A late switch for post - tetanic potentiation : Once again it ’ s Ca

Many prior studies have documented the role of calcium in the induction of posttetanic potentiation (PTP). In this issue Habets & Borst (2006) now elucidate the effect of Ca during the expression of PTP. Ingenious fine-tuning of the Caimaging allowed them to measure Ca in a largely unperturbed system: the calyx of Held nerve terminal. They first loaded fluorescent Ca-indicator dyes via a wholecell patch pipette for a few minutes and then removed the pipette. These nerve terminals pre-loaded with Ca-indicators were then stimulated via their afferent axons and imaged. By using a very sensitive, cooled, and low noise photon-multiplier tube (PMT), a judicious choice of low noise Ca-indicators to increase the signal-tonoise ratio, and a pinhole imaging only the focal plane to reduce the background even further, they resolve the time-course of the Ca rise during a single presynaptic action potential (AP). Due to this technical improvement they found that after PTP induction a presynaptic AP triggers a 15% larger Ca influx added onto an increased basal Ca level. Assuming an increased vesicle pool size and a 3.5 power-law between Ca and release the authors estimate that this surprising increase in Ca influx is sufficient to explain the amount of enhanced transmitter release during PTP. It is also noteworthy that this increase in Ca influx occurred over a similar time-course as PTP.