Properties and functional roles of hyperpolarization-gated currents in guinea-pig retinal rods

The inward rectification induced by membrane hyperpolarization was studied in adult guinea-pig rods by the perforated-patch-clamp technique. CsCl blocked the rectification observed in both voltage- and current-clamp recordings at voltages negative to −60 mV, while BaCl 2 blocked the inward relaxation observed at voltages positive to −60 mV. The current activated at −90 mV had a low selectivity between sodium and potassium and reversed at −31.0 mV. These observations suggest that two inward rectifiers are present in guinea-pig rods: a hyperpolarization-activated ( I h ) and a hyperpolarization-deactivated ( I kx ) current. The functional roles of I h and I kx were evaluated by stimulating rods with currents sinusoidally modulated in time. Rods behave like bandpass amplifiers, with a peak amplification of 1.5 at about 2 Hz. For hyperpolarizations that mainly gate I kx , amplification and phase shifts are fully accounted for by a rod membrane analogue model that includes an inductance. For hyperpolarizations that also gate I h , a harmonic distortion became apparent. Bandpass filtering and amplification of rod signals, associated with I h and I kx gating by membrane hyperpolarization, are strategically located to extend, beyond the limits imposed by the slow phototransductive cascade, the temporal resolution of signals spreading to the rod synapse.