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Pharmacological Research 191 (2023) 106746
Available online 29 March 2023
1043-6618/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Role of neuronal nicotinic acetylcholine receptors in
cannabinoid dependence
☆
Belle Buzzi
a
,
*
, Eda Koseli
a
, Lauren Moncayo
a
, Mohammed Shoaib
b
, M Imad Damaj
a
a
Department of Pharmacology and Toxicology, Virginia Commonwealth University, Richmond, VA, USA
b
Newcastle, UK
ARTICLE INFO
Keywords:
Nicotine
Nicotinic acetylcholine receptors
Smoking
Cannabis
THC
Cannabis use disorder (CUD)
ABSTRACT
Cannabis is among the most widely consumed psychoactive drugs around the world and cannabis use disorder
(CUD) has no current approved pharmacological treatment. Nicotine and cannabis are commonly co-used which
suggests there to be overlapping neurobiological actions supported primarily by the co-distribution of both re-
ceptor systems in the brain. There appears to be strong rationale to explore the role that nicotinic receptors play
in cannabinoid dependence. Preclinical studies suggest that the ɑ7 nAChR subtype may play a role in modulating
the reinforcing and discriminative stimulus effects of cannabinoids, while the ɑ4β2 * nAChR subtype may be
involved in modulating the motor and sedative effects of cannabinoids. Preclinical and human genetic studies
point towards a potential role of the ɑ5, ɑ3, and β4 nAChR subunits in CUD, while human GWAS studies strongly
implicate the ɑ2 subunit as playing a role in CUD susceptibility. Clinical studies suggest that current smoking
cessation agents, such as varenicline and bupropion, may also be benecial in treating CUD, although more
controlled studies are necessary. Additional behavioral, molecular, and mechanistic studies investigating the role
of nAChR in the modulation of the pharmacological effects of cannabinoids are needed.
1. Introduction
Cannabis is among the most widely consumed psychoactive sub-
stances worldwide. An estimated 4% of the world population aged
15–64 used cannabis at least once in 2019 (United Nations Ofce on
Drugs and Crime. [93].). In the United States, cannabis represents the
most abused federally illicit drug with 18% of the population reporting
past use (2020 NSDUH Detailed Tables | [12].). Studies estimate that
approximately 10–30% of the 48 million Americans who use cannabis
are likely to develop cannabis use disorder (CUD) [34,47]. Those that
are diagnosed with CUD exhibit a continued problematic pattern of use
despite negative consequences, which causes signicant distress or
impairment in cognitive function [63]. CUD is also characterized by the
development of tolerance, and withdrawal upon cessation [34].
Δ−9-Tetrahydrocannabinol (THC) is the main addictive component
in Cannabis sativa that acts in the central nervous system (CNS) by
binding to cannabinoid receptors (CB) in different brain areas to pro-
duce its ‘high,’ characterized by mild euphoria, relaxation, and
perceptual and cognitive alterations [29]. Additionally, clinical studies
have shown a similar array of withdrawal symptoms following absti-
nence from repeated oral THC ingestion or smoking cannabis [9,31].
Cannabis withdrawal during abstinence is comparative to that of to-
bacco or nicotine cessation, with reports of increased craving to resume
intake [18]. Despite the obvious societal implications of cannabis abuse,
mechanisms underlying THC dependence remain widely unknown, and
there are currently no FDA-approved medications for the treatment of
marijuana use disorder. With the dramatic shift in the legal, political,
and cultural landscape surrounding cannabis over the last decade, it is
likely that the number of individuals with CUD will increase in the
coming years as cannabis becomes more widely accepted and the
perceived risks continue to decline. Effective treatments for CUD are
clearly in need.
2. Neurobiology of cannabinoid dependence
The various pharmacological and behavioral effects of THC and
☆
The multifaceted activities of nervous and non-nervous neuronal nicotinic acetylcholine receptors in physiology and pathology. Eds: Dr Cecilia Gotti, Prof
Francesco Clementi, Prof Michele Zoli
* Correspondence to: Department of Pharmacology and Toxicology, Virginia Commonwealth University, 410 North 12th Street, PO BOX 980613, Richmond, VA
23298-0613, USA.
E-mail address: buzzib@vcu.edu (B. Buzzi).
Contents lists available at ScienceDirect
Pharmacological Research
journal homepage: www.elsevier.com/locate/yphrs
https://doi.org/10.1016/j.phrs.2023.106746
Received 21 December 2022; Received in revised form 15 March 2023; Accepted 27 March 2023
Pharmacological Research 191 (2023) 106746
2
cannabinoid agonists are mediated by the endocannabinoid system. The
endocannabinoid system is composed of two CB receptors (CB1 and
CB2) and two main endogenous ligands anandamide (AEA) and 2-arach-
idonoyl-glycerol (2-AG) and the associated enzymes responsible for
their synthesis and degradation. AEA and 2-AG are synthesized on de-
mand by their respective biosynthetic enzymes post-synaptically and
travel retrogradely to bind and activate pre-synaptic CB1 and CB2.
Additionally, endocannabinoid tone is highly regulated by several spe-
cic biosynthetic and metabolic enzymes. For example, AEA is synthe-
sized by N-acyl-phosphatidylethanolamine-specic phospholipase D
(NAPE-PLD) and later degraded by fatty acid amide hydrolase (FAAH).
Whereas diacylglycerol-lipases (DAGL
α
and DAGLβ) synthesize 2-AG
which is metabolized by monoacylglycerol-lipase (MAGL) [70].
2.1. Neurocircuitry of cannabinoid dependence
Both G
i
-coupled G-protein coupled receptors, CB1 and CB2 receptors
are understood to have specialized roles due to the high expression of
CB2 on peripheral immune cells and the primary expression of CB1 on
neurons in spinal cord and brain. Therefore, the effects of THC (among
other cannabinoids) and subsequent abuse potential are thought to be
mediated through central CB1 receptors. For example, activation of CB1
by ligands such as THC and its active metabolites activate the meso-
limbic reward pathway that regulates components of drug dependence
and withdrawal [70]. CB1 is highly expressed in cortical regions, with
the majority of mRNA levels detected in glutamatergic neurons [32,58].
Additionally, high levels of CB1 are also detected in GABAergic cells of
the striatum [43,51]. Therefore, CB1 is highly expressed in cells with
projections in dopaminergic pathways involved in drug abuse. In fact,
several CNS tissues and dopaminergic pathways are implicated in
cannabinoid tolerance, rewarding, and withdrawal-induced aversive--
like effects [26].
2.2. Interactions between nicotinic and cannabinoid systems in the brain
The endocannabinoid system shares anatomical and functional
similarities with nicotinic receptors. For example, rats tolerant to THC
show an increased endocannabinoid tone in limbic regions, such as the
amygdala, that express high levels of
α
7 nicotinic receptors [23,75].
Additionally,
α
7 expression overlaps in the cerebral cortex where
tolerance develops to THC metabolism following repeated administra-
tion [75,90]. Indeed, repeated activation of cannabinoid receptors by
agonists produces changes in brain regions with high expression of
several nicotinic receptor subtypes. Upon repeated activation, down-
regulation of CB1 occurs in the substantia nigra, a tissue with high
distribution of
α
5,
α
6 and β3 nicotinic receptor subunits. Although it is
clear the neurobiology of the endocannabinoid system interacts with the
nicotinic system in the brain, the exact relationship between nicotinic
receptor subtypes and cannabinoid dependence is not well organized.
3. Role of neuronal nicotinic receptors (nAChRs) in cannabinoid
dependence
A number of epidemiological and longitudinal studies showed that a
large number of cannabis users also identify as cigarette smokers, with
some suggesting that tobacco use may contribute to an increased inci-
dence of cannabis use disorder [4,67,69]. In addition, cannabis and
tobacco are noted as one of the most concurrently used drugs of abuse by
adolescents and young adults throughout the world [68]. The co-use of
tobacco and cannabis is frequent among cannabis users (40–78% use a
tobacco product) and can occur simultaneously, sequentially or asyn-
chronously [61,73]. However, clinical and preclinical studies investi-
gating this co-use/abuse are challenging because of the diversity in
tobacco and cannabis use patterns and cannabis commercial products.
While the relationship between smoking and cannabis use is compli-
cated, it does suggest a possible interaction between the nicotinic and
endocannabinoid neuronal systems. Although several studies have
sought to determine the role of the endocannabinoid system in nicotine
dependence, the question on how the nicotinic cholinergic system plays
a role in cannabinoid dependence is not well studied [14,25,44,59]. This
review seeks to outline and examine the current preclinical and clinical
studies available on the role of neuronal nAChRs in cannabinoid
dependence, as well as discuss what is further needed to advance our
understanding of the interplay between both systems.
3.1. The nicotinic cholinergic system
The primary psychoactive ingredient with addictive properties in
tobacco is nicotine. Nicotine acts as an agonist for the pentameric
ligand-gated ion channel receptors known as nicotinic acetylcholine
receptors (nAChRs). Agonist binding to these receptors causes a
conformational change in the ion channel, allowing positively charged
ions to move across it and therefore a depolarization of the plasma
membrane. These nAChRs are distributed throughout the central and
peripheral nervous system; however, this review will focus on the
neuronal nAChR system [92]. Although nAChRs may contain a variety
of functional subtypes with differing compositions of ɑ (2−10) and β
subunits (2−4), the main nAChR subtypes responsible for nicotine’s
rewarding effects are ɑ4β2 * (the asterisk denotes the possible presence
of other subunits) as the primary heteromeric receptor subtype and ɑ7 as
the primary homomeric receptor subtype. nAChR kinetics and func-
tional properties vary in relation to subunit composition. In particular,
α
4β2 * nAChRs display a high afnity for ACh and nicotine and lower
Ca2 +-permeability. On the other hand,
α
7-nAChRs have a relatively
low afnity for ACh and nicotine and are highly Ca2 +-permeable [28].
Additionally, heteromeric receptor subtypes are thought to contain 2
ligand binding sites between ɑ and β subunits, while homomeric re-
ceptor subtypes contain 5 identical ligand binding sites [92].
A crosstalk between the nicotinic and cannabinoid systems is sub-
stantiated by the overlapping distribution of cannabinoid and nicotinic
acetylcholine receptors in many brain regions involved in drug reward
and dependence, such as the VTA, which project to the nucleus
accumbens (NAc), and to other limbic structures, such as the amygdala,
hippocampus, and prefrontal cortex. Activation of nAChRs within the
VTA stimulates the ring rate and bursting activity of dopamine neurons
and dopamine release, specically in the NAc, a common mechanism
underlying drug-induced reward and reinforcement [22,54,65]. In this
area, nAChRs are located not only on cell bodies of dopamine neurons
but also on GABAergic cell bodies and terminals and on presynaptic
nerve endings of glutamatergic neurons [13]. Studies have additionally
implicated the role of cannabinoids on altering glutamate transmission,
indicating another cross-over of both systems [16].
3.2. Preclinical studies
3.2.1. nAChR involvement in cannabinoid reward and reinforcement
Self-administration paradigms in preclinical models serve as a means
of interpreting the potential reinforcing effects of drugs of abuse [62].
Self-administration models in animals use operant conditioning in
which, typically, a lever press results in the delivery of a reinforcer, such
as a drug. Drugs may be delivered using intravenous, oral, inhalation, or
insufation routes, among others. Intravenous (i.v.) self-administration
of Δ9-THC has been developed in non-human primates [40,83], and in
rodents [81,82]. I.v. self-administration was reported for other CB1 re-
ceptor agonists as well, such as WIN55,212–2, in rodents [3,41,80,89].
Recently, nicotine injections were reported to increase i.v.
self-administration of WIN55,212–2 and THC in adult rats, implicating a
potential role of nAChRs in cannabinoid self-administration [82]. As
both nAChRs and CB1Rs are colocalized in several brain areas involved
in reward, questions remain about the crosstalk mechanisms of both
systems in a model of cannabinoid self-administration. However, the
nAChR subtypes mediating this interaction is unknown.
B. Buzzi et al.
Pharmacological Research 191 (2023) 106746
3
Both ɑ4β2 * and ɑ7 nAChRs are highly expressed in regions of the
brain involved in reward, such as the mesolimbic dopaminergic system,
however Solinas and others suggest that the homomeric ɑ7 nAChRs, and
not ɑ4β2 * nAChRs, modulate the abuse-related behaviors and neuro-
chemical changes of cannabinoid dependence [79]. Using a relatively
selective ɑ7 antagonist, methyllycaconitine (MLA), they found that
systemic injections of MLA signicantly decreased intravenous
self-administration of CB1 agonist WIN55,212–2 in rats, while dihy-
dro-β-erythroidine (DHβE), a nAChR selective β2 * antagonist, had no
effect on WIN55,212–2 self-administration. Furthermore, MLA was able
to block THC-induced increases in dopamine levels in the nucleus
accumbens (NAc) shell, while DHβE did not. Similarly, another study
found that neuronal increase of kynurenic acid (KYNA), a non-selective
ɑ7 nAChR negative allosteric modulator (NAM), by a selective kynur-
enine 3-monooxygenase (KMO) inhibitor Ro 61–8048, signicantly
decreased i.v. self-administration of WIN55,212–2 and THC in rats and
squirrel monkeys, respectively [42]. Ro 61–8048 additionally prevented
a relapse-like effect induced by a priming dose of WIN55,212–2 in rats.
Neurochemically, Ro 61–8048 was able to block THC and WIN55,
212–2-induced elevations of dopamine in the NAc shell in rats. Pre-
treatment with ɑ7 nAChR positive allosteric modulators galantamine
and PNU120596 was able to reverse all of these behavioral and neuro-
chemical blockades by Ro 61–8048, further suggesting a role of ɑ7
nAChRs in cannabinoid intake and relapse in animal models [42].
Conditioned place preference (CPP) is another behavioral model
used to assess the potential rewarding effects of drugs using classical
conditioning learning paradigm [52]. In preclinical research, the basic
premise involves pairing the drug of interest, the unconditioned stim-
ulus, with an initially neutral environment, the conditioned stimulus,
typically either a dark side or a light side of a box, with differing tactile
stimuli, over several conditioning days. The subjects’ preference is then
tested in a drug-free state where they are allowed to freely explore both
sides over a period of time, typically measuring preference as the time
spent on the drug-paired side compared to either the vehicle-paired side
or the baseline preference. One study explored the effect of seven-week
exposure to nicotine, using either electronic cigarette (e-cig) delivery or
cigarette (cig) delivery, on THC-induced expression of preference using
the CPP paradigm in Balb/c mice [66]. They found that pre-exposure to
nicotine, using both e-cig and cig delivery increased sensitivity to THC’s
rewarding effects using a sub-threshold dose of THC that did not cause
an expression of preference in both naive mice and those pre-exposed to
air for seven weeks [66]. Additionally, they observed an increase in
ΔFosB in the NAc, as well as an increase in the AMPA receptor
GluA1/GluA2–3 subunit ratio in the groups that received pre-exposure
to nicotine, hypothesizing that these changes may be responsible for
the increased sensitivity to THC’s rewarding effects [66]. However, the
nAChR subtypes mediating the effects of nicotine are not known.
3.2.2. nAChR involvement in cannabinoid drug discrimination
Drug discrimination in animals is a preclinical paradigm that uses
operant conditioning to model the subjective, pharmacological, and
discriminative properties of drugs [94]. In short, the model uses drugs as
discriminative stimuli in order to signal the delivery of a reinforcer and
measures the ability of the animal to identify the drug of choice when
compared to a vehicle or other drugs [30].
The studies that examined the role of ɑ7 nAChRs versus non-ɑ7
nAChRs in cannabinoid self-administration also evaluated the ability of
ɑ7 and non-ɑ7 allosteric modulators to alter the discriminative prop-
erties of cannabinoids. In rats that were able to discriminate the effects
of both THC and WIN55,212–2, the ɑ7 antagonist MLA dose-
dependently blocked the discriminative stimulus of THC and
WIN55,212–2, while DHβE was not effective [79]. Ro 61–8048, a KMO
inhibitor that results in an increase in the ɑ7 negative allosteric modu-
lator KYNA, was conversely not able to block the discriminative effects
of WIN55,212–2 in rats but was able to cause a rightward shift of the
dose-response curve of the discriminative effects of THC in squirrel
monkeys, indicating a possible decrease in potency following Ro
61–8048 administration [42]. This study demonstrates a potential role
in the ɑ7 nAChR in modulating the subjective effects of CB1R agonists in
rats and squirrel monkeys, however additional studies are needed to
further conrm this.
In a separate study, researchers sought to determine whether both
the nAChR and muscarinic cholinergic systems (mAChR) are involved in
the modulation of cannabinoid discrimination in rats and to elucidate
the potential mechanism [79]. Both the nAChR agonist nicotine and the
mAChR agonist pilocarpine were able to potentiate the discriminative
stimulus effects of low doses of THC, without decreasing rates of
responding. After determining that nAChRs play a role in the modula-
tion of THC discrimination, they used the CB1 antagonist rimonabant to
reverse this potentiation, which was only successful in nicotine, and not
pilocarpine-induced potentiation. The ability of the CB1 antagonist to
block nicotine-induced potentiation of THC discrimination suggests that
this potentiation by nicotine may be due to release of endocannabinoids.
To further delineate this theory, a selective fatty acid amide hydrolase
(FAAH) inhibitor, URB-597, which increases levels of the endocanna-
binoid anandamide, signicantly increased nicotine-induced potentia-
tion of THC discrimination, while URB-597 had no effect on
pilocarpine-induced potentiation of THC discrimination, conrming
the idea that nicotinic receptor activation potentiates THC discrimina-
tion via endocannabinoid release [79].
3.2.3. nAChR involvement in cannabinoid withdrawal
Chronic cannabinoid use followed by discontinuation has been re-
ported to cause cannabinoid withdrawal syndrome in both humans and
animals, a major factor behind users not being able to quit successfully
[9,26]. There are currently no approved medications to treat cannabis
use disorder or cannabis withdrawal. Early investigations on THC and
nicotinic system interaction showed that repeated co-administration of
nicotine and THC led to an attenuation in tolerance to THC but with
enhancement of somatic signs of precipitated THC withdrawal in CD-1
mice [85]. As nicotine and cannabinoids are often concurrently used,
the question remains on how the nicotinic system may play a role in
cannabinoid withdrawal, including the specic receptor subtypes
involved.
Nicotinic receptors containing
α
3,
α
5 and β4 subunits were investi-
gated for their role in cannabis withdrawal. Human genetic studies
showed that genetic variation in the
α
5/
α
3/β4 nAChR subunit gene
cluster signicantly increases the risk of tobacco addiction. In particular,
polymorphisms in the
α
5 subunit gene (CHRNA5) present compelling
evidence for a genetic contribution to tobacco addiction vulnerability
and smoking-related diseases ([5]; for review see [6]). In addition, ro-
dent studies implicated these subunits in nicotine withdrawal [37,36].
Given these ndings, our recent study investigated the role of these
subunits in THC withdrawal [24]. We found that a selective ɑ3β4
antagonist, AulB, and a partial ɑ3β4 agonist, AT-1001, both
dose-dependently attenuated precipitated THC withdrawal somatic
signs in C57BL/6 J mice. We also observed that ɑ5 KO mice showed a
decrease in precipitated THC withdrawal somatic signs, implicating ɑ5,
ɑ3, and β4 nAChR subunits in THC withdrawal. Additionally, other
studies showed that the ɑ6 nAChR subunit, expressed in dopaminergic
neurons which co-assembles with β4 and β2 containing receptor sub-
types [17,45] may play a role in THC withdrawal. ɑ6 KO mice and mice
given BulA, an ɑ6β4 antagonist, showed reduced precipitated THC
withdrawal somatic signs, implicating the ɑ6 nAChR subunit in THC
withdrawal. However, ɑ7 and β2 KO mice showed no differences in the
intensity of THC of somatic signs of withdrawal. While these studies
indicate an important role of the ɑ5, ɑ3, ɑ6, and β4 nAChR subunits in
THC withdrawal, clear limitations exist in the rodent model of precipi-
tated withdrawal in contrast to spontaneous cessation in humans.
Therefore, the extent to which these symptoms offer translational rele-
vance is unclear. Likewise, THC plasma levels were not reported in these
studies, so it is unknown if these doses used for THC administration are
B. Buzzi et al.
Pharmacological Research 191 (2023) 106746
4
correlated to human THC intake.
3.2.4. nAChR involvement in adverse behavioral effects of cannabinoids
Cannabis use comes with several side effects, some of which may be
undesirable. Side effects are comprised of behavioral and physical
manifestations. As an example, cannabinoids are known to cause a
disruption in working memory, attention impairments, increased anxi-
ety, insomnia, etc. [10,15]. One theory as to why cannabinoid users tend
to also use nicotine, and vice versa, is that they may both decrease these
adverse effects and increase the reinforcing effects of one another [25,
85]. A study found that chronic nicotine pretreatment reduced the
anxiogenic-like effects that are induced by high THC doses in both the
elevated plus maze (EPM) test and social interaction (SI) test in rats,
with the study suggesting that chronic nicotine may induce neuroplastic
alterations that modulate the response to THC exposure [50].
In addition to increased anxiety-like behavior, repeated cannabinoid
use is speculated to cause working memory impairments in both pre-
clinical and clinical models [20,48]. One study found that THC
decreased working memory in a non-matching-to-position model in rats
and that administration of ɑ7 NAM had no effect on this, while in
squirrel monkeys, a THC-induced decrease in working memory using a
delayed matching-to-sample model was reversed by the same ɑ7 NAM,
suggesting a possible role of the ɑ7 nAChR in the modulation of
THC-induced changes in working memory [42].
Cannabinoid use also causes a myriad of motor impairment effects,
including hypolocomotion, catalepsy, and ataxia, the latter of which can
be undesirable, especially when considering cannabinoids for treatment
and/or therapy of motor disorders [55]. Interestingly, chronic nicotine
exposure attenuated acute THC-induced hypolocomotion in adult rats,
raising the possibility that chronic nicotine sensitizes neurons in the
mesolimbic dopaminergic system, thus altering the responses to THC on
this system [57]. Additionally, it is known that repeated nicotine
exposure causes an increase in endocannabinoids in the VTA and NAc,
which may also play a role in this modulation [27].
In addition, acute THC administration is known to cause cerebellar
ataxia, among other motor effects, largely due to a population of CB1
receptors in the cerebellum [71,77]. The impact of nicotine on canna-
binoids motor suppressant effects was investigated early on since there
is a population of nAChRs expressed in the cerebellum as well [19].
Indeed, intracerebellar micro infusion of THC in CD-1 male mice caused
ataxia, which was dose-dependently attenuated by micro infusions of
nicotine [77]. Central administration of hexamethonium, a
non-selective peripheral nAChR antagonist, was able to block this
attenuation, demonstrating a role of the nicotinic receptors in this
modulation of THC-induced ataxia. RJR-2403, a selective ɑ4β2 * nAChR
agonist, was also able to dose-dependently attenuate this ataxia, which
was then blocked by the selective β2 * nAChR antagonist DhβE, impli-
cating specically that receptor subtype in this attenuation of
THC-induced ataxia. MLA, a selective ɑ7 nAChR antagonist, was also
tested against both nicotine and RJR, ultimately having no effect on the
attenuation, showing that ɑ7 plays no role in nicotine attenuation of
THC-induced ataxia [77].
After conrmation that the ɑ4β2 nAChR plays a direct role in nico-
tine’s attenuation of THC-induced ataxia, the mechanism of this atten-
uation was further explored by the same team [78]. nAChR activation
has been implicated in long-term potential (LTP) via the nitric
oxide-cGMP signaling pathway, and there are high nitric oxide synthase
(NOS) levels in the cerebellum [33]. As nicotine-induced modulation of
locomotor activity has been previously shown to be blocked by inhibi-
tion of nitric oxide synthase (NOS) and CB1 agonists have been shown to
inhibit NOS in the cerebellum [76], the same group sought to explore the
role of this system in THC cerebellar ataxia [78]. Using intracerebellar
infusions of sodium nitroprusside (SNP), a nitric oxide (NO) donor,
THC-induced ataxia was attenuated, and RJR attenuation of
THC-induced ataxia was even further enhanced when infused with SNP,
revealing a possible additive effect, or potential shared mechanism, of
increased NO and ɑ4β2 receptor activation on attenuation of
THC-induced ataxia. Furthermore, S-methylisothiourea (SMT), a NOS
inhibitor, inhibited RJR-induced attenuation of THC-induced ataxia as
well as increased THC-induced ataxia when given alone. As NO donors
act on guanylyl cyclase (GC) to increase muscle relaxation, the re-
searchers then looked at increasing levels of GC activation on nicotine
attenuation of THC-induced ataxia. Intracerebellar infusions of iso-
liquiritigenin (ISO), a GC activator, and ODQ, a GC inhibitor, respec-
tively enhanced and antagonized RJR attenuation of THC-induced
ataxia [78]. Additionally, NOx levels were conrmed to be lowered after
acute THC administration in mouse cerebellar tissue using the uoro-
metric DAN method, and increased after both nicotine and RJR
administration, showing a clear role of NO donors and GC activation on
RJR attenuation of THC- induced ataxia [78]. Overall, nicotine seems to
play a role via ɑ4β* 2 nAChR subtypes through increased NO and GC
activation, to attenuate the ataxia caused by acute administration of
THC. All preclinical studies are summarized in Table 1A.
3.3. Human studies
3.3.1. Clinical studies
Many tobacco users also misuse cannabis, and the concurrent use of
these two substances can add to the health burden experienced by such
cannabis users and may increase the potential for substance-induced
harm and disorders above that caused by using one substance on its
own. Therefore, recent studies explored the possible effectiveness of
medications used in smoking cessation, such as varenicline and bupro-
pion, for the treatment of cannabis use disorder (CUD). Varenicline, an
α
4β2 subtype partial agonist and a full agonist at the
α
3β4 and
α
7 sub-
types [56], is the current most effective approved pharmacotherapy for
tobacco smoking cessation (see [39] for review).
Initially, a case series reported reductions in amount of enjoyment of
cannabis and self-reported cannabis use among ve cannabis- and
nicotine-dependent individuals receiving varenicline for 12 weeks [60].
In a later laboratory study, Herrmann et al. [35] demonstrated that
varenicline reduced tobacco use, craving, and negative affect in tobac-
co/cannabis co-users, although it had no effect on cannabis relapse. In
addition, a small pilot trial reported reduced cannabis craving, cannabis
use, and tobacco use when varenicline was given to individuals with
opioid use disorder for four weeks [1]. Finally, a recent proof-of-concept
pilot randomized trial assessed the initial efcacy of varenicline
(6-weeks trial) in cannabis-using individuals and found signicant de-
creases in self-reported cannabis withdrawal and greater rates of
self-reported abstinence [53].
A small number of studies looked at the efcacy of bupropion, an
antidepressant, and a smoking cessation agent [91] for the treatment of
cannabis dependence. In a double-blind and placebo-controlled study
looking at bupropion-sustained release in a 13-week outpatient treat-
ment program, there was no evidence for a signicant increase in the
probability of abstinence from cannabis or a reduction of the severity of
cannabis dependence and cannabis withdrawal symptoms [11]. How-
ever, bupropion was found to be effective in reducing symptoms expe-
rienced in marijuana withdrawal in a double-blind, placebo-controlled
study with chronic marijuana users [64]. All clinical studies are sum-
marized in Table 1B.
3.3.2. Human genetic studies
Based on several twin and family heritability studies, the heritability
of CUD initiation is estimated to be 30–70% which vary largely due to
environment factors [49,87,88]. Recent genome-wide association
studies (GWASs) of CUD have identied variants reaching genome-wide
signicance, with one locus on chromosome 8, tagged by a cis-eQTL for
CHRNA2 (encoding a nicotinic acetylcholine receptor), has been
robustly identied [21]. In addition, they found that individuals with
CUD show reduced expression of CHRNA2 in the cerebellum compared
to the controls. The locus on chromosome 8 was recently conrmed in a
B. Buzzi et al.
Pharmacological Research 191 (2023) 106746
5
large GWAS meta-analysis [38]. The comparison of the expression
proles in the Allan Brain Atlas [7], shows a negative correlation on the
cerebellar cortex and cerebellar nuclei between CHRNA2 and CNR1
(cannabinoid receptor 1) gene expressions. These ndings suggest that
CHRNA2 expression which encodes for the
α
2 nAChR subunit may play
a functional role in susceptibility to developing CUD. Interestingly
CHRNA2 gene has been identied as one of the risk loci for nicotine
dependence and smoking behavior [46] and
α
2 nAChR KO mice show
reduced precipitated nicotine withdrawal signs [72]. Finally, in an as-
sociation study using the Gene and Environment (SAGE)
European-American and African-American datasets, our group investi-
gated the correlation between cannabis dependence and nAChRs [24].
The results of the study suggested that the following genes that code
nAChRs subunits were associated with CUD;
α
3 (CHRNA3) with toler-
ance,
α
5 (CHRNA5) with withdrawal, and
α
6 (CHRNA6) with both
tolerance and withdrawal. In addition, the correlation between CHRNA3
and CHRNA5 genes and CUD was signicant using the DSM-IV criteria
[24].
4. Looking ahead
Cannabis remains the most abused federally illegal drug in the
United States and is heavily used around the world. CUD, which is
characterized by continuous use of cannabis (or other cannabinoids)
despite negative consequences, has yet to have an approved pharma-
cological treatment. As nicotine is often co-used with cannabis and with
the overlapping distribution of CBRs and nAChRs in brain regions
involved in substance abuse, the role that the nicotinic system plays in
cannabis use and abuse represents a high area of research interest.
Preclinical studies suggest that the ɑ7 nAChR subtype may play a
role in modulating both the reinforcing and discriminative stimulus ef-
fects of cannabinoids, and in the modulation of THC-induced decrease in
working memory, using both behavioral and neurochemical assays.
Further studies exploring the role of this nAChR subtype in cannabinoid
drugs seeking, reinstatement, and choice preferences are necessary. For
that, developing selective ɑ7 NAMs with drug-like properties for clinical
use is needed. While some preclinical studies listed here used orthosteric
ɑ7 antagonists, this use may be problematic in clinical applications as
early rodent studies suggest that these drugs may cause working mem-
ory impairments [2]. Alternatively, ɑ7 partial agonists could be tested in
preclinical cannabis animal models for their potential efcacy. Several
ɑ7 partial agonists that were tested in clinical studies for schizophrenia.
Such as Encenicline and ABT-126 are possible candidates [84]. KMO
inhibitors, which increase levels of the ɑ7 NAM KYNA, is currently being
investigated as a treatment for other substances of abuse in preclinical
models [74,86], and for schizophrenia in clinical trials [8]. However, it
will be important to be further explored in its role in CUD. While the ɑ7
nAChR has been implicated in the rewarding effects of cannabinoids, via
endocannabinoid release, the ɑ4β2 * nAChR has been implicated in the
attenuation of cannabinoid induced motor impairments, via increasing
NO levels, suggesting that there may be differential roles of the neuronal
nicotinic acetylcholine system in the behavioral and neurochemical
modulation of the effects of cannabinoids.
Both preclinical and human genetic studies indicate a potential role
of ɑ5, ɑ3, and β4 nAChR subunits in CUD phenotypes, including toler-
ance and withdrawal. Human GWAS studies strongly implicated the ɑ2
subunit as playing a possible functional role in both susceptibility to
CUD and in nicotine withdrawal. Clearly the role of CHRNA2 gene needs
to be elucidated further in terms of its role in cannabinoid pharmacology
and dependence. As there is no current selective ligand for the ɑ2
subunit-containing subtypes, there is a need to determine not only which
set of subunits play a role in cannabinoid dependence but also the
location of these subtypes that are thought to be correlated with
increased susceptibility of CUD. Additionally, as both ɑ3β4 and ɑ5
containing subunits were implicated as playing a role THC withdrawal
in preclinical and genetic studies, these should also be considered a
potential target to further explore in CUD.
Smoking cessation agents that target nicotinic receptors, such as
Table 1A
Preclinical studies.
Behavioral Test Species Strain Sex Drugs Exposure Result Reference
THC Elevated Plus Maze Rats Sprague-Dawley M/
F
Nicotine 14 days ↓ [50]
THC Social Interaction Rats Sprague-Dawley M/
F
Nicotine 14 days ↓ [50]
Somatic Signs of Precipitated
THC Withdrawal
Mice C57BL/6 J M ɑ3β4 antagonist AulB, ɑ3β4 partial agonist AT-1001, ɑ6β4
antagonist BulA
Single
dose
↓ [24]
Self-administration of THC
and WIN55,212–2
Rats, Squirrel
Monkeys
Sprague-Dawley,
Long-Evans,
M KMO inhibitor Ro 61–8048, β2 * antagonist DHβE, ɑ7
antagonist MLA, Nicotine, ɑ7 PAMs galantamine and
PNU120596
Mixed ↓/↑ [42,79,
82]
Drug Discrimination of THC
and WIN55,212–2
Rats Sprague-Dawley M Nicotine, Pilocarpine, ɑ7 antagonist MLA, β2 * antagonist
DHβE
Single
dose
↓/↑ [79]
Rotorod (THC Induced
Ataxia)
Mice CD-1 M Nicotine, ɑ3β4 agonist RJR-2403, Single
dose
↓ [77]
THC CPP Mice Balb/c M Nicotine 7 weeks ↑ [66]
THC Locomotor Activity Rats Sprague-Dawley M/
F
Nicotine 14 days ↑ [57]
↓ =reduced response; ↑ =increased response; ↔ =no reliable response, ↓/↑ =mixed responses
Table 1B
Clinical studies.
Outcomes Drug of Abuse Sex Drug Tested Exposure Result Reference
Usage Nicotine and/or Cannabis M Varenicline (Chantix®) 2–6 weeks ↓ [60]
Mood, Cravings, Sleep, Usage Tobacco and Cannabis Co-users M/F Varenicline, Nabilone 16 days ↓/↑ [35]
Usage, Cravings, Withdrawal Co-occurring Cannabis and Tobacco use M/F Varenicline 4 weeks ↓ [1]
Usage, Withdrawal Cannabis M/F Varenicline 6 weeks ↔ [53]
Craving, Withdrawal, Sleep, Cognition Cannabis M/F Bupropion
(Zyban® SR)
21 days ↓/↑ [64]
Usage and Withdrawal Cannabis M/F Bupropion, Nefazodone 13 weeks ↔ [11]
↓ =reduced response; ↑ =increased response; ↔ =no reliable response, ↓/↑ =mixed responses
B. Buzzi et al.
Pharmacological Research 191 (2023) 106746
6
varenicline and bupropion, have been investigated in the treatment of
CUD and while the pilot studies showed promising results, large and
controlled clinical trials are essential. Overall, further behavioral studies
and molecular mechanisms of the role that nAChRs play in the modu-
lation of the rewarding and aversive effects of cannabinoids are needed.
CRediT authorship contribution statement
B. Buzzi: Conceptualization, Visualization, Writing - original draft,
Writing - review & editing. E. Koseli: Visualization, Writing - original
draft, Writing - review & editing. L. Moncayo: Writing - original draft,
Writing - review & editing. M. Shoaib: Writing - original draft, Writing -
review & editing. M.I. Damaj: Conceptualization, Supervision, Funding
acquisition, Writing - original draft, Writing - review & editing.
Declarations of Competing Interest
No conict of interests.
Data Availability
No data was used for the research described in the article.
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