Threshold shifts and enhancement of cortical evoked responses after noise exposure in rats

doi:10.1016/S0378-5955(99)00175-6

Hearing Research

Volume 139, Issues 1–2, January 2000, Pages 59-68

https://doi.org/10.1016/S0378-5955(99)00175-6 Get rights and content

Abstract

The effect of exposure to various types of noise (broadband, high-frequency or low-frequency) was studied in adult pigmented rats. 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. Exposure to noise with intensities ranging from 105 to 120 dB for 1 h produced only temporary threshold shifts (TTS). Exposure to broadband noise produced TTS throughout the whole frequency range of the rat’s hearing, mostly expressed at frequencies of maximal hearing sensitivity (16–32 kHz). Hearing loss produced by high- or low-frequency noise exposure was related to the spectral characteristics of the noise. The exposure to high-intensity noise may also result in amplitude enhancement of the MLR. This phenomenon was seen mainly after broadband noise exposure and occurred in response to both low-frequency and high-frequency test stimuli. High-frequency and low-frequency noise produced amplitude enhancement mainly at frequencies which corresponded to the maximum exposure energy. In contrast to the relatively similar values of TTS obtained in different rats under the same conditions of noise exposure, great inter-individual variability was found in the MLR amplitude enhancement. In all rats the dynamics of recovery functions for amplitude enhancement were different from those for MLR thresholds. The data indicate that whereas post-exposure TTS are related to peripheral changes, the post-exposure MLR amplitude enhancement is most probably connected with a change in the processing of auditory information in the central nervous system.

Introduction

Changes in auditory function after noise exposure are traditionally interpreted as the result of mechanical damage to the cochlear structures (for review see e.g. Syka, 1989, Axelsson et al., 1996). However, accumulated evidence exists that changes produced by acoustic trauma also comprise changes in the higher levels of the auditory system. It was demonstrated that partial or complete inner ear injury may induce functional and morphological reorganization of auditory nuclei (Taniguchi and Saito, 1978, Reale et al., 1987, Robertson and Irvine, 1989, Møller, 1990, Popelář et al., 1994). Acoustic overstimulation, as an example of partial and temporary deafferentation, may result in enhanced spontaneous and evoked-unit activity in the cochlear nucleus or inferior colliculus (Henderson and Møller, 1975, Lonsbury-Martin and Martin, 1981, Willott and Lu, 1982, Salvi et al., 1982, Salvi et al., 1992, Kaltenbach et al., 1998, Zhang and Kaltenbach, 1998), in the enhancement of evoked response amplitudes in the inferior colliculus (Willott and Henry, 1974, Bock and Saunders, 1976, Salvi et al., 1990, Szczepaniak and Møller, 1996) or increased amplitudes of middle latency responses (MLR) recorded at the auditory cortex (Popelář et al., 1987, Syka et al., 1994).

In our previous works (Popelář et al., 1987, Syka et al., 1994), MLR amplitude enhancement was studied in guinea pigs. Guinea pigs may be characterized as animals with a relatively broad frequency hearing range, which also includes low frequencies (even below 100 Hz). Their best sensitivity of hearing spans from 8 to 12 kHz. These studies showed that MLR amplitude enhancement in guinea pigs after noise exposure was found only when low-frequency tones were used as test stimuli. Therefore it would be of interest to study the phenomenon of MLR amplitude enhancement under similar experimental conditions in animals with hearing range shifted to high frequencies. Rats represent such a species, with the frequency hearing range spanning from approximately 800 Hz to 65 kHz, with the best sensitivity between 8 and 32 kHz.

The aim of the present study was to evaluate the effects of exposure to various types of noise on the MLR recorded in the auditory cortex in the rat. The rat was selected as an experimental animal also because it is a widely used animal model for the study of the structure and function of the central nervous system. However, in contrast to the guinea pig, chinchilla or gerbil, data concerning the influence of noise exposure on hearing function in rat are very scarce (Lenoir et al., 1979, Borg, 1982, Szczepaniak and Møller, 1996).

Section snippets

Materials and methods

Experiments were performed on 28 adult (3–5-month-old) female pigmented rats (strain Long-Evans) weighing 250–300 g. The MLR were recorded with chronically implanted teflon-coated platinum iridium ball electrodes (ball diameter 0.5 mm) fixed on the surface of the primary auditory cortex. The reference electrode was placed in the neck muscles. All electrodes were soldered to pins of a connector socket mounted on the skull with stainless steel screws and acrylic resin. During the implantation,

Results

In all investigated rats noise exposure produced temporary threshold shifts (TTS). The magnitude and pattern of hearing loss were dependent on the spectral characteristics of the noise and its intensity, and were also related to hearing sensitivity at different frequencies. Exposure to 120 dB SPL broadband noise produced TTS throughout the whole frequency range of the rat’s hearing, mostly expressed at frequencies of the rat’s maximal hearing sensitivity (8–32 kHz). The maximal TTS value of

Discussion

The above results demonstrate that 1 h noise exposure with intensities of 105–120 dB SPL produces in adult rats temporary threshold shifts which last for about 2 weeks. Similar effects were observed by Lenoir et al. (1979), who exposed rats to 120 dB SPL white noise for 30 min. In our study it was demonstrated that the magnitude and pattern of hearing loss were dependent on the spectral characteristics of the noise and its intensity and were also related to the hearing sensitivity of the rat at

Acknowledgements

This research was supported by grants from the Ministry of Health 4747-3 and from the Grant Agency of the Czech Republic 309/97/0830. The authors wish to thank Dr Jiřı́ Popelář for his assistance and comments in different stages of the experiments.

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Threshold shifts and enhancement of cortical evoked responses after noise exposure in rats

Abstract

Introduction

Section snippets

Materials and methods

Results

Discussion

Acknowledgements

Hear. Res.

Hear. Res.

Hear. Res.

Hear. Res.

Dev. Brain Res.

Electroenceph. Clin. Neurophysiol.

Exp. Neurol.

Neurosci. Lett.

Effects of low and high frequency noise bands in producing a physiologic correlate of loudness recruitment in mice

Trans. Am. Acad. Ophthal. Otol.

Effects of noise on cortical evoked potentials in rats

Act. Nerv. Super.

Alteration of central auditory processing of brief stimuli; A review and a neural model

J. Acoust. Soc. Am.

Potentials evoked by sound in the auditory cortex of the cat

Am. J. Physiol.