Figure 1. Melatonin induces a cGMP-dependent cation current from carp retinal Rod-ON-BCs.

A, a Lucifer yellow-filled Rod-ON-BC that is characterized by an enlarged characteristic terminal bulb (arrowhead). Scale bar, 10 μm. B, melatonin (MEL) of 100 nm induced a sustained inward current from the Rod-ON-BC shown in A, voltage-clamped at −60 mV, with a long delay. The current was almost completely blocked by lunzindole (LUZ) of 100 nm. C, melatonin-induced inward current was blocked by addition of 100 nm 4-P-PDOT. D, current was suppressed by addition of 3 mm Cd²⁺ in a reversible manner. E, puff of 8-Br-cGMP (1 mm) on the dendrites of a Rod-ON-BC for 50 s induced a sustained inward current (inset; scale bars, 4 pA pF⁻¹, 20 s). I–V relationships of 8-Br-cGMP- (grey) and melatonin- (black) induced currents, from two Rod-ON-BCs, obtained using a voltage ramp from −80 mV to 40 mV for 500 ms. Both curves show slight outward rectification with an E_rev of 2.0 mV and 1.5 mV, respectively. F, internal infusion of 2 mm cGMP induced a sustained inward current, and addition of 100 nm melatonin to the bath solution failed to induce any discernable current. G, puff of 3 mm glutamate (Glu) to the dendrites of a Rod-ON-BC induced an outward current. In the presence of 100 nm melatonin, the glutamate current much increased in size.