Quantal secretion and nerve-terminal cable properties at neuromuscular junctions in an amphibian (Bufo marinus)

The effect of a conditioning depolarizing current pulse (80-200 micros) on quantal secretion evoked by a similar test pulse at another site was examined in visualized motor-nerve terminal branches of amphibian endplates (Bufo marinus). Tetrodotoxin (200 nM) and cadmium (50 microM) were used to block voltage-dependent sodium and calcium conductances. Quantal release at the test electrode was depressed at different distances (28-135 microm) from the conditioning electrode when the conditioning and test pulses were delivered simultaneously. This depression decreased when the interval between conditioning and test current pulses was increased, until, at an interval of approximately 0.25 ms, it was negligible. At no time during several thousand test-conditioning pairs, for electrodes at different distances apart (28-135 microm) on the same or contiguous terminal branches, did the electrotonic effects of quantal release at one electrode produce quantal release at the other. Analytic and numerical solutions were obtained for the distribution of transmembrane potential at different sites along terminal branches of different lengths for current injection at a point on a terminal branch wrapped in Schwann cell, in the absence of active membrane conductances. Solutions were also obtained for the combined effects of two sites of current injection separated by different time delays. This cable model shows that depolarizing current injections of a few hundred microseconds duration produce hyperpolarizations at approximately 30 microm beyond the site of current injection, with these becoming larger and occurring at shorter distances the shorter the terminal branch. Thus the effect of a conditioning depolarizing pulse at one site on a subsequent test pulse at another more than approximately 30 microm away is to substantially decrease the absolute depolarization produced by the latter, provided the interval between the pulses is less than a few hundred microseconds. It is concluded that the passive cable properties of motor nerve terminal branches are sufficient to explain the effects on quantal secretion by a test electrode depolarization of current injections from a spatially removed conditioning electrode.