Selective alpha7 nicotinic receptor activation by AZD0328 enhances cortical dopamine release and improves learning and attentional processes

Targeting α7 nicotinic acetylcholine receptors and their protein interactions in Alzheimer's disease drug development

The decades-old cholinergic hypothesis of Alzheimer's disease (AD) led to clinical testing and FDA approval of acetylcholinesterase inhibitor drugs. Subsequently, the α7 nicotinic acetylcholine receptor (α7nAChR) was proposed as a new drug target for enhancing cholinergic neurotransmission. Nearly simultaneously, soluble amyloid β1-42 (Aβ42 ) was shown to bind α7nAChR with picomolar affinity to activate kinases that hyperphosphorylate tau, the precursor to tau-containing tangles. Multiple biopharmaceutical companies explored α7nAChR as a drug target for AD, mostly to enhance neurotransmission. Directly targeting α7nAChR proved to be a drug development challenge. The ultra-high-affinity interaction between Aβ42 and α7nAChR posed a significant hurdle for direct competition in the AD brain. The receptor rapidly desensitizes, undermining efficacy of agonists. Drug discovery approaches therefore included partial agonists and allosteric modulators of α7nAChR. After substantial effort, numerous drug candidates were abandoned due to lack of efficacy or drug-related toxicities. As alternatives, proteins interacting with α7nAChR were sought. In 2016, a novel nAChR regulator was identified, but no drug candidates have emerged from this effort. In 2012, the interaction of filamin A with α7nAChR was shown to be critical to Aβ42 's toxic signaling via α7nAChR, presenting a new drug target. The novel drug candidate simufilam disrupts the filamin A-α7nAChR interaction, reduces Aβ42 's high-affinity binding to α7nAChR, and suppresses Aβ42 's toxic signaling. Early clinical trials of simufilam showed improvements in experimental CSF biomarkers and indications of cognitive improvement in mild AD patients at 1 year. Simufilam is currently in phase 3 clinical trials as a disease-modifying treatment for AD.

Nicotinic acetylcholine receptors: Key targets for attenuating neurodegenerative diseases

Nicotinic acetylcholine receptors (nAChRs) are master regulators of immune functions via the cholinergic anti-inflammatory pathway and are expressed in microglia, the brain's resident immune cells. There is an extensive dialogue between the neurons and the glial cells around them from which microglia are tasked with monitoring, nurturing, and defending their microenvironment. Dysregulation of any of these processes can have devastating and long-lasting consequences involving microglia-mediated neuroinflammation associated with neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and Huntington's disease, amongst others. Disease-associated microglia acquire a distinguishing phenotype that emphasizes scavenging and defence functions while nurturing and repairing functions become muted. Attempts to resolve this critical imbalance remain a key focus of research. Furthermore, cholinergic modulation of neuroinflammation represents a promising avenue for treatment.

An overview of the active clinical trials for Parkinson's disease psychosis

Tweetable abstract An overview of the active clinical trials for Parkinson's disease psychosis. In this article, we review the drugs currently undergoing clinical testing for Parkinson's disease psychosis and offer some perspectives on the treatment of the condition.

Nicotine-like discriminative and aversive effects of two α4β2-selective nicotine agonists, ispronicline and metanicotine

An attempt to determine the receptor selective nature of some of nicotine's behavioral effects was undertaken through the evaluation of the ability of two nicotinic α4β2*-selective receptor agonists to produce nicotine-like effects and modify rates of responding in a discrimination assay and in an aversive stimulus assay. A group of eight rats was trained to discriminate the presence of 1 mg/kg nicotine base. Another group of 4-6 rats was trained to report the aversive effects of nicotine by selecting a lever that produced one food pellet over a second lever that produced two food pellets and an intravenous injection of nicotine. Ispronicline and metanicotine, two α4β2*-selective receptor agonists, increased selection of the nicotine-appropriate lever in a dose-related manner, up to a maximum of approximately 75%. The α4β2*-selective receptor antagonist, dihydro-beta-erythroidine blocked both the discriminative stimulus effects and the rate-suppressing effects of ispronicline, metanicotine, and small, but not large doses of nicotine. The nonselective antagonist, mecamylamine, antagonized the discriminative stimulus effects of each of the three nicotine agonists as well as the rate-decreasing effects of nicotine and metanicotine. Mecamylamine did not modify the rate-decreasing effects of ispronicline. Both ispronicline and metanicotine as well as nicotine were avoided in the drug + food vs. food choice situation. The receptor-selective nature of ispronicline and metanicotine was hereby confirmed in a behavioral assay, as were earlier reports that the discriminative stimulus effects of relatively small doses of nicotine are likely mediated by activity at the α4β2* nicotine receptor.

Therapeutic Targeting of α7 Nicotinic Acetylcholine Receptors

The α7-type nicotinic acetylcholine receptor is one of the most unique and interesting of all the members of the cys-loop superfamily of ligand-gated ion channels. Since it was first identified initially as a binding site for α-bungarotoxin in mammalian brain and later as a functional homomeric receptor with relatively high calcium permeability, it has been pursued as a potential therapeutic target for numerous indications, from Alzheimer disease to asthma. In this review, we discuss the history and state of the art for targeting α7 receptors, beginning with subtype-selective agonists and the basic pharmacophore for the selective activation of α7 receptors. A key feature of α7 receptors is their rapid desensitization by standard "orthosteric" agonist, and we discuss insights into the conformational landscape of α7 receptors that has been gained by the development of ligands binding to allosteric sites. Some of these sites are targeted by positive allosteric modulators that have a wide range of effects on the activation profile of the receptors. Other sites are targeted by direct allosteric agonist or antagonists. We include a perspective on the potential importance of α7 receptors for metabotropic as well as ionotropic signaling. We outline the challenges that exist for future development of drugs to target this important receptor and approaches that may be considered to address those challenges. SIGNIFICANCE STATEMENT: The α7-type nicotinic acetylcholine receptor (nAChR) is acknowledged as a potentially important therapeutic target with functional properties associated with both ionotropic and metabotropic signaling. The functional properties of α7 nAChR can be regulated in diverse ways with the variety of orthosteric and allosteric ligands described in this review.

The human-specific duplicated α7 gene inhibits the ancestral α7, negatively regulating nicotinic acetylcholine receptor-mediated transmitter release

Gene duplication generates new functions and traits, enabling evolution. Human-specific duplicated genes in particular are primary sources of innovation during our evolution although they have very few known functions. Here we examine the brain function of one of these genes (CHRFAM7A) and its product (dupα7 subunit). This gene results from a partial duplication of the ancestral CHRNA7 gene encoding the α7 subunit that forms the homopentameric α7 nicotinic acetylcholine receptor (α7-nAChR). The functions of α7-nAChR in the brain are well defined, including the modulation of synaptic transmission and plasticity underlying normal attention, cognition, learning, and memory processes. However, the role of the dupα7 subunit remains unexplored at the neuronal level. Here, we characterize that role by combining immunoblotting, quantitative RT-PCR and FRET techniques with functional assays of α7-nAChR activity using human neuroblastoma SH-SY5Y cell variants with different dupα7 expression levels. Our findings reveal a physical interaction between dupα7 and α7 subunits in fluorescent protein-tagged dupα7/α7 transfected cells that negatively affects normal α7-nAChR activity. Specifically, in both single cells and cell populations, the [Ca²⁺]_i signal and the exocytotic response induced by selective stimulation of α7-nAChR were either significantly inhibited by stable dupα7 overexpression or augmented after silencing dupα7 gene expression with specific siRNAs. These findings identify a new role for the dupα7 subunit as a negative regulator of α7-nAChR-mediated control of exocytotic neurotransmitter release. If this effect is excessive, it would result in an impaired synaptic transmission that could underlie the neurocognitive and neuropsychiatric disorders associated with α7-nAChR dysfunction.

Neurocognitive effects of atypical antipsychotics in patients with first-episode schizophrenia

Introduction: Cognitive impairment is a core feature of schizophrenia. The effects of atypical antipsychotics on the cognitive functions of patients with first-episode schizophrenia have not been comprehensively investigated so far. This study aims to compare neurocognitive effects of risperidone, olanzapine, and aripiprazole for first-episode schizophrenia.Methods: The study was a multicenter, randomized, open-label clinical trial. 546 patients were randomly divided into three medication groups, and followed up for 1 year. Cognitive performance was evaluated with a neuropsychological test battery. The Clinical trials.gov ID of the study is NCT01057849.Results: At 6 months, treatment resulted in significant improvements in all three groups in most cognitive domains except verbal learning and memory. At 12 months, three treatment groups had further improvements in three cognitive domains, but visual learning and memory performance dropped back to baseline.Conclusion: All three atypical antipsychotics tested in the study can potentially improve cognitive performance in first-episode schizophrenia, but no significant difference in the degree of improvement was found between drugs.

Crosstalk between the M1 muscarinic acetylcholine receptor and the endocannabinoid system: A relevance for Alzheimer's disease?

Alzheimer's disease (AD) is a neurodegenerative disorder which accounts for 60-70% of the 50 million worldwide cases of dementia and is characterised by cognitive impairments, many of which have long been associated with dysfunction of the cholinergic system. Although the M1 muscarinic acetylcholine receptor (mAChR) is considered a promising drug target for AD, ligands targeting this receptor have so far been unsuccessful in clinical trials. As modulatory receptors to cholinergic transmission, the endocannabinoid system may be a promising drug target to allow fine tuning of the cholinergic system. Furthermore, disease-related changes have been found in the endocannabinoid system during AD progression and indeed targeting the endocannabinoid system at specific disease stages alleviates cognitive symptoms in numerous mouse models of AD. Here we review the role of the endocannabinoid system in AD, and its crosstalk with mAChRs as a potential drug target for cholinergic dysfunction.

Activators of α7 nAChR as Potential Therapeutics for Cognitive Impairment

The α7 nicotinic acetylcholine receptor (nAChR) is a promising target for the treatment of cognitive deficits associated with psychiatric and neurological disorders, including schizophrenia and Alzheimer's disease (AD). Several α7 nAChR agonists and positive allosteric modulators (PAMs) have demonstrated procognitive effects in preclinical models and early clinical trials. However, despite intense research efforts in the pharmaceutical industry and academia, none of the α7 nAChR ligands has been approved for clinical use. This chapter will focus on the α7 nAChR ligands that have advanced to clinical studies and explore the reasons why these agents have not met with unequivocal clinical success.

Radioiodinated 9-fluorenone derivatives for imaging α7-nicotinic acetylcholine receptors

A series of 9H-fluoren-9-one substituents were synthesized and evaluated for imaging cerebral α7-nAChRs. Meta-iodine substituted 9-fluorenone 5 with high binding affinity (K _i = 9.3 nM) and selectivity was radiolabeled with ¹²⁵I. Fully in vitro and in vivo studies of [¹²⁵I]5 have been performed. [¹²⁵I]5 exhibited well brain uptake with a peak concentration of 7.5 ± 0.9% ID/g in mice brains. Moreover, ex vivo autoradiography studies and micro single-photon emission computed tomography (micro-SPECT/CT) dynamic imaging in mice confirmed its in vivo imaging properties. Besides, molecular docking and MD studies were also performed to interpret the binding mechanisms of the two series of ligands towards α7-nAChRs. To conclude, the meta-iodine substituted 9-fluorenone [¹²⁵I]5 could be a promising tracer for imaging α7-nAChRs.

Human umbilical cord blood cells restore vascular integrity in injured rat brain and modulate inflammation in vitro

Aim: Traumatic brain injury is a complex condition consisting of a mechanical injury with neurovascular disruption and inflammation with limited clinical interventions available. A growing number of studies report systemic delivery of human umbilical cord blood (HUCB) as a therapy for neural injuries. Materials & methods: HUCB cells from five donors were tested to improve blood-brain barrier integrity in a traumatic brain injury rat model at a dose of 2.5 × 10⁷ cells/kg at 24 or 72 h postinjury and for immunomodulatory activity in vitro. Results & Conclusion: We observed that cells delivered 72 h postinjury significantly restored blood-brain barrier integrity. HUCB cells reduced the amount of TNF-α and IFN-γ released by activated primary rat splenocytes, which correlated with the expression of COX2 and IDO1.

Dyskinesia and brain-derived neurotrophic factor levels after long-term levodopa and nicotinic receptor agonist treatments in female mice with near-total unilateral dopaminergic denervation

BACKGROUND

The treatment of Parkinson's disease is often complicated by levodopa-induced dyskinesia (LID). Nicotinic acetylcholine receptor agonists can alleviate LID in animal models but may be less effective in conditions of severe dopaminergic denervation. While the mechanisms of LID remain incompletely understood, elevated corticostriatal levels of the brain-derived neurotrophic factor (BDNF) have been suggested to play a role. Here, female mice with near-total unilateral 6-hydroxydopamine-induced nigrostriatal lesions were chronically treated with levodopa, and the effects of the α7 nicotinic receptor partial agonist AZD0328 and nicotine on LID were assessed. At the end of the experiment, BDNF protein levels in the prefrontal cortex and striatum were measured.

RESULTS

Five-day treatments with three escalating doses of AZD0328 and a 10-week treatment with nicotine failed to alleviate LID. BDNF levels in the lesioned striatum correlated positively with LID severity, but no evidence was found for a levodopa-induced elevation of corticostriatal BDNF in the lesioned hemisphere. The nicotine treatment decreased BDNF levels in the prefrontal cortex but had no effect on striatal BDNF.

CONCLUSIONS

The findings suggest that treatment of LID with nicotinic agonists may lose its effectiveness as the disease progresses, represent further evidence for a role for BDNF in LID, and expand previous knowledge on the effects of long-term nicotine treatment on BDNF.

Assessing the acute effects of CDP-choline on sensory gating in schizophrenia: A pilot study

Deficient sensory gating (SG) in schizophrenia is associated with functional outcome and offers a therapeutic target as it is linked to the altered function/expression of the α7 nicotinic acetylcholine receptors (nAChRs). This study analyzed the effects of citicoline (CDP-choline), a supplement with α7 nAChRs agonist properties, on SG in a sample of schizophrenia (SZ) patients. Using a randomized, placebo-controlled, double-blind design the dose-dependent (500 mg, 1000 mg, 2000 mg) and baseline-dependent (deficient versus normal suppressors) effects of CDP-choline on SG were examined using the P50 event-related potential (ERP) index of SG. Overall analysis failed to demonstrate treatment effects but CDP-choline improved SG (500 mg) in the deficient SZ subgroup by increasing suppression of the S2 P50 amplitude. These findings tentatively support α7 nAChR dysfunction in the expression of SG deficits and suggest further trials to assess the effects of sustained α7 nAChR activation on SG with low doses of CDP-choline.

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.

Nicotinic alpha 7 receptor agonists EVP-6124 and BMS-933043, attenuate scopolamine-induced deficits in visuo-spatial paired associates learning

Agonists at the nicotinic acetylcholine alpha 7 receptor (nAChR α7) subtype have the potential to treat cognitive deficits in patients with Alzheimer’s disease (AD) or schizophrenia. Visuo-spatial paired associates learning (vsPAL) is a task that has been shown to reliably predict conversion from mild cognitive impairment to AD in humans and can also be performed by nonhuman primates. Reversal of scopolamine-induced impairment of vsPAL performance may represent a translational approach for the development of nAChR α7 agonists. The present study investigated the effect of treatment with the acetylcholinesterase inhibitor, donepezil, or three nAChR α7 agonists, BMS-933043, EVP-6124 and RG3487, on vsPAL performance in scopolamine-treated cynomolgus monkeys. Scopolamine administration impaired vsPAL performance accuracy in a dose- and difficulty- dependent manner. The impairment of eventual accuracy, a measure of visuo-spatial learning during the task, was significantly ameliorated by treatment with donepezil (0.3 mg/kg, i.m.), EVP-6124 (0.01 mg/kg, i.m.) or BMS-933043 (0.03, 0.1 and 0.3 mg/kg, i.m.). Both nAChR α7 agonists showed inverted-U shaped dose-effect relationships with EVP-6124 effective at a single dose only whereas BMS-933043 was effective across at least a 10 fold dose/exposure range. RG3487 was not efficacious in this paradigm at the dose range examined (0.03–1 mg/kg, i.m.). These results are the first demonstration that the nAChR α7 agonists, EVP-6124 and BMS-933043, can ameliorate scopolamine-induced cognitive deficits in nonhuman primates performing the vsPAL task.

The current agonists and positive allosteric modulators of α7 nAChR for CNS indications in clinical trials

The alpha-7 nicotinic acetylcholine receptor (α7 nAChR), consisting of homomeric α7 subunits, is a ligand-gated Ca²⁺-permeable ion channel implicated in cognition and neuropsychiatric disorders. Enhancement of α7 nAChR function is considered to be a potential therapeutic strategy aiming at ameliorating cognitive deficits of neuropsychiatric disorders such as Alzheimer's disease (AD) and schizophrenia. Currently, a number of α7 nAChR modulators have been reported and several of them have advanced into clinical trials. In this brief review, we outline recent progress made in understanding the role of the α7 nAChR in multiple neuropsychiatric disorders and the pharmacological effects of α7 nAChR modulators used in clinical trials.

Design and synthesis of a novel series of (1'S,2R,4'S)-3H-4'-azaspiro[benzo[4,5]imidazo[2,1-b]oxazole-2,2'-bicyclo[2.2.2]octanes] with high affinity for the α7 neuronal nicotinic receptor

We describe an efficient and convergent synthesis of a series of (1'S,2R,4'S)-3H-4'-azaspiro[benzo[4,5]imidazo[2,1-b]oxazole-2,2'-bicyclo[2.2.2]octanes] displaying potency for the α7 nicotinic acetylcholine receptor (nAChR) and good selectivity vs. the related 5-HT_3A receptor.

Preclinical abuse liability assessment of ABT-126, an agonist at the α7 nicotinic acetylcholine receptor (nAChR)

ABT-126 is a nicotinic acetylcholine receptor (nAChR) agonist that is selective for the α7 subtype of the receptor. nAChRs are thought to play a role in a variety of neurocognitive processes and have been a pharmacologic target for disorders with cognitive impairment, including schizophrenia and Alzheimer's disease. As part of the preclinical safety package for ABT-126, its potential for abuse was assessed. While the involvement of the α4β2 subtype of the nicotinic receptor in the addictive properties of nicotine has been demonstrated, the role of the α7 receptor has been studied much less extensively. A number of preclinical assays of abuse potential including open-field, drug discrimination and self-administration were employed in male rats. ABT-126 had modest effects on locomotor activity in the open-field assay. In nicotine and d-amphetamine drug discrimination assays, ABT-126 administration failed to produce appreciable d-amphetamine-like or nicotine-like responding, suggesting that its interoceptive effects are distinct from those of these drugs of abuse. In rats trained to self-administer cocaine, substitution with ABT-126 was similar to substitution with saline, indicating that it lacks reinforcing effects. No evidence of physical dependence was noted following subchronic administration. Overall, these data suggest that ABT-126 has a low potential for abuse. Together with other literature on this drug class, it appears that drugs that selectively activate α7 nAChRs are not likely to result in abuse or dependence.

BMS-933043, a Selective α7 nAChR Partial Agonist for the Treatment of Cognitive Deficits Associated with Schizophrenia

The therapeutic treatment of negative symptoms and cognitive dysfunction associated with schizophrenia is a significant unmet medical need. Preclinical literature indicates that α7 neuronal nicotinic acetylcholine (nACh) receptor agonists may provide an effective approach to treating cognitive dysfunction in schizophrenia. We report herein the discovery and evaluation of 1c (BMS-933043), a novel and potent α7 nACh receptor partial agonist with high selectivity against other nicotinic acetylcholine receptor subtypes (>100-fold) and the 5-HT_3A receptor (>300-fold). In vivo activity was demonstrated in a preclinical model of cognitive impairment, mouse novel object recognition. BMS-933043 has completed Phase I clinical trials.

Tropisetron sensitizes α7 containing nicotinic receptors to low levels of acetylcholine in vitro and improves memory-related task performance in young and aged animals

Tropisetron, a 5-HT₃ receptor antagonist commonly prescribed for chemotherapy-induced nausea and vomiting also exhibits high affinity, partial agonist activity at α7 nicotinic acetylcholine receptors (α7 nAChRs). α7 nAChRs are considered viable therapeutic targets for neuropsychiatric disorders such as Alzheimer's disease (AD). Here we further explored the nAChR pharmacology of tropisetron to include the homomeric α7 nAChR and recently characterized heteromeric α7β2 nAChR (1:10 ratio) and we evaluated its cognitive effects in young and aged animals. Electrophysiological studies on human nAChRs expressed in Xenopus oocytes confirmed the partial agonist activity of tropisetron at α7 nAChRs (EC₅₀ ∼2.4 μM) with a similar effect at α7β2 nAChRs (EC₅₀ ∼1.5 μM). Moreover, currents evoked by irregular pulses of acetylcholine (40 μM) at α7 and α7β2 nAChRs were enhanced during sustained exposure to low concentrations of tropisetron (10 and 30 nM) indicative of a "priming" or co-agonist effect. Tropisetron (0.1-10 mg/kg) improved novel object recognition performance in young Sprague-Dawley rats and in aged Fischer rats. In aged male and female rhesus monkeys, tropisetron (0.03-1 mg/kg) produced a 17% increase from baseline levels in delayed match to sample long delay accuracy while combination of non-effective doses of donepezil (0.1 mg/kg) and tropisetron (0.03 and 0.1 mg/kg) produced a 24% change in accuracy. Collectively, these animal experiments indicate that tropisetron enhances cognition and has the ability to improve the effective dose range of currently prescribed AD therapy (donepezil). Moreover, these effects may be explained by tropisetron's ability to sensitize α7 containing nAChRs to low levels of acetylcholine.

Design and synthesis of a novel series of 4-heteroarylamino-1'-azaspiro[oxazole-5,3'-bicyclo[2.2.2]octanes as α7 nicotinic receptor agonists 2. Development of 4-heteroaryl SAR

Quinuclidine-containing spirooxazolines, as described in the previous report in this series, were demonstrated to have utility as α7 nicotinic acetylcholine receptor (α7 nAChR) partial agonists. In this work, the SAR of this chemotype was expanded to include an array of diazine heterocyclic substitutions. Many of the heterocyclic analogs were potent partial agonists of the α7 receptor, selective against other nicotinic receptors and the serotinergic 5HT_3A receptor. (1'S,3'R,4'S)-N-(6-phenylpyrimidin-4-yl)-4H-1'-azaspiro[oxazole-5,3'-bicyclo[2.2.2]octan]-2-amine, a potent and selective α7 nAChR partial agonist, was demonstrated to improve cognition in the mouse novel object recognition (NOR) model of episodic memory.

Development of 4-Heteroarylamino-1'-azaspiro[oxazole-5,3'-bicyclo[2.2.2]octanes] as α7 Nicotinic Receptor Agonists

We describe the synthesis of quinuclidine-containing spiroimidates and their utility as α7 nicotinic acetylcholine receptor (nAChR) partial agonists. A convergent synthetic route allowed for rapid SAR investigation and provided a diverse set of fused 6,5-heteroaryl analogs. Two potent and selective α7 nAChR partial agonists, (1'S,3'R,4'S)-N-(7-bromopyrrolo[2,1-f][1,2,4]triazin-4-yl)-4H-1'-azaspiro[oxazole-5,3'-bicyclo[2.2.2]octan]-2-amine (20) and (1'S,3'R,4'S)-N-(7-chloropyrrolo[2,1-f][1,2,4]triazin-4-yl)-4H-1'-azaspiro[oxazole-5,3'-bicyclo[2.2.2]octan]-2-amine (21), were identified. Both agonists improved cognition in a preclinical rodent model of learning and memory. Additionally, 5-HT3A receptor SAR suggested the presence of a steric site that when engaged led to significant loss of affinity at that receptor.

Cholinergic System and Post-translational Modifications: An Insight on the Role in Alzheimer's Disease

BACKGROUND

Alzheimer's disease (AD) is the most common form of old age dementia. The formation of amyloid plaques (Aβ), neurofibrillary tangles and loss of basal forebrain cholinergic neurons are the hallmark events in the pathology of AD.

LITERATURE REVIEW

Cholinergic system is one of the most important neurotransmitter system involved in learning and memory which preferentially degenerates in the initial stages of AD. Activation of cholinergic receptors (muscarinic and nicotinic) activates multiple pathways which result in post translational modifications (PTMs) in multiple proteins which bring changes in nervous system. Cholinergic receptors-mediated PTMs "in-part" substantially affect the biosynthesis, proteolysis, degradation and expression of many proteins and in particular, amyloid precursor protein (APP). APP is subjected to several PTMs (proteolytic processing, glycosylation, sulfation, and phosphorylation) during its course of processing, resulting in Aβ deposition, leading to AD. Aβ also alters the PTMs of tau which is a microtubule associated protein. Therefore, post-translationally modified tau and Aβ collectively aggravate the neuronal loss that leads to cholinergic hypofunction.

CONCLUSION

Despite the accumulating evidences, the interaction between cholinergic neurotransmission and the physiological significance of PTM events remain speculative and still needs further exploration. This review focuses on the role of cholinergic system and discusses the significance of PTMs in pathological progression of AD and highlights some important future directions.

Development of spiroguanidine-derived α7 neuronal nicotinic receptor partial agonists

We describe the synthesis of quinuclidine-containing spiroguanidines and their utility as α7 neuronal nicotinic acetylcholine receptor (nAChR) partial agonists. The convergent synthetic route developed for this study allowed for rapid SAR investigation and provided access to a structurally diverse set of analogs. A potent and selective α7 nAChR partial agonist, N-(6-methyl-1,3-benzoxazol-2-yl)-3',5'-dihydro-4-azaspiro[bicyclo[2.2.2]octane-2,4'-imidazole]-2'-amine (BMS-910731, 16), was identified. This compound induced immediate early genes c-fos and Arc in a preclinical rodent model of α7 nAChR-derived cellular activation and plasticity. Importantly, the ability to incorporate selectivity for the α7 nACh receptor over the 5-HT_3A receptor in this series suggested a significant difference in steric requirements between the two receptors.

Alpha7 nicotinic acetylcholine receptor agonists and PAMs as adjunctive treatment in schizophrenia. An experimental study

Nicotine has been found to improve cognition and reduce negative symptoms in schizophrenia and a genetic and pathophysiological link between the α7 nicotinic acetylcholine receptors (nAChRs) and schizophrenia has been demonstrated. Therefore, there has been a large interest in developing drugs affecting the α7 nAChRs for schizophrenia. In the present study we investigated, in rats, the effects of a selective α7 agonist (PNU282987) and a α7 positive allosteric modulator (PAM; NS1738) alone and in combination with the atypical antipsychotic drug risperidone for their utility as adjunct treatment in schizophrenia. Moreover we also investigated their utility as adjunct treatment in depression in combination with the SSRI citalopram. We found that NS1738 and to some extent also PNU282987, potentiated a subeffective dose of risperidone in the conditioned avoidance response test. Both drugs also potentiated the effect of a sub-effective concentration of risperidone on NMDA-induced currents in pyramidal cells of the medial prefrontal cortex. Moreover, NS1738 and PNU282987 enhanced recognition memory in the novel object recognition test, when given separately. Both drugs also potentiated accumbal but not prefrontal risperidone-induced dopamine release. Finally, PNU282987 reduced immobility in the forced swim test, indicating an antidepressant-like effect. Taken together, our data support the utility of drugs targeting the α7 nAChRs, perhaps especially α7 PAMs, to potentiate the effect of atypical antipsychotic drugs. Moreover, our data suggest that α7 agonists and PAMs can be used to ameliorate cognitive symptoms in schizophrenia and depression.

Selective activation of α7 nicotinic acetylcholine receptors augments hippocampal oscillations

Neural α7 nicotinic acetylcholine receptors (α7 nAChRs) emerged as a potential pharmacologic target for treating cognitive deficits in schizophrenia and Alzheimer's disease. Experiments modeling these dysfunctions, as well as clinical evidence, demonstrate the relatively consistent procognitive effects of α7 nAChR agonists. One preclinical observation supporting the procognitive role of α7 nAChRs is their ability to modulate neuronal network oscillations closely associated with learning and memory, especially hippocampal oscillations. Due to the high degree of structural similarity between α7 nACh and 5-HT receptors, the majority of α7 nAChR agonists to date also act as 5-HT3 antagonists. To address this confounding property and determine the relevance of α7 nAChR agonist binding to 5-HT3 receptors in modulating hippocampal activity, we tested two well-described α7 nAChR agonists, PNU-282987 and FRM-17874, in mice lacking α7 nAChRs (α7 knock-out, α7KO) using the brainstem simulation-elicited hippocampal theta oscillation assay. Under urethane anesthesia both agonists at equivalent doses demonstrated efficacy in wild-type (WT) mice, significantly enhancing theta power and theta phase-gamma amplitude coupling as compared to saline treated control mice. These effects are comparable to those seen with drugs clinically used to treat Alzheimer's disease. Although α7KO mice showed no alterations in elicited hippocampal oscillations, both α7 nAChR agonists failed to enhance theta power or theta phase - gamma amplitude coupling in these mice. Our findings demonstrate that selective activation of α7 nAChRs can modulate hippocampal oscillation, and these receptors are the primary targets of the tested agonists, PNU-282987 and FRM-17874 and likely underlies their observed procognitive activity.

Understanding the Bases of Function and Modulation of α7 Nicotinic Receptors: Implications for Drug Discovery

The nicotinic acetylcholine receptor (nAChR) belongs to a superfamily of pentameric ligand-gated ion channels involved in many physiologic and pathologic processes. Among nAChRs, receptors comprising the α7 subunit are unique because of their high Ca(2+) permeability and fast desensitization. nAChR agonists elicit a transient ion flux response that is further sustained by the release of calcium from intracellular sources. Owing to the dual ionotropic/metabotropic nature of α7 receptors, signaling pathways are activated. The α7 subunit is highly expressed in the nervous system, mostly in regions implicated in cognition and memory and has therefore attracted attention as a novel drug target. Additionally, its dysfunction is associated with several neuropsychiatric and neurologic disorders, such as schizophrenia and Alzheimer's disease. α7 is also expressed in non-neuronal cells, particularly immune cells, where it plays a role in immunity, inflammation, and neuroprotection. Thus, α7 potentiation has emerged as a therapeutic strategy for several neurologic and inflammatory disorders. With unique activation properties, the receptor is a sensitive drug target carrying different potential binding sites for chemical modulators, particularly agonists and positive allosteric modulators. Although macroscopic and single-channel recordings have provided significant information about the underlying molecular mechanisms and binding sites of modulatory compounds, we know just the tip of the iceberg. Further concerted efforts are necessary to effectively exploit α7 as a drug target for each pathologic situation. In this article, we focus mainly on the molecular basis of activation and drug modulation of α7, key pillars for rational drug design.

Synthesis and biological activities of indolizine derivatives as alpha-7 nAChR agonists

Human α7 nicotinic acetylcholine receptor (nAChR) is a promising therapeutic target for the treatment of schizophrenia accompanied with cognitive impairment. Herein, we report the synthesis and agonistic activities of a series of indolizine derivatives targeting to α7 nAChR. The results show that all synthesized compounds have affinity to α7 nAChR and some give strong agonistic activity, particularly most active agonists show higher potency than control EVP-6124. The docking and structure-activity relationship studies provide insights to develop more potent novel α7 nAChR agonists.

Identification and in vitro pharmacological characterization of a novel and selective α7 nicotinic acetylcholine receptor agonist, Br-IQ17B

Aim:

Alpha7-nicotinic acetylcholine receptor (α7 nAChR) is a ligand-gated Ca²⁺-permeable ion channel implicated in cognition and neuropsychiatric disorders. Activation of α7 nAChR improves learning, memory, and sensory gating in animal models. To identify novel α7 nAChR agonists, we synthesized a series of small molecules and characterized a representative compound, Br-IQ17B, N-[(3R)-1-azabicyclo[2,2,2]oct-3-yl]-5-bromoindolizine-2-carboxamide, which specifically activates α7 nAChR.

Methods:

Two-electrode voltage clamp (TEVC) recordings were primarily used for screening in Xenopus oocytes expressing human α7 nAChR. Assays, including radioisotope ligand binding, Western blots, whole-cell recordings of hippocampal culture neurons, and spontaneous IPSC recordings of brain slices, were also utilized to evaluate and confirm the specific activation of α7 nAChR by Br-IQ17B.

Results:

Br-IQ17B potently activates α7 nAChR with an EC₅₀ of 1.8±0.2 μmol/L. Br-IQ17B is selective over other subtypes such as α4β2 and α3β4, but it blocks 5-HT_3A receptors. Br-IQ17B displaced binding of the α7 blocker [³H]-MLA to hippocampal crude membranes with a K_i of 14.9±3.2 nmol/L. In hippocampal neurons, Br-IQ17B evoked α7-like currents that were inhibited by MLA and enhanced in the presence of the α7 PAM PNU-120596. In brain slice recordings, Br-IQ17B enhanced GABAergic synaptic transmission in CA1 neurons. Mechanistically, Br-IQ17B increased ERK1/2 phosphorylation that was MLA-sensitive.

Conclusion:

We identified the novel, potent, and selective α7 agonist Br-IQ17B, which enhances synaptic transmission. Br-IQ17B may be a helpful tool to understand new aspects of α7 nAChR function, and it also has potential for being developed as therapy for schizophrenia and cognitive deficits.

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.

cAMP-dependent protein kinase inhibits α7 nicotinic receptor activity in layer 1 cortical interneurons through activation of D1/D5 dopamine receptors

KEY POINTS

Protein kinases can modify the function of many proteins including ion channels. However, the role of protein kinase A in modifying nicotinic receptors in the CNS has never been investigated. We showed through whole-cell recordings of layer 1 prefrontal cortical interneurons that α7 nicotinic responses are negatively modulated by protein kinase A. Furthermore, we show that stimulation of dopamine receptors can similarly attenuate α7 nicotinic responses through the activation of protein kinase A. These results suggest how the interaction of the cholinergic and dopaminergic systems may influence neuronal excitability in the brain.

ABSTRACT

Phosphorylation of ion channels, including nicotinic acetylcholine receptors (nAChRs), by protein kinases plays a key role in the modification of synaptic transmission and neuronal excitability. α7 nAChRs are the second most prevalent nAChR subtype in the CNS following α4β2. Serine 365 in the M3-M4 cytoplasmic loop of the α7 nAChR is a phosphorylation site for protein kinase A (PKA). D1/D5 dopamine receptors signal through the adenylate cyclase-PKA pathway and play a key role in working memory and attention in the prefrontal cortex. Thus, we examined whether the dopaminergic system, mediated through PKA, functionally interacts with the α7-dependent cholinergic neurotransmission. In layer 1 interneurons of mouse prefrontal cortex, α7 nicotinic currents were decreased upon stimulation with 8-Br-cAMP, a PKA activator. In HEK 293T cells, dominant negative PKA abolished 8-Br-cAMP's effect of diminishing α7 nicotinic currents, while a constitutively active PKA catalytic subunit decreased α7 currents. In brain slices, the PKA inhibitor KT-5720 nullified 8-Br-cAMP's effect of attenuating α7 nicotinic responses, while applying a PKA catalytic subunit in the pipette solution decreased α7 currents. 8-Br-cAMP stimulation reduced surface expression of α7 nAChRs, but there was no change in single-channel conductance. The D1/D5 dopamine receptor agonist SKF 83822 similarly attenuated α7 nicotinic currents from layer 1 interneurons and this attenuation of nicotinic current was prevented by KT-5720. These results demonstrate that dopamine receptor-mediated activation of PKA negatively modulates nicotinic neurotransmission in prefrontal cortical interneurons, which may be a contributing mechanism of dopamine modulation of cognitive behaviours such as attention or working memory.

Targeting of α7 Nicotinic Acetylcholine Receptors in the Treatment of Schizophrenia and the Use of Auditory Sensory Gating as a Translational Biomarker

Accumulating evidence suggests that the α7 subtype of nicotinic acetylcholine receptors (nAChRs) plays a key role in inflammatory processes, thought to be involved in the pathophysiology of neuropsychiatric diseases, such as schizophrenia and Alzheimer's disease. Preclinical and clinical studies showed that the diminished suppression of P50 auditory evoked potentials in patients with schizophrenia may be associated with a decreased density of α7 nAChRs in the brain. This points to a role for auditory sensory gating (P50) as a translational biomarker. A number of agonists and positive allosteric modulators (PAMs) for α7 nAChR promoted beneficial effects in animal models with sensory gating and cognitive deficits. Additionally, several clinical studies showed that α7 nAChR agonists could improve suppression in auditory P50 evoked potentials, as well as cognitive deficits, and negative symptoms in patients with schizophrenia. Taken together, α7 nAChR presents as an extremely attractive therapeutic target for schizophrenia. In this article, the author discusses recent findings on α7 nAChR agonists such as DMXB-A, RG3487, TC-5619, tropisetron, EVP-6124 (encenicline), ABT-126, AQW051 and α7 nAChR PAMs such as JNJ-39393406, PNU- 120596 and AVL-3288 (also known as UCI-4083), and their potential as therapeutic drugs for neuropsychiatric diseases, such as schizophrenia.

The α7 nAChR selective agonists as drug candidates for Alzheimer's disease

The nicotinic acetylcholine receptors (nAChRs) are ion channels distribute in the central or peripheral nervous system. They are receptors of the neurotransmitter acetylcholine and activation of them by agonists mediates synaptic transmission in the neuron and muscle contraction in the neuromuscular junction. Current studies reveal relationship between the nAChRs and the learning and memory as well as cognation deficit in various neurological disorders such as Alzheimer's disease, Parkinson's disease, schizophrenia and drug addiction. There are various subtypes in the nAChR family and the α7 nAChR is one of the most abundant subtypes in the brain. The α7 nAChR is significantly reduced in the patients of Alzheimer's disease and is believed to interact with the Aβ amyloid. Aβ amyloid is co-localized with α7 nAChR in the senile plaque and interaction between them induces neuron apoptosis and reduction of the α7 nAChR expression. Treatment with α7 agonist in vivo shows its neuron protective and procognation properties and significantly improves the learning and memory ability of the animal models. Therefore, the α7 nAChR agonists are excellent drug candidates for Alzheimer's disease and we summarized here the current agonists that have selectivity of the α7 nAChR over the other nAChR, introduced recent molecular modeling works trying to explain the molecular mechanism of their selectivity and described the design of novel allosteric modulators in our lab.

Safety and clinical effects of EVP-6124 in subjects with Alzheimer's disease currently or previously receiving an acetylcholinesterase inhibitor medication

Alzheimer's disease (AD) is a prevalent and currently incurable brain disease whose impact will continue to rise as the population ages. With limited treatment options, a variety of experimental therapies are currently in clinical trials. EVP-6124 (encenicline) is an α7 nicotinic acetylcholine receptor partial agonist under investigation for the symptomatic treatment of AD. EVP-6124 activates the α7 nicotinic acetylcholine receptor at low nanomolar brain concentrations and improves memory performance in rats. Treatment with EVP-6124 in Phase I and II trials involving patients with mild-to-moderate AD was well tolerated and showed statistically significant improvements compared with placebo on cognitive and functional measures. Two Phase III trials under the title COGNITIV AD will assess the efficacy and tolerability of EVP-6124 in patients with mild-to-moderate AD. Based on the completed clinical trials and proposed mechanism of action, EVP-6124 would appear to be a good candidate for therapy in combination with cholinesterase inhibitors.

[Neuroenhancement]

BACKGROUND

Cognitive enhancement or neuroenhancement describes the increase in cognitive performance in humans by means of psychotropic drugs or brain stimulation methods, such as transcranial magnetic stimulation (TMS).

PROBLEM

This article discusses the potential of pharmacological cognitive enhancement with some of the most common drugs.

METHODS

A selective literature search was performed taking into account the most important groups of substances (i.e. caffeine, nicotine, stimulants including modafinil, and acetylcholine esterase inhibitors) for which studies on the pharmacological elevation of cognitive performance in healthy subjects are available.

RESULTS

The extent of the effects that can be pharmacologically achieved is essentially genetically determined. Some of the best-characterized polymorphisms are described here. Pharmacological enhancement of cognitive performance is currently possible with all of the compounds described here and caffeine and nicotine are used by millions of people without the explicit intention of most consumers of cognitive enhancement.

DISCUSSION

Clinical neuroscientists are required to share their expertise to a greater extent in the social discourse on cognitive enhancement in the future in order to influence opinion-forming and decision-making processes.

CDP-choline: effects of the procholine supplement on sensory gating and executive function in healthy volunteers stratified for low, medium and high P50 suppression

Diminished auditory sensory gating and associated neurocognitive deficits in schizophrenia have been linked to altered expression and function of the alpha-7 nicotinic acetycholinergic receptor (α7 nAChR), the targeting of which may have treatment potential. Choline is a selective α7 nAChR agonist and the aim of this study was to determine whether cytidine 5'-diphosphocholine (CDP-choline), or citicoline, a dietary source of choline, increases sensory gating and cognition in healthy volunteers stratified for gating level. In a randomized, placebo-controlled, double-blind design involving acute administration of low, moderate doses (500 mg, 1000 mg) of CDP-choline, 24 healthy volunteers were assessed for auditory gating as indexed by suppression of the P50 event-related potential (ERP) in a paired-stimulus (S1, S2) paradigm, and for executive function as measured by the Groton Maze Learning Task (GMLT) of the CogState Schizophrenia Battery. CDP-choline improved gating (1000 mg) and suppression of the S2 P50 response (500 mg, 1000 mg), with the effects being selective for individuals with low gating (suppression) levels. Tentative support was also shown for increased GMLT performance (500 mg) in low suppressors. These preliminary findings with CDP-choline in a healthy, schizophrenia-like surrogate sample are consistent with a α7 nAChR mechanism and support further trials with choline as a pro-cognitive strategy.

Methyllycaconitine- and scopolamine-induced cognitive dysfunction: differential reversal effect by cognition-enhancing drugs

There is a growing body of evidence pointing to the pivotal role of alpha-7 nicotinic acetylcholine receptor (α7 nAchR) dysfunction in cognitive disorders such as Alzheimer’s disease or schizophrenia. This study was undertaken to establish and characterize an in vivo model for cognitive disorder secondary to the blockade of α7 nAChR by its specific antagonist, methyllycaconitine (MLA). The results show that MLA elicited cognitive dysfunction as assessed by reduced spontaneous alternation of mice in the T-maze. The maximal effect of MLA produced 25–30% reduction in the spontaneous alternation of mice, a level comparable with that induced by the muscarinic antagonism of scopolamine. Donepezil and galantamine fully reversed both MLA and scopolamine-induced cognitive dysfunction. However, the ED₅₀ of donepezil and galantamine was significantly shifted to the left in the MLA- compared to scopolamine-treated mice (0.0005 and 0.002 mg/kg for donepezil; 0.0003 and 0.7 mg/kg for galantamine). Moreover, memantine elicited marked reversion of cognitive dysfunction (up to 70%) in MLA-treated mice while only a weak reversal effect at high dose of memantine (less than 20%) was observed in scopolamine-treated mice. The above findings indicate that MLA-induced cognitive dysfunction in the mouse is highly sensitive and more responsive to the current procognitive drugs than the traditional scopolamine-based assay. Thus, it can be of value for the preclinical screening and profiling of cognition-enhancing drugs.

The alpha-7 nicotinic receptor partial agonist/5-HT3 antagonist RG3487 enhances cortical and hippocampal dopamine and acetylcholine release

RATIONALE

Alpha-7 nicotinic acetylcholine receptor (nAChR) agonists may ameliorate cognitive deficits in schizophrenia, in part, because of their ability to enhance dopaminergic and cholinergic neurotransmission.

OBJECTIVES

In the current study, the effects of partial nAChR agonist and 5-HT3 receptor antagonist RG3487 (previously R3487/MEM3454) on dopamine (DA) and acetylcholine (ACh) effluxes in rat prefrontal cortex (mPFC) and hippocampus (HIP) were investigated in awake, freely moving rats.

RESULTS

R3487/MEM3454, at doses of 0.1-10 mg/kg, s.c., enhanced DA and ACh effluxes in rat mPFC and (HIP), with a peak effect at 0.3- to 0.6-mg/kg doses, producing a bell-shaped dose-response curve. Pretreatment with the selective nAChR antagonist, methyllycaconitine (1.0 mg/kg), completely blocked RG3487-induced (0.45 mg/kg) DA but not ACh efflux, while the selective 5-HT3 receptor agonist 1-(m-chlorophenyl)-biguanide (1.0 mg/kg) partially inhibited cortical ACh but not DA efflux. RG3487 (0.45 mg/kg) combined with atypical antipsychotic drug (APD) risperidone (0.1 mg/kg), but not typical APD haloperidol (0.1 mg/kg), induced a significantly greater increase in HIP ACh efflux. Their combined effect on DA efflux was additive. RG3487, combined with other atypical APDs, namely aripiprazole (0.3 mg/kg), olanzapine (1.0 mg/kg), and quetiapine (30 mg/kg), also produced additive effects on DA efflux.

CONCLUSIONS

These results suggest that RG3487 enhances DA efflux by nAChR stimulation, whereas ACh efflux is primarily mediated via 5-HT3 receptor antagonism, and that RG3487 alone or as augmentation may improve cognitive impairment in schizophrenia.

Effect of alpha7 nicotinic acetylcholine receptor agonists on attentional set-shifting impairment in rats

RATIONALE

Attentional set shifting, a measure of executive function, is impaired in schizophrenia patients. Current standard of care has little therapeutic benefit for treating cognitive dysfunction in schizophrenia; therefore, novel drugs and animal models for testing novel therapies are needed. The NMDA receptor antagonist, MK-801, produces deficits in a rat maze-based set-shifting paradigm, an effect which parallels deficits observed on tests of executive function in schizophrenia patients. Alpha7 nicotinic acetylcholine receptor (nAChR) agonists, currently under clinical development by several companies, show promise in treating cognitive symptoms in schizophrenia patients and can improve cognition in various animal models.

OBJECTIVES

The objectives of the present study were to determine whether the MK-801 deficit in set shifting could be reproduced in a drug discovery setting and to determine whether cognitive improvement could be detected for the first time in this task with alpha7 nAChR agonists.

RESULTS

The data presented here replicate findings that a systemic injection of the NMDA receptor antagonist MK-801 can induce a deficit in set shifting in rats. Furthermore, the deficit could be reversed by the atypical antipsychotic clozapine as well as by several alpha7 nAch receptor agonists (SSR-180711, PNU-282987, GTS-21) with varying in vitro properties.

CONCLUSIONS

Results indicate that the MK-801 set-shift assay is a useful preclinical tool for measuring prefrontal cortical function in rodents and can be used to identify novel mechanisms for the potential treatment of cognitive deficits in schizophrenia.

Evaluating the role of the alpha-7 nicotinic acetylcholine receptor in the pathophysiology and treatment of schizophrenia

The group of schizophrenia disorders affects approximately 1% of the population and has both genetic and environmental etiologies. Sufferers report various behavioral abnormalities including hallucinations and delusions (positive symptoms), reduced joy and amotivation (negative symptoms), plus inattention and poor learning (cognitive deficits). Despite the heterogeneous symptoms experienced, most patients smoke. The self-medication hypothesis posits that patients smoke to alleviate symptoms, consistent with evidence for nicotine-induced enhancement of cognition. While nicotine acts on multiple nicotinic acetylcholine receptors (nAChRs), the primary target of research is often the homomeric α7 nAChR. Given genetic linkages between schizophrenia and this receptor, its association with P50 sensory gating deficits, and its reduced expression in post-mortem brains, many have attempted to develop α7 nAChR ligands for treating schizophrenia. Recent evidence that ligands can be orthosteric agonists or positive allosteric modulators (PAMs) has revitalized the hope for treatment discovery. Herein, we present evidence regarding: (1) pathophysiological alterations of α7 nAChRs that might occur in patients; (2) mechanistic evidence for the normal action of α7 nAChRs; (3) preclinical studies using α7 nAChR orthosteric agonists and type I/II PAMs; and (4) where successful translational testing has occurred for particular compounds, detailing what is still required. We report that the accumulating evidence is positive, but that greater work is required using positron emission tomography to understand current alterations in α7 nAChR expression and their relationship to symptoms. Finally, cross-species behavioral tasks should be used more regularly to determine the predictive efficacy of treatments.

Radiochemical synthesis and in vivo evaluation of [18F]AZ11637326: an agonist probe for the α7 nicotinic acetylcholine receptor

INTRODUCTION

The alpha-7 nicotinic acetylcholine receptor (α7 nAChR) is key in brain communication and has been implicated in the pathophysiology of diseases of the central nervous system. A positron-emitting radioligand targeting the α7 nAChR would enable better understanding of a variety of neuropsychiatric illnesses, including schizophrenia and Alzheimer's disease, and could enhance the development of new drugs for these and other conditions. We describe our attempt to synthesize an α7 nAChR-selective radiotracer for positron emission tomography (PET).

METHODS

We prepared the high-affinity (K(d) = 0.2 nM) α7 nAChR agonist, 5'-(2-[(18)F]fluorophenyl)spiro[1-azabicyclo-[2.2.2]octane]-3,2'-(3'H)furo[2,3-b]pyridine, [(18)F]AZ11637326, in two steps, a nucleophilic fluorination followed by decarbonylation. We studied [(18)F]AZ11637326 in rodents, including mice lacking α7 nAChR, and in non-human primates.

RESULTS

[(18)F]AZ11637326 was synthesized in a non-decay-corrected radiochemical yield of 3% from the end of synthesis (90 min) with a radiochemical purity >90% and average specific radioactivity of 140GBq/μmol (3,781 mCi/μmol). Modest rodent brain uptake was observed (2-5% injected dose per gram of tissue, depending on specific activity), with studies comparing CD-1 and α7 nAChR null mice indicating an element of target-specific binding. Blocking studies in non-human primates did not reveal specific binding within the brain.

CONCLUSION

Despite the high affinity and target selectivity of AZ11637326 for α7 nAChR in vitro and encouraging rodent studies, receptor-mediated binding could not be demonstrated in non-human primates. Further structural optimization of compounds of this class will be required for them to serve as suitable radiotracers for PET.

The functional tumor necrosis factor-α (308A/G) polymorphism modulates attentional selection in elderly individuals

There has been increasing interest in understanding the role of inflammatory processes for cognitive functions in aging using molecular genetic approaches. Though this has mostly been evaluated in pathological aging, little is known about the relevance for cognitive functions in healthy aging in humans. On the basis of behavioral data and neurophysiological data (event-related potentials and time-frequency decomposition) we show that the A-allele of the functional tumor necrosis factor (TNF)-α -308 A/G polymorphism confers dysfunction in a number of cognitive processes: prolonged attentional selection indexed by a delayed P1/N1 complex, an increased P3a, which is interpreted as an enhanced distractibility by nonrelevant stimuli and compromised response selection mechanisms, as indexed by a reduced frontocentral N2. Time-frequency analyses show that allelic variations further exert their effects by modulating alpha and beta frequency oscillations. On a neurobiological level, these effects might be because of the interaction of TNF-α with glutamatergic neural transmission by which TNF-α is known to boost apoptotic mechanisms in elderly individuals.