The Mammalian Circadian Clock in the Suprachiasmatic Nuclei is Reset In Vitro by cAMP

A circadian clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus controls the daily behavioral and physiological rhythms of mammals. While the mammalian circadian system has been the focus of research for many years, very little work has been directed at understanding its underlying biochemical mechanisms. In these experiments we used the hypothalamic brain slice technique to investigate these mechanisms, focusing specifically on the intrinsic resetting properties of the circadian clock of the rat. We monitored a primary expression of the clock or pacemaker, the circadian rhythm of electrical activity of SCN neurons. This rhythm continues the oscillatory pattern seen in vivo for up to 60 hr in vitro, with an activity peak near midday that shows very little variation among SCN from different rats. The stability of the rhythm in vitro enabled us to use the time of peak activity to monitor the phase of the underlying pacemaker. Bath application of membrane-soluble cAMP analogs in 1 hr pulses induced robust advances in the phase of the rhythm that remained stable for 2 cycles. This effect depended on the phase of the pacemaker at the time of treatment: the peak was maximally advanced (4-6 hr) by treatments during the middle of the subjective day (projected from the donor's cycle); treatments during most of the subjective night and early subjective day induced no phase changes. Half-maximal phase resetting was induced at 1 x 10(-10) M concentrations of active cAMP analog.