Developmental profile of ionotropic glutamate receptors in the vestibular nucleus of rats: analysis of miniature synaptic currents and Fos expression

Abstract

In mammals, ionotropic glutamate receptors are important for signal transmission within the adult vestibular nucleus. However, little is known about the contribution of these receptors during postnatal development of the vestibular nucleus. Here we show the developmental expression and functional profiles of two types of ionotropic glutamate receptors, viz. AMPA and NMDA receptors, within the spinal vestibular nucleus (SpV) of Sprague-Dawley rats. Whole-cell patch clamp recordings in brainstem slices of P5-21 rats revealed that both AMPA receptor- and NMDA receptor-mediated currents contributed equally to the miniature excitatory postsynaptic current (mEPSC) on or before P9. At P21, the AMPA receptor-mediated currents became the dominant component of the mEPSC. Triple immunofluorescence also demonstrated the co-expression of NMDA (NR1 subunit) and AMPA (GluR1, GluR2/3 or GluR4 subunit) receptors in SpV neurons that were functionally activated by off-vertical axis rotation (OVAR). Otolithic origin was confirmed by labyrinthectomized and stationary controls. OVAR-activated SpV neurons were first identified at P7 rats by the expression of Fos protein. While ~80% of Fos-immunoreactive neurons co-expressed NR1 and GluR1 (or GluR2/3) throughout postnatal development and in adult rats, Fos-immunoreactive neurons co-expressing NR1 and GluR4 decreased from ~90% in P7 to ~45% in adult rats. Given the major involvement of AMPA receptor-mediated synaptic transmission in adult SpV, our results document changes in contributions of GluR subunits to the maturation of the SpV network.