Localized detection of action potential‐induced presynaptic calcium transients at a Xenopus neuromuscular junction

1 Action potential (AP)‐induced fluorescence transients were measured, using Ca 2+ indicators and a spot‐detection method, at single nerve terminals of a cultured Xenopus neuromuscular junction preparation with simultaneous measurement of neurotransmitter release. 2 Transients obtained using the low affinity Ca 2+ indicator Oregon Green™ 488 BAPTA‐5N (OGB‐5N) exhibited rapid rising ( t 1/2 (time at which one‐half of the peak fluorescence was attained) = 0.54 ms) and decaying ( T fast = 1.9 ms) phases. The higher affinity indicator Oregon Green™ 488 BAPTA‐2 (OGB‐2) produced transients with significantly slower kinetics ( t 1/2 = 2 ms; T slow = 73 ms). 3 Tetanic stimulation elicited distinct increases in fluorescence in response to each AP. Each OGB‐5N fluorescence increase was more rapid than those observed using OGB‐2. Furthermore, a smaller proportion of residual fluorescence at the end of the train was observed using OGB‐5N. 4 When OGB‐5N was used, a significant [Ca 2+ ] increase was observed prior to the release of neurotransmitter. This was not observed when OGB‐2 was used. 5 We conclude that the use of localized optical detection coupled with low affinity Ca 2+ indicators can help elucidate rapid changes in presynaptic [Ca 2+ ] dynamics underlying evoked neurotransmitter release.