Extrasynaptic distribution of NMDA receptors in cochlear inner hair cell afferent signaling complex.

OBJECTIVE: The distribution and role of NMDA receptors is unclear in the afferent signaling complex of the cochlea. The present study aimed to examine the distribution of NMDA receptors in cochlear afferent signaling complex of the adult mouse, and their relationship with ribbon synapses of inner hair cells (IHCs) and GABAergic efferent terminals of the lateral olivocochlear (LOC).

METHODS: Immunofluorescence staining in combination with confocal microscopy was used to investigate the distribution of glutamatergic NMDA and AMPA receptors in afferent terminals of SGNs, and their relationship with ribbon synapses of IHCs and GABAergic efferent terminals of LOC.

RESULTS: Terminals with AMPA receptors along with Ribbons of IHC formed afferent synapses in the basal pole of IHCs, and those with NMDA receptors were mainly distributed longitudinally in the IHCs nuclei region. Significant difference was found in the distribution of NMDA and AMPA receptors in IHC afferent signaling complex (P<0.05). Some GABAergic terminals colocalized with NMDA receptors at the IHC nucleus region (P>0.05).

CONCLUSION: There is significant difference in the distribution of NMDA and AMPA receptors in cochlear afferent signaling complex. NMDA receptors are present in the extra-synaptic region of ribbon synapses of IHCs, and they are related to GABA efferent terminals of the afferent signaling complex.