Optical recordings of Ca2+ signaling activities from identified inner ear cells in cochlear slices and hemicochleae

One of the major obstacles hindering the progress of studies on mammalian cochlear physiology is the inaccessibility of inner ear cells located in a complex structure of the bony labyrinth. We describe here a technique to record cellular fluorescent signals from any identified inner ear cells in cochlear slices and hemicochleae. Cochlear slices were obtained from postnatal rats (P0-P7) before the cochlea completely ossify, and hemicochleae were cut from older animals (P7-adult). Individual inner ear cells were visually identified using infrared differential interference contrast or oblique illumination optics. Techniques were developed for either bulk-loading cells or loading selected single cells with Ca(2+) indicator dyes, and for maintaining functional viability of cochlear slices/hemicochleae for recordings. Robust and reliable responses of ligand-gated receptors were recorded from individual inner ear cells (e.g. hair cells, spiral ganglion neurons etc.) for at least 24 h after slices/hemicochleae were cut by an oscillating tissue slicer. The technique described here allowed direct observations of [Ca(2+)](i) activities from multiple cells simultaneously in situ, thus providing a feasible way to study the intercellular communication or networking activities from identified cells in the inner ear.