To identify which type of dopamine receptor is involved in the dopamine-mediated reduction in
I h in human rods, we applied various agonists and antagonists of the dopamine receptors. In control solution, hyperpolarization to −100 mV from a V
h of −40 mV induced
I h (
Fig. 2A, thin line). Peak amplitude was −320 pA. Quinpirole (20 μM), a D2 dopamine agonist, reduced
I h by 34% (
Fig. 2A, thick line). A similar reduction (32% ± 5%) was obtained in six rods. Quinpirole decreased the peak amplitude of
I h at all voltage steps between −100 mV and −50 mV (
Fig. 2B, filled circles).
We also examined the effects of the D1 agonist SKF-38393 on
I h. SKF-38393 (100 μM) changed neither the
I h induced by hyperpolarization to −100 mV (
Fig. 2C) nor its I-V relationship (
Fig. 2D). To confirm whether dopamine reduces
I h through D2 receptors, we tested the effects of dopamine on
I h by adding 20 μM sulpiride, a D2 dopamine antagonist, to the bath. The
I h induced by hyperpolarizing voltage steps from a V
h of −60 mV in the solution containing 20 μM sulpiride is shown in
Figure 2E. Adding both 20 μM sulpiride and 20 μM dopamine did not affect
I h significantly (
Fig. 2F), suggesting that sulpiride blocked the reduction of
I h by dopamine. These results suggest that the action of dopamine on
I h is mediated by D2 receptors and not by D1 receptors.