VOLTAGE-DEPENDENT AND TIME-DEPENDENT POTASSIUM CONDUCTANCES ENHANCE THE FREQUENCY-RESPONSE OF HORIZONTAL CELLS IN THE TURTLE RETINA

The contribution of voltage- and time-dependent potassium conductances to visual information processing in the distal turtle retina was stud ied in the isolated retina preparation. The effects of specific potass ium channel blockers; tetraethylammonium (TEA) and 4-aminopyridine (4- AP) on the membrane potential and photoresponses of L-cones and L-type horizontal cells were monitored with intracellular microelectrodes. B oth drugs produced a large depolarization of the L-type horizontal cel ls though the effect of 4-AP was more transient than that of TEA. Whil e TEA produced response augmentation associated with negligible change s in the kinetics of the photoresponses, 4-AP induced profound changes in response kinetics which were seen as an overshoot of the resting p otential at stimulus offset and a pronounced slowing down in the retur n of the membrane potential toward the prestimulus level. The effects of TEA on horizontal cells could be accounted for by the action of the drug on cone photoreceptors. The effects of 4-AP on the horizontal ce lls could not be attributed to an indirect action mediated by either t he cone photoreceptors or by GABAergic and/or glycinergic neurons in t he inner retina. These results suggest that voltage- and time-dependen t potassium conductances act to speed up the recovery of the turtle ho rizontal cell membrane potential from the effects of bright light stim uli. Such a role was supported by the effects of potassium channel blo ckers on the frequency response curves of horizontal cells: the corner frequency was reduced on the average by 25%.