MELATONIN EFFECTS THE OXYTOCIN AND PROLACTIN RESPONSES TO STRESS IN MALE RATS

Since the pineal-neurohypophysial interactions are now well establishe d and oxytocin secretion is known to be a component of the neuroendocr ine response to the majority of stressful stimuli, the present experim ents were undertaken to estimate whether melatonin modifies the respon se of oxytocinergic neurons to the immobilization stress. Oxytocin (OT ) content in the hypothalamus and neurohypophysis as well as plasma le vel of OT, prolactin (PRL) and adrenocorticotropin (ACTH) were studied after melatonin treatment in sham-operated or pinealectomized male ra ts. In sham-operated rats, melatonin diminished the hypothalamic OT co ntent as well as plasma OT and PRL concentrations, but was without eff ect on neurohypophysial OT and plasma ACTH levels in otherwise not tre ated rats. In both wehicle- or melatonin-treated rats, food and water deprivation did not affect the OT, PRL and ACTH secretion. Under stres s conditions, however, pituitary OT storage was diminished in vehicle- treated rats and melatonin augmented this response of OT to stress. Me latonin also diminished the PRL and ACTH secretion into the blood in s tressed rats. In pinealectomized animals neither hypothalamo-neurohypo physial OT content nor plasma OT, PRL or ACTH concentrations were modi fied by melatonin treatment in animals otherwise not treated or in tho se deprived of food and water for 24 hrs. However, melatonin increased the pituitary oxytocin content as well as plasma OT and ACTH concentr ations in immobilized animals. Plasma PRL concentration was diminished after melatonin treatment in stressed rats. The results suggest that the response of oxytocinergic neurons to immobilization stress is augm ented by melatonin. The effect of melatonin on the OT, PRL and ACTH se cretion is modified by pinealectomy.