Circadian fluctuations of cAMP content in the suprachiasmatic nucleus and the anterior hypothalamus of the rat.

Signal transduction by second messengers has long been hypothesized to be involved in the circadian system in the suprachiasmatic nucleus (SCN) of the mammalian brain. The present study reports that the concentration of adenosine-3',5'-monophosphate (cyclic AMP or cAMP) in the SCN region exhibited a circadian fluctuation with two peaks during a day under constant conditions. While a sharp peak at around the end of the subjective night was observed only in the SCN region, the other peak in the late subjective day was also found in the anterior hypothalamic area. The distinct cAMP peak at the late subjective night contrasts with the daytime peaks in electrical and metabolic activity rhythms of the SCN.