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

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Abstract
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 L. 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. When the SB-SD synapse is bathed in 30 .mu.M choline, there is no measurable direct postsynaptic effect at the SB-SD synapse. After several hours of choline incubation, an increase is observed in the size of SB-elicited junction potentials recorded from the SD. When the synapse is bathed in 300 .mu.M choline, a short-term decrease is seen in the amplitudes of junction potentials (JP) 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 .mu.M choline. After several hours incubation in 300 .mu.M choline, the amplitudes of JP elicited by the SB on the SD increase. The long-term effect of exogenous choline is blocked by the choline-uptake inhibitor, hemicholinium 3. 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 JP recorded from the SD.