Voltage and calcium activated currents in mammalian cone photoreceptors

Purpose: Voltage and calcium activated channels in the photoreceptors of lower vertebrates are known to shape the light responses. The membrane properties of the inner segments of mammalian photoreceptors are largely unknown, although they are thought to be coupled to rods and to other cones through gap junctions. I wanted to identify and characterize the membrane currents activated by voltage, and by calcium influx and elucidate their role in shaping the light responses. I also wanted to monitor coupling between cones under dark and light adapted conditions. Methods: Intact cone photoreceptors from in vitro tree Shrew retinas were recorded from using the whole-cell patch clamp technique. Neurobiotin was included in the recording electrode to monitor tracer coupling between the cells. Results: Neurobiotin tracer coupling was not observed in either dark or light adapted preparations. This result was supported by analysis of capacitive transients recorded under voltage clamp, showing that single photoreceptors were largely isopotential. An inwardly rectifying current, activated by hyperpolarization, was completely blocked by 5mM external Cs. An outward rectifier was also observed, that appeared similar to the delayed rectifier potassium current found in most neurons. A sustained inward calcium current was activated by depolarization positive to -40mV. This current was insensitive to 20μM nifedipine, and was completely abolished by replacing external calcium with cobalt. A large calcium-activated chloride current was also observed. Conclusions: Mammalian cone photoreceptors did not appear to be electrically coupled in either dark or light adapted retinas. Cone photoreceptors displayed an array of voltage-activated currents similar to rod and cone photoreceptors of lower vertebrates. In particular, the calcium current was sustained, similar to the L-type channels seen in bipolar cells and photoreceptors of lower vertebrates, but was insensitive to dihydropyridine antagonists selective for L-type channels.