We have isolated δ-conotoxin EVIA (δ-EVIA)_, a conopeptide in Conus ermineus venom that contains 32 amino acid residues and a six-cysteine/four-loop framework similar to that of previously described ω-, δ-, μO-, and κ-conotoxins. However, it displays low sequence homology with the latter conotoxins. δ-EVIA inhibits Na⁺ channel inactivation with unique tissue specificity upon binding to receptor site 6 of neuronal Na⁺ channels. Using amphibian myelinated axons and spinal neurons, we showed that δ-EVIA increases the duration of action potentials by inhibiting Na⁺ channel inactivation. δ-EVIA considerably enhanced nerve terminal excitability and synaptic efficacy at the frog neuromuscular junction but did not affect directly elicited muscle action potentials. The neuronally selective property of δ-EVIA was confirmed by showing that a fluorescent derivative of δ-EVIA labeled motor nerve endings but not skeletal muscle fibers. In a heterologous expression system, δ-EVIA inhibited inactivation of rat neuronal Na⁺ channel subtypes (rNa_V1.2a, rNa_V1.3, and rNa_V1.6) but did not affect rat skeletal (rNa_V1.4) and human cardiac muscle (hNa_V1.5) Na⁺ channel subtypes. δ-EVIA, in the range of concentrations used, is the first conotoxin found to affect neuronal Na⁺ channels without acting on Na⁺ channels of skeletal and cardiac muscle. Therefore, it is a unique tool for discriminating voltage-sensitive Na⁺ channel subtypes and for studying the distribution and modulation mechanisms of neuronal Na⁺ channels, and it may serve as a lead to design new drugs adapted to treat diseases characterized by defective nerve conduction.

The attainment of strong pharmacological effects with oligonucleotides is hampered by inefficient access of these molecules to their sites of action in the cytosol or nucleus. Attempts to address this problem with lipid or polymeric delivery systems have been only partially successful. Here, we describe a novel alternative approach involving the use of a non-toxic small molecule to enhance the pharmacological effects of oligonucleotides. The compound Retro-1 was discovered in a screen for small molecules that reduce the actions of bacterial toxins and has been shown to block the retrograde trafficking pathway. We demonstrate that Retro-1 can also substantially enhance the effectiveness of antisense and splice switching oligonucleotides in cell culture. This effect occurs at the level of intracellular trafficking or processing and is correlated with increased oligonucleotide accumulation in the nucleus but does not involve the perturbation of lysosomal compartments. We also show that Retro-1 can alter the effectiveness of splice switching oligonucleotides in the in vivo setting. These observations indicate that it is possible to enhance the pharmacological actions of oligonucleotides using non-toxic and non-lysosomotropic small molecule adjuncts.