Effects of Mg2+ on the stimulation-induced changes in transmitter release at the frog neuromuscular junction.

Four components of stimulation-induced changes in neurotransmitter release are known as the synaptic plasticity at the frog neuromuscular junction. These are: fast and slow facilitation, augmentation and potentiation, classified by their decay time constants after repetitive nerve stimulation. Most experiments support the view that fast facilitation is caused by residual Ca2+. However, the causes of the other three components are not clear. We have studied electrophysiologically the effect of Mg2+ on these three components. Transmitter release was estimated by the amplitudes of endplate potential (EPP) and by the frequencies of miniature endplate potential (MEPP). The increase in the transmitter release by nerve stimulation is described as the product of four components. The magnitude of potentiation of MEPP frequencies after a tetanic nerve stimulation (100 Hz, 5000 times) increased markedly in high Mg2+ concentrations. Conversely, the magnitude of augmentation (MEPP frequencies and EPP amplitude) decreased in the higher Mg2+ Ringer solution.