CALCIUM-ACTIVATED POTASSIUM CONDUCTANCES IN RETINAL GANGLION-CELLS OFTHE FERRET

Patch-clamp recordings were made from isolated and intact retinal gang lion cells (RGCs) of the ferret to examine the calcium-activated potas sium channels expressed by these neurons and to determine their functi onal role in the generation of spikes and spiking patterns. Single-cha nnel recordings from isolated neurons revealed the presence of two cal cium-sensitive potassium channels that had conductances of 118 and 22 pS. The properties of these two channels were shown to be similar to t hose ascribed to the large-conductance calcium-activated potassium cha nnel (BKCa) and small-conductance calcium-activated potassium channel (SKCa) channels in other neurons. Whole cell recordings from isolated RGCs showed that apamin and charybdotoxin (CTX), specific blockers of the SKCa and BKCa channels, respectively, resulted in a shortening of the time to threshold and a reduction in the hyperpolarization after t he spike. Addition of these blockers also resulted in a significant in crease in spike frequency over a wide range of maintained depolarizati ons. Similar effects of apamin and CTX were observed during current-cl amp recordings from intact alpha and beta ganglion cells, morphologica lly identified after Lucifer yellow filling. About 20% of these neuron s did not exhibit a sensitivity to either blocker, suggesting the pres ence of functionally distinct subgroups of alpha and beta RGCs on the basis of their intrinsic membrane properties. The expression of these calcium-activated potassium channels in the majority of alpha and beta cells provides a meats by which the activity of these output neurons could be modulated by retinal neurochemicals.