Many people experience severe depression.
One part of the treatment of depression is to give antidepressants, which work for many people.
Antidepressant drugs work to keep the neurotransmitters serotonin and norepinephrine around in the synapse for a much longer time.
They do not allow these neurotransmitters to reenter the presynaptic neuron.
The cells that produce these neurotransmitters start in the brain stem and travel widely through the forebrain and limbic system.