Developmental changes in voltage-activated potassium currents of rat retinal ganglion cells

Ca2(+)-independent voltage-activated potassium currents were investigated during the differentiation of rat retinal ganglion cells. Whole-cell patch-clamp recordings of Ca2(+)-independent voltage-activated potassium currents and their individual current components, i.e. a sustained, tetraethylammonium-sensitive current, a transient, 4-aminopyridine-sensitive current, and a slowly decaying current that was blocked by Ba2+, revealed distinct ontogenetic modifications in current densities and in activation and inactivation parameters. All three current types were expressed simultaneously at embryonic day 17/18 and were present in all retinal ganglion cells thereafter without showing any significant changes until the end of the first postnatal week. Ca2(+)-independent voltage-activated potassium current densities then increased strongly from postnatal day 8 onwards. Tetraethylammonium-sensitive current density increased about eightfold from 74 pA/pF in embryonic stages to 586 pA/pF in adult cells, whereas the transient potassium currents blocked by 4-aminopyridine increased only about 2.5-fold from 174 pA/pF to 442 pA/pF. The Ba2(+)-sensitive current increased simultaneously from 35 pA/pF to 332 pA/pF. The much higher increase in the sustained current components during retinal ganglion cell differentiation accounted for the changes in decay kinetics of Ca2(+)-independent voltage-activated potassium current observed in later postnatal stages. Alterations in current densities were paralleled by pronounced changes in current kinetics. From postnatal day 8 onwards, activation of Ca2(+)-independent voltage-activated potassium current was right-shifted for about 10 mV owing to a shift in tetraethylammonium-sensitive current-activation, whereas activation of other K+ components remained unaltered. Tetraethylammonium-sensitive current steady-state inactivation was incomplete at all developmental stages. About 50% of the tetraethylammonium-sensitive current elicited by a depolarization to +36 mV did not inactivate after prepulse potentials positive to -10 mV. In contrast, transient potassium current blocked by 4-aminopyridine almost fully inactivated during embryonic stages, whereas in adult retinal ganglion cells about 40% of this current component did not inactivate after prepulse potentials positive to -20 mV. Parallel investigation of the resting membrane potential during retinal ganglion cells differentiation showed an exponential increase from -3 mV at embryonic day 15/16 when no voltage-activated ion currents were expressed to a final value of -58 mV at postnatal day 8. These results show that fundamental potassium current modifications occur relatively late in retinal ganglion cell development and only after the resting potential is at its final value.