In this report we describe an interesting form of plasticity in climbing fiber responses evoked in Purkinje cells of the rabbit cerebellum by periodic vestibular stimulation. The cerebellar nodulus receives a vestibular as well as a visual climbing fiber afferent input from the contralateral inferior olive.1 The vestibular input to the inferior olive originates from the ipsilateral medial and descending vestibular nuclei and terminates within the β-nucleus.11,13 This projection to the β-nucleus from secondary vestibular neurons is GABAergic.3,9,10 This means that vestibularly evoked activity of neurons in the β-nucleus is controlled by modulation of an inhibitory input, rather than by modification of an excitatory input. We have recorded the climbing fiber responses of single Purkinje cells in the uvula-nodulus (lobules 9c, 9d and 10) of rabbits and we have characterized the climbing fiber response of each Purkinje cell from which we recorded by testing its sensitivity to otolithic stimulation, semicircular canal stimulation and optokinetic stimulation. Vestibularly evoked climbing fiber responses recorded from the left nodulus were stationary. They were repeatedly evoked, for tens of minutes, by sinusoidally rotating the rabbit about the longitudinal axis onto its left side (Fig. 1B). When the vestibular stimulus was stopped the discharge of these climbing fiber responses returned to non-periodic spontaneous levels. However, the vestibularly evoked climbing fiber discharge of approximately 5% of Purkinje cells was not stationary. In these Purkinje cells, climbing fiber responses were initially evoked when the rabbit was rolled onto its left side (removal of inhibition from the right β-nucleus), but these climbing fiber responses decreased during repeated sinusoidal vestibular roll stimulation. When the vestibular stimulation was stopped, a slow oscillatory discharge of climbing fiber responses appeared. This oscillatory discharge had a frequency that was equal to the previous vestibular stimulus frequency and it lasted 200-300 s.
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