U.S. scientists have found primitive emotion-like behavior in fruit fly, Drosophila melanogaster.
According to researchers, their findings could provide new insights into the neurological basis of attention deficit hyperactivity disorder (ADHD).
Most of the genes found in the fruit fly, also referred to as the vinegar fly, are found in humans as well, including those neurons that produce brain chemicals like dopamine and serotonin.
During the study, researchers from California Institute of Technology (Caltech) found that a series of brief but brisk air puffs, delivered in rapid succession, caused flies to run around their test chamber in what they called a "frantic manner."
This behavior persisted for several minutes after the last of the puffs.
"Even after the flies had 'calmed down' they remained hypersensitive to a single air puff," said David Anderson, Caltech's Seymour Benzer Professor of Biology and a Howard Hughes Medical Institute investigator.
The team led by postdoctoral fellow Tim Lebestky developed an automated machine-vision-based system to track the movement of the flies, and derived a simple mathematical model to fit the movement data and to extract metrics that described various aspects of the flies' responses under different conditions.
They found that flies with the dopamine-receptor mutation were hypersensitive to the air puffs, and took much longer to calm down than did "normal" flies without the mutation.
It is often assumed that because individuals with ADHD are hyperactive and easily distracted, they have difficulty learning.
The ways the mutant flies respond to the air puffs is, moreover, "reminiscent of the way in which individuals with ADHD display hypersensitivity to environmental stimuli and are more easily aroused by such influences," says Anderson.
Importantly, ADHD has been genetically linked to abnormalities of the dopamine system in humans, further strengthening the analogy between the mutant flies and this psychiatric disorder.
The Caltech group showed, however, that hyperactivity and learning disabilities are not causally related in flies bearing the dopamine receptor mutation, thereby disproving the theory-at least in flies.
"We could separately 'rescue' the hyperactivity and learning deficits in a completely independent manner," says Anderson, "by genetically restoring the dopamine receptor to different regions of the fly's brain."
Thus, in dopamine-receptor-mutant flies, hyperactivity does not seem to cause learning deficits. Instead, these two "symptoms" reflect independent effects of the mutation that manifest themselves in distinct brain regions.
"The finding that flies exhibit emotion-like behaviors that are controlled by some of the same brain chemicals as in humans opens up the possibility of applying the powerful genetics of this 'model organism' to understanding how these chemicals influence behavior through their actions on specific brain circuits," said Anderson.
The study is published in journal Neuron.