Morphologic changes in the vestibular nerves and nuclei after labyrinthectomy in the cat: a case for the neurotrophin hypothesis in vestibular compensation.

Morphological changes in the vestibular nerves and superior vestibulocular neurons (SVON) after unilateral labyrinthectomy in cats revealed a progressive loss of axons in the ipsilateral vestibular nerve (35%) and synaptic profiles (SP) on ipsilateral SVON (60%) up to a 1-year survival period. Although the ipsilateral vestibular nerve showed further degeneration (45%) at 2 years post ablation, the number of SP on ipsilateral SVON increased to 60% of normal (40% loss). These SP likely represent sprouting from crossing commissural or cerebellar pathways. Contralateral vestibular nerves at 1 and 2 years post ablation revealed normal numbers and size spectrum, but the number of SP contacting the contralateral SVON at 8 weeks, 1 and 2 years paralleled the levels of SP found on ipsilateral SVON. The symmetry in adjustment of SP on the SVON of both sides of the brainstem after ablation may be explained by the neurotrophin hypothesis.