Dopamine modulates the voltage response of human rod photoreceptors by inhibiting the h current

Purpose. The h current (I _h) is a hyperpolarization-activated current that plays important roles in the physiological functions of different types of cells. In the retina of lower vertebrates, I _h contributes to the rod responses to light stimuli by bringing the membrane potential back to the dark level in the presence of continuous light. The purpose of this study was to determine how dopamine modulates I _h in human rods and regulates voltage responses. Methods. A patch-clamp recording technique was used on surgically excised human retinas to investigate the effects of dopamine on the I _h of isolated rods. Dopamine was applied in the superfusate. Results. Dopamine reversibly decreased the amplitude of the I _h induced by hyperpolarizing voltage steps from a holding potential of -60 mV. At a voltage step of -100 mV, 20 μM dopamine decreased the amplitude of I _h. The D2 dopamine agonist quinpirole inhibited I _h, but the D1 agonist SKF-38393 had no effect. Dopamine-induced reduction of I _h amplitude was blocked by the D2 dopamine antagonist sulpiride. Under current-clamp conditions, an injection of hyperpolarizing current steps to rods produced voltage responses that exhibited a gradual decay. Adding dopamine to the superfusate inhibited the decay in the voltage responses. Quinpirole also inhibited the voltage decay, whereas SKF-38393 was ineffective. Conclusions. Dopamine reduced I _h through a D2 receptor and inhibited the gradual decay in the voltage response through a D2 receptor, indicating that dopamine slows the recovery phase of responses to light stimuli by inhibiting I _h in human rods.