Hyperpolarization-Activated Current (Ih) in Ganglion-Cell Photoreceptors
Figure 1
Light responses in isolated ipRGCs and evidence for expression of the hyperpolarization-activated inwardly-rectifying current (Ih).
A, B) Light-evoked responses recorded from dissociated ipRGCs, retrolabeled by injection of rhodamine labeled latex microspheres into the suprachiasmatic nucleus. A) Voltage clamp recording of a typical ipRGC. Cell was held at −73 mV and given a 4 s flash of white light (black bar), triggering a large inward current. B) Current clamp recording from a different ipRGC. A 1 s light flash (black bar) depolarized the cell, causing spiking that persisted several minutes after termination of the light stimulus. C–F) Evidence for the presence of Ih. C) Hyperpolarizing the membrane in 4 s steps evoked an instantaneous current response that, at membrane potentials negative to −83 mV, was followed by a slowly activating inward current. Bath-application of 3 mM Cs+ to block Ih abolished the slow component. D) Group data (N = 6) plotting the total current (squares; Itot), the Cs+-insensitive current (circles; ICs), and the difference between them (triangles; Itot-ICs), which isolates the Cs+-sensitive, presumptive Ih current. E, F) Bath-application of the alternative Ih blocker 100 µM ZD7288 likewise abolished the slow component (N = 4).