[3H]Dopamine release by d-amphetamine from striatal synaptosomes of reserpinized rats

The injection of reserpine, 5 mg/kg i.p. (ipRes), the regimen employed by a majority of investigators, results in synaptosomal and vesicular preparations which are incompletely reserpinized as determined by [3H]dopamine ([3H]DA) accumulation. Reserpine administered by the subcutaneous route, 5 mg/kg (scRes), appears to produce complete reserpinization. Release of [3H]DA by d-amphetamine (Amph) was observed from striatal synaptosomes prepared both from normal rats and those pretreated with reserpine intraperitoneally but not from those injected subcutaneously. In the more completely reserpinized scRes synaptosomes, so little [3H]DA had accumulated that release by Amph was not measurable, indicating that if a labile, reserpine-resistant, extravesicular DA storage pool releasable by Amph is present under these conditions, it must be extremely small. In scRes monoamine oxidase (MAO)-inhibited preparations, Amph released preloaded [3H]DA located in the cytosol in the absence of functional vesicles. Although chromatographic analysis of the superfusate from ipRes striatal synaptosomes showed that significant amounts of preloaded [3H]DA were released by Amph, the level of dihydroxyphenylacetic acid was not increased over controls, indicating that Amph releases only DA and not its metabolite and is also acting as a MAO inhibitor. No [3H]DA could be released by Amph from superfused hyposmotically shocked normal or ipRes synaptosomes, suggesting that an intact membrane is required for Amph-induced release.