U.S. scientists have uncovered evidence of a primitive emotion-like behavior in the fruit fly, and their finding may facilitate studies of the relationship between a corrective medicine and a learning disability.

The evidence, published by researchers with the California Institute of Technology (Caltech) in the December issue of the journal Neuron, may be relevant to the relationship between the neuro-transmitter dopamine and attention deficit hyperactivity disorder (ADHD).

Caltech researchers said that the brain of the fruit fly Drosophila contains only about 20,000 neurons, or nerve cells, and has long been considered enough for the studies of the genetic basis of such behavior as learning, memory, courtship and metabolism otherwise known as circadian rhythm.

But they are not yet sure whether the Drosophila brain could also be used to study the genetic basis of "emotional" behavior.

David Anderson, a Caltech professor of biology and a Howard Hughes Medical Institute investigator, said that such studies are important because it is believed that abnormalities in these types of behavior may underlie many psychiatric disorders.

The Caltech research team, led by post-doctoral fellow Tim Lebestky, found that a series of brief but brisk air puffs, delivered in rapid succession, caused fruit flies to run around their test chamber in what Anderson described as a "frantic manner."

Researchers said that this behavior persisted for several minutes after the last of the puffs and that even after the flies had calmed down, they remained hypersensitive to a single air puff.

Lebestky said what is surprising about this result "is that previous studies in both fruit flies and vertebrates had suggested that dopamine promotes activity, but our experiments uncovered a function of dopamine in the opposite direction."

And since removing the receptor causes hypersensitivity to the air puffs, these findings "suggested that dopamine actively inhibits the hyperactivity response," Lebestky added.

Researchers said the ways the mutant flies respond to the air puffs is reminiscent of the way in which individuals with ADHD display hypersensitivity to environmental stimuli and are more easily aroused by such influences.

There can also be another possible link: some individuals with ADHD display learning disabilities, researchers said.

According to the Caltech study, this finding in fruit flies raises the possibility that hyperactivity and learning disabilities also may not be causally linked in humans with ADHD.

If so, it ultimately may prove more effective to develop drugs to treat these two symptoms separately, rather than trying to cure them both with the broad-spectrum pharmaceuticals currently available, which have many undesirable side effects.