Antagonizing α7 nicotinic receptors with methyllycaconitine (MLA) potentiates receptor activity and memory acquisition

Targeting α7-nAChR by galantamine mitigates reserpine-induced fibromyalgia-like symptoms in rats: Involvement of cAMP/PKA, PI3K/AKT, and M1/M2 microglia polarization

Fibromyalgia (FM) is a pain disorder marked by generalized musculoskeletal pain accompanied by depression, fatigue, and sleep disturbances. Galantamine (Gal) is a positive allosteric modulator of neuronal nicotinic acetylcholine receptors (nAChRs) and a reversible inhibitor of cholinesterase. The current study aimed to explore the therapeutic potential of Gal against reserpine (Res)-induced FM-like condition along with investigating the α7-nAChR's role in Gal-mediated effects. Rats were injected with Res (1 mg/kg/day; sc) for 3 successive days then Gal (5 mg/kg/day; ip) was given alone and with the α7-nAChR blocker methyllycaconitine (3 mg/kg/day; ip), for the subsequent 5 days. Galantamine alleviated Res-induced histopathological changes and monoamines depletion in rats' spinal cord. It also exerted analgesic effect along with ameliorating Res-induced depression and motor-incoordination as confirmed by behavioral tests. Moreover, Gal produced anti-inflammatory effect through modulating AKT1/AKT2 and shifting M1/M2 macrophage polarization. The neuroprotective effects of Gal were mediated through activating cAMP/PKA and PI3K/AKT pathways in α7-nAChR-dependent manner. Thus, Gal can ameliorate Res-induced FM-like symptoms and mitigate the associated monoamines depletion, neuroinflammation, oxidative stress, apoptosis, and neurodegeneration through α7-nAChR stimulation, with the involvement of cAMP/PKA, PI3K/AKT, and M1/M2 macrophage polarization.

Neuropharmacological Potential of Diterpenoid Alkaloids

This study provides a narrative review of diterpenoid alkaloids (DAs), a family of extremely important natural products found predominantly in some species of Aconitum and Delphinium (Ranunculaceae). DAs have long been a focus of research attention due to their numerous intricate structures and diverse biological activities, especially in the central nervous system (CNS). These alkaloids originate through the amination reaction of tetra or pentacyclic diterpenoids, which are classified into three categories and 46 types based on the number of carbon atoms in the backbone structure and structural differences. The main chemical characteristics of DAs are their heterocyclic systems containing β-aminoethanol, methylamine, or ethylamine functionality. Although the role of tertiary nitrogen in ring A and the polycyclic complex structure are of great importance in drug-receptor affinity, in silico studies have emphasized the role of certain sidechains in C13, C14, and C8. DAs showed antiepileptic effects in preclinical studies mostly through Na⁺ channels. Aconitine (1) and 3-acetyl aconitine (2) can desensitize Na⁺ channels after persistent activation. Lappaconitine (3), N-deacetyllapaconitine (4), 6-benzoylheteratisine (5), and 1-benzoylnapelline (6) deactivate these channels. Methyllycaconitine (16), mainly found in Delphinium species, possesses an extreme affinity for the binding sites of α7 nicotinic acetylcholine receptors (nAChR) and contributes to a wide range of neurologic functions and the release of neurotransmitters. Several DAs such as bulleyaconitine A (17), (3), and mesaconitine (8) from Aconitum species have a drastic analgesic effect. Among them, compound 17 has been used in China for decades. Their effect is explained by increasing the release of dynorphin A, activating the inhibitory noradrenergic neurons in the β-adrenergic system, and preventing the transmission of pain messages by inactivating the Na⁺ channels that have been stressed. Acetylcholinesterase inhibitory, neuroprotective, antidepressant, and anxiolytic activities are other CNS effects that have been investigated for certain DAs. However, despite various CNS effects, recent advances in developing new drugs from DAs were insignificant due to their neurotoxicity.

Dexmedetomidine attenuates lipopolysaccharide-induced inflammation through macrophageal IL-10 expression following α7 nAchR activation

Dexmedetomidine, a highly selective α₂-adrenoceptor agonist, has been recently reported to alleviate systemic inflammatory response induced by lipopolysaccharide (LPS), in addition to its sedative, analgesic, bradycardic and hypotensive properties. This study aimed to illustrate the molecular mechanisms underlying dexmedetomidine-induced anti-inflammation. In the LPS-pretreated mice, subcutaneous injection of dexmedetomidine reduced the spleen weight as well as serum and spleen expression of proinflammatory cytokines, tumor necrosis factor-α (TNF-α), interleukin (IL)-6 and IL-1β, and increased serum and spleen expression of IL-10, a known anti-inflammatory cytokine. In addition, dexmedetomidine-attenuated proinflammatory cytokine reduction was entirely inhibited by selective α7 nicotinic acetylcholine receptor (nAChR) antagonist methyllycaconitine but not α₂-adrenoceptor antagonist yohimbine. Dexmedetomidine also increased macrophageal IL-10 expression in the presence and absence of LPS, which was also attenuated by methyllycaconitine but not yohimbine. Furthermore, the stimulatory effect of dexmedetomidine on the expression of IL-10 was also reduced by the α7 nAChR gene silencer siRNA/α7 nAChR. Lastly, pretreatment with the IL-10 neutralizing antibody reversed dexmedetomidine-supressed expression of proinflammatory cytokines. Our findings illustrate that dexmedetomidine-induced anti-inflammation is through macrophageal expression of IL-10 following activation of α7 nAchRs but not α₂-adrenoceptors.

α7 nicotinic acetylcholine receptors in the hippocampal circuit: taming complexity

Cholinergic innervation of the hippocampus uses the neurotransmitter acetylcholine (ACh) to coordinate neuronal circuit activity while simultaneously influencing the function of non-neuronal cell types. The α7 nicotinic ACh receptor (nAChR) subtype is highly expressed throughout the hippocampus, has the highest calcium permeability compared with other subtypes of nAChRs, and is of high therapeutic interest due to its association with a variety of neurological disorders and neurodegenerative diseases. In this review, we synthesize research describing α7 nAChR properties, function, and relationship to cognitive dysfunction within the hippocampal circuit and highlight approaches to help improve therapeutic development.

Type I and type II positive allosteric modulators of α7 nicotinic acetylcholine receptors induce antidepressant-like activity in mice by a mechanism involving receptor potentiation but not neurotransmitter reuptake inhibition. Correlation with mTOR intracellular pathway activation

The aim of this study is to determine whether type I and type II positive allosteric modulators (PAMs) of α7 nicotinic acetylcholine receptors (nAChRs) induce antidepressant-like activity in mice after acute, subchronic, and chronic treatments, and to assess whether α7-PAMs inhibit neurotransmitter transporters and activate mTOR (mammalian target of rapamycin) and/or ERK (extracellular signal-regulated protein kinases) signaling. The forced swim (FST) and tail suspension (TST) test results indicated that NS-1738 (type I PAM), PNU-120596 and PAM-2 (type II PAMs) induce antidepressant-like activity after subchronic treatment, whereas PAM-2 was also active after chronic treatment. Methyllycaconitine (α7-antagonist) inhibited the observed effects, highlighting the involvement of α7 nAChRs in this process. Drug interaction studies showed synergism between PAM-2 and bupropion (antidepressant), but not between PAM-2 and DMXBA (α7-agonist). The studied PAMs showed no high affinity (< 1 µM) for the human dopamine, serotonin, and noradrenaline transporters, suggesting that transporter inhibition is not the underlying mechanism for the observed activity. To assess whether mTOR and ERK signaling pathways are involved in the activity of α7-PAMs, the phosphorylation status of key signaling nodes was determined in prefrontal cortex and hippocampus from mice chronically treated with PAM-2. In conclusion, the antidepressant-like activity of type I and type II PAMs is mediated by a mechanism involving α7 potentiation but not α7 desensitization or neurotransmitter transporter blockade, and is correlated with activation of both mTOR and ERK signaling pathways. These results support the view that α7-PAMs might be clinically used to ameliorate depression disorders .

Palonosetron/Methyllycaconitine Deactivate Hippocampal Microglia 1, Inflammasome Assembly and Pyroptosis to Enhance Cognition in a Novel Model of Neuroinflammation

Since westernized diet-induced insulin resistance is a risk factor in Alzheimer's disease (AD) development, and lipopolysaccharide (LPS) coexists with amyloid β (Aβ)1-42 in these patients, our AD novel model was developed to resemble sporadic AD by injecting LPS into high fat/fructose diet (HFFD)-fed rats. The neuroprotective potential of palonosetron and/or methyllycaconitine, 5-HT3 receptor and α7 nAChR blockers, respectively, was evaluated after 8 days of daily administration in HFFD/LPS rats. All regimens improved histopathological findings and enhanced spatial memory (Morris Water Maze); however, palonosetron alone or with methyllycaconitine promoted animal performance during novel object recognition tests. In the hippocampus, all regimens reduced the expression of glial fibrillary acidic protein and skewed microglia M1 to M2 phenotype, indicated by the decreased M1 markers and the enhanced M2 related parameters. Additionally, palonosetron and its combination regimen downregulated the expression of ASC/TMS1, as well as levels of inflammasome downstream molecules and abated cleaved caspase-1, interleukin (IL)-1β, IL-18 and caspase-11. Furthermore, ACh and 5-HT were augmented after being hampered by the insult. Our study speculates that blocking 5-HT3 receptor using palonosetron overrides methyllycaconitine to combat AD-induced neuroinflammation and inflammasome cascade, as well as to restore microglial function in a HFFD/LPS novel model for sporadic AD.

The protective effect of α7-nACh receptor and its interaction with 5-HT1B/1D receptors in acute intestinal ischemia-reperfusion injury in rats

Over the past decades, great attention has been given to the nervous system modulating effects on the immune response in inflammation-associated injuries, such as acute intestinal ischemia-reperfusion (IR). Recently, we proved the anti-inflammatory and antioxidant effects of 5-hydroxytryptamine (5-HT)1B/1D receptors in intestinal IR injury in rats. Also, the alpha7 nicotinic acetylcholine (α7-nACh) receptor has anti-inflammatory effects in different inflammation-associated injuries. Starting from these premises, we aimed to examine the function of the α7-nACh receptors and the functional interactions between the anti-inflammatory and antioxidant effects of α7-nACh and 5-HT1B/1D receptors in acute intestinal IR injury. To confirm the expression and localization of α7-nACh receptors on the ileum nerves, an immunofluorescence-based method was applied. Then, intestinal IR injury was induced by 30-min occlusion of superior mesenteric artery and reperfusion for 2 h in rats. Acute systemic administration of α7-nACh receptor agonist PNU-282987 and antagonist methyllycaconitine, and 5-HT1B/1D receptors agonist (sumatriptan) and antagonist (GR127, 935) were used in the model of intestinal IR injury. Finally, biochemical and histological parameters were assessed. Α7-nACh receptors were expressed by 9% on the ileum nerves. Likewise, activation of the α7-nACh receptor showed anti-inflammatory and antioxidant effects in intestinal IR injury but not as well as 5-HT1B/1D receptors. Interestingly, 5-HT1B/1D receptors via attenuation of glutamate (Glu) release indirectly activated the α7-nACh receptor and its protective effects against inflammation and oxidative stress. The protective effect of the α7-nACh receptor on intestinal IR injury was activated indirectly through the 5-HT1B/1D receptors' modulatory impact on Glu release.

Cynandione A and PHA-543613 inhibit inflammation and stimulate macrophageal IL-10 expression following α7 nAChR activation

Cynandione A, an acetophenone isolated from Cynanchum Wilfordii Radix, attenuates inflammation. The present study aimed to study the mechanisms underlying cynandione A-induced antiinflammation. Treatment with cynandione A and the specific α7 nicotinic acetylcholine receptor (α7 nAChR) agonist PHA-543613 remarkably reduced overexpression of proinflammatory cytokines, including tumor necrosis factor (TNF)-α, interleukin (IL)-6 and IL-1β in lipopolysaccharide (LPS)-treated RAW264.7 cells and primary peritoneal macrophages, and endotoxemic mice. Both cynandione A and PHA-543613 also stimulated IL-10 expression in naïve and LPS-treated macrophages and endotoxemic mice. Cynandione A- and PHA-543613-inhibited proinflammatory cytokine expression was completely blocked by the α7 nAChR antagonist methyllycaconitine and the IL-10 antibody. The stimulatory effect of cynandione A and PHA-543613 on IL-10 expression were suppressed by methyllycaconitine and knockdown of α7 nAChRs using siRNA/α7 nAChR. Cynandione A significantly stimulated STAT3 phosphorylation, which was attenuated by methyllycaconitine and the IL-10 neutralizing antibody. The STAT3 activation inhibitor NSC74859 also blocked cynandione A-inhibited proinflammatory cytokine expression. Taken together, our results, for the first time, demonstrate that cynandione A and PHA-543613 inhibit inflammation through macrophageal α7 nAChR activation and subsequent IL-10 expression.

Modified Snake α-Neurotoxin Averts β-Amyloid Binding to α7 Nicotinic Acetylcholine Receptor and Reverses Cognitive Deficits in Alzheimer's Disease Mice

Alzheimer's disease (AD) is the most common cause of senile dementia and one of the greatest medical, social, and economic challenges. According to a dominant theory, amyloid-β (Aβ) peptide is a key AD pathogenic factor. Aβ-soluble species interfere with synaptic functions, aggregate gradually, form plaques, and trigger neurodegeneration. The AD-associated pathology affects numerous systems, though the substantial loss of cholinergic neurons and α7 nicotinic receptors (α7AChR) is critical for the gradual cognitive decline. Aβ binds to α7AChR under various experimental settings; nevertheless, the functional significance of this interaction is ambiguous. Whereas the capability of low Aβ concentrations to activate α7AChR is functionally beneficial, extensive brain exposure to high Aβ concentrations diminishes α7AChR activity, contributes to the cholinergic deficits that characterize AD. Aβ and snake α-neurotoxins competitively bind to α7AChR. Accordingly, we designed a chemically modified α-cobratoxin (mToxin) to inhibit the interaction between Aβ and α7AChR. Subsequently, we examined mToxin in a set of original in silico, in vitro, ex vivo experiments, and in a murine AD model. We report that mToxin reversibly inhibits α7AChR, though it attenuates Aβ-induced synaptic transmission abnormalities, and upregulates pathways supporting long-term potentiation and reducing apoptosis. Remarkably, mToxin demonstrates no toxicity in brain slices and mice. Moreover, its chronic intracerebroventricular administration improves memory in AD-model animals. Our results point to unique mToxin neuroprotective properties, which might be tailored for the treatment of AD. Our methodology bridges the gaps in understanding Aβ-α7AChR interaction and represents a promising direction for further investigations and clinical development.

Cynandione A Alleviates Neuropathic Pain Through α7-nAChR-Dependent IL-10/β-Endorphin Signaling Complexes

Cynandione A, an acetophenone isolated from Cynanchum Wilfordii Radix, exhibits antineuropathic pain effect. This study further explored the target molecule and signaling mechanisms underlying cynandione-A-induced antineuropathic pain. Intrathecal injection of cynandione A significantly attenuated mechanical allodynia in neuropathic rats and substantially increased spinal expression of IL-10 and β-endorphin but not dynorphin A. Cynandione A treatment also enhanced expression of IL-10 and β-endorphin but not α7 nicotinic acetylcholine receptors (nAChRs) in cultured microglia. The IL-10 antibody attenuated cynandione-A-induced spinal or microglial gene expression of β-endorphin and mechanical allodynia, whereas the β-endorphin antiserum blocked cynandione-A-induced mechanical antiallodynia but not spinal or microglial IL-10 gene expression. The α7 nAChR antagonist methyllycaconitine significantly reduced cynandione-A-induced mechanical antiallodynia and spinal or microglial expression of IL-10 and β-endorphin. Furthermore, cynandione A stimulated microglial phosphorylation of PKA, p38, and CREB in an α7-nAChR-dependent manner, and treatment with their inhibitors attenuated cynandione-A-induced mechanical antiallodynia and spinal or microglial expression of IL-10 and β-endorphin. In addition, cynandione A stimulated spinal phosphorylation of the transcription factor STAT3, which was inhibited by methyllycaconitine, the PKA activation inhibitor or IL-10 antibody. The STAT3 inhibitor NSC74859 also abolished cynandione-A-induced mechanical antiallodynia and spinal expression of β-endorphin. These findings suggest that cynandione A suppresses neuropathic pain through α7-nAChR-dependent IL-10/β-endorphin signaling pathway in spinal microglia.

Potentiation of cognitive enhancer effects of Alzheimer's disease medication memantine by alpha7 nicotinic acetylcholine receptor agonist PHA-543613 in the Morris water maze task

RATIONALE

There are controversial pieces of evidence whether combination therapies using memantine and cholinesterase inhibitors are beneficial over their monotreatments. However, results of preclinical studies are promising when memantine is combined with agonists and allosteric modulators of the alpha7 nicotinic acetylcholine receptor (nAChR).

OBJECTIVES

Here, we tested the hypothesis that cognitive enhancer effects of memantine can be potentiated through modulating alpha7 nAChRs in a scopolamine-induced amnesia model.

METHODS

Monotreatments, as well as co-administrations of selective alpha7 nicotinic acetylcholine receptor agonist PHA-543613 and memantine were tested in the Morris water maze task in rats. The efficacy of the co-administration treatment was observed on different domains of spatial episodic memory.

RESULTS

Low dose of memantine (0.1 mg/kg) and PHA-543613 (0.3 mg/kg) successfully reversed scopolamine-induced short-term memory deficits both in monotreatments and in co-administration. When recall of information from long-term memory was tested, pharmacological effects caused by co-administration of subeffective doses of memantine and PHA-543613 exceeded that of their monotreatments.

CONCLUSION

Our results further support the evidence of beneficial interactions between memantine and alpha7 nAChR ligands and suggest a prominent role of alpha7 nAChRs in the procognitive effects of memantine.

Implications of Oligomeric Amyloid-Beta (oAβ42) Signaling through α7β2-Nicotinic Acetylcholine Receptors (nAChRs) on Basal Forebrain Cholinergic Neuronal Intrinsic Excitability and Cognitive Decline

Neuronal and network-level hyperexcitability is commonly associated with increased levels of amyloid-β (Aβ) and contribute to cognitive deficits associated with Alzheimer's disease (AD). However, the mechanistic complexity underlying the selective loss of basal forebrain cholinergic neurons (BFCNs), a well-recognized characteristic of AD, remains poorly understood. In this study, we tested the hypothesis that the oligomeric form of amyloid-β (oAβ₄₂), interacting with α7-containing nicotinic acetylcholine receptor (nAChR) subtypes, leads to subnucleus-specific alterations in BFCN excitability and impaired cognition. We used single-channel electrophysiology to show that oAβ₄₂ activates both homomeric α7- and heteromeric α7β2-nAChR subtypes while preferentially enhancing α7β2-nAChR open-dwell times. Organotypic slice cultures were prepared from male and female ChAT-EGFP mice, and current-clamp recordings obtained from BFCNs chronically exposed to pathophysiologically relevant level of oAβ₄₂ showed enhanced neuronal intrinsic excitability and action potential firing rates. These resulted from a reduction in action potential afterhyperpolarization and alterations in the maximal rates of voltage change during spike depolarization and repolarization. These effects were observed in BFCNs from the medial septum diagonal band and horizontal diagonal band, but not the nucleus basalis. Last, aged male and female APP/PS1 transgenic mice, genetically null for the β2 nAChR subunit gene, showed improved spatial reference memory compared with APP/PS1 aged-matched littermates. Combined, these data provide a molecular mechanism supporting a role for α7β2-nAChR in mediating the effects of oAβ₄₂ on excitability of specific populations of cholinergic neurons and provide a framework for understanding the role of α7β2-nAChR in oAβ₄₂-induced cognitive decline.

Higher levels of α7 nicotinic receptors, but not choline acetyltransferase, in the dorsolateral prefrontal cortex from a sub-group of patients with schizophrenia

It has been suggested the study of sub-groups within the syndrome of schizophrenia will assist in elucidating the complex pathophysiology of the syndrome. Hence, we have studied a number of cholinergic markers in the cortex from a sub-group of subjects with schizophrenia that have a marked decrease in levels of muscarinic M1 receptors (MRDS). The displacement of [³H]NMS by cortical extracts was used to measure tissue anticholinergic load, [¹²⁵I]α bungarotoxin binding was used to measure levels of the α7 nicotinic receptor (CHRNA7) and western blotting was used to measure levels of choline acetyltransferase (ChAT) 68 and 82 as well as synaptosome nerve-associated protein 25 (SNAP25). In comparing schizophrenia, MRDS and non-MRDS to controls, there were no differences in levels of ChAT 68 or 82, SNAP 25 or cholinergic load in BA 9. However, levels of CHRNA7 were higher in BA 9, but not BA 6 or 44, from subjects with MRDS. These data argue that there is no change in cholinergic innovation (measured using ChAT), presynaptic neurons (measured using SNAP25) or cholinergic load in schizophrenia, MRDS or non-MRDS. However, increased levels of CHRNA7 may be contributing to a breakdown in cholinergic homeostasis in BA 9, but not BA 6 or 44, in subjects with MRDS.

α7 nicotinic acetylcholine receptors as therapeutic targets in schizophrenia: Update on animal and clinical studies and strategies for the future

Schizophrenia is a devastating mental illness and its effective treatment is among the most challenging issues in psychiatry. The symptoms of schizophrenia are heterogeneous ranging from positive symptoms (e.g., delusions, hallucinations) to negative symptoms (e.g., anhedonia, social withdrawal) to cognitive dysfunction. Antipsychotics are effective at ameliorating positive symptoms in some patients; however, they are not reliably effective at improving the negative symptoms or cognitive impairments. The inability to address the cognitive impairments is a particular concern since they have the greatest long-term impact on functional outcomes. While decades of research have been devoted to the development of pro-cognitive agents for schizophrenia, to date, no drug has been approved for clinical use. Converging behavioral, neurobiological, and genetic evidence led to the identification of the α7-nicotinic acetylcholine receptor (α7-nAChR) as a therapeutic target several years ago and there is now extensive preclinical evidence that α7-nAChR ligands have pro-cognitive effects and other properties that should be beneficial to schizophrenia patients. However, like the other pro-cognitive strategies, no α7-nAChR ligand has been approved for clinical use in schizophrenia thus far. In this review, several topics are discussed that may impact the success of α7-nAChR ligands as pro-cognitive agents for schizophrenia including the translational value of the animal models used, clinical trial design limitations, confounding effects of polypharmacy, dose-effect relationships, and chronic versus intermittent dosing considerations. Determining the most optimal pharmacologic strategy at α7-nAChRs: agonist, positive allosteric modulator, or potentially even receptor antagonist is also discussed. article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

Nicotine Suppresses the Invasiveness of Human Trophoblasts by Downregulation of CXCL12 Expression through the Alpha-7 Subunit of the Nicotinic Acetylcholine Receptor

Smoke exposure during pregnancy has detrimental effects upon numerous fetal and neonatal outcomes. Nicotine (the main component of tobacco) has been suggested to affect placental development. During placental development, efficient invasion by trophoblasts is required for establishment of the fetus–maternal circulation. In this study we explored the regulation of trophoblast invasion by nicotine. An immortalized first trimester extravillous trophoblast cell line (HTR-8/SVneo cells) was used for all the experiments, which were treated by nicotine, methyllycaconitine, and C-X-C motif chemokine ligand 12 (CXCL12). Total RNA and protein were used to study the expressions of nicotinic acetylcholine receptors (nAChRs), and transwell assay was used to study invasiveness. Changes of RNA expression due to nicotine treatment were detected by RNA sequence. Level of CXCL12 mRNA was verified by quantitative PCR. We showed that HTR-8/SVneo expressed subunits α2–4, α7, α9, β1, and β2 of nAChRs. Nicotine downregulated CXCL12 expression and inhibited trophoblast invasion. Methyllycaconitine, as an antagonist of the α7 homopolymer, blocked the inhibitory effect of nicotine. CXCL12 could rescue the nicotine-induced inhibitory effect on invasion of HTR-8/SVneo cells. These results suggest that the α7 subunit of the nAChR has important roles in modulating trophoblast invasion through CXCL12.