A radioenzymatic assay was used to determine the concentration of choline in the hemolymph of the terrestrial mollusk Limax maximus maintained on choline-enriched and choline-deficient diets. The levels of free choline in the slug's hemolymph varied with the availability of choline in the diet. The concentration of choline in the plasma of choline-enriched animals averaged 5.5 μM while the mean circulating choline level of choline-deprived slugs was 3.3 μM. The difference in choline levels between the two groups was statistically significant. Neurophysiological measurements of transmission at the isolated cholinergic synapses between the salivary burster neuron (SB) and the salivary duct muscle (SD) showed that this synapse was sensitive to fluctuations in the external choline concentration within the physiological range. Increases in exogenous choline from 0.5 to 2.0 μM, from 2.5 to 5.0 μM, and from 5.0 to 15.0 μM were followed by increases in the amplitudes of SB-elicited junction potentials recorded from the SD by a focal extracellular electrode. Thus the SB-SD synapse is sensitive to the fluctuations in exogenous choline that can occur with a change in diet.
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