Repeated administration of alpha7 nicotinic acetylcholine receptor (nAChR) agonists, but not positive allosteric modulators, increases alpha7 nAChR levels in the brain

The role of nicotinic acetylcholine receptors in the pathophysiology and pharmacotherapy of autism spectrum disorder: Focus on α7 nicotinic receptors

Postmortem studies have revealed that brains of individuals with autism spectrum disorder (ASD) exhibit abnormalities in various components of the cholinergic system including cholinergic receptors, projections, and nuclei. Deletions in the 15q13.3 region which encompasses CHRNA7, the gene that encodes the α7-nACh receptor, have been linked to various neurodevelopmental disorders, including ASD. In addition, the involvement of α7-nACh receptors in biological phenomena known to play a role in the pathophysiology of ASD such as cognitive functions, learning, memory, neuroinflammation, and oxidative stress, as well as the excitation-inhibition balance in neuronal circuits and maternal immune activation have been reported in previous studies. Furthermore, evolving preclinical and clinical literature supports the potential therapeutic benefits of using selectively acting cholinergic compounds, particularly those targeting the α7-nACh receptor subtype, in the treatment of ASD. This study reviews the previous literature on the involvement of nACh receptors in the pathophysiology of ASD and focuses on the α7-nACh receptor as a potential therapeutic target.

N1H- and N1-Substituted Phenylguanidines as α7 Nicotinic Acetylcholine (nACh) Receptor Antagonists: Structure-Activity Relationship Studies

We previously reported that N-(3-chlorophenyl)guanidine (1) represents a novel α7 nicotinic ACh (nACh) receptor antagonist chemotype. In the present study, a small series of compounds was synthesized with the intent to investigate the structure-activity relationship (SAR). Preliminary data suggested that the N-methyl analog of 1, 2, was several times more potent. Therefore, the chloro group at the aryl 3-position of 1 and its N₁-methyl counterpart 2 were replaced with a number of substituents considering the electronic, lipophilic, and steric nature of the substituents. The potencies of the compounds to inhibit acetylcholine (ACh)-induced responses were obtained in Xenopus laevis oocytes expressing human α7 nicotinic ACh receptors (nAChRs) using a two-electrode voltage-clamp assay. We found that the nature of the 3-position substituents had relatively little (i.e., <10-fold) effect on potency, and the presence of an N₁-isopropyl substituent was tolerated. Here, we report the first SAR investigation of this novel α7 nAChR antagonist chemotype.

The Spinal α7-Nicotinic Acetylcholine Receptor Contributes to the Maintenance of Cancer-Induced Bone Pain

Introduction

Cancer-induced bone pain (CIBP) is acknowledged as a multifactorial chronic pain that tortures advanced cancer patients, but existing treatment strategies for CIBP have not been satisfactory yet. Investigators have demonstrated that the activation of α7-nAChRs exerts analgesic effects in some chronic pain models. However, the role of spinal α7-nAChRs in CIBP remains unknown. This study was designed to investigate the role of α7-nAChRs in a well-established CIBP model induced by Walker 256 rat mammary gland carcinoma cells.

Methods

The paw withdrawal threshold (PWT) of the ipsilateral hind paw was measured using von Frey filament. The expressions of spinal α7-nAChRs and NF-κB were measured with Western blotting analysis. Immunofluorescence was employed to detect the expression of α7-nAChRs and co-expressed of α7-nAChRs with NeuN or GFAP or Iba1.

Results

Experiment results showed that the expression of spinal α7-nAChRs was significantly downregulated over time in CIBP rats, and in both CIBP rats and sham rats, most of the α7-nAChRs located in neurons. Behavioral data suggested PNU-282,987, a selective α7-nAChRs agonist, dose-dependently produced analgesic effect and positive allosteric modulator could intensify its effects. Further, repeated administration of PNU-282,987 reversed the expression of α7-nAChRs, inhibited the nuclear factor kappa B (NF-κB) signaling pathway, and attenuates CIBP-induced mechanical allodynia state as well.

Conclusion

These results suggest that the reduced expression of spinal α7-nAChRs contributes to the maintenance of CIBP by upregulating NF-κB expression, which implying a novel pharmacological therapeutic target for the treatment of CIBP.

(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.

Positive allosteric modulation of the α7 nicotinic acetylcholine receptor as a treatment for cognitive deficits after traumatic brain injury

Cognitive impairments are a common consequence of traumatic brain injury (TBI). The hippocampus is a subcortical structure that plays a key role in the formation of declarative memories and is highly vulnerable to TBI. The α7 nicotinic acetylcholine receptor (nAChR) is highly expressed in the hippocampus and reduced expression and function of this receptor are linked with cognitive impairments in Alzheimer's disease and schizophrenia. Positive allosteric modulation of α7 nAChRs with AVL-3288 enhances receptor currents and improves cognitive functioning in naïve animals and healthy human subjects. Therefore, we hypothesized that targeting the α7 nAChR with the positive allosteric modulator AVL-3288 would enhance cognitive functioning in the chronic recovery period of TBI. To test this hypothesis, adult male Sprague Dawley rats received moderate parasagittal fluid-percussion brain injury or sham surgery. At 3 months after recovery, animals were treated with vehicle or AVL-3288 at 30 min prior to cue and contextual fear conditioning and the water maze task. Treatment of TBI animals with AVL-3288 rescued learning and memory deficits in water maze retention and working memory. AVL-3288 treatment also improved cue and contextual fear memory when tested at 24 hr and 1 month after training, when TBI animals were treated acutely just during fear conditioning at 3 months post-TBI. Hippocampal atrophy but not cortical atrophy was reduced with AVL-3288 treatment in the chronic recovery phase of TBI. AVL-3288 application to acute hippocampal slices from animals at 3 months after TBI rescued basal synaptic transmission deficits and long-term potentiation (LTP) in area CA1. Our results demonstrate that AVL-3288 improves hippocampal synaptic plasticity, and learning and memory performance after TBI in the chronic recovery period. Enhancing cholinergic transmission through positive allosteric modulation of the α7 nAChR may be a novel therapeutic to improve cognition after TBI.

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.

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.

Wheel running during chronic nicotine exposure is protective against mecamylamine-precipitated withdrawal and up-regulates hippocampal α7 nACh receptors in mice

BACKGROUND AND PURPOSE

Evidence suggests that exercise decreases nicotine withdrawal symptoms in humans; however, the mechanisms mediating this effect are unclear. We investigated, in a mouse model, the effect of exercise intensity during chronic nicotine exposure on nicotine withdrawal severity, binding of α4β2*, α7 nicotinic acetylcholine (nAChR), μ-opioid (μ receptors) and D2 dopamine receptors and on brain-derived neurotrophic factor (BDNF) and plasma corticosterone levels.

EXPERIMENTAL APPROACH

Male C57Bl/6J mice treated with nicotine (minipump, 24 mg·kg-1 ·day-1 ) or saline for 14 days underwent one of three concurrent exercise regimes: 24, 2 or 0 h·day-1 voluntary wheel running. Mecamylamine-precipitated withdrawal symptoms were assessed on day 14. Quantitative autoradiography of α4β2*, α7 nAChRs, μ receptors and D2 receptor binding was performed in brain sections of these mice. Plasma corticosterone and brain BDNF levels were also measured.

KEY RESULTS

Nicotine-treated mice undertaking 2 or 24 h·day-1 wheel running displayed a significant reduction in withdrawal symptom severity compared with the sedentary group. Wheel running induced a significant up-regulation of α7 nAChR binding in the CA2/3 area of the hippocampus of nicotine-treated mice. Neither exercise nor nicotine treatment affected μ or D2 receptor binding or BDNF levels. Nicotine withdrawal increased plasma corticosterone levels and α4β2* nAChR binding, irrespective of exercise regimen.

CONCLUSIONS AND IMPLICATIONS

We demonstrated for the first time a profound effect of exercise on α7 nAChRs in nicotine-dependent animals, irrespective of exercise intensity. These findings shed light onto the mechanism underlining the protective effect of exercise on the development of nicotine dependence.

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.

Alpha-7 nicotinic acetylcholine receptor agonist treatment in a rat model of Huntington's disease and involvement of heme oxygenase-1

Neuroinflammation is a common element involved in the pathophysiology of neurodegenerative diseases. We recently reported that repeated alpha-7 nicotinic acetylcholine receptor (α7nAChR) activations by a potent agonist such as PHA 543613 in quinolinic acid-injured rats exhibited protective effects on neurons. To further investigate the underlying mechanism, we established rat models of early-stage Huntington's disease by injection of quinolinic acid into the right striatum and then intraperitoneally injected 12 mg/kg PHA 543613 or sterile water, twice a day during 4 days. Western blot assay results showed that the expression of heme oxygenase-1 (HO-1), the key component of the cholinergic anti-inflammatory pathway, in the right striatum of rat models of Huntington's disease subjected to intraperitoneal injection of PHA 543613 for 4 days was significantly increased compared to the control rats receiving intraperitoneal injection of sterile water, and that the increase in HO-1 expression was independent of change in α7nAChR expression. These findings suggest that HO-1 expression is unrelated to α7nAChR density and the increase in HO-1 expression likely contributes to α7nAChR activation-related neuroprotective effect in early-stage Huntington's disease.

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.

Perinatal nicotine treatment induces transient increases in NACHO protein levels in the rat frontal cortex

The nicotinic acetylcholine receptor (nAChR) regulator chaperone (NACHO) was recently identified as an important regulator of nAChR maturation and surface expression. Here we show that NACHO levels decrease during early postnatal development in rats. This decrease occurs earlier and to a greater degree in the frontal cortex (FC) compared with the hippocampus (HIP). We further show that rats exposed to nicotine during pre- and postnatal development exhibit significantly higher NACHO levels in the FC at postnatal day (PND) 21, but not at PND60. Repeated exposure to nicotine selectively during early (PND8-14) or late (PND54-60) postnatal stages did not affect NACHO protein levels in the FC or HIP, neither did exposure to high doses of the selective α7 nAChR agonists SSR180711, A-582941, or PNU-282987. However, we found significantly increased NACHO protein levels in the FC of PND36 rats after a single exposure to a combination of nicotine and the type II α7 nAChR positive allosteric modulator (PAM) PNU-120596, but not the type I PAM AVL-3288. These findings suggest that exposure to nAChR agonism affects NACHO protein levels, and that this effect is more pronounced during pre- or early postnatal development. The effect of PNU-120596 further suggests that the increase in NACHO expression is caused by activation rather than desensitization of nAChRs.

The human CHRNA7 and CHRFAM7A genes: A review of the genetics, regulation, and function

The human α7 neuronal nicotinic acetylcholine receptor gene (CHRNA7) is ubiquitously expressed in both the central nervous system and in the periphery. CHRNA7 is genetically linked to multiple disorders with cognitive deficits, including schizophrenia, bipolar disorder, ADHD, epilepsy, Alzheimer's disease, and Rett syndrome. The regulation of CHRNA7 is complex; more than a dozen mechanisms are known, one of which is a partial duplication of the parent gene. Exons 5-10 of CHRNA7 on chromosome 15 were duplicated and inserted 1.6 Mb upstream of CHRNA7, interrupting an earlier partial duplication of two other genes. The chimeric CHRFAM7A gene product, dupα7, assembles with α7 subunits, resulting in a dominant negative regulation of function. The duplication is human specific, occurring neither in primates nor in rodents. The duplicated α7 sequence in exons 5-10 of CHRFAM7A is almost identical to CHRNA7, and thus is not completely queried in high throughput genetic studies (GWAS). Further, pre-clinical animal models of the α7nAChR utilized in drug development research do not have CHRFAM7A (dupα7) and cannot fully model human drug responses. The wide expression of CHRNA7, its multiple functions and modes of regulation present challenges for study of this gene in disease. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

The therapeutic potential of α7 nicotinic acetylcholine receptor (α7 nAChR) agonists for the treatment of the cognitive deficits associated with schizophrenia

Homomeric α7 nicotinic acetylcholine receptors (α7 nAChRs) have implications in the regulation of cognitive processes such as memory and attention, and have shown promise as a therapeutic target for the treatment of the cognitive deficits associated with schizophrenia. Multiple α7 nAChR agonists have entered human trials; however, unfavorable side effects and pharmacokinetic issues have hindered the development of a clinical α7 nAChR agonist. Currently, EVP-6124 is in phase III clinical trials, and several other α7 nAChR agonists (GTS-21 and AQW051) are in earlier stages of development. This review will summarize the recent advances and failures of α7 nAChR agonists in clinical trials for the treatment of the aforementioned pathology.

Diverse strategies targeting α7 homomeric and α6β2* heteromeric nicotinic acetylcholine receptors for smoking cessation

Preclinical studies suggest that a diversity of nicotinic acetylcholine receptors (nAChRs) with different sensitivities to nicotine may contribute to tobacco addiction. Using rodent intravenous nicotine self-administration as a preclinical model with good predictive validity for therapeutic efficacy for tobacco cessation, investigators have identified heteromeric α6β2* and homomeric α7 nAChRs as promising novel therapeutic targets to promote smoking abstinence (*denotes possible assembly with other subunits). The data suggest that diverse strategies that target these subclasses of nAChRs, namely inhibition of α6β2* nAChRs and stimulation of α7 nAChRs, will support tobacco cessation. α6β2* nAChRs, members of the high-affinity family of β2* nAChRs, function similarly to α4β2* nAChRs, the primary target of the FDA-approved drug varenicline, but have a much more selective neuroanatomical pattern of expression in catecholaminergic nuclei. Although activation of β2* nAChRs facilitates nicotine self-administration, stimulation of α7 nAChRs appears to negatively modulate both nicotine reinforcement and β2* nAChR function in the mesolimbic dopamine system. Although challenges and caveats must be considered in the development of therapeutics that target these nAChR subpopulations, an accumulation of data suggests that α7 nAChR agonists, partial agonists, or positive allosteric modulators and α6β2* nAChR antagonists, partial agonists, or negative allosteric modulators may prove to be effective therapeutics for tobacco cessation.

Perinatal hypoxia-ischemia reduces α 7 nicotinic receptor expression and selective α 7 nicotinic receptor stimulation suppresses inflammation and promotes microglial Mox phenotype

Inflammation plays a central role in neonatal brain injury. During brain inflammation the resident macrophages of the brain, the microglia cells, are rapidly activated. In the periphery, α7 nicotinic acetylcholine receptors (α7R) present on macrophages can regulate inflammation by suppressing cytokine release. In the current study we investigated α7R expression in neonatal mice after hypoxia-ischemia (HI). We further examined possible anti-inflammatory role of α7R stimulation in vitro and microglia polarization after α7R agonist treatment. Real-time PCR analysis showed a 33% reduction in α7R expression 72 h after HI. Stimulation of primary microglial cells with LPS in combination with increasing doses of the selective α7R agonist AR-R 17779 significantly attenuated TNFα release and increased α7R transcript in microglial cells. Gene expression of M1 markers CD86 and iNOS, as well as M2 marker CD206 was not influenced by LPS and/or α7R agonist treatment. Further, Mox markers heme oxygenase (Hmox1) and sulforedoxin-1 (Srx1) were significantly increased, suggesting a polarization towards the Mox phenotype after α7R stimulation. Thus, our data suggest a role for the α7R also in the neonatal brain and support the anti-inflammatory role of α7R in microglia, suggesting that α7R stimulation could enhance the polarization towards a reparative Mox phenotype.

Effects of α7 positive allosteric modulators in murine inflammatory and chronic neuropathic pain models

Agonists and positive allosteric modulators (PAMs) of α7 nicotinic acetylcholine receptors (nAChRs) are currently being considered as novel therapeutic approaches for managing cognitive deficits in schizophrenia and Alzheimer's disease. Though α7 agonists were recently found to possess antinociceptive and anti-inflammatory properties in rodent models of chronic neuropathic pain and inflammation, the effects of α7 nAChRs PAMs on chronic pain and inflammation remain largely unknown. The present study investigated whether PAMs, by increasing endogenous cholinergic tone, potentiate α7 nAChRs function to attenuate inflammatory and chronic neuropathic pain in mice. We tested two types of PAMS, type I (NS1738) and type II (PNU-120596) in carrageenan-induced inflammatory pain and chronic constriction injury (CCI) neuropathic pain models. We found that both NS1738 and PNU-120596 significantly reduced thermal hyperalgesia, while only PNU-120596 significantly reduced edema caused by a hind paw infusion of carrageenan. Importantly, PNU-120596 reversed established thermal hyperalgesia and edema induced by carrageenan. In the CCI model, PNU-120596 had long-lasting (up to 6 h), dose-dependent anti-hyperalgesic and anti-allodynic effects after a single injection, while NS1738 was inactive. Systemic administration of the α7 nAChR antagonist MLA reversed PNU-120596's effects, suggesting the involvement of central and peripheral α7 nAChRs. Furthermore, PNU-120596 enhanced an ineffective dose of selective agonist PHA-543613 to produce anti-allodynic effects in the CCI model. Our results indicate that the type II α7 nAChRs PAM PNU-120596, but not the type I α7 nAChRs PAM NS1738, shows significant anti-edematous and anti-allodynic effects in inflammatory and CCI pain models in mice.

Differential immediate and sustained memory enhancing effects of alpha7 nicotinic receptor agonists and allosteric modulators in rats

The α7 nicotinic acetylcholine receptor (nAChR) is a potential target for the treatment of cognitive deficits in patients with schizophrenia, ADHD and Alzheimer's disease. Here we test the hypothesis that upregulation of α7 nAChR levels underlies the enhanced and sustained procognitive effect of repeated administration of α7 nAChR agonists. We further compare the effect of agonists to that of α7 nAChR positive allosteric modulators (PAMs), which do not induce upregulation of the α7 nAChR. Using the social discrimination test as a measure of short-term memory, we show that the α7 nAChR agonist A-582941 improves short-term memory immediately after repeated (7× daily), but not a single administration. The α7 nAChR PAMs PNU-120596 and AVL-3288 do not affect short-term memory immediately after a single or repeated administration. This demonstrates a fundamental difference in the behavioral effects of agonists and PAMs that may be relevant for clinical development. Importantly, A-582941 and AVL-3288 increase short-term memory 24 hrs after repeated, but not a single, administration, suggesting that repeated administration of both agonists and PAMs may produce sustained effects on cognitive performance. Subsequent [¹²⁵I]-bungarotoxin autoradiography revealed no direct correlation between α7 nAChR levels in frontal cortical or hippocampal brain regions and short-term memory with either compound. Additionally, repeated treatment with A-582941 did not affect mRNA expression of RIC-3 or the lynx-like gene products lynx1, lynx2, PSCA, or Ly6H, which are known to affect nAChR function. In conclusion, both α7 nAChR agonists and PAMs exhibit sustained pro-cognitive effects after repeated administration, and altered levels of the α7 nAChR per se, or that of endogenous regulators of nAChR function, are likely not the major cause of this effect.