Figure 5. The properties of I_h in the different cell types.

A, I_h was elicited by 800 ms-long hyperpolarising voltage steps from a holding potential of −40 mV to the range of −50 to −120 mV, in 10 mV steps. Current traces recorded before and after the application of 10 μm ZD7288. The ZD7288-sensitive current was obtained by digital subtraction. B, the activation curves of I_h in the investigated cell types, calculated from tail current amplitudes. C, the current–voltage (I–V) relation of I_h in the different cell types obtained by plotting steady-state currents. Note that at physiologically relevant potentials (positive to −90 mV), a significantly larger amplitude of I_h was activated in PCs than in interneurons. D, the activation kinetics (Tau_h) of I_h plotted against membrane potential for PCs, O-R cells and OLM cells obtained from single exponential fits. The time course of activation was generally more rapid in PCs than in interneurons over the entire voltage range. E and F, two components of the activation kinetics could be clearly identified with double exponential fits in all cell types. Fast (Tau₁, E) and slow time constant (Tau₂, F) of I_h activation as a function of voltage for PCs, O-R cells and OLM cells. In each cell type the fast time constant was about 5–8 times more rapid than the slow time constant of activation at a given potential. G, the fraction of the fast exponential component (Tau₁) as a function of voltage. Note that in PCs the fraction of Tau₁ was predominant over the entire voltage range, while in interneurons the fast component becomes predominant only at hyperpolarised potentials (below −100 mV) but even at −120 mV it represents only 70% of the total current amplitude. H, to determine the reversal potential of the current, I_h was fully activated with a voltage pulse to −120 mV and this was followed by steps to different test potentials (from −110 mV to −40 mV in 10 mV increments). I, the open-channel I–V plot was constructed from the tail current amplitudes measured at each test potential and the reversal potential of I_h was extrapolated from these plots. J, the estimated maximal conductance values of I_h for each cell of the different cell types (crosses). Circles indicate median values. There was no significant difference in the maximal conductance of I_h between the investigated cell types. In all plots PCs are shown in blue, O-R cells in red and OLM cells in green.