Elimination of the fast transient in superior cervical ganglion neurons with expression of KV4.2W362F:: Molecular dissection of IA

Electrophysiological and molecular studies have revealed considerable heterogeneity in voltage-gated K+ currents and in the subunits that underlie these channels in mammalian neurons. At present, however, the relationship between native K+ currents and cloned subunits is poorly understood. In the experiments here, a molecular genetic approach was exploited to define the molecular correlate of the fast transient outward K+ current, I-Af, in sympathetic neurons and to explore the functional role of I-Af in shaping action potential waveforms and controlling repetitive firing patterns. Using the biolistic gene gun, cDNAs encoding a dominant negative mutant Kv4.2 alpha-subunit (Kv4.2W362F) and enhanced green fluorescent protein (EGFP) were introduced into rat sympathetic neurons in vitro. Whole-cell voltage-clamp recordings obtained from EGFP-positive cells revealed that I-Af is selectively eliminated in cells expressing Kv4.2W362F, demonstrating that Kv4 alpha-subunits underlie I-Af in sympathetic neurons. In addition, I-Af density is increased significantly in cells overexpressing wild-type Kv4.2. In cells expressing Kv4.2W362F, input resistances are increased and (current) thresholds for action potential generation are decreased, demonstrating that I-Af plays a pivotal role in regulating excitability. Expression of Kv4.2W362F and elimination of I-Af also alters the distribution of repetitive firing patterns observed in response to a prolonged injection of depolarizing current. The wild-type superior cervical ganglion is composed of phasic, adapting, and tonic firing neurons. Elimination of I-Af increases the percentage of adapting cells by shifting phasic cells to the adapting firing pattern, and increased I-Af density reduces the number of adapting cells.