Recovery of structure and function of inner ear afferent synapses following kainic acid excitotoxicity

Hear Res. 1997 Mar;105(1-2):65-76. doi: 10.1016/s0378-5955(96)00188-8.


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.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Auditory Threshold / drug effects
  • Auditory Threshold / physiology
  • Chinchilla
  • Cochlea / drug effects
  • Cochlea / innervation
  • Cochlea / physiopathology
  • Evoked Potentials, Auditory, Brain Stem / drug effects
  • Excitatory Amino Acid Agonists / administration & dosage
  • Excitatory Amino Acid Agonists / toxicity*
  • Female
  • Hair Cells, Auditory, Inner / cytology
  • Hair Cells, Auditory, Inner / drug effects
  • Hair Cells, Auditory, Inner / pathology
  • Hearing Loss, Sensorineural / chemically induced
  • Hearing Loss, Sensorineural / physiopathology
  • Kainic Acid / administration & dosage
  • Kainic Acid / toxicity*
  • Male
  • Microscopy, Confocal
  • Neurons, Afferent / cytology
  • Neurons, Afferent / drug effects*
  • Otoacoustic Emissions, Spontaneous / drug effects
  • Round Window, Ear / drug effects
  • Synapses / drug effects*
  • Vestibulocochlear Nerve / cytology
  • Vestibulocochlear Nerve / drug effects*


  • Excitatory Amino Acid Agonists
  • Kainic Acid