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Music, mice and microscopic imaging combine to provide new insights into the effects of environmental chemicals on hearing loss.
Researchers at the Beckman Institute for Advanced Science and Technology found that early exposure to an environmental chemical called polychlorinated biphenyls, or PCBs, made it more difficult for mice to recover from noise-related trauma later in life.
Their paper appears in the Journal of Neuroscience.
PCBs are carcinogenic compounds formerly used in industrial and consumer products. Although they were banned in the United States in 1979 and have not been used industrially for decades, their very stable chemical structure makes them difficult to remove from the environment. Exposure continues to this day and is most common through consumption of contaminated fish. In particular, exposure to PCBs can be harmful to a developing fetus.
“The most sensitive time in pregnancy for these types of developmental exposures is usually early in pregnancy, in the first trimester,” said Dr. Daniel Llano, an associate professor in the Department of Molecular and Integrative Physiology at the University of Illinois Urbana-Champaign and the corresponding author on the paper. “But PCBs, as chemical entities, are very permeable to all kinds of membranes. They can cross the placenta and they can get into the brain. That makes them particularly dangerous during all stages of pregnancy.”
The foundation for the research was laid several years ago by Susan Schantz, emeritus professor in the Department of Comparative Biosciences who studied the effects of PCBs on the developing auditory system. She found that rats treated with PCBs had epileptic seizures in response to certain noise levels.
This led to a collaboration between Schantz, Llano and Baher Ibrahim, a research scientist in the Department of Molecular and Integrative Physiology and the paper’s lead author. Although the researchers had known for years that PCBs altered hearing, they wanted to understand how.
The researchers suspected that if someone exposed to PCBs in the womb suffered an acoustic trauma later in life (for example, from a noisy occupation or even from a recreational activity such as a rock concert), their hearing may not recover as well. would usually be expected.
Instead of continuing to study the auditory cortex, the researchers shifted focus to a lower brain region known as the inferior colliculus. There they found damage as a result of the combination of printed circuit boards and sound.
They studied individual neurons in the animals’ brains using a technique called multiphoton imaging.
“This is a unique microscope and we’re one of the few labs in the world doing this specific kind of imaging in this brain region,” Llano said. “People have shown in the past that if you expose a mouse to loud noise, the neurons in the lower colliculus become hyperresponsive over time. But if you combine the two toxic exposures, PCBs and noise, and use our imaging technique, hyperexcitability went away.”
The researchers also saw that the neurons became hypoexcitable – an entirely new finding.
The research team used chemical analysis to better understand the mechanisms behind these changes. Their findings centered around a common cellular process called oxidative stress, which automatically releases oxygen radicals — or highly reactive chemicals — when cells are sick or exposed to toxins. Oxygen radicals are eliminated by an intrinsic system in the body.
Mice with higher levels of the intrinsic protection system sustained less damage to the inferior colliculus, suggesting that PCBs and noise may cause excess oxidative stress in the inferior colliculus, suppressing the auditory system’s ability to recover from acoustic trauma.
“On its own, exposure to PCBs in the womb can cause only a moderate degree of hearing loss,” Llano said. “But that exposure to PCBs creates a particular vulnerability for later hearing loss. And so someone who is exposed to PCBs during development and has significant occupational or recreational noise exposure later in life may be more than expected when it comes to hearing.”
Compared to many toxicology studies, which look at a single exposure, the Beckman research team accounted for two exposures — PCBs and noise — experienced in separate cases over the course of a lifetime.
“What we found were effects that would not have been predictable based on the separate effects of the individual exposures,” Llano said. “You can’t necessarily predict the effects of combined exposure to two environmental factors by adding up the effects of one of them separately. I think that’s one of the more interesting things about this study, and hopefully it can be used as a model for future toxicological studies.”
The research team will continue to investigate the links between PCB exposure, noise exposure and hearing loss. While oxidative stress appears to be an important mediator of the effect, researchers have yet to measure the oxygen radicals in the tissues to confirm this.
“Dr. Llano and I have now co-supervised a graduate student and three postdoctoral fellows who continued this research,” Schantz said. “We are about to embark on the next phase of our collaboration, supervising a fourth postdoc who will study the microvasculature of key brain regions after exposure to PCBs and noise.”
These research questions will be further explored right where the research began: at the Beckman Institute.
“This is the kind of research that can only happen with Beckman,” Llano said. “Dr. Schantz and I are in completely different departments. Our home departments are in completely different locations on campus, but our Beckman offices are 10 feet apart. That’s why it has become very easy for us to work together. I think this is the kind of study that would be nearly impossible in a different environment where we didn’t have easy access to each other’s ideas and the different interactions that exist here.”
More information:
Baher A. Ibrahim et al, Developmental exposure to polychlorinated biphenyls prevents recovery from noise-induced hearing loss and disrupts the functional organization of the inferior colliculus, The Journal of Neuroscience (2023). DOI: 10.1523/JNEUROSCI.0030-23.2023
Quote: Fetal exposure to PCBs found to affect hearing health later in life (2023, June 5) Retrieved June 5, 2023 from https://medicalxpress.com/news/2023-06-fetal-exposure-pcbs-affect-health. html
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