Effect of blockade of choline uptake by hemicholinium-3 on synaptic output.

The decrease in amplitude of endplate potentials that occurs with high frequency nerve stimulation at the rat phrenic-diaphragm preparation is greater after blockade of choline uptake by hemicholinium-3. The effect is mainly presynaptic and occurs after presumably only a small fraction of the total number of releasable quanta has been discharged. Moreover, when the phrenic nerve is stimulated with a rapid sequence of short tetanic trains, the decrease of the amplitude of both the "first" and the "last" endplate potential of each train which is usually monoexponential becomes not only greater but also biphasic. The effect on the "first" endplate potentials is particularly large. This can be interpreted as further evidence that newly synthesized acetylcholine preferentially replenishes the immediately available store of quanta released by nerve stimulation.