Characteristics and physiological role of hyperpolarization activated currents in mouse cold thermoreceptors

Hyperpolarization-activated currents (I-h) are mediated by the expression of combinations of hyperpolarization-activated, cyclic nucleotide-gated (HCN) channel subunits (HCN1-4). These cation currents are key regulators of cellular excitability in the heart and many neurons in the nervous system. Subunit composition determines the gating properties and cAMP sensitivity of native I-h currents. We investigated the functional properties of I-h in adult mouse cold thermoreceptor neurons from the trigeminal ganglion, identified by their high sensitivity to moderate cooling and responsiveness to menthol. All cultured cold-sensitive (CS) neurons expressed a fast activating I-h, which was fully blocked by extracellular Cs+ or ZD7288 and had biophysical properties consistent with those of heteromeric HCN1-HCN2 channels. In CS neurons from HCN1((-/-)) animals, I-h was greatly reduced but not abolished. We find that I-h activity is not essential for the transduction of cold stimuli in CS neurons. Nevertheless, I-h has the potential to shape the excitability of CS neurons. First, I-h blockade caused a membrane hyperpolarization in CS neurons of about 5 mV. Furthermore, impedance power analysis showed that all CS neurons had a prominent subthreshold membrane resonance in the 5-7 Hz range, completely abolished upon blockade of I-h and absent in HCN1 null mice. This frequency range matches the spontaneous firing frequency of cold thermoreceptor terminals in vivo. Behavioural responses to cooling were reduced in HCN1 null mice and after peripheral pharmacological blockade of I-h with ZD7288, suggesting that I-h plays an important role in peripheral sensitivity to cold.