Effect of cholinesterase inhibitors on synaptic potentials of the frog neuromuscular junction

The action of neostigmine and armin on the parameters of synaptic potentials of the m. cutaneus pectoris of the frog was investigated. Both drugs caused an approximately equal increase in amplitude of miniature end-plate potentials (MEPP) and end-plate potentials (EPP), and lengthened the rise time and the half-decay time of EPP, but the effect of armin developed faster. The presynaptic action of both inhibitors was demonstrated (an increase in the frequency of MEPP and in the quantum composition of EPP), but it developed much more slowly than the postsynaptic effects; it was more marked in the case of neostigmine. Administration of armin after preliminary contact with neostigmine increased the duration of EPP somewhat, but did not potentiate the presynaptic effects. No significant changes were observed when neostigmine was given after armin. The possible mechanism of the presynaptic effects of the cholinesterase inhibitors is discussed.