Developmental expression of NMDA receptor subunits in rat vestibular nuclear neurons responsive to off-vertical axis rotation

The aim of the present study is to determine the developmental profile of NMDA receptor subunits in otolith-related vestibular nuclear neurons. Combined immuno- and in situ hybridization histochemistry experiments were performed in conscious Sprague-Dawely rats (P5 to adult) that were subjected to off-vertical axis rotation (OVAR). Neuronal activation within the vestibular nuclei was defined by the expression of Fos protein. Control animals, viz. labyrinthectomized rats subjected to OVAR and normal rats that remained stationary, showed only a few sporadically scattered Fos-positive neurons. Functionally activated Fos-positive neurons were first found in the vestibular nuclei at P7. During postnatal development, the number of Fos-positive neuron in the vestibular nuclei and its subnuclei increased with age and reached the maximum level in adult. Further, the expression of NMDA receptor subunits in Fos-positive neurons was assessed by immunocytochemistry. Nearly all Fos-positive neurons were found to co-express NR1 or NR2A/B subunits. The expression of mRNAs of NMDA receptor subunits in Fos-positive neurons was also evaluated by in situ hybridization technique using 35S-labeled mRNA probes. From P7 to adult, the granule density of both NR1 and NR2B mRNAs was found to decrease, while that of NR2A mRNA was found to increase. Our findings suggest that dynamic change of the expression of NMDA receptor subunits during development may affect the maturation of central otolith neurons in the coding of horizontal head movements. . Supported by HK RG