Metabolism and significance of acetylcholine in the brain of the adult housefly, Musca domestica L.

Abstract

Methods are described for the determination of [ 14C] acetylcholine formed in the head tissues of the adult housefly in vivo following [ 14C] acetate injection. Acetylcholine estimated radiometrically and that assayed pharmacologically were in good agreement. This confirms that this ester largely, if not entirely, accounts for the pharmacological activity of head extracts as assayed with the frog rectus abdominis muscle preparation. The data indicated that the acetylcholine content of the brain is due to a steady state of synthesis and hydrolysis with a minimum turnover rate of 6·1 × 10 −5 μmoles acetylcholine/min/head. Turnover was apparently reduced in insects poisoned with acetylcholinesterase inhibitors. These and other observations are interpreted as showing that the ACh system of the brain is highly compartmentalized, synthesis from choline and intracellular acetate being controlled by the availability of choline arising from acetylcholinesterase action. No correlation was found between acetylcholine turnover in vivo and the incidence of light on the compound eye. Turnover was apparently unaffected by surgical elimination of possible nerve activity originating in the thoracic ganglion. It was, apparently, reduced during cyclopropane anaesthesia. Current problems of insect neurophysiology are critically discussed.