Dendro‐somatic distribution of calcium‐mediated electrogenesis in Purkinje cells from rat cerebellar slice cultures

Abstract

1 The role of Ca²⁺ entry in determining the electrical properties of cerebellar Purkinje cell (PC) dendrites and somata was investigated in cerebellar slice cultures. Immunohistofluorescence demonstrated the presence of at least three distinct types of Ca²⁺ channel proteins in PCs: the α_1A subunit (P/Q type Ca²⁺ channel), the α_1G subunit (T type) and the α_1E subunit (R type). 2 In PC dendrites, the response started in 66 % of cases with a slow depolarization (50 ± 15 ms) triggering one or two fast (∼1 ms) action potentials (APs). The slow depolarization was identified as a low-threshold non-P/Q Ca²⁺ AP initiated, most probably, in the dendrites. In 16 % of cases, this response propagated to the soma to elicit an initial burst of fast APs. 3 Somatic recordings revealed three modes of discharge. In mode 1, PCs display a single or a short burst of fast APs. In contrast, PCs fire repetitively in mode 2 and 3, with a sustained discharge of APs in mode 2, and bursts of APs in mode 3. Removal of external Ca²⁺ or bath applications of a membrane-permeable Ca²⁺ chelator abolished repetitive firing. 4 Tetraethylammonium (TEA) prolonged dendritic and somatic fast APs by a depolarizing plateau sensitive to Cd²⁺ and to ω-conotoxin MVII C or ω-agatoxin TK. Therefore, the role of Ca²⁺ channels in determining somatic PC firing has been investigated. Cd²⁺ or P/Q type Ca²⁺ channel-specific toxins reduced the duration of the discharge and occasionallyinduced the appearance of oscillations in the membrane potential associated with bursts of APs. 5 In summary, we demonstrate that Ca²⁺ entry through low-voltage gated Ca²⁺ channels, not yet identified, underlies a dendritic AP rarelyeliciting a somatic burst of APs whereas Ca²⁺ entry through P/Q type Ca²⁺ channels allowed a repetitive firing mainly by inducing a Ca²⁺-dependent hyperpolarization.