Mechanisms of hair cell tuning

Neuronal proteins custom designed by alternative splicing

The evolution of alternative splicing in eukaryotes greatly expanded the number of functionally distinct proteins that could be produced from a finite gene pool. Extensive in the brains of higher organisms, alternative splicing might be the primary mechanism for generating the spectrum of protein activities that support complex brain functions. Alternative splicing is controlled at the level of individual neurons to custom design proteins for optimal performance. The expression profiles of splice isoforms are modified during development and as neuronal activity changes. Alternative splicing can lead to incremental, long lasting changes in ion channel and receptor activities, independent of changes in gene transcription. Recent studies of tissue-specific splicing factors are revealing how coordinated alterations in alternative splicing of RNA transcripts control synaptic function.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

Neural Sensitivity to Interaural Envelope Delays in the Inferior Colliculus of the Guinea Pig

Interaural time differences (ITDs) are important cues for mammalian sound localization. At high frequencies, sensitivity to ITDs, which are conveyed only by the envelope of the waveforms, has been shown to be poorer than sensitivity to ITDs at low frequencies, which are conveyed primarily by the fine structure of the waveforms. Recently, human psychophysical experiments have demonstrated that sensitivity to envelope-based ITDs in high-frequency transposed tones can be equivalent to low-frequency fine-structure–based ITD sensitivity. Transposed tones are designed to provide high-frequency auditory nerve fibers (ANFs) with similar temporal information to that provided by low-frequency tones. We investigated neural sensitivity to ITDs in high-frequency transposed and sinusoidally amplitude modulated (SAM) tones, in the inferior colliculus of the guinea pig. Neural sensitivity to ITDs in transposed tones was found to be greater than that to ITDs in SAM tones; in response to transposed tones, neural firing rates were more modulated as a function of ITD and discrimination thresholds were found to be lower than those in response to SAM tones. Similar to psychophysical findings, ITD discrimination of single neurons in response to transposed tones for rates of modulation <250 Hz was comparable to neural discrimination of ITDs in low-frequency tones. This suggests that the neural mechanisms that mediate sensitivity to ITDs at high and low frequencies are functionally equivalent, provided that the stimuli result in appropriate temporal patterns of action potentials in ANFs.

Extrasynaptic localization of inactivating calcium-activated potassium channels in mouse inner hair cells

Auditory hair cells from nonmammalian vertebrates are electrically tuned to specific sound frequencies primarily by the interactions of voltage-gated calcium channels and calcium-activated potassium (BK) channels colocalized at synaptic active zones. Mammalian inner hair cells are not electrically tuned and, yet, BK channels are also thought to reside at active zones. Using patch-clamp recordings and immunofluorescence, we characterized BK channel expression in mouse inner hair cells. Unexpectedly, these channels have inactivating currents and are clustered near the apex of the cell away from synaptic sites near the base. These results indicate a novel function of BK channels in mammalian inner hair cells and provide a framework for future research.

Regulation of K+ flow by a ring of negative charges in the outer pore of BKCa channels. Part II: Neutralization of aspartate 292 reduces long channel openings and gating current slow component

Neutralization of the aspartate near the selectivity filter in the GYGD pore sequence (D292N) of the voltage- and Ca²⁺-activated K⁺ channel (MaxiK, BKCa) does not prevent conduction like the corresponding mutation in Shaker channel, but profoundly affects major biophysical properties of the channel (Haug, T., D. Sigg, S. Ciani, L. Toro, E. Stefani, and R. Olcese. 2004. J. Gen. Physiol. 124:173–184). Upon depolarizations, the D292N mutant elicited mostly gating current, followed by small or no ionic current, at voltages where the wild-type hSlo channel displayed robust ionic current. In fact, while the voltage dependence of the gating current was not significantly affected by the mutation, the overall activation curve was shifted by ∼20 mV toward more depolarized potentials. Several lines of evidence suggest that the mutation prevents population of certain open states that in the wild type lead to high open probability. The activation curves of WT and D292N can both be fitted to the sum of two Boltzmann distributions with identical slope factors and half activation potentials, just by changing their relative amplitudes. The steeper and more negative component of the activation curve was drastically reduced by the D292N mutation (from 0.65 to 0.30), suggesting that the population of open states that occurs early in the activation pathway is reduced. Furthermore, the slow component of the gating current, which has been suggested to reflect transitions from closed to open states, was greatly reduced in D292N channels. The D292N mutation also affected the limiting open probability: at 0 mV, the limiting open probability dropped from ∼0.5 for the wild-type channel to 0.06 in D292N (in 1 mM [Ca²⁺]_i). In addition to these effects on gating charge and open probability, as already described in Part I, the D292N mutation introduces a ∼40% reduction of outward single channel conductance, as well as a strong outward rectification.

Localization and developmental expression of BK channels in mammalian cochlear hair cells

The expression of Slo channels (alpha subunits of BK channels) was investigated in the developing mouse cochlea using a polyclonal antibody against the C-terminal part of the protein (residues 1098-1196). The first BK channel immunoreactivity was observed in the cochlea at E18, where it was localized within the cytoplasm of cells lining the area of the organ of Corti and the spiral ganglion. There was an increase of immunoreactivity in all cells bordering the scala media (supporting and hair cells of the organ of Corti, the stria vascularis and the Reissner's membrane) in the following stages (postnatal day [P] 0 and P6). From P12 to adult, a strong membranous labeling, increasing with age, appeared in inner hair cells. The distribution of BK channels was mainly observed as dense elongated plaques localized in the lateral membrane below the cuticular plate. In addition, a more discrete immunolabeling for BK channels, as punctuated dots, was observed in the synaptic area of inner hair cells. This dual localization of BK channels within inner hair cells was confirmed by a different technique using a fluorescently labeled high-affinity ligand of these channels: IbTX-D19C-Alexa488. We demonstrated under patch clamp experiments that this fluorescent toxin conserved its native property, i.e. to reversibly inhibit BK currents in isolated inner hair cells. The fluorescent toxin, both in living or fixed tissues, also showed a preferential binding to mature inner hair cells with a similar subcellular distribution described above using immunocytochemical technique. Overall, our present results confirm the appearance of membranous BK channels around P12 in mouse inner hair cells, an age at which the auditory system becomes functional. The expression of BK channels in mature inner hair cells, near the site of mechanical-transduction, might serve to limit receptor potential attenuation due to the space constant, and thus permitting these sensory cells to function as fast and sensitive transducers.

Identification of Chicken Transmembrane Channel-like (TMC) genes: Expression analysis in the cochlea

Mutations of the human gene encoding transmembrane channel-like protein (TMC)1 cause dominant and recessive nonsyndromic hearing disorders, suggesting that this protein plays an important role in the inner ear. In this study, we cloned chicken Tmc2 (GgTmc2) from a cochlear cDNA library and we annotated four additional TMC family members: GgTmc1, GgTmc3, GgTmc6, and GgTmc7. All chicken TMCs possess the defining TMC signature motif and display high conservation of their genomic structure when compared with other vertebrate TMC genes. GgTmc1 is localized on the chicken sex chromosome Z at a locus that displays conserved synteny with the loci of mammalian orthologues residing on autosomes. In contrast, the locus of GgTmc2 does not exhibit conserved synteny with its mammalian orthologues. Because murine TMC1 and TMC2 are restrictively expressed in cochlear hair cells, we determined the expression of the chicken orthologues in the basilar papilla, the avian equivalent of the organ of Corti. While GgTmc2 was present throughout the basilar papilla and in other tissues, GgTmc1 transcript was detected specifically in the basal portion of the basilar papilla and was not detectable in any other tissue or organ studied. GgTmc3 and GgTmc6 were detectable in all organs analyzed. Antibody labeling revealed that GgTmc2 is predominantly associated with the lateral membranes of hair and supporting cells. The expression of GgTmc2 by both cell types was further confirmed by RT-PCR using isolated cells. This expression and subcellular localization of GgTmc2 is in agreement with the proposed potential role of this novel class of transmembrane proteins in ion transport.

Spontaneous low-frequency voltage oscillations in frog saccular hair cells

Spontaneous membrane voltage oscillations were found in 27 of 130 isolated frog saccular hair cells. Voltage oscillations had a mean peak-to-peak amplitude of 23 mV and a mean oscillatory frequency of 4.6 Hz. When compared with non-oscillatory cells, oscillatory cells had significantly greater hyperpolarization-activated and lower depolarization-activated current densities. Two components, the hyperpolarization-activated cation current, I(h), and the K(+)-selective inward-rectifier current, I(K1), contributed to the hyperpolarization-activated current, as assessed by the use of the I(K1)-selective inhibitor Ba(2+) and the I(h)-selective inhibitor ZD-7288. Five depolarization-activated currents were present in these cells (transient I(BK), sustained I(BK), I(DRK), I(A), and I(Ca)), and all were found to have significantly lower densities in oscillatory cells than in non-oscillatory cells (revealed by using TEA to block I(BK), 4-AP to block I(DRK), and prepulses at different voltages to isolate I(A)). Bath application of either Ba(2+) or ZD-7288 suppressed spontaneous voltage oscillations, indicating that I(h) and I(K1) are required for generating this activity. On the contrary, TEA or Cd(2+) did not inhibit this activity, suggesting that I(BK) and I(Ca) do not contribute. A mathematical model has been developed to test the interpretation derived from the pharmacological and biophysical data. This model indicates that spontaneous voltage oscillations can be generated when the electrophysiological features of oscillatory cells are used. The oscillatory behaviour is principally driven by the activity of I(K1) and I(h), with I(A) playing a modulatory role. In addition, the model indicates that the high densities of depolarization-activated currents expressed by non-oscillatory cells help to stabilize the resting membrane potential, thus preventing the spontaneous oscillations.

Genetic Models of Mechanotransduction: The NematodeCaenorhabditis elegans

Mechanotransduction, the conversion of a mechanical stimulus into a biological response, constitutes the basis for a plethora of fundamental biological processes such as the senses of touch, balance, and hearing and contributes critically to development and homeostasis in all organisms. Despite this profound importance in biology, we know remarkably little about how mechanical input forces delivered to a cell are interpreted to an extensive repertoire of output physiological responses. Recent, elegant genetic and electrophysiological studies have shown that specialized macromolecular complexes, encompassing mechanically gated ion channels, play a central role in the transformation of mechanical forces into a cellular signal, which takes place in mechanosensory organs of diverse organisms. These complexes are highly efficient sensors, closely entangled with their surrounding environment. Such association appears essential for proper channel gating and provides proximity of the mechanosensory apparatus to the source of triggering mechanical energy. Genetic and molecular evidence collected in model organisms such as the nematode worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse highlight two distinct classes of mechanically gated ion channels: the degenerin (DEG)/epithelial Na+channel (ENaC) family and the transient receptor potential (TRP) family of ion channels. In addition to the core channel proteins, several other potentially interacting molecules have in some cases been identified, which are likely parts of the mechanotransducing apparatus. Based on cumulative data, a model of the sensory mechanotransducer has emerged that encompasses our current understanding of the process and fulfills the structural requirements dictated by its dedicated function. It remains to be seen how general this model is and whether it will withstand the impiteous test of time.

Magnesium treatment for sudden hearing loss

Magnesium treatment has been repeatedly shown to reduce the incidence of both temporary and permanent noise-induced hearing loss. We hypothesized that it might also improve the permanent threshold shift in patients with acute-onset hearing loss. In a prospective, randomized, double-blind, placebo-controlled trial, 28 patients with idiopathic sudden sensorineural hearing loss were treated with either steroids and oral magnesium (study group) or steroids and a placebo (control group). Compared to the controls, the magnesium-treated group had a significantly higher proportion of patients with improved hearing (>10 dB hearing level) across all frequencies tested, and a significantly greater mean improvement in all frequencies. Analysis of the individual data confirmed that more patients treated with magnesium experienced hearing improvement, and at a larger magnitude, than control subjects. Magnesium is a relatively safe and convenient adjunct to steroid treatment for enhancing the improvement in hearing, especially in the low-tone range, in patients with sudden sensorineural hearing loss.

Deletion of the Ca2+-activated potassium (BK) alpha-subunit but not the BKbeta1-subunit leads to progressive hearing loss

The large conductance voltage- and Ca2+-activated potassium (BK) channel has been suggested to play an important role in the signal transduction process of cochlear inner hair cells. BK channels have been shown to be composed of the pore-forming alpha-subunit coexpressed with the auxiliary beta1-subunit. Analyzing the hearing function and cochlear phenotype of BK channel alpha-(BKalpha-/-) and beta1-subunit (BKbeta1-/-) knockout mice, we demonstrate normal hearing function and cochlear structure of BKbeta1-/- mice. During the first 4 postnatal weeks also, BKalpha-/- mice most surprisingly did not show any obvious hearing deficits. High-frequency hearing loss developed in BKalpha-/- mice only from approximately 8 weeks postnatally onward and was accompanied by a lack of distortion product otoacoustic emissions, suggesting outer hair cell (OHC) dysfunction. Hearing loss was linked to a loss of the KCNQ4 potassium channel in membranes of OHCs in the basal and midbasal cochlear turn, preceding hair cell degeneration and leading to a similar phenotype as elicited by pharmacologic blockade of KCNQ4 channels. Although the actual link between BK gene deletion, loss of KCNQ4 in OHCs, and OHC degeneration requires further investigation, data already suggest human BK-coding slo1 gene mutation as a susceptibility factor for progressive deafness, similar to KCNQ4 potassium channel mutations.

Hair-cell versus afferent adaptation in the semicircular canals

The time course and extent of adaptation in semicircular canal hair cells was compared to adaptation in primary afferent neurons for physiological stimuli in vivo to study the origins of the neural code transmitted to the brain. The oyster toadfish, Opsanus tau, was used as the experimental model. Afferent firing-rate adaptation followed a double-exponential time course in response to step cupula displacements. The dominant adaptation time constant varied considerably among afferent fibers and spanned six orders of magnitude for the population ( approximately 1 ms to >1,000 s). For sinusoidal stimuli (0.1-20 Hz), the rapidly adapting afferents exhibited a 90 degrees phase lead and frequency-dependent gain, whereas slowly adapting afferents exhibited a flat gain and no phase lead. Hair-cell voltage and current modulations were similar to the slowly adapting afferents and exhibited a relatively flat gain with very little phase lead over the physiological bandwidth and dynamic range tested. Semicircular canal microphonics also showed responses consistent with the slowly adapting subset of afferents and with hair cells. The relatively broad diversity of afferent adaptation time constants and frequency-dependent discharge modulations relative to hair-cell voltage implicate a subsequent site of adaptation that plays a major role in further shaping the temporal characteristics of semicircular canal afferent neural signals.

Development of Transient Outward Currents Coupled With Ca2+-Induced Ca2+Release Mediates Oscillatory Membrane Potential in Ascidian Muscle Cells

Isolated ascidian Halocynthia roretzi blastomeres of the muscle lineage exhibit muscle cell-like excitability on differentiation despite the arrest of cell cleavage early in development. This characteristic provides a unique opportunity to track changes in ion channel expression during muscle cell differentiation. Here, we show that the intrinsic membrane property of ascidian cleavage-arrested muscle-type cells becomes oscillatory by expressing transient outward currents ( Ito) activated by Ca2+-induced Ca2+release (CICR) in a maturation-dependent manner. In current-clamp mode, most day 4 (72 h after fertilization) cleavage-arrested muscle cells exhibited an oscillatory membrane potential of –20 mV at 15 Hz, whereas most day 3 (48 h after fertilization) cells exhibited a spiking pattern. In voltage-clamp mode, the day 4 cells exhibited prominent transient outward currents that were not present in day 3 cells. Itowas abolished by the application of 10 mM caffeine, implying that CICR was involved in Itoactivation. Itowas based on K+efflux and sensitive to tetraethylammonium and some Ca2+-activated K+channel inhibitors. We found a 60-pS single channel conductance that was activated by local Ca2+release in ascidian muscle cell. Voltage-clamp recording with an oscillatory waveform as a command pulse showed that CICR-activated K+currents were activated during the falling phase of the membrane potential oscillation. These results suggest that developmental expression of CICR-activated K+current plays a role in the maturation of larval locomotion by modifying the intrinsic membrane excitability of muscle cells.

Ca(2+) and K(+) (BK) channels in chick hair cells are clustered and colocalized with apical-basal and tonotopic gradients

Electrical resonance is a mechanism used by birds and many vertebrates to discriminate between frequencies of sound, and occurs when the intrinsic oscillation in the membrane potential of a specific hair cell corresponds to a specific stimulus sound frequency. This intrinsic oscillation results from an interplay between an inward Ca(2+) current and the resultant activation of a hyperpolarizing Ca(2+)-activated K(+) current. These channels are predicted to lie in close proximity owing to the fast oscillation in membrane potential. The interplay of these channels is widespread in the nervous system, where they perform numerous roles including the control of synaptic release, burst frequency and circadian rhythm generation. Here, we used confocal microscopy to show that these two ion channels are clustered and colocalized in the chick hair cell membrane. The majority of Ca(2+) channels were colocalized while the proportion of colocalized BK channels was markedly less. In addition, we report both an apical-basal gradient of these clusters in individual hair cells, as well as a gradient in the number of clusters between hair cells along the tonotopic axis. These results give physical confirmation of previous predictions. Since the proportion of colocalized channels was a constant function of Ca(2+) channels, and not of BK channels, these results suggest that their colocalization is determined by the former. The molecular mechanisms underpinning their clustering and colocalization are likely to be common to other neuronal cells.

Two Distinct Types of Noisy Oscillators in Electroreceptors of Paddlefish

Our computational analyses and experiments demonstrate that ampullary electroreceptors in paddlefish ( Polyodon spathula) contain 2 distinct types of continuously active noisy oscillators. The spontaneous firing of afferents reflects both rhythms, and as a result is stochastically biperiodic (quasiperiodic). The first type of oscillator resides in the sensory epithelia, is recorded as approximately 26 Hz and ±70 μV voltage fluctuations at the canal skin pores, and gives rise to a noisy peak at fe≈ 26 Hz in power spectra of spontaneous afferent firing. The second type of oscillator resides in afferent terminals, is seen as a noisy peak at fa≈ 30–70 Hz that dominates the power spectra of spontaneous afferent firing, and corresponds to the mean spontaneous firing rate. Sideband peaks at frequencies of fa± feare consistent with epithelia-to-afferent unidirectional synaptic coupling or, alternatively, nonlinear mixing of the 2 oscillatory processes. External stimulation affects the frequency of only the afferent oscillator, not the epithelial oscillators. Application of temperature gradients localized the feand faoscillators to different depths below the skin. Having 2 distinct types of internal oscillators is a novel form of organization for peripheral sensory receptors, of relevance for other hair cell sensory receptors.

Neural processing of amplitude-modulated sounds

Amplitude modulation (AM) is a temporal feature of most natural acoustic signals. A long psychophysical tradition has shown that AM is important in a variety of perceptual tasks, over a range of time scales. Technical possibilities in stimulus synthesis have reinvigorated this field and brought the modulation dimension back into focus. We address the question whether specialized neural mechanisms exist to extract AM information, and thus whether consideration of the modulation domain is essential in understanding the neural architecture of the auditory system. The available evidence suggests that this is the case. Peripheral neural structures not only transmit envelope information in the form of neural activity synchronized to the modulation waveform but are often tuned so that they only respond over a limited range of modulation frequencies. Ascending the auditory neuraxis, AM tuning persists but increasingly takes the form of tuning in average firing rate, rather than synchronization, to modulation frequency. There is a decrease in the highest modulation frequencies that influence the neural response, either in average rate or synchronization, as one records at higher and higher levels along the neuraxis. In parallel, there is an increasing tolerance of modulation tuning for other stimulus parameters such as sound pressure level, modulation depth, and type of carrier. At several anatomical levels, consideration of modulation response properties assists the prediction of neural responses to complex natural stimuli. Finally, some evidence exists for a topographic ordering of neurons according to modulation tuning. The picture that emerges is that temporal modulations are a critical stimulus attribute that assists us in the detection, discrimination, identification, parsing, and localization of acoustic sources and that this wide-ranging role is reflected in dedicated physiological properties at different anatomical levels.

Quantitative analysis of stereociliary arrays on vestibular hair cells

We have developed a method for quantifying the number, spacing, and distribution of stereocilia on the apical surface of hair cells using spatial autocorrelation analysis and statistics for directional data. Here, we illustrate the method using idealized hair bundles, and we apply it to scanning micrographs of turtle hair cells from the utricle and posterior canal, and to freeze-fracture preparations of bullfrog saccule. The analysis suggests three common features of stereociliary bundles. First, bundle geometries form a continuum from 'loose' to 'tight' rather than two distinct groups. Second, interciliary spacing along the three hexagon axes is not equal; spacing is usually widest along the hexagon axis closest to the bundle's axis of bilateral symmetry (the presumptive excitatory axis). Third, spacing between stereocilia changes with distance from the kinocilium. All three features will influence predictions of the tip link tensions that accompany bundle deflection.

Afferent synaptic transmission in a hair cell organ: pharmacological and physiological analysis of the role of the extended refractory period

One feature of neuronal discharge proposed to play a role in coding temporal information is the relative refractory period that follows each action potential. In neurons innervating hair cells, there is an extended refractory period that can last </=100 ms. We have taken a pharmacological approach to examine the extended refractory period in the Xenopus lateral line organ. We show that each action potential in the afferent fiber, whether generated spontaneously or through an antidromic electrical pulse, decreases the probability of subsequent afferent discharge for a period of </=100 ms. We show that the extended refractory period can be modulated with drugs that alter glutamatergic transmission between the hair cell and the afferent fiber. The extended refractory period can be enhanced by perfusion with agents that reduce synaptic activity. These agents include blockers of voltage-dependent transmitter release, such as cobalt, as well as glutamate receptor antagonists, such as CNQX and kynurenic acid. Conversely, perfusion with agents that increase synaptic activity through activation of the glutamate receptors, such as AMPA or kainate, reduces the magnitude of suppression during the extended refractory period. The extended refractory period is greatly reduced by iberiotoxin and tetraethylammonium (TEA), indicating it may be mediated in large part by a calcium-dependent potassium channel. The ability to modulate the extended refractory period with changes in synaptic input suggests a simple, dynamic mechanism by which strong input (i.e., large or frequent excitatory postsynaptic potentials) can be strengthened and weak inputs weakened.

Organ of Corti kinematics

The internal workings of the organ of Corti and their relation to basilar membrane motion are examined with the aid of a simple kinematic model. It is shown that, due to the lever system embodied in the organ of Corti, there is a significant transformer gain between basilar membrane and cilia displacements. While this transformation is nonlinear, linear response prevails in the narrow physiologically relevant operating range of the ciliary transducer. The model also simulates cilia deflection when the mechanical stimulus is the length change of outer hair cells.

Splice variants: a homology modeling approach

Splice variants play an important role within the cell in both increasing the proteome diversity and in cellular function. Splice variants are also associated with disease states and may play a role in their etiology. Information about splice variants has, until now, mostly been derived from the primary transcript or through cellular studies. In this study information from the transcript and other studies is combined with tertiary structure information derived from homology models. Through this method we have determined that it is possible to effectively model splice variants. Forty models of splice variants for fourteen proteins were produced. Analysis of the models shows that deletions produce superior model validation values. Additions to sequences where there is little homology become increasingly difficult to model with increasing sequence length. Many of the splicing events are associated with post-translational modification either in the N-terminal region by changing the signal peptide or by affecting the number or availability of glycosylation sites. Often the alternative exon combinations are associated with loss or gain of whole structural units, as opposed to just changing small loop regions. Losing part of the secondary structure may destabilize neighboring parts of the same secondary structure. Detailed analysis is given of four biomedically relevant proteins (Beta-site Amyloid Precursor Protein Cleaving enzyme (BACE), Interleukin-4, Frataxin and Hereditary hemochromatosis protein) and their associated splice variant models. The visualization of these possible structures provides new insights about their functionality and the possible etiology of associated diseases.

Sensation is painless

Emily Dickinson declared: 'After great pain, a formal feeling comes'. This formal feeling begins when sensory neurons are activated by noxious stimuli, such as stepping on a tack. Recently, Seymour Benzer's group identified sensory neurons in Drosophila larvae that mediate aversive responses to noxious heat and mechanical stimuli. Thresholds for behavioral and nerve responses are elevated by mutations in the painless gene, which encodes a TRP ion channel protein. Painless thus joins an elite group of TRPs implicated in sensory transduction in insects, nematodes, mammals and fish.

Voltage-gated outward K currents in frog saccular hair cells

A biophysical analysis of the voltage-gated K (Kv) currents of frog saccular hair cells enzymatically isolated with bacterial protease VIII was carried out, and their contribution to the cell electrical response was addressed by a modeling approach. Based on steady-state and kinetic properties of inactivation, two distinct Kv currents were found: a fast inactivating IA and a delayed rectifier IDRK. IA exhibited a strongly hyperpolarized inactivation V(1/2) (-83 mV), a relatively rapid single exponential recovery from inactivation (taurec of approximately 100 ms at -100 mV), and fast activation and deactivation kinetics. IDRK showed instead a less-hyperpolarized inactivation V(1/2) (-48 mV), a slower, double-exponential recovery from inactivation (taurec1 approximately 490 ms and taurec2 approximately 4,960 ms at -100 mV), and slower activation and deactivation kinetics. Steady-state activation gave a V(1/2) and a k of -46.2 and 8.2 mV for IA and -48.3 and 4.2 mV for IDRK. Both currents were not appreciably blocked by bath application of 10 mM TEA, but were inhibited by 4-AP, with IDRK displaying a higher sensitivity. IDRK also showed a relatively low affinity to linopirdine, being half blocked at approximately 50 microM. Steady-state and kinetic properties of IDRK and IA were described by 2nd- and 3rd-order Hodgkin-Huxley models, respectively. The goodness of our quantitative description of the Kv currents was validated by including IA and IDRK in a theoretical model of saccular hair cell electrical activity and by comparing the simulated responses with those obtained experimentally. This thorough description of the IDRK and IA will contribute toward understanding the role of these currents in the electrical response on this preparation.

Alternative splicing of N- and C-termini of a C. elegans ClC channel alters gating and sensitivity to external Cl- and H+

CLH-3 is a meiotic cell cycle-regulated ClC Cl- channel that is functionally expressed in oocytes of the nematode Caenorhabditis elegans. CLH-3a and CLH-3b are alternatively spliced variants that have identical intramembrane regions, but which exhibit striking differences in their N- and C-termini. Structural and functional studies indicate that N- and C-terminal domains modulate ClC channel activity. We therefore postulated that alternative splicing of CLH-3 would alter channel gating and physiological functions. To begin testing this hypothesis, we characterized the biophysical properties of CLH-3a and CLH-3b expressed heterologously in HEK293 cells. CLH-3a activates more slowly and requires stronger hyperpolarization for activation than CLH-3b. Depolarizing conditioning voltages dramatically increase CLH-3a current amplitude and induce a slow inactivation process at hyperpolarized voltages, but have no significant effect on CLH-3b activity. CLH-3a also differs significantly in its extracellular Cl- and pH sensitivity compared to CLH-3b. Immunofluorescence microscopy demonstrated that CLH-3b is translationally expressed during all stages of oocyte development, and furthermore, the biophysical properties of the native oocyte Cl- current are indistinguishable from those of heterologously expressed CLH-3b. We conclude that CLH-3b carries the oocyte Cl- current and that the channel probably functions in nonexcitable cells to depolarize membrane potential and/or mediate net Cl- transport. The unique voltage-dependent properties of CLH-3a suggest that the channel may function in muscle cells and neurones to regulate membrane excitability. We suggest that alternative splicing of CLH-3 N- and C-termini modifies the functional properties of the channel by altering the accessibility and/or function of pore-associated ion-binding sites.

Beta1 subunits facilitate gating of BK channels by acting through the Ca2+, but not the Mg2+, activating mechanisms

The beta1 subunit of BK (large conductance Ca2+ and voltage-activated K+) channels is essential for many key physiological processes, such as controlling the contraction of smooth muscle and the tuning of hair cells in the cochlea. Although it is known that the beta1 subunit greatly increases the open probability of BK channels, little is known about its mechanism of action. We now explore this mechanism by using channels in which the Ca2+- and Mg2+-dependent activating mechanisms have been disrupted by mutating three sites to remove the Ca2+ and Mg2+ sensitivity. We find that the presence of the beta1 subunit partially restores Ca2+ sensitivity to the triply mutated channels, but not the Mg2+ sensitivity. We also find that the beta1 subunit has no effect on the Mg2+ sensitivity of WT BK channels, in contrast to its pronounced effect of increasing the apparent Ca2+ sensitivity. These observations suggest that the beta1 subunit increases open probability by working through the Ca2+-dependent, rather than Mg2+-dependent, activating mechanisms, and that the action of the beta1 subunit is not directly on the Ca2+ binding sites, but on the allosteric machinery coupling the sites to the gate. The differential effects of the beta1 subunit on the Ca2+ and Mg2+ activation of the channel suggest that these processes act separately. Finally, we show that Mgi2+ inhibits, rather than activates, BK channels in the presence of the beta1 subunit for intermediate levels of Cai2+. This Mg2+ inhibition in the presence of the beta1 subunit provides an additional regulatory mechanism of BK channel activity.

The afferent synapse of cochlear hair cells

Mechanosensory hair cells of the cochlea must serve as both transducers and presynaptic terminals, precisely releasing neurotransmitter to encode acoustic signals for the postsynaptic afferent neuron. Remarkably, each inner hair cell serves as the sole input for 10-30 individual afferent neurons, which requires extraordinary precision and reliability from the synaptic ribbons that marshal vesicular release onto each afferent. Recent studies of hair cell membrane capacitance and postsynaptic currents suggest that the synaptic ribbon may operate by simultaneous multi-vesicular release. This mechanism could serve to ensure the accurate timing of transmission, and further challenges our understanding of this synaptic nano-machine.

A ring of eight conserved negatively charged amino acids doubles the conductance of BK channels and prevents inward rectification

Large-conductance Ca2+-voltage-activated K+ channels (BK channels) control many key physiological processes, such as neurotransmitter release and muscle contraction. A signature feature of BK channels is that they have the largest single channel conductance of all K+ channels. Here we examine the mechanism of this large conductance. Comparison of the sequence of BK channels to lower-conductance K+ channels and to a crystallized bacterial K+ channel (MthK) revealed that BK channels have a ring of eight negatively charged glutamate residues at the entrance to the intracellular vestibule. This ring of charge, which is absent in lower-conductance K+ channels, is shown to double the conductance of BK channels for outward currents by increasing the concentration of K+ in the vestibule through an electrostatic mechanism. Removing the ring of charge converts BK channels to inwardly rectifying channels. Thus, a simple electrostatic mechanism contributes to the large conductance of BK channels.

Genetics of sensory mechanotransduction

The molecular mechanisms for the transduction of light and chemical signals in animals are fairly well understood. In contrast, the processes by which the senses of touch, balance, hearing, and proprioception are transduced are still largely unknown. Biochemical approaches to identify transduction components are difficult to use with mechanosensory systems, but genetic approaches are proving more successful. Genetic research in several organisms has demonstrated the importance of cytoskeletal, extracellular, and membrane components for sensory mechanotransduction. In particular, researchers have identified channel proteins in the DEG/ENaC and TRP families that are necessary for signaling in a variety of mechanosensory cells. Proof that these proteins are components of the transduction channel, however, is incomplete.

Contribution of BK Ca2+-activated K+ channels to auditory neurotransmission in the Guinea pig cochlea

Large-conductance calcium-activated potassium (BK) channels are known to play a prominent role in the hair cell function of lower vertebrates where these channels determine electrical tuning and regulation of neurotransmitter release. Very little is known, by contrast, about the role of BK channels in the mammalian cochlea. In the current study, we perfused specific toxins in the guinea pig cochlea to characterize the role of BK channels in cochlear neurotransmission. Intracochlear perfusion of charybdotoxin (ChTX) or iberiotoxin (IbTX) reversibly reduced the compound action potential (CAP) of the auditory nerve within minutes. The cochlear microphonics (CM at f1 = 8 kHz and f2 = 9.68 kHz) and their distortion product (DPCM at 2f1-f2) were essentially not affected, suggesting that the BK specific toxins do not alter the active cochlear amplification at the outer hair cells (OHCs). We also tested the effects of these toxins on the whole cell voltage-dependent membrane current of isolated guinea pig inner hair cells (IHCs). ChTX and IbTX reversibly reduced a fast outward current (activating above -40 mV, peaking at 0 mV with a mean activation time constant tau ranging between 0.5 and 1 ms). A similar block of a fast outward current was also observed with the extracellular application of barium ions, which we believe permeate through Ca2+ channels and block BK channels. In situ hybridization of Slo antisense riboprobes and immunocytochemistry demonstrated a strong expression of BK channels in IHCs and spiral ganglion and to a lesser extent in OHCs. Overall, our results clearly revealed the importance of BK channels in mammalian cochlear neurotransmission and demonstrated that at the presynaptic level, fast BK channels are a significant component of the repolarizing current of IHCs.

Static material properties of the tectorial membrane: a summary

The tectorial membrane (TM) is a polyelectrolyte gel. Hence, its chemical, electrical, mechanical, and osmotic properties are inextricably linked. We review, integrate, and interpret recent findings on these properties in isolated TM preparations. The dimensions of the TM in alligator lizard, chick, and mouse are sensitive to bath ion concentrations of constituents normally present in the cochlear fluids - an increase in calcium concentration shrinks the TM, and an increase in sodium concentration swells the TM in a manner that depends competitively on the calcium concentration. The sodium-induced swelling is specific; it does not occur with other alkali metal cations. We interpret these findings as due to competitive binding of sodium and calcium to TM macromolecules which causes a change in their conformation that leads to a change in mechanical properties. In mouse TM, decreasing the bath pH below 6 or increasing it above 7 results in swelling of the TM. Electric potential measurements are consistent with the notion that the swelling is caused by a pH-driven increase in positive fixed charge at low pH and an increase in the magnitude of the negative fixed charge at high pH which is consistent with the known protonation pattern of TM macromolecules. Increasing the osmotic pressure of the bathing solution with polyethylene glycol shrinks the TM and decreasing the ionic strength of the bathing solution swells the TM. Both results are qualitatively consistent with predictions of a polyelectrolyte gel model of the TM.

Exocytosis at the ribbon synapse of retinal bipolar cells studied in patches of presynaptic membrane

The distribution of exocytic sites and ion channels in the synaptic terminal of retinal bipolar cells was investigated by measuring capacitance and conductance changes in cell-attached patches of presynaptic membrane. Patch depolarization evoked capacitance and conductance increases that were inhibited by blocking Ca(2+) influx or loading the terminal with EGTA. The increase in capacitance declined as the depolarization approached the reversal potential for Ca(2+), indicating that it was a result of Ca(2+)-dependent exocytosis. The conductance increase was caused by K(Ca) channels that were also activated by Ca(2+) influx. Two observations indicated that sites of exocytosis and endocytosis colocalized with clusters of Ca(2+) channels and K(Ca) channels; the initial rate of exocytosis was correlated with the activation of K(Ca) channels, and exocytosis did not occur in the 41% of patches lacking this conductance. Electron microscopy demonstrated that there were approximately 16 vesicles docked to the plasma membrane at each active zone marked by a ribbon, but vesicles were also attached to the rest of the membrane at a density of 1.5/microm(2). The density of ribbons was 0.10 +/- 0.02/microm(2), predicting that approximately 43% of cell-attached patches would lack an active zone. The density of Ca(2+) channel clusters assayed by capacitance and conductance responses was therefore similar to the density of ribbons. These results are consistent with the idea that Ca(2+) channel clusters were colocalized with ribbons but do not exclude the possibility that calcium channels also occurred at other sites. The wide distribution of vesicles docked to the plasma membrane suggests that exocytosis might also be triggered by the spread of Ca(2+) from Ca(2+) channel clusters.

Effects of permeant ion concentrations on the gating of L-type Ca2+ channels in hair cells

We determined the gating and permeation properties of single L-type Ca(2+) channels, using hair cells and varying concentrations (5-70 mM) of the charge carriers Ba(2+) and Ca(2+). The channels showed distinct gating modes with high- and low-open probability. The half-activation voltage (V(1/2)) shifted in the hyperpolarizing direction from high to low permeant ion concentrations consistent with charge screening effects. However, the differences in the slope of the voltage shifts (in VM(-1)) between Ca(2+) (0.23) and Ba(2+) (0.13), suggest that channel-ion interaction may also contribute to the gating of the channel. We examined the effect of mixtures of Ba(2+) and Ca(2+) on the activation curve. In 5 mM Ca(2+), the V(1/2) was, -26.4 +/- 2.0 mV compared to Ba(2+), -34.7 +/- 2.9 mV, as the charge carrier. However, addition of 1 mM Ba(2+) in 4 mM Ca(2+), a molar ratio, which yielded an anomalous-mole fraction effect, was sufficient to shift the V(1/2) to -34.7 +/- 1.5 mV. Although Ca(2+)-dependent inactivation of the L-type channels in hair cells can yield the present findings, we provide evidence that the anomalous gating of the channel may stem from the closed interaction between ion permeation and gating.

Variation in large-conductance, calcium-activated potassium channels from hair cells along the chicken basilar papilla

The mechanism for electrical tuning in non-mammalian hair cells rests within the widely diverse kinetics of functionally distinct, large-conductance potassium channels (BK), thought to result from alternative splicing of the pore-forming alpha subunit and variable co-expression with an accessory beta subunit. Inside-out patches from hair cells along the chicken basilar papilla revealed 'tonotopic' gradations in calcium sensitivity and deactivation kinetics. The resonant frequency for the hair cell from which the patch was taken was estimated from deactivation rates, and this frequency reasonably matched that predicted from the originating cell's tonotopic location. The rates of deactivation for native BK channels were much faster than rates reported for cloned chicken BK channels including both alpha and beta subunits. This result was surprising since patches were pulled from hair cells in the apical half of the papilla where beta subunits are most highly expressed. Heterogeneity in the properties of native chicken BK channels implies a high degree of molecular variation and hinders our ability to identify those molecular constituents.

Seasonal plasticity of peripheral auditory frequency sensitivity

Female midshipman fish (Porichthys notatus) use the auditory sense to detect and locate vocalizing males during the breeding season. Detection of conspecific vocal signals is essential to their reproductive success and can evoke strong phonotactic responses in gravid females but not in spent females that have released all of their eggs. Here, we test the hypothesis that seasonal variation in reproductive state affects the neurophysiological response properties of the peripheral auditory system in female midshipman fish. Iso-intensity responses of eighth nerve afferents from the sacculus, the main auditory end organ of the inner ear, to individual tones were measured for spike rate and vector strength (VS) of synchronization. Most auditory saccular units in reproductive, summer females showed robust temporal encoding up to 340 Hz, whereas nonreproductive winter females showed comparable encoding only up to 100 Hz. The dramatic upward shift in temporal encoding among summer fish was paralleled by increases in best frequency (BF), maximum evoked spike rate at BF, VS values at BF, and the percentage of units that showed significant VS to iso-intensity tones >140 Hz. Reproductive summer females were best suited to encode the higher harmonic components of male advertisement calls. This first demonstration of a natural cyclicity in peripheral auditory frequency sensitivity among vertebrates may represent, in this case, an adaptive plasticity of the female midshipman's auditory system to enhance the acquisition of auditory information needed for mate identification and localization during the breeding season.

Chick hair cells do not exhibit voltage-dependent somatic motility

It is generally believed that mechanical amplification by cochlear hair cells is necessary to enhance the sensitivity and frequency selectivity of hearing. In the mammalian ear, the basis of cochlear amplification is believed to be the voltage-dependent electromotility of outer hair cells (OHCs). The avian basilar papilla contains tall and short hair cells, with the former being comparable to inner hair cells, and the latter comparable to OHCs, based on their innervation patterns. In this study, we sought evidence for somatic electromotility by direct measurements of voltage-dependent length changes in both tall and short hair cells at nanometre resolution. Microchamber and whole-cell voltage-clamp techniques were used. Motility was measured with a photodiode-based measurement system. Non-linear capacitance, an electrical signature of somatic motility, was also measured to complement motility measurement. Significantly, chick hair cells did not exhibit somatic motility nor express non-linear capacitance. The lack of somatic motility suggests that in avian hair cells the active process resides elsewhere, most likely in the hair cell stereocilia.

Expression of Ca2+-activated BK channel mRNA and its splice variants in the rat cochlea

Voltage-activated K(+) channels are important for shaping the receptor potentials of cochlear hair cells. In particular, the functional maturation of inner hair cells in mice around the onset of hearing coincides with the expression of a large, fast K(+) conductance, probably mediated by Ca(2+)-activated K(+) (BK) channels. In hearing organs of lower vertebrates, frequency tuning depends on BK-type K(+) channels with different kinetics. Kinetics are varied by alternative splicing of the channels' alpha subunits and combination with modulating beta subunits. It is unclear whether similar mechanisms "fine tune" mammalian hair cells. We used various polymerase chain reaction (PCR) approaches to screen rat cochleae for splice variants of BK-type alpha subunits. We isolated mainly minimal variants and only occasionally splice variants with additional inserts. We conclude that alpha subunits with different kinetics are not substantially used in the rat cochlea. However, we isolated six variants differing in their extreme C-terminal sequences, which may be involved in the targeting of the channel protein. By using reverse transcriptase-PCR, we demonstrated also the expression of transcripts for several beta subunits. In situ hybridization experiments revealed strict coexpression of alpha with beta1 transcripts. In inner hair cells, strong labeling emerged shortly before the onset of hearing. Labeling of outer hair cells appeared later and generally weaker. Thus, our molecular data confirm electrophysiological results that suggested that BK channels underlie the large K(+) conductance in inner hair cells of mammals. Extensive splicing of BK channel transcripts, however, does not seem to be used in mammalian hair cells as is done in lower vertebrates.

Frog saccular hair cells dissociated with protease VIII exhibit inactivating BK currents, K(V) currents, and low-frequency electrical resonance

Outward K currents and electrical resonance of frog (Rana esculenta) saccular hair cells isolated enzymatically with bacterial protease VIII were investigated using the perforated patch-clamp method. Under voltage-clamp conditions we identified two K currents, a voltage-dependent K (K(V)) current, and a partially inactivating iberiotoxin-sensitive K (BK) current. The K(V) current activated at a membrane potential of approximately -50 mV (from a holding potential of -70 mV). Its activation rate was rather slow, having a time constant in the range 5-8 ms at 0 mV. The K(V) current was resistant to tetraethylammonium (10 mM), but was inhibited by 4-aminopyridine (1 mM). A striking feature of the BK current was its inactivation; this was monoexponential and had fast kinetics (tau(inact)=2.7 ms +/-1.2, at -10 mV; n=8). Inactivation of the current was incomplete, a residual sustained component remaining. This varied considerably among hair cells (mean ratio between peak transient and sustained component was 1.22+/-0.18, range 0.53-1.8; n=8). In current-clamp mode steady depolarizing current pulses evoked membrane potential oscillatory responses, with mean frequencies varying between 30 and 100 Hz for membrane potentials from -60 to -40 mV (n=18). Most hair cells (14/18) exhibited damped oscillations, and in the remainder a few initial damped oscillations were succeeded by smaller, undamped voltage oscillations. The peak quality factor and the characteristic frequency assessed on 14 cells displaying only damped oscillatory responses were 2.4+/-1.3 and 59+/-39 Hz, respectively. In contrast, papain-dissociated frog saccular hair cells possess solely a sustained BK current, and exhibited significantly higher resonant frequencies and quality factors. In conclusion, the K currents and the electrical resonance of hair cells dissociated in protease VIII differ markedly from those dissociated with papain, but are similar to those reported for in situ preparations, suggesting that our dissociation procedure preserves the electrophysiological profile of in situ frog saccular hair cells.

Energy integration describes sound-intensity coding in an insect auditory system

We investigate the transduction of sound stimuli into neural responses and focus on locust auditory receptor cells. As in other mechanosensory model systems, these neurons integrate acoustic inputs over a fairly broad frequency range. To test three alternative hypotheses about the nature of this spectral integration (amplitude, energy, pressure), we perform intracellular recordings while stimulating with superpositions of pure tones. On the basis of online data analysis and automatic feedback to the stimulus generator, we systematically explore regions in stimulus space that lead to the same level of neural activity. Focusing on such iso-firing-rate regions allows for a rigorous quantitative comparison of the electrophysiological data with predictions from the three hypotheses that is independent of nonlinearities induced by the spike dynamics. We find that the dependence of the firing rates of the receptors on the composition of the frequency spectrum can be well described by an energy-integrator model. This result holds at stimulus onset as well as for the steady-state response, including the case in which adaptation effects depend on the stimulus spectrum. Predictions of the model for the responses to bandpass-filtered noise stimuli are verified accurately. Together, our data suggest that the sound-intensity coding of the receptors can be understood as a three-step process, composed of a linear filter, a summation of the energy contributions in the frequency domain, and a firing-rate encoding of the resulting effective sound intensity. These findings set quantitative constraints for future biophysical models.

Functional analysis of whole cell currents from hair cells of the turtle posterior crista

Controlled currents were used to study possible functions of voltage-sensitive, outwardly rectifying conductances. Results were interpreted with linearized Hodgkin-Huxley theory. Because of their more hyperpolarized resting potentials and lower impedances, type I hair cells require larger currents to be depolarized to a given voltage than do type II hair cells. "Fast" type II cells, so-called because of the fast activation of their outward currents, show slightly underdamped responses to current steps with resonant (best) frequencies of 40-85 Hz, well above the bandwidth of natural head movements. Reflecting their slower activation kinetics, type I and "slow" type II cells have best frequencies of 15-30 Hz and are poorly tuned, being critically damped or overdamped. Linearized theory identified the factors responsible for tuning quality. Our fast type II hair cells show only modestly underdamped responses because their steady-state I-V curves are not particularly steep. The even poorer tuning of our type I and slow type II cells can be attributed to their slow activation kinetics and large conductances. To study how ionic currents shape response dynamics, we superimposed sinusoidal currents of 0.1-100 Hz on a small depolarizing steady current intended to simulate resting conditions in vivo. The steady current resulted in a slow inactivation, most pronounced in fast type II cells and least pronounced in type I cells. Because of inactivation, fast type II cells have nearly passive response dynamics with low-frequency gains of 500-1,000 Momega. In contrast, type I and slow type II cells show active components in the vestibular bandwidth and low-frequency gains of 20-100 and 100-500 Momega, respectively. As there are no differences in the responses to sinusoidal currents for fast type II cells from the torus and planum, voltage-sensitive currents are unlikely to be responsible for the large differences in gains and response dynamics of afferents innervating these two regions of the peripheral zone. The low impedances and active components of type I cells may be related to the low gains and modestly phasic response dynamics of calyx-bearing afferents.

Slo1 tail domains, but not the Ca2+ bowl, are required for the beta 1 subunit to increase the apparent Ca2+ sensitivity of BK channels

Functional large-conductance Ca(2+)- and voltage-activated K(+) (BK) channels can be assembled from four alpha subunits (Slo1) alone, or together with four auxiliary beta1 subunits to greatly increase the apparent Ca(2+) sensitivity of the channel. We examined the structural features involved in this modulation with two types of experiments. In the first, the tail domain of the alpha subunit, which includes the RCK2 (regulator of K(+) conductance) domain and Ca(2+) bowl, was replaced with the tail domain of Slo3, a BK-related channel that lacks both a Ca(2+) bowl and high affinity Ca(2+) sensitivity. In the second, the Ca(2+) bowl was disrupted by mutations that greatly reduce the apparent Ca(2+) sensitivity. We found that the beta1 subunit increased the apparent Ca(2+) sensitivity of Slo1 channels, independently of whether the alpha subunits were expressed as separate cores (S0-S8) and tails (S9-S10) or full length, and this increase was still observed after the Ca(2+) bowl was mutated. In contrast, beta1 subunits no longer increased Ca(2+) sensitivity when Slo1 tails were replaced by Slo3 tails. The beta1 subunits were still functionally coupled to channels with Slo3 tails, as DHS-I and 17 beta-estradiol activated these channels in the presence of beta1 subunits, but not in their absence. These findings indicate that the increase in apparent Ca(2+) sensitivity induced by the beta1 subunit does not require either the Ca(2+) bowl or the linker between the RCK1 and RCK2 domains, and that Slo3 tails cannot substitute for Slo1 tails. The beta1 subunit also induced a decrease in voltage sensitivity that occurred with either Slo1 or Slo3 tails. In contrast, the beta1 subunit-induced increase in apparent Ca(2+) sensitivity required Slo1 tails. This suggests that the allosteric activation pathways for these two types of actions of the beta1 subunit may be different.

Regional analysis of whole cell currents from hair cells of the turtle posterior crista

The turtle posterior crista is made up of two hemicristae, each consisting of a central zone containing type I and type II hair cells and a surrounding peripheral zone containing only type II hair cells and extending from the planum semilunatum to the nonsensory torus. Afferents from various regions of a hemicrista differ in their discharge properties. To see if afferent diversity is related to the basolateral currents of the hair cells innervated, we selectively harvested type I and II hair cells from the central zone and type II hair cells from two parts of the peripheral zone, one near the planum and the other near the torus. Voltage-dependent currents were studied with the whole cell, ruptured-patch method and characterized in voltage-clamp mode. We found regional differences in both outwardly and inwardly rectifying voltage-sensitive currents. As in birds and mammals, type I hair cells have a distinctive outwardly rectifying current (I(K,L)), which begins activating at more hyperpolarized voltages than do the outward currents of type II hair cells. Activation of I(K,L) is slow and sigmoidal. Maximal outward conductances are large. Outward currents in type II cells vary in their activation kinetics. Cells with fast kinetics are associated with small conductances and with partial inactivation during 200-ms depolarizing voltage steps. Almost all type II cells in the peripheral zone and many in the central zone have fast kinetics. Some type II cells in the central zone have large outward currents with slow kinetics and little inactivation. Although these currents resemble I(K,L), they can be distinguished from the latter both electrophysiologically and pharmacologically. There are two varieties of inwardly rectifying currents in type II hair cells: activation of I(K1) is rapid and monoexponential, whereas that of I(h) is slow and sigmoidal. Many type II cells either have both inward currents or only have I(K1); very few cells only have I(h). Inward currents are less conspicuous in type I cells. Type II cells near the torus have smaller outwardly rectifying currents and larger inwardly rectifying currents than those near the planum, but the differences are too small to account for variations in discharge properties of bouton afferents innervating the two regions of the peripheral zone. The large outward conductances seen in central cells, by lowering impedances, may contribute to the low rotational gains of some central-zone afferents.

Transgenic and gene targeting studies of hair cell function in mouse inner ear

Despite the rapid discovery of a large number of genes in sensory hair cells of the inner ear, the functional roles of these genes in hair cells remain largely undetermined. Recent advances in transgenic and gene targeting technologies in mice have offered unprecedented opportunities to genetically manipulate the expression of these genes and to study their functional roles in hair cells in vivo. Transgenic analyses have revealed the presence of hair-cell-specific promoters in the genes encoding Math1, myosin VIIa, Pou4f3, and the alpha9 subunit of the acetylcholine receptor (alpha9 AChR). Targeted inactivation using embryonic stem cell technology and transgenic expression studies have revealed the roles of several genes involved in hair cell lineage (Math1), differentiation (Pou4f3), mechanotransduction (Myo1c, and Myo7a), electromotility (Prestin), and efferent modulation (Chrna9, encoding alpha9 AChR). Although many of these genes also play roles in other tissues, inactivation of these genes in hair cells alone will soon be possible by using the Cre-loxP system. Also imminent is the development of genetic methods to inactivate genes specifically in mouse hair cells at a desired time, by using inducible systems established in other types of neurons. Combining these types of manipulation of gene expression will enable hearing researchers to elucidate some of the fundamental and unique features of hair cell function such as mechanotransduction, frequency tuning, active mechanical amplification, and efferent modulation.

Evolution of structure and function of the hearing organ of lizards

Following their origin during the early Cretaceous, the lizards radiated early into a number of families. This radiation was accompanied by a diversification in the structure of the inner ear. The morphology of the auditory basilar papilla is family-specific, with large variations in a number of parameters. At the physiologic level, this wide variation does not result in an equivalent range of physiologic parameters. This review considers the possible influence of various morphologic features on function, and correlates these features with physiologic response parameters. Anatomical variety that does not result in significant changes in the inputs to the brain is "neutral" with regard to selection pressures. This independence apparently removed evolutionary constraints and led to some of the great variety of auditory papillae seen. Other anatomical features are more important and do produce significant effects at the level of the auditory nerve.

Mechanism of magnesium activation of calcium-activated potassium channels

Large-conductance (BK type) Ca(2+)-dependent K(+) channels are essential for modulating muscle contraction and neuronal activities such as synaptic transmission and hearing. BK channels are activated by membrane depolarization and intracellular Ca(2+) and Mg(2+) (refs 6-10). The energy provided by voltage, Ca(2+) and Mg(2+) binding are additive in activating the channel, suggesting that these signals open the activation gate through independent pathways. Here we report a molecular investigation of a Mg(2+)-dependent activation mechanism. Using a combined site-directed mutagenesis and structural analysis, we demonstrate that a structurally new Mg(2+)-binding site in the RCK/Rossman fold domain -- an intracellular structural motif that immediately follows the activation gate S6 helix -- is responsible for Mg(2+)-dependent activation. Mutations that impair or abolish Mg(2+) sensitivity do not affect Ca(2+) sensitivity, and vice versa. These results indicate distinct structural pathways for Mg(2+)- and Ca(2+)-dependent activation and suggest a possible mechanism for the coupling between Mg(2+) binding and channel opening.

Temporal encoding for auditory computation: physiology of primary afferent neurons in sound-producing fish

Many fish rely on sounds for communication, yet the peripheral structures containing the hair cells are simple, without the morphological specializations that facilitate frequency analysis in the mammalian cochlea. Despite this, neurons in the midbrain of sound-producing fish (Pollimyrus) have complex receptive fields, extracting features from courtship sounds. Here we present an analysis of the initial encoding of sounds by the primary afferents and demonstrate that the representation of sound undergoes a substantial transformation as it ascends to the midbrain. Afferents were isolated as they coursed from the sacculus through the medulla. Tones (100 Hz-1.2 kHz) elicited synchronized spikes [vector strength (VS) >0.9] on each stimulus cycle [coefficient of variation (CV) <1.1], with little spike rate adaptation. Most afferents (67%) were spontaneously active and began synchronizing 10 dB below rate threshold. Rate thresholds for the most sensitive afferents (65 dB) were close to behavioral thresholds. The distribution of characteristic frequencies and best sensitivities was matched to the spectrum of sounds of this species and to its audiogram. Three clusters of afferents were identified, one including afferents that generated spike bursts and had v-shaped response areas (bursters), and two others that included entrained afferents with broad response areas (entrained types I and II). All afferents encoded the timing of clicks within click trains with time-locked spikes, and none showed selectivity for interclick intervals. Understanding the computations that yield complex receptive fields is an essential goal for auditory neuroscience, and these data on primary encoding advance this goal, allowing a comparison of inputs with feature-extracting midbrain neurons.

Developmental regulation of neuronal K(Ca) channels by TGFbeta1: an essential role for PI3 kinase signaling and membrane insertion

TGFbeta1 is a target-derived factor responsible for the developmental expression of large-conductance Ca(2+)-activated K(+) (K(Ca)) channels in ciliary neurons of the chick ciliary ganglion. The acute effects of TGFbeta1 on K(Ca) channels are mediated by posttranslational events and require activation of the MAP kinase Erk. Here we show that TGFbeta1 evokes robust phosphorylation of Akt/PKB, a protein kinase dependent on the products of phosphatidylinositol 3-OH kinase (PI3K). TGFbeta1-evoked stimulation of K(Ca) channels is blocked by the PI3K inhibitors wortmannin and LY294002. These drugs also inhibit TGFbeta1 effects on Akt/PKB phosphorylation but have no effect on TGFbeta1-evoked Erk activation. Application of the MEK1 inhibitor PD98059 blocked TGFbeta1 effects on Erk but had no effect on Akt/PKB phosphorylation. These results indicate that PI3K and Erk represent parallel signaling cascades activated by TGFbeta1 in ciliary neurons. The effects of TGFbeta1 on functional expression of K(Ca) are blocked by the microtubule inhibitors colchicine and nocodazole, by botulinum toxins A and E, and by brefeldin-A, an agent that disrupts the Golgi apparatus. These data indicate that translocation of a membrane protein, possibly Slowpoke (SLO), is required for the acute posttranslational effects of TGFbeta1 on K(Ca) channels. Confocal immunofluorescence studies with three different SLO antisera showed robust expression of SLO in multiple intracellular compartments of embryonic day 9-13 ciliary neurons, including the cell nucleus. These data suggest that TGFbeta1 evokes insertion of SLO channels into the plasma membrane as a result of signaling cascades that entail activation of Erk and PI3K.

New disguises for an old channel: MaxiK channel beta-subunits

Ca(2+)-activated K(+) channels of large conductance (MaxiK or BK channels) control a large variety of physiological processes, including smooth muscle tone, neurosecretion, and hearing. Despite being coded by a single gene (Slowpoke), the diversity of MaxiK channels is great. Regulatory b-subunits, splicing, and metabolic regulation create this diversity fundamental to the adequate function of many tissues.