Rhythmic multiunit neural activity in slices of hamster suprachiasmatic nucleus reflect prior photoperiod.

The suprachiasmatic nucleus (SCN) is an endogenous circadian pacemaker, and SCN neurons exhibit circadian rhythms of electrophysiological activity in vitro. In vivo, the functional state of the pacemaker depends on changes in day length (photoperiod), but it is not known if this property persists in SCN tissue isolated in vitro. To address this issue, we prepared brain slices from hamsters previously entrained to light-dark (LD) cycles of different photoperiods and analyzed rhythms of SCN multiunit neuronal activity using single electrodes. Rhythms in SCN slices from hamsters entrained to 8:16-, 12:12-, and 14:10-h LD cycles were characterized by peak discharge rates relatively higher during subjective day than subjective night. The mean duration of high neuronal activity was photoperiod dependent, compressed in slices from the short (8:16 and 12:12 LD) photoperiods, and decompressed (approximately doubled) in slices from the long (14:10 LD) photoperiod. In slices from all photoperiods, the mean phase of onset of high neuronal activity appeared to be anchored to subjective dawn. Our results show that the electrophysiological activity of the SCN pacemaker depends on day length, extending previous in vivo data, and demonstrate that this capacity is sustained in vitro.