Impact of the activation rate of the hyperpolarization- activated current I_h on the neuronal membrane time constant and synaptic potential duration

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_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_h conductance is well understood, the role of the I_h kinetics remains unclear. Here, we use a model of the I_h current model with either fast or slow kinetics to determine its influence on the membrane time constant ( τ _m) of a CA1 pyramidal cell model. Our simulation results show that the I_h with fast kinetics decreases τ _m and attenuates and shortens the excitatory postsynaptic potentials more than the slow I_h . We conclude that the I_h activation kinetics is able to modulate τ _m and the temporal properties of excitatory postsynaptic potentials (EPSPs) in CA1 pyramidal cells. To elucidate the mechanisms by which I_h kinetics controls τ _m , we propose a new concept called “time scaling factor”. Our main finding is that the I_h kinetics influences τ _m by modulating the contribution of the I_h derivative conductance to τ _m .