Effective Perturbations by Phenobarbital on I(Na), I(K(erg)), I(K(M)) and I(K(DR)) during Pulse Train Stimulation in Neuroblastoma Neuro-2a Cells

Phenobarbital (PHB, Luminal Sodium(®)) is a medication of the barbiturate and has long been recognized to be an anticonvulsant and a hypnotic because it can facilitate synaptic inhibition in the central nervous system through acting on the γ-aminobutyric acid (GABA) type A (GABA(A)) receptors. However, to what extent PHB could directly perturb the magnitude and gating of different plasmalemmal ionic currents is not thoroughly explored. In neuroblastoma Neuro-2a cells, we found that PHB effectively suppressed the magnitude of voltage-gated Na(+) current (I(Na)) in a concentration-dependent fashion, with an effective IC(50) value of 83 µM. The cumulative inhibition of I(Na), evoked by pulse train stimulation, was enhanced by PHB. However, tefluthrin, an activator of I(Na), could attenuate PHB-induced reduction in the decaying time constant of I(Na) inhibition evoked by pulse train stimuli. In addition, the erg (ether-à-go-go-related gene)-mediated K(+) current (I(K(erg))) was also blocked by PHB. The PHB-mediated inhibition on I(K(erg)) could not be overcome by flumazenil (GABA antagonist) or chlorotoxin (chloride channel blocker). The PHB reduced the recovery of I(K(erg)) by a two-step voltage protocol with a geometrics-based progression, but it increased the decaying rate of I(K(erg)), evoked by the envelope-of-tail method. About the M-type K(+) currents (I(K(M))), PHB caused a reduction of its amplitude, which could not be counteracted by flumazenil or chlorotoxin, and PHB could enhance its cumulative inhibition during pulse train stimulation. Moreover, the magnitude of delayed-rectifier K(+) current (I(K(DR))) was inhibited by PHB, while the cumulative inhibition of I(K(DR)) during 10 s of repetitive stimulation was enhanced. Multiple ionic currents during pulse train stimulation were subject to PHB, and neither GABA antagonist nor chloride channel blocker could counteract these PHB-induced reductions. It suggests that these actions might conceivably participate in different functional activities of excitable cells and be independent of GABA(A) receptors.