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 GABAA receptor, enhancing the GABA-elicited chloride current (IGABA). In this work, we studied the influence of competitive antagonists of the GABAA 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 GABAA receptors in isolated Purkinje cells of the rat cerebellum. The IGABA was measured using the patch-clamp technique and a system of fast application. The effects of the drugs on IGABA 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 GABAA receptor antagonists (p < 0. 001).