By David N. Leff
When a male fruit fly (Drosophila melanogaster) feels the urge to proposition a female, he says it with music.
"There's one early report," observed molecular geneticist Jay Hirsh, "that the tone of the male courtship song is altered under circadian gene function." Circadian (the word means "around the day") defines the 24-hour body-clock periodicity that influences every form of life on earth. (See BioWorld Today, Nov. 21, 1994, p. 1.)
Hirsh, a professor of biology at the University of Virginia in Charlottesville, made the added point, "In the past year or two, a whole cascade of circadian genes has come out, which are all exquisitely involved in regulating one another."
Fruit fly geneticists have given this cascade of genes names that match their clock-like functions, to wit: period, cycle, clock, timeless and doubletime. "They're all conserved very well," Hirsh pointed out, "up into higher vertebrates including us, where the circadian genes have similar, although not identical, functions."
He and his co-workers have discovered an unexpected function in the flies, which may have socio-medical implications, if duplicated in human circadian genes. Their article in today's Science, dated Aug. 13, 1999, tells that story. Its title: "Requirement of circadian genes for cocaine sensitization in Drosophila." Hirsh is its senior author.
"The overall finding of this paper," he told BioWorld Today, "is that genes involved in circadian function in flies often have non-circadian roles in responses to the drug of abuse, cocaine. Those roles," he continued, "are in a process called sensitization to cocaine. Animals or flies given repeated pulsatile doses of that psychostimulant will develop an enhanced response to it. This is very counterintuitive - just the opposite of tolerance to alcohol. It's sometimes called reverse tolerance.
"The importance of sensitization," Hirsh went on, "is that it's been linked to the addictive process in humans, as well as - potentially - to diseases, such as schizophrenia. So there's some real interest in understanding sensitization."
How he and his co-authors hit upon this cocaine connection to fruit fly circadian genes is a story in intralaboratory interaction: "It came from another study," he recounted, "that is currently submitted from this Science paper's lead author, graduate student Rozi Andretic. She found that there are circadian changes in dopamine receptor responsiveness. [The neurotransmitter dopamine presides over the brain's reward center.]
"And my technician Sara Chaney, the article's other co-author, Hirsh continued, "found that using the same sort of assay system as a function of cocaine sensitization, we could also see changes in dopamine receptor responsiveness. Then it was Rozi's thought to investigate, since there was a common link here, whether the circadian genes themselves would have any role in regulating cocaine sensitization."
Fruit Flies Respond To Freebase Hits
Whereupon, the team began systematically studying mutated strains of Drosophila's chronometrically-named palette of circadian genes. "What we do," Hirsh recounted, "is expose the flies repeatedly to volatilized freebase - or crack - cocaine, by boiling it off from a hot wire. Then we videotape the flies, very carefully, watch them, and quantitate their behaviors. That's how we get our data."
That data, like a bill of indictment, suggested that those insects' circadian genes may be guilty of inciting susceptibility to cocaine addiction. The mutated ones did not become sensitized to the drug. The functionally intact ones did.
"In a clinical sense," Hirsh pointed out, "there is no immediate application of these findings. Of course, all this now has to be demonstrated in vertebrates. There are many animal models where this can be done at present. We have some collaborations ongoing in mice. Long range," he went on, "we hope it would open up the field of drug abuse to thinking about some totally new players that could likely be involved in modulating response in the addictive process. And the overriding point, perhaps," he concluded, "would be to help in the realization that addiction is likely to be a disease state rather than a criminal behavior."
"This exciting new research," commented Alan Leshner, director of the National Institute on Drug Abuse (NIDA), "has given us a clue to the specific genetic mechanisms that influence vulnerability to addiction. They could become the basis for predicting who is most at risk for addiction, and thus become a major aid in preventing this national health problem." (NIDA co-funded Hirsh's project, together with the National Institute of General Medical Sciences.)
Nipping Cocaine Craving Before It Buds
On another front of the cocaine-addiction wars, a French biotech firm, Paris-based Laboratoire Bioprojet, has developed a compound that binds to dopamine's third receptor (D3), and activates it - but only partially. This receptor resides in the brain's nucleus acumbens, the key region of addictive dependence. All products that induce dependence in humans, whether cocaine, morphine, heroin, alcohol or nicotine, ramp up dopamine release in this D3-rich area.
Molecular pharmacologist Pierre Sokoloff, of INSERM - France's National Institute of Health and Medical Research - discovered D3 a decade ago. Now he is senior author of a paper in a recent issue of Nature, dated July 22, 1999, titled: "Selective inhibition of cocaine-seeking behavior by a partial dopamine D3 receptor agonist."
The article reports in vivo testing of the compound in cocaine-dependent rats, trained to self-inject the drug by pressing a lever. At each fix, a light came on, a cue that prompted the animals to press the lever even when it yielded no cocaine. Human addicts who try to break the habit often relapse, long after withdrawal, at the sight of a spoon, a syringe or a long-forgotten setting reminiscent of their onetime addiction.
Administration of D3 in low doses clearly attenuated, but did not abolish, the rats' compulsive drug-seeking behavior in response to the light signal. But they didn't try to self-dose the dopamine-receptor compound, which is not itself addictive. But that diminished craving occurred only in rats given D3 before they had any cocaine in their system. Then the compound did not quash the craving for more. So its use may be limited to ex-addicts trying to resist a relapse. Bioprojet is planning to start clinical trials soon.