Optical imaging of subcellular fluctuations within hair cells.

Although the transduction process has been well studied in hair cells, the possible presence of mechanical perturbations in the hair cell soma has not been explored in nonmammalian species. Hair cell mechanotransduction involves rapid biophysical events that remain difficult to observe in intact tissue. We developed a label-free optical method to image active motility within the soma during both spontaneous and mechanically driven hair bundle motion. Localized light-intensity fluctuations were detected at distinct focal planes, particularly near the periphery and basal pole of the soma. These optical signals exhibited spectral components that matched those of the hair bundle and were substantially reduced when mechanotransduction channel gating was disrupted, indicating that the somatic activity reflects physiological processes linked to mechanotransduction. Activity hotspots consistently aligned with regions of ionic channels and synaptic contacts, and strong stimulation produced phase-locked somatic responses that diminished after tip-link disruption. These findings parallel reports of mechanical correlates of neuronal activity and support the presence of an optical signature of transduction within the soma. Our results demonstrate that wide-field, label-free imaging can resolve intrinsic optical events in semi-intact sensory epithelia, offering a promising approach for noninvasive studies of hair cell and afferent-neuron signaling.