title: Sensory transduction at the frog semicircular canal: how hair cell membrane potential controls junctional transmission.
abstract: At the frog semicircular canals, the afferent fibers display high spontaneous activity (mEPSPs), due to transmitter release from hair cells. mEPSP and spike frequencies are modulated by stimulation that activates the hair cell receptor conductance. The relation between receptor current and transmitter release cannot be studied at the intact semicircular canal. To circumvent the problem, we combined patch-clamp recordings at the isolated hair cell and electrophysiological recordings at the cytoneural junction in the intact preparation. At isolated hair cells, the K channel blocker tetraethylammonium (TEA) is shown to block a fraction of total voltage-dependent K-conductance (IKD) that depends on TEA concentration but not on membrane potential (V m). Considering the bioelectric properties of the hair cell, as previously characterized by this lab, a fixed fractional block of IKD is shown to induce a relatively fixed shift in V m, provided it lies in the range -30 to -10 mV. The same concentrations of TEA were applied to the intact labyrinth while recording from single afferent fibers of the posterior canal, at rest and during mechanical stimulation. At the peak of stimulation, TEA produced increases in mEPSP rate that were linearly related to the shifts produced by the same TEA concentrations (0.1-3 mM) in hair cell V m (0.7-5 mV), with a slope of 29.8 Hz/mV. The membrane potential of the hair cell is not linearly related to receptor conductance, so that the slope of quantal release vs. receptor conductance depends on the prevailing V m (19.8 Hz/nS at -20 mV; 11 Hz/nS at -10 mV). Changes in mEPSP peak size were negligible at rest as well as during stimulation. Since ample spatial summation of mEPSPs occurs at the afferent terminal and threshold-governed spike firing is intrinsically nonlinear, the observed increases in mEPSP frequency, though not very large, may suffice to trigger afferent spike discharge.
authors.affiliation: Dipartimento di Scienze della Vita e Biotecnologie, Ferrara University Ferrara, Italy.
authors.forename: Marta
authors.initials: M
authors.lastname: Martini
authors.affiliation: Dipartimento di Scienze della Vita e Biotecnologie, Ferrara University Ferrara, Italy.
authors.forename: Rita
authors.initials: R
authors.lastname: Canella
authors.affiliation: Dipartimento di Scienze della Vita e Biotecnologie, Ferrara University Ferrara, Italy.
authors.forename: Gemma
authors.initials: G
authors.lastname: Rubbini
authors.affiliation: Centre of Neuroscience and DISTA, Insubria University Varese, Italy.
authors.forename: Riccardo
authors.initials: R
authors.lastname: Fesce
authors.affiliation: Dipartimento di Scienze della Vita e Biotecnologie, Ferrara University Ferrara, Italy.
authors.forename: Maria Lisa
authors.initials: ML
authors.lastname: Rossi
keywords: TEA effect; hair cell currents; mEPSP release; membrane potential; receptor conductance; semicircular canal; sensory processing