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

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.