L-type and R-type Ca(2+) currents were detected in frog semicircular canal hair cells. The former was noninactivating and nifedipine-sensitive (5 microM); the latter, partially inactivated, was resistant to omega-conotoxin GVIA (5 microM), omega-conotoxin MVIIC (5 microM), and omega-agatoxin IVA (0.4 microM), but was sensitive to mibefradil (10 microM). Both currents were sensitive to Ni(2+) and Cd(2+) (>10 microM). In some cells the L-type current amplitude increased almost twofold upon repetitive stimulation, whereas the R-type current remained unaffected. Eventually, run-down occurred for both currents, but was prevented by the protease inhibitor calpastatin. The R-type current peak component ran down first, without changing its plateau, suggesting that two channel types generate the R-type current. This peak component appeared at -40 mV, reached a maximal value at -30 mV, and became undetectable for voltages > or =0 mV, suggestive of a novel transient current: its inactivation was indeed reversibly removed when Ba(2+) was the charge carrier. The L-type current and the R-type current plateau were appreciable at -60 mV and peaked at -20 mV: the former current did not reverse for voltages up to +60 mV, the latter reversed between +30 and +60 mV due to an outward Cs(+) current flowing through the same Ca(2+) channel. The physiological role of these currents on hair cell function is discussed.
1. The mechanism of transmitter release at the cytoneural junction of the frog posterior canal was investigated by recording intracellularly subthreshold postsynaptic potentials (EPSPs), and performing a statistical analysis of time intervals and peak amplitudes. In single units EPSPs display highly variable size, so it is not clear whether they are generated by the release of single quanta of transmitter and whether large ones represent giant events, multiquantal events, or the random summation of independent unitary events. 2. In units with low resting EPSP rates, peak amplitudes and time intervals between EPSPs were measured directly. Peak amplitude histograms were continuous, unimodal and well fitted by log normal distributions. Time-interval histograms were well described by single exponentials. 3. At high EPSP rates (either at rest or during experimental treatments), where single events overlapped extensively, peak amplitude histograms were skewed markedly towards high values. Under these conditions, the EPSP waveform was estimated by autoregressive fit to the autocorrelation of the recorded signal. The fit was used to build a Wiener filter, for sharpening the original signal, before computing time-interval and peak amplitude histograms. This yielded consistent log normal peak amplitude distributions with no 'excess' skewness, similar to those obtained with low resting rates. 4. After sharpening by the Wiener filter, shoulders or small second peaks in amplitude distributions were observed only at the highest EPSP rates (> 300 s1). The number of 'multiquantal' events was reduced by Wiener filtering, and was in general consistent with the expectation that more than one independent event occurred within the duration of the single event. This suggests that the events are uniquantal, random and independent, i.e. miniature EPSPs (mEPSPs). reduced peak response during facilitation, but suppressed its waning. 6. In the presence of ATP a consistent though transient increase in resting mEPSP rate was observed in about 50% of units. ATP effect was absent in all fibres where efferent stimulation produced inhibition and present in all fibres under facilitatory efferent control. In these fibres, efferent facilitation, measured after the effect of ATP had vanished, was reduced with respect to facilitation in control solution. The effects of ATP were mimicked by its analogue adenosine-5'-0-3-thiotriphosphate (ATP-y-S).MS 2556, pp. 17-35 17 M. L. Rossi, M. Martini, B. Pelucchi and R. Fesce 7. Our results suggest that at the cytoneural junction under the conditions investigated here: (1) mEPSPs are due to the release of single quanta of transmitter by the hair cell; (2) no giant mEPSPs occur; (3) single quanta are still released independently when external calcium levels are modified; (4) the rate of transmitter release, but not its basic mechanism, is affected by [Ca2+]0, ATP and rotational stimuli as well as by efferent-induced modifications in hair cell membrane conductances; and (5) ATP appears to act at the basal po...
Potassium-current inactivation and recovery kinetics are pivotal in sustaining dynamic processing of time-varying sensory signals in hair cells. We report a detailed analysis of K(+)-currents in isolated hair cells from the frog crista ampullaris. The single components were dissected using a novel procedure based on their differential kinetic properties: The fast IA component exhibited two processes of inactivation removal; the persistent I (KD) component (I (KV) + I (KCa)), unexpectedly displayed partial inactivation, removed by negative potentials with particularly slow, delayed kinetics. The physiological relevance of these observations was investigated by imposing sinusoidal membrane potential changes to mimic receptor response to hair bundle deflection. The excitatory phase elicited extra-currents (hysteresis) only if the off phase went sufficiently negative to remove IA inactivation. Native, resting hair cells are depolarised by receptor current; thus, voltage continuously modulates I(KD), whereas IA only transiently ensues when the receptor current vanishes (zero-current potential approximately -70 mV) and polarisation removes IA inactivation.
6. Summation of IAHP was demonstrated under voltage-clamp conditions when the depolarizing steps were repeated sufficiently close to one another. Under current-clamp conditions the threshold depolarizing charge for action potential discharge significantly increased with progressive pulse numbers in the train, suggesting that an opposing conductance was accumulating with repetitive firing. This frequency-dependent spike firing ability was eliminated by pharmacological inhibition of the slow IAHP. 7. The IAHP was significantly activated by a single action potential; it was turned on cumulatively by Ca2+ load during successive action potential discharge and acted to further limit cell excitability.Influx of calcium via voltage-dependent channels leads to activation of a heterogeneous class of ion channels including two types of K+ channel, the so-called BK
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