Pattern of spatial coding in otolith neurons of rat vestibular nucleus during postnatal development

Abstract

To determine the time course of maturation of central otolith neurons in coding horizontal spatial orientations, immunocytochemical and in situ hybridization experiments were performed in conscious Sprague-Dawley rats (aged 6 days to adult) subjected to sinusoidal linear acceleration along different head axes on the horizontal plane. Neuronal activation within the vestibular nuclei was defined by the expression of c-fos mRNA and/or Fos protein in response to horizontal linear acceleration. Control animals, viz labyrinthectomized rats subjected to accelerations and normal rats that remained stationary, showed only a few sporadically scattered labeled neurons. In normal experimental rats, the expression of c-fos in the vestibular nuclei was first observed in P7 rats that were stimulated along the interaural axis. No labeling was evident throughout the vestibular nuclei when P7 rats were stimulated along the naso-occipital axis. This indicates that at P7, the rat's capacity to process information about head movement is limited to the interaural direction. The expression of c-fos elicited by acceleration along the naso-occipital axis emerged from P9 onwards. The number of Fos-positive neurons evoked by stimulation along different head axis on the horizontal plane increased progressively with age. The adult pattern in which stimulation along different head axes excited comparable number of responsive neurons was observed at P21. Our finding suggests that during the postnatal period studied, central otolith neurons gradually achieve their capacity in coding horizontal head movements.