1. Action potentials have been recorded extracellularly from individual preganglionic nerve terminals in the lumbar sympathetic chain of frogs (cf. Koketsu & Nishi, 1968).2. The action potentials were unaffected by changes in external Mg(2+) or Ca(2+) concentration, but were attenuated or even abolished by acetylcholine or carbachol. The effect on the presynaptic action potential was transient, in contrast to the blockade of synaptic transmission caused by these drugs, which as is well known persists indefinitely in their presence.3. In the presence of tubocurarine (5 x 10(-5)M), acetylcholine and carbachol had no effect on the presynaptic action potential.4. The action potentials were reduced by an increase in external K(+) concentration and abolished at between 9 and 16 mM K(+). Synaptic transmission persisted until the amplitude of the action potential was reduced to less than one third of its control value.5. It is concluded that although the presynaptic nerve is endowed with acetylcholine receptors, they are not the source of the long lasting blockade of synaptic transmission caused by cholinergic substances.
Acetylcholine Action Potentials