Molecular identity, ontogeny, and cAMP modulation of the hyperpolarization-activated current in vestibular ganglion neurons

Abstract

Properties, developmental regulation, and cAMP modulation of the hyperpolarization-activated current ( Ih) were investigated by the whole cell patch-clamp technique in vestibular ganglion neurons of the rat at two postnatal stages (P7–10 and P25–28). In addition, by RT-PCR and immunohistochemistry the identity and distribution of hyperpolarization-activated and cyclic nucleotide-gated channel (HCN) isoforms that generate Ih were investigated. Ih current density was larger in P25–28 than P7–10 rats, increasing 410% for small cells (<30 pF) and 200% for larger cells (>30 pF). The half-maximum activation voltage ( V1/2) of Ih was −102 mV in P7–10 rats and in P25–28 rats shifted 7 mV toward positive voltages. At both ages, intracellular cAMP increased Ih current density, decreased its activation time constant (τ), and resulted in a rightward shift of V1/2 by 9 mV. Perfusion of 8-BrcAMP increased Ih amplitude and speed up its activation kinetics. Ih was blocked by Cs+, zatebradine, and ZD7288. As expected, these drugs also reduced the voltage sag caused with hyperpolarizing pulses and prevented the postpulse action potential generation without changes in the resting potential. RT-PCR analysis showed that HCN1 and HCN2 subunits were predominantly amplified in vestibular ganglia and end organs and HCN3 and HCN4 to a lesser extent. Immunohistochemistry showed that the four HCN subunits were differentially expressed (HCN1 > HCN2 > HCN3 ≥ HCN4) in ganglion slices and in cultured neurons at both P7–10 and P25–28 stages. Developmental changes shifted V1/2 of Ih closer to the resting membrane potential, increasing its functional role. Modulation of Ih by cAMP-mediated signaling pathway constitutes a potentially relevant control mechanism for the modulation of afferent neuron discharge.