Serotonin and Parkinson's disease: On movement, mood, and madness.

An appreciation of the multiple roles that serotonin (5-HT) may play in Parkinson's disease (PD) has increased in recent years. Early pathological studies in PD demonstrated nonselective reductions of 5-HT in brain tissue but little correlation to comorbidities such as dyskinesia and mood disturbance. This, combined with treatment failures using serotonergic drugs in comparison to levodopa, meant the field was largely neglected until recently. The multitude of subtypes of 5-HT receptors in the brain and an increased understanding of the potential function 5-HT may play in modulating other neurotransmitter systems, including dopamine, GABA, and glutamate, have meant an expansion in efforts to develop potential serotonergic drugs for both motor and nonmotor symptoms in PD. However, several unanswered questions remain, and future studies need to focus on correlating changes in 5-HT neurotransmission in both pathological and in vivo imaging studies with a full clinical phenotype.