Decoding the Deadly Secret of Snake Venom

The world’s animals have developed an incredible variety of venoms. But how?

Coral Snake Mimic
This is a close mimic of the coral snake, but the real version has a singular venom. Flickr user Joe Townsend

When he returned home to France after a stay in Costa Rica in 1983, Jean-Pierre Rosso carried back an unusual souvenir—a vial of deadly snake venom. Three decades later, after painstaking chemical and neurological analyses, Rosso and colleagues report that two toxins used by Costa Rican coral snakes act like no others, offering new insight into the astonishing array of chemical weapons that have evolved in the world’s animals.When Rosso’s team, led by Pierre Bougis, a biochemist at France’s National Center for Scientific Research, identified the six toxins within the venom, four of them worked as expected, causing paralysis in rodents and other effects. But two were puzzling because they triggered seizures instead.

The first step to understanding the mysterious toxins was to obtain more of the stuff to study in the lab. “I asked many times, ‘Can we get more venom?’” recalls Bougis. But his Costa Rican collaborators, who had initially milked the rare reptile, always replied: “We don’t have snakes.” So the team had to synthesize the toxins, which took a full decade.

The planet is home to more than 100,000 animals with venom, much of which is only now being characterized by scientists. There are not only snakes, spiders and scorpions, but also snails, fish, caterpillars, lizards, squid and even a few mammals, including the platypus, the short-tailed shrew and the slow loris, the world’s only venomous primate.

Because of the great variety, scientists suspect that the adaptation evolved not once but many times. A venomous jellyfish or sea anemone probably came first, maybe 500 million years ago, and venom arose in snakes some 65 million years ago, followed by monotremes (such as the platypus) 46 million years ago. “If we find complex life on other planets,” says Bryan Fry, head of the venom evolution laboratory at the University of Queensland in Australia, “I bet there’s going to be something venomous there.”

Especially if that alien life depends on amino acids. Venom toxins, it turns out, are strings of these basic biological molecules, called peptides or proteins, depending on their size. Scientists speculate that the toxins in venoms weren’t created by animals from scratch but are instead slightly altered versions of everyday peptides and proteins. A simple gene mutation can turn them into toxic weapons.

The French researchers don’t know where the coral snake toxins come from, but once they got hold of enough material, they figured out where the toxins go. The team radioactively tagged the synthetic toxins and applied them to isolated bits of rat brain. The compounds bound so tightly to receptors for a neurotransmitter called GABA that the neurons became overly excited.

Intriguingly, such receptors are involved in human disorders such as epilepsy and chronic pain. Bougis is determined to continue studying the toxins’ interactions with neurons, hoping it will lead to a new understanding of the disorders and perhaps treatments—even if the work takes another decade. “I French, we say, tête dure,” he laughs, “hard-headed.”

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