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 alkaloidsmorphine, 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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