The source of choline for acetylcholine synthesis in a sympathetic ganglion

Choline (Ch) and acetylcholine (ACh) uptake and release were measured by a combination of tracer and bioassay techniques in perfused superior cervical ganglia of the cat during rest and repetitive preganglionic stimulation. The uptake of labelled ACh as such was small; but when Ch (methyl-3H labelled) was present at physiological concentration (1.5 μg/ml) in the perfusion fluid, its incorporation into the ACh and free Ch pools of the ganglion proceeded linearly in the absence of stimulation, was accelerated by stimulation, and was inhibited by hemicholinium but not by hexamethonium. Up to 85% of ganglionic ACh could be replaced by labelled ACh during a 1-h stimulation period. On subsequent perfusion with fluid containing unlabelled Ch, labelled Ch was lost but labelled ACh was retained and could be released by stimulation, either as ACh in the presence or as Ch in the absence of eserine. The output of label during stimulation was approximately doubled by eserine but was unaffected by hemicholinium, which, however, prevented the formation and release of newly synthesized ACh. It appears that about half the Ch formed from released ACh is immediately recaptured and resynthesized into ACh. Newly synthesized ACh rapidly gains access to the releasable transmitter pool and may be preferentially released.