In the vestibular periphery neurotransmission between hair cells and primary afferent nerves occurs via specialized ribbon synapses. Type I vestibular hair cells (HCI) make synaptic contacts with calyx terminals, which enclose most of the HCI basolateral surface. To probe synaptic transmission, whole cell patch clamp recordings were made from calyx afferent terminals isolated together with their mature HCI from gerbil crista. Neurotransmitter release was measured as excitatory postsynaptic currents (EPSCs) in voltage clamp. Spontaneous EPSCs were classified as simple or complex. Simple events exhibited a rapid rise-time and a fast monoexponential decay (time constant < 1 ms). Remaining events, constituting ~40% of EPSCs, showed more complex characteristics. Extracellular Sr2+ greatly increased EPSC frequency and EPSCs were blocked by the AMPA receptor blocker NBQX. The role of presynaptic Ca2+ channels was assessed by applying the L-type Ca2+ channel blocker nifedipine (20 µM), which reduced EPSC frequency. In contrast the L-type Ca2+ channel opener BAYK8644 increased EPSC frequency. Cyclothiazide increased the decay time constant of averaged simple EPSCs by approximately 2 fold. The low-affinity AMPA receptor antagonist γ-D-glutamylglycine (2 mM) reduced the proportion of simple EPSCs relative to complex events, indicating glutamate accumulation in the restricted cleft between HCI and calyx. In crista slices EPSC frequency was greater in central compared to peripheral calyces which may be due to greater numbers of presynaptic ribbons in central hair cells. Our data support a role for L-type Ca2+ channels in spontaneous release and demonstrate regional variations in AMPA-mediated quantal transmission at the calyx synapse.
- Type I hair cell
- AMPA Receptor
- Copyright © 2016, Journal of Neurophysiology