Allopregnanolone reduces the blocking properties of competitive antagonists of GABA(A) receptor

The orthostheric site of the GABA_A receptor binds both the agonist (GABA) and the competitive antagonists bicuculline (Bic) and gabazine (GBZ). The endogenous neurosteroid allopregnanolone (Allo) is known to alter the conformation of the orthosteric site of the GABA_A receptor, increasing its affinity for GABA. The aim of this study was to investigate the effect of Allo on the blocking properties of Bic and GBZ. Experiments were conducted on isolated rat cerebellar Purkinje cells, in which the GABA-induced chloride current (I_GABA) was recorded using the patch-clamp method. The effect of Allo on the blocking properties of Bic and GBZ was studied using two approaches. First, we used functionally equivalent GABA concentrations to induce I_GABA in the absence or presence of 1 μM Allo and measured the magnitude of the I_GABA block induced by 0.5 μM GBZ or 5 μM Bic. It was shown that, in the presence of Allo, blocking effect was reduced for GBZ from 63 to 31 % (p < 0.05), and for Bic from 87 to 56 % (p < 0.001). The second approach was to study the behavior of dose-effect curve for antagonist (GBZ or Bic) in the presence of Allo. Allo caused the right shift of both dose-effect curves with the increase in the IC₅₀ values from 0.22 to 0.60 μM for GBZ (p < 0.001), and from 1.1 to 2.4 μM for Bic (p < 0.05). Results suggest that Allo decreases the blocking properties of competitive antagonists Bic and GBZ of the GABA_A receptor.