Factors promoting the release of picrotoxin from the trap in the GABA(A) receptor pore

Picrotoxin (PTX), a convulsant of plant origin, has been used in many studies as research tool. PTX is the open channel blocker of the GABA_A receptor (GABA_AR). Being in the pore, PTX initiates transfer of the channel to the closed state and thus it falls into the "trap". The consequence of this PTX trapping is so-called aftereffect, i.e. continuation of the blockade of the GABA-induced chloride current (I_GABA) after removal of PTX from the external solution. The present work shows that the positive allosteric modulators (PAMs) of the GABA_A receptor, allopregnanolone (Allo) and zolpidem (Zolp) as well as a high concentration of GABA shortened the PTX aftereffect. Experiments were carried out on isolated Purkinje neurons of the rat cerebellum using the whole-cell patch-clamp method. I_GABA was induced by applications of 5 μM GABA (EC₃₀) for 1 s with 30 s intervals. 50 μM PTX completely blocked I_GABA, and recovery upon PTX washout occurred with a time constant (τ_rec) of 20.2 min. 1 μM Allo reduced the blocking effect of PTX by 30% and accelerated the recovery of I_GABA by almost 10 times (τ_rec = 2.4 min). 0.5 μM Zolp did not change the I_GABA block in the presence of PTX but accelerated the recovery of I_GABA by more than 3 times (τ_rec = 5.6 min). Increasing the GABA concentration to 20 μM did not change the blocking effect of PTX, but accelerated recovery by 6 times (τ_rec = 3.3 min). The mechanism of the shortening of the PTX aftereffect is presumably the expansion of the GABA_AR pore in the presence of PAMs and a high concentration of the agonist and, as a consequence, the escape of PTX from the "trap". The work describes new pharmacological properties of Allo and Zolp.