Expression of AMPA, kainate, and NMDA receptor subunits in cochlear and vestibular ganglia.

Glutamate is believed to be the principal afferent neurotransmitter in the peripheral auditory and vestibular systems. In this report, we present a comprehensive molecular analysis of ionotropic glutamate receptor gene expression in the cochlear and vestibular ganglia of the rat. Fourteen glutamate receptor subunits were studied: GluR1-4 (including flip and flop variants), GluR5-7, KA1&2, NR1, and NR2A-D. Reverse transcription of RNA followed by DNA amplification with the polymerase chain reaction was used for the initial analysis. Immunocytochemistry and in situ hybridization with subunit-specific oligonucleotides were subsequently used for cellular localization of receptor expression. AMPA (GluR2-4), kainate (GluR5&6 and KA1&2), and NMDA receptor (NR1 and NR2A-D) subunit expression was detected. Based on the relative amounts of mRNA detected by in situ hybridization, the predominant receptors expressed by cochlear and vestibular ganglion cells appear to be GluR2, GluR3, GluR4, GluR5, and NR1. At a moderate level were GluR6, NR2B, and NR2D. KA1, KA2, NR2A, and NR2C mRNAs were also expressed in ganglion cells, but at lower levels. Only the AMPA receptor subunit GluR1 and the kainate receptor subunit GluR7 were not found to be expressed in vestibulocochlear neurons. These studies suggest that functional AMPA, kainate, and NMDA receptors are present at the hair cell/vestibulocochlear nerve synapse.