Low-frequency neuromuscular depression is a consequence of a reduction in nerve terminal Ca²⁺ currents at mammalian motor nerve endings

BACKGROUND: The decline in voluntary muscle contraction during low-frequency nerve stimulation is used clinically to assess the type and degree of neuromuscular block. The mechanism underlying this depression is unknown. METHODS: Simultaneous electrophysiological measurements of neurotransmitter release and prejunctional Ca²⁺ currents were made at mouse neuromuscular junctions to evaluate the hypothesis that decreases in nerve terminal Ca²⁺ currents are responsible for low-frequency depression. RESULTS: Under conditions generally used to measure Ca²⁺ currents at the neuromuscular junction, increasing the frequency of nerve stimulation briefly from 0.017 to 0.1-1 Hz caused a simultaneous reduction in the release of the neurotransmitter acetylcholine to 52.2 ± 4.4% of control and the Ca²⁺ current peak to 75.4 ± 2.0% of control (P < 0.001, n = 5 experiments for both measurements, mean ± SEM for all data). In conditions used for train-of-four monitoring (4 stimuli, 2 Hz), neurotransmitter release declined to 42.0 ± 1.0% of control and the Ca²⁺ current peak declined to 75.8 ± 3.3% of control between the first and fourth stimulus (P < 0.001, n = 7 experiments for both measurements). Depression in acetylcholine release during train-of-four protocols also occurred in the absence of neuromuscular-blocking drugs. DISCUSSION: The results demonstrate that neuromuscular depression during train-of-four monitoring is due to a decline in nerve terminal Ca²⁺ currents, hence reducing the release of acetylcholine. As similar processes may come into play at higher stimulation frequencies, agents that antagonize the decline in Ca currents could be used to treat conditions in which neuromuscular depression can be debilitating.