Dioxin's fowl deed: misshapen brains.
Eight years ago, scouting for changes induced by dioxins in wildlife, Diane S. Henshel opened the skull of a heron she had hatched from an egg collected near a dioxin-spewing pulp and paper mill. At most, Henshel had expected to find subtle biochemical changes. Instead, the Indiana University neurobiologist recalls, she "found this brain vastly different from any I had seen--grossly different, left to right." When she looked at the brain of a heron from a clean site, it fit her original expectations--confirming that something had been very wrong with the first one.
Since then, she has found unusually large left brains in other dioxin-tainted herons, cormorants, and eagles. Yet when she exposed the eggs of white leghorn chickens to TCDD, the most potent dioxin, she noticed no such changes.
A few years ago, while using a photo of those chicken brains to illustrate the lack of an effect, she saw that there might be a subtle asymmetry after all. So she injected a new set of eggs with TCDD, between 10 and 1,000 picograms per gram of egg. Then, throughout the birds' incubation and after hatching, she studied their brains.
Now, she reports millimeter-scale asymmetries in dioxin-treated chickens--effects not seen in unexposed fowl. Though affected structures again appear larger on the left side of the brain, this may simply reflect less growth of equivalent sites on the right, Henshel observes. She and her colleagues in Bloomington report their findings in the just-published July Environmental Health Perspectives.
At all doses, TCDD altered the tectum, the brain's relay station for auditory and visual signals, beginning early in development. High doses changed the forebrain--critical to motor function and integrated thinking--early in development, but even low doses triggered asymmetries at a later stage. Since the tectum undergoes a growth spurt earlier than the forebrain, Henshel notes, these data suggest that sensitivity to dioxin may be heightened in regions undergoing expansive growth.
Her group now has preliminary data linking these asymmetries to a host of subtle behavioral changes in chicks, even those with the low exposures.
"I have found gross deformities in [dioxin-exposed birds]--twisted beaks, missing eyes, clubbed feet," says Michael Gilbertson of the International Joint Commission in Windsor, Ontario. "What's interesting here," he says, "are the structural deformities in the brain," because they seem consistent with reports of cognitive problems in children who were exposed in the womb to dioxinlike compounds (SN: 9/14/96, p. 165).
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|Title Annotation:||neurobiologist Diane S. Henshel reports that exposure to dioxin caused brain asymmetry in herons, chickens and other birds|
|Article Type:||Brief Article|
|Date:||Aug 30, 1997|
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