A CGMP-GATED CURRENT CAN CONTROL EXOCYTOSIS AT CONE SYNAPSES

The voltage-gated Ca2+ current in cone photoreceptors operates over on ly a small part of the physiological voltage range produced by light a nd, consequently, appears insufficient for controlling transmitter rel ease. We have used a whole-cell voltage clamp to measure membrane curr ent and the capacitance change produced by exocytosis in solitary cone and rod photoreceptors isolated from the salamander retina. In both t ypes of photoreceptor, Ca2+ influx through voltage-gated Ca2+ channels initiated exocytosis. In addition, Ca2+ influx through a cGMP-gated c hannel in the inner segment and synaptic processes of cones also initi ated exocytosis. The cGMP-gated current sustained exocytosis over the entire physiological voltage range.