Cyclic AMP and protein kinase A rhythmicity in the mammalian suprachiasmatic nuclei

The levels of cyclic AMP and protein kinase A, as well as the activity of t his enzyme, were measured in the hamster suprachiasmatic nuclei at differen t time points throughout the daily or circadian cycle. Significant diurnal Variations for levels of AMPc and the catalytic subunit of protein kinase A and the activity of this enzyme were found. All of these parameters tended to increase throughout the nocturnal phase, reaching higher values at the end of the night and the beginning of the day and minimal values around the time of lights off. This rhythmicity appears to be under exogenous control , since constant darkness abolished fluctuations throughout the circadian c ycle. In vitro incubation in the presence of melatonin during the day signi ficantly decreased cyclic AMP levels and basal protein kinase A activity in the SCN, while neither neuropeptide Y nor light pulses affected these para meters. These results suggest a significant diurnal regulation of the cycli c AMP-dependent system in the hamster circadian clock. (C) 2000 Elsevier Sc ience B.V. All rights reserved.