Brain serotonin transporter binding in depressed patients with bipolar disorder using positron emission tomography

Context: Depression in bipolar disorder is clinically indistinguishable from that observed in major depressive disorder. As in major depression, selective serotonin reuptake inhibitors targeting brain serotonin transporters are first-line treatments for bipolar depression. Associations of serotonin transporter promoter polymorphisms and bipolarity have been reported; however, research on alterations in serotonergic neurotransmission in bipolar depression remains scant. Objectives: To assess in vivo brain serotonin transporter binding potential (BP1, proportional to serotonin transporter number) in patients with bipolar depression and controls and to examine the relationship between serotonin transporter binding and genotype. Design: Case-control study. Setting: University hospital. Participants: A sample of 18 medication-free patients with bipolar depression and 41 controls. Main Outcome Measures: In vivo brain serotonin transporter binding was measured using positron emission tomography and radiolabeled trans-1,2,3,5,6,10-β-hexahydro-6-[4-(methylthio) phenyl]pyrrolo-[2,1-a]-isoquinoline ([¹¹C](+)-McNeil 5652). Participants were genotyped assessing biallelic and triallelic 5-HTTLPR polymorphisms. Results: Patients with bipolar disorder had 16% to 26% lower serotonin transporter BP1 in the midbrain, amygdala, hippocampus, thalamus, putamen, and anterior cingulate cortex. Triallelic 5-HTTLPR genotypes were unrelated to serotonin transporter BP1. Conclusions: Lower serotonin transporter BP1 in bipolar depression overlaps with that observed in major depression and suggests that serotonergic dysfunction is common to depressive conditions.