Changes in the levels of cyclic nucleotides in rat brain during the sleep-wakefulness cycle

Changes in the levels of adenosine 3',5'-monophosphate (cAMP) and guanosine 3',5'-monophosphate (cGMP) in the discrete regions of rat brain during the sleep-wakefulness cycle were studied using a microwave fixation method. Rats were restrained in a specially designed frame in which they could move their heads and extremities freely and take food and water ad libitum. The rats were acclimatized to the microwave applicator for 1 h a day for 5-7 days, and then they were sacrificed by microwave irradiation of the heads under polygraphic monitoring. This procedure made it possible to obtain tissue for the analysis of cyclic nucleotides at different times during the sleep-wakefulness cycle. The content of cAMP in the various regions of the brain except the cerebellum decreased during sleep. In the hippocampus, midbrain, pons-medulla and cerebellum, the level of cGMP was highest during wakefulness, whereas in the striatum it was highest during paradoxical sleep. In the midbrain and pons-medulla, the level of cGMP was higher during paradoxical sleep than during slow-wave sleep. These changes of cyclic nucleotide contents may reflect the changes in the release of putative neurotransmitters during the sleep-wakefulness cycle. Our findings suggest that cyclic nucleotides may play some roles in the regulation of the sleep-wakefulness cycle.