The Role of Serotonin Transporters in Tourette Syndrome and OCD

Tourette Syndrome is a disorder characterized by repeating motor movements and/or verbal tics that are not easily suppressed. It is common for people with Tourette Syndrome to have additional conditions such as ADHD (Attention Deficit Hyperactivity Disorder) and anxiety disorder. The simultaneous presence of Tourette Syndrome and OCD (Obsessive Compulsive Disorder) has been of interest to some researchers as of late. Conditions like OCD are often treated with SSRIs (Selective Serotonin Reuptake Inhibitors) which are drugs that influence the nervous system by changing the behavior of neurotransmitters, the chemical messengers that neurons release to communicate. SSRIs cause an increase in the amount of neurotransmitter serotonin available for transmission between neurons by restricting the amount that can be absorbed for reuse after release. Since less serotonin is being resorbed, neurotransmitter is left in the cleft between two neurons longer and has more opportunity to affect the next neuron. The success of this mode of treatment for OCD implies that the cause of this disorder may be related to the serotonergic system.

A study published in early 2019 by Müller-Vahl et al. sought to gain clarity on the role of the serotonergic system in Tourette Syndrome patients by examining availability of the proteins responsible for the reuptake and recycling of serotonin known as serotonin transporters. In addition to this main objective, the authors had three other goals. Firstly, they were interested in how the presence of a condition like OCD would affect these proteins’ capacity to function. Secondly, they wanted to see whether there was a perceivable difference between the functional capacity of these proteins in patients with Tourette Syndrome and OCD compared to patients with only OCD. Lastly, they wanted to examine the influence of a specific SSRI called escitalopram on the functional capacity of these proteins.

The study involved injecting neuroimaging molecules, or radioactive ligands, with a very strong affinity for serotonin transporter proteins into 33 participants. Some participants had both Tourette Syndrome and OCD, some had only Tourette Syndrome, some had only OCD, and some were “healthy” lacking both Tourette Syndrome and OCD. To view the activity of the proteins, measurements of the presence of the radioactive molecules were taken using SPECT (Single-Photon Emission Computed Tomography) immediately after injection for up to five hours. Measurements were taken again 12 to 16 weeks later after treatment with escitalopram. The purpose of these measurements was to gauge the effectiveness of serotonin transporters in critical areas of the brain. Mapping software was used to produce images of the serotonin transporter activity in each participant as well as a statistical analysis of the results.

The study found that serotonin transporter activity is elevated in the caudate and midbrain of individuals that have Tourette Syndrome and OCD relative to individuals with neither condition, individuals with only Tourette Syndrome, and individuals with only OCD. More transporter activity means that there is more reuptake of serotonin and less available for communication between neurons. Because this elevation was not found in participants with only Tourette Syndrome, it was concluded that alterations in the serotonergic system are related to Tourette Syndrome only when combined with OCD. Importantly, they clarified that alterations in the serotonergic system are not the primary cause of Tourette Syndrome. While this particular study did not support the notion that alterations in the serotonergic system are related to OCD, previous research referenced in this study found there to be increased serotonin transporter activity in early onset OCD. The fact that the sample size of participants with early onset OCD in this study was relatively small may account for these findings not being replicated. If the previous study was accurate, we can conclude that there is a connection between the etiology of early onset OCD and Tourette Syndrome combined with OCD but not late onset OCD on its own. Finally, treatment with the SSRI escitalopram significantly reduced the serotonin transporter activity in all critical areas of the brain in participants with only OCD and participants with both Tourette Syndrome and OCD. The difference in activity induced by the drug was much larger than the difference that exists naturally between untreated patients and healthy individuals, proving escitalopram to be a highly effective treatment method.

Source: Müller-Vahl, K. R., Szejko, N., Wilke, F., Jakubovski, E., Geworski, L., Bengel, F., & Berding, G. (2019). Serotonin transporter binding is increased in Tourette syndrome with Obsessive Compulsive Disorder. Scientific reports9(1), 1-10. https://www.nature.com/articles/s41598-018-37710-4