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National Institute for Physiological Sciences, Okazaki, Japan.

1. Electrophysiological properties of ionic channels of isolated or cultured cochlear ganglion (CG) neurones from chick embryo were studied under voltage-clamp conditions using a patch electrode. 2. Tetrodotoxin-sensitive Na+ current was activated by a step depolarization more positive than -40 mV, and was inactivated rapidly. 3. Outward-going K+ current was activated by step depolarization to membrane potentials more positive than -62 mV. 4. Two types of Ca2+ currents were demonstrated, an inactivating and a non-inactivating type. The inactivating type was activated by step depolarizations more positive than -69 mV and was inactivated rapidly. The non-inactivating type was activated by step depolarizations more positive than -52 or -41 mV depending on the external divalent cation species. 5. The I-V relationship and the activation kinetics of the non-inactivating type Ca2+ channel was shifted in a positive direction along the voltage axis by 12 mV when extracellular 2.5 mM-Sr2+ or Ba2+ were replaced by Ca2+. This shift was not observed in the inactivating type Ca2+ channel. 6. The amplitude of peak current through the inactivating type Ca2+ channel was in the order of Ca2+ greater than Sr2+ greater than Ba2+. The order of relative permeability through the non-inactivating type estimated from the tail current amplitude was Ba2+ greater than Sr2+ greater than Ca2+. 7. After 5 days in culture, glutamate (30 microM), aspartate (100 microM), kainate (100 microM) and N-methyl-D-aspartic acid (NMDA; 100 microM) elicited ionic currents. The glutamate response was depressed by 1 mM-Mg2+ in a voltage-dependent manner at negative membrane potentials and was almost extinguished by amino-phosphonovalerate (APV) (0.1 mM). The major subtype of glutamate receptor could be of the NMDA type. 8. The permeability of the NMDA receptor channel to Na+ and Li+ was estimated from the reversal potential and was 1.0 and 0.7 compared with that of Cs+, respectively. 9. Divalent cations were more permeable than the monovalent cations through the NMDA receptor channel: PCa greater than or equal to PBa greater than PSr greater than PCs.

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