Opiate binding differentially associated with oxytocin and vasopressin nerve endings from porcine neurohypophyses

We examined opioid binding in fractions with disconnected nerve endings (secretosomes) which were prepared from porcine neurohypophyses by centrifugation in a discontinuous Percoll gradient. Specific (= displaceable) binding was observed with 3H-etorphine and with 3H-diprenorphine, two ligands with low selectivity for distinct opiate receptor subclasses. No displaceable binding was found with the prototypic mu- and delta-ligands 3H-dihydromorphine and 3H-(D-Ala, D-Leu) enkephalin. Displacement of 3H-diprenorphine binding was almost absent with the selective mu- and delta-ligands morphiceptin and ICI-174864. Partial displacement occurred with the selective kappa-ligand U-50488 and with dynorphin. Binding of 3H-etorphine was stereo-specific. 3H-diprenorphine binding was saturable with a KD between 2 and 4 nM. Maximum of opiate binding activity was detected in the fractions with accumulated secretosomes. By autoradiography specific 3H-diprenorphine binding is shown to be mainly associated with secretosomes. In imunocytochemical preparations an oxytocin antibody was immunoreactive in 85% of the secretosomes in the fraction with highest opiate binding. These fractions in radioimmunoassays exhibited the largest contents in oxytocin and low vasopressin levels. The data therefore suggest that in the porcine neurohypophysis opioid binding sites of the kappa-type occur in secretory endings presumably of the oxytocin type.