The expression of methiopropamine-induced locomotor sensitization requires dopamine D2, but not D1, receptor activation in the rat

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Methiopropamine (MPA) is a structural analog to methamphetamine and is categorized as a novel psychoactive substance that needs to be controlled. However, no study has been performed to determine whether MPA actually develops an addiction-like behavior similar to those arising from other psychomotor stimulants. Thus, we attempted to determine whether MPA produces locomotor sensitization in a manner similar to amphetamine. In the first experiment, rats were pre-exposed to either saline or one of three different doses of MPA (0.2, 1.0, or 5.0 mg/kg, IP) with a total of four injections, respectively. After a 2-week withdrawal period, when they were challenged with the same dose of MPA, only the group that was pre-exposed to high dose of MPA (5.0 mg/kg) showed sensitized locomotor activity. In the second experiment, all rats were pre-exposed to MPA (5.0 mg/kg) only. Interestingly, the expression of MPA-induced locomotor sensitization was inhibited by a pre-injection of a dopamine D2 receptor antagonist, eticlopride (0.05 mg/kg, IP), though not by a dopamine D1 receptor antagonist, SCH23390 (0.01 mg/kg, IP). These results suggest that repeated injection of MPA in the rat provokes certain neuronal changes involving specific, likely D2, dopamine receptor-mediated pathways that contribute to the expression of MPA-induced locomotor sensitization.

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