Monoamine dysfunction and the pathophysiology and treatment of depression

Alterations in noradrenergic and serotonergic function in the central nervous system (CNS) have been implicated in the pathophysiology of depression and the mechanism of action of antidepressant drugs. Based on changes in norepinephrine and serotonin metabolism in the CNS, it has been postulated that subgroups of patients with differential responses to norepinephrine and serotonin reuptake inhibitors may exist. Alpha-methylparatyrosine (AMPT), which causes rapid depletion of brain catecholamines, has been used as a noradrenergic probe to test the hypothesis that changes in neurotransmission through the catecholamine system may underlie the therapeutic response to norepinephrine reuptake inhibitors. Brain serotonin is dependent on plasma levels of the essential amino acid tryptophan. Rapid tryptophan depletion, in the form of a tryptophan-free amino acid drink, has been used as a serotonergic probe to identify therapeutically responsive subsets of patients. Using these probes, we have recently examined the behavioral effects of reduced concentrations of brain monoamines on depressed patients treated with a variety of serotonin selective reuptake inhibitors (SSRIs) or the relatively norepinephrine-selective antidepressant desipramine, during 3 different states: drug-free and depressed; in remission on antidepressant drugs; and drug-free in remission. The results of a series of investigations confirm the importance of monoamines in the mediation of depressed mood, but also suggest that other brain neural systems may have more of a primary role than previously thought in the pathophysiology of depression. Noradrenergic and serotonergic probes may be used in time to identify subsets of depressed patients to determine which patients might respond differentially to the new selective norepinephrine reuptake inhibitors or SSRIs.