Voltage‐gated and chemically gated ionic channels in the cultured cochlear ganglion neurone of the chick.

Abstract

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.