Expression of hyperpolarization-activated current (I_h) in zonally defined vestibular calyx terminals of the crista

Calyx terminals make afferent synapses with type I hair cells in vestibular epithelia and express diverse ionic conductances that influence action potential generation and discharge regularity in vestibular afferent neurons. Here we investigated the expression of hyperpolarization-activated current (I_h) in calyx terminals in central and peripheral zones of mature gerbil crista slices, using whole cell patch-clamp recordings. Slowly activating I_h was present in >80% calyces tested in both zones. Peak I_h and half-activation voltages were not significantly different; however, I_h activated with a faster time course in peripheral compared with central zone calyces. Calyx I_h in both zones was blocked by 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride (ZD7288; 100 µM), and the resting membrane potential became more hyperpolarized. In the presence of dibutyryl-cAMP (dB-cAMP), peak I_h was increased, activation kinetics became faster, and the voltage of half-activation was more depolarized compared with control calyces. In current clamp, calyces from both zones showed three different categories of firing: spontaneous firing, phasic firing where a single action potential was evoked after a hyperpolarizing pulse, or a single evoked action potential followed by membrane potential oscillations. In the absence of I_h, the latency to peak of the action potential increased; I_h produces a small depolarizing current that facilitates firing by driving the membrane potential closer to threshold. Immunostaining showed the expression of HCN2 subunits in calyx terminals. We conclude that I_h is found in calyx terminals across the crista and could influence conventional and novel forms of synaptic transmission at the type I hair cell-calyx synapse.NEW & NOTEWORTHY Calyx afferent terminals make synapses with vestibular hair cells and express diverse conductances that impact action potential firing in vestibular primary afferents. Conventional and nonconventional synaptic transmission modes are influenced by hyperpolarization-activated current (I_h), but regional differences were previously unexplored. We show that I_h is present in both central and peripheral calyces of the mammalian crista. I_h produces a small depolarizing resting current that facilitates firing by driving the membrane potential closer to threshold.