Fig. 8.

The shifting of the voltage dependence of steady-state I_h activation modulates synaptic gain. A: example of an experiment where native g_h was blocked with ZD7288 and then reconstituted with virtual g_h at 3 different levels of V_1/2. Each set of records shows the same 6.5-s segment from a longer, 40-s sequence. The dynamic clamp command signal (bottom) was constructed to contain converging strong (300% g_syn) and weak (60% g_syn) synapses, firing in a noisy burst pattern at average frequencies of 1 Hz. The inward virtual I_h generated by the dynamic clamp (middle) became larger as V_1/2 was moved from −83.6 to −63.6 mV. Note that inward currents have a positive sign, because they follow the standard convention for current clamp data. As a consequence of activating more I_h at rest, the shifting of V_1/2 in the depolarizing direction also depolarized V_rest by a few millivolts. This can be seen in the membrane potential records (top). In these data, one can also see changes in the number of action potentials driven by summation of weak EPSPs (asterisks): 3 spikes when V_1/2 = −83.6 mV, 5 spikes when V_1/2 = −73.6 mV, and 8 spikes when V_1/2 = −63.6 mV. This indicates that shifting V_1/2 in the depolarizing direction has the effect of increasing synaptic gain. B: grouped data summarizing the effect of shifting V_1/2 upon synaptic gain. In each cell, the gain values were normalized to the gain observed when V_1/2 = −73.6 mV. The n for each measurement is denoted in the bars.