Pharmacology of hearing and ototoxicity

Bumetanide for neonatal seizures: no light in the pharmacokinetic/dynamic tunnel

In his editorial, Kevin Staley criticizes our recent work demonstrating the lack of effect of bumetanide in a novel model of neonatal seizures. The main points in our response are that (1) our work is on an asphyxia model, not one on "hypercarbia only"; (2) clinically relevant parenteral doses of bumetanide applied in vivo lead to concentrations in the brain parenchyma that are at least an order of magnitude lower than what would be sufficient to exert any direct effect—even a transient one—on neuronal functions, including neonatal seizures; and (3) moreover, bumetanide's molecular target in the brain is the Na‐K‐2Cl cotransporter NKCC1, which has vital functions in neurons, astrocytes, and oligodendrocytes as well as microglia. This would make it impossible even for highly brain‐permeant NKCC1 blockers to specifically target depolarizing and excitatory actions of γ‐aminobutyric acid in principal neurons of the brain, which is postulated as the rationale of clinical trials on neonatal seizures.

Is Intraperitoneal Tobramycin Ototoxic in CAPD Patients?

In 40 CAPD patients treated for peritonitis, the authors did a prospective study of ototoxic effects of intraperitoneal tobramycin. They evaluated cochlear function in pure-tone threshold audiograms over a range of frequencies from 250–10, 000 Hz, in the speech-reception threshold test and in the speech-discrimination test. These tests were performed within 48 hours of initiation of tobramycin treatment and within 2 or 3 weeks of the drug's discontinuation. With the aminoglycoside doses used in this study, no statistical difference between the mean baseline and mean follow-up hearing levels was seen in these 40 patients. However, according to the standard criteria of ototoxicity, the hearing in 10 of 40 patients (25%) deteriorated after tobramycin, while it improved in seven patients (17.5%). In the remaining 23 (57.5%), hearing remained stable. With respect to the risk factors for ototoxicity such as advanced age, increased duration of treatment, elevated plasma aminoglycoside levels, concomitant treatment with other ototoxic drugs, pre-existing hearing loss, renal dysfunction and hyperthermia, no statistically significant difference was demonstrated between the patients with deteriorated, stable or improved hearing. The results of this study do not confirm that tobramycin given intraperitoneally to CAPD patients produces auditory toxicity. The hearing deterioration observed in 10 patients may be due to synergistic factors. The improvement observed in 7 patients could not be explained.

Predicting the Need for a Tier II Ototoxicity Study From Early Renal Function Data

History has established that many drugs, such as the antibiotics, chemotherapies, and loop diuretics, are capable of inducing both nephrotoxicity and ototoxicity. The exact mechanisms by which cellular damage occurs remain to be fully elucidated. Monitoring the indices of renal function conducted in the Food and Drug Administration's prescribed set of early investigational new drug (IND)-enabling studies may be the first signs of ototoxicity properties of the new drug candidate. In developing improved and efficacious new molecular entities, it is critically necessary to understand the cellular and molecular mechanisms underlying the potential ototoxic effects as early in the drug development program as possible. Elucidation of these mechanisms will facilitate the development of safe and effective clinical approaches for the prevention and amelioration of drug-induced ototoxicity prior to the first dose in man. Biomarkers for nephrotoxicity in early tier I or tier II nonclinical IND-enabling studies should raise an inquiry as to the need to conduct a full auditory function assay early in the game to clear the pipeline with a safer candidate that has a higher probability of continued therapeutic compliance once approved for distribution.

Effect of Corticosteroid on Salicylate-Induced Morphological Changes of Isolated Cochlear Outer Hair Cells

Our previous studies showed that pretreatment with corticosteroids, which inhibits release of arachidonic acid (precursor of prostaglandins and leukotrienes), partially prevented salicylate-induced hearing loss in vivo. The purpose of this study was to determine the effect of pretreatment with corticosteroid (dexamethasone sodium phosphate) on isolated cochlear outer hair cells (OHCs) exposed to salicylate in vitro. Isolated OHCs from the chinchilla cochlea were exposed to salicylate with or without pretreatment with dexamethasone. Images were stored and analyzed on the Image program. The OHCs exposed to salicylate demonstrated a significant shortening in cell length. The OHCs exposed to salicylate after pretreatment with dexamethasone exhibited no significant change in cell length. We conclude that corticosteroid treatment of isolated OHCs is effective in blocking the morphological changes induced by salicylate. This study gives additional evidence that salicylate ototoxicity is mediated by alteration in the levels of arachidonic acid metabolites.

Ototoxicity: the hidden menace

Otolaryngology, although considered a surgical specialty, also covers many diseases that are not cured by surgery. These are treated medically and thus the otolaryngologist should have a good knowledge of drug treatments. It also entails ability to recognize, when an ENT symptom may be caused by one of the patient's medications, particularly as this is easily remedied by changing the drug. Although most of us know the common drugs that can cause otological side effects, there are many others that we may not be aware of. Here we have tried to consolidate a list of some commonly used drugs having otological side effects.

Ototoxic drugs and noise

Drugs that produce tinnitus can be subdivided into those which produce temporary or permanent hearing loss and those which apparently do not cause any hearing loss. The tinnitus occurring with drugs of the first group is probably secondary to the hearing loss. However, most of the drugs that produce tinnitus without an accompanying hearing loss probably do so because of their effect on biogenic amines in the central nervous system and/or as an extension of their proconvulsant side-effects. A pre-existing cochlear impairment is the underlying factor in most patients who experience tinnitus. Not only can ototoxic drugs or high levels of noise produce cochlear impairment but the interaction of the two can place humans in more jeopardy than when exposed to either agent alone. Chloramphenicol has little ototoxic potential when administered systemically in humans. However, our studies show that when chloramphenicol is combined with noise exposure in rats, considerably more cochlear damage results than from the noise alone (chloramphenicol alone does no produce any cochlear damage). We are presently conducting more detailed studies of this ototoxic interaction to determine whether it occurs with other antibiotics (such as erythromycin) which are also commonly considered to have minimal ototoxicity.

Myotonia congenita

Myotonia is a symptom of many different acquired and genetic muscular conditions that impair the relaxation phase of muscular contraction. Myotonia congenita is a specific inherited disorder of muscle membrane hyperexcitability caused by reduced sarcolemmal chloride conductance due to mutations in CLCN1, the gene coding for the main skeletal muscle chloride channel ClC-1. The disorder may be transmitted as either an autosomal-dominant or recessive trait with close to 130 currently known mutations. Although this is a rare disorder, elucidation of the pathophysiology underlying myotonia congenita established the importance of sarcolemmal chloride conductance in the control of muscle excitability and demonstrated the first example of human disease associated with the ClC family of chloride transporting proteins.

Distinguishing cochlear pathophysiology in 4-aminopyridine and furosemide treated ears using a nonlinear systems identification technique

To test the adequacy of physiologic indices derived from a third-order polynomial model quantifying cochlear mechano-electric transduction (MET), 24 Mongolian gerbils were exposed to either 250-mM glucose (control), 150-mM 4-aminopyridine (4-AP), or 30-mM furosemide solutions applied to the round window (RW) membrane. The cochlear microphonic (CM) was recorded from the RW in response to 68- and 88-dB SPL Gaussian noise. A nonlinear systems identification technique (NLID) provided the frequency-domain parameters and physiologic indices of the polynomial model of MET. The control group showed no change in both compound action potential (CAP) thresholds and CM. Exposure to 4-AP and furosemide resulted in a similar elevation in CAP thresholds and a reduction in CM. However, the polynomial model of MET showed different changes. The operating point, slope, and symmetry of the MET function, the polynomial model parameters, and related nonlinear coherences differed between the experimental groups. It is concluded that the NLID technique is sensitive and specific to alterations in the cochlear physiology.

Mice lacking the basolateral Na-K-2Cl cotransporter have impaired epithelial chloride secretion and are profoundly deaf

In chloride-secretory epithelia, the basolateral Na-K-2Cl cotransporter (NKCC1) is thought to play a major role in transepithelial Cl(-) and fluid transport. Similarly, in marginal cells of the inner ear, NKCC1 has been proposed as a component of the entry pathway for K(+) that is secreted into the endolymph, thus playing a critical role in hearing. To test these hypotheses, we generated and analyzed an NKCC1-deficient mouse. Homozygous mutant (Nkcc1(-/-)) mice exhibited growth retardation, a 28% incidence of death around the time of weaning, and mild difficulties in maintaining their balance. Mean arterial blood pressure was significantly reduced in both heterozygous and homozygous mutants, indicating an important function for NKCC1 in the maintenance of blood pressure. cAMP-induced short circuit currents, which are dependent on the CFTR Cl(-) channel, were reduced in jejunum, cecum, and trachea of Nkcc1(-/-) mice, indicating that NKCC1 contributes to cAMP-induced Cl(-) secretion. In contrast, secretion of gastric acid in adult Nkcc1(-/-) stomachs and enterotoxin-stimulated fluid secretion in the intestine of suckling Nkcc1(-/-) mice were normal. Finally, homozygous mutants were deaf, and histological analysis of the inner ear revealed a collapse of the membranous labyrinth, consistent with a critical role for NKCC1 in transepithelial K(+) movements involved in generation of the K(+)-rich endolymph and the endocochlear potential.

Electron microscopic temporal bone histopathology in experimental pneumococcal meningitis

Bacterial meningitis is one of the most common causes of acquired profound sensorineural deafness in children. Measurement of hearing and examination of the cochlea is limited in patients suffering from acute meningitis. A rabbit model of pneumococcal meningitis was developed to identify the temporal bone histopathologic changes that occur in meningogenic labyrinthitis caused by Streptococcus pneumoniae. Light microscopy was previously performed on temporal bones from acutely meningitic rabbits with profound hearing loss as determined electrophysiologically. Extensive inflammation of the cochlea with endolymphatic hydrops was observed. The organ of Corti, however, showed preserved architecture in the majority of these animals. In order to further investigate these findings, a protocol was used to create meningitic rabbits with hearing loss ranging from early high-frequency loss to profound deafness. The temporal bones from 7 rabbits were examined by transmission electron microscopy. In cases of mild hearing loss, partial degeneration of the inner row of outer hair cells, as well as edema of efferent cochlear nerve endings and marginal cells of the stria vascularis, was seen. With increasing degrees of hearing loss, the remainder of the organ of Corti and intermediate cells of the stria showed ultrastructural abnormalities. Spiral ganglion cells and basal cells of the stria vascularis remained intact in all subjects. This study provides unique information regarding the histology and pathophysiology of meningogenic deafness. The clinical implications of these findings are discussed, with an emphasis on potentially reversible changes and therapeutic intervention.

Effect of leukotriene inhibitor on salicylate induced morphologic changes of isolated cochlear outer hair cells

Our previous studies have shown that salicylate ototoxicity is associated with decreased levels of prostaglandins (PGs) and increased levels of leukotrienes (LTs) in the perilymph. Other studies have demonstrated that salicylate ototoxicity is associated with decreased cochlear blood flow, reversible changes in isolated cochlear outer hair cells (OHCs), and decreased otoacoustic emission. We have shown that pretreatment with an LT inhibitor prevents salicylate induced hearing loss, a decrease in cochlear blood flow and changes in otoacoustic emissions. The objectives of the current study were to determine the effect of exposure of salicylate and LTs on the morphology of isolated OHSc and to determine the effect of LT inhibitors on salicylate induced morphologic changes of isolated OHCs. Isolated OHCs from chinchilla cochlea were exposed to different test solutions. The groups included sodium salicylate (10 mM) with or without pretreatment with an LT inhibitor (L-663, 536, 30 microM), 0.1 or 1.0 microM solution of LTC4, LTD4, LTE4, and two control solutions, standard bathing solution (SBS) or leukotriene inhibitor alone. Osomolality of all solutions were kept at 305 +/- 5 mmolkg-1. The OHCs were observed under an inverted microscope. Images were stored onto a computer and analyzed later. OHCs exposed to the salicyalate developed a decrease in mean cell length. The exposure of OHCs to LTC4, LTD4, and LTE4 also demonstrated a similar decrease in mean cell length. Cells in the control SBS or LT inhibitor alone groups did not show any change. OHCs exposed to salicylate in the presence of the LT inhibitor did not exhibit morphologic changes. This study suggest that arachidonic acid metabolites, especially an increase in the concentration of LTs, seem to play an important role in the pathogenesis of salicylate ototoxicity.

Recording of brainstem evoked potentials and their association with gentamicin in neonates

Brainstem auditory evoked potential (BAEP) recording was used to screen presymptomatically the hearing of 200 neonates treated with ampicillin (100 mg/kg daily) and gentamicin sulfate (5 mg/kg daily). The study included 130 male and 70 female neonates; post-conceptional age ranged from 34 to 57 weeks (mean 42.36 weeks). We divided neonates into 2 groups according to duration of antibiotic treatment; group 1 consisted of 179 patients who were treated with antibiotic agents for < or = 7 days. Although 15 (8.4%) in this group initially manifested abnormal BAEP recordings, only 8 of these brain-damaged neonates (4.5%) (6 with peripheral and 2 with central dysfunction) later manifested abnormal recordings. Group 2 consisted of 21 neonates who were treated for 10 to 30 days; BAEP recordings were abnormal in 7 patients (33.3%) (4 with peripheral and 3 with central dysfunction). We conclude that BAEP is indicated only for neonates treated with gentamicin sulfate for > 10 days. In this group, infants so treated usually have underlying disease or severe infection, including birth asphyxia, hypoxia, sepsis, and meningoencephalitis, all of which are clinically significant indicators of high risk for auditory pathway dysfunction.

Mechanism of cisplatin ototoxicity: antioxidant system

The dose and duration limiting toxic effects of cisplatin are ototoxicity and nephrotoxicity. While several studies have attempted to shed some light on the causes of nephrotoxicity, the reasons for ototoxicity induced by cisplatin are poorly understood. Therefore, this investigation was undertaken to delineate the potential mechanisms underlying cisplatin ototoxicity. The role of glutathione (GSH), oxidized glutathione (GSSG) and malondialdehyde levels, and antioxidant enzyme activities [superoxide dismutase, catalase, GSH peroxidase, and GSH reductase] were examined in cochlear toxicity following an acute dose of cisplatin. Male Wistar rats were treated with various doses of cisplatin. Pretreatment auditory brain stem evoked responses (ABR) were performed and then post-treatment ABRs and endocochlear potentials were also performed after three days. Acute cochlear toxicity (ototoxicity) was evidenced as elevated hearing thresholds and prolonged wave I latencies in response to various stimuli (clicks and tone bursts at 2, 8, 16 and 32 kHz) on ABRs. The endocochlear potentials were reduced (50% control) in cisplatin-treated rats as compared to control animals. The rats were sacrificed and cochleae isolated. The GSH, GSSG and malondialdehyde levels, and antioxidant enzyme activities were determined. Cisplatin ototoxicity correlated with a decrease in cochlear GSH [0.45 +/- 0.012 nmol/mg] after cisplatin administration compared to 0.95-012 nmol/mg in control cochleae (P < 0.05). Superoxide dismutase, catalase activities and malondialdehyde levels were significantly increased in the cochleae of cisplatin injected rats. Cochlear GSH-peroxidase and GSH reductase activity significantly decreased after cisplatin administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Ototoxicity in vitro: effects of neomycin, gentamicin, dihydrostreptomycin, amikacin, spectinomycin, neamine, spermine and poly-L-lysine

The effects that the aminoglycoside-aminocyclitol antibiotics amikacin, dihydrostreptomycin, gentamicin, neomycin, and spectinomycin, the neomycin fragment neamine, and the polybasic compounds spermine and poly-L-lysine, have on outer hair cells in cochlear cultures prepared from the early post-natal mouse have been assessed using both scanning and transmission electron microscopy (SEM and TEM). The antibiotics were used at concentrations ranging from 0.25-1.0 mM, spermine from 10 microM to 3.0 mM, and poly-L-lysine from 0.05-2 microM. Qualitative assessment of apical surface damage allows the antibiotics to be ranked in the following order: neomycin > gentamicin > dihydrostreptomycin > amikacin > neamine > spectinomycin. At a concentration of 1 mM spectinomycin is essentially non-toxic and the effects of neamine are marginal. Poly-L-lysine and spermine also cause surface damage, with poly-L-lysine being substantially more toxic than any of the antibiotics, and spermine ranking, on the basis of SEM observations, between dihydrostreptomycin and amikacin. TEM indicates that although all toxic compounds cause damage to the apical surface of the hair cell, only neomycin, poly-L-lysine and spermine induce the formation of whorls of tightly packed membrane resembling myelin within the apical surface lesions to any great extent. Apical-surface changes induced by dihydrostreptomycin and amikacin are simply large distensions of the cell filled with cytoplasmic organelles of normal appearance. Although the effects of the aminoglycoside antibiotics are largely limited to the apical surface of the cell, poly-L-lysine induces complete necrosis of the cell, and spermine causes a dramatic increase in cytoplasmic electron density and condensation of the nuclear chromatin.

Freeze fracture analysis of apical membranes in cochlear cultures: differences between basal and apical-coil outer hair cells and effects of neomycin

Previous studies have shown that exposure of cochlear cultures to the ototoxic aminoglycoside antibiotic neomycin leads to the rapid formation of numerous membrane filled protrusions on the apical surface of the hair cells but not on the surrounding supporting cells, and that hair cells in basal-coil cultures are more sensitive to these effects of neomycin than those in the distal end of apical-coil cultures. Freeze-fracture has been used to examine and compare the apical surfaces of hair cells and supporting cells in basal and apical-coil cultures in order to look for features that may explain the differential sensitivity of the various cell types to neomycin, and to characterize the membrane type that forms in response to neomycin and compare it with the normal apical membrane of the hair cell. The apical surface of the highly responsive basal-coil outer hair cells differs significantly from the apical surfaces of apical-coil outer hair cells and supporting cells in both regions of the cochlea in both surface area and the number and density of endocytotic vesicles associated with this surface. Basal-coil hair cells have an average of 120 +/- 39 vesicles per cell surface and a density of 3.5 +/- 0.89 vesicles per microns 2, whereas apical-coil hair cells have 14.8 +/- 15.8 vesicles/cell surface and density of 0.73 +/- 0.72 vesicles per microns 2. There are no significant differences in intramembrane particle (IMP) density on the apical surfaces of all the cell types examined, and qualitative observations of filipin-treated specimens indicate that cholesterol densities are also similar. The membrane that accumulates in response to neomycin treatment at the apical pole of the hair cell is IMP free, does not respond to filipin, and fractures in a manner that is indicative of a high content of unsaturated phospholipid in a fluid phase, and is therefore different in several respects from the normal apical surface of the hair cell. The results of this study suggest that apical surface associated endocytotic vesicle numbers may determine the differential sensitivity of apical and basal-coil hair cells to neomycin, and that neomycin may interfere with some aspect of phospholipid metabolism or membrane turnover in sensory hair cells.

Pharmacokinetics of furosemide in endolymph

To investigate the pharmacokinetics of organic anions in the endolymph of the guinea pig, 100 mg/kg furosemide, an organic anion, was intravenously given to measure the concentration in the cochlear endolymph by high-performance liquid chromatography with fluorescence detection. In the endolymph, the concentration of the furosemide increased slowly for 1 hr to 1.6 micrograms/ml and gradually declined thereafter. Pretreatment with 200 mg/kg probenecid, an anion transport inhibitor, had no effect on the furosemide elimination in the endolymph except on the concentration at 2 hr. This was contrary to the drastic change observed in the perilymph of the scala tympani by the same pretreatment. Analogous to the effect in the endolymph, probenecid showed no change in the concentration of the serum, while a pronounced gradient of furosemide concentration existed between them. The present results suggest that the furosemide passively transfers from blood to the endolymph at a relatively low penetrability.

Cochlear permeability of neomycin and gentamicin: an immunohistochemical study

Aminoglycoside antibiotics (AGAs) target specifically the cochleo-vestibular hair cells, but with varied ototoxicity. Differences in their penetration and clearance rates into the membranous labyrinth may play a role. This in turn may be related to a difference in the number of amine groups, the cationic nature, as well as the molecular weight and size of the AGA molecule. Immunohistochemical labeling techniques were used to study the pathways of gentamicin and neomycin from the perilymph into cochlear tissues and target cells. The more cochleotoxic AGA, neomycin, penetrated into cochlear tissues faster than the less cochleotoxic AGA, gentamicin.

Effects on cochlear morphology of repeated insults to the stria vascularis

The interrelationship of stria vascularis and organ of Corti integrity was investigated. Strial morphology was altered by repeated injections of ethacrynic acid in the chinchilla. Although prolonged temporary strial damage was created, neither strial atrophy nor organ of Corti damage resulted.

Cochlear cultures as a model system for studying aminoglycoside induced ototoxicity

Light and electron microscopy have been used to evaluate the effects of treating mouse cochlear cultures with the ototoxic aminoglycoside antibiotic neomycin sulphate at concentrations of 0.2 mM and greater for periods of up to 1 hour. Neomycin rapidly induces the formation of numerous, membrane filled blisters on the apical surfaces of the sensory hair cells. Such morphological damage is restricted to the hair cells, and is not observed on the surfaces of supporting cells within the organ of Corti. Hair cells in apical-coil cultures are less sensitive than those in basal-coil cultures, and, at any given point along the cochlea, outer hair cells appear to be more extensively damaged by neomycin than inner hair cells. These morphological effects of neomycin are considerably more severe when the drug is applied in calcium/magnesium free saline, and can be blocked by elevating the saline concentration of either calcium or magnesium. The effects can also be blocked by lowering the temperature to 4 degrees C, but not by either K+ depolarization or the lectin Concanavalin A. The potential value of this culture system as a model for studying aminoglycoside induced ototoxicity is discussed.

Furosemide alters organ of corti mechanics: evidence for feedback of outer hair cells upon the basilar membrane

A widely held hypothesis of mammalian cochlear function is that the mechanical responses to sound of the basilar membrane depend on transduction by the outer hair cells. We have tested this hypothesis by studying the effect upon basilar membrane vibrations (measured by means of either the Mössbauer technique or Doppler-shift laser velocimetry) of systemic injection of furosemide, a loop diuretic that decreases transduction currents in hair cells. Furosemide reversibly altered the responses to tones and clicks of the chinchilla basilar membrane, causing response-magnitude reductions that were largest (up to 61 dB, averaging 25-30 dB) at low stimulus intensities at the characteristic frequency (CF) and small or nonexistent at high intensities and at frequencies far removed from CF. Furosemide also induced response-phase lags that were largest at low stimulus intensities (averaging 77 degrees) and were confined to frequencies close to CF. These results constitute the most definitive demonstration to date that mechanical responses of the basilar membrane are dependent on the normal function of the organ of Corti and strongly implicate the outer hair cells as being responsible for the high sensitivity and frequency selectivity of basilar membrane responses. A corollary of these findings is that sensorineural hearing deficits in humans due to outer hair cell loss reflect pathologically diminished vibrations of the basilar membrane.

Sensory-evoked potentials in rats chronically exposed to trichloroethylene: predominant auditory dysfunction

Sensory-evoked potentials (EPs) were studied in male Long-Evans and Fischer-344 rats in order to characterize the electrophysiological consequences of chronic inhalation exposure to trichloroethylene (TCE). Groups of ten Long-Evans rats were exposed to air or 1600 ppm or 3200 ppm TCE for twelve weeks and evaluated periodically with a multisensory test battery. Brainstem auditory-evoked response (BAER) amplitudes were depressed by TCE, whereas somatosensory and visual potentials remained normal. The effects on BAERs, which varied with tone intensity and frequency, suggested that TCE causes a predominantly high-frequency hearing loss. Comparable effects were obtained in both strains of rats and were like those previously observed following exposure to toluene.

Furosemide has no effect on endocochlear potential and tuning properties of primary afferent fibres in the pigeon inner ear

In the pigeon no influence of high doses (135 mg/kg) of furosemide on endocochlear potential and sound evoked activity in single auditory nerve fibres was found. This finding contrasts strongly to results in mammals.

Screening procedure for assessment of ototoxicity in the common marmoset

Detection of drug-induced ototoxicity in safety evaluation studies of novel chemical entities is rarely attempted. Where such examinations are included, they usually rely on reflex testing. The Brainstem Auditory Evoked Response can be measured with the use of externally positioned electrodes, and it monitors electrophysiologic responses to sound from the cochlear nerve and associated structures of the 8th cranial nerve. These responses have been reproducibly measured in sedated marmosets and the method shown to be a sensitive detector of hearing loss caused by loop diuretics or aminoglycoside antibiotics. additionally, where hearing damage is reversible, recovery can also be monitored. It is proposed that where a sensitive marker for ototoxicity is considered necessary as part of a multifaceted study investigating in vivo drug safety, this procedure may prove advantageous over existing methods.

Ototoxic and nephrotoxic drugs inhibit agonist-induced inositol phosphate formation in rat brain synaptoneurosomes

Neomycin (an aminoglycoside antibiotic), ethacrynate (a loop diuretic), cisplatin (an anticancer drug) and mercuric chloride are chemically unrelated drugs which present similar ototoxic and nephrotoxic properties. We have found that all these molecules inhibit inositol phosphate turnover induced by carbachol or glutamate in rat brain synaptoneurosomes. Since this second messenger system appears to be a key mechanism for cell functioning and even survival, our observations raise the possibility that the expression of the specific toxicity of these compounds may result from excessive inhibition of the phosphoinositide cascade.

Antimicrobial therapy for pediatric patients

New antimicrobial agents are being introduced for clinical use at an increasingly rapid rate. This has required physicians continually to review relevant data and determine unique properties that might guide selection among any new antibiotics as well as older ones. Efficacy, potential toxicity, and comparative cost (in that order) generally guide selection. The present comprehensive review examines currently available antibiotics along with some under investigation, emphasizing these three basic areas of consideration.

Multimodal effects of acute exposure to toluene evidenced by sensory-evoked potentials from Fischer-344 rats

Male Fischer-344 rats were exposed by inhalation to 500, 2000, 5000, 8000 and 16000 ppm toluene for 30 min in two experiments. Exposures up to 8000 ppm in Experiment 1 caused concentration-related changes in the click-elicited brainstem auditory-evoked response (CBAER), flash-evoked potential (FEP) and somatosensory-evoked potential (SEP). Latencies of CBAER components were prolonged and amplitudes of late components were increased by toluene. Toluene did not detectably alter the latencies of FEP or SEP components. Early FEP component-amplitudes were increased and late component-amplitudes were decreased; toluene also induced a poststimulus oscillation in the FEP. Most component-amplitudes of the SEP were substantially increased, but N2P2 amplitude appeared to be more sensitive than other components to depressant effects of the solvent. The same effects on the CBAER were observed in Experiment 2, but a more substantial increase in the amplitudes of late components elicited by tone pips suggested that frequency-dependent cochlear irritation might underlie previously observed subchronic ototoxicity. These effects were increased by exposure to 16000 ppm toluene. Effects like those observed in Experiment 1 were noted on the FEP, but the oscillations were less with exposure to 16000 than 8000 ppm. Changes in the SEP were evident within 2 minutes of exposure onset, and amplitudes increased over the course of about 15 min, leveling off or decreasing thereafter. The amplitude of the N2P2 component was again less influenced than other components during exposure to 8000 ppm and was reduced to less than baseline amplitude by 16000 ppm. Effects of concentration and rates of development and recovery were systematically related to SEP component latency. Toluene appears to have both enhancing and inhibiting effects on neural pathways serving sensory systems, depending on the modality and the site of generation of the components within modalities. A particular balance between these properties might relate to the hedonic characteristics of this abused solvent.

Toxicity studies with quinine hydrochloride

Three-month studies in the rat, a rat embryo-toxicity study and a specific study to investigate ototoxicity were carried out with quinine hydrochloride. The results of these studies suggest an acceptable daily intake of 40 mg quinine hydrochloride for an adult. There were no indications of teratogenic effects and no indications of interference with auditory function in rats receiving up to 200 mg/kg.

Load responsiveness of protein synthesis in adult mammalian myocardium: role of cardiac deformation linked to sodium influx

Exposure of adult mammalian myocardium to increased hemodynamic loads augments cardiac protein synthesis, ultimately leading to hypertrophy of the affected chamber. This established relationship between loading conditions and protein synthesis was examined in terms of two questions. First, is there a basic difference between the anabolic effect of a passive load imposed on diastolic myocardium and that of an active load generated by systolic myocardium? This issue was addressed by measuring [3H]phenylalanine incorporation into muscle protein in either quiescent or contracting ferret papillary muscles, set at known isometric lengths. Myocardial protein synthesis increased in proportion to total muscle tension in each case, with an equivalent relation describing both quiescent and contracting muscles. Synthesis of two contractile proteins, actin and myosin heavy chain, were enhanced by muscle loading. Thus, a quantitative rather than qualitative difference between the anabolic effects of diastolic and systolic loading was demonstrated. Second, since increased sodium influx is an initial cellular response requisite to the growth-inducing activity of many substances, and since sodium entry through stretch-activated ion channels is stimulated by deformation of the sarcolemma, does cardiac deformation during increased loading promote sodium influx as a signal to increase anabolic activity? In either quiescent or contracting papillary muscles, the rate of 24Na+ uptake was found to increase with load. Streptomycin, a cationic blocker of the mechanotransducer ion channels, was without effect on protein synthesis in stimulated but slack muscles; however, it inhibited, in a dose-related manner, the augmented protein synthesis otherwise observed in contracting muscles developing tension. At 500 microM, streptomycin did not reduce active tension, but it did reduce the synthesis of both actin and myosin heavy chain. In a second pharmacologic approach, inotropic agents were chosen which uniformly increased muscle tension development but which had contrasting effects on sodium influx. Protein synthesis increased in the presence of Na+ influx enhancers, monensin or veratridine; however, protein synthesis decreased in the presence of amiloride, a sodium influx inhibitor. Thus, myocardial protein synthesis varied directly with sodium influx despite the positive inotropic effect observed with each of these agents. In addition, inhibition of protein synthesis by ouabain demonstrated that activation of the Na+ pump is required for the anabolic effect of load.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative vestibular toxicity of dibekacin, habekacin and cisplatin

The vestibular toxicity of two aminoglycoside antibiotics, dibekacin sulfate and habekacin sulfate, and of a drug with potent antimitotic activity, cisplatin (cis-diamminedichloroplatinum) has been investigated in both rats and frogs. In rats, chronic intraperitoneal injection of a saline solution of dibekacin (50 mg/kg/day), habekacin (50 mg/kg/day), cisplatin (0.5 mg/kg/day) for 4 weeks and of cisplatin (1 mg/kg/day) for 5 weeks, produced no behavioral vestibular disorders and the righting reflex could be elicited at any time. In frogs, the spontaneous discharge was recorded from individual fibres of the ampullary nerve of the horizontal semicircular canal before and after acute administration of the drugs, dissolved in Ringer, into the perilymph of the inner ear near the horizontal ampulla. Following injection of 1 microliter of solutions containing 10 micrograms or 20 micrograms of dibekacin, 20 micrograms or 50 micrograms of habekacin, 0.5 micrograms, 2.5 micrograms or 10 micrograms of cisplatin, the spontaneous discharge decreased in a number of fibres and was sometimes completely abolished. The vestibular toxicity of the three drugs tested is discussed with respect to that of aminosides whose ototoxicity is well known.

Effect of cisplatin on hair cell morphology and lateral wall Na,K-ATPase activity

The dose-response ototoxic effects of cisplatin were studied in guinea pigs. Loss of Preyer reflex and suppression of the N1 amplitude occurred in cisplatin-treated animals and was described as dose-related. Drug-induced hair cell damage, as observed with scanning electron microscopy, occurred sporadically throughout the turns of the cochlea and the incidence increased with dose. Na,K-ATPase activity in the lateral wall tissues was not significantly different between treatment groups. The results reported here indicate that cisplatin ototoxicity was dose-dependent, but was not directly related to Na,K-ATPase activity in the lateral wall.

Differential effects of benzodiazepines on cochlear and auditory nerve responses

The influence exerted by chlordiazepoxide or midazolam upon auditory nerve compound action potential (cAP) and cochlear microphonic (CM) has been analyzed in chronic as well as in acutely prepared guinea pigs. Pre-receptorial variables were carefully controlled. The benzodiazepines dissociated the cochlear recorded potentials, increasing the cAP amplitude, in response to clicks, and decreasing the CM area, produced by a coherent pure tone pip. Both responses were dose related. A direct effect upon the cochlea was eliminated by local infusion of the drugs. It was also demonstrated to be a specific benzodiazepinic action because the use of an antagonist, Ro 15-1788, abolished the effect. Benzodiazepines could have increased the GABAergic activity at the pontine origins of the olivo-cochlear bundle or in the reticulo-cochlear fibers. These are the only central pathways that could be responsible for the effects obtained at the cochlea or auditory nerve levels. We suggest that this is the cause of the withdrawal of inhibitory tonus from the primary afferent fibers mediated by the efferent system (lateral superior olive), as may occur during dishabituation. It may also be the cause of the CM decrement, but the effect in this case would be exerted mainly through another set of efferent fibers (trapezoid body nucleus).

Vestibulo-ocular reflex (VOR) in guinea pigs. Impairment induced by aminoglycoside antibiotics

The vestibulo-ocular reflexes (VORs) were studied in guinea pigs receiving daily administration of aminoglycoside antibiotics. The vestibular epithelia were also examined by scanning electron microscope technique (SEM). The treatment with aminoglycosides led to varying degrees of VORs according to (i) the type of aminoglycoside drug; (ii) the duration of the treatment and, (iii) the sensitivity of the various vestibular receptors. Gentamicin caused an earlier and severe reduction of the VOR gain. Dibekacin also caused evident damage, but the onset of its action was delayed. Both drugs affected mainly the vertical responses. Tobramycin and netilmicin altered the VORs slightly. Histological examination revealed damage to the sensory epithelia corresponding to the observed VOR impairments.

Comparative ototoxicity of furosemide and piretanide

The purpose of these studies was to compare the ototoxicity of piretanide to that of furosemide in the chinchilla. Chinchillas weighing 400 to 700 g were anesthetized with ketamine plus pentobarbital. Endocochlear potential (EP) was measured continuously by the round window approach using glass microelectrodes. Piretanide or furosemide injected through a jugular vein catheter in adults range from 10-100 mg/kg. Little or no change in EP was noted with doses below 15 mg/kg of either diuretic. The overall pattern of decline of EP was rather similar in piretanide-treated and furosemide-injected animals, and it was interesting to find that the dose-ototoxicity response curve for piretanide was similar to that for furosemide. The diuretic effect of equal ototoxic doses of either diuretic was equivalent. Both diuretics cause a decrease of endocochlear potential when applied locally to the round window membrane of the chinchilla as well as by systemic administration. These findings suggest that piretanide and furosemide have approximately the same propensity for ototoxicity.

Comparative ototoxicity of netilmicin, gentamicin, and tobramycin in cats

Netilmicin, a semisynthetic aminoglycoside antibiotic, is less ototoxic in a variety of species than other aminoglycosides currently in therapeutic use. In this study, mixed-breed cats (four/group) were given daily sc injections of netilmicin (20, 40, and 80 mg/kg), gentamicin (20 and 40 mg/kg), or tobramycin (20, 40, and 80 mg/kg) for up to 30 weeks or until ototoxicity was observed. The animals were examined throughout the study for effects on cochlear and vestibular function. Hematologic, serum chemical, and drug-serum (24-hr postdose) assays were performed at approximate monthly intervals during the dosing period. The cochleae, kidneys, and liver were examined microscopically. The mean number of dose days required to produce vestibulotoxic effects, demonstrated by impaired righting reflex or locomotor ataxia, was from 41 to 61 in cats dosed with tobramycin (40 and 80 mg/kg) or gentamicin. No vestibular dysfunction was observed in any of the netilmicin 20-mg/kg-dosed cats, in two cats each of the tobramycin 20-mg/kg and netilmicin 40-mg/kg groups, and in one netilmicin 80-mg/kg-dosed animal. Histologic examination of the cochleae revealed degeneration of the hair cells and supporting sensory structures in the majority of cats dosed with gentamicin at 20 and 40 mg/kg and tobramycin at 40 and 80 mg/kg. Less than 50% of the tissues from cats of the tobramycin 20-mg/kg and netilmicin 40- and 80-mg/kg-dosed groups had similar degenerative cochlear changes. No cochlear damage was noted in any of the cats given netilmicin at 20 mg/kg. Results of the clinical laboratory determinations were generally unremarkable. Proximal tubular degeneration was the principal finding observed in the kidneys of the animals. Under the conditions of this study, at least a twofold (vestibular) to fourfold (cochlear) relative safety margin for ototoxicity was established in favor of netilmicin over tobramycin and gentamicin.

Metachromasia as the key to aminoglycoside ototoxicity. Preliminary report

In in-vitro experiments, it was shown that aminoglycosides, in contrast to other antibiotics, inhibited the metachromatic reaction using o-toluidine blue as a basic dye and heparin-Na as a polysulfate-polysaccharide substrate. The inhibitory concentrations were inversely proportional to the free amino groups of aminoglycosides tested. Taking into consideration (a) that the aminoglycosides antagonize the presence of Ca++ and (b) that mechanisms of ototoxicity involve ionic alterations in endolymph, we suggest that: aminoglycoside ototoxicity could be the result of the reaction between aminoglycosides and anionic polyelectrolytes, of the group of polysulfated polysaccharides, in a metachromatic process.

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.

The phenotype in partial 13q trisomies, apropos of a familial (13;15)(q22;q26) translocation

A 12 month-old male patient with a karyotype 46,XY,-15,+der(15),t(13;15)(q22;q26)pat is presented. His stillborn sib showed malformations compatible with the 13q deletion syndrome, probably due to a 46,XY,der(13) karyotype. Phenotypic analysis of 41 cases from the literature with partial distal 13q (D13q) trisomies indicate that the segment 13q22----qter in trisomy with or without another concomitant aneusomy is sufficient to produce the majority of the trisomy 13 syndrome features, some of which (cleft palate, increased HbF and projections in PMN) are present in different non-overlapping partial 13q trisomies. About 82% of the D13q trisomies are inherited, more frequently from the mother.

Reversible hearing loss after meningitis. Prospective assessment using auditory evoked responses

Hearing loss (HL) was prospectively studied in 236 children with meningitis using brainstem auditory evoked responses. Hearing loss was detected in 38 (16.1%) in the acute phase of the disease and in 10 (5.2%) of 193 patients who were tested 6 months later. Hearing loss was more frequent and more severe in patients with bacterial meningitis (25.4% in the acute phase, and 8% 6 months later) than in patients with viral and tuberculous meningitis. In half of the affected cases the auditory lesion was bilateral. Follow-up was possible in 32 patients with early postmeningitic HL. Ten (31.2%) had permanent HL while the remaining 22 (68.7%) recovered normal hearing. Severe initial losses tended to be permanent, whereas minimal and intermediate losses were generally reversible and patients recovered completely in 1 to 6 months. The high incidence of HL in bacterial meningitis suggests that any one of the existing methods of auditory screening should be performed in all patients during the convalescent period.

Discrimination of auditory from nonauditory toxicity by reflex modulation audiometry: effects of triethyltin

The evaluation of toxicity in sensory systems presents particular problems because of the need to distinguish specific sensory loss from other toxic effects. It has recently been shown that modulation of reflex behavior by low-intensity test stimuli can be used to provide a rapid assessment of sensory acuity and of ototoxic hearing loss. In this report the ability of the method to distinguish the known neuromuscular consequences of triethyltin (TET) from hearing loss is demonstrated. Rats treated with TET bromide (30 mg/liter) in their drinking water for 3 weeks showed a profound decrease in acoustic startle reflex amplitudes which was apparent prior to overt neuromuscular weakness and subsequent hindlimb paralysis. Following cessation of treatment, startle levels recovered to preexposure values. Despite these large changes in startle reflex amplitudes, TET treatment did not alter the ability of pure tone stimuli to modulate reflex behavior. These results indicate that TET as administered in these studies disrupted neuromuscular function but did not affect hearing.

Aminoglycoside ototoxicity in the chick (Gallus domesticus) inner ear: I. The effects of kanamycin and netilmicin on the basilar papilla

A single dose (100 mg/kg of egg weight) of kanamycin or netilmicin was injected into the yolk sacs of 7-day-old chick (Gallus domesticus) embryos. Embryos were collected every 24 hours and processed for light and transmission electron microscopy. Morphologic study of the medial basilar papilla disclosed that both kanamycin and netilmicin are toxic to the hair cells in this region. Intoxication was manifested by an increased number of dense osmiophilic bodies, swollen mitochondria, agglomerated chromatin, and occasional disorganization of the kinocilium basal bodies. The cytologic changes observed in the hair cells of embryos injected with netilmicin and kanamycin were similar. However, mitochondrial damage was more severe in the chicks after kanamycin than after netilmicin injection. Some of the cytologic alterations described here are comparable to those already reported for aminoglycoside-intoxicated hair cells in several mammalian species. This study and previous work indicate that the chick embryo provides a satisfactory developmental model for testing ototoxicity of drugs in vivo.

Effect of "high ceiling" diuretics on active salt transport in the cortical thick ascending limb of Henle's loop of rabbit kidney. Correlation of chemical structure and inhibitory potency

The group of "high ceiling" diuretics consists of a variety of chemically different potent diuretic and saluretic substances. Appart from a few exemptions direct evidence for an action of these substances in the thick ascending limb of the loop of Henle (TAL) is still lacking. For furosemide, we have reported recently that it inhibits most likely the Na+-2 Cl--K+ cotransport system present in the lumen membrane of the TAL. The present study tests: 1. whether other "high ceiling" diuretics have a similar site and mode of action, and 2. how modifications of the furosemide molecule alter the inhibitory potency. Isolated cortical TAL (cTAL) segments (n = 185) of rabbit kidneys were perfused in vitro. The equivalent short circuit current (Isc = transepithelial PD/transepithelial resistance), as a measure of active salt transport was correlated to the dose of 64 substances. Several diuretics, such as 2-aminomethyl-4-(1,1-dimethyl-ethyl)-6-iodophenol hydrochloride (MK 447), hydrochlorothiazide, muzolimine, etozoline, tizolimide, amiloride, and triamterene were ineffective both from the lumen and basolateral side at concentrations as high as 10(-4) - 10(-3) mol X 1(-1). The phenoxyacetic acids ethacrynic acid, indacrinone (MK 196), and to less an extend tienilic acid were inhibitory active. They differed from furosemide in one or more of the following criteria: delayed onset, incomplete reversibility, stronger action from the bath, different slope of the dose response curve. Similarly, 1-ozolinone acted stronger from the bath. In contrast, the diuretics of the furosemide type and related compounds (bumetanide and piretanide) showed rapid onset and complete reversibility of inhibition. These substances acted stronger from the lumen. The individual positions in the benzyl ring of the diuretics were differently affected by substitutions, leading to parallel shifts in the dose response curves with halfmaximal inhibition at concentrations ranging between 8 X 10(-8) to greater than 10(-4) mol X 1(-1). For these substances the calculated Hill coefficients were close to unity: 0.96 +/- 0.05. We conclude that the so called "high ceiling" or "loop" diuretics consist of at least 3 groups: 1. drugs that do not interfere with the active salt transport in the cTAL segment, 2. drugs that interfere by so far not characterised mechanisms, and 3. drugs of the furosemide type which inhibit the Na+-2 Cl--K+ cotransport system in the lumen membrane of the cTAL segment.

Drug interactions and the elderly

The elderly take more drugs than the young, and are therefore more susceptible to adverse drug interactions. Prescribing the minimum number of agents is recommended. Drug absorption, metabolism, and excretion may be altered by various interactions. Dose-response relationships of oral anticoagulants, oral hypoglycemics, anticonvulsants, steroid hormones, theophylline, and tricyclic antidepressants are significantly affected by changes in drug metabolism. Proper adjustment of dosage permits the safe, concomitant use of agents that interact with these drugs. Pharmacodynamic interactions also occur in the elderly. A common example is the additive effect of several drugs with central nervous system depressant activity, causing oversedation.

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.

The pharmacology of ototoxic drugs

This article briefly reviews the nature of the toxic effects of drugs on the inner ear and the incidence of ototoxic side effects in man. There follows a more detailed discussion of the most important groups of ototoxic drugs which are identified as the aminoglycoside antibiotics, the "loop" diuretics, quinine and chloroquine, the salicylates and some antitumour drugs. Attention is drawn to the synergistic interaction between aminoglycoside antibiotics and "loop" diuretics and the predisposition to ototoxicity if the drugs are given to subjects with renal impairment. The comparative ototoxicological potential of individual aminoglycosides is discussed and their toxic effects on the kidney and the neuromuscular junction summarized. The importance of an understanding of the pharmacokinetics of aminoglycosides both in relation to toxicity and the rational control of therapy is emphasized.