Quantification of synaptic density changes in the medial vestibular nucleus of the cat following vestibular neurectomy

The synaptic density in the medial vestibular nuclei of the cat was analyzed after section of the ipsilateral vestibular nerve, using an anti-synaptophysin antibody as a nerve terminal marker. Synaptic areas of nuclei ipsilateral (test) and contralateral (control) to the lesion were measured to quantify synaptic loss (one week post-lesion) and synaptic site reoccupation during vestibular compensation (3 weeks and 5 months post-lesion). The results indicate that only 35% of the immunoreactive synaptic profiles in the media) vestibular nucleus were due to vestibular nerve input: the mean synaptic profile loss in the deafferented nucleus one week post-lesion was 35.25% (± 13.83). This loss was rapidly compensated overtime since it was reduced to 14% (± 13.25) 3 weeks post-lesion, corresponding to a synaptic restoration of approximately 60%. The synaptic density in the deafferented nucleus 5 months after neurectomy was not significantly different from that of the intact vestibular nuclei. These results suggest that vestibular neurectomy is followed by an axonal sprouting in the partially deafferented medial vestibular nuclei, which raises the question concerning the origins and the functional role of the new axon terminals in vestibular compensation.