Pathological gambling (PG) is classified as an impulse control disorder. The mechanisms underlying the behaviour are poorly understood but may include dysfunction of mesolimbic structures in association with dysregulation of the dopamine (DA) neurotransmitter system. The disorder is more prevalent in Parkinson's disease patients taking a combination of dopamine agonists and levodopa (approximately 8%) and this suggests that dopamine may prime the behaviour. Therefore, alterations in the DA receptors arising from genetic variations in the DA receptor genes are likely to be involved in this phenomenon. Dopaminergic agents currently used to treat Parkinson's disease are mainly D2 and D3 receptor agonists. Therefore, we propose to investigate polymorhisms on D2 and D3 receptor genes, using tagged haplotypes (35 makers on D2 gene and 30 markers on D3) in Parkinson's Disease patients that will take part in a Positron Emission Tomography (PET) study using the ligand [C11] raclopride (Problem Gambling in Parkinson Disease: A human model of dopamine dysregulation).