Sleep and serotonin: an unfinished story.

Serotonin (5-HT) was first believed to be a true neuromodulator of sleep because the destruction of 5-HT neurons of the raphe system or the inhibition of 5-HT synthesis with p-chlorophenylalanine induced a severe insomnia which could be reversed by restoring 5-HT synthesis. However the demonstration that the electrical activity of 5-HT perikarya and the release of 5-HT are increased during waking and decreased during sleep was in direct contradiction to this hypothesis. More recent experiments suggest that the release of 5-HT during waking may initiate a cascade of genomic events in some hypnogenic neurons located in the preoptic area. Thus, when 5-HT is released during waking, it leads to an homeostatic regulation of slow-wave sleep.