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Chick cochlear hair cell exocytosis mediated by dihydropyridine-sensitive calcium channels

Maria Spassova *, Marc D. Eisen †, James C. Saunders † and Thomas D. Parsons *

1. A semi-intact preparation of the chick basilar papilla was developed to study calcium-dependent neurotransmitter release by tall hair cells (avian equivalent of cochlear inner hair cells).
2. Tall hair cell depolarization resulted in changes in cell membrane capacitance (C_m) that reflected cell surface area increases following synaptic vesicle exocytosis and provided a surrogate measure of neurotransmitter release. Both calcium current (I_Ca) and C_m were reversibly blocked by cobalt, and exhibited a similar bell-shaped dependency on voltage with a peak response around -10 mV.
3. Pharmacological agents selective for L-type calcium channels were employed to assess the role of this channel type in neurotransmitter exocytosis. Nimodipine, a dihydropyridine (DHP) antagonist, suppressed I_Ca and blocked C_m. Conversely, the DHP agonist Bay K 8644 increased both I_Ca and C_m amplitude nearly 3-fold. These findings suggest that chick tall hair cell neurotransmitter release is mediated by calcium influx through L-type calcium channels.

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