The team's earlier animal studies demonstrated that the toxin caused cognitive deficits, but could not distinguish between learning and memory disorders. The most recent studies have shown that the learning of new tasks is severely affected.
"When we used pretrained rats, we did not see the dramatic effect on memory that we saw on spatial learning," said Dr. Edward Levin, lead investigator of the study, associate professor of psychiatry and director of the Integrated Toxicology Program at Duke. "We confirmed that the toxin affected the learning aspect of their behavior."
Levin prepared his findings for presentation at a poster session Monday at the annual Society of Toxicology Meeting in Cincinnati. His collaborators on the study include assistant botany professor JoAnn Burkholder and botany research associate Howard Glasgow of North Carolina State University, and Duke associate professor of neurobiology Donald Schmechel. Funding for the research was provided by the National Oceanic and Atmospheric Administrations' Sea Grant College Program, and in part by the National Science Foundation.
Levin began conducting experiments on the effects of the toxic organism Pfiesteria piscicida because people who had been exposed to it exhibited dramatic mental impairment, serious disorientation, emotionality, immune problems and dermatological problems. Since Burkholder identified Pfiesteria in 1988, it has also been implicated in about 30 percent of all fish kills in North Carolina. The organism virtually melts the fishes' flesh away, killing them within minutes.
The Duke experiments involved injecting rats with the Pfiesteria. The researchers then conducted classic animal experiments in two stages, measuring the effects of the toxin on the rats' ability to learn and remember a task.
For the first stage of the experiment, scientists trained a non-injected group of rats over 18 sessions on a radial arm maze -- a platform with planks radiating from it. At the end of each plank was half a Fruit Loop. Once eaten, the food was not replaced, and the rats learned quickly that it was not worth their effort to go down the same plank twice. When the researchers tried to train rats to perform the task after injection with the toxin, they learned at a significantly slower rate than rats that had not been injected. In contrast, when the rats first were trained on the maze and then injected, they performed the tasks just as well.
Then, in the second stage of the experiment, the researchers had the rats learn a different task. They baited only three of the planks, each day changing which three were baited. The rats were allowed to find the food, and the researchers counted how many errors they made and how long they took. They repeated this exercise five times a session, baiting the same three planks. The injected rats, though they remembered their pre-injection training, showed very poor ability to learn the new task.
"The high dose group was retarded in their learning," said Levin. "And the effect was very persistent, even 10 weeks after the injection."
Future research, said Levin, will involve trying to determine what kind of learning is affected, whether it is only spatial learning or also involves other types of tasks. He also plans to work with chemists to isolate the toxic chemical in the algae.
Pfiesteria is a bizarre creature, possessing at least 20 different forms. It photosynthesizes like a plant. When threatened it can turn from a tiny dinoflagellate -- a cell with a tail -- into a large amoeba and engulf its predator. While lying dormant in cyst form in sediment, it senses the presence of fish and emerges into the water to poison them.