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 Vh 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.

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