Development of a synaptosomal model to determine drug-induced in vivo changes in GABA-levels of nerve endings in 11 brain regions of the rat

An experimental procedure was developed which allowed the simultaneous measurement of GABA in synaptosomes from 11 regions of one rat brain. Synaptosomal fractions were prepared by conventional subcellular fractionation procedures and characterized by electron microscopy. Post-mortem increases of GABA during removal and dissection of brain tissue, homogenization and fractionation procedures could be sufficiently minimized by rapid processing of the tissue at low temperatures and inclusion of 3-mercaptopropionic acid (1 mM) in the homogenizing medium. Experiments with addition of aminooxyacetic acid (AOOA, 1 mM) to the homogenizing medium indicated that GABA was not being degraded during synaptosome preparation. The presence of exogenous GABA (1 mM) did not alter the GABA levels in the organelles, indicating that no significant redistribution of GABA occurred during subcellular fractionation. On the basis of these findings, it was suggested that synaptosomal fractions could be used as a model to monitor indirectly the drug-induced changes in GABA levels of nerve endings in discrete brain areas of the intact animal. In vivo experiments with AOAA (30 mg/kg i.p.) and valproic acid (VPA, 200 mg/kg i.p.) showed that both drugs caused differential effects on synaptosmal GABA levels in different brain regions. Although AOAA was more potent than VPA in increasing GABA in whole tissue of most brain regions, significant increases of synaptosomal GABA levels after AOAA were only determined in olfactory bulbs and frontal cerebral cortex. In contrast, VPA induced significant synaptosomal GABA increases in olfactory bulbs, hypothalamus, superior and inferior colliculus, substantia nigra, and cerebellum. The data indicate that the synaptosomal model can provide useful information on the in vivo effects of drugs on GABA levels in nerve terminals and their ability to exert this effect in specific brain areas.