Low- and high-voltage-activated calcium currents: their relationship to the site of neurotransmitter release in an identified neuron of Helisoma.

In this study we have characterized two types of Ca2+ currents in identified neuron B5 of Helisoma and have examined the relationship between these currents and neurotransmitter release. Neuron B5 contains low-voltage-activated (LVA) and high-voltage-activated (HVA) Ca2+ currents. These currents have distinct electrophysiological and pharmacological properties. To gain access to the site of neurotransmitter release, we used a model system in which somata that do not extend neurites assume the role of neurotransmitter release. Before somata gain the ability to release neurotransmitter, they contain LVA and HVA Ca2+ currents. After 3 days of culture, when spherical somata have gained the secretory capacity, only the HVA Ca2+ current is present. Experiments were also performed when neurite extension was permitted. These data indicate that neurons with processes have a differential distribution of Ca2+ currents. The soma, which does not release neurotransmitter, contains both LVA and HVA Ca2+ currents, while distal secretory processes contain only HVA current.