Suprachiasmatic nucleus, secondary synchronizing stimuli and the central neural control of circadian rhythms.

The introduction of a secondary synchronizing stimulus, deprivation of water for 22 h each day, with water available during the morning only, to intact adult female rats produces a burst of activity and drinking behavior during and around the time water is available. Normal circadian rhythms in activity and drinking behavior persist through the deprivation period, however, and the morning burst of activity and drinking behavior ceases with the cessation of deprivation schedule. These data indicate that the induced rhythm in activity and drinking behavior is a diurnal, not a circadian rhythm. This view is supported by the fact that a free-running circadian rhythm in activity and drinking behavior persists through a separate period of introduction of the water deprivation schedule even though a diurnal rhythm is once again induced. Suprachiasmatic nucleus ablation abolishes circadian rhythms in activity, drinking and learned behavior, but does not alter the deprivation schedule-induced rhythms.