Inhibition of the norepinephrine transporter improves behavioral flexibility in rats and monkeys

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Abstract
Poor cognitive control, including reversal learning deficits, has been reported in children with attention deficit hyperactivity disorder, in stimulant-dependent humans, and in animal models of these disorders; these conditions have each been associated with abnormal catecholaminergic function within the prefrontal cortex. In the current studies, we sought to explore how elevations in extracellular catecholamine levels, produced by pharmacological inhibition of catecholamine reuptake proteins, affect behavioral flexibility in rats and monkeys. Adult male Long–Evans rats and vervet monkeys were trained, respectively, on a four-position discrimination task or a three-choice visual discrimination task. Following systemic administration of pharmacological inhibitors of the dopamine and/or norepinephrine membrane transporters, rats and monkeys were exposed to retention or reversal of acquired discriminations. In accordance with our a priori hypothesis, we found that drugs that inhibit norepinephrine transporters, such as methylphenidate, atomoxetine, and desipramine, improved reversal performance in rats and monkeys; this was mainly due to a decrease in the number of perseverative errors. Interestingly, the mixed dopamine and norepinephrine transporters inhibitor methylphenidate, if anything, impaired performance during retention in both rats and monkeys, while administration of the selective dopamine transporter inhibitor GBR-12909 increased premature responses but did not alter reversal learning performance. Our results suggest that pharmacological inhibition of the membrane norepinephrine, but not membrane dopamine, transporter is associated with enhanced behavioral flexibility. These data, combined with earlier reports, may indicate that enhanced extracellular catecholamine levels in cortical regions, secondary to norepinephrine reuptake inhibition, improves multiple aspects of inhibitory control over responding in rats and monkeys.