Serotonergic loss underlying apathy in Parkinson’s disease

This scientific commentary refers to ‘The prominent role of serotonergic degeneration in apathy, anxiety and depression in de novo Parkinson’s disease’, by Maillet et al. (doi:10.1093/brain/aww162) .Disentangling the contributions of various neurotransmitter systems to specific clinical manifestations of Parkinson’s disease is important for understanding their pathogenesis and for developing effective treatments. The clinical phenomenology of Parkinson’s disease encompasses a range of motor and non-motor disorders, of which neuropsychiatric presentations such as depression, anxiety, dementia, impulse control disorders and apathy are among the most deleterious for patients’ and carers’ quality of life. Apathy, characterized by diminished goal-oriented behaviour, cognition, interests, and emotional expression (Starkstein, 2012), is common in Parkinson’s disease, affecting ∼40% of patients in cross-sectional studies. It is under-recognized (Gallagher et al. , 2010) and often confused with dementia or depression, of which it can also be an integral part (Pagonabarraga et al. , 2015). Understanding its distinct nature can help improve families’ approaches to dealing with apathy and ensure its appropriate management. The underlying pathology of apathy is poorly understood, although clinical and imaging findings support the role of dopaminergic dysfunction in apathy in patients with (Magnard et al. , 2016; Wen et al. , 2016;) and without Parkinson’s disease (Starkstein, 2012; Pagonabarraga et al. , 2015). In this issue of Brain , Maillet and co-workers use PET molecular imaging to …