Evidences for calcium-dependent inactivation of calcium current at the frog motor nerve terminal

Assessment of calcium-dependent inactivation of calcium current in nerve terminals is not feasible due to technical reasons. Perineural measurement of calcium-flow, however, might be utilized as indirect means to evaluate synaptic currents. Using perineural recording from frog neuromuscular junction, supra-threshold stimuli applied to motor nerve in paired-pulse manner with varying inter-pulse intervals (5-50 ms) are demonstrated in this study to cause paired-pulse depression (PPD) of Ca2+-current. PPD of Ca2+-flow was reduced at lower extracellular Ca2+ concentrations, in BAPTA-AM and EGTA-AM treated preparations and after replacing extracellular Ca2+ with Sr2+. Using perineural measurement of calcium current as an indirect model to investigate synaptic ionic activity, our findings demonstrate that PPD may be attributed to calcium-dependent inactivation of Ca2+-current, which may serve as negative feedback in response to massive Ca2+ entry to motor nerve terminals. A putative sensor of Ca2+-current is also proposed in this study.