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Australian Frog—First Vertebrate to Make Poison

Laura Lewis
for National Geographic News
May 17, 2002
 
Scientists have discovered a wild Australian frog stocked with its own ammo and prepared for chemical warfare.

The frog, of the genus Pseudophryne, secretes a unique poisonous alkaloid from its skin, which offers protection from would-be predators.



This poisonous system of defense is not in itself surprising. Researchers have shown that certain frogs secrete the poisons that they obtain by eating a diet high in such poisons, which belong to a class of compounds known as alkaloids. But the new study represents the first documented case of a vertebrate producing its own alkaloids—essentially manufacturing the poisons on its own.

"You may think after 30 years of looking at frog skins for alkaloids there wouldn't be any surprises left," says John W. Daly, a biochemist with the National Institutes of Health in Bethesda, Maryland, who led the research. But the discovery of the self-initiated poison production, he said, "was completely unexpected."

The finding was reported in April in the Journal of Natural Products.

During many years of research, Daly and his team previously discovered that certain frogs from tropical forests all over the world have skin alkaloids.

Alkaloids are complex organic compounds of nitrogen, usually from plants, that can be very poisonous. Their various forms include such drugs as cocaine, quinine, strychnine, morphine, and nicotine.

The unique alkaloids of frog skin protect the amphibians from predators and perhaps from skin infections by microbes and fungi. Snakes, for example, often prey on frogs but spit them out upon tasting the poison.

Jerrold Meinwald, the Goldwin Smith Professor of Chemistry at Cornell University in Ithaca, New York, said he was pleased to learn that frogs have evolved the ability to manufacture inborn chemical defense. "I would have been surprised if there weren't frogs to do the necessary chemistry that insects do all the time. It makes perfectly good sense," he said.

Eventual Medical Impacts?

Daly and his colleagues first learned that Pseudophryne had unique alkaloids in its skin when they were studying the frog in the late 1980s. The researchers named the new class of alkaloids "pseudophrynamines," after the frog's genus.

The pseudophrynamines in the frogs' skin were "complex structures with no precedent in nature." Daly assumed that the alkaloids, like those of other alkaloids found in frog skins, originated from the diet. Yet a set of experiments conducted in Australia and analyzed at NIH over the past three years proved this early assumption wrong.

The research team knew that the Australian frog also possessed another type of alkaloid known as pumiliotoxins, which were linked with diet. Studies of poison frogs in South America and Madagascar showed that frogs raised in captivity did not naturally have alkaloids in their skin, but were able to accumulate them into skin glands when given to them in their diet.

Daly planned experiments with Ben Smith, a biologist at Adelaide University in Australia, to analyze the skin alkaloids of Australian frogs caught in the wild and the skin alkaloids of other frogs of the same lineage that were raised in captivity.

"I didn't suspect that they made [pseudophrynamines] because I hadn't found any other frog that was making an alkaloid," Daly said.

He thought that the frogs acquired the poison by eating a specific insect native to Australia that contained the alkaloids, and theorized that the Pseudophryne frogs, which are nocturnal, must have fed on certain nocturnal insects that contained the alkaloids.

When the researchers examined skin samples from frogs in the wild that had eaten their usual diet, levels of pumiliotoxins dominated and only traces of pseudophrynamines were found. The captive frogs that had been subject to an alkaloid-free diet, however, showed high levels of pseudophrynamines and no pumiliotoxins.

Daly concluded that Pseudophryne made their own pseudophrynamines but obtained pumiliotoxins in their skin by eating insects with high concentrations of these alkaloids. If they eat a diet heavy in pumiliotoxin alkaloids, he observed, then the frogs do not need to make pseudophrynamine to defend themselves.

From Poisons to Pain Killers

The studies of pseudophrynamines and similar substances in nature are of considerable interest because of their possible applications to medicine and biomedical research, such as in the production of heart stimulants, local anesthetics, and pain killers.

"We know historically that so many drugs come from plant alkaloids—morphine, codeine, caffeine, cocaine, nicotine," Daly said.

Meinwald believes there could be "all kinds of wonderful roles" for skin alkaloids. "Things don't occur in nature for no reason," he said. "If an organism is making it, it must have value for life."

One factor that makes skin alkaloids from frogs potentially useful for medical purposes is the presence of so-called nicotinic receptors, which play a major role in pain perception, protection from stroke, cognitive loss in Alzheimer's disease, and anxiety.

Properties of pseudophrynamine also show promise for chemotherapy treatment, said Thomas Spande, a research chemist in Daly's group at NIH. The frogs' ability to produce pseudophrynamines and not pump them out of their bodies could be a model for better techniques of chemotherapy, in which it's important for cells not to purge the chemical agents, he explained.

The team has also speculated that the frog skin alkaloids might have antibiotic activity. According to Spande, the researchers are collaborating with a group at American University in Washington, D.C., to investigate possible antifungal and antibacterial prospects of pseudophrynamines.

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