Positive and negative modulation of nicotinic receptors

Allosteric Activation of α7 Nicotinic Acetylcholine Receptors by Novel 2-Arylamino-thiazole-5-carboxylic Acid Amide Derivatives for the Improvement of Cognitive Deficits in Mice

Enhancing α7 nAChR function serves as a therapeutic strategy for cognitive disorders. Here, we report the synthesis and evaluation of 2-arylamino-thiazole-5-carboxylic acid amide derivatives 6-9 that as positive allosteric modulators (PAMs) activate human α7 nAChR current expressed in Xenopus ooctyes. Among the 4-amino derivatives, a representative atypical type I PAM 6p exhibits potent activation of α7 current with an EC50 of 1.3 μM and the maximum activation effect on the current over 48-fold in the presence of acetylcholine (100 μM). The structure-activity relationship (SAR) analysis reveals that the 4-amino group is crucial for the allosteric activation of α7 currents by compound 6p as the substitution of 4-methyl group results in its conversion to compound 7b (EC50 = 2.1 μM; max effect: 58-fold) characterized as a typical type I PAM. Furthermore, both 6p and 7b are able to rescue auditory gating deficits in mouse schizophrenia-like model of acoustic startle prepulse inhibition.

Screening for new ligands of the MB327-PAM-1 binding site of the nicotinic acetylcholine receptor

Intoxications with organophosphorus compounds (OPCs) effect a severe impairment of cholinergic neurotransmission that, as a result of overstimulation may lead to desensitization of nicotinic acetylcholine receptors (nAChRs) and finally to death due to respiratory paralysis. So far, therapeutics, that are capable to address and revert desensitized neuromuscular nAChRs into their resting, i.e. functional state are still missing. Still, among a class of compounds termed bispyridinium salts, which are characterized by the presence of two pyridinium subunits, constituents have been identified, that can counteract organophosphate poisoning by resensitizing desensitized nAChRs. According to comprehensive modeling studies this effect is mediated by an allosteric binding site at the nAChR termed MB327-PAM-1 site. For MB327, the most prominent representative of the bispyridinium salts and all other analogues studied so far, the affinity for the aforementioned binding site and the intrinsic activity measured in ex vivo and in in vivo experiments are distinctly too low, to meet the criteria to be fulfilled for therapeutic use. Hence, in order to identify new compounds with higher affinities for the MB327-PAM-1 binding site, as a basic requirement for an enhanced potency, two compound libraries, the ChemDiv library with 60 constituents and the Tocriscreen Plus library with 1280 members have been screened for hit compounds addressing the MB327-PAM-1 binding site, utilizing the [2H6]MB327 MS Binding Assay recently developed by us. This led to the identification of a set of 10 chemically diverse compounds, all of which exhibit an IC50 value of ≤ 10 µM (in the [2H6]MB327 MS Binding Assay), which had been defined as selection criteria. The three most affine ligands, which besides a quinazoline scaffold share similarities with regard to the substitution pattern and the nature of the substituents, are UNC0638, UNC0642 and UNC0646. With binding affinities expressed as pKi values of 6.01 ± 0.10, 5.97 ± 0.05 and 6.23 ± 0.02, respectively, these compounds exceed the binding affinity of MB327 by more than one log unit. This renders them promising starting points for the development of drugs for the treatment of organophosphorus poisoning by addressing the MB327-PAM-1 binding site of the nAChR.

The Antinociceptive Activity of (E)-3-(thiophen-2-yl)- N -(p-tolyl)acrylamide in Mice Is Reduced by (E)-3-(furan-2-yl)- N -methyl- N -(p-tolyl)acrylamide Through Opposing Modulatory Mechanisms at the α7 Nicotinic Acetylcholine Receptor

BACKGROUND

The primary objective of this study was to characterize the pharmacological and behavioral activity of 2 novel compounds, DM497 [(E)-3-(thiophen-2-yl)- N -(p-tolyl)acrylamide] and DM490 [(E)-3-(furan-2-yl)- N -methyl- N -(p-tolyl)acrylamide], structural derivatives of PAM-2, a positive allosteric modulator of the α7 nicotinic acetylcholine receptor (nAChR).

METHODS

A mouse model of oxaliplatin-induced neuropathic pain (2.4 mg/kg, 10 injections) was used to test the pain-relieving properties of DM497 and DM490. To assess possible mechanisms of action, the activity of these compounds was determined at heterologously expressed α7 and α9α10 nAChRs, and voltage-gated N-type calcium channel (Ca V 2.2) using electrophysiological techniques.

RESULTS

Cold plate tests indicated that 10 mg/kg DM497 was able to decrease neuropathic pain in mice induced by the chemotherapeutic agent oxaliplatin. In contrast, DM490 induced neither pro- nor antinociceptive activity but inhibited DM497's effect at equivalent dose (30 mg/kg). These effects are not a product of changes in motor coordination or locomotor activity. At α7 nAChRs, DM497 potentiated whereas DM490 inhibited its activity. In addition, DM490 antagonized the α9α10 nAChR with >8-fold higher potency than that for DM497. In contrast, DM497 and DM490 had minimal inhibitory activity at the Ca V 2.2 channel. Considering that DM497 did not increase the mouse exploratory activity, an indirect anxiolytic mechanism was not responsible for the observed antineuropathic effect.

CONCLUSIONS

The antinociceptive activity of DM497 and the concomitant inhibitory effect of DM490 are mediated by opposing modulatory mechanisms on the α7 nAChR, whereas the involvement of other possible nociception targets such as the α9α10 nAChR and Ca V 2.2 channel can be ruled out.

Contribution of the α5 nAChR Subunit and α5SNP to Nicotine-Induced Proliferation and Migration of Human Cancer Cells

Nicotine in tobacco is known to induce tumor-promoting effects and cause chemotherapy resistance through the activation of nicotinic acetylcholine receptors (nAChRs). Many studies have associated the α5 nicotinic receptor subunit (α5), and a specific polymorphism in this subunit, with (i) nicotine administration, (ii) nicotine dependence, and (iii) lung cancer. The α5 gene CHRNA5 mRNA is upregulated in several types of cancer, including lung, prostate, colorectal, and stomach cancer, and cancer severity is correlated with smoking. In this study, we investigate the contribution of α5 in the nicotine-induced cancer hallmark functions proliferation and migration, in breast, colon, and prostate cancer cells. Nine human cell lines from different origins were used to determine nAChR subunit expression levels. Then, selected breast (MCF7), colon (SW480), and prostate (DU145) cancer cell lines were used to investigate the nicotine-induced effects mediated by α5. Using pharmacological and siRNA-based experiments, we show that α5 is essential for nicotine-induced proliferation and migration. Additionally, upon downregulation of α5, nicotine-promoted expression of EMT markers and immune regulatory proteins was impaired. Moreover, the α5 polymorphism D398N (α5SNP) caused a basal increase in proliferation and migration in the DU145 cell line, and the effect was mediated through G-protein signaling. Taken together, our results indicate that nicotine-induced cancer cell proliferation and migration are mediated via α5, adding to the characterization of α5 as a putative therapeutical target.

Signaling of nicotinic acetylcholine receptors in mononuclear phagocytes

Nicotinic acetylcholine receptors are not only expressed by the nervous system and at the neuro-muscular junction but also by mononuclear phagocytes, which belong to the innate immune system. Mononuclear phagocyte is an umbrella term for monocytes, macrophages, and dendritic cells. These cells play pivotal roles in host defense against infection but also in numerous often debilitating diseases that are characterized by exuberant inflammation. Nicotinic acetylcholine receptors of the neuronal type dominate in these cells, and their stimulation is mainly associated with anti-inflammatory effects. Although the cholinergic modulation of mononuclear phagocytes is of eminent clinical relevance for the prevention and treatment of inflammatory diseases and neuropathic pain, we are only beginning to understand the underlying mechanisms on the molecular level. The purpose of this review is to report and critically discuss the current knowledge on signal transduction mechanisms elicited by nicotinic acetylcholine receptors in mononuclear phagocytes.

Recent Advances in the Discovery of Nicotinic Acetylcholine Receptor Allosteric Modulators

Positive allosteric modulators (PAMs), negative allosteric modulators (NAMs), silent agonists, allosteric activating PAMs and neutral or silent allosteric modulators are compounds capable of modulating the nicotinic receptor by interacting at allosteric modulatory sites distinct from the orthosteric sites. This survey is focused on the compounds that have been shown or have been designed to interact with nicotinic receptors as allosteric modulators of different subtypes, mainly α7 and α4β2. Minimal chemical changes can cause a different pharmacological profile, which can then lead to the design of selective modulators. Experimental evidence supports the use of allosteric modulators as therapeutic tools for neurological and non-neurological conditions.

α1 Proline 277 Residues Regulate GABAAR Gating through M2-M3 Loop Interaction in the Interface Region

Cys-loop receptors are a superfamily of transmembrane, pentameric receptors that play a crucial role in mammalian CNS signaling. Physiological activation of these receptors is typically initiated by neurotransmitter binding to the orthosteric binding site, located at the extracellular domain (ECD), which leads to the opening of the channel pore (gate) at the transmembrane domain (TMD). Whereas considerable knowledge on molecular mechanisms of Cys-loop receptor activation was gathered for the acetylcholine receptor, little is known with this respect about the GABA_A receptor (GABA_AR), which mediates cellular inhibition. Importantly, several static structures of GABA_AR were recently described, paving the way to more in-depth molecular functional studies. Moreover, it has been pointed out that the TMD-ECD interface region plays a crucial role in transduction of conformational changes from the ligand binding site to the channel gate. One of the interface structures implicated in this transduction process is the M2-M3 loop with a highly conserved proline (P277) residue. To address this issue specifically for α₁β₂γ_2L GABA_AR, we choose to substitute proline α₁P277 with amino acids with different physicochemical features such as electrostatic charge or their ability to change the loop flexibility. To address the functional impact of these mutations, we performed macroscopic and single-channel patch-clamp analyses together with modeling. Our findings revealed that mutation of α₁P277 weakly affected agonist binding but was critical for all transitions of GABA_AR gating: opening/closing, preactivation, and desensitization. In conclusion, we provide evidence that conservative α₁P277 at the interface is strongly involved in regulating the receptor gating.

Computational analysis of non-competitive antagonist arylguanidine-α7 nAChR complexes

meta-Chlorophenylguanidine (1) is a non-competitive α7 nicotinic acetylcholine receptor (nAChR) antagonist. Here we examined the hydrogen bond donor role of the anilinic N₁-H on the inhibitory effect of 1 by preparing its N₁-CH₃ counterpart 2. Analog 2 was found to be at least as potent as 1 as a non-competitive α7 nAChR antagonist in a patch-clamp assay. To establish a structural basis for the mode of interaction of guanidines 1 and 2, we generated 100 homology models of the hα7 nAChR. This was followed by Connolly surface (SYBYL-X2.1) and blind docking (AutoDock 4.1) studies to identify eight possible binding pockets, two of which were supported by empirical data and employed in our docking studies. The optimized model-ligand complexes were analyzed using a Hydropathic INTeractions (HINT) analysis in order to compare and contrast different binding pockets and modes. We identified a potential allosteric binding site and distinct rotameric binding modes for 1 and 2 at α7 nAChRs. These differences in the binding orientations minimized the importance of an anilinic NH function for the antagonist activity at nACh receptors.

(E)-3-Furan-2-yl-N-p-tolyl-acrylamide and its Derivative DM489 Decrease Neuropathic Pain in Mice Predominantly by α7 Nicotinic Acetylcholine Receptor Potentiation

The main objective of this study was to determine whether (E)-3-furan-2-yl-N-p-tolyl-acrylamide (PAM-2) and its structural derivative DM489 produce anti-neuropathic pain activity using the streptozotocin (STZ)- and oxaliplatin-induced neuropathic pain animal models. To assess possible mechanisms of action, the pharmacological activity of these compounds was determined at α7 and α9α10 nicotinic acetylcholine receptors (nAChRs) and Ca_V2.2 channels expressed alone or coexpressed with G protein-coupled GABA_B receptors. The animal results indicated that a single dose of 3 mg/kg PAM-2 or DM489 decreases STZ-induced neuropathic pain in mice, and chemotherapy-induced neuropathic pain is decreased by PAM-2 (3 mg/kg) and DM489 (10 mg/kg). The observed anti-neuropathic pain activity was inhibited by the α7-selective antagonist methyllycaconitine. The coadministration of oxaliplatin with an inactive dose (1 mg/kg) of PAM-2 decreased the development of neuropathic pain after 14, but not 7, days of cotreatment. The electrophysiological results indicated that PAM-2 potentiates human (h) and rat (r) α7 nAChRs with 2-7 times higher potency than that for hCa_V2.2 channel inhibition and an even greater difference compared to that for rα9α10 nAChR inhibition. These results support the notion that α7 nAChR potentiation is likely the predominant molecular mechanism underlying the observed anti-nociceptive pain activity of these compounds.

3-Furan-2-yl-N-p-tolyl-acrylamide, a highly selective positive allosteric modulator of α7 nicotinic receptors, produces anxiolytic-like activity in mice

BACKGROUND

Several lines of investigations support the idea that nicotinic acetylcholine receptors modulate neuronal pathways involved in anxiety and depression.

AIMS

The purpose of this study was to determine whether 3-furan-2-yl-N-p-tolyl-acrylamide, a highly selective positive allosteric modulator of α7 nicotinic acetylcholine receptors, influences anxiety-like behaviour in mice, and to determine the modulatory activity of 3-furan-2-yl-N-p-tolyl-acrylamide on mice pretreated with either nicotine or selective α7-agonists (i.e. PNU-282987 or (2.4)-dimethoxybenzylidene anabaseine dihydrochloride).

METHODS

The elevated plus maze and novelty suppressed feeding tests were selected to evaluate 3-furan-2-yl-N-p-tolyl-acrylamide and other nicotinic ligands on anxiety-like behaviour in mice.

RESULTS

The results indicated that: (a) 3-furan-2-yl-N-p-tolyl-acrylamide induces anxiolytic-like activity at 0.5 (elevated plus maze) and 1.0 (novelty suppressed feeding) mg/kg, respectively, after acute treatment, whereas its efficacy is increased after chronic treatments (i.e. active at 0.1 mg/kg; elevated plus maze). This is the first time showing anxiolytic-like activity elicited by 3-furan-2-yl-N-p-tolyl-acrylamide, contrary to the lack of activity for PNU-120596 (0.1 mg/kg); (b) the anxiolytic-like activity of 0.5 mg/kg 3-furan-2-yl-N-p-tolyl-acrylamide is inhibited by methyllycaconitine, a selective α7-antagonist, suggesting that α7 nicotinic acetylcholine receptors are involved in this process; (c) 0.5 mg/kg 3-furan-2-yl-N-p-tolyl-acrylamide reverses the anxiogenic effects induced by 0.1 mg/kg nicotine but not by 10.0 mg/kg PNU-282987; and (d) inactive doses of both 3-furan-2-yl-N-p-tolyl-acrylamide (0.1 mg/kg) and (2.4)-dimethoxybenzylidene anabaseine dihydrochloride (1.0 mg/kg) produce anxiolytic-like effects, suggesting drug interactions, probably synergistic.

CONCLUSIONS

Our findings indicated that anxiolytic-like activity is mediated by α7 nicotinic acetylcholine receptors, supporting the concept that these receptors modulate anxiety processes. The results indicating that the chronic treatment with 3-furan-2-yl-N-p-tolyl-acrylamide is more efficient than the acute treatment in eliciting anxiolytic-like activity, and that 3-furan-2-yl-N-p-tolyl-acrylamide reverses the anxiogenic effects induced by nicotine, might be of therapeutic importance during smoking cessation.

Allosteric Modulation of Ionotropic Glutamate Receptors: An Outlook on New Therapeutic Approaches To Treat Central Nervous System Disorders

The allosteric targeting of ionotropic glutamate receptors (iGluRs) is a valuable approach for treating various central nervous system (CNS) disorders. In this frame, this Innovations provides a summary of the state-of-the art in the development of allosteric modulators for iGluRs and offers an outlook regarding innovative strategies for treating neurological diseases.

Molecular function of the novel α7β2 nicotinic receptor

The α7 nicotinic receptor is a promising drug target for neurological and inflammatory disorders. Although it is the homomeric member of the family, a novel α7β2 heteromeric receptor has been discovered. To decipher the functional contribution of the β2 subunit, we generated heteromeric receptors with fixed stoichiometry by two different approaches comprising concatenated and unlinked subunits. Receptors containing up to three β2 subunits are functional. As the number of β2 subunits increases in the pentameric arrangement, the durations of channel openings and activation episodes increase progressively probably due to decreased desensitization. The prolonged activation episodes conform the kinetic signature of α7β2 and may have an impact on neuronal excitability. For activation of α7β2 receptors, an α7/α7 binding-site interface is required, thus indicating that the three β2 subunits are located consecutively in the pentameric arrangement. α7-positive allosteric modulators (PAMs) are emerging as novel therapeutic drugs. The presence of β2 in the pentamer affects neither type II PAM potentiation nor activation by an allosteric agonist whereas it impairs type I PAM potentiation. This first single-channel study provides fundamental basis required to decipher the role and function of the novel α7β2 receptor and opens doors to develop selective therapeutic drugs.

Molecular interactions of type I and type II positive allosteric modulators with the human α7 nicotinic acetylcholine receptor: an in silico study

The binding site locations and structural components for type I and type II positive allosteric modulators (PAMs) of the α7 nicotinic acetylcholine receptor (nAChR) have not been fully characterized yet. In this regard, homology models of the human α7 nAChR and hα7/m5-HT_3A chimera, built using the crystal structure of the serotonin type 3A receptor (5-ΗΤ_3ΑR), were used for molecular docking and molecular dynamics simulations to study the molecular interactions of selected type I (5-hydroxyindol, NS-1738, and LY-2087101) and type II (PNU-120596, PAM-2, and TBS-516) PAMs. The docking results indicate: (1) a site located in the extracellular domain (ECD) for type I PAMs such as NS-1738 and LY-2087101, but not for 5-HI; (2) an overlapping site in the ECD-transmembrane domain (TMD) junction for all studied PAMs. Additional docking results on the hα7/m5-HT_3A chimera supported experimental results indicating that the ECD site might be relevant for type I PAM activity; and (3) two TMD sites, an intrasubunit site that recognizes type II PAMs, and an intersubunit pocket with high specificity for 5-HI (type I PAM). The in silico α7TSLMF mutant results support the view that M1-Ser223 and M3-Ile281 are key residues for the interaction of PAM-2 and PNU-120596 with the intrasubunit cavity. Our in silico results are in agreement with experimental data showing that the intrasubunit cavity is relevant for the activity of type II PAMs, and suggest that the ECD-TMD junction and intersubunit sites could be significant for the activity of type I PAMs.

Designing selective modulators for the nicotinic receptor subtypes: challenges and opportunities

Nicotinic receptors are membrane proteins involved in several physiological processes. They are considered suitable drug targets for various CNS disorders or conditions, as shown by the large number of compounds which have entered clinical trials. In recent years, nonconventional agonists have been discovered: positive allosteric modulators, allosteric agonists, site-specific agonists and silent desensitizers are compounds able to modulate the receptor interacting at sites different from the orthodox one, or to desensitize the receptor without prior opening. While these new findings can further complicate the pharmacology of these proteins and the design and optimization of ligands, they undoubtedly offer new opportunities to find drugs for the many therapeutic indications involving nicotinic receptors.

Development of MS Binding Assays targeting the binding site of MB327 at the nicotinic acetylcholine receptor

The bispyridinium compound MB327 has been shown previously to have a positive pharmacological effect against poisoning with organophosphorous compounds (OPCs). The mechanism by which it exerts its therapeutic effect seems to be directly mediated by the nicotinic acetylcholine receptor (nAChR). In the present study, the development of mass spectrometry based binding assays (MS Binding Assays) for characterization of the binding site of MB327 at the nAChR from Torpedo californica is described. MS Binding Assays follow the principle of radioligand binding assays, but do not, in contrast to the latter, require a radiolabeled reporter ligand, as the readout is in this case based on mass spectrometric detection. For [²H₆]MB327, a deuterated MB327 analogue employed as reporter ligand in the MS Binding Assays, an LC-ESI-MS/MS method was established allowing for its fast and reliable quantification in samples resulting from binding experiments. Using centrifugation for separation of non-bound [²H₆]MB327 from target-bound [²H₆]MB327 in saturation and autocompetition experiments (employing native MB327 as competitor) enabled reliable determination of specific binding. In this way, the affinities for [²H₆]MB327 (K_d=15.5±0.9μmolL^-1) and for MB327 (K_i=18.3±2.6μmolL^-1) towards the nAChR could be determined for the first time. The almost exactly matching affinities for MB327 and [²H₆]MB327 obtained in the MS Binding Assays are in agreement with potencies previously found in functional studies. In summary, our results demonstrate that the established MS Binding Assays represent a promising tool for affinity determination of test compounds towards the binding site of MB327 at the nAChR.

Understanding structure-function relationships of the human neuronal acetylcholine receptor: insights from the first crystal structures of neuronal subunits

Nicotinic ACh receptors (nAChRs) are the best studied members of the superfamily of pentameric ligand-gated ion channels (pLGICs). Neuronal nAChRs regulate neuronal excitability and neurotransmitter release in the nervous system and form either homo- or hetero-pentameric complexes with various combinations of the 11 neuronal nAChR subunits (α2-7, α9, α10 and β2-4) known to exist in humans. In addition to their wide distribution in the nervous system, neuronal nAChRs have been also found in immune cells and many peripheral tissues. These nAChRs are important drug targets for neurological and neuropsychiatric diseases (e.g. Alzheimer's, schizophrenia) and substance addiction (e.g. nicotine), as well as in a variety of diseases such as chronic pain, auditory disorders and some cancers. To decipher the functional mechanisms of human nAChRs and develop efficient and specific therapeutic drugs, elucidation of their high-resolution structures is needed. Recent studies, including the X-ray crystal structures of the near-intact α4β2 nAChR and of the ligand-binding domains of the α9 and α2 subunits, have advanced our knowledge on the detailed structure of the ligand-binding sites formed between the same and different subunits and revealed many other functionally important interactions. The aim of this review is to highlight some of the structural and functional findings of these studies and to compare them with recent breakthrough findings on other pLGIC members and earlier data from their homologous ACh-binding proteins.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc.

IPPA08 allosterically enhances the action of imidacloprid on nicotinic acetylcholine receptors

Our previous study showed that IPPA08, a cis-configuration neonicotinoid compound with unique oxabridged substructure, acted as a specific synergist to neonicotinoid insecticides targeting nicotinic acetylcholine receptors (nAChRs). Heteropentamer nAChRs have diverse characteristics and can form canonical and noncanonical subunit interfaces. While canonical interfaces have been exploited as targets of many drugs, noncanonical interfaces have received less attention. In this study, the mechanism of IPPA08 synergism was evaluated on hybrid nAChRs consisting of three α1 subunits from the brown planthopper and two rat β1 subunits (Nlα1/rβ2) expressed in Xenopus oocytes. IPPA08 alone evoked inward currents, but only at very high concentrations, greater than 1 mM. However, at concentrations below 200 μM, IPPA08 slowed the decay of inward currents evoked by imidacloprid, but not by acetylcholine, and also increased the sensitivity of Nlα1/rβ2 to imidacloprid. Both modulations by IPPA08 were concentration-dependent in the same concentration range of 10-150 μM. Experimentally induced mutations in canonical (α+/β-) and noncanonical (β+/α-) interfaces of Nlα1/rβ2 receptors were also examined to evaluate the presence of possible binding sites for IPPA08 on the receptors. Our results showed that mutations in the canonical interfaces affected only the potency of IPPA08 as an agonist, while mutations in the noncanonical interfaces affected only the synergistic action of IPPA08. Based on these results, we propose that at low concentrations IPPA08 can act as a positive allosteric modulator of noncanonical interfaces, and likely slow the decay of currents through stabilizing the open-channel state caused by the action of imidacloprid on canonical interfaces.

Phytomedicine-Modulating oxidative stress and the tumor microenvironment for cancer therapy

In spite of the current advances and achievements in systems biology and translational medicinal research, the current strategies for cancer therapy, such as radiotherapy, targeted therapy, immunotherapy and chemotherapy remain palliative or unsatisfactory due to tumor metastasis or recurrence after surgery/therapy, drug resistance, adverse side effects, and so on. Oxidative stress (OS) plays a critical role in chronic/acute inflammation, carcinogenesis, tumor progression, and tumor invasion/metastasis which is also attributed to the dynamic and complex properties and activities in the tumor microenvironment (TME). Re-educating or reprogramming tumor-associated stromal or immune cells in the TME provides an approach for restoring immune surveillance impaired by disease in cancer patients to increase overall survival and reduce drug resistance. Herbal medicines or plant-derived natural products have historically been a major source of anti-cancer drugs. Delving into the lore of herbal medicine may uncover new leads for anti-cancer drugs. Phytomedicines have been widely documented to directly or indirectly target multiple signaling pathways and networks in cancer cells. A combination of anti-cancer drugs and polypharmacological plant-derived extracts or compounds may offer a significant advantage in sensitizing the efficacy of monotherapy and overcoming drug-induced resistance in cancer patients. This review introduces several phytochemicals and phytoextracts derived from medicinal plants or dietary vegetables that have been studied for their efficacy in preclinical cancer models. We address the underlying modes of action of induction of OS and deregulation of TME-associated stromal cells, mediators and signaling pathways, and reference the related clinical investigations that look at the single or combination use of phytochemicals and phytoextracts to sensitize anti-cancer drug effects and/or overcome drug resistance.

Positive allosteric modulators of α7 nicotinic acetylcholine receptors affect neither the function of other ligand- and voltage-gated ion channels and acetylcholinesterase, nor β-amyloid content

The activity of positive allosteric modulators (PAMs) of α7 nicotinic acetylcholine receptors (AChRs), including 3-furan-2-yl-N-p-tolyl-acrylamide (PAM-2), 3-furan-2-yl-N-o-tolylacrylamide (PAM-3), and 3-furan-2-yl-N-phenylacrylamide (PAM-4), was tested on a variety of ligand- [i.e., human (h) α7, rat (r) α9α10, hα3-containing AChRs, mouse (m) 5-HT3AR, and several glutamate receptors (GluRs)] and voltage-gated (i.e., sodium and potassium) ion channels, as well as on acetylcholinesterase (AChE) and β-amyloid (Aβ) content. The functional results indicate that PAM-2 inhibits hα3-containing AChRs (IC50=26±6μM) with higher potency than that for NR1aNR2B and NR1aNR2A, two NMDA-sensitive GluRs. PAM-2 affects neither the activity of m5-HT3ARs, GluR5/KA2 (a kainate-sensitive GluR), nor AChE, and PAM-4 does not affect agonist-activated rα9α10 AChRs. Relevant clinical concentrations of PAM-2-4 do not inhibit Nav1.2 and Kv3.1 ion channels. These PAMs slightly enhance the activity of GluR1 and GluR2, two AMPA-sensitive GluRs. PAM-2 does not change the levels of Aβ42 in an Alzheimer's disease mouse model (i.e., 5XFAD). The molecular docking and dynamics results using the hα7 model suggest that the active sites for PAM-2 include the intrasubunit (i.e., PNU-120596 locus) and intersubunit sites. These results support our previous study showing that these PAMs are selective for the α7 AChR, and clarify that the procognitive/promnesic/antidepressant activity of PAM-2 is not mediated by other targets.

Anticonvulsants Based on the α-Substituted Amide Group Pharmacophore Bind to and Inhibit Function of Neuronal Nicotinic Acetylcholine Receptors

Although the antiepileptic properties of α-substituted lactams, acetamides, and cyclic imides have been known for over 60 years, the mechanism by which they act remains unclear. I report here that these compounds bind to the nicotinic acetylcholine receptor (nAChR) and inhibit its function. Using transient kinetic measurements with functionally active, nondesensitized receptors, I have discovered that (i) α-substituted lactams and cyclic imides are noncompetitive inhibitors of heteromeric subtypes (such as α4β2 and α3β4) of neuronal nAChRs and (ii) the binding affinity of these compounds toward the nAChR correlates with their potency in preventing maximal electroshock (MES)-induced convulsions in mice. Based on the hypothesis that α-substituted amide group is the essential pharmacophore of these drugs, I found and tested a simple compound, 2-phenylbutyramide. This compound indeed inhibits nAChR and shows good anticonvulsant activity in mice. Molecular docking simulations suggest that α-substituted lactams, acetamides, and cyclic imides bind to the same sites on the extracellular domain of the receptor. These new findings indicate that inhibition of brain nAChRs may play an important role in the action of these antiepileptic drugs, a role that has not been previously recognized.

Exploring the positive allosteric modulation of human α7 nicotinic receptors from a single-channel perspective

Enhancement of α7 nicotinic receptor (nAChR) function by positive allosteric modulators (PAMs) is a promising therapeutic strategy to improve cognitive deficits. PAMs have been classified only on the basis of their macroscopic effects as type I, which only enhance agonist-induced currents, and type II, which also decrease desensitization and reactivate desensitized nAChRs. To decipher the molecular basis underlying these distinct activities, we explored the effects on single-α7 channel currents of representative members of each type and of less characterized compounds. Our results reveal that all PAMs enhance open-channel lifetime and produce episodes of successive openings, thus indicating that both types affect α7 kinetics. Different PAM types show different sensitivity to temperature, suggesting different mechanisms of potentiation. By using a mutant α7 receptor that is insensitive to the prototype type II PAM (PNU-120596), we show that some though not all type I PAMs share the structural determinants of potentiation. Overall, our study provides novel information on α7 potentiation, which is key to the ongoing development of therapeutic compounds.

Muscle-Type Nicotinic Receptor Blockade by Diethylamine, the Hydrophilic Moiety of Lidocaine

Lidocaine bears in its structure both an aromatic ring and a terminal amine, which can be protonated at physiological pH, linked by an amide group. Since lidocaine causes multiple inhibitory actions on nicotinic acetylcholine receptors (nAChRs), this work was aimed to determine the inhibitory effects of diethylamine (DEA), a small molecule resembling the hydrophilic moiety of lidocaine, on Torpedo marmorata nAChRs microtransplanted to Xenopus oocytes. Similarly to lidocaine, DEA reversibly blocked acetylcholine-elicited currents (I_ACh) in a dose-dependent manner (IC₅₀ close to 70 μM), but unlike lidocaine, DEA did not affect I_ACh desensitization. I_ACh inhibition by DEA was more pronounced at negative potentials, suggesting an open-channel blockade of nAChRs, although roughly 30% inhibition persisted at positive potentials, indicating additional binding sites outside the pore. DEA block of nAChRs in the resting state (closed channel) was confirmed by the enhanced I_ACh inhibition when pre-applying DEA before its co-application with ACh, as compared with solely DEA and ACh co-application. Virtual docking assays provide a plausible explanation to the experimental observations in terms of the involvement of different sets of drug binding sites. So, at the nAChR transmembrane (TM) domain, DEA and lidocaine shared binding sites within the channel pore, giving support to their open-channel blockade; besides, lidocaine, but not DEA, interacted with residues at cavities among the M1, M2, M3, and M4 segments of each subunit and also at intersubunit crevices. At the extracellular (EC) domain, DEA and lidocaine binding sites were broadly distributed, which aids to explain the closed channel blockade observed. Interestingly, some DEA clusters were located at the α-γ interphase of the EC domain, in a cavity near the orthosteric binding site pocket; by contrast, lidocaine contacted with all α-subunit loops conforming the ACh binding site, both in α-γ and α-δ and interphases, likely because of its larger size. Together, these results indicate that DEA mimics some, but not all, inhibitory actions of lidocaine on nAChRs and that even this small polar molecule acts by different mechanisms on this receptor. The presented results contribute to a better understanding of the structural determinants of nAChR modulation.

The Antinociceptive and Antiinflammatory Properties of 3-furan-2-yl-N-p-tolyl-acrylamide, a Positive Allosteric Modulator of α7 Nicotinic Acetylcholine Receptors in Mice

BACKGROUND

Positive allosteric modulators (PAMs) facilitate endogenous neurotransmission and/or enhance the efficacy of agonists without directly acting on the orthosteric binding sites. In this regard, selective α7 nicotinic acetylcholine receptor type II PAMs display antinociceptive activity in rodent chronic inflammatory and neuropathic pain models. This study investigates whether 3-furan-2-yl-N-p-tolyl-acrylamide (PAM-2), a new putative α7-selective type II PAM, attenuates experimental inflammatory and neuropathic pains in mice.

METHODS

We tested the activity of PAM-2 after intraperitoneal administration in 3 pain assays: the carrageenan-induced inflammatory pain, the complete Freund adjuvant-induced inflammatory pain, and the chronic constriction injury-induced neuropathic pain in mice. We also tested whether PAM-2 enhanced the effects of the selective α7 agonist choline in the mouse carrageenan test given intrathecally. Because the experience of pain has both sensory and affective dimensions, we also evaluated the effects of PAM-2 on acetic acid-induced aversion by using the conditioned place aversion test.

RESULTS

We observed that systemic administration of PAM-2 significantly reversed mechanical allodynia and thermal hyperalgesia in inflammatory and neuropathic pain models in a dose- and time-dependent manner without motor impairment. In addition, by attenuating the paw edema in inflammatory models, PAM-2 showed antiinflammatory properties. The antinociceptive effect of PAM-2 was inhibited by the selective competitive antagonist methyllycaconitine, indicating that the effect is mediated by α7 nicotinic acetylcholine receptors. Furthermore, PAM-2 enhanced the antiallodynic and antiinflammatory effects of choline, a selective α7 agonist, in the mouse carrageenan test. PAM-2 was also effective in reducing acetic acid-induced aversion in the conditioned place aversion assay.

CONCLUSIONS

These findings suggest that the administration of PAM-2, a new α7-selective type II PAM, reduces the neuropathic and inflammatory pain sensory and affective behaviors in the mouse. Thus, this drug may have therapeutic applications in the treatment and management of chronic pain.

Multiple binding sites in the nicotinic acetylcholine receptors: An opportunity for polypharmacolgy

For decades, the development of selective compounds has been the main goal for chemists and biologists involved in drug discovery. However, diverse lines of evidence indicate that polypharmacological agents, i.e. those that act simultaneously at various protein targets, might show better profiles than selective ligands, regarding both efficacy and side effects. On the other hand, the availability of the crystal structure of different receptors allows a detailed analysis of the main interactions between drugs and receptors in a specific binding site. Neuronal nicotinic acetylcholine receptors (nAChRs) constitute a large and diverse family of ligand-gated ion channels (LGICs) that, as a product of its modulation, regulate neurotransmitter release, which in turns produce a global neuromodulation of the central nervous system. nAChRs are pentameric protein complexes in such a way that expression of compatible subunits can lead to various receptor assemblies or subtypes. The agonist binding site, located at the extracellular region, exhibits different properties depending on the subunits that conform the receptor. In the last years, it has been recognized that nAChRs could also contain one or more allosteric sites which could bind non-classical nicotinic ligands including several therapeutically useful drugs. The presence of multiple binding sites in nAChRs offers an interesting possibility for the development of novel polypharmacological agents with a wide spectrum of actions.

Desformylflustrabromine (dFBr) and [3H]dFBr-Labeled Binding Sites in a Nicotinic Acetylcholine Receptor

Desformylflustrabromine (dFBr) is a positive allosteric modulator (PAM) of α4β2 and α2β2 nAChRs that, at concentrations >1 µM, also inhibits these receptors and α7 nAChRs. However, its interactions with muscle-type nAChRs have not been characterized, and the locations of its binding site(s) in any nAChR are not known. We report here that dFBr inhibits human muscle (αβεδ) and Torpedo (αβγδ) nAChR expressed in Xenopus oocytes with IC50 values of ∼ 1 μM. dFBr also inhibited the equilibrium binding of ion channel blockers to Torpedo nAChRs with higher affinity in the nAChR desensitized state ([(3)H]phencyclidine; IC50 = 4 μM) than in the resting state ([(3)H]tetracaine; IC50 = 60 μM), whereas it bound with only very low affinity to the ACh binding sites ([(3)H]ACh, IC50 = 1 mM). Upon irradiation at 312 nm, [(3)H]dFBr photoincorporated into amino acids within the Torpedo nAChR ion channel with the efficiency of photoincorporation enhanced in the presence of agonist and the agonist-enhanced photolabeling inhibitable by phencyclidine. In the presence of agonist, [(3)H]dFBr also photolabeled amino acids in the nAChR extracellular domain within binding pockets identified previously for the nonselective nAChR PAMs galantamine and physostigmine at the canonical α-γ interface containing the transmitter binding sites and at the noncanonical δ-β subunit interface. These results establish that dFBr inhibits muscle-type nAChR by binding in the ion channel and that [(3)H]dFBr is a photoaffinity probe with broad amino acid side chain reactivity.

The influence of allosteric modulators and transmembrane mutations on desensitisation and activation of α7 nicotinic acetylcholine receptors

Acetylcholine activates nicotinic acetylcholine receptors (nAChRs) by binding at an extracellular orthosteric site. Previous studies have described several positive allosteric modulators (PAMs) that are selective for homomeric α7 nAChRs. These include type I PAMs, which exert little or no effect on the rate of receptor desensitisation, and type II PAMs, which cause a dramatic loss of agonist-induced desensitisation. Here we report evidence that transmembrane mutations in α7 nAChRs have diverse effects on receptor activation and desensitisation by allosteric ligands. It has been reported previously that the L247T mutation, located toward the middle of the second transmembrane domain (at the 9′ position), confers reduced levels of desensitisation. In contrast, the M260L mutation, located higher up in the TM2 domain (at the 22′ position), does not show any difference in desensitisation compared to wild-type receptors. We have found that in receptors containing the L247T mutation, both type I PAMs and type II PAMs are converted into non-desensitising agonists. In contrast, in receptors containing the M260L mutation, this effect is seen only with type II PAMs. These findings, indicating that the M260L mutation has a selective effect on type II PAMs, have been confirmed both with previously described PAMs and also with a series of novel α7-selective PAMs. The novel PAMs examined in this study have close chemical similarity but diverse pharmacological properties. For example, they include compounds displaying effects on receptor desensitisation that are typical of classical type I and type II PAMs but, in addition, they include compounds with intermediate properties.

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A series of novel positive allosteric modulators (PAMs) is described.

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The series of PAMs display differing effects on α7 nAChR desensitisation.

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Transmembrane mutations influencing PAM activity are examined.

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Transmembrane mutations can convert PAMs into agonists.

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Identification of a mutation with differing effects on type I and type II PAMs.

Subunit interfaces contribute differently to activation and allosteric modulation of neuronal nicotinic acetylcholine receptors

Neuronal nicotinic acetylcholine receptors (nAChRs) are widely distributed in the nervous system and are implicated in many normal and pathological processes. The structural determinants of allostery in nAChRs are not well understood. One class of nAChR allosteric modulators, including the small molecule morantel (Mor), acts from a site that is structurally homologous to the canonical agonist site but exists in the β(+)/α(-) subunit interface. We hypothesized that all nAChR subunits move with respect to each other during channel activation and allosteric modulation. We therefore studied five pairs of residues predicted to span the interfaces of α3β2 receptors, one at the agonist interface and four at the modulator interface. Substituting cysteines in these positions, we used disulfide trapping to perturb receptor function. The pair α3Y168-β2D190, involving the C loop region of the β2 subunit, mediates modulation and agonist activation, because evoked currents were reduced up to 50% following oxidation (H2O2) treatment. The pair α3S125-β2Q39, below the canonical site, is also involved in channel activation, in accord with previous studies of the muscle-type receptor; however, the pair is differentially sensitive to ACh activation and Mor modulation (currents decreased 60% and 80%, respectively). The pairs α3Q37-β2A127 and α3E173-β2R46, both in the non-canonical interface, showed increased currents following oxidation, suggesting that subunit movements are not symmetrical. Together, our results from disulfide trapping and further mutation analysis indicate that subunit interface movement is important for allosteric modulation of nAChRs, but that the two types of interfaces contribute unequally to receptor activation.

Optogenetic studies of nicotinic contributions to cholinergic signaling in the central nervous system

Molecular manipulations and targeted pharmacological studies provide a compelling picture of which nicotinic receptor subtypes are where in the central nervous system (CNS) and what happens if one activates or deletes them. However, understanding the physiological contribution of nicotinic receptors to endogenous acetylcholine (ACh) signaling in the CNS has proven a more difficult problem to solve. In this review, we provide a synopsis of the literature on the use of optogenetic approaches to control the excitability of cholinergic neurons and to examine the role of CNS nicotinic ACh receptors (nAChRs). As is often the case, this relatively new technology has answered some questions and raised others. Overall, we believe that optogenetic manipulation of cholinergic excitability in combination with some rigorous pharmacology will ultimately advance our understanding of the many functions of nAChRs in the brain.

Dementia praecox redux: a systematic review of the nicotinic receptor as a target for cognitive symptoms of schizophrenia

Most individuals with schizophrenia suffer some cognitive dysfunction: such deficits are predictive of longer-term functioning; and current dopamine-blocking antipsychotics have made little impact on this domain. There is a pressing need to develop novel pharmacological agents to tackle this insidious but most disabling of problems. The acetylcholinergic system is involved in cognitive and attentional processing, and its metabotropic and nicotinic receptors are widespread throughout the brain. Deficits in acetylcholinergic functioning occur in schizophrenia, and high rates of tobacco smoking have been posited to represent a form of self-medication. The nicotinic acetylcholine receptor (nAChR) has emerged as a putative target to improve cognitive deficits in schizophrenia, and this study systematically reviewed the emerging data. Nineteen studies were identified, covering three compound classes: agonists at the α7 and α 4β2 nAChRs, and positive allosteric modulators. Overall data are underwhelming: some studies showed significant improvements in cognition but as many studies had negative findings. It remains unclear if this represents drug limitations or nascent study methodology problems. The literature is particularly hindered by variability in inclusion of smokers, generally small sample sizes, and a lack of consensus on cognitive test batteries. Future work should evaluate longer-term outcomes, and, particularly, the effects of concomitant cognitive training.

Structurally similar allosteric modulators of α7 nicotinic acetylcholine receptors exhibit five distinct pharmacological effects

Activation of nicotinic acetylcholine receptors (nAChRs) is associated with the binding of agonists such as acetylcholine to an extracellular site that is located at the interface between two adjacent receptor subunits. More recently, there has been considerable interest in compounds, such as positive and negative allosteric modulators (PAMs and NAMs), that are able to modulate nAChR function by binding to distinct allosteric sites. Here we examined a series of compounds differing only in methyl substitution of a single aromatic ring. This series of compounds includes a previously described α7-selective allosteric agonist, cis-cis-4-p-tolyl-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide (4MP-TQS), together with all other possible combinations of methyl substitution at a phenyl ring (18 additional compounds). Studies conducted with this series of compounds have revealed five distinct pharmacological effects on α7 nAChRs. These five effects can be summarized as: 1) nondesensitizing activation (allosteric agonists), 2) potentiation associated with minimal effects on receptor desensitization (type I PAMs), 3) potentiation associated with reduced desensitization (type II PAMs), 4) noncompetitive antagonism (NAMs), and 5) compounds that have no effect on orthosteric agonist responses but block allosteric modulation (silent allosteric modulators (SAMs)). Several lines of experimental evidence are consistent with all of these compounds acting at a common, transmembrane allosteric site. Notably, all of these chemically similar compounds that have been classified as nondesensitizing allosteric agonists or as nondesensitizing (type II) PAMs are cis-cis-diastereoisomers, whereas all of the NAMs, SAMs, and type I PAMs are cis-trans-diastereoisomers. Our data illustrate the remarkable pharmacological diversity of allosteric modulators acting on nAChRs.

Bilirubin modulates acetylcholine receptors in rat superior cervical ganglionic neurons in a bidirectional manner

Autonomic dysfunction as a partial contributing factor to cardiovascular instability in jaundiced patients is often associated with increased serum bilirubin levels. Whether increased serum bilirubin levels could directly inhibit sympathetic ganglion transmission by blocking neuronal nicotinic acetylcholine receptors (nAChRs) remains to be elucidated. Conventional patch-clamp recordings were used to study the effect of bilirubin on nAChRs currents from enzymatically dissociated rat superior cervical ganglia (SCG) neurons. The results showed that low concnetrations (0.5 and 2 μM) of bilirubin enhanced the peak ACh-evoked currents, while high concentrations (3 to 5.5 µM) of bilirubin suppressed the currents with an IC₅₀ of 4 ± 0.5 μM. In addition, bilirubin decreased the extent of desensitization of nAChRs in a concentration-dependent manner. This inhibitory effect of bilirubin on nAChRs channel currents was non-competitive and voltage independent. Bilirubin partly improved the inhibitory effect of forskolin on ACh-induced currents without affecting the action of H-89. These data suggest that the dual effects of enhancement and suppression of bilirubin on nAChR function may be ascribed to the action mechanism of positive allosteric modulation and direct blockade. Thus, suppression of sympathetic ganglionic transmission through postganglionic nAChRs inhibition may partially contribute to the adverse cardiovascular effects in jaundiced patients.

Pharmacological and molecular studies on the interaction of varenicline with different nicotinic acetylcholine receptor subtypes. Potential mechanism underlying partial agonism at human α4β2 and α3β4 subtypes

To determine the structural components underlying differences in affinity, potency, and selectivity of varenicline for several human (h) nicotinic acetylcholine receptors (nAChRs), functional and structural experiments were performed. The Ca2+ influx results established that: (a) varenicline activates (μM range) nAChR subtypes with the following rank sequence: hα7>hα4β4>hα4β2>hα3β4>>>hα1β1γδ; (b) varenicline binds to nAChR subtypes with the following affinity order (nM range): hα4β2~hα4β4>hα3β4>hα7>>>Torpedo α1β1γδ. The molecular docking results indicating that more hydrogen bond interactions are apparent for α4-containing nAChRs in comparison to other nAChRs may explain the observed higher affinity; and that (c) varenicline is a full agonist at hα7 (101%) and hα4β4 (93%), and a partial agonist at hα4β2 (20%) and hα3β4 (45%), relative to (±)-epibatidine. The allosteric sites found at the extracellular domain (EXD) of hα3β4 and hα4β2 nAChRs could explain the partial agonistic activity of varenicline on these nAChR subtypes. Molecular dynamics simulations show that the interaction of varenicline to each allosteric site decreases the capping of Loop C at the hα4β2 nAChR, suggesting that these allosteric interactions limit the initial step in the gating process. In conclusion, we propose that in addition to hα4β2 nAChRs, hα4β4 nAChRs can be considered as potential targets for the clinical activity of varenicline, and that the allosteric interactions at the hα3β4- and hα4β2-EXDs are alternative mechanisms underlying partial agonism at these nAChRs.

Synthesis, in vitro and in vivo studies, and molecular modeling of N-alkylated dextromethorphan derivatives as non-competitive inhibitors of α3β4 nicotinic acetylcholine receptor

9 N-alkylated derivatives of dextromethorphan are synthesized and studied as non-competitive inhibitors of α3β4 nicotinic acetylcholine receptors (nAChRs). In vitro activity towards α3β4 nicotinic acetylcholine receptor is determined using a patch-clamp technique and is in the micromolar range. Homology modeling, molecular docking and molecular dynamics of ligand-receptor complexes in POPC membrane are used to find the mode of interactions of N-alkylated dextromethorphan derivatives with α3β4 nAChR. The compounds, similarly as dextromethorphan, interact with the middle portion of α3β4 nAChR ion channel. Finally, behavioral tests confirmed potential application of the studied compounds for the treatment of addiction.

The Concise Guide to PHARMACOLOGY 2013/14: ligand-gated ion channels

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full.

Ligand-gated ion channels are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets.

It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Harnessing allostery: a novel approach to drug discovery

Allostery is the most direct and efficient way for regulation of biological macromolecule function, ranging from the control of metabolic mechanisms to signal transduction pathways. Allosteric modulators target to allosteric sites, offering distinct advantages compared to orthosteric ligands that target to active sites, such as greater specificity, reduced side effects, and lower toxicity. Allosteric modulators have therefore drawn increasing attention as potential therapeutic drugs in the design and development of new drugs. In recent years, advancements in our understanding of the fundamental principles underlying allostery, coupled with the exploitation of powerful techniques and methods in the field of allostery, provide unprecedented opportunities to discover allosteric proteins, detect and characterize allosteric sites, design and develop novel efficient allosteric drugs, and recapitulate the universal features of allosteric proteins and allosteric modulators. In the present review, we summarize the recent advances in the repertoire of allostery, with a particular focus on the aforementioned allosteric compounds.

Antidepressant activity in mice elicited by 3-furan-2-yl-N-p-tolyl-acrylamide, a positive allosteric modulator of the α7 nicotinic acetylcholine receptor

The objective of the current study is to determine whether 3-furan-2-yl-N-p-tolyl-acrylamide (PAM-2), a positive allosteric modulator of α7 nicotinic receptors (AChRs), produces antidepressant-like behavior in mice, and reactivates desensitized α7 AChRs expressed in CH3-α7 cells. Mice from both sexes were injected (i.p.) with PAM-2 (1.0mg/kg) on a daily basis for three weeks. Forced swim tests (FSTs) were performed on Day 1 and Day 7 to determine the acute and subchronic effects of PAM-2, respectively, and on Days 14 and 21 to determine its chronic activity. To examine the residual effects after drug treatment, a withdrawal period of two more weeks was continued with FSTs performed on Day 28 and 35. Our results indicate that: (1) PAM-2 does not induce acute antidepressant effects in male or female mice, (2) PAM-2 induces antidepressant effects in mice from both sexes after one (subchronic) and two (chronic) weeks, whereas at the third week (chronic), the antidepressant effect is decreased in male and increased in female mice. Since PAM-2 does not influence the locomotor activity of mice, the observed antidepressant activity is not driven by nonspecific motor-stimulant actions, (3) the residual antidepressant effect mediated by PAM-2 after one week of treatment cessation is observed only in female mice, and finally the Ca(2+) influx results indicate that (4) PAM-2 can reactivate desensitized α7 AChRs. Our results clearly indicate that PAM-2 elicits antidepressant activity, probably by enhancing the activity of the endogenous neurotransmitter acetylcholine on α7 AChRs, without inducing receptor desensitization, and that this activity is gender-dependent. This is the first time that an antidepressant activity is described for an α7 PAM, supporting further studies as potential therapeutic medications for depressive states.

Molecular interactions between mecamylamine enantiomers and the transmembrane domain of the human α4β2 nicotinic receptor

To characterize the binding sites of mecamylamine enantiomers on the transmembrane domain (TMD) of human (h) (α4)3(β2)2 and (α4)2(β2)3 nicotinic acetylcholine receptors (AChRs), we used nuclear magnetic resonance (NMR), molecular docking, and radioligand binding approaches. The interactions of (S)-(+)- and (R)-(-)-mecamylamine with several residues, determined by high-resolution NMR, within the hα4β2-TMD indicate different modes of binding at several luminal (L) and nonluminal (NL) sites. In general, the residues sensitive to each mecamylamine enantiomer are similar at both receptor stoichiometries. However, some differences were observed. The molecular docking experiments were crucial for delineating the location and orientation of each enantiomer in its binding site. In the (α4)2(β2)3-TMD, (S)-(+)-mecamylamine interacts with the L1 (i.e., between positions -3' and -5') and L2 (i.e., between positions 16' and 20') sites, whereas the β2-intersubunit (i.e., cytoplasmic end of two β2-TMDs) and α4/β2-intersubunit (i.e., cytoplasmic end of α4-TM1 and β2-TM3) sites are shared by both enantiomers. In the (α4)3(β2)2-TMD, both enantiomers bind with different orientations to the L1' (closer to ring 2') and α4-intrasubunit (i.e., at the cytoplasmic ends of α4-TM1 and α4-TM2) sites, but only (R)-(-)-mecamylamine interacts with the L2' (i.e., closer to ring 20') and α4-TM3-intrasubunit sites. Our findings are important because they provide, for the first time, a structural understanding of the allosteric modulation elicited by mecamylamine enantiomers at each hα4β2 stoichiometry. This advancement could be beneficial for the development of novel therapies for the treatment of several neurological disorders.

Reducing cannabinoid abuse and preventing relapse by enhancing endogenous brain levels of kynurenic acid

In the reward circuitry of the brain, α-7-nicotinic acetylcholine receptors (α7nAChRs) modulate effects of Δ(9)-tetrahydrocannabinol (THC), marijuana's main psychoactive ingredient. Kynurenic acid (KYNA) is an endogenous negative allosteric modulator of α7nAChRs. Here we report that the kynurenine 3-monooxygenase (KMO) inhibitor Ro 61-8048 increases brain KYNA levels and attenuates cannabinoid-induced increases in extracellular dopamine in reward-related brain areas. In the self-administration model of drug abuse, Ro 61-8048 reduced the rewarding effects of THC and the synthetic cannabinoid WIN 55,212-2 in squirrel monkeys and rats, respectively, and it also prevented relapse to drug-seeking induced by reexposure to cannabinoids or cannabinoid-associated cues. The effects of enhancing endogenous KYNA levels with Ro 61-8048 were prevented by positive allosteric modulators of α7nAChRs. Despite a clear need, there are no medications approved for treatment of marijuana dependence. Modulation of KYNA offers a pharmacological strategy for achieving abstinence from marijuana and preventing relapse.

Positive modulation of the α9α10 nicotinic cholinergic receptor by ascorbic acid

BACKGROUND AND PURPOSE

The activation of α9α10 nicotinic cholinergic receptors (nAChRs) present at the synapse between efferent olivocochlear fibres and cochlear hair cells can prevent acoustic trauma. Hence, pharmacological potentiators of these receptors could be useful therapeutically. In this work, we characterize ascorbic acid as a positive modulator of recombinant α9α10 nAChRs.

EXPERIMENTAL APPROACH

ACh-evoked responses were analysed under two-electrode voltage-clamp recordings in Xenopus laevis oocytes injected with α9 and α10 cRNAs.

KEY RESULTS

Ascorbic acid potentiated ACh responses in X. laevis oocytes expressing α9α10 (but not α4β2 or α7) nAChRs, in a concentration-dependent manner, with an effective concentration range of 1-30 mM. The compound did not affect the receptor's current-voltage profile nor its apparent affinity for ACh, but it significantly enhanced the maximal evoked currents (percentage of ACh maximal response, 240 ± 20%). This effect was specific for the L form of reduced ascorbic acid. Substitution of the extracellular cysteine residues present in loop C of the ACh binding site did not affect the potentiation. Ascorbic acid turned into a partial agonist of α9α10 nAChRs bearing a point mutation at the pore domain of the channel (TM2 V13'T mutant). A positive allosteric mechanism of action rather than an antioxidant effect of ascorbic acid is proposed.

CONCLUSIONS AND IMPLICATIONS

The present work describes one of the few agents that activates or potentiates α9α10 nAChRs and leads to new avenues for designing drugs with potential therapeutic use in inner ear disorders.

Physostigmine and galanthamine bind in the presence of agonist at the canonical and noncanonical subunit interfaces of a nicotinic acetylcholine receptor

Galanthamine and physostigmine are clinically used cholinomimetics that both inhibit acetylcholinesterase and also interact directly with and potentiate nAChRs. As with most nAChR-positive allosteric modulators, the location and number of their binding site(s) within nAChRs are unknown. In this study, we use the intrinsic photoreactivities of [(3)H]physostigmine and [(3)H]galanthamine upon irradiation at 312 nm to directly identify amino acids contributing to their binding sites in the Torpedo californica nAChR. Protein sequencing of fragments isolated from proteolytic digests of [(3)H]physostigmine- or [(3)H]galanthamine-photolabeled nAChR establish that, in the presence of agonist (carbamylcholine), both drugs photolabeled amino acids on the complementary (non-α) surface of the transmitter binding sites (γTyr-111/γTyr-117/δTyr172). They also photolabeled δTyr-212 at the δ-β subunit interface and γTyr-105 in the vestibule of the ion channel, with photolabeling of both residues enhanced in the presence of agonist. Furthermore, [(3)H]physostigmine photolabeling of γTyr-111, γTyr-117, δTyr-212, and γTyr-105 was inhibited in the presence of nonradioactive galanthamine. The locations of the photolabeled amino acids in the nAChR structure and the results of computational docking studies provide evidence that, in the presence of agonist, physostigmine and galanthamine bind to at least three distinct sites in the nAChR extracellular domain: at the α-γ interface (1) in the entry to the transmitter binding site and (2) in the vestibule of the ion channel near the level of the transmitter binding site, and at the δ-β interface (3) in a location equivalent to the benzodiazepine binding site in GABA(A) receptors.