The potentiating activity of benzodiazepine site of the GABA(A) receptor is inhibited by competitive antagonists of orthosteric site

Benzodiazepines (BDZs) are widely-prescribed drugs that act as positive allosteric modulators of GABA_A receptor, enhancing the GABA-elicited chloride current (I_GABA). In this work, we studied the influence of competitive antagonists of the GABA_A receptor gabazine (GBZ), bicuculline (Bic), and amiloride (Ami) on the potentiating effect of the agonist of BDZ site zolpidem (Zolp). These antagonists bind to their own sites, which partially overlap with the orthosteric site. The experiments were carried out on native GABA_A receptors in isolated Purkinje cells of the rat cerebellum. The I_GABA was measured using the patch-clamp technique and a system of fast application. The effects of the drugs on I_GABA were assessed by the change in the EC₅₀ value for GABA dose-effect curve constructed in the ranges of 0.5–100 μM GABA. Changes in EC₅₀ values as a percentage relative to the control were calculated. 0.5 μM Zolp shifted the GABA curve to the left and decreased the EC₅₀ by 54 % (from 4.8 μM to 2.2 μM). Competitive antagonists shifted the GABA curve to the right and increased the EC₅₀ to 72.6 μM (0.5 μM GBZ), 25.5 μM (500 μM Ami) and 28.8 μM (5 μM Bic). With the addition of Zolp, these EC₅₀ values decreased by 21–25 % and were 56.8 μM (GBZ), 19.2 μM (Ami), and 22.7 μM (Bic), respectively. The results show that the potentiating effect of Zolp is reduced by half in the presence of competitive GABA_A receptor antagonists (p < 0. 001).