Cerebellar LTD facilitates but is not essential for long-term adaptation of the vestibulo-ocular reflex

The induction of cerebellar long-term depression (LTD) at the parallel fibre-Purkinje cell synapse is selectively blocked in L7-PKCi transgenic mice, rendering these mice unable to adaptively modify their vestibulo-ocular reflex (VOR) during visuo-vestibular training for a few hours. Despite this deficit, their eye movement performance as well as their general motor behaviour appears unaffected. This combination suggests that, in the long term, residual forms of plasticity in the vestibulo-cerebellar circuitry can compensate for the absence of cerebellar LTD. To investigate whether LTD-deficient mice exhibit motor learning in the long run, we subjected L7-PKCi transgenic mice to visuo-vestibular training paradigms that were aimed at either increasing or decreasing the VOR response in the course of eight consecutive days. During the increasing paradigm, the VOR gain of transgenic mice increased significantly, while VOR gain decreased and VOR phase-lead increased during the decreasing paradigm. The impact of these long training periods on the VOR was significantly smaller in LTD-deficient mice than in wild type littermates. Thus, while LTD may be necessary for short-term VOR adaptation, it facilitates but is not required for long-term adaptation of the VOR.