UC Davis Children's Center Collaborates on Research Demonstrating that Noncoplanar PCBs Cause Brain Development Abnormalities in Rodents
Scientists at the University of California at Davis Children’s Center, working with researchers at the University of California at San Francisco, have shown that certain types of PCBs that are widely dispersed in the environment and human tissues cause significant developmental and auditory abnormalities in rat pups. The mothers of the pups were exposed to noncoplanar PCBs in their food during pregnancy and when the pups were nursing. Previously this class of PCBs has not been considered hazardous. The PCB level in the blood of the pups is roughly equivalent to that in the blood of breast-fed babies of mothers exposed to extremely high levels of PCBs from contaminated fish, soil, water and air in high-risk environments.
The study, published in Proceedings of the National Academy of Sciences (online April 25, 2007), shows that the PCBs altered the rat pups’ auditory cortex, the part of the brain that processes sound. The neural circuitry of this region was disorganized and the nerve cells had a decreased capacity to change, or “learn,” in response to sound. The capacity of the brain to change in response to stimuli contributes to the development of cognitive function, and since the auditory cortex is the first sensory region to develop, researchers say its abnormal development in the rat pups could be just a hint of more widespread effects of PCB exposure.
Results show that the brains of pups raised without exposure to PCBs developed normally, but the auditory cortex of the pups exposed to noncoplanar PCBs was profoundly altered and their brain’s ability to interpret what they were hearing was distorted. The balance of inhibitory and excitatory signaling between nerve cells, which contributes to the response of the brain to stimuli, was disrupted. This may have important implications for children’s health, because evidence shows an imbalance in brain signaling in children with some developmental disorders such as autism. In these children, the auditory cortex responds abnormally to sound and some scientists believe this is the basis of the conditions. While these findings do not demonstrate that noncoplanar PCBs cause developmental disorders, the researchers say that given its severe impact in the rat pups and the prevalence of noncoplanar PCBs and related chemicals in the environment, this issue warrants serious attention.
The research was led by Tal Kenet, PhD working in the lab of Michael Merzenich PhD, Professor of Otolaryngology at UCSF and a member of the Keck Center for Integrative Neuroscience. The finding builds on recent cell culture studies by co-author Isaac Pessah, PhD, Professor of Molecular Biosciences at UC Davis and Principal Investigator of the UC Davis Center for Children’s Environmental Health. Pessah’s studies have shown that noncoplanar PCBs significantly influence chemical and electrical signaling between neurons that affects brain development and learning. The Center has been investigating the biological basis for autism and related neurodevelopmental disorders.
Polychlorinated biphenyls (PCBs), used mostly as coolants and lubricants beginning in the 1930s and were banned in 1977. Early studies focused on coplanar PCBs, which were shown to pose a serious health risk, but recent studies have shown that noncoplanar PCBs are particularly stable, less susceptible to degradation in the environment, and are more prevalent in environmental and human tissue samples than the coplanar forms. The current study focused on PCB95, a typical noncoplanar PCB prevalent in the environment.
This finding, coupled with other factors -- particularly an increase in the rate and duration of breast feeding in the United States, epidemiologic evidence of negative effects on cognitive function in children, and lab evidence in rats suggests the need for studies in human populations to determine the risks of exposure to noncoplanar PCBs, especially in infants who may have a genetic predisposition to developmental disorders based on their family history.
PCB95 is closely related in its chemical structure to polybrominated diphenyl ether (PBDE), which has been used in large quantities in the past 25 years, mostly in fire retardation treatments in home and office furniture and in electronics. Pessah says the UC Davis Center has studies of PBDEs which have not yet been published but indicates that the current finding “could be just the tip of the iceberg.”
The study was funded by the Environmental Protection Agency, the National Institute of Environmental Health Sciences, Cure Autism Now, the UC Davis M.I.N.D. Institute, the Jane Coffin Childs Foundation for Medical Research and the Sandler Program in Basic Sciences.
Kenet T, Froemke RC, Schreiner CE, Pessah IN, Merzenich MM 2007. Perinatal exposure to a noncoplanar polychlorinated biphenyl alters tonotopy, receptive fields, and plasticity in rat primary auditory cortex. Proc Natl Acad Sci U S A. 2007 Apr 25; [Epub ahead of print] 104(18):7646-7651.
Full Article in Proceedings of the National Academy of Sciences (free open access)