EFFECT OF PINEALECTOMY AND MELATONIN ON VASOPRESSIN-POTENTIATED PASSIVE-AVOIDANCE IN RATS

The pineal indoleamine, melatonin, and the hypothalamic neuropeptide, vasopressin, facilitate passive avoidance behaviour in rats. The simil arity of the effects suggests that interaction might occur between the two substances. Therefore, the effect of intraperitoneally applied va sopressin and/or melatonin on one-trial learning passive avoidance beh aviour was studied in pinealectomized rats. Intraperitoneal treatment with 500 ng vasopressin 1 hr before the retention test increased passi ve avoidance latency of sham-operated rats. In pinealectomized rats, a n identical amount of vasopressin was ineffective. In sham-operated ra ts, melatonin blocked the effect of vasopressin. It is concluded that vasopressin needs a regulated pineal function for developing effects i n passive avoidance behaviour.