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Level: Advanced

Numbers of Slides: 20

Abstract:

Rats were exposed to the organic solvent ethyl benzene at 0, 300, 400 and 550 ppm in air for 8 hours/day for 5 consecutive days. Three to six weeks after the exposure, auditory function was tested by measuring distortion product otoacoustic emissions (DPOAEs) and compound action potentials (CAPs). In addition, outer hair cell (OHC) loss was quantified by histological examination. Generally, the frequency region found to be affected after ethyl benzene exposure was similar with all three evaluation methods. However, at 400 ppm ethyl benzene, CAP threshold shifts were statistically significant and in line with OHC loss, but DPOAE measurements merely showed a trend for loss. Thus, DPOAEs seem to be less sensitive than CAP and OHC counts.

Level: Advanced

Number of Slides: 32

Abstract:

INTRODUCTION: Carboplatin is a antitumour drug which selectively alters the micromechanical function of the inner hair cells (IHCs) of the organ of Corti. Data from an earlier study (Wake et al, 1996) from a chinchilla model support the hypothesis that carboplatin administration not only disrupts the IHC function but affects the efferent feedback loop to the cochlea, causing an amplification of the otoacoustic emission responses. The present study was designed to verify the OAE amplification-issue, using distortion product emissions (DPOAEs) in a wider bandwidth range.
MATERIALS AND METHODS: Carboplatin (Paraplatin,10 mg/ml, Brystol Myers) was administered by a 30 min low infusion. Pre and 72-hour post DPOAE and ABR recordings were acquired from a group of 12 Sprague -Dawley rats (mean weight 360 ± 35 gr). The animals were anesthetized with a ketamine-atropin anesthesia administered in two consecutive phases.The DPOAE responses (cubic distortion products) were recorded with 4 asymmetrical protocols P1= 60-50 ; P2 = 50-40 ; P3= 40-30 and P4= 30-20 dB SPL, in the frequency range from 5.0 to 16 kHz. ABR responses were obtained for bipolar clicks and tone-pips at the frequencies 8.0, 10.0, 20.0 and 30 kHz using stimuli in the range from 100 to 30 dB SPL . Wave-III was used to identify shifts in hearing threshold. Four animals randomly selected, underwent a SEM evaluation assessment. A repeated measures model was used for the analysis of the DPOAE responses .
RESULTS: ABR threshold shifts of 20 dB were observed in the frequencies from 20 to 30 kHz and shifts of 10 dB in the frequencies 8.0 and 10.0 kHz. The comparison of pre and post treatment DPOAE responses did not revealed any significant changes for protocols P1, P2 and P4. Border line differences were observed for the P3 protocol.
CONCLUSIONS: The findings from the rat model contrast the data available in the literature from a gerbil animal model and suggest that different efferent mechanisms might suppress or enhance the activity of the OHCs of the organ of Corti across various species.

Level: Advanced

Numbers of Slides: 32

Abstract:

INTRODUCTION: Recent studies (Harel et al,1997) have suggested that ketamine compounds increase the amplitude level of the TEOAE and DPOAE responses in a gerbil animal model. Since the major application of the OAEs is the detection of sensorineural hearing losses, expressed by alterations of the OAE responses, it is important to define the time window in which the anesthesia effect on the OAEs is not present.
MATERIALS AND METHODS: The anesthesia effect on the OAE recordings was evaluated in 3 groups of 64 Sprague -Dawley rats (mean weight 225 ± 20 gr). Thirty due animals were tested with TEOAEs, 27 animals with DPOAEs and 5 animals with both OAE types.The anaesthetic (equal volumes of ketamine mixed with xylazine or atropine and saline solution) was administered in two consecutive phases. In phase one, the animal received an intra-peritoneal dose (1 ml / kg of body weight) and upon the first signs of muscular relaxation (phase two) a second half-volume dose was administered subcutaneously. The TEOAEs were recorded according to a nonlinear protocol and were evoked by a 63.5 dB SPL click stimulus. The DPOAEs (cubic distortion products) were evoked by two different asymmetrical protocols with primaries set at 60-50 and 50-40 dB SPL. The OAE responses were recorded in 10 min intervals, starting 5 min after the first dose and ending at 60 min post treatment.
RESULTS: An analysis of the data of both OAE types with a repeated measures model indicated the following: (1) There is a decrease of the TEOAE response level, the TEOAE correlation value and the S/N ratios in the tested frequencies 12-14 min after the administration of the anesthesia, but these differences were not distinguishable from random variation (not statistically significant). (2) The DPOAE responses evoked by a 50-40 protocol showed the highest S/N decrease , but the time differences were also not significant.
CONCLUSIONS: These findings from the rat model contrast the data available in the literature from a gerbil animal model and suggest that the previous data were treated, erroneously, by statistical methods which did not consider time as a variable.

Level: Advanced

Number of Slides: 52

Abstract:

A summary from Invited lectures in the period 2003-2004. In order to maintain the high resolution of images and figures the pdf file is large and requires 8 min downloading time (56K modem). 

Level: Advanced

Number of Slides: 32

Abstract:

Both, a genetic or acquired neonatal thyroid hormone (TH) deficiency may result in a profound mental disability that is often accompanied by deafness. The existence of various TH-sensitive periods during inner ear development and general success of delayed, corrective TH-treatment was investigated by treating pregnant and lactating rats with the goitrogen methimazole (MMI). We observed that for development of normal hearing thyroid hormone is only required within a crucial time between the onset of fetal thyroid gland function and the onset of hearing at P12. Within this time, however, any postponement in the rise of TH-plasma levels, as can be brought about by treating lactating mothers with MMI, leads to permanent hearing defects of the adult offspring. The severity of hearing defects that were measured in 3- to 9-month-old offspring could be increased with each additional day of TH-deficiency during this critical period. Unexpectedly, the active cochlear process, assayed by DPOAE measurements, and speed of auditory brainstem responses, which both until now were not thought to be controlled by TH, proved to be TH-dependent processes that were damaged by a delay of TH-supply within this critical time. In contrast, no significant differences in the gross morphology and innervation of the organ of Corti or myelin gene expression in the auditory system, detected as MPB and PLP mRNA using Northern blot approach, were observed when TH-supply was delayed for few days. These classical TH-dependent processes, however, were damaged when TH-supply was delayed for several weeks. These surprising results may suggest the existence of different TH-dependent processes in the auditory system: those which respond to corrective TH supply (e.g. innervation and morphogenesis of the organ of Corti) and those which do not, but require T3-activity during a very tight time window (e.g. active cochlear process, central processes).