Age Dependent Changes and Acoustic Trauma in the Spiral Organ of the Guinea Pig

Susceptibility to noise exposure during postnatal development in rats

Susceptibility to noise-induced hearing loss was studied during maturation in 20 female pigmented rats (strain Long-Evans). Young rats, 3, 4, 5 and 6-7 weeks old, were exposed for 1 h to a broad-band noise with an intensity of 120 dB SPL. The thresholds and amplitudes of middle latency responses (MLR) recorded from electrodes implanted on the surface of the auditory cortex were analyzed before and after noise exposure. The results were compared with data from our previous study, in which the effects of broad-band noise exposure on MLR were investigated in adult rats [Syka, J. and Rybalko, N. (2000) Hear. Res. 139, 59-68]. The hearing thresholds of 3-7 week old rats before noise exposure were within the normal adult range. Noise exposure in young rats produced an adult-like pattern with an elevation of hearing thresholds. One-two weeks post-exposure a recovery of MLR thresholds was observed, though full recovery only occurred in the low frequency range. Recovery of hearing thresholds in the high frequency range depended on the age of the animal at the time of exposure. In all animals aged less that 6-7 weeks, exposure resulted in a permanent threshold shift in the range of 4-32 kHz. The mean values of permanent threshold shifts at 16 kHz (the frequency of maximal hearing loss) were 53.0+/-4.5, 47.6+/-9.6, 37.5+/-7.5 and 27+/-10 dB for rats exposed at 3, 4, 5 and 6-7 weeks of age, respectively. Similar to adult rats, young rats exposed to noise exhibited an enhancement of MLR amplitudes. This amplitude enhancement was more pronounced in the high frequency range. In several rats exposed at 3-5 weeks of age, the recovery period to normal amplitudes was substantially prolonged and lasted 4-8 weeks in comparison with 1-2 weeks in adult rats. These results demonstrate a greater susceptibility to noise exposure in rats during the first 5 postnatal weeks.

Cochlear damage in guinea pigs following contralateral sound stimulation with and without gentamicin

The effect of a minimally damaging sound exposure and a sub-ototoxic dose of gentamicin on cochlear hair cells contralateral to the sound exposure was evaluated. The cochleae of pigmented guinea pigs exposed to an 8 kHz pure tone at 116 dB SPL for 1 h and/or 50 mg/kg/day of gentamicin for 10 consecutive days and repeated after an interval of 3 weeks, were used for this purpose. Hair cell loss was found to have occurred in the contralateral cochleae following the sound exposure alone. The occurrence of potentiation, synergism and differential synergism between the agents in the contralateral ears was also seen. Possible explanations for these phenomena are proposed.

The effect of gentamicin-induced hair cell loss on the tight junctions of the reticular lamina

Freeze fracture has been used to examine junctional complexes within the gentamicin damaged guinea pig organ of Corti during the period of structural reorganisation associated with hair cell loss. When examined up to 28 days after a regime of chronic gentamicin administration, no significant alterations to the pattern or nature of the complex tight junctions associated with the hair cell apex could be detected. In contrast, marked abnormalities of the junctions between adjacent supporting cells, and evidence of formation of new gap junctions on the lateral membranes of Deiter's cell was seen. The use of lanthanum as an electron dense tracer in thin sections also showed that no breakdown of permeability barriers occurred in the reticular lamina. These results support the concept of a controlled response by the organ of Corti, with conservation of the tight junction of the hair cell and maintenance of permeability barriers. Early changes to the membranes of the stereocilia and lateral cisternae of the outer hair cells were also identified. These may indicate direct effects of gentamicin upon membrane structure.

Synergistic interactions of gentamicin and pure tones causing cochlear hair cell loss in pigmented guinea pigs

The effects of single and repeated combinations of gentamicin and sound on Preyer reflex and cochlear hair cells in pigmented guinea pigs have been examined. Gentamicin at 50 mg/kg for 10 days and an 8 kHz pure tone (116 dB for 60 min) cause little or no hair cell loss when given alone. In combination, hair cell loss occurs with the gentamicin and sound acting synergistically. If the pure tone is given on the first of a 10-day gentamicin course there is considerable loss of hair cells in the basal coil; if given on the 10th day the loss is several times less. Repeating the treatments after 3 weeks increases the hair cell loss and the synergistic effect. A possible explanation for the differential synergistic effect is discussed.

Cochlear hair cell damage from intermittent noise exposure in young and adult guinea pigs

A comparative study of age-dependent damage to cochlea from intermittent noise exposure was carried out on five-week-old and one-year-old pigmented guinea pigs. Hair cell loss in the organ of Corti was studied after five weeks' exposure to a pure tone stimulus (95 dB SPL at 2 kHz, one hour per day for five weeks). The noise-induced damage was sharply limited to the 7- and 11-mm marks from the apex. Damage was more marked in younger guinea pigs and was distinct from natural age-induced cell loss. When the 7- to 11-mm zone was further analyzed, outer hair cell damage appeared highly significant in both age groups but more severe in younger animals. Inner hair cell damage in this area was severe in both groups but statistically insignificant.

Psychoacoustical and audiometric prediction of auditory disability for different frequency responses at listener-adjusted presentation levels

Audiometric prediction of word identification scores has typically used one fixed presentation level for all subjects in the sample, with presentation in quiet and a wide range of hearing impairment among the listeners; under such conditions it is hardly surprising that moderate to good predictions are found. To see if prediction is possible under clinically relevant conditions, that is, on a homogeneous clinical sample of new hearing-aid candidates and to listener-adjusted levels, as would obtain in use of a hearing aid. In addition to audiometric variables, we employed a clinical approximation to the psychoacoustic tuning curve. We tested speech identification (FAAF) performance both with a 'rising'(+9 dB/octave) and with a 'flat' frequency response. Prediction of performance in the 'flat' condition was only good when a full set of audiometric frequencies entered the multiple-regression formula, each with its own weighting. Audiometric prediction for the 'rising' frequency response was particularly poor. Thus, the fairly good predictability from thresholds found traditionally for word identification scores or other disability measures appears to be a special case, depending partly on the wide range of hearing levels employed. Within our clinical sample the predictive power of formulae based on the mean of all thresholds or of mid-frequency thresholds alone (as used in compensation schemes) or on a priori combinations of thresholds (such as slopes) was generally poor. However, a three-parameter model taking account separately of low (0.25 kHz) and high-frequency (greater than 2.0 kHz) thresholds was effective. This and other audiometric descriptions were valuably supplemented by a psychoacoustic measure of frequency resolution at 2 kHz. In particular, such supplementation here allowed a satisfactory level of prediction to be achieved for speech heard with a +9 dB/octave frequency response, which the audiogram alone did not. The limitations of the prediction paradigm are discussed and several conceptual and statistical problems not previously emphasised in the audiological literature are illustrated in relation to the data.

The questionable relation between cochlear pigmentation and noise-induced hearing loss

Some evidence has suggested that susceptibility to noise-induced hearing loss may be inversely proportional to the amount of melanin in the cochlea. However, published data have not been consistent in demonstrating this relation, and some results may be contaminated by lack of genetic control in experimental animals. In this investigation, noise-induced hearing loss was evaluated in pigmented and albino C57BL/6J mice that differed only in their ability to produce melanin. Surface-recorded electric responses to sound were used to assess auditory sensitivity. Results indicated no difference between the two groups either in pre-exposure thresholds or in magnitude of noise-induced threshold shift.

Age-dependent changes in susceptibility to ototoxic hearing loss

Several species undergo age-dependent changes in susceptibility to noise-induced and drug-induced hearing loss. In this experiment 18-day-old mice were given 200 mg/kg kanamycin plus varying doses of bumetanide (20, 30 or 40 mg/kg). Thresholds of the scalp-recorded compound action potential of the auditory nerve indicated that diuretic doses of 30 and 40 mg/kg produced significant loss of hearing sensitivity. Thirty-eight-day-old mice were then treated with 200 mg/kg kanamycin plus 40 mg/kg bumetanide and demonstrated no loss of hearing sensitivity. These results confirm the existence of age-dependent changes in susceptibility to ototoxic hearing loss in mice. Furthermore, the treatment, consisting of two injections on the same day, offers promise as a means of defining the critical period of susceptibility in greater detail than has been possible with aminoglycosides alone.

Integrated development and maturation of the hearing system. A critical review article

The understanding of development and maturation of the auditory system is essential for many reasons, including the practical aspects of auditory behaviour, testing, and teaching a hearing-impaired child to communicate effectively. The study of maturation of the auditory system is gaining increasing importance also because it should help us to interpret correctly certain aspects of auditory behaviour in infants. When studying the auditory system we should not be concerned solely with development of function in isolation; the emphasis is on integrated development. Understanding a process of integrated development enables us to understand the peculiarities of auditory behaviour in infants. Sound became the raw material of human language for good reasons. It is the only medium which made it possible to transmit efficiently complex information encoded in human language. Phylogenetically, hearing in vertebrates is a late development, and because of this, in certain unfavourable circumstances the hearing system is more vulnerable than other phylogenetically older systems. Within the auditory system, high-frequency hearing is also phylogenetically a late development, and therefore more vulnerable to certain unfavourable metabolic influences. In all species there is a 'best frequency' range, usually the one which is most vital for communication. In humans the best frequency range is the one which is most important for transmitting speech sounds. This is already noticeable in newborn and very young infants, that is, we can obtain the best reactions in the frequencies which are important for speech. Higher sensitivity for perception of patterns is already developed in newborn and young infants (and acquires a special significance). Direction detection and localisation of sound source develops gradually. It is fully developed only when the auditory pathway matures and when the function is well integrated with the maturing motor system. The ears of the young are more vulnerable to noise damage and therefore a variety of protective mechanisms exist. The quality of the sound environment of the infant is important and can be decisive for further development. It is highly desirable to detect hearing impairment as soon as possible after birth. Lack of stimulation of the auditory system, even when it functions only partially, or along a very narrow channel of communication, may lead to permanent complete loss of function of the auditory centre. But this could be prevented by early stimulation, even when it is possible only via the narrow channel of communication which fortunately is present in the great majority of cases of even severely hearing-impaired children.

Patterns of cochlear degeneration following gentamicin administration in both old and young guinea pigs

This study aims to compare experimentally the effect of gentamicin administration on the cochleae of guinea pigs of different ages. While it is frequently cited that age may represent a predisposing factor towards aminoglycoside ototoxicity in clinical reports, little experimental work has been carried out on this topic. Two groups of albino guinea pigs, either 4 weeks or 24 weeks old, were used. Animals were injected subcutaneously for 10 consecutive days with gentamicin at a dose of either 50 or 125 mg/kg/day, or with an equivalent volume of saline to act as controls. The survival period after the last injection was either 4 or 12 weeks. On being killed, both bullae were removed and fixed in osmium tetroxide, and prepared of phase-contrast microscopy by the surface preparation technique. The entire hair cell population of one cochlea from each animal was recorded onto cochleograms, enabling detailed graphical and statistical analysis. Both age groups display extremely similar patterns of outer and inner hair cell loss at the higher dose; the lower dose of gentamicin was not ototoxic in this study. Hair cell loss is predominantly basal, and tends to be maximally concentrated in two areas of the base of the cochlea. While mortality is much higher in the older animals, there is no difference in the response of the two age groups to the ototoxic effects of gentamicin.

Further studies of the effects of continuous white noise of moderate intensity (70--80 dB SPL) on the cochlea in young guinea pigs. Time course and distribution of hair cell degeneration

Guinea pigs aged one week were exposed to white noise at a maximum of 76 dB SPL for 7 days and were then killed 3, 8 and 16 weeks later for histological examination of the cochlea by the surface preparation method. Appreciable increases in outer hair cell losses were observed in the apical turn 3/3 1/2, chiefly in the outer two rows, between the 3rd and 8th week, but not between the 8th and 16th week. No significant losses were seen in control groups corresponding to 3- and 8-week periods, although in the control group of 16 weeks' survival, small deficits, attributable to natural ageing, were seen in the apical half-turn, 3 1/2.

Acoustic trauma effects from doubling the exposure time

Guinea pigs were used to test acoustic trauma effects from doubling the exposure time. Previously, two frequencies (20 kHz and 4 kHz) were used in conjunction with each other (ie double exposure) to see whether a prior exposure of the cochlea to one frequency would alter the extent of damage caused by a secondary frequency, when compared with damage caused by either frequency alone. A reduced area of damage was attained after double exposures. To complete the series, the same two frequencies were used separately, but the exposure time was doubled. The results from the 20 kHz series surprisingly indicated that a reduced area of damage was obtained after doubling the exposure time. No significant differences were found in the 4 kHz series.

Hereditary deafness in the cat. An electron microscopic study of the stria vascularis and Reissner's membrane

The auditory pathway potentials have been recorded from a series of 35 cats, and the stria vascularis and Reissner's membrane examined with the electron microscope. The ages varied from the first postnatal day to at least 6 years. 30 of the animals had white coats, and 45 of the ears lacked potentials. The basal processes of the marginal cells fail to develop in pathological ears, and appear displaced towards the endolymphatic surface by a layer of grossly swollen, chromophobe cells. These features are already present at the 3 day stage, and persist until the third week. Thereafter, increasing electron density of all cell types is found, with progressive reduction in strial thickness. Swelling of the capillary endothelial cells and descent of Reissner's membrane are found in the first coil at 2 weeks. Obliteration of capillaries, and development of vacuolar cytoplasmic outgrowths from the vestibular membrane mesothelial cells occur at later stages. These findings are discussed and compared with the pathological changes produced by a variety of endogenous 2nd exogenous factors in both man and animals.