The Role of the Serotonergic System in REM Sleep Behavior Disorder

Study Objectives: REM sleep behavior disorder (RBD) can be induced by antidepressants, especially serotonin reuptake inhibitors (SSRI), thus a role of the serotonergic system in the pathogenesis of RBD has been proposed. However, the serotonergic system integrity in idiopathic RBD (iRBD) is still unknown. We aimed to study brain stem serotonergic system integrity, by means of I-123-FP-CIT-SPECT, in a group of iRBD patients as compared to normal subjects. Design: Single-center, prospective observational study. Setting: University hospital. Patients or Participants: Twenty iRBD outpatients and 23 age-matched normal controls. Measurements and Results: The diagnosis of RBD was determined clinically and confirmed by means of overnight, laboratory-based video-polysomnography. Both iRBD patients and normal subjects underwent I-123-FP-CIT-SPECT as a marker of dopamine transporter (DAT) at basal ganglia level and of serotonin transporter (SERT) at brainstem and thalamus levels. I-123-FP-CIT-SPECT images were analyzed and compared between iRBD patients and controls by means of both region of interest analysis at basal ganglia, midbrain, pons and thalamus levels, and voxel-based analysis, taking into account age and the use of SSRI as confounding factors. No difference in I-123-FP-CIT-SPECT specific to non-displaceable binding ratios (SBR) values was found between iRBD and normal subjects at brainstem and thalamus levels while iRBD patients showed lower SBR values in all basal ganglia nuclei (P < 0.0001) compared to controls. Conclusions: These results suggest that the serotonergic system is not directly involved in RBD pathogenesis while confirming nigro-striatal dopaminergic deafferentation in iRBD.