molecules
Review
Is the Antidepressant Activity of Selective Serotonin Reuptake
Inhibitors Mediated by Nicotinic Acetylcholine Receptors?
Hugo R. Arias 1, *,† , Katarzyna M. Targowska-Duda 2,† , Jesús García-Colunga 3,†
1
2
3
4
*
†
Citation: Arias, H.R.;
Targowska-Duda, K.M.;
García-Colunga, J.; Ortells, M.O. Is
the Antidepressant Activity of
Selective Serotonin Reuptake
Inhibitors Mediated by Nicotinic
Acetylcholine Receptors? Molecules
2021, 26, 2149. https://doi.org/
10.3390/molecules26082149
Academic Editor: Dina Manetti
Received: 12 March 2021
Accepted: 5 April 2021
Published: 8 April 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
and Marcelo O. Ortells 4
Department of Pharmacology and Physiology, Oklahoma State University College of Osteopathic Medicine,
Tahlequah, OK 74464, USA
Department of Biopharmacy, Medical University of Lublin, 20-093 Lublin, Poland; katarzyna.duda@umlub.pl
Departamento de Neurobiología Celular y Molecular, Instituto de Neurobiología, Campus Juriquilla,
Universidad Nacional Autónoma de México, Querétaro 76230, Mexico; garciacolunga@unam.mx
Facultad de Medicina, Universidad de Morón, CONICET, Morón 1708, Argentina; mortells@retina.ar
Correspondence: hugo.arias@okstate.edu; Tel.: +1-918-525-6324; Fax: +1-918-280-2515
These authors contributed equally to this work.
Abstract: It is generally assumed that selective serotonin reuptake inhibitors (SSRIs) induce antidepressant activity by inhibiting serotonin (5-HT) reuptake transporters, thus elevating synaptic
5-HT levels and, finally, ameliorates depression symptoms. New evidence indicates that SSRIs
may also modulate other neurotransmitter systems by inhibiting neuronal nicotinic acetylcholine
receptors (nAChRs), which are recognized as important in mood regulation. There is a clear and
strong association between major depression and smoking, where depressed patients smoke twice as
much as the normal population. However, SSRIs are not efficient for smoking cessation therapy. In
patients with major depressive disorder, there is a lower availability of functional nAChRs, although
their amount is not altered, which is possibly caused by higher endogenous ACh levels, which
consequently induce nAChR desensitization. Other neurotransmitter systems have also emerged
as possible targets for SSRIs. Studies on dorsal raphe nucleus serotoninergic neurons support the
concept that SSRI-induced nAChR inhibition decreases the glutamatergic hyperstimulation observed
in stress conditions, which compensates the excessive 5-HT overflow in these neurons and, consequently, ameliorates depression symptoms. At the molecular level, SSRIs inhibit different nAChR
subtypes by noncompetitive mechanisms, including ion channel blockade and induction of receptor
desensitization, whereas α9α10 nAChRs, which are peripherally expressed and not directly involved
in depression, are inhibited by competitive mechanisms. According to the functional and structural
results, SSRIs bind within the nAChR ion channel at high-affinity sites that are spread out between
serine and valine rings. In conclusion, SSRI-induced inhibition of a variety of nAChRs expressed
in different neurotransmitter systems widens the complexity by which these antidepressants may
act clinically.
Keywords: selective serotonin reuptake inhibitors; antidepressants; nicotinic acetylcholine receptors;
noncompetitive antagonists; neuronal pathways; molecular modeling
published maps and institutional affiliations.
1. Introduction
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Major depressive disorder (MDD), also known as clinical depression, is a neuropsychiatric disorder characterized by depressed mood, sleep and cognitive disturbances, anxiety,
anhedonia or decreased interest in enjoyable activities, and suicidal thoughts, among other
symptoms [1]. Although the etiology of MDD and related depression disorders is not fully
understood, it is very complex and depends on neuronal, genetic, biological, cognitive,
sociocultural, and psychological aspects [2–4].
Previous studies have determined that the malfunctioning of certain brain regions
could be one important aspect in the development of MDD [5]. The most important brain
abnormalities associated with depression are located in the prefrontal neocortex, limbic
Molecules 2021, 26, 2149. https://doi.org/10.3390/molecules26082149
https://www.mdpi.com/journal/molecules
Molecules 2021, 26, 2149
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system, basal ganglia, and brain stem regions. In animal models of depression, as well
as in depressed patients, the size and number of synapses are significantly reduced in
several brain areas, including the nucleus accumbens (NAc), amygdala, cingulate cortex,
–
hippocampus, and prefrontal cortex (PFC) [2–4].
Several hypotheses have been described and proposed in order to elucidate the
etiology of MDD, but each one includes various neurotransmitter systems [6,7]. One
of the first and most extensive depression hypotheses is associated with the decrease
in extracellular neurotransmitter levels, such as serotonin (5-HT), dopamine (DA), and
noradrenaline (NA) [8]. In this regard, substances that might elevate monoamine levels
would be beneficial for depression treatment. Since the 1950s, MDD patients have been
treated with monoamine oxidase inhibitors, which block monoamine reuptake and/or
metabolism (e.g., selegiline) [9,10]. Among monoamine reuptake inhibitors, selective
serotonin reuptake inhibitors (SSRIs; molecular structures are presented in Figure 1) have
been classified as some of the most effective substances [7].
NH
O
H2N
NH
O
F
F
F
F
F
Fluoxetine
O
Cl
Sertraline
Nor-fluoxetine
H
N
Cl
F
F
O
O
O
N
N
F
Paroxetine
Citalopram
Figure 1. Molecular structures of clinically used selective serotonin reuptake inhibitors (SSRIs) that
also inhibit various nicotinic acetylcholine receptor (nAChR) subtypes.
The cholinergic–adrenergic hypothesis proposes that the underlying cause of depres–
sion is an imbalance between the content of acetylcholine (ACh) and NA, producing an
overstimulation of the cholinergic system over the noradrenergic system [11]. Cholinergic
neurotransmission, which is mediated by muscarinic and nicotinic receptors, has been
related to various neurophysiological processes, such as attention, learning and memory, mood, and appetite changes, as well as pathological conditions, including anxiety
disorders and MDD [12–14]. Higher than normal extracellular levels of ACh and its
– have been observed in patients with depression [14–16].
hydrolysis metabolite, choline,
–
Moreover, treatment with physostigmine, an acetylcholinesterase (AChE) inhibitor that
increases the levels of synaptic ACh, elicited depression symptoms in humans [17–19] and
–
depression-like effects in rodents [20].
Mounting evidence supports the view that other neurotransmitter circuits and pathways are involved in the clinical activity of SSRIs, including those modulated by nicotinic
ACh receptors (nAChRs). This review focuses on functional, preclinical, therapeutic, and
structural studies that support the activity of SSRIs on different nAChR subtypes.
2. Selective Serotonin Reuptake Inhibitors (SSRIs)
To date, the most accepted and best characterized mechanism of action for SSRIs is
centered on the inhibition of serotonin transporters (SERT) located at presynaptic terminals.
The first action of SSRIs is to inhibit SERT, increasing the synaptic levels of 5-HT up
to seven-fold [21–23]. This elevated 5-HT concentration, in turn, activates a variety of
postsynaptic 5-HT receptors in several brain areas, with result in changes in serotoninergic
Molecules 2021, 26, 2149
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signaling, which is believed to attenuate depression conditions [24,25]. Furthermore, high
extracellular concentrations of 5-HT trigger a negative feedback mechanism involving
5-HT1A autoreceptors, which regulate 5-HT levels in the synaptic cleft. A high endogenous
concentration of 5-HT may be sufficient for inhibiting nAChRs [26,27].
SSRIs are extensively used medications for the treatment of MDD and other neuropsychiatric maladies, including obsessive–compulsive disorders, panic disorders, alcoholism,
obesity, migraines, and chronic pain [25,28,29]. For example, sertraline and paroxetine
are prescribed for the treatment of panic attacks, obsessive–compulsive disorders, posttraumatic stress disorders, social anxiety disorders, and premenstrual dysphoric disorders,
a severe form of premenstrual syndrome (https://www.webmd.com/corporate/default.
htm). Fluoxetine is also employed in the treatment of eating disorders and slow-channel
congenital myasthenic syndromes [29,30]. A common feature in both conditions is that of
higher ACh concentrations compared to those in the normal population [20,30,31]. In slowchannel congenital myasthenic syndromes, high levels of ACh open slow-channel mutant
nAChRs with long-lasting durations, allowing more cation influx into the endplate after
an early onset of progressive muscle weakness [30,32]. Therefore, fluoxetine restores the
pathological condition of congenital myasthenic syndromes by decreasing both the channel
opening frequency and channel open time, as well as increasing the channel closed time.
Although SSRIs are used in many clinical conditions, the exact mechanisms underlying
their clinical effects are not very clear. A plausible mechanism is that SSRIs, in addition to
inhibiting SERT (e.g., fluoxetine inhibitory potency (IC50 ) = 6–17 nM [33–35]), modulate
other targets, including a variety of nAChRs.
3. Nicotinic Acetylcholine Receptor Functions Modulate Depression States
nAChRs are members of the pentameric ligand-gated ion channel superfamily that
also comprises 5-HT type 3, glycine, and γ-aminobutyric acid type A receptors. nAChRs
are involved in a variety of physiological and pathological processes in both neuronal
and non-neuronal tissues. In general, presynaptic nAChRs modulate the release of a
variety of neurotransmitters, including those implicated in depression [36,37], whereas
postsynaptic nAChRs mediate stimulatory transmission across the central and peripheral
nervous systems [38,39].
The malfunctioning of nAChRs (by excessive or decreased activity) expressed in
circuits regulating mood has been implicated in the development of anxiety states and
MDD [15,40,41]. As part of the same concept, an imbalance between the cholinergic and
noradrenergic systems—the so-called “cholinergic–adrenergic hypothesis of depression”—
where the former system is more sensitive or more stimulated than the latter might cause
MDD to develop [11]. In this regard, nAChR inhibition exerted by structurally different
antidepressants, including SSRIs, could be relevant for their clinical efficacy [15].
The involvement of nAChRs in MDD has also been supported by studies using different brain imaging techniques. For instance, single-photon-emission computed tomography
studies showed that the densities of β2* nAChRs (* indicates that other different subunits
are potentially present) in the brains of patients with MDD and of healthy subjects were
similar [42]. These results indicate that the number of nAChRs is not an important factor in the development of MDD. Nevertheless, a lower availability of β2* nAChRs was
determined in the cortex, thalamus, cerebellum, striatum, hippocampus, amygdala, and
brainstem in patients with either depression [42] or bipolar depression [43] compared
to healthy subjects. This lower receptor availability could be caused by higher levels of
endogenous Ach, which may result in receptor desensitization [42], a closed state that is
unresponsive to ACh or other agonists. Moreover, positron emission tomography studies
showed that Parkinson’s patients with depression symptoms had decreased levels of α4β2*
nAChRs within subcortical regions, such as the putamen (involved in reward) and midbrain (where hypoactivity of 5-HT, DA, and/or NA has been related to MDD), as well as
in cortical regions, including the anterior cingulate cortex and occipital cortex [44]. These
Molecules 2021, 26, 2149
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data support the notion that altered nAChR-induced neurotransmission plays a role in the
development of MDD.
4. Relationship between Smoking and Depression
Preclinical and clinical studies support the involvement of nAChRs in MDD [15,20,45,46].
Although the causality between nicotine exposure and depression remains unclear, clinical
evidence supports a bidirectional association and dose-dependent relationship between
smoking and depression [47]. For example, depressed patients smoke twice as much as
the average population, probably to control their mood symptoms. People smoking more
than twenty cigarettes per day double their risk of subsequent depression during their
lives. Paradoxically, however, the chronic use of nicotine may lead to the development of
depression symptoms, probably through a process involving nicotine-induced receptor
desensitization [14,40,48,49]. This evidence highlights the correlation between smoking
habits and increased risks of depression disorders, which, in turn, will increase the chances
of using antidepressants in the long term.
A proposed mechanism by which smoking induces depression is that the excess of
nicotine activates nAChRs and, consequently, the cholinergic signaling, thus breaking
the balance between the cholinergic and noradrenergic systems [14,40,50,51]. Depression
states, in turn, may increase the rate of smoking due to mood changes, which are positively
correlated with the quantity of tobacco used. Additionally, chronic smoking leads to
desensitization and upregulation of high-affinity β2* nAChRs (i.e., α4β2*, α6β2*) (see
more details below), which, in turn, could contribute to depression symptoms [31,52].
In this regard, smokers and nonsmokers might respond differently to antidepressants,
but the effects of smoking on treatment outcomes have rarely been examined and are
therefore unclear. Depression symptoms in adults were more attenuated when an SSRI was
combined with mecamylamine, a nonselective nAChR antagonist with antidepressant-like
activity [53], compared to the SSRI alone [54]. Other analyses, however, have shown that
smokers respond less to this drug combination than nonsmokers [55].
Additional studies also highlighted the importance of the cholinergic system in the
development of depression. For example, augmenting ACh content by decreasing the
AChE activity using the inhibitor physostigmine or virally delivered shRNAs produced
depression-like behavior in rodents [20]. This is in accordance with the hypothesis that
subjects with depression have higher endogenous ACh content, which may overstimulate
the cholinergic system [42]. On the other hand, structurally different nAChR antagonists
also produce antidepressant-like effects, probably by limiting nAChR function [14,45,46].
Anhedonia, a main symptom in major depression, has been linked to dysfunctions in
the brain reward system, particularly when the DA content is decreased [56]. Therefore,
a biochemical hypothesis of nicotine dependence [57] could also explain its relationship
with nicotine-modulating neuronal pathways and depression. The initial and intrinsic biochemical basis of tobacco addiction should be found in the mechanisms by which nicotine
promotes different effects on nAChRs. According to this model, nAChR desensitization and
upregulation, but not activation, are compatible molecular processes that effectively cause
nicotine dependence [58]. In addition to the classical desensitization process, low agonist
concentrations can induce desensitization even without nAChR activation, a process called
“high-affinity desensitization” [59]. This is a slow process that, in the prolonged presence
of low concentrations of nicotine (~0.5 µM), preferentially affects α4β2* compared to α7*
nAChRs. Nicotine accumulation during a smoking day is enough to produce nAChR desensitization. At nicotine concentrations present in chronic smokers’ brains, α4β2* nAChRs
are preferably desensitized and subsequently upregulated in contrast to α7* nAChRs.
Since presynaptic α4β2* nAChRs regulate GABA release onto dopaminergic neurons, a
possibility is that nicotine-induced nAChR desensitization decreases GABAergic inhibition
of dopaminergic neurons, and the consequent disinhibition finally promotes rewarding
effects. After chronic nicotine use, however, the subsequent upregulation of α4β2* nAChRs
enhances the inhibitory action of GABA on dopaminergic neurons, and consequently, even
Molecules 2021, 26, 2149
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more nicotine is needed to restore at least a basal level of reward, leading to a vicious
circle of nicotine-induced α4β2* desensitization (reward) and upregulation (enhanced
reward inhibition).
To test this hypothesis, the effect of nicotine at smoking concentrations (0.3 µM) on
GABAergic transmission was determined in pyramidal neurons from the medial PFC
(mPFC), which modulates the brain’s reward circuitry [60]. The results clearly indicated
that nicotine inhibits GABAergic postsynaptic currents by activating non-α7—probably
α4β2* and α6β2*—nAChRs, whereas the role of α7* nAChRs in this process was less clear.
These results contrasted with those of studies in the hippocampus, where α7* nAChRs
were the main subtype involved in nicotine-induced modulation [61]. Our study also
showed that nicotine increased membrane conductance, probably by activating postsynaptic α7* nAChRs, and decreased current frequency, corroborating the role of presynaptic β2*
nAChRs in GABA release. In other words, our study supported the activation of pre- and
post-synaptic α7* and β2* nAChRs in GABAergic modulation, while the possibility that
nAChR desensitization is also involved in this process could not be ruled out [60].
There is also comorbidity between depression and anxiety conditions and nicotine addiction [41], highlighting the relationship between these psychiatric disorders and nAChR
functioning. Although different antidepressants are successfully used in the treatment
of both depression and anxiety, only bupropion and nortriptyline produced long-term
beneficial effects when used for smoking cessation, whereas fluoxetine was effective only
in smokers with strong depressive symptoms [62,63]. Moreover, non-selective antidepressants, such as monoamine oxidase inhibitors, did not produce benefits in smoking cessation
therapy [62].
5. Preclinical Studies with Combinations of SSRIs and Nicotinic Ligands
There is experimental evidence showing that nicotinic ligands modulate the antidepressant-like activity of SSRIs. For instance, microdialysis analyses evidenced that nicotine
elevates 5-HT levels in the frontal cortex during the initial 15 min, while nicotine in the
presence of fluoxetine increases and prolongs the release of 5-HT for at least 2 h, and
these effects were blocked by pretreatment with mecamylamine [64]. Additional studies
determined that nicotine-evoked 5-HT release depends on α7 nAChR activation [46]. For
instance, the combined administration of the α7-selective agonist PNU-282987 at a dose
(30 mg/kg) that causes full receptor occupancy [65] with a subactive dose of citalopram
(3 mg/kg), which inhibits ~50% of the SERT but is unable to induce antidepressant-like
activity in mice per se [46,66], causes antidepressant-like effects. Since PNU-282987 alone
showed no antidepressant-like activity, it is possible that the combination of α7 nAChR
activation and partial SERT inhibition was responsible for the observed antidepressant-like
effects, confirming nAChRs as targets for the treatment of major depression.
Although the administration of nicotine alone showed no effect in the tail suspension
test (i.e., a very useful animal test to determine the antidepressant-like activity of drugs) and
citalopram alone produced a slight decrease in immobility, the co-administration of both
drugs resulted in a robust antidepressant-like activity [67]. Moreover, mecamylamine, but
not dihydro-β-erythroidine (DHβE) (antagonist with relative selectivity for β2* nAChRs,
including α4β2* and α6β2*), increased the antidepressant-like effect of citalopram [67],
suggesting that these nAChR subtypes are not involved in the antidepressant activity
of SSRIs. The latter coincides with Ca2+ influx experiments where citalopram had the
lowest inhibitory potency for α4β2 nAChRs compared to other SSRIs [68] (Table 1). These
results contrasted with the evidence that both mecamylamine and DHβE enhanced the
antidepressant-like activity of imipramine (a tricyclic antidepressant that blocks both NE
and DA reuptake transporters), suggesting that both functionally different antidepressants
target distinct nAChR subtypes.
Molecules 2021, 26, 2149
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Table 1. Inhibitory potency (IC50) of SSRIs for different nAChR subtypes.
nAChR
Subtype
Method
IC50 (µM)
References
Paroxetine
Citalopram
Voltage-clamp
Ca2+ influx
Ca2+ influx
Ca2+ influx
1.83
4.4 ± 0.6
8.6 ± 2.3
19.0 ± 4.2
[69]
[70]
[70]
[68]
Fluoxetine
Voltage-clamp
Nor-fluoxetine
Zimelidine
Voltage-clamp
Voltage-clamp
0.37 b
0.45 ± 0.03 c
0.13 b
0.67 b
[71]
[27]
[71]
[71]
hα3β4
Fluoxetine
Paroxetine
Citalopram
Ca2+ influx
Ca2+ influx
Ca2+ influx
2.0 ± 0.4
2.6 ± 0.3
5.1 ± 1.3
[70]
[70]
[68]
rα3β4
Fluoxetine
Voltage-clamp
0.64 ± 0.03 c
[27]
rα4β4
Fluoxetine
Voltage-clamp
(+100–200 µM Zn2+ )
0.3 ± 0.045 d
0.17 ± 0.018 d
[72]
rα3β4α5
Fluoxetine
Paroxetine
Sertraline
efflux
efflux
86 Rb+ efflux
2.5
4.9
3.1
[73]
[73]
[73]
Sertraline
Citalopram
Voltage-clamp
Ca2+ influx
Ca2+ influx
Voltage-clamp
Ca2+ influx
Ca2+ influx
Ca2+ influx
Ca 2+ influx
1.6
4.7 ± 0.83
4.9 ± 1.0
5.27
8.6 ± 2.0
9.1 ± 0.6
10.0 ± 0.81
18.8 ± 1.1
[74]
[75]
[70]
[69]
[70]
[75]
[75]
[68]
Hippocampal
rα7 *
Nor-fluoxetine
Fluoxetine
Escitalopram
Voltage-clamp
Voltage-clamp
Voltage-clamp
0.82 ± 0.04 a
0.66 ± 0.06 a
28.9 ± 5.1
[76]
[76]
[76]
hα3β2
Fluoxetine
Voltage-clamp
4.14
[69]
rα9α10
Citalopram
Voltage-clamp
7.5 ± 0.9
[68]
efflux
Voltage-clamp (0 mV)
Voltage-clamp
(+100–200 µM Zn2+ )
Voltage-clamp
Voltage-clamp
2.1
2.2
0.45± 0.04 d
0.19 ± 0.02 d
0.22 b
0.3 e
[73]
[26]
1.8 ± 0.6
0.22 ± 0.01
0.17 ± 0.08
[77]
hα4β2
rα2β4
SSRI
Fluoxetine
Fluoxetine
hα7
Paroxetine
86 Rb+
86 Rb+
86 Rb+
mα1β1γδ
Fluoxetine
[72]
[71]
[27]
hα1β1γδ
Fluoxetine
Ca2+ influx:
5 min pre-incubation
240 min pre-incubation
1440 min pre-incubation
hα1β1εδ
Fluoxetine
Ca2+ influx
0.85 b
0.55 b
[30]
mα1β1γδ
Nor-fluoxetine
Voltage-clamp
0.07 b
[71]
4.8 ± 0.6
[77]
hα1β1γδ
Paroxetine
Ca2+
influx: 5 min
pre-incubation
Molecules 2021, 26, 2149
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Table 1. Cont.
nAChR
Subtype
SSRI
mα1β1γδ
Paroxetine
Sertraline
86 Rb+
Habenular
mα3β4 *
Citalopram
Patch-clamp
Hippocampal
nAChRs
nAChRs from
superior cervical
ganglion
Citalopram
Fluoxetine
Fluoxetine
Method
86 Rb+
efflux
efflux
Nicotine-evoked
NA release
Nicotine-evoked
NA release
Ca2+ influx
IC50 (µM)
References
5.6
3.5
[73]
[73]
7.6 ± 1.0
[68]
0.93
[78]
0.57
[78]
2.0
[69]
h, human. r, rat. m, mouse. * denotes the potential existence of other subunit(s). The ratio between the agonistinduced response in the presence of the antidepressant and the control agonist-induced response at the respective
concentration of the antidepressant (a 20, b 10, c 2, d 5, and e 1 µM, respectively).
6. SSRIs Inhibit nAChRs at Clinical Concentrations
SSRIs inhibit a variety of nAChR subtypes with distinct potencies (Table 1) and
by different mechanisms [26,27,30,70,71,74,77]. This functional inhibition may reduce the
exacerbated cholinergic activity observed during depression conditions, which is consistent
with the cholinergic hypothesis of depression [11,17–20].
Although Ca2+ influx results established that fluoxetine is the most potent SSRI in
inhibiting a variety of nAChR subtypes, the use of different functional assays did not
give only one value, but a range of values for each SSRI. Table 1 shows that the activity
of each SSRI also depends on the studied nAChR subtype. For example, the potency
of both fluoxetine and paroxetine was practically the same in the hα3β4 nAChR, while
two-fold-lower values were determined for citalopram in the same receptor subtype.
Considering a single nAChR subtype, the calculated IC50 values depended on the type of
SSRI molecule. For example, the following inhibitory potency rank order was determined
at hα4β2 nAChRs (IC50 ’s in µM): fluoxetine (4.4 ± 0.6) > paroxetine (8.6 ± 2.3) > citalopram
(19.0 ± 4.2) (Table 1).
Although, in general, SSRIs inhibit nAChRs through noncompetitive mechanisms,
the inhibition at the α9α10 nAChRs is mediated by a competitive mechanism. Voltageclamp results revealed that citalopram inhibits ACh-evoked α9α10 currents in a voltageindependent and competitive manner [68]. This inhibitory mechanism was also observed
in structurally different antidepressants, such as imipramine [79]. Since α9α10 nAChRs are
not expressed in the brain, but preferably in peripheral immunocompetent cells [80], the
inhibitory mechanism observed at this receptor is not relevant for the clinical efficacy of
SSRIs in depression, but might be related to their anti-inflammatory activity [81].
An important aspect for determining whether SSRIs inhibit nAChRs at clinically
relevant concentrations is the calculation of the plasma/brain ratio after drug treatment.
The determined levels of SSRIs in plasma were generally lower than those in the brain.
For example, after administration of 40 mg/day fluoxetine for 30 days, the plasma concentration could reach 0.29–0.97 µM, and in some patients, up to 1.6 µM [82,83]. A wide
range of brain-to-plasma ratios (2.6–20) has been calculated for fluoxetine [84,85], which
is concurrent with the relatively higher brain concentration (13 µM) [86]. Post-mortem
studies in individuals that had been taking citalopram showed brain concentrations of
4.7 ± 3.3 µM, which was approximately four-fold higher than that in plasma [87]. Accordingly, it is feasible that different nAChR subtypes can be inhibited—at least partially—by
SSRIs (Table 1). By meticulously determining both the inhibitory activity of SSRIs, such as
fluoxetine, sertraline, paroxetine, and citalopram, at α4β2 nAChRs and the free brain level
of each SSRI, a functional inhibition of only ~2.5% was calculated, a percentage that was
Molecules 2021, 26, 2149
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10-fold lower than that estimated to be necessary to produce a noticeable antidepressantlike activity [88]. In comparison, the calculated free mecamylamine concentration in the
brain supported ∼20% inhibition of the α4β2* nAChR function [88]. Although these results suggest that α4β2 nAChRs are not important for the antidepressant-like activity of
SSRIs, there are other nAChR candidates that might be involved. For instance, habenular
α3β4* and hippocampal α7* nAChRs (Table 1), as well as α6β2* nAChRs, are expressed
in the brain reward system. Although no information on the effect of SSRIs at α6β2*
nAChRs is currently available, there is experimental evidence indicating that modulation
of α6* nAChRs by LF-3-88, a novel partial agonist that would pharmacologically act like a
competitive antagonist, induces antidepressant-like effects in several mouse models [89].
7. Neuronal Pathways Involved in the Antidepressant Activity of SSRIs
Since activation of presynaptic nAChRs modulates the release of several neurotransmitters involved in mood regulation, the malfunctioning of these receptors expressed in
different neuronal pathways might be related to mood disorders [31,33]. Considering that
SSRIs inhibit different nAChRs (Table 1), the simplest contemplated mechanism by which
SSRIs might regulate nicotinic pathways involves the inhibition of presynaptic nAChRs.
According to the cholinergic hypothesis of depression, SSRIs might recover the unbalance
of cholinergic signaling by direct inhibition of presynaptic nAChRs expressed in brain
regions associated with depression [15,90]. Nevertheless, new evidence supports the notion
that the mechanism of action of SSRIs is more complex than we previously thought, and
involves many neurotransmitter systems.
The dorsal raphe nucleus (DRN) is a serotoninergic center involved in physiological
functions related to cognition, mood, and emotion. Glutamatergic hyperstimulation (e.g.,
under stress conditions), increasing 5-HT content, and the subsequent deficit at limbic
projections have been considered as potential mechanisms that underlie depression conditions [91]. Thus, SSRI-induced inhibition of nAChRs expressed in DRN might decrease
5-HT content, thus alleviating this condition.
Figure 2 shows a scheme of a DRN serotoninergic neuron and its modulation by
other neurotransmitter systems. Under normal conditions, glutamatergic excitatory afferents from pyramidal neurons of the mPFC directly activate the DRN, thus increasing
extracellular 5-HT, which is, in turn, regulated by somatodendritic 5-HT1A autoreceptors.
Under stress conditions, however, glutamatergic neurotransmission is hyperactivated,
flooding the DRN with 5-HT (i.e., so-called “5-HT flooding”), but attenuating 5-HT outflow.
In fact, higher-than-normal levels of 5-hydroxyindoleacetic acid (5-HIAA; major 5-HT
metabolite) have been found in the cerebrospinal fluid (CSF) of subjects with depression,
corresponding to the 5-HT flooding hypothesis. An excessive stimulation of 5-HT2A Rs in
the mPFC increases glutamatergic input, whereas an excessive stimulation of postsynaptic
5-HT1A Rs in the hippocampus is associated with a deficit at corticolimbic projection sites,
thus compromising neuroplastic processes. Since NE and histamine are also increased
during stress, 5-HT neurons are inhibited by NE transmission from the locus coeruleus and
by histaminergic transmission.
Molecules 2021, 26, 2149
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Figure 2. Scheme depicting the functioning of a dorsal raphe nucleus (DRN) serotoninergic neuron
under stress conditions that induce depression disorders (modified from [91]). In normal conditions,
glutamatergic excitatory afferents from pyramidal neurons of the medial prefrontal cortex (mPFC)
directly activate the DRN, thus increasing extracellular serotonin (5-HT), which is regulated by
somatodendritic 5-HT1A autoreceptors, decreasing the excessive release in projection areas such as
the mPFC and hippocampus. Under stress conditions, however, glutamatergic neurotransmission
called “5 HT flooding”)
is hyperactivated, flooding the DRN with 5-HT (i.e., so-called “5-HT flooding”), but concomitantly
attenuating 5-HT outflow. An excessive stimulation of 5-HT2A Rs in the mPFC increases glutamatergic
input, whereas an excessive stimulation of postsynaptic 5-HT1A Rs in the hippocampus is associated
with a deficit at corticolimbic projection sites, thus compromising neuroplastic processes. Based on
this scheme, it is possible that SSRIs alleviate depression symptoms through two processes involving
nAChRs: (1) inhibiting presynaptic nAChRs at glutamatergic afferents, which decreases 5-HT flood– postsynaptic nAChRs, which attenuates DRN activity [7,92–94].
ing to the DRN, and (2) inhibiting
Since noradrenaline (NE) and histamine are also increased during stress/depression conditions, 5-HT
locus coeruleus
through α2
neuron
firing is inhibited
by NE transmission from the locus coeruleus through α2-heteroceptors
and by histamine transmission through histamine-1 receptors. Serotonin transporters are located
presynaptically. CSF: cerebrospinal fluid; 5-HIAA: 5-hydroxyindoleacetic acid.
Molecules 2021, 26, 2149
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Experimental results indicated that ~75% of the DRN neurons that project to the
NAc, an area involved in brain reward, are serotoninergic, and are, in turn, modulated
by nAChRs [90]. Activation (by nicotine) of postsynaptic nAChRs (i.e., α4β2* and α7*
nAChRs) increased the firing frequency of DRN serotoninergic neurons, whereas activation of presynaptic α4β2* nAChRs augmented glutamate (Glu) release from excitatory
glutamatergic afferents to the DRN [95–98]. Based on this scheme, it is possible that SSRIs
alleviate depression symptoms through two processes (see Figure 2): (1) by inhibiting presynaptic nAChRs at glutamatergic afferents, which decreases 5-HT flooding at DRN neurons,
and (2) by inhibiting postsynaptic nAChRs, which attenuates DRN activity. Activation of
presynaptic α4β2* nAChRs also increased 5-HT release in serotoninergic projections to the
NAc, which might be related to the anxiolytic and addictive activity of nicotine [93].
Other neurotransmitter systems modulated by nAChRs are also involved in depression, which adds to the complexity of SSRI-modulating actions. For instance, MDD is
associated with reductions in inhibitory GABAergic transmission in the occipital cortex,
anterior cingulate, and dorsomedial/dorsolateral PFC, as well as with an excessive increase
in Glu release or changes in glutamate receptors (GluR )properties in the hippocampus
and PFC [7,93]. There are contrasting explanations of how fluoxetine may improve information processing in these impaired neuronal networks. Komlósi et al. [94] suggested
that fluoxetine can suppress excitatory glutamatergic transmission without changing the
output of GABAergic neurons, whereas Van Dyke et al. [92] indicated that chronic fluoxetine potentiates excitatory synapses by activating 5-HT1B Rs, and may induce deleterious
effects, such as long-term depression and decreased long-term memory. On the other hand,
Méndez et al. [99] concluded that acute fluoxetine decreases GABAergic transmission in
the hippocampus through presynaptic mechanisms, independently of its effect on SERT.
Interestingly, cholinergic signaling via nAChRs in pyramidal cells and GABAergic interneurons modulates inhibitory circuits in the hippocampus, which is crucial for information
processing [100]. Different SSRIs inhibit hippocampal nAChRs (Table 1), and this activity disinhibits GABAergic neurons, finally improving the symptoms in MDD. In other
words, SSRI-induced inhibition of nAChRs expressed in both glutamatergic and GABAergic neurons—regulating excitatory and inhibitory responses, respectively—is considered
fundamental in maintaining neuronal circuit oscillations, which seem to be disrupted
during depression [101].
The rostromedial tegmental (RMTg) nucleus, which contributes to monoaminergic
responses to stressing events and is involved in depression, receives cholinergic inputs
from the laterodorsal and pedunculopontine tegmental nuclei that activate presynaptic
α7 nAChRs, thus finally increasing Glu release in the lateral habenula (LHb). The LHb
has been considered to play a role in the development of pain and depression, two highly
prevalent disorders that coexist in ~60% of patients [102]. In turn, RMTg neurons exert an
inhibitory effect mediated by GABA on dopaminergic neurons in the ventral tegmental area
(VTA), which is part of the brain reward system [103,104]. In the VTA, nicotine increased
postsynaptic current amplitude, which was blocked by methyllycaconitine, indicating the
involvement of α7 nAChRs in the reward mechanism [103]. Based on this scenario, it
is possible to suggest that SSRIs, by inhibiting α7 nAChRs, may decrease the inhibitory
activity of GABAergic neurons in the reward system.
The LHb is connected to the limbic system, whereas the medial habenula (MHb) is
connected to the interpeduncular nucleus. The habenulo–interpeduncular pathway is
considered a second—not the least important—reward circuitry that modulates, directly
and indirectly, the mesolimbic dopaminergic reward system. Interestingly, this cholinergic pathway is a major player in the aversive effects of nicotine [103,104]. In particular,
the MHb expresses α3β4* nAChRs, which are considered targets for the anti-addictive
activity of coronaridine congeners, including natural alkaloids, such as ibogaine and its
active metabolite noribogaine, and the synthetic derivative 18-methoxycoronaridine. Our
functional results showed that both coronaridine alkaloids and synthetic derivatives noncompetitively inhibit α3β4 nAChRs expressed in heterologous cells and the MHb [105,106].
Molecules 2021, 26, 2149
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It has been demonstrated that ibogaine and noribogaine induce antidepressant-like activity
in rodents [107], probably through inhibition of habenular α3β4* nAChRs. Since citalopram also inhibits habenular α3β4* nAChRs [68] (Table 1), it is obvious to conclude that
the observed inhibition may be involved, at least partially, in the clinical efficacy of SSRIs.
8. SSRIs Bind Distinct nAChR Subtypes with Different Affinities
Although we know that SSRIs inhibit different nAChR subtypes, the relationship between inhibitory potency, binding affinities, and clinical efficacy is less clear. Thus, the binding affinities of SSRIs and other antidepressants at various nAChRs in different conformational states were determined through radioligand competition experiments [70,77,108,109].
More specifically, the influence of several SSRIs on the maximal binding of either [3 H]imipramine (a known tricyclic antidepressant and noncompetitive antagonist (NCA) of
nAChRs [71,110]) to a variety of nAChR subtypes or [3 H]TCP (a known NCA of Torpedo nAChRs [111]) to Torpedo nAChRs in the resting (toxin-bound) and desensitized
(agonist-bound) states was determined. The studied SSRIs completely inhibited the specific
binding of both [3 H]imipramine [77] and [3 H]TCP [70], respectively. The calculated binding affinities (Ki ) for desensitized nAChRs were generally higher than those in the resting
state (Table 2). In the desensitized state, fluoxetine inhibited [3 H]imipramine binding to
α4β2 AChRs with higher affinity than with α3β4 (~5-fold) and α7 (~10-fold) nAChRs,
respectively (Table 2). Comparing the binding affinity of different SSRIs, the following rank
order was determined (Ki ’s in µM) for α4β2: fluoxetine (1.0 ± 0.1) > citalopram (4.1 ± 0.3)
> paroxetine (6.7 ± 0.9), and for α3β4: citalopram (1.8 ± 0.1) > fluoxetine (4.8 ± 0.5) >
paroxetine (6.9 ± 0.6), respectively (Table 2). These results support a direct interaction of
SSRIs with NCA sites located in the nAChR ion channel (see Section 10).
Table 2. Binding affinities (Ki ) of SSRIs with resting and desensitized nAChRs.
Radioligand
[3 H]Imipramine
[3 H]TCP
nAChR Subtype
SSRI
hα4β2
Ki (µM) a
Reference
Resting State
Desensitized State
Fluoxetine
Paroxetine
Citalopram
3.2 ± 0.4
16.1 ± 1.3
ND
1.0 ± 0.1
6.7 ± 0.9
4.1 ± 0.3
[70]
[70]
[68]
hα3β4
Citalopram
Fluoxetine
Paroxetine
ND
14.9 ± 1.4
24.0 ± 1.8
1.8 ± 0.1
4.8 ± 0.5
6.9 ± 0.6
[68]
[70]
[70]
hα7
Fluoxetine
Paroxetine
16.9 ± 1.4
8.9 ± 0.8
11.0 ± 1.0
8.7 ± 0.6
[70]
[70]
Torpedo
α1β1γδ
Fluoxetine
Paroxetine
1.9 ± 0.2
34 ± 2
0.96 ± 0.04
2.5 ± 0.1
[77]
[77]
a
The inhibition constant (Ki ) represents the ligand affinity for the tested radioligand binding site(s) in the desensitized (agonist-bound)
and resting (bungarotoxin-bound) states, respectively. h, human.
9. SSRIs Interact with nAChRs in Different Conformational States
Although SSRIs generally inhibit nAChRs in a noncompetitive manner, the intrinsic
inhibitory mechanisms are less clear. nAChRs exist in three allosteric transitions between
different conformational states: a resting state, an active open-channel state, and several
desensitized states, each with a non-conducting ion channel [38,112]. Even though the
functional properties of SSRIs have not been studied on all existent nAChR subtypes and
conformational states, the experimental evidence indicates that their activity is dependent
on the conformational state. For instance, fluoxetine allosterically modulates nAChRs
by interacting with the resting (closed), active (open), and desensitized (non-conducting)
receptor states [27,30,70]. Particularly, fluoxetine promotes a faster nAChR desensitization and has stronger inhibitory potency at higher concentrations, suggesting a higher
affinity to desensitized nAChRs [26,77] in accordance with the radioligand binding results
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(Table 2). This pharmacological characteristic is common to other structurally different
antidepressants [110], suggesting a shared mode of action.
While the pharmacological activity of fluoxetine with nAChRs is subtype- and statedependent, we can summarize its basic functional effects as follow: (a) Fluoxetine reduces
the frequency of channel opening, (b) increases channel closed times, and (c) decreases
channel open times, without (d) altering single-channel conductance [27,30]. Accordingly,
fluoxetine activity at the whole-cell level results in increased nAChR desensitization and the
reduction of agonist-induced current amplitude, ultimately reducing Ca2+ entry [27,30,69].
Based on this evidence, we can suggest a probable mechanism for the clinical activity
of SSRIs. In depression conditions, where higher concentrations of ACh are expected, most
nAChRs are likely in the desensitized state. As shown previously, this conformational
(closed) state is powerfully inhibited by antidepressants and maintained for a longer time,
thus synergistically reducing nAChR-related signaling, which may ultimately be beneficial
for the improvement of MDD and other depression disorders.
10. Characterization of SSRI Binding Sites for Different nAChR Subtypes
Molecular docking simulations have been used to determine the molecular interactions
of SSRIs with a variety of nAChR subtypes [70,77,108,109,113,114]. In this regard, based on
the crystal structure of hα4β2 (PDB id: 5KXI) [115] and Torpedo nAChR molecular models
(PDB id: 2BG9) [116], several neuronal nAChRs were constructed, including the α4β2,
α3β4, α7, and α9α10 subtypes [68,70,108,109,113,117,118]. SSRIs and other antidepressants
were subsequently docked to each nAChR model [68,70,117,118], and molecular dynamics
simulations were performed to determine the stability of each interaction.
Docking simulations indicated that fluoxetine interacts within the middle portion
of the ion channel of each nAChR subtype, except the α9α10 subtype. Considering that
each subunit has one transmembrane M2 segment, the ion channel is formed by five M2
segments [119]. Although the ion channel is highly conserved among species, differences
are also apparent among nAChR subunit sequences [119], producing variations in the
nAChR ion channel structure. For example, in the α4β2 nAChR (Figure 3), the amino
acid rings are named: outer or extracellular (position 20′ ), nonpolar (position 17′ ), valine
(position 13′ ), leucine (position 9′ ), serine (position 6′ ), threonine (position 2′ ), intermediate
(position -2′ ), and cytoplasmic or inner (position -5′ ), whereas in the α7 nAChR, the polar
rings (positions 2′ and 6′ ) are switched: The threonine ring occurs at position 6′ , and the
serine ring at position 2′ .
The molecular docking of fluoxetine suggested that this molecule interacts with
residues lining the channel lumen located between amino acid rings from position 6′ up to
position 13′ of each studied nAChR subtype [70]. The orientations of docked fluoxetine in
the α4β2 (Figure 3) and α7 [76] nAChR models are essentially the same. The amino group
and the aromatic ring containing the trifluoromethyl moiety of fluoxetine interact through
van der Waals contacts with valine residues at position 13′ . Moreover, the oxygen atom
forms a hydrogen bond with the hydroxyl group provided by the α4-Ser251 residue at
position 10′ (Figure 3).
The docking orientation of fluoxetine in the α3β4 nAChR ion channel is slightly
different because the α4 subunit carries additional phenylalanine residues at position
13′ , forming the valine/phenylalanine ring together with an α3 subunit. In particular, the
position of the fluoxetine aminoaryl moiety in this ion channel is exchanged with the benzyl
ring position that was suggested in other nAChR subtypes. Thus, both aromatic rings
of the molecule bind to the three phenylalanine residues at position 13′ (i.e., β4-Phe253),
probably through π–π interactions [117].
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Figure 3. Molecular docking of fluoxetine (in yellow) and imipramine (in green), both in the protonated state, within
α4β2 nAChR
the α4β2 nAChR ion channel (modified from [70]). (A) Side view of the overlapping binding sites for both ligands that
interact with the middle portion of the
ion lumen
channel.
(inion
green)
and fluoxetine
(in interact
yellow) mainly
interact with the M2
of (B)
theImipramine
α4β2 nAChR
channel.
Both ligands
transmembrane segments forming the lumen of the α4β2 nAChR ion channel. Both
van
3′ ligands interact mainly through
6′
der Waals contacts with a domain formed between the valine (VAL) (position 13′ ) and serine (SER) (position 6′ ) rings. In
addition,
the black arrow indicates
the
hydrogen
bond
formed
between
the oxygen
atomin
ofligand
fluoxetine
andare
thepresented
hydroxyl
group
of α4
0′). For
clarity,
one β2
subunit
is hidden.
Residues
involved
binding
group of α4-Ser251 (position 10′ ). For clarity, one β2 subunit is hidden. Residues involved in ligand binding are presented
in stick mode (gray), whereas ligands are rendered either in the ball (A) or stick mode (B). All non-polar hydrogen atoms
are hidden.
The docking orientation of fluoxetine in the α3β4 nAChR ion channel is slightly
different
because
α4 subunit carries
additional phenylalanine
residues
position
13′
In the
case ofthe
escitalopram
(i.e., (S)-citalopram),
two luminal
bindingatsites
at α3β4
nAChRs were characterized [68]. A high-affinity binding site
was located
between positions
α3 subunit.
In particular,
the po5′ and 16′ —in the middle of the ion channel—toward the extracellular ion channel’s mouth
(Figure 4A). In addition, a low-affinity binding site for escitalopram was located closer
site
to the cytoplasmic side, between positions –3′ and 6′ . Although the
3′ high-affinity
(i.e., β4
corresponds to that
found
for fluoxetine
in the α3β4 nAChR (see above), few differences
π–π
interactions
[1
between both antidepressants were detected. For instance,
but not
fluoxetine,
, twoescitalopram,
luminal binding
sites
at α3β4
established a cation–π interaction with α3-F255 (position 14′ ) and interacted with β4-T254
(position
12′16′—
) (Figure 4B). Interestingly, fluoxetine —
formed a π–π interaction with β4-Phe255
′ and
’
(position 13′ ), which coincides with that for coronaridine congeners [105]. The binding site
′
′
for coronaridine congeners, located between positions
– ′ 6 and
6′ 13 [105], overlaps with the
site corresponds
for SSRIs. to that found for fluoxetine in the α3β4 nAChR (see above), few
site
Since fluoxetine displaced [3 H]imipramine binding to different nAChRs (Table 2), the
molecular docking of imipramine
was compared
that for fluoxetine 4′
at both α4β2 and
–π interaction
withtoα3
α3β4
nAChRs
[70,117].
The
results
suggested
that
imipramine
interacts
with
a site
located
with β4 T254 (position 12′) (
4B). Interestingly, fluoxetine formed a π–π
interaction
in
the
middle
portion
of
the
ion
channel.
A
more
detailed
analysis
indicated
that
with β4 Phe255 (position 13′), which coincides with that for coronaridine congeners in
[1 the
case of α4β2 nAChRs, imipramine interacts with residues that form the valine
and
leucine,
6′
3′
but not the serine, rings via van der Waals contacts [118], whereas in the α3β4 nAChR,
imipramine interacts with several residues between the valine/phenylalanine (e.g., its
aromatic rings form π–π interactions with three β4-Phe253 residues at position 13′ ) and
leucine rings [117]. These docking results agree with radioligand binding data, suggesting
that the SSRI binding site in different nAChR subtypes overlaps the imipramine locus in
both desensitized and resting states [70,77].
Molecules 2021, 26, 2149
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the (α3) (β4)
Figure 4. Docking sites for S-(+)-citalopram (escitalopram) in the
(α3)3 (β4)2 nAChR model (modified
from [68]). (A) Escitalopram docked at two luminal sites (surface model): a high-affinity site located
located closer to the extracellular ion channel’s mouth (blue) and a low
closer to the extracellular ion channel’s
mouth
(blue)
α3 (white)
and β4
(darkand
gr a low-affinity site located closer to the
cytoplasmic side (red). The α3 (white) and β4 (dark gray) subunits
are represented
as solid ribbons.
3′
6′
Dotted3′ lines indicate the positions of the Gly (position -3′ ), Ser (position 6′ ), and Val (position
interaction
α3
4′) and theat
interaction
with β4 luminal
13′ ) rings along the ion–πchannel.
(B)with
Detailed
view of escitalopram
the high-affinity
(position 12′).
′
site, showing the cation–π interaction with α3-F255 (position 14 ) and the interaction with β4-T254
(position 12′ ).
the molecular
docking
of imipramine
was compared
to that
for fluoxetine
at bothindicated
α4β2
Additional
molecular
docking studies
using the
α9α10
nAChR model
and
α3β4
nAC
that escitalopram interacts with orthosteric binding sites, but not with luminal sites [68]
(Figure 5A).
in the case of α4β2 nAChRs, imipramine interacts with residues
So far, this is the only nAChR subtype where the SSRI-induced inhibition is mediated
α3β4
by a competitive mechanism. The orthosteric sites for escitalopram are located at the interface between α10(+) (principal component) and α9(−) or another α10(−) (complementary
π–π interactions with three β4
component), but not at the α9(+) and α10(−) interface [68]. To study these interactions in
3′
more details, in silico mutations were performed on key residues at the orthosteric binding
site, transforming α9α10 into α3β4 and vice versa. The docking results indicated that the
mutations
did not
abolish escitalopram
binding
theα9α10
α9α10
mutantmodel
or enable
escitalopram
Additional
molecular
docking studies
usingtothe
nAChR
indicated
that
binding to the α3β4 mutant. To further investigate these intriguing findings, the role of the
β9-β10 loop was assessed. In this regard, the orthosteric binding sites from α3, α9, and
α10 were superposed with escitalopram docked at α10(+) (Figure 5B). The superposition
showed that the β9-β10 loop, which contains the Cys pair typical of α-subunits, is widely
open in α10, allowing escitalopram to fit into the binding site [68]. This loop is closer to the
binding pocket in α9, and even closer in α3, compared to α10, causing escitalopram to overlap with the main-chain atoms of these subunits when it is docked to α10(+). Additional
structural details may support the observed differences. More specifically, α10-R186 might
force the β9-β10 loop in α10 to be set apart from the receptor agonist pocket (Figure 5C).
Looking from the β9 sheet (Figure 5D), the positively charged α10-R186—in its extended
mode in the model—points to the middle of the β9-β10 loop, contrary to the homologous
hydrophobic α9-V186 and α3-Y184 residues. The homologous position at α7 nAChRs
is another positively charged residue, α7-K182, but unlike to α10-R186, it is readily de-
Molecules 2021, 26, 2149
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protonated when partially buried within proteins [120,121]. These structural differences
might account for a unique orthosteric binding pocket at α9α10 nAChRs, which allows the
accommodation of molecules larger than nicotine or Ach, as observed in α10 (Figure 5C).
At the ion channel level, however, there is no structural difference that might explain the
absence of SSRI-induced α9α10 nAChR ion channel blockade. It might be the case that
an initial competitive inhibition of α9α10 nAChR prevails over channel blocking, but the
latter mechanism can be reached at higher concentrations, as suggested in the experimental
studies [68].
the h(α9)2(α10)3 nAChR model
Figure 5. Docking sites for S-(+)-citalopram (escitalopram) in the h(α9)2(α10)3 nAChR model
(modified
Escitalopram
(light blue
surface
model) component)
interacted with
three possible
orthostericfrom
sites [68]).
located(A)
at the
interface between
the α10(+)
(principal
and α9(
orthosteric
sites
located
at
the
interface
between
the
α10(+)
(principal
component)
and α9(−) (or
another α10(
α10 (white) and α9 (dark gr
)
Orthosteric
binding
sites
at
the
superposed
α3(+)
(red),
α9(+)
another α10(−)) (complementary component) subunits. The α10 (white) and α9 (dark gray)
subunits
(black),
α10(+)
(blue),
and
α9(
are represented as solid ribbons. (B) Orthosteric binding sites at the superposed α3(+) (red), α9(+)
molecular surface. The β9 β10 loop at
α3 and α9 subunits
(black), α10(+) (blue), and α9(−) (white) subunits. Escitalopram is shown as sticks surrounded by
than that at α10, and consequently
the
surface.
The
β9-β10
at theoverlap
α3 andthe
α9ligand
subunits
is closer
to theasreceptor
center than
sitemolecular
in α3 and α9.
The α3
and
α9 β9loop
β10 loops
when
is docked
in the α9α10
consequently,
there
is no roombetween
for escitalopram
fit subunits
in the agonist
site in α3
that at α10, and
) Amino
acid sequence
comparison
α3, α9, andtoα10
at the binding
level of the
β9
β10
loop.
Blue:
ids
identified
as
causing
different
β9
β10
loop
conformations.
(
and α9. The α3- and α9-β9-β10 loops overlap the ligand when it is docked as in the α9α10 receptor.
α10
(C) Amino acid sequence comparison between α3, α9, and α10 subunits at the level
of the β9-β10
α9 V186, and α3
chain volume differences in the β9
loop.
Blue:
Amino
acids
identified
as
causing
different
β9-β10
loop
conformations.
(D)
Side-chain
uld force the α10 β9 β10 loop to
view showing differences in the occupied volume of side chains at the α10-R186, α9-V186, and
α3-Y184 positions, respectively. The side-chain volume differences in the β9-sheet would force the
α10-β9-β10 loop to be set apart from the binding pocket.
een α10(+) (principal component) and α9(−) or another α10(−) (complementary component), but not at the α9(+) and α10(
ng site, transforming α9α10 into α3β4 and vice versa. The docking results
Molecules 2021, 26, 2149
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11. Concluding Remarks
Mounting evidence supports the idea that the therapeutic activity of many commonly
used antidepressants, including SSRIs, is partially mediated through the inhibition of
nAChRs, thus counteracting the overactivity of the cholinergic system, which leads to
depressive symptoms. New data highlight the importance of nAChRs expressed in different
neuronal pathways in the regulation of 5-HT-induced effects. DRN serotoninergic neurons
have been studied with enough detail to ascribe a potential modulatory activity of nAChRs,
especially under stress/depression conditions. The evidence that SSRIs also inhibit a
variety of nAChRs expressed in different neurotransmitter systems increases the complexity
by which these antidepressants may act clinically. Although “nicotinic modulation” of
depression has been indirectly recognized, the development of specific ligands, such as
inverse agonists and partial agonists, of therapeutic value is still lacking.
Author Contributions: H.R.A., K.M.T.-D., and J.G.-C. equally contributed to the writing: original
draft preparation, review, and editing. M.O.O., writing—review and editing. All authors have read
and agreed to the published version of the manuscript.
Funding: This research was funded by the Dirección General de Asuntos del Personal Académico
(DGAPA), UNAM, México: PAPIIT-IN202420 and PASPA (to J.G.-C.), National Science Center, Poland
(SONATA funding, UMO-2013/09/D/NZ7/04549) (to K.M.T-D. (PI) and H.R.A. (Co-PI)), and by an
OVPR Pilot/Seed Grant (OSU-CHS) (to H.R.A.).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available on request from the
corresponding author.
Acknowledgments: We are grateful to Elizabeth Vázquez-Gómez for the technical assistance.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or
in the decision to publish the results.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
DSM-IV. Diagnostic and Statistical Manual of Mental Disorders IV-TR; American Psychiatric Press: Washington, DC, USA, 2020.
Duman, R.S.; Aghajanian, G.K. Synaptic Dysfunction in Depression: Potential Therapeutic Targets. Science 2012, 338, 68–72.
[CrossRef]
Salerian, A.J.; Altar, C.A. The Prefrontal Cortex Influence over Subcortical and Limbic Regions Governs Antidepressant Response
by N=H/(M+R). Psychiatry Res. 2012, 204, 1–12. [CrossRef]
Zurkovsky, L.; Taylor, W.D.; Newhouse, P.A. Cognition as a Therapeutic Target in Late-Life Depression: Potential for Nicotinic
Therapeutics. Biochem. Pharmacol. 2013, 86, 1133–1144. [CrossRef] [PubMed]
Pandya, M.; Altinay, M.; Malone, D.A.; Anand, A. Where in the Brain Is Depression? Curr. Psychiatry Rep. 2012, 14, 634–642.
[CrossRef]
Drago, A.; Crisafulli, C.; Sidoti, A.; Serretti, A. The Molecular Interaction between the Glutamatergic, Noradrenergic, Dopaminergic and Serotoninergic Systems Informs a Detailed Genetic Perspective on Depressive Phenotypes. Prog. Neurobiol. 2011, 94,
418–460. [CrossRef]
Pacher, P.; Kecskemeti, V. Trends in the Development of New Antidepressants. Is There a Light at the End of the Tunnel? Curr.
Med. Chem. 2004, 11, 925–943. [CrossRef]
Goldberg, J.S.; Bell, C.E.; Pollard, D.A. Revisiting the Monoamine Hypothesis of Depression: A New Perspective. Perspect. Med.
Chem. 2014, 6, 1–8. [CrossRef]
Ferguson, J.T. Treatment of Reserpine-Induced Depression with a New Analeptic: Phenidylate. Ann. N. Y. Acad. Sci. 1955, 61,
101–107. [CrossRef]
Freis, E.D. Mental Depression in Hypertensive Patients Treated for Long Periods with Large Doses of Reserpine. N. Engl. J. Med.
1954, 251, 1006–1008. [CrossRef]
Janowsky, D.S.; el-Yousef, M.K.; Davis, J.M.; Sekerke, H.J. A Cholinergic-Adrenergic Hypothesis of Mania and Depression. Lancet
1972, 2, 632–635. [CrossRef]
Levin, E.D. Complex Relationships of Nicotinic Receptor Actions and Cognitive Functions. Biochem. Pharmacol. 2013, 86,
1145–1152. [CrossRef]
Molecules 2021, 26, 2149
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
17 of 21
Kenney, J.W.; Gould, T.J. Modulation of Hippocampus-Dependent Learning and Synaptic Plasticity by Nicotine. Mol. Neurobiol.
2008, 38, 101–121. [CrossRef]
Mineur, Y.S.; Picciotto, M.R. Nicotine Receptors and Depression: Revisiting and Revising the Cholinergic Hypothesis. Trends
Pharmacol. Sci. 2010, 31, 580–586. [CrossRef]
Shytle, R.D.; Silver, A.A.; Lukas, R.J.; Newman, M.B.; Sheehan, D.V.; Sanberg, P.R. Nicotinic Acetylcholine Receptors as Targets
for Antidepressants. Mol. Psychiatry 2002, 7, 525–535. [CrossRef]
Steingard, R.J.; Yurgelun-Todd, D.A.; Hennen, J.; Moore, J.C.; Moore, C.M.; Vakili, K.; Young, A.D.; Katic, A.; Beardslee, W.R.;
Renshaw, P.F. Increased Orbitofrontal Cortex Levels of Choline in Depressed Adolescents as Detected by in Vivo Proton Magnetic
Resonance Spectroscopy. Biol. Psychiatry 2000, 48, 1053–1061. [CrossRef]
Janowsky, D.S.; el-Yousef, M.K.; Davis, J.M. Acetylcholine and Depression. Psychosom. Med. 1974, 36, 248–257. [CrossRef]
Risch, S.C.; Cohen, R.M.; Janowsky, D.S.; Kalin, N.H.; Murphy, D.L. Mood and Behavioral Effects of Physostigmine on Humans
Are Accompanied by Elevations in Plasma Beta-Endorphin and Cortisol. Science 1980, 209, 1545–1546. [CrossRef]
Risch, S.C.; Cohen, R.M.; Janowsky, D.S.; Kalin, N.H.; Sitaram, N.; Gillin, J.C.; Murphy, D.L. Physostigmine Induction of
Depressive Symptomatology in Normal Human Subjects. Psychiatry Res. 1981, 4, 89–94. [CrossRef]
Mineur, Y.S.; Obayemi, A.; Wigestrand, M.B.; Fote, G.M.; Calarco, C.A.; Li, A.M.; Picciotto, M.R. Cholinergic Signaling in the
Hippocampus Regulates Social Stress Resilience and Anxiety- and Depression-like Behavior. Proc. Natl. Acad. Sci. USA 2013, 110,
3573–3578. [CrossRef]
Dailey, J.W.; Yan, Q.S.; Mishra, P.K.; Burger, R.L.; Jobe, P.C. Effects of Fluoxetine on Convulsions and on Brain Serotonin as
Detected by Microdialysis in Genetically Epilepsy-Prone Rats. J. Pharmacol. Exp. Ther. 1992, 260, 533–540.
Perry, K.W.; Fuller, R.W. Effect of Fluoxetine on Serotonin and Dopamine Concentration in Microdialysis Fluid from Rat Striatum.
Life Sci. 1992, 50, 1683–1690. [CrossRef]
Nagayasu, K.; Kitaichi, M.; Nishitani, N.; Asaoka, N.; Shirakawa, H.; Nakagawa, T.; Kaneko, S. Chronic Effects of Antidepressants
on Serotonin Release in Rat Raphe Slice Cultures: High Potency of Milnacipran in the Augmentation of Serotonin Release. Int. J.
Neuropsychopharmacol. 2013, 16, 2295–2306. [CrossRef]
Elhwuegi, A.S. Central Monoamines and Their Role in Major Depression. Prog. Neuropsychopharmacol. Biol. Psychiatry 2004, 28,
435–451. [CrossRef]
Fitzgerald, K.T.; Bronstein, A.C. Selective Serotonin Reuptake Inhibitor Exposure. Top. Companion Anim. Med. 2013, 28, 13–17.
[CrossRef] [PubMed]
García-Colunga, J.; Miledi, R. Blockage of Mouse Muscle Nicotinic Receptors by Serotonergic Compounds. Exp. Physiol. 1999, 84,
847–864. [CrossRef] [PubMed]
García-Colunga, J.; Awad, J.N.; Miledi, R. Blockage of Muscle and Neuronal Nicotinic Acetylcholine Receptors by Fluoxetine
(Prozac). Proc. Natl. Acad. Sci. USA 1997, 94, 2041–2044. [CrossRef]
Gunasekara, N.S.; Noble, S.; Benfield, P. Paroxetine. An Update of Its Pharmacology and Therapeutic Use in Depression and a
Review of Its Use in Other Disorders. Drugs 1998, 55, 85–120. [CrossRef] [PubMed]
Wong, D.T.; Bymaster, F.P.; Engleman, E.A. Prozac (Fluoxetine, Lilly 110140), the First Selective Serotonin Uptake Inhibitor and an
Antidepressant Drug: Twenty Years since Its First Publication. Life Sci. 1995, 57, 411–441. [CrossRef]
Deflorio, C.; Catalano, M.; Fucile, S.; Limatola, C.; Grassi, F. Fluoxetine Prevents Acetylcholine-Induced Excitotoxicity Blocking
Human Endplate Acetylcholine Receptor. Muscle Nerve 2014, 49, 90–97. [CrossRef]
Staley, J.K.; Krishnan-Sarin, S.; Cosgrove, K.P.; Krantzler, E.; Frohlich, E.; Perry, E.; Dubin, J.A.; Estok, K.; Brenner, E.;
Baldwin, R.M.; et al. Human Tobacco Smokers in Early Abstinence Have Higher Levels of β2* Nicotinic Acetylcholine Receptors than Nonsmokers. J. Neurosci. 2006, 26, 8707–8714. [CrossRef]
Chaouch, A.; Müller, J.S.; Guergueltcheva, V.; Dusl, M.; Schara, U.; Rakocević-Stojanović, V.; Lindberg, C.; Scola, R.H.; Werneck,
L.C.; Colomer, J.; et al. A Retrospective Clinical Study of the Treatment of Slow-Channel Congenital Myasthenic Syndrome. J.
Neurol. 2012, 259, 474–481. [CrossRef]
Bliziotes, M.; Eshleman, A.; Burt-Pichat, B.; Zhang, X.-W.; Hashimoto, J.; Wiren, K.; Chenu, C. Serotonin Transporter and Receptor
Expression in Osteocytic MLO-Y4 Cells. Bone 2006, 39, 1313–1321. [CrossRef]
Nencetti, S.; Demontis, G.C.; Mazzoni, M.R.; Betti, L.; Banti, I.; Rossello, A.; Lapucci, A. 3-[(Aryl)(4-Fluorobenzyloxy)Methyl]Piperidine Derivatives: High-Affinity Ligands for the Serotonin Transporter. J. Pharm. Pharmacol. 2007, 59, 1439–1445. [CrossRef]
Wang, S.; Chen, Y.; Liu, X.; Xu, X.; Liu, X.; Liu, B.-F.; Zhang, G. Synthesis and Evaluation of Novel 2,3-Dihydrobenzo[b][1,4]Dioxinand Indolealkylamine Derivatives as Potential Antidepressants. Arch. Pharm. 2014, 347, 32–41. [CrossRef]
Lendvai, B.; Vizi, E.S. Nonsynaptic Chemical Transmission through Nicotinic Acetylcholine Receptors. Physiol. Rev. 2008, 88,
333–349. [CrossRef]
Marchi, M.; Grilli, M. Presynaptic Nicotinic Receptors Modulating Neurotransmitter Release in the Central Nervous System:
Functional Interactions with Other Coexisting Receptors. Prog. Neurobiol. 2010, 92, 105–111. [CrossRef] [PubMed]
Changeux, J.-P. The Nicotinic Acetylcholine Receptor: The Founding Father of the Pentameric Ligand-Gated Ion Channel
Superfamily. J. Biol. Chem. 2012, 287, 40207–40215. [CrossRef] [PubMed]
Yakel, J.L. Cholinergic Receptors: Functional Role of Nicotinic ACh Receptors in Brain Circuits and Disease. Pflüg. Arch. Eur. J.
Physiol. 2013, 465, 441–450. [CrossRef] [PubMed]
Molecules 2021, 26, 2149
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
64.
18 of 21
Picciotto, M.R.; Mineur, Y.S. Molecules and Circuits Involved in Nicotine Addiction: The Many Faces of Smoking. Neuropharmacology 2014, 76 Pt B, 545–553. [CrossRef]
Picciotto, M.R.; Brunzell, D.H.; Caldarone, B.J. Effect of Nicotine and Nicotinic Receptors on Anxiety and Depression. Neuroreport
2002, 13, 1097–1106. [CrossRef] [PubMed]
Saricicek, A.; Esterlis, I.; Maloney, K.H.; Mineur, Y.S.; Ruf, B.M.; Muralidharan, A.; Chen, J.I.; Cosgrove, K.P.; Kerestes, R.;
Ghose, S.; et al. Persistent β2*-Nicotinic Acetylcholinergic Receptor Dysfunction in Major Depressive Disorder. Am. J. Psychiatry
2012, 169, 851–859. [CrossRef]
Hannestad, J.O.; Cosgrove, K.P.; DellaGioia, N.F.; Perkins, E.; Bois, F.; Bhagwagar, Z.; Seibyl, J.P.; McClure-Begley, T.D.; Picciotto,
M.R.; Esterlis, I. Changes in the Cholinergic System between Bipolar Depression and Euthymia as Measured with [123I]5IA
Single Photon Emission Computed Tomography. Biol. Psychiatry 2013, 74, 768–776. [CrossRef]
Meyer, P.M.; Strecker, K.; Kendziorra, K.; Becker, G.; Hesse, S.; Woelpl, D.; Hensel, A.; Patt, M.; Sorger, D.; Wegner, F.; et al.
Reduced α4β2*-Nicotinic Acetylcholine Receptor Binding and Its Relationship to Mild Cognitive and Depressive Symptoms in
Parkinson Disease. Arch. Gen. Psychiatry 2009, 66, 866–877. [CrossRef]
Andreasen, J.T.; Henningsen, K.; Bate, S.; Christiansen, S.; Wiborg, O. Nicotine Reverses Anhedonic-like Response and Cognitive
Impairment in the Rat Chronic Mild Stress Model of Depression: Comparison with Sertraline. J. Psychopharmacol. Oxf. Engl. 2011,
25, 1134–1141. [CrossRef]
Andreasen, J.T.; Redrobe, J.P.; Nielsen, E.Ø. Combined α7 Nicotinic Acetylcholine Receptor Agonism and Partial Serotonin
Transporter Inhibition Produce Antidepressant-like Effects in the Mouse Forced Swim and Tail Suspension Tests: A Comparison
of SSR180711 and PNU-282987. Pharmacol. Biochem. Behav. 2012, 100, 624–629. [CrossRef]
Gentile, A.; Bianco, A.; Nordström, A.; Nordström, P. Use of Alcohol, Drugs, Inhalants, and Smoking Tobacco and the Long-Term
Risk of Depression in Men: A Nationwide Swedish Cohort Study from 1969–2017. Drug Alcohol Depend. 2021, 221, 108553.
[CrossRef]
Glassman, A.H.; Helzer, J.E.; Covey, L.S.; Cottler, L.B.; Stetner, F.; Tipp, J.E.; Johnson, J. Smoking, Smoking Cessation, and Major
Depression. JAMA 1990, 264, 1546–1549. [CrossRef]
Kalman, D.; Morissette, S.B.; George, T.P. Co-Morbidity of Smoking in Patients with Psychiatric and Substance Use Disorders.
Am. J. Addict. 2005, 14, 106–123. [CrossRef]
Mendelsohn, C. Smoking and Depression—A Review. Aust. Fam. Physician 2012, 41, 304–307.
Tsuang, M.T.; Francis, T.; Minor, K.; Thomas, A.; Stone, W.S. Genetics of Smoking and Depression. Hum. Genet. 2012, 131, 905–915.
[CrossRef]
Piirtola, M.; Kaprio, J.; Baker, T.B.; Piasecki, T.M.; Piper, M.E.; Korhonen, T. The Associations of Smoking Dependence Motives
with Depression among Daily Smokers. Addict. Abingdon Engl. 2021. [CrossRef] [PubMed]
Bacher, I.; Wu, B.; Shytle, D.R.; George, T.P. Mecamylamine—A Nicotinic Acetylcholine Receptor Antagonist with Potential for
the Treatment of Neuropsychiatric Disorders. Expert Opin. Pharmacother. 2009, 10, 2709–2721. [CrossRef] [PubMed]
George, T.P.; Sacco, K.A.; Vessicchio, J.C.; Weinberger, A.H.; Shytle, R.D. Nicotinic Antagonist Augmentation of Selective Serotonin
Reuptake Inhibitor-Refractory Major Depressive Disorder: A Preliminary Study. J. Clin. Psychopharmacol. 2008, 28, 340–344.
[CrossRef] [PubMed]
Weinberger, A.H.; McKee, S.A.; Picciotto, M.R.; Mazure, C.M. Examining Antidepressant Drug Response by Smoking Status:
Why Is It Important and How Often Is It Done? J. Psychopharmacol. Oxf. Engl. 2011, 25, 1269–1276. [CrossRef]
Belujon, P.; Grace, A.A. Dopamine System Dysregulation in Major Depressive Disorders. Int. J. Neuropsychopharmacol. 2017, 20,
1036–1046. [CrossRef]
Ortells, M.O.; Barrantes, G.E. Tobacco Addiction: A Biochemical Model of Nicotine Dependence. Med. Hypotheses 2010, 74,
884–894. [CrossRef] [PubMed]
Fenster, C.P.; Hicks, J.H.; Beckman, M.L.; Covernton, P.J.; Quick, M.W.; Lester, R.A. Desensitization of Nicotinic Receptors in the
Central Nervous System. Ann. N. Y. Acad. Sci. 1999, 868, 620–623. [CrossRef]
Giniatullin, R.; Nistri, A.; Yakel, J.L. Desensitization of Nicotinic ACh Receptors: Shaping Cholinergic Signaling. Trends Neurosci.
2005, 28, 371–378. [CrossRef]
Cuevas-Olguin, R.; Esquivel-Rendon, E.; Vargas-Mireles, J.; Barajas-López, C.; Salgado-Delgado, R.; Saderi, N.; Arias, H.R.;
Atzori, M.; Miranda-Morales, M. Nicotine Smoking Concentrations Modulate GABAergic Synaptic Transmission in Murine
Medial Prefrontal Cortex by Activation of α7* and β2* Nicotinic Receptors. Eur. J. Neurosci. 2020, 51, 781–792. [CrossRef]
Wanaverbecq, N.; Semyanov, A.; Pavlov, I.; Walker, M.C.; Kullmann, D.M. Cholinergic Axons Modulate GABAergic Signaling
among Hippocampal Interneurons via Postsynaptic α7 Nicotinic Receptors. J. Neurosci. 2007, 27, 5683–5693. [CrossRef]
Hughes, J.R.; Stead, L.F.; Hartmann-Boyce, J.; Cahill, K.; Lancaster, T. Antidepressants for Smoking Cessation. Cochrane Database
Syst. Rev. 2014, CD000031. [CrossRef] [PubMed]
Brown, R.A.; Abrantes, A.M.; Strong, D.R.; Niaura, R.; Kahler, C.W.; Miller, I.W.; Price, L.H. Efficacy of Sequential Use of
Fluoxetine for Smoking Cessation in Elevated Depressive Symptom Smokers. Nicotine Tob. Res. 2014, 16, 197–207. [CrossRef]
[PubMed]
Ribeiro, E.B.; Bettiker, R.L.; Bogdanov, M.; Wurtman, R.J. Effects of Systemic Nicotine on Serotonin Release in Rat Brain. Brain Res.
1993, 621, 311–318. [CrossRef]
Molecules 2021, 26, 2149
65.
66.
67.
68.
69.
70.
71.
72.
73.
74.
75.
76.
77.
78.
79.
80.
81.
82.
83.
84.
85.
86.
87.
88.
19 of 21
Redrobe, J.P.; Nielsen, E.Ø.; Christensen, J.K.; Peters, D.; Timmermann, D.B.; Olsen, G.M. α7 Nicotinic Acetylcholine Receptor
Activation Ameliorates Scopolamine-Induced Behavioural Changes in a Modified Continuous Y-Maze Task in Mice. Eur. J.
Pharmacol. 2009, 602, 58–65. [CrossRef]
Andreasen, J.T.; Nielsen, E.Ø.; Christensen, J.K.; Olsen, G.M.; Peters, D.; Mirza, N.R.; Redrobe, J.P. Subtype-Selective Nicotinic
Acetylcholine Receptor Agonists Enhance the Responsiveness to Citalopram and Reboxetine in the Mouse Forced Swim Test. J.
Psychopharmacol. Oxf. Engl. 2011, 25, 1347–1356. [CrossRef] [PubMed]
Popik, P.; Kozela, E.; Krawczyk, M. Nicotine and Nicotinic Receptor Antagonists Potentiate the Antidepressant-like Effects of
Imipramine and Citalopram. Br. J. Pharmacol. 2003, 139, 1196–1202. [CrossRef]
Arias, H.R.; Jin, X.-T.; Gallino, S.; Peng, C.; Feuerbach, D.; García-Colunga, J.; Elgoyhen, A.B.; Drenan, R.M.; Ortells, M.O.
Selectivity of (±)-Citalopram at Nicotinic Acetylcholine Receptors and Different Inhibitory Mechanisms between Habenular
α3β4* and α9α10 Subtypes. Neurochem. Int. 2019, 131, 104552. [CrossRef]
Chen, M.-F.; Huang, Y.-C.; Long, C.; Yang, H.-I.; Lee, H.-C.; Chen, P.-Y.; Hoffer, B.J.; Lee, T.J.-F. Bimodal Effects of Fluoxetine on
Cerebral Nitrergic Neurogenic Vasodilation in Porcine Large Cerebral Arteries. Neuropharmacology 2012, 62, 1651–1658. [CrossRef]
Arias, H.R.; Feuerbach, D.; Targowska-Duda, K.M.; Russell, M.; Jozwiak, K. Interaction of Selective Serotonin Reuptake Inhibitors
with Neuronal Nicotinic Acetylcholine Receptors. Biochemistry 2010, 49, 5734–5742. [CrossRef]
López-Valdés, H.E.; García-Colunga, J. Antagonism of Nicotinic Acetylcholine Receptors by Inhibitors of Monoamine Uptake.
Mol. Psychiatry 2001, 6, 511–519. [CrossRef]
García-Colunga, J.; Vázquez-Gómez, E.; Miledi, R. Combined Actions of Zinc and Fluoxetine on Nicotinic Acetylcholine Receptors.
Pharm. J. 2004, 4, 388–393. [CrossRef]
Fryer, J.D.; Lukas, R.J. Antidepressants Noncompetitively Inhibit Nicotinic Acetylcholine Receptor Function. J. Neurochem. 1999,
72, 1117–1124. [CrossRef] [PubMed]
Maggi, L.; Palma, E.; Miledi, R.; Eusebi, F. Effects of Fluoxetine on Wild and Mutant Neuronal α7 Nicotinic Receptors. Mol.
Psychiatry 1998, 3, 350–355. [CrossRef] [PubMed]
Feuerbach, D.; Lingenhöhl, K.; Dobbins, P.; Mosbacher, J.; Corbett, N.; Nozulak, J.; Hoyer, D. Coupling of Human Nicotinic
Acetylcholine Receptors α7 to Calcium Channels in GH3 Cells. Neuropharmacology 2005, 48, 215–227. [CrossRef]
Duarte, Y.; Rojas, M.; Canan, J.; Pérez, E.G.; González-Nilo, F.; García-Colunga, J. Different Classes of Antidepressants Inhibit the
Rat α7 Nicotinic Acetylcholine Receptor by Interacting within the Ion Channel: A Functional and Structural Study. Molecules
2021, 26, 998. [CrossRef]
Arias, H.R.; Feuerbach, D.; Bhumireddy, P.; Ortells, M.O. Inhibitory Mechanisms and Binding Site Location for Serotonin Selective
Reuptake Inhibitors on Nicotinic Acetylcholine Receptors. Int. J. Biochem. Cell Biol. 2010, 42, 712–724. [CrossRef] [PubMed]
Hennings, E.C.; Kiss, J.P.; De Oliveira, K.; Toth, P.T.; Vizi, E.S. Nicotinic Acetylcholine Receptor Antagonistic Activity of
Monoamine Uptake Blockers in Rat Hippocampal Slices. J. Neurochem. 1999, 73, 1043–1050. [CrossRef]
Arias, H.R.; Vázquez-Gómez, E.; Hernández-Abrego, A.; Gallino, S.; Feuerbach, D.; Ortells, M.O.; Elgoyhen, A.B.; García-Colunga,
J. Tricyclic Antidepressants Inhibit Hippocampal α7* and α9α10 Nicotinic Acetylcholine Receptors by Different Mechanisms. Int.
J. Biochem. Cell Biol. 2018, 100, 1–10. [CrossRef]
Peng, H.; Ferris, R.L.; Matthews, T.; Hiel, H.; Lopez-Albaitero, A.; Lustig, L.R. Characterization of the Human Nicotinic
Acetylcholine Receptor Subunit alpha (α) 9 (CHRNA9) and alpha (α) 10 (CHRNA10) in Lymphocytes. Life Sci. 2004, 76, 263–280.
[CrossRef]
Caiaffo, V.; Oliveira, B.D.R.; de Sá, F.B.; Evêncio Neto, J. Anti-inflammatory, Antiapoptotic, and Antioxidant Activity of Fluoxetine.
Pharmacol. Res. Perspect. 2016, 4. [CrossRef]
Goodnick, P.J. Pharmacokinetics of Second Generation Antidepressants: Fluoxetine. Psychopharmacol. Bull. 1991, 27, 503–512.
[PubMed]
Henry, M.E.; Schmidt, M.E.; Hennen, J.; Villafuerte, R.A.; Butman, M.L.; Tran, P.; Kerner, L.T.; Cohen, B.; Renshaw, P.F. A Comparison of Brain and Serum Pharmacokinetics of R-Fluoxetine and Racemic Fluoxetine: A 19-F MRS Study. Neuropsychopharmacology
2005, 30, 1576–1583. [CrossRef]
Karson, C.N.; Newton, J.E.; Livingston, R.; Jolly, J.B.; Cooper, T.B.; Sprigg, J.; Komoroski, R.A. Human Brain Fluoxetine
Concentrations. J. Neuropsychiatry Clin. Neurosci. 1993, 5, 322–329. [PubMed]
Renshaw, P.F.; Guimaraes, A.R.; Fava, M.; Rosenbaum, J.F.; Pearlman, J.D.; Flood, J.G.; Puopolo, P.R.; Clancy, K.; Gonzalez, R.G.
Accumulation of Fluoxetine and Norfluoxetine in Human Brain during Therapeutic Administration. Am. J. Psychiatry 1992, 149,
1592–1594.
Bolo, N.R.; Hodé, Y.; Nédélec, J.F.; Lainé, E.; Wagner, G.; Macher, J.P. Brain Pharmacokinetics and Tissue Distribution in Vivo
of Fluvoxamine and Fluoxetine by Fluorine Magnetic Resonance Spectroscopy. Neuropsychopharmacology 2000, 23, 428–438.
[CrossRef]
Nedahl, M.; Johansen, S.S.; Linnet, K. Reference Brain/Blood Concentrations of Citalopram, Duloxetine, Mirtazapine and
Sertraline. J. Anal. Toxicol. 2018, 42, 149–156. [CrossRef] [PubMed]
Weber, M.L.; Hofland, C.M.; Shaffer, C.L.; Flik, G.; Cremers, T.; Hurst, R.S.; Rollema, H. Therapeutic Doses of Antidepressants
Are Projected Not to Inhibit Human α4β2 Nicotinic Acetylcholine Receptors. Neuropharmacology 2013, 72, 88–95. [CrossRef]
Molecules 2021, 26, 2149
89.
90.
91.
92.
93.
94.
95.
96.
97.
98.
99.
100.
101.
102.
103.
104.
105.
106.
107.
108.
109.
110.
111.
112.
113.
20 of 21
Yu, L.F.; Brek Eaton, J.; Zhang, H.K.; Sabath, E.; Hanania, T.; Li, G.N.; van Breemen, R.B.; Whiteaker, P.; Liu, Q.; Wu, J.; et al.
The Potent and Selective α4β2*/α6*-Nicotinic Acetylcholine Receptor Partial Agonist 2-[5-[5-((S)Azetidin-2-Ylmethoxy)-3Pyridinyl]-3-Isoxazolyl]Ethanol Demonstrates Antidepressive-like Behavior in Animal Models and a Favorable ADME-Tox
Profile. Pharmacol. Res. Perspect. 2014, 2, e00026. [CrossRef] [PubMed]
Dulawa, S.C.; Janowsky, D.S. Cholinergic Regulation of Mood: From Basic and Clinical Studies to Emerging Therapeutics. Mol.
Psychiatry 2019, 24, 694–709. [CrossRef]
Coplan, J.D.; Gopinath, S.; Abdallah, C.G.; Berry, B.R. A Neurobiological Hypothesis of Treatment-Resistant DepressionMechanisms for Selective Serotonin Reuptake Inhibitor Non-Efficacy. Front. Behav. Neurosci. 2014, 8, 189. [CrossRef]
Van Dyke, A.M.; Francis, T.C.; Chen, H.; Bailey, A.M.; Thompson, S.M. Chronic Fluoxetine Treatment in Vivo Enhances Excitatory
Synaptic Transmission in the Hippocampus. Neuropharmacology 2019, 150, 38–45. [CrossRef]
Luscher, B.; Shen, Q.; Sahir, N. The GABAergic Deficit Hypothesis of Major Depressive Disorder. Mol. Psychiatry 2011, 16, 383–406.
[CrossRef] [PubMed]
Komlósi, G.; Molnár, G.; Rózsa, M.; Oláh, S.; Barzó, P.; Tamás, G. Fluoxetine (Prozac) and Serotonin Act on Excitatory Synaptic
Transmission to Suppress Single Layer 2/3 Pyramidal Neuron-Triggered Cell Assemblies in the Human Prefrontal Cortex. J.
Neurosci. 2012, 32, 16369–16378. [CrossRef]
Chang, B.; Daniele, C.A.; Gallagher, K.; Madonia, M.; Mitchum, R.D.; Barrett, L.; Vezina, P.; McGehee, D.S. Nicotinic Excitation of
Serotonergic Projections from Dorsal Raphe to the Nucleus Accumbens. J. Neurophysiol. 2011, 106, 801–808. [CrossRef] [PubMed]
Galindo-Charles, L.; Hernandez-Lopez, S.; Galarraga, E.; Tapia, D.; Bargas, J.; Garduño, J.; Frías-Dominguez, C.; Drucker-Colin,
R.; Mihailescu, S. Serotoninergic Dorsal Raphe Neurons Possess Functional Postsynaptic Nicotinic Acetylcholine Receptors.
Synapse 2008, 62, 601–615. [CrossRef]
Garduño, J.; Galindo-Charles, L.; Jiménez-Rodríguez, J.; Galarraga, E.; Tapia, D.; Mihailescu, S.; Hernandez-Lopez, S. Presynaptic
α4β2 Nicotinic Acetylcholine Receptors Increase Glutamate Release and Serotonin Neuron Excitability in the Dorsal Raphe
Nucleus. J. Neurosci. 2012, 32, 15148–15157. [CrossRef] [PubMed]
Tucci, S.A.; Genn, R.F.; File, S.E. Methyllycaconitine (MLA) Blocks the Nicotine Evoked Anxiogenic Effect and 5-HT Release in
the Dorsal Hippocampus: Possible Role of α7 Receptors. Neuropharmacology 2003, 44, 367–373. [CrossRef]
Méndez, P.; Pazienti, A.; Szabó, G.; Bacci, A. Direct Alteration of a Specific Inhibitory Circuit of the Hippocampus by Antidepressants. J. Neurosci. 2012, 32, 16616–16628. [CrossRef]
Griguoli, M.; Cherubini, E. Regulation of Hippocampal Inhibitory Circuits by Nicotinic Acetylcholine Receptors. J. Physiol. 2012,
590, 655–666. [CrossRef]
Chamberland, S.; Topolnik, L. Inhibitory Control of Hippocampal Inhibitory Neurons. Front. Neurosci. 2012, 6, 165. [CrossRef]
Zhang, G.-F.; Zhang, G.-F.; Zhang, G.-F. The Lateral Habenula: Role in Chronic Pain and Depression. Transl. Perioper. Pain Med.
2020, 7, 271–278.
Castillo-Rolón, D.; Ramírez-Sánchez, E.; Arenas-López, G.; Garduño, J.; Hernández-González, O.; Mihailescu, S.; Hernández-López,
S. Nicotine Increases Spontaneous Glutamate Release in the Rostromedial Tegmental Nucleus. Front. Neurosci. 2020, 14, 604583.
[CrossRef]
Elmer, G.I.; Palacorolla, H.; Mayo, C.L.; Brown, P.L.; Jhou, T.C.; Brady, D.; Shepard, P.D. The Rostromedial Tegmental Nucleus
Modulates the Development of Stress-Induced Helpless Behavior. Behav. Brain Res. 2019, 359, 950–957. [CrossRef]
Arias, H.R.; Targowska-Duda, K.M.; Feuerbach, D.; Jozwiak, K. Coronaridine Congeners Inhibit Human α3β4 Nicotinic
Acetylcholine Receptors by Interacting with Luminal and Non-Luminal Sites. Int. J. Biochem. Cell Biol. 2015, 65, 81–90. [CrossRef]
Arias, H.R.; Jin, X.; Feuerbach, D.; Drenan, R.M. Selectivity of Coronaridine Congeners at Nicotinic Acetylcholine Receptors and
Inhibitory Activity on Mouse Medial Habenula. Int. J. Biochem. Cell Biol. 2017, 92, 202–209. [CrossRef]
Rodrı Guez, P.; Urbanavicius, J.; Prieto, J.P.; Fabius, S.; Reyes, A.L.; Havel, V.; Sames, D.; Scorza, C.; Carrera, I. A Single
Administration of the Atypical Psychedelic Ibogaine or Its Metabolite Noribogaine Induces an Antidepressant-Like Effect in Rats.
ACS Chem. Neurosci. 2020, 11, 1661–1672. [CrossRef]
Arias, H.R.; Fedorov, N.B.; Benson, L.C.; Lippiello, P.M.; Gatto, G.J.; Feuerbach, D.; Ortells, M.O. Functional and Structural
Interaction of (-)-Reboxetine with the Human α4β2 Nicotinic Acetylcholine Receptor. J. Pharmacol. Exp. Ther. 2013, 344, 113–123.
[CrossRef]
Arias, H.R.; Ortells, M.O.; Feuerbach, D. (-)-Reboxetine Inhibits Muscle Nicotinic Acetylcholine Receptors by Interacting with
Luminal and Non-Luminal Sites. Neurochem. Int. 2013, 63, 423–431. [CrossRef]
Gumilar, F.; Arias, H.R.; Spitzmaul, G.; Bouzat, C. Molecular Mechanisms of Inhibition of Nicotinic Acetylcholine Receptors by
Tricyclic Antidepressants. Neuropharmacology 2003, 45, 964–976. [CrossRef]
Arias, H.R.; Bhumireddy, P.; Bouzat, C. Molecular Mechanisms and Binding Site Locations for Noncompetitive Antagonists of
Nicotinic Acetylcholine Receptors. Int. J. Biochem. Cell Biol. 2006, 38, 1254–1276. [CrossRef]
Calimet, N.; Simoes, M.; Changeux, J.-P.; Karplus, M.; Taly, A.; Cecchini, M. A Gating Mechanism of Pentameric Ligand-Gated
Ion Channels. Proc. Natl. Acad. Sci. USA 2013, 110, E3987–E3996. [CrossRef]
Arias, H.R.; Feuerbach, D.; Targowska-Duda, K.M.; Aggarwal, S.; Lapinsky, D.J.; Jozwiak, K. Structural and Functional Interaction of (±)-2-(N-Tert-Butylamino)-3′ -Iodo-4′ -Azidopropiophenone, a Photoreactive Bupropion Derivative, with Nicotinic
Acetylcholine Receptors. Neurochem. Int. 2012, 61, 1433–1441. [CrossRef]
Molecules 2021, 26, 2149
21 of 21
114. Arias, H.R.; Gumilar, F.; Rosenberg, A.; Targowska-Duda, K.M.; Feuerbach, D.; Jozwiak, K.; Moaddel, R.; Wainer, I.W.; Bouzat, C.
Interaction of Bupropion with Muscle-Type Nicotinic Acetylcholine Receptors in Different Conformational States. Biochemistry
2009, 48, 4506–4518. [CrossRef]
115. Morales-Perez, C.L.; Noviello, C.M.; Hibbs, R.E. X-Ray Structure of the Human α4β2 Nicotinic Receptor. Nature 2016, 538,
411–415. [CrossRef] [PubMed]
116. Unwin, N. Refined Structure of the Nicotinic Acetylcholine Receptor at 4Å Resolution. J. Mol. Biol. 2005, 346, 967–989. [CrossRef]
117. Arias, H.R.; Targowska-Duda, K.M.; Feuerbach, D.; Sullivan, C.J.; Maciejewski, R.; Jozwiak, K. Different Interaction between
Tricyclic Antidepressants and Mecamylamine with the Human α3β4 Nicotinic Acetylcholine Receptor Ion Channel. Neurochem.
Int. 2010, 56, 642–649. [CrossRef] [PubMed]
118. Arias, H.R.; Rosenberg, A.; Targowska-Duda, K.M.; Feuerbach, D.; Jozwiak, K.; Moaddel, R.; Wainer, I.W. Tricyclic Antidepressants
and Mecamylamine Bind to Different Sites in the Human α4β2 Nicotinic Receptor Ion Channel. Int. J. Biochem. Cell Biol. 2010, 42,
1007–1018. [CrossRef]
119. Targowska-Duda, K.M.; Arias, H.R.; Jozwiak, K. Application of in Silico Methods to Support Experimental Data: Interactions of
Antidepressants with Nicotinic Acetylcholine Receptors. Open Conf. Proc. J. 2013, 4, 11–22.
120. Harms, M.J.; Schlessman, J.L.; Chimenti, M.S.; Sue, G.R.; Damjanović, A.; García-Moreno, B. A Buried Lysine That Titrates with a
Normal pKa : Role of Conformational Flexibility at the Protein-Water Interface as a Determinant of pKa Values. Protein Sci. Publ.
Protein Soc. 2008, 17, 833–845. [CrossRef] [PubMed]
121. Harms, M.J.; Schlessman, J.L.; Sue, G.R.; García-Moreno, E.B. Arginine Residues at Internal Positions in a Protein Are Always
Charged. Proc. Natl. Acad. Sci. USA 2011, 108, 18954–18959. [CrossRef]