Modulation of (+)-[3H]pentazocine binding to guinea pig cerebellum by divalent cations.
The ability of cations to modulate the binding of the sigma 1 receptor-selective ligand (+)-[3H]pentazocine to guinea pig cerebellum was investigated. Di- and trivalent cations biphasically inhibited (+)-[3H]pentazocine binding, revealing multiple affinity states. The rank order of potency of these cations (based on the high affinity component of inhibition) was Zn2+ > Co2+ >> La3+ = Ni2+ = Cd2+ = Mn2+ = Gd2+ > Ba2+ = Sr2+ >> Mg2+ > Ca2+. The inhibition of 1,3-[3H]di(2-tolyl)guanidine binding to the sigma 2 receptor by these cations differed qualitatively and quantitatively from their effects on (+)-[3H]pentazocine binding. Although monovalent cations decreased the Kd for (+)-[3H]pentazocine binding, divalent cations split (+)-[3H]pentazocine binding into low and high affinity components. The Bmax of the high affinity component decreased with increasing divalent cation concentrations. Both mono- and divalent cations significantly reduced the rate of association of (+)-[3H]pentazocine with the sigma 1 receptor without altering the dissociation rate. (+)-[3H]Pentazocine binding was not altered by guanine nucleotides or by treatment with cholera or pertussis toxins. However, nonselective cation channel blockers (cinnarizine, hydroxyzine, prenylamine, amiodarone, and proadifen) potently inhibited (+)-[3H]pentazocine binding. These results indicate that physiologically relevant concentrations of divalent cations allosterically modulate (+)-[3H]pentazocine binding to the sigma 1 receptor, to reveal multiple affinity states. These sites do not represent sigma 1 to sigma 2 subtype interconversion or ternary complex formation with guanine nucleotide-binding proteins. However, the rank order of cation potency and the inhibition of binding by cation channel blockers is consistent with a potential role for sigma receptors as constituents of cation channels.
D000818:Animals; D001667:Binding, Competitive; D002413:Cations, Divalent / Q000378:metabolism*; D002531:Cerebellum / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000378:metabolism; D006150:Guanine Nucleotides / Q000494:pharmacology; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects; D010423:Pentazocine / Q000378:metabolism*; D017480:Receptors, sigma / Q000378:metabolism
D002413:Cations, Divalent; D006150:Guanine Nucleotides; D007473:Ion Channels; D017480:Receptors, sigma; D019204:GTP-Binding Proteins; D010423:Pentazocine