Use of the mouse vas deferens to determine mu, delta, and kappa receptor affinities of opioid antagonists.
The present study was designed to identify a single smooth muscle preparation possessing mu, delta, and kappa receptors that can be used in the development of opioid selective antagonists. In vitro studies with the mouse vas deferens indicated that the delta selective agonists, DPLPE and DSLET, had potent agonist activity (ED50 approximately 1 nM) to inhibit the twitch response. The mu selective agonists, normorphine and fentanyl, also inhibited the twitch response in the mouse vas deferens, but were approx 100-fold less potent than the delta selective agonists, consistent with the enrichment of this preparation with delta receptors. U50,488, a kappa selective agonist, also inhibited the twitch response with a potency similar to that of the mu agonists. Naloxone, MR 2266, and WIN 44,441 all antagonized the agonist activity of U50,488 with antagonist dissociation constants distinct from those calculated using mu or delta receptor agonists. To confirm the presence of all three opioid receptors in this preparation, we examined a series of 14 phenylpiperidine opioid antagonists. An excellent correlation was observed between affinities of these piperidine opioid antagonists at mu and kappa receptors determined via radioligand binding studies, and affinities determined by blockade of fentanyl- or U50,488-induced twitch inhibition. Of the piperidine opioid antagonists studied, two possessed relatively high kappa receptor antagonist affinity. Furthermore, the study of an enantiomeric pair of an N-substituted 4-phenylpiperidine derivative demonstrated differences in absolute configuration to be more important for binding at mu and delta than kappa receptors. Thus, we have established the presence of kappa, in addition to the known mu and delta receptors, in the mouse vas deferens, and identified certain piperidines to have high kappa receptor antagonist affinity.