At the time of this study, there was a hypothesis that dysfunctions in serotonin (a neurotransmitter responsible for mood, cognition, reward, and memory) were associated with OCD; however, this was not proven.
This study, led by Drs. Simpson, Liebowitz, and Laruelle, aimed to test the serotonin transporter protein in 11 OCD patients with comorbid depression and 11 healthy controls by using positron emission tomography (PET) and a specific serotonin radiotracer called [11C]McN 5652 . The results found no difference between the OCD group and the control group for serotonin transporter protein availability in the caudate nucleus (a brain region associated with movement and learning) and other subcortical and limbic regions of the brain, contrary to the hypothesis. However, the authors noted that more research into other features of the serotonin system in OCD are necessary.
*Simpson, H.B., Lombardo, I., Slifstein, M., Huang, H.Y., Hwang, D-R., Abi-Dargham, A., Liebowitz, M.R., & Laruelle, M. (2003). Serotonin transporters in obsessive-compulsive disorder: A positron emission tomography study with [11C]McN 5652. Biological Psychiatry, 54(12), 1414-1421. doi:10.1016/S0006-3223(03)00544-4