MONOETHYLCHOLINE AS A FALSE TRANSMITTER PRECURSOR AT THE FROG AND MOUSE NEUROMUSCULAR-JUNCTIONS

Monoethylcholine (MECH) enters motor nerve terminals where it is made into acetylmonoethylcholine (AMECH). AMECH opens endplate channels for about half of the average duration observed when they are opened by a cetylcholine (AGH). Therefore when AMECH is present in a quantum the e ndplate currents decay more rapidly. MECH has been used to measure qua ntal turnover in motor nerve terminals. We find that the incorporation of AMECH into quanta is blocked by vesamicol, an inhibitor of ACH tra nsport into synaptic vesicles. AMECH is incorporated more rapidly when acetylcholinesterase is inhibited, when the choline uptake inhibitor, hemicholinium-3, is present or when extracellular Na+ (required for a ctive CH uptake) is replaced with methylamine. This suggests that in t he absence of these inhibitors CH obtained from released ACH is recycl ed. Therefore, experiments on the rate of incorporation of MECH are mi sleading unless CH recycling is prevented. Previous work also suggeste d that MECH is incorporated at a faster rate into those quanta which a re released by stimulation than into those released spontaneously. We conclude that quanta released spontaneously and following nerve stimul ation probably come from the same pool. The distribution of t(1/2)'s d uring the incorporation of MECH can be accounted for in the framework of recent studies of the recycling of synaptic vesicles. We conclude t hat false transmitter is a valuable tool for studying the loading of q uanta, but that there are several complications to be considered when trying to use it to measure the turnover of the population of quanta.