H-Current

The recent cloning of the genes encoding the channel subunits underlying the h-current (Ih) by three different groups has renewed interest in the functional role of this unusual conductance (Gauss et al. 1998; Ludwig et al. 1998; Santoro et al. 1998). This often maligned current (termed If for “funny” in cardiac tissue and IQ for “queer” in the hippocampus) is activated by hyperpolarization beyond approximately −50 to −70 mV, does not inactivate, is carried by Na+ and K+ ions, and slowly depolarizes the cell toward its equilibrium potential of approximately −30 mV (reviewed by Pape 1996). Although relatively small in maximal conductance, this current can form a significant portion of the total membrane conductance at subthreshold membrane potentials, precisely the region of membrane voltage in which synaptic and nonsynaptic computations occur, and therefore it may contribute to the responsiveness and pattern of activity generated by neurons. The ability of neurotransmitters to modulate the voltage dependence of Ih through the production of cyclic nucleotides imparts an additional level of flexibility.