FIG. 1.

Typical hyperpolarization-activated inward currents in calyx terminals. A Whole-cell currents elicited in response to hyperpolarizing steps in voltage clamp with voltage protocol shown below. The cell was held at −79 mV and stepped from −139 to −74 mV in incrementing 5 mV steps of 650 ms duration. A capacitive transient was followed by an instantaneous current (I _ins) at the start of the voltage step and a slowly developing inward current that approached steady state (I _end) towards the end of the step. At return to −79 mV, tail currents peaked within 40 ms. Tail currents are shown over a longer time course in a different cell (inset). B Instantaneous (open circles) and steady-state (closed circles) IV relationships for the cell in A (left). Current activated by hyperpolarization was obtained by subtracting I _ins from I _end (right). C IV relationship for I _end − I _ins for nine calyx terminals (mean ± SEM). D Hyperpolarization-activated current does not inactivate during a 2-s voltage pulse to −139 mV.