CALCIUM CURRENTS IN DISSOCIATED COCHLEAR NEURONS FROM THE CHICK-EMBRYO AND THEIR MODIFICATION BY NEUROTROPHIN-3

Calcium entry through voltage-dependent channels Flay a critical role in neuronal development. Using patch-clamp techniques we have identifi ed the components of the macroscopic Ca2+ current in acutely-isolated chick cochlear ganglion neurons and analysed their functional expressi on throughout embryonic development. With Ba2+ as a charge carrier, th e currents exhibited two main components, both with a high activation threshold but differing in their inactivation kinetics. One component showed inactivation with a time constant around 100 ms (transient) whe reas the other hardly inactivated (sustained). The currents were sensi tive to omega-Conotoxin GVIA and dihydropyridines, blocked by 20 mu M Cd2+, but unaffected by omega-Agatoxin IVA. In a few cases, only with Ca2+ as a charge carrier, an additional component with low activation threshold and fast inactivation (time constant of 20 ms), was observed . Currents were first detected at day 7 of embryonic development. Curr ent density (amplitude/cell capacitance) increased through embryonic d ay 9, when early synaptic contacts are established, and decreased ther eafter to lower steady values. The effect of neurotrophin-3, a neurotr ophic factor required for survival and differentiation of cochlear gan glion neurons, was also examined. Neurons isolated at embryonic day 7 or day 11 and maintained two days in culture with 2 ng/ml neurotrophin -3 showed a substantial increase in Ca2+ current density, particularly in the transient component. These findings indicate that the expressi on of neuronal Ca2+ channels is predominant at the time of synapse for mation between transducing hair cells and their primary afferents. Bes ides its effects on survival and neuritogenesis, neurotrophin-3 enhanc es the expression of Ca2+ channels in cultured neurons. Taken together these results suggest that the functional expression of Ca2+ channels is regulated during embryonic development of cochlear neurons by the release of neurotrophin-3 from the differentiating sensory epithelium of the cochlea. (C) 1997 IBRO. Published by Elsevier Science Ltd.