Activity of nucleus raphe pallidus neurons across the sleep-waking cycle in freely moving cats

The activity of serotonin-containing nucleus raphe pallidus (RPA) units was recorded by means of movable 32 or 62 microns diameter insulated nichrome wires in freely moving cats. RPA units displayed a slow, rhythmic discharge rate during waking (mean = 5.3 spikes/s) and showed no significant change in activity during slow-wave sleep. However, these neurons showed a large decrease in activity during REM sleep (mean = 1.2 spikes/s). In contrast with results from studies on serotonin-containing nucleus raphe dorsalis units, RPA neurons showed no relationship to the occurrence of sleep spindles, and were not inhibited by low doses of LSD.