Properties and Pharmacology of a Ttx-Insensitive Na+ Current in Neurones of the Jellyfish Cyanea Capillata

Abstract

Neurones of the motor nerve net of the jellyfish Cyanea capillata were axotomized, and voltage-clamped using the whole-cell configuration of the patch-clamp technique. Outward currents were blocked by a combination of extracellular 4-aminopyridine (4-AP) and intracellular Cs⁺, tetraethylammonium (TEA⁺) and 4-AP. Under normal conditions, the inward current consisted of a fast, transient current which could be abolished by removal of extracellular Na⁺ and whose reversal potential was dependent on the extracellular Na⁺ concentration. This current was completely insensitive to tetrodotoxin (TTX), saxitoxin (STX) and conotoxin GIIIA but could be blocked by extracellular Cd²⁺, lidocaine, W7 [N-(6 aminohexyl)-5-chloro-1-napthalenesulphonamide] and verapamil. Inactivation was voltage-dependent with a V_h of −15mV, and was unaffected by veratridine, batrachotoxin (BTX), sea anemone toxin and scorpion (Leiurus) venom. Reactivation required repolarization to a negative membrane potential for 12 ms for half-maximal reactivation. In the absence of extracellular Na⁺ no inward current was visible unless [Ca²⁺]_o was elevated. In Na⁺-free, 95 mmoll⁻¹ Ca²⁺ saline, a slightly slower, inward current was recorded. This current is believed to be the Ca²⁺ current that underlies synaptic transmission. 7. These findings are discussed with reference to synaptic transmission in these cells and the evolution of ion channels.