Vol. 11, No. 4, 2005 Page 4
Pregnant women concerned about the mercury content of fish now have another reason to be cautious about eating seafood: a new animal study shows that maternal exposure to even "safe" levels of a naturally-occurring toxin in seafood, domoic acid, may cause lasting behavioral changes in offspring.
Edward Levin and colleagues injected pregnant rats with varying doses of domoic acid and then measured their offspring's behavior during a variety of tests measuring movement, learning, and memory. They report that exposed rats showed greater initial activity in a maze test than control rats, followed by a more rapid decline in activity levels. While male rats normally outscore females on spatial discrimination learning tasks, this difference was diminished in exposed rats. Also, both male and female rats showed increased vulnerability to scopolamine, a drug that induces amnesia. The researchers say this suggests that the rats "had less functional reserve" to aid in performing tasks that call on memory.
Levin concludes, "Brief, low-dose domoic acid exposure in rats during gestational development results in subtle neurobehavioral impairments that persist into adolescence and adulthood. Furthermore, long-lasting effects on locomotor activity and cognitive function occurred at levels having no clinically evident consequences for the animals." This finding, he says, indicates that "we may need to re-evaluate monitoring of waters, shellfish and fish to make sure that the most sensitive parts of the human population are protected from toxic exposure to domoic acid."
Domoic acid is produced by algae called Pseudo-nitzchia, and the toxin accumulates in shellfish that eat the algae. Currently, officials close fisheries when domoic acid levels reach 20 parts per million in animal tissues, the level the FDA designated as unsafe when the toxin was first discovered in the 1990s. However, the rats in Levin's study were exposed to levels lower than the FDA threshold.
Earlier animal studies using far higher doses of domoic acid revealed that the toxin can cause extensive damage to the hippocampus, a brain region involved in learning and memory. Human adults exposed to high levels of the toxin can suffer "amnesic shellfish poisoning," resulting in a permanent loss of short- term memory.
"Persisting behavioral consequences of prenatal domoic acid exposure in rats," E. D. Levin, K. Pizarro, W. G. Pang, J. Harrison, and J. S. Ramsdell, Neurotoxicology and Teratology, July 26, 2005 (epub ahead of print). Address: Edward Levin, Department of Psychiatry and Behavioral Sciences, Duke University Medical Center, Box 3412 DUMC, Durham, NC 27710.
"Prenatal exposure to marine toxin causes lasting damage," news release, Duke University Medical Center, September 6, 2005.