Serotonin: its place today in sleep preparation, triggering or maintenance.

Serotonin (5-HT) is involved in sleep in two different ways. First, when released during waking by the axonal nerve endings, it influences the synthesis of hypnogenic substances in specific brain targets. Such a synthesis might be in keeping with the waking qualitative aspects. As an example, the hypnogenic CLIP peptide (ACTH18-39) is synthesized when stressful events occur during wakefulness. Second, when released during sleep within the nucleus raphe dorsalis (nRD) by dendrites of 5-HT neurons, it contributes to 5-HT perikarya silencing through an auto-inhibitory process. Nitric oxide, co-synthesized with 5-HT, may act in synergy with this amine at both mentioned levels. Regarding the triggered hypnogenic substances, they induce sleep through acting on two components within the nRD: (1) the 5-HT component; its silencing is necessary to remove the gating effect exerted on phasic sleep events (ponto-geniculo-occipital, PGO, waves); (2) a substance P component; its silencing is necessary, at least, to alleviate the tonic influence exerted on somatic muscles. These two components may constitute the brain "sleep switch-on" mechanism allowing wake/sleep alternation. Pharmacological procedures influencing this switch may be determinant for treating insomniac patients. Serotonin appears thus to be involved in sleep preparation, triggering and maintenance.