Common causes of occluded ears and temporary hearing loss like childhood ear infections and build-up of ear wax, or even the common cold–where everything sounds muffled, can lead to changes in the hearing system. Even short-term blockages of this kind can lead to remarkable auditory changes, altering the behavior and structure of nerve cells that relay information from the ear to the brain, according to a new University at Buffalo (UB) study.
The research, published online in the December 1, 2016 edition of the Journal of Neuroscience, investigated what happened when mice had their ears surgically blocked for a period of several days to “dampen” hearing. The investigators saw that some significant changes do occur within a few days. “What’s still unclear, however, is whether the cells return to their normal state when acoustic conditions return to normal,” said Matthew Xu-Friedman, PhD, the lead researcher and an associate professor of biological sciences in UB’s College of Arts and Sciences. We see in our research that the cells do seem to mostly bounce back, but we don’t yet know whether they completely recover.”
According to a recent announcement from UB, the changes the research team observed had to do with neurotransmitters — chemicals that help send signals from the ear to the brain. In mice whose ears were blocked, cells in the auditory nerve started to use their supplies of neurotransmitter more freely. They depleted their reserves of these chemicals rapidly each time a new auditory signal came in, and they decreased the amount of space within the cells that housed sac-like structures called vesicles — biological storage tanks where neurotransmitter chemicals are kept.
“When it’s quiet, the demands on the auditory nerve cells are not as great,” said Xu-Friedman in the announcement. “So it makes sense that you would see these changes: You no longer need as much neurotransmitter, so why invest in a lot of storage? If you’re not that active, you don’t need a big gas tank. And you’re not as afraid to use up what you have. This is one plausible explanation for what we observed.”
The changes in cellular structure and behavior were the opposite of what Xu-Friedman’s team saw in a previous study that placed mice in a consistently noisy environment. In that project, the mice’s auditory nerve cells started to economize their resources, conserving supplies of neurotransmitter while increasing the storage capacity for the chemicals.
“It looks like these effects are two sides of the same coin, and they might be the first hints of a general rule that nerve cells regulate their connections based on how active they are,” Xu-Friedman says.
In the more recent study, cellular changes began to reverse themselves when the mice’s ears were unplugged. When a treatment was “undone, the cells started to go back to what they were like before. Yet, it’s not clear to the researchers if the cells completely recover, so the team needs to do more research to see if that’s the case. Xu-Friedman also wants to study what happens when cells are repeatedly exposed to conductive hearing loss, as happens in some small children who get recurring ear infections. Xu-Friedman’s co-authors on the paper were first author Xiaowen Zhuang, a UB PhD student in biological sciences, and Wei Sun, PhD, UB associate professor of Communicative Disorders and Sciences. The research was suported by the National Science Foundation.
Source: University at Buffalo
Image credit: Hua Yang, University at Buffalo