Effects of funnel web spider toxin on Ca currents in neurohypophysial terminals

Funnel web spider toxin (FTX) is reportedly a specific blocker of P-type Ca²⁺ channels. The effects of FTX on the Ca²⁺ currents of isolated neurohypophysial nerve terminals of the rat were investigated using the ‘whole-cell’ patch-clamp technique. Both the transient and long-lasting Ca²⁺ current components were maximally elicited by depolarization from a holding potential equal to the normal terminal resting potential (−90 mV). Externally applied FTX inhibited the high-voltage-threshold, transient component of the Ca²⁺ current in a concentration-dependent manner, with a half-maximal inhibition at a dilution of approximately 1: 10000. FTX also shifted the peak current of the I–V relationship by + 10 mV. The long-lasting Ca²⁺ current component, which is sensitive to L-type Ca²⁺ channel blockers, was insensitive to FTX. The transient current, which is sensitive to ω-conotoxin GVIA, was completely blocked by FTX. These results suggest that there could be a novel, inactivating Ca²⁺ channel in the rat neurohypophysial terminals which is affected by both N-type and P-type Ca²⁺ channel blockers.