Separation of synaptosomes storing catecholamines and gamma-aminobutyric acid in rat corpus striatum

We have examined the separation of synaptosomes prepared from rat corpus striatum labeled with radioactive catecholamine and gamma-aminobutyric acid (GABA) in linear continuous sucrose density gradients. Catecholamine containing synaptosomes localized in a denser region of gradients than did those containing GABA. The extent of separation was dependent on the amount of tissue placed on the gradient but was independent of incubation time or of isotopic label. The best separation was obtained by centrifuging gradients for a short duration. The profile of radioactivity in the gradients represented bound GABA and bound catecholamine, except for the least dense area of the gradient which contained soluble isotope. Electron microscopic evaluation of gradient fractions indicated that free mitochondria predominated in the area of peak monoamine oxidase activity and that the largest number of synaptosomal profiles occurred in the fraction containing the maximum amount of potassium and labeled catecholamine and GABA. Variations in synaptosomal morphology in different gradient fractions were noted.