Kv3 channels modulate calcium signals induced by fast firing patterns in the rat retinal ganglion cells

Expression of non-inactivating K(v)3.1/K(v)3.2 potassium channels determines fast-spiking phenotype of many types of neurones including retinal ganglion cells (RGCs); furthermore K(v)3 channels regulate neurotransmitter release from presynaptic terminals. In the present study we investigated how inhibition of K(v)3 channel by low TEA concentrations modifies firing properties and Ca2+ influx in the rat RGCs. Experiments were performed on the whole-mount retinal preparations from 4 to 6 weeks old Wistar rats using simultaneous whole cell patch clamp and intracellular Ca2+ measurements in combination with single-cell RT-PCR. In response to 500-ms depolarization step the RGCs demonstrated fast firing tonic behaviour with a mean frequency of spiking 61 +/- 5 Hz (n = 28). All of the tonic cells tested (n = 9) expressed specific mRNA for either K(v)3.1 or K(v)3.2 or for both channels. Bath applications of TEA (250 mu M, 500 mu M and 1 mM) modified firing patterns dose-dependently as follows: firing frequency was decreased, mean action potential (AP) half-width increased and mean amplitude of after hyperpolarization was reduced. The amplitude of the Ca2+ signals induced by the cells firing was linearly dependent on number of APs with a mean slope of 7.3 +/- 0.9 nM per one AP (n = 8). APs widening by TEA increased the slope of the amplitude vs. AP number plots in a dose-dependent manner: 250 mu M of TEA increased the mean slope value to 9.5 +/- 1.2 nM/AP, 500 mu M to 12.4 +/- 2.4 nM/AP and 1 mM to 13.2 +/- 2.9 nM/AP (n = 6). All these parameters, as well as the cells firing properties, were significantly different from controls and from each other except between 500 mu M and 1 mM. This is consistent with the pharmacological properties of K(v)3.1/K(v)3.2 channels: the TEA IC50 is in the range 150-300 mu M with almost complete block at 1 mM. This suggests that K(v)3.1/K(v)3.2 channels underlie the fast firing of the rat RGCs and provide at a given firing frequency 1.8-fold restriction Ca2+ influx, thus protecting the cells from its cytotoxic action. (c) 2012 Elsevier Ltd. All rights reserved.