The efficacy of γ-aminobutyric acid type A receptor (GABA AR) subtype-selective positive allosteric modulators in blocking tetramethylenedisulfotetramine (TETS)-induced seizure-like behavior in larval zebrafish with minimal sedation

Paige C Mundy,Brandon Pressly,Dennis R Carty,Bianca Yaghoobi,Heike Wulff,Pamela J Lein

doi:10.1016/j.taap.2021.115643

Paige C Mundy, Brandon Pressly + Show 4 more

Open Access

https://doi.org/10.1016/j.taap.2021.115643

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Abstract

The chemical threat agent tetramethylenedisulfotetramine (TETS) is a γ-aminobutyric acid type A receptor (GABA AR) antagonist that causes life threatening seizures. Currently, there is no specific antidote for TETS intoxication. TETS-induced seizures are typically treated with benzodiazepines, which function as nonselective positive allosteric modulators (PAMs) of synaptic GABAARs. The major target of TETS was recently identified as the GABAAR α2β3γ2 subtype in electrophysiological studies using recombinantly expressed receptor combinations. Here, we tested whether these in vitro findings translate in vivo by comparing the efficacy of GABAAR subunit-selective PAMs in reducing TETS-induced seizure behavior in larval zebrafish. We tested PAMs targeting α1, α2, α2/3/5, α6, ß2/3, ß1/2/3, and δ subunits and compared their efficacy to the benzodiazepine midazolam (MDZ). The data demonstrate that α2- and α6-selective PAMs (SL-651,498 and SB-205384, respectively) were effective at mitigating TETS-induced seizure-like behavior. Combinations of SB-205384 and MDZ or SL-651,498 and 2–261 (ß2/3-selective) mitigated TETS-induced seizure-like behavior at concentrations that did not elicit sedating effects in a photomotor behavioral assay, whereas MDZ alone caused sedation at the concentration required to stop seizure behavior. Isobologram analyses suggested that SB-205384 and MDZ interacted in an antagonistic fashion, while the effects of SL-651,498 and 2–261 were additive. These results further elucidate the molecular mechanism by which TETS induces seizures and provide mechanistic insight regarding specific countermeasures against this chemical convulsant.

Highlights

Tetramethylenedisulfotetramine (TETS) is considered a credible chemical threat agent that poses a serious public health risk to military personnel and civilians (Jett and Spriggs, 2020)
Despite a worldwide ban on its production, TETS is still available on the black market as a rodenticide, and the weaponization of TETS together with accidental and suicidal poisonings has caused a significant number of human deaths (Li et al, 2014; Zhang et al, 2011; Zhou et al, 2011)
At 5 dpf, zebrafish were exposed to 4 μM TETS, a concentration previously shown to induce seizure-like activity in Tropical 5D wildtype zebrafish larvae (Bandara et al, 2020) for 20 min, and subsequently treated with varying concentrations of GABAAR subtype-selective positive allosteric modulators (PAMs)

Summary

Introduction

Tetramethylenedisulfotetramine (TETS) is considered a credible chemical threat agent that poses a serious public health risk to military personnel and civilians (Jett and Spriggs, 2020). The GABAAR subtype selectivity of TETS has been previously explored using whole-cell patch-clamp analysis of recombi nantly expressed human synaptic and extrasynaptic GABAARs (Pressly et al, 2018). From these experiments, the major target of TETS was proposed to be α2β3γ2 (Pressly et al, 2018), which makes up 15–20% of the GABAARs in the mammalian brain (Rudolph and Knoflach, 2011). TETS inhibits α2β3γ2 receptors by binding to the non-competitive antagonist site in the channel pore (Pressly et al, 2020). All other tested GABAARs were significantly less sensitive except the α6β3γ2 isoform, which was blocked by sub-micromolar TETS concentrations (Pressly et al, 2018)

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