Serotonergic refers to the neurotransmitter serotonin, the neurons in the brain that produce serotonin, and the neurons that receive and respond to it. This system is distributed throughout the brain and is involved in regulating many different functions. In psychopharmacology, “serotonergic” refers to drugs that affect the serotonergic system, e.g. selective serotonin reuptake inhibitors (SSRIs). These drugs work by blocking the reuptake of serotonin into neurons, thereby increasing its availability in the synaptic cleft and enhancing its effects on downstream neurons.