serotonin agonist
any agent that increases the affinity for, or availability of, serotonin at various serotonin receptors in the brain or peripheral tissues. Commonly used indirect agonists are the SSRIs (e.g., fluoxetine, citalopram), which work by blocking the presynaptic reuptake of serotonin, thereby increasing the availability of serotonin at postsynaptic receptor sites. Other serotonin agonists exert their effects directly at the receptor site; for example, the triptans are direct agonists at receptor subtypes 5-HT1B and 5-HT1D. The anxiolytic agent buspirone is a partial agonist at the postsynaptic 5-HT1A receptor, whereas the serotonin-like hallucinogens (e.g., LSD) act as partial agonists at 5-HT2A receptors.