Exogenous choline augments transmission at an identified cholinergic synapse in terrestrial mollusk Limax maximus

A variety of pharmacological tests indicate that the neuromuscular junction between the salivary burster neuron (SB) and the salivary duct muscle (SD) in the terrestrial mollusk Limax maximus is cholinergic. These include the effects of curare, atropine, and hexamethonium. Exogenously applied choline can act both presynaptically and postsynaptically at the synapse between the SB and SD. a) When the SB-SD synapse is bathed in 30 μM choline, there is no measurable direct postsynaptic effect at the SB-SD synapse. After several hours of choline incubation, however, an increase is observed in the size of SB-elicited junction potentials recorded from the SD. b) When the synapse is bathed in 300 μM choline, a short-term decrease is seen in the amplitudes of junction potentials (JPs) recorded from the SD. This effect is interpreted as competition between choline and acetylcholine (ACh) for postsynaptic receptor sites on the SD, since the response of the denervated SD to ACh is diminished in the presence of 300 μM choline. c) After several hours incubation in 300 μM choline, the amplitude of JPs elicited by the SB on the SD increase. The long-term effect of exogenous choline is blocked by the choline-uptake inhibitor, hemicholinium 3. We conclude that exogenous choline is taken up by the presynaptic terminals of the SB and converted to ACh. Transmitter output rises with growing transmitter stores, producing an increase in the size of SB-elicited JPs recorded from the SD.