Postnatal expression of EAAC1 and glutamate receptor subunits in vestibular nuclear neurons responsive to vertical linear acceleration

Both glutamate receptors and transporters are known to be important in the postsynaptic regulation of glutamate neurotransmission. However, the maturation profile of glutamate transporter EAAC1 and glutamate receptor subunits (NR1, NR2A and NR2B; and GluR 1-4) in functionally activated saccule-related vestibular nuclear neurons of postnatal rats remains unclear. In the present study, conscious Sprague-Dawley rats (P4 to adult) were subjected to sinusoidal linear acceleration along the vertical plane. Neuronal activation was denoted by the expression of Fos protein. In control experiments, labyrinthectomized animals subjected to stimulation and normal animals that remain stationary showed only sporadically scattered Fos labeled neurons. Functionally activated neurons were also studied for co-localization with NMDA receptor subunits, AMPA receptor subunits or EAAC1. During postnatal development, the proportion of Fos/EAAC1 double-labeled neurons within individual vestibular nuclei progressively decreased with age and constituted about one-third of the Fos-labeled neurons in adult rats. However, comparable proportions of Fos/NMDA or Fos/AMPA double-labeled neurons were observed in each age group. About 70-80% of Fos-labeled neurons in the spinal vestibular nucleus, medial vestibular nucleus and group x subnucleus expressed either NMDA or AMPA receptor subunits. However, in subnucleus y 70% of the total Fos-labeled neurons expressed NMDA while only 10-30% of Fos-labeled neurons expressed AMPA receptor subunits. Triple-immunofluorescence data further showed that some Fos-labeled neurons co-expressed EAAC1 and NMDA or AMPA receptor subunits. Our results suggest that neurons in the different vestibular subnuclei differ in post-synaptic processing of gravity-related vertical spatial information.