Transient Ca²⁺‐permeable AMPA receptors in postnatal rat primary auditory neurons

Abstract

Fast excitatory transmission in the nervous system is mostly mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors whose subunit composition governs physiological characteristics such as ligand affinity and ion conductance properties. Here, we report that AMPA receptors at inner hair cell (IHC) synapses lack the GluR2 subunit and are transiently Ca2+-permeable before hearing onset as evidenced using agonist-induced Co2+ accumulation, Western blots and GluR2 confocal microscopy in the rat cochlea. AMPA (100 microM) induced Co2+ accumulation in primary auditory neurons until postnatal day (PND) 10. This accumulation was concentration-dependent, strengthened by cyclothiazide (50 microM) and blocked by GYKI 52466 (80 microM) and Joro spider toxin (1 microM). It was unaffected by D-AP5 (50 microM), and it could not be elicited by 56 mM K+ or 1 mM NMDA + 10 microM glycine. Western blots showed that GluR1 immunoreactivity, present in homogenates of immature cochleas, had disappeared by PND12. GluR2 immunoreactivity was not detected until PND10 and GluR3 and GluR4 immunoreactivities were detected at all the ages examined. Confocal microscopy confirmed that the GluR2 immunofluorescence was not located postsynaptically to IHCs before PND10. In conclusion, AMPA receptors on maturing primary auditory neurons differ from those on adult neurons. They are probably composed of GluR1, GluR3 and GluR4 subunits and have a high Ca2+ permeability. The postsynaptic expression of GluR2 subunits may be continuously regulated by the presynaptic activity allowing for variations in the Ca2+ permeability and physiological properties of the receptor.

Developmental Expression of Ca2+-Permeable AMPA Receptors Underlies Depolarization-Induced Long-Term Depression at Mossy Fiber–CA3 Pyramid Synapses

“…For example, synaptic CP-AMPARs occur in layer 5 pyramids until postnatal day 16 after which CI-AMPARs dominate transmission (Kumar et al, 2002). Similar developmental switches happen at inner hair cell synapses (Eybalin et al, 2004), Xenopus retinotectal synapses (Aizenman et al, 2002), and chicken forebrain synapses (Migues et al, 2007). Moreover, developmental increases in the ratio of GluR2 to other AMPAR subunits occur throughout the CNS (Pellegrini-Giampietro et al, 1992;Pickard et al, 2000;Zhu et al, 2000).…”

“…1 B, bottom panel). Thus, the contribution of native CP-AMPARs to MF-PYR transmission is transient, being limited to early developmental time points with subsequent replacement by CI-AMPARs as reported for various other synapses between principal neurons (Aizenman et al, 2002;Kumar et al, 2002;Eybalin et al, 2004).…”

“…However, recent findings indicate GluR2-lacking, CP-AMPARs are assembled in principal neurons and become synaptically incorporated under certain conditions (Aizenman et al, 2002;Kumar et al, 2002;Eybalin et al, 2004;Ju et al, 2004;Terashima et al, 2004;Bagal et al, 2005;Harms et al, 2005;Ogoshi et al, 2005;Shin et al, 2005;Thiagarajan et al, 2005;Clem and Barth, 2006;Plant et al, 2006;Sutton et al, 2006). One compelling line of evidence suggests that CP-AMPARs are transiently expressed by principal neurons early in development.…”

Cochlear Kainate Receptors

“…We examined the distribution of kainate receptor subunits in the cochlea by comparing immunolabel for those receptor subunits with that of GluA2, an AMPA receptor subunit present in the afferent nerve terminals (Matsubara et al 1996;Eybalin et al 2004). Immunolabel for both the AMPA and kainate receptor subunits was found in intense puncta at the base of the inner hair cells (Fig.…”

“…Immunohistochemical analysis of ionotropic receptors in the cochlea has always been challenging. There are several reports of AMPA receptor localization in the cochlea (Matsubara et al 1996;Knipper et al 1997;Eybalin et al 2004) though none of kainate receptors.…”