Secretion of prolactin in response to serotonin requires an intact hypothalamo-pituitary axis in the ewe

The effects on prolactin secretion and the subsequent site of action of serotonin (5-HT) were examined in ovariectomized hypothalamo-pituitary intact (HP-intact) and ovariectomized hypothalamo-pituitary disconnected (HPD) ewes. The administration of 5 mg per animal of 5-HT increased plasma prolactin concentrations within 20 min by 2- to 3-fold in HP-intact ewes, but had no effect in HPD ewes. Injection of vehicle (saline) did not alter plasma prolactin concentrations in either group. These data indicate that 5-HT is involved in the control of prolactin secretion in sheep and that this action is exerted at the level of the hypothalamus and not directly at the anterior pituitary gland.