Upstream eventReduction, Ionotropic GABA receptor chloride channel conductance
Reduction, Neuronal synaptic inhibition
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Directness||Weight of Evidence||Quantitative Understanding|
|Binding to the picrotoxin site of ionotropic GABA receptors leading to epileptic seizures||directly leads to||Strong||Strong|
|guinea pig||Cavia porcellus||Strong||NCBI|
Life Stage Applicability
How Does This Key Event Relationship Work
A decline in conductance through chloride channels in iGABARs causes a reduction in GABA-mediated inhibition of neuronal synaptic signalling, which is reflected as decreased frequency and amplitude of iGABAR-mediated spontaneous inhibitory postsynaptic currents or abolishment of GABA-induced firing action (Newland and Cull-Candy 1992). For instance, whole-cell in vitro recordings in the rat basolateral amygdala (BLA) showed that RDX reduces the frequency and amplitude of GABAA receptor mediated spontaneous inhibitory postsynaptic currents (sIPSCs) and the amplitude of GABA-evoked postsynaptic currents, whereas in extracellular field recordings from the BLA, RDX induced prolonged, seizure-like neuronal discharges (Williams et al. 2011). These pieces of cellular level evidence support that binding to the GABAA receptor convulsant site is the primary mechanism of seizure induction by RDX and that the key event of reduction of GABAergic inhibitory transmission in the amygdala is involved in the generation of RDX-induced seizures.
Weight of Evidence
Chloride channels play an important role in regulating neuronal excitability, especially in the context of fast synaptic inhibition mediated by GABAA receptors. But in order for chloride channels to reduce excitability, chloride driving force must be maintained to keep a dynamic balancing of chloride influx and efflux, which also involves a variety of other ion species (Prescott 2014). If chloride regulation is compromised, the efficacy of fast synaptic inhibition can be compromised with adverse effects such as reduced neuronal inhibition.
Empirical Support for Linkage
The GABAA receptor is part of a larger GABA/drug receptor-Cl− ion channel macromolecular complex. An integral part of this complex is the Cl− channel. The binding sites localized in or near the Cl− channel for GABA, benzodiazepines, barbiturates, picrotoxin and anesthetic steroids modulate receptor response to GABA stimulation. The GABA-binding site is directly responsible for opening the Cl− channel. Electrophysiological studies of the GABAA-receptor complex indicate that it mediates an increase in membrane conductance with an equilibrium potential near the resting level of −70 mV. This conductance increase is often accompanied by a membrane hyperpolarization, resulting in an increase in the firing threshold and, consequently, a reduction in the probability of action potential initiation, causing neuronal inhibition (Olsen and DeLorey 1999). This reduction in membrane resistance is accomplished by the GABA-dependent facilitation of Cl− ion influx through a receptor-associated channel.
Channel blockers, such as the convulsant compound picrotoxin, cause a decrease in mean channel open time. Picrotoxin acts on the gating process of the GABAA receptor channel and works by preferentially shifting opening channels to the briefest open state (1 msec). Experimental convulsants like pentylenetetrazol and the cage convulsant t-butyl bicyclophosphorothionate (TBPS) act in a manner similar to picrotoxin, preventing Cl− channel permeability (Macdonald and Olsen 1994).
Uncertainties or Inconsistencies
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.
Quantitative Understanding of the Linkage
Is it known how much change in the first event is needed to impact the second? Yes, but very few studies reported changes in both events. One examples is Williams et al. (2011), where whole-cell in vitro recordings in the rat basolateral amygdala (BLA) showed that RDX reduced the frequency and amplitude of spontaneous GABAA receptor–mediated inhibitory postsynaptic currents and the amplitude of GABA-evoked postsynaptic currents, whereas in extracellular field recordings from the BLA, RDX induced prolonged, seizure-like neuronal discharge.
Are there known modulators of the response-response relationships? There is no known modulator that acts in between chloride channel conductance decrease and neuronal inhibition reduction, even though there are many other players such as potassium-chloride cotransporters and sodium-potassium-chloride cotransporters that may affect chloride flux/homeostasis and electrochemical gradient (Prescott 2014), leading to changes in postsynpatic neuronal inhibition.
Are there models or extrapolation approaches that help describe those relationships? No.
Evidence Supporting Taxonomic Applicability
iGABARs and synaptic neurons are present in animals throughout the animal kingdom, therefore this event is applicable to a wide range of species from earthworm to humans. This relationship has been shown directly in rats ( Williams et al. 2011) and guinea pig (Juarez et al. 2013).
Juarez E H, Ochoa-Cortes F, Miranda-Morales M, Espinosa-Luna R, Montano L M, Barajas-Lopez C. Selectivity of antagonists for the Cys-loop native receptors for ACh, 5-HT and GABA in guinea-pig myenteric neurons. Auton Autacoid Pharmacol 2013; 34(1-2):1-8.
Macdonald R L, Olsen R W. GABAA receptor channels. Annu. Rev. Neurosci. 1994;17:569–602.
Newland C F, Cull-Candy S G. On the mechanism of action of picrotoxin on GABA receptor channels in dissociated sympathetic neurones of the rat. J Physiol 1992; 447: 191–213.
Olsen R W, DeLorey T M. Chapter 16. GABA and Glycine: GABA Receptor Physiology and Pharmacology. In: Siegel GJ, Agranoff BW, Albers RW, et al. (Eds), Basic Neurochemistry: Molecular, Cellular and Medical Aspects (6th edition), Philadelphia: Lippincott-Raven; 1999.
Prescott S A. Chloride channels. In: Jaeger D and Jung R (Eds.), Encyclopedia of Computational Neuroscience, Springer, New York, 2014. pp.1-4.
Williams L R, Aroniadou-Anderjaska V, Qashu F, Finne H, Pidoplichko V, Bannon D I et al. 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 2011; 119(3):357-363.