Developmental responsiveness of rat dorsomedial cell column neurons to horizontal and vertical linear acceleration

To chart the postnatal time course by which 3-D spatial reference of gravity information is encoded in the dorsomedial cell column (DMCC) of the inferior olive, conscious Sprague-Dawley rats (P6–adult) were subjected to linear acceleration along the vertical or horizontal plane. Neuronal activation was discerned by the expression of Fos protein in cell nuclei of the DMCC. Fos-labeled neurons were rarely found in control rats. In experimental rats, DMCC neurons that showed c-fos expression in response to vertical stimulation were observed first at P9 while those to horizontal interaural stimulation were observed from P11 onwards. No functionally activated DMCC neuron was found with horizontal stimulation along the antero-posterior axis. Extracellular recordings in decerebrate rats subjected to otolith stimulation on the horizontal plane also confirmed that the best response orientations of DMCC neurons were confined to the transverse direction. Altogether, these indicate that otolith-related DMCC neurons only process information about head movements in the coronal plane. The expression profile of ionotropic glutamate receptor subunits in these developing neurons was also examined by combining immuno- and hybridization histochemistry experiments. In each age group studied, about 95-98% of Fos-labeled neurons responsive to horizontal interaural stimulation or vertical stimulation expressed NMDA (NR1, NR2A, NR2B) or AMPA (GluR2/3) receptor subunits. While the granule density of both NR2A and NR2B mRNAs decreased from P6 onwards, that of NR1 mRNA increased with age, peaked at P11 and reached the adult level by P21. Our findings suggest that dynamic changes in the expression of glutamate receptor subunits contribute to the maturation of otolith-related DMCC neurons in the coding of head movements in the coronal plane. Supported by HK RG