1521-0103/344/1/113–123$25.00
THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS
Copyright ª 2013 by The American Society for Pharmacology and Experimental Therapeutics
http://dx.doi.org/10.1124/jpet.112.197905
J Pharmacol Exp Ther 344:113–123, January 2013
Functional and Structural Interaction of (2)-Reboxetine with the
Human a4b2 Nicotinic Acetylcholine Receptor
Hugo R. Arias, Nikolai B. Fedorov, Lisa C. Benson, Patrick M. Lippiello, Greg J. Gatto,
Dominik Feuerbach, and Marcelo O. Ortells
Department of Medical Education, College of Medicine, California Northstate University, Elk Grove, California (H.R.A); Preclinical
Research, Targacept, Inc., Winston Salem, North Carolina (N.B.F., L.C.B., P.M.L., G.J.G.); Neuroscience Research, Novartis
Institutes for Biomedical Research, Basel, Switzerland (D.F., M.O.O.); and Faculty of Medicine (D.F.) and CONICET (M.O.O.),
University of Morón, Argentina
Received June 28, 2012; accepted September 25, 2012
Introduction
Selective norepinephrine reuptake inhibitors (SNRIs) have
been used with great success to treat the symptoms of depressive
disorders and other related neuropsychiatric illnesses, including
panic attacks, narcolepsy, and attention deficit hyperactivity
disorder (reviewed in Gorman and Kent, 1999). Structurally,
SNRIs can be classified as secondary amine tricyclic antidepressants (e.g., desipramine) and non-tricyclic antidepressants
(e.g., reboxetine). Reboxetine, a racemate of (2)-R,R- and (1)-(S,
S)-([2-[a[2-ethoxyphenoxy]benzyl]-morpholine], is the first commercially available SNRI developed specifically as a first-line
therapy for depressive disorders (Hajos et al., 2004). Although
reboxetine is currently in use in several European countries, it
has not been approved by the Food and Drug Administration for
use in the United States.
From the mechanistic point of view, SNRIs inhibit norepinephrine reuptake transporters, increasing the synaptic
This research was supported by grants from the Consejo Nacional de
Investigaciones Científicas y Técnicas (to M.O.).
dx.doi.org/10.1124/jpet.112.197905.
desensitized ha4b2 nAChR ion channels. Patch-clamp results
also indicate that (2)-reboxetine progressively inhibits the ha4b2
nAChR with two-fold higher potency at the end of one-second
application of agonist, compared with the peak current. The
molecular docking studies show that (2)-reboxetine blocks the ion
channel at the level of the imipramine locus, between M2 rings 69
and 149. In addition, we found a (2)-reboxetine conformer that
docks in the helix bundle of the a4 subunit, near the middle region.
According to molecular dynamics simulations, (2)-reboxetine
binding is stable for both sites, albeit less stable than imipramine.
The interaction of these drugs with the helix bundle might alter
allostericaly the functionality of the channel. In conclusion, the
clinical action of (2)-reboxetine may be produced (at least
partially) by its inhibitory action on ha4b2 nAChRs.
concentration of norepinephrine, thereby enhancing the activity of postsynaptic norepinephrine receptors (Hajos et al.,
2004). Nevertheless, SNRIs also behave as noncompetitive
antagonists (NCAs) of several nicotinic acetylcholine receptors
(nAChRs) (Hennings et al., 1999; Izaguirre et al., 1997; Miller
et al., 2002; Rana et al., 1993; reviewed in Arias et al., 2006).
nAChRs are members of the Cys-loop ligand-gated ion channel
superfamily, which also includes types A and C g-aminobutyric
acid, type 3 serotonin, and glycine receptors (reviewed in Arias
et al., 2006; Arias, 2010; Albuquerque et al., 2009). The
antidepressant-induced inhibition of one or more nAChR
subtypes might be related to their therapeutic actions. This is
in agreement with the cholinergic-adrenergic hypothesis, which
states that the hyperactivity or hypersensitivity of the cholinergic system over the adrenergic system can lead to depressed
mood states (reviewed in Shytle et al., 2002). This hypothesis is
also supported by epidemiologic results showing a higher rate of
smoking in depressed patients than in the general population
(reviewed in Picciotto et al., 2002) and by animal behavior
studies indicating that nicotinic agonists enhance the antidepressant activity of reboxetine (Andreasen et al., 2011).
ABBREVIATIONS: ACh, acetylcholine; BS, binding saline; BTx, k-bungarotoxin; CCh, carbamylcholine; IC50, ligand concentration that inhibits
50% binding or ion flux; Kd, dissociation constant; Ki, inhibition constant; nAChR, nicotinic acetylcholine receptor; NCA, noncompetitive antagonist;
nH, Hill coefficient; (2)-reboxetine, (2)-R,R-([2-[a[2-ethoxyphenoxy]benzyl]-morpholine]); k- RT, room temperature; SNRI, selective norepinephrine
reuptake inhibitor.
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ABSTRACT
The interaction of the selective norepinephrine reuptake inhibitor
(2)-reboxetine with the human a4b2 nicotinic acetylcholine receptor (nAChR) in different conformational states was studied by
several functional and structural approaches. Patch-clamp and
Ca21-influx results indicate that (2)-reboxetine does not activate
ha4b2 nAChRs via interaction with the orthosteric sites, but
inhibits agonist-induced ha4b2 activation by a noncompetitive
mechanism. Consistently, the results from the electrophysiologybased functional approach suggest that (2)-reboxetine may act
via open channel block; therefore, it is capable of producing a usedependent type of inhibition of the ha4b2 nAChR function. We
tested whether (2)-reboxetine binds to the luminal [3H]imipramine
site. The results indicate that, although (2)-reboxetine binds with
low affinity to this site, it discriminates between the resting and
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Arias et al.
Materials and Methods
3
Materials. [ H]Imipramine hydrochloride (47.5 Ci/mmol) and
[3H]cytisine hydrochloride (35.6 Ci/mmol) were obtained from PerkinElmer Life Sciences Products, Inc. (Boston, MA) and stored at -20°C.
(2)-Reboxetine mesylate, acetylcholine chloride (ACh), imipramine
hydrochloride, carbamylcholine chloride (CCh), pepstatin A, leupeptin,
aprotinin, benzamidine, phenylmethylsulfonyl fluoride, and polyethylenimine were purchased from Sigma-Aldrich (St. Louis, MO). Geneticin
(G418) and hygromycine B and (6)-epibatidine were obtained from
Tocris Bioscience (Ellisville, MO). k-Bungarotoxin (k-BTx) was obtained
from Biotoxins Incorporated (St. Cloud, FL). Fetal bovine serum and
trypsin/EDTA were purchased form Gibco BRL (Paisley, UK). Salts were
of analytical grade.
Ca21 Influx Measurements in HEK293-ha4b2 Cells. Ca21
influx experiments were performed using the HEK293-ha4b2 cell
line, which was cultured as explained previously (Arias et al., 2010b;
Arias, 2010). In brief, cells were cultured in a 1:1 mixture of Dulbecco’s
modified Eagle medium containing 3.7 g/l NaHCO3, 1.0 g/l sucrose,
supplemented with stable glutamine (L-AlanyL-L-Glutamine, 524 mg/l),
and Ham’s F-12 Nutrient Mixture containing 1.176 g/l NaHCO3 and
supplemented with 10% (v/v) fetal bovine serum, Geneticin (G418; 0.2
mg/ml), and hygromycine B (0.2 mg/ml). The cells were maintained
at 37°C, 5% CO2, and 95% relative humidity and were passaged
every 3 days by detaching the cells from the cell culture flask by
washing with phosphate-buffered saline and brief incubation (∼3
minutes) with trypsin (0.5 mg/ml)/EDTA (0.2 mg/ml). After cell
culturing, 5 104 cells per well were seeded 48 hours before the
Ca21 influx experiment on black poly-L-lysine 96-well plates (Costar,
Corning Inc., New York) and incubated at 37°C in a humidified
atmosphere (5% CO2/95% air), as previously described (Arias et al.,
2010b; Arias, 2010; Michelmore et al., 2002). Sixteen hours before the
experiment, the medium was changed to 1% bovine serum albumin
(BSA) in HEPES-buffered salt solution (HBSS) (130 mM NaCl, 5.4 mM
KCl, 2 mM CaCl2, 0.8 mM MgSO4, 0.9 mM NaH2PO4, 25 mM glucose,
20 mM HEPES; pH, 7.4). On the day of the experiment, the medium
was removed by flicking the plates and replaced with 100 ml HBSS/1%
BSA containing 2 mM Fluo-4 (Molecular Probes, Eugene, OR) in the
presence of 2.5 mM probenecid (Sigma, Buchs, Switzerland). The
cells were then incubated at 37°C in a humidified atmosphere (5%
CO2/95% air) for 1 hour. Plates were flicked to remove excess of Fluo-4,
washed twice with HBSS/1% BSA, and finally refilled with 100 ml of
HBSS containing different concentrations of (2)-reboxetine and were
preincubated for 5 minutes. Plates were then placed in the cell plate
stage of the fluorimetric imaging plate reader (FLIPR, PC, Molecular
Devices, Sunnyvale, CA). A baseline consisting of 5 measurements of
0.4 seconds each was recorded. (6)-Epibatidine (0.1 mM) was then
added from the agonist plate to the cell plate using the 96-tip pipettor
simultaneously to fluorescence recordings for a total length of 3
minutes. The laser excitation and emission wavelengths were 488 and
510 nm, at 1 W, with a CCD camera opening of 0.4 seconds.
A 1:1 ratio of the alpha4 and the beta2 gene was used for the
generation of the HEK293-human nAChRalpha4beta2 cell line.
Experiments were conducted at 37°C. The stoichiometries of a4b2
in this experimental setting in HEK293 cells was 82% (a4(3)b2(2)). The
heterogeneous stoichiometry in the cell line results in a biphasic
concentration response curve for acetylcholine in the calcium influx
assay (Feuerbach et al., unpublished observations). The strong
prevalence of the LS stoichiometry traps the existence of the much
smaller amount of HS (18%), but in concentration response curves
with a higher number of assessment points (e.g., 16 instead of 8), the
HS fraction becomes evident. The stoichiometry of alpha4beta2
receptors have not been investigated as extensively in the SHEP
cells as in the HEK293 cells. However, the EC50 and the nH value for
ACh may suggest a higher percentage of HS, and the mixed population will be seen more easily.
Electrophysiological Measurements in SHEP1-ha4b2 Cells.
For electrophysiology-based assays, we used the subclonal human
epithelial ha4b2 cells (kindly provided by Dr. Ortrud Steinlein,
Institute of Human Genetics, University Hospital, Ludwig-Maximilians-Universität, Munich, Germany) and subcloned pcDNA3.1zeocin and pcDNA3.1-hygromycin vectors, respectively, into native
nAChR-null SHEP1 cells to create the stably transfected, monoclonal
subclonal human epithelial (SHEP1)-ha4b2 cell line heterologously
expressing ha4b2 nAChRs. Cell cultures were maintained at low
passage numbers (1–26 from frozen stocks to ensure the stable expression of the phenotype) in complete medium augmented with 0.5 mg/ml
zeocin and 0.4 mg/ml hygromycin (to provide a positive selection of
transfectants) and passaged once weekly by splitting the justconfluent cultures 1:50 to maintain cells in proliferative growth.
Reverse-transcriptase polymerase chain reaction, immunofluorescence,
radioligand-binding assays, and isotopic ion flux assays were conducted recurrently to confirm the stable expression of a4b2 nAChRs
as message, protein, ligand-binding sites, and functional receptors.
After removal from the incubator, the medium was aspirated, and
the dissociation medium Accumax (ThermoFisher Scientific, Pittsburgh, PA) was added to the cells for 5 minutes. After the cells were
lifted from the plate, new media was added and the cells were
transferred to a 15 ml conical tube and centrifuged at 1000 rpm for 2
minutes. The supernatant was aspirated, and the cells were resuspended in 2 ml of external solution from which cells were placed in the
Dynaflow chip mount on the stage of an inverted microscope (Carl Zeiss
Inc., Thornwood, NY). On average, 5 minutes was necessary before the
whole-cell recording configuration was established. To avoid modification of the cell conditions, a single cell was recorded per single load. To
evoke fast responses (1 second), agonists were applied using a Dynaflow
system (Cellectricon, Inc., Gaithersburg, MD), in which each channel
delivered pressure-driven solutions at either 50 or 150 psi.
Conventional whole-cell current recordings, together with a computer-controlled Dynaflow system for fast application and removal of
agonists, were used in these studies. In brief, the cells were placed in a
silicon chip bath mount on an inverted Zeiss microscope. Cells chosen
for analysis were continuously perfused with standard external solution. Glass microelectrodes 3–5 MV resistance were used to form tight
seals (1 GV) on the cell surface until suction was applied to convert to
conventional whole-cell recording. The cells were then voltage-clamped
at holding potentials of 260 mV, and ion currents in response to application of ligands were measured. Whole-cell currents recorded with
an Axon 700A amplifier were filtered at 1 kHz and sampled at 5 kHz by
an ADC board 1440 (Molecular Devices, Sunnyvale, CA) and stored on
the hard disk of a PC computer. Whole-cell access resistance was less
than 20 MV. Whole-cell currents were acquired using a Clampex 10
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Considering these results, neuronal nAChRs could be
potential targets for enhancing the beneficial actions elicited
by SNRIs. An interesting example is the attenuation of
nicotine self-administration in rats elicited by reboxetine
(Rauhut et al., 2002) and desipramine (Paterson et al., 2008),
supporting a potential therapeutic use of these antidepressants for the treatment of nicotine addiction. Thus, a better
understanding of the interaction of SNRIs with different
nAChR subtypes is crucial to the development of new and
safer antidepressant therapies or even for novel clinical
uses. In this regard, we have characterized the interaction
of (2)-reboxetine with the human (h) a4b2 nAChR, the most
abundant nAChR subtype in the brain, in different conformational states. To accomplish this, we used structural
and functional approaches, including radioligand binding
assays using the noncompetitive antagonist [3H]imipramine
and the agonist [3H]cytisine, Ca21-influx and patch-clamp
measurements, and molecular docking and dynamics
studies.
Reboxetine and ha4b2 nAChRs
conformational states was studied. In this regard, ha4b2 nAChR
membranes (1.5 mg/ml) were suspended in BS buffer with 15 nM [3H]
imipramine in the presence of 0.1 mM (6)-epibatidine (desensitized/
agonist-bound state) or 0.1 mM k-BTx (resting/k-BTx-bound state) or,
alternatively, with 9.1 nM [3H]cytisine in the absence of any ligand,
and preincubated for 30 minutes at RT. k-BTx is a high-affinity
competitive antagonist that maintains the nAChRs in the resting
(closed) state (Moore and McCarthy, 1995). Nonspecific binding was
determined in the presence of 100 mM imipramine ([3H]imipramine
experiments) and 1 mM CCh ([3H]cytisine experiments).
The total volume was divided into aliquots, and increasing concentrations of the ligand under study were added to each tube and incubated
for 2 hours at RT. nAChR-bound radioligand was then separated from
free ligand by a filtration assay using a 48-sample harvester system with
GF/B Whatman filters (Brandel Inc., Gaithersburg, MD), previously
soaked with 0.5% polyethylenimine for 30 minutes. The membranecontaining filters were transferred to scintillation vials with 3 ml of BioSafe II (Research Product International Corp, Mount Prospect, IL), and
the radioactivity was determined using a Beckman LS6500 scintillation
counter (Beckman Coulter, Inc., Fullerton, CA).
The concentration-response data were curve-fitted by nonlinear
least squares analysis using the Prism software. The corresponding
IC50 values were calculated using the following equation:
u 5 1=½l 1 ð½L=IC50ÞnH
ð1Þ
where u is the fractional amount of the radioligand bound in the
presence of inhibitor at a concentration [L], compared with the
amount of the radioligand bound in the absence of inhibitor (total
binding). IC50 is the inhibitor concentration at which u 5 0.5 (50%
bound), and nH is the Hill coefficient. The observed IC50 values from
the competition experiments described above were transformed into
inhibition constant (Ki) values using the Cheng-Prusoff relationship
(Cheng and Prusoff, 1973):
Ki 5 IC50=1 1 ð½½3 Himipramine=Kdimipramine Þ
ð2Þ
where [[3H]imipramine] is the initial concentration of [3H]imipramine
and Kdimipramine corresponds to the inhibition constant of [3H]
imipramine (0.83 mM) (Arias et al., 2010b). The calculated Ki and
nH values are summarized in Table 2.
Molecular Docking of (2)-Reboxetine and Imipramine
Within the ha4b2 nAChR Ion Channel. The neuronal ha4b2
nAChR was built by homology modeling using the electron microscopy
structure of the Torpedo nAChR determined at ∼4 Å resolution (PDB
2BG9) (Miyazawa et al., 2003; Unwin, 2005) as the template and
using the programs Modeler 9.8 (Sali and Blundell, 1993) and SWIFT
MODELER (Mathur et al., 2011). The ha4b2 nAChR structure was
energy minimized using molecular mechanics using the program
NAMD (Phillips et al., 2005), the CHARMM force field (Brooks et al.,
2009), and the software VEGA ZZ as interface (Pedretti et al., 2004).
The energy minimization was performed by fixing the backbone atoms
to their original positions, to avoid distorting the secondary structure.
(2)-Reboxetine, in the protonated (P) and neutral (N) states, was first
modeled using the VEGA ZZ program (Pedretti et al., 2004).
Minimization and partial charge calculations were done using the
MOPAC program as implemented in VEGA ZZ and using the
semiempirical AM1 method. Subsequently, these molecules were docked
into the nAChR ion channel using AutoDock Vina. The same protocol
was used for imipramine.
The automatic docking procedure was used to investigate the
binding modes of (2)-reboxetine, in the neutral and protonated states,
and imipramine in the nAChR model of the whole receptor. The
parameters used with AutoDock Vina were exhaustiveness of 570
and maximum number of modes of 20. Although the default and
recommended value for the exhaustiveness parameter is eight, we
used the maximum value that our computational system (an AMD 6
six core processor computer with 8Gb RAM) allowed. Although the
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(Molecular Devices Inc., San Diego, CA). Concentration-response
profiles were fit and analyzed using Prism 5.0 (GraphPad Software
Inc., San Diego, CA). The experimental data are presented as the
mean 6 S.E.M., and comparisons of different conditions were analyzed
for statistical significance using Student’s t tests. All experiments were
performed at room temperature (RT; 22°C). More than 90% of the
cells responded to ACh, and every cell presenting a measurable
current was taken into account. All drugs were prepared daily from
stock solutions.
For determination of potential agonist-like properties of (2)-reboxetine in this study, we first used 13 concentrations of ACh (0.1–1000 mM)
to construct dose-response curves, followed by reboxetine (0.1–1000 mM).
Then, one additional application of ACh (0.1–1000 mM) was performed
to assure stability of responses or the existence of any residual inhibition
and/or facilitation. To determine the extent of potential competitive
types of inhibition, we applied five consecutive applications of 10 mM
ACh (10 mM, 30-second intervals) and then preapplied (2)-reboxetine
(3-200 mM, 2-second preapplication before coapplication) and measured
peak current amplitude and the tail portion of the current. To determine
the potential for non-competitive use-dependent inhibition, we used
a procedure described by us and others previously (Fedorov et al., 2009;
Giniatullin et al., 2000). In brief, 10 consecutive pulses of ACh (10 mM,
1-second duration with 30-second intervals) were applied to assure
a stable baseline recording followed by 10 pulses of ACh in the presence
of 1 or 10 mM (2)-reboxetine to induce use-dependent inhibition of
responses by (2)-reboxetine, followed by application of ACh alone to
evaluate recovery from inhibition.
The standard external solution contained 120 mM NaCl, 3 mM
KCl, 2 mM MgCl2, 2 mM CaCl2, 25 mM D-glucose, and 10 mM HEPES
and was adjusted to a pH of 7.4 with TRIS base. In the experiments,
ACh was applied as an agonist without atropine, because our
experimental data showed that 1 mM atropine sulfate did not affect
ACh-induced currents (not shown) and because atropine itself has
been reported to block nAChRs (Liu et al., 2008). For all conventional
whole-cell recordings, TRIS electrodes were used and filled with
solution containing 110 mM TRIS phosphate dibasic, 28 mM TRIS
base, 11 mM EGTA, 2 mM MgCl2, 0.1 mM CaCl2, and 4 mM Mg-ATP
(pH, 7.3). To initiate whole-cell current responses, ACh was delivered
by moving cells from the control solution to agonist-containing
solution and back so that solution exchange occurred within 50
milliseconds (based on 10%–90% peak current rise times). Intervals
between drug applications (0.5–1 minutes) were adjusted specifically
to ensure the stability of receptor responsiveness (without functional
rundown), and the selection of pipette solutions used in most of the
studies described here was made with the same objective.
Preparation of nAChR-Containing Membranes. To prepare
cell membranes in large quantities, the method of Arias et al. (2010)
was used. In brief, HEK293-ha4b2 cells were cultured separately in
suspension using nontreated Petri dishes (150 mm 15 mm). After
culturing the cells for ∼3 weeks, cells were harvested by gently
scraping and were centrifuged at 1000 rpm for 5 minutes at 4°C using
a Sorvall Super T21 centrifuge. Cells were resuspended in binding
saline (BS) buffer (50 mM Tris-HCl, 120 mM NaCl, 5 mM KCl, 2 mM
CaCl2, 1 mM MgCl2; pH, 7.4), containing 0.025% (w/v) sodium azide
and a cocktail of protease inhibitors including, leupeptin, bacitracin,
pepstatin A, aprotinin, benzamidine, and phenylmethylsulfonylfluoride. The suspension was maintained on ice and homogenized using
a Polytron PT3000 (Brinkmann Instruments Inc., Westbury, NY), and
then centrifuged at 10,000 rpm for 30 minutes at 4°C. The pellet was
finally resuspended in BS buffer containing 20% sucrose (w/v) using
the Polytron and briefly (6 15 s) sonicated (Branson Ultrasonics Co.,
Danbury, CT) to assure maximum homogenization. Total protein was
determined using the bicinchoninic acid protein assay (ThermoFisher
Scientific, Rockford, IL). The ha4b2 nAChR membrane preparation
was stored at 280°C in 20% sucrose until required.
Radioligand Binding Experiments Using nAChRs in Different Conformational States. The effect of (2)-reboxetine on [3H]
imipramine and [3H]cytisine binding to ha4b2 nAChRs in different
115
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Arias et al.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
N
+i 5 1 ∂2i
RMSD 5
N
(3)
where N is the number of atoms from the ligand and ∂2i is the distance
between the corresponding ligand atoms obtained at each step and the
starting conformation. RMSD values represent the inter-molecular
conformational changes and the rotation and translation of the whole
molecule. The RMSD was calculated using the program VEGA ZZ.
Results
(2)-Reboxetine-Induced Inhibition of (6)-Epibatidine-Mediated Ca21 Influx in HEK293-ha4b2 Cells. The
potency of (6)-epibatidine to activate ha4b2 nAChRs was
first determined by assessing the fluorescence change in
HEK293-ha4b2 cells caused by an increase in intracellular
Ca21 after agonist stimulation (Fig. 1A). Increased concentrations of (6)-epibatidine activate the ha4b2 nAChR with
potency EC50 5 30 6 5 nM. The observed potency is in the
same concentration range as that previously determined using
cell lines expressing the same nAChR type (Arias et al., 2010b;
Arias, 2010; Gerzanich et al., 1995). (6)-Epibatidine-induced
nAChR activation is blocked by preincubation with (2)-reboxetine (Fig. 1A), with inhibitory potency IC50 5 16 6 1 mM
(Table 1). The fact that the nH values for (2)-reboxetine are
higher than unity (Table 1) indicates that the blocking process
is produced in a cooperative manner. In turn, this suggests that
(2)-reboxetine inhibits the nAChR by interacting with more
than one binding site or that there are different inhibitory
mechanisms. To determine the mechanism of inhibition elicited
by (2)-reboxetine on the ha4b2 nAChR, different concentrations of (2)-reboxetine were added to one (6)-epibatidine
concentration (Fig. 1B). The results indicate that (2)-reboxetine
Fig. 1. (A) (2)-Reboxetine-induced inhibition of (6)-epibatidine–evoked
calcium influx in HEK293-ha4b2 cells. The (6)-epibatidine–evoked
calcium influx in HEK293-ha4b2 cells (j) was inhibited by preincubation
(5 min) with several concentrations of (2)-reboxetine (u), followed by
addition of 0.1 mM (6)-epibatidine. Response was normalized to the
maximal (6)-epibatidine response, which was st as 100%. (B) Pretreatment with 1 (m), 10 (d), or 100 (¤ ) mM (2)-reboxetine inhibits
(6)-epibatidine–induced calcium influx in HEK293-ha4b2 cells in
a dose-dependent and noncompetitive manner. The plots are representative of 28 (j), 6 (u), and 3 (m, d, ¤) determinations, respectively, where
the error bars represent the S.D. values. The calculated IC50 and nH values
are summarized in Table 1.
inhibits the ha4b2 nAChR in a dose-response manner and by
a noncompetitive mechanism.
Lack of (2)-Reboxetine-Induced nAChR Activation
Determined by Electrophysiology-Based Assays. To
evaluate the ability of (2)-reboxetine to activate ha4b2
nAChRs, a wide range of (2)-reboxetine concentrations was
applied to SHEP1-ha4b2 cells. The experimental design and
results of these experiments are presented in Fig. 2. We
initially applied ACh and followed this with an application of
(2)-reboxetine and then one additional application of ACh
(Fig. 2A). Under these experimental conditions, we were able
to obtain a robust full dose response of ACh-induced ha4b2
nAChR activation (0.1–1000 mM; Fig. 2, B and C). Subsequent
applications of (2)-reboxetine revealed no apparent current
induction even at the highest concentrations used (1 mM; Fig.
2C). An additional application of ACh confirmed the presence
of functional receptors with full recovery of the initial responses
to ACh (Fig. 2, A and B). This result strongly suggests that
(2)-reboxetine is not an agonist of a4b2 nAChRs (i.e., it does
not interact with the orthosteric sites of the receptor).
(2)-Reboxetine Inhibits ha4b2 nAChR Function
in a Dose-Dependent Manner. To determine whether
(2)-reboxetine inhibis ha4b2 nAChR function, we first used
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software searched the whole receptor structure, the time used for each
run was in the order of minutes. To achieve this performance,
however, no flexible residues were allowed in the receptor models.
For each molecule [i.e., neutral and protonated (2)-reboxetine and
imipramine], the program provides theoretical estimations of their
affinities to the ha4b2 nAChR. The program gives the best 20 poses
found for each run. However, before giving this final output, it
performs an internal clustering in such a way that multiple similar
results are trimmed. Nevertheless, in each output, several of the best
20 conformers are still superposed. From every cluster of superposed
conformations, we selected the one with the highest binding affinity
according to the Vina program. In this way, the number of binding
sites found in every run is defined by the number of conformers
remaining after this selection process.
To test the stability of (2)-reboxetine in their predicted docking
sites, a 10-ns molecular dynamics was performed at 300K using the
program NAMD, the CHARMM force field, and the software VEGA ZZ
as interface. For comparison purposed, the same protocol was applied
to imipramine. For this aim, the ha4b2 nAChR model was hydrated
with a 10 Å minimum thick shell using the program solvate 1.0
(Grubmuller et al., 1996); this also added the appropriated number of
Cl2 and Na1 to neutralize the system, which was fully minimized
using NAMD. During the molecular dynamics simulation and to
reduce computation time, all residues and water molecules outside
a 20 Å radius sphere and centered on the corresponding conformer
were restricted to their original positions, whereas those within this
sphere were free to move. The same size sphere was used to implement
a spherical periodic boundary condition. To estimate the root mean
square deviation (RMSD) (see Eq. 3) values with respect to the initial
structure, the conformations during the simulation were extracted
every 10-ps from the simulation trajectory of 10-ns total time.
Reboxetine and ha4b2 nAChRs
TABLE 1
Inhibitory potency (IC50) of (2)-reboxetine for the ha4b2 nAChR
Method
Conformational State
IC50
nHa
mM
Ca2+ influxb
16.0 6 1.0 1.45 6 0.09
5 min preincubation Mix of activated
and desensitized
states
Patch clampc
Amplitude peak
Mainly
40.0 6 0.7 1.10 6 0.40
measurements
activated state
Mainly desensitized 21.0 6 0.8 1.20 6 0.30
Tail current
state
measurements,
2-s preincubation
a
Hill coefficient.
Values obtained from Fig. 1Aa and Fig. 3b, respectively.
b,c
increased the concentration range of (2)-reboxetine to 0.3200 mM (Fig. 3). In this case, (2)-reboxetine produced a robust
inhibition of the ha4b2 nAChR function (Fig. 3A; Table 1). A
relatively greater portion of the inhibitory action produced by
(2)-reboxetine on ha4b2 nAChR function occurred at the end
of the coapplication but not before that. As a result of this
selective inhibition, the current kinetics was substantially
modified by (2)-reboxetine. More specifically, the peak
current amplitude (IC50 ∼40 mM) was inhibited to a lesser
extent than the steady-state (tail) currents (IC50 ∼21 mM) at
the final ACh application (Table 2). Changes in current
kinetics consisted of a 3.2-fold increase in the rate of current
decay, from t 5 0.47 seconds in controls to t 5 0.15 seconds in
the presence of 200 mM (2)-reboxetine (Figs. 3C,D). Thus, our
results indicate that the inhibition produced by (2)-reboxetine depends on the time of exposure, suggesting a time- and/
or use-dependent mechanism of action via open-channel
blockade. This pattern of functional changes resembles that
recently published for AMPA-type glutamate receptors (Zaitsev
et al., 2011). Therefore, the next question was to find out
whether (2)-reboxetine produces open channel blockade.
(2)-Reboxetine Inhibits ha4b2 nAChR Function in a
Use-Dependent Manner. To determine whether (2)-reboxetine inhibits ha4b2 nAChR function by an open-channel
Fig. 2. (2)-Reboxetine does not activate ha4b2 nAChRs. (A) Plot representing the flow of experiments, with consecutive applications of increasing
concentrations of ACh, starting with an ACh control after the first ACh application (s), followed by (2)-reboxetine (u), and ACh again (d) during the
second ACh application (n = 5). Note a full recovery of responses to ACh after application of 0.1–1000 mM (2)-reboxetine, suggesting the absence of any
residual inhibition. (B) Results from (A) on a logarithmic scale. Increasing ACh and (2)-reboxetine concentrations (1-s pulses) were delivered every 30 s
(symbols the same as in A). Note typical biphasic dose-response to ACh control (C) fitted with dual Hill equation and absence of responses to application
of (2)-reboxetine. (C) Representative dose-response curves at concentrations of 0.1, 1, 10, 100, and 1000 mM ACh. Open bar above curves indicates time
of ACh application. Calibration bars are 200 pA and 2 s. (D) Representative curve of application of 1000 mM (2)-reboxetine demonstrates lack of any
apparent or aberrant currents after application of (2)-reboxetine. Open bar above curve represents exact timing of (2)-reboxetine application.
Calibration bars are 40 pA and 2 s. The results shown are the mean (6 S.E.M.) of four cells, where some error bars lie within the symbol size.
Downloaded from jpet.aspetjournals.org at Novartis Global on January 7, 2013
a protocol of preapplication of (2)-reboxetine followed by coapplication of a single concentration of ACh with increasing
concentrations of (2)-reboxetine. In the first set of experiments, we used a concentration range of (2)-reboxetine from
0.001-1 mM (n 5 3, not shown). In this concentration range,
(2)-reboxetine did not produce significant inhibition of a4b2
receptor function. In the second set of experiments, we
117
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Arias et al.
TABLE 2
Binding affinity of (2)-reboxetine for the [3H]imipramine binding site in
the ha4b2 nAChR ion channel
Desensitized State
Ki
a
nH
Resting State
b
mM
18 6 1
Ki
a
nH
b
mM
0.87 6 0.05
38 6 3
0.77 6 0.05
a
Ki values were obtained in the presence of 0.1 mM (6)-epibatidine (desensitized
state) or 0.1 mM k-BTx (resting state) (Fig. 5A), according to Eq. 2.
b
Hill coefficient.
blocking mechanism, we used a protocol for use-dependent
inhibition commonly implemented for well-known open channel
blockers of nAChRs (Fedorov et al., 2009; Giniatullin et al.,
2000). The protocol consisted of 10 consecutive applications of
ACh at a single concentration in controls to establish a stable
baseline, followed by 10 consecutive applications of the same
Radioligand Binding Experiments Using ha4b2
nAChRs in Different Conformational States. Because
(2)-reboxetine inhibits ha4b2 nAChRs by a noncompetitive
mechanism and considering that a noncompetitive mechanism can be produced by a luminal interaction, the effect
of (2)-reboxetine on [3H]imipramine binding to ha4b2 nAChRs
in different conformational states was studied. The [3H]imipramine competition binding results show that the highest concentration of (2)-reboxetine (200 mM) inhibits 75%–85% of the
specific binding of [3H]imipramine to the ha4b2 nAChR ion
channel (Fig. 5A), whereas the same concentration range does
not inhibit [3H]cytisine binding to the ha4b2 nAChR agonist sites. This supports the view that (2)-reboxetine is not an
agonist/competitive antagonist but a noncompetitive antagonist.
The observed Ki values indicate that (2)-reboxetine interacts with the [3H]imipramine binding site with relatively
low affinity and that it discriminates between the resting and
desensitized states (Table 2). The fact that the calculated nH
values for (2)-reboxetine are close to unity (Table 2) indicates
Downloaded from jpet.aspetjournals.org at Novartis Global on January 7, 2013
Fig. 3. Functional inhibition of ha4b2 nAChR responses by (2)-reboxetine. (A) Full dose-response inhibition mediated by (2)-reboxetine (i.e.,
3–200 mM) on the peak current amplitude (j) and tail current (u)
measurements, respectively. (B) Representative examples of inhibition
produced by increasing concentrations of (2)-reboxetine. Note that peak
current amplitude was inhibited to a lesser extent than the tail current,
leading to a profound change in current kinetics [t values of 0.43 and 0.15 s
for control and (2)-reboxetine, respectively]. Bars above the plot indicate
time of application of ACh (black) and (2)-reboxetine (white), respectively.
Calibration under curves is 100 pA and 1 s. (C) Scaled curves of ha4b2
nAChR responses to ACh in the absence (control) and in the presence of
(2)-reboxetine (200 mM), respectively, exemplify the profound changes in
current kinetics. Vertical dashed line indicates the time point for tail
current measurements. Time scale 1-s. Bars above curves show timing of
ACh (black) and (2)-reboxetine (white) applications, respectively.
concentration of ACh in the presence of a single concentration of
(2)-reboxetine. To address recovery from inhibition (in terms of
reversibility), 10 additional applications of ACh were performed
at the end of the protocol. The results of these experiments are
shown in Fig. 4. In the first set of experiments, we used 10 mM
ACh and 1 mM (2)-reboxetine (Fig. 4A). The first coapplication
of 1 mM (2)-reboxetine and ACh did not produce significant
inhibition of ha4b2 nAChR responses (∼4%; t . 0.05; Student
t test). Repetition of ACh applications in the presence of
(2)-reboxetine (10 times) resulted in a modest yet significant
(t , 0.05; one-way ANOVA) slow onset inhibition during
stimulation (t . 20000 pulses). After application of 10 pulses,
the amplitude of responses was inhibited by 15% 6 4%. In the
second set of experiments, we used 10 mM (2)-reboxetine to
evaluate the concentration dependency of this use-dependent
type of inhibition. As can be seen in Fig. 4A, at this
concentration, the first application of ACh in the presence of
(2)-reboxetine was inhibited by 11%, whereas the last (10th
application) was inhibited by 26%. Overall, one-way ANOVA
showed significant differences between control applications and 1
and 10 mM (2)-reboxetine and significant differences between 1
and 10 mM applications of (2)-reboxetine (P , 0.05). In addition,
the inhibitory action of (2)-reboxetine was readily reversible.
Washout of 10 mM (2)-reboxetine resulted in fast recovery of
responses to ACh with a time constant of t 5 1.3 applications
after applications of ACh during washout (Fig. 4A). These results
suggest that (2)-reboxetine can produce a use-dependent type of
inhibition of ha4b2 nAChR function at concentrations as low as 1
mM and in a dose-dependent manner (e.g., compare 1 mM with 10
mM in Fig. 4).
Reboxetine and ha4b2 nAChRs
neutral and protonated states, has several putative binding
sites in two different receptor regions of the ha4b2 nAChR
(Table 3). One locus is situated in the ion-channel, near the
cytoplasmic side between M2 residue rings 6´ and 14´
(Table 3, Fig. 6A–D). The docking experiment of imipramine
gave the same result as (2)-reboxetine, where their docking
sites are superposed. The same binding site within the ha4b2
nAChR ion channel was previously found (Arias et al., 2010b).
Molecular dynamics simulations of these (2)-reboxetine and
imipramine conformers (Fig. 7A) showed that they slightly
depart from their original docked positions, but after having
reached their new orientation after ∼0.5 nanoseconds,
imipramine remains stable for the remaining 9.5 nanosecond.
that (2)-reboxetine inhibits [3H]imipramine binding in a
noncooperative manner. In turn, this result suggests that
(2)-reboxetine inhibits radioligand binding in a steric fashion and, consequently, that (2)-reboxetine and imipramine
interact with a single binding site.
Molecular Docking of (2)-Reboxetine and Imipramine Within the ha4b2 nAChR Ion Channel. Molecular
docking simulations showed that (2)-reboxetine, in the
TABLE 3
ha4b2 nAChR residues involved in the binding of (2)-reboxetine to the transmembrane region and the corresponding
theoretical binding affinities
For the transmembrane regions M1, M3, and M4 the total number of different residues involved in the binding of reboxetine are represented. For
the transmembrane region M2, the relative position of the residue within this segment is shown.
(2)-Reboxetine
M1
M2
M3
M4
Total
Ki
mM
Bundle a4 subunit
Ion-channel
a
RR
RR
RR
RR
RR
Na
N
P
P
P
RR Na
RR P
2
2
4
4
2
6
9
10
12
13
14
15
1
16
18
19
21
22
1
2
2
2
2
2
2
Conformer with the highest affinity that is also shown in Figure 6.
1
1
1
1
4
4
4
6
4
3
4
10
6
12
11
6
0.4
0.7
0.8
1.4
1.6
8
8
0.6
1.4
Downloaded from jpet.aspetjournals.org at Novartis Global on January 7, 2013
Fig. 5. Effect of (2)-reboxetine on (A) [3H]imipramine and (B) [3H]cytisine
binding to ha4b2 nAChRs. ha4b2 nAChR membranes (1.5 mg/ml) were
equilibrated (2 h) with (A) 15 nM [3H]imipramine in the presence of 0.1 mM
(6)-epibatidine (u) (desensitized/agonist-bound state) or 0.1 mM k-BTx
(s) (resting/k-BTx-bound state), or with (B) 9.1 nM [3H]cytisine (d), and
increased concentrations of (2)-reboxetine. Nonspecific binding was
determined in the presence of 100 mM imipramine (A) or 1 mM CCh (B).
Each plot is the combination of three separated experiments each one
performed in triplicate, where the error bars represent the S.D. values.
The IC50 and nH values were obtained from plots in (A) by nonlinear leastsquares fit according to Eq. 1. Subsequently, the Ki values were calculated
using Eq. 2. The Ki and nH values are summarized in Table 2.
Fig. 4. Use-dependent block of responses to repetitive applications of ACh
(10 mM) in the presence of 1 and 10 mM (2)-reboxetine. (A) Experimental
design and results. After achieving a stable baseline with ACh in controls
(, and d, 5 min), ACh was applied 10 times in the presence of 1 (e) and 10
(u) mM (2)-reboxetine (5 min). This was followed by application of 10 mM
ACh alone (n, washout 5 min). (2)-Reboxetine blocked responses to ACh
in a use-dependent manner, seen as a progressive increase in the extent
of inhibition during repeated applications of ACh in the presence
(2)-reboxetine. (2)-Reboxetine blocked ∼15% and ∼30% of the current
with a time constant of 20,000 and 4.4 applications (1 mM and 10 mM,
respectively), in a partially reversible manner.
Receptor Region
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Arias et al.
On the other hand, (2)-reboxetine has a greater mobility from
this final site. Thus, despite the fact that (2)-reboxetine binds
to the same locus as imipramine, its binding is less stable. In
both cases, these molecules completely block the ion channel.
The other locus where both (2)-reboxetine and imipramine
bind is within the a4 subunit transmembrane helix bundle. At
this position these drugs do not block the channel, but they
can presumably alter the luminal structure of the channel,
decreasing ion flux. In this case, molecular dynamics show
that imipramine also changes slightly from its original
docking site (as determined by the docking simulation) and
maintains the new orientation without any major deviation.
On the other hand, (2)-reboxetine deviates more from the
original docking site, but it remains there in a more stable
way than in the ion channel lumen. In this regard, it seems
that (2)-reboxetine might allosterically promote changes in
the channel properties with more potency, including desensitization, than would a direct block of the channel.
Discussion
The inhibitory activity of reboxetine on nAChRs (Arias
et al., 2006; Miller et al., 2002) might be relevant to the
observed preclinical and clinical actions elicited by this nontricyclic SNRI (Andreasen et al., 2011; Hajos et al., 2004; Rauhut
et al., 2002). In this regard, the interaction of (2)-reboxetine
with the ha4b2 nAChR in different conformational states
was characterized. To this end, functional and structural
approaches were used, including radioligand binding assays,
Ca21 influx and patch-clamp methods, and molecular docking
and dynamics studies.
Downloaded from jpet.aspetjournals.org at Novartis Global on January 7, 2013
Fig. 6. Molecular interactions between
(2)-reboxetine and ha4b2 nAChR ion
channel residues. (A) General side view
of the ha4b2 nAChR parallel to the
membrane plane, with (2)-reboxetine
docked at the imipramine binding site
within the channel lumen (blue surface)
and at the interior of the helix bundle of
the a4 subunit (red surface). (B) Cytoplasmic view of the ha4b2 nAChR with
the same conformers as in (A). (C)
Detailed cytoplasmic view of (2)-reboxetine (blue surface) docked at the
imipramine binding site. (D) Detailed
side view parallel to the membrane
plane of (2)-reboxetine (blue surface)
docked at the imipramine binding
site. (E) Detailed cytoplasmic view of
(2)-reboxetine (red surface) docked at
the a-helix bundle. (F) Detailed side
view parallel to the membrane plane
of (2)-reboxetine (red surface) docked
at the a-helix bundle. M1, M3, and M4
denote residues belonging to the corresponding transmembrane regions.
R denotes the ring number of the M2
transmembrane segment.
Reboxetine and ha4b2 nAChRs
121
To determine the effect of (2)-reboxetine on (6)-epibatidineactivated Ca21 influx in HEK293-ha4b2 cells, a preincubation
protocol was used (Fig. 1). The results indicate that (2)-reboxetine
inhibits ha4b2 nAChRs with relatively lower potency (Table 1)
than that for human muscle AChRs (∼2–4 mM; paper in preparation). To our knowledge, this is the first time that the
inhibitory potency of (2)-reboxetine for ha4b2 nAChRs has been
demonstrated. Of interest, reboxetine was found to inhibit 86Rb1
efflux from thalamic synaptosomes containing primarily a4b2*
nAChRs with IC50 of 0.65 mM (Miller et al., 2002). The observed
large difference for the inhibitory potencies on the a4b2*
nAChRs is commonly attributed to intrinsic methodological
differences in assay conditions regarding direct (electrophysiology) versus indirect (fluorescence-based) type of
measurements and properties of signal carriers (86Rb1
versus fluorescent dye).
To confirm and expand the results from the fluorescencebased screening assays, we also investigated the excitatory
and/or inhibitory actions of (2)-reboxetine on ha4b2 nAChRs
function with use of electrophysiology-based approaches on
SHEP1-ha4b2 cells, which provide direct measurements of
ion flow through the ion channel. First, we found that
applications of (2)-reboxetine over a wide concentration
range (0.1–1000 mM) did not produce any measurable
currents. We did not find any hint of transient and/or aberrant
currents, which would be apparent in cases of a mixture of
excitatory and inhibitory action at certain concentrations.
This suggests the following possibilities: (1) (2)-reboxetine
does not interact with the orthosteric site of the ha4b2 nAChR
in an agonist-like fashion, (2) it interacts with the orthosteric
sites in a competitive antagonist fashion, and (3) the potency
of the inhibitory action of (2)-reboxetine greatly exceeds
(leftward shift) the potency of agonist-like actions. To address
these possibilities, we first determined the inhibitory action
of (2)-reboxetine and found that (2)-reboxetine appeared
to be an inhibitor of ha4b2 nAChR function. Of interest,
(2)-reboxetine exhibited a different magnitude of inhibition
that was dependent on the time and exposure of the open
channel state during applications of ACh with (2)-reboxetine.
The amplitude of the peak current showed an IC50 of ∼40 mM,
whereas the steady-state portion of the current (i.e., the tail
current) showed an IC50 of ∼21 mM (Table 1). This pharmacological pattern resembles that for the recently described
AMPA receptor blockers (Zaitsev et al., 2011). Consistently,
the difference in potencies was associated with significant
changes in the current kinetics of ACh-induced activation of
ha4b2 nAChRs, which were found to be 3.2-fold faster in
the presence of 200 mM (2)-reboxetine (t 5 0.471 s versus t 5
0.147 s). There are two main types of inhibition of nAChR
function (competitive and noncompetitive), based on the
affinity of an antagonist for the nAChR orthosteric sites.
These results suggested that at low concentrations, the
inhibitory action of (2)-reboxetine was primarily mediated
by a noncompetitive type of inhibition, presumably via openchannel block.
To confirm the potential open-channel blocking mechanism,
we investigated the extent of use-dependent inhibition, which
represents a well-known feature of open-channel blockers of
nAChRs (Fedorov et al., 2009; Giniatullin et al., 2000). We
found that (2)-reboxetine was capable of producing small yet
significant use-dependent inhibition in a concentrationdependent manner (1 mM and 10 mM). Of importance, even
at the lowest concentrations of (2)-reboxetine used (1 mM), we
were able to demonstrate significant use-dependent inhibition (by 15%) during only 10 consecutive agonist applications.
It is known that use-dependent inhibition is dependent on the
Downloaded from jpet.aspetjournals.org at Novartis Global on January 7, 2013
Fig. 7. Molecular dynamics of (2)-reboxetine and imipramine binding to the ha4b2 nAChR. (A) RMSD values of (2)-reboxetine (light gray line) and
imipramine (black line) of a 10-ns dynamics simulation performed at their docking sites within the (A) ha4b2 nAChR ion channel and the (B) a4-subunit
transmembrane helix bundle.
122
Arias et al.
and neuronal-type nAChRs (Arias et al., 2010b; Arias et al.,
2010c; Arias et al., 2010d; Gumilar and Bouzat, 2008; LopezValdes and Garcia-Colunga, 2001). This evidence suggests
that there is a basic mechanistic motif for structurally
different antidepressants mediating the inhibition of distinct
nAChRs.
Authorship Contributions
Participated in research design: Arias, Fedorov, Benson, Gatto,
Lippiello, Feuerbach, Ortells.
Conducted experiments: Arias, Fedorov, Benson, Feuerbach,
Ortells.
Contributed new reagents or analytic tools: Fedorov, Gatto.
Performed data analysis: Arias, Fedorov, Benson, Lippiello,
Feuerbach, Ortells.
Wrote or contributed to the writing of the manuscript: Arias,
Fedorov, Benson, Gatto, Lippiello, Feuerbach, Ortells.
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number of uses (i.e., the number of openings and/or the time of
exposure of channels in the open state to the antagonist). The
number of potential opening events in the brain is quite large,
far exceeding the 10 applications used in our model (due to
channel flickering and opening of channels as a result of
spontaneous and/or evoked endogenous transmitter release).
Thus, it could be speculated that the continuous presence of
a pharmacologically relevant concentration of (2)-reboxetine
(∼1 mM) in the system, the noncompetitive, use-dependent
inhibitory action described here, would dominate other types
of potential interactions. This also calls into question the
mechanistic differentiation between functional desensitization and open-channel blocked states of receptors. In this
regard, although our results suggest that (2)-reboxetine at
low concentrations mainly acts via an open channel blocking
mechanism, (2)-reboxetine–induced nAChR desensitization
cannot be completely ruled out.
Because the inhibitory action of (2)-reboxetine on agonistinduced ha4b2 nAChR activation is mediated by a noncompetitive mechanism (see Figs. 1B and 2B), we tested whether
(2)-reboxetine binds to the nAChR lumen by [3H]imipramine
competition binding experiments. The results indicate that
(2)-reboxetine binds with ∼2-fold higher affinity to the
desensitized ha4b2 nAChR, compared with the resting
nAChR (see Table 2). Considering that the nH values from
these competition experiments are close to unity, (2)-reboxetine may be interacting with the imipramine binding site in
a steric fashion. Consistent with these results, (2)-reboxetine
does not bind to the ha4b2 nAChR agonist sites (see Fig. 5B).
Previous experiments using rat whole-brain membranes also
showed that reboxetine binds with very low affinity to the
nAChR agonist sites (Miller et al., 2002). Previous results
from our laboratory indicate that tricyclic antidepressants
and selective serotonin reuptake inhibitors (Arias et al.,
2010b; Arias, 2010) bind to overlapping sites in a domain
formed between the serine (position 69) and valine (position
139) rings of the ha4b2 nAChR ion channel. In this regard, our
radioligand binding (Table 2) and docking (see Fig. 6) results
support the view that (2)-reboxetine also binds to the same
binding domain as that for these structurally different
antidepressants.
The plasma concentration of (2)-reboxetine in patients
receiving long-term treatment (6 months) can reach values as
high as ∼1 mM (Ohman et al., 2003). New insights into the
potential use-dependent mechanism of inhibition demonstrated in the present study using electrophysiology-based
assays suggest that (2)-reboxetine at concentrations as low as
1 mM is capable of producing small, yet significant, concentration and use-dependent inhibition of ha4b2 nAChR
function. Although the clinical concentration of reboxetine
and magnitude of effect was found not within physiologically
relevant margins, our results suggest that (2)-reboxetine can
inhibit ha4b2 nAChRs in a clinically significant concentration
range by blocking the a4b2 nAChR ion channel through the
interaction with the tricyclic antidepressant locus.
Our results support the idea that the clinical action of
reboxetine is mediated, at least partially, by a noncompetitive
inhibitory (blocking) mechanism on ha4b2 nAChRs. Inhibitory mechanisms have also been reported for tricyclic
antidepressants, bupropion, and selective serotonin reuptake
inhibitors in muscle- (Arias et al., 2009; Arias et al., 2010a;
Gumilar et al., 2003; Lopez-Valdes and Garcia-Colunga, 2001)
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