CHARACTERIZATION OF THE BINDING OF [H-3] (+)-PENTAZOCINE TO SIGMA-RECOGNITION SITES IN GUINEA-PIG BRAIN

The selective sigma compound (+)-pentazocine was radiolabeled and its binding characteristics in guinea pig brain membranes were investigated. [H-3](+)-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-degrees-C and was greater than 90% of total binding. Equilibrium was reached by 120 min and dissociation was complete by 420 min, with a t1/2 of 121 min. Li+, Ca2+ and Mg2+ 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 alpha diastereomers of pentazocine and cyclazocine were more potent than their corresponding (-) enantiomers and beta diastereomers. The rank order of potency for the sigma reference agents to displace [H-3](+)-pentazocine binding was similar to that reported using the [H-3]sigma ligands dextromethorphan, 1,3-di(2-tolyl)guanidine and (+)-3-(3-hydroxyphenyl)-N-propylpiperidine. Haloperidol, (+)-pentazocine, (+)-3-(3-hydroxyphenyl)-N-propylpiperidine and rimcazole were competitive inhibitors of binding to the [H-3](+)-pentazocine-defined sigma recognition site, suggesting that these different structural classes of compounds all bind to a single molecular entity.