Lucifer Yellow Slows Voltage‐Gated Na+ Current Inactivation in a Light‐Dependent Manner in Mice

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Abstract
Lucifer Yellow CH (LY), a membrane‐impermeant fluorescent dye, has been used in electrophysiological studies to visualize cell morphology, with little concern about its pharmacological effects. We investigated its effects on TTX‐sensitive voltage‐gated Na+ channels in mouse taste bud cells and hippocampal neurons under voltage‐clamp conditions. LY applied inside cells irreversibly slowed the inactivation of Na+ currents upon exposure to light of usual intensities. The inactivation time constant of Na+ currents elicited by a depolarization to −15 mV was increased by fourfold after a 5 min exposure to halogen light of 3200 lx at source (3200 lx light), and sevenfold after a 1‐min exposure to 12 000 lx light. A fraction of the Na+ current became non‐inactivating following the exposure. The non‐inactivating current was ≈ 20 % of the peak total Na+ current after a 5 min exposure to 3200 lx light, and ≈ 30 % after a 1 min exposure to 12 000 lx light. Light‐exposed LY shifted slightly the current‐voltage relationship of the peak Na+ current and of the steady‐state inactivation curve, in the depolarizing direction. A similar light‐dependent decrease in kinetics occurred in whole‐cell Na+ currents of cultured mouse hippocampal neurones. Single‐channel recordings showed that exposure to 6500 lx light for 3 min increased the mean open time of Na+ channels from 1.4 ms to 2.4 ms without changing the elementary conductance. The pre‐incubation of taste bud cells with 1 mM dithiothreitol, a scavenger of radical species, blocked these LY effects. These results suggest that light‐exposed LY yields radical species that modify Na+ channels.