Forced activity alters sleep cycle periodicity and dorsal raphe discharge rhythm

Single-cell activity recorded from the brain stem dorsal raphe nucleus (DRN) of cats showed a regular firing pattern during wakefulness (2-3 spikes/s), decreased activity during slow-wave or synchronized sleep (1 spike/s), and cessation of regular discharge during desynchronized sleep (0.05 spikes/s). DRN discharge was negatively correlated with the onset and maintenance of the polycyclic desynchronized sleep rhythm. These findings were derived from analyses of DRN firing rate sampled during different behavioral states. The present time-course analyses of DRN discharge described for the first time the period, amplitude, and phase characteristics of DRN activity relative to the timing of behavioral states that comprise the complete sleep cycle. The time course of both DRN discharge and the occurrence of wakefulness, slow-wave (S) sleep, transition, and desynchronized (D) sleep were behaviorally manipulated. Forced locomotor activity imposed by a treadmill task shortened the period length of the sleep cycle and the duration of the DRN discharge cycle. The magnitude and direction of these shifts in period length were consistent across multiple sleep cycles, and there was a high degree of coherence between the period length of the sleep cycle and the DRN discharge profiles. The DRN discharge rate also displayed phase dependence within S and D sleep. These results support the hypothesis that the DRN has a physiological role in regulating the timing of the ultradian sleep cycle.