SUMMARY1. Outer hair cells from the cochlea of the guinea-pig were isolated and their motile properties studied in short-term culture by the whole-cell variant of the patch recording technique.2. Cells elongated and shortened when subjected to voltage steps. Cells from both high-and low-frequency regions of the cochlea responded with an elongation when hyperpolarized and a shortening when depolarized. The longitudinal motion of the cell was measured by a differential photosensor capable of responding to motion frequencies 0-40 kHz.3. Under voltage clamp the length change of the cell was graded with command voltage over a range + 2 ,tm (approximately 4 % ofthe length) for cells from the apical turns of the cochlea. The mean sensitivity of the movement was 2 11 nm/pA injected current, or 19'8 nm/mV membrane polarization. 4. The kinetics of the cell length change during a voltage step were measured. Stimulated at their basal end, cells from the apical (low-frequency) cochlear turns responded with a latency of between 120 and 255 gs. The cells thereafter elongated exponentially by a process which could be characterized by three time constants, one with value 240 ,us, and a second in the range 1-3-2-8 ms. A third time constant with a value 20-40 ms characterized a slower component which may represent osmotic changes.5. Consistent with the linearity shown to voltage steps, sinusoidal stimulation of the cell generated movements which could be measured at frequencies above 1 kHz. The phase of the movement relative to the stimulus continued to grow with frequency, suggesting the presence of an absolute delay in the response of about 200 gs.6. The electrically stimulated movements were insensitive to the ionic composition of the cell, manipulated by dialysis from the patch pipette. The responses occurred when the major cation was K+ or Na+ in the pipette. Loading the cell with ATPfree solutions or calcium buffers did not inhibit the response.7. It is concluded that interaction between actin and myosin, although present in the cell, is unlikely to account for the cell motility. Instead, it is proposed that outer hair cell motility is associated with structures in the cell cortex. The implications for cochlear mechanics of such force generation in outer hair cells are discussed. 11-2

Normal hearing depends on sound amplification within the mammalian cochlea. The amplification, without which the auditory system is effectively deaf, can be traced to the correct functioning of a group of motile sensory hair cells, the outer hair cells of the cochlea. Acting like motor cells, outer hair cells produce forces that are driven by graded changes in membrane potential. The forces depend on the presence of a motor protein in the lateral membrane of the cells. This protein, known as prestin, is a member of a transporter superfamily SLC26. The functional and structural properties of prestin are described in this review. Whether outer hair cell motility might account for sound amplification at all frequencies is also a critical question and is reviewed here.

SUMMARY1. Whole-cell currents were measured in outer hair cells isolated from each turn of the organ of Corti of the guinea-pig.2. The slope input conductances at -70 mV of the cells ranged from 3-6 to 51 nS depending on the length of the cell. Shorter cells from the basal turns of the cochlea had the highest values. The membrane time constant of the cells varied from 3 to 0 2 ms from the apex to the base.3. Irrespective of the position of the cells along the cochlea, three distinct currents were found. Each type of current was found in approximately the same proportion in all cells.4. An outward K+ current was present which activated at potentials more positive than -35 mV. The current was sensitive to tetraethylammonium (30 mM), quinidine (100 AM) and nifedipine (50 /tM). It could be removed by replacing external Ca21 with Ba2+ or Mg2+. The current was also removed by substituting Nat or Cst for Kt pipette solution. This outwardly rectifying current appears similar to the calciumactivated K+ current described in other hair cells.5. The main current present at membrane potentials from -90 mV to -50 mV was a second voltage-activated K+ current. It was 50 % activated at -80 mV, and relaxed with a time constant of 20-40 ms on hyperpolarization to -120 mV. Near rest the kinetics were essentially time-independent, but depended upon the external K+ concentration. The current was blocked by 5 mm external Cs+.6. This current was highly selective for K+. Measured from reversal of the tail currents, the permeability ratio PK PNa was approximately 30: 1. Depolarization of the cell, presumed to lead to an elevation of intracellular calcium, produced a prolonged activation of the current. 7. A third current found in the cells was a cation current. By external ion replacement, the selectivity sequence was determined to be Ca2+ > Na'~K+ > choline+ > NMDG+ (respective permeabilities relative to Na: 2 9, 1P0, 0 99, 0-63 and 0 37). This current * Present address:

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This composite article is intended to give the experts in the field of cochlear mechanics an opportunity to voice their personal opinion on the one mechanism they believe dominates cochlear amplification in mammals. A collection of these ideas are presented here for the auditory community and others interested in the cochlear amplifier. Each expert has given their own personal view on the topic and at the end of their commentary they have suggested several experiments that would be required for the decisive mechanism underlying the cochlear amplifier. These experiments are presently lacking but if successfully performed would have an enormous impact on our understanding of the cochlear amplifier.

SUMMARY1. Responses to light were recorded from bipolar cells in the retina of the dogfish, Scyliorhinus canicula, under dark-adapted conditions. The identity of the cells was confirmed by Procion Yellow staining.2. More than 95 % of the bipolar cells sampled were of the type which depolarized to a spot of light. These are termed depolarizing bipolar cells. In most cells, illumination of the surround had little effect on the responses elicited from the central receptive field.3. The mean flash sensitivity of the depolarizing bipolar cells was 270 mV/Rh** (where Rh** signifies rhodopsin photoisomerization per rod for full field illumination).4. The mean flash sensitivity of horizontal cells under the same conditions was 8 mV/Rh**. In a limited sample of hyperpolarizing bipolar cells the highest flash sensitivity was 42 mV/Rh**.5. The high flash sensitivity of the depolarizing bipolar cells indicates a large voltage gain at its synapse with rods. On the assumption of a rod flash sensitivity of 2 mV/Rh** the mean gain at the synapse was 135, but for some cells the gain was in excess of 500.6. Responses of depolarizing bipolar cells to dim flashes could be approximated by the impulse response of a 12-16 stage low-pass filter, whereas horizontal cell responses could be fitted by a low-pass filter of six sections. The implied filter at the rod-bipolar cell synapse is tuned to the higher frequency components of rod signals, thereby improving temporal resolution in the rod pathway.7. Depolarizing bipolar cell responses to test flashes are reduced by weak background illumination (less than 0-1 Rh**/sec). This desensitization, which would not be expected to affect rod responses, could be explained by a shift in the operating point to a less sensitive region of the intensity-response curve as a result of the large depolarization elicited by the background.8. The results of current injection into the cell in darkness and during the response to light are consistent with the release by rod terminals of a transmitter which closes ionic channels in a conductance path having a reversal potential of -8 mV, transmitter release being suppressed by light.

Ribbon-type synapses in inner hair cells of the mammalian cochlea encode the complexity of auditory signals by fast and tonic release through fusion of neurotransmitter-containing vesicles. At any instant, only about 100 vesicles are tethered to the synaptic ribbon, and about 14 of these are docked to the plasma membrane, constituting the readily releasable pool. Although this pool contains about the same number of vesicles as that of conventional synapses, ribbon release sites operate at rates of about two orders of magnitude higher and with submillisecond precision. How these sites replenish their vesicles so efficiently remains unclear. We show here, using two-photon imaging of single release sites in the intact cochlea, that preformed vesicles derived from cytoplasmic vesicle-generating compartments participate in fast release and replenishment. Vesicles were released at a maximal initial rate of 3 per millisecond during a depolarizing pulse, and were replenished at a rate of 1.9 per millisecond. We propose that such rapid resupply of vesicles enables temporally precise and sustained release rates. This may explain how the first auditory synapse can encode with indefatigable precision without having to rely on the slow, local endocytic vesicle cycle.

It is thought that the sensitivity of mammalian hearing depends on amplification of the incoming sound within the cochlea by a select population of sensory cells, the outer hair cells. It has been suggested that these cells sense displacements and feedback forces which enhance the basilar membrane motion by reducing the inherent damping of the cochlear partition. In support of this hypothesis, outer hair cells show membrane-potential-induced length changes at acoustic rates. This process has been termed 'reverse transduction'. For amplification, the forces should be large enough to move the basilar membrane. Using a displacement-sensitive interferometer, we tested this hypothesis in an isolated cochlea while stimulating the outer hair cells with current passed across the partition. We show here that the cochlear partition distorts under the action of electrically driven hair cell length changes and produces place-specific vibration of the basilar membrane of a magnitude comparable to that observed near auditory threshold (about 1 nm). Such measurements supply direct evidence that cochlear amplification arises from the properties of the outer hair cell population.