Presynaptic calcium stores modulate afferent release in vestibular hair cells

Hair cells, the mechanoreceptors of the acoustic and vestibular system, are presynaptic to primary afferent neurons of the eighth nerve
and excite neural activity by the release of glutamate. In the present work, the role played by intracellular Ca2+ stores in afferent
transmission was investigated, at the presynaptic level, by monitoring changes in the intracellular Ca2+ concentration ([Ca2+]i) in
vestibular hair cells, and, at the postsynaptic level, by recording from single posterior canal afferent fibers. Application of 1–10 mM
caffeine to hair cells potentiated Ca2+ responses evoked by depolarization at selected Ca2+ hot spots, and also induced a graded increase
in cell membrane capacitance (delta Cm), signaling exocytosis of the transmitter. Ca2+ signals evoked by caffeine peaked in a region located
about 10 nanometer from the base of the hair cell. [Ca2+]i increases, similarly localized, were observed after 500 msec depolarizations, but not with
50 msec depolarizations, suggesting the occurrence of calcium-induced calcium release (CICR) from the same stores. Both Ca2+and delta Cm
responses were inhibited after incubation with ryanodine (40 microM) for 8 –10 min. Consistent with these results, afferent transmission was
potentiated by caffeine and inhibited by ryanodine both at the level of action potentials and of miniature EPSPs (mEPSPs). Neither
caffeine nor ryanodine affected the shape and amplitude of mEPSPs, indicating that both drugs acted at the presynaptic level. These
results strongly suggest that endogenous modulators of the CICR process will affect afferent activity elicited by mechanical stimuli in the
physiological frequency range.