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Basis of Tetrodotoxin's Selectivity in Blockage of Squid Axons

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Abstract

The blockage of nerve activity by tetrodotoxin is unusually potent and specific. Our experiments were designed to distinguish whether its specificity of action was based on the identification of ions, the direction of cation flow, or differences in the early transient and late steady conductance pathways. Alkali cations were substituted for sodium in the sea water, bathing an "artificial node" in a voltage-clamped squid axon. When tetrodotoxin was added to the artificial sea waters at a concentration of 100 to 150 mM, it was found to always block the flow of cations through the early transient channel, both inward and outward, but it never blocked the flow of ions using the late steady pathway. We conclude that the selectivity of tetrodotoxin is based on some difference in these two channels.
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... Characterizing resistance in TTX-bearing species is straightforward because the mode and action of TTX is well understood (Furukawa et al., 1959;Hille, 2001Hille, , 1975Moore et al., 1967;Narahashi, 1974;Narahashi et al., 1964). Tetrodotoxin selectively binds to and blocks the ion-conducting pore of voltage-gated sodium ion channels (VGSCs). ...
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The natural history and pharmacology of tetrodotoxin (TTX) has long intrigued biologists. This toxin has a remarkable distribution that spans two domains of life (Bacteria and Eukarya). Within Eukaryotes, TTX has only been identified in animals but is known to be present in over five-dozen species of phylogenetically distant Metazoans. Despite decades of work, the origin and biosynthetic pathways of TTX remain unresolved. Investigations in puffer fishes and salamanders have provided insights into the acquisition of auto-resistance to TTX through the evolution of voltage-gated sodium ion channels (VGSCs) that have reduced binding affinity for TTX. To date there have been no studies of these proteins in tetrodotoxic Blue-Ringed Octopuses. Here we report data demonstrating that the Greater Blue-ringed Octopus (Hapalochlaena lunulata) expresses a VGSC (HlNaV1) gene with mutations that reduce the channel's TTX-binding affinity and likely render the organism TTX resistant. We identified three amino-acid substitutions in the TTX-binding site of HlNaV1 that likely confer TTX-resistance to both the channel and the organism. These substitutions are associated with organismal TTX-resistance in other TTX-bearing taxa and are convergent with substitutions that have evolved in fish, salamanders, and some TTX-resistant invertebrates.
... Toxins influencing Na v channel function by interacting with the pore region TTX and saxitoxin (STX). TTX and STX are naturally occurring guanidinium toxins that potently interact with the Na v channel pore region and cork the Na + permeation pathway (Furukawa et al., 1959;Narahashi et al., 1964;Moore et al., 1967;Narahashi, 1974;Hille, 1975bHille, , 2001. TTX played an important role in the biochemical purification of the Na v channel protein (Agnew et al., 1978;Miller et al., 1983) and in characterizing its selectivity filter (Terlau et al., 1991;Lipkind and Fozzard, 2008). ...
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Chapter
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Chapter
Marine toxins such as TTX and STX have aroused the interest of physiologists, biochemists, pharmacologists, and biological warfare people, as well as writers of spy and mystery stories. In the hands of the neuroscientist they have been turned into helpful instruments vital for the study of nerve function and excitable membranes. In this chapter on modes of action, I shall limit myself to the studies of TTX and STX on nerve and muscle. On these tissues they have been studied extensively and in some depth. Neither the historical development nor chemistry of these compounds will be discussed, because there are a number of excellent papers available covering the subject (Kao, 1966). Before discussing the mode of action of TTX and STX, one must say something about the nature of the nerve impulse.
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Interest in the poisons tetrodotoxin and saxitoxin was initially aroused because of deaths which occurred following careless or innocent consumption of puffer-fish or shellfish. From ancient times it was well known to the Japanese and Chinese that it was hazardous to eat the ovaries and eggs of the puffer-fish (tetrodons, also known as fugu or swellfish), but rather than forsake the fish which is apparently prized by oriental gourmets, chefs were specially trained and licensed to prepare nontoxic fugu. Restaurants which were fortunate enough to have the services of such a cook could include the item in their menu (see Chap. 6). Natural predators were no doubt aware of puffer toxicity, and this must have been very useful in the struggle for survival of the species. One of the earliest pharmacological uses of tetrodotoxin would have been as a poison in suicide, and perhaps homicide, attempts (Kao, 1964).
Article
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Article
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