The GABA transporter and the regulation of the GABAA receptor
ã-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian brain. The major receptor for GABA in the mammalian brain is the GABAA receptor, which is located primarily on postsynaptic membranes. This receptor is a member of a superfamily of ligand gated ion channels whose members also include the nicotinic acetylcholine receptor, and the glycine receptor. The major goal of my research has been to determine what role, if any, the postsynaptic accumulation of GABA plays in the regulation of the GABAA receptor. To investigate the regulation of the postsynaptic GABAA receptor, an appropriate model was needed. The synaptoneurosome preparation was chosen, as this subcellular brain preparation contains both pre- and postsynaptic membrane-derived vesicles associated with one another, and a functional GABAA receptor. The strategy chosen to manipulate GABA levels was to either inject the animals from which the synaptoneurosomes would be prepared with agents that would alter GABA levels in the brain in vivo, or add GABA directly to the synaptoneurosomes preparation. GABA levels within the synaptoneurosomes or in the bathing medium of the preparation were assayed by reverse phase high performance liquid chromatography, while the activity of the GABAA receptor was assayed by measuring muscimol-stimulated chloride flux in the synaptoneurosomes preparation. The results of this study suggest that while extracellular GABA levels appear to be responsible for the desensitization of the receptor, it is the intracellular level of GABA which controls the concentration of GABA present in the extracellular compartment, thus establishing a basal level of extracellular GABA. Furthermore, it appears that the bidirectional GABA transporter is responsible for mediating the equilibrium between the internal and external GABA pools. To determine if elevated GABA levels may bring about a decrease in GABAA receptor function via second messenger systems, the inositol phosphate and cAMP second messenger systems were examined. The data suggest that the cAMP system may be involved in regulating the GABAA receptor.