Potentiation of GABA_A receptor agonists by GABA uptake inhibitors in the rat ventral midbrain

Whole-cell patch recordings were made from dopamine-containing neurons in the ventral tegmental area (VTA) and substantia nigra zona compacta (SNC). Isoguvacine evoked an outward current (at -60 mV) in a concentration-dependent manner with an EC₅₀ of 62 ± 8 μM. The γ-aminobutyric acid (GABA) uptake inhibitor 1-(2(((diphenylmethylene)imino)oxy)ethyl)-1,2,5,6-tetrahydro-3-pyridine- carboxylic acid hydrochloride (NO 711) (3 μM) shifted the isoguvacine concentration-response curve to the left, with a new EC₅₀ of 22 ± 4 μM. L-Arginine (3 mM) also shifted the isoguvacine concentration-response curve to the left, with a new EC₅₀ of 29 ± 5 μM. L-Arginine (3 mM) increased the currents evoked by GABA (100 μM) and muscimol (1 μM) by 208% and 261%, respectively. The GABA uptake inhibitor 4,5,6,7,-tetrahydroisoxazolo[4,5-c]-pyridin-3-ol hydrobromide (THPO) (300 μM) not only mimicked but also occluded the ability of L-arginine (3 mM) to potentiate currents evoked by isoguvacine. Equimolar replacement of Na⁺ with choline increased GABA-evoked currents, suggesting that a low Na⁺ concentration has an inhibitory effect on GABA transport. Low Na⁺ concentration (25 mM) inhibited isoguvacine currents but still occluded the potentiating effects of L-arginine. We conclude that GABA uptake inhibitors potentiate the actions of the GABA_A receptor agonists, isoguvacine and muscimol, probably because they are effective substrates for GABA transporters in the ventral midbrain.