Melatonin affects the oxytocin and prolactin responses to stress in male rats

Since the pineal-neurohypophysial interactions are now well established and oxytocin secretion is known to be a component of the neuroendocrine response to the majority of stressful stimuli, the present experiments were undertaken to estimate whether melatonin modifies the response of oxytocinergic neurons to the immobilization stress. Oxytocin (OT) content in the hypothalamus and neurohypophysis as well as plasma level of OT, prolactin (PRL) and adrenocorticotropin (ACTH) were studied after melatonin treatment in sham-operated or pinealectomized male rats. In sham-operated rats, melatonin diminished the hypothalamic OT content as well as plasma OT and PRL concentrations, but was without effect on neurohypophysial OT and plasma ACTH levels in otherwise not treated rats. In both wehicle- or melatonin-treated rats, food and water deprivation did not affect the OT, PRL and ACTH secretion. Under stress conditions, however, pituitary OT storage was diminished in vehicle-treated rats and melatonin augmented this response of OT to stress. Melatonin also diminished the PRL and ACTH secretion into the blood in stressed rats. In pinealectomized animals neither hypothalamo-neurohypophysial OT content nor plasma OT, PRL or ACTH concentrations were modified by melatonin treatment in animals otherwise not treated or in those deprived of food and water for 24 hrs. However, melatonin increased the pituitary oxytocin content as well as plasma OT and ACTH concentrations 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 augmented by melatonin. The effect of melatonin on the OT, PRL and ACTH secretion is modified by pinealectomy.