Lack of response in the release of oxytocin and vasopressin from isolated neurohypophyses to dopamine, met-enkephalin and leu-enkephalin.

We investigated the effects of dopamine, met-enkephalin and leu-enkephalin on basal and ouabain-stimulated release of oxytocin and vasopressin from isolated neurointermediate lobes. The present study revealed that neurohypophyseal hormone release was not affected by dopamine, neither from lobes of untreated rats nor from those of rats with dopamine-deficiency (pretreated with alpha-methyl-p-tyrosine-methylester). Likewise, metoclopramide, a dopamine antagonist, was unable to alter the neurohypophyseal hormone release. Our results also indicate that the opioid peptides met-enkephalin and leu-enkephalin do not influence spontaneous or ouabain-stimulated oxytocin and vasopressin release, which is in accordance with our findings that naloxone under our experimental conditions is also ineffective.