Multiple calcium signalling pathways in squid giant presynaptic terminals

This chapter considers the recent studies that use the fluorescent Ca²⁺ indicator, fura-2, to further characterize presynaptic Ca²⁺ signaling. The fluorescent Ca indicator, fura-2, can be used to measure the changes in presynaptic Ca concentration associated with action potentials. When using either a photomultiplier tube or a video camera to detect fura-2 fluorescence, trains of presynaptic action potentials produce rapid rises in Ca concentration that reach a peak level of a few nM per action potential and which decay very slowly, over hundreds of seconds. However, these measured rises are not those responsible for triggering transmitter release because injection of the Ca buffer, EGTA, blocks them in Ca concentration but not transmitter release. EGTA injection blocks synaptic augmentation, a form of synaptic plasticity which increases the amount of release produced by an action potential, suggesting that the measured Ca rises mediate augmentation. The Ca²⁺ signal for neurotransmitter release must be localized to escape detection in imaging experiments. Other experiments suggest that this localization occurs as a consequence of a close spatial association between Ca²⁺ channels and the Ca²⁺ receptors which trigger release. The chapter concludes with the observation that the transmitter release is mediated by a rise in Ca concentration which is too localized to be detected with fura-2 measurements, while augmentation is mediated by a more widespread and detectable Ca signal.