T-type calcium channels are thought to transform neuronal output to a burst mode by generating low voltage-activated (LVA) calcium currents and rebound burst discharge. In this study we assess the expression pattern of the three different T-type channel isoforms (Cav3.1, Cav3.2, and Cav3.3) in cerebellar neurons and focus on their potential role in generating LVA spikes and rebound discharge in deep cerebellar nuclear (DCN) neurons. We detected expression of one or more Cav3 channel isoforms in a wide range of cerebellar neurons and selective expression of different isoforms in DCN cells. We further identify two classes of large-diameter DCN neurons that exhibit either a strong or weak capability for rebound discharge, despite the ability to generate LVA spikes when calcium currents are pharmacologically isolated. By correlating the Cav3 channel expression pattern with the electrophysiological profile of identified DCN cells, we show that Cav3.1 channels are expressed in isolation in DCN-burst cells, whereas Cav3.3 is expressed in DCN-weak burst cells. Cav3.1-expressing DCN cells correspond to excitatory or GABAergic neurons, whereas Cav3.3-expressing cells are non-GABAergic. The Cav3 class of LVA calcium channels is thus expressed in specific combinations in a wide range of cerebellar neurons but contributes to rebound burst discharge in only a select number of cell classes.Purkinje cells ͉ cerebellum ͉ rebound discharge

Spike output in many neuronal cell types is affected by low-voltage-activated T-type calcium currents arising from the Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3 channel subtypes and their splice isoforms. The contributions of T-type current to cell output is often proposed to reflect a differential distribution of channels to somatic and dendritic compartments, but the subcellular distribution of the various rat T-type channel isoforms has not been fully determined. We used subtype-specific Ca(v)3 polyclonal antibodies to determine their distribution in key regions of adult Sprague-Dawley rat brain thought to exhibit T-type channel expression, and in particular, dendritic low-voltage-activated responses. We found a selective subcellular distribution of Ca(v)3 channel proteins in cell types of the neocortex and hippocampus, thalamus, and cerebellar input and output neurons. In general, the Ca(v)3.1 T-type channel immunolabel is prominent in the soma/proximal dendritic region and Ca(v)3.2 immunolabel in the soma and proximal-mid dendrites. Ca(v)3.3 channels are distinct in distributing to the soma and over extended lengths of the dendritic arbor of particular cell types. Ca(v)3 distribution overlaps with cell types previously established to exhibit rebound burst discharge as well as those not recognized for this activity. Additional immunolabel in the region of the nucleus in particular cell types was verified as corresponding to Ca(v)3 antigen through analysis of isolated protein fractions. These results provide evidence that different Ca(v)3 channel isoforms may contribute to low-voltage-activated calcium-dependent responses at the somatic and dendritic level, and the potential for T-type calcium channels to contribute to multiple aspects of neuronal activity.

Purkinje cells (PCs) generate the sole output of the cerebellar cortex and govern the timing of action potential discharge from neurons of the deep cerebellar nuclei (DCN). Here, we examine how voltage-gated Kv1 K ϩ channels shape intrinsically generated and synaptically controlled behaviors of PCs and address how the timing of DCN neuron output is modulated by manipulating PC Kv1 channels. Kv1 channels were studied in cerebellar slices at physiological temperatures with Kv1-specific toxins. Outside-out voltage-clamp recordings indicated that Kv1 channels are present in both somatic and dendritic membranes and are activated by Na ϩ spike-clamp commands. Whole-cell current-clamp recordings revealed that Kv1 K ϩ channels maintain low frequencies of Na ϩ spike and Ca-Na burst output, regulate the duration of plateau potentials, and set the threshold for Ca 2ϩ spike discharge. Kv1 channels shaped the characteristics of climbing fiber (CF) responses evoked by extracellular stimulation or intracellular simulated EPSCs. In the presence of Kv1 toxins, CFs discharged spontaneously at ϳ1 Hz. Finally, "Kv1-intact" and "Kv1-deficient" PC tonic and burst outputs were converted to stimulus protocols and used as patterns to stimulate PC axons and synaptically activate DCN neurons. We found that the Kv1-intact patterns facilitated short-latency and high-frequency DCN neuron rebound discharges, whereas DCN neuron output timing was markedly disrupted by the Kv1-deficient stimulus protocols. Our results suggest that Kv1 K ϩ channels are critical for regulating the excitability of PCs and CFs and optimize the timing of PC outputs to generate appropriate discharge patterns in postsynaptic DCN neurons.

The modification of first-spike latencies by low-threshold and inactivating K ϩ currents (I A ) have important implications in neuronal coding and synaptic integration. To date, cells in which first-spike latency characteristics have been analyzed have shown that increased hyperpolarization results in longer first-spike latencies, producing a monotonic relationship between first-spike latency and membrane voltage. Previous work has established that cerebellar stellate cells express members of the K v 4 potassium channel subfamily, which underlie I A in many central neurons. Spike timing in stellate cells could be particularly important to cerebellar output, because the discharge of even single spikes can significantly delay spike discharge in postsynaptic Purkinje cells. In the present work, we studied the first-spike latency characteristics of stellate cells. We show that first-spike latency is nonmonotonic, such that intermediate levels of prehyperpolarization produce the longest spike latencies, whereas greater hyperpolarization or depolarization reduces spike latency. Moreover, the range of first-spike latency values can be substantial in spanning 20 -128 ms with preceding membrane shifts of Ͻ10 mV. Using patch clamp and modeling, we illustrate that spike latency characteristics are the product of an interplay between I A and lowthreshold calcium current (I T ) that requires a steady-state difference in the inactivation parameters of the currents. Furthermore, we show that the unique first-spike latency characteristics of stellate cells have important implications for the integration of coincident IPSPs and EPSPs, such that inhibition can shift first-spike latency to differentially modulate the probability of firing.

No abstract