Impact of the activation rate of the hyperpolarization-activated current $I_{\rm h}$ on the neuronal membrane time constant and synaptic potential duration

TL;DR

This study investigates how the kinetics of the hyperpolarization-activated current ($I_h$) affects the membrane time constant and synaptic potential duration in neurons, revealing that faster kinetics shorten EPSPs and modulate neuronal responsiveness.

Contribution

The paper introduces the concept of 'time scaling factor' to explain how $I_h$ kinetics influence the membrane time constant and synaptic potential dynamics in CA1 pyramidal cells.

Findings

Fast $I_h$ kinetics decrease the membrane time constant ($ au_m$).

Fast $I_h$ kinetics shorten and attenuate EPSPs more than slow kinetics.

$I_h$ kinetics modulate $ au_m$ by affecting the $I_h$ derivative conductance.

Abstract

The temporal dynamics of membrane voltage changes in neurons is controlled by ionic currents. These currents are characterized by two main properties: conductance and kinetics. The hyperpolarization-activated current ($I_{\rm h}$) strongly modulates subthreshold potential changes by shortening the excitatory postsynaptic potentials and decreasing their temporal summation. Whereas the shortening of the synaptic potentials caused by the $I_{\rm h}$ conductance is well understood, the role of the $I_{\rm h}$ kinetics remains unclear. Here, we use a model of the $I_{\rm h}$ current model with either fast or slow kinetics to determine its influence on the membrane time constant ($\tau_m$) of a CA1 pyramidal cell model. Our simulation results show that the $I_{\rm h}$ with fast kinetics decreases $\tau_m$ and attenuates and shortens the excitatory postsynaptic potentials more than the slow $I_{\rm h}$. We conclude that the $I_{\rm h}$ activation kinetics is able to modulate $\tau_m$ and the temporal properties of excitatory postsynaptic potentials (EPSPs) in CA1 pyramidal cells. In order to elucidate the mechanisms by which $I_{\rm h}$ kinetics controls $\tau_m$, we propose a new concept called "time scaling factor". Our main finding is that the $I_{\rm h}$ kinetics influences $\tau_m$ by modulating the contribution of the $I_{\rm h}$ derivative conductance to $\tau_m$.