Dynorphin (1-8) inhibits stimulated release of oxytocin but not vasopressin from isolated neurosecretory endings of the rat neurohypophysis.

The effects of the opioid peptide dynorphin (1-8) on oxytocin and vasopressin release at the level of isolated neurosecretory endings were investigated. Neurosecretory endings prepared by homogenization and centrifugation were placed on a filter and constantly superfused. Stimulated hormone release was evoked by potassium depolarization (30 mM) and simultaneous increase of the osmolarity (20 mosmol/1). Stimulation resulted in two peaks of hormone release--a short first peak and a longer second one. Addition of dynorphin (1-8) (10(-7) M) to the superfusion buffer significantly diminished the first peak of oxytocin release and totally abolished the second. There was no effect of dynorphin (1-8) on vasopressin release.