The research aim to study in detail the motor, cognitive and affective modifications of Parkinsonian patients (compared to control subjects) at a moderate stage of the disease and to analyze by positron emission tomography (PET) imaging with the specific radioligand [11C]SB207145 the expression levels of this 5-HT4 receptor. At the same time, the study will monitor the impact of the 5-HTTLPR polymorphisms of Serotonin Transporter (SERT) and Brain-derived neurotrophic factor (BDNF) Val66Met on the expression of the 5-HT4 receptor in subjects.

Inclusion Criteria:

• Man or woman : Age between ≥ 50 and ≤ 85 years old
• For women: postmenopausal
• Affiliated to a social security scheme or similar;
• Having given written consent to participate in the free and informed study.
• Level of study: ≥ 6 years of schooling
• only for healthy volunteer : No history of neurological or psychiatric disease
• Only for patient : Having a clinically established or probable diagnosis of Parkinson's disease according to the criteria defined by the Movement Disorder Society at a Hoehn and Yahr stage ≤ 3 (in on dopa conditions) and a duration of evolution of the disease between 2 and 8 years.
• Only for patient : Having a MoCA (Montreal Cognitive Assessment) ≥ 20/30.

Exclusion Criteria:

• Diagnosis other than Parkinson's disease ( for patient only)
• Treated with specific serotonin inhibitors (SSRIs), specific noradrenaline inhibitors (SNRIs), tricyclic antidepressants, monoamine oxidase inhibitors (MAOIs) and neuroleptics in the last 3 months
• Use of recreational drugs interfering with the serotonergic system (ecstasy, MDMA) or opioids (cannabis, opiates) in the last 3 months or chronic use
• Contraindication to magnetic resonance imaging (MRI) or positron emission tomography (PET)
• Participation in the last year to a study using ionizing radiation or concomitant participation in another research project involving the human being that may interfere with the results or the conclusions of this study
• Exceeding the annual amount of compensation authorized for participation in research protocols
• Deprivation of liberty by judicial or administrative decision, person subject to a legal protection measure
• BMI ≥ 35kg/m2
• Presence of depression (BDI-2 score ≥21), anxiety (Parkinson Anxiety Scale (PAS) score ≥14) or apathy (STARKSTEIN score ≥14) - (for healthy volunteers only)
• Cognitive impairment (MOCA score ≤26) (for healthy volunteers only)
• Current or past neurological or psychiatric pathology (for healthy volunteers only)
• Serious and progressive medical pathology
• Current or past dependence on any addictive substance according to DSM-V (The Diagnostic and Statistical Manual of Mental Disorders) criteria (alcohol, cannabis, hallucinogens, inhalants, opiates, sedatives, stimulants), except tobacco and caffeine. (for healthy volunteers only)