Developmental responsiveness of vestibular-related brainstem neurons to horizontal rotational and translational motions

Abstract

To investigate the distribution of functionally activated vestibular-related brainstem neurons during postnatal development, combined immuno-/hybridization histochemistry for c-fos expression was performed in Sprague-Dawley rats (P1–21; adult). Conscious animals were subjected to rotational or translational stimulus which activates hair cells on the horizontal semicircular canals or utricle, respectively. Neuronal activation within brainstem nuclei was defined by the expression of c-fos . Labyrinthectomized controls and normal stationary controls showed only a few sporadically scattered Fos-expressing neurons. During postnatal development, Fos-labeled vestibular nuclear (VN) neurons activated by cycles of constant angular acceleration and deceleration were observed by P4 while those by sinusoidal translational movement were not identifiable until P7. In both stimulation paradigms, an age-dependent increase in Fos-labeled neurons was observed in the VN and prepositus hypoglossal nucleus (PrH), the latter being a downstream area related to vestibulo-oculomotor function. In P7–9 rats, Fos-labeled neurons activated by canal or utricle input were evenly distributed throughout the rostro-caudal length of the VN complex (except the dorsal part of lateral VN) as well as groups x and y. At this age, PrH neurons were only activated with rotational stimulus and those activated with translational stimulus were not found until P11. In P21 and adult rats, the overlapping distribution of canal- and utricle-related Fos-labeled neurons was limited to the middle part of medial VN and caudal part of spinal VN. The rostral part of spinal VN was found to be constituted with neurons activated by translational motion. In the medial VN and PrH, a vast majority of the neurons was activated by rotational stimulus and only a small proportion was by translational stimulus. Taken together, we have demonstrated that canal- and otolith-related brainstem neurons that encode horizontal rotational and translational movements are histologically segregated and exhibit different developmental time frame. Acknowledgement: Supported by HKRGC.