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

1. The role of Ca2+ Entry in determining thf 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 alpha(1A) subunit (P/Q type Ca2+ channel), the alpha(1G) subunit (T type) and the alpha(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 (similar to 1 ms) action potentials (APs). The slow depolarization was identified as a low-threshold non-P/Q Ca2+ AT initiated, most probably: in thc 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 As 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 omega-conotoxin MVII C or omega-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 occasionally induced 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 rarely eliciting 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.