Postnatal co-expression of AMPA and NMDA receptors in utricle-related vestibular nuclear neurons of rats

Abstract

Glutamate is a major excitatory neurotransmitter within the vestibular nucleus. We hypothesize that different classes of glutamate receptors mature with different time scales to result in the different postsynaptic responses observed with postnatal development. To address this, Sprague–Dawley rats (P5 to adult) were subjected to off-vertical axis rotation (OVAR), a scanning stimulus that sequentially activates all directional polarization of hair cells on the utricle. Functionally activated central vestibular neurons were identified by Fos immunoreactivity. These neurons were also evaluated for co-expression of NMDA (NR1) and AMPA (GluR1, GluR2, GluR2/3 or GluR4) receptor subunits. In control animals (both labyrinthectomized rats subjected to OVAR and normal rats kept stationary), only a few sporadic Fos-positive neurons were found in the vestibular nuclei complex, thus confirming the otolithic origin of Fos expression in the test animals. Utricle-related neurons were first found within the vestibular nuclei in P7 rats. Around 85% of Fos-positive neurons in the vestibular nuclear complex at P7 co-expressed NMDA and AMPA receptor subunits. This percentage decreased progressively with age until 70% in adults. Among the remaining Fos-positive neurons in each age group of the rats tested, twothird of them only expressed NR1 or AMPA receptor subunit and the remaining one-third did not express any of these ionotropic receptors. These findings provide the histological basis for cross-modulation of NMDA and AMPA receptors leading to change in synaptic efficiacy during maturation. Acknowledgement: Supported by HK RGC grant.