Abstract
Mechanosensory hair cells release glutamate at ribbon synapses to excite postsynaptic afferent neurons, via AMPA-type ionotropic glutamate receptors (AMPARs). However, type II afferent neurons contacting outer hair cells in the mammalian cochlea were thought to differ in this respect, failing to show GluA immunolabeling and with many “ribbonless” afferent contacts. Here it is shown that antibodies to the AMPAR subunit GluA2 labeled afferent contacts below inner and outer hair cells in the rat cochlea, and that synaptic currents in type II afferents had AMPAR-specific pharmacology. Only half the postsynaptic densities of type II afferents that labeled for PSD-95, Shank, or Homer were associated with GluA2 immunopuncta or presynaptic ribbons, the “empty slots” corresponding to ribbonless contacts described previously. These results extend the universality of AMPAergic transmission by hair cells, and support the existence of silent afferent contacts.
Footnotes
↵1 The authors declare no conflicting financial interests.
↵3 This research was supported by National Institute on Deafness and Other Communication Disorders (NIDCD) Grants R01-DC-006476 (to E.G.), R01-DC-011741 (to P.F.), P30-DC-005211 (to the Center for Hearing and Balance), T32-DC-000023 (to C.W.), and F31-DC-010948 (to C.W.); Grant NS050274, a Multiphoton Imaging Core grant (to Hopkins Neuroscience); the John Mitchell, Jr. Trust; and the David M. Rubenstein Fund for Hearing Research (to Department of Otolaryngology–Head and Neck Surgery). This research also was supported in part by the Intramural Research Program of the National Institutes of Health, NIDCD (to C.W.).
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