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



  • 1Action potential (AP)-induced fluorescence transients were measured, using Ca2+ indicators and a spot-detection method, at single nerve terminals of a cultured Xenopus neuromuscular junction preparation with simultaneous measurement of neurotransmitter release.
  • 2Transients obtained using the low affinity Ca2+ indicator Oregon Green™ 488 BAPTA-5N (OGB-5N) exhibited rapid rising (t1/2(time at which one-half of the peak fluorescence was attained) = 0.54 ms) and decaying (Tfast= 1.9 ms) phases. The higher affinity indicator Oregon Green™ 488 BAPTA-2 (OGB-2) produced transients with significantly slower kinetics (t1/2= 2 ms; Tslow= 73 ms).
  • 3Tetanic 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.
  • 4When OGB-5N was used, a significant [Ca2+] increase was observed prior to the release of neurotransmitter. This was not observed when OGB-2 was used.
  • 5We conclude that the use of localized optical detection coupled with low affinity Ca2+ indicators can help elucidate rapid changes in presynaptic [Ca2+] dynamics underlying evoked neurotransmitter release.