Transient expression of NMDA receptors during rearrangement of AMPA-receptor-expressing fibers in the developing inner ear.

A major reorganization of afferent and efferent nerve terminals, concomitant to significant neuronal cell loss and pruning of superfluous fibers, takes place during the development of the organ of Corti, prior to the onset of hearing. We examined the spatio/temporal distribution of subtype-specific AMPA- and N-methyl-d-aspartate (NMDA)-selective glutamate receptor proteins in postnatal inner ears from rats during this critical period. From the first postnatal day onwards, GluR2/3 receptor subtypes appeared in nerve endings of afferent fibers associated with inner and outer hair cells. During the following 2 weeks, GluR2/3 receptors were downregulated in exchange for GluR4 receptors. In parallel efferents projecting from the medial olivocochlear complex to the outer hair cells underwent synaptogenesis and efferents projecting from the lateral olivocochlear complex to the inner hair cells appeared to change contacts to the dendrites of afferents. Concomitant to these events, NMDA receptor subtypes NR1 and NR2A transiently appeared in hair cells as well as afferent and efferent fibers. Recently, we described a temporary expression of the neurotrophin receptor trkB in hair cells, coincident to the growth (GAP-43) and synaptogenesis (synaptophysin) of efferents. Here, we show that trkB was expressed together with NR1 receptors in hair cells in high spatio/temporal correlation with the rearrangement of afferents and efferents. Cochlea NMDA receptors may, therefore, be a part of the mechanism by which, in addition to neurotrophic activity, the mature phenotype of cochlea neurons is acquired through activity-dependent processes.