Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor symptoms such as tremor, rigidity and slowness of movement. In later stages of the disease, when pharmacological treatment becomes less efficient, deep brain stimulation (DBS) of the subthalamic nucleus (STN) becomes a treatment option. Although motor symptoms improve significantly by DBS, a number of operated patients experience severe side effects, mostly related to mood, cognition or behavior. These adverse effects are most likely mediated through the serotonin (5-HT) system. Additionally, a dysfunction of the 5-HT system is implied in the pathophysiology of PD. PD patients are therefore regarded as 'vulnerable' to experiencing mood, cognitive and emotional problems due to changes in 5-HT activity.~To elucidate whether STN DBS is indeed the trigger for psychiatric and cognitive problems to arise in the PD patient, the 5-HT levels in PD patients implanted with STN DBS will be manipulated. In order to do this, the investigators will make use of the tryptophan (TRP) depletion method, an established research paradigm. In TRP depletion, the brain is depleted of TRP, the precursor of 5-HT, which consequently leads to lowered 5-HT levels. In both the normal and 5-HT depleted condition, mood- and cognitive parameters of the PD patients both with the STN stimulation on (ON) and off (OFF) will be assessed.~The goal is to get more insight into the effects of STN DBS in PD patients with a 5-HT vulnerabililty and the effects on 5-HT related mood and cognitive behaviour. This way, possible risk factors for and mechanisms underlying psychiatric side effects of STN DBS can be identified. The study is an intervention study with a placebo controlled, randomized cross-over design.
The purpose of this study is to assess the effect of tryptophan depletion on mood and behavior in Parkinson's disease (PD) patients treated with deep brain stimulation (DBS) of the subthalamic nucleus (STN). By doing this, the investigators hope to be able to identify risk factors for and mechanisms underlying psychiatric side effects of STN DBS. The study will be an intervention study with a placebo controlled, randomized cross-over design.