The Journal of Neuroscience, June 7, 2006, ():
The Role of BKCa Channels in Electrical Signal Encoding in the Mammalian Auditory Periphery
J. Neurosci. Oliver et al.
26: 6181
Supplemental data
Files in this Data Supplement:
- supplemental material
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Supplementary Figure 1: Receptor potential-like voltage responses of BKα-/- IHCs show increased DC and decreased AC components.
A, Transduction currents were mimicked by injecting AC currents shaped according to the transducer characteristics. Current waveforms were derived by passing sinusoids through a second-order Boltzmann function generally used to describe the transfer function hair cells (left panel; Kros et al., 1992). Since data on transduction currents in adult IHCs were not available, we used published transducer characteristics from neonatal mammalian hair cells (Kros et al., 1992) with about 10 % of the conductance active at rest (Kros et al., 1992; Kros, 1996).
B, The resulting asymmetric and saturating transducer-like currents (right panel) were applied to IHCs at a frequency of 2 kHz (representing the lower end of mouse hearing and close to the characteristic frequencies for the apical IHCs studied here) in 25 ms bursts at various degrees of saturation (Fig. 3B). Currents used had saturating amplitudes of 1nA. This is somewhat below the maximum transducer conductance of neonatal IHCs (13 nS) estimated by Kros (Kros, 1996), which is equivalent to a transducer current of 1.9 nA at the apical driving force of ≈145 mV supplied by membrane potential and endocochlear potential. However, injection of saturating currents of 2 nA yielded comparable results to those shown here. Recently, larger transducer currents have been reported from mature IHCs in the gerbil hemicochlea (Jia et al., 2005, Assoc Res Otolaryngol, 169). Thus, it is likely that receptor potentials in vivo are larger than the RP-like voltage reponses shown in C.
C, Representative voltage responses of current-clamped IHCs to injection of the current waveform shown in panel B recorded from a BKα+/+ mouse (left) and a BKα-/-mouse (right).
The voltage responses of BKα+/+ IHCs to these transducer-like stimuli (Fig. 3C) closely resembled RPs recorded from IHCs in vivo with sharp electrodes (Palmer and Russell, 1986). For small stimuli, responses were sinusoidal, for increasingly larger and more asymmetrical currents they showed aprominent DC component. The DC component was step-like with fast onset and offset and tended to saturate for the largest current stimuli. Responses from BKα-/- IHCs differed in several aspects (Fig. 3C). For small stimuli, the DC component had a slower onset, and for larger stimuli showed an initial depolarizing voltage transient, similar to the responses to depolarizing current steps.
D, Summary of voltage responses from 5 BKα+/+ and 7 BKα-/- IHCs shows that RP-like responses depolarize the IHC stronger in the absence of BKCa. Voltage amplitudes were measured at the end of each 25 ms stimulus episode.
E, DC components of the RP-like voltage responses were determined as the difference between (DC) voltage at the end of each stimulus episode and baseline voltage.
F, AC components of the RP-like responses, obtained as the amplitude of the sinusoidal voltage response at the end of each stimulus episode, are decreased in the BKα-/- IHCs.
G, The ratio of AC to DC components is decreased in BKα-/- IHCs at all degrees of stimulus saturation.
- supplemental material
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Supplementary Figure S2:
A: DPOAE thresholds in BKα-/- ears
Cochlear threshold sensitivity was monitored via distortion product otoacoustic emissions (DPOAEs), which assess cochlear function upstream of the IHC and AN. Since BKα-/- mice show a slowly progressive hearing loss(Ruttiger et al., 2004), all measurements were confined to young animals. Cochlear sensitivity, measured via isoresponse contours for DPOAEs, is slightly reduced at all test frequencies in BKα-/- vs. BKα+/+. However, there was significant overlap in DPOAE data from individual BKα-/- vs. +/+ animals.
B: Dynamic range of BKα-/- AN fibers.
AN fibers from BKα-/- ears show no abnormalities in dynamic range. As schematized in the inset, the dynamic range of each fiber is measured from the rate-vs.-level function for tone bursts at the characteristic frequency. Dynamic range is calculated as the difference in stimulus levels (in dB) between that eliciting 10% and that eliciting 90% of the saturated driven rate (driven rate = maximum rate - spontaneous rate). Up-arrows signify that the rate-vs.-level function did not saturate at the highest level presented. Fibers with spontaneous rate = 0 were plotted as 0.05 sp/s.
C Phase-locking in BKα-/- mice.
A standard measure of phase-locking is the synchronization index (Johnson, 1980), which is extracted from post-zero-crossing histograms of spike times referred to the positive zero crossings of the pure-tone stimulus (inset), i.e. the beginning of each stimulus period: the higher the value, the more closely time-locked the spike-times are with the stimulus phase. To measure maximum synchrony, the level of the stimulus is increased until the synchronization index saturates (Johnson, 1980). The maximum synchronization indices obtained from 3 BKα-/- and 4 BKα+/+ fibers are shown in comparison to published data from the CBA/CaJ AN (Taberner and Liberman, 2005) and guinea pig AN fibers (dashed lines; (Palmer and Russell, 1986). The stimulus frequency (2 kHz) was chosen to match the transducer-like currents used for the patch-clamp recordings.
The data show that synchrony is not eliminated by the loss of BKCa channels but may be reduced.
