Suppression by an h current of spontaneous Na⁺ action potentials in human cone and rod photoreceptors

PURPOSE. The sense of vision in humans is robust, and visual flickering is rarely experienced. To investigate this mechanism, electrophysiological and molecular biological techniques were used on human cone and rod photoreceptors. METHODS. Voltage-gated currents were recorded using the patch-clamp technique on isolated human cones, and especially their voltage-gated Na⁺ currents were analyzed in detail. Whether Na⁺ channel transcripts could be detected in single photoreceptors using RT-PCR was also examined, to test the expression of voltage-gated Na⁺ channels in cones and/or rods. RESULTS. Under current-clamp conditions, blocking h currents (hyperpolarization-activated cationic currents) with Cs⁺, Tl ⁺, or ZD7288 hyperpolarized the resting potentials of cones and rods by ∼10 to 15 mV, and surprisingly generated spontaneous action potentials. The spontaneous spikes were blocked by 1 μM tetrodotoxin, but not by 1 mM Co²⁺, suggesting that they were Na⁺ spikes rather than Ca²⁺ spikes. Under voltage-clamp conditions, application of Cs ⁺ and ZD7288 markedly decreased the steady inward current through the h channel. This is consistent with Cs⁺-induced hyperpolarization under a current-clamp condition. SCN2 Na⁺ channel was observed in both cones and rods by single-cell RT-PCR analysis, suggesting that human photoreceptors express the SCN2 Na⁺ channel. CONCLUSIONS. The data confirmed that voltage-gated Na⁺ channels were expressed not only in human rods but also in cones by electrophysiological and molecular biological experiments. These results suggest that the h current may contribute to preventing visual flickering by inhibiting the generation of spontaneous Na ⁺ spikes in human photoreceptors.