Hyperpolarization-activated cyclic nucleotide-gated channels in mouse vomeronasal sensory neurons

Hyperpolarization-activated currents (I-h) are present in several neurons of the central and peripheral nervous system. However, I-h in neurons of the vomeronasal organ (VNO) is not well characterized. We studied the properties of I-h in sensory neurons from acute slices of mouse VNO. In voltage-clamp studies, I-h was identified by the characteristic kinetics of activation, voltage dependence, and blockage by Cs+ or ZD-7288, two blockers of the I-h. Forskolin, an activator of adenylyl cyclase, shifted the activation curve for I-h to less negative potentials. A comparison of I-h properties in VNO neurons with those of heterologously expressed hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, together with RT-PCR experiments in VNO, indicate that I-h is caused by HCN2 and/or HCN4 subunits. In current-clamp recordings, blocking I-h with ZD-7288 induced a hyperpolarization of 5.1 mV, an increase in input resistance, a decrease in the sensitivity to elicit action potentials in response to small current injections, and did not modify the frequency of action potentials elicited by a large current injection. It has been shown that in VNO neurons some pheromones induce a decrease in cAMP concentration, but the physiological role of cAMP is unknown. After application of blockers of adenylyl cyclase, we measured a hyperpolarization of 5.1 mV in 11 of 14 neurons, suggesting that basal levels of cAMP could modulate the resting potential. In conclusion, these results show that mouse VNO neurons express HCN2 and/or HCN4 subunits and that I-h contributes to setting the resting membrane potential and to increase excitability at stimulus threshold.