Genetic Variation in Serotonin Transporter Alters Resting Brain Function in Healthy Individuals Hengyi Rao, Seth J. Gillihan, Jiongjiong Wang, Marc Korczykowski, Geena Mary V. Sankoorikal, Kristin A. Kaercher, Edward S. Brodkin, John A. Detre, and Martha J. Farah Background: Perfusion functional magnetic resonance imaging (fMRI) was used to investigate the effect of genetic variation of the human serotonin transporter (5-HTT) gene (5-HTTLPR, SLC6A4) on resting brain function of healthy individuals. Methods: Twenty-six healthy subjects, half homozygous for the 5-HTTLPR short allele (s/s group) and half homozygous for the long allele (l/l group), underwent perfusion functional and structural magnetic resonance imaging during a resting state. The two genotype groups had no psychiatric illness and were similar in age, gender, and personality scores. Results: Compared with the l/l group, the s/s group showed significantly increased resting cerebral blood flow (CBF) in the amygdala and decreased CBF in the ventromedial prefrontal cortex. The effect of functional modulation in these regions by 5-HTTLPR genotype cannot be accounted for by variations in brain anatomy, personality, or self-reported mood. Conclusions: The 5-HTTLPR genotype alters resting brain function in emotion-related regions in healthy individuals, including the amygdala and ventromedial prefrontal cortex. Such alterations suggest a broad role of the 5-HTT gene in brain function that may be associated with the genetic susceptibility for mood disorders such as depression. Key Words: Amygdala, ASL perfusion fMRI, cerebral blood flow, depression, ventromedial orbitofrontal cortex R ecent advances in integrating noninvasive functional neu- roimaging with genetics have enabled investigators to explore the associations between specific genes and the neural pathways that mediate individual differences in both normal and abnormal human behaviors, particularly those re- lated to negative affect (for a review, see Hariri et al. 2006; Hariri and Holmes 2006; Hariri and Weinberger 2003; Wurtman 2005). Previous studies (Ansorge et al. 2004; Graspar et al. 2003; Lesch et al. 1996; Lotrich and Pollock 2004) have demonstrated the critical role of the serotonin neurotransmitter system in the development of emotional circuitry and the onset of mood disorders. Specifically, a polymorphism in the human serotonin transporter (5-HTT) gene (5-HTTLPR or SLC6A4) associated with 5-HTT protein expression and function has been shown to modulate the influence of stressful life events on depression (Caspi et al. 2003; Kendler et al. 2005) and the responses of the amygdala to negative stimuli. Evidence from several independent groups (Bertolino et al. 2005; Canli et al. 2005; Furmark et al. 2004; Hariri et al. 2002, 2005; Heinz et al. 2005) utilizing blood oxygenation level-dependent (BOLD) functional magnetic reso- nance imaging (fMRI) consistently indicates increased activation in the amygdala in response to negative stimuli in healthy individuals who carry the short allele (s) versus healthy individ- uals carrying the homozygous long alleles (l). Such reactivity to negatively valenced stimuli is an important aspect of psycholog- ical and neural function, one that BOLD fMRI is well suited to measure. However, whether and how the 5-HTTLPR genotype affects the brain’s ongoing activity between the occurrences of negative external stimuli, arguably comprising the majority of its function- ing, is still unknown. The present study therefore was designed to investigate the possibility of genetically driven differences in brain function during a resting baseline condition as a function of serotonin transporter genotype. Such information may be critical in interpreting the different manifestations of BOLD responses to aversive stimuli in long and short allele carrier groups. For example, Canli et al. (2005) have shown that apparent genotypic differences in response to negatively valenced pictures may actually result from different responses to the affectively neutral pictures of the baseline condition. Valid and reliable inferences of resting amygdala activity cannot be derived from BOLD fMRI studies per se, as BOLD fMRI measures only relative changes in neural activity. For this reason, we used arterial spin labeled (ASL) perfusion fMRI to measure resting brain function in two homozygous (s/s and l/l) groups. Using magnetically labeled arterial blood water as an endogenous tracer (Detre et al. 1992), ASL perfusion fMRI has been reported to provide reliable quan- tification of absolute cerebral blood flow (CBF) (in milliliters of blood per 100 g of tissue per minute), excellent reproducibility over long time periods, and reduced across-subject variability (Aguirre et al. 2002; Parkes et al. 2004; Wang et al. 2003). These features suggest that ASL perfusion fMRI provides a sensitive technique for reliable visualization of brain function during the resting state as well as during task performance. The regions of interests (ROIs) in the present study include the amygdala and ventromedial prefrontal cortex (VMPFC). There is considerable evidence showing 5-HTTLPR genotype effects on both amygdala structure and function (for a review, see Hariri et al. 2006; Hariri and Holmes 2006; Hariri and Weinberger 2003). Abnormally elevated resting amygdala blood flow and metabolism in depressed patients relative to control subjects have been consistently reported, and increased resting From the Center for Functional Neuroimaging (HR, JW, MK, JAD), Depart- ment of Neurology and Radiology, and Center for Cognitive Neuro- science (SJG, JAD, MJF), University of Pennsylvania, Philadelphia, Penn- sylvania; Center for Neurobiology and Behavior (GMVS, KAK, ESB), Department of Psychiatry, University of Pennsylvania School of Medi- cine, Philadelphia, Pennsylvania; and Center for Functional Brain Imag- ing (HR), Department of Psychology, Sun Yat-Sen University, Guang- zhou, China. Address reprint requests to Hengyi Rao, Ph.D., Department of Neurology, University of Pennsylvania, 3W Gates, 3400 Spruce Street, Philadelphia, PA 19104; E-mail: hengyi@mail.med.upenn.edu. Received July 20, 2006; revised November 21, 2006; accepted November 22, 2006. BIOL PSYCHIATRY 2007;62:600 – 606 0006-3223/07/$32.00 doi:10.1016/j.biopsych.2006.11.028 © 2007 Society of Biological Psychiatry