Properties, developmental regulation, and cAMP modulation of the hyperpolarization-activated current (I(h)) 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 I(h) were investigated. I(h) 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 (V(1/2)) of I(h) was -102 mV in P7-10 rats and in P25-28 rats shifted 7 mV toward positive voltages. At both ages, intracellular cAMP increased I(h) current density, decreased its activation time constant (τ), and resulted in a rightward shift of V(1/2) by 9 mV. Perfusion of 8-BrcAMP increased I(h) amplitude and speed up its activation kinetics. I(h) 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 V(1/2) of I(h) closer to the resting membrane potential, increasing its functional role. Modulation of I(h) by cAMP-mediated signaling pathway constitutes a potentially relevant control mechanism for the modulation of afferent neuron discharge.

Almanza A, Navarrete F, Vega R, Soto E. Modulation of voltagegated Ca 2ϩ current in vestibular hair cells by nitric oxide. J Neurophysiol 97: 1188Neurophysiol 97: -1195Neurophysiol 97: , 2007. First published December 20, 2006; doi:10.1152/jn.00849.2006. The structural elements of the nitric oxide-cyclic guanosine monophosphate (NO-cGMP) signaling pathway have been described in the vestibular peripheral system. However, the functions of NO in the vestibular endorgans are still not clear. We evaluated the action of NO on the Ca 2ϩ currents in hair cells isolated from the semicircular canal crista ampullaris of the rat (P14 -P18) by using the whole cell and perforated-cell patch-clamp technique. The NO donors 3-morpholinosydnonimine (SIN-1), sodium nitroprusside (SNP), and ( In the presence of Nethylmaleimide (NEM), a sulfhydryl alkylating agent that prevents the S-nitrosylation reaction, the SNP effect on the Ca 2ϩ current was significantly diminished. These results demonstrated that NO inhibits in a voltage-independent manner the voltage-activated Ca 2ϩ current in rat vestibular hair cells by the activation of a cGMP-signaling pathway and through a direct action on the channel protein by a Snitrosylation reaction. The inhibition of the Ca 2ϩ current by NO may contribute to the regulation of the intracellular Ca 2ϩ concentration and hair-cell synaptic transmission.

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