By Sonja J. Pyott, Ph.D.
The development of the auditory system begins in the womb and culminates in a newborn’s ability to hear upon entering the world. While the age at which hearing begins varies across mammals, the sensory structures of the inner ears are active before the onset of hearing. This activity instructs the maturation of the neural connections between the inner ear and brain, an essential component of the proper development of hearing. However, we still know very little about the mechanisms regulating the activity of these sensory structures and their neural connections, specifically during the critical period just before the onset of hearing.
In our paper, “mGluR1 enhances efferent inhibition of inner hair cells in the developing rat cochlea,” soon to be published in an upcoming issue of The Journal of Physiology, we investigate the role of glutamate, a neurotransmitter, in regulating activity of the sensory structures and their connections in the inner ear before the start of hearing.
Neurotransmitters assist in the communication between neurons and are typically classified as either excitatory or inhibitory based on their action. Excitatory action results in stimulation; inhibitory action assists in the calming of the brain. Our research found that although glutamate typically excites activity, it also elicits inhibitory activity. This dual role for glutamate occurs because it activates two distinct classes of glutamate receptors: ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs).
Importantly, this dual activation balances excitatory and inhibitory activity of the sensory structures, a balance of which is likely important in the final refinement of the neural connections between the inner ear and brain prior to the onset of hearing.
As part of future research, we will further investigate the role of mGluRs, one the distinct classes of glutamate receptors, in the development of hearing. We will also investigate if mGluRs balance excitatory and inhibitory activity in the adult inner ear, similar to its role prior to the onset of hearing. Insights into these mechanisms may identify new ways to modulate activity and prevent congenital or acquired hearing loss.
Study coauthor Sonja J. Pyott, Ph.D., was a 2007 and 2008 Hearing Health Foundation Emerging Research Grants recipient.