Immunogold labeling for GluR4 was confined to synaptic sites, represented by puncta in immunofluorescence. The relative numbers of puncta changed with a gradient similar to that of GLAST labeling. VGLUT1 labeling occurred in IHCs but showed no clear cochleotopic gradient. These data suggest that both the density of innervation and the activity levels of glutamatergic synapses may be involved in modulating regional expression of GLAST.