The evening rise in the rat pineal N-acetyltransferase activity under various photoperiods

The evening rise in the rat pineal N-acetyltransferase (NAT) activity was studied under various photoperiods. After the onset of darkness, NAT increased more rapidly under longer than under shorter photoperiods. Since following the evening administration of isoproterenol the rates of NAT induction and NAT responses to various doses were similar in short and in long photoperiods, it is suggested that the different times courses of the NAT increase may be due to the time of noradrenaline release from pineal nerve endings and to the quantity of the released noradrenaline.