, 2011). In this study, we directly investigate Ca2+’s role in regulating adaptation in mammalian auditory hair cells. In voltage-clamped Neratinib price hair cells, adaptation manifests itself in two
ways, as a time-dependent decrease in current amplitude during mechanical stimulation and as a shift in the peak current-displacement (I–X) plot. We developed piezo-coupled devices that allow stimulation rates up to 30 kHz producing rise times as fast as 11 μs, resulting in very fast adaptation time constants. Clamp speeds averaging 28 μs and output filtering up to 100 kHz also allow for better resolution of adaptation kinetics than previously possible. Here, we used 50 ms step stimulations from −170 nm to 600 nm to measure both fast and slow adaptation processes in rat MG-132 in vivo cochlear outer hair cells (OHCs; Figure 1A). Current-displacement plots for the peak and steady state responses illustrate the adaptation shift (Figure 1B). Double exponential fits to each MET current response produced time constants ranging between 0.1 and 5 ms for bundle deflections eliciting up to ∼75% of the maximal current (Figures 1C and 1E). Larger stimulations required three time constants (Figures 1C and 1D) with the third time constant ranging between 8 and 50 ms (Figure 1E). The two faster time
constants likely underlie fast adaptation, as the sensitivity, operating range, and kinetics are most consistent with previous reports (Kennedy et al., 2003, Ricci et al., 2005 and Waguespack et al., 2007). The two time constants likely reflect the faster stimulus rise time rather than the existence of multiple mechanisms, given that the absolute values of these time constants do not change, but rather, the proportion of each varies with stimulus intensity. The slowest time constant may
represent saturation of fast adaptation or recruitment of a distinct slower process. This mechanism contributes at most 30% of the total adaptation observed at maximal stimulations, with no contribution at stimulation levels eliciting less than 75% of the maximal current (Figure 1F), in agreement with other reports in mammals (Kennedy et al., 2003, Ricci et al., 2005 and Waguespack et al., why 2007). In contrast, low-frequency cells show near 100% motor adaptation contribution for maximal stimulations and 50% motor adaptation with 50% maximum stimulations (Wu et al., 1999). Thus, mammalian data are consistent with the hypothesis that fast adaptation is the predominant mode of adaptation in mammalian auditory hair cells. Depolarization reverses the MET current and eliminates Ca2+ entry into stereocilia, and thus, provides a means to assess whether Ca2+ is driving adaptation (Assad et al., 1989 and Crawford et al., 1989).