Raphe nuclei

The raphe nuclei are a moderate-size cluster of nuclei found in the brain stem. Their main function is to release serotonin to the rest of the brain. Selective serotonin reuptake inhibitor antidepressants are believed to act in these nuclei, as well as at their targets.

The raphe nuclei have a vast impact upon the central nervous system. Many of the neurons in the nuclei are serotonergic; i.e., contain serotonin, a type of monoamine neurotransmitter and are modulated through fibrous pathways in the midbrain.

Projections from the raphe nuclei also terminate in the dorsal horn of spinal gray matter where they regulate the release of enkephalins, which inhibit pain sensation.

The raphe nuclei provide feedback to the suprachiasmatic nuclei, thus contributing in circadian rhythms in animals. The SCN transmits to the raphe nuclei via the dorsomedial hypothalamus nucleus altering serotonin levels for sleep/wake states. The raphe nuclei will then transmit feedback to the SCN about the animal's vigilance and levels of alertness. This reciprocal feedback between the two structures provides an adaptable yet stable basis of circadian rhythms.