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 delta1 and delta2 glutamate receptors share low homology with the other three types, and no clear response or ligand binding has been obtained from cells transfected with delta alone or in combination with other ionotropic receptors. Studies of mice lacking expression of delta2 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 delta receptors may be functionally important in the inner ear and prompted us to investigate the expression of delta 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 delta1 in the organ of Corti is intense and restricted to the inner hair cells, whereas delta1 is expressed in all spiral ganglion neurons as well as in their satellite glial cells. In the vestibular end organ, delta1 was highly expressed in both hair cell types and also was expressed in the vestibular ganglion neurons. The prominent expression of delta1 in inner hair cells and in type I and type II vestibular hair cells suggests a functional role in hair cell neurotransmission.