Vasopressin and oxytocin release as affected by constant light in pinealectomized male rats

The effect of continuous lighting on vasopressin (AVP) and oxytocin (OT) levels in the hypothalamus, neurointermediate lobe (NIL) and blood plasma was studied in normal, sham operated and pinealectomized male rats. Under normal light conditions (L:D 12:12) the OT and AVP content in the hypothalamus and the NIL was not changed 8 days after pinealectomy, but it was significantly diminished 8 weeks after the surgery. When normal animals were exposed to constant light for 8 days, the AVP and OT content in the hypothalamus and NIL was significantly diminished, whereas plasma OT level was increased. Exposure of pinealectomized animals to constant light for 8 days resulted in the diminution of neurohypophysial AVP as well as OT but did not change significantly the hypothalamic and plasma levels of both hormones. Exposure of animals with intact pineal gland to the constant light for 8 weeks did not change the OT and AVP levels in the hypothalamo-neurohypophysial system and in the blood plasma. After pineal removal, however, the pinealectomy-induced decrease of OT and AVP content was not observed in the hypothalamus, whereas the plasma level of both hormones was even higher than that observed in not-pinealectomized animals kept at constant light conditions. These results suggest that the response of vasopressinergic and oxytocinergic neurons to continuous lighting is changing when the time of exposure of animals to such conditions is prolonged; these effects depend on the presence of intact pineal gland.