Effects of adenosine on Ca2+ entry in the nerve terminal of the frog neuromuscular junction

This study aimed to test whether nerve-evoked and adenosine-induced synapti c depression are due to reduction in Ca2+ entry in nerve terminals of the f rog neuromuscular junction. Nerve terminals were loaded with the fluorescen t Ca2+ indicator fluo 3 (fluo 3-AM) or loaded with dextran-coupled Ca2+ gre en-1 transported from the cut end of the nerve. Adenosine (10-50 mu M) did not change the resting level of Ca2+ in the presynaptic terminal, whereas i t induced large Ca2+ responses in perisynaptic Schwann cells, indicating th at adenosine was active and might have induced changes in the level of Ca2 in the nerve terminal. Ca2+ responses in nerve terminals could be induced by nerve stimulation (0.5 or 100 Hz for 100 ms) over several hours. In the presence of adenosine (10 mu M), the size and duration of the nerve-evoked Ca2+ responses were unchanged. When extracellular Ca2+ concentration was lo wered to produce the same reduction in transmitter release as the applicati on of adenosine, Ca2+ responses induced by nerve stimulations were reduced by 40%. This indicates that changes in Ca2+ responsible for the decrease in release should have been detected if the mechanism of adenosine depression involved partial block of Ca2+ influx. Ca2+ responses evoked by prolonged high frequency trains of stimuli (50 Hz for 10 or 30 s), which caused profo und depression of transmitter release, were sustained during the whole dura tion of the stimulation, and adenosine had no effect on these responses. Th ese data indicate that neither adenosine induced synaptic depression nor st imulation-induced synaptic depression are caused by reductions in Ca2+ entr y into the presynaptic terminal in the frog neuromuscular junction.