L-type calcium channels couple membrane depolarization in neurons to numerous processes including gene expression, synaptic efficacy, and cell survival. To establish the contribution of L-type calc...

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L-type calcium channels: the low down.

▪ Abstract Mechanosensory hair cells of the vertebrate inner ear contribute to acoustic tuning through feedback processes involving voltage-gated channels in the basolateral membrane and mechanotransduction channels in the apical hair bundle. The specific number and kinetics of calcium-activated (BK) potassium channels determine the resonant frequency of electrically tuned hair cells. Kinetic variation among BK channels may arise through alternative splicing of slo gene mRNA and combination with modulatory β subunits. The number of transduction channels and their rate of adaptation rise with hair cell response frequency along the cochlea's tonotopic axis. Calcium-dependent feedback onto transduction channels may underlie active hair bundle mechanics. The relative contributions of electrical and mechanical feedback to active tuning of hair cells may vary as a function of sound frequency.

Mechanisms of hair cell tuning.

Progressive hearing loss is common in the human population, but little is known about the molecular basis. We report a new N-ethyl-N-nitrosurea (ENU)-induced mouse mutant, diminuendo, with a single base change in the seed region of Mirn96. Heterozygotes show progressive loss of hearing and hair cell anomalies, whereas homozygotes have no cochlear responses. Most microRNAs are believed to downregulate target genes by binding to specific sites on their mRNAs, so mutation of the seed should lead to target gene upregulation. Microarray analysis revealed 96 transcripts with significantly altered expression in homozygotes; notably, Slc26a5, Ocm, Gfi1, Ptprq and Pitpnm1 were downregulated. Hypergeometric P-value analysis showed that hundreds of genes were upregulated in mutants. Different genes, with target sites complementary to the mutant seed, were downregulated. This is the first microRNA found associated with deafness, and diminuendo represents a model for understanding and potentially moderating progressive hair cell degeneration in hearing loss more generally.

/pdf/an-enu-induced-mutation-of-mir-96-associated-with-2mgc4znowb.pdf

An ENU-induced mutation of miR-96 associated with progressive hearing loss in mice.

Purines appear to be the most primitive and widespread chemical messengers in the animal and plant kingdoms The evidence for purinergic signalling in plants, invertebrates and lower vertebrates is reviewed Much is based on pharmacological studies, but important recent studies have utilized the techniques of molecular biology and receptors have been cloned and characterized in primitive invertebrates, including the social amoeba Dictyostelium and the platyhelminth Schistosoma, as well as the green algae Ostreococcus, which resemble P2X receptors identified in mammals This suggests that contrary to earlier speculations, P2X ion channel receptors appeared early in evolution, while G protein-coupled P1 and P2Y receptors were introduced either at the same time or perhaps even later The absence of gene coding for P2X receptors in some animal groups [eg in some insects, roundworms (Caenorhabditis elegans) and the plant Arabidopsis] in contrast to the potent pharmacological actions of nucleotides in the same species, suggests that novel receptors are still to be discovered

Evolutionary origins of the purinergic signalling system

'Vestibular compensation': neural plasticity and its relations to functional recovery after labyrinthine lesions in frogs and other vertebrates.

Activation of the efferent system in the isolated frog labyrinth: Effects on the afferent EPSPs and spike discharge recorded from single fibers of the posterior nerve

1. Electrical responses in hair cells located in the peripheral regions and in the central region of the frog crista ampullaris were investigated in thin slice preparations by using the whole-cell configuration of the patch-clamp technique. 2. Hair cells from the peripheral regions exhibited mostly a club-like shape and had an average resting potential of -46 mV, whereas cells from the central region had mostly a cylindrical shape and a more negative resting potential (-57 mV). 3. Voltage-clamp recordings revealed that ionic conductances differed in the two epithelial regions. Cells from the peripheral regions exhibited a transient K+ current of A-type (IA) in conjunction with a slow rectifier outward K+ current (IK). Cells from the central region showed little or no IA and generated an IK together with an inward rectifier K+ current (IIR). In both regions, hair cells showed a rapidly activating Ca(2+)-dependent outward K+ current (IK(Ca)) that rapidly inactivated to reach a steady-state level during 150-ms test pulses. 4. IA activated close to -60 mV and was inhibited by 12 mM 4-aminopyridine (4-AP). The time course of this current showed time to peak values of 3-4 ms at 0 mV. Inactivation was fast and almost voltage-independent. The decay time constant was approximately 35 ms at 0 mV. 5. IK was recruited close to -60 mV and activated slowly, reaching peak values in approximately 100 ms at 0 mV. It showed no evidence of inactivation during 150-ms test pulses and it was insensitive to 4-AP. 6. IIR activated at membrane potentials more negative than -90 mV and was blocked by exposure to 6 mM Cs+ or to a K(+)-free medium. This current showed an outward relaxation at potentials more negative than -140 mV, an effect that disappeared after exposure to a Na(+)-free medium. 7. IK(Ca) was recruited close to -40 mV and was inhibited by exposure to a Ca(2+)-free external medium or to 0.5 mM Cd2+. The time to peak of this current was approximately 3 ms at 0 mV and inactivation was very fast and almost independent from the membrane potential. The decay time constant was approximately 4 ms at 0 mV. 8. IK and IA were prominent in hair cells from the peripheral regions, whereas IK accounted for most of the membrane conductance in cells from the central region. The contribution of IK(Ca) was comparable in cells from both epithelial regions.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential expression of potassium currents by hair cells in thin slices of frog crista ampullaris

The effect of glutamate on the frog semicircular canal

Pre- and postsynaptic excitatory action of glutamate agonists on frog vestibular receptors

Some properties of Ca2+ currents in hair cells isolated from frog semicircular canals by enzymatic or mechanical treatment were studied by using the whole-cell configuration of the patch-clamp technique After blocking the large outward K+ currents by substituting Cs+ for K+ and adding tetraethylammonium to the pipette filling solution, voltage- and time-dependent inward currents were clearly detectable in the presence of 4 mM Ca2+ in the extracellular solution Ca2+ current was recruited at test potentials more positive than -60 mV, showed a rapid activation, and exhibited no inactivation during 150-ms depolarizing pulses The maximal amplitude was attained at about -20 mV, with an average value of about 80 pA When Ca2+ in the extracellular solution was replaced with Ba2+, the magnitude of inward currents increased about twofold Ba2+ currents were blocked more effectively by Cd2+ than by Ni2+, were suppressed by 05 microM omega-conotoxin, and were virtually unaffected by amiloride The dihydropyridine Bay K 8644 caused a marked voltage-dependent increase in inward currents The present data suggest that hair cells from frog crista ampullaris are endowed with a homogeneous population of Ca2+ channels having several properties similar to those described for neuronal L channels Since these channels are recruited in a range of potentials close to the resting level, it is suggested that they subserve the control of both resting and evoked transmitter release from the basal pole of the hair cells

http://www.jvr-web.org/Download/Volume_02/Number_1/v02_n1_a3.pdf

Ivo Prigioni

Papers

Activation of the efferent system in the isolated frog labyrinth: Effects on the afferent EPSPs and spike discharge recorded from single fibers of the posterior nerve

Differential expression of potassium currents by hair cells in thin slices of frog crista ampullaris

The effect of glutamate on the frog semicircular canal

Pre- and postsynaptic excitatory action of glutamate agonists on frog vestibular receptors

Calcium currents in solitary hair cells isolated from frog crista ampullaris.