Olivocochlear efferent plasticity during development and reorganization during hearing loss
Zachary, Stephen Paul
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Inner hair cells (IHCs) are the primary sensory receptors of the auditory system. Before the onset of hearing, immature IHCs have numerous cholinergic efferent synapses that are eliminated in the second postnatal week. These efferents, which are functionally inhibitory, exist during an important period of cellular and circuit maturation. Efferent inhibition is brought about via the gating of postsynaptic nicotinic acetylcholine receptors (nAChRs), which flux calcium to activate calcium-dependent potassium (SK) channels. Whole-cell recordings revealed that calcium-induced calcium release from internal stores and voltage-gated calcium channel function also contribute to the calcium signal that shapes SK channel activation. These additional calcium sources underlie a plasticity mechanism that encodes recent IHC depolarization in cholinergic responses. Thus, the activity of the IHC modulates the strength of efferent inhibition. Recordings from aged animals were also performed to investigate whether efferent neurons re-innervate IHCs during aging. Functional efferent synaptic activity was observed in aged animals, and the degree of efferent innervation increased alongside afferent loss, outer hair cell death, and threshold elevation. Such efferent synapses were inhibitory and utilized the same ionic mechanisms found in early postnatal IHCs. These data are the first recordings from aged hair cells and show that the damaged cochlea assumes features reminiscent of development.