Role of serotonin in the pathophysiology of depression: Focus on the serotonin transporter

Considerable evidence has accrued in the last two decades to support the hypothesis that alterations in serotonergic neuronal function in the central nervous system occur in patients with major depression. These findings include the following: (a) reduced cerebrospinal fluid (CSF) concentrations of 5-hydroxyindoleacetic acid (5-HIAA), the major metabolite of serotonin (5- HT) in drug-free depressed patients; (b) reduced concentrations of 5-HT and 5-HIAA in postmortem brain tissue of depressed and (or) suicidal patients; (c) decreased plasma tryptophan concentrations in depressed patients and a profound relapse in remitted depressed patients who have responded to a serotonergic antidepressant when brain tryptophan availability s reduced; (d) in general, all clinically efficacious antidepressants augment 5-HT neurotransmission following chronic treatment; (e) clinically efficacious antidepressant action by all inhibitors of 5-HT uptake; (f) increases in the density of 5-HT₂ binding sites in postmortem brain tissue of depressed patients and suicide victims, as well as in platelets of drug-free depressed patients; (g) decreased number of 5-HT transporter (determined with [³H]imipramine or [³H]paroxetine) binding sites in postmortem brain tissue of suicide victims and depressed patients and in platelets of drug-free depressed patients. In our studies, this reduction in platelet 5-HT transporter binding is not due to prior antidepressant treatment or hypercortisolemia and is not observed in mania, Alzheimer disease, schizophrenia, panic disorder, fibromyalgia, or atypical depression. In a pilot study, this deficit predicted treatment response to an experimental antidepressant. These findings support the hypothesis that alterations in 5- HT neurons play a role in the pathophysiology of depression.