CELLULAR AND SUBCELLULAR-LOCALIZATION OF AMPA-SELECTIVE GLUTAMATE RECEPTORS IN THE MAMMALIAN PERIPHERAL VESTIBULAR SYSTEM

The cellular and subcellular distribution of AMPA-selective glutamate receptors in the mammalian peripheral vestibular system was examined u sing antibodies against peptides corresponding to the C-terminal porti ons of AMPA receptor subunits: GluR1, GluR2/R3 and GluR4. The light an d electron microscopic immunocytochemical studies were carried out on Vibratome sections of rat and guinea pig vestibular sensory epithelia and ganglia. In the epithelium, GluR1 subunit immunoreactivity appeare d as accumulations of patches outlining the base-lateral periphery of the type I sensory cells. The GluR1-immunoreactive microareas were pos tsynaptically distributed on the membranes of calyceal afferent fibers . GluR2/R3 immunoreactivity was present in the sensory cells. GluR4 wa s not detected. In the vestibular ganglion, the neurons were densely s tained with antibodies to GluR2/R3 and GluR4. The fibroblasts and the Schwann cells were also intensely stained with antibodies to GluR2/R3 and GluR4. In the sensory cells, the AMPA receptors, GluR2/R3, may fun ction as (1) autoreceptors controlling afferent neurotransmitter relea se or (2) 'postsynaptic' receptors activated by the neurotransmitter r elease of the afferent calyx. The detection of GluR1 at postsynaptic s ites in the afferent fibers provides anatomical evidence for the role of glutamate as a neurotransmitter of sensory cells. In the ganglion n eurons, GluR2/R3 and GluR4 may represent reserve intracytoplasmic pool s of receptor subunits in transit to the postsynaptic sites. In the Sc hwann cells, GluR2/R3 and GluR4 may be involved in neuronal-glial sign alling at the nodes of Ranvier.