Neurophysiological Impact and Modeling-independent elucidation of inactivation pathways in A-type K+ channels

Poster presented at Society for Neuroscience Abstract: A-type voltage-gated K+ channels auto-regulate their function by undergoing fast inactivation. Independent of molecular mechanisms, this inactivation can proceed after channel opening (open-state inactivation, OSI) or from a closed state prior to opening (closed-state inactivation, CSI). We hypothesize that the specific neurophysiological roles of A-type Kv channels depend on whether they undergo OSI, CSI or both (CSI+OSI). To explore these possibilities, we introduced Markov kinetic schemes of the A-type Kv4 conductance into a computational model of the hippocamcal CA1 neuron assuming either CSI or CSI+OSI and compared the properties of the somatic and dendritic action potentials (APs). Relative to the impact of CSI, the main differential effects of CSI+OSI are: 1. 15% less attenuation of back-propagating APs 2. Shorter latency to the first somatic spike 3. Exaggerated activity-dependent spike broadening and peak attenuation in somatic AP trains 4. The inter-spike intervals of AP trains initially increases before it is shortened (CSI generates monotonic shortening). The outcome of these simulations thus motivated the development of a simple, modeling-independent method to conclusively elucidate the preferred pathways of inactivation in two distinct A-type Kv channels -Kv3.4 and Kv4.2- expressed in heterologous cells and specific neurons. We applied two voltage-clamp pulse protocols- single- and double-pulse (modest conditioning step followed by a strong test step) - to obtain three pieces of critical information: development macroscopic single-pulse inactivation, the rate of double-pulse inactivation and the voltage-dependence of the time constant of macroscopic inactivation. Consistent with OSI, the rate of Kv3.4 inactivation precisely superimposes on the profile of the Kv3.4 current evoked by a single-pulse and the time constant vs. voltage relation decreases monotonically and levels off. By contrast, in ternary Kv4.2 channels, the rate of Kv4.2 inactivation is asynchronous, peaking earlier relative to the profile of the Kv4.2 single-pulse current and the time constant vs. voltage relation displays a \u27J-shape\u27 profile. Thus, Kv4.2 inactivation occurs uncoupled from channel opening, indicating CSI. Furthermore, removing KChIP1 from the Kv4 ternary complex or adding DPP10a to Kv4.2 channels produces a CSI+OSI phenotype. This procedure unambiguously establishes contrasting pathways of inactivation in neuronal A-type Kv channels, and provides a simple tool to correlate regulation of ionic conductance and neurophysiological activity