Marine organisms produce a wide array of toxins, many of which are not only structurally unusual, but also show potent and interesting modes of action. Since the discovery of tetrodotoxin, a pufferfish toxin, as a potent and selective blocker of Na+ channels in 1964, it has been widely used as a research tool in pharmacological and physiological research. This has led to the identification of a number of important biological functions for Na+ channels. In recent years, much biological research has been carried out at molecular and cellular levels, and therefore selective inhibitors of enzymes and selective antagonist/agonists of receptors and channels have become increasingly important