Meeting Abstract

63.5  Thursday, Jan. 6  Coevolution Between Bark Scorpion Pain-inducing Toxins and Grasshopper Mice Pain Receptors ROWE, A.H.*; ROWE, M.P.; ZAKON, H.H.; Univ. of Texas, Austin; Sam Houston State Univ., Huntsville; Univ. of Texas, Austin ahrowe@mail.utexas.edu

Because ion channels are necessary for neuronal transmission and muscle contraction, many animals have evolved chemical weapons that disrupt channel function. Disruption of channel function imposes strong selection on the victim, so it is not surprising that some victims evolve resistance to chemical weapons. Bark scorpion (Centruroides) venom contains Na⁺ and K⁺ channel-specific toxins that cause pain, paralysis, seizures and death. Painful stings force predators to drop these scorpions, allowing them to escape. Grasshopper mice (Onychomys) prey on scorpions. We measured grasshopper mice sensitivity to pain-inducing toxins by injecting venom samples into their hind paws and measuring the duration of paw licking for 15 minutes; house mice (Mus) were used as a control. Grasshopper mice licked their paws significantly less than house mice, suggesting the former have evolved insensitivity to pain-inducing toxins. To test that other painful stimuli elicit paw licking, we injected formalin into the hind paws of grasshopper mice and house mice. There was no difference between species in the duration of paw licking, suggesting that insensitivity to pain-inducing toxins is not an inability of grasshopper mice to perceive pain. Electrophysiological analyses demonstrated that bark scorpion toxins bind Nav1.7, a Na⁺ channel expressed in mammalian pain-sensing neurons that functions in pain perception. We sequenced the gene encoding Nav1.7 from grasshopper mice and compared the sequence with orthologs from other mammals. We identified four amino acid substitutions at highly conserved positions in grasshopper mice Nav1.7. We are currently testing whether these amino acid substitutions reduce the effects of toxins on Nav1.7.