Individual outer hair cells isolated from guinea pig cochleae were observed in vitro during the application of solutions that are known to cause hair cells to shorten. Solutions containing high potassium, which depolarizes cells, were applied in the form of potassium gluconate. The initial response was a shortening, followed by an elongation, after which the hair cells nearly resumed their original length. Solutions containing the presumed efferent neurotransmitter acetylcholine also caused an initial shortening, occasionally followed by an elongation, where a cell either returned to normal or exceeded its original length. Solutions containing cationized ferritin caused some cells to shorten and caused others to lengthen. The results indicate that the hair cell response to a chemical stimulus can be bidirectional. Moreover, the initial response of an individual cell may depend not only on the stimulus but also on the physiological state of the hair cell or the original location of the hair cell along the length of the sensory epithelium when it was in the cochlea.

Acetylcholine↗

PubMed1988

Volume and length changes in outer hair cells of the guinea pig after potassium-induced shortening.

Measurements of volume and length were made on isolated guinea pig cochlear outer hair cells (OHCs) in an attempt to establish whether OHC shortening was accompanied by changes in cell volume. A sustained shortening in response to an isotonic 100 mM KCl solution was found to be accompanied by a significant increase in OHC volume. The application of hypotonic solutions had a similar effect. When testing solutions with unchanged or reduced [K+] [Cl-] products in order to avoid loading the cells with chloride ions and producing concomitant water influx, the most frequent response was a rapid shortening followed by an elongation beyond the original cell length. These findings of a sustained and a spontaneously reversible shortening suggest that the potassium-induced response may consist of two components: a rapid one, which may be reversible in the presence of the stimulus, and a second, slower, component resulting in sustained shortening.

Animals↗

PubMed1988

Effects of caffeine and tetracaine on outer hair cell shortening suggest intracellular calcium involvement.

Outer hair cell (OHC) shortening has previously been induced in vitro by the application of solutions containing high potassium (a depolarizing agent), acetylcholine (a suggested efferent transmitter) and cationized ferritin (a positively charged macromolecule), as well as by electrical current. The application of caffeine, which causes contractures in skeletal and smooth muscle by releasing calcium from intracellular stores to activate actin and myosin interaction, also causes shortening of OHCs. Tetracaine, which interferes with calcium movement in muscle and non-muscle cells, blocks potassium-induced and caffeine-induced shortening of OHCs, but does not block electrically-induced shortening. Sodium dantrolene which is an inhibitor of intracellular calcium release in skeletal muscle does not block potassium-induced OHC shortening. Immunocytochemical studies using antibodies to muscle-like contractile and regulatory proteins on unfixed, freeze-dried OHCs demonstrate the co-localization of calmodulin with actin throughout the OHC cytoplasm. These results support the ideas that in OHCs, intracellular calcium release is involved in the activation of shortening and that an actin-mediated cell shape change may be regulated by calmodulin in a manner similar to that which occurs in contraction of smooth muscle.

Actins↗

PubMed1988

Glutaraldehyde induces cell shape changes in isolated outer hair cells from the inner ear.

Individual isolated outer hair cells (OHCs) from the cochlea were maintained in a collagen gel and viewed in the light microscope. They were observed during fixation and processing for transmission electron microscopy and individual cells were selected for observation in the electron microscope. Application of glutaraldehyde at several concentrations caused OHCs to become shorter. Shrinkage occurred during dehydration but there was no further change during infiltration with the epoxy resin. Ultrastructural analysis of isolated cells fixed with glutaraldehyde and postfixed with osmium tetroxide showed that these cells were similar to cells fixed in the intact cochlea. The glutaraldehyde-induced cell shape change is similar to the shortening seen in intact OHCs in response to the application of solutions containing high potassium or caffeine. Application of glutaraldehyde to cells pretreated with potassium or caffeine caused further shortening. Glutaraldehyde-induced cell shape change was not blocked by the application of tetracaine, which did prevent potassium-induced and caffeine-induced shortening. Glutaraldehyde-induced cell shape change was not stopped by short treatment with N-ethylmaleimide, which did inhibit potassium-induced shortening. Results from these experiments suggest that the glutaraldehyde-induced OHC shape change is not caused by an effect on the membrane or by calcium activation of a contractile response. Shortening may be caused by shrinkage due to cross-linking of proteins.

Aldehydes↗

PubMed1988

Ultrastructural correlates of inner ear sensory cell shortening.

Ultrastructural analysis was performed on isolated outer hair cells of the inner ear, to further investigate the mechanisms involved in cell shortening. Hair cells were fixed either in their presumed normal, 'relaxed', state or after application of solutions containing high concentrations of potassium to induce shortening, or 'contraction'. Ultrastructural analysis showed that shortened outer hair cells had a more undulated, or wavy, cell membrane as compared to normal cells. Outer hair cell shortening was not accompanied by the formation of vacuoles in the cytoplasm or appreciable swelling of the subsurface cisternae. The results further support the idea that a contractile apparatus associated with the lateral wall could be responsible for outer hair cell motility, and that the mechanism resembles that involved in smooth muscle contraction.

Animals↗

PubMed1987

Motility in auditory sensory cells.

Three different procedures were employed to induce contraction of isolated cochlear outer hair cells. When cells were permeabilized and exposed to calcium and ATP both a shortening of cell length (14%) and a decrease in cell diameter (12-16%) was observed indicating a longitudinal and a circumferential contraction. When intact cells were treated with positively charged ferritin a shortening of about 10% was observed together with an increase in cell diameter. Application of solutions containing high potassium concentrations gave similar but more rapid results as compared with positively charged ferritin. The observed responses of inner ear sensory cells together with their morphological and immunohistochemical characteristics further support the idea that the hearing organ acts as a motile system.

Adenosine Triphosphate↗

PubMed1986

Mechanisms of movement in outer hair cells and a possible structural basis.

Isolated outer hair cells were found to slowly shorten when subjected to a solution that would induce contraction in a muscle fibre. Two possible mechanisms underlying this behaviour emerge from ultrastructural and immunocytochemical investigations. Antibody labelling at the electron microscopic level demonstrates that actin is present not only in the stereocilia and in the cuticular plate but also along the wall of outer hair cells, between the plasma membrane and the subsurface fenestrated cisternae. The latter are interconnected by regularly spaced pillars, resembling those seen between the T-tubules and sarcoplasmic reticulum in muscle fibres. Contraction also results from the application of positively charged macromolecules to the bathing solution. This implies sensitivity of the membrane-associated complex (the cortex system) to an electrical current. A second contractile system may reside in the cytoplasm, where calmodulin is present in contracted hair cells. This protein is a calcium-binding control protein for contraction-like events in smooth muscle and non-muscle cells. The unique presence of the cortex system in outer hair cells, and its absence in inner hair cells, indicates a functional significance that relates to a motor function of outer hair cells in hearing.

Actins↗

PubMedDate not supplied

Investigating routes to chaos in the guinea-pig cochlea using the continuous wavelet transform and the short-time Fourier transform.

The continuous wavelet transform (CWT) and the short-time Fourier transform (STFT) were used to analyze the time course of cellular motion in the guinea pig inner ear. The velocity responses of individual outer hair cells and Hensen's cells to amplitude modulated (AM) acoustical signals applied to the ear canal displayed characteristics typical of nonlinear systems, such as the generation of spectral components at harmonics of the carrier frequency. Nonlinear effects were particularly pronounced at the highest stimulus levels, where half-harmonic (and sometimes quarter-harmonic) components were also seen. The generation of these components was consistent with the behavior of a dynamical system entering chaos via a period-doubling route. A negative-stiffness Duffing oscillator model yielded period-doubling behavior similar to that of the experimental data. We compared the effectiveness of the CWT and the STFT for analyzing the responses to AM stimuli. The CWT (calculated using a high-Q Morlet-wavelet basis) and the STFT were both useful for identifying the various spectral components present in the AM velocity response of the cell. The high-Q Morlet wavelet CWT was particularly effective in distinguishing the lowest frequency components present in the response, since its frequency resolution is appreciably better than the STFT at low frequencies. Octave-band-based CWTs (using low-Q Morlet, Meyer, and Daubechies 4-tap wavelets) were largely ineffective in analyzing these signals, inasmuch as the frequency spacing between neighboring spectral components was far less than one octave.

Animals↗

PubMedDate not supplied

Lead (Pb2+) modulation of potassium currents of guinea pig outer hair cells.

Outer hair cells (OHC) are mechanosensitive sensory cells of the inner ear cochlea and are involved in modulating the activity of inner hair cells in the transduction of an acoustic stimulus. Potassium (K(+)) currents play an important role in the sensory transduction process. K(+) currents were recorded from acutely dissociated OHC obtained from the guinea pig organ of Corti. The whole-cell patch clamp technique was employed. We identified a channel that exhibited outward current of the delayed rectifier type (Kv). Kv channels mediating inward currents carried by potassium ions were also identified and took on the appearance of a previously described inwardly rectifying current. Lead (Pb(2+)) acetate at concentrations of 0.1, 1.0, 10, and 100 microM was bath applied. Time to activation for outward-going current was not affected by Pb(2+). The time course of Pb(2+) effects was seen as a dose-dependent reduction of K(+) current over time, with very little or no recovery after washout. Pb(2+) inhibited the outward Kv relative current with values of 0.10, 0.14, 0.18, and 0.30 at 0.1, 1.0, 10, and 100 microM, respectively. Pb(2+) did not modulate time to activation, peak current, or inactivation of inward I(K). The effects of Pb(2+) on the potassium currents of OHC are not remarkable and therefore OHC are probably not a major cause of purported peripheral hearing loss observed in Pb(2+)-exposed animals and humans.

Animals↗

PubMedDate not supplied

Mercury (Hg2+) suppression of potassium currents of outer hair cells.

The heavy metal mercury (Hg(2+)) is an insidious environmental pollutant that causes toxic effects on sensory systems. It is well known that the group IIB divalent cation Hg(2+) is an inhibitor of the group I monovalent potassium (K(+)) cation pore-forming channel in several biological preparations. Here, we used the whole cell patch clamp technique on freshly isolated outer hair cells (OHCs) of the guinea pig cochlea to record outward K(+) currents and inward K(+) currents treated with mercuric chloride (HgCl(2)). HgCl(2) affected K(+) currents in a voltage- and dose-dependent manner. The effects of HgCl(2) at 1.0-100 microM are more pronounced on onset peak current than on steady-state end current. HgCl(2) depolarized also the resting membrane potential. Although the effect of HgCl(2) at 1.0 microM was partially washed out over several minutes, the effects at 10 and 100 microM were irreversible to washout. Since K(+) channels of OHCs are targets for HgCl(2) ototoxicity, this may lead to auditory transduction problems, including a loss in hearing sensitivity. A better understanding of fundamental mechanisms underlying K(+) channelopathies in OHCs due to HgCl(2) poisoning may lead to better preventive or therapeutic agents.

Animals↗

PubMedDate not supplied

A method for studying the effects of neurochemicals on long-term compensation in unilaterally labyrinthectomized rats.

A new method has been developed to study the influence of drugs and toxicants on long-term recovery of dynamics in the horizontal vestibulo-ocular reflex of the rat after hemilabyrinthectomy (HL). HL was performed by injecting sodium arsanilate into the middle ear. The lesion was confirmed by histology. Eye movements elicited by sinusoidal vestibular stimulation, in both light and darkness, were recorded by a search-coil technique and then analyzed by a computer program created with virtual instrument soft-ware, which calculated the gain of the slow-phase velocity (SPV) and the saccades para-meters (frequency, amplitude, and peak velocity) to the lesioned side and to the intact side separately. During the 2-10 week period after HL, repeated analysis of the spontaneous long-term recovery of such parameters revealed a slight but significant reduction of the post-HL asymmetry between SPV gain to the lesioned side and to the intact side. During the follow-up period, a post-HL increase of the phase lead remained unchanged. The reduced number of saccades/min was not completely restored. To test the usefulness of the experimental model for neurochemical investigation of such adaptation, we administered baclofen and toluene to rats 8-12 wk after hemilabyrinthectomy. Baclofen, a specific GABAB agonist, immediately restored the symmetry of SPV gain. By contrast, toluene, which has some effects on the central vestibular system that are related to GABAB transmission, aggravated the asymmetry in both the SPV gain and the number of saccades. We suggest that the experimental model would be useful for studying neurochemical mechanisms in vestibular adaptation processes.

Adaptation, Physiological↗

PubMedDate not supplied

Outer hair cells isolated from the organ of corti exposed to increased hydrostatic pressure.

A model using outer hair cells isolated from the guinea pig organ of Corti was used to study the effects of changes in hydrostatic pressure. Outer hair cells were placed in a closed chamber and the pressure was raised to levels corresponding to pressures measured inside the cochlea or higher. No changes in cell shape could be detected using either videomicroscopy or confocal microscopy. No clear changes were observed using a potentiometric vital dye.

Animals↗