I(h) "run-up" in rat neocortical neurons and transiently rat or human HCN1-expressing HEK293 cells.

Hyperpolarization-activated cyclic nucleotide-gated ion channels (HCN) are key determinants of CNS functions. Here we describe an increase in hyperpolarization-activated current (I(h)) at the beginning of whole-cell recordings in rat layer 5 cortical neurons. For a closer investigation of this I(h) increase, we overexpressed the predominant layer 5 rat subunit HCN1 in HEK293 cells. We characterized the resulting I(h) in the cell-attached and whole-cell configurations. Breaking into whole-cell configuration led to about a 30% enhancement of rat HCN1-mediated I(h) accompanied by a depolarizing shift in voltage dependence and an accelerated time course of activation. This current enhancement is not species specific; for human HCN1, the current similarly increases in amount and kinetics. Although the changes were bound to cytosolic solution exchange, they were independent of cAMP, ATP, GTP, and the phosphate group donor phosphocreatine. Together, these data provide a characterization of heterologous expression of rat HCN1 and suggest that cytosolic contents suppress I(h). Such a mechanism might constitute a reserve in h-channel function in vivo.