Imaging electrical resonance in hair cells
The mechanosensory hair cells of many auditory receptor organs are tuned by an electrical resonance that increases their responses to stimulation over a narrow band of frequencies. The small oscillations of membrane potential characteristic of this phenomenon have previously been detectable only through intracellular electrode measurements, which are laborious and preclude analysis at the level of an entire sensory organ. We employed a voltage-sensitive dye to detect electrical resonance in the hair cells of an intact preparation of the bullfrog's sacculus, a receptor organ sensitive to low-frequency seismic and auditory stimuli. Imaging revealed distinct populations of hair cells whose resonant response varied with the frequency of transepithelial electrical stimulation. Most of the hair cells in the saccular epithelium in vitro were electrically tuned to stimulation at 25 50 Hz. The frequency dependence of the fluorescence signal was sensitive to pharmacological blockade of large-conductance Ca2+-sensitive K+ channels and to enzymatic digestion. With an elevated concentration of Ca2+, we observed transient fluorescence signals that probably represented individual action potentials. These stroboscopic imaging and analysis techniques provide a general approach for studying subthreshold oscillations in electrically excitable cells.
The upper images, which portray the peak relative fluorescence changes (ΔF/F) for the indicated stimulus frequencies, delineate hair cells. The calibration is shown at the right. In the lower panels, hair cells are apparent—especially for stimulation at 25 Hz and 50 Hz—in images showing the phase of the fluorescence response based on a least-squares fit. The pixels with phases between π/8 radian and π/8 radian are colored dark blue to visually threshold the images; the color axis wraps around at ±π radians.
