Characterize The Modulatory Effects Of Dopamine D2/D3 Receptor Agonist And Antagonist Drugs On Compulsive Behaviors

3 groups of subjects (healthy controls, OCD subjects and stimulant-dependent subjects) will receive pramipexole (1.5 mg, single dose), amisulpride (400 mg, single dose) or placebo in a cross-over, double-blind, placebo-controlled design. Effects of compulsive behaviour will be assessed using fMRI and cognitive testing. Assess biomarkers including cardiovascular responses and plasma levels. All groups studied on 3 separate occasions following screening, with at least a week intervening between consecutive assessments. The procedures to be adopted for study assessment will be identical on each occasion.