Effects of extracellular δ-aminolaevulinic acid on sodium currents in acutely isolated rat hippocampal CA1 neurons

The effects of delta-aminolaevulinic acid (ALA) on voltage-gated sodium channel (VGSC) currents (I-Na) in acutely isolated hippocampal CA1 neurons from 10- to 12-day-old Wistar rats were examined by using the whole-cell patch-clamp technique under voltage-clamp conditions. ALA from 0.01 mu m to 20 mu m was applied to the recorded neurons. Low concentrations of ALA (0.01-1.0 mu m) increased I-Na amplitude, whereas high concentrations of ALA (5.0-20.0 mu m) decreased it. The average I-Na amplitude reached a maximum of 117.4 +/- 3.9% (n = 9, P < 0.05) with 0.1 mu m ALA, and decreased to 78.1 +/- 3.8% (n = 13, P < 0.05) with 10 mu m ALA. ALA shifted the steady-state activation and inactivation curves of I-Na in the hyperpolarizing direction with different V-0.5, suggesting that ALA could depress the opening threshold of the voltage-gated sodium channel (VGSC) and thus increase the excitability of neurons through facilitating the opening of VGSC. The time course of recovery from inactivation was significantly prolonged at both low and high concentrations of ALA, whereas either low or high concentrations of ALA had no significant effect on the attenuation of I-Na during stimulation at 5 Hz, indicating that the effect of ALA on VGSC is state-independent. Furthermore, we found that application of ascorbic acid, which blocks pro-oxidative effects in neurons, could prevent the increase of I-Na amplitude at low concentrations of ALA. Baclofen, an agonist of GABAb receptors, induced some similar effects to ALA on VGSC, whereas bicuculline, an antagonist of GABAa receptors, could not prevent ALA-induced effects on VGSC. These results suggested that ALA regulated VGSC mainly through its pro-oxidative effects and GABAb receptor-mediated effects.