Tracking presynaptic Ca2+ dynamics during neurotransmitter release with Ca2+-activated K+ channels

Neurotransmitter release during action potentials is thought to require transient, localized [Ca2+](i) as high as hundreds of micromolar near presynaptic release sites. Most experimental attempts to characterize the magnitude and time course of these Ca2+ domains involve optical methods that sample large volumes, require washout of endogenous buffers and often affect Ca2+ kinetics and transmitter release. Endogenous calcium-activated potassium (K-Ca) channels colocalize with presynaptic Ca2+ channels in Xenopus nerve-muscle cultures. We used these channels to quantify the rapid, dynamic changes in [Ca2+](i) at active zones during synaptic activity. Confirming Ca2+-domain predictions, these K-Ca channels revealed [Ca2+](i) over 100 mu M during synaptic activity and much faster buildup and decay of Ca2+ domains than shown using other techniques.