Ca2+ dynamics at the frog motor nerve terminal

• Published in: Pflügers Archiv 2000-07, Vol.440 (3), p.351-365
• Main Authors: Suzuki, S, Osanai, M, Murase, M, Suzuki, N, Ito, K, Shirasaki, T, Narita, K, Ohnuma, K, Kuba, K, Kijima, H
• Format: Article
• Language: English
• Subjects: Animals; Calcium - pharmacokinetics; Chelating Agents - pharmacology; Computer simulation; Egtazic Acid - analogs & derivatives; Egtazic Acid - pharmacology; Fluorescent Dyes; Indoles; Magnesium - pharmacology; Membrane Potentials - drug effects; Membrane Potentials - physiology; Microscopy, Fluorescence; Motor Neurons - metabolism; Neurology; Neuronal Plasticity - physiology; Presynaptic Terminals - metabolism; Ranidae; Synaptic Transmission - drug effects; Synaptic Transmission - physiology
• ISSN: 0031-6768; 1432-2013
• DOI: 10.1007/s004240000278

Rises in free [Ca2+]i in response to various tetanic stimuli (Ca2+ transient) in frog motor nerve terminals were measured by recording fluorescence changes of Ca2+ indicators and analyzed in relation to short-term synaptic plasticity. Ca2+ transients reached a plateau after 10-20 impulses at 100 Hz and decayed in a three-exponential manner, in which the fast component was predominant. The plateau and fast component of the Ca2+ transient were elevated infralinearly with an increase in tetanus frequency. Computer simulation showed that the Ca2+ transients estimated from fluorescence changes faithfully reflect the true changes in [Ca2+]i except for the initial 20 ms. A slow Ca2+ chelator, EGTA, loaded into the nerve terminal, decreased the magnitude of both the fast and slow components of facilitation of transmitter release and the time constant of the former. A fast Ca2+ chelator, BAPTA, decreased the magnitude of fast facilitation but slightly increased its time constant. These results suggest that Ca2+ transients in the frog motor nerve terminals are primarily caused by Ca2+ entry and are dissipated by three components, in which the rate of the fast component is equivalent to that of free Ca2+ diffusion. The residual Ca2+ in the nerve terminals after stimulation accounts for the fast component of facilitation.