Compartmentalization of choline and acetylcholine metabolism in cultured sympathetic neurons

To determine the relative contribution of cell bodies and distal axons to the production of acetylcholine, we used retinoic acid to induce a cholinergic phenotype in compartmented cultures of rat sympathetic neurons. When [H-3]choline was given to cell bodies/proximal axons for 24 h, 98% of the radiolabel was recovered as choline, phosphocholine, CDP-choline and phosphatidylcholine, whereas only 1 to 2% of the radiolabel was incorporated into acetylcholine. Choline taken up by cell bodies and transported to axons is poorly utilized for acetylcholine biosynthesis. In contrast, when distal axons were supplied with [H-3]choline, 11% of the radiolabel was recovered in acetylcholine after 24 h, the remainder being incorporated into precursors/metabolites of phosphatidylcholine. The lack of acetylcholine synthesis in cell bodies/proximal axons could not be ascribed to an absence of choline acetyltransferase activity in this region of the neurons, since the specific activity of this enzyme was similar in cell bodies/proximal axons and distal axons. The rate of choline uptake by distal axons (15.3 +/- 4.4 nmol/5 min/mg protein) was similar to 10-fold greater than by cell bodies/proximal axons (1.6 +/- 0.8 nmol/5 min/mg protein). Moreover, choline uptake into distal axons was inhibited by 74.5% by hemicholinium-3, and by 80.1% by removal of Na+ from the medium. In contrast, choline uptake by cell bodies/proximal axons was not significantly inhibited by hemicholinium-3 or Na+ removal. These results suggest that the majority of axonal acetylcholine is synthesized in distal axons/axon terminals from choline taken up by a high-affinity choline transporter in distal axons.