Modulation of GABA_A receptors by benzodiazepines and barbiturates is autonomous of PKC activation

Previous studies have suggested that activation of calcium-phospholipid-dependent protein kinase (PKC) enhances benzodiazepine (BZD)- and pentobarbital (PB)- mediated potentiation of α₁β₁γ₂ GABA_A receptors (GABA_A-Rs). To delineate the underlying mechanism(s), voltage-clamp recordings were performed on recombinant α₁β₁γ₂ GABA_A receptors functionally expressed in Xenopus laevis oocytes. GABA_A-Rs were tested for their sensitivity to diazepam and PB before and after incubation in phorbol 12-myristate 13-acetate (PMA). PMA (25 nM) significantly attenuated the GABA_A current (p<0.05, n=12-19) up to 90%. PMA treatment, however, did not alter the sensitivity to diazepam or pentobarbital. Similar results were obatined with recombinant α₁β₂γ₂ GABA receptors. These data suggest that PKC activation does not alter the allosteric modulation of GABA_A-Rs by benzodiazepines and barbiturates and is consistent with the observation from other studies in oocytes that PMA decreases the amplitude of the GABA-activated currents via receptor internalization rather than modification of receptor kinetics.