Characterization of the binding of [ 3H](+)-pentazocine to σ recognition sites in guinea pig brain

Abstract

The selective σ compound (+)-pentazocine was radiolabeled and its binding characteristics in guinea pig brain membranes were investigated. [ 3H](+)-Pentazocine bound to a single high-affinity site with a K D of 2.9 nM and a B max of 1998 fmol/mg protein. Saturation was achieved at a ligand concentration of 15 nM. Maximal specific binding was observed at 37°C and was greater than 90% of total binding. Equilibrium was reached by 120 min and dissociation was complete by 420 min, with a t 1,2 of 121 min. Li +, Ca 2+ and Mg 2+ inhibited binding at high concentrations, and binding was insensitive to adenyl and guanyl nucleotides. Stereoselectivity was observed for the inhibition of binding by benzomorphans, 3-(3-hydroxyphenyl)-N-propylpiperidine and butaclamol, and the (+) enantiomers and α diastereomers of pentazocine and cyclazocine were more potent than their corresponding (−) enantiomers and β diastereomers. The rank order of potency for the σ reference agents to displace [ 3H](+)-pentazocine binding was similar to that reported using the [ 3H]σ ligands dextromethorphan, 1,3-di(2-tolyl)guuanidine and (+)-3-(3-hydroxyphenyl)-N-propylpiperidine. Haloperidol, (+)-pentazocine, (+)-3-(3-hydroxyphenyl)-N-propylpiperidine and rimcazole were competitive inhibitors of binding to the [ 3H](+)-pentazocine-defined σ recognition site, suggesting that these different structural classes of compounds all bind to a single molecular entity.