Delayed rectifier K⁺ currents, I_K, are encoded by Kv2 α-subunits and regulate tonic firing in mammalian sympathetic neurons

Previous studies have revealed the presence of four kinetically distinct voltage-gated K⁺ currents, I_Af, I_As, I_K, and I_SS, in rat superior cervical ganglion (SCG) neurons and demonstrated that I_K and I_SS are expressed in all cells, whereas I_Af and I_As are differentially distributed. Previous studies have also revealed the presence of distinct components of I_Af encoded by α-subunits of the Kv1 and Kv4 subfamilies. In the experiments described here, pore mutants of Kv2.1 (Kv2.1W365C/Y380T) and Kv2.2 (Kv2.2W373C/Y388T) that function as Kv2 subfamily-specific dominant negatives (Kv2.1DN and Kv2.2DN) were generated to probe the functional role(s) of Kv2 α-subunits. Expression of Kv2.1DN or Kv2.2DN in human embryonic kidney-293 cells selectively attenuates Kv2.1 - or Kv2.2-encoded K⁺ currents, respectively. Using the Biolistics Gene Gun, cDNA constructs encoding either Kv2.1DN or Kv2.2DN [and enhanced green fluorescent protein (EGFP)] were introduced into SCG neurons. Whole-cell recordings from EGFP-positive Kv2.1DN or Kv2.2DN-expressing cells revealed selective decreases in I_K. Coexpression of Kv2.1DN and Kv2.2DN eliminates I_K in most (75%) SCG cells and, in the remaining (25%) cells, I_K density is reduced. Together with biochemical data revealing that Kv2.1 and Kv2.2 a-subunits do not associate in rat SCGs, these results suggest that Kv2.1 and Kv2.2 form distinct populations of I_K channels, and that Kv2 a-subunits underlie (most of) I_K in SCG neurons. Similar to wild-type cells, phasic, adapting, and tonic firing patterns are evident in SCG cells expressing Kv2.1 DN or Kv2.2DN, although action potential durations in tonic cells are prolonged. Expression of Kv2.2DN also results in membrane depolarization, suggesting that Kv2.1- and Kv2.2-encoded I_K channels play distinct roles in regulating the excitability of SCG neurons.