DURHAM, N.C. -- The toxic marine organism Pfiesteria piscicida, which has killed millions of fish and sickened people in North Carolina and Maryland, also interferes with the learning process in rats, according to a series of studies completed at Duke University Medical Center.
The findings, which appear in the December issue of the journal Environmental Health Perspectives, are the first published results of scientific experiments showing exposure to Pfiesteria toxin can cause learning deficits.
In the laboratory experiments, toxins produced by the one-celled organism appear to interfere with learning new tasks, but don't impair the memory of previously learned tasks, according to Edward Levin, lead investigator of the study.
Pfiesteria represents a growing human health concern and hazard to economically important marine estuaries along the Atlantic coastal waters of North Carolina and Maryland, where it caused major fish kills in recent years.
Levin and co-investigators Donald Schmechel from Duke, JoAnn Burkholder, Howard Glasgow and Nora Deamer-Melia from North Carolina State University, Virginia Moser from the U.S. Environmental Protection Agency, and G. Jean Harry from the National Institute of Environmental Health Sciences collaborated in the study. The researchers were prompted to begin the animal study after several scientists accidentally exposed to the organism in a research laboratory experienced a range of worrisome symptoms, including dramatic mental impairment, serious disorientation, emotionality, immune problems and skin sores.
Since then, doctors at Duke, Johns Hopkins, and the University of Maryland found that some people who had been exposed to Pfiesteria before or during a Pfiesteria-related fish kill showed evidence of neurological problems. Other people have reported burning skin and respiratory irritation, followed by problems with concentration.
Funding for the research was provided by grants from the National Oceanic and Atmospheric Administration's Sea Grant College Program, the National Sea Grant Marine Biotechnology Program and the National Science Foundation. Support was also provided by the Duke University department of psychiatry and Integrated Toxicology Program, the N.C. State department of botany, the N.C. Agricultural Research Service, the U.S. Environmental Protection Agency and the National Institute of Environmental Health Sciences.
Marine biologist Burkholder, a co-investigator of the current study, discovered Pfiesteria in 1988. After intense study, she and her co-workers at North Carolina State determined the organism is a type of dinoflagellate, which forms plankton, tiny marine organisms that provide food for many small marine animals. Most dinoflagellates are harmless. However, some dinoflagellates are toxic, producing powerful nerve poisons to deter fish from eating them. Pfiesteria, in contrast, uses its toxins to attack and eat fish flesh.
For the current study, Burkholder provided water samples from her laboratory that contained active, fish-killing Pfiesteria toxin.
The researchers conducted the experiments in two stages, each designed to measure the effects of the toxin on the rats' ability to learn and remember a task. First, scientists injected 26 rats with aquarium water containing Pfiesteria toxin and tested their ability to remember the maze in comparison to 26 rats injected with aquarium water without the toxin. Then they trained the rats for 18 sessions on a wagon-wheel shaped radial maze consisting of a platform with planks radiating from it. At the end of each plank was a food reward. Once eaten, the food was not replaced. The control rats learned quickly that it was not worth their effort to go down the same plank twice, but the Pfiesteria-exposed rats showed significantly slower learning.
In the second stage of the project, the researchers pre-trained a different group of rats, then injected the toxin. These rats remembered the maze as well as the control rats. But then the researchers changed the task by baiting only three of the planks, the Pfiesteria-exposed rats showed poor ability to learn the new task.
"The rats exposed to Pfiesteria toxin were significantly retarded in their learning," said Levin, associate professor of psychiatry and director of the Integrated Toxicology Program at Duke. "The effect persisted throughout the 10 weeks the animals were retested after the injection."
At the same time, researchers tested each rat using a battery of tests in which they observe sensory and motor responses to new sights and sounds. During these tests, the Pfiesteria-exposed animals mainly appeared normal, but they did show a lower ability to get used to new surroundings, a process known as habituation.
"The functional battery of tests supports our other results showing that Pfiesteria causes a learning impairment in otherwise healthy animals," Levin said.
The animals were then sacrificed under anesthesia and researchers at NIEHS performed a preliminary assessment of a variety of brain structures, but did not identify the source of the Pfiesteria-induced learning impairment.
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Journal
Environmental Health Perspectives