Identifying the binding site(s) for antidepressants on the Torpedo nicotinic acetylcholine receptor: [3H]2-azidoimipramine photolabeling and molecular dynamics studies

In Vivo Photoadduction of Anesthetic Ligands in Mouse Brain Markedly Extends Sedation and Hypnosis

Photoaffinity ligands are best known as tools used to identify the specific binding sites of drugs to their molecular targets. However, photoaffinity ligands have the potential to further define critical neuroanatomic targets of drug action. In the brains of WT male mice, we demonstrate the feasibility of using photoaffinity ligands in vivo to prolong anesthesia via targeted yet spatially restricted photoadduction of azi-m-propofol (aziPm), a photoreactive analog of the general anesthetic propofol. Systemic administration of aziPm with bilateral near-ultraviolet photoadduction in the rostral pons, at the border of the parabrachial nucleus and locus coeruleus, produced a 20-fold increase in the duration of sedative and hypnotic effects compared with control mice without UV illumination. Photoadduction that missed the parabrachial-coerulean complex also failed to extend the sedative or hypnotic actions of aziPm and was indistinguishable from nonadducted controls. Paralleling the prolonged behavioral and EEG consequences of on target in vivo photoadduction, we conducted electrophysiologic recordings in rostral pontine brain slices. Using neurons within the locus coeruleus to further highlight the cellular consequences of irreversible aziPm binding, we demonstrate transient slowing of spontaneous action potentials with a brief bath application of aziPm that becomes irreversible on photoadduction. Together, these findings suggest that photochemistry-based strategies are a viable new approach for probing CNS physiology and pathophysiology.SIGNIFICANCE STATEMENT Photoaffinity ligands are drugs capable of light-induced irreversible binding, which have unexploited potential to identify the neuroanatomic sites of drug action. We systemically administer a centrally acting anesthetic photoaffinity ligand in mice, conduct localized photoillumination within the brain to covalently adduct the drug at its in vivo sites of action, and successfully enrich irreversible drug binding within a restricted 250 µm radius. When photoadduction encompassed the pontine parabrachial-coerulean complex, anesthetic sedation and hypnosis was prolonged 20-fold, thus illustrating the power of in vivo photochemistry to help unravel neuronal mechanisms of drug action.

(-)-Reboxetine inhibits muscle nicotinic acetylcholine receptors by interacting with luminal and non-luminal sites

The interaction of (-)-reboxetine, a non-tricyclic norepinephrine selective reuptake inhibitor, with muscle-type nicotinic acetylcholine receptors (AChRs) in different conformational states was studied by functional and structural approaches. The results established that (-)-reboxetine: (a) inhibits (±)-epibatidine-induced Ca(2+) influx in human (h) muscle embryonic (hα1β1γδ) and adult (hα1β1εδ) AChRs in a non-competitive manner and with potencies IC50=3.86±0.49 and 1.92±0.48 μM, respectively, (b) binds to the [(3)H]TCP site with ~13-fold higher affinity when the Torpedo AChR is in the desensitized state compared to the resting state, (c) enhances [(3)H]cytisine binding to the resting but activatableTorpedo AChR but not to the desensitized AChR, suggesting desensitizing properties, (d) overlaps the PCP luminal site located between rings 6' and 13' in the Torpedo but not human muscle AChRs. In silico mutation results indicate that ring 9' is the minimum structural component for (-)-reboxetine binding, and (e) interacts to non-luminal sites located within the transmembrane segments from the Torpedo AChR γ subunit, and at the α1/ε transmembrane interface from the adult muscle AChR. In conclusion, (-)-reboxetine non-competitively inhibits muscle AChRs by binding to the TCP luminal site and by inducing receptor desensitization (maybe by interacting with non-luminal sites), a mechanism that is shared by tricyclic antidepressants.

GABA binding to an insect GABA receptor: a molecular dynamics and mutagenesis study

RDL receptors are GABA-activated inhibitory Cys-loop receptors found throughout the insect CNS. They are a key target for insecticides. Here, we characterize the GABA binding site in RDL receptors using computational and electrophysiological techniques. A homology model of the extracellular domain of RDL was generated and GABA docked into the binding site. Molecular dynamics simulations predicted critical GABA binding interactions with aromatic residues F206, Y254, and Y109 and hydrophilic residues E204, S176, R111, R166, S176, and T251. These residues were mutated, expressed in Xenopus oocytes, and their functions assessed using electrophysiology. The data support the binding mechanism provided by the simulations, which predict that GABA forms many interactions with binding site residues, the most significant of which are cation-π interactions with F206 and Y254, H-bonds with E204, S205, R111, S176, T251, and ionic interactions with R111 and E204. These findings clarify the roles of a range of residues in binding GABA in the RDL receptor, and also show that molecular dynamics simulations are a useful tool to identify specific interactions in Cys-loop receptors.

Computational methods for the discovery of mood disorder therapies

INTRODUCTION

Despite the significant progress, research is still needed to reveal details of the complex and dynamic chemical processes operating in the central nervous system (CNS) and their relationship to psychological effects such as mood disorders. The incidence of behavioral depression is widely spread worldwide, with an estimated 14.8 million adults diagnosed yearly in the United States alone. The efficacy of current antidepressants on 50 - 60% of patients, their slow onset of action and the prevalence of adverse side effects highlight the need for developing a new generation of improved antidepressants. Computational methods have the potential to aid in the discovery of mood modulators.

AREAS COVERED

This review contains three main sections: historical evolution of marketed antidepressants, physicochemical and structural properties of antidepressant compounds reported in the ChEMBL database and recent efforts in the design and discovery of antidepressants using computational methods. The authors provide details of the computational methods employed, from chemoinformatic analyses to molecular modeling.

EXPERT OPINION

While there have been numerous and important findings in depression research, the high cost and time spent on research into new therapies for brain disorders is a risky undertaking. Computational methodologies can be employed to speed up the discovery of new antidepressants and to detect new sources of chemical compounds with potential antidepressant activity. Compound collections containing compounds already approved in the pharmaceutical and food industries that cover the property space and complement the structural space of CNS drugs represent a promising starting point for the discovery of new antidepressant agents.

Structure-activity relationship of ibogaine analogs interacting with nicotinic acetylcholine receptors in different conformational states

The interaction of ibogaine analogs with nicotinic acetylcholine receptors (AChRs) in different conformational states was studied by functional and structural approaches. The results established that ibogaine analogs: (a) inhibit (±)-epibatidine-induced Ca²⁺ influx in human embryonic muscle AChRs with the following potency sequence (IC(50) in μM): (±)-18-methylaminocoronaridine (5.9±0.3)∼(±)-18-methoxycoronaridine (18-MC) (6.8±0.8)>(-)-ibogaine (17±3)∼(+)-catharanthine (20±1)>(±)-albifloranine (46±13), (b) bind to the [³H]TCP binding site with higher affinity when the Torpedo AChR is in the desensitized state compared to that in the resting state. Similar results were obtained using [³H]18-MC. These and docking results suggest a steric interaction between TCP and ibogaine analogs for the same site, (c) enhance [³H]cytisine binding to resting but not to desensitized AChRs, with desensitizing potencies (apparent EC₅₀) that correlate very well with the pK(i) values in the desensitized state, and (d) there are good bilinear correlations between the ligand molecular volumes and their affinities in the desensitized and resting states, with an optimal volume of ∼345 ų for the ibogaine site. These results indicate that the size of the binding sites for ibogaine analogs, located between the serine and nonpolar rings and shared with TCP, is an important structural feature for binding and for inducing desensitization.

Interaction of ibogaine with human alpha3beta4-nicotinic acetylcholine receptors in different conformational states

The interaction of ibogaine and phencyclidine (PCP) with human (h) alpha3beta4-nicotinic acetylcholine receptors (AChRs) in different conformational states was determined by functional and structural approaches including, radioligand binding assays, Ca2+ influx detections, and thermodynamic and kinetics measurements. The results established that (a) ibogaine inhibits (+/-)-epibatidine-induced Ca2+ influx in h(alpha)3beta4 AChRs with approximately 9-fold higher potency than that for PCP, (b) [3H]ibogaine binds to a single site in the h(alpha)3beta4 AChR ion channel with relatively high affinity (Kd = 0.46 +/- 0.06 microM), and ibogaine inhibits [3H]ibogaine binding to the desensitized h(alpha)3beta4 AChR with slightly higher affinity compared to the resting AChR. This is explained by a slower dissociation rate from the desensitized ion channel compared to the resting ion channel, and (c) PCP inhibits [3H]ibogaine binding to the h(alpha)3beta4 AChR, suggesting overlapping sites. The experimental results correlate with the docking simulations suggesting that ibogaine and PCP interact with a binding domain located between the serine (position 6') and valine/phenylalanine (position 13') rings. This interaction is mediated mainly by van der Waals contacts, which is in agreement with the observed enthalpic contribution determined by non-linear chromatography. However, the calculated entropic contribution also indicates local conformational changes. Collectively our data suggest that ibogaine and PCP bind to overlapping sites located between the serine and valine/phenylalanine rings, to finally block the AChR ion channel, and in the case of ibogaine, to probably maintain the AChR in the desensitized state for longer time.

Interaction of selective serotonin reuptake inhibitors with neuronal nicotinic acetylcholine receptors

We compared the interaction of fluoxetine and paroxetine, two selective serotonin reuptake inhibitors (SSRIs), with the human (h) alpha4beta2, alpha3beta4, and alpha7 nicotinic acetylcholine receptors (AChRs) in different conformational states, using Ca(2+) influx, radioligand binding, and molecular docking approaches. The results established that (1) fluoxetine was more potent than paroxetine in inhibiting agonist-activated Ca(2+) influx on halpha4beta2 and halpha7 AChRs, whereas the potency of both SSRIs was practically the same in the halpha3beta4 AChR. [corrected] (2) SSRIs bind to the [(3)H]imipramine locus with a [corrected] higher affinity when the AChRs are in the desensitized states compared to the resting states. (3) The different receptor specificity for fluoxetine determined by their inhibitory potencies or binding affinities suggests different modes of interaction when the AChR is in the closed or activated state. (4) Neutral and protonated fluoxetine interacts with a binding domain located in the middle of the AChR ion channel. In conclusion, SSRIs inhibit the most important neuronal AChRs with potencies and affinities that are clinically relevant by binding to a luminal site that is shared with tricyclic antidepressants.

Interaction of 18-methoxycoronaridine with nicotinic acetylcholine receptors in different conformational states

The interaction of 18-methoxycoronaridine (18-MC) with nicotinic acetylcholine receptors (AChRs) was compared with that for ibogaine and phencyclidine (PCP). The results established that 18-MC: (a) is more potent than ibogaine and PCP inhibiting (+/-)-epibatidine-induced AChR Ca(2+) influx. The potency of 18-MC is increased after longer pre-incubation periods, which is in agreement with the enhancement of [(3)H]cytisine binding to resting but activatable Torpedo AChRs, (b) binds to a single site in the Torpedo AChR with high affinity and inhibits [(3)H]TCP binding to desensitized AChRs in a steric fashion, suggesting the existence of overlapping sites. This is supported by our docking results indicating that 18-MC interacts with a domain located between the serine (position 6') and valine (position 13') rings, and (c) inhibits [(3)H]TCP, [(3)H]ibogaine, and [(3)H]18-MC binding to desensitized AChRs with higher affinity compared to resting AChRs. This can be partially attributed to a slower dissociation rate from the desensitized AChR compared to that from the resting AChR. The enthalpic contribution is more important than the entropic contribution when 18-MC binds to the desensitized AChR compared to that for the resting AChR, and vice versa. Ibogaine analogs inhibit the AChR by interacting with a luminal domain that is shared with PCP, and by inducing desensitization.

Tricyclic antidepressants and mecamylamine bind to different sites in the human alpha4beta2 nicotinic receptor ion channel

The interaction of tricyclic antidepressants with the human (h) alpha4beta2 nicotinic acetylcholine receptor in different conformational states was compared with that for the noncompetitive antagonist mecamylamine by using functional and structural approaches. The results established that: (a) [(3)H]imipramine binds to halpha4beta2 receptors with relatively high affinity (K(d)=0.83+/-0.08 microM), but imipramine does not differentiate between the desensitized and resting states, (b) although tricyclic antidepressants inhibit (+/-)-epibatidine-induced Ca(2+) influx in HEK293-halpha4beta2 cells with potencies that are in the same concentration range as that for (+/-)-mecamylamine, tricyclic antidepressants inhibit [(3)H]imipramine binding to halpha4beta2 receptors with affinities >100-fold higher than that for (+/-)-mecamylamine. This can be explained by our docking results where imipramine interacts with the leucine (position 9') and valine (position 13') rings by van der Waals contacts, whereas mecamylamine interacts electrostatically with the outer ring (position 20'), (c) van der Waals interactions are in agreement with the thermodynamic results, indicating that imipramine interacts with the desensitized and resting receptors by a combination of enthalpic and entropic components. However, the entropic component is more important in the desensitized state, suggesting local conformational changes. In conclusion, our data indicate that tricyclic antidepressants and mecamylamine efficiently inhibit the ion channel by interacting at different luminal sites. The high proportion of protonated mecamylamine calculated at physiological pH suggests that this drug can be attracted to the channel mouth before binding deeper within the receptor ion channel finally blocking ion flux.

Different interaction between tricyclic antidepressants and mecamylamine with the human alpha3beta4 nicotinic acetylcholine receptor ion channel

The interaction of tricyclic antidepressants (TCAs) with the human (h)alpha3beta4 nicotinic acetylcholine receptor (AChR) in different conformational states was compared with that for mecamylamine by using functional and structural approaches including, Ca(2+) influx, radioligand binding, and molecular docking. The results established that: (a) [(3)H]imipramine binds to a single site with relatively high affinity (K(d) = 0.41 +/- 0.04 microM), (b) imipramine inhibits [(3)H]imipramine binding to the resting/kappa-bungarotoxin-bound AChR (K(i) = 0.68 +/- 0.08 microM) with practically the same affinity as to the desensitized/epibatidine-bound AChR (K(i) = 0.83 +/- 0.08 microM), suggesting that TCAs do not discriminate between these conformational states, and (c) although TCAs (IC(50) approximately 1.8-2.7 microM) and mecamylamine (IC(50) = 3.3 +/- 0.4 microM) inhibit (+/-)-epibatidine-induced Ca(2+) influx with potencies in the same concentration range, TCAs (K(i) approximately 1-3.6 microM), but not mecamylamine (apparent IC(50) approximately 0.2 mM), inhibit [(3)H]imipramine binding to halpha3beta4 AChRs in different conformational states. This is explained by our docking results where imipramine, in the neutral and protonated states, interacts with the leucine (position 9') and valine/phenylalanine (position 13') rings, whereas protonated mecamylamine (>99% at physiological pH) interacts with the outer ring (position 20'). Our data indicate that TCAs bind to overlapping sites located between the serine and valine/phenylalanine rings in the halpha3beta4 AChR ion channel, whereas protonated mecamylamine can be attracted to the channel mouth before blocking ion flux by interacting with a luminal site in its neutral state.

Interaction of lipids and ligands with nicotinic acetylcholine receptor vesicles assessed by electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy is a powerful technique that permits the study of membrane-embedded proteins in its lipid environment by assessing the interaction of spin labels with the protein in its natural environment (i.e., native membranes) or in reconstituted systems prepared with exogenous lipid species. Nicotinic acetylcholine receptors (AChRs) contain a large surface in intimate contact with the lipid membrane. AChRs, members of the Cys-loop receptor superfamily, have essential functional roles in the nervous system and its malfunctioning has been considered as the origin of several neurological diseases including Alzheimer's disease, drug addiction, depression, and schizophrenia. In this regard, these receptors have been extensively studied as therapeutic targets for the action of several drugs. The majority of the marketed medications bind to the neurotransmitter sites, the so-called agonists. However, several drugs, some of them still in clinical trials, interact with non-competitive antagonist (NCA) binding sites. A potential location for these binding sites is the proper ion channel, blocking ion flux and thus, inhibiting membrane depolarization. However, several NCAs also bind to the lipid-protein interface, modulating the AChR functional properties. The best known examples of these NCAs are local and general anesthetics. Several endogenous molecules such as free fatty acids and neurosteroids also bind to the lipid-protein interface, probably mediating important physiological functions. Phospholipids, natural components of lipid membranes interacting with the AChR, are also essential to maintain the structural and functional properties of the AChR. EPR studies showed that local anesthetics bind to the lipid-protein interface by essentially the same dynamic mechanisms found in lipids, and that local and general anesthetics preferably decrease the phospholipid but not the fatty acid interactions with the AChR. This is consistent with the existence of annular and non-annular lipid domains on the AChR.

Interaction of bupropion with muscle-type nicotinic acetylcholine receptors in different conformational states

To characterize the binding sites and the mechanisms of inhibition of bupropion on muscle-type nicotinic acetylcholine receptors (AChRs), structural and functional approaches were used. The results established that bupropion (a) inhibits epibatidine-induced Ca(2+) influx in embryonic muscle AChRs, (b) inhibits adult muscle AChR macroscopic currents in the resting/activatable state with approximately 100-fold higher potency compared to that in the open state, (c) increases the desensitization rate of adult muscle AChRs from the open state and impairs channel opening from the resting state, (d) inhibits binding of [(3)H]TCP and [(3)H]imipramine to the desensitized/carbamylcholine-bound Torpedo AChR with higher affinity compared to the resting/alpha-bungarotoxin-bound AChR, (e) binds to the Torpedo AChR in either state mainly by an entropy-driven process, and (f) interacts with a binding domain located between the serine (position 6') and valine (position 13') rings, by a network of van der Waals, hydrogen bond, and polar interactions. Collectively, our data indicate that bupropion first binds to the resting AChR, decreasing the probability of ion channel opening. The remnant fraction of open ion channels is subsequently decreased by accelerating the desensitization process. Bupropion interacts with a luminal binding domain shared with PCP that is located between the serine and valine rings, and this interaction is mediated mainly by an entropy-driven process.

Is the inhibition of nicotinic acetylcholine receptors by bupropion involved in its clinical actions?

In this mini review we will focus on those molecular and cellular mechanisms exerted by bupropion (BP), ultimately leading to the antidepressant and anti-nicotinic properties described for this molecule. The main pharmacological mechanism is based on the fact that BP induces the release as well as inhibits the reuptake of neurotransmitters such as a dopamine (DA) and norepinephrine (NE). Additional mechanisms of action have been also determined. For example, BP is a noncompetitive antagonist (NCA) of several nicotinic acetylcholine receptors (AChRs). Based on this evidence, the dual antidepressant and anti-nicotinic activity of BP is currently considered to be mediated by its stimulatory action on the DA and NE systems as well as its inhibitory action on AChRs. Considering the results obtained in the archetypical mouse muscle AChR, a sequential mechanism can be hypothesized to explain the inhibitory action of BP on neuronal AChRs: (1) BP first binds to AChRs in the resting state, decreasing the probability of ion channel opening, (2) the remnant fraction of open ion channels is subsequently decreased by accelerating the desensitization process, and (3), BP interacts with a binding domain located between the serine (position 6') and valine (position 13') rings that is shared with the NCA phencyclidine and other tricyclic antidepressants. This new evidence paves the way for further investigations using AChRs as targets for the action of safer antidepressants and novel anti-addictive compounds.

Pharmacological and neurotoxicological actions mediated by bupropion and diethylpropion

The antiappetite agent diethylpropion (DEP), and the antidepressant and antismoking aid compound bupropion (BP), not only share the same structural motif but also present similar mechanisms of action in the CNS. For example, both drugs induce the release as well as inhibit the reuptake of neurotransmitters such as a dopamine (DA) and norepinephrine (NE). In general, they produce mild side effects, including reversible psychomotor alterations mostly in geriatric patients (by BP), or moderate changes in neurotransmitter contents linked to oxidative damage (by DEP). Therefore, attention must be paid during any therapeutic use of these agents. Regarding the interaction of BP with the DA transporter, residues S359, located in the middle of TM7, and A279, located close to the extracellular end of TM5, contribute to the binding and blockade of translocation mediated by BP, respectively. Additional mechanisms of action have also been determined for each compound. For example, BP is a noncompetitive antagonist (NCA) of several nicotinic acetylcholine receptors (AChRs). Based on this evidence, the dual antidepressant and antinicotinic activity of BP is currently considered to be mediated by its stimulatory action on DA and NE systems as well as its inhibitory action on AChRs. Considering the results obtained in the archetypical mouse muscle AChR, a sequential mechanism can be hypothesized to explain the inhibitory action of BP on neuronal AChRs: (1) BP first binds to AChRs in the resting state, decreasing the probability of ion channel opening, (2) the remnant fraction of open ion channels is subsequently decreased by accelerating the desensitization process, and finally (3) BP interacts with a binding domain located between the serine (position 9') and valine (position 13') rings that is shared with the NCA phencyclidine and other tricyclic antidepressants. The homologous location in the alpha3beta4 AChR is between the serine and valine/phenylalanine rings. This new evidence opens a window for further investigation using AChRs as targets for the action of safer antidepressants and novel antiaddictive compounds.