Proceedings of The Physiological Society

Future Physiology (Leeds, UK) (2017) Proc Physiol Soc 39, PC22

Poster Communications

Spontaneous activity drives functional maturation in developing outer hair cells

A. Hendry1, F. Ceriani1, J. Jeng1, D. Simmons2, W. Marcotti1

1. University of Sheffield, Sheffield, United Kingdom. 2. Baylor University, Waco, Texas, United States.

In mammals, the sense of hearing relies on mechanoelectrical transduction performed by the primary sensory receptor inner hair cells (IHCs) and the outer hair cells (OHCs), in response to acoustic stimuli. Before the onset of hearing, spontaneous activity generated in the cochlea is thought to be crucial for the refinement of tonotopic maps in the auditory pathway, as well as maturation of sensory cells. This activity has so far been ascribed uniquely to IHCs, which are known to exhibit spontaneous calcium (Ca2+) action potentials before becoming mature sensory receptors. OHCs, the highly specialised sensory cells conferring fine tuning and high sensitivity to the mammalian cochlea, have not thus far been found to exhibit this property. How these specialised sensory cells and their innervation patterns are refined during pre-hearing stages of development is currently unknown. Whole-cell, cell attached patch-clamp recordings and 2-photon calcium imaging were performed from OHCs of C57B, Cx30 (-/-) and Cav1.3 (-/-) mice. OHCs were studied in acutely dissected cochleae from postnatal day 0 (P0) to P13, where the day of birth is P0. Recordings were performed using near-physiological solutions (1.3 mM extracellular Ca2+). Animal work was performed following UK regulation procedures. We show that, similarly to IHCs, immature OHCs exhibit spontaneous Ca2+ spikes, which were associated with action potential firing. This activity, which is mediated by L-type Ca2+ channels, is present during the first postnatal week, before OHCs become electromotile and fully acquire their mature ion channel profile, and disappears progressively from base to apex. We demonstrate that spontaneous Ca2+ spikes are required for OHC functional differentiation into sensory receptors. We also show that spontaneous release of ATP from Deiters' cells directly coordinates the firing activity of adjacent OHCs. In knockout mice for connexin 30, in which ATP release from connexin hemichannels in non-sensory cells is largely reduced, the normal maturation of OHC afferent and efferent innervation pattern is impaired. Our results indicate that, before the onset of OHC function, the acquisition of the mature physiological characteristics of OHCs and their neuronal connectivity is coordinated by experience-independent electrical activity involving both sensory and non-sensory cells of the mammalian cochlea.

Where applicable, experiments conform with Society ethical requirements