Evidence that the rapid binding of newly accumulated noradrenaline within synaptosomes involves synaptic vesicles

When rat brain synaptosomes were incubated with [³H]noradrenaline for 1 min and then exposed to osmotic shock, only about 20% of the newly accumulated [³H]noradrenaline was released. It would appear that most, but possibly not all of the newly accumulated [³H]noradrenaline is rapidly bound to some particulate cytoplasmic constituent within the synaptosome. [³H]Dopamine and [³H]5-hydroxytryptamine were also rapidly bound within synaptosomes but [³H]glycine and [³H]γ-aminobutyric acid were not. Reserpinization (5 mg/kg, i.p., 24 h before preparation) only slightly reduced the initial rate of [³H]noradrenaline uptake by synaptosomes. However, when reserpinized synaptosomes were osmotically shocked, most of the newly accumulated radioactivity was released; this radioactivity was identified chromatographically as [³H]noradrenaline. On the basis of the findings with reserpinized preparations, it seems likely that (1) the rapid intrasynaptosomal binding involves synaptic vesicles and (2) the neuronal membrane transport system itself may be capable of driving the uptake of noradrenaline by nerve-terminals. The rapid vesicular binding observed may not be essential for the accumulation of the amine by presynaptic terminals during brief exposures.