Compartmentalization of antagonistic Ca2+ signals in developing cochlear hair cells

MARCELO J. MOGLIE; JUAN D. GOUTMAN; ANA BELÉN ELGOYHEN; PAUL ALBERT FUCHS

Córdoba

Congreso; XXXIII annual meeting of the Argentinean Society for Neuroscience; 2018

Sociedad Argentina de Investigación en Neurociencias

The normal maturation of the auditory pathway relies on a critical developmental period characterized by the firing of sensory-independent action potentials by cochlear inner hair cells (IHCs). Spiking activity produces the influx of Ca2+ through voltage-gated channels which in turn triggers the synaptic release of glutamate onto dendrites of the auditory nerve, leading to the propagation of the spontaneous activity throughout the auditory system. On the other hand, IHCs are the postsynaptic target of efferent cholinergic neurons from the brainstem. At this synapse, Ca2+ entry through nicotinic α9α10 receptors is coupled to the activation of Ca2+-dependent K+ channels to hyperpolarize the IHC. Thus, efferent Ca2+ influx is inhibitory, opposing the excitatory Ca2+ signal produced during IHC firing. The aim of our work was to investigate the mechanisms that allow segregation of excitatory versus inhibitory Ca2+ effects within the small and diffusionally compact IHCs. Electrophysiological recordings combined with swept?field confocal Ca2+ imaging experiments revealed the existence of multiple efferent Ca2+ entry hotspots which were closely positioned to afferent Ca2+ entry sites within a single IHC. This finding was confirmed by IHC reconstructions at a nanometer scale using serial section electron micrographs (EM), suggesting that efferent Ca2+ spread could invade afferent synapses. However, recordings from postsynaptic boutons of auditory nerve neurons showed that even high frequency stimulation of efferent fibers failed to cross-activate and trigger the synaptic release of glutamate.Efficient compartmentalization of Ca2+ signals was accomplished by: i) sub-synaptic cisterns revealed in EM reconstructions, juxtaposed to cholinergic contacts; ii) a fast Ca2+ extrusion pathway mediated by SERCA pumps; iii) and a very strong Ca2+ buffering in IHC cytoplasm. Thus, efferent fibers maintain its inhibitory signature and modulate spontaneous activity in the developing IHC.