Episodic ataxia type-2 (EA2) is caused by mutations in P/Q-type voltage-gated calcium channels that are expressed at high densities in cerebellar Purkinje cells. Because P/Q channels support neurotransmitter release at many synapses, it is believed that ataxia is caused by impaired synaptic transmission. Here we show that in ataxic P/Q channel mutant mice, the precision of Purkinje cell pacemaking is lost such that there is a significant degradation of the synaptic information encoded in their activity. The irregular pacemaking is caused by reduced activation of calcium-activated potassium (K(Ca)) channels and was reversed by pharmacologically increasing their activity with 1-ethyl-2-benzimidazolinone (EBIO). Moreover, chronic in vivo perfusion of EBIO into the cerebellum of ataxic mice significantly improved motor performance. Our data support the hypothesis that the precision of intrinsic pacemaking in Purkinje cells is essential for motor coordination and suggest that K(Ca) channels may constitute a potential therapeutic target in EA2.

A key component of recent theories on cerebellar function is rebound firing in neurons of the deep cerebellar nuclei (DCN). Despite the robustness of this phenomenon in vitro, in vivo studies have provided little evidence for its prevalence. Here we show that under physiological conditions, in vitro or in vivo, mice or rat DCN neurons rarely show rebound firing, a finding that necessitates a critical re-evaluation of recent cerebellar models.A vast amount of cortical and sensory information that converges onto the cerebellum is integrated by cerebellar Purkinje cells and subsequently conveyed to the neurons of the deep cerebellar nuclei (DCN) 1 . DCN neurons further process this information and generate the major output of the cerebellum, encoding the computational outcome of the cerebellar circuitry in their rate and temporal pattern of activity.A stereotypic biophysical feature of DCN neurons is that they are capable of rebound depolarization 2-4 . Following a strong hyperpolarization their membrane potential briefly rebounds to a more depolarized level resulting in a transient increase in their firing rate; a phenomenon termed rebound firing 4,5 . Given the inhibitory GABAergic nature of Purkinje cell synapses onto DCN neurons, and primarily on the basis of this stereotypic biophysical property in vitro, rebound firing has been extensively incorporated into recent theories of cerebellar function 6-8 . Several functional roles, from timing to encoding information and mediating plasticity have been assigned to rebound depolarization and firing 6-10 . However, even though rebound firing is robust when examined using intracellular recordings in vitro, there is little direct evidence in support of its physiological prevalence in vivo 11-13 . We investigated this discrepancy. DCN rebound depolarization is most likely mediated by low-threshold T-type calcium channels 3-5 . Factors that determine the extent of rebound are average membrane potential prior to hyperpolarization, and the level and duration of hyperpolarization 2,4,5 . Using acutely prepared rat cerebellar slices, we designed our experiments to replicate these factors as close to their physiological parameters as possible (see Supplemental data). We avoided intracellular recordings because they inevitably alter the membrane input resistance, and the cytosolic ionic composition. Therefore, we monitored the activity of DCN neurons extracellularly to preserve their baseline firing rate and the true reversal potential of their GABAergic inputs. Two sets of experiments were done to mimic strong hyperpolarizations that may occur under NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript physiological conditions. First, with excitatory transmission blocked, GABAergic synaptic inputs were stimulated using a train of 10 electrical pulses at 100 Hz (Fig. 1a). The strength of the stimulation was adjusted such that it efficiently paused firing in the target cell (average pause duration = 199.3 ± 9.1 ms, n = 39 cells). Using this paradigm...

The orchestration of simple motor tasks by the cerebellum results in coordinated movement and the maintenance of balance. The cerebellum integrates sensory and cortical information to generate the signals required for the coordinated execution of simple motor tasks. These signals originate in the firing rate of Purkinje cells, each of which integrates sensory and cortical information conveyed by granule cell synaptic inputs. Given the importance of the granule cell input-Purkinje cell output function for cerebellar computation, this algorithm was determined. Using several stimulation paradigms, including those that mimicked patterns of granule cell activity similar to those observed in vivo, we quantified the poststimulus maximum firing rate and number of extra spikes in response to granule cell synaptic input. Both of these parameters linearly encoded the strength of synaptic input when inhibitory synaptic transmission was blocked. This linear algorithm was independent of the location or temporal pattern of synaptic input. With inhibitory synaptic transmission intact, the maximum firing rate, but not the number of extra spikes, encoded the strength of granule cell synaptic input. Furthermore, the maximum firing rate of Purkinje cells linearly encoded the strength of synaptic input whether or not the activation of granule cells resulted in a pause in Purkinje cell firing. On the basis of the data presented, we propose that Purkinje cells encode the strength of granule cell synaptic input in their maximum firing rate with a linear algorithm.

At the center of the computational cerebellar circuitry are Purkinje cells, which integrate synaptic inputs from >150,000 granule cell inputs. Traditional theories of cerebellar function assume that all granule cell inputs are comparable. However, it has recently been suggested that the two anatomically distinct granule cell inputs, ascending and parallel fiber, have different functional roles. By systematically examining the efficacy of patches of granule cells with photostimulation, we found no differences in the efficacy of the two inputs in driving the activity of, or in producing postsynaptic currents in, Purkinje cells in cerebellar slices in vitro. We also found that the activity of Purkinje cells was significantly increased upon stimulation of lateral granule cells in vivo. Moreover, when we estimated parallel fiber and ascending apparent unitary EPSC amplitudes using photostimulation in cerebellar slices in vitro, we found them to be indistinguishable. These results are inconsistent with differential functional roles for these two inputs. Instead, our data support theories of cerebellar computation that consider granule cell inputs to be functionally comparable.

Purkinje cells, the sole output of the cerebellar cortex, encode the timing signals required for motor coordination in their firing rate and activity pattern. Dendrites of Purkinje cells express a high density of P/Q-type voltage-gated calcium channels and fire dendritic calcium spikes. Here we show that dendritic subthreshold Kv1.2 subunit-containing Kv1 potassium channels prevent generation of random spontaneous calcium spikes. With Kv1 channels blocked, dendritic calcium spikes drive bursts of somatic sodium spikes and prevent the cell from faithfully encoding motor timing signals. The selective dendritic function of Kv1 channels in Purkinje cells allows them to effectively suppress dendritic hyperexcitability without hindering the generation of somatic action potentials. Further, we show that Kv1 channels also contribute to dendritic integration of parallel fibre synaptic input. Kv1 channels are often targeted to soma and axon and the data presented support a major dendritic function for these channels.