Dendritic Ca²⁺-activated K⁺ conductances regulate electrical signal propagation in an invertebrate neuron

Activity-dependent changes in the short-term electrical properties of neurites were investigated in the anterior pagoda (AP) cell of leech. Imaging studies revealed that backpropagating Na⁺ spikes and synaptically evoked EPSPs caused Ca²⁺ entry through low-voltage-activated Ca²⁺ channels that are distributed throughout the neurites. Voltage-clamp recordings from the soma revealed a TEA-sensitive outward current that was reduced when Ca²⁺ entry was blocked with Co²⁺ or when the intracellular concentration of free Ca²⁺ was reduced by a high-affinity Ca^2- buffer. Ca²⁺ released in the neurite from a caged Ca²⁺ compound caused a hyperpolarization of the membrane potential. These data imply that the AP cell expresses Ca²⁺- activated K⁺ conductances, and that these conductances are present in the neurites. When the Ca²⁺activated K⁺ current was reduced through the block of Ca²⁺ entry, backpropagating Na⁺ spikes and synaptically evoked EPSPs increased in amplitude. Hence, the activity-dependent changes in the intracellular [Ca²⁺] together with the Ca²⁺-activated K⁺ conductances participate in the regulation of dendritic signal propagation.