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.