Pharmacologic inhibition of twin-pulse facilitation of release of transmitter quanta at the mouse neuromuscular junction

1. The frequency (F, s(-1)) of miniature endplate potentials and the quantal content (m) of endplate potentials were simultaneously measured intracellularly at mouse diaphragm endplates in a bath solution that contained 0.6 mM Ca2+ ions and 5 mM Mg2+ ions. 2. Twin pulses at 4-ms intervals gave the quantal contents of the first (mi) and second (m2) responses, The ratio of m2/ml was taken as an indicator of the temporal facilitation of the release of transmitter. 3. Lead ions (Pb2+; 10 mu M), bis(o-aminophenoxy) ethane-N, N,N',N'-tetraacetic acid (BAPTA; loaded for 60 min at 200 mu M), and chlortetracycline (CTC; loaded for 30 min at 80 mu M) reduced the values of F and m-2/m1. Pb2+ ions and CTC reduced the value of m, whereas BAPTA did not. omega-Agatoxin (omega AGT; 10 ng/ml) reduced the value of m without affecting F or m2/m1. 4. These results suggest that synaptic facilitation is modifiable by agents that can affect systems which buffer intracellular levels of Ca2+ ions. Copyright (C) 1997 Elsevier Science Inc.