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Transient outward currents in cochlear ganglion neurons of the chick embryo
D N Sheppard1, M A Valverde, J Represa
1Departamento de Bioquímica y Biología Molecular y Fisiología, Facultad de Medicina, Universidad de Valladolid, Spain.
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 (Ito,f) had a threshold for activation of -61 +/- 5.5 mV; steady-state inactivation was voltage-dependent between -90 and -60 mV, with half-inactivation near -75 mV. The slowly inactivating outward current (Ito,s) showed an activation threshold of 34 +/- 4 mV. Half-inactivation was at -67 +/- 3 mV. Ito,f was blocked by 4-aminopyridine which did not affect Ito,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. Ito,f was strongly inhibited by removing Ca2+ from the extracellular bathing solution. Cobalt ions inhibited Ito,f in a dose-dependent manner between 2 and 20 mM. The inhibitory effect of Co2+ was voltage-dependent, displaying a bell-shaped inhibition curve as a function of membrane voltage, maximal inhibition occurring between -20 and 0 mV. Ca2+ removal did not affect Ito,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.
Insights
Cochlear neurons possess two distinct transient outward currents (Ito,f and Ito,s), identified through patch-clamp recordings and pharmacological analysis. These currents likely contribute to initial sound transmission in the auditory system.
Area of Science:
- Neuroscience
- Auditory Physiology
- Ion Channel Research
Background:
- Cochlear ganglion neurons are crucial for auditory signal transduction.
- Understanding ion channel function is key to deciphering neuronal excitability and signal processing.
Purpose of the Study:
- To characterize the kinetic and pharmacological properties of transient outward currents in chick embryo cochlear neurons.
- To differentiate between distinct outward currents based on their inactivation kinetics and drug sensitivity.
Main Methods:
- Patch-clamp electrophysiology was used to record membrane currents from isolated cochlear ganglion neurons.
- Voltage-step protocols were applied to study current activation and inactivation.
- Pharmacological agents (4-aminopyridine, tetraethylammonium, cobalt ions) and ionic manipulations (Ca2+ removal) were employed to probe current properties.
Main Results:
- Two transient outward currents, Ito,f and Ito,s, with distinct inactivation time constants (<30 ms and >100 ms) were identified.
- Ito,f showed specific sensitivity to 4-aminopyridine and Ca2+ ions, while Ito,s was unaffected by these.
- Tetraethylammonium modulated the non-inactivating outward current but not the transient currents.
Conclusions:
- Kinetic and pharmacological data strongly support the existence of two separate transient outward currents in cochlear neurons.
- These identified currents, Ito,f and Ito,s, are proposed to play significant roles in the initial stages of auditory signal transmission.