By binding to the receptor, the venom molecules cause it to change shape and allow ions to pass through a cell's membrane. This in turn sends pain messages to the brain.
The research team says other tarantula species and venomous spiders may also use neurotoxins in this way to ward off predators or competitors.
"This would parallel mechanisms adopted by numerous plant species to deter predatory mammals through the production of chemical irritants," the team wrote.
In peppers, the defense mechanism appears to be targeted specifically at predators that don't benefit the plant. Bird species that are effective at dispersing the seeds inside peppers seem to be immune to the effects of capsaicin.
Inflicting pain is only one function of spider venom, however, scientists point out.
Venom injected via a spider's fangs acts in various other ways, such as to kill or immobilize prey and to begin the process of digesting its meal.
Wayne Hodgson, head of the Monash Venom Research Laboratory at Monash University in Australia, says animal venoms are made up of a cocktail of toxins with different actions, which when combined are often more effective than single compounds.
"Biological activity is often complex, and individual toxins may have a subtle role," he said. "However, there is certainly a synergistic effect when these toxins act in concert."
Hodgson says a number of these toxins appear to be designed to cause or intensify feelings of pain.
"I am not surprised that this tarantula venom may also have that capability," he said. "Causing pain in predators is certainly one way to avoid being eaten."
The study of such spider venoms is extremely important in the development of new therapeutic drugs, Hodgson adds.
"These animals have evolved their venoms over many millions of years, and they contain highly potent and selective toxins which can target physiological processes in humans," he said. (Related: "Toxic Snail Venoms Yielding New Painkillers, Drugs" [June 14, 2005].)
For instance, scientists have discovered that the venom of the Chilean rose tarantula contains toxin molecules that have a similar action as anesthetics.
Researchers at the University at Buffalo in New York found that the compounds block the mechanical activity of some cells, a discovery which may lead to new treatments for conditions ranging from muscular dystrophy to irregular heartbeat.
And a recent French study described two new toxins from the Trinidad chevron tarantula—the same species used in the chili study—that are active against plasmodia, the parasite that causes malaria. The research raises hopes of new treatments for the disease.
The growing interest in spiders from the medical research community has even spawned dedicated suppliers such as Spider Pharm in Arizona, which sells a wide variety of spider venoms. (Related story: "Spider-Venom Profits to Be Funneled Into Conservation" [August 12, 2004].)
"Remembering that these animals represent little chemistry labs," Hodgson said, "we would be stupid not to explore their potential."
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