Mechanisms contributing to synaptic Ca2+ signals and their heterogeneity in hair cells.
Sound coding at hair cell ribbon synapses is tightly regulated by Ca(2+). Here, we used patch-clamp, fast confocal Ca(2+) imaging and modeling to characterize synaptic Ca(2+) signaling in cochlear inner hair cells (IHCs) of hearing mice. Submicrometer fluorescence hotspots built up and collapsed at the base of IHCs within a few milliseconds of stimulus onset and cessation. They most likely represented Ca(2+) microdomains arising from synaptic Ca(2+) influx through Ca(V)1.3 channels. Synaptic Ca(2+) microdomains varied substantially in amplitude and voltage dependence even within single IHCs. Testing putative mechanisms for the heterogeneity of Ca(2+) signaling, we found the amplitude variability unchanged when blocking mitochondrial Ca(2+) uptake or Ca(2+)-induced Ca(2+) release, buffering cytosolic Ca(2+) by millimolar concentrations of EGTA, or elevating the Ca(2+) channel open probability by the dihydropyridine agonist BayK8644. However, we observed substantial variability also for the fluorescence of immunolabeled Ca(V)1.3 Ca(2+) channel clusters. Moreover, the Ca(2+) microdomain amplitude correlated positively with the size of the corresponding synaptic ribbon. Ribbon size, previously suggested to scale with the number of synaptic Ca(2+) channels, was approximated by using fluorescent peptide labeling. We propose that IHCs adjust the number and the gating of Ca(V)1.3 channels at their active zones to diversify their transmitter release rates.
Frank
Thomas
T
InnerEarLab, Department of Otolaryngology and Center for Molecular Physiology of the Brain, University of Göttingen, 37099 Göttingen, Germany.
Khimich
Darina
D
Neef
Andreas
A
Moser
Tobias
T
D000818:Animals; D020746:Calcium Channels, L-Type / Q000502:physiology*; D020013:Calcium Signaling; D003600:Cytosol; D006199:Hair Cells, Auditory, Inner / Q000502:physiology*; D006309:Hearing / Q000502:physiology; D051379:Mice; D008928:Mitochondria; D009435:Synaptic Transmission
C485995:Cacna1d protein, mouse; D020746:Calcium Channels, L-Type