Sensory hair fusion in vestibular sensory cells after gentamycin exposure. A transmission and scanning electron microscope study

Mechanisms and Impact of Aminoglycoside-Induced Vestibular Deficits

PURPOSE

Acquired vestibulotoxicity from hospital-prescribed medications such as aminoglycoside antibiotics affects as many as 40,000 people each year in North America. However, there are no current federally approved drugs to prevent or treat the debilitating and permanent loss of vestibular function caused by bactericidal aminoglycoside antibiotics. This review will cover our current understanding of the impact of, and mechanisms underlying, aminoglycoside-induced vestibulotoxicity and highlight the gaps in our knowledge that remain.

CONCLUSIONS

Aminoglycoside-induced vestibular deficits have long-term impacts on patients across the lifespan. Additionally, the prevalence of aminoglycoside-induced vestibulotoxicity appears to be greater than cochleotoxicity. Thus, monitoring for vestibulotoxicity should be independent of auditory monitoring and encompass patients of all ages from young children to older adults before, during, and after aminoglycoside therapy.

Hair cell uptake of gentamicin in the developing mouse utricle

Intratympanic injection of gentamicin has proven to be an effective therapy for intractable vestibular dysfunction. However, most studies to date have focused on the cochlea, so little is known about the distribution and uptake of gentamicin by the counterpart of the auditory system, specifically vestibular hair cells (HCs). Here, with a combination of in vivo and in vitro approaches, we used a gentamicin-Texas Red (GTTR) conjugate to investigate the mechanisms of gentamicin vestibulotoxicity in the developing mammalian utricular HCs. In vivo, GTTR fluorescence was concentrated in the apical cytoplasm and the cellular membrane of neonatal utricular HCs, but scarce in the nucleus of HCs and supporting cells. Quantitative analysis showed the GTTR uptake by striolar HCs was significantly higher than that in the extrastriola. In addition, the GTTR fluorescence intensity in the striola was increased gradually from 1 to 8 days, peaking at 8-9 days postnatally. In vitro, utricle explants were incubated with GTTR and candidate uptake conduits, including mechanotransduction (MET) channels and endocytosis in the HC, were inhibited separately. GTTR uptake by HCs could be inhibited by quinine, a blocker of MET channels, under both normal and stressed conditions. Meanwhile, endocytic inhibition only reduced GTTR uptake in the CoCl₂ hypoxia model. In sum, the maturation of MET channels mediated uptake of GTTR into vestibular HCs. Under stressed conditions, MET channels play a pronounced role, manifested by channel-dependent stress enhanced GTTR permeation, while endocytosis participates in GTTR entry in a more selective manner.

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.

Uptake of fluorescent gentamicin by peripheral vestibular cells after systemic administration

Objective

In addition to cochleotoxicity, systemic aminoglycoside pharmacotherapy causes vestibulotoxicity resulting in imbalance and visual dysfunction. The underlying trafficking routes of systemically-administered aminoglycosides from the vasculature to the vestibular sensory hair cells are largely unknown. We investigated the trafficking of systemically-administered gentamicin into the peripheral vestibular system in C56Bl/6 mice using fluorescence-tagged gentamicin (gentamicin-Texas-Red, GTTR) imaged by scanning laser confocal microscopy to determine the cellular distribution and intensity of GTTR fluorescence in the three semicircular canal cristae, utricular, and saccular maculae at 5 time points over 4 hours.

Results

Low intensity GTTR fluorescence was detected at 0.5 hours as both discrete puncta and diffuse cytoplasmic fluorescence. The intensity of cytoplasmic fluorescence peaked at 3 hours, while punctate fluorescence was plateaued after 3 hours. At 0.5 and 1 hour, higher levels of diffuse GTTR fluorescence were present in transitional cells compared to hair cells and supporting cells. Sensory hair cells typically exhibited only diffuse cytoplasmic fluorescence at all time-points up to 4 hours in this study. In contrast, non-sensory cells rapidly exhibited both intense fluorescent puncta and weaker, diffuse fluorescence throughout the cytosol. The numbers and size of fluorescent puncta in dark cells and transitional cells increased over time. There is no preferential GTTR uptake by the five peripheral vestibular organs’ sensory cells. Control vestibular tissues exposed to Dulbecco’s phosphate-buffered saline or hydrolyzed Texas Red had negligible fluorescence.

Conclusions

All peripheral vestibular cells rapidly take up systemically-administered GTTR, reaching peak intensity 3 hours after injection. Sensory hair cells exhibited only diffuse fluorescence, while non-sensory cells displayed both diffuse and punctate fluorescence. Transitional cells may act as a primary pathway for trafficking of systemic GTTR from the vasculature to endolymph prior to entering hair cells.

Temporal and spatial distribution of gentamicin in the peripheral vestibular system after transtympanic administration in guinea pigs

BACKGROUND AND OBJECTIVE

Transtympanic administration of gentamicin is effective for treating patients with intractable vertigo. This study explored the spatial and temporal distribution of gentamicin in vestibular end-organs after transtympanic administration.

METHODS

Thirty guinea pigs were transtympanically injected with gentamicin conjugated to Texas Red (GTTR) and their vestibular end-organs examined after various survival periods. Another 9 guinea pigs received GTTR at different doses. Nine animals received Texas Red only and served as controls. We used confocal microscopy to determine the cellular distribution of GTTR in semicircular canal cristae, as well as the utricular and saccular maculae.

RESULTS

The most intense GTTR labeling was present in the saccule compared to other vestibular end-organs. GTTR fluorescence was detected predominantly in type I hair cells, type II hair cells and transitional cells after a single transtympanic dose of GTTR (0.1 mg/ml, 0.05 ml), while only weak fluorescence was observed in non-sensory cells such as supporting cells, dark cells and lumenal epithelial cells. Transitional cells displayed intense GTTR fluorescence in the supra-nuclear regions 24 h after transtympanic injection that was retained for at least 4 weeks. A decreasing spatial gradient of GTTR fluorescence was observed sensory epithelial regions containing central type I to peripheral type I and then type II hair cells in the crista ampullaris, and from striolar to extra-striolar hair cells within the vestibular macula. GTTR fluorescence extended from being restricted to the apical cytoplasm at lower doses to the entire cell body of type I hair cells with increasing dose. GTTR fluorescence reached peak intensities for individual regions of interest within the cristae and maculae between 3 and 7 days after transtympanic injection.

CONCLUSION

The saccular uptake of GTTR is greater than other vestibular end-organs after transtympanic injection in the semicircular canals.

The prevention of noise induced hearing loss in children

Increasingly, our acoustic environment is filled with amplified sound sources (e.g., MP3 players, video game stations, and sports/entertainment venues). There is serious concern and also some controversy about the risks of acoustic trauma in children. This overview provides some basic information on the physiological mechanisms that lead to noise induced hearing loss, a survey of various studies that, on balance, indicates that there is cause for concern, and finally a discussion on measures that can help to prevent noise induced hearing loss in children. This paper is designed for public health and other healthcare professions (ENT, audiologists, family doctors, and pediatricians) who should understand the risks of noise induced hearing loss and its prevention.

The effect of explantation and neomycin on hair cells and supporting cells in organotypic cultures of the adult guinea-pig utricle

Recent reports suggest that immature hair bundles are observed following aminoglycoside-induced hair-cell loss in the mammalian utricle in vitro as well as in vivo. It is therefore important to document the initial morphological changes associated with both culturing and aminoglycoside application so that degeneration can be clearly distinguished from regeneration. In this study, utricles from adult guinea pigs were maintained in culture for either 3 or 8 days, half being exposed to neomycin for days 2 and 3. They were then processed for microscopical examination and compared with control utricles from animals of the same age. The numbers of hair-cell and supporting-cell nuclei were counted and hair-cell morphology assessed. Bundles were classified as having either stepped (SHB) or unstepped (UHB) stereocilia, and their density determined. The numbers of hair-cell, but not supporting-cell, nuclei declined significantly compared with controls in both untreated and treated explants, the greatest reduction occurring 5 days after neomycin administration. The density of SHBs also declined but there was no significant change in UHB density, resulting in a residual population of hair bundles of more immature appearance in both untreated and treated utricles in vitro than in vivo. Although degenerative events such as hair-cell ejection from, or retraction into, the sensory epithelium were observed, no evidence of regeneration was found.

Systemic absorption of gentamicin ear drops

Aminoglycoside-containing topical preparations are commonly used in the treatment of chronic suppurative otitis media and otitis externa. We report a case where systemic absorption of topical Gentisone HC occurred producing a serum gentamicin level of 6.2 micrograms/ml.

Round window membrane. Structure function and permeability: a review

The ultrastructure of the round window membrane of humans, monkeys, felines, and rodents discloses three basic layers: an outer epithelium, a middle core of connective tissue, and an inner epithelium. Interspecies variations are mainly in terms of thickness, being thinnest in rodents and thicker in humans. Morphologic evidence suggests that the layers of the round window participate in absorption and secretion of substances to and from the inner ear, and that the entire membrane could play a role in the defense system of the ear. Different substances, including antibiotics, local anesthetics, and tracers such as cationic ferritin, horseradish peroxidase, and 1 mu latex microspheres, are placed in the middle ear side traverse the membrane. Cationic ferritin and 1 micron microspheres placed in perilymph become incorporated by the inner epithelial cells of the membrane. Permeability is selective; factors include size, concentration, liposolubility, electrical charge, and thickness of the membrane. Passage of substances through the round window membrane is by different pathways, the nature of which is seemingly decided at the outer epithelium of the round window membrane.

Damage and recovery of hair cells in fish canal (but not superficial) neuromasts after gentamicin exposure

Recent evidence demonstrating the presence of two types of sensory hair cell in the ear of a teleost fish (Astronotus ocellatus, the oscar) indicates that hair cell heterogeneity may exist not only in amniotic vertebrates but also in anamniotes. Here we report that a similar heterogeneity between hair cell types may also occur in the other mechanosensory organ of the oscar, the lateral line. We exposed oscars to the aminoglycoside (ototoxic) antibiotic gentamicin sulfate and found damaged sensory hair cells in one class of the lateral line receptors, the canal neuromasts, but not in the other class, the superficial neuromasts. This effect was not due to the canal environment. Moreover, new ciliary bundles on hair cells of the canal neuromasts were found after, and during, gentamicin exposure. The pattern of hair cell destruction and recovery in canal neuromasts is similar to that of type I-like hair cells found in the striolar region of the utricle and lagena of the oscar after gentamicin treatment. These results suggest that the hair cells in the canal and superficial neuromasts may be similar to type I-like and type II hair cells, respectively, in the fish ear.

Ototoxic antibiotics: a review

Fifty years of experience with aminoglycoside antibiotics has confirmed their usefulness in many infections with Gram-negative bacteria. The ototoxic action has been investigated clinically in patients and in numerous animal studies. The increased knowledge about molecular structure, pharmacology and pharmacokinetics as well as information of the site of action in the labyrinth has resulted in reduced risks for severe toxic damage in kidneys and in the ear. Nevertheless, a number of cases of severe hearing loss and a few cases of deafness still occur every year. Further studies on modified molecular structure as well as site of action at the molecular level and ways of avoiding toxicity will make this group of antibiotics still more useful.

Heterogeneity of sensory hair cells in a fish ear

The ultrastructure of sensory hair cells in the utricle of the cichlid fish, Astronotus ocellatus, the oscar, was studied by transmission electron microscopy of serial ultrathin sections from different regions of the epithelium. Two distinctly different types of hair cell were found, one located in the striolar region of the epithelium and the other in the extrastriolar region. Striolar hair cells have a well-defined perinuclear cisterna located just below the nucleus, and large perinuclear mitochondria. Synaptic bodies of striolar cells are small and located in clusters, while those in extrastriolar cells are relatively large and individually dispersed. The extrastriolar hair cell closely resembles the amniote type II hair cell. On the basis of these data, and consistent with earlier studies, it appears that the striolar hair cells closely resemble amniote type I hair cells in many significant ways. Thus we have called them type I-like cells. The extrastriolar hair cells appear to be typical of eighth nerve mechanoreceptors commonly described for fish and closely resemble the amniote type II hair cell.

The effect of endolymphatic sac obliteration on vestibular sensory hair bundles. A high-resolution scanning electron microscopic investigation

A tannic acid-osmium staining technique and high-resolution scanning electron microscopy were used to demonstrate changes in the glycocalyx and ciliary interconnections of the vestibular sensory cells of guinea pigs after extradural obliteration of the endolymphatic sac and duct. Three months after the obliteration, it was possible to observe degeneration in the glycocalyx and the ciliary interconnections as well as the tip links. These findings suggest that the endolymphatic hydrops causes an endolymphatic ionic imbalance which affects the glycocalyx and ciliary interconnections resulting in further morphological changes of the cilia. The tip links, which are believed to be involved in sensory cell transduction, also seem to be affected.

Effect of gentamicin on vestibular ganglion

The morphological changes of the vestibular ganglion induced by gentamicin (GM) were investigated using light, scanning and transmission electron microscopy. The guinea pigs were injected 4 mg of GM into the middle ear for 5 days. The vestibular ganglion was observed up to 4 weeks after the treatment. Three days after the treatment the degenerative change was first noted in the vestibular ganglion. The degenerative process started from the destruction of mitochondrial cristae and vacuolization of the cytoplasm of the Schwann cell. The cytoplasmic organelles in the ganglion cell gradually deteriorated with swelling of the endoplasmic cisterns as well as Golgi apparatus. At the later stage, the myelin sheath around the ganglion cell disappeared and the number of the cells reduced. These changes of the ganglion cell may be due to the direct action of GM.

Effect of gentamicin on the carbohydrates of the vestibular end organs: an investigation by the use of FITC-lectins

The effect of gentamicin on the glycoconjugates in the vestibular end organs is demonstrated using FITC-lectins. Five milligrams of gentamicin dissolved in 0.1 ml. saline was injected in a single dose into the middle ear of adult guinea pigs. Seven days after the injection, the fluorescent reactivity of Wheat germ agglutinin (WGA), Abrus precatorius agglutinin (APA), Concanavaline A (Con A), and Latyrus odoratus agglutinin (LOA) was decreased in the apical epithelial cell surface as well as in the gelatinous layer of the otolithic membrane of the maculae. In the cupula of the crista ampullaris, the reactivity for WGA and Con A was reduced. The otoconia, however, showed no detectable changes. This indicates that gentamicin may induce an altered carbohydrate metabolism resulting in a decrease of N-acetyl-glucosamine, mannose, galactose and glucose in the glycocalyx lining the epithelial cells as well as in the gelatinous layer of the otolithic membrane or cupula in the vestibular end organs.

Initial changes in the sensory hair-cell membrane following aminoglycoside administration in a guinea pig model

This study demonstrates the initial changes affecting the sensory hair-cell plasma membranes in the vestibular end organs of gentamicin-treated guinea pigs by using a ruthenium red staining technique. First, 0.1 ml of a solution containing 5 mg gentamicin sulfate was injected into the middle ear. After 7 days, the sensory hair cell cilia were observed to be degenerating. The various stages of this degeneration process were classified into two types: the decrease in glycocalyx was designated type I fusion, while type II fusion was characterized by a bleb formation of the plasma membrane of the sensory hair cells, followed by a decrease in glycocalyx. The latter mechanism allowed plasma membrane contact, with subsequent fusion of the plasma membrane of neighboring sensory hair-cell cilia. The material also illustrates the degeneration of ciliary actin filaments. These findings suggest that the aminoglycoside affects both the glycocalyx and the plasma membrane, and that the decrease in glycocalyx may be the first sign of sensory hair-cell fusion.

The effect of gentamicin on the glycocalyx and the ciliary interconnections in vestibular sensory cells: a high resolution scanning electron microscopic investigation

Changes in glycocalyx structure and ciliary interconnections of the vestibular sensory cells are demonstrated after gentamicin administration. A special high resolution scanning electron microscope and a tannic acid-osmium staining technique giving an almost three dimensional view were used to achieve this purpose. Guinea pigs were injected with a single dose of 5 mg of gentamicin directly into the middle ear. Seven days after the injection, it was possible to observe the degenerative process of the glycocalyx and the ciliary interconnections. The first detectable change was a disarrangement of the cilia with a loosening of the interconnections. The ciliary membrane presented with an irregular appearance. The tip links connecting the tips of the stereocilia to their neighbours were also affected showing elongation or even disappearance. In the later stages of the degeneration process, the sensory hairs presented with different degrees of fusion whereafter they finally disappeared totally. These findings suggest that the glycocalyx acts to maintain a normal stability and shape of the ciliary membrane and to keep the regular distance between cilia in order to maintain the arrangement of the whole ciliary bundle. Gentamicin probably affects the glycocalyx and the ciliary interconnections resulting in a disarrangement, detachment and fusion of cilia. The tip links, which are suggested to be involved in sensory cell transduction, seem to be also affected by gentamicin.

Sensory hair fusion and glycocalyx changes following gentamicin exposure in the guinea pig vestibular organs

This study demonstrates the mechanism of sensory hair fusion and its relationship to glycocalyx on the hair cells in the vestibular epithelia of gentamicin-treated guinea pigs, using a ruthenium red staining technique. 0.1 ml of a solution containing 50 mg gentamicin sulphate per 1 ml was injected once into the middle ear. After 7 days, various stages of sensory hair degeneration were observed. The glycocalyx was clearly visualized by ruthenium red. The gentamicin-induced hair damage started with a local decrease in or loss of glycocalyx from neighbouring sensory hairs, followed by formation of local attachment areas of membrane sensory hairs. These findings suggest that the glycocalyx of the sensory cell may play an important role in separating the cilia as well as maintaining the organization of the bundle. Therefore, the decrease in or loss of glycocalyx from the hair bundle of sensory cells affected by gentamicin, either directly or secondarily, can be considered as closely related to the fusion of the sensory hairs.

Experimental studies on round window structure: function and permeability

Current research and an overall review of 12 years of round window membrane studies is presented. The approach, rationale, and concepts that have evolved from the studies are described. An ultrastructural study of the round window membrane of rhesus monkeys disclosed three basic layers: an outer epithelium, a middle core of connective tissue, and an inner epithelium. Morphologic evidence in monkeys, cats, and chinchillas suggests that these layers of the round window participate in absorption and secretion of substances to and from the inner ear, and that the entire membrane could play a role in the defense system of the ear. Cationic ferritin, horseradish peroxidase, 1-micron latex spheres, and neomycin-gold spheres placed in the middle ear of these experimental animals were observed to traverse the round window membrane through pinocytotic vesicles. Three-micron latex spheres and anionic ferritin were not incorporated by the membrane. Cationic ferritin and 1-micron latex spheres placed in perilymph were incorporated by the inner epithelial cells, suggesting absorptive capabilities of the round window membrane. Cationic ferritin was observed within the mesothelial cells underlying the scala tympani side of the basilar membrane, suggesting a role for these cells in the inner-ear defense system. A review of the subject and a general perspective from the author's viewpoint are discussed.

Sensory hair fusion and glycocalyx changes after gentamicin exposure in the guinea pig

This study demonstrates the mechanism of sensory hair fusion and its relationship to glycocalyx on the inner ear sensory cells of gentamicin-treated guinea pigs, using the ruthenium red staining technique. 0.5 ml containing 5 mg gentamicin sulfate solution was injected in a single dose into the middle ear. After 7 days, various stages of sensory hair degeneration could be observed. The glycocalyx was clearly visualized by ruthenium red. The degeneration starts with a decrease in or even complete loss of glycocalyx in the sensory hairs, with subsequent local adhesion of the plasma membrane in neighbouring hairs. These findings suggest that the glycocalyx of the sensory cell may play an important role in separating the cilia, yet keeping them in a bundle.

A scanning electron microscopic study of vestibular organ malformation following prenatal gamma irradiation

Pregnant CBA/CBA mice were exposed to 1 and 2 Gy whole-body gamma irradiation on the 13th and 16th gestational days, respectively. The litters were born on the 21st day of gestation and were tested for vestibular function at the age of 1 month. The animals were then sacrificed and their inner ears were analyzed by scanning electron microscopy. No disturbances of vestibular function were noted in the animals studied. However, the cristae ampullares showed severe malformations as regards their gross shape, with irregularities of their outer contours. Type I hair cells seemed to be more severely changed than Type II hair cells, with fusion of sensory hairs, giant hair formation and bulging of the cuticular plate. In certain sites the hair cells were totally missing. These derangements were usually located in the central areas of the cristae ampullares and in the striolar portion of the maculae utriculi. The morphological damage found showed a dose-dependent, time-related pattern.

Effects of noise and ototoxic drugs at the cellular level in the cochlea: a review

Currently available information concerning the cellular mechanisms involved in acoustic trauma and aminoglycoside ototoxicity is reviewed to shed some new light on the cellular events that may be related to functional impairment of the auditory organ. Based on the available data, the following postulations can be made concerning the cellular mechanisms involved. 1) The macromolecular disruption of the stereocilia and cuticular plates is the initial cellular event in acoustic trauma. This disruption would affect the micromechanics of the transduction process, leading to temporary threshold shift. Further cellular impairment would involve basic cellular functions such as the protein, lipid, and glucose synthesis needed for cell repair and survival, and such impairment would result in permanent cell injury or cell death, leading to permanent threshold shift. 2) It can be postulated that the cellular mechanisms involved in aminoglycoside ototoxicity include two events. The early event is the reversible blockage of the transduction channels from the endolymph side of the hair cells. The later event is the interference in such cellular functions as protein and/or phospholipid synthesis because of binding of aminoglycoside to the phospholipids and/or protein, leading to cell death. The latter event may be facilitated by penetration or membrane-mediated internalization of the aminoglycoside from the perilymph side of the hair cell.

Aminoglycoside binding sites in the cochlea as revealed by neomycin-gold labelling

Neomycin/bovine serum albumin/gold was used as a probe to detect the binding sites of aminoglycosides on the thin sections of the cochlea embedded in Spurr. The binding sites were mainly located on the stereocilia, the cuticular plate of hair cells, the head plates of Deiters' cells, fibrous structures in pillar cells, in the spiral limbus and tectorial membrane and basilar membrane, plasma membranes, mitochondria and the chromatin of various kinds of cells. Triphosphoinositide, acidic glycosaminoglycans, and RNA were considered to be responsible for the binding activity.

Distribution of gentamicin by immunofluorescence in the guinea pig inner ear

We studied the distribution of gentamicin in the inner ear, brain and kidney of the guinea pig following intraperitoneal administration or perfusion of gentamicin through the perilymphatic space. The resulting histopathological changes were examined by immunofluorescence using antigentamicin antiserum. After perfusion of gentamicin through the perilymphatic space, specific fluorescence was found in the cochlea, and was especially prominent in the outer hair cells, basilar membrane and basilar crest. Although no fluorescence was observed in the cochlea following intraperitoneal administration of high doses of gentamicin, type I hair cells in the vestibule were seen to be selectively stained with the antibody. Furthermore, some of the vestibular ganglion cells, Purkinje cells and unidentified nuclei in the brain stem were also stained. In particular, fine granules showing relatively intense fluorescence were recognized in the cytoplasm of the stained cells. In the cortex of kidney, only proximal tubular cells were stained with intense fluorescence. Our results suggest that the aminoglycoside antibiotics have two sites of action: one is the cell membrane of the sensory hair cells and the other is the cytoplasm.

Gentamicin tympanoclysis: effects on the vestibular secretory cells

Gentamicin application to the middle ear may relieve Meniere's disease, presumably by reducing endolymph secretion by the dark cells. To explore this possibility, the ears of adult cats were treated with daily intratympanic infusions of gentamicin until ataxia occurred. The temporal bones of these cats were then examined using electron microscopy. A 3 per cent solution of gentamicin resulted in ataxia after four treatments. Acute dark cell damage to basal infoldings seen after one month persisted at six months. Treatment with a 0.3 per cent solution required 15 and 21 days to effect ataxia and resulted in a similar damage pattern. Treatment with the 0.3 per cent solution for 13 days (i.e., before ataxia developed) resulted in subtle early lesions in the basal infoldings. Such lesions might affect the rate of endolymph secretion.

Preservation of the Human Cochlea

Human cochleas processed with a varying interval between death and initial fixation were examined by transmission and scanning electron microscopy. The specimens were fixed by paraformaldehyde-glutaraldehyde fixatives in buffer. Cochleas fixed less than two hours postmorten showed excellent morphology in general. With increasing time lag between death and fixation the results became more variable. In some specimens fixed up to six hours postmorten, however, the morphology still was well preserved, but as the interval increased, a greater variability was introduced. The conclusion is that specimens fixed up to six hours postmortem can show excellent electron microscopic morphology. Limited information can be obtained up to 12 hours postmortem, after which the results tend to be unreliable.

Vestibular hair cell pathology following low-dose irradiation during embryonic development

Pregnant CBA/CBA mice (total gestational age 20-21 days) were whole-body irradiated on the 12th, 13th and 16th gestational days with single doses of 1 and 2 Gy using a 60Co source. Litters were taken for morphological analysis 14-21 days after birth. Irradiation on the 12th and 13th gestational days was thus performed prior to morphologic inner ear organogenesis and cytodifferentiation. In approximately 20% of the total postnatal vestibular hair cell population cytological alterations were evident: giant cytoplasmic protrusions, sensory hair fusion and disintegration of the cuticular plate. The other parts of the hair cells remained intact, including nerve endings. Morphometry was not performed. Hair cells type I are more vulnerable to ionizing irradiation during cytodifferentiation than hair cells of type II. All investigated animals showed clinically a normal balance and a normal righting reflex.

Clinical monitoring of the effects of gentamicin by electrocochleography

Eight patients receiving prolonged treatment with gentamicin for bacterial endocarditis were monitored for possible ototoxicity using transtympanic electrocochleography as an adjunct to pure tone audiometry, vestibular function tests and serum gentamicin levels. An immediate effect of intravenous gentamicin on the cochlea, shown by changes in the whole nerve action potential and/or cochlear microphonic was recorded in seven patients, none of whom had experienced vestibular or auditory symptoms with gentamicin prior to testing. Two patients subsequently developed evidence of vestibular dysfunction, and a high frequency sensorineural hearing loss occurred in a third individual. There have been no previous report of the immediate effects of gentamicin on the human cochlea to date. The significance of these findings in routine clinical monitoring of ototoxicity is discussed in the light of clinical and animal evidence for the possible mode of action of aminoglycosides on the auditory and vestibular apparatus.

Ultrastructural studies of stereocilia in noise-exposed rabbits

In rabbits with noise-induced, permanent hearing loss of more than 20 dB the stereocilia of the inner hair cells (IHCs) showed widespread signs of damage: fracturing, folding, fusion, and formation of giant hairs. Damage to the stereocilia on outer hair cells (OHCs) was much less pronounced in corresponding regions. On damaged OHCs the stereocilia were usually missing, except in the apical turn where fusion of stereocilia and formation of giant stereocilia occurred. On both IHCs and OHCs the 'neck' of the stereocilium appeared to be a site of fracture of the actin skeleton. On IHCs the actin cores of the former stereocilia were enclosed in a bulge of the apical cytoplasm to form a giant hair, with portions of the stereocilia projecting from the bulge. Inside the giant hairs profiles of the original stereocilia could be recognized, and these often retained their original circular form at least at the base of a giant hair. In the distal portion of the giant hair the actin filaments frequently formed a single thick bundle where the individual stereocilia could not be distinguished. Material had condensed between the surface membrane enclosing the former stereocilia and the actin cores. A thin kinocilium was often identified on IHCs. together with giant hairs possibly indicating a regenerative activation of the cell. It is suggested that fracture and fusion of stereocilia are due to different pathophysiological mechanisms, and that the actin of damaged cilia may undergo a slow reorganization and regeneration.

Freeze-fracturing of vestibular sensory epithelia in a strain of the waltzing guinea pig

The waltzing guinea pig exhibits genetically induced inner ear disease which develops into deafness and vestibular disorder. The type I hair cell in vestibular end organs shows specific pathology, featuring protrusion of the hair cell apex, sensory hair fusion and an intracellular actin filament rod. With the freeze-fracturing technique, consecutive stages of degeneration of the apical part of the type I hair cell are shown. Different possible mechanisms underlying these changes are discussed. It is probable that the genetically induced prenatally developed hair cell pathology is related to a pathological development of actin filaments and their connections in the apical cytoskeleton.

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.

Aminoglycoside ototoxicity

Aminoglycoside antibiotics are frequently employed in the treatment of serious infections caused by aerobic gram negative bacilli. The use of these potent antibacterial agents is limited by the risks of ototoxicity and nephrotoxicity. Aminoglycosides are excreted by glomerular filtration at a rate proportional to the serum concentration. Impaired renal excretion reduces the rate of clearance from the serum. Utilizing information about aminoglycoside pharmacokinetics, the susceptibility of infecting pathogens to aminoglycosides, and risk factors for ototoxicity, physicians can attempt to optimize the administratioin of an aminoglycoside to maximize the therapeutic efficacy and to minimize the risk of ototoxicity. Periodic assessments of renal function and of aminoglycoside levels in the serum are essential to guide therapy. The otolaryngologist and audiologist must be able to provide information about ototoxicity to medical colleagues using these drugs. They should also be prepared to evaluate and follow patients who develop sensorineural inner ear dysfunction during or after a course of therapy with an aminoglycoside antibiotic.

Extracorporeal preservation. Organ culture of the post-natal mammalian inner ear

Postnatal (newborn and mature) inner ear organs from CBA/CBA mouse and guinea pig were analysed concerning hair cell survival in an in vitro system. After only a few days in the artificial surroundings a transformation of hair cell characteristics occurred, in the form of either loss of sensory hairs (cochlear hair cells) or hair fusion (vestibular hair cells), although the hair cell itselt survived for a considerably longer time. Intracellular myelin figures became evident after 2--3 days in culture. However, a considerable individual variation among hair cells was observed concerning the ultrastructure of the cell at this stag in vitro. Completely normal hair cells could in rare cases be recognised after 1 week in organ culture (cochlear inner hair cells of the CBA/CBA mouse). Loss of the surface structures of the hair cells is likely to constitute an irreversible transformation not in agreement with the true hair cell characteristics.

Gentamicin-induced mitochondrial damage in inner ear sensory cells of the lizard Calotes versicolor

Earlier morphological and histochemical studies on the effects on inner ear sensory cells caused by aminoglycoside antibiotics have failed to give sufficient information of the genesis of these effects. The present study was focused on the phases of progessive mitochondrial changes in sensory cells of the the lizard basilar papilla induced by consecutive large doses of gentamicin. The mitochondria react by swelling, changes in the configuration and number of the cristae and formation of matrical inclusions. Myelin figures are a consistent finding in degenerating cells after gentamicin exposure. These are shown to be derived from changed mitochondria. The final product is an "onion-like" structure which is built of primitive membranes. There is a marked difference in reaction to the damage between individual mitochondria in the same cell. This difference is also evident between individual sensory cells in the same specimen. By studying the phases of the mitochondrial breakdown process in the sensory cell, some additional information on the changes in cell metabolism caused by ototoxic drugs, may be extracted.

Organ culture of the postnatal mouse Crista ampullaris. Part I

The 2 day post partum mouse Crista ampullaris was cultured "in vitro" for 7 days. The differentiation into secretory and sensory areas was preserved. The ultrastructure of the dark cells of the secretory region was well maintained. The hair cell morphologically was considerably changed though the hair cells themselves survived 7 days in culture.

Gentamicin-induced changes in RNA content in sensory and ganglionic cells in the hearing organ of the lizard Calotes versicolor. A cytochemical and morphological investigation

The effect of short-term administration of gentamicin on RNA content in isolated sensory and ganglionic cells in the hearing organ of the lizard Calotes versicolor was investigated. The effects of the drug on this biochemical parameter are compared with ultrastructural changes in sensory cells from the same material. In the first test group, daily intraperitoneal injections for 3 to 6 days resulted in a 30-50% decrease of the amount of RNA in sensory and ganglionic cells. No constant ultrastructural changes were recorded in the sensory cells at this point in time. A second test group received 3 or 5 injections of gentamicin and the animals were then left untreated for 21 days. The reduced RNA content persisted in both sensory cells and in ganglionic cells from animals that received 5 injections, whereas the content of RNA in ganglionic cells from lizards that received 3 injections was restored almost to control values at the end of this 21-day period. In the second test group, definite morphological derangements were observed in the sensory cells. The correlation between cytochemical and morphological findings as well as possible modes of action of gentamicin on inner ear ganglionic and sensory cell RNA content are discussed.

Atoxyl-induced damage to the sensory cells in the organ of Corti in the guinea pig cochlea

Atoxyl-induced damage to the inner and the outer hair cells in the guinea pig cochlea starts in the apical coil and spreads successively downward toward the round window. Vesicular degeneration of both the inner and the outer hair cells and the cell organelles occurs. The degeneration is probably caused by an alteration in ionic composition of the endolymph resulting from primary changes of the stria vascularis (vesiculation and rejection of degenerated marginal and intermediate cells) as reported by Anniko and Wersäll (1975). However, a direct action of atoxyl on the sensory cells of the organ of Corti cannot be excluded.

Toxic effects of gentamicin on the basilar papilla in the lizard Calotes versicolor. A surface study

Gentamicin in a dose of 100 mg and in some cases 150 mg per kg bodyweight and day was given intraperitoneally to healthy lizards, belonging to the species Calotes Versicolor. The animals were injected for 7, 14 and 21 days. After completed injections the animals were sacrificed and their hearing organ, the basilar papilla, was processed for scanning electron microscopy. Animals treated for 7 days did not show any significant surface damage in the basilar papilla. When gentamicin was administered for 14 days the normal appearance of the surface structure was lost. The ventral (apical) type A cells were relatively intact while the type B cell-population in the dorsal (basal) part of the organ showed sensory hair fusions and cytoplasmic herniations. Lizards treated for 21 days showed a severely damaged basilar papilla. The ventral (apical) type A cells still only were moderately damaged with some hair fusions and cytoplasmic herniations while the dorsal (basal) type B cells were more or less destroyed. Only occasional cells were left and some of these were severely damaged. The surface of the dorsal (basal) part of the organ instead was covered by supporting cells thus forming a sort of scar tissue.

Ototoxicity of topically applied gentamicin using a statistical analysis of electrophysiological measurement

Ototoxicity of topically applied gentamicin was studied in guinea pigs. 0.3% gentamicin was instilled in one side of the middle ear cavity and Ringer's solution was instilled in the other side, and the difference in the cochlear microphonics measured with the round window electrode was analysed statistically. Instillation of Ringer's solution in the middle ear cavity for 1 day did not cause any significant sensorineural hearing loss, but on the 3rd day of instillation significantly reduced responses were observed, compared with the responses from non-treated ears, followed by a partial recovery starting on the 4th day. When gentamicin 0.3% was instilled into the middle ear cavity, significant deafness occurred 24 hours later, and highly significant deafness on the 2nd and 3rd day. The usage of gentamicin ear drops of the current formula should be discouraged until a better formula is provided.