Positive allosteric modulators as an approach to nicotinic acetylcholine receptor-targeted therapeutics: advantages and limitations

Advances in the In vitro and In vivo pharmacology of Alpha4beta2 nicotinic receptor positive allosteric modulators

Receptors containing α4 and β2 subunits are a major neuronal nicotinic acetylcholine receptor (nAChR) subtype in the brain. This receptor plays a critical role in nicotine addiction, with potential smoking cessation therapeutics producing modulation of α4β2 nAChR. In addition, compounds that act as agonists at α4β2 nAChR may be useful for the treatment of pathological pain. Further, as the α4β2 nAChR has been implicated in cognition, therapeutics that act as α4β2 nAChR agonists are also being examined as treatments for cognitive disorders and neurological diseases that impact cognitive function, such as Alzheimer's disease and schizophrenia. This review will cover the molecular in vitro evidence that allosteric modulators of the α4β2 neuronal nAChR provide several advantages over traditional α4β2 nAChR orthosteric ligands. Specifically, we explore the concept that nAChR allosteric modulators allow for greater pharmacological selectivity, while minimizing potential deleterious off-target effects. Further, here we discuss the development and preclinical in vivo behavioral assessment of allosteric modulators at the α4β2 neuronal nAChR as therapeutics for smoking cessation, pathological pain, as well as cognitive disorders and neurological diseases that impact cognitive function. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

Behavioral and Molecular Basis of Cholinergic Modulation of Pain: Focus on Nicotinic Acetylcholine Receptors

Nicotinic acetylcholine receptors (nAChRs) have emerged as a novel therapeutic strategy for pain and inflammatory disorders. In particular, α4β2∗, α7, and α9α10 nAChR subtypes have been investigated as potential targets to treat pain. The nAChRs are distributed on the pain transmission pathways, including central and peripheral nervous systems and immune cells as well. Several agonists for α4β2∗ nAChR subtypes have been investigated in multiple animal pain models with promising results. However, studies in human indicated a narrow therapeutic window for α4β2∗ agonists. Furthermore, animal studies suggest that using agonists for α7 nAChR subtype and antagonists for α9α10 nAChR subtypes are potential novel therapies for chronic pain management, including inflammatory and neuropathic pain. More recently, alternative nAChRs ligands such as positive allosteric modulators and silent agonists have shown potential to develop into new treatments for chronic pain.

Evidence for positive allosteric modulation of cognitive-enhancing effects of nicotine in healthy human subjects

RATIONALE

Cognitive benefits of nicotinic acetylcholine receptor (nAChR) agonists are well established but have generally been of small magnitude and uncertain clinical significance. A way of raising the effect size may be to facilitate agonist-induced responses by co-administering a nAChR positive allosteric modulator (PAM).

OBJECTIVE

The aim was to test whether galantamine, a PAM at several nAChR subtypes, can potentiate the cognitive-enhancing effects of nicotine.

METHODS

Twenty-six adult never-smokers were treated, in a double-blind counterbalanced sequence, with nicotine (7 mg/24 h, transdermally) and galantamine (4 mg, p.o.) combined, nicotine alone, galantamine alone, and double placebo. A low dose of galantamine was chosen to minimize acetylcholinesterase inhibition, which was verified in blood assays. In each condition, participants were tested with three cognitive tasks.

RESULTS

Nicotine significantly improved reaction time (RT) and signal detection in a visuospatial attention task and the Rapid Visual Information Processing Task. Galantamine did not modulate these effects. A trend toward RT reduction by galantamine correlated with acetylcholinesterase inhibition. In a change detection task, there were no effects of nicotine or galantamine alone on accuracy or RT. However, both drugs combined acted synergistically to reduce RT. This effect was not associated with acetylcholinesterase inhibition.

CONCLUSIONS

A pattern consistent with allosteric potentiation of nicotine effects by galantamine was observed on one of six performance measures. This may reflect specific nAChR subtype involvement, or additional pharmacological actions of galantamine may have overshadowed similar interactions on other measures. The finding suggests that allosteric potentiation of nAChR agonist-induced cognitive benefits is possible in principle.

Anti-Inflammatory Effect of Alpha7 Nicotinic Acetylcholine Receptor Modulators on BV2 Cells

OBJECTIVE

The development of neuroinflammation shares numerous risk factors and involves many complex interactions which contribute to disease pathology. An important cell type in neuroinflammation is the active microglia cell - the resident immune cell of the CNS. There is increasing need to understand how these pathways related to neuroinflammation work and how they can be regulated. Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated receptors and widely distributed in the brain. The α7 nAChR is a penta-homomeric receptor and is one of the nAChRs expressed in microglia. This study was first designed to characterize the effects of lipopolysaccharide (LPS) on BV2 culture cells, a cell line of murine microglia origin, on release of inflammatory markers and to characterize the inhibitory effects of α7 nAChR modulators in these cells.

METHODS

First, the BV2 cell cultures were functionally validated by exposing them to LPS for 4-24 h and then examining the release of tumor necrosis factor-alpha (TNF-α) using ELISA and nitric oxide (NO) release using the Griess assay, respectively. Next, α7 nAChR modulators with different pharmacological profiles were applied dose-dependently to study their effects on LPS-induced release of NO and TNF-α.

RESULTS

The time-course and dose-response curve revealed that LPS dose-dependently activated (EC50 = 2.5 ng/mL) BV2 cells releasing TNF-α at 4 h, followed by release of NO that occurred first at 8-h time point. The α7 nAChR subunit mRNA was identified in the BV2 cells. The pharmacology studies showed the α7 nAChR selective modulators NS6740 and TQS reduced NO and cytokine release from BV2 cell cultures.

CONCLUSION

We here identified the dose- and time-dependent effects of LPS in BV2 cell cultures on several inflammatory readouts and showed that α7 nAChR modulators with little or no ion channel activity inhibited this anti-inflammatory mechanism.

Impact of modulation of the α7 nicotinic acetylcholine receptor on nicotine reward in the mouse conditioned place preference test

RATIONALE

The α7 nicotinic acetylcholine receptor (nAChR) has been implicated as a target in modulating nicotine reward. However, the effect of pharmacological agents that have been shown to alter the channel properties of the α7 nAChR is not well understood in nicotine reward.

OBJECTIVES

This study aimed to investigate the impact of α7 nAChR pharmacological modulation on nicotine conditioned place preference (CPP) in mice by using positive allosteric modulators (PAMs) and a silent agonist.

METHODS

The effect of the orthosteric α7 nAChR full agonist PNU282987 (1.3 and 9 mg/kg, s.c.), Type I α7 PAM NS1738 (1 and 10 mg/kg; i.p.), the Type II α7 PAM PNU120596 (0.3, 1, and 3 mg/kg, i.p.), and the α7 silent agonist NS6740 (1 and 3 mg/kg, i.p) on nicotine CPP was measured in mice. Mice were conditioned with either saline or nicotine (0.5 mg/kg) for 3 days in the CPP paradigm.

RESULTS

The α7 full orthosteric agonist PNU282987 and the Type II α7 nAChR PAM PNU120596 reduced nicotine CPP, while the silent agonist NS6740 and Type I PAM NS1738 had no effect. The effects of PNU282987 and PNU120596 did not have an effect on morphine CPP.

CONCLUSIONS

Taken together, our results suggest that modulation of the α7 nAChR can play important roles in nicotine CPP in mice. In addition, the Type II α7 nAChR PAM PNU120596 attenuated nicotine reward suggesting that endogenous acetylcholine/choline tone is sufficient to reduce nicotine CPP. These findings highlight a beneficial effect of using α7 nAChR PAMs in nicotine reward.

The α7 nicotinic acetylcholine receptor positive allosteric modulator prevents lipopolysaccharide-induced allodynia, hyperalgesia and TNF-α in the hippocampus in mice

BACKGROUND

Previous studies have shown that α7 nicotinic acetylcholine receptor (nAChR) has a critical role in the regulation of pain sensitivity and neuroinflammation. However, pharmacological effects of α7 nAChR activation in the hippocampus on neuroinflammatory mechanisms associated with allodynia and hyperalgesia remain unknown. We have determined the effects of 3a,4,5,9b-tetrahydro-4-(1-naphthalenyl)-3H-cyclopentan[c]quinoline-8-sulfonamide (TQS), an α7 nAChR positive allosteric modulator, on lipopolysaccharide (LPS)-induced allodynia and hyperalgesia in mice. We also evaluated the effects of TQS on immunoreactivity of microglial marker ionized-calcium binding adaptor molecule 1 (Iba-1), phospho-nuclear factor-κB (p-NF-κB p65), tumor necrosis factor-alpha (TNF-α), and norepinephrine (NE) level.

METHODS

Mice were treated with (0.25, 1 or 4 mg/kg, ip) followed by LPS (1 mg/kg, ip) administration. Allodynia and hyperalgesia were determined using von Frey filaments and hot plate respectively. Immunoreactivity of Iba-1, p-NF-κB p65, and TNF-α, were measured in the hippocampus using immunofluorescence assay. Hippocampal NE level was evaluated using high performance liquid chromatography.

RESULTS

LPS administration resulted in allodynia and hyperalgesia in mice after six h. Systemic administration of TQS prevented LPS-induced allodynia and hyperalgesia. TQS pretreatment significantly decreased the immunoreactivity of Iba-1, p-NF-κB, and TNF-α in CA1 and DG regions of the hippocampus. In addition, TQS reversed LPS-induced NE reduction in the hippocampus.

CONCLUSIONS

Taken together, our results suggest that TQS prevented LPS-induced allodynia and hyperalgesia, upregulation of TNF-α expression and NE level reduction involving microglial α7 nAChR in part in the hippocampus. Therefore, these findings highlight the important effects of α7 nAChR allosteric modulator against symptoms of inflammatory pain.

Alpha7 nicotinic acetylcholine receptors and neural network synaptic transmission in human induced pluripotent stem cell-derived neurons

The α7 nicotinic acetylcholine receptor has been extensively researched as a target for treatment of cognitive impairment in Alzheimer's disease and schizophrenia. Investigation of the α7 receptor is commonly performed in animals but it is critical to increase the biologically relevance of the model systems to fully capture the physiological role of the α7 receptor in humans. For example most humans, in contrast to animals, express the hybrid gene CHRFAM7A, the product of which modulates α7 receptor activity. In the present study, we used human induced pluripotent stem cell (hiPSC) derived neurons to establish a humanized α7 model. We established a cryobank of neural stem cells (NSCs) that could reproducibly be matured into neurons expressing neuronal markers and CHRNA7 and CHRFAM7A. The neurons responded to NMDA, GABA, and acetylcholine and exhibited synchronized spontaneous calcium oscillations. Gene expression studies and application of a range of α7 positive allosteric modulators (PNU-120595, TQS, JNJ-39393406 and AF58801) together with the α7 agonist PNU-282987 during measurement of intracellular calcium levels demonstrated the presence of functional α7 receptors in matured hiPSC-derived neuronal cultures. Pharmacological α7 activation also resulted in intracellular signaling as measured by ERK 1/2 phosphorylation and c-Fos protein expression. Moreover, PNU-120596 increased the frequency of the spontaneous calcium oscillations demonstrating implication of α7 receptors in human synaptic networks activity. Overall, we show that hiPSC derived neurons are an advanced in vitro model for studying human α7 receptor pharmacology and the involvement of this receptor in cellular processes as intracellular signaling and synaptic transmission.

NAChR α4β2 Subtype and their Relation with Nicotine Addiction, Cognition, Depression and Hyperactivity Disorder

BACKGROUND

Neuronal α4β2 nAChRs are receptors involved in the role of neurotransmitters regulation and release, and this ionic channel participates in biological process of memory, learning and attention. This work aims to review the structure and functioning of the α4β2 nAChR emphasizing its role in the treatment of associated diseases like nicotine addiction and underlying pathologies such as cognition, depression and attention-deficit hyperactivity disorder.

METHODS

The authors realized extensive bibliographic research using the descriptors "Nicotine Receptor α4β2" and "cognition", "depression", "attention-deficit hyperactivity disorder", besides cross-references of the selected articles and after analysis of references in the specific literature.

RESULTS

As results, it was that found 179 relevant articles presenting the main molecules with affinity to nAChR α4β2 related to the cited diseases. The α4β2 nAChR subtype is a remarkable therapeutic target since this is the most abundant receptor in the central nervous system.

CONCLUSION

In summary, this review presents perspectives on the pharmacology and therapeutic targeting of α4β2 nAChRs for the treatment of cognition and diseases like nicotine dependence, depression and attention-deficit hyperactivity disorder.

Flavonoids as positive allosteric modulators of α7 nicotinic receptors

The use of positive allosteric modulators (PAM) of α7 nicotinic receptors is a promising therapy for neurodegenerative, inflammatory and cognitive disorders. Flavonoids are polyphenolic compounds showing neuroprotective, anti-inflammatory and pro-cognitive actions. Besides their well-known antioxidant activity, flavonoids trigger intracellular pathways and interact with receptors, including α7. To reveal how the beneficial actions of flavonoids are linked to α7 function, we evaluated the effects of three representative flavonoids -genistein, quercetin and the neoflavonoid 5,7-dihydroxy-4-phenylcoumarin- on whole-cell and single-channel currents. All flavonoids increase the maximal currents elicited by acetylcholine with minimal effects on desensitization and do not reactivate desensitized receptors, a behaviour consistent with type I PAMs. At the single-channel level, they increase the duration of the open state and produce activation in long-duration episodes with a rank order of efficacy of genistein > quercetin ≥ neoflavonoid. By using mutant and chimeric α7 receptors, we demonstrated that flavonoids share transmembrane structural determinants with other PAMs. The α7-PAM activity of flavonoids results in decreased cell levels of reactive oxygen species. Thus, allosteric potentiation of α7 may be an additional mechanism underlying neuroprotective actions of flavonoids, which may be used as scaffolds for designing new therapeutic agents.

Type I-like behavior of the type II α7 nicotinic acetylcholine receptor positive allosteric modulator A-867744

Cognitive impairment often involves the decreased expression or hypofunction of alpha 7-type nicotinic acetylcholine receptors (α7 nAChRs). Agonists or positive allosteric modulators (PAMs) of α7 nAChRs are known to be potential treatments for dementias, different neurodegenerative disorders, pain syndromes and conditions involving inflammation. In some of these conditions, it is desirable to maintain the temporal precision of fast cholinergic events, while in others, this temporal precision is unnecessary. For this reason, the optimal therapeutic effect for distinct indications may require PAMs with different mechanisms of action. The two major mechanisms are called “type I”, which are compounds that augment α7 nAChR-mediated currents but maintain their characteristic fast kinetics; and “type II”, which are compounds that produce augmented and prolonged currents. In this study, we performed a kinetic analysis of two type II PAMs of the α7 nAChR: PNU-120596 and A-867744, using a fast perfusion method that allowed high temporal resolution. We characterized the type of modulation produced by the two compounds, the state-dependence of the modulatory action, and the interaction between the two compounds. We found fundamental differences between the modulation mechanisms by PNU-120596 and A-867744. Most importantly, during brief agonist pulses, A-867744 caused a strikingly type I-like modulation, while PNU-120596 caused a type II-like prolonged activation. Our results demonstrate that specific compounds, even though all labeled as type II PAMs, can behave in completely different ways, including their onset and offset kinetics, state preference, and single channel open time. Our results emphasize that subtle details of the mechanism of action may be significant in assessing the therapeutic applicability of α7 nAChR PAM compounds.

Reversal of Nicotine Withdrawal Signs Through Positive Allosteric Modulation of α4β2 Nicotinic Acetylcholine Receptors in Male Mice

Introduction

Nicotine withdrawal symptoms are important factors in determining the relapse rate to tobacco smoking and drugs that diminish these symptoms would potentially have a higher success rate as smoking cessation aids. Unlike US Food and Drug administration approved smoke cessation aids (nicotine and varenicline) which act as nicotinic acetylcholine receptors (nAChRs) agonists, desformylflustrabromine (dFBr) acts as a nAChR positive allosteric modulator with higher selectivity to the α4β2 nAChR. In animal studies, dFBr was well tolerated and reduced intravenous nicotine self-administration. In this study, we use behavioral test in mouse model of spontaneous nicotine withdrawal to assess the effect of dFBr on nicotine withdrawal symptoms.

Methods

Spontaneous nicotine withdrawal in nicotine-dependent ICR male mice was established 18-24 h after termination (minipump removal) of 14 days infusion of nicotine. After that (day 15), spontaneous signs of nicotine withdrawal were examined in the following order: anxiety-like behaviors, somatic signs, and then hyperalgesia using previously published behavioral protocols. Fifteen minutes before withdrawal signs testing, mice received a subcutaneous acute injection of vehicle or dFBr at the doses of 0.02, 0.1, and 1 mg/kg to determine the effect of dFBr on nicotine withdrawal symptoms.

Results

dFBr produced dose-dependent reversal of nicotine withdrawal signs in mouse model of spontaneous nicotine withdrawal.

Implications

Positive allosteric modulators of nAChR such as dFBr reduce nicotine withdrawal symptoms supporting the potential clinical use of this novel class of nAChR-based therapeutics as smoking cessation aid.

1-(2',5'-Dihydroxyphenyl)-3-(2-fluoro-4-hydroxyphenyl)-1-propanone (RGM079): A Positive Allosteric Modulator of α7 Nicotinic Receptors with Analgesic and Neuroprotective Activity

Acetylcholine α7 nicotinic receptors are widely expressed in the brain, where they are involved in the central processing of pain as well as in neuropsychiatric, neurodegenerative, and inflammatory processes. Positive allosteric modulators (PAMs) show the advantage of allowing the selective regulation of different subtypes of acetylcholine receptors without directly interacting with the agonist binding site. Here, we report the preparation and biological activity of a fluoro-containing compound, 1-(2',5'-dihydroxyphenyl)-3-(2-fluoro-4-hydroxyphenyl)-1-propanone (8, RGM079), that behaves as a potent PAM of the α7 receptors and has a balanced pharmacokinetic profile and antioxidant properties comparable or even higher than well-known natural polyphenols. In addition, compound RGM079 shows neuroprotective properties in Alzheimer's disease (AD)-toxicity related models. Thus, it causes a concentration-dependent neuroprotective effect against the toxicity induced by okadaic acid (OA) in the human neuroblastoma cell line SH-SY5Y. Similarly, in primary cultures of rat cortical neurons, RGM079 is able to restore the cellular viability after exposure to OA and amyloid peptide Aβ_1-42, with cell death almost completely prevented at 10 and 30 μM, respectively. Finally, compound RGM079 shows in vivo analgesic activity in the complete Freund's adjuvant (CFA)-induced paw inflammation model after intraperitoneal administration.

Heteromeric Neuronal Nicotinic Acetylcholine Receptors with Mutant β Subunits Acquire Sensitivity to α7-Selective Positive Allosteric Modulators

Homomeric α7 nicotinic acetylcholine receptors (nAChR) have an intrinsically low probability of opening that can be overcome by α7-selective positive allosteric modulators (PAMs), which bind at a site involving the second transmembrane domain (TM2). Mutation of a methionine that is unique to α7 at the 15' position of TM2 to leucine, the residue in most other nAChR subunits, largely eliminates the activity of such PAMs. We tested the effect of the reverse mutation (L15'M) in heteromeric nAChR receptors containing α4 and β2, which are the nAChR subunits that are most abundant in the brain. Receptors containing these mutations were found to be strongly potentiated by the α7 PAM 3a,4,5,9b-tetrahydro-4-(1-naphthalenyl)-3H-cyclopentan[c]quinoline-8-sulfonamide (TQS) but insensitive to the alternative PAM 1-(5-chloro-2,4-dimethoxyphenyl)-3-(5-methylisoxazol-3-yl)-urea. The presence of the mutation in the β2 subunit was necessary and sufficient for TQS sensitivity. The primary effect of the mutation in the α4 subunit was to reduce responses to acetylcholine applied alone. Sensitivity to TQS required only a single mutant β subunit, regardless of the position of the mutant β subunit within the pentameric complex. Similar results were obtained when β2L15'M was coexpressed with α2 or α3 and when the L15'M mutation was placed in β4 and coexpressed with α2, α3, or α4. Functional receptors were not observed when β1L15'M subunits were coexpressed with other muscle nAChR subunits. The unique structure-activity relationship of PAMs and the α4β2L15'M receptor compared with α7 and the availability of high-resolution α4β2 structures may provide new insights into the fundamental mechanisms of nAChR allosteric potentiation. SIGNIFICANCE STATEMENT: Heteromeric neuronal nAChRs have a relatively high initial probability of channel activation compared to receptors that are homomers of α7 subunits but are insensitive to PAMs, which greatly increase the open probability of α7 receptors. These features of heteromeric nAChR can be reversed by mutation of a single residue present in all neuronal heteromeric nAChR subunits to the sequence found in α7. Specifically, the mutation of the TM2 15' leucine to methionine in α subunits reduces heteromeric receptor channel activation, while the same mutation in neuronal β subunits allows heteromeric receptors to respond to select α7 PAMs. The results indicate a key role for this residue in the functional differences in the two main classes of neuronal nAChRs.

Chemical conversion of nicotinamide into type I positive allosteric modulator of α7 nAChRs

Structural modifications of nicotinamide, a form of vitamin B3, gave rise to a series of compounds (8aa-8ce) that exhibit activities as type I positive allosteric modulators (PAMs) of human α7 nAChR expressed in Xenopus oocytes in two-electrode voltage clamp assay. The compound 8ai was a potent and efficacious PAM with an EC₅₀ = 3.34 ± 1.13 μM and the maximum activation effect of α7 current over 1474 ± 246% in the presence of acetylcholine (100 μM). It is highly specific to α7 nAChR over other subtypes of nAChR and 5-HT_3A receptors. The structure-activity relationship analysis identified a key skeleton of nicotinamide nucleus critical for biological activity. Taken together, the 8ai as a type I PAM of α7 nAChR may be beneficial for improvement of cognitive deficit.

The α7 nicotinic acetylcholine receptor positive allosteric modulator attenuates lipopolysaccharide-induced activation of hippocampal IκB and CD11b gene expression in mice

We have reported that 3a,4,5,9b-tetrahydro-4-(1-naphthalenyl)-3H-cyclopentan[c]quinoline-8-sulfonamide (TQS), α7 nicotinic acetylcholine receptor (nAChR) positive allosteric modulator (PAM) reduces lipopolysaccharide (LPS)-induced hyperalgesia and allodynia in mice. The objective of the present study was to determine the effects of TQS on LPS-induced activation of hippocampal inhibitor of κB (IκB) and cluster of differentiation 11b (CD11b) gene expression involving hyperalgesia and allodynia in mice. We also examined the effects of TQS on microglial phenotype following LPS administration. Pretreatment of TQS (4 mg/kg) reduced the expressions of IκB and CD11b mRNA. Pretreatment of methyllycaconitine (3 mg/kg), an α7 nAChR antagonist, reversed TQS-induced decrease in IκB and CD11b mRNA expressions in the hippocampus indicating the involvement of α7 nAChR. In addition, TQS (4 mg/kg) reversed the LPS-induced microglial morphological changes. These results suggest that TQS reduces LPS-induced IκB and CD11b gene expression and microglial activation associated with hyperalgesia and allodynia by targeting microglial α7 nAChR in the hippocampus.

Facilitation and inhibition of firing activity and N-methyl-D-aspartate-evoked responses of CA1 hippocampal pyramidal cells by alpha7 nicotinic acetylcholine receptor selective compounds in vivo

Alpha7 nicotinic acetylcholine receptors (nAChRs) are promising novel targets for the treatment of neurocognitive disorders. Although the cognitive enhancer potential of alpha7 nAChR agonists and positive allosteric modulators (PAMs) has been confirmed in several preclinical animal models, there are only sparse in vivo electrophysiological data on their effects on the firing activity and excitability of neurons. The present study investigated and compared local effects of alpha7 nAChR agonist PHA-543613 and PAMs PNU-120596 and NS-1738 on the spontaneous and N-methyl-D-aspartate-evoked (NMDA-evoked) firing rate of rat CA1 hippocampal pyramidal cells, in vivo. Furthermore, effects of alpha7 nAChR antagonist methyllycaconitine (MLA) and GABA were also tested. Results showed substantially different effects of the alpha7 nAChR agonist and PAMs. While PNU-120596 and NS-1738 predominantly and significantly increased both spontaneous and NMDA-evoked firing rate of the neurons, application of PHA-543613 resulted in almost equal distribution of facilitatory and inhibitory effects. The increase of the NMDA-evoked firing rate exerted by NS-1738 was superadditive over the sum of the single effects of NMDA and NS-1738. The simultaneous application of alpha7 nAChR agonist PHA-543613 and PAM NS-1738 resulted in additive increase of both spontaneous and NMDA-evoked firing rate. However, NS-1738 counteracted inhibitory effects of PHA-543613 in 5 out of 6 neurons, resulting in a synergistic potentiation of their firing responses to NMDA. Our results suggest that alpha7 nAChR PAMs increase neuronal excitability more potently than agonists, while the remarkable occurrence of inhibitory effects of PHA-543613 (possibly originating from receptor desensitization) implies that agonists may exert neuroprotective effects.

Discovery of an intrasubunit nicotinic acetylcholine receptor-binding site for the positive allosteric modulator Br-PBTC

Nicotinic acetylcholine receptor (nAChR) ligands that lack agonist activity but enhance activation in the presence of an agonist are called positive allosteric modulators (PAMs). nAChR PAMs have therapeutic potential for the treatment of nicotine addiction and several neuropsychiatric disorders. PAMs need to be selectively targeted toward certain nAChR subtypes to tap this potential. We previously discovered a novel PAM, (R)-7-bromo-N-(piperidin-3-yl)benzo[b]thiophene-2-carboxamide (Br-PBTC), which selectively potentiates the opening of α4β2*, α2β2*, α2β4*, and (α4β4)₂α4 nAChRs and reactivates some of these subtypes when desensitized (* indicates the presence of other subunits). We located the Br-PBTC-binding site through mutagenesis and docking in α4. The amino acids Glu-282 and Phe-286 near the extracellular domain on the third transmembrane helix were found to be crucial for Br-PBTC's PAM effect. E282Q abolishes Br-PBTC potentiation. Using (α4^E282Qβ2)₂α5 nAChRs, we discovered that the trifluoromethylated derivatives of Br-PBTC can potentiate channel opening of α5-containing nAChRs. Mutating Tyr-430 in the α5 M4 domain changed α5-selectivity among Br-PBTC derivatives. There are two kinds of α4 subunits in α4β2 nAChRs. Primary α4 forms an agonist-binding site with another β2 subunit. Accessory α4 forms an agonist-binding site with another α4 subunit. The pharmacological effect of Br-PBTC depends both on its own and agonists' occupancy of primary and accessory α4 subunits. Br-PBTC reactivates desensitized (α4β2)₂α4 nAChRs. Its full efficacy requires intact Br-PBTC sites in at least one accessory and one primary α4 subunit. PAM potency increases with higher occupancy of the agonist sites. Br-PBTC and its derivatives should prove useful as α subunit-selective nAChR PAMs.

Allosteric Agonism of α7 Nicotinic Acetylcholine Receptors: Receptor Modulation Outside the Orthosteric Site

Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop superfamily of ligand-gated ion channels. Typically, channel activation follows the binding of agonists to the orthosteric binding sites of the receptor. α7 nAChRs have a very low probability of channel activation, which can be reversed by the binding of α7 selective positive allosteric modulators (PAMs) to putative sites within the transmembrane domains. Although typical PAMs, like PNU-120596, require coapplication of an orthosteric agonist to produce large channel activations, some, like GAT107 and B-973B [(S)-3-(3,4-difluorophenyl)-N-(1-(6-(4-(pyridin-2-yl)piperazin-1-yl)pyrazin-2-yl)ethyl)propanamide], are characterized as allosteric activating PAMs, which also bind to an allosteric activation (AA) site in the extracellular domain and activate the α7 ion channel by themselves. We had previously characterized N,N-diethyl-N'-phenylpiperazine analogs with various functions. In this work, we docked members of this family to a homology model of the α7 receptor extracellular domain. The compound 1,1-diethyl-4(naphthalene-2-yl)piperazin-1-ium (2NDEP) a weak partial agonist, showed particularly favorable docking and binding energies at the putative AA site of the receptor. We hypothesized that 2NDEP could couple with PAMs through the AA site. This hypothesis was tested with the α7 mutant C190A, which is not activated by orthosteric agonists but is effectively activated by GAT107. The results showed that 2NDEP acts as an allosteric agonist of α7C190A when coapplied with the PAM PNU-120596. Also, the allosteric activity was nearly abolished upon coapplication with the AA site-selective antagonist 2,3,5,6MP-TQS (cis-trans-4-(2,3,5,6-tetramethylphenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide), consistent with AA site involvement. Overall, our findings show a novel mode of agonism through an allosteric site in the extracellular domain of α7 nAChR.

Decreases in smoking during treatment for methamphetamine-use disorders: preliminary evidence

Despite high rates of smoking (70-90%) and the severely negative impact of smoking on physical and mental health, only 12% of individuals receiving stimulant-use disorder treatment also receive smoking-cessation treatment. The aim of this investigation was to examine the effect of a contingency management (CM) intervention targeting methamphetamine (MA) use on cigarette smoking. Sixty-one adults with MA-use disorders who were smokers were assigned to CM or standard psychosocial treatment. Rates of smoking-negative breath samples (carbon monoxide <3 ppm) were compared between the two groups while controlling for baseline carbon monoxide level, marijuana use, MA use, and time. This subgroup of mostly male (59%) participants included 44 participants in the CM group and 17 participants in the standard psychosocial treatment. Tobacco smoking participants who received CM targeting MA use were 140% (odds ratio: 2.395; 95% confidence interval: 1.073-5.346) more likely to submit a smoking-negative breath sample relative to standard psychosocial treatment during the treatment period, holding constant several other prespecified covariates. This study provides evidence that a behavioral treatment for MA use results in reductions in cigarette smoking in adults with MA-use disorder.

Combining CDP-choline and galantamine, an optimized α7 nicotinic strategy, to ameliorate sensory gating to speech stimuli in schizophrenia

Neural α7 nicotinic acetylcholine receptor (nAChR) expression and functioning deficits have been extensively associated with cognitive and early sensory gating (SG) impairments in schizophrenia (SCZ) patients and their relatives. SG, the suppression of irrelevant and redundant stimuli, is measured in a conditioning-testing (S₁-S₂) paradigm eliciting electroencephalography-derived P50 event-related potentials (ERPs), the S₂ amplitudes of which are typically suppressed relative to S₁. Despite extensive reports of nicotine-related improvements and several decades of research, an efficient nicotinic treatment has yet to be approved for SCZ. Following reports of SG improvements in low P50 suppressing SCZ patients and healthy participants with the α7 agonist, CDP-choline, this pilot study examined the combined modulatory effect of CDP-choline (500 mg) and galantamine (16 mg), a nAChR positive allosteric modulator and acetylcholinesterase inhibitor, on SG to speech stimuli in twenty-four SCZ patients in a randomized, double-blind and placebo-controlled design. As expected, in low P50 suppressors CDP-choline/galantamine (vs. Placebo) improved rP50 and dP50 scores by increasing inhibitory mechanisms as reflected by S₂P50 amplitude reductions. Results also suggest a moderating role for auditory verbal hallucinations in treatment response. These preliminary findings provide supportive evidence for the involvement of α7 nAChR activity in speech gating in SCZ and support additional trials, examining different dose combinations and repeated doses of this optimized and personalized targeted α7 cholinergic treatment for SG dysfunction in subgroups of SCZ patients.

Neurobiological and Neurophysiological Mechanisms Underlying Nicotine Seeking and Smoking Relapse

Tobacco-related morbidity and mortality continue to be a significant public health concern. Unfortunately, current FDA-approved smoking cessation pharmacotherapies have limited efficacy and are associated with high rates of relapse. Therefore, a better understanding of the neurobiological and neurophysiological mechanisms that promote smoking relapse is needed to develop novel smoking cessation medications. Here, we review preclinical studies focused on identifying the neurotransmitter and neuromodulator systems that mediate nicotine relapse, often modeled in laboratory animals using the reinstatement paradigm, as well as the plasticity-dependent neurophysiological mechanisms that facilitate nicotine reinstatement. Particular emphasis is placed on how these neuroadaptations relate to smoking relapse in humans. We also highlight a number of important gaps in our understanding of the neural mechanisms underlying nicotine reinstatement and critical future directions, which may lead toward the development of novel, target pharmacotherapies for smoking cessation.

Curare alkaloids from Matis Dart Poison: Comparison with d-tubocurarine in interactions with nicotinic, 5-HT3 serotonin and GABAA receptors

Several novel bisbenzylisoquinoline alkaloids (BBIQAs) have recently been isolated from a Matis tribe arrow poison and shown by two-electrode voltage-clamp to inhibit mouse muscle nicotinic acetylcholine receptors (nAChR). Here, using radioligand assay with Aplysia californica AChBP and radioiodinated α-bungarotoxin ([125I]-αBgt), we show that BBIQA1, BBIQA2, and d-tubocurarine (d-TC) have similar affinities to nAChR orthosteric site. However, a competition with [125I]-αBgt for binding to the Torpedo californica muscle-type nAChR revealed that BBIQAs1, 2, and 3 are less potent (IC50s = 26.3, 8.75, and 17.0 μM) than d-TC (IC50 = 0.39 μM), while with α7 nAChR in GH4C1 cells, BBIQA1 was less potent that d-TC (IC50s = 162 μM and 7.77 μM, respectively), but BBIQA2 was similar (IC50 = 5.52 μM). In inhibiting the Ca2+ responses induced by acetylcholine in Neuro2a cells expressing the mouse adult α1β1εδ nAChR or human α7 nAChR, BBIQAs1 and 2 had similar potencies to d-TC (IC50s in the range 0.75-3.08 μM). Our data suggest that BBIQA1 and BBIQA2 can inhibit adult muscle α1β1εδ nAChR by both competitive and noncompetitive mechanisms. Further experiments on neuronal α3β2, α4β2, and α9α10 nAChRs, expressed in Xenopus laevis oocytes, showed that similar potencies for BBIQAs1, 2, and d-TC. With α3β2γ2 GABAAR currents were almost completely inhibited by d-TC at a high (100 μM) concentration, but BBIQAs1 and 2 were less potent (only 40-50% inhibition), whereas in competition with Alexa Fluor 546-α-cobratoxin for binding to α1β3γ2 GABAAR in Neuro2a cells, d-TC and these analogs had comparable affinities. Especially interesting effects of BBIQAs1 and 2 in comparison with d-TC were observed for 5-HT3AR: BBIQA1 and BBIQA2 were 5- and 87-fold less potent than d-TC (IC50 = 22.63 nM). Thus, our results reveal that these BBIQAs differ from d-TC in their potencies towards certain Cys-loop receptors, and we suggest that understanding the reasons behind this might be useful for future drug design.

Macroscopic and Microscopic Activation of α7 Nicotinic Acetylcholine Receptors by the Structurally Unrelated Allosteric Agonist-Positive Allosteric Modulators (ago-PAMs) B-973B and GAT107

B-973 is an efficacious type II positive allosteric modulator (PAM) of α7 nicotinic acetylcholine receptors that, like 4BP-TQS and its active isomer GAT107, can produce direct allosteric activation in addition to potentiation of orthosteric agonist activity, which identifies it as an allosteric activating (ago)-PAM. We compared the properties of B-973B, the active enantiomer of B-973, with those of GAT107 regarding the separation of allosteric potentiation and activation. Both ago-PAMs can strongly activate mutants of α7 that are insensitive to standard orthosteric agonists like acetylcholine. Likewise, the activity of both ago-PAMs is largely eliminated by the M254L mutation in the putative transmembrane PAM-binding site. Allosteric activation by B-973B appeared more protracted than that produced by GAT107, and B-973B responses were relatively insensitive to the noncompetitive antagonist mecamylamine compared with GAT107 responses. Similar differences are also seen in the single-channel currents. The two agents generate unique profiles of full-conductance and subconductance states, with B-973B producing protracted bursts, even in the presence of mecamylamine. Modeling and docking studies suggest that the molecular basis for these effects depends on specific interactions in both the extracellular and transmembrane domains of the receptor.

Menthol acts as a positive allosteric modulator on nematode levamisole sensitive nicotinic acetylcholine receptors

The ongoing and widespread emergence of resistance to the existing anti-nematodal pharmacopeia has made it imperative to develop new anthelminthic agents. Historically, plants have been important sources of therapeutic compounds and offer an alternative to synthetic drugs. Monoterpenoids are phytochemicals that have been shown to produce acute toxic effects in insects and nematodes. Previous studies have shown nicotinic acetylcholine receptors (nAChRs) to be possible targets for naturally occurring plant metabolites such as carvacrol and carveol. In this study we examined the effects of monoterpenoid compounds on a levamisole sensitive nAChR from Oesophagostomum dentatum and a nicotine sensitive nAChR from Ascaris suum. We expressed the receptors in Xenopus laevis oocytes and used two-electrode voltage-clamp to characterize the effect of various compounds on these cys-loop receptors. At 100 μM the majority of these compounds acted as antagonists. Interestingly, further experiments revealed that both 0.1 μM and 10 μM menthol potentiated acetylcholine and levamisole responses in the levamisole sensitive receptor but not the nicotine sensitive receptor. We also investigated the effects of 0.1 μM menthol on the contractility of A. suum somatic muscle strips. Menthol produced significant potentiation of peak contractions at each concentration of acetylcholine. The positive allosteric modulatory effects of menthol in both in vivo and in vitro experiments suggests menthol as a promising candidate for combination therapy with cholinergic anthelmintics.

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We screened 12 monoterpenoid compounds to search for a potential anthelmintic.

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Menthol acted as a PAM on levamisole sensitive nematode nAChRs in vitro & in vivo.

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Carvacrol produced antagonism on both levamisole and nicotine sensitive nAChRs.

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Monoterpenoid compounds can be used as an adjunct with cholinergic anthelmintics.

Natural Negative Allosteric Modulators of 5-HT₃ Receptors

Chemotherapy-induced nausea and vomiting (CINV) remain the most common and devastating side-effects associated with cancer chemotherapy. In recent decades, several lines of research emphasize the importance of 5-hydroxytryptamine3 (5-HT3; serotonin) receptors in the pathogenesis and treatment of CINV. 5-HT₃ receptors are members of ligand-gated ion channels that mediate the rapid and transient membrane-depolarizing effect of 5-HT in the central and peripheral nervous system. These receptors play important roles in nausea and vomiting, as well as regulation of peristalsis and pain transmission. The development of antagonists for 5-HT₃ receptor dramatically improved the treatment of CINV in cancer patients. In fact, the most common use of 5-HT₃ receptor antagonists to date is the treatment of nausea and vomiting. In recent years, there has been an increasing tendency to use natural plant products as important therapeutic entities in the treatment of various diseases. In this article, we examined the results of earlier studies on the actions of natural compounds on the functional properties of 5-HT₃ receptors. It is likely that these natural modulators of 5-HT₃ receptors can be employed as lead structures for the synthesis of therapeutic agents for treating CINV in future clinical studies.

B-973, a Novel α7 nAChR Ago-PAM: Racemic and Asymmetric Synthesis, Electrophysiological Studies, and in Vivo Evaluation

We report here the total synthesis of B-973 (five steps), a recently identified α7 nAChR ago-PAM, its enantiomeric resolution, and its electrophysiological characterization in Xenopus oocytes to identify (-)-B-973B as the bioactive enantiomer. The asymmetric synthesis of B-973B was accomplished in 99% ee, and X-ray crystallography studies revealed its absolute "S" stereochemistry. B-973B was effective in attenuating pain behavior and decreasing paw edema (formalin test), and its analgesic effects were mediated through α7 nAChR.

Nicotine improves probabilistic reward learning in wildtype but not alpha7 nAChR null mutants, yet alpha7 nAChR agonists do not improve probabilistic learning

Cognitive impairments, e.g., reward learning, are present in various psychiatric disorders and warrant treatment. Improving reward-related learning could synergistically enhance psychosocial treatments and cognition generally. A critical first step is to understand the mechanisms underlying reward learning. The dopamine system has been implicated in such learning, but less known is how indirect activation of this system may affect reward learning. We determined the role of alpha7 nicotinic acetylcholine receptors (nAChR) on a probabilistic reversal learning task (PRLT) in mice that includes reward and punishment. Male alpha7 knockout (KO), heterozygous (HT), and wildtype (WT) littermate mice (n = 84) were treated with vehicle, 0.03, or 0.3 mg/kg nicotine. Two cohorts of C57BL/6NJ male mice were treated with various alpha7 nAChR ligands, including the full agonists PNU282877 and AR-R-17779, the positive allosteric modulator CCMI, the partial agonist SSR180711, and the antagonist methyllycaconitine. All mice were then tested in the PRLT. Nicotine (0.3 mg/kg) significantly improved initial reward learning in alpha7 WT and HT mice but did not improve learning in KO mice, suggesting an involvement of the alpha7 nAChR in the pro-learning effects of nicotine. Neither alpha7 nAChR treatments (PNU282987, AR-R-17779, CCMI, SSR180711, nor methyllycaconitine) affected mouse PRLT performance however. Nicotine improved reward learning via a mechanism that may include alpha7 nAChRs. This improvement unlikely relied solely on alpha7 nAChRs however, since no alpha7 nAChR ligand improved reward learning in normal mice. Future assessments of the effects of other nAChR subtypes on reward learning are needed.

The Antinociceptive and Anti-Inflammatory Properties of the α7 nAChR Weak Partial Agonist p-CF3 N,N-diethyl-N'-phenylpiperazine

Chronic pain and inflammatory diseases can be regulated by complex mechanisms involving α7 nicotinic acetylcholine receptors (nAChRs), making this subtype a promising drug target for anti-inflammatory therapies. Recent evidence suggests that suchtreatment of inflammatory pain may rely on metabotropic-like rather than ionotropic activation of the α7 receptor subtype in non-neuronal cells. We previously identified para-trifluoromethyl (p-CF3) N,N-diethyl-N'-phenylpiperazinium (diEPP) iodide to be among the compounds classified as silent agonists, which are very weak α7 partial agonists that are able to induce positive allosteric modulator (PAM)-sensitive desensitization. Such drugs have been shown to selectively promote α7 ionotropic-independent functions. Therefore, we here further investigated the electrophysiological profile of p-CF3 diEPP and its in vivo antinociceptive activity using Xenopus oocytes expressing α7, α4β2, or α3β4 nAChRs. The evoked currents confirmed p-CF3 diEPP to be α7-selective with a maximal agonism 5% that of acetylcholine (ACh). Coapplication of p-CF3 diEPP with the type II PAM 4-naphthalene-1-yl-3a,4,5,9b-tetrahydro-3-H-cyclopenta[c]quinoline-8-sulfonic acid amide (TQS) produced desensitization that could be converted to PAM-potentiated currents, which at a negative holding potential were up to 13-fold greater than ACh controls. Voltage-dependence experiments indicated that channel block may limit both control ACh and TQS-potentiated responses. Although no p-CF3 diEPP agonist activity was detected for the heteromeric nAChRs, it was a noncompetitive antagonist of these receptors. The compound displayed remarkable antihyperalgesic and antiedema effects in in vivo assays. The antinociceptive activity was dose and time dependent. The anti-inflammatory components were sensitive to the α7-selective antagonist methyllycaconitine, which supports the idea that these effects are mediated by the α7 nAChR.

Insights Into Nicotinic Receptor Signaling in Nicotine Addiction: Implications for Prevention and Treatment

BACKGROUND

Nicotinic acetylcholine receptors (nAChRs) belong to the Cys-loop ligandgated ion-channel (LGIC) superfamily, which also includes the GABA, glycine, and serotonin receptors. Many nAChR subunits have been identified and shown to be involved in signal transduction on binding to them of either the neurotransmitter acetylcholine or exogenous ligands such as nicotine. The nAChRs are pentameric assemblies of homologous subunits surrounding a central pore that gates cation flux, and they are expressed at neuromuscular junctions throughout the nervous system.

METHODS AND RESULTS

Because different nAChR subunits assemble into a variety of pharmacologically distinct receptor subtypes, and different nAChRs are implicated in various physiological functions and pathophysiological conditions, nAChRs represent potential molecular targets for drug addiction and medical therapeutic research. This review intends to provide insights into recent advances in nAChR signaling, considering the subtypes and subunits of nAChRs and their roles in nicotinic cholinergic systems, including structure, diversity, functional allosteric modulation, targeted knockout mutations, and rare variations of specific subunits, and the potency and functional effects of mutations by focusing on their effects on nicotine addiction (NA) and smoking cessation (SC). Furthermore, we review the possible mechanisms of action of nAChRs in NA and SC based on our current knowledge.

CONCLUSION

Understanding these cellular and molecular mechanisms will lead to better translational and therapeutic operations and outcomes for the prevention and treatment of NA and other drug addictions, as well as chronic diseases, such as Alzheimer's and Parkinson's. Finally, we put forward some suggestions and recommendations for therapy and treatment of NA and other chronic diseases.

Effects of alpha-7 nicotinic allosteric modulator PNU 120596 on depressive-like behavior after lipopolysaccharide administration in mice

Evidence suggests that α7 nicotinic acetylcholine receptor (α7 nAChR) in the central nervous system has a critical role in the regulation of microglial function and neuroinflammation associated with the pathophysiology of major depressive disorder. The objectives of the present study were to determine the effects of PNU 120596, an α7 nAChR positive allosteric modulator (PAM), on depressive-like behavior and expression of ionized calcium binding adaptor molecule 1 (Iba-1), a microglial marker, in male C57BL/6J mice following lipopolysaccharide (LPS) administration, an animal model for depressive-like behavior. Forced swim test (FST), tail suspension test (TST), and sucrose preference test were used to determine the effects of PNU 120596 on depressive-like behavior, measured by increased immobility time or decreased sucrose preference. We also examined the effects of PNU 120596 on Iba-1 expression by using Western blot analysis and immunofluorescence staining in the hippocampus and prefrontal cortex, the brain regions implicated in major depressive disorder. Administration of LPS (1 mg/kg, i.p.) significantly increased immobility time during FST and TST and decreased sucrose preference. The PNU 120596 (1 or 4 mg/kg, i.p.) dose-dependently prevented LPS-induced depressive-like behavior during FST, TST, and sucrose preference test. The PNU 120596 (1 or 4 mg/kg) alone did not show any significant alteration on immobility time and sucrose preference. Pretreatment of methyllycaconitine (3 mg/kg, i.p.), an α7 nAChR antagonist, significantly prevented the antidepressant-like effects of PNU (4 mg/kg). Similarly, the PNU 120596 (4 mg/kg, i.p.) significantly reduced LPS-induced increased expression of Iba-1 in the hippocampus or prefrontal cortex. Overall, these results suggest that PNU 120596 reduces LPS-induced depressive-like behavior and microglial activation in the hippocampus and prefrontal cortex in mice. Therefore, α7 nAChR PAMs could be developed as potential therapeutic utility for the treatment of major depressive disorder in humans.

Identification by virtual screening and functional characterisation of novel positive and negative allosteric modulators of the α7 nicotinic acetylcholine receptor

Several previous studies have demonstrated that the activity of neurotransmitters acting on ligand-gated ion channels such as the nicotinic acetylcholine receptor (nAChR) can be altered by compounds binding to allosteric modulatory sites. In the case of α7 nAChRs, both positive and negative allosteric modulators (PAMs and NAMs) have been identified and have attracted considerable interest. A recent study, employing revised structural models of the transmembrane domain of the α7 nAChR in closed and open conformations, has provided support for an inter-subunit transmembrane allosteric binding site (Newcombe et al 2017). In the present study, we have performed virtual screening of the DrugBank database using pharmacophore queries that were based on the predicted binding mode of PAMs to α7 nAChR structural models. A total of 81 compounds were identified in the DrugBank database, of which the 25 highest-ranked hits corresponded to one of four previously-identified therapeutic compound groups (carbonic anhydrase inhibitors, cyclin-dependent kinase inhibitors, diuretics targeting the Na⁺-K⁺-Cl^- cotransporter, and fluoroquinolone antibiotics targeting DNA gyrase). The top-ranked compound from each of these four groups (DB04763, DB08122, furosemide and pefloxacin, respectively) was tested for its effects on human α7 nAChR expressed in Xenopus oocytes using two-electrode voltage-clamp electrophysiology. These studies, conducted with wild-type, mutant and chimeric receptors, resulted in all four compounds exerting allosteric modulatory effects. While DB04763, DB08122 and pefloxacin were antagonists, furosemide potentiated ACh responses. Our findings, supported by docking studies, are consistent with these compounds acting as PAMs and NAMs of the α7 nAChR via interaction with a transmembrane site.

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Identification of α7 nAChR positive and negative allosteric modulators.

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Furosemide is a positive allosteric modulator of α7 nAChRs.

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DB04763, DB08122 and pefloxacin are negative allosteric modulators of α7 nAChRs.

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Modulation of α7 nAChRs by an allosteric transmembrane site.

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.

Pharmacological modulation of the α7 nicotinic acetylcholine receptor in a mouse model of mecamylamine-precipitated nicotine withdrawal

RATIONALE

Recent preclinical data has implicated the α7 nicotinic acetylcholine receptor (nAChR) as a target in modulating nicotine reward. However, the role of the channel properties of the α7 nAChR in nicotine withdrawal is unknown.

OBJECTIVES

This study aimed to investigate the impact of α7 nAChR pharmacological modulation on mecamylamine-precipitated nicotine withdrawal behaviors in mice by using positive allosteric modulators (PAMs).

METHODS

The effect of the orthosteric α7 nAChR full agonist PNU282987 (1, 3, 9 mg/kg, s.c.), type I α7 PAM NS1738 (1 and 10 mg/kg; i.p.) and the type II α7 PAM PNU120596 (3 and 9 mg/kg, i.p.) on anxiety-like behavior, somatic signs, and hyperalgesia was measured in mice undergoing mecamylamine-precipitated nicotine withdrawal. Mice were infused with 24 mg/kg/day nicotine or saline for 14 days using s.c. osmotic minipumps. Nicotine withdrawal signs were precipitated upon administration of the non-selective nAChR antagonist mecamylamine (3.5 mg/kg, i.p.).

RESULTS

Anxiety-like behavior in nicotine withdrawn mice was only attenuated by PNU282987 in a dose-related fashion. Somatic signs were reduced by PNU282987 and NS1738. PNU120596 was the only compound that reversed precipitated nicotine withdrawal-induced hyperalgesia.

CONCLUSIONS

Taken together, our results suggest that modulation of the α7 nAChR can play important roles in mecamylamine-precipitated nicotine withdrawal behaviors in mice. In addition, the effects of PAMs in this study suggest that endogenous acetylcholine/choline tone is sufficient to attenuate some aspects of precipitated nicotine withdrawal. These findings highlight a beneficial effect of using α7 nAChR PAMs in some aspects of precipitated nicotine withdrawal.

Nicotinic acetylcholine receptors at the single-channel level

Over the past four decades, the patch clamp technique and nicotinic ACh (nACh) receptors have established an enduring partnership. Like all good partnerships, each partner has proven significant in its own right, while their union has spurred innumerable advances in life science research. A member and prototype of the superfamily of pentameric ligand-gated ion channels, the nACh receptor is a chemo-electric transducer, binding ACh released from nerves and rapidly opening its channel to cation flow to elicit cellular excitation. A subject of a Nobel Prize in Physiology or Medicine, the patch clamp technique provides unprecedented resolution of currents through single ion channels in their native cellular environments. Here, focusing on muscle and α7 nACh receptors, we describe the extraordinary contribution of the patch clamp technique towards understanding how they activate in response to neurotransmitter, how subtle structural and mechanistic differences among nACh receptor subtypes translate into significant physiological differences, and how nACh receptors are being exploited as therapeutic drug targets.

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/.

Aminobenzisoxazole compounds as agonists of α7 nicotinic acetylcholine receptors: a patent evaluation (WO 2017027600)

INTRODUCTION

alpha 7 subtype nicotinic acetylcholine receptor (α7 nAChR) ligands, that is, ligands that interact with the orthosteric or allosteric binding sites of α7 nAChR, hold great potential for several therapeutic applications. Numerous compounds have been designed targeting α7 nAChR but most of them cannot be used therapeutically for various reasons. Areas covered: The patent application describes a series of germinal substituted aminobenzisoxazole compounds as α7 nAChR ligands. These compounds were claimed as potential therapeutics for treating and/or improving cognitive function. All of the (R)-stereoisomer presented high binding activities for α7 nAChR and several compounds displayed excellent selectivity over 5-HT₃R. Expert opinion: The privileged structure-derived modification via bioisosterism and scaffold hopping is an important approach for seeking novel α7 nAChR ligands. The claimed germinal substituted aminobenzisoxazole derivatives with low tPSA values as well as low number of hydrogen bond donors and acceptors are supposed to have sufficient BBB penetration. Although there is a lack of essential biological data and the molecular mechanisms are not clear, these compounds stand for a new type of α7 nAChR ligands and deserve further studies.

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.

Diversity of Nicotinic Acetylcholine Receptor Positive Allosteric Modulators Revealed by Mutagenesis and a Revised Structural Model

By combining electrophysiological and computational approaches we have examined a series of positive allosteric modulators (PAMs) acting on the human α7 nicotinic acetylcholine receptor (nAChR). Electrophysiological studies have focused on three α7-selective PAMs (A-867744, TBS-516, and TQS) that display similar effects on wild-type α7 nAChRs. In addition to potentiating agonist-evoked responses, all three compounds reduce receptor desensitization and, consequently, are classed as type II PAMs. Despite having similar effects on wild-type receptors, A-867744 was found to have profoundly differing effects on mutated receptors compared with TBS-516 and TQS, a finding that is consistent with previous studies indicating that A-867744 may have a different mechanism of action compare with other α7-selective type II PAMs. Due to evidence that these PAMs bind within the α7 nAChR transmembrane region, we generated and validated new structural models of α7. Importantly, we have corrected a previously identified error in the transmembrane region of the original cryo-electron microscopy Torpedo model; the only pentameric ligand-gated ion channel imaged in a native lipid membrane. Real-space refinement was used to generate closed and open conformations on which the α7 models were based. Consensus docking with an extended series of PAMs with chemical similarity to A-867744, TBS-516, and TQS suggests that all bind to a broadly similar intersubunit transmembrane site. However, differences in the predicted binding of A-867744, compared with TBS-516 and TQS, may help to explain the distinct functional effects of A-867744. Thus, our revised structural models may provide a useful tool for interpreting functional effects of PAMs.

LY2087101 and dFBr share transmembrane binding sites in the (α4)3(β2)2 Nicotinic Acetylcholine Receptor

Positive allosteric modulators (PAMs) of nicotinic acetylcholine receptors (nAChRs) have potential therapeutic application in neuropathologies associated with decrease in function or loss of nAChRs. In this study, we characterize the pharmacological interactions of the nAChRs PAM, LY2087101, with the α4β2 nAChR using mutational and computational analyses. LY2087101 potentiated ACh-induced currents of low-sensitivity (α4)3(β2)2 and high-sensitivity (α4)2(β2)3 nAChRs with similar potencies albeit to a different maximum potentiation (potentiation I _max  = ~840 and 450%, respectively). Amino acid substitutions within the α4 subunit transmembrane domain [e.g. α4Leu256 and α4Leu260 within the transmembrane helix 1 (TM1); α4Phe316 within the TM3; and α4Gly613 within TM4] significantly reduced LY2087101 potentiation of (α4)3(β2)2 nAChR. The locations of these amino acid residues and LY2087101 computational docking analyses identify two LY2087101 binding sites: an intrasubunit binding site within the transmembrane helix bundle of α4 subunit at the level of α4Leu260/α4Phe316 and intersubunit binding site at the α4:α4 subunit interface at the level of α4Leu256/α4Ile315 with both sites extending toward the extracellular end of the transmembrane domain. We also show that desformylflustrabromine (dFBr) binds to these two sites identified for LY2087101. These results provide structural information that are pertinent to structure-based design of nAChR allosteric modulators.

The role of alpha7 nicotinic acetylcholine receptors in inflammatory bowel disease: involvement of different cellular pathways

INTRODUCTION

Autonomic imbalance plays a pivotal role in the pathophysiology of inflammatory bowel diseases (IBD). The central nervous system (CNS) cooperates dynamically with the immune system to regulate inflammation through humoral and neural pathways. In particular, acetylcholine (Ach), the main neurotransmitter in the vagus nerve, decreases the production of pro-inflammatory cytokines through a mechanism dependent on the α7 nicotinic Ach receptors (α7nAChRs). Areas covered: Here, we review the evidence for involvement of the cholinergic anti-inflammatory pathway (CAP) in IBD. We also elaborate the role of α7nAChRs and subsequent cellular pathways in CAP. Finally, we review potential therapeutic implications of modulators of these receptors. Expert opinion: Alpha7nAChR modulators possess both cognitive improving and anti-inflammatory properties. Although, these agents demonstrated therapeutic benefits in experimental models, their efficacy has not always been translated in clinical trials. Thus, development of more specific α7nAChR ligands as well as more experimental studies and better controlled trials, especially in the field of IBD, are encouraged for a progress in this field.

New Insights on Neuronal Nicotinic Acetylcholine Receptors as Targets for Pain and Inflammation: A Focus on α7 nAChRs

BACKGROUND

Nicotine and nicotinic acetylcholine receptors (nAChRs) have been explored for the past three decades as targets for pain control. The aim of this review is to introduce readers particularly to α7 nAChRs in a perspective of pain and its modulation.

METHODS

Developments for α7 nAChR modulators and recent animal studies related to pain are reviewed.

RESULTS

Accumulating evidences suggest that selective ligands for α7 nAChRs hold promise in the treatment of chronic pain conditions as they lack many of side effects associated with other nicotinic receptor types.

CONCLUSION

This review provides the reader recent insights on α7 nAChRs from structure and function to the latest findings on the pharmacology and therapeutic targeting of these receptors for the treatment of pain and inflammation.

Counteracting desensitization of human α7-nicotinic acetylcholine receptors with bispyridinium compounds as an approach against organophosphorus poisoning

Irreversible inhibition of acetylcholinesterase (AChE) resulting in accumulation of acetylcholine and overstimulation of muscarinic and nicotinic receptors accounts for the acute toxicity of organophosphorus compounds (OP). Accordingly, the mainstay pharmacotherapy against poisoning by OP comprises the competitive muscarinic acetylcholine receptor antagonist atropine to treat muscarinic effects and, in addition, oximes to reactivate inhibited AChE. A therapeutic gap still remains in the treatment of desensitized nicotinic acetylcholine receptors following OP exposure. Hereby, nicotinic effects result in paralysis of the central and peripheral respiratory system if untreated. Thus, these receptors pose an essential target for therapeutic indication to address these life-threatening nicotinic symptoms of the cholinergic crisis. Identification of ligands regulating dynamic transitions between functional states by binding to modulatory sites appears to be a promising strategy for therapeutic intervention. In this patch clamp study, the ability of differently substituted bispyridinium non-oximes to "resensitize" i.e. to recover the activity of desensitized human homomeric α7-type nAChRs stably transfected in CHO cells was investigated and compared to the already described α7-specific positive allosteric modulator PNU-120596. The structures of these bispyridinium analogues were based on the lead structure of the tert-butyl-substituted bispyridinium propane MB327, which has been shown to have a positive therapeutic effect due to a non-competitive antagonistic action at muscle-type nAChRs in vivo and has been found to have a positive allosteric activity at neuronal receptors in vitro. Prior to test compounds, desensitization of hα7-nAChRs was verified by applying an excess of nicotine revealing activation at low, and desensitization at high concentrations. Thereby, desensitization could be reduced by modulation with PNU-120596. Desensitization was further verified by dose-response profiles of agonists, carbamoylcholine and epibatidine in the absence and presence of PNU-120596. Although less pronounced than PNU-120596 and the lead structure MB327, bispyridinium compounds, particularly those substituted at position 3 and 4, resensitized the nicotine desensitized hα7-nAChRs in a concentration-dependent manner and prolonged the mean channel open time. In summary, identification of more potent compounds able to restore nAChR function in OP intoxication is needed for development of a putative efficient antidote.