Abstract
During a critical developmental period, cochlear inner hair cells (IHCs) exhibit sensory-independent activity, featuring action potentials in which Ca(2+) ions play a fundamental role in driving both spiking and glutamate release onto synapses with afferent auditory neurons. This spontaneous activity is controlled by a cholinergic input to the IHC, activating a specialized nicotinic receptor with high Ca(2+) permeability, and coupled to the activation of hyperpolarizing SK channels. The mechanisms underlying distinct excitatory and inhibitory Ca(2+) roles within a small, compact IHC are unknown. Making use of Ca(2+) imaging, afferent auditory bouton recordings, and electron microscopy, the present work shows that unusually high intracellular Ca(2+) buffering and "subsynaptic" cisterns provide efficient compartmentalization and tight control of cholinergic Ca(2+) signals. Thus, synaptic efferent Ca(2+) spillover and cross-talk are prevented, and the cholinergic input preserves its inhibitory signature to ensure normal development of the auditory system.