Elementary properties of Ca_V1.3 Ca²⁺ channels expressed in mouse cochlear inner hair cells

Abstract

Mammalian cochlear inner hair cells (IHCs) are specialized to process developmental signals during immature stages and sound stimuli in adult animals. These signals are conveyed onto auditory afferent nerve fibres. Neurotransmitter release at IHC ribbon synapses is controlled by L-type CaV1.3 Ca2+ channels, the biophysics of which are still unknown in native mammalian cells. We have investigated the localization and elementary properties of Ca2+ channels in immature mouse IHCs under near-physiological recording conditions. CaV1.3 Ca2+ channels at the cell pre-synaptic site co-localize with about half of the total number of ribbons present in immature IHCs. These channels activated at about −70 mV, showed a relatively short first latency and weak inactivation, which would allow IHCs to generate and accurately encode spontaneous Ca2+ action potential activity characteristic of these immature cells. The CaV1.3 Ca2+ channels showed a very low open probability (about 0.15 at −20 mV: near the peak of an action potential). Comparison of elementary and macroscopic Ca2+ currents indicated that very few Ca2+ channels are associated with each docked vesicle at IHC ribbon synapses. Finally, we found that the open probability of Ca2+ channels, but not their opening time, was voltage dependent. This finding provides a possible correlation between presynaptic Ca2+ channel properties and the characteristic frequency/amplitude of EPSCs in auditory afferent fibres.

Ribeye is required for presynaptic CaV1.3a channel localization and afferent innervation of sensory hair cells

“…Presynaptic spheres in retinal bipolar cells contain the proteins Piccolo, Bassoon and Ribeye, but do not contain L-type Ca 2+ channel 1 subunit (Regus-Leidig et al, 2009). Furthermore, in inner hair cells of developing mouse cochlea only about 50% of synaptic ribbons colocalize with Ca V 1.3 (Zampini et al, 2010). Alternatively, the Ribeye aggregates at the plasma membrane may direct the clustering of Ca V 1.3a.…”

Lack of Brain-Derived Neurotrophic Factor Hampers Inner Hair Cell Synapse Physiology, But Protects against Noise-Induced Hearing Loss

“…For double labeling studies, both antibodies were simultaneously incubated for identical time periods. Sections were viewed by an AX70 Olympus microscope as previously described (Zampini et al, 2010). For whole mount preparations hair cell region was imaged by z-stacking, 3 dimensionally deconvoluted (ADVMLE, cellSens, Olympus) and displayed as maximum intensity projection over z.…”

An Improved Model for the Rate–Level Functions of Auditory-Nerve Fibers

“…From these studies, we estimate a physiological range of the membrane potential of ϳ15 mV, from approximately Ϫ45 to Ϫ30 mV or slightly more positive. Third, the dependence of the open probability of the voltage-gated Ca V 1.3 channels at IHC active zones on voltage is well described by a Boltzmann equation, according to which the channels reach halfmaximum open probability at approximately Ϫ34 mV, with a slope of 6.1 mV [Zampini et al (2010), their Fig. 3F].…”

“…This current gives rise to the resting membrane potential of the IHC, which is relatively depolarized at approximately Ϫ45 mV or more (Russell and Sellick, 1978;Goodman et al, 1982;Russell and Cowley, 1983;Palmer and Russell, 1986;Ashmore, 2009). This resting membrane potential in turn results in a nonzero open probability of voltage-gated Ca V 1.3 Ca 2ϩ channels associated with the individual active zones of the IHC (Robertson and Paki, 2002;Brandt et al, 2003;Zampini et al, 2010), and hence in Ca 2ϩ signals mediating exocytosis. The AA model assumes that this resting stimulus and the equivalent physiological stimulus generated by the sound sum arithmetically.…”