Developmental changes in ionotropic glutamate receptor subunits in rat inferior olivary neurons responsive to linear accelerations

Abstract

Sinusoidal oscillations of linear acceleration along the horizontal or vertical planes were applied to stimulate otolith organs in the inner ear of rats at different postnatal ages. Using Fos expression as an indicator of neuronal recruitment into otolith-related neural circuit, we investigated the time at which responsive neurons in the inferior olive (IO) were first detected. Neurons in subnucleus dorsomedial cell column (DMCC) were activated by vertical stimulation and horizontal (interaural) stimulation from P9 and P11, respectively. By P13, neurons in the β subnucleus of IO (IOβ) became responsive to horizontal stimulation along the interaural and antero-posterior directions. By P21, neurons in the rostral IOβ became also responsive to vertical stimulation while those in the caudal IOβ remained responsive to horizontal stimulation only. The expression of NR1 subunit of the NMDA receptor and the GluR2/3 subunit of the AMPA receptor was observed in nearly all functionally activated neurons within DMCC and IOβ. Our whole-cell patch-clamp findings further indicated that glutamate receptor-mediated miniature excitatory postsynaptic currents of IOβ neurons showed postnatal increase in amplitude, reaching the adult level by P14. These neurons also exhibited subthreshold oscillations in membrane potential as from P14. Taken together, our results indicate that the IO encodes spatial orientations in three dimensions and ionotropic glutamate receptors in the IO play an active role in postnatal coding of gravity-related information.