Cellular and Molecular Life Sciences (2021) 78:5341–5370
https://doi.org/10.1007/s00018-021-03846-2
Cellular and Molecular Life Sciences
REVIEW
Electrophysiology of ionotropic GABA receptors
Erwan Sallard1,2
· Diane Letourneur3,4
· Pascal Legendre5
Received: 21 December 2020 / Revised: 2 April 2021 / Accepted: 23 April 2021 / Published online: 1 June 2021
© The Author(s) 2021
Abstract
GABAA receptors are ligand-gated chloride channels and ionotropic receptors of GABA, the main inhibitory neurotransmitter in vertebrates. In this review, we discuss the major and diverse roles GABAA receptors play in the regulation of neuronal
communication and the functioning of the brain. GABAA receptors have complex electrophysiological properties that enable
them to mediate different types of currents such as phasic and tonic inhibitory currents. Their activity is finely regulated by
membrane voltage, phosphorylation and several ions. GABAA receptors are pentameric and are assembled from a diverse
set of subunits. They are subdivided into numerous subtypes, which differ widely in expression patterns, distribution and
electrical activity. Substantial variations in macroscopic neural behavior can emerge from minor differences in structure
and molecular activity between subtypes. Therefore, the diversity of GABAA receptors widens the neuronal repertoire of
responses to external signals and contributes to shaping the electrical activity of neurons and other cell types.
Keywords Synaptic receptor · GABAA subtypes · Neurotransmitter · Neuronal inhibition · Phasic currents · Tonic activity
Introduction
γ-Aminobutyric acid (GABA) is one of the main neurotransmitters in virtually all Metazoans. It is the most widely distributed inhibitory neurotransmitter in the central nervous
system of mature vertebrates, being present in around 30%
of the synapses [1, 2].
GABA-mediated signals can be transduced in receiving
cells through metabotropic or ionotropic receptors located at
the plasma membrane. GABAB receptors are heterodimeric
metabotropic receptors coupled to potassium and calcium
* Erwan Sallard
erwan.sallard@ens.psl.eu
1
École Normale Supérieure de Paris, Biology Department,
PSL Research University, 45 rue d’Ulm, 75005 Paris, France
2
Department of Human Medicine, Faculty of Health, Center
for Biomedical Education and Research (ZBAF), Institute
for Virology and Microbiology, Witten/Herdecke University,
58453 Witten, Germany
3
INSERM U1015, Gustave Roussy, Villejuif, France
4
Master de Biologie, École Normale Supérieure de Lyon,
Université Claude Bernard Lyon I, Université de Lyon,
69342 Lyon Cedex 07, France
5
Sorbonne Université, UPMC Univ Paris 06, INSERM,
CNRS, Neurosciences Paris Seine, Institut de Biologie Paris
Seine (NPS, IBPS), 75005 Paris, France
channels through G proteins. On the other hand, GABAA
receptors are GABAergic pentameric chloride channels [3],
i.e. ionotropic GABA receptors, belonging to the cys-loop
family of ion channels [4]. Certain ionotropic GABA receptors were initially termed GABAC receptors [5], but they are
now classified as a subset of GABAA receptors [6].
GABAA receptors are found in all major taxa of bilateral
metazoans [7] and are among the most abundant neurotransmitter receptors [8]. They are mainly found in neurons in
synaptic, perisynaptic, and extrasynaptic locations. They can
also be found in non-neuronal cells as well as outside of the
nervous system.
The monomers constituting GABAA pentameric receptors
are drawn out of a large set of different subunits. Consequently, GABAA receptors can be made of different combinations of subunits and are subdivided into subtypes: each
subtype corresponds to a combination of subunits. Subtypes
can differ in their electrophysiology and their pharmacology.
The total number of functional subtypes existing in vivo may
reach several hundreds [9], although most receptors belong
to one or two dozens of major subtypes [10].
The conductance and current time-course of GABAergic
receptor-channels depend on their subunits composition,
the ionic conditions of their environment and the patterns
of GABA exposure they face. In particular, the differential
regulation, abundance and localization of GABAA receptor
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subtypes enable a very precise modulation and a great diversity of responses to GABA signals across the brain and the
organism, facilitating the numerous biological processes in
which GABAA receptors are involved.
Despite the complexity and biological significance of
GABAA receptors, recent reviews on their electrophysiology are missing. In this review, we summarize the current
knowledge on the electrophysiology of the different GABAA
receptor subtypes and show how their properties explain the
role they play in cellular and cerebral activity. We mainly
focus on mammals and on neuronal GABAA receptors.
Related subjects such as GABAA receptor pharmacology or
pathology will not be covered in this review; other reviews
have already been published about pharmacology [11–20],
the regulation of subunits gene expression [21–23], subunit intracellular trafficking [24, 25], non-neuronal GABAA
receptors [26–29], and pathology [30–32].
Diversity and structure of GABAA receptors
GABAA receptor subunits diversity
Mammals have 19 genes coding for a GABAA receptor subunit, classified in classes (α, β, γ, ρ, θ, ε, π and δ) and isoforms (α1–6, β1–3, γ1–3 and ρ1–3) [33, 34]. The diversity
of subunits is further increased by alternative splicing, to
which 9 out of 19 subunits are submitted [34, 35]. Alternative splicing enables the β2, γ2, γ3 and ρ1 subunit genes
to produce at least two mature proteins, but differences in
receptor activity depending on alternative splicing have been
reported only between the γ2L and γ2S isoforms of the γ2
subunit; the main function of alternative splicing is believed
to be the regulation of subunit expression [34].
Hundreds of millions of years ago, duplications of an
ancestral gene led to the emergence of the different classes
of subunits, and later to the different isoforms of a same
class. The amino acid identity rate between isoforms of the
same class ranges between 70 and 80%, whereas it typically
lies between 30 and 40% for subunits of different classes
[36].
Most subunits have remained very stable in the last tens
of millions of years of vertebrate evolution. For example, the
amino acid identity rate between mouse and human ortholog
subunits is above 90% (except ρ3, with 84% identity). The
conservation is not only structural but also functional: crustacean β subunits have been shown to functionally replace
human β subunits in chimeric receptors [7, 37]. Likewise,
deletion of a single GABAA receptor subunit often results
only in a mild pathology or no phenotype at all [38–42],
showing that different subunits can substitute each other.
These facts highlight the strong selective pressures applied
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on GABAA receptors, and suggest it has a substantial biological importance.
Subunits assemble into receptors whose structure
is finely regulated
Mature subunits are integral plasma membrane proteins.
Human subunits are 420–632 amino acid long and weigh
between 52 and 59 kDa [34, 36]. They contain an extracellular N-terminal segment of 220–250 residues, which can
be glycosylated; 4 transmembrane domains termed M1, M2,
M3 and M4; and a short extracellular C-terminal segment
(Fig. 1a).
A functional receptor contains five subunits (Fig. 1a),
assembled in a circle around a chloride-permeable pore
delineated by the M2 transmembrane domain of all five
subunits [8]. Each monomer is thus in contact with two other
subunits, respectively at its “principle”, or + , and “complementary”, or −, faces.
The study of structure–function relationships of GABAA
receptors has long been hindered by the substantial diversity
of conformations, making it difficult to stabilize a receptor
population in a single state [43], and cryo-EM structures of
GABAA receptors were only recently resolved (reviewed in
[44]). GABA-binding sites are located at interfaces between
subunits on the surface of the extracellular part of the receptor. GABA binding induces a conformation change, typically
when all binding sites are occupied, through a concerted
rotation of the extracellular domains of the five monomers [45]. This rotation is translated to the transmembrane
domains and results in variations of the opening of the central pore (Fig. 1b). Based on models of GABAA receptor
structure, Rossokhin predicted that the diameter of the channel pore is mainly controlled by residues—2′, 9′ and 20′ of
the M2 transmembrane domain [46]. The ring formed by
the 9′ M2 residues of the five subunits is termed activation
gate. In the main closed conformation, the diameter of the
ring is only 2.5–3.4 Å. GABA-induced conformation change
prompts the side chains of the activation ring residues to
shift away from the central pore, whose diameter increases
up to 3.4–7.6 Å, allowing ions to flow through the channel.
The most frequent subunit composition of GABAA receptors is 2 α, 2 β and 1 γ subunit [47, 48]. In this case, the 2
α and 2 β subunits in the receptor are most often, but not
always, of the same isoform [48–53] and the receptor carries
two GABA binding sites [54] located at the β+/α− subunit interfaces [44]. However, other stoichiometries can be
observed [55]: ε, π and δ subunits can replace a γ subunit,
and a θ subunit can replace a β subunit [33]. In addition,
binary receptors (receptors containing two different classes
of subunits instead of three, such as the αβ subtypes) are able
to assemble in vitro. They were also described in vivo, but
the evidence of their existence often comes from indirect
Electrophysiology of ionotropic GABA receptors
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Fig. 1 GABAA receptor structure and gating. a Structure of
an isolated GABAA receptor
subunit and of a mature receptor. Five subunits, potentially
belonging to different classes
or isoforms, assemble into a
channel permeable to chloride
and bicarbonate ions. Ions flow
through GABAA receptors down
their electrochemical gradient.
b Control of GABAA receptor
conductance by the receptor
conformation. The central pore
can be either closed or open as
a result of agonist fixation on
the receptor. In this example,
the receptor is viewed from the
extracellular space and the subunits are ordered as in the most
abundant subtypes, namely αβγ.
The presence of two GABA
binding sites corresponds to
ternary receptors (composed
of subunits from three different
classes)
methods [56] or study of γ2 or δ deficient mice [57, 58]: it
is therefore possible that they are artefactual or accidental.
ρ subunits usually assemble in homopentamers or heteropentamers [59, 60], with five GABA binding sites per
receptor [54], i.e. one at each subunit interface. However, in
rare cases, ρ subunits assemble with α1 and/or γ2 subunits
[61, 62].
GABAA receptors are classified into hundreds
of subtypes
Due to the substantial subunit diversity, there are numerous
subtypes of the GABAA receptor, characterized by the combination of subunits they contain (e.g. α1β2γ3) [6].
GABAA receptors are expressed in numerous cell types,
most notably in neurons. The expression patterns of the different subunits determine which subtypes are assembled, as
well as their cellular and subcellular location and their abundance. Numerous cells coexpress multiple GABAA subunits
isoforms [48–53]. Consequently, a single cell can express
several GABAA receptor subtypes [56, 63, 64].
The most abundant subtype in the mammalian nervous system is α1β2γ2, possibly accounting for 60% of
all GABAA receptors [65]. The other major subtypes are
α2β3γ2 and α3β3γ2 [66], while α4–6βγ2, α6β2–3δ, α4β2–3δ
and ρ1–3 are less abundant. Minor subtypes whose physiological existence in vivo is deemed likely include α1βδ,
α2β1γ1, αβε, αβπ, αβθ, α1α6βγ2, α1α6βδ and αβγ3 [10,
66]. Though it is unlikely that all possible subunit combinations exist and assemble into functional receptors, the large
number of possible combinations of subunits expressed by
a given cell explains that the number of existing functional
subtypes is estimated to be as high as 500 [9]. Most of these
subtypes are expected be very rare compared to the most
abundant ones [66] and to be limited to a specific tissue,
brain area or developmental phase, but they may still exert
non-neglectable effects on brain function because of the high
abundance of GABAA receptors compared to most neuronal
receptors [9] and because rare subtypes can be selectively
enriched in certain neurons or synapses [12].
GABAA receptor ion conductance selectivity
GABAA receptors are ion channels. Different ions can flow
down their electrochemical gradient through the pore from
one side of the membrane to the other, generating currents.
The intensity, i.e. the net charge transfer per time unit, of a
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transmembrane ion current is the product of the ionic electrochemical gradient by the number and conductance of
open channels.
∑
dq
ΔEion Gion .
= I = Nopen
dt
ions
(1)
Nopen is the number of open channels at the considered
membrane; it depends on numerous parameters, notably
GABA concentration. For each ion, ΔEion designates the
cognate transmembrane electrochemical gradient and Gion
the conductance of a single open GABAA channel.
The conductance of GABAA receptors varies widely for
different ions: we will study in this paragraph how this selectivity shapes the currents mediated by GABAA receptors.
GABAA receptor chloride conductance
In most circumstances in the adult brain, K+ coupled secondary active Cl− transporters [67] generate an outwardly
directed chloride electrochemical gradient at the neuron
plasma membrane [68]. Consequently, GABA-induced
opening of GABAA receptors induces a chloride influx
(Fig. 1a), thus a hyperpolarization of the plasma membrane
and an inhibition of action potential generation. This is the
most frequent mechanism of inhibitory neurotransmission,
which occurs in a large proportion of brain synapses.
However, during development [69], GABAA receptors
mediate chloride efflux due to an inverted electrochemical
gradient [70], and thus often activate the neuron [70–73].
Such a mechanism has also been observed in slices prepared
from adult brains [74], but these findings have been questioned because neurons in slices seem to have a higher intracellular chloride concentration than in vivo. This artifact
may produce a non-physiological inversion of the chloride
electrochemical gradient [75].
Membrane resistivity (ρ) is equal to the transmembrane
potential (ΔE) divided by the intensity (I) of the current
passing through a unit of membrane surface: ρ = ΔE × S/I
(where S is the area of the membrane domain). The lower
the resistivity is, the more intense is the transmembrane
current generated by a given voltage gradient. The opening of channels, regardless if they mediate depolarizing or
hyperpolarizing currents at the resting membrane potential,
decreases membrane resistivity and promotes the dissipation of action potentials, a phenomenon termed shunting
inhibition. Indeed, it enables ions to respond to the electrochemical gradient created by the action potential with local
transmembrane flows instead of the expected longitudinal
flows that are necessary for propagating the action potential
to other membrane locations. Depolarizing currents mediated by GABAA receptors can interfere with action potentials propagation through shunting [76] or inactivation
13
of voltage-dependent Na+ channels [77], and can thus be
inhibitory instead of excitatory [78, 79]. Conversely, hyperpolarizations mediated by GABAA receptors may result in
neuronal activation through rebound spikes [80, 81].
The GABAA receptor-mediated depolarizations observed
during development can induce an influx of calcium through
voltage-dependent channels [80, 82], favoring neurite outgrowth [83], synaptogenesis [84], neuron migration [85],
and neuron survival [86].
Permeability of GABAA receptors to ions other
than chloride
Although chloride ions account for most of GABAA receptor
conductance in vivo [3], GABAA receptors are also permeable to bicarbonate, formate, propionate, acetate, cyanide
and halides [3, 4, 87]. Differences in GABAA receptor conductance and permeability to these anions are caused by
a selectivity filter relying on recognition sites [3] and the
limited pore diameter.
Because of CO 2 production and conversion in
HCO3− inside the cell, bicarbonate flows mediated by
GABAA receptors are outwardly directed (Fig. 1a), and thus
depolarize the cell membrane. In standard physiological conditions, the ratio of HCO3− vs Cl− permeability of GABAA
receptors is comprised between 0.2 and 0.3 [88], i.e. chloride
currents have a greater amplitude than bicarbonate currents
and the net effect is hyperpolarization. However, prolonged
GABA exposure can occasionally turn hyperpolarizing currents into depolarizing ones [89], notably during epileptic
seizures or in hypothalamic hamartoma neurons [90]. This
phenomenon probably indicates a dissipation of the chloride
gradient [91, 92] when the channel remains open for long
periods. Under these conditions, the bicarbonate efflux may
become larger than the chloride influx and GABAA receptors
may transiently become excitatory [72].
Intrinsic electrophysiological parameters
of the different GABAA receptor subtypes
The impact of GABAA receptors on cellular electrophysiology mostly depends on the time-course and amplitude of
the chloride currents evoked by exposure to an agonist such
as GABA. Typically, the current time-course following the
introduction of an agonist in the receptor’s environment consists of an initial current peak, followed by a decrease in
amplitude (Fig. 2a). This decrease ends with a stabilization
at lower amplitude or a complete closure of the receptors,
depending on agonist concentration, duration of exposure
to the agonist, and on the receptor subtype. The effects of
GABAA receptors on neuronal activity are determined primarily by the charge transfer (the product of the current
Electrophysiology of ionotropic GABA receptors
a
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b
Fig. 2 General characteristics of GABAA receptor-mediated currents. a Typical GABAA receptor current time-course and kinetic electrophysiological parameters. b Dependence of GABAA receptor-mediated current intensity on agonist concentration for different Hill coefficients
intensity with its duration) [93], but also by its synchronization with other electrical inputs, hence the importance of the
shape of the current time-course [94, 95]. The current intensity at a certain time point is the product of the conductance
of a single channel by the chloride electrochemical gradient
and the number of open receptors (Eq. 1), which depends
on kinetic parameters governing channel gating as well as
receptor and agonist concentrations. Indeed, the effects of
an agonist are dose-dependent: for example, the proportion
of open receptors as a function of the GABA concentration follows an allosteric pattern (Fig. 2b), characterized by
a concentration threshold at which the proportion of open
receptors sharply increases.
In most physiological situations, the cell response to a
GABA input is driven by the joint action of dozens or hundreds or receptors that perceive the signal. The macroscopic
scale (receptor populations) therefore appears more suited
than the microscopic scale (single channels) to understand
the biological consequences of GABAA receptor electrophysiology and is easier to study, which explains that most
available information on GABAA receptor electrophysiology covers macroscopic features. Nevertheless, macroscopic
properties always emerge from the integration of single
channel activity characteristics.
The electric activity of GABAA receptors can be understood with a series of parameters that describe and explain
their responses to the various patterns of agonist exposure.
In this section, we will describe the macroscopic electrophysiological parameters that determine the activity of populations of GABAA receptors in response to GABA, and
we will highlight the differences between subtypes in that
regard.
Electrophysiological parameters can be subdivided into
kinetic parameters (activation time, desensitization time,
deactivation time…) and so-called functional parameters
(conductance, EC50, Hill coefficient…), which describe
respectively the current time-course and its dependence on
agonist concentration. As will be exemplified below, some
of these parameters are not independent.
Tools for the study of GABAA receptor
electrophysiology
Before detailing the electrophysiological parameters of
GABAA receptors, we will discuss two conceptual tools
that were instrumental in the discovery and understanding
of these parameters: modeling and the use of channel modulating drugs.
Modeling
Numerous models have been used to interpret the growing
amount of data on GABAA receptor electrophysiology, predict receptor activity, and understand the microscopic origin
of macroscopic parameters and large-scale behavior of channel populations.
Most of the models used are Markovian. Markovian models are discrete (time is discontinuous) and memory-free: the
next step of evolution of a model depends only on its present
state, and not on past states. Markovian models describe the
activity of a channel with a series of states, each of them
having its own conductance and being supposed to correspond to a specific receptor conformation. Transition rates
can be a function of environmental parameters such as transmembrane potential or agonist concentrations. Transition
rates indicate the kinetics of channel activity as well as states
occupancy at equilibrium. For example, in the model showed
in Fig. 3a, if p is the proportion of receptors in the open
state at a given time point, a proportion pkc of the receptor
population transitions to the closed state before the next time
point, while a proportion (1 − p)ko of receptor transitions to
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a
c
Fig. 3 Markovian models of receptor channel activity. a Simplest
Markovian model of a receptor channel, with two unitary states: open
(O) and closed (C). The transition rates describing channel opening
and closure are respectively ko and kc. Both transition rates are a function of the agonist concentration [A]. b Application of the two-states
model to steady-state situations. K is the equilibrium constant, and
depends on agonist concentration. c MWC model. L, r and K are constants depending on receptor structure and [G] is the GABA concentration. The open states are O (no GABA bound), OG (one GABA)
and OG2 (two GABA); likewise, C, CG and CG2 are the close states
the open state. At equilibrium, the net flux is null, meaning
that pkc = (1 − p)ko, and therefore p = ko/(kc + ko).
Certain models specifically describe systems at equilibrium. This simplification allows the replacement of transition rates by equilibrium constants. In our example model,
the equilibrium constant K would follow the equation
K = peq/(1 − peq) = ko/kc (Fig. 3b).
The most classic model used to describe GABAA receptor activity is the Monod–Wyman–Changeux two-state
concerted transition model (MWC) [96]. This equilibrium
model takes into account one open and one closed conformations, both of which can be bound to zero, one or two
GABA molecules (Fig. 3c). The MWC model requires only
four parameters to entirely describe the receptor response
to an agonist, which explains its attractiveness. It accurately
describes GABAA activity at the macroscopic scale, and its
activation by numerous agonists and combination of agonists
[47, 96–98]. This model has confirmed the presence of two
GABA binding sites on ternary receptors [97].
Other models (Table 1) complexify the MWC model to
fit more accurately to certain experimental data, such as the
flip model, which includes a transient pre-sensitized closed
conformation with high agonist affinity [96, 99]. More complex models [100–104] consider a greater number of states.
Although generally more accurate than simpler models,
complex models often lack predictive value since it is difficult to accurately estimate all the parameters of the model
[96].
Complex models usually stem from the study of single
channels, whose complex behavior requires several states to
be accurately described, whereas fewer states are necessary
to describe the properties of GABAA receptors activity at
the synaptic or cellular scale. Despite the likely existence of
several conformations, the two-conformations MWC models
provide accurate predictions at a macroscopic scale. This
indicates that the receptor conformations can be separated
into two groups with minor electrophysiological differences
within each group [96, 105], or that some of them are occupied only transiently and are not involved in the steady-state
circumstances that the MWC model is used to describe.
Electrophysiological parameters can be calculated as a
function of transition rates and state occupancies in certain
models. Modeling also helps to understand the interdependence between parameters, or to calculate parameters
inaccessible to direct experimentation. For example, Chang
and Weiss [106] estimated that ρ-containing receptors open
when they bind GABA in at least 3 of their 5 binding sites.
Channel modulating drugs
The activity of GABAA receptors is modulated by subtypeselective agonists and antagonists belonging to different
chemical families such as benzodiazepines or barbiturates
and including anesthetic, anxiolytic or antiepileptic drugs.
Since primary and transfected cells can often express several subtypes simultaneously, it is necessary to distinguish
from which subtype come the currents measured during
electrical recordings. Subtype-selective channel modulating
drugs facilitate in situ subtype identification [10].
For example, Lindquist and Birnir performed a patchclamp single-channel study of GABAA extrasynaptic receptors on dentate gyrus granule neurones and identified three
types of electrophysiological signatures [107]. Receptors of
the first, second and third type were potentiated respectively
by zolpidem, flumazenil and THDOC, indicating that they
belong respectively to α1βγ2, α4βγ2 and αβδ subtypes.
Table 1 Examples of Markovian steady-state models used for GABAA receptor study
Model
States
Conditions of application
MWC
Flip model
Resting-activated-desensitized
Chang and Weiss model
Close, open (both bound to 0, 1 or 2 GABA)
Resting, resting bound to GABA, pre-sensitized bound, open bound
Resting, open, desensitized (all bound to 0, 1 or 2 GABA)
Close (bound to 0–3 GABA), open
Macroscopic scale
Macroscopic scale
Desensitizing receptors
ρ-Containing receptors
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Electrophysiology of ionotropic GABA receptors
Channel modulating drugs have also been used to study
GABAA receptors structure and to map subtypes expression
in the brain. The benzodiazepine molecules [3H]Ro15-4513
and diazepam bind respectively all γ-containing subtypes
and α1,2,3,5βγ receptors. Hence, under competition from
diazepam, [3H]Ro15-4513 binds only α4,6βγ subtypes.
Using this property, Korpi et al. mapped α4,6βγ receptors
by autoradiography of [3H]Ro15-4513 in mouse brains and
showed that they were overexpressed in δ−/− mice [108].
This proved that γ subunits can partly substitute δ.
Etomidate is a general anesthetic binding GABAA receptors on their GABA binding sites and potentiating their
response to GABA. This potentiation involves comparable
allosteric shifts between αβδ and αβγ subtypes, showing that
αβδ subtypes carry two GABA binding sites as had been
previously described for αβγ subtypes [109].
Kinetic parameters
Activation time
The delay between a change in GABA concentration and
channel response varies between receptor subtypes [110].
The channel opening occurs in two phases: first, agonists
bind to the receptor; second, the receptor conformation
changes and the central pore opens. Thus, the delay in
GABAA receptor opening upon GABA introduction in its
environment depends both on GABA binding kinetics [111]
and on the kinetics of receptor conformational change upon
GABA fixation [111]. GABA fixation kinetics is determined
by GABA concentration [75, 100, 105], following an allosteric pattern [112], and by the receptor affinity for GABA.
At a given GABA concentration, the characteristic time of
the GABA binding kinetics is theoretically inversely proportional to the receptor affinity for GABA: this relation has
been observed in β2γ2S-containing subtypes [75]. However,
at saturating GABA concentrations (in the order of 10 mM
[112]), the conformational change kinetics becomes the limiting factor. This may explain why the δ subunit lowers the
reactivity of the receptor compared to γ2 [113] despite a
higher affinity. The activation time, i.e. the time to go from
10 to 90% of open channels after introduction of GABA, is
0.46 ms for α1β3γ2L at a GABA concentration of 1 mM,
versus 2.4 ms for α1β3δ [114].
The activation times provided in Tables 2, 3 and 4 are
calculated at saturating GABA concentration, thus should
reflect the kinetics of conformation transition. In association
with β3 and γ2L subunits, the rank order of α subunits in
terms of activation time is α1 < α2 < α4 < α6 < α5 < α3, with
α3 being three times slower than α1 [110–112, 114]. However, when associated with β1 and γ2, α2 confers an activation twice faster than α1 [100]. αβ subtypes [115], and even
more ρ-containing subtypes [116], display an extremely slow
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activation time lasting several tens of milliseconds. Receptors containing the ε subunit display slow activation in vivo
[117]. However, this property has never been studied with
recombinant ε-containing receptors, thus it may be conferred
by another subunit.
The measured activation time depends on the size of the
experimental settings. For example, macro-patches, in which
several receptors gathered on a micrometric membrane
domain are studied, display faster activation than wholecell recordings [112]. It may be due to the time required for
GABA to diffuse to all GABAA receptors, which is longer
for larger experimental models, or to cytoplasmic proteins
that may modulate GABAA receptor kinetics in whole-cell
studies [112].
Desensitization
Upon long exposure to GABA, GABAA receptors lose
their ability to open: this process is called desensitization
(Fig. 2a). This mechanism is also observed when receptors
are exposed to GABA transients at a high frequency and is
then termed rundown (Fig. 4a) [118]. Desensitization prevents the transmission of abnormally strong inhibitory signals during synaptic communication: if GABA is chronically
present in the synapse rather than by interspaced bursts, as it
is under normal conditions, postsynaptic GABAA receptors
will close [119].
In desensitized conformations, the M2 transmembrane
domains are tilted and their -2′ residues obstruct the intracellular end of the central pore of GABAA channels [46].
The five monomers constituting the channel can transition
between open and desensitized conformations independently
from one another [120].
During prolonged GABA exposure, the proportion of
open channels sometimes follows a decreasing exponential
curve over time [75], thereby a characteristic time of desensitization (td) can be calculated. In other cases, the observed
desensitization curve can be described as the addition of
several exponential components [112, 121, 122] (Fig. 4b).
In this situation, a single td can be computed as the average
of the characteristic times of each component weighted on
the amplitude of the related current (this is the procedure followed to calculate the td shown in Tables 2, 3 and 4). Consequently, the calculated value of td depends on the duration of
GABA exposure [111]. In long exposures, slow components
gain in relative importance and the calculated td increases. In
addition, td depends strongly on GABA concentration [75]:
the curve of desensitization rate as a function of GABA concentration follows approximately an allosteric pattern. When
enough data are available for calculations, the td presented in
Tables 2, 3 and 4 are the asymptote of this curve at infinite
GABA concentration.
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a
b
Fig. 4 Characteristics of GABAA receptor desensitization. a Rundown. When GABAA receptors are exposed to frequent GABA transients, a portion of the receptor population desensitizes at each transient and does not recover before the next exposure, resulting in a
decrease in peak amplitude. b Biphasic desensitization. The current
decay upon desensitization can often be modeled as the sum of two
decreasing exponential components. A global desensitization rate can
be computed from the characteristic time (τ) of each component.
The characteristic time of desensitization is usually in the
order of 1 s, despite important variations. αβγ receptors containing the α4 subunit desensitize two to three times faster
than those containing the α3 or α5 subunit, with α1 and
α2 desensitizing almost as fast as α4, and α6 conferring an
intermediary desensitization time [75, 111, 112]. However,
when the receptor includes a δ subunit, α6 appears to confer
faster desensitization than α1 [123]. β2 confers a slightly
faster desensitization than β3, with β1 as an intermediate
[124] or giving results similar to β2 [100]. Desensitization of
γ2-containing receptors is two or three times faster than that
of δ-containing receptors [113, 123, 125], but substantially
slower than that of ε-containing receptors [126].
Certain subtypes do not desensitize or desensitize only
partially. For example, homomeric ρ-containing receptors
are almost insensitive to desensitization [5, 116, 127, 128].
Unlike γ-containing subtypes, receptors containing the δ
subunit desensitize only partially [101]: α1β3δ currents
decrease by only 11.6% upon 6 s exposure to saturating
GABA concentration [123]. The α subunit plays a lesser
but still notable role in determining the extent of desensitization (Table 5): γ-containing subtypes desensitize incompletely only if they contain the α3 or α5 subunit [111], with
a steady-state current in prolonged GABA exposure of up to
30% of the peak amplitude. α1β3 receptors containing two α
and three β subunits desensitize more extensively than receptors containing three α and two β subunits [129].
Desensitization can be followed by a refractory period
[114], during which receptors are unable to reopen. The
length of this period is modeled as a function of several
decreasing exponentials, indicating that GABAA receptors
can adopt several closed conformations [114, 130], whose
relative proportions depend on GABA concentration [130].
In vivo, the fast components of the refractory period disappear at low GABA concentration. In these conditions, desensitization is less extensive but its effects last longer [130,
131]. The length of the refractory period varies between subtypes, with receptors containing α4 or α5 being the fastest
to recover (with a characteristic time of 25 ms following a 5
ms exposure to GABA), followed by α2 and α3 (about 100
ms), then α1 (about 200 ms) and finally α6 (about 400 ms)
[111]. Nonetheless, characteristic times heavily depend on
experimental procedures and method of calculation [112].
The study of desensitization prompted the development
of complex, non-steady-state models of GABAA receptor
activity since the MWC model does not describe receptor desensitization and therefore applies well only to nondesensitizing subtypes or to short time-frames of activation.
These limitations are tackled by three-state models such as
the resting-activated-desensitized model (Table 1) [132].
Other models describe the different components of desensitization or refractory periods with several desensitized states
[114, 120, 133]; the desensitization times and durations of
refractory period components are represented by the transition rates respectively to and from these states. Using one
of these models, Gielen et al. [120] proposed that the fast
component of desensitization corresponds to receptors with
two of their five monomers adopting a desensitized conformation, while the slow component corresponds to receptors
with at least three desensitized monomers.
Speed, extent and relative proportions of exponential
components of desensitization are critical in shaping the
time-course of the inhibitory post-synaptic current (IPSC)
mediated by GABAA receptors and strongly differ between
subtypes [102, 106, 114, 134, 135]. For example, α6β3δ
has no fast component, unlike α6β3γ2L [101, 123]. Consequently, despite the similarity of the characteristic time
of their slow component of desensitization, α6β3δ appears
to desensitize less extensively than α6β3γ2L and is less
sensitive to the duration of GABA exposure. As a result,
α6β3δ transmits inhibitory signals whose intensity is
roughly proportional to the duration of GABA exposure,
13
Electrophysiology of ionotropic GABA receptors
while α6β3γ2L-mediated signals do not depend on exposure
duration. This exemplifies that minor differences between
subtypes at the microscopic scale can be amplified at larger
scales. Furthermore, the time-course of IPSCs determines
the strength and timing of neuronal inhibition, so limited
differences in desensitization properties can have dramatic
effects in the response of neuronal circuits to high-frequency
inhibitory signals [101, 136].
Deactivation
GABAA receptors spontaneously release their ligands and
close several milliseconds or tens of milliseconds after a
drop in GABA concentration. The event of deactivation puts
an end to the transmission of a hyperpolarizing signal during synaptic communication, after a brief GABA exposure
(synaptic GABA bursts usually last no longer than a few milliseconds). Contrary to desensitized channels, deactivated
channels can reopen without a refractory period.
Deactivation has been observed to be biphasic in vitro
[100, 103, 111, 114, 122, 133] and in vivo [105, 137];
the relative proportions of the two components can vary
depending on the receptor subtype. These two components
may correspond to two conformations, one bound to two
GABA molecules (the slow component) and the other to a
5349
single one (the fast component) [102], but this hypothesis
is debated [114]. Deactivation can sometimes be triphasic
[56], once again depending on the subtype and probably
on recording procedures, because certain protocols do not
enable the recording of very fast components [112].
Homomeric ρ-containing receptors deactivate quickly
[116]. Among α-containing subtypes, those containing α4
are the fastest to close, followed by α5 (1.5 times slower),
α1 (2.5 times slower than α4), then α2 (5 times slower than
α4), and finally α6 (6 times slower than α4) and α3 (about
10 times slower than α4). The γ2 subunit confers a slower
deactivation than δ [138, 139]. The β2 subunit confers a
substantially faster deactivation than β3 [140].
Most studies on deactivation have been conducted on
whole-cell or multichannel patches, which should give
results close to what is observed in vivo at the synaptic or
cell scale. However, deactivation is much faster in singlechannel (0.2–25 ms) than in channel cluster or whole cell
(5–680 ms) recordings [141, 142]. This discrepancy likely
stems from sequential closing and opening events of a single
channel (Fig. 5), which are integrated in a single current
time-course by whole-cell or multichannel patches recordings [105, 143]. It exemplifies the apparent gap between the
complex macroscopic properties of GABAA receptors and
the single-channel activity they emerge from.
Fig. 5 Relationships between
microscopic and macroscopic
kinetics of channel opening.
Deactivation is a macroscopic
property resulting from the
integration of single-channel
currents time-courses. Its
characteristic time (τ) is longer
than mean channel open time
due to possible channel reopening events and is increased by
desensitization due to the possible reactivation of desensitized
channels
13
5350
E. Sallard et al.
The deactivation time increases in proportion with the
extent of desensitization [102, 123, 135], maybe due to a
high GABA affinity of desensitized states [135]. GABA is
thus retained on desensitized or pre-desensitized receptors
for tens of milliseconds, even when all GABA has been
cleared from the extracellular space [144]. If the receptor is
still bound to GABA when the refractory period ends, it is
able to reopen immediately. When numerous receptors are
considered together, this process results in a prolongation of
the phase in which the current decreases, which is described
as a slower deactivation (Fig. 5). Consequently, the fast
components of desensitization prolong GABAA receptor
responses to GABA and therefore synaptic communication
while decreasing the current amplitude [102, 131].
Functional parameters
Conductance
A single GABAA channel can display several conductance
states, remarkably conserved between species and cell types,
ranging around respectively 12, 20, 30 and 45 pS for the
most frequent subtypes. The main conductance state is 30
pS, meaning that this is the state adopted by the channel in
most circumstances when it is open [4]. The conductance
states may correspond to different channel conformations.
Most GABAA receptor subtypes display very similar conductance values (Tables 2, 3, 4) [145, 146]. However, the
main conductance state of γ-containing receptors (~ 30 pS)
[125] is higher than that of δ-containing receptors (~ 22 pS)
[125], ρ-containing receptors (7 or 8 pS) [147, 148] and αβ
binary subtypes (11–20 pS) [56].
GABA EC50
The curve of GABAA receptor peak current intensity as a
function of GABA concentration follows an allosteric pattern (Fig. 2b), described by the equation of Hill:
I
Imax
=
[A]h
,
EC50h + [A]h
(2)
where I is the current intensity, Imax its asymptote at infinite
agonist concentration, [A] the agonist concentration, and h
the Hill coefficient (see “Hill coefficient”).
EC50 is the GABA concentration at which the current
peak reaches 50% of its maximal amplitude. The EC50 partially reflects the receptor affinity for GABA: in general, the
lower the EC50, the higher the affinity. However, the EC50
also depends on gating parameters such as the speed or maximal probability of channel opening. This can be exemplified
by a simple stochastic model with a single binding site and
a single opening state.
13
Kon
𝛽
A + R ⇌ AR ⇌ AR ∗,
Koff
𝛼
(3)
where A is an agonist that binds to a receptor site R, A + R is
the vacant state of the receptor site, AR the occupied state,
AR* the open state, Kon and Koff, respectively the association and dissociation constants of the agonist, and β and α
are respectively the opening rate and closing rate constants
of the channel. In this model, the true affinity of the receptor for its ligand is the microscopic dissociation constant
Kd = Koff/Kon. The receptor efficacy is defined as the equilibrium constant of the closed-open isomerization E = β/α.
In this model, the apparent affinity EC50 verifies EC50 = Kd/
(1 + E).
The receptor efficacy is related with Pomax, the maximum fraction of receptors in the active state, by the relation Pomax = E/(1 + E) = 1 − (EC50/Kd). Pomax can only
be estimated at saturating GABA concentration using nonstationary noise analysis [149]. However, the presence of
several open states can render the estimation of receptor
efficacy very challenging. Hence, the true affinity and efficacy of GABAA receptors are often inaccessible to accurate
measurements and the EC50 is used as a measure of the
apparent affinity.
When certain receptors of a cluster open for the first time
upon GABA exposure, others may be already closed, due to
fast desensitization. It results in a smoothened shape of the
current time-course through decreased peak amplitude and
spreading of charge transfer over time. This phenomenon is
stronger at high GABA concentrations, which explains why
Pomax is often substantially inferior to 1 and EC50 is close
to the true affinity Kd.
GABA EC50 is up to 100-fold lower than the concentration (termed BC50) at which the probability of occupancy
of the GABA binding sites is equal to 0.5 [150], and than
the concentration at which the half-maximal activation rate
is reached [100, 112]. This discrepancy is explained by differences in GABA affinity between receptor conformations
[106] and by the influence of desensitization on EC50, as
confirmed by the small difference between EC50 and BC50
in the non-desensitizing ρ1 subtype [106]. It indicates that
the EC50 is not a purely thermodynamic parameter but
partially depends on receptor kinetics. Macroscopic parameters of GABAA receptor electrophysiology are complex and
emerge from the interconnection of the numerous properties
of single channels.
Significant differences in GABA EC50 can be observed
between GABAA receptor subtypes, covering 2 orders of
magnitude from around 0.5 µM to around 50 µM. Discrepancies are also observed for the same subtype in different studies, due to differences in the cell type where the receptor was
expressed, in the species of origin of the receptor subunits
Electrophysiology of ionotropic GABA receptors
or in experimental design (Tables 2, 3, 4). It nonetheless
appears that the α subunit plays a crucial role in determining
the receptor’s affinity for GABA (Table 5). The rank order
of increasing GABA EC50 is α6 < α1 ~ α2 < α4 < α5 < α3
when these subunits are associated with β3 and γ2S [151],
but α6 < α4 ~ α5 < α1 < α2 < α3 in association with β3 and
γ2L [111]. The β isoform affects GABA affinity to a limited degree, with the β3 isoform conferring a one to fourfold lower GABA EC50 than β1 and β2 (Table 6). The γ1
subunit confers a higher EC50 than γ2 when these subunits
are associated with α5, but in association with α3, the relationship is inverted. Several studies indicate a low EC50
in receptors containing the γ2S or γ3 subunits compared
with the γ2L splice variant (Table 7). Receptors containing
the δ subunit consistently display an EC50 around 5 times
lower than γ2-containing receptors. ρ-containing subtypes
are around 7 times more sensitive to GABA than the most
abundant ternary receptors [147]. Subtypes containing two
different isoforms of the same class generally show intermediate properties between the two corresponding subtypes
containing only one of these two isoforms [125, 152–157].
Binary receptors have a very low GABA EC50 [146, 158],
around 1 µM, which is as low as the most GABA sensitive
ternary receptors. Binary α1β3 receptors containing two α
and three β subunits have a lower GABA EC50 than α1β3
receptors with three α and two β subunits [129].
Hill coefficient
The Hill coefficient of a GABAA receptor depends on
its number of GABA binding sites and on cooperativity
between GABA binding sites, the process by which the binding of GABA to one site increases the affinity or accessibility of the other site(s) for GABA. The higher the number of
GABA binding sites or the extent of cooperativity are, the
higher the Hill coefficient is. For Hill coefficients higher than
1, there is a threshold in GABA concentration where the
opening probability of the receptor dramatically increases
over a limited concentration span. The higher the Hill coefficient, the steeper the threshold (Fig. 2b).
In experimental systems with low temporal resolution,
fast desensitization can decrease the resolution of the current peak, which might lead to errors in calculations of Hill
coefficients. To the best of our knowledge, this phenomenon has not been tested with GABAA receptors, but it was
described in the glycine receptor [159] which belongs to the
same receptor superfamily as GABAA receptors.
Most GABAA receptor subtypes have very similar Hill
coefficients of around 1.5. However, binary αβ receptors
and ε-containing receptors display low cooperativity [126,
146, 160, 161] with Hill coefficients below 1. On the opposite, ρ-containing subtypes display higher Hill coefficients
than the most abundant ternary receptors [106, 128, 162],
5351
probably due to the presence of 5 GABA-binding sites
instead of 2 [54].
Subunits interplay in the integration
of subtype‑specific electrophysiologycal properties
Due to the remarkable evolutionary conservation of the
sequence of GABAA receptor ortholog subunits, the electrophysiological properties of a certain subtype are expected to
be similar in different species. To our knowledge, receptors
of the same subtype but different species have never been
compared in the same study. Tables 2, 3 and 4 highlight
significant differences in the main parameters of certain subtypes between species, but important intraspecies differences
are also observed, sometimes even when the subunits are
expressed in the same cell line. Indeed, experimental procedures and measurement techniques can have a substantial
influence on the results [112]: it can therefore be difficult to
compare different subtypes. A further complication is that
many authors did not distinguish between the splice variants
γ2S and γ2L. Still, conclusions can be drawn by comparing
subtypes studied in the same article with the same protocols
or when several articles give very similar results on the same
subtype.
The electrophysiological properties of a ternary subtype
are mainly determined by the α subunit isoform and the class
of its “third subunit class” (Table 5, 6, 7). Few differences
are observed between β or γ isoforms. Some of the differences observed between β isoforms are thought to stem from
their differential distribution and association with different
isoforms of the α class. For example, preferential association
of β3 with α2 and α3 instead of α1 is likely to explain the
slower kinetics of the β3-containing subtypes as compared
with the other β-containing subtypes [193]. Moreover, the
electrophysiology of a subtype does not result in a straightforward manner from the addition of the properties conferred by its subunits because of the complex interactions
between these subunits: the properties of a subunit depend
on its environment within the receptor.
Binary αβ receptors display low GABA EC50, conductance and Hill coefficient, and activate slowly. In most
cases, they desensitize slowly and not extensively, except
the α1β3 subtype which desensitizes as fast and extensively
as α1β3γ2L [114]. Despite substantial differences with ternary receptors, binary receptors helped to understand the
molecular basis of ternary receptors electrophysiology. For
example, using αβ2 subtypes, Olander et al. [115] showed
that differences in activation and desensitization kinetics
between α isoforms are determined by their transmembrane
and intracellular domains. However, this study didn’t identify the cause of variations in deactivation kinetics, since the
different αβ2 subtypes investigated didn’t differ significantly
in that parameter. Similarly, the extent of desensitization
13
5352
E. Sallard et al.
Table 2 Main electrophysiological parameters of the different GABAA receptor subtypes in standard ionic conditions (subunits expressed in
Xenopus oocytes)
GABAA
receptor
subtype
Single-channel
conductance (pS)
GABA EC50 (µM)
30 [100]
4.5 [167]
6 [100]
α1β1γ1
α1β1γ2
Hill coefficient
Characteristic time of
desensitization td (s)
Characteristic time
of deactivation (ms)
Activation time (ms)
1.2 [100]
0.492 [100]
([GABA] = 1 mM)
0.6 [103]
([GABA] = 100
µM)
20.5 [100]
17.4 [103]
1.1 [100]
1 [103] (γ2S splice variant)
1.6 [110] (γ2S splice
variant)
2.12 [152]
1.4 [110] (γ2S splice
variant)
220 [110] (γ2S
splice variant)
1.2 [167]
α1β1γ3
0.6 [167]
α1β2γ1
0.67 [167] (human γ1, rat
α1 and β2, expressed in
293 cells)
α1β2γ2
32 [152]
27.9 [56] (γ2S
splice variant)
5 [172]
7 [110] (γ2S splice variant)
1.71 [177]
α1β2γ3
30.2 [178]
1.3 [167]
α1β2δ
6.71 [179]
α1β3γ1
2.1 [167]
α1β3γ2
28 [56] (γ2S splice
variant)
27.5 [180]
1.7 [158] (γ2L splice
1.7 [167]
variant)
2.9 [151] (γ2S splice
variant)
7.6 [181] (γ2L splice
variant)
14 [112] (γ2L splice variant)
36.2 [158] (γ2L splice
variant)
14.1 [122] (γ2L splice
variant)
α1β3δ
27.5 [180]
7.4 [181]
α2β1γ2
30 [100]
17 [184]
37 [100]
68.6 [111] (γ2L
0.424 [111] (average
splice variant)
of 1 and 3 mM of
143 [112] (γ2L
GABA, γ2L splice
splice variant)
variant)
0.95 [112] (γ2L splice 82.5 [122] (γ2L
splice variant)
variant, 1 mM of
GABA)
63.4 [181]
86.8 [123]
1.8 [100]
0.449 [100]
([GABA] = 1 mM)
198.7 [100]
82 (type 1) or 51
(type 2) [103]
0.5 [100]
0.5 [103] (γ2S splice variant)
0.391 [111] (average
of 1 and 3 mM of
GABA, γ2L splice
variant)
110.6 [111] (γ2L
splice variant)
0.735 [111] (γ2L splice variant)
α2β1ε
11.2 [185]
α2β2γ2
1.47 [177]
α2β3γ2
5.2 [151] (γ2S splice
variant)
α3β1γ2
15.1 [167] (human α3, rat
γ2 and β1, expressed in
293 cells)
α3β2γ2
1.5 [110] (γ2S splice
130 [187]
variant)
15.1 [167] (human α3, rat
1.1 [187]
γ2 and β2, expressed in
293 cells)
75 [110] (γ2S splice variant)
2.21 [177]
3.2 [110] (γ2S splice
variant)
680 [110] (γ2S
splice variant)
α3β3γ2
48 [151] (γ2S splice variant)
0.701 [111] (average
of 1 and 3 mM of
GABA, γ2S splice
variant)
188.8 [111] (γ2L
splice variant)
α4β1γ2
1.40 [177]
α4β1δ
0.30 [188]
0.17 [177]
α4β2γ2
3.9 [189]
1.4 [167]
α4β2δ
0.25 [179]
α4β3γ2
7.6 [151] (γ2S splice
variant)
15 [112] (γ2L splice variant)
13
0.55 [101] (γ2L splice variant)
0.603 [111] (γ2L splice variant)
1.57 [188]
1.788 [111] (γ2L splice variant)
174 [177]
0.408 [111] (average
of 1 and 3 mM of
GABA, γ2L splice
variant)
0.711 [112] (γ2L
splice variant, 1
mM of GABA)
24.8 [111] (γ2L
splice variant)
109 [112] (γ2L
splice variant)
0.951 [111] (γ2L splice variant)
Electrophysiology of ionotropic GABA receptors
5353
Table 2 (continued)
Single-channel
conductance (pS)
GABA EC50 (µM)
Hill coefficient
α4β3δ
0.35 [188]
1.8 [188]
α5β1γ2
5.6 [167]
α5β2γ2
5.8 [187]
4.2 [167]
0.47 [177]
1.5 [187]
4.9 [190]
1.9 [190]
GABAA
receptor
subtype
α5β2γ3
α5β3γ2
24.9 [180]
11.6 [151] (γ2S splice
variant)
α6β1γ2
0.5 [167]
α6β2γ1
0.4 [167]
α6β2γ2
5.2 [156] (γ2S splice
variant)
1.4 [189]
0.34 [172]
0.50 [179] (γ2S splice
variant)
α6β2δ
0.52 [156] (γ2S splice
variant)
1.2 [167]
0.21 [179]
α6β3γ2
1 [151] (γ2S splice variant)
Characteristic time of
desensitization td (s)
Characteristic time
of deactivation (ms)
Activation time (ms)
1.315 [111] (average
of 1 and 3 mM of
GABA, γ2L splice
variant)
41.5 [111] (γ2L
splice variant)
1.247 [111] (γ2L splice variant)
0.596 [111] (average
of 1 and 3 mM of
GABA, γ2L splice
variant)
163.8 [111] (γ2L
splice variant)
1.052 [111] (γ2L splice variant)
α6β3δ
340.4 [123]
ρ1
1.7 [116]
3.5 [116]
14.5 [116] (30 µM of
GABA)
154 [116]
ρ2
2.6 [116]
3.3 [116]
8.2 [116] (30 µM of
GABA)
180 [116]
Roman: rat subunits; bold: human subunits; bold italics: mouse subunits. The Hill coefficient, GABA EC50, main conductance state and characteristic times of activation, deactivation and desensitization of the most abundant subtypes in vivo, i.e. ternary subtypes and ρ homopentamers,
are presented. Information on other subtypes can be found in [13, 103, 112, 114, 116, 123, 125, 130, 146, 152–157, 176, 180, 182, 191, 192]
of αβγ receptors was not matched by the corresponding αβ
subtypes, with α1β2 desensitizing incompletely, contrary to
α1β2γ. This confirms that novel properties emerge from the
new subunits interactions introduced by γ or δ subunits in
ternary receptors.
Receptors made of ρ subunits react very slowly to changes
of GABA concentration, do not desensitize and deactivate
quickly; they have low conductance and GABA EC50 but a
high Hill coefficient.
The response of GABAA receptors to particular GABA
inputs can be predicted from the parameters that have been
described so far. For example, due to similar conductance,
faster activation and slower deactivation, α2β1γ2 mediates tenfold higher charge transfers than α1β1γ2 when it is
activated [100], but the higher GABA EC50 of the former
means that stronger inputs are necessary to activate it.
Several studies found a great consistency between the
electrophysiological activity of in vitro recombinant and
in vivo endogenous GABAA receptors [60, 102, 103, 137,
152, 154, 194] (reviewed in [195]). The parameter values
given above should therefore be relevant enough in physiological contexts to explain the activity and function of
GABAA receptors in the nervous system.
GABAA receptors mediate different types
of currents at the neuronal scale
As receptors of the most abundant inhibitory neurotransmitter,
GABAA receptors are expressed in most neurons and influence their electric activity. Depending on the neuronal type,
GABAA receptors can be located in the dendrites, the cell body
and/or the axons. They are highly enriched at post-synaptic
sites [194, 196], where they usually mediate inhibitory neurotransmission, but can also be found extrasynaptically and
perisynaptically. Despite the preferential synaptic localization
of α1, α2, α3, and especially γ2 [197], no GABAA receptor
subunit has yet been found exclusively in synapses [196, 198].
Receptors containing the α4, α5, α6, γ1, γ3 or δ subunits, as
well as binary αβ subtypes, are mainly extrasynaptic [56, 196,
13
5354
E. Sallard et al.
Table 3 Main electrophysiological parameters of the different GABAA receptor subtypes in standard ionic conditions (subunits expressed in
mouse L929 cells)
GABAA
receptor
subtype
Single-channel conductance (pS)
α1β1γ1
GABA EC50 (µM)
Hill coefficient
Characteristic time of
desensitization td (s)
Characteristic time of Activation time (ms)
deactivation (ms)
5.2 [165]
α1β1γ2
30 [125] (γ2L splice 7.1 [146]
variant)
5.2 [168] (γ2L splice
29.3 [145] (γ2S splice
variant)
variant)
5.1 [169] (γ2L splice
variant)
6.2 [125] (γ2L splice
variant)
α1β1δ
22 [125]
α1β2γ2
29 [145] (γ2S splice
variant)
11 [171]
α1β3γ2
25.9 [114]
26.8 [130]
14 [171]
11.6 [130] (γ2L
splice variant)
15.5 [111] (γ2L
splice variant)
15.5 [182] (γ2L
splice variant)
1.48 [111] (γ2L
splice variant)
1.5 [182] (γ2L splice
variant)
0.462 [114] ([GABA] = 1 76.1 [114] (γ2L
mM, γ2L splice variant)
splice variant)
0.46 [114] (γ2L splice
variant)
α1β3δ
23.8 [114]
26.7 [130]
2.8 [130]
4.4 [182]
1.4 [182]
1.26 [114] ([GABA] = 1
mM)
42.8 [114]
2.4 [114]
α1β3ε
24 [161] (rat α1 and
0.8 [161] (rat α1 and
β3, and human ε, in
β3, and human ε,
L929 cells)
in L929 cells)
51.8 [133] (γ2L
splice variant)
0.9 [133] (γ2L splice
variant)
1.7 [146]
1.9 [168] (γ2L splice
variant)
1.9 [169] (γ2L splice
variant)
1.4 [125] (γ2L splice
variant)
5 [125] (γ2L splice
variant)
6.0 [145] (γ2S splice
variant)
No desensitization
observed [125]
0.9 [161] (rat α1 and
β3, and human ε,
in L929 cells)
α2β3γ2
25 [111] (γ2L splice
variant)
α3β3γ2
35.8 [111] (γ2L
splice variant)
α4β3γ2
10.7 [111] (γ2L
splice variant)
2.57 [113]
1.3 [113]
2.5 [113] ([GABA] = 100
µM)
α4β3δ
0.5 [113]
1.3 [113]
4.8 [113] ([GABA] = 100
µM)
α5β1γ2
32.8 [157] (γ2L
splice variant)
26 [124] (γ2L splice
variant)
1.69 [157] (γ2L
splice variant)
5.7 [157] (γ2L splice
variant)
6 [124] (γ2L splice
variant)
9.4 [111] (γ2L splice
variant)
1.54 [157] (γ2L
splice variant)
α5β3γ2
22 [133] (γ2L splice
variant)
α5β3γ3
26.9 [191]
1.5 [191]
1.6 [191]
α5β3π
23.8 [191]
1.3 [191]
1.4 [191]
α6β2γ2
2 [171]
α6β2δ
0.2 [171]
α6β3γ2
2 [171] (γ2L splice
variant)
2.25 [111] (γ2L
splice variant)
α6β3δ
0.3 [171]
400 [125]
1.04 [111] (γ2L
splice variant)
Roman: rat subunits; bold: human subunits; italics: bovine subunits. The Hill coefficient, GABA EC50, main conductance state and characteristic times of activation, deactivation and desensitization of the most abundant subtypes in vivo, i.e. ternary subtypes and ρ homopentamers, are
presented. Information on other subtypes can be found in [13, 103, 112, 114, 116, 123, 125, 130, 146, 152–157, 176, 180, 182, 191, 192]
13
Electrophysiology of ionotropic GABA receptors
5355
Table 4 Main electrophysiological parameters of the different GABAA receptor subtypes in standard ionic conditions (subunits expressed in
Xenopus oocytes)
GABAA receptor subtype
GABA EC50 (µM)
Hill coefficient
α1β1γ1
25 [163]
41 [164]
75 [166]
α1β1γ2
9.8 [170] (γ2S splice variant)
19.9 [126] (γ2S splice variant)
1.36 [126] (γ2S splice variant)
α1β1δ
4.9 [170]
α1β1ε
4.0 [126]
0.85 [126]
α1β2γ2
41 [153]
16 [132] (γ2L splice variant)
45.8 [47, 173, 174]
34 [97] (γ2L splice variant)
55 [175]
20 [155]
20 [163]
4.61 [176] (γ2L splice variant)
1.39 [153]
1.59 [174]
1.57 [47, 173]
1.38 [97] (γ2L splice variant)
1.4 [175]
1.38 [176] (γ2L splice variant)
α1β3γ2
8 [163]
α2β1γ1
39.8 [183]
α2β1γ2
30.6 [183]
α3β1γ1
114 [163]
α3β1γ2
240 [154]
208 [163]
200 [186] (γ2S splice variant)
98 [164]
α3β1γ3
32 [163]
α3β1ε
2.3 [186]
α3β1θ
81 [186]
α3β2γ2
487 [154]
11 [163]
α3β3γ2
28 [163]
α5β1γ2
17 [154]
15 [163]
α5β2γ2
14 [154]
α5β3γ1
24 [163]
α5β3γ2
3 [163] (γ2L splice variant)
α5β3γ3
2 [163]
α6β2γ2
6.7 [153]
1.6 [155]
0.82 [153]
ρ1
0.81 [106]
2.83 [106]
ρ2
1.19 [162]
2.17 [162]
Roman: rat subunits; bold: human subunits; italics: bovine subunits; bold italics: mouse subunits. The Hill coefficient, GABA EC50, main conductance state and characteristic times of activation, deactivation and desensitization of the most abundant subtypes in vivo, i.e. ternary subtypes
and ρ homopentamers, are presented. Information on other subtypes can be found in [13, 103, 112, 114, 116, 123, 125, 130, 146, 152–157, 176,
180, 182, 191, 192]
197, 199], although the α4, α5 and α6 subunits can also be
found in synapses at medium concentrations.
Due in part to the diversity in electrophysiology of the different
subtypes, GABAA receptors mediate several types of currents in
neurons. These currents, and by extension the subtypes involved
in their generation, have different biological functions.
Phasic activity
Post-synaptic GABAA receptors primarily have a phasic
activity: they mediate intense currents of regulated duration
upon binding GABA molecules released in the synaptic cleft
by the pre-synaptic neuron. Phasic receptors are involved
in synaptic communication and are the main mediators of
inhibitory signals [200]. The most abundant synaptic receptors of the mature brain belong to subtypes containing the
γ2 subunit associated with β subunits and α1 or α2 [114,
201]. Their electrophysiological properties (see Tables 5, 6,
7) ensure the speed and specificity of inhibitory post-synaptic currents (IPSCs) generated in response to pre-synaptic
GABA release, ensuring efficient neuronal communication.
Indeed, the GABA concentration reaches 2-5 mM in the
13
5356
E. Sallard et al.
Table 5 Properties conferred by the “first subunit class” of the receptor
isoform
α1
GABA EC50
Desensitization
Refractory period
Deactivation
Activation
α2
α3
α4
α5
α6
Medium
Medium
Complete and
Complete and
moderately fast moderately fast
Very high
Slow and incomplete
Medium
Fast and complete
Medium
Slow and incomplete
Long
Moderately fast
Fast
Medium
Very slow
Slow
Short
Very fast
Medium
Short
Fast
Moderately slow
Very low
Medium
speed and
complete
Very long
Slow
Medium
Medium
Moderately slow
Moderately fast
Table 6 Properties conferred by the “second subunit class” of the
receptor
Isoform
β1
β2
β3
GABA EC50
Desensitization
Deactivation
Medium
Medium
Medium
Medium
Medium
Moderate
Moderately slow
Moderately slow
synaptic cleft during a GABA burst [134, 202], a concentration much higher than the EC50 of any GABAA receptor subtype (Tables 2, 3, 4). Low affinity receptors are thus
activated by GABA bursts without being activated by background GABA concentration (Fig. 7a), which ensures that
receptor activation is dependent on pre-synaptic inputs. The
quick activation of most phasic receptors increases the speed
at which neuronal networks communicate, and allows the
receptors to respond to GABA bursts despite the extremely
fast clearance of GABA in the synapse (whose time constant
is only 0.3–0.6 ms) [134].
The transmission of a strong inhibitory signal requires
maximization of charge transfer. This is ensured by the high
receptor concentration (there are usually tens to hundreds of
receptors per inhibitory synapse) [56, 73] and the maintenance of the IPSC for relatively long times [102]. Indeed, the
slow or intermediate deactivation of phasic receptors enables
IPSCs to last several tens of milliseconds. In addition, slow
deactivation is associated with fast and complete desensitization, which explains the common observation of IPSC
depression upon high frequency stimulation, a phenomenon
termed rundown (Fig. 4a) [203, 204]. This process protects
neurons against pathological excessive inhibition in case of
defective clearance of GABA, or excessive GABA emission
in the synaptic cleft.
Different GABAA receptor subtypes can be located at different synapses of the same neuron. This phenomenon enables precise modulation of neurotransmission because identical signals received at different synapses will not induce
the same phasic current in the postsynaptic neuron [196,
205, 206]. Several subtypes can even be expressed in the
same synapse [207–210]. In such a case, the post-synaptic
current triggered by GABA is a composite of the currents
mediated by the individual subtypes.
Ternary subtypes (αβγ or αβδ) and ρ-containing subtypes
are coexpressed at the axon terminals of retinal bipolar cells
[208, 210] and mediate different types of phasic currents:
ternary receptors mediate fast and brief currents, whereas
ρ-containing receptors mediate long and delayed currents
that can be explained by their slow activation and lack of
desensitization [211]. Since ρ-containing subtypes have the
lowest GABA EC50 (Tables 2, 3, 4), it is possible that at
low GABA concentration, only the slow and long currents
take place. Therefore, the diversity of phasic receptors facilitates complex and diverse responses to GABA inputs in a
dose-dependent manner [5, 212]. In retinal bipolar cells, the
contribution of different GABAA receptor subtypes to the
generation of inhibitory currents varies between cell subpopulations [213, 214], probably because of differences in
Table 7 Properties conferred by the “third subunit class” of the receptor
Isoform
γ1
γ2L
Conductance
GABA EC50
Medium
Medium
Hill coefficient
Desensitization
Deactivation
Activation
Medium
Medium
Medium
Medium (γ2L variant) or moderate Moderate
(γ2S variant)
Medium
Medium
Complete and moderately fast
Medium
Fast
13
γ3
δ
ε
π
Low
Low
Low
Medium
Medium
Moderate
Medium
Slow and incomplete
Moderately fast
Slow
Low
Fast
Medium
Electrophysiology of ionotropic GABA receptors
5357
Fig. 6 Effects of spillover on
GABAA receptor-mediated postsynaptic currents. Phasic activity occurs upon the fixation of
GABA by GABAA receptors on
the post-synaptic neuron. Spillovers from nearby synapses can
desensitize part of the receptors
and decrease the amplitude of
the next IPSC
abundance of the considered subtypes. Consequently, the
IPSCs follow different times-courses in different cell subpopulations [215], leading to differences in cell function
(reviewed in [5]). This exemplifies that the characteristics of
an IPSC depend on the GABAA receptor subtype by which
it is mediated and that different types of neurons can vary in
their phasic activity as a consequence of different subtypes
expression patterns (reviewed in [216]).
During development, phasic currents are longer and less
intense than in the adult brain [56, 73, 200, 217]. Indeed, the
α1 and α2 subunits are not yet expressed at high levels and
most synaptic receptors belong to the α3βγ2 subtypes, which
display very slow deactivation (Tables 2, 3, 4).
Sometimes, GABA escapes from a synapse (a process
termed spillover) and reaches a nearby synapse. The micromolar GABA concentration in the receiving synapse induces
a very faint IPSC, and desensitizes a portion of the GABAA
receptors present there, which enter refractory periods that
can last several seconds. If millimolar GABA bursts occur
in the synapse before the end of the refractory period, the
resulting phasic current will be diminished by rundown
(Fig. 6) [131]. Since GABA bursts frequently occur in synapses, the net effect of spillovers is often excitatory despite
the faint IPSC they directly induce. Spillovers are rare in
physiological conditions in the adult brain since highly efficient GABA uptake systems usually ensure efficient clearance of GABA in synapses; however, these systems can be
downregulated [218, 219].
Whether GABAA receptor-mediated currents are excitatory or inhibitory depends in certain circumstances on
receptor location and timing relative to depolarizing signals
[220, 221], hence the importance of the brevity and spatial
confinement of phasic currents [198]. Phasic currents are
brief and often induced at a single synapse, allowing them
to regulate neuronal and cerebral activity very finely and
to generate a substantial diversity of responses to similar
inputs, in part due to GABAA receptor subtype diversity.
Among other functions, GABAA receptor phasic activity
is required in the generation of θ and γ frequency network
oscillations, under strict temporal control [198].
Tonic activity
Certain GABAA receptors, mostly extrasynaptic, mediate
chronic low-amplitude currents termed tonic currents [198].
Their activity is less temporally and spatially restricted than
that of phasic receptors [198]. They are involved in longrun (seconds to minutes) modulation of neuronal activity
and respond to GABA spillover or to non-synaptic release
of GABA. They belong mostly to subtypes containing
the δ subunit and the α4, α5 or α6 subunits [64, 114, 196,
222] or to ρ-containing subtypes [62], although certain
γ2-containing subtypes can mediate tonic currents (reviewed
in [201]). The characteristics of tonic activity are related to
the electrochemical properties of the GABAA receptor subtypes involved [223]: these subtypes display a high affinity
to GABA (Tables 2, 3, 4), allowing a response to low extrasynaptic GABA concentration; and a low conductance as
well as incomplete desensitization, enabling channel groups
to deliver limited but steady currents (Fig. 7b). Furthermore,
the amplitude of current mediated by δ-containing receptors
at saturating GABA concentrations can be multiplied up to
20 times by the agonist etomidate [109], showing that the
maximal open probability in the absence of agonists Pomax
of the concerned subtypes is no higher than 5%, and explaining the low amplitude of the tonic currents they mediate.
γ-containing subtypes desensitize extensively when
exposed to high GABA concentrations, but the low GABA
concentrations which elicit tonic activity may not desensitize them to such an extent as to abolish tonic currents [114,
195]. Deactivation does not hamper tonic activity, because
GABA presence is maintained over long periods, allowing
closed receptors to reopen; deactivation even ensures that
tonic currents remain at a low amplitude. Finally, the slow
activation of tonic receptors, caused both by low GABA concentrations and the electrophysiological properties of the
13
5358
E. Sallard et al.
a
b
Fig. 7 Comparison of phasic and tonic currents. a Phasic current. 1:
due to the high GABA EC50 of most phasic receptors, background
GABA transients do not activate GABAA receptors. 2: fast activation.
3: high amplitude current peak. 4: slow deactivation. 5: rundown. b
Tonic current. 1: slow activation. 2: steady-state, low-amplitude current with limited desensitization
subtypes involved, participates in shaping tonic currents.
At subsaturating GABA concentrations, the current peak is
truncated, resulting in stable currents reaching a steady state
without going through sharp peaks after changes in GABA
concentration [112].
Receptors that usually mediate phasic currents can at
times display a low-level tonic activity, including in synaptic
locations, due to a very low but strictly positive probability
of opening at GABA concentrations close to zero [132, 224].
For example, even at standard extracellular GABA concentration, the amplitude of tonic currents through α1β2γ2L
can reach 1% of the amplitude of phasic currents [224].
Moreover, it has been proposed that certain subtypes can
mediate both phasic and tonic currents not only depending
on their location or on GABA concentration, but also on
whether both or only one of their two GABA-binding sites
are occupied [175].
Tonic charge transfer can have a significant effect on neuronal communication: it has stronger inhibitory effects than
phasic inhibition in certain neuron types such as cerebellar
granule cells and hippocampal pyramidal cells [73, 200, 222,
225]. This demonstrates that the steadiness of tonic currents
can compensate for their lack of intensity compared to phasic currents in generating a strong response [222].
Tonic activity has been predicted to have different effects
than phasic activity on neuronal network activity, notably a
decrease of rhythmicity and synchronicity between neurons
[226, 227]. At a wider scale, models of cerebellar activity
predict that tonic inhibition increases the number of motor
patterns that can be stored in the cerebellum [228, 229].
During development, tonic activity is mainly activated
by action potential-induced GABA release in the extracellular space [73, 200, 217]; however, non-vesicular and
acetylcholine-induced, action potential-independent GABA
release plays the major role in tonic inhibition in the adult
cerebellum [137, 200]. This developmental transition can
be related to the increasing wrapping of synapses by glial
cells [230], which progressively prevents GABA spillover
[200]. It is not only the source of tonic currents that varies
during development, but also their amplitude. For example,
tonic currents are detected in hippocampal pyramidal cells
only in embryonic life [231], while they increase over time
in cerebellar granule cells [73, 200]. Spatial discrepancies
can also be caused by variations in extracellular GABA concentration between brain areas [225].
Tonic currents can modify the information conveyed
by neurons [70, 232]. Unlike phasic currents, they do not
determine the information conveyed by a single message but
rather subtly modulate the information transmitted by the
neuron over long time frames encompassing several communication events. Tonic inhibition is a form of regional-scale
neuromodulation, offering a negative feedback to periods of
intense synaptic activity during which successive spillovers
increase the extracellular GABA concentration [198]. The
long duration of tonic currents enables them to mitigate the
oscillatory patterns often induced by phasic communication
[226] and thus to desynchronize the activity of neighbor
neurons [227].
Since the various tonic GABAA receptor subtypes differ
in their EC50, slight increases in GABA extracellular concentration can progressively recruit a new receptor population. It widens the repertoire of potential tonic activity, since
limited and finely regulated changes in GABA concentration
can lead to the selective activation or inhibition of only part
of the receptors mediating tonic currents [225].
Most transporters responsible for the clearance of GABA
out of the extracellular space are unable to lower GABA
concentration below 0.4 µM [233], a concentration higher
than the EC50 of several α6-containing subtypes (Tables 2,
3, 4). Some receptors are thus continually activated and
exert a function of leakage channel, as experimentally confirmed in the rat hippocampus [234]. Rather than being
involved in the transmission of inhibitory signals, such
receptors are believed to regulate membrane resistivity
13
Electrophysiology of ionotropic GABA receptors
5359
[78]. If this hypothesis is correct, these receptors control
the time window over which synaptic integration occurs
as well as the neuronal gain (the ratio between received
and emitted currents), and thus the action potential firing
rate [225, 232]. This exemplifies that the functions accomplished by a GABAA receptor subtype are determined by its
electrophysiology.
Perisynaptic GABAA receptors
Perisynaptic GABAA receptors are activated by GABA spillover out of the synaptic cleft. They account for the slowcomponent inhibition of biphasic GABA-mediated response
[235], a phenomenon in which the GABA released in a synapse first activates phasic post-synaptic receptors, then spills
over in the extracellular space and activates perisynaptic
receptors. GABA diffuses in a much greater volume in the
extracellular space than in the synaptic cleft. Consequently,
it reaches perisynaptic receptors with a delay compared
to synaptic receptors and at a much lower concentration.
Therefore, the biphasic GABA-mediated response consists
of a localized, fast and intense phasic current followed by
a diffuse, low-level and less cell-specific hyperpolarization.
Perisynaptic receptors thus prolong phasic activity. They
also take part in tonic inhibition [73] and spillover-mediated
heterosynaptic modulation [232], whereby GABA spillover
out of a synapse inhibits nearby neurons uninvolved in the
considered synapse (reviewed in [236]).
Presynaptic inhibition
GABAA receptors can be found on the axons of certain neuron types, where they sometimes mediate depolarizations,
with either inhibitory [237, 238] or excitatory [78, 239]
effects on downstream synaptic activity.
In the process of presynaptic inhibition, GABA released
at axo-axonic synapses activates GABAA receptors on the
axon of the receiving neuron and prevents the propagation
of synchronous action potentials [240] (Fig. 8). Unlike synaptic and perisynaptic inhibitions [241], presynaptic inhibition requires a precise synchronization of the excitatory
signal with the chloride influx mediated by the GABAA
receptor [198]. However, this does not necessarily imply
that the GABA input must be synchronized with the action
potentials: indeed, in certain retinal cells where ρ-containing
subtypes are exclusively located in axons [213], the longlasting signals mediated by these subtypes ensure an efficient
presynaptic inhibition which allows a small delay between
the inhibitory input and the action potentials.
The functions of presynaptic inhibition remain poorly
understood [78]. However, the release of GABA by axons of
hippocampal mossy fiber [242, 243] or cerebellar basket and
stellar cells [244] suggests that one function of presynaptic
Fig. 8 Presynaptic inhibition of action potentials mediated by
GABAA receptors. Inhibitory currents mediated by GABAA receptors
can inhibit simultaneous action potentials, and allow different axonal
ramifications to convey different signals
inhibition is self-inhibition and tissular negative feedback
on neuronal activity. In addition, presynaptic inhibition enables the axonal ramifications of a neuron to convey different
messages if only part of the ramifications receive a presynaptic inhibitory signal (Fig. 8). This mechanism increases
the complexity and specificity of neuronal communication
[245]. For example, the presynaptic inhibition mediated by
ρ-containing subtypes in the retina increases the dynamic
range of brain responses to light intensity [246].
Control of GABAA receptor electrophysiology
GABAA receptor electrical activity is determined by GABA
concentration and its intrinsic electrophysiological properties, but also by its chemical and cellular environment, sometimes in a subtype-specific manner.
GABAA receptor control by phosphorylation
GABAA receptors are subject to phosphorylation by several kinases such as protein kinase A (PKA) and C (PKC).
Phosphorylation by PKC inhibits GABAA receptors, probably by endocytosis [247] or through an inhibition of the
non-desensitizing fraction of GABAA receptors [248]. Both
increases [249, 250] and decreases [147, 251–253] in currents mediated by GABAA receptors have been reported
upon phosphorylation by PKA, probably because it can
simultaneously speed up desensitization while slowing down
deactivation [254, 255]. In the retina, PKA mediates dopaminergic signals which result in a potentiation of GABAA
receptors through a decrease in GABA EC50 [128]. Phosphorylation may explain the differences in electrophysiology
between γ2S and γ2L containing receptors, since the only
difference between these two splice forms is a cytoplasmic
insertion of 8 residues in γ2L containing a PKC phosphorylation site [34].
13
5360
Receptors of different neurotransmitters can colocalize at post-synaptic sites and be activated by presynaptic
co-release of several neurotransmitters. The simultaneous
activation of different postsynaptic receptors often induces
a partial occlusion of their respective currents. This crosstalk has been proposed to represent a fast adaptive process
in controlling signal transmission [256]. Negative cross-talk
was demonstrated between GABAA receptors and dopamine
receptors [256], P2X receptors [257] and glycine receptors
[258]. The inhibition of GABAA receptors by glycine receptors depends on phosphatase 2B activity, while kinase activity is necessary for recovery of GABAA receptors from this
inhibition [259].
Glutamate excitatory neurotransmission mediated by
NMDA receptors induces calcineurin-dependent γ2 subunit
dephosphorylation, leading to a reversible and local dispersion of postsynaptic GABAA receptor clusters [260]. Dispersion lowers receptors’ EC50, activation and desensitization
times, and slows down their deactivation [261–263]. Consequently, receptors transition from synaptic phasic activity to
extrasynaptic tonic activity upon dephosphorylation.
Voltage dependency
Although GABAA receptors are generally voltage-independent, a few subtypes are voltage-sensitive in rare situations
[123, 240, 264]. Voltage seems to affect current peak amplitude but not kinetic properties (desensitization and deactivation) of the α1β3δ, α5β3π and ρ1 subtypes [106, 123, 191,
192], unlike the α6β3δ subtype whose kinetic properties are
affected but not other parameters [123]. Voltage dependency
can also be observed in channels that are rare or never found
in vivo, such as α1β3 or α6β3 [123].
Ionic modulation
GABA A receptors are sensitive to ionic modulation: a
change in the concentration of certain ions, in either the
extracellular or the cytosolic compartment, can modify their
activity. The nature or extent of the modulation is subtypespecific (Fig. 9).
Extracellular Ca2+ does not affect the activity of ternary
receptors [87, 265], but it increases the amplitude of currents mediated by ρ-containing subtypes [266]. Intracellular
Ca2+ has varying effects on many subtypes, depending on
its origin and concentration. Ca2+ coming from the extracellular space does not affect GABAA receptor opening [267],
whereas Ca2+ originating from intracellular compartments
activates GABA A receptors at low concentrations, but
inhibits it at high concentrations [268, 269]. This may be
a consequence of receptor phosphorylation by PKC [248]
or dephosphorylation by calcineurin [260], because these
enzymes are activated by intracellular calcium. Ca2+ control
13
E. Sallard et al.
Fig. 9 Ionic modulation of the GABAA receptor. Ion flows are represented with black arrows, activation with green arrows and inhibition
with red arrows. Dashed lines indicate subtype specificity
of GABAA receptors is involved in the long-run (seconds to
minutes) modulation of neuronal activity [268].
Extracellular Zn2+ is released during synaptic activity by
numerous neurons, particularly in the limbic and neocortical regions [270] and during development. It decreases the
opening frequency of certain GABAA receptor subtypes
[271] (reviewed in [4]). αβ receptors are up to 3400-fold
more sensitive to Zn2+ than αβγ and αβδ receptors [113,
272], while ρ [266], αβπ [191] and αβε [126, 171] subtypes
have an intermediate sensitivity. In addition, α1 confers a
higher zinc sensitivity than the other α subunits [124, 126,
171, 182, 189, 273]. The discrepancy in sensitivity between
αβγ and αβ subtypes is explained by the position of the Zn2+
binding sites in the receptor (two between α and β subunits
and one in the ion pore), which are disturbed by γ subunits
[182, 274]. Additionally, αβ subtypes comprised of two α
and three β subunits contain a third zinc-binding site at the
β-β interface, explaining their higher sensitivity compared
with αβγ subtypes and the very rare αβ subtypes containing
three α and two β subunits [129]. Zinc increases the EC50 of
α1β1δ but not α1β1, and the Hill coefficient of α1β1 while it
decreases the Hill coefficient of α1β1δ [273]. Zn2+ sensitivity displays very similar characteristics in vivo and in vitro
(reviewed in [5]). Zn2+ may take part in the control of tonic
currents in hippocampal pyramidal neurons, where up to
10% of extrasynaptic GABAA receptors may belong to αβ
subtypes [180].
All GABAA receptor subtypes seem to be insensitive to
intracellular pH [275] (reviewed in [4]), but certain subtypes
are sensitive to extracellular pH because of their two extracellular proton-binding sites [170, 275]. Conductance is negatively correlated with extracellular pH for α1β1, α1β2γ2 and
α1β1δ [170, 276], and positively for ρ subtypes [277, 278],
while α1β1γ2S is pH-insensitive [170] and the curves of
α1β1γ2Sδ, α4β3γ2, α4β3δ and α1β2γ2S conductance as a
function of extracellular pH are bell-shaped [73, 170]. Extracellular protons slow the deactivation of the α1β3δ subtype
Electrophysiology of ionotropic GABA receptors
and increase its opening probability in the absence of GABA
without affecting its desensitization, resulting in an increase of
currents at low pH [181]. On the contrary, α1β2γ2 desensitizes
less and slower at low pH [276], while its activation is slowed,
and deactivation is shifted toward its slow component. GABAA
receptor inhibition at high pH is explained by a faster closure
of the channel [279].
In vivo, GABAA channel charge transfer is generally negatively correlated with extracellular pH [170, 280]. During
synaptic transmission, a transient acidosis of the synaptic cleft
is followed by a durable increase in pH [281]. This phenomenon synergizes with the concomitant variations of GABA
concentration and helps to induce a fast and massive opening
of GABAA receptors during the first milliseconds of synaptic
transmission, followed by the progressive closure of the channels when protons and GABA are cleared from the synaptic
cleft.
NMDA, the glutamate receptor mediating most of the excitatory signals in the central nervous system, is inhibited by
low pH [282]. Thus, the opposite effects of pH on the inhibitory GABAA receptor and the excitatory NMDA receptor may
explain why neuronal activity and excitability increase with
extracellular pH [283]. High regional electrical activity elicits
an acidosis: the pH-sensitivity of GABAA receptor and NMDA
receptor thus creates a negative feedback against hyperactivity. This process relies notably on tonic currents mediated by
δ-containing receptors [181]. Other pH regulation processes
can also modulate neurone activity through GABAA receptors
[280]. Extracellular pH can itself be modified by bicarbonate
transport across GABAA receptors [284], making them broadcasters of neuromodulatory signals in addition to their function
of chloride channel.
Furthermore, acidity displays protective effects against
Zn2+ inhibition in α1β1 receptors [273], showing that the different types of ionic modulation to which GABAA receptors
are submitted are not independent.
The ionic modulation of the GABAA receptor is involved
in the long-term depression or potentiation of inhibitory currents characteristic of neuronal plasticity, and may play a role
in hypomagnesia-induced hyperexcitability [285], anoxic and
ischaemic conditions [286] or epileptogenesis [287].
Conclusion
GABAA receptors are highly complex and are able to generate a vast repertoire of electric responses to different inputs
under the control of various types of regulations. Their
influence on cellular electric activity at different scales of
time and space can to a great extent be predicted from the
5361
structural and electrophysiological properties of the different subtypes, whose diversity provides additional modularity, robustness and specialization to the GABAA receptor
system.
GABAA receptor agonists and antagonists are increasingly used in medicine to treat various diseases, from epilepsy to insomnia and others. Improved knowledge of the
electrophysiology, pharmacology and expression patterns
of GABAA receptor subtypes has led to the development of
subtype-specific drugs that limit side effects. Further studies on GABAA receptor electrophysiology are warranted in
order to develop new treatments, particularly in the fields of
GABAA receptors structure–function relationships, interactions with intracellular proteins, and electrical properties
of lesser-known subtypes such as those containing π or θ
subunits.
Acknowledgements We are grateful to Thibault Pébrier1, Philibert
Courau1 and María Castro Scherianz1 for their helpful review and comments of this article. 1 École Normale Supérieure de Paris, Biology
department, PSL research university, 45 rue d’Ulm 75005 Paris, France
Author contributions ES: conceptualization, data collection, curation
and analysis, draft writing, manuscript review. DL: manuscript review.
PL: supervision, manuscript review.
Funding Open Access funding enabled and organized by Projekt
DEAL.
Data availability All data relevant to this review is included in the text,
references and figs.
Code availability Not applicable.
Declarations
Conflict of interest None.
Ethics approval Not applicable.
Consent to participate Not applicable.
Consent for publication Not applicable.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article’s Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
the article’s Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
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