1. The recapture and re-use of
choline formed by the hydrolysis of released
acetylcholine (ACh) was studied in the superior cervical ganglion of the cat using
radioactive tracer techniques. The
ganglion's ACh store was labelled by perfusion, during preganglionic nerve stimulation, with
Krebs solution containing [(3)H]
choline.2. Preganglionic stimulation (5 Hz for 20 min) of ganglia containing [(3)H]ACh released similar amounts of radioactivity when perfusion was with
neostigmine-
choline-Krebs or with
hemicholinium-Krebs. This indicated that
neostigmine does not increase transmitter release.3. The amount of radioactivity collected from stimulated ganglia during perfusion with
choline-Krebs was 39% of the amount of radioactivity collected during perfusion with medium containing
neostigmine or hemi-cholinium. This difference in release was almost (85%) accounted for at the end of the experiment by extra radioactive ACh in the ganglia perfused with
choline-Krebs. It is concluded that during preganglionic nerve stimulation approximately 50-60% of endogenously produced
choline is recaptured for ACh synthesis; thus, during activity preganglionic nerve terminals appear selectively to accumulate
choline.4. However, chronically decentralized ganglia accumulated as much
choline as did acutely decentralized ganglia, and this was interpreted as indicating that at rest preganglionic nerve terminals do not selectively accumulate
choline.5. Increased exogenous
choline concentration increased the amount of radioactivity collected during nerve stimulation in the absence, but not the presence, of an
anticholinesterase agent. The spontaneous efflux of radioactivity was little affected by changes in external
choline levels. It is concluded that exogenous
choline and
choline made available from released transmitter compete for uptake into nerve terminals.