Benzopyrene exposure linked to learning, memory deficits in zebrafish

CORVALLIS, Ore. – A new study suggests that exposure to benzopyrene impairs adult learning in zebrafish, adding to mounting evidence that the chemical compound and others like it may cause intellectual and memory deficits in humans.

“There was no doubt that exposure to even a low-level concentration of benzopyrene affected the learning and memory behaviors in the fish,” said Robyn Tanguay, a molecular toxicologist in Oregon State University’s College of Agricultural Sciences. “We detected neurotoxicity at chemical exposure levels that don’t produce other, more visible signs of toxicity such as body deformities.”

The study is published in the journal Neurotoxicology and Teratology: Developmental benzo[a]pyrene (B[a]P) exposure impacts larval behavior and impairs adult learning in zebrafish. The National Institutes of Health helped fund the research.

Benzopyrene is a polycyclic aromatic hydrocarbon, known as a PAH, which forms naturally as the result of almost any type of combustion, from a wood stove to an automobile engine, cigarette or a coal-fired power plant. Many PAHs are associated with air pollution, are known to be carcinogenic and are the subject of extensive research at OSU and elsewhere around the world.

Zebrafish have been found in recent years to be an excellent model for biomedical research because they reproduce rapidly, and their embryonic genetics and biological systems bear many similarities to humans.

The OSU researchers exposed zebrafish embryos to benzopyrene and then monitored their activities from larval stage to adult. They noted hyperactivity in zebrafish larvae and recorded learning and memory deficits in fish they raised to adulthood.

The researchers tested learning and memory in adult zebrafish in experiments involving custom-built shuttle boxes. The boxes are designed to condition the fish to swim away from the blue-lighted side to avoid a mild electrical stimulation. The fish exposed to benzopyrene, upon being in the blue-lighted, or unsafe side, were slower to swim to the dark, safe side, even when prodded by the mild shock.

“Zebrafish are very intelligent and are able to make complex decisions,” said Tanguay, director of the Sinnhuber Aquatic Research Lab at OSU. “We were conditioning the fish to make the correct decision to be on the safe side where they wouldn’t be punished. Some of the exposed fish never moved to the safe side.”

In a related publication involving a chemical compound that isn’t a PAH but is also the subject of research worldwide, Tanguay concluded that bisphenol A, or BPA, weakens sperm function in adult zebrafish.

Bisphenol A has gained attention in recent years due to its large-scale production and widespread use in plastic products. Humans are exposed to BPA in low concentrations when it leaches from food and drink containers, including baby bottles. The U.S. Food and Drug Administration has deemed the chemical safe when used at the current levels occurring in food containers and packages.

The zebrafish in this study were exposed to BPA at Wenzhou University during three different developmental stages, resulting in a reduction of sperm volume, density, motility and velocity.

The results of the bisphenol A study, a collaboration with Wenzhou University in China, are consistent with reports of reduced sperm motility and sperm counts in adult mice or rats exposed to low concentrations of BPA during their development, Tanguay said.

“We found a more profound effect on the sperm at a low BPA concentration rather than at a higher concentration,” Tanguay said. “That’s common in chemicals that disrupt endocrine systems. The next step is to explain the mechanism that leads to disruption.”