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Depolarization amplitude and Ca2+-inflow control the time course of quantal releases at murine motor nerve terminals


Dr J. Dudel, as above.


In order to test whether the time courses of quantal releases after a depolarization pulse are affected by the depolarization amplitude, time courses were measured for small depolarization pulses that elicited close to threshold releases and for large depolarizations that elicited releases approaching saturation level. Diaphragms of young mice were excised and superfused with Bretag’s solution at 18°C. Synaptic currents were elicited and recorded through a perfused macropatch pipette. Releases elicited by threshold depolarizations rose earlier than releases elicited by saturation depolarizations. The short delays in the rising phases of release after large depolarizations may be due to the shift of Ca2+ currents flowing during the pulse to tail currents. After its peak, release decayed with a time constant τ. For saturation depolarizations τ was about 0.3 ms, and for threshold depolarizations τ increased up to 0.8 ms. In order to differentiate between the effects of variations in Ca2+ inflow and in depolarization, the amplitudes of large depolarization pulses were held constant while the amount of release was depressed by halving the Ca2+ concentration at the terminal. The time course of the lowered releases was slightly delayed while τ remained at 0.3 ms as typical for saturation depolarizations. Double pulse facilitation unexpectedly revealed a short phase of depression of release after the pulse. This depression may contribute to the rapid decay (τ) of release after large depolarizations. The dependence of τ on depolarization amplitude indicates that the final phase of the time course of release is largely controlled by the amplitude of the preceding depolarization.

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