Characterization of GABAA receptor-mediated 36chloride uptake in rat brain synaptoneurosomes

gamma-Aminobutyric acid (GABA) receptor-mediated 36chloride (36Cl-) uptake was measured in synaptoneurosomes from rat brain. GABA and GABA agonists stimulated 36Cl- uptake in a concentration-dependent manner with the following order of potency: Muscimol greater than GABA greater than piperidine-4-sulfonic acid (P4S)greater than 4,5,6,7-tetrahydroisoxazolo-[5,4-c]pyridin-3-ol (THIP) = 3-aminopropanesulfonic acid (3APS) much greater than taurine. Both P4S and 3APS behaved as partial agonists, while the GABAB agonist, baclofen, was ineffective. The response to muscimol was inhibited by bicuculline and picrotoxin in a mixed competitive/non-competitive manner. Other inhibitors of GABA receptor-opened channels or non-neuronal anion channels such as penicillin, picrate, furosemide and disulfonic acid stilbenes also inhibited the response to muscimol. A regional variation in muscimol-stimulated 36Cl- uptake was observed; the largest responses were observed in the cerebral cortex, cerebellum and hippocampus, moderate responses were obtained in the striatum and hypothalamus and the smallest response was observed in the pons-medulla. GABA receptor-mediated 36Cl- uptake was also dependent on the anion present in the media. The muscimol response varied in media containing the following anions: Br- greater than Cl- greater than or equal to NO3- greater than I- greater than or equal to SCN- much greater than C3H5OO- greater than or equal to ClO4- greater than F-, consistent with the relative anion permeability through GABA receptor-gated anion channels and the enhancement of convulsant binding to the GABA receptor-gated Cl- channel.