Melatonin, pinealectomy, and release of neurohypophysial hormones: in vitro studies.

The effect of melatonin as well as pinealectomy on the basal and K(+)-evoked release of vasopressin and oxytocin from the neurointermediate lobes in vitro was determined. Pineal removal resulted in a diminution of vasopressin and oxytocin release from the neurointermediate lobes in vitro. Melatonin (10(-3) or 10(-6) M/l) increased vasopressin and oxytocin release from neurointermediate lobes of sham-operated rats. Nevertheless, when pinealectomized rats served as donors of the neurointermediate lobes, melatonin (10(-3) or 10(-6) M/l) increased vasopressin release under basal conditions. For the same tissue, melatonin did not affect the oxytocin release either under basal conditions or during depolarization due to excess potassium. When 10(-7) M/l melatonin was used, no changes in either vasopressin or oxytocin release were observed in vitro.