Tarantula bites activate the same pain receptor as chilli peppers.
Credit: iStockphoto
SYDNEY: What's the difference between a tarantula bite and chewing on a chilli? Probably less than you thought, according to a new U.S. study.
"The central finding of this paper is that we've determined how a spider, this one tarantula, elicits a pain response by synthesising molecules," said co-author David Julius from the University of California in San Francisco. "These small peptides activate this one receptor … that is also activated by the pungent ingredients in chilli peppers."
In the study, which is published today in the British journal Nature, the researchers purified the venom from a tarantula native to the West Indies and identified three components that interacted with the chilli pain receptor. These closely related molecules were found to be a kind of peptide, the building blocks of amino acids, which the researchers named vanillotoxins.
Vanillotoxins belong to a larger group of peptides which all target the capsaicin receptor, TRPV1, according to Julius. Capsaicin is the chemical in chilli peppers and other plants that activates pain and the burning sensation familiar to curry lovers.
The peptides were injected into the hind paws of normal mice and other mice that had been genetically engineered to lack the receptor, said Julius. Mice with the TRP receptors flinched and licked at the paw after separate injections of capsaicin and spider venom. The paw also swelled up, indicating that the pain and inflammation pathway had been sparked. The control mice showed a minimal reaction.
Plants and animals alike deter predators by producing chemical irritants - capsaicin and vanillotoxins, respectively, explained Julius. The mechanisms for pain production, however, remain to be discovered.
While most toxins work by blocking different types of channels to induce pain and, in some cases, paralysis, the research team predicted vanillotoxins would activate the channels instead. The study confirmed this hypothesis, finding that these toxins follow an 'excitatory' pathway. "That's unusual about this family of toxin molecules," said Julius.
The next step is to research how these channels actually activate pain. "In the long run, people want to know something about how these channels work because, one, they're fascinating little machines, but also they are involved in pain response," said Julius.
"So, one of the goals, at least from a clinical perspective, is to identify ways to modulate these receptors in some context of inflammatory pain, where blocking these channels and preventing them from acting may have some therapeutic benefit."
