Expression of hyperpolarization-activated current (Ih) 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 (Ih ) in calyx terminals in central and peripheral zones of mature gerbil crista slices using whole cell patch clamp recordings. Slowly activating Ih was present in > 80 % calyces tested in both zones. Peak Ih and half-activation voltages were not significantly different, however Ih activated with a faster time course in peripheral compared to central zone calyces. Calyx Ih in both zones was blocked by ZD7288 (100 µM) and the resting membrane potential became more hyperpolarized. In the presence of dibutyryl-cAMP (dB-cAMP), peak Ih 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 3 different categories of firing: spontaneous firing, phasic firing where a single action potential was evoked following a hyperpolarizing pulse, or a single evoked action potential followed by membrane potential oscillations. In the absence of Ih , the latency to peak of the action potential increased; Ih produces a small depolarizing current which facilitates firing by driving the membrane potential closer to threshold. Immunostaining showed the expression of HCN2 subunits in calyx terminals. We conclude that Ih 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.