["The Journal of Physiology, EarlyView. ", "\nAbstract figure legend Cerebellar dysfunction can give rise to various movement disorders. Misfiring cerebellar neurons often underlie this dysfunction. Because Purkinje cells form inhibitory synapses onto cerebellar nuclei cells, it has been presumed that spike activity changes in Purkinje cells cause inverse spike activity changes in the downstream nuclei cells. This study tests this relationship by investigating whether there is a systemic relationship between Purkinje cell and nuclei cell spike activity changes in mouse models for cerebellar movement disorders. We use a database of in vivo electrophysiological recordings from different mouse models for cerebellar disease. Based on their anatomical connectivity, we expected that if there was an increase in Purkinje cell firing rate, we would observe a decrease in nuclei cell firing rate and vice versa (a negative correlation). We also anticipated that if there was an increase in Purkinje cell firing irregularity, there would be an increase in the nuclei cell firing irregularity (a positive correlation). However, our results did not show any significant correlation between Purkinje cell and nuclei firing rate changes. The correlations regarding steady‐state Purkinje cell and nuclei cell firing irregularity were variable. Overall, our results do not show a consistent relationship between Purkinje cell and nuclei cell spike activity changes in mouse models of the disease.\n\n\n\n\nAbstract\nCerebellar dysfunction can cause ataxia, dystonia, and tremor. Cerebellar nuclei neurons, the main cerebellar output neurons, exhibit distinct spike patterns in mouse models for different movement disorders. However nuclei spike pattern changes often arise from misfiring, miswiring, or degenerating Purkinje cells, which form the predominant input onto nuclei cells. It is often assumed that changes in Purkinje cell spike patterns cause inverse changes in nuclei cell spike patterns because Purkinje cells form GABAergic synapses onto cerebellar nuclei cells. We test this hypothesis by investigating whether a systematic relationship between spike patterns in Purkinje and nuclei cells exists. We analysed parameters relating to spike rate and irregularity from single‐cell in vivo electrophysiological recordings of cerebellar cells in five mouse models for cerebellar movement disorders. We examined whether changes in steady‐state Purkinje cell spike patterns could predict nuclei cell spike pattern changes. We found that some parameters for steady‐state spike irregularity were positively correlated between Purkinje and nuclei cells, but no – especially no inverse – relationship was observed between steady‐state Purkinje and nuclei cell spike rate. We also did not observe an increased steady‐state nuclei cell spike rate in mice with silenced or degenerating Purkinje cells. Because we find no systematic relationship between steady‐state Purkinje and nuclei spike activity, it is not possible to reliably predict disease‐associated nuclei spike patterns based on single‐cell Purkinje recordings. Moreover, normal Purkinje cell spike patterns can mask disease‐causing spike patterns in cerebellar nuclei cells, underscoring the importance of studying cerebellar nuclei cell function in cerebellar disease.\n\n\n\n\n\n\n\n\n\nKey points\n\nThis study tests the hypothesis that cerebellar Purkinje cell spike rates inversely change with nuclei cell spike rates in cerebellar disease models.\nWe found no reliable correlation in steady‐state spike rate parameters between Purkinje and nuclei cells; across mouse models differences in Purkinje cell firing rate were not reliably associated with differences in nuclei cell firing rate.\nWe observed a positive correlation for steady‐state spike irregularity between Purkinje and nuclei cells, but also found nuclei cell irregularity changes in mice without Purkinje cell spike irregularity.\nThe findings emphasize the need to directly study nuclei cell function in cerebellar disease, as Purkinje cell spike patterns do not reliably correlate with nuclei cell changes or even mask disease states.\nThe complex relationship may stem from nuclei cell's intrinsic adaptive properties, developmental compensation, or converging Purkinje cell inputs.\n\n\n"]