Animal defenses neurotoxins

Many alkaloids fall into the class of spedflc modulators and have been modified during evolution in such a way that they mimic endogenous ligands, hormones, or substrates [1,3,18,19]. We have termed this selection process evolutionary molecular modeling [12,13,19,23]. Many alkaloids are strong neurotoxins that were selected for defense against animals [2,3,19]. Table 1.1 summarizes the potential neuronal targets that can be affected by alkaloids. Extensive reviews on this topic have been published [2,3,19]. 

Marine dinoflagellates produce a number of toxins, such as saxitoxin, surugatoxin, tetrodotoxin, and gonyautoxin, that affect ion channels (Table IV). These algae are eaten by some copepods, fish, and molluscs that also store these neurotoxins 4,17,28,29,494,495). As a consequence, these animals have acquired chemical defense compounds, which they can use against predators. 

Of the 25 animal phyla, almost half are worms. Thus, it is not at all surprising that some worms contain toxins. The nemertines are a phylum of over 800 known species which resemble flatworms but are active predators on crustaceans and other worms. This phylum is exceptionally toxic among the various worm phyla. The Heteronemertine side possesses peptide toxins which appear to be only defensive, as these animals have no means of injecting a venom. The peptides include neurotoxins, which enhance excitability of nerve membranes, and cytolysins, which permeabilize and destroy cell membranes. Members of the Hoplonemertine class inject a venom into their prey using a mineralized stylet located in their proboscis, which is also used to immobilize the prey. Their toxins are alkaloids similar to nicotine which in minute amounts paralyze crustaceans and annelid worms and primarily activate nicotinic acetylcholine receptors. Another well-known worm toxin is nereistoxin, a disulfide-containing alkaloid which also binds to nicotinic... 

The occurrence of TTX in flatworm led scientists to extensive studies on nemertines, resulting in detection of TTX for the first time in two species of ribbon worm Linens fuscoviridis and Tubulanus punctatus, with the highest score of 503 and 540 MU/g, respectively (Miyazawa et al., 1988). In addition to TTX, toxic TDA-like substance was detected from these ribbon worms. Kem (1976) isolated four polypeptides from the toxic ribbon worm Cere-bratulus lacteus that showed a selective toxicity variety of animals, suggesting that the toxin is also used for a defensive purpose. Subsequently, the toxic mucus secretion of C. lacteus was shown to consist of cytotoxins and neurotoxins (Kem and Blumental, 1978). 

This same species subsequently yielded a new indohc alkaloid, isopteropodine, already known in the terrestrial environment, which may be derived from the animal s food (Martin, Sanduja, and Alam, 1986). A neurotoxin derived from bromotryptamine was isolated from the species Cal-liostoma canaliculatum from the kelp forests of California (Monterey Bay). This substance, used for defense against predators, acts on potassium channels (Kelley et al, 2003 Wolters, Jayawickrama, and Sweedler, 2005). 

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