Interaction Between Allopregnanolone and Amiloride Binding Sites on the GABA Receptor.

Allopregnanolone (Allo) is a positive allosteric modulator of the GABA receptor, and amiloride (Ami) is a competitive antagonist of the GABA receptor. The purpose of this work was to study the combined effect of Allo and Ami on functional activity of GABA receptor. The GABA-induced chloride current (I) was measured in isolated Purkinje cells of rat cerebellum using the patch-clamp technique and a system of fast application. Our results indicate that Allo suppresses the inhibitory effect of Ami on I, the IC value of Ami concentration-response curve was increased from 164 to 547 µM (P < 0.001) in the presence of Allo. Next, GABA concentration-response curves (EC = 5.8 µM) were constructed in the presence of Allo (EC = 1.2 µM), Ami (EC = 25.5 µM), and the combination of Allo+Ami (EC = 3.2 µM). Changes in EC values as a percentage relative to the control were calculated. The blocking effect of Ami is reduced in the presence of Allo (340% vs 150%, P < 0.01) and the potentiating effect of Allo does not change in the presence of Ami (78% vs 87%, P > 0.05). The results suggest that there is an allosteric relationship between the Allo and Ami binding sites on GABA receptor that operates in one direction, from Allo sites to Ami site, but not vice versa.