Serotonin in Parkinson's disease

Parkinson's disease is a chronic neurodegenerative disorder characterized by the motor symptoms of bradykinesia, tremor, rigidity and postural instability. However, non-motor symptoms such as chronic fatigue, depression, dementia and sleep disturbances are also frequent and play a significant role with negative consequences in the quality of life of patients with Parkinson's disease. Although the progressive dopaminergic denervation is the cardinal pathology in the brains of patients with Parkinson's disease, others systems such as the serotonergic are affected as well. Over the last decade, several lines of evidence suggest that a progressive and non-linear loss of serotonergic terminals takes place in Parkinson's disease, though this is at a slower pace compared to the dopaminergic loss. Several studies have indicated that serotonergic dysfunction in Parkinson's disease is associated with the development of motor and non-motor symptoms and complications. Here, we aim to review the current evidence with regards to the serotonergic pathology in Parkinson's disease and its relevance to the development of clinical symptoms. We are primarily revising in vivo human studies from research with positron emission tomography molecular imaging.