Depolarization Differentially Affects Allosteric Modulation by Neurotoxins of Scorpion α‐Toxin Binding on Voltage‐Gated Sodium Channels

Abstract

Abstract:Voltage-gated sodium channels serve as a target for many neurotoxins that bind to several distinct, allosterically interacting receptor sites. We examined the effect of membrane potentials (incited by increasing external K~concentrations) on the binding modulation by veratridine, brevetoxin, and tetrodotoxin of the scorpion a-toxin AaH II to receptor site 3 on sodium channels of rat brain synaptosomes. Depolarization is shown to differentially modulate neurotoxin effects on AaH II binding: Veratridine increase is potentiated, brevetoxin's inhibitory effect is reduced, and tetrodotoxin enhancement is evident mainly at resting membrane potential (5 mM K~). Both tetrodotoxin and veratridine apparently reverse the inhibition of AaH II binding by brevetoxin at resting membrane potential, but only veratridine is able to partially restore AaH II binding at 0 mV (135 mM K~). Thus, the allosteric interactions are grouped into two categories, depending on the membrane potential. Under depolarized conditions, the cooperative effects among veratridine and brevetoxin on AaH II binding fit the previously described two-state conformational model. At resting membrane potential, additional interactions are revealed, which may be explained by assuming that toxin binding induces conformational changes on the channel structure, in addition to being state-dependent. Our results provide a new insight into neurotoxin action and the complex dynamic changes underlying allosteric coupling of neurotoxin receptor sites, which may be related to channel gating. Key Words: Allosteric interactionsNeurotoxins-Sodium channels-Scorpion a-toxinBrevetoxin.

Development of Resistance to the Effect of Local Anesthetic Agents Administered Via Various Routes Due to Single or Multiple, Previous Scorpion Bites: A Proposed Hypothesis and Reporting a Yet Unrecognized Phenomenon

“…Recently on the lines of our findings and supporting our hypothesis, more evidence has emerged, where, Issin A [23] has reported a case of "Local anesthetic resistance in a young woman with history of scorpion bite" [24]. We are of the opinion as more and more researchers and clinicians become sensitized and aware, about our hypothesis, they will be able to correlate, it with their own experiences and report this elusive phenomenon.…”

“…The more the number of bites and more recent the bite, more the chances of failure, patchy/incomplete the block, delayed onset of sensory and/or motor blockade, thus requiring either replacement with balanced general anesthesia or supplementation with some adjuvant like sedation. We firmly believe, it has immunological basis and the mechanism of this 'resistance' involves possible 'competitive antagonism' at the receptor site, the 6 th segment of domain IV of alpha subunit of the sodium channels in the peripheral nerves, the common binding site for both the scorpion venom and the local anesthetics agents(IV-S6) [1,24,25].…”

Effect of Depolarization on Binding Kinetics of Scorpion α-Toxin Highlights Conformational Changes of Rat Brain Sodium Channels

“…Lqh-II is highly similar in effects and binding properties to previously studied scorpion R-toxins (e.g., LqTx or Lqq-V from Leiurus qinquestriatus qinquestriatus, 15,21,26,[30][31][32] and is almost identical in sequence to Aah-II, from Androctonus australis hector (18,29,33,38). Synaptosomes are capable of retaining a resting membrane potential due to a passive K + efflux, which can be modulated in the range of -55 to 0 mV by changing the external K + concentration (32,33,(39)(40)(41).…”

“…The saturable binding of these R-toxins is reduced by ∼90% upon membrane depolarization with 90-135 mM K + (membrane potential of ∼0 mV; 29,32,33,39,48). It has been demonstrated that the decrease in binding affinity in high external [K + ] was due to its effect on membrane depolarization and not a direct effect on scorpion R-toxin binding, as equivalent changes in R-toxin affinity occurred when membrane potential was depolarized by various methods (21,30,32,33).…”

“…In turn, the bioactive surface of site 3 toxins contains positively charged residues (22,24,25). Binding of scorpion R-toxins has been useful in probing subtle conformational changes at the channel surface that occur during voltage-dependent gating as well as during modulation provoked by other groups of neurotoxins (15,16,(26)(27)(28)(29). However, the structural basis of the voltage dependency in binding of site 3 toxins and the mechanism by which binding to channel external regions affects fast inactivation, which is presumably mediated by intracellular protein components, is still elusive.…”