Synaptic transmission at the inner hair cell (IHC) afferent synapse, the first synapse in the auditory pathway, is specialized for rapid and reliable signaling. Here we investigated the properties of a hyperpolarization-activated current (Ih), expressed in the afferent dendrite of auditory nerve fibers, and its role in shaping postsynaptic activity. We used whole cell patch-clamp recordings from afferent dendrites directly where they contact the IHC in excised postnatal rat cochlear turns. Excitatory postsynaptic potentials (EPSPs) of variable amplitude (1–35 mV) were found with 10–90% rise times of about 1 ms and time constants of decay of about 5 ms at room temperature. Current–voltage relations recorded in afferent dendrites revealed Ih. The pharmacological profile and reversal potential (−45 mV) indicated that Ih is mediated by hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels. The HCN channel subunits HCN1, HCN2, and HCN4 were found to be expressed in afferent dendrites using immunolabeling. Raising intracellular cAMP levels sped up the activation kinetics, increased the magnitude of Ih and shifted the half activation voltage (Vhalf) to more positive values (−104 ± 3 to −91 ± 2 mV). Blocking Ih with 50 μM ZD7288 resulted in hyperpolarization of the resting membrane potential (∼4 mV) and slowing the decay of the EPSP by 47%, suggesting that Ih is active at rest and shortens EPSPs, thereby potentially improving rapid and reliable signaling at this first synapse in the auditory pathway.
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