Unusually Slow Spike Frequency Adaptation in Deep Cerebellar Nuclei Neurons Preserves Linear Transformations on the...

Purkinje cells (PCs) are spontaneously active neurons of the cerebellar cortex that inhibit glutamatergic projection neurons within the deep cerebellar nuclei (DCN) that provide the primary cerebellar output. Brief reductions of PC firing rapidly increase DCN neuron firing. However, prolonged reductions of PC inhibition, as seen in some disease states, certain types of transgenic mice, during optogenetic suppression of PC firing, and in acute slices of the cerebellum, do not lead to large, sustained increases in DCN firing. Here we test whether DCN neurons undergo spike frequency adaptation that could account for these properties. We perform current-clamp recordings at near physiological temperature in acute brain slices from mice of both sexes to examine how DCN neurons respond to prolonged depolarizations. DCN neuron adaptation is exceptionally slow and bidirectional. A depolarizing current step evokes large initial increases in firing that decay to ∼20% of the initial increase within ∼10 s. We find that spike frequency adaptation in DCN neurons is mediated by a novel mechanism that is independent of the most promising candidates, including calcium entry and Na⁺-activated potassium channels mediated by Slo2.1 and Slo2.2 Slow adaptation allows DCN neurons to gradually and bidirectionally adapt to prolonged currents but to respond linearly to current injection on rapid timescales. This suggests that an important consequence of slow adaptation is that DCN neurons respond linearly to the rate of PC firing on rapid timescales but adapt to slow firing rate changes of PCs on long timescales.SIGNIFICANCE STATEMENT Excitatory neurons in the cerebellar nuclei provide the primary output from the cerebellum. This study finds that these neurons exhibit very slow bidirectional spike frequency adaptation that has important implications for cerebellar function. This mechanism allows neurons in the cerebellar nuclei to adapt to long-lasting changes in synaptic drive while also remaining responsive to short-term changes in excitatory or inhibitory drive.