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1 UMR 7060 Laboratoire de Neurobiologie des Reseaux Sensorimoteurs, CNRS - Universite Rene Descartes (Paris 5), Paris, France
* To whom correspondence should be addressed. E-mail: nivi{at}ccr.jussieu.fr.
Vestibular compensation after a unilateral labyrinthectomy leads to nearly complete disappearance of the static symptoms triggered by the lesion. However, the dynamic vestibular reflexes associated with head movements remain impaired. Since the contralesional labyrinth plays a prominent role in the generation of these dynamic responses, intracellular recordings of contralesional medial vestibular nucleus neurons (MVNn) were done after one month of compensation. Their firing properties and cell type were characterized at rest, and their response dynamics investigated using step, ramp, and sinusoidal current stimulations. The sensitivity of the contralesional MVNn firing rates to applied current was increased, which, along with increased phase leads, suggests that significant changes in active conductances occurred. We found an increased proportion of the phasic type B neurons relative to the tonic type A neurons in the contralesional MVN. In addition, the remaining contralesional type A MVNn response dynamics tended to approach those of type B MVNn. Thus, the contralesional MVNn in general showed more phasic response dynamics than control MVNn. Altogether, the firing properties of MVNn are differentially modified on the ipsilesional and contralesional sides of the brainstem one month after unilateral labyrinthectomy. Ipsilesional MVNn acquire more "type A-like" tonic membrane properties, which would contribute to the stabilization of the spontaneous activity that recovers in the deafferented neurons during vestibular compensation. The bilateral increase in the sensitivity of MVNn and the acquisition of more "B-like" phasic membrane properties by contralesional MVNn should promote the restoration of the vestibular reflexes generated by the remaining, contralesional labyrinth.
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