Recovery of structure and function of inner ear afferent synapses following kainic acid excitotoxicity.
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. The nerve endings went through a sequence of swelling, degeneration and recovery over a 3-5 day period at higher frequency. 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.
Zheng
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Hearing Research Laboratories, Department of Communicative Disorders and Sciences, State University of New York at Buffalo 14214, USA.
Henderson
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Hu
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McFadden
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SL
D000818:Animals; D001309:Auditory Threshold / Q000187:drug effects / Q000502:physiology; D002682:Chinchilla; D003051:Cochlea / Q000187:drug effects / Q000294:innervation / Q000503:physiopathology; D016057:Evoked Potentials, Auditory, Brain Stem / Q000187:drug effects; D018690:Excitatory Amino Acid Agonists / Q000008:administration & dosage / Q000633:toxicity*; D005260:Female; D006199:Hair Cells, Auditory, Inner / Q000166:cytology / Q000187:drug effects / Q000473:pathology; D006319:Hearing Loss, Sensorineural / Q000139:chemically induced / Q000503:physiopathology; D007608:Kainic Acid / Q000008:administration & dosage / Q000633:toxicity*; D008297:Male; D018613:Microscopy, Confocal; D009475:Neurons, Afferent / Q000166:cytology / Q000187:drug effects*; D017084:Otoacoustic Emissions, Spontaneous / Q000187:drug effects; D012405:Round Window, Ear / Q000187:drug effects; D013569:Synapses / Q000187:drug effects*; D000159:Vestibulocochlear Nerve / Q000166:cytology / Q000187:drug effects*
D018690:Excitatory Amino Acid Agonists; D007608:Kainic Acid