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The role of dopamine (DA) pathways in the pathophysiology of depressive disorder is poorly understood. However, because DA plays a key role in motivational behavior, it is important to study in a disorder characterized by anhedonia, lack of energy and psychomotor retardation. A recently developed dietary manipulation (`tyrosine (TYR) depletion') offers a novel method to assess the role of DA in major depression. We studied 1.5 euthymic women with a past history of recurrent depression, who received a 74 g amino-acid drink lacking TYR and phenylalanine (PHE) (TYR-free) and a balanced (BAL) amino acid drink on two separate occasions in a double-blind, random-order, crossover design. Plasma prolactin levels rose following the TYR-free drink relative to the balanced mixture, while performance on a spatial recognition memory task was impaired. However, relative to the BAL drink, the TYR-free drink did not lower objective or subjective measures of mood. We conclude that as in healthy volunteers, TYR depletion in euthymic subjects, with a past history of major depression, attenuated DA function, as reflected in increased plasma prolactin levels and decreased spatial memory performance. However ratings of depression were unaffected, suggesting that disruption of dopaminergic function by this manipulation does not induce a lowering of mood in individuals vulnerable to depression. [ABSTRACT FROM AUTHOR]

Amphetamines are highly addictive drugs that have pronounced effects on emotional and cognitive behavior in humans. These effects are mediated through their potent dopaminergic agonistic properties. Dopamine has also been implicated in the modulation of responses of the ‘reward circuit’ in animal and human studies. In this study we use functional magnetic resonance imaging (fMRI) to identify the brain circuitry involved in the psychostimulant effect of methamphetamine in psychostimulant-naïve human subjects. Seven healthy volunteers were scanned in a 3T MR imaging system. They received single-blind intravenous infusions of methamphetamine (0.15 mg/kg), and rated their experience of ‘mind-racing’ on a button press throughout the experiment. Data were analyzed with statistical parametric mapping methods. Amphetamine administration activated the medial orbitofrontal cortex, the rostral part of the anterior cingulate cortex, and the ventral striatum. Ratings of ‘mind-racing’ after methamphetamine infusion correlated with activations in the rostral part of the anterior cingulate cortex and in the ventral striatum. In addition, activations in the medial orbitofrontal cortex were independent of motor and related responses involved in making the ratings. These findings indicate that the first administration of a psychostimulant to human subjects activates classical reward circuitry. Our data also support recent hypotheses suggesting a central role for the orbitofrontal cortex in drug reinforcement and the development of addiction. [ABSTRACT FROM AUTHOR]