Sigma receptor binding affinity and inhibition of apomorphine-induced climbing.

Several relatively selective compounds with affinity for the sigma binding site were assessed for their ability to inhibit apomorphine-induced climbing in the mouse. Although, the majority of compounds inhibited apomorphine-induced climbing, there was no correlation between the ability to inhibit climbing and potency in sigma binding assays using [3H]1,3-di-o-tolylguanidine (DTG) or [3H](+)-pentazocine as ligands. The potency of the compounds to inhibit binding to muscarinic M1 or M2 receptors correlated with the potency to inhibit apomorphine-induced climbing. However, several of the compounds that inhibit climbing had microM affinity at muscarinic receptors. Whether these concentrations were achieved in vivo is unclear. Our data suggest that sigma activity per se is not responsible for inhibition of apomorphine-induced climbing.

Characterization of the binding of [3H](+)-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. [3H](+)-Pentazocine bound to a single high-affinity site with a KD of 2.9 nM and a Bmax 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 [3H](+)-pentazocine binding was similar to that reported using the [3H]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 [3H](+)-pentazocine-defined sigma recognition site, suggesting that these different structural classes of compounds all bind to a single molecular entity.

Competitive interactions at [3H]1,3-di(2-tolyl)guanidine (DTG)-defined sigma recognition sites in guinea pig brain.

In saturation binding experiments, (+)pentazocine, (+)3-(3-hydroxyphenyl)-N-propylpiperidine (3-PPP), haloperidol and rimcazole did not inhibit the binding of [3H]DTG in a purely competitive fashion. Although Scatchard analysis indicated that [3H]DTG bound to a single site, the inhibition curves of some, but not all, reference compounds exhibited Hill coefficients of less than 0.8. The Scatchard data were consistent with a model of hyperbolic competitive inhibition of binding to the [3H]DTG-defined sigma site, although other possibilities such as negative cooperativity or binding to two sites cannot be definitively excluded. Compounds from numerous pharmacological and structural classes inhibited the binding of [3H]DTG, suggesting that interactions of [3H]DTG with other receptors may have confounded the Scatchard analysis of the binding of [3H]DTG to sigma recognition sites.

Lack of correlation between sigma binding potency and inhibition of contractions in the mouse vas deferens preparation.

The existence of sigma receptors in the mouse, rat and guinea pig vasa deferentia has previously been proposed, although drug effects are inconsistent and generally occur only at high concentrations. The purpose of the present study was to evaluate lower, physiologically relevant concentrations of ligands for possible sigma effects on electrically stimulated twitch contractions in the mouse vas deferens (MVD). Putative sigma agonists and antagonists all inhibited 0.1 Hz electrically stimulated twitch contractions in nM concentrations. Inhibitory activity plateaued between 20 and 60% for all compounds except 1,3-di(2-tolyl)guanidine (DTG), which had a shallow concentration-effect curve. Subsequent to the plateau, higher concentrations (30 microM) of rimcazole and haloperidol fully inhibited electrically stimulated twitch contractions. There was no correlation between inhibitory potency or maximal effect in the MVD and binding potency at sigma sites in either MVD or guinea pig brain. The inhibitory effects of R(+)-3-(3-hydroxyphenyl)-N-1-propylpiperidine ((+)3-PPP) or DTG on electrically stimulated twitch contractions were not antagonized by the putative sigma antagonists DTG, haloperidol, rimcazole or BMY-14802, nor by alpha 2-adrenoceptor, dopamine D1, dopamine D2 or opiate antagonists. Although the mechanism of sigma ligand effects in the MVD has not been established, the data caution against a presumption that effects of sigma ligands on electrically stimulated twitch contractions in this preparation are mediated by sigma receptors.