Serotonergic loss in motor circuitries correlates with severity of action-postural tremor in PD

Objective: The underlying pathophysiology of tremor in Parkinson disease (PD) is unclear; however, it is known that tremor does not appear to be as responsive to dopaminergic medication as bradykinesia or rigidity. It is suggested that serotonergic dysfunction could have a role in tremor development. Methods: Using C-11-DASB PET, a marker of serotonin transporter binding, and clinical observations, we have investigated function of serotonergic terminals in 12 patients with tremor-predominant and 12 with akinetic-rigid PD. Findings were compared with those of 12 healthy controls. Results: Reductions of C-11-DASB in caudate, putamen, and raphe nuclei significantly correlated with tremor severity on posture and action, but not with resting tremor. The tremor-predominant group also showed reductions of C-11-DASB in other regions involved in motor circuitry, including the thalamus and Brodmann areas 4 and 10. Conclusions: Our findings support a role for serotonergic dysfunction in motor circuitries in the generation of postural tremor in PD.