The Glutamate Receptor Subunit δ1 Is Highly Expressed in Hair Cells of the Auditory and Vestibular Systems

Abstract

In the inner ear, fast excitatory synaptic transmission is mediated by ionotropic glutamate receptors, including AMPA, kainate, and NMDA receptors. The recently identified ␦1 and ␦2 glutamate receptors share low homology with the other three types, and no clear response or ligand binding has been obtained from cells transfected with ␦ alone or in combination with other ionotropic receptors. Studies of mice lacking expression of ␦2 show that this subunit plays a crucial role in plasticity of cerebellar glutamatergic synapses. In addition, these mice show a deficit in vestibular compensation. These findings and the nature of glutamatergic synapses between vestibulocochlear hair cells and primary afferent dendrites suggest that ␦ receptors may be functionally important in the inner ear and prompted us to investigate the expression of ␦ receptors in the cochlea and peripheral vestibular system. Reverse transcription and DNA amplification by PCR combined with immunocytochemistry and in situ hybridization were used. Our results show that the expression of ␦1 in the organ of Corti is intense and restricted to the inner hair cells, whereas ␦1 is expressed in all spiral ganglion neurons as well as in their satellite glial cells. In the vestibular end organ, ␦1 was highly expressed in both hair cell types and also was expressed in the vestibular ganglion neurons. The prominent expression of ␦1 in inner hair cells and in type I and type II vestibular hair cells suggests a functional role in hair cell neurotransmission. Key words: cochlea; vestibular end organ; hair cells; spiral ganglion neurons; PCR; in situ hybridization; Western blot; immunocytochemistryAuditory and vestibular stimuli are detected by hair cells in the inner ear and are transmitted to the brain by way of the auditory and vestibular nerves. Of the two types of hair cells found in the organ of Corti, inner hair cells (IHC s) and outer hair cells (OHC s), IHC s are the primary transducers of sensory information. IHC s form chemical synapses with dendrites of type I spiral ganglion neurons (SGNs) that constitute 90 -95% of the SGNs. OHC s synapse on the remaining type II SGNs (Spoendlin, 1972;Berglund and Ryugo, 1987). The vestibular epithelium contains two types of sensory hair cells. T ype I cells (V HCI) are flaskshaped and surrounded by an afferent nerve caly x, whereas type

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