Modeling sensitization to stimulants in humans -: An [11C] raclopride/positron emission tomography study in healthy men

Context: In animals, repeated exposure to stimulant drugs leads to an enhanced drug-induced psychomotor response and increased dopamine release. This phenomenon, known as sensitization, may confer vulnerability to drug addiction or drug-induced psychosis in humans. A similar phenomenon, referred to as endogenous sensitization, is also believed to play a role in the emergence of positive symptoms in patients with schizophrenia. Objective: To determine whether behavioral and neurochemical sensitization occur in healthy individuals after limited exposure to amphetamine in the laboratory. Design: Open-label, 1-year follow-up of repeated amphetamine administration in healthy volunteers. Setting: Department of Psychiatry, McGill University, and McConnell Brain Imaging Center, Montreal Neurological Institute. Participants: Ten healthy men (mean +/- SD age, 25.8 +/- 1.8 years). Intervention: Three single doses of amphetamine (dextroamphetamine sulfate, 0.3 mg/kg by mouth) were administered on days 1, 3, and 5. Main Outcome Measures: Using positron emission tomography and [C-11] raclopride, we measured dopamine release in response to amphetamine on the first exposure (day 1) and 14 days and 1 year after the third exposure. Results: The initial dose of amphetamine caused dopamine release in the ventral striatum (a reduction in [C-11] raclopride binding). Consistent with a sensitization-like phenomenon, 14 and 365 days after the third dose of amphetamine there was a greater psychomotor response and increased dopamine release (a greater reduction in [C-11] raclopride binding), relative to the initial dose, in the ventral striatum, progressively extending to the dorsal caudate and putamen. A high novelty-seeking personality trait and self-rating assessments indicating impulsivity predicted proneness to sensitization. Conclusions: Sensitization to stimulants can be achieved in healthy men in the laboratory. This phenomenon is associated with increased dopamine release and persists for at least 1 year.