Caption: Action of selective serotonin reuptake inhibitors (SSRIs) at a chemical synapse, computer artwork. SSRIs are antidepressants that regulate levels of serotonin (the neurotransmitter that controls our moods) within the nervous system. Chemical synapses are the junctions between any two nerves. The drugs (pink) work at the synapses to block the reuptake of serotonin (orange beads) to the presynaptic cell (top). This increases the amount available to the postsynaptic cell downstream (bottom).