Acetylcholine in the choline-deficient housefly, Musca domestica

The distribution of choline metabolites has been studied in the nervous system of housefly larvae reared on diets containing various amounts of choline or on diets containing either β-methylcholine (β-MCh) or N-dimethylethylcholine (DMECh). Adult houseflies were obtained from larvae reared on diets containing at least 4·6 times the level of choline in the basic diet or on diets containing the two choline analogues. The distribution of choline metabolites has been studied in the heads and thoraces plus abdomina of these adults.

Acetylcholine was found to be concentrated in the larval nervous system where it was synthesized preferentially to other choline metabolites when the amount of choline available to the insect was severely restricted. The acetylcholine content of the adult was between six and nine times that of the larval nervous system and approximately 70 per cent was concentrated in the head. The amount of acetylcholine present in flies obtained from larvae fed on diets containing the lowest amount of choline which allowed adults to develop was 230 pmole/insect. Flies obtained from larvae fed on diets with added β-MCh contained 150 pmole of acetylcholine/insect but no detectable acetyl-βMCh. Flies obtained from larvae fed on diets with added DMECh contained 43 pmole of acetylcholine and 127 pmole of acetyl-DMECh/insect.

It is concluded that the two choline analogues spare the choline requirement of the housefly by two different mechanisms. β-MCh displaces choline from the lipids of non-nervous tissue making more available for uptake into the nervous system where sufficient acetylcholine is synthesized for adult development. DMECh is a complete replacement for choline; the acetyl-DMECh acting as a neurotransmitter in place of acetylcholine.