Evidence that amphetamine and Na⁺ gradient reversal increase striatal synaptosomal dopamine synthesis through carrier-mediated efflux of dopamine

Amphetamine (AMPH) releases dopamine (DA) from striatal synaptosomes and concomitantly increases DA synthesis. Since AMPH may release DA through carrier-mediated diffusion via reversal of the DA uptake system, the increase in DA synthesis might depend on a functioning uptake carrier. Consistent with such a mechanism, the uptake inhibitors nomifensine (NMF) and benztropine (BZT) completely prevented the AMPH-induced increase in DA synthesis at concentrations known to inhibit DA uptake. Changes in the Na⁺ gradient across the synaptosomal membrane also promote DA release, since DA and Na⁺ are cotransported by the neuronal uptake carrier. Incubation of synaptosomes in medium containing decreasing Na⁺ increased DA synthesis inversely proportional to Na⁺ over the range 128 to 20 mM. Similarly, incubations in the presence of 10^-4 M ouabain to inhibit Na⁺, K⁺-ATPase and allow intracellular accumulation of Na⁺ also increased DA synthesis. These changes in DA synthesis could also be prevented by BZT and were non-additive with the AMPH-induced increase in DA synthesis. However, a concentration of ouabain (10^-6 M) which by itself did not increase DA synthesis, and does not promote DA release, potentiated the AMPH-induced increase in DA synthesis. Further, the increased DA synthesis promoted by all three manipulations was only marginally dependent on the presence of Ca²⁺ in the incubation medium. However, at 5 and 10 mM Na⁺, a second component of increased DA synthesis was observed which was insensitive to BZT, but was prevented by Ca²⁺ removal. These results suggest that the increase in DA synthesis, and presumably DA release promoted by AMPH, lowered Na⁺, and ouabain, depend on the availability of the DA carrier at the internal face of the neuronal membrane and the intracellular content of Na⁺. The second component of increased DA synthesis which is evident at 5 and 10 mM Na⁺ is discussed in terms of a possible Ca²⁺-mediated change in DA synthesis which is independent of the DA carrier.