The stiffness of the outer hair cell (OHC) lateral wall, measured by the micropipette aspiration technique, is non-linear, decreasing from the ciliary pole (stiffness parameter Sp 1.83+/-0.13 nN/microm n=10) towards the cell base (Sp 1.14+/-0.16 nN/microm, n=10) irrespective of the cochleoapical or cochleobasal origin of the cells. The length of the aspirated lateral wall segment was related exponentially to the duration of the applied negative pressure (6 cm H2O) in the synaptic region of the OHCs whereas an active, sigmoid component was observed between 30 and 60 s in the supranuclear regions. A significant increase of the midlateral wall stiffness (to 1.91+/-0.23 nN/microm; n=10) was observed in calcium-free medium and the sigmoid component of the response of the lateral wall was abolished. Salicylate (5 mM) had no significant effect on the active sigmoid behaviour of the lateral wall (n=10). Gadolinium (5 mM), a non-specific cation channel blocker, increased the stiffness of the lateral wall and attenuated the active component (n=10). The motor protein prestin thus does not seem to be involved in the active stiffness regulation seen in this study. A role for the cortical cytoskeleton in the regulation of stiffness seems reasonable according to our model. The mechanism may involve calcium-dependent metabolic modification of cytoskeletal or membrane proteins.

Otosclerosis is an inflammatory disease associated with persistent measles virus (MV) infection of the otic capsule. The nature of sensorineural hearing loss (SNHL) related to otosclerosis can be due to the chronic TNF-alpha release from the foci. TNF-alpha enters the inner ear fluid spaces in histologically active stages of otosclerosis and may cause outer hair cell functional disorder and subsequent SNHL without morphological changes of the organ of Corti. On the contrary, non-otosclerotic stapes ankylosis being a non-inflammatory disease is not harmful for hair cells. Theoretically, SNHL should not associate to this type of stapes fixation. Stapes footplates (N = 248) were examined by hematoxylin-eosin staining and corresponding MV-, OPG- and TNF-alpha-specific RT-PCR. Anti-measles IgG levels of serum specimens were measured by ELISA. Preoperative audiological results were correlated with otosclerotic and non-otosclerotic histopathologies. Among patients with stapes fixation, we found 93 active and 67 inactive otosclerosis, and 88 non-otosclerotic stapes ankylosis. MV could only be detected in otosclerotic stapes footplates. Audiometry revealed bone conduction threshold elevation toward the high frequencies in otosclerotic patients, which was associated to the duration of hearing loss. OPG mRNA expression was significantly lower in the TNF-alpha positive specimens, which was independent from virus positivity. In about one-third of stapes fixations, the etiology is non-otosclerotic stapes ankylosis. Histologic otosclerosis exhibits a strong correlation with MV presence in the bone as a sign of persistent MV infection and related inflammation with TNF-alpha release. This causes SNHL in the function of time. Non-otosclerotic stapes fixations do not cause high-frequency SNHL.

To review our current knowledge of the pathologic bone metabolism in otosclerosis and to discuss the possibilities of non-surgical, pharmacological intervention. Otosclerosis has been suspected to be associated with defective measles virus infection, local inflammation and consecutive bone deterioration in the human otic capsule. In the early stages of otosclerosis, different pharmacological agents may delay the progression or prevent further deterioration of the disease and consecutive hearing loss. Although effective anti-osteoporotic drugs have become available, the use of sodium fluoride and bisphosphonates in otosclerosis has not yet been successful. Bioflavonoids may relieve tinnitus due to otosclerosis, but there is no data available on long-term application and effects on sensorineural hearing loss. In the initial inflammatory phase, corticosteroids or non-steroidal anti-inflammatory drugs may be effective; however, extended systemic application may lead to serious side effects. Vitamin D administration may have effects on the pathological bone loss, as well as on inflammation. No information has been reported on the use of immunosuppressive drugs. Anti-cytokine targeted biological therapy, however, may be feasible. Indeed, one study on the local administration of infliximab has been reported. Potential targets of future therapy may include osteoprotegerin, RANK ligand, cathepsins and also the Wnt-β-catenin pathway. Finally, anti-measles vaccination may delay the progression of the disease and potentially decrease the number of new cases. In conclusion, stapes surgery remains to be widely accepted treatment of conductive hearing loss due to otosclerosis. Due to lack of solid evidence, the place of pharmacological treatment targeting inflammation and bone metabolism needs to be determined by future studies.

Acetylcholine (ACh) and GABA, the main neurotransmitters of the efferent innervation of the outer hair cells (OHCs), are assumed to regulate the efficacy of the cochlear amplifier through a variety of mechanisms. The recently described stretch-induced changes of the lateral wall stiffness (regulatory stiffness response) and the stretch-induced slow cell motility of OHCs may be important regulatory mechanisms in this process. We found that ACh in cochleobasal OHCs significantly reduces the stiffness of the lateral wall but increases the regulatory stiffness response and stretch-induced slow cell motility. Qualitatively similar cellular responses were evoked by GABA in cochleoapical OHCs. The effects of ACh could be inhibited by strychnine, the specific inhibitor of the alpha(9) ACh receptors expressed in OHCs, whereas the effects of GABA could be blocked by bicuculline, a specific GABA(A) receptor antagonist. In the absence of extracellular Ca(2+) the effects of ACh and GABA on the regulatory stiffness response were reduced, indicating the involvement of Ca(2+) in the control of this process. Based on our results we suggest that efferent innervation protects the organ of Corti against high sound intensities and supports adaptation by modification of the micromechanical properties of OHCs. This could be governed by ACh and GABA indirectly, via the potentiation of stretch-induced cell shortening in a Ca(2+)-dependent manner, rather than by a direct stiffness regulation-related mechanism.

No abstract

Among the cells of the inner ear, the outer hair cells (OHCs) are the most important targets of noise-induced effects, being the most sensitive cell types. The aim of this study was to examine the effects of noise (50 Hz-20 kHz, 80 dB sound pressure level, 14 days) on intracellular calcium levels and on the expression pattern of purinoceptors in the membrane of the OHCs of the guinea pig and to measure the stiffness changes of the lateral membrane of these cells. In noise-exposed animals, the resting intracellular calcium concentration increased compared to nontreated animals and was slightly higher in the cells of the basal (219 +/- 29 nM: ) than in the apical (181 +/- 24 nM: ) turns of the cochlea. After application of 180 muM: adenosine triphosphate, the intracellular calcium level rose by 60 +/- 22 nM: in cells from the apical and by 44 +/- 10 nM: in cells from the basal turns, significantly less than in nontreated animals. Expression of the P(2X1), P(2X2), P(2X4), P(2X7), P(2Y1) and P(2Y4) receptor subtypes was suppressed, while expression of the P(2Y2) subtype did not decrease in either of the two preparations. In parallel with the increase in intracellular calcium concentration, the stiffness of the lateral wall of the OHCs was increased. Noise-induced changes in intracellular calcium homeostasis and subsequently in the calcium-dependent regulatory mechanisms may modify OHC lateral wall stiffness and may lead to reduction of the efficacy of the cochlear amplifier.

Among the supporting cells, Deiters cells are in intimate contact with outer hair cells (OHCs) in the inner ear. The aim of this study was to characterize the outward rectifying K+ current of Deiters cells in conjunction with cellular morphological characteristics. In the majority of cells, the K+ current had a biphasic inactivation kinetics (tau1 and tau2 were 2,735+/-90 (n=77) and 160+/-14 ms (n=72), respectively). The rapidly inactivating current component was more sensitive to Charybdotoxin (ChTx, 10 nM) block whereas the slowly inactivating current could be blocked more efficiently by tetraethylammonium (1 mM). All these point toward the existence of two distinct potassium channel types in these cells. Deiters cells attached to shorter OHCs had more voluminous, whereas those attached to longer OHCs had lanky cell bodies. The inactivation kinetics was slower in cells having corpulent cell bodies due to the increased proportion of the slowly inactivating current component (0.736+/-0.033, n=27) as compared to the one determined for lanky cells (0.522+/-0.023, n=36). The average peak K+ current was higher in Deiters cells connected to OHCs (5,417+/-541 pA, n=40) than in isolated ones (3,527+/-410, n=37). Deiters cells having different cell shapes and showing different K+ channel expression may contribute to the active mechanism of the cochlea to various degrees.

Combined mechanical and chemical stimulation evoked greater OHC shortening than mechanical stimulation alone. Both forms of stimulation resulted in reversible shortening. Electromotility was measured using low voltage (+/- 35 mV) and higher voltage (up to +/- 240 mV) electrical pulses mimicking the receptor potential at different stages of cell shortening. The magnitude of electromotility decreased simultaneously with slow motile shortenings of OHCs. Irrespective of the character of the stimulus (mechanical or mechanical + chemical), the decrease in the magnitude of electromotility was dependent on the degree of cell shortening. Ocadaic acid, a protein phosphatase inhibitor, blocked slow motility and decreased the magnitude of electromotility.