title: Antagonists show GTP-sensitive high-affinity binding to the sigma-1 receptor.
abstract: OBJECTIVE
  Sigma-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.
  
  
  METHODS
  The affinities of sigma-1 receptor ligands was assessed using radioligand saturation and competition binding of [³H]-(+)-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.
  
  
  RESULTS
  Sigma-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
  Sigma-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.
authors.affiliation: Department of Pharmacy and Pharmacology, University of Bath, Bath, UK.
authors.forename: J M
authors.initials: JM
authors.lastname: Brimson
authors.forename: C A
authors.initials: CA
authors.lastname: Brown
authors.forename: S T
authors.initials: ST
authors.lastname: Safrany
mesh_terms: D001665:Binding Sites; D001667:Binding, Competitive; D002118:Calcium / Q000378:metabolism; D020013:Calcium Signaling / Q000502:physiology; D002478:Cells, Cultured; D002772:Cholera Toxin / Q000494:pharmacology; D016257:Fura-2 / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism*; D006801:Humans; D010423:Pentazocine / Q000494:pharmacology; D011485:Protein Binding; D017480:Receptors, sigma / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D013498:Suramin / Q000378:metabolism; D000097605:Sigma-1 Receptor
chemical_list: D002118:Calcium; D002772:Cholera Toxin; D016257:Fura-2; D019204:GTP-Binding Proteins; D006160:Guanosine Triphosphate; D010423:Pentazocine; D017480:Receptors, sigma; D013498:Suramin; D000097605:Sigma-1 Receptor