REGULATION OF NMDA RECEPTOR SUBUNIT MESSENGER-RNA EXPRESSION IN THE GUINEA-PIG VESTIBULAR NUCLEI FOLLOWING UNILATERAL LABYRINTHECTOMY

The localization of neurons expressing mRNAs for the NR1 and NR2A-D su bunits of the glutamatergic NMDA receptor was examined by non-radioact ive in situ hybridization throughout the guinea pig vestibular nuclei. After deafferentation of the vestibular nuclei by unilateral labyrint hectomy, modifications of the mRNA distributions were followed for 30 days. A quantitative analysis was performed in the medial vestibular n ucleus by comparison of the labelled neurons in the ipsi-and contra-la teral nuclei. In vestibular nuclei, the NR1 subunit mRNA was found in various populations of neurons. The NR2A and NR2C subunit mRNAs were l ess widely distributed, whereas little NR2D mRNA was detected and only rare cells contained NR2B mRNA. NR1 and NR2A-D mRNAs were colocalized in some but not other neuronal types. Twenty hours after the lesion, there was a transient ipsilateral increase of NR1 mRNA level in the me dial vestibular nucleus, followed by a decrease 48 h after the lesion and, at 3 days, by recovery to the control level. An ipsilateral incre ase in the mRNA level of NR2C subunit was detected 20 h after lesion a nd maintained at 48 h. No significant changes were apparent in NR2A, N R2B and NR2D mRNA levels. The distributions and the differential signa l intensities of NR2A-D mRNAs suggest various subunit organizations of the NMDA receptors in different neurons of the vestibular nuclei. Neu ronal plasticity reorganizations in the vestibular nuclei following un ilateral labyrinthectomy appear to include only changes in NR1 and NR2 C mRNA levels modifying the functional diversity of the NMDA receptor in the ipsilateral medial vestibular nucleus neurons. The transient ch anges in NR1 and the NR2C subunit mRNA expressions in response to sens ory deprivation are consistent with an active role for NMDA receptors in the appearance and development of the vestibular compensatory proce ss.