Induced variation of serotonin in Drosophila melanogaster and its relation to learning performance

In insects the level of serotonin can be modified through the alimentary tract. Drosophila fed on chemically defined media to which has been added a synthesis inhibitor or an immediate precursor of this neurotransmitter have their serotonin level decreased or increased, respectively. (A 4- to 5-day treatment is optimal). Such treated flies, if tested in a procedure of Pavlovian conditioning, have different learning performances. With the same duration of starvation, nearly the same threshold for sucrose solutions, but different amounts of serotonin, the flies (blindly trained) showed conditioned inhibition of the tarsal (or proboscis-extension) reflex according to the amount of serotonin. The highest level of this biogenic amine gave the best performance.