Hearing research
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The present study was conducted to examine the re-establishment of IHC/VIII nerve synapses following kainic acid (KA) excitotoxicity and to discern if the re-organized afferent could render not only a normal auditory threshold but also a normal supra-threshold function. KA (60 mM) applied to the intact round window membrane in chinchilla destroyed postsynaptic endings of the auditory nerve, depressed the input-output (I/O) functions of auditory evoked potentials (EVP) and produced an average loss of sensitivity of over 80 dB at 4, 8, and 16 kHz, with less substantial losses (40-60 dB) at lower frequencies. However, there was no significant difference in 2f1-f2 distortion-product otoacoustic emissions (DPOAE) before and after the application of KA. ⋯ Auditory sensitivity and supra-threshold response returned accordingly. In contrast, complete recovery at lower frequencies (1 and 2 kHz) required more than 5 days. The results provide strong evidence that (1) excitotoxically damaged cochlear afferent neurons can recover and render both a normal EVP threshold and EVP I/O function and (2) afferent innervation to IHCs is not necessary for DPOAE generation.
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Cisplatin (CDDP) is a widely used chemotherapeutic agent. Unfortunately, CDDP is highly ototoxic. We tested D-methionine (D-Met), a sulfur containing compound, as an otoprotectant in male Wistar rats. ⋯ D-Met provided excellent otoprotection even at the lowest level with complete otoprotection obtained for the 300 mg/kg dosing as measured by both ABR and SEM. D-Met also markedly reduced weight loss and mortality. All animals receiving D-Met (15/15) survived to the end of the study period as opposed to only 5/10 of the treated controls.
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The time course of recovery of compound action potential (CAP) thresholds was observed in individual adult pigeons after severe acoustic trauma. Pigeons were overstimulated with a tone of 0.7 kHz and 136-142 dB SPL presented to one ear for 1 h under general anesthesia. Recovery of CAP audiograms was monitored at regular intervals after trauma. ⋯ However, the onset of recovery is delayed and the time course is slower than after destruction of short (abneural) hair cells alone. Furthermore recovery is incomplete, both functionally and morphologically. There are residual permanent hearing losses and regeneration of short (abneural) hair cells is incomplete.
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Comparative Study
Physiology of the young adult Fischer 344 rat inferior colliculus: responses to contralateral monaural stimuli.
This study was designed to establish the young adult (3 month) Fischer 344 (F344) rat as a model of inferior colliculus (IC) physiology, providing a baseline for analysis of changes in single unit responses as the animals age and for the study of noise induced hearing loss. The response properties of units localized to the central nucleus of the IC (CIC) and those localized to the external cortex of the IC (ECIC) were compared in order to better characterize differences between these two subnuclei in the processing of simple auditory stimuli. In vivo extracellular single unit recordings were made from IC neurons in ketamine/xylazine anesthetized young adult F344 rats. ⋯ Differences between CIC and ECIC units included a higher percentage of nonmonotonic RIFs and lower percentage of onset temporal response patterns in the CIC than in the ECIC. Some properties which have been previously used as hallmarks for differentiation between CIC and ECIC units, namely broader tuning and longer first spike latencies in the ECIC, did not reach statistical significance in this study. These may reflect species differences and/or the highly variable and largely overlapping sets of responses evident in the large sample size used in this study.
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Comparative Study
Responses of young and aged Fischer 344 rat inferior colliculus neurons to binaural tonal stimuli.
The inferior colliculus (IC) is one nucleus of the central auditory system which displays age-related changes. Inputs to the IC use primarily the amino acid neurotransmitters glutamate and gamma-aminobutryic acid (GABA). Neurochemical and anatomical studies of the Fischer 344 (F344) rat IC have shown decreases in GABA and GABA receptor levels (see Caspary et al., 1995 for review). ⋯ Although there was some shift in the distribution of binaural RIF shapes with age, it was not statistically significant. The shift included a reduction in the percentage of units classified as E/I (excited by contralateral stimulation/ipsilaterally inhibited during binaural stimulation), but an increase with age in the percentage of units classified as E/f (excited by contralateral stimulation/ further facilitated by the addition of low intensity ipsilateral stimulation, but inhibited by higher intensity ipsilateral stimulation). Despite the role of inhibitory neurotransmission in binaural processing in the IC, age-related neurochemical deficits in the IC do not appear to result in a major deficit in the processing of simple binaural stimuli in F344 rats.