Comparison of DABA and GABA Transport into Plasma Membrane Vesicles Derived from Synaptosomes

Abstract: Transport of GABA by a high-affinity transport system ( K _m? 10^?5 M) is thought to terminate the action of this postulated neurotransmitter. 2,4-Diaminobutyric acid (DABA), a structural analogue, is taken up by neuronal elements and inhibits GABA uptake. Localization of ³H]DABA by auto-radiography has been used to identify neurons with the GABA high-affinity transport system. After reconstitution of lysed synaptosomal fractions in potassium salts, transfer of these membrane vesicles to sodium salts produces sodium and potassium ion gradients which drive ³H]GABA and ³H]DABA transport. For each, transport requires external sodium, is abolished by ionophores that dissipate the Na⁺ gradient, and is enhanced by conditions which make the intravesicular electromotive force more negative. Some characteristics of the transport of these substances, however, differ. For example, external chloride is required for GABA, but not DABA, transport. Internal potassium is required for DABA, but not GABA, transport. DABA is a competitive inhibitor ( K _i? 0.6 MM) of GABA transport into membrane vesicle and synaptosomes. GABA, however, is a feeble inhibitor of DABA uptake into the membrane vesicles. These differences suggest that the two substances are transported by different mechanisms and possibly by different carriers. In addition to these experiments, using enzymatic-fluorometric techniques, it was shown that the artificially imposed ion gradients drive net chemical transport of GABA into the vesicles.