In order to clarify the effects of endogenous opiate peptides on the vasopressin system, we have investigated the presence of different opiate receptor subtypes in the neurohypophysis by radioreceptor assay and autoradiography. [3H]-etorphine binding to membrane preparations revealed the presence of high- and low-affinity binding sites (KD, 1.2 nM and 8.1 nM). Displacement of [3H]-etorphine by opiate receptor subtype-specific ligands gave the following results: (1) the preferential mu agonists DAGO (Tyr-D-Ala-Gly-NMe-Phe-Gly-oL) and the tetrapeptide morphiceptin did not displace etorphin; (2) the preferential sigma receptor agonists DADLE (D-Ala2, D-Leu5-enkephalin) or DSTLE (D-Ser2, Leu5, Thr6-enkephalin) and β-endorphin, a preferential agonist of the epsilon receptor, displaced [3H]-etorphine from its low-aiïinity site only, and (3) dynorphin 1–8, a preferential kappa agonist, displaced [3H]-etorphine from its high-affinity binding site. Film autoradiography of neurohypophyseal sections incubated with [3H]-etorphine showed a displacement of 30% of the labeled ligand by unlabeled dynorphin 1–8. Exposure of rat neurointermediate lobes in organ culture to dynorphin 1–8 caused a small but significant stimulation of vasopressin release. These results demonstrate the existence of dynorphin 1–8 sensitive opiate receptors of the kappa subtype in the neurohypophysis and their possible involvement in vasopressin release.

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