Contribution of Kv3 potassium channels to signal processing by electrosensory neurons

Abstract

Gamma-frequency burst discharge in pyramidal cells of the apteronotid electrosensory lateral line lobe (ELL) is necessary for signal processing. Bursting is dependent on an interaction between somatic and apical dendritic currents, in which spike broadening in the apical dendrite potentiates a somatic afterpotential that follows each rapid somatic spike. Somatic spike repolarization must be consistent in order for the afterpotential to be expressed soon after. The work presented in this thesis describes how the expression and differential subcellular distribution of two Kv3-type K+ channels in ELL pyramidal cells may contribute to this mechanism of burst discharge.I cloned a family of Kv3 channels from an apteronotid brain cDNA library and demonstrated that two of these channels, homologues of the mammalian subtypes Kv3.1 and Kv3.3, are expressed in ELL pyramidal cells. Immunohistochemical analysis demonstrated that the AptKv3.3 K+ channel is distributed throughout the dendrites of pyramidal cells while the AptKv3.1 channel is restricted in its expression to pyramidal cell somata, basilar dendrites and proximal apical dendrites. Heterologous expression of each channel in HEK 293 cells indicated that AptKv3.3 encodes a high-threshold inactivating K + current while AptKv3.1 encodes a high-threshold K+ current which does not display inactivation upon prolonged membrane depolarization. Based on these results as well as pharmacological analysis of native ELL pyramidal cells, I propose that AptKv3.3 mediates spike repolarization in the apical dendrite and inactivation of the channel during repetitive firing allows spike broadening. In contrast, AptKv3.1 likely contributes towards rapid and consistent spike repolarization in the cell soma. Therefore, the expression and differential distribution of these two Kv3 channels in ELL pyramidal cells may underlie the compartmental differences in spike repolarzation that is necessary for burst discharge.The extensive dendritic localization of AptKv3.3 observed in ELL pyramidal cells as well as in other hindbrain neurons has not previously been demonstrated for members of the Kv3 family of K+ channels. The differential localization of AptKv3.1. AptKv3.3 and possibly AptKv3.3 splice variants that I have identified presents an opportunity to examine the molecular mechanisms of Kv3 channel targeting in neurons. Preliminary data is presented which provides the foundation for future studies on channel targeting