A study in deaf mice, key to improving cochlear implants

2023-06-27 20:17:30

protein mutations connection 26 are the most common cause of hearing loss at birth, accounting for more than 25% of genetic hearing loss worldwide. To understand how these mutations lead to deafness and whether they affect the development of the auditory system, a team from the University Johns Hopkins has created a mouse model of congenital deafness that is completely devoid of connexin 26 in the supporting cells of the organ of Corti in the cochlea. The study is published in PLOS Biology.

Without connexin 26, the supporting cells cannot communicate properly with each other, and the mice had profound hearing loss. Next, the researchers focused their experiments on mice less than two weeks old, when the ear canal is still closed, and in the period just after it opens, when the first sounds can be detected.

The results indicated that the loss of connection 26 it did not affect various aspects of auditory development. First, they found that the supporting cells generated spontaneous activity that led to normal firing of hair cells—the sensory cells of the inner ear—and normal firing of neurons in the brain’s sound-processing centers.

This early, sound-independent activity is necessary to train the auditory system to respond correctly to sounds of different frequencies.

Second, just after hearing onset, the various pitches were correctly assigned to separate subregions of the midbrain and auditory cortex, as measured by the small neural responses that persist in these deaf animals.

The problem was that spontaneous activity, and everything that followed, ceased once the ear canal opened up and the auditory system began to depend on sound for its activity.

It also suggests that early intervention may be important to avoid compensatory increased sensitivity.

Thereafter, the neurons’ sensitivity to sounds increased dramatically, similar to the «hypercussia» that often accompanies hearing loss.

The preservation of this early spontaneous activity, and the training it induces, against deafness-causing mutations may prepare the auditory system to respond well to cochlear prostheses, thus improving the efficacy of early therapeutic interventions. It also suggests that early intervention may be important to avoid compensatory increased sensitivity.

“Our results show that the spontaneous activity in the developing auditory system of mice lacking a critical protein for hearing. This may allow sound processing centers in the brain to mature so that they can correctly interpret the responses produced by cochlear implants in patients with this form of congenital deafness,” the researchers explain.