Naloxone reduces the fenfluramine-induced prolactin release in man

In order to ascertain whether there is a relation between opioids and the serotoninergic system in prolactin (PRL) secretion increase, we investigated in seven healthy men (21 to 26 years of age) the effect of naloxone, a specific opioid antagonist, on PRL secretion induced by fenfluramine, a drug that stimulates serotonin release and inhibits its re-uptake. We observed that subjects receiving fenfluramine (60 mg orally) had a significantly (P less than 0.001) higher increase in PRL plasma levels than the controls receiving placebo. In all subjects naloxone infusion at a dose of 15 mg caused a significant reduction (P less than 0.0005) in the PRL response to fenfluramine. Higher doses of naloxone (30 mg) do not further inhibit the PRL secretion induced by fenfluramine. These results suggest that naloxone may interact with opiate receptors on serotonin neurons thereby reducing the synthesis and release of serotonin. It seems that in man, therefore, there is an interplay between opiates and the serotoninergic system in the facilitatory influence on PRL release.