TYPE Review

PUBLISHED 12 January 2023

DOI 10.3389/fpsyt.2022.1060949

GABAA receptor subtypes and

OPEN ACCESS benzodiazepine use, misuse, and
abuse
EDITED BY
James K. Rowlett,
University of Mississippi Medical Center,
United States

REVIEWED BY
Elif Engin1,2*
Miroslav M. Savic,
University of Belgrade, Serbia 1
Stress Neurobiology Laboratory, Division of Basic Neuroscience, McLean Hospital, Belmont, MA,
Dai N. Stephens, United States, 2 Department of Psychiatry, Harvard Medical School, Boston, MA, United States
University of Sussex, United Kingdom

*CORRESPONDENCE
Elif Engin Benzodiazepines have been in use for over half a century. While they remain highly
eengin@mclean.harvard.edu
prescribed, their unfavorable side-effect profile and abuse liability motivated a search
SPECIALTY SECTION
for alternatives. Most of these efforts focused on the development of benzodiazepine-
This article was submitted to
Addictive Disorders, like drugs that are selective for specific GABAA receptor subtypes. While there is
a section of the journal ample evidence that subtype-selective GABAA receptor ligands have great potential
Frontiers in Psychiatry
for providing symptom relief without typical benzodiazepine side-effects, it is less
RECEIVED 04 October 2022 clear whether subtype-selective targeting strategies can also reduce misuse and
ACCEPTED 29 December 2022
PUBLISHED 12 January 2023
abuse potential. This review focuses on the three benzodiazepine properties that are
relevant to the DSM-5-TR criteria for Sedative, Hypnotic, or Anxiolytic Use Disorder,
CITATION
Engin E (2023) GABAA receptor subtypes and namely, reinforcing properties of benzodiazepines, maladaptive behaviors related
benzodiazepine use, misuse, and abuse. to benzodiazepine use, and benzodiazepine tolerance and dependence. We review
Front. Psychiatry 13:1060949.
existing evidence regarding the involvement of different GABAA receptor subtypes
doi: 10.3389/fpsyt.2022.1060949
in each of these areas. The reviewed studies suggest that α1-containing GABAA
COPYRIGHT
© 2023 Engin. This is an open-access article
receptors play an integral role in benzodiazepine-induced plasticity in reward-related
distributed under the terms of the Creative brain areas and might be involved in the development of tolerance and dependence
Commons Attribution License (CC BY). The use, to benzodiazepines. However, a systematic comparison of the contributions of
distribution or reproduction in other forums is
permitted, provided the original author(s) and
all benzodiazepine-sensitive GABAA receptors to these processes, a mechanistic
the copyright owner(s) are credited and that understanding of how the positive modulation of each receptor subtype might
the original publication in this journal is cited, in contribute to the brain mechanisms underlying each of these processes, and a
accordance with accepted academic practice.
No use, distribution or reproduction is
definitive answer to the question of whether specific chronic modulation of any given
permitted which does not comply with these subtype would result in some or all of the benzodiazepine effects are currently lacking
terms. from the literature. Moreover, how non-selective benzodiazepines might lead to the
maladaptive behaviors listed in DSM and how different GABAA receptor subtypes
might be involved in the development of these behaviors remains unexplored.
Considering the increasing burden of benzodiazepine abuse, the common practice
of benzodiazepine misuse that leads to severe dependence, and the current efforts
to generate side-effect free benzodiazepine alternatives, there is an urgent need
for systematic, mechanistic research that provides a better understanding of the
brain mechanisms of benzodiazepine misuse and abuse, including the involvement
of specific GABAA receptor subtypes in these processes, to establish an informed
foundation for preclinical and clinical efforts.

KEYWORDS

benzodiazepines (BDZs), drug abuse, GABAA receptor, withdrawal, tolerance, reward,

dependence

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Engin 10.3389/fpsyt.2022.1060949

1. Introduction The above studies, many of which were carried out in genetically
modified mice due to a lack of subtype-specific pharmacological
Benzodiazepines (BDZs) have been in use since 1960s and are still agents, demonstrated the possibility of developing subtype-specific
prescribed at high rates with over 90 million prescriptions dispensed agents that would have efficacy for specific indications without the
in the US alone each year (1). In 2015, one in eight US adults reported undesirable effects of BDZs. Efforts to develop subtype-selective
BDZ use within the past year, further illuminating the widespread use GABAAR modulators have yielded a large number of drugs in the
of BDZs. Studies from other countries indicate comparable rates of last 30 years [For recent reviews, see (31, 32)]. While no truly
prescribed or non-medical BDZ use despite some variation in rates subtype-specific drug has been developed to date, several compounds
and in the specific subpopulations (e.g., the elderly) where BDZ use with subtype-selective affinity or subtype-selective efficacy have been
is most common (2–8). investigated in preclinical studies for their behavioral effects, with
BDZs achieve their therapeutic effects through the allosteric a few of them also making it to clinical trials. The below sections
modulation of gamma amino butyric acid type A receptors aim to answer the question of whether these subtype-selective
(GABAARs). GABAARs are postsynaptic pentameric complexes, compounds would have reduced abuse and dependence liability
with the subunits comprising the pentamere drawn from a subunit compared to classical BDZs by summarizing relevant findings from
repertoire of at least 19 subunits (α1-6, β1-3, γ1-3, δ, ε, θ, π, ρ1-3). preclinical studies.
Most GABAARs in the brain are composed of 2 α, 2 β and one γ or
δ subunit, with the specific subunit composition influencing receptor
kinetics, subcellular localization, and anatomical distribution of the
receptor in the brain, as well as its pharmacological properties 2. Benzodiazepine abuse and misuse
with regards to its modulation by different drug classes (9–12).
GABA binding to binding sites at the interface of α and β subunits DSM-5-TR (33) criteria for Sedative, Hypnotic, or Anxiolytic Use
leads to the opening of the chloride channel at the center of the Disorder (pp. 620–621) focus on a number of problematic drug-
pentamere, allowing chloride movement between the intracellular related behaviors many of which can also be studied in preclinical
and extracellular spaces. In the adult brain, this usually results work. The criteria can be roughly categorized as those that indicate
in chloride influx to the cell and hyperpolarization, while in the loss of control over use (i.e., using the drug in larger doses or
immature brain [and possibly in the mature brain under certain for a longer time than intended, continuing use despite negative
pathological conditions; (13)], the opening of the channel leads to consequences, failed attempts to reduce or stop use), expenditure
chloride efflux and depolarization. BDZ binding sites are distinct of significant time and effort for drug related activities, often at the
from the GABA-binding site and are located at the interface of the expense of other desirable activities (e.g., time/effort/money spent in
α and γ subunits on GABAARs containing the α1, α2, α3, or α5 acquiring the drug, recovering from drug effects, giving up on other
subunits (α1GABAAR, α2GABAAR, α3GABAAR, and α5GABAAR). activities in favor of using the drug, not being able to focus on
Thus, BDZs bind a subset of GABAARs, at a site distinct from the other activities due to craving, failure to fulfill major obligations at
GABA-binding site, and their effect is to increase the frequency of work, home, school, other social settings, due to drug use), risky
chloride channel opening at a given GABA concentration, causing a drug use (e.g., recurrent use in physically hazardous situations such
leftward shift in the GABA dose-response curve without altering the as driving under the influence, taking risks to acquire the drug),
maximal response. and pharmacological criteria (i.e., development of tolerance and
BDZs have anxiolytic, sedative, hypnotic, amnestic, withdrawal). Having only 2 of the 11 listed symptoms is sufficient for
anticonvulsant, myorelaxant effects (9). While this heterogenous diagnosis, with the presence of 2–3 symptoms considered “mild”, 4–5
effect profile has made it possible for BDZs to be used for symptoms “moderate”, and 6 or more symptoms “severe”.
a wide range of indications and in different settings, the While the DSM criteria outline the typical behavioral
desired effects in one setting are often viewed as undesired presentations of BDZ abuse and misuse, research on medical
side-effects in another setting (e.g., sedation and anterograde and non-medical use of BDZs reveals the most common reasons
amnesia are highly desirable effects when BDZs are used underlying BDZ abuse and misuse.
in a peri-surgical setting but are highly undesirable when While some recreational users of BDZs use BDZs alone for
they are used as anxiolytics in the treatment of generalized their alcohol-like euphoric effects, BDZs are more often abused
anxiety disorder). in combination with other drugs, most commonly opioids, to
Considering the apparent functional relevance of the subunit supplement the high (34–36). These users typically use BDZs
composition of GABAARs to receptor properties and anatomical at higher doses than the common therapeutic range (37) and
location, it was postulated that the different behavioral effects as suggested by the recent popularity of fast-acting designer
of BDZs may be mediated by their positive modulation of BDZs in illicit drug markets, they may prefer faster and shorter
different GABAAR subtypes. Findings from early studies acting BDZs (38). Another common use of BDZs among illicit
indeed indicated that BDZ modulation of α1GABAARs is polydrug users is to use the BDZs as a way of managing the
required for the sedative effects (14), while BDZ modulation anxiety and irritability commonly experienced as a part of the
of α2GABAARs is required for the anxiolytic-like effects of withdrawal from the primary drug when regular access is disrupted,
BDZs (15). Continued work in this area not only confirmed and or managing anxiety experienced due to co-occurring psychiatric
further expanded the association of specific behavioral effects conditions (34, 39).
with specific GABAAR subtypes (16–25), but uncovered new, Misuse of BDZs in medical settings involves the use of BDZs for
previously unappreciated indications for subtype-selective GABAAR different indications, at different doses, and/or for longer periods of
modulation (26–30). time than recommended. Off-label prescription of BDZs, particularly

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for indications such as post-traumatic stress disorder, obsessive- 3. GABAAR subtypes and
compulsive and related disorders, and mood disorders is common reward-related effects of BDZs
(40–43). While this is a concern, it should be noted that off-label
prescription of medications for different indications than those Based on the DSM criteria provided above, it is possible to
approved is common practice for many drugs and is not specific inquire into the rewarding effects of BDZs at multiple levels. For any
to BDZs. compound to be used by choice or abused, it should first serve as a
The second concern with BDZ misuse is patients using BDZs at reinforcer, that is, its administration should increase the likelihood
higher doses that recommended, particularly when used long-term. of behaviors that preceded it and/or were causally linked to it.
While well-documented development of tolerance to the effects of The simplest form of this would be a preference for BDZs over
BDZs would support the expectation that patients would escalate alternatives when the two come at equal and negligible cost. For
dose with long-term use, there has been relatively little empirical instance, rodents drink more from the bottle containing the water-
evidence to support consistent escalation of BDZ dose, even among soluble BDZ midazolam, when midazolam and water are provided
long-term BDZ users (44, 45). This may be due to the fact that in a two-bottle choice setup in their home-cages (24, 58–60). A
tolerance develops primarily to the sedative effect of BDZs, which related concept is drug-seeking behavior: BDZs support associative
is often viewed as an undesirable side-effect by individuals who take learning in a conditioned place preference paradigm where animals
BDZs for anxiety-related indications, while tolerance to the anxiolytic spend more time in the BDZ-associated chamber of a two-chamber
effect is either small and delayed or non-existent in humans (46). apparatus during the drug-free test session (61). The second level
While off-label use or dose escalation do not seem to be major would be the question of willingness to expend effort to acquire
concerns for BDZ misuse, extended use is a significant issue. Current the drug. BDZs are self-administered in tests where animals have
recommended length of treatment with BDZs is 2–4 weeks, with to engage in operant behaviors (e.g., press a lever) to receive the
no BDZ approved for use for more than 4 months. Yet, many drug (34, 62) and increase the level of effort the animals are willing
patients are prescribed BDZs for months, years, decades, sometimes to expend to receive a brain stimulation reward in intracranial self-
indefinitely (47–53). More alarmingly, while the number of new BDZ stimulation (ICSS) studies [i.e., reward enhancement; (24, 63, 64)].
prescriptions remained stable between 2005 and 2015, there was These two levels are linked to the value of BDZs as reinforcers and
a 50% increase in renewed prescriptions during the same period, thus, the question of reward (see Section 3.3 for possible issues with
suggesting a specific increase in this problematic, longer-term use this interpretation). However, DSM criteria go further than this and
(54). Aside from continued need for therapeutic relief, withdrawal include many maladaptive consequences of BDZ abuse, including the
symptoms are the primary reason for long-term BDZ use. devaluation of natural reinforcers (e.g., food, sex) and giving these
In 2020, FDA issued a requirement to update the Boxed Warning up in favor of BDZs, engaging in risky behaviors under the influence
on BDZs, indicating that following chronic use of BDZs over of or in order to acquire BDZs, and the neglect of responsibilities
several days or weeks, abrupt cessation or dose reduction of BDZs (e.g., poor parental behavior) due to BDZ abuse. As many of these
can cause severe withdrawal symptoms, including seizures (1). behaviors may depend on the reinforcing value of the drug, with
Indeed, studies indicate that withdrawal symptoms can continue stronger reinforcers causing more maladaptive behaviors, we will be
for months, even years (55). In a recent Internet study, 60–85% covering maladaptive behaviors under the general heading of reward-
of individuals reported having moderate to very severe symptoms related behaviors. However, it should be noted that interactions with
in different life domains while tapering off BDZs, with 54% of specific properties of drugs may influence each of these categories
them reporting suicidal thoughts (55). The challenges involved in differentially. For instance, alcohol and stimulant use have different
discontinuing BDZs were present even when tapering was done effects on the disinhibition of sexual behaviors and risk-taking (65).
in a clinical setting where the withdrawal symptoms were closely At the level of simple preference, the preference of rodents for the
managed (56, 57). midazolam-containing liquid in two-bottle choice experiments has
In summary, two major reasons for BDZ abuse and misuse are been shown to depend on midazolam binding to α1 and α2GABAARs
the reward-related effects of BDZs, mainly related to abuse, and (24, 59). These studies employed mice with point mutations that
physical dependence, as defined by the presence of a withdrawal make the targeted subunit insensitive to BDZs (14–16, 18, 19). While
syndrome upon discontinuation, which is the primary underlying mice with point mutations on the α3 or α5 subunits continued to
factor for misuse, with likely involvement in abuse as well. As prefer midazolam-containing solution, this preference was abolished
noted earlier, the efforts to develop GABAAR subtype selective in mice with mutated α1 or α2 subunits.
compounds have been motivated by the idea of developing In support of the integral role of α1 modulation in the
GABAergic therapeutics without the unfavorable side-effect profile pleasurable effects of BDZs, α1-preferring compound zolpidem is
of classical BDZs. Thus, a highly significant question is whether self-administered by non-human primates (NHPs) and has higher
GABAAR subtype-specific compounds, if developed, would have the reinforcement value than non-selective BDZs, such as diazepam
same abuse and misuse liability as BDZs. To start answering this or midazolam, in self-administration tests (66–69). Comparison
question, we review evidence regarding the involvement of specific between zolpidem and midazolam is particularly relevant, as early
GABAAR subtypes in behaviors relevant to the 3 main domains studies suggest that short-acting BDZs act as stronger reinforcers
of DSM-5-TR criteria for Sedative, Hyponotic, and Anxiolytic than longer acting BDZs [(66, 70, 71); see (72) for a comparison
Use disorder: Reward-related effects of BDZs which support of pharmacokinetic properties of commonly used BDZs]. Zolpidem,
persistent drug-seeking, development of maladaptive behaviors as a rapidly eliminated BDZ modulator, might owe its reinforcing
associated with BDZ use, and development of BDZ tolerance value to its fast action as well as its receptor selectivity. Thus, a
and withdrawal. comparison with a rapidly eliminated non-selective BDZ, such as

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midazolam, isolates the role of receptor selectivity as a determinant effects of diazepam in ICSS (63), while such involvement was not
of reinforcement value. found for reward-facilitation by midazolam (24), leaving the question
Self-administration of zolpidem demonstrates that α1-binding of α3 involvement unresolved.
may be sufficient to sustain self-administration. Another relevant In summary, there is evidence that α1, α2, and possibly α3
question is whether α1-binding is necessary. For instance, the subunits contribute to the reward-related effects of BDZs, with no
2-bottle choice experiments above indicate that α1-binding involvement of α5GABAARs (77).
might be necessary for midazolam preference. Some studies (73)
indeed suggest that α1-sparing compounds do not maintain self-
administration in NHPs, agreeing with the necessity of α1-binding.
Others (68, 69) suggest that sparing α1 is not sufficient to eliminate 3.1. Maladaptive behaviors linked to BDZ use
self-administration. Shinday et al. (69) elucidate the importance and GABAAR subtypes
of drug history in this process, where α1-sparing compounds
maintained self-administration in animals trained with midazolam, The main maladaptive behaviors noted in DSM for Sedative,
but not in animals trained with cocaine. As subjective stimulus Hypnotic, and Anxiolytic Use Disorder can be categorized as those
properties of BDZs were shown to be primarily mediated by α1 in that represent abandoning natural rewards or responsibilities in favor
drug discrimination tests (67), this finding is unlikely to be a result of of the drug and those that represent risky behaviors while using or to
the subjective similarities between the effects of an α1-sparing drug acquire the drug.
and the training compound midazolam. Drug history was found Devaluation of natural rewards (e.g., food, sex, caring for one’s
to be important in the reinforcing effects of BDZs in humans as offspring, socializing) is a common consequence of drug addiction
well, where non-selective BDZs were found to be more reinforcing and has been investigated through animal models for different
in individuals with histories of sedative use and moderate alcohol classes of drugs of abuse (79–83), often comparing drug responses
consumption (34). It is possible that previous chronic exposure to responses to palatable foods, such as sucrose. These experiments
to GABAergic compounds causes changes in the expression and usually take the form of providing a sucrose solution while the
trafficking of GABAAR subtypes (74) and/or plasticity involving animals are anticipating a drug reward. This leads to a comparison
other systems (75, 76), such that α1-sparing compounds can activate of the stronger drug reward with the now weaker, devalued natural
brain circuitry involved in the experience of reward at a level that reward. The effects of BDZs in this commonly used natural reward
can maintain self-administration (see below for a more detailed devaluation task have not been investigated. However, some early
discussion of plastic changes following long-term exposure to studies found a paradoxical role of drugs of abuse, including BDZs,
GABAergic drugs). in conditioned taste aversion (CTA) tasks (83). CTA tasks involve the
In cases where α1-sparing compounds are self-administered, pairing of a new, palatable food (e.g., a sucrose or saccharin solution)
efficacy at α2/3 seems critical for the maintenance of self- with an illness-inducing agent, such as lithium chloride. After this,
administration, based on reports that BDZ self-administration animals avoid the consumption of the illness-associated stimulus.
in NHPs is not influenced by the co-administration of an α5- If the illness-associated stimulus is delivered intra-orally without
selective negative modulator (77) and that compounds with reduced operant behavior on the part of the animal, it is accompanied by
efficacy at α2/3 do not maintain self-administration (73). A role suppressed ingestion responses, as well as active rejection responses
for α2GABAARs in reinforcing properties of BDZs has also been such as gaping (84). The fact that preceding a palatable gustatory
substantiated by ICSS studies in mice, where mice with mutated stimulus with a drug of abuse that is regularly self-administered
α2 subunits that render this subunit insensitive to the effects of by animals leads to reduced consumption of this stimulus was
BDZs no longer showed the reward-facilitating effects of diazepam perplexing. Moreover, in the intra-oral delivery setting, the animals
or midazolam (24, 63). Similar to findings with self-administration suppressed ingestion but showed no active rejection responses in
of α1-sparing compounds by NHPs, Schwienteck et al. (78) reported this case, suggesting that the gustatory stimulus was not necessarily
that low-efficacy positive allosteric modulators with some selectivity considered “aversive”. This type of suppression of response to natural
for α2/3 lead to weak reward-facilitation in ICSS, suggesting that reward has instead been considered a form of natural reward
high-potency modulation of α2GABAARs might be both necessary devaluation, where the animals show reduced interest in the natural
and sufficient for self-administration and reward-facilitation effects. stimulus that was previously linked with a BDZ or other drug of
The demonstration of a role for α1GABAARs in ICSS has been abuse, because the stimulus is now considered less rewarding (i.e.,
less straightforward. While some studies suggested that α1-binding is devalued) compared to the greater reward of the drug (83). This
may be necessary (24) and sufficient (78), others noted negligible reduction of interest in palatable gustatory stimuli due to BDZ
involvement of α1GABAARs in reward-facilitation effects (63). The pairing cannot be attributed to an aversive effect of BDZs, as these
differences in findings may be due to variability in dose ranges compounds are readily self-administered, or to an overall suppression
employed in different studies, as highly sedative compounds such of appetite, as BDZs are otherwise known to increase food intake (85),
as zolpidem can non-selectively reduce responding in ICSS giving further supporting the likelihood of a natural reward devaluation due
the impression of reduced reward-facilitation, as well as to the to reward comparison effect.
variability in the drug histories of the animals in each study, as Caring for offspring can be conceived of as a natural reward
the studies involve sequential testing with multiple drugs. Finally, and as a translational measure of carrying out responsibilities. While
while the lack of α2- or α3-specific agents prevents conclusions evidence suggests that acute or sub-chronic administration of BDZs
regarding the individual contribution of each subtype to BDZ reward causes impairments in maternal behavior and fragmented care for
in pharmacological studies, the gene-targeted mouse studies suggest the offspring (86, 87), no studies to our knowledge investigated the
a possible involvement of α3GABAARs in the reward-enhancing question of maternal care in a free choice setting where the dams are

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provided with a choice to self-administer BDZs or care for offspring. to predictive cues from the reward itself (93). Importantly, drugs
As noted, studies also used acute or brief administration of BDZs of abuse continue to cause increased firing even after repeated
which does not represent a drug use disorder scenario. presentations, counter to the normal functioning of the brain reward
Overall, there is some support for the idea that BDZs might lead system (60). Another important property of drugs of abuse is that
to devaluation of natural rewards, however, this question has not been they can induce long-lasting plasticity after even a single exposure
systematically studied. Moreover, there is no information about the (94). While the specific type of plasticity observed in the VTA
specific GABAAR subtypes that might be involved in this process depends on the mechanism of action of the specific drug of abuse, the
to clarify whether the targeting of the specific GABAAR subtype overall effect is to cause increased dopamine release into the nucleus
might reduce the liability of natural reward devaluation compared to accumbens (NAc) and a priming of the VTA dopamine system that
non-selective BDZs. makes it more likely to respond to similar stimuli in the future.
Acute administration of BDZs causes behavioral disinhibition BDZ actions on the mesocorticolimbic dopamine system are
and increased sensitivity to recent rewards, leading to increased similar to other drugs of abuse. Specifically, BDZs increase dopamine
risky decision-making (86, 88, 89). Strikingly, the facilitatory effects release from the VTA onto target mesolimbic structures through
of BDZs on risky decision-making seemed limited to individuals a disinhibition mechanism, where BDZ binding to the GABAARs
with drug abuse histories and to relatively high doses of BDZs expressed on the VTA GABAergic interneurons leads to inhibition
(89, 90), characteristics often observed in recreational BDZ users. of the interneurons and the subsequent increased activation of the
Indeed, there is some evidence that polydrug users who also abuse dopaminergic projection neurons (59, 60). Such a disinhibition-based
BDZs engage in more risky behaviors compared to non-BDZ-using mechanism is shared by some other drugs of abuse, such as opioids
polydrug users (91, 92). Thus, there is some evidence that BDZ use (95). In addition, like other drugs of abuse, a single injection of
may be associated with increased risk-taking behaviors, however, the BDZs can cause VTA synaptic plasticity in the form of increased ratio
brain mechanisms of BDZ-induced risk-taking are mostly unknown. of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)
One study showed that administration of lorazepam was linked to N-methyl-D-aspartate (NMDA) receptor-mediated excitatory
to reduced activation of the amygdala and the medial prefrontal currents in the VTA for at least 3 days post-injection (96). BDZ
cortex and increased activation of the insular cortex during risky binding to α1GABAARs seems to be both necessary and sufficient
decision making [i.e., choosing of risky options over safe ones; (90)]. to induce BDZ-induced disinhibition and excitatory plasticity of the
However, the study involved the administration of low doses of VTA dopamine neurons, as these effects were abolished in α1H101R
lorazepam which did not cause changes in risk-taking behaviors, mice that have BDZ-insensitive α1 subunits and the same effects
which complicates the interpretation of the changes in brain activity. could be induced by the α1-preferring GABAAR modulator zolpidem
There have also been no studies to date investigating the involvement (59, 96). These physiological data provide a mechanistic explanation
of different GABAAR subtypes in the promotion of risk-taking by for the above behavioral findings noting self-administration of α1-
BDZs. As all BDZ-sensitive GABAARs are expressed in the cortex preferring compounds and reduced ability of α1-sparing compounds
and the amygdala, the findings from the Arce et al. (90) study also to sustain self-administration.
do not provide any clues as to which subtype(s) may be critical Studies in rodents also point to the possibility of α2GABAAR
for the observed risk-promoting effects of BDZs. More relevant involvement in BDZ-induced reward. α2GABAARs are expressed
translationally is also the question of whether these acute effects are at negligible levels in the VTA, suggesting their involvement in
exacerbated upon chronic use, as is the case in the DSM definitions of BDZ reward may be through a different node in the brain reward
Sedative, Hypnotic, and Anxiolytic Use Disorder, and how they might system. Due to the high expression of α2GABAARs in the NAc,
promote a cycle of risk-taking and drug use. one possibility is that α2GABAARs mediate BDZ reward not by
Overall, BDZs are self-administered and have been shown to influencing dopamine release from VTA to target structures, but
facilitate reward effects in different species and a few studies by modulating the effects of dopamine on those target structures
investigated which GABAAR subtypes may be involved in these such as the NAc. Viral-mediated knockdown of α2GABAARs in the
effects. However, maladaptive behavioral patterns observed in NAc was indeed sufficient to abolish midazolam preference in a two-
Sedative, Hypnotic, and Anxiolytic Use Disorder have not been bottle choice drinking task (24). As α2GABAARs are expressed on
studied in animal models, despite the availability of validated both D1+ and D2+ medium spiny neurons (MSNs) of the NAc
models from studies of other drugs of abuse. Thus, the question (97), it is difficult to speculate on an exact mechanism by which
of whether specific GABAAR subtype(s) may play a central role in α2GABAARs of NAc regulate BDZ reward. Recent work suggests that
the progression of BDZ use from self-administration to a cycle of α2GABAARs on D2+ NAc MSNs may be involved in the regulation
self-destructive behaviors remains open. of stress resiliency (29). As the effects of stress and subjective reward
from BDZs seem to be closely linked (58, 98), it is possible that
the α2GABAAR inhibitory regulation of D2+ MSNs plays a role
3.2. Brain mechanisms of BDZ reward in BDZ reward as well. Furthermore, α1GABAARs are expressed
at high levels in the parvalbumin positive (PV+) interneurons of
Drugs of abuse achieve their rewarding effects similarly to natural the NAc, which have been shown to play a significant role in
rewards, by increasing dopaminergic neurotransmission from the motivated behaviors and the effects of drugs of abuse (99). The role of
ventral tegmental area (VTA) to its mesolimbic target structures. α1GABAARs in regulating the activity of this pivotal cell population
While unexpected natural rewards initially cause increased dopamine indicates a second possible venue through which α1GABAARs might
firing in the VTA, after repeated presentation, the firing shifts be involved in the reward-related effects of BDZs.

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3.3. Issues related to interpretation and and mask or supplement reward-related properties of the BDZs or
translation of findings from animal studies of subtype-selective compounds.
BDZ reward A final significant point is the comparability of the findings
across species, and ultimately, the translatability of the findings
Studies using animal models provide a rich opportunity to to humans. Studies suggest many cross-species similarities in the
understand pharmacological and brain mechanisms far beyond what expression of different GABAARs in brain areas relevant for the
could be achieved through studies in humans alone. However, like experience and processing of reward. For instance, high levels of α2
every modeling attempt, they come with certain possible confounds and α4, moderate-to-high levels of α1, and low-to-moderate levels
and alternative explanations that complicate the interpretation of of α3 expression in the striatum is observed in rodents (101–103),
findings within each model. Moreover, it is not clear whether findings NHPs (104, 105), and humans (106). However, while α5 expression
from animal models can be directly translated to humans and caution is undetectable in the striatum in rodents (101–103), studies report
should be exercised when drawing translational conclusions. high levels of α5 in the NHP (105) and human (106) striatum. In
In animal models, the multitude of behavioral effects induced by the prefrontal cortex, while α5 expression is largely limited to layer
BDZs often complicate the interpretation of results as purely reward- 5, with low expression in other layers in rodents (101), the expression
related. For instance, the two-bottle choice experiments where the is more diffuse across layers in humans, with high expression in layers
rodents are presented with a bottle of water and a bottle of midazolam 4, 5 and 6, moderate expression in layers 2 and 3, and low expression
mixture may be affected by the sedative and amnestic effects of in layer 1 (107). Thus, through strong expression in the striatum and
BDZs as well as their pleasurable subjective effects. Sedation may more pronounced expression in the prefrontal cortex, α5 is more
place a limit on drinking from the midazolam-containing bottle, as likely to have a role in reward processes in NHPs and humans than
midazolam is fast-acting and highly sedative. The bottle placement is in rodents. In this sense, the finding that the co-administration of an
randomized every 24-h in these types of experiments, but amnestic α5-selective negative allosteric modulator did not influence triazolam
effects may make it difficult for mice to learn which bottle has the self-administration in rhesus monkeys is highly relevant, suggesting
pleasure-inducing liquid within the 24 h where the bottles remain that this subunit does not play an integral role in the maintenance
put. Drugs affecting certain combinations of GABAARs may appear of self-administration despite its dense expression in relevant brain
more preferred compared to other combinations due to increased areas in this species.
pleasurable effects, or due to a reduction in sedative and/or amnestic Based on the above-noted differences in GABAAR expression
effects, or when pleasurable effects and sedation and/or amnestic in different species, it is important to reemphasize here is that
effects are mediated by the same receptor subtype, the pleasurable while all of the two-bottle choice, CPP, and ICSS studies reviewed
effects might be masked by the other effects. Similarly, while findings above were conducted in rodents, all of the self-administration
from the conditioned place preference test are often interpreted studies were conducted in NHPs. This adds another layer of
as drug-seeking behavior, they depend on the animal’s ability to complexity to comparative interpretation of the findings where
associate the context with the subjective effects of the drug during differing task demands of different behavioral paradigms is also
the training sessions and then retrieve this memory during the combined with possible species differences. Unfortunately, data on
test session. Drugs with amnestic effects may interfere with this GABAAR expression in other relevant brain areas, such as the VTA,
process. Drugs affecting specific receptor subtype combinations is missing in NHPs and humans, further adding to the uncertainty of
with reduced amnestic effects may look like they induce more the translatability of findings.
drug-seeking behavior, purely due to better memory rather than
increased reward, or again, reward-like effects might be masked by
amnestic effects. ICSS, on the other hand, can be sensitive to the 4. Tolerance to BDZ effects, BDZ
anticonvulsant effects of BDZs (100), as electrical stimulation of the withdrawal, and GABAAR subtypes
forebrain can induce seizure activity. As anticonvulsant effects of
BDZs are largely mediated by α1GABAARs (9), sparing binding to 4.1. BDZ tolerance
this subunit could increase ICSS thresholds (i.e., reduce apparent
reward-facilitation by the compound) because of increased seizure Tolerance occurs at different rates for the different behavioral
susceptibility independent of any reward-related effects. effects of BDZs, with rapid development of tolerance to the sedative
Self-administration studies often involve training with a drug and hypnotic effects, followed by the anticonvulsant effects (46,
that easily supports the acquisition of the operant behavior (e.g., 57, 108–113). Tolerance to the anxiolytic effects is delayed and
cocaine), and then the ability of different drugs to maintain self- inconsistent in animal studies (114–118) and seems to be rare or non-
administration is tested. However, as noted in the above sections, for existent in humans (46, 109, 110, 119, 120). Similarly, amnestic effects
most drugs of abuse, even a single exposure can lead to long-lasting of BDZs do not seem to be attenuated during chronic treatment (111,
plastic effects in the brain reward circuitry. Moreover, we have noted 121–124). Lack of tolerance to amnestic effects can be considered
that although there are points of convergence in the overall effects of a disadvantage, as amnestic effects are an undesirable side-effect of
drugs of abuse on the brain, the specific nature of these plastic effects BDZs in most of their uses, particularly in case of elderly patients who
depends on the properties of the drug. Based on this information, take BDZs long-term, often for sleep problems (125–127).
perhaps it is not surprising that Shinday et al. (69) found that the drug A few studies addressed the question of whether the chronic
history of the animal determines whether an α1-sparing compound modulation of specific GABAAR subtypes would lead to the same
will maintain self-administration behavior or not. Thus, whether the type of tolerance to specific behavioral effects as non-selective
animals received other compounds prior to testing and the specific BDZs. Vinkers et al. (118) investigated the sedative, anxiolytic, and
properties of these compounds have the potential to affect outcomes hypothermic effects of acute diazepam in mice treated chronically

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with diazepam, bretazenil [partial, non-selective GABAAR positive least sedative tolerance can be circumvented through the use of
allosteric modulator (PAM)], zolpidem (α1-preferring PAM), or α1GABAAR-selective compounds.
TPA023 (α2/3 preferring PAM). Tolerance was observed to all three
effects in chronic dizepam treated animals. In bretazenil treated mice,
cross-tolerance to anxiolytic and hypothermic effects were observed,
although there was no tolerance to sedative effects. Most strikingly, 4.2. BDZ withdrawal
zolpidem-treated mice showed full tolerance only to the hypothermic
effects of diazepam, while no tolerance to any of the effects was Tolerance and dependence are often viewed as related
observed in TPA023-treated mice. phenomena, both stemming from compensatory changes in
At first sight, the finding that zolpidem did not lead to sedative the affected receptors and systems over prolonged exposure.
tolerance is particularly surprising, as the sedative effects of BDZs However, experimental evidence suggests that the development of
are mediated primarily by the α1GABAARs, raising the expectation BDZ tolerance is not an indication that the individual will experience
that sedative tolerance would also be observed with a compound physical dependence to BDZs. On the contrary, BDZ tolerance and
that is selective for α1GABAARs. However, studies conducted on withdrawal seem to be independent phenomena where withdrawal
mice with mutations that render specific GABAAR subunits BDZ- symptoms can be observed in behavioral domains where no tolerance
insensitive indicate that BDZ-binding to α5GABAARs is required for was observed and vice versa (64, 142). This behavioral distinction
the development of tolerance to the sedative effects of BDZs (128). In between tolerance and withdrawal is accompanied by distinct
wild-type mice, the development of tolerance to the sedative effects molecular effects of long-term exposure to BDZs vs. discontinuation
of diazepam was associated with a decrease in the expression of α5 of treatment [e.g., (143)].
subunits in the dentate gyrus. In the context of these findings, lack of Common BDZ withdrawal symptoms include agitation, anxiety,
tolerance to zolpidem’s sedative effects can be attributed to its lack of mood swings, muscle tension and spasms, feeling of “pins and
affinity for the α5GABAARs. In line with this, chronic treatment with needles”, perceptual sensitivity to light and sound, and seizures.
a non-selective BDZ can cause cross-tolerance to the sedative effects Severe withdrawal can involve hallucinations and paranoid delusions,
of zolpidem (129), presumably due to the fact that chronic BDZ depersonalization, and can be fatal (57, 144, 145). Withdrawal
exposure has led to changes in dentate gyrus α5GABAAR expression symptoms appear within 2–3 days of cessation for short-acting
during this time. Chronic BDZ treatment also causes a reduction in BDZs and 5–10 days for longer-acting BDZs (137). Severe symptoms
the expression of α1GABAARs in cortex (74, 130), which may also can mostly be avoided by gradual discontinuation over 6–8 weeks.
play a role in sedative tolerance. However, even with managed discontinuation, it is estimated that up
Overall, studies suggest that α1-preferring compounds may cause to half of the patients develop some level of withdrawal symptoms
little or no sedative tolerance compared to non-selective BDZs, (145). For instance, in a study where patients were withdrawn
however, findings are far from unequivocal (66, 74, 131–135). from BDZs with individually calculated and managed withdrawal
Moreover, as α1-preferring compounds are used long-term primarily parameters over 2 weeks, with clinical monitoring every 48 h
for their hypnotic effects, the more clinically relevant question is including physical examination and intensive psychological support
whether tolerance develops to their hypnotic effects through chronic and psychoeducation, 6 out of 9 long-term lorazepam users failed to
use. While some animal studies suggest that tolerance develops to discontinue the drug (57), demonstrating the significant challenge
the sleep-promoting effects of zolpidem over chronic administration imposed by withdrawal symptoms to discontinuation of BDZs.
(136), clinical work suggests less tolerance to the hypnotic effects of Not surprisingly, particularly for patients who have been using
zolpidem compared to non-selective BDZs, at least at lower doses BDZs long-term (i.e., more than 6 months) or at high doses (e.g.,
(137, 138). equivalent of 100 mg diazepam per day or more), hospitalization
Additionally, despite demonstrating tolerance to the anxiolytic- during the withdrawal period and pharmacological management of
like properties of diazepam, the Vinkers et al. (118) study suggests the symptoms is recommended (145, 146).
that α2/3-selective compounds may provide anxiety relief even Overall, withdrawal symptoms upon BDZ discontinuation are
chronically, without any apparent tolerance to the anxiolytic effects. common and serious. Withdrawal symptoms are the main driver
As α1-sparing compounds also do not cause sedation, this would of BDZ misuse and can contribute to abuse where users may start
be the ideal scenario for a long-term, effective anxiolytic. However, using BDZs primarily for their positive effects as outlined above, but
while previous studies suggested that α1GABAARs are required for are drawn into an abuse cycle as the primary motivator behind use
the sedative effects of BDZs, recent preclinical work suggests that switches to the avoidance of withdrawal symptoms (147).
at high occupancy levels, BDZ binding to α3GABAARs may be Despite the clear significance of withdrawal symptoms and the
sufficient to produce sedation (25). These preclinical findings also availability of tools, such as gene-targeted mouse models and some
help to explain clinical findings that MK-409, a compound with pharmacological compounds with at least some selectivity for specific
selective efficacy at the α2/3GABAARs caused sedation in healthy GABAAR subtypes, the role of specific GABAAR subtypes in BDZ
volunteers (139). withdrawal symptoms has been addressed in only a few studies. Work
The most relevant aspect of tolerance development to BDZ in NHPs has demonstrated withdrawal signs after the discontinuation
misuse and abuse would be the escalation of dose over use in order of α1-preferring compounds and the recapitulation of flumazenil
to attain the previous levels of pharmacological effect. However, (non-selective BDZ antagonist) precipitated withdrawal by α1-
studies show that escalation to higher doses over long-term use selective antagonists (73, 113, 129). However, these studies included
is rare with BDZs (44, 45, 140, 141). In summary, tolerance to measurement of only a small subset of typical BDZ withdrawal
specific effects of BDZs does not constitute a major problem from symptoms and it is not clear whether α1-preferring agents might
the perspective of BDZ misuse and there is some evidence that at engender only a subset of withdrawal symptoms. Similarly, the

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duration or severity of withdrawal symptoms were not evaluated for instance, are causally involved in the development of a
systematically in comparison to non-selective BDZs, leaving open withdrawal syndrome following the cessation of chronic BDZ
the possibility that withdrawal from α1-preferring compounds might treatment (156). When the drug is withdrawn at the end of
be milder, at least on certain symptoms, and/or briefer than that chronic BDZ treatment, there is often an asymptomatic refractory
from non-selective BDZs. Finally, there is some evidence that period of 3 to 5 days before the symptoms begin. Even for
discontinuation of α2/3-selective compounds may not result in a longest-acting BDZs, this refractory period is too long to be
BDZ-like withdrawal syndrome (73, 129). explained by the gradual clearance of the drug. During this
refractory period, glutamatergic synapses go through a number of
plastic changes with the insertion of AMPA receptors into the
synapse and their subsequent phosphorylation, leading to increased
4.3. Brain mechanisms of BDZ tolerance and AMPA/NMDA transmission ratio (157–163). Treatment with AMPA
withdrawal (but not NMDA) receptor antagonists during the refractory period
abolishes the development of the withdrawal syndrome (164–167),
While it is tempting to assume that tolerance and withdrawal demonstrating the causal involvement of this type of plasticity
result simply from a compensatory mechanism whereby the cell- in excitatory synapses in the development of the withdrawal
surface expression of the targeted receptor is reduced, BDZ tolerance symptoms. A reduction in NMDA receptor expression and function
and withdrawal seem to involve not only changes in GABAAR is observed secondary to this enhancement of AMPA-mediated
expression and function, but more complicated mechanisms that go conductance (167) and the administration of NMDA receptor
beyond the GABAergic system. antagonists during the symptomatic portion of the withdrawal
Starting with the GABAergic changes, several studies reported period can ameliorate symptoms (164). Even more strikingly, it
changes in the expression levels of mRNAs for GABAAR receptor was demonstrated that the co-administration of an NMDA receptor
subunits upon chronic BDZ administration and discontinuation. As antagonist during chronic lorazepam administration can abolish
these changes have been thoroughly reviewed elsewhere (148) and tolerance to the anticonvulsant effects of lorazepam, although
seem to be complex and dependent on the brain area investigated, an overall reduction of BDZ-binding sites was observed in
the specific BDZs employed, length and dose of administration, NMDA antagonist administered animals similar to controls (109),
and whether the measures are taken at the end of the chronic suggesting that glutamatergic mechanisms may be more important
administration period or following withdrawal, we will provide only for the development of tolerance and dependence than changes in
a brief synopsis of the most common findings here. GABAAR expression.
The most common changes following chronic administration of The involvement of other systems and receptors [e.g., nitric
BDZs are in expression of the α1 and α4 subunits (74, 149–154). oxide, (168); adenosine, (169); neuropeptide systems (170)] in
While the findings have been mixed in terms of the presence of an the development of BDZ tolerance and/or withdrawal has been
effect, where effects were found, they were often in the direction suggested, however, it is not clear whether the changes in these
of a reduction in α1 expression and an increase in α4 expression. systems are essential for tolerance/dependence development or
Reduction in α1 expression in the cortex and the hippocampus secondary to the observed changes in the glutamatergic and
has also been reported following withdrawal from chronic BDZs GABAergic systems.
(124, 154). In experiments conducted in rat cerebellar granule cells, Despite the well-established essential role of excitatory synaptic
5-day exposure of the cells to diazepam resulted in a decrease in plasticity in the development of BDZ tolerance and withdrawal and
α1 expression similar to the above in vivo studies (143). Withdrawal close interactions between the glutamatergic and GABAergic systems,
of diazepam, however, led to both a decrease in α1 and an increase it is not known whether chronic modulation of specific GABAAR
in α4, suggesting discrete effects of chronic exposure and tolerance subtypes may lead to more rapid or enhanced glutamatergic
on GABAAR subunit expression. Withdrawal from zolpidem, an plasticity. An understanding of these interactions would be
α1-preferring compound, led to similar changes in α1 and α4 essential for predicting dependence liability of subunit-specific
expression as diazepam exposure in vitro (155). Similar reductions GABAAR modulators. Similarly, it is not clear how GABAAR
in α1 expression (in addition to α3 expression) were observed in subtypes may interact with other neurotransmitter systems in a way
the somatosensory cortex of mice following chronic exposure to that might exacerbate the observed tolerance and dependence
zolpidem in vivo (74). An important conclusion of these findings symptoms, even if those neurotransmitter systems are not
is that the changes observed in GABAAR subunit expression are causally involved in the development of BDZ tolerance or BDZ
not limited to the subunits that are modulated by a given drug. We withdrawal syndrome.
observe changes in the expression of the α4 subunit, which BDZs
do not bind, following BDZ exposure and withdrawal, as well as
changes in the α3 and α4 subunits following chronic exposure to an
α1-preferring compound (74, 143, 155). Thus, for chronic exposure 4.4. Issues related to interpretation and
or withdrawal following a subtype-specific compound, we cannot translation of findings from animal studies of
assume that the GABAAR changes will be limited to the GABAAR BDZ tolerance and withdrawal
subtype that is affected by this compound.
In addition to the above complex changes taking place in While hippocampal plasticity, which has been the focus of
the GABAARs, BDZ tolerance and withdrawal involve other most studies related to BDZ withdrawal, is likely to be involved
neurotransmitter systems in the brain. The glutamatergic system in the development of several withdrawal symptoms, it is highly
and synaptic plasticity involving NMDA and AMPA receptors, likely that the development of tolerance to different behavioral

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effects of BDZs and the development of different withdrawal in interaction with early life adversity (190, 191). In cocaine-
symptoms following BDZ discontinuation involve different brain dependent individuals, GABRA2 SNPs were associated with cocaine
areas. Similarly, different GABAAR subtypes might be involved in cue reactivity (192). The involvement of α2GABAARs in some,
different withdrawal symptoms. Thus, behavioral studies covering but not all, effects of cocaine has also been confirmed in rodent
all common withdrawal symptoms and systematically investigating studies (191, 193, 194). Finally, long-term exposure to cocaine
the development of each following chronic modulation of a specific was found to cause changes in the expression of α2GABAARs in
GABAAR subtype followed by drug discontinuation are needed. the hippocampi of rodents (195), however, this finding was not
If only specific symptoms develop following discontinuation confirmed in postmortem studies of hippocampi from individuals
of a GABAAR subtype-specific modulation, this can also be with cocaine use disorder (187). Others found that cocaine
used as an opportunity to study the brain mechanisms of use disorder was associated with disruptions in several GABA-
specific withdrawal symptoms. The studies reviewed above, related genes in the postmortem dorsolateral prefrontal cortex,
while informative, have not undertaken a detailed study of the including GABRA1 and GABRA4. Interestingly, no changes were
withdrawal phenomenon and its mechanisms, and during a time observed in genes related to glutamate signaling, emphasizing the
other areas of neuroscience and neuropharmacology research special role of GABARs in the pathophysiology of substance use
have seen an explosion of new findings with unprecedented disorders (196).
detail, our understanding of BDZ withdrawal has progressed
relatively little since the early studies conducted in 1990s and
early 2000s. 6. Conclusions and directions
As seen, our knowledge regarding the involvement of specific
GABAAR subtypes in all areas relevant to BDZ misuse and abuse, that
5. GABAARs in alcohol and other is, reward processes, drug-related maladaptive behaviors, tolerance,
substance use disorders and withdrawal, is characterized by gaps and a lack of systematic and
mechanistic studies. Due to its central role in both BDZ misuse and
GABAARs are expressed heavily in most brain regions involved BDZ abuse, an understanding of the mechanisms of BDZ withdrawal
in the effects of drugs of abuse and modulate the activity of brain and how each GABAAR subtype is involved in the initiation and
circuits involved in the behavioral effects of drugs (171). As such, continuation of the withdrawal syndrome is particularly important.
it is not surprising that different GABAARs have been implicated Research so far suggests that α1-sparing compounds would be
in the effects, use, and abuse of other drugs. Of these, alcohol is highly desirable as anxiolytics, as they have the potential to provide
arguably the most relevant for discussion here due to its shared anxiolysis without sedation and seem to have reduced abuse and
GABAergic mechanism. misuse liability due to the apparent role of α1GABAARs in both
Similar to BDZs, alcohol achieves most of its behavioral the reward-related effects of BDZs and the development of a
and subjective effects through positive allosteric modulation BDZ withdrawal syndrome upon cessation. However, some studies
of GABAARs. Unlike BDZs, however, at high concentrations, suggest the possible involvement of other GABAAR subtypes in
alcohol modulates all GABAARs in an unselective manner, these processes as well and it is not clear whether abolishing action
whereas at low concentrations (i.e., “social” drinking), synaptic at the α1GABAARs is sufficient to overcome potential for abuse
GABAARs are mostly insensitive to alcohol’s effects, whereas and misuse. Considering the increasing burden of BDZ abuse, the
extrasynaptic, BDZ-insensitive GABAARs containing the δ common practice of BDZ misuse resulting in severe BDZ dependence
subunits are highly sensitive to these low alcohol concentrations in many patients, and the current efforts to produce subtype-
(172, 173). With chronic exposure, extrasynaptic responsiveness specific GABAAR modulators as alternatives to classical BDZs,
to ethanol decreases while synaptic responsiveness increases, there is an urgent need for systematic and mechanistic research in
with a concurrent relocation of α4GABAARs from extrasynaptic this area.
to synaptic locations (174). Changes in the expression and
trafficking of other GABAARs, some of them similar to those
observed with BDZ exposure, are also observed following chronic
Author contributions
exposure to ethanol in animal models (175–178). In humans,
EE conducted the literature search and wrote the manuscript.
several studies identified associations between GABRA2 gene
(encoding the α2 subunit of the GABAAR) variations and alcohol
use disorder (179–184). However, GABRA2 single nucleotide Funding
polymorphisms (SNPs) failed to reach significance on genome-
wide association studies (GWAS) using more conservative This work was supported by Eunice Kennedy Schriver National
analysis methods (185, 186). Still, GABRA2 gene expression Institute of Child Health and Human Development/National
was reduced in the hippocampi of alcohol dependent individuals in Institutes of Health (NIH) Grant R01HD104656 (EE).
postmortem analyses (187). Others have found associations between
polymorphisms in GABRA1 and GABRA6 genes and alcohol
dependence (188, 189), however, again, these genes were not hits in Conflict of interest
GWAS studies.
Polymorphisms in the GABRA2 gene have also been implicated EE has received compensation as a consultant from Sensorium
in stimulant (cocaine) and opioid (heroin) use disorders, particularly Therapeutics in the last year.

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Publisher’s note organizations, or those of the publisher, the editors and the reviewers.
Any product that may be evaluated in this article, or claim that may
All claims expressed in this article are solely those of the be made by its manufacturer, is not guaranteed or endorsed by the
authors and do not necessarily represent those of their affiliated publisher.

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