In vitro assessment of the effect of methylene blue on voltage-gated sodium channels and action potentials in rat hippocampal CA1 pyramidal neurons

Methylene blue (MB) is a vital dye to allow better visualization and marker of parathyroid glands. The compound causes a toxic encephalopathy in clinical observations and some neuronal adverse effects in experimental studies. Of neurotoxic effects, reduced field excitatory postsynaptic potentials (fEPSPs) in hippocampal slice cultures and apoptosis induced in neurons by MB, suggest that MB may affect electrophysiological properties in neurons. Consequently, studies were undertaken to characterize the effects of MB on voltage-gated sodium currents (INa) in hippocampal CA1 neurons. MB was tested at a clinically-relevant concentration (10 μM), of which as a surgical marker of the parathyroid glands, and other concentrations (0.25 μM, 1 μM, and 100 μM). The results showed that MB reduced the amplitude of INa at the concentrations of 10 μM and 100 μM. No significant changes were found with the other two concentrations of MB. 10 μM of MB did not produce a shift in the activation–voltage curve of INa but produced a hyperpolarizing shift in the inactivation–voltage curve of INa and delayed the recovery of INa from inactivation. Action potential (AP) properties and the pattern of repetitive firing were examined using whole-cell current-clamp recordings. Peak amplitude, overshoot and maximum velocity of depolarization (Vmax) of the evoked single AP decreased in the presence of the 10 μM MB solution. The rate of repetitive firing also decreased. The results suggest MB as a surgical marker of the parathyroid glands, may cause sodium channel inhibition on neurons in the nervous system.