Another neurotransmitter is serotonin (5HT is the abbreviation- derived from the chemical name, 5-hydroxytryptamine). 5HT is largely found in the gastrointestinal tract where it regulates the muscles there. However, I think its functions in the brain are even more interesting. Serotonin is largely produced by the Raphe Nuclei in the brain and have functions throughout cortex. The context you have most likely heard about 5HT in the past is because most anti-depressants target serotonin. Selective Serotonin Reuptake Inhibitors (SSRIs) block the reuptake of serotonin and thus keep serotonin in the synapse for longer allowing for greater/prolonged activation of serotonin receptors. Other anti-psychotics also target the serotonin system.