INTRACELLULAR-RECORDINGS FROM SUPPORTING CELLS IN THE GUINEA-PIG COCHLEA - DC POTENTIALS

Supporting cells and hair cells from the low-frequency region of the guinea pig cochlea were studied in vivo using intracellular recording and horseradish-peroxidase (HRP) marking techniques. The response of third- and fourth-turn support cells to tone bursts is composed of a number of components: an AC component at the frequency of the stimulating tone, harmonic components, a DC component present at the onset of the stimulating tone (the early DC), a slowly developing depolarization, and a slowly decaying afterpotential. The early CD of support-cell responses is generally less than or equal to that in the adjacent organof Corti fluids [at the best frequency (BF) for an 80 or 90 dB sound pressure level (SPL) stimulus the average early DC of support-cell responses is 0.9 times that of the adjacent fluids; n = 71], and both are less than that seen in the hair cells [average early DC of inner hair-cell (IHC) responses at the same sound levels is 14.2 times that in the adjacent organ fluids, n = 15; average early DC of outer hair-cell (OHC) responses is 11.5 times that in nearby organ fluids, n = 2]. The end DC, magnitude of the DC response shortly before signal end, in responses of support cells deep into Corti's organ (e.g., pillar, inner phalangeal, border cells] is often greater than that recorded in the potentials of the adjacent organ fluids (e.g., for an 80 or 90 dB SPL stimulus at the BF with a 30 ms steady-state time, the average end DC of the support cells deep into the organ is 2 times that of the adjacent organ fluids, n = 42). In contrast, the end DC for the responses of peripheral support cells - the Hensen's cells - generally equals, or is smaller than, the extracellular-fluid counterpart (for an 80 or 90 dB SPL stimulus, the average end DC of Hensen's cells is 0.9 times that of the nearby, outer-tunnel fluids, n = 29). Thus a difference exists across support-cell type with respect to support-cell end DC vis-a-vis that of the adjacent organ of Corti fluids. A slowly increasing depolarization is often present in moderate and high-level support-cell responses. It is not normally present in IHC or OHC responses. Magnitude of the slowly increasing depolarization, the slow DC, is dependent on stimulus duration and stimulus level. The largest slow DC seen was 1.8 mV. The slow DC rises in an asymptotic manner, and the average time constant of the rise is roughly 11 ms (n = 10; for 80- and 90-dB tones at the BF with a 30-ms steady-state time). A slowly decaying afterpotential is present in support-cell responses and is absent in the potentials of the organ fluids. The decay has an exponential time course with an average time constant of 2.3 s (n = 5; 270-Hz stimulus at 90 dB SPL with a steady-state time of 1,000 ms). The magnitude of the afterdecay is dependent on input level. The early-DC component of the support-cell response appears to be a reflection of hair-cell-generated nonlinearities, whereas the slow-DC component and afterdecay appear to originate within the support cells themselves. The slow DC and afterdecay are reminiscent of slow potentials seen in the CNS neuroglia and suggest that the auditory support cells play a role in buffering the extracellular environment of the hair cells and/or the eighth-nerve dendritic endings.