Recycling process of acetylcholine in the rat muscle controlled in a short-term memory manner

The recycling process of acetylcholine (ACh) was studied in reference to muscle potentials evoked by sciatic stimuli in the presence of neostigmine. Wistar rats around 20 days old were anesthetized with urethane. Muscle potentials were recorded with concentric electrodes from their exposed medial gastrocnemius muscles before and after the injection of neostigmine. Attention was paid to changes in muscle potentials evoked by a set of 3 stimuli, and comparison was made mainly between successive two sets. Three potentials before neostigmine treatment were similar in amplitude. When a set of 3 stimuli was delivered 8 min after it, the first potential was as large as that before it. The others were greatly depressed. The first potential evoked by a set of 3 stimuli 10 min after neostigmine treatment showed a similar amplitude to the control level. The other potentials were again reduced severely. However, none of them were depressed so much when 3 stimuli were delivered 11 min after the drug application. When two sets of 3 stimuli were given with an interval of one minute, it was repeatedly observed in all animals that muscle potentials evoked by the second set were influenced by the first set. The second potential evoked by the second set was not so much depressed as seen when the interval was two or more minutes. Thus, a striking contrast was observed between successive two sets of 3 stimuli with an interval of one minute. These results indicate that the recycling process of ACh was activated following impulse transmission in rat muscles, and suggest that there exists such a neural mechanism as can exert an influence on the process in a short-term memory manner.