Poison Dart Frogs

This topic submitted by Vera L Figueiredo ( figueivl@miamioh.edu) at 8:49 PM on 6/14/06.

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My discussion topic is on the biochemistry of Poison Dart Frogs. I feel that this topic is important because we can learn a lot about how these organisms have evolved the ability to produce poisons by aquiring the alkaloids from their diet. Poison Dart Frogs, like other amphibians, are important indicator species. I hope to teach the class how dart frogs produce toxins, how they use them, and what the toxins do to their predators.


Introduction:
Poison dart frogs are scientifically classified in the order of Anura, and the Dendrobatidae family. There are nine genera and over one hundred known species of the Dendrobatidae family. In Costa Rica eight species of dart frogs can be found of three different genera: Colostethus, Dendrobates, and Phyllobates. Each species are found in different locations of the country. The species Colostethus talamancae commonly known as the rocket frog can be found on the southeast Atlantic region of the country while the Colostethus nubicola, Colostethus flotator, Dendrobates granuliferus, and Phyllobates vittatus are all found on the southwest Pacific region. The species Dendrobates pumilo and Phyllobates lugubris are common on the Atlantic versant or mountainside while the Dendrobates auratus occurs on both the Atlantic and Pacific versants and is the most widely distributed species of dart frogs in Costa Rica (Boston 2006).

Biology:
Poison dart frogs are generally found on tropical rain forest floors and are commonly found in two geographical regions: Central and South America but the Dendrobates auratus are also found on the Hawaiian Islands because they were introduced there by man. Dart frogs have a life span of about ten to fifteen years in the wild but have been found to live to about twenty years when in captivity. This order of amphibians is generally small in size ranging from 0.5 inches (1cm) to 2.5 inches (6cm) long depending on the individuals age and species. Dart frogs are diurnal organisms that spend their lives in solitude. They are also territorial animals and territorial disputes can often lead to aggressive behavior (Dorcas 2000).

Poison dart frogs breed through the rainy season. These organisms lay their eggs on leaves because the high humidity provides the proper environment for the developing eggs. In some dart frog species the male will tend to the eggs and the hatched tadpoles while in other species the female is the primary care giver to the offspring. When the tadpoles hatch they maneuver themselves onto their parentsÕ back. The parent then carries the offspring to a pool of water or a water-filled bromeliad where the offspring will continue to develop. In some genera of dart frogs the female attends to the developing tadpoles by providing them with nutritive, unfertilized eggs. There are some genera such as the Colostethus that do not tend to their offspring after hatching. Their offspring will hitchhike on the parentsÕ back to the nearest water source and are then abandoned to survive on their own (Hamlett 2006).

These organisms are often brightly colored. Different species will vary in color but most are bright red, orange, yellow, or green and black. The Colostheus genera are generally a dull, brown color. The bright colors expressed by a majority of these organisms serve to warn predators of their toxins which they acquire from the alkaloids found in their prey. Their diet in the wild is mainly based on Myrimicine ants and Coccinellid beetles but they have also been known to eat termites, millipedes, mites, and mosquitoes (Hamlett 2006).

Native American Use:
The common name for the poison arrow dart frog originated from the native Indians that depend on these organisms. The Choc— and Ember‡ Choc— of western Columbia dip blow-gun darts into the toxic secretions of these frogs. The Choc— then use the darts in a blowgun as a method of hunting for food. Only three species are used for their poisons by the Choc— tribes. The species Phyllobates aurtotaenia are used by the northern Choc— of Columbia, Phyllobates bicolor are used the southern Choc—, and the Phyllobates terribilis are used by the Choc— in the Cauca valley of western Columbia. Although only the toxins of these three species are used on blow darts the name stuck with the entire Dendrobatidae family (Dorcas 2000).

Venom vs. Poison:
Before discussing where the poisons in the dart frogs are found and where they come from it is beneficial to understand the difference between venoms and poisons. A venomous organism has an active delivery system. The toxins are stored in venom glands and they are injected into the predator or prey via fangs, pinchers, or spines. A venomous organism usually uses their toxins to protect themselves or to help catch their food.

A poisonous organism has a passive delivery system. The toxins are usually stored in their skin usually eaten or absorbed by the predator therefore it is not directly injected into the blood stream but is absorbed through the digestive system. The toxins of a poisonous organism are usually for protection only. If the predator ingests the poisons it may get sick or even die depending on the potency of the toxin.

Biochemistry:
The glandular epithelium of dart frogs produces potentially lethal secretions through their poison glands. Theses poison glands are exocrine glands which can secrete mucus, saliva, oils, digestive enzymes, and other substances. The secretions are released onto the epithelial surface through ducts or tubes. The epithelial surface is a layer of tissue that covers the external surface of the body (Starr 1998).

All Dendrobatidae have poison glands but only a few of them are toxic enough to kill a person. The toxins are composed of lipophilic alkaloids which can be extremely poisonous when found in large quantities. These organic compounds are composed of substances such as carbon, hydrogen, nitrogen, and oxygen that are able to dissolve more easily in lipids than in water. There are over 300 alkaloids found in these frogs which are derived from the Myrimicine ants and Coccinellid beetles they consume. Dart frogs will store the toxic alkaloid molecules in their glands without being harmed (Patocka 1999). The Myrimicine ants and Coccinellid beetles they consume can synthesize the alkaloids as well as acquire them from plants they feed on. This is one explanation for why dart frogs in captivity lack their toxic defense. The theory that the alkaloids come from their food source is also supported when we look at dart frogs introduced on the Hawaiian Islands. The make up of the dart frog alkaloids on the Hawaiian Islands are different than in Central and South America because they are feeding on different species of insects (Carr 2000).

Poison dart frogs produce different types of neurotoxins which mainly affect the nerve cells or neurons usually by interacting with the membrane proteins and ion channels. With so many species of dart frogs there many neurotoxins found in this family of frogs but I will only discuss the three major types of neurotoxins: batrachotoxins, pumiliotoxins, and histrionicotoxins. Each of these toxins is found in different species of dart frogs and originates from their diet (Patocka 1999).

Batrachotoxin is an extremely potent nerve poison that occurs in all Phyllobates species. Phyllobates get this type of toxin from the alkaloids found in the Coccinellid beetles they consume. Wild-caught Phyllobates can contain anywhere between 700 to 1,900 micrograms of batrachotoxins. One gram of this toxin can kill 1,000 full grown, adult humans. A predator that consumes a Phyllobate will endure symptoms such as strong muscle contractions, violent convulsions, salivation, heart arrhythmia, heart fibrillation, heart failure and death (Patocka 1999). Batrachotoxins affect the voltage-dependent sodium channels. Normally, during a nerve impulse the cell depolarizes by the opening of sodium channels. At this point the cell goes through a refractory period where the nerve is unresponsive to a second stimulus until the cell becomes repolarized. To repolarize the cell the sodium channel goes through an inactivation phase to make the sodium channels impermeable to sodium. The toxins force the channels to remain open and cause the cell membrane to become depolarized after allowing a large amount of sodium ions to enter the channel. Therefore the cell remains in the refractory phase causing a block in the transmission of nerve impulses and may stop the heart from functioning (2002).

Pumiliotoxin is a cardiac stimulant and is commonly only 100 to 1000 times less toxic than batrachotoxins. Pumiliotoxin is found in all species of Dendrobates and Phyllobates. These species get the toxin from the alkaloids found in the ants they consume. Pumiliotoxins can be divided into three groups, Pumiliotoxins A, B, C where A and B are more toxic than C (Patocka 1999). Pumiliotoxins can affect the voltage-dependent calcium channels in the heart or skeletal muscles. This channel works very similar to that of a sodium channel but this toxin forces the release of intercellular calcium ions and also prevents the re-accumulation of calcium ions (2002). Therefore, small doses of Pumiliotoxins A and B can cause hyperactivity, difficulty in locomotion, hypersensitivity to stimuli, partial paralysis, convulsions and even death (Patocka 1999).

Histrionicotoxins occur in Dendrobatidae and they get their toxins from the Myrimicine ants they consume. These toxins block acetylcholine which is a neurotransmitter that carries messages between the nerves and muscles. Histrionicotoxins also block end-plate receptors preventing action potentials jumping from nerve to muscle cells. This toxin also affects the potassium channels forcing them to remain open or closed. This can cause action potentials to be prolonged which increases the time of muscle contractions (Patocka 1999).

Why are they important?:
Poison dart frogs are extremely fascinating and important organisms on this planet. These organisms consume insects which can potentially help control diseases. It is also possible that the alkaloids ingested by poison arrow dart frogs may become useful for medical purposes. The components of most of the toxins are similar to that of cocaine and morphine allowing for the development of an improved pain killer. In 1998 Abbot laboratories in North Chicago were developing an experimental drug for AlzheimerÕs and came across the chemical epibatidine. They then converted the chemical into the drug ABT-594 and found that it is a more powerful painkiller than morphine without any of the side affects, non-addictive as well as less toxic (Cameron 1998).

Dangers:
A Global Amphibian Assessment which surveyed the planetÕs amphibian species found that about a third of the worldÕs five thousand seven hundred forty three amphibian species are currently threatened. It is difficult to determine why there has been such a decline in amphibian populations but one theory is the emergence of the deadly chytrid fungal disease. Chytridiomycosis is a fatal disease of amphibians whose sporangia grow in the keratin layer of the skin. It is not yet well known why the disease is fatal but one possible explanation is that the sporangia may release fatal toxins that are absorbed by the semi-permeable skin. It is also possible that the disease damages the epidermis which in turn affects water and electrolyte balance (2006).

Conclusion:
Poison arrow dart frogs are extremely small organisms with potent and potentially lethal poisons which this species has evolved the ability to store for protection. They are a useful organism for example, like many other amphibian species, dart frogs play an important role in controlling pests. Their poisons continue to be an important aspect of the hunting habits of the Choc— and Ember‡ Choc— of western Columbia. They can also play a major role in assisting human kind with medical advances yet they are threatened with dangers that can potentially seize their existence and any chance for medical advances. Protection, conservation and a better knowledge base of these and many other organisms may be the only prevention to their extinction.

References:
Berti, J. 2005. Poison Dart Frogs. Toxic Animals around the World. University of Wisconsin-Eau Claire. http://www.uwec.edu/piercech/animals/frog.htm

Boston, M. 2006. Oas Safari: Jewels of the Forest. Costa Rica Inn Keepers Association. http://www.lookout-inn.com/articles/jewels.php

Cameron, J. Brawley, K. 1998. ABT-594. Department of Chemistry. University of Aberdeen. http://www.abdn.ac.uk/chemistry/abt/

Carr, R. 2000. The Ecological Significance of Lipophilic Alkaloids in the Dendrobatidae. Department of Entomology. Colorado State. www.colostate.edu

Dorcas, M. 2000. Poison Dart Frogs and Their Toxins. Animal Physiology. Davidson College. http://www.bio.davidson.edu

Hamlett, L. 2006. Poison Arrow Frogs. Nashville Zoo at Grassmere. http://www.nashvillezoo.org/blfrog.htm

Patocka, J. Wulff, K.S. Marini Palomeque, M. V. 1999. Dart Poison Frogs an Their Toxins. Department of Toxicology, Military Academy Masaryk University. Applied Sciences and Analysis. 1057-9419

Starr, C. Taggart, R. 1998. Biology: The Unity and Diversity of Life. Eight Edition Wadsworth Publishing Company. Belmont CA.

2002. Sodium Channels. Ion Channels in Biological Membranes. http://www.chemsoc.org/ExemplarChem/entries/2002/Tim_Smith/channels/sodium/

2006. National Zoo Scientists Help to Create a Safety Net for Frogs, which are Declining Around the World at Alarming Rates. Conservation and Science. National Zoological Park.


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