USE OF THE MOUSE VAS-DEFERENS TO DETERMINE MU-RECEPTOR, DELTA-RECEPTOR, AND KAPPA-RECEPTOR AFFINITIES OF OPIOID ANTAGONISTS

The present study was designed to identify a single smooth muscle prep aration 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, D PLPE and DSLET, had potent agonist activity (ED(50)approximate to 1 nM ) to inhibit the twitch response. The mu selective agonists, normorphi ne and fentanyl, also inhibited the twitch response in the mouse vas d eferens, but were approx 100-fold less potent than the delta selective agonists, consistent with the enrichment of this preparation with del ta receptors. U50,488, a kappa selective agonist, also inhibited the t witch response with a potency similar to that of the mu agonists. Nalo xone, MR 2266, and WIN 44,441 all antagonized the agonist activity of U50,488 with antagonist dissociation constants distinct from those cal culated using mu or delta receptor agonists. To confirm the presence o f all three opioid receptors in this preparation, we examined a series of 14 phenylpiperidine opioid antagonists. An excellent correlation w as 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 p ossessed relatively high kappa receptor antagonist affinity. Furthermo re, the study of an enantiomeric pair of an N-substituted 4-phenylpipe ridine derivative demonstrated differences in absolute configuration t o be more important for binding at mu and delta than kappa receptors. Thus, we have established the presence of kappa, in addition to the kn own mu and delta receptors, in the mouse vas deferens, and identified certain piperidines to have high kappa receptor antagonist affinity.