REGIONAL DIFFERENCES IN THE SYNAPTOSOMAL UPTAKE OF ³H--AMINOBUTYRIC ACID AND ¹⁴C-GLUTAMATE AND POSSIBLE ROLE OF EXCHANGE PROCESSES

¹ Laboratorio di Biologia Cellulare, Via Romagnosi 18A (G.L. and J.C.) and Istituto di Farmacologia, Università Cattolica (A.B. and M.R.), Rome, Italy

1) The uptake of ³H--aminobutyric acid (³H-GABA) and ¹⁴C-glutamate was studied using different subcellular preparations enriched in synaptosomes, from seven rat brain areas. 2) In all the areas the following order of decreasing uptake was observed: synaptosomes purified from incubated crude synaptosomal fractions > crude synaptosomal fractions > purified synaptosomes > mitochondnia purified from incubated crude fractions > purified mitochondnia. 3) Uptake was much higher in synaptosomes than in mitochondnia, and the ratio between GABA and glutamate uptakes was different in synaptosomes and mitochondria. 4) GABA was taken up more by synaptosomes from diencephalic areas than from other brain areas, whereas the uptake of glutamate was highest in telencephalic regions. The regional heterogeneity of uptake may reflect heterogeneity of the regional distribution of GABA-storing and glutamate-storing nerve terminals, respectively. 5) A good parallelism was present between regional distribution of endogenous synaptosomal GABA or glutamate and regional patterns of uptake, and also between endogenous GABA/glutamate concentration ratios, and GABA/glutamate uptake ratios. 6) The possible role of homoexchange in the determination of the apparent uptake levels was investigated : a) by studying the uptake of ³H-GABA and ¹⁴C-glutamate in synaptosomes preloaded with unlabeled GABA or glutamate, or prepared from amino-oxyacetic-treated animals ; and b) by comparing the uptake of radioactivity to the net uptake of GABA and glutamate measured chromatographically. These experiments agreed in showing that homoexchange plays a significant role in the uptake of ¹⁴C-glutamate, but not of ³H-GABA. This finding raised some doubt regarding the hypothesis that re-uptake may terminate the synaptic action of glutamate in vivo.