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Adenosine 5′‐monophosphate enhances the dark and isoproterenol‐induced rise in rat pineal N‐acetyltransferase activity

Abstract: Exposure of rats to light during darkness or blockade of the pineal β‐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 β‐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.