Mu opioid antagonist properties of a cyclic somatostatin octapeptide in vivo: Identification of mu receptor-related functions

We have shown previously that D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Pen-Thr-NH₂ (CTP) produces selective antagonism of mu, but not delta or kappa, opioid receptor-selective ligands in the guinea pig ileum and mouse vas deferens bioassays, and in radioligand binding assays using homogenized rat brains. In the present study we characterized the agonist and opioid antagonist profile of CTP in analgesic (hot-plate test, abdominal stretch test) and in gastrointestinal assays (transit time test) in mice. CTP was a potent antagonist of the supraspinal and spinal analgesic effects of the mu selective agonist [MePhe³, D-Pro⁴]morphiceptin (PL017) in both assays. The gastrointestinal antitransit actions of PL017 were also antagonized by CTP at both supraspinal and spinal sites. CTP did not alter the effects of the kappa agonist trans-3,4-dichloro-N-methyl-N-(1-pyrrolidinyl)cyclohexyl)benz eneacetamine in any test. Surprisingly, CTP also antagonized the analgesia produced by i.c.v. and intrathecal administration of [D-Pen²,D-Pen⁵]enkephalin (DPDPE), a highly delta selective agonist, in both analgesic tests. Differential antagonism of DPDPE, but not PL017, by the delta selective antagonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH in the hot-plate test indicates that PL017 and DPDPE may act at separate receptors to produce analgesia (mu and delta, respectively). In contrast, CTP did not reverse the gastrointestinal antitransit effects of intrathecal DPDPE. Schild analysis of the interactions of CTP with supraspinal mu and delta agonists in the hot-plate test indicated that although CTP antagonized PL017 in a competitive fashion (Schild slope = -1.0), the interaction of CTP with DPDPE was not competitive (Schild slope = -0.5). Naloxone, a nonselective opioid antagonist, produced competitive antagonism of both PL017 and DPDPE. In summary, we have shown that CTP is a competitive mu antagonist in vivo, in agreement with results obtained previously in vitro. In addition, CTP also blocked delta receptor-mediated analgesia, but not antitransit actions. The incongruity in delta antagonism seen in analgesic but not in the gastrointestinal assay, or in any in vitro test, might be explained best by a possible coupling of a certain population of mu and delta receptors that mediate analgesia. These findings also confirm the previously reported roles of mu, delta and kappa receptors in analgesia and in regulation of gastrointestinal function.