Thiamine and Cholinergic Transmission in the Electric Organ of Torpedo

Abstract

The electrogenic tissue of Torpedo was found to phosphorylate in vitro external [14C]thiamine by a saturable process. The rate of this metabolisaton was increased when acetate, an efficient precursor of acetylcholine (ACh) synthesis in this tissue, was added to the incubation medium, thus increasing the turnover of ACh. Nerve stimulation did not release significant amounts of the previously accumulated [14C]thiamine. Exogenous thiamine modified the size of the electroplaque potential (e.p.p.). At concentrations higher than 10(-3)M the nerve-electroplaque transmission was depressed after a transient increase of the e.p.p. Such a depression was due to a strong decrease of the ACh release. At concentrations equal to or lower than 10(-3)M, thiamine affected transmission in a rather complex fashion. ACh release was decreased or increased depending on concentration, time of application, and mode of stimulation. Oxythiamine, a structural antimetabolite of thiamine, affected the transmission in a very characteristic manner at 10(-5)M and higher concentrations. The amplitude of the e.p.p. was increased and, more strikingly, its duration was prolonged. These changes were not due to an inhibition of cholinesterase activity but to an enhancement of the evoked release of ACh either on single-impulse or repetitive stimulation. Another antimetabolite, pyrithiamine, had no effect on the transmission nor on ACh release. From this and our previous work, it is proposed that thiamine is involved, directly or indirectly, in the process of ACh release. The possible mechanisms of this involvement are discussed.