The lack of utility of the rat vas deferens as a functional bioassay for sigma ligands

Characterization of [3H]nisoxetine binding in rat vas deferens membranes: modulation by sigma and PCP ligands

Sigma (sigma) and phencyclidine (PCP) receptor ligands, apart from their main effects on sigma receptors and NMDA receptor-mediated neurotransmission, have been found to interact with catecholamine systems in several central and peripheral tissues. In the present study the binding profile of [3H]nisoxetine ([3H]NIS), a selective marker of the noradrenaline transporter, has been characterized in rat vas deferens membranes to further study its modulation by a number of characteristic sigma and PCP ligands. The binding of [3H]NIS was found to be of high affinity (Kd = 1.63 +/- 0.36 nM), saturable, sodium-dependent and to a single population of binding sites (nH = 1.003 +/- 0.017). The maximal binding capacity was 1,625 +/- 500 fmol/mg of protein. Kinetic experiments gave a k(+1) of 3.9 x 10(7) min(-1)M(-1) and a k(-1) of 0.005 min(-1). The [3H]NIS binding was totally inhibited, with IC50 values in the micromolar range, by all the sigma and PCP ligands tested, with the following order of potency: haloperidol > dextromethorphan > dizocilpine > dextrorphan > (+)-3-PPP > PCP > tenocyclidine. This order correlates well with that described in other tissues using [3H]desmethylimipramine. The inhibition by all these compounds, except that of (+)-3-PPP, was competitive. These results suggest that sigma and PCP ligands bind, at low micromolar concentrations, to a site in the noradrenaline transporter that is labelled by [3H]NIS.

Involvement of sigma-receptors in the increase in contraction of mouse vas deferens induced by exogenous ATP

The effects of sigma-receptor ligands on the twitch contraction elicited by the exogenous application of adenosine 5'-triphosphate (ATP) in the unstimulated mouse vas deferens were studied. (-)-Pentazocine, 1,3-di(2-tolyl)guanidine(DTG) and two pairs of optical isomers of 3-(3-hydroxyphenyl)-N-(1-propyl)piperidine(3-PPP) and N-allylnormetazocine (SKF-10,047) potentiated the exogenous application of ATP-induced twitch-type contraction in a concentration-dependent manner, while (+)-pentazocine did not affect it. The order of potentiating ability was: (+)-3-PPP > (-)pentazocine > (-)-SKF-10,047> DTG > (-)-3-PPP > (+)-SKF-10,047. On the other hand, haloperidol and rimcazole, putataive sigma-receptor antagonists, suppressed this twitch contraction. In addition, these antagonists significantly blocked the (+)-3-PPP- and (-)-pentazocine-induced potentiation at concentrations which did not affect contractions per se. These findings indicate that the exogenous application of ATP-induced twitch contraction in the mouse vas deferens is regulated by sigma-receptors. In addition, the present ranking order suggests that the sigma-receptor potentiating the ATP-induced twitch contraction at post-junctional sites may differ from the sigma 1- and/or sigma 2-receptor subtypes.

Effects of haloperidol and reduced haloperidol on binding to sigma sites

The s.c. administration of a single dose of 0.1 mg/kg of reduced haloperidol to guinea pigs produced a marked inhibition of the binding of [3H]dextromethorphan and [3H]3-(3-hydroxyphenyl)-N-(n-propyl)piperidine ([3H](+)-3-PPP) to brain. The inhibition was still evident 10 days later, and it was accompanied by residual brain levels of reduced haloperidol, and much lower levels of haloperidol. Scatchard and computer-assisted analysis demonstrated that the inhibition was due to a reduction in the number of binding sites without changes in the affinity. In the rat, haloperidol and reduced haloperidol also produced a rapid inhibition of binding to sigma sites. Interestingly, the brain of the reduced haloperidol-treated rats contained both haloperidol and reduced haloperidol, but the levels of reduced haloperidol in the haloperidol-treated rats were undetectable. However, the inhibition observed was of comparable magnitude, indicating that the haloperidol remaining in the brain is also inhibitory. In vitro experiments showed that the inhibition produced by haloperidol and reduced haloperidol was apparently competitive, but when brain membranes were preincubated with either drug, the inhibition was noncompetitive. By contrast, the inhibition produced by dextromethorphan was always competitive. Moreover, the inhibition produced by haloperidol and reduced haloperidol could not be reversed by washing. This investigation strongly suggests that the inhibition observed after the administration of haloperidol or reduced haloperidol is not a classic agonist-induced receptor down-regulation. The results indicated that the inhibition produced is a complex phenomenon, and suggest the formation of a slowly reversible or irreversible complex with reduced haloperidol or haloperidol.

Correlation between potentiation of neurogenic twitch contraction and benzomorphan sigma receptor binding potency in the mouse vas deferens

The effects of sigma receptor ligands on the neurogenic twitch contraction in the ddY mouse vas deferens were studied. In functional studies, (+)-N-allylnormetazocine ((+)-SKF-10,047) and (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3-PPP) potentiated neurogenic twitch contractions in a concentration-dependent manner. The potentiation by each (+) enantiomer was significantly more potent than that by the respective (-) enantiomer. In addition, haloperidol and (+/-)-pentazocine also potentiated neurogenic twitch contractions. The order of potentiating ability was: haloperidol > (+/-)-pentazocine > (+)-3-PPP > (-)-3-PPP > (+)-SKF-10,047 > (-)-SKF-10,047. In contrast, other sigma receptor ligands, 1,3-di(2-tolyl)guanidine (DTG) and rimcazole, suppressed this twitch contraction. In addition, rimcazole significantly antagonized the (+)-SKF-10,047-induced potentiation at concentrations which did not affect contractions per se. Furthermore, binding studies showed that the kinetic parameters and the inhibitory potencies of sigma receptor ligands for the binding of [3H](+)-SKF-10,047 in the mouse vas deferens were similar to those in the guinea pig brain. The order of potency of sigma receptor ligands to potentiate the neurogenic twitch contraction in the mouse vas deferens was significantly correlated with the potency to inhibit [3H](+)-SKF-10,047 binding in both mouse vas deferens and guinea pig brain. These results indicate that sigma receptor ligands regulate the neurogenic twitch contraction, which is mediated by rimcazole-sensitive benzomorphan-type sigma receptors.