EFFECTS OF MG2-INDUCED CHANGES IN TRANSMITTER RELEASE AT THE FROG NEUROMUSCULAR-JUNCTION( ON THE STIMULATION)

Four components of stimulation-induced changes in neurotransmitter rel ease are known as the synaptic plasticity at the frog neuromuscular ju nction. These are: fast and slow facilitation, augmentation and potent iation, classified by their decay time constants after repetitive nerv e stimulation. Most experiments support the view that fast facilitatio n is caused by residual Ca2+ However, the causes of the other three co mponents are not clear. We have studied electrophysiologically the eff ect of Mg2+ on these three components. Transmitter release was estimat ed by the amplitudes of endplate potential (EPP) and by the frequencie s of miniature endplate potential (MEPP). The increase in the transmit ter release by nerve stimulation is described as the product of four c omponents. The magnitude of potentiation of MEPP frequencies after a t etanic nerve stimulation (100 Hz, 5000 times) increased markedly in hi gh Mg2+ concentrations. Conversely, the magnitude of augmentation (MEP P frequencies and EPP amplitude) decreased in the higher Mg2+ Ringer s olution.