Adenosine 5'-monophosphate enhances the dark and isoproterenol-induced rise in rat pineal N-acetyltransferase activity.

Exposure of rats to light during darkness or blockade of the pineal beta-adrenoceptor in stimulated pineal glands results in a rapid fall in pineal N-acetyltransferase (NAT) activity. Maintenance of a high level of NAT activity requires continuous stimulation of the pineal beta-adrenoceptors. It is not known what factors in the pinealocyte are responsible for this rapid inactivation of NAT activity. In the present study we have attempted to investigate a possible regulatory role of 5'-AMP on pineal NAT activity. The results show that 5'-AMP further enhances dark-induced as well as isoproterenol-induced NAT activity by approximately 3-fold but does not alter unstimulated daytime NAT activity. Theophylline, an inhibitor of 5'-AMP synthesis, when administered early in the dark phase, caused a rise in pineal cAMP with a concomitant fall in pineal NAT activity. These findings indicate that 5'-AMP could play a role in the activation of pineal NAT. The possibility that the rapid inactivation of NAT is due to a rapid removal of 5'-AMP by, for example, phosphorylation, remains to be investigated.