Contribution of vesicular and cytosolic dopamine to the increased striatal dopamine efflux elicited by intrastriatal injection of SKF38393

Abstract

Like dexamphetamine, SKF38393 induces an increase in striatal dopamine efflux which is insensitive for tetrodotoxin, Ca 2+ independent and prevented by a dopamine transporter inhibitor. The dexamphetamine-induced striatal dopamine efflux originates from both the reserpine-sensitive vesicular dopamine pool and the alpha-methyl-para-tyrosine-sensitive cytosolic dopamine pool. Given the similarities between dexamphetamine and SKF38393, we hypothesized that both types of pool also contribute to the striatally applied SKF38393-induced dopamine efflux. Using in vivo microdialysis technique, we analysed the contribution of these pools to the SKF38393-induced striatal dopamine efflux in freely moving rats. The increase of dopamine efflux induced by 1.5 μg SKF38393 was largely prevented by either reserpine (5 mg/kg i.p., given 24 h earlier) or alpha-methyl-para-tyrosine (250 mg/kg i.p., given 2 h earlier), showing that both the vesicular dopamine pool and the cytosolic dopamine pool contribute to the SKF38393-induced increase in striatal dopamine efflux. The sum of the amounts of dopamine that was sensitive to either reserpine or alpha-methyl-para-tyrosine, was greater than 100%, namely 137.6% of the basal dopamine level and 143.9% of the SKF38393-induced dopamine level, suggesting that striatally applied SKF38393 promotes the redistribution of dopamine from vesicles to the cytosol, and vice versa. The finding that the combined treatment of reserpine and alpha-methyl-para-tyrosine only inhibited the SKF38393-induced striatal dopamine efflux till 86.0% of the control, is ascribed to the notion that SKF38393 can also inhibit the re-uptake of dopamine. The latter conclusion has far-reaching consequences for studies in which the effects of SKF38393 are simply ascribed to its dopamine D 1 receptor stimulation capacity.