Frog defense: make snakes yawn.

Frog defense: Make snakes yawn

The Chinese say it's the year of the rabbit, but for medicine and biology it may be the year of the African clawed frog. In August, one researcher showed that this frog's skin contains a previously unidentified class of microbe-killing peptides, a finding that might lead to better treatment of burns, cystic fibrosis and other human ills (SN: 8/8/87, p.85). Now, two zoologists report that other compounds excreted by the frog's skin appear to trigger uncontrollable fits of yawning and gaping in snakes that try to eat the frogs.

Imbalances in these peptide and indoleamine compounds have in the past been linked to human nervous system disorders, say George T. Barthalmus and William J. Zielinski at North Carolina State University in Raleigh, who did the work with American northern water snakes. And because the snakes' behavior resembles the involuntary muscle contractions of people suffering from Parkinson's disease and tardive dyskinesia, which is caused by the long-term administration of antipsychotic drugs, they believe their discovery could improve the understanding and treatment of these problems.

Scientists have known for some time that frogs produce chemicals in their skin that are usually found only in the nervous systems of other vertebrates. "What's so bizarre,' says Barthalmus, "is that no one's ever wondered what in the world these [neurochemicals] are doing in the skin of the frog.'

Because the African clawed frog is one of the oldest frog species, he suspects that its skin neurochemicals, like the antimicrobial peptides found this summer, represent a primitive chemical defense system against predators. More advanced amphibians, he notes, use complex mixtures of similar chemicals, but these either taste awful or make their predators sick. The evolutionary advantage of the more advanced compounds, he says, may have been that they act faster and have more enduring effects than do the neurotoxins: Once a predator bites an advanced amphibian, it leaves the amphibian alone, whereas the water snakes in Barthalmus's lab repeatedly attack the African clawed frogs even though the snakes are seized with mouth-wrenching contractions each time.

Barthalmus would like to see whether snakes living near the clawed frogs in Africa are resistant to the frogs' neurotoxins. If so, he says, their resistance could suggest ways to "formulate antipsychotic drugs that don't produce side effects or even ways to treat tardive dyskinesia, which has been basically untreatable.'