Metabolism of Acetylcholine in the Nervous System of Aplysia Californica. IV. Studies of an Identified Cholinergic Axon

[3H]Choline, injected directly into the major axon of the identified cholinergic neuron R2, was readily incorporated into [3H]acetylcholine. Its metabolic fate was similar to that of [3H]choline injected into the cell body of R2. Over the range injected, we found that the amounts of acetylcholine formed were proportional to the amounts injected; the synthetic capability was not exceeded even when 88 pmol of [3H]choline were injected into the axon. Newly synthesized acetylcholine moved within the axon with the kinetics expected of diffusion. We could not detect any selective orthograde or retrograde transport from the site of the injection. In contrast, as indicated by experiments with colchicine, 30% of the [3H]acetylcholine formed after intrasomatic injection was selectively exported from the cell body and transported along the axon. Most of the [3H]acetylcholine was recovered in the soluble fraction after both intra-axonal and intrasomatic injection of [3H]choline; only a small fraction was particulate. The significance of large amounts of soluble acetylcholine in R2 is uncertain, and some may occur physiologically. The concentrations of choline introduced by intraneuronal injection into both cell body and axon were, however, greater than those normally available to choline acetyltransferase in the cholinergic neuron; nevertheless, these large concentrations were efficiently converted into the transmitter. The synthetic capacity of the neuron supplied with injected choline may exceed the capacity of storage vesicles and of the axonal transport process.