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Relationship: 666

Title

A descriptive phrase which clearly defines the two KEs being considered and the sequential relationship between them (i.e., which is upstream, and which is downstream). More help

Binding at picrotoxin site, iGABAR chloride channel leads to Reduction, Ionotropic GABA receptor chloride channel conductance

Upstream event

The causing Key Event (KE) in a Key Event Relationship (KER). More help

Binding at picrotoxin site, iGABAR chloride channel

Downstream event

The responding Key Event (KE) in a Key Event Relationship (KER). More help

Reduction, Ionotropic GABA receptor chloride channel conductance

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes.Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

AOP Name	Adjacency	Weight of Evidence	Quantitative Understanding	Point of Contact	Author Status	OECD Status
Binding to the picrotoxin site of ionotropic GABA receptors leading to epileptic seizures in adult brain	adjacent	High	High	Ping Gong (send email)	Open for citation & comment	WPHA/WNT Endorsed

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) that help to define the biological applicability domain of the KER.In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER. More help

Term	Scientific Term	Evidence	Link
human	Homo sapiens	High	NCBI
zebrafish	Danio rerio	High	NCBI
rat	Rattus norvegicus	High	NCBI
Drosophila melanogaster	Drosophila melanogaster	High	NCBI

Sex Applicability

An indication of the the relevant sex for this KER. More help

Life Stage Applicability

An indication of the the relevant life stage(s) for this KER. More help

Key Event Relationship Description

Provides a concise overview of the information given below as well as addressing details that aren’t inherent in the description of the KEs themselves. More help

Acting as the major inhibitory neurotransmitter receptors, the ionotropic GABA receptors (iGABARs) are ligand-gated ion channels (LGICs) (Carpenter et al. 2013). Upon binding of an agonist (e.g., GABA), the iGABAR opens and increases the intraneuronal concentration of chloride ions, thus hyperpolarizing the cell and inhibiting the transmission of the nerve action potential. iGABARs also contain many other modulatory binding pockets that differ from the agonist-binding site. The picrotoxin-binding site is a noncompetitive channel blocker site located at the cytoplasmic end of the transmembrane channel (Olsen 2015). Binding to this pocket blocks GABA-induced chloride current, hence reduces chloride conductance.

Evidence Collection Strategy

Include a description of the approach for identification and assembly of the evidence base for the KER. For evidence identification, include, for example, a description of the sources and dates of information consulted including expert knowledge, databases searched and associated search terms/strings. Include also a description of study screening criteria and methodology, study quality assessment considerations, the data extraction strategy and links to any repositories/databases of relevant references.Tabular summaries and links to relevant supporting documentation are encouraged, wherever possible. More help

Evidence Supporting this KER

Addresses the scientific evidence supporting KERs in an AOP setting the stage for overall assessment of the AOP. More help

Biological Plausibility

Addresses the biological rationale for a connection between KEupstream and KEdownstream. This field can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured. More help

The mechanisms for noncompetitive picrotoxin site binding-induced reduction in chloride conductance have been investigated intensively for several decades. The consensus has been reached with ample support of computational and experimental evidence. Noncompetitive channel blockers fit the 2' to 9' pore region forming hydrogen bonds with the T6' hydroxyl and hydrophobic interactions with A2', T6' and L9' alkyl substituents (Chen et al. 2006), which is the primary binding site in the chloride channel lumen lined by five TM2 segments, thereby blocking the channel. Recent evidence suggests that there also exists a secondary modulatory pocket at the interface between the ligand-binding domain and the transmembrane domain of the iGABAR (Carpenter et al. 2013). It is believed that the two mechanisms mediate the blockage of chloride conductance (Yoon et al. 1993; Carpenter et al. 2013).

Empirical Evidence

Provides specific (citable) evidence that supports the idea of a change in the upstream KE (KEupstream) leading to, or being associated with, a subsequent change in the downstream KE (KEdownstream), assuming the perturbation of KEupstream is sufficient. More help

Numerous pharmacological and computational studies have lent strong support for this relationship. For instance, picrotoxin, applied intracellularly, was capable of blocking GABA-activated chloride current (Akaike et al. 1985). Recently, a computational study using homology modeling, docking and molecular dynamics simulation methods revealed that difference in binding affinity of fipronil with different iGABARs may lead to differential toxicity (potency) (Zheng et al. 2014).

Uncertainties and Inconsistencies

Addresses inconsistencies or uncertainties in the relationship including the identification of experimental details that may explain apparent deviations from the expected patterns of concordance. More help

As a heteropentameric receptor, the iGABAR consists of five protein subunits arranged around a central pore that form an ion channel through the membrane. The subunits are drawn from a pool of 19 distinct gene products, including six alpha, three beta, and three gamma subunits. The high diversity of subunit genes, in combination with alternative splicing and editing, leads to an enormous variety and, consequently, variability in function and sensitivity. This constitutes the main source of uncertainties.

Known modulating factors

This table captures specific information on the MF, its properties, how it affects the KER and respective references.1.) What is the modulating factor? Name the factor for which solid evidence exists that it influences this KER. Examples: age, sex, genotype, diet 2.) Details of this modulating factor. Specify which features of this MF are relevant for this KER. Examples: a specific age range or a specific biological age (defined by...); a specific gene mutation or variant, a specific nutrient (deficit or surplus); a sex-specific homone; a certain threshold value (e.g. serum levels of a chemical above...) 3.) Description of how this modulating factor affects this KER. Describe the provable modification of the KER (also quantitatively, if known). Examples: increase or decrease of the magnitude of effect (by a factor of...); change of the time-course of the effect (onset delay by...); alteration of the probability of the effect; increase or decrease of the sensitivity of the downstream effect (by a factor of...) 4.) Provision of supporting scientific evidence for an effect of this MF on this KER. Give a list of references. More help

Quantitative Understanding of the Linkage

Captures information that helps to define how much change in the upstream KE, and/or for how long, is needed to elicit a detectable and defined change in the downstream KE. More help

Is it known how much change in the first event is needed to impact the second? There is no study that quantitatively measured both receptor binding affinity and inhibition of chloride flux.

Are there known modulators of the response-response relationships? There is no known modulator that acts in between receptor binding and channel blocking, even though there are many binding sites other than the picrotoxin-binding sites that may affect chloride conductance.

Are there models or extrapolation approaches that help describe those relationships? No, however, there exist computational models based on 3D structure modeling that have been used to predict the binding affinity of ligands/chemicals at specific pockets of the ion channel (Yoon et al. 1993; Zheng et al. 2014).

Response-response Relationship

Provides sources of data that define the response-response relationships between the KEs. More help

Time-scale

Information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). More help

Known Feedforward/Feedback loops influencing this KER

Define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits. More help

Domain of Applicability

A free-text section of the KER description that the developers can use to explain their rationale for the taxonomic, life stage, or sex applicability structured terms. More help

Due to the universal existence of iGABARs in the animal kingdom, it would be a very long list of studies that provide supporting evidence with regard to taxonomic applicability of this key event relationship. The following are two examples: Williams et al. (2011) determined the binding affinity of RDX to the picrotoxin-binding site and the blockage of GABA_A receptor-mediated currents in the rat amygdala; Grolleau and Sattelle (2000) reported a complete blocking of inward current by 100 μM picrotoxin in the wild-type RDL (iGABAR) of Drosophila melanogaster.

References

List of the literature that was cited for this KER description. More help

Akaike N. Hattori K, Oomura Y, Carpenter D0. 1985. Bicuculline and picrotoxin block γ-aminobutyric acid-gated Cl-conductance by different mechanisms. Experientia 41:70-71.

Carpenter TS, Lau EY, Lightstone FC. 2013. Identification of a possible secondary picrotoxin-binding site on the GABA(A) receptor. Chem Res Toxicol. 26(10):1444-54.

Chen L, Durkin KA, Casida J. 2006. Structural model for gamma-aminobutyric acid receptor noncompetitive antagonist binding: widely diverse structures fit the same site. Proc Natl Acad Sci USA, 103(13):5185-5190.

Grolleau F, Sattelle DB. 2000. Single channel analysis of the blocking actions of BIDN and fipronil on a Drosophila melanogaster GABA receptor (RDL) stably expressed in a Drosophila cell line. Br J Pharmacol. 130(8):1833-42.

Olsen RW. 2015. Allosteric ligands and their binding sites define γ-aminobutyric acid (GABA) type A receptor subtypes. Adv Pharmacol. 73:167-202.

Williams LR, Aroniadou-Anderjaska V, Qashu F, Finne H, Pidoplichko V, Bannon D I et al. 2011. RDX binds to the GABA(A) receptor-convulsant site and blocks GABA(A) receptor-mediated currents in the amygdala: a mechanism for RDX-induced seizures. Environ Health Perspect. 119(3):357-363.

Yoon KW, Covey DF, Rothman SM. 1993. Multiple mechanisms of picrotoxin block of GABA-induced currents in rat hippocampal neurons. J Physiol. 464:423-39.

Zheng N, Cheng J, Zhang W, Li W, Shao X, Xu Z, Xu X, Li Z. 2014. Binding difference of fipronil with GABA(A)Rs in fruitfly and zebrafish: Insights from homology modeling, docking, and molecular dynamics simulation studies. J Agric Food Chem. 62:10646-53.