Scientists found that small fish larvae lacking a functional fmr1 gene — the fish equivalent to the human FMR1 gene that, when mutated, causes fragile X syndrome — show changes in specific brain circuits that induce extreme sensitivity to sound.
“Loud noises often cause sensory overload and anxiety in people with autism and Fragile X syndrome — sensitivity to sound is common to both conditions,” Lena Constantin, PhD, a scientist at the Queensland Brain Institute and first author of the study, said in a press release.
“We think the brain is transmitting more auditory information because it is being filtered and adjusted differently,” Constantin said.
The team’s findings were reported in “Altered brain-wide auditory networks in a zebrafish model of fragile X syndrome,” a study published in the journal BMC Biology.
Although extreme sensitivity and abnormal responses to certain sounds are features of both fragile X and autism, the reasons why these patients have such unusual responses are still poorly understood.
Now, to explore the neurological mechanisms that might cause increased sensitivity to certain sounds, researchers in Australia studied the patterns of brain activity of small fish larvae that had been genetically modified to lack a functional fmr1 gene, as a model of fragile X.
They specifically picked these animals because their whole brain can be viewed under a microscope, and the activity of each brain cell can be assessed individually in real-time.
The investigators used this method to measure the animals’ brain activity in different circumstances, such as exposing them to sound bursts and showing them movies. According to Constantin, movies worked as a visual stimuli to the animals, mimicking signals induced by movement or a predator.
While the researchers did not find any difference in brain activity in healthy fish and in those lacking fmr1 upon exposure to visual stimuli, animals lacking the gene responded in a very different way when presented with a sudden burst of sound.
After seeing how loud noises dramatically changed brain activity, the investigators designed an experiment to evaluate how fish lacking the fmr1 gene responded to 12 different volumes of auditory stimuli, compared with healthy animals.
This experiment revealed that animals lacking the gene were able to detect and respond to much lower sound volumes compared with healthy fish, and have much greater sound sensitivity.
“We demonstrated that fmr1 [deficient] larvae are hypersensitive to sound … and identified four sub-cortical brain regions [below the cerebral cortex] with more plentiful responses and/or greater response strengths to auditory stimuli,” the researchers wrote.
According to the team, two brain regions — the thalamus and the hindbrain — were particularly active when fish lacking the fmr1 gene were exposed to auditory signals.
“The fish with Fragile X mutations had more connections between different regions of their brain and their responses to the sounds were more plentiful in the hindbrain and thalamus,” Constantin said.
Notably, the thalamus is a brain region that works as a central hub, controlling how sensory information gathered by other parts of the body is relayed to other regions of the brain, while the hindbrain is important to coordinate behavioral and motor responses, as well as sleep and heart rate.
“Using the zebrafish, we’ve been able to see a lot more detail and for the first time, seen more activity in the hindbrain which we’re keen to explore further,” said Constantin.
“We hope that by discovering fundamental information about how the brain processes sound, we will gain further insights into the sensory challenges faced by people with Fragile X syndrome and autism,” she added.
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