Transient outward currents in cochlear ganglion neurons of the chick embryo

Abstract

Cochlear ganglion neurons were isolated from chick embryos and membrane currents recorded using the patch-clamp technique. Depolarizing voltage steps elicited transient outward currents whose inactivation was best fitted by a double-exponential function with time constants < 30 ms and > 100 ms. The fast inactivating transient outward current ( I to,f) had a threshold for activation of −61 ± 5.5mV; steady-state inactivation was voltage-dependent between −90 and −60 mV, with half-inactivation near −75 mV. The slowly inactivating outward current ( I to,s) showed an activation threshold of 34 ± 4mV. Half-inactivation was at −67 ± 3mV. I to,f was blocked by 4-aminopyridine which did not affect I to,s. The effect was concentration- and voltage-dependent. Tetraethylammonium had no effect on either fast or slow transient currents but reduced the amplitude of the non-inactivating outward current in a dose-dependent manner. I to,f was strongly inhibited by removing Ca 2+ from the extracellular bathing solution. Cobalt ions inhibited I to,f in a dose-dependent manner between 2 and 20 mM. The inhibitory effect of Co 2+ was voltage-dependent, displaying a bell-shaped inhibition curve as a function of membrane voltage, maximal inhibition occurring between −20 and 0 mV. Ca 2+ removal did not affect I to,s and partially reduced the amplitude of the steady-state current. These results provide kinetic and pharmacological evidence for the presence of two distinct transient outward currents in cochlear neurons. These currents may play a role in the first synaptic relay of sound transmission.