Interactions of δ-Conotoxins with Alkaloid Neurotoxins Reveal Differences Between the Silent and Effective Binding Sites on Voltage-Sensitive Sodium Channels

The delta-conotoxin-TxVIA from Conus textile (delta TxVIA) is a mollusk-specific conotoxin that slows sodium channel inactivation exclusively in mollusk neuronal membranes but reveals high-affinity binding to both mollusk (effective binding) and rat brain (silent binding) neuronal membranes, despite not having any toxic effect in vertebrates in vivo and in vitro. Using binding studies with radioactive delta TxVIA we demonstrate that a different mollusk-specific conotoxin, delta-conotoxin-GmVIA from the venom of Conus gloriamaris, possesses "silent" and effective binding properties in rat brain and mollusk sodium channels, respectively. Binding studies and electrophysiological tests with both vertebrate muscle and insect neuronal preparations have indicated that the silent binding sites of delta TxVIA are highly conserved in a wide range of distinct vertebrate and insect sodium channels. Direct probing of receptor site 2 by a tritiated derivative of batrachotoxin ([3H]BTX-B) revealed that [3H]BTX-B binding in mollusk sodium channels is of high affinity with no addition of enhancing ligands, unlike [3H]BTX-B binding in rat brain. In contrast to the negative allosteric modulation of delta TxVIA binding by veratridine, delta TxVIA is not able to affect the binding of [3H]BTX-B in mollusk neuronal membranes but reduces [3H]BTX-B binding in rat brain in the presence of alpha-scorpion toxins. The latter finding indicates the existence of a pharmacological distinction between the silent and effective binding sites of delta TxVIA and points out possible functionally important structural differences between molluscan and rat brain sodium channels.