Mu-receptors in the C57BL mouse vas deferens

Actions of opioids on the dorsal root potential of the isolated spinal cord preparation of the neonate rat

The effects of opioids were studied on the dorsal root-dorsal root evoked potential (DRP) of the neonatal hemisected spinal cord of the rat in vitro. Applications of [D-Ala2,Met5]enkephalinamide (DAME), [D-Ala2,D-Leu5]enkephalin (DADL), Leu5 enkephalin, Met5 enkephalin, Dynorphin 1-9 and normorphine produced dose-dependent depressions of the dorsal root potential. The depressant effects of these agents were antagonised by naloxone but not by the highly selective delta-opioid receptor antagonist ICI 174864. Compounds acting on the kappa opioid receptor had only weak and inconsistent inhibitory effects on the dorsal root potential. Morphine had antagonist properties on the dorsal root potential, as did the kappa agonists ethylketocyclazocine and bremazocine, but not U50488 and tifluadom. The depressant actions of opioids on the dorsal root potential thus appeared to be mediated by actions on the mu receptor type. However, only mu agonists with relatively high intrinsic activity were able to reduce the dorsal root potential. Other mu agonists, including morphine, acted as antagonists. Actions on receptors for the delta and kappa agonists, in contrast, appeared to have little influence on this response. The antagonist actions of certain kappa agonists were probably due to their high affinity for, but low efficacy at mu receptors.

The contribution of intrinsic activity to the action of opioids in vitro

The effects of opioids were compared in five field-stimulated isolated tissue models, the guinea-pig ileum and vasa deferentia from rat, rabbit and mice of the Alderley Park and C57BL/6 strains. Although the mu-receptor agonist [D-Ala2, MePhe4, Gly-ol5] enkephalin appeared to act at similar receptors in the guinea-pig ileum, rat vas deferens, mouse vas deferens and C57BL/6 mouse vas deferens preparations, its potency varied considerably between these preparations. Similar potency differences were also observed with the kappa-agonist, ethylketocyclazocine. It is proposed that these variations in potency reflect differences in the number of spare receptors present in each model. The finding that some drugs which have agonist activity in the more sensitive preparations behave as antagonists in the less sensitive tissues supports this proposal and highlights the importance of intrinsic activity in determining the action of opioids. Many of the prototypic opioid agonists were found to be either partial agonists (eg. morphine and bremazocine) or to possess affinity for more than one receptor type (eg. ethylketocyclazocine, Mr 2034).