Some individuals who are depressed have an abnormally small amount of the neurotransmitter serotonin at certain synapses in the brain.  One category of antidepressant drugs is called selective serotonin reuptake inhibitors.  Can you deduce what action this drug has at a synapse to increase the amount of serotonin present in the synaptic cleft?  Another type of antidepressant inhibits the enzymes that naturally break down serotonin at the synaptic cleft.  Can you explain how this might reduce the symptoms of depression?