EFFECT OF ACUTE AND CHRONIC MORPHINE ON OXYTOCIN CONCENTRATION IN DIFFERENT BRAIN-AREAS AND PLASMA OF MALE-RATS

Acute morphine-HCl (20 mg/kg i.p.) failed to modify oxytocin concentration in the hypothalamus, the hippocampus, the septum and in plasma of male rats. In contrast, chronic morphine (10 mg/kg i.p. twice daily for 7 days) decreased oxytocin levels by 42% 90 min after the last administration in the hippocampus, but not in the other brain tissues and plasma. Twelve h after the last morphine treatment oxytocin levels were normal in the hippocampus but were decreased by 38% in the hypothalamus. Similar results were obtained 90 min after the last treatment when the opioid antagonist naloxone (5 mg/kg i.p.) was given 45 min before sacrifice. Naloxone increased plasma oxytocin concentration by 314% in chronic morphine-treated rats. The results show that chronic morphine differentially influences oxytocin concentration in brain areas of male rats and provide further support for the existence of an opioid-oxytocin interaction in the central nervous system.