Dendritic HCN channels shape excitatory postsynaptic potentials at the inner hair 3 cell afferent synapse in the mammalian cochlea 4 5

24 Synaptic transmission at the inner hair cell (IHC) afferent synapse, the first synapse in the 25 auditory pathway, is specialized for rapid and reliable signaling. Here we investigated 26 the properties of a hyperpolarization activated current (Ih) expressed in the afferent 27 dendrite of auditory nerve fibers, and its role in shaping postsynaptic activity. 28 We used whole-cell patch-clamp recordings from afferent dendrites directly where they 29 contact the IHC in excised postnatal rat cochlear turns. Excitatory postsynaptic potentials 30 (EPSPs) of variable amplitude (1-35 mV) were found with 10-90% rise time of ~ 1 ms 31 and time constant of decay of ~ 5 ms at room temperature. Current voltage relations 32 recorded in afferent dendrites revealed a hyperpolarization-activated current (Ih). The 33 pharmacological profile and reversal potential (-45 mV) indicated that Ih is mediated by 34 hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels. The HCN 35 channel subunits HCN1, HCN2 and HCN4 were found to be expressed in afferent 36 dendrites using immunolabeling. Raising intracellular cAMP levels sped up the 37 activation kinetics and increased the magnitude of Ih and shifted the half activation 38 voltage (Vhalf) to more positive values (-104 ± 3 mV to -91 ± 2 mV). Blocking Ih with 50 39 μM ZD7288 resulted in hyperpolarization of the resting membrane potential (~ 4 mV) 40 and slowing the decay of the EPSP by 47 %, suggesting that Ih is active at rest and 41 shortens EPSPs, thereby potentially improving rapid and reliable signaling at this first 42 synapse in the auditory pathway. 43 44