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Voltage-gated sodium channel NaV1.8 regulates transmission of pain signals to the brain. While NaV1.8 has the potential to serve as a drug target, the molecular mechanisms that shape NaV1.8 gating are not completely understood, particularly mechanisms that couple activation to inactivation. Interactions between toxin producing animals and their predators provide a novel approach for investigating NaV structure-function relationships. Arizona bark scorpions produce Na+ channel toxins that initiate pain signaling. However, in predatory grasshopper mice, toxins inhibit NaV1.8 currents and block pain signals. A screen of synthetic peptide toxins predicted from bark scorpion venom showed that peptide NaTx36 inhibited Na+ current recorded from a recombinant grasshopper mouse NaV1.8 channel (OtNaV1.8). Toxin NaTx36 hyperpolarized OtNaV1.8 activation, steady-state fast inactivation, and slow inactivation. Mutagenesis revealed that the first gating charge in the domain I (DI) S4 voltage sensor and an acidic amino acid (E) in the DII SS2 - S6 pore loop are critical for the inhibitory effects of NaTx36. Computational modeling showed that a DI S1 - S2 asparagine (N) stabilizes the NaTx36 - OtNaV1.8 complex while residues in the DI S3 - S4 linker and S4 voltage sensor form electrostatic interactions that allow a toxin glutamine (Q) to contact the first S4 gating charge. Surprisingly, the models predicted that NaTx36 contacts amino acids in the DII S5 - SS1 pore loop instead of the SS2 - S6 loop; the DII SS2 - S6 loop motif (QVSE) alters the conformation of the DII S5 - SS1 pore loop, enhancing allosteric interactions between toxin and the DII S5 - SS1 pore loop. Few toxins have been identified that modify NaV1.8 gating. Moreover, few toxins have been described that modify sodium channel gating via the DI S4 voltage sensor. Thus, NaTx36 and OtNaV1.8 provide tools for investigating the structure-activity relationship between channel activation and inactivation gating, and the connection to alternative pain phenotypes.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.anbehav.2005.08.003 Byline: Ashlee H. Rowe, Matthew P. Rowe Abstract: Predators should benefit from assessing the risks posed by prey that differ in their dangerousness. Assessment might come at any phase of the predatory sequence: search/recognition, pursuit/attack and handling/subjugation. Grasshopper mice are voracious carnivores in North American deserts, feeding almost exclusively on arthropods. In certain regions, grasshopper mice co-occur with extremely neurotoxic bark scorpions, whose sting can prove lethal to vertebrates. We presented three different prey items to wild-caught grasshopper mice at several field sites in the southwestern U.S. The three items, listed in increasing order of dangerousness, were: laboratory crickets, Acheta domesticus, having few or no defences; two sister species of Vaejovis spp., a non-neurotoxic genus of scorpion; and two sister species of toxic Centruroides. Grasshopper mice made no distinctions among the prey in either the recognition or pursuit phases of the encounter, attacking crickets and both genera of scorpions with little hesitation. There were, however, significant differences in how the mice handled the three different prey types, with Centruroides requiring significantly more effort to subdue. The difficulties that mice had in dispatching Centruroides were not related to the neurotoxic components of these scorpions' venoms, to which the mice are resistant. Instead, the difficulties appeared to result from other constituents of Centruroides' venom that cause intense, short-term pain. Author Affiliation: (a) Department of Zoology, North Carolina State University, Raleigh, U.S.A. (a ) Department of Biology, Appalachian State University, Boone, NC, U.S.A. Article History: Received 1 December 2004; Revised 3 March 2005; Accepted 29 August 2005 Article Note: (miscellaneous) MS. number: A10052R

Voltage-gated sodium channel NaV1.8 regulates transmission of pain signals to the brain. While NaV1.8 has the potential to serve as a drug target, the molecular mechanisms that shape NaV1.8 gating are not completely understood, particularly mechanisms that couple activation to inactivation. Interactions between toxin producing animals and their predators provide a novel approach for investigating NaV structure-function relationships. Arizona bark scorpions produce Na+ channel toxins that initiate pain signaling. However, in predatory grasshopper mice, toxins inhibit NaV1.8 currents and block pain signals. A screen of synthetic peptide toxins predicted from bark scorpion venom showed that peptide NaTx36 inhibited Na+ current recorded from a recombinant grasshopper mouse NaV1.8 channel (OtNaV1.8). Toxin NaTx36 hyperpolarized OtNaV1.8 activation, steady-state fast inactivation, and slow inactivation. Mutagenesis revealed that the first gating charge in the domain I (DI) S4 voltage sensor and an acidic amino acid (E) in the DII SS2 – S6 pore loop are critical for the inhibitory effects of NaTx36. Computational modeling showed that a DI S1 – S2 asparagine (N) stabilizes the NaTx36 – OtNaV1.8 complex while residues in the DI S3 – S4 linker and S4 voltage sensor form electrostatic interactions that allow a toxin glutamine (Q) to contact the first S4 gating charge. Surprisingly, the models predicted that NaTx36 contacts amino acids in the DII S5 – SS1 pore loop instead of the SS2 – S6 loop; the DII SS2 – S6 loop motif (QVSE) alters the conformation of the DII S5 – SS1 pore loop, enhancing allosteric interactions between toxin and the DII S5 – SS1 pore loop. Few toxins have been identified that modify NaV1.8 gating. Moreover, few toxins have been described that modify sodium channel gating via the DI S4 voltage sensor. Thus, NaTx36 and OtNaV1.8 provide tools for investigating the structure-activity relationship between channel activation and inactivation gating, and the connection to alternative pain phenotypes. [ABSTRACT FROM AUTHOR]

Background: Among scorpion species, the Buthidae produce the most deadly and painful venoms. However, little is known regarding the venom components that cause pain and their mechanism of action. Using a paw-licking assay (Mus musculus), this study compared the pain-inducing capabilities of venoms from two species of New World scorpion (Centruroides vittatus, C. exilicauda) belonging to the neurotoxin-producing family Buthidae with one species of non-neurotoxin producing scorpion (Vaejovis spinigerus) in the family Vaejovidae. A pain-inducing a-toxin (CvIV4) was isolated from the venom of C. vittatus and tested on five Na+ channel isoforms. Principal Findings: C. vittatus and C. exilicauda venoms produced significantly more paw licking in Mus than V. spinigerus venom. CvIV4 produced paw licking in Mus equivalent to the effects of whole venom. CvIV4 slowed the fast inactivation of Nav1.7, a Na+ channel expressed in peripheral pain-pathway neurons (nociceptors), but did not affect the Nav1.8-based sodium currents of these neurons. CvIV4 also slowed the fast inactivation of Nav1.2, Nav1.3 and Nav1.4. The effects of CvIV4 are similar to Old World a-toxins that target Nav1.7 (AahII, BmK MI, LqhIII, OD1), however the primary structure of CvIV4 is not similar to these toxins. Mutant Nav1.7 channels (D1586A and E1589Q, DIV S3-S4 linker) reduced but did not abolish the effects of CvIV4. Conclusions: This study: 1) agrees with anecdotal evidence suggesting that buthid venom is significantly more painful than non-neurotoxic venom; 2) demonstrates that New World buthids inflict painful stings via toxins that modulate Na+ channels expressed in nociceptors; 3) reveals that Old and New World buthids employ similar mechanisms to produce pain. Old and New World &agr;-toxins that target Nav1.7 have diverged in sequence, but the activity of these toxins is similar. Pain-inducing toxins may have evolved in a common ancestor. Alternatively, these toxins may be the product of convergent evolution. [ABSTRACT FROM AUTHOR]

Voltage-gated sodium channel Na V 1.8 regulates transmission of pain signals to the brain. While Na V 1.8 has the potential to serve as a drug target, the molecular mechanisms that shape Na V 1.8 gating are not completely understood, particularly mechanisms that couple activation to inactivation. Interactions between toxin producing animals and their predators provide a novel approach for investigating Na V structure-function relationships. Arizona bark scorpions produce Na + channel toxins that initiate pain signaling. However, in predatory grasshopper mice, toxins inhibit Na V 1.8 currents and block pain signals. A screen of synthetic peptide toxins predicted from bark scorpion venom showed that peptide NaTx36 inhibited Na + current recorded from a recombinant grasshopper mouse Na V 1.8 channel (OtNa V 1.8). Toxin NaTx36 hyperpolarized OtNa V 1.8 activation, steady-state fast inactivation, and slow inactivation. Mutagenesis revealed that the first gating charge in the domain I (DI) S4 voltage sensor and an acidic amino acid (E) in the DII SS2 - S6 pore loop are critical for the inhibitory effects of NaTx36. Computational modeling showed that a DI S1 - S2 asparagine (N) stabilizes the NaTx36 - OtNa V 1.8 complex while residues in the DI S3 - S4 linker and S4 voltage sensor form electrostatic interactions that allow a toxin glutamine (Q) to contact the first S4 gating charge. Surprisingly, the models predicted that NaTx36 contacts amino acids in the DII S5 - SS1 pore loop instead of the SS2 - S6 loop; the DII SS2 - S6 loop motif (QVSE) alters the conformation of the DII S5 - SS1 pore loop, enhancing allosteric interactions between toxin and the DII S5 - SS1 pore loop. Few toxins have been identified that modify Na V 1.8 gating. Moreover, few toxins have been described that modify sodium channel gating via the DI S4 voltage sensor. Thus, NaTx36 and OtNa V 1.8 provide tools for investigating the structure-activity relationship between channel activation and inactivation gating, and the connection to alternative pain phenotypes., Competing Interests: TE-A was employed by Amsaal Venom Farm after data was collected for this project. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 George, Lopez-Mateos, Abd El-Aziz, Xiao, Kline, Bao, Raza, Stockand, Cummins, Fornelli, Rowe, Yarov-Yarovoy and Rowe.)