Vulnerability of the organ of Corti in poisoning

I. Experimental Approach and Histological Criteria for Evaluating Drug Effects

The rationale and methodology for transtympanic drug injection have been established. By the use of phase contrast and Normarski differential interference contrast microscopy to study “surface preparations” of both cochlear and vestibular sensory structures, the degenerative cellular changes which resulted from the localized administration of aminoglycoside antibiotics have been examined toward the selection of an anatomical criterion for the measurement of ototoxicity. The “collapse” cell has been selected for this purpose because of its positive identification, persistence and inability to recover.

III. Comparative Quantitative Analysis

Hypotheses to explain the selective ototoxicity of aminoglycoside antibiotics may be grouped into two categories: 1) a partition effect and 2) innate hair cell selectivity. The results of this study give evidence that, in fact, both mechanisms may interplay. This conclusion is induced from the observations that following the transtympanic administration of streptomycin, dihydrostreptomycin and neomycin: 1) All three drugs elicited nearly the same topographic pattern and degree of damage in the vestibular compartment. 2) All three drugs elicited nearly the same topographic pattern but widely different degrees of damage in the cochlear compartment (neomycin > > streptomycin > dihydrostreptomycin). 3) Neomycin caused the same devastating cochlear hair cell damage as reported for its systemic administration. 4) Dihydrostreptomycin was significantly more vestibulotoxic than cochleotoxic, and streptomycin was both significantly more cochleotoxic than vestibulotoxic and significantly more cochleotoxic than was dihydrostreptomycin. The latter observations are discussed with regard to contrasts with the literature reporting the drug effects following systemic administration.

Aminoguanidine reduces cisplatin ototoxicity

Cisplatin is known to cause high-frequency neurosensory hearing loss. While reactive oxygen species have been shown to play a role, reactive nitrogen species have been implicated, but not proven to be involved, in cisplatin ototoxicity. The purpose of the present study was to investigate the role of nitric oxide (*NO) in cisplatin ototoxicity by administering aminoguanidine (AG), a relatively specific inhibitor of inducible nitric oxide synthase (iNOS), in conjunction with cisplatin. Rats were injected with cisplatin, AG, or both. Auditory brainstem evoked responses (ABR) were measured before and 3 days after cisplatin administration. The cochlear tissue was then assayed for *NO and malondialdehyde. Cisplatin alone caused significant ABR threshold shifts at all stimuli tested, whereas AG alone caused no shifts. There was a significant reduction in threshold shift for clicks and 16 kHz tone bursts (but not 32 kHz) when AG was given with cisplatin. The malondialdehyde concentration (but not the *NO concentration) in the AG/cisplatin group was significantly lower than that of the cisplatin group. This suggests that AG reduces cisplatin ototoxicity by directly scavenging hydroxyl radicals. The iNOS pathway may play a role in the generation of free radicals and hearing loss resulting from cisplatin administration, but this conclusion was not supported by our data.

Nitric oxide synthase inhibitor reduces the apoptotic change in the cisplatin-treated cochlea of guinea pigs

Cisplatin is known to cause inner ear damage. The role of nitric oxide (NO) in the cochlea of the guinea pigs after injections of cisplatin or a combination of cisplatin and NO synthase (NOS) inhibitor [N(G)-nitro-L-arginine methyl ester (L-NAME)] i.p. was examined by means of immunohistochemistry. Three days after injection, the cochleas were examined immunohistochemically for single-stranded DNA (ssDNA). We found that ssDNA was expressed in the stria vascularis and spiral ganglion cells of the cisplatin-treated cochlea. In the L-NAME/cisplatin-treated cochlea, the number of cells that exhibited positive staining for ssDNA was markedly reduced. High NO levels lead to inner ear dysfunction under pathological conditions. Our results indicate that NO mediates the ototoxicity of cisplatin.

Nitric oxide synthase inhibitor suppresses the ototoxic side effect of cisplatin in guinea pigs

Cisplatin is known to cause inner ear damage (ototoxicity). The role of inducible nitric oxide synthase (iNOS) in the cochlea of guinea pigs after injections of cisplatin or a combination of cisplatin and NOS inhibitor (NG-nitro-L-arginine methyl ester, L-NAME) i.p. was examined electro-and immunohistochemically. The auditory brain stem responses (ABR) were measured prior to injection and 3 days after the injection. Three days after injection, the cochleas were examined immunohistochemically for iNOS. We found that iNOS was expressed in the cisplatin- and L-NAME/ cisplatin-treated cochlea. The threshold shift of ABR was significant in the cisplatin group, whereas it was decreased in the L-NAME/cisplatin group. iNOS catalyzed high NO levels lead to inner ear dysfunction. Our results indicate that iNOS mediates the ototoxicity of cisplatin.

Gentamicin-induced alterations of succinic dehydrogenase activity in the organ of Corti as revealed by non-decalcified frozen sections of the guinea pig's cochlea

The distribution of succinic dehydrogenase (SDH) activity and the changes of SDH activity after injection of gentamicin (GM) were observed in the organ of Corti using non-decalcified frozen sections of the guinea pig's cochlea. The distribution of SDH activity was found to increase from the apex to the basal turn. At each turn, SDH activity of the inner hair cells, the inner supporting cells and the nerve endings surrounding the supporting cell and on the hair cells presented a greater activity than that found in the outer hair cells, adjacent Deiter's cells and associated nerve endings. It was further observed that GM had a greater effect on SDH activity in the basal turn than the other turns. At each turn, a more sensitive area of response to GM was found on the nerve endings one each hair cell, especially on the outer hair cells of the basal turn.

Cochleovestibular toxicity related to dichloromethotrexate

A 69-year-old man receiving monotherapy for lung cancer with dichloromethotrexate (DCM), and without exposure to other chemotherapeutic agents or known ototoxic drugs, developed profound cochleovestibular dysfunction. The initial presentation was vestibular. This resolved, but unilateral hearing loss ensued and continued to progress to a total of 80 db loss in the low frequency range and 40 db in the high range, despite discontinuation of the drug. Because DCM may be used in combination with the known ototoxin cisplatin, the potential of DCM as an ototoxin should also be considered in such patients.

Long-term ototoxic effects of neomycin applied topically in the middle ear: a morphological study in the guinea pig

Neomycin was instilled daily, uni- or bilaterally, into the middle ear of guinea pigs for three months. The cochleae were examined, by light and electron microscopy, six months after the end of treatment. The organs of Corti of the treated ears were completely destroyed, and in the most advanced lesions, were substituted by a single layer of very thin flat cells. In the spiral ganglion only some glial cells and a few neurons could be observed. All surviving neurons were myelinated, and their ultrastructure was greatly altered, with disorganization of the rough endoplasmic reticulum and ribosomes. The basilar membrane almost disappeared, losing it amorphous and filamentous components. The spiral limbus and the stria vascularis were atrophic and were also covered, in the final stages, by flat elongated cells. In view of its morphological characteristics, this epithelium may arise from the displacement of the interdental cells and perhaps from the cochlear surface of Reissner's membrane.

The ototoxic potential of EMLA. A new local anesthetic for the tympanic membrane

Instillation of EMLA, a new local anesthetic, into the middle ear of the guinea pig caused severe morphological damage to the organ of Corti in the first 4 mm from the round window. Further up the cochlea, only derangements of the stereocilia were found. The extent of morphological damage was the same, whether the agent was administered once or several times. The ototoxic potential of EMLA was obvious and is probably due to direct damage in areas where present in high concentrations. Over a short distance of approximately 0.1 mm there is a transition from a total destruction of the organ of Corti to a completely normal morphology.

Effects of X-ray irradiation on hearing in guinea pigs

Effects of X-ray irradiation on the hearing with CM and ABR to both air- and bone-conduction stimuli were studied continuously from immediately after the irradiation to a few weeks later. At 2, 4, and 6 krad, only slight conductive hearing impairment was found, but at 8 krad or more, CM and ABR disappeared abruptly, after having increased briefly. Advanced sensorineural hearing loss appeared at about 10 hours after irradiation ended at 8 krad, at about 6 hours at 10 krad, and at about 3 hours at 12 krad. More severe hearing impairment was observed at higher frequencies and the endocochlear d.c. potential (EP) decreased at about 7 hours after 10 krad irradiation. Histologically, no discernible changes in the hair cells or sensory hairs were found with a scanning electron microscope at about 6 hours after 10 krad irradiation, while with a transmission electron microscope, the outer hair cells in the basal coil of the cochlea were found to be mostly destroyed. However, no changes could be found in the inner hair cells or other supporting cells.

Link between functional and morphological changes in the inner ear--functional changes produced by ototoxic agents and their interactions

Common potentials used to evaluate cochlear function are the ac cochlear potential (ACCP), N1 and the positive dc endocochlear potential (EP). The ACCP is an electrical analogue of the sound stimulus; its source is the electrical activity of the cochlear hair cells. N1 is a volume conductor recorded action potential of the auditory nerve. The EP is the positive polarization of the middle compartment of the cochlea (scala media) with respect to the other compartments (the scalae tympani and vestibuli); the stria vascularis is apparently responsible for the EP. Generally, ototoxic drugs and very intense broad-band noise affect the basal portion of the cochlea first and, because of tonotopic organization, the ACCP responses to high frequency pure tones are affected before those to the low frequencies. However, the correlation between the effect of an ototraumatic agent on the ACCP and its effect on cochlear morphology is not always reliable. The correlations between changes in N1 and EP and in cochlear morphology are even less precise. Also discussed will be the cochlear effects of noise and the ototoxic interactions between drug/drug, noise/drug, and noise/drug/otitis media.

Differences in the cochlear degeneration pattern in the guinea pig as a result of gentamicin and cis-platinum intoxication

This study gives the results of a series of studies on inner ears carried out with the help of microdissections and surface preparations. The total degeneration patterns of the cochlea of the guinea pig due to two different ototoxic drugs were investigated with this technique. Clear differences between the results of the ototoxic drugs (gentamicin and cis-platinum) in the inner ear were observed. Gentamicin has a 'typical starting point' of degeneration on the basilar membrane. From this 'degeneration point' the loss of hair cells progressed towards the round window (fast) and the apex (slowly). The stria vascularis showed no signs of degeneration due to gentamicin. Reissner's membrane, on the other hand, showed cellular vacuolization of the endolymphatic cells. Cis-platinum (DDP) showed no 'degeneration point'; the loss of hair cells was found over the complete length of the basilar membrane, with a preference for the basilar turn. The stria vascularis, on the other hand, showed severe degenerative changes due to DDP, whereas Reissner's membrane showed no change. In using the microdissection technique we were in a position to discover these differences between the two ototoxic drugs.

Structure side-effect sorting of drugs. VI. Ototoxicities

From a literature survey, over 130 (about 7.8%) drugs and chemicals have been associated with ototoxicities. The major classes are basic aminoglycoside and other antibiotics, anti-inflammatory drugs, antimalarials, beta-blockers, antineoplastic agents, heavy metals, diuretics, some topical agents and various miscellaneous drugs. Possible mechanisms of action are presented and discussed. These include inhibition of protein synthesis, the glycolytic cycle, the TCA cycle, energy utilization, energy generation and the respiratory system within the mitochondria membrane of the hair cell, and also alteration of the permeability of the endolymphatic membrane or alteration of the excretion system for the basic aminoglycosides in the lateral wall of the membranous cochlea. The relative rank order of ototoxicity and reactivity toward mucopolysaccharides of five aminoglycosides is found to be related to the number of basic groups in each molecule.

The effects of intracochlear and systemic furosemide on the properties of single cochlear nerve fibres in the cat

1. Tuning properties and spontaneous discharge rate of single cochlear fibres in the anaesthetized cat were determined during short- and long-term poisoning of the cochlea by locally and systemically applied furosemide.2. With intra-arterial administration of furosemide, short-term reversible elevation occurred of the low threshold sharply tuned ;tip' segment of the frequency threshold (;tuning') curve (f.t.c.) by up to 40 db, without substantial changes in the threshold of the low frequency ;tail' segment of the f.t.c. These changes could occur in part without changes in the spontaneous activity and entirely without changes in the maximal evoked activity. These effects were observed in all fibres examined, the characteristic frequencies of which ranged from 3.5 to 31 kHz.3. Intracochlear administration of furosemide in 0.9 mM concentrations produced similar changes, but these were not reversible.4. The changes correlated with the depression of the amplitude of the gross cochlear action potential. The cochlear microphonic potential, however, was either unchanged, or only slightly reduced.5. In long-term furosemide poisoning of the cochlea, fibres with anomalous response properties were found alongside fibres having normal tuning. The former exhibited either reduced excitability of the low threshold tip segment, or a tip segment attenuated in both excitability and threshold.6. It is concluded that the selective effects of furosemide on the tip segment of cochlear fibre f.t.c.s offer further evidence for a physiologically vulnerable ;second filter' in the cochlea. The selective influence of the furosemide on the low threshold tip segment provides support for the hypothesis that the normal f.t.c. is generated by two largely independent processes: one vulnerable, low threshold and sharply tuned, and the other less vulnerable, but high threshold and more broadly tuned.7. The findings, obtained with an agent known to produce reversible impairment of hearing in man, provide direct physiological evidence in support of the hypothesis that in sensorineural hearing loss of cochlear origin the frequency selectivity of cochlear nerve fibres is impaired.

Cochlear damage from chlorhexidine in guinea pigs

The disinfectant chlorhexidine has been extensively used as preoperative skin sterilizing agent. In the early 1970s it was suspected from clinical experience that inner ear damage with deafness might result from chlorhexidine accidentally introduced into the middle ear during preoperative skin disinfection. In order to study if chlorhexidine by local application in the middle ear has any ototoxic effect resulting in morphological changes in the organ of Corti, the tympanic cavity of guinea pigs was exposed to two different concentrations of the substance. Two different solvents were used and the duration of exposure was varied. The animals were sacrificed 2, 3, 4 or 10 weeks after exposure. In surface preparations of the organ of Corti, damage was seen in almost all exposed ears, related in extent to the concentration of chlorhexidine, the duration of exposure, and the time lapse after exposure. Furthermore, it was found that inner ear damage often extended beyond the organ of Corti and pathological changes were also observed in the mucosal lining of the tympanic cavity.

Cis-diamminedichloroplatinum (II) ototoxicity in the guinea pig

Cochleas from 12 guinea pigs were evaluated using light, scanning, and transmission electron microscopy after systemic administration of cis-diamminedichloroplatinum (cis-DDP). Administration of cis-DDP resulted in loss of the Preyer reflex and degeneration of outer hair cells (OHC) with increased dose. The OHC degeneration was most pronounced in the basal turns of the cochlea with greatest severity in the inner row. Ultrastructural evidence of OHC degeneration included dilatation of the parietal membranes, softening of the cuticular plate, increased vacuolization and increased numbers of lysosome-like bodies in the apical portion of the cell. Supporting cells appeared more sensitive than OHC. Alteration of supporting cell ultrastructure preceded detectable change in OHC. Injury to the supporting cells was noted with intracellular vesiculation and increased autophagocytosis.