With their large, hairy bodies and long legs, tarantulas are an arachnophobe’s worst nightmare. For pain researchers, however, these outsized spiders are a dream come true: Their venom contains a cocktail of toxins, each of which activates pain-sensing nerve fibres in different ways, and researchers in the United States have now identified one such toxin that will help them to better understand pain, and could also lead to treatments for the chronic pain associated with irritable bowel syndrome.
Physical pain signals are transmitted from the body to the brain by specialised sensory neurons called nociceptors. These pain-sensing neurons have cell bodies located just outside the spinal cord, and possess a single conductive fibre that splits in two, with one branch extending out towards the skin surface, and the shorter one entering the back of the cord.
Nociceptors synthesize a wide variety of membrane proteins that are sensitive to different types of painful stimuli – such as excessive mechanical pressure, painfully hot and cold temperatures, and the noxious chemicals that spill out of damaged cells. These proteins form pores that span the nerve cell membrane, which open in response to the appropriate stimulus, allowing electrical current, in the form of sodium, potassium, and other ions, to flow in or out of the cell. This triggers nervous impulses that are propagated into the spinal cord and then relayed up to the brain.
Different subsets of nociceptors respond to different combinations of stimuli, depending on which protein sensors are embedded in their nerve terminal membrane, and animal venoms act on these proteins in some way or another. Tetrodotoxin for example, isolated the puffer fish, blocks the sodium channels that all nerve cells use to generate nervous impulses, causing paralysis and death, and has been invaluable for helping researchers determine how impulses generated, and for understanding the role that sodium channels play in pain transmission.
In 2010, David Julius of the University of California in San Francisco and his colleagues reported that a toxin isolated from the venom of the Earth Tiger tarantula acts by binding to the external portion of the heat-sensitive TRPV1 protein and trapping the pore in its open state.
For their latest study, Julius and his colleagues collected about 100 venoms from different spider, scorpion and centipede species, and added them one by one to sensory neurons isolated from rats and mice and kept alive in Petri dishes. They then used calcium imaging to determine which ones evoke increases in calcium ion concentration – which is indicative of neuronal activity – within the cells.
One of the venoms they tested – from the Togo starburst tarantula (Heteroscodra maculata) – excited subsets of sensory neurons. The researchers analysed the venom, and found that it contains two toxins, which they named delta-theraphotoxin-Hm1a and delta-theraphotoxin-Hm1b. They also found that a synthetic version of Hm1a evoked robust responses in cultured sensory neurons, showing that it is an active component of the venom, and that tetrodotoxin blocks the increases in calcium ion concentration it produces, suggesting that the toxin acts on sodium channels.
Nociceptors express several different of sodium channels, but pain researchers have focused their attention on three subtypes called Nav1.7, Nav1.8 and Nav1.9, mutations in which are associated with congenital insensitivity to pain and various persistent pain syndromes. But Julius and his colleagues used compounds that selectively block each sodium channel subtype to establish that Hm1a acts on the Nav1.1 channel, which has previously been implicated in autism, epilepsy, and Alzheimer’s Disease, but not in pain.
Antibody staining further revealed that Nav1.1 is found primarily in A-delta fibres, nociceptors with medium diameter, insulated fibres that respond selectively to mechanical pressure. Confirming this, the addition of small amounts of Hm1a to skin preparations increased the firing rate of these cells, as measured by microelectrodes, suggesting that the Nav1.1 channel contributes to mechanical pain.
Indeed, injecting the toxin into the hind paws of mice immediately produced furious licking or biting of the injected paw; it also hypersensitized the animals to mechanical, but not thermal, stimuli, and did not produce the inflammation associated with activation of uninsulated, small diameter nociceptors.
Hypersensitivity to mechanical stimuli plays a role in the chronic abdominal pain characteristic of irritable bowel syndrome (IBS), so the researchers examined gut-nerve preparations from mice and found, once again, that administering Hma1 to the preparations increased the firing rate of mechanically-responsive cells, and also significantly reduced their activation threshold.
Together, these results reveal a hitherto unrecognised role for Nav1.1 in nociceptors that are sensitive to mechanical pain. The researchers also used genetic engineering to characterise the region of the Nav1.1 protein which binds the tarantula toxin; this could help researchers to develop new drugs that alleviate the chronic pain experienced by IBS sufferers by selectively blocking the channel.
Osteen, J. D., et al. (2016). Selective spider toxins reveal a role for the Nav1.1 channel in mechanical pain. Nature, DOI: 10.1038/nature17976 [Abstract]