The Role of Spontaneous Electrical Activity in the Developing Auditory System
The auditory system of the chicken has been extensively used in comparative studies of the evolution and development of hearing. The auditory systems of both humans and chickens are at comparable stages at birth, partially but not fully developed. Also, both species start to hear in the second-third of the gestation period, making the chick cochlea an attractive model system to study the developmental mechanisms of hair cells. In addition, avian species can regenerate hearing, an ability lost in mammals.
The avian equivalent of the mammalian organ of Corti is the basilar papilla. The two hair cell types found in the chicken basilar papilla are tall and short hair cells, with tall hair cells similar to mammalian inner hair cells, and short hair cells similar to mammalian outer hair cells.
Calcium ions are fundamental to the most important roles in sensory processing mechanisms in vertebrate hair cells, including avian and mammalian, such as mechanotransduction, synaptic release, and frequency selectivity. Moreover, it is well established that calcium ions are instrumental to hair cell development, for instance, by supporting the ability to produce spontaneous electrical activity (spontaneous action potentials), one of the hallmarks of development.
This developmentally regulated spontaneous electrical activity also reappears in regenerating chick hair cells, suggesting that the process of regeneration may partly mimic developmental processes. This is an important implication to keep in mind when trying to regenerate the sensory epithelium in mammals.
Auditory hair cells fire spontaneous action potentials before the onset of hearing, which may be a major determinant of synaptic formation and ensuing establishment of proper tonotopic maps along auditory axes during development. Once synapses are established between hair cells and spiral ganglia neurons, the spontaneous action potentials cease, highlighting the importance of spontaneous action potentials to the development of the auditory inner ear.
Specific ion currents have been identified as major players supporting this electrical activity in developing and regenerating chick hair cells. This research, published in Frontiers in Cell Neuroscience in January 2022, details the role of small-conductance, calcium-ion-activated potassium channels (SK) in developing and regenerating chick hair cells.
Analysis of the functional expression of the SK current showed that most dramatic changes occurred between embryonic day 8 and embryonic day 16. Further, the SK current is present in developing tall hair cells and contributes to the patterning of spontaneous electrical activity, and the SK current reappears in regenerating hair cells. This study provides direct evidence that the functional expression of the SK current may contribute to the patterning of spontaneous action potentials in developing and regenerating chick tall hair cells.
2008 Emerging Research Grants scientist Snezana Levic, Ph.D., is a lecturer in physiology at Brighton and Sussex Medical School in the U.K. This is adapted from her paper “SK Current, Expressed During the Development and Regeneration of Chick Hair Cells, Contributes to the Patterning of Spontaneous Action Potentials,” published in Frontiers in Cell Neuroscience in January 2022.