Poison may be a means for some animals to defend themselves, but the success of this defensive plan is conditional on the poison cooks not tasting from their own poison.
A dangerous neurotoxin
Many poisonous animals protect themselves from predators with toxins that cause paralysis and cardiac arrest by binding to voltage-gated sodium channels, crucial proteins that mediate electrical impulses in neurons, muscle, and the heart.
Batrachotoxin, for example, is an extremely potent sodium channel toxin found in poisonous Pitohui birds in New Guinea as well as several poison frog species in Colombia. The golden poison frog Phyllobates terribilis is estimated to carry 1 milligram of batrachotoxin in its skin glands, enough to kill between 10 and 20 humans.
But despite their having voltage-gated sodium channels of their own, Pitohui birds and poison frogs obtain batrachotoxin from the insects they eat and store the poison for long periods. How, then, do these animals avoid poisoning themselves?
One possibility is that the animals have evolved batrachotoxin-resistant sodium channels. “However, there haven’t been any functional studies of poison frog or Pitohui sodium channels, so whether batrachotoxin-bearing animals rely on changes within their sodium channels or alternative resistance mechanisms remains unclear,” says UCSF Professor Daniel L. Minor, Jr.
|The golden poison frog, Phyllobates terribilis|
A team of researchers from the University of California, San Francisco (UCSF), Stanford University, and the California Academy of Sciences (CAS) has uncovered new clues as to how poisonous frogs and birds avoid intoxicating themselves.
Their study, which will be published August 5 in the Journal of General Physiology (JGP), suggests that, rather than evolving resistant versions of the toxin’s target protein, the animals produce “toxin sponges” that can mop up the poison and prevent it from exerting its deadly effects.
For the study, the team isolated sodium channels from both Pitohui birds and P. terribilis. They determined that they were highly sensitive to batrachotoxin. Frog sodium channels were, for example, sensitive to batrachotoxin levels more than 10 times below those found in P. terribilis in the wild.
Contrary to what was indicated by a previous study, this study showed that the golden poisonous frog contains membrane proteins sensitive and non-resistant to “batrachotoxin”.
Thus, intrinsic resistance to toxins is not a matter of sodium channel mutations in these organisms. How do these organisms avoid their own toxins?
|A poisonous Pitohui bird|
Absorbent Sponge Proteins
Here, the team believed that there was some mechanism in the way of allowing batrachotoxin to target sodium channels. The team found that a protein known as Saxiphilin acts as a sponge absorbent that can absorb saxitoxin, another neurotoxin similar to batrachotoxin.
However, the study has not yet identified any spongy proteins that can absorb the batrachotoxin poison and prevent its binding to sodium channels in poisonous golden frogs and in “Pitohui” birds, which is what Minor and his team intend to study in the future.
Minor concludes that this study “These sequestration strategies might not only offer a general means of toxin protection, but could also act in pathways involved in safely transporting and concentrating toxins in key defensive organs such as the skin. Understanding these pathways may lead to the discovery of antidotes against various toxic agents.”