these ligands and found antagonists bind to high-and low-affinity states not distinguished by agonists. EXPERIMENTAL APPROACHThe affinities of sigma-1 receptor ligands was assessed using radioligand saturation and competition binding of [ 3 H]-(+)-pentazocine to permeabilized MDA-MB-468 cells. This was compared with the effect of ligands on metabolic activity using an MTS-based assay and calcium signalling using cells loaded with the calcium dye, Fura-2. KEY RESULTSSigma-1 receptor antagonists, but not agonists, show GTP-and suramin-sensitive high-affinity binding. Functional responses (calcium signalling and metabolic activity), while associated with sigma-1 receptor binding, required binding to an unidentified, low-affinity target. CONCLUSIONS AND IMPLICATIONSSigma-1 receptors are coupled to G proteins. This interaction is only observed when analysing antagonist binding. The identity of the G protein remains to be resolved. The concept of agonist and antagonist at the sigma-1 receptor needs to be revisited. Abbreviations(+)-3-PPP, (+)-3-(3-hydroxyphenyl)-N-1-(propyl)piperidine; [Ca 2+ ], free concentration of Ca 2+ ions; DMT, N,Ndimethyltryptamine; IPAG, 1-(4-iodophenyl)-3-(2-adamantyl)guanidine; SKF-10047, N-allylnormetazocine IntroductionThe search for understanding of the sigma-1 receptor has been a long, mysterious and as yet incomplete journey. The receptor has mostly been studied within the CNS. However, more recent work over the past decade has shown the receptor to be abundant in neuronal, and non-neuronal tissues (Vilner et al., 1995) leading to its study in cancer biology (Spruce et al., 2004).The sigma receptor was proposed as a novel fourth opioid receptor in 1976 to account for the behavioural effects of N-allylnormetazocine (SKF-10047), which could not be accounted for by the m (morphine) or k (ketocyclazocine) receptors (Martin et al., 1976). SKF-10047 was defined as an agonist as it raised pulse and respiration rates, as well as body temperature and caused dilatation of pupils in beagles. Interestingly, m-receptor agonists lowered pulse and respiration rates, as well as body temperature and produced classic BJP British Journal of Pharmacology DOI:10.1111DOI:10. /j.1476DOI:10. -5381.2011 772 British Journal of Pharmacology (2011) 164 772-780The Authors British Journal of Pharmacology © 2011 The British Pharmacological Society pinpoint pupils. Sigma-1 receptor antagonists were defined as agents that were able to prevent these responses. Later studies (e.g. Spruce et al., 2004) suggested that biochemically, agonists do not cause activation of the receptor but can block the effects of antagonists which do. More recently, data obtained using mice in which the sigma-1 receptor has been knocked out further cloud the picture; expression o...

Activation of the sigma-1 receptor by haloperidol metabolites facilitates brain-derived neurotrophic factor secretion from human astroglia

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