Ascorbic acid and striatal transport of [3H] 1-methyl-4-phenylpyridine (MPP+) and [3H] dopamine

The inhibition of uptake of [3H] dopamine and [3H] 1-methyl-4-phenylpyridine (MPP+) was examined in mouse striatal synaptosomal preparations. Kinetic analysis indicated that ascorbic acid is a noncompetitive inhibitor of [3H] MPP+ uptake. No inhibition of [3H] dopamine uptake is observed. The dopamine uptake blockers, GBR-12909, cocaine, and mazindol strongly inhibit (IC50 less than 1 uM) both [3H] dopamine and [3H] MPP+ transport. Nicotine, its metabolites, and other tobacco alkaloids are weak inhibitors (IC50 greater than 1 mM) except 4-phenylpyridine and lobeline, which are moderate inhibitors (IC50 = 3 to 40 uM) of both [3H] dopamine and [3H] MPP+ uptake. These similarities in potencies are in agreement with the suggestion that [3H] MPP+ and [3H] dopamine are transported by the same carrier. The differences observed in the alteration of dopaminergic transport and mazindol binding by ascorbic acid suggest that ascorbic acid's effects on [3H] MPP+ transport are related to translocation and/or dissociation processes occurring subsequent to the initial binding event.