Opiate receptors in the rat vas deferens

The nature of the opiate receptor population in the rat vas deferens (RVD) has been examined by evaluating the interaction of a range of antagonists with prototypic mu-, kappa- and sigma-opioid agonists in the tissue. Ke values for 5 antagonists against normorphine in the isolated mouse vas deferens showed excellent correlation with Ke values obtained against the mu-agonist RX783030 in the RVD. RX783030 could be effectively antagonised by naltrexone in the RVD but not by the sigma-antagonist ICI 154129 whereas D-Ala2,D-Leu5-enkephalin required both antagonists to yield parallel shifts of its dose response. The lack of agonist activity of morphine is a result of the low intrinsic activity of this agent in the RVD. The kappa-agonists ethylketocyclazocine, tifluadom and U50488 also showed antagonist properties in the RVD. These results can be rationalised by postulating that the RVD contains a mu-receptor population with a high intrinsic activity requirement together with some sigma-receptors. It is not necessary to propose the existence of a novel epsilon-receptor in order to rationalise the data reported.