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REPORT

I_h Without K_ir in Adult Rat Retinal Ganglion Cells

¹Section of Neurobiology, Physiology, and Behavior and ²Department of Ophthalmology and Vision Science, University of California, Davis, California

Submitted 27 November 2006; accepted in final form 14 February 2007

Antisera directed against hyperpolarization-activated mixed-cation ("I_h") and K⁺ ("K_ir") channels bind to some somata in the ganglion cell layer of rat and rabbit retina. Additionally, the termination of hyperpolarizing current injections can trigger spikes in some cat retinal ganglion cells, suggesting a rebound depolarization arising from activation of I_h. However, patch-clamp studies showed that rat ganglion cells lack inward rectification or present an inwardly rectifying K⁺ current. We therefore tested whether hyperpolarization activates I_h in dissociated, adult rat retinal ganglion cell somata. We report here that, although we found no inward rectification in some cells, and a K_ir-like current in a few cells, hyperpolarization activated I_h in roughly 75% of the cells we recorded from in voltage clamp. We show that this current is blocked by Cs⁺ or ZD7288 and only slightly reduced by Ba²⁺, that the current amplitude and reversal potential are sensitive to extracellular Na⁺ and K⁺, and that we found no evidence of K_ir in cells presenting I_h. In current clamp, injecting hyperpolarizing current induced a slowly relaxing membrane hyperpolarization that rebounded to a few action potentials when the hyperpolarizing current was stopped; both the membrane potential relaxation and rebound spikes were blocked by ZD7288. These results provide the first measurement of I_h in mammalian retinal ganglion cells and indicate that the ion channels of rat retinal ganglion cells may vary in ways not expected from previous voltage and current recordings.