the theory that depression is caused by a deficit in the production or uptake of the amine compounds serotonin, norepinephrine, and dopamine. This theory has been used to explain the effects of monoamine oxidase inhibitors but is now regarded as too simplistic. A related notion, the monoamine neurotransmitter theory, proposes that runner’s high is the result of an increase of norepinephrine and serotonin with exercise.