The Raphe nuclei (singular = Raphe nucleus) are a cluster of nuclei in the hindbrain which release a neurotransmitter called serotonin (5-HT). The serotonin molecules get sent all over the brain, where they're needed for all sorts of roles. 'Selective serotonin reuptake inhibitor' (SSRI) antidepressants are believed to act in these nuclei, as well as at their targets. Many opioids (drugs that calm people down) can act on the Raphe nuclei to secrete too much 5-HT.