Physiology, pharmacology and plasticity at the inner hair cell synaptic complex

This report summarizes recent neuropharmacological data at the IHC afferent/efferent synaptic complex: the type of Glu receptors and transporter involved and the modulation of this fast synaptic transmission by the lateral efferents. Neuropharmacological data were obtained by coupling the recording of cochlear potentials and single unit of the auditory nerve with intra-cochlear applications of drugs (multi-barrel pipette). We also describe the IHC afferent/efferent functioning in pathological conditions. After acoustic trauma or ischemia, acute disruption of IHC-auditory dendrite synapses are seen. However, a re-growth of the nerve fibres and a re-afferentation of the IHC were completely done 5 days after injury. During this synaptic repair, multiple presynaptic bodies were commonly found, either linked to the membrane or “floating” in ectopic positions. In the meantime, the lateral efferents directly contact the IHCs. The demonstration that NMDA receptors blockade delayed the re-growth of neurites suggests a neurotrophic role of NMDA receptors in pathological conditions.