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Physiology

Channelled pain

It seems an unlikely connection. But as they report in this issue, David Julius and colleagues have identified a common factor in the way that a tarantula spider and a chilli plant defend themselves (J. Siemens et al. Nature 444, 208–212; 2006).

The particular tarantula is Psalmopoeus cambridgei (pictured), a native of the West Indies. From investigations of the spider's venom, Julius and colleagues have identified new members of the 'inhibitor cysteine knot', or ICK, family of peptide toxins.

The newly isolated peptides are dubbed 'vanillotoxins' because of their action on the TRPV1 vanilloid receptor, and they cause pain and inflammation when injected into mice. TRPV1 is a member of the TRP group of cell-membrane ion channels and was first identified through its sensitivity to capsaicin, the red-hot component of chilli peppers. Unlike almost every other ICK peptide, however, vanillotoxins activate rather than inhibit their target — in this case, TRPV1 channels in sensory neurons.

Credit: N. UPTON/NATUREPL.COM

An obvious question is whether the excitatory effect on TRPV1 means that the vanillotoxins have a different mechanism of action from the other ICK peptides. A comparison with ICK peptides that have similar amino-acid sequences — hanatoxins and heteroscodratoxins, which inhibit voltage-gated potassium channels — suggests that they don't. For instance, two of the vanillotoxins can also inhibit voltage-gated potassium channels; like hanatoxins and heteroscodratoxins, they impede the channel mechanism so that larger changes in voltage are required for the channel to open. Parenthetically, then, these data lend further support to the idea that TRP channels and voltage-gated potassium channels are structurally related.

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