Raphe unit activity in freely moving cats: lack of diurnal variation.

Activity of presumed serotonin-containing neurons in the dorsal raphe nucleus of the cat was recorded across the sleep--waking cycle every 4 h, for 24 or 48 h. These neurons showed no diurnal variation in activity independent of behavioral state. This suggests that reports of diurnal fluctuations in serotonin metabolism are due either to differential patterns of sleep--wakefulness across the 24 h cycle and/or to a lack of tight coupling between neuronal activity and neurotransmitter metabolism.