Impact of the activation rate of the hyperpolarization- activated current

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 ($$\hbox {I}_{\mathrm {h}}$$) 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 $$\hbox {I}_{\mathrm {h}}$$ conductance is well understood, the role of the $$\hbox {I}_{\mathrm {h}}$$ kinetics remains unclear. Here, we use a model of the $$\hbox {I}_{\mathrm {h}}$$ 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 $$\hbox {I}_{\mathrm {h}}$$ with fast kinetics decreases $$\tau _{m}$$ and attenuates and shortens the excitatory postsynaptic potentials more than the slow $$\hbox {I}_{\mathrm {h}}$$. We conclude that the $$\hbox {I}_{\mathrm {h}}$$ activation kinetics is able to modulate $$\tau _{m}$$ and the temporal properties of excitatory postsynaptic potentials (EPSPs) in CA1 pyramidal cells. To elucidate the mechanisms by which $$\hbox {I}_{\mathrm {h}}$$ kinetics controls $$\tau _{m}$$, we propose a new concept called “time scaling factor”. Our main finding is that the $$\hbox {I}_{\mathrm {h}}$$ kinetics influences $$\tau _{m}$$ by modulating the contribution of the $$\hbox {I}_{\mathrm {h}}$$ derivative conductance to $$\tau _{m}$$