Efferent neurons from the superior olivary complex release ACh to hyperpolarize outer hair cells (OHCs) in the mammalian cochlea. OHC hyperpolarization results from the opening of calcium-dependent (SK) potassium channels, triggered by calcium influx through nicotinic cholinergic receptors. Within the hair cell an endoplasmic reticulum that is found in close apposition to the efferent contact has been proposed to serve as a synaptic calcium store. For example, perfusion of ryanodine into perilymph enhanced the potency of efferent inhibition (Sridhar et al. 1997), and OHCs isolated from the guinea-pig showed some sensitivity to ryanodine and caffeine (Evans et al. 2000). Here we examine the effect of compounds acting on ryanodine-sensitive stores for their ability to modulate the cholinergic response of OHCs in an organ of Corti preparation excised from the rat cochlea.

Rat pups (3-4 weeks old) were anaesthetized with pentobarbitone and decapitated as approved by the Animal Care and Use Committee. An apical turn of the organ of Corti was removed to a recording chamber on the stage of a compound microscope. Patch pipettes were used for whole-cell recordings from hair cells as described previously (Glowatzki & Fuchs, 1999). ACh alone or in combination with test compounds was applied from a perfusion array. Membrane potential of the OHC was held at -30 mV in most instances. The response to ACh consisted of an outward current up to 600 pA in amplitude. The average value was well maintained during a 20 s application of ACh, although the current fluctuated markedly about that mean. When ACh was applied in the presence of 0.5-1.0 µM ryanodine the evoked current was 60 ± 29 % (S.E.M.) larger in six OHCs. When 100 µM ryanodine was applied the current evoked by ACh was reduced by 39 ± 4 % (6 cells). Application of 1 mM caffeine caused an initial augmentation of 14 ± 4 % (3 cells), followed within 8 min by inhibition (65 ± 6 %) of the ACh-evoked current (4 cells). These results demonstrate a consistent picture of sensitivity to compounds that act on calcium-induced calcium release via ryanodine receptors, supporting this mechanism as a contributory factor in the hair cell’s cholinergic response.

This work was supported by NIDCD DC01508.