Developmental Ototoxicity

Susceptibility of immature spiral ganglion neurons to aminoglycoside-induced ototoxicity is mediated by the TRPV1 channel in mice

Aminoglycoside antibiotics are among the most common agents that can cause sensorineural hearing loss. From clinical experience, premature babies, whose inner ear is still developing, are more susceptible to aminoglycoside-induced ototoxicity, which is echoed by our previous study carried out in organotypic cultures. This study aimed to investigate whether a nonselective cation channel, TRPV1, contributes to the susceptibility of immature spiral ganglion neurons (SGNs) to the damage caused by aminoglycosides. Through western blotting and immunofluorescence, we found that the TRPV1 expression levels were much higher in immature SGNs than in their mature counterparts. In postnatal day 7 cochlear organotypic cultures, AMG-517 reduced reactive oxygen species generation and inhibited SGN apoptosis under aminoglycoside challenge. However, in adult mice, AMG-517 did not ameliorate the ABR threshold increase at high frequencies (16 kHz and 32 kHz) after aminoglycoside administration, and the SGNs within the cochleae had no morphological changes. By further regulating the function of TRPV1 in primary cultured SGNs with its inhibitor AMG-517 and agonist capsaicin, we demonstrated that TRPV1 is a major channel for aminoglycoside uptake: AMG-517 can significantly reduce, while capsaicin can significantly increase, the uptake of GTTR. In addition, TRPV1 knockdown in SGNs can also significantly reduce the uptake of GTTR. Taken together, our results demonstrated that aminoglycosides can directly enter immature SGNs through the TRPV1 channel. High expression of TRPV1 contributes to the susceptibility of immature SGNs to aminoglycoside-induced damage. The TRPV1 inhibitor AMG-517 has the potential to be a therapeutic agent for preventing aminoglycoside-induced ototoxicity in immature SGNs.

In vivo overexpression of X-linked inhibitor of apoptosis protein protects against neomycin-induced hair cell loss in the apical turn of the cochlea during the ototoxic-sensitive period

Aminoglycoside-induced cochlear ototoxicity causes hair cell (HC) loss and results in hearing impairment in patients. Previous studies have developed the concept of an ototoxicity-sensitive period during which the cochleae of young mice are more vulnerable to auditory trauma than adults. Here, we compared neomycin-induced ototoxicity at the following four developmental ages in mice: postnatal day (P)1–P7, P8–P14, P15–P21, and P60–P66. We found that when neomycin was administered between P8 and P14, the auditory brainstem response threshold increase was significantly higher at low frequencies and HC loss was significantly greater in the apical turn of the cochlea compared to neomycin administration during the other age ranges. Quantitative real-time PCR (qPCR) data revealed that the expression of apoptotic markers, including Casp3 and Casp9, was significantly higher when neomycin was injected from P8 to P14, while the expression of the X-linked inhibitor of apoptosis protein (XIAP) gene was significantly higher when neomycin was injected from P60 to P66. Because XIAP expression was low during the neomycin-sensitive period, we overexpressed XIAP in mice and found that it could protect against neomycin-induced hearing loss at low frequencies and HC loss in the apical turn of the cochlea. Altogether, our findings demonstrate a protective role for XIAP against neomycin-induced hearing loss and HC loss in the apical turn of the cochlea during the ototoxic-sensitive period, and suggest that apoptotic factors mediate the effect of neomycin during the ototoxic-sensitive period.

Neurodevelopmental and neurofunctional outcomes in children with congenital diaphragmatic hernia

The objective of this review was to provide a critical overview of our current understanding on the neurocognitive, neuromotor, and neurobehavioral development in congenital diaphragmatic hernia (CDH) patients, focusing on three interrelated clinical issues: (1) comprehensive outcome studies, (2) characterization of important predictors of adverse outcome, and (3) the pathophysiological mechanism contributing to neurodevelopmental disabilities in infants with CDH. Improved survival for CDH has led to an increasing focus on longer-term outcomes. Neurodevelopmental dysfunction has been recognized as the most common and potentially most disabling outcome of CDH and its treatment. While increased neuromotor dysfunction is a common problem during infancy, behavioral problems, hearing impairment and quality of life related issues are frequently found in older children and adolescence. Intelligence appears to be in the low normal range. Patient and disease specific predictors of adverse neurodevelopmental outcome have been defined. Imaging studies have revealed a high incidence of structural brain abnormalities. An improved understanding of the pathophysiological pathways and the neurodevelopmental consequences will allow earlier and possibly more targeted therapeutic interventions. Continuous assessment and follow-up as provided by an interdisciplinary team of medical, surgical and developmental specialists should become standard of care for all CDH children to identify and treat morbidities before additional disabilities evolve and to reduce adverse outcomes.

Risk factors for elevation of ABR threshold in NICU-treated infants

OBJECTIVE

Several risk factors for hearing impairment among infants treated in the neonatal intensive care unit (NICU) have been reported, but there have been few studies that show the correlation strength between the risk factors in NICU-treated infants and hearing impairment in childhood. The aim of this study was to clarify the relationship between risk factors in NICU-treated infants and a deteriolation of auditory brainstem response (ABR) threshold in their childhood.

METHODS

One hundred one NICU-treated infants with ABR threshold of 50 dBnHL or more underwent 2nd ABR test at 20 months after delivery. Multiple regression analysis was performed with ABR threshold change as an objective variable and risk factors as explanatory variables.

RESULTS

Two ABR tests of the 101 infants resulted in that 7 showed an elevation of ABR threshold by 20 dB, 70 showed a drop of ABR threshold by 20 dB, and 24 showed no significant change. Multiple regression analysis revealed that the factors contributing to the elevation of ABR threshold were congenital diaphragmatic hernia, severe respiratory disease, and a high C-reactive protein (CRP) level.

CONCLUSIONS

In the infants treated in NICU, an incidence of ABR threshold of 50 dBnHL or more was 9.0%, and 6.9% of the infants with the ABR threshold abnormality showed a significant elevation of ABR threshold in their childhood. Factors significantly related to an elevation of ABR threshold were a history of congenital diaphragmatic hernia, severe respiratory disease, and elevation of CRP. In infants with such factors, periodical examination of hearing is required.

Aminoglycoside ototoxicity

PURPOSE OF REVIEW

To summarize mechanisms of ototoxicity associated with aminoglycoside antibiotics and discuss possible protective strategies.

RECENT FINDINGS

Studies in the past 15 years have demonstrated that aminoglycoside ototoxicity is mediated by an apoptotic form of cell death which employs caspase-dependent pathways. Reactive oxygen species have been demonstrated in the sensory epithelium after aminoglycoside administration and methods of blocking reactive oxygen species in the cochlea have been attempted, but not found to be uniformly effective in protecting against cell loss or threshold shift. Aspirin has recently been studied in a human chemoprevention trial in China, and while data suggest the possibility of protection, there was a significant increase in gastrointestinal bleeds associated with aspirin use.

SUMMARY

There are currently no recommendations for pretreatment or posttreatment therapies to attenuate ototoxicity associated with aminoglycoside antibiotics. Our understanding of the mechanisms of ototoxicity has improved and apoptotic pathways are clearly responsible for hair cell demise. Further studies are necessary before significant improvement in hearing outcome can be expected after use of ototoxic antibiotics.

Risk factors for sensorineural hearing loss in survivors with severe congenital diaphragmatic hernia

Recent improvements in perinatal management have improved the prognosis in patients with severe congenital diaphragmatic hernia (CDH). However, in surviving patients with severe CDH, hearing loss has sometimes been reported to occur during the follow-up period. Although some of the risk factors for developing sensorineural hearing loss (SNHL) have been reported in CDH, no definitive risk factors have yet been reported. We, therefore, investigated the risk factors regarding postnatal management in patients with severe CDH. In 16 surviving patients with severe CDH, which had all been detected antenatally, and whose lung-to-thoracic ratio was less than 0.2, four patients demonstrated late onset SNHL, which occurred between 1.5 and 5 years of age. The risk factors for SNHL regarding the postnatal treatment for CDH were analyzed between the four patients with SNHL and the remaining 12 patients without SNHL, regarding such factors as the use of ototoxic drugs, neuromuscular blocking agents, high-frequency oscillation (HFO), and inhaled nitric oxide, the duration of hypocapnia, hypoxia, severe acidosis, severe alkalosis, and mechanical ventilation. In addition, the types of neuromuscular blocking agents were also analyzed, including the administration of pancuronium bromide (PB) and vecuronium bromide (VB). The patients with SNHL were found to have a significantly higher risk than the patients without SNHL regarding the duration of loop diuretics usage and the duration of usage of both mechanical ventilation and HFO. Furthermore, all four patients with SNHL used PB. In contrast, none of the five patients using VB developed SNHL The duration and cumulative dose of PB used in the patients with severe CDH showed a significant correlation to the occurrence of SNHL. Although this study was retrospective, based on our data, the prolonged use of PB, in addition to the duration of treatment by loop diuretics, mechanical ventilation, and HFO usage, might, thus, be suggested to be a possible risk factor for late onset SNHL in patients with severe CDH.

Melatonin as pharmacologic support in burn patients: a proposed solution to thermal injury-related lymphocytopenia and oxidative damage

OBJECTIVE

To review the data that support the clinical use of melatonin in the treatment of burn patients, with special emphasis on the stimulation of the oxidative defense system, the immune system, circadian rhythm of sleep/wakefulness, and the reduction in the toxicity of therapeutic agents used in the treatment of burn victims.

DATA SOURCE

A MEDLINE/PubMed search from 1975 to July 2006 was conducted.

STUDY SELECTION

The screening of the literature was examined using the key words: burn patients, lymphocytopenia, skin oxidative stress, antioxidant, melatonin, and free radicals.

DATA EXTRACTION AND SYNTHESIS

Thermal injury often causes damage to multiple organs remote from the original burn wound and may lead to multiple organ failure. Animal models and burn patients exhibit elevated free radical generation that may be causative in the local wound response and in the development of burn shock and distant organ injury. The suppression of nonspecific resistance and the disturbance in the adaptive immune system makes burn patients vulnerable to infections. Moreover, there is loss of sleep and the toxicity produced by drugs habitually used in the clinic for burn patients. Melatonin is a powerful antioxidant and is a potent protective agent against damage after experimental thermal injury. Some actions of melatonin as a potential supportive pharmacologic agent in burn patients include its: role as a scavenger of both oxygen and nitrogen-based reactants, stimulation of the activities of a variety of antioxidative enzymes, reduction in proinflammatory cytokines, inhibition of adhesion molecules, chronobiotic effects, and reduction in the toxicity of the drugs used in protocols to treat thermal injury patients.

CONCLUSIONS

These combined actions of melatonin, along with its low toxicity and its ability to penetrate all morphophysiologic membranes, could make it a ubiquitously acting and highly beneficial molecule in burn patients.

The results of newborn hearing screening by means of transient evoked otoacoustic emissions

OBJECTIVE

The incidence of sensorineural hearing loss is between 1 and 3 per 1000 in healthy neonates and 2-4 per 100 in high-risk children. Transient evoked otoacoustic emissions (TEOAEs) represent a method which can be applied to all newborns prior to hospital discharge, enabling early identification of hearing loss. The aim of the study was to evaluate the results of newborn hearing screening by means of TEOAEs.

METHODS

Between 01.10.2002 and 30.09.2003, 5601 newborns born in the University Hospital in Poznan, Poland were screened with ERO SCAN (MAICO). Healthy neonates were screened in the second or third day of life and children treated in pathology unit--when their general condition was stable. The risk factors of hearing loss were recorded in a questionnaire. Children who failed the screening test or had risk factors of hearing impairment were referred to the outpatient clinic for further evaluation.

RESULTS

Risk factors were identified in 739 newborns. The most often risk factors were: use of ototoxic drugs, low Apgar score and prematurity. Positive test result was obtained in 219 (3.91%) children unilaterally and in 137 (2.45%) bilaterally. In healthy children the prevalence of positive results was 3.56% and in high-risk infants 24.9%. The relative risk of positive test results was the highest in infants with positive family history (RR=7.5), congenital malformations (RR=6.7) and low Apgar score (RR=5). Of the group of 912 children, who were referred to the specialist, only 218 turned up to be assessed during the observation period and had the additional otoacoustic emission test performed. There was not any significant difference in the percentage of children with and without risk factors who turned up for the second test and in whom the result was positive (39.7% versus 40.3%). In 41.9% children with risk factors whose screening test was negative, the second exam gave positive result.

CONCLUSIONS

The incidence of positive results in newborn hearing screening is much higher than the prevalence of hearing loss in general population and these results need verification by more precise methods. However, TEOAEs enable to select children who should be referred for audiological evaluation.

Deafness due to A1555G mitochondrial mutation without use of aminoglycoside

OBJECTIVES/HYPOTHESIS

The objective was to clarify the characteristics of deafness associated with the A1555G mutation within mitochondrial 12S ribosomal RNA gene in the absence of aminoglycoside exposure.

STUDY DESIGN

Clinical and genetic studies in family members with the A1555G mitochondrial mutation were performed.

METHODS

The subjects were 123 maternally related members of a large Japanese family with the A1555G mutation. All subjects had no previous history of exposure to aminoglycosides. Hearing disability and handicap, tinnitus, and medical histories were analyzed by interviews in all of the subjects, genetic testing was performed in 41 subjects, and pure-tone audiometry was conducted in 26 subjects with hearing disability and handicap.

RESULTS

The A1555G mutation was detected in a homoplasmic form (meaning that all the mitochondrial DNA carries the mutation) in all 41 subjects who were screened. The risk for developing postlingual hearing loss was likely to be much higher in the present subjects than in the general population. Both the severity and age at onset of the phenotype were similar in affected subjects within the same sibling group. Pure-tone averages were significantly worse in subjects who developed hearing loss before age 10 years than in those who developed hearing loss later.

CONCLUSION

The present study demonstrated that the prevalence of deafness in individuals with the A1555G mitochondrial mutation was likely to be high even in the absence of aminoglycoside exposure and clearly showed the association of severe to profound hearing loss with the onset of hearing loss before age 10 years.

The need for long-term audiologic follow-up of neonatal intensive care unit (NICU) graduates

OBJECTIVE

To evaluate the adequacy of newborn hearing screening in the identification of hearing loss in post-neonatal intensive care unit (NICU) infants.

METHODS

Eighty-two post-NICU infants who had initially passed automated auditory brainstem response (AABR) screening were studied prospectively between November 1997 and July 1999. Tympanometry and transient evoked otoacoustic emissions (TEOAE) were used to evaluate middle ear status and screen the hearing of subjects when they were seen routinely in the Mary L. Johnson Infant Development Clinic, where NICU graduates are followed at our institution. TEOAEs were not performed in subjects with abnormal tympanometry, defined as negative pressures greater than 200 daPa or flat tympanograms.

RESULTS

Of the 82 subjects, 31 (37%) had abnormal tympanometry in at least one ear, with 24 (29%) exhibiting abnormal values bilaterally. Two subjects were identified with delayed-onset or previously undiagnosed sensorineural hearing loss. One had a history of persistent pulmonary hypertension (PPHN) and extracorporeal membrane oxygenation. The other infant had no risk factors for sensorineural hearing loss.

CONCLUSIONS

Our data indicate that newborn hearing screening programs may not provide adequate vigilance for NICU graduates. The high incidence of abnormal middle ear status and the identification of delayed-onset hearing loss in an infant without known risk factors highlights the need for close audiologic and speech/language follow-up in the post-NICU population.

Antibiotic prophylaxis with cefazolin and gentamicin in cardiac surgery for children less than ten kilograms

OBJECTIVE

Antibiotic prophylaxis is recommended in pediatric cardiac surgery, but no data concerning the current antibiotic regimen were available.

DESIGN

Prospective study from April to June 2000.

SETTING

University hospital operating room and postoperative intensive care unit.

PARTICIPANTS

Nineteen consecutive infants less than 10 kg with normal renal function undergoing cardiac surgery with cardiopulmonary bypass longer than 30 minutes.

INTERVENTIONS

Intravenous administration of cefazolin, 40 mg/kg, and gentamicin, 5 mg/kg, at induction of anesthesia; followed by cefazolin, 35 mg/kg every 8 hours, and gentamicin, 2 mg/kg every 12 hours, over 48 hours.

MEASUREMENTS AND MAIN RESULTS

Levels of serum antibiotics were measured: cefazolin (microbiologic) and gentamicin (fluorescence immunoassay) with 8 intraoperative and 5 postoperative samplings. Intraoperatively, cefazolin levels decreased from 166 +/- 44 (mean +/- standard deviation) down to 54 +/- 16 microg/mL and gentamicin from 20.8 +/- 9.5 down to 5.9 +/- 1.5 microg/mL. The postoperative trough levels were 12 +/- 7, 15 +/- 10, and 19 +/- 22 microg/mL for cefazolin and 1.1 +/- 0.5, 0.8 +/- 0.4, and 0.8 +/- 0.9 microg/mL for gentamicin.

CONCLUSIONS

Antibiotic serum levels are consistent with satisfactory efficacy, but intraoperative gentamicin peak levels appeared too high.

Melatonin: reducing the toxicity and increasing the efficacy of drugs

Melatonin (N-acetyl-5-methoxytryptamine) is a molecule with a very wide phylogenetic distribution from plants to man. In vertebrates, melatonin was initially thought to be exclusively of pineal origin recent studies have shown, however, that melatonin synthesis may occur in a variety of cells and organs. The concentration of melatonin within body fluids and subcellular compartments varies widely, with blood levels of the indole being lower than those at many other sites. Thus, when defining what constitutes a physiological level of melatonin, it must be defined relative to a specific compartment. Melatonin has been shown to have a variety of functions, and research in the last decade has proven the indole to be both a direct free radical scavenger and indirect antioxidant. Because of these actions, and possibly others that remain to be defined, melatonin has been shown to reduce the toxicity and increase the efficacy of a large number of drugs whose side effects are well documented. Herein, we summarize the beneficial effects of melatonin when combined with the following drugs: doxorubicin, cisplatin, epirubicin, cytarabine, bleomycin, gentamicin, ciclosporin, indometacin, acetylsalicylic acid, ranitidine, omeprazole, isoniazid, iron and erythropoietin, phenobarbital, carbamazepine, haloperidol, caposide-50, morphine, cyclophosphamide and L-cysteine. While the majority of these studies were conducted using animals, a number of the investigations also used man. Considering the low toxicity of melatonin and its ability to reduce the side effects and increase the efficacy of these drugs, its use as a combination therapy with these agents seems important and worthy of pursuit.

Methods to identify and characterize developmental neurotoxicity for human health risk assessment. I: behavioral effects

Alterations in nervous system function after exposure to a developmental neurotoxicant may be identified and characterized using neurobehavioral methods. A number of methods can evaluate alterations in sensory, motor, and cognitive functions in laboratory animals exposed to toxicants during nervous system development. Fundamental issues underlying proper use and interpretation of these methods include a) consideration of the scientific goal in experimental design, b) selection of an appropriate animal model, c) expertise of the investigator, d) adequate statistical analysis, and e) proper data interpretation. Strengths and weaknesses of the assessment methods include sensitivity, selectivity, practicality, and variability. Research could improve current behavioral methods by providing a better understanding of the relationship between alterations in motor function and changes in the underlying structure of these systems. Research is also needed to develop simple and sensitive assays for use in screening assessments of sensory and cognitive function. Assessment methods are being developed to examine other nervous system functions, including social behavior, autonomic processes, and biologic rhythms. Social behaviors are modified by many classes of developmental neurotoxicants and hormonally active compounds that may act either through neuroendocrine mechanisms or by directly influencing brain morphology or neurochemistry. Autonomic and thermoregulatory functions have been the province of physiologists and neurobiologists rather than toxicologists, but this may change as developmental neurotoxicology progresses and toxicologists apply techniques developed by other disciplines to examine changes in function after toxicant exposure.

Ototoxicity caused by aminoglycosides is ameliorated by melatonin without interfering with the antibiotic capacity of the drugs

The production of free radicals seems to be involved in the mechanisms of ototoxicity. Aminoglycosides produce ototoxicity, which can be determined through distortion product otoacoustic emissions (OAEs) that measure the activity of the outer hair cells of the organ of Corti. An ototoxic chart was obtained in rats using gentamicin or tobramycin. Together with this treatment, the animals ingested melatonin in the drinking water, or melatonin was injected subcutaneously or intramuscularly. The distortion product OAEs were determined over a prolonged period of time for each of the groups. The effect of melatonin on the antibiotic capacity of the aminoglycosides used was also studied. Antibiograms inoculated with Escherichia coli or Pseudomonas aeruginosa and treated with gentamicin or tobramycin in the presence or absence of melatonin at quantities from pharmacological to physiological doses were performed. The ototoxicity produced by gentamicin and tobramycin was maximal from days 3 to 5 post-treatment, returning to normal values in 2 wk. When melatonin was present, the recovery was at day 5 post-treatment, independently of the means of administration of the pineal product. The antibiograms showed that melatonin had no effect on the antibiotic capacity. It is concluded that the ototoxicity caused by gentamicin and tobramycin is ameliorated by melatonin and that the pineal hormone does not interfere with the antibiotic capacity of these antibiotics.

Ototoxicity: an argument for evaluation of the cochlea in safety testing in animals

The cochlea is one of the more common targets for toxic effects, yet current toxicologic screening in animals does not routinely evaluate the cochlea as a potential target organ. Although histopathologic sections are routinely taken from the eye and the optic nerve and tract and most studies include at least 1 section through the nasal cavity and olfactory mucosa, the cochlea is not histopathologically examined in routine toxicity studies. Unfortunately, routine clinical examinations frequently miss ototoxicity because rodents and other species can lose most of their high-frequency hearing and still respond to most ambient noises. Ototoxicity as a deficiency in toxicologic screening can be remedied by using well-established histopathologic and behavioral methods or electrophysiologic methods, such as brain stem auditory evoked responses (BAERs). Once the equipment is in place, BAERs can be obtained quickly and easily for ototoxicity screening (approximately 15 minutes for paired testing of 2 rats and 30 minutes each for dogs). BAERs also can be used in virtually all mammalian species. Three or 4 probe frequencies (eg, 4, 8, 16, and 32 kHz), representing different areas of the cochlea, can be tested in a few minutes with subcutaneous electrodes under short-acting chemorestraint. Given the availability of several approaches to screening for ototoxicity and the importance of the auditory function in human health, safety tests of chemicals and drugs should include an effective screening test for ototoxicity.

Aminoglycosides activate oxygen metabolites production in the cochlea of mature and developing rats

The ototoxicity of antibiotics, particularly of aminoglycosides, is a well-known undesirable side effect which may be based on a free radical mechanism. We studied the effect of different antibiotics in the production of reactive oxygen species in freshly isolated cochleas of mature and 2-10 weeks old developing rats. Phorbol myristate acetate induced the release of reactive oxygen species after a lag time close to 30 s and declined back to basal values in 10-20 min. The rate of reactive oxygen species production correlated inversely to the age in 2-10 weeks old rats. The study of a set of antibiotics showed that a very low concentration of gentamicin and streptomycin (10-100 ng/ml) enhanced the effect of phorbol myristate acetate. At the above-indicated concentrations, ciprofloxacin did not modify phorbol myristate acetate-induced activation. These results show the enhancement by aminoglycosides of reactive oxygen species production in cochlear tissues, particularly in developing rats.

Age-related sensitivity to cisplatin ototoxicity in gerbils

This study was undertaken to define the developmental period of maximal sensitivity to cisplatin ototoxicity in gerbils. Five groups were established based upon post-natal age (P) at exposure to cisplatin, P10 (n = 8), P14 (n = 8), P18 (n = 6), P22 (n = 7) and P42 (n = 7). Animals were given cisplatin, 1 mg/kg/day intraperitoneal for 4 days. In the first four groups, P10, P14, P18 and P22, distortion product otoacoustic emissions were measured at 45 days of age, when responses were expected to be developmentally stable. Distortion product grams and input-output functions were measured. There was a statistically significant difference only between P14 and P42 (P<0.01). There was a significant interaction of age and frequency in the P14 group only (P<0.01). A secondary analysis compared distortion product grams of P14 animals, exposed to cisplatin, and age-matched saline-treated animals (n = 6). There was a significant treatment effect. In summary, there was an effect of age on the cisplatin ototoxicity in gerbils. Also, there was an effect of the frequency on DPOAE levels in P14 gerbils. These data support the presence of a 'sensitive' period to cisplatin ototoxicity in gerbils.

Cisplatin ototoxicity in developing gerbils

This experimental study was undertaken to investigate the dose-related effect of cisplatin exposure in young gerbils (2 weeks of age) and explore the relationship between different methods used to monitor auditory function after exposure to cisplatin. Four groups of animals, including a control group, were used. The treatment groups, D1 (n = 6), D2 (n = 7) and D3 (n = 6), received one, two, and three doses of cisplatin (5 mg/kg/dose), respectively, at weekly intervals. Treated animals were first exposed to cisplatin at 2 weeks of age. Distortion product otoacoustic emissions (DPOAE) and auditory brainstem responses (ABR) were measured in treated and control animals at 6 weeks of age. The effects of dose and frequency on the DPOAE amplitude, as well as the relationship between the DPOAE and the ABR thresholds were analyzed. Animals in the D1 and D3 groups demonstrated significant elevation of DPOAE and ABR thresholds. Interestingly, animals in the D2 group demonstrated a bimodal distribution of DPOAE and ABR responses, with four animals severely affected and three not showing an effect. A tendency for a bimodal distribution of DPOAE and ABR responses was also observed in the D1 group, at frequencies below 8 kHz.

Development of blood-labyrinth barrier in the semicircular canal ampulla of the rat

Cationic polyethyleneimine (PEI) administered intravenously was transported to anionic sites on the capillary and subepithelial basal laminae (BL) in the vestibular labyrinth. Therefore, changes in the PEI distribution on the BL reflect changes in the transport system in the vestibular labyrinth. A 0.1% PEI solution was administered intravenously (7.5 ml/kg) to developing (1, 4, 7, 14 days after birth) and adult rats in order to investigate the development of the macromolecular transport in the ampulla of the semicircular canal as a function of age. After 1 h, the bony labyrinth was removed and embedded in Epoxy resin. Ultrathin sections of the ampulla were then examined with a transmission electron microscope. In the subepithelial BL in the dark cell area and capillary BL in the crista ampullaris, the PEI distribution in both 1- and 4-day-old rats was markedly increased compared to that in either 7-, 14-day or adult rats. In the sensory cells in 1-, 4-day or 7-day-old rats, PEI density and area was significantly greater than in the adult rats. These findings suggest that the macromolecular transport system in the developing rat ampulla becomes mature by 14 days after birth and that the maturation of its transport system in the ampulla is strongly associated with that in the stria vascularis.

The Effects of Kanamycin Injection into the Fetal Lamb Cochlea

Mammalian auditory hair cells have minimal capacity for repair or regeneration after a variety of insults, including acoustic trauma and aminoglycoside exposure. Although fetal tissues have a greater potential for repair and regeneration than adult tissues, there have been no reported studies on fetal hair cell response to injury in mammals. The purpose of this research was to investigate the effects of local application of kanamycin on fetal lamb cochlear hair cells.

Eleven fetal lambs in the early third trimester underwent kanamycin injection through the left round-window membrane. The right ear served as a control. Click-evoked compound action potentials (CAPs) were serially recorded in 8 fetuses. CAPs were observed in all control ears. None of the 8 kanamycin-injected ears had measurable CAPs on postoperative day 1. One kanamycin-injected ear demonstrated definite CAPs, beginning on postoperative day 6. Hair cells were found to be intact in 6 of 9 kanamycin-treated ears. Hair cells were missing only in animals that went into premature labor.

The presence of intact hair cells despite the loss of measurable CAPs in kanamycin-perfused lamb cochleae was striking. This finding may indicate that the fetal auditory epithelium is relatively resistant to aminoglycoside injury or may be capable of prompt repair or regeneration. Further studies on the effects of aminoglycoside injury in the fetal cochleae seem to be warranted.

Role of gravity in the development of posture and locomotion in the neonatal rat

This report describes the early motor behaviour in the neonatal rat in relation with the maturation of sensory and motor elements of the central nervous system (CNS). The role of vestibular information during the week before (E14-21) and the 2 weeks after (P0-15) birth will be considered. There is a rostro-caudal gradient in the maturation of posture and locomotion with a control of the head and forelimbs during the first postnatal week and then a sudden acceleration in the functional maturation of the hindlimb. At birth, the neonatal rat is blinded and deaf; despite the immaturity of the other sensory systems, the animal uses its olfactory system to find the mother nipple. Vestibular development takes place between E8 and P15. Most descending pathways from the brainstem start to reach the lumbar enlargement of the spinal cord a few days before birth (reticulo-, vestibulospinal pathways as well as the serotonergic and noradrenergic projections); their development is not completed until the end of the second postnatal week. At birth, in an in vitro preparation, a locomotor activity can be evoked by perfusing excitatory amino acids and serotonin over the lumbar region. The descending pathways which trigger the activity of the CPG are also partly functional. At the same age both air stepping and swimming can be induced. Complex locomotion such as walking, trotting and galloping start later because it requires the maturation of the vestibular system, descending pathways and postural reflex regulation. The period around birth is critical to properly define how the vestibular information is essential for the structuring of the motor behaviour. Different types of experiments (hypergravity, microgravity) are planned to test this hypothesis.

Development of the blood-labyrinth barrier in the rat

Systemically administered cationic polyethyleneimine (PEI) passes through the capillary endothelial cell and attaches to anionic sites on the capillary basal lamina (BL). Thus, the distribution of PEI on the BL reflects the changes in the endothelial cell transport system. A 0.1% PEI solution was administered by intravenous injection (7.5 ml/kg) to developing (4, 7, 11, 14, 21 days after birth) and adult rats to evaluate the development of endothelial cell transport in the cochlear capillary as related to age. One hour later, the bony labyrinth was removed and embedded in Epoxy resin. Ultrathin sections of the cochlear lateral wall were then viewed with a transmission electron microscope. The distribution of PEI in the capillary BL in the stria vascularis of the 4-, 7- and 11-day-old rats was significantly greater compared to the adult rats. The distribution of PEI in the capillary BL in the spiral ligament of the 4- and 7-day-old rats was also significantly greater compared to the adult rats. These findings suggest that the endothelial cell transport is more robust in the developing rat cochlea and that the blood-labyrinth barrier becomes mature by 14 days after birth in rats.

Reactive oxygen species in chick hair cells after gentamicin exposure in vitro

Reactive oxygen species have been invoked as a causative agent of cell death in many different developmental and pathological states. The presence of free radicals and their importance of hair cell death due to aminoglycosides is suggested by a number of studies that have demonstrated a protective effect of antioxidants. By using dichlorofluorescin (DCFH) a fluorescent compound that is a reporter of reactive oxygen species, we have shown that free radicals are rapidly produced by avian hair cells in vitro after exposure to gentamicin. In addition, free radical scavengers, catalase and glutathione, were tested with DCFH fluorescent imaging for their ability to quench the production of reactive oxygen species in hair cells after drug exposure. Both free radical scavengers were very effective in suppressing drug-induced production of free radicals. Next, we investigated the ability of these antioxidants to preserve the structural integrity of hair cells after exposure to gentamicin. We were not able to detect any attenuation of the hair cell loss using antioxidants in conjunction with gentamicin. This result must be qualified by the fact that the antioxidants used were not effective over long-term gentamicin exposure. Therefore, methodological constraints prevented adequately testing possible protective effects of the free radical scavengers in this model system.

Protection of auditory neurons from aminoglycoside toxicity by neurotrophin-3

Hearing is conveyed from the auditory receptors, the hair cells in the organ of Corti, to the brain via the spiral ganglion neurons. Damage or loss of either spiral ganglion neurons or hair cells causes hearing impairment. Such hearing disorders are often permanent and can be caused by therapeutic agents, such as aminoglycoside antibiotics and cisplatin, or by aging, loud sounds, infections and mechanical injury (1). Brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), members of the neurotrohin family of neurotrophic factors that also include nerve growth factor (NGF) and neurotrophin-4/5 (NT-4), are important in development of the neuronal components of the inner ear. We report here that the loss of target innervation and the degeneration of approximately 90% of the adult spiral ganglion neurons caused by aminoglycoside toxicity can be prevented by infusion of the neurotrophic factor, neurotrophin-3 (NT-3) in the membranous labyrinth in guinea pigs. The potency of NT-3 in protecting spiral ganglion neurons from degenerating suggests that neurotrophins may be useful for the treatment of hearing disorders.