Responsiveness of inferior olivary neurons to horizontal linear acceleration in developing rats

Abstract

ong, Sassoon Road, Hong Kong, China. To investigate the coding of gravity-related horizontal orientations in the brain, Sprague–Dawley rats (P5—adult) were subjected to sinusoidal linear acceleration along either the antero-posterior or transverse axis on the horizontal plane. Neuronal activation within the vestibular nucleus and its downstream relay the inferior olive (IO) was indicated by the expression of Fos protein. The few sporadically scattered Foslabeled neurons found in both labyrinthectomized controls and static controls constituted an insignificant background in the assessment of otolithic activation of Fos expression. During postnatal development, vestibular nuclear neurons activated by horizontal transverse movement were detectable only from P7 onwards. Those responsive to horizontal anteroposterior movement were only observed from P9 onwards. In the IO, however, Fos expression was not observed until P11. These findings illustrate that the postnatal onset of recognition of gravity-related horizontal orientations differs between relay stations of the vestibulo-olivary pathway. In adult rats, random distribution of Fos-labeled neurons that differed in response vectors was observed in various vestibular subnuclei. On the other hand, evidence of a gravity-related topographic spatial organization along the rostrocaudal dimension of the IO was observed. In the dorsomedial cell column (DMCC) which is located in the rostral end of the IO, neurons responded only to transverse stimulation. More caudally, most neurons in the rostral half of the beta nucleus (IO ) responded to transverse stimulation. Neurons in the caudal IO , however, responded only to anterior–posterior stimulation. This adult distribution pattern emerged during the third postnatal week. Before this period, DMCC neurons already showed the adult pattern; rostral IO neurons were not responsive to both directions but caudal IO responded to stimulations in both transverse and antero-posterior directions. The emergence of gravity-related dichotomy in an intermediate relay station suggests anatomical segregation in the maturation of central processing of orientation-specific information. Acknowledgement: Supported by HK RGC grant.