Meeting Report for: The 5th Satellite Meeting to the SfN: Nicotinic Acetylcholine Receptors as Therapeutic Targets

The interaction of a small molecule made in one cell with a large receptor made in another is the signature event of cell signaling. Understanding the structure and energy changes associated with agonist activation is important for engineering drugs, receptors and synapses. The nicotinic acetylcholine receptor (AChR) is a approximately 300kD ion channel that binds the neurotransmitter acetylcholine (ACh) and other cholinergic agonists to elicit electrical responses in the central and peripheral nervous systems. This mini-review is in two sections. First, general concepts of skeletal muscle AChR operation are discussed in terms of energy landscapes for conformational change. Second, adult vs. fetal AChRs are compared with regard to interaction energies between ACh and agonist-site side chains, measured by single-channel electrophysiology and molecular dynamics simulations. The five aromatic residues that form the core of each agonist binding site can be divided into two working groups, a triad (led by alphaY190) that behaves similarly at all sites and a coupled pair (led by gammaW55) that has a large influence on affinity only in fetal AChRs. Each endplate AChR has 5 homologous subunits, two of alpha(1) and one each of beta, delta, and either gamma (fetal) or (adult). These nicotinic AChRs have only 2 functional agonist binding sites located in the extracellular domain, at alphadelta and either alphagamma or alpha subunit interfaces. The receptor undergoes a reversible, global isomerization between structures called C and O. The C shape does not conduct ions and has a relatively low affinity for ACh, whereas O conducts cations and has a higher affinity. When both agonist sites are empty (filled only with water) the probability of taking on the O conformation (PO) is low, <10(-6). When ACh molecules occupy the agonist sites the C-->O opening rate constant and C<-->O gating equilibrium constant increase dramatically. Following a pulse of ACh at the nerve-muscle synapse, the endplate current rises rapidly to reach a peak that corresponds to PO approximately 0.96.

The aim of the present study was to determine the impact of alpha5 nicotinic acetylcholine receptor (nAChR) subunit deletion in the mouse on the development and intensity of nociceptive behavior in various chronic pain models. The role of alpha5-containing nAChRs was explored in mouse models of chronic pain, including peripheral neuropathy (chronic constriction nerve injury, CCI), tonic inflammatory pain (the formalin test) and short and long-term inflammatory pain (complete Freund's adjuvant, CFA and carrageenan tests) in alpha5 knock-out (KO) and wild-type (WT) mice. The results showed that paw-licking time was decreased in the formalin test, and the hyperalgesic and allodynic responses to carrageenan and CFA injections were also reduced. In addition, paw edema in formalin-, carrageenan- or CFA-treated mice were attenuated in alpha5-KO mice significantly. Furthermore, tumor necrosis factor-alpha (TNF-alpha) levels of carrageenan-treated paws were lower in alpha5-KO mice. The antinociceptive effects of nicotine and sazetidine-A but not varenicline were alpha5-dependent in the formalin test. Both hyperalgesia and allodynia observed in the CCI test were reduced in alpha5-KO mice. Nicotine reversal of mechanical allodynia in the CCI test was mediated through alpha5-nAChRs at spinal and peripheral sites. In summary, our results highlight the involvement of the alpha5 nAChR subunit in the development of hyperalgesia, allodynia and inflammation associated with chronic neuropathic and inflammatory pain models. They also suggest the importance of alpha5-nAChRs as a target for the treatment of chronic pain.

Nicotinic acetylcholine receptors (nAChRs) are receptors for the neurotransmitter acetylcholine and are members of the 'Cys-loop' family of pentameric ligand-gated ion channels (LGICs). Acetylcholine binds in the receptor extracellular domain at the interface between two subunits and research has identified a large number of nAChR-selective ligands, including agonists and competitive antagonists, that bind at the same site as acetylcholine (commonly referred to as the orthosteric binding site). In addition, more recent research has identified ligands that are able to modulate nAChR function by binding to sites that are distinct from the binding site for acetylcholine, including sites located in the transmembrane domain. These include positive allosteric modulators (PAMs), negative allosteric modulators (NAMs), silent allosteric modulators (SAMs) and compounds that are able to activate nAChRs via an allosteric binding site (allosteric agonists). Our aim in this article is to review important aspects of the pharmacological diversity of nAChR allosteric modulators and to describe recent evidence aimed at identifying binding sites for allosteric modulators on nAChRs.

Nicotinic acetylcholine receptors (nAChRs) are expressed widely in the CNS, and mediate both synaptic and perisynaptic activities of endogenous cholinergic inputs and pharmacological actions of exogenous compounds (e.g., nicotine and choline). Behavioral studies indicate that nicotine improves such cognitive functions as learning and memory, however the cellular mechanism of these actions remains elusive. With help from newly developed biosensors and optogenetic tools, recent studies provide new insights on signaling mechanisms involved in the activation of nAChRs. Here we will review alpha7 nAChR's action in the tri-synaptic pathway in the hippocampus. The effects of alpha7 nAChR activation via either exogenous compounds or endogenous cholinergic innervation are detailed for spontaneous and evoked glutamatergic synaptic transmission and synaptic plasticity, as well as the underlying signaling mechanisms. In summary, alpha7 nAChRs trigger intracellular calcium rise and calcium-dependent signaling pathways to enhance glutamate release and induce glutamatergic synaptic plasticity.

Currently, there are no medications that target core deficits of social communication and restrictive, repetitive patterns of behavior in persons with autism spectrum disorders (ASDs). Adults with Down syndrome (DS) display a progressive worsening of adaptive functioning, which is associated with Alzheimer's disease (AD)-like histopathological changes in brain. Similar to persons with ASDs, there are no effective medication strategies to prevent or retard the progressive worsening of adaptive functions in adults with DS. Data suggest that the alpha7-subunit containing nicotinic acetylcholine receptor (alpha7nAChR) is implicated in the pathophysiology and serves as a promising therapeutic target of these disorders. In DS, production of the amyloidogenic Abeta1-42 peptide is increased and binds to the alpha7nAChR or the lipid milieu associated with this receptor, causing a cascade that results in cytotoxicity and deposition of amyloid plaques. Independently of their ability to inhibit the complexing of Abeta1-42 with the alpha7nAChR, alpha7nAChR agonists and positive allosteric modulators (PAMs) also possess procognitive and neuroprotective effects in relevant in vivo and in vitro models. The procognitive and neuroprotective effects of alpha7nAChR agonist interventions may be due, at least in part, to stimulation of the PI3K/Akt signaling cascade, cross-talk with the Wnt/beta-catenin signaling cascade and both transcriptional and non-transcriptional effects of beta-catenin, and effects of transiently increased intraneuronal concentrations of Ca(2+) on metabolism and the membrane potential. Importantly, alpha7nAChR PAMs are particularly attractive medication candidates because they lack intrinsic efficacy and act only when and where endogenous acetylcholine is released or choline is generated.

Nicotinic acetylcholine receptors (nAChRs) are widely distributed throughout the central nervous system, being expressed in neurons and non-neuronal cells, where they participate in a variety of physiological responses like memory, learning, locomotion, attention, among others. We will focus on the alpha7 nAChR subtype, which has been implicated in neuroprotection, synaptic plasticity and neuronal survival, and is considered as a potential therapeutic target for several neurological diseases. Oxidative stress and neuroinflammation are currently considered as two of the most important pathological mechanisms common in neurodegenerative diseases such as Alzheimer, Parkinson or Huntington diseases. In this review, we will first analysed the distribution and expression of nAChR in mammalian brain. Then, we focused on the function of the alpha7 nAChR subtype in neuronal and non-neuronal cells and its role in immune responses (cholinergic anti-inflammatory pathway). Finally, we will revise the anti-inflammatory pathway promoted via alpha7 nAChR activation that is related to recruitment and activation of Jak2/STAT3 pathway, which on the one hand inhibits NF-kappaB nuclear translocation, and on the other hand, activates the master regulator of oxidative stress Nrf2/HO-1. This review provides a profound insight into the role of the alpha7 nAChR subtype in microglia and point out to microglial alpha7/HO-1 pathway as an anti-inflammatory therapeutic target.

Copy number variants (CNVs) have been implicated in multiple neuropsychiatric conditions, including autism spectrum disorder (ASD), schizophrenia, and intellectual disability (ID). Chromosome 15q13 is a hotspot for such CNVs due to the presence of low copy repeat (LCR) elements, which facilitate non-allelic homologous recombination (NAHR). Several of these CNVs have been overrepresented in individuals with neuropsychiatric disorders; yet variable expressivity and incomplete penetrance are commonly seen. Dosage sensitivity of the CHRNA7 gene, which encodes for the alpha7 nicotinic acetylcholine receptor in the human brain, has been proposed to have a major contribution to the observed cognitive and behavioral phenotypes, as it represents the smallest region of overlap to all the 15q13.3 deletions and duplications. Individuals with zero to four copies of CHRNA7 have been reported in the literature, and represent a range of clinical severity, with deletions causing generally more severe and more highly penetrant phenotypes. Potential mechanisms to account for the variable expressivity within each group of 15q13.3 CNVs will be discussed.

The alpha-7 subtype of the nicotinic acetylcholine receptor (alpha7-nAChR) is fundamental to physiology; it mediates various brain functions and represents an important target for drug discovery. Exploration of the brain nicotinic acetylcholine receptors (nAChRs) using positron-emission tomography (PET) will make it possible to better understand the important role of this receptor and to study its involvement in schizophrenia, bipolar disorder, Alzheimer's and Parkinson's diseases, drug dependence, inflammation and many other disorders and simplify the development of nicotinic drugs for treatment of these disorders. Until recently, PET imaging of alpha7-nAChRs has been impeded by the absence of good radiotracers. This review describes various endeavors to develop alpha7-nAChR PET tracers by several research groups including the author's group. Most initial PET tracers for imaging alpha7-nAChRs did not exhibit suitable imaging properties due to their low specific binding. Newly discovered [(18)F]ASEM is the first highly specific alpha7-nAChR radioligand and in 2014 it was translated to human PET imaging.

The nervous system must balance excitatory and inhibitory input to constrain network activity levels within a proper dynamic range. This is a demanding requirement during development, when networks form and throughout adulthood as networks respond to constantly changing environments. Defects in the ability to sustain a proper balance of excitatory and inhibitory activity are characteristic of numerous neurological disorders such as schizophrenia, Alzheimer's disease, and autism. A variety of homeostatic mechanisms appear to be critical for balancing excitatory and inhibitory activity in a network. These are operative at the level of individual neurons, regulating their excitability by adjusting the numbers and types of ion channels, and at the level of synaptic connections, determining the relative numbers of excitatory versus inhibitory connections a neuron receives. Nicotinic cholinergic signaling is well positioned to contribute at both levels because it appears early in development, extends across much of the nervous system, and modulates transmission at many kinds of synapses. Further, it is known to influence the ratio of excitatory-to-inhibitory synapses formed on neurons during development. GABAergic inhibitory neurons are likely to be key for maintaining network homeostasis (limiting excitatory output), and nicotinic signaling is known to prominently regulate the activity of several GABAergic neuronal subtypes. But how nicotinic signaling achieves this and how networks may compensate for the loss of such input are important questions remaining unanswered. These issues are reviewed.

Cholinergic signaling via the nicotinic acetylcholine receptors (nAChRs) in the mesolimbic circuitry is involved in the rewarding effects of abused drugs such as cocaine and opioids. In mouse studies, nonselective nAChR antagonist mecamylamine blocks cocaine-induced conditioned place preference (CPP) and behavioral sensitization. Among subtype-selective nAChR antagonists, the beta2-selective antagonist dihydrobetaerythroidine and alpha7 antagonist methyllycaconitine (MLA), but not MLA alone prevent behavioral sensitization to cocaine. Since the role of the alpha3beta4 nAChR subtype in the rewarding and behavioral effects of cocaine is unknown, the present study investigated the effect of two potent and selective alpha3beta4 nAChR ligands, AT-1001 and AT-1012, on the acquisition of cocaine-induced CPP and behavioral sensitization in mice. At 5-30mg/kg, cocaine produced robust CPP, whereas behavioral sensitization of locomotor activity was only observed at the higher doses (20-30mg/kg). Pretreatment with AT-1001 (1-10mg/kg) or AT-1012 (3-10mg/kg) blocked CPP induced by 5mg/kg cocaine, but not by 30mg/kg cocaine. Lower doses of AT-1001 (0.3-1mg/kg) and AT-1012 (1-3mg/kg) did not affect the increase in locomotor activity induced by 5 or 30mg/kg cocaine. But AT-1001, at these doses, blocked locomotor sensitization induced by 30mg/kg cocaine. These results indicate that the alpha3beta4 nAChR play a role in the rewarding and behavioral effects of cocaine, and that selective alpha3beta4 nAChR ligands can attenuate cocaine-induced behavioral phenomena. Since the selective alpha3beta4 nAChR functional antagonist AT-1001 has also been shown to block nicotine self-administration in rats, the present results suggest that alpha3beta4 nAChRs may be a target for the treatment of cocaine addiction as well as for cocaine-nicotine comorbid addiction.

Schizophrenia is a chronic disease that has been hypothesized to be linked to neurodevelopmental abnormalities. Schizophrenia patients exhibit impairments in basic sensory processing including sensory gating deficits in P50 and mismatch negativity (MMN). Neuronal nicotinic acetylcholine receptor (nAChR) agonists have been reported to attenuate these deficits. Gestational exposure of rats to methylazoxymethanol acetate (MAM) at embryonic day 17 leads to developmental disruption of the limbic-cortical system. MAM exposed offspring show neuropathological and behavioral changes that have similarities with those seen in schizophrenia. In this study, we aimed to assess whether N40 auditory sensory gating (the rodent form of P50 gating) and MMN deficits as measures of auditory evoked potential (AEP) electroencephalography (EEG) are present in MAM rats and whether nAChR agonists could attend the deficit. E17 male MAM and sham rats were implanted with cortical electrodes at 2 months of age. EEG recordings evaluating N40 gating and MMN paradigms were done comparing effects of vehicle (saline), nicotine and the alpha7 agonist ABT-107. Deficits were seen for MAM rats compared to sham animals in both N40 auditory sensory gating and MMN AEP recordings. There was a strong trend for N40 deficits to be attenuated by both nicotine (0.16mg/kg i.p. base) and ABT-107 (1.0mg/kg i.p. base). MMN deficits were significantly attenuated by ABT-107 but not by nicotine. These data support the MAM model as a useful tool for translating pharmacodynamic effects in clinical medicine studies of novel therapeutic treatments for schizophrenia.

Recent published findings have shown that many proteins discovered in the immune system and residing on immune cells with well established immune-related functions are also found in neurons of the central nervous system. These studies have uncovered a rich variety of neuronal functions attributed to these immune proteins. This review will focus on two key interacting protein complexes that previously were known for adaptive immune reactions, the major histocompatability complex and the T-cell receptor complex. We will review where these immune proteins are expressed in the CNS and their neuronal function.

Neuronal nicotinic acetylcholine receptors (nAChRs) play an important role in a variety of modulatory and regulatory processes including neurotransmitter release and synaptic transmission in various brain regions of the central nervous system (CNS). Glutamate is the principal excitatory neurotransmitter in the brain and the glutamatergic system participates in the pathophysiology of several neuropsychiatric disorders. Underpinning the importance of nAChRs, many studies demonstrated that nAChRs containing the alpha7 subunit facilitate glutamate release. Here, we review the currently available body of experimental evidence pertaining to alpha7 subunit containing nAChRs in their contribution to the modulation of glutamatergic neurotransmission, and we highlight the role of alpha7 in synaptic plasticity, the morphological and functional maturation of the glutamatergic system and therefore its important contribution in the modulation of neural circuits of the CNS.

Anxiety disorders are a group of crippling mental diseases affecting millions of Americans with a 30% lifetime prevalence and costs associated with healthcare of $42.3 billion. While anxiety disorders show high levels of co-morbidity with smoking (45.3% vs. 22.5% in healthy individuals), they are also more common among the smoking population (22% vs. 11.1% in the non-smoking population). Moreover, there is clear evidence that smoking modulates symptom severity in patients with anxiety disorders. In order to better understand this relationship, several animal paradigms are used to model several key symptoms of anxiety disorders; these include fear conditioning and measures of anxiety. Studies clearly demonstrate that nicotine mediates acquisition and extinction of fear as well as anxiety through the modulation of specific subtypes of nicotinic acetylcholine receptors (nAChRs) in brain regions involved in emotion processing such as the hippocampus. However, the direction of nicotine's effects on these behaviors is determined by several factors that include the length of administration, hippocampus-dependency of the fear learning task, and source of anxiety (novelty-driven vs. social anxiety). Overall, the studies reviewed here suggest that nicotine alters behaviors related to fear and anxiety and that nicotine contributes to the development, maintenance, and reoccurrence of anxiety disorders.

Aggression is frequently comorbid with neuropsychiatric conditions and is a predictor of worse outcomes, yet current pharmacotherapies are insufficient and have debilitating side effects, precluding broad use. Multiple models of aggression across species suggest that the nicotinic acetylcholine receptor (nAChR) agonist nicotine has anti-aggressive (serenic) properties. Here we demonstrate dose-dependent serenic effects of acute nicotine administration in three distinct mouse strains: C57BL/6, BALB/c, and CD1. While acute nicotine administration (0.25mg/kg) modestly reduced solitary homecage locomotion, this could not account for nicotine's serenic effects since social encounters eliminated the hypolocomotor effect, and nicotine did not alter social interaction times. Pretreatment with the homomeric (alpha7 subunit) nAChR antagonist methyllycaconitine (5mg/kg), but not the heteromeric (beta2 or beta4 subunit-containing) nAChR antagonist dihydro-beta-erythroidine (DHbetaE, 3mg/kg), blocked the serenic effects of nicotine. By contrast, pretreatment with DHbetaE blocked the effect of acute nicotine administration on locomotion, uncoupling nicotine's serenic and hypolocomotor effects. Finally, the alpha7 nAChR partial agonist GTS-21 reduced aggression in C57BL/6 mice. These results support the idea that acute nicotine administration has serenic effects and provide evidence for specificity of this effect distinct from effects on locomotion. Furthermore, pharmacological studies suggest that activation of alpha7 nAChRs underlies the serenic effects of nicotine. Further studies of nAChRs could enhance understanding of the neurobiology of aggression and may lead to the development of novel, more specific treatments for pathological aggression.

Myasthenia gravis (MG) is an organ-specific autoimmune disease characterized by muscle fatigability. In most cases, it is mediated by autoantibodies targeting muscle nicotinic acetylcholine receptors (AChRs) at the neuromuscular junction. Experimental autoimmune myasthenia gravis (EAMG) is an animal model for MG, which is usually induced by immunization with AChR purified from fish electric organ. Pathological autoantibodies to AChRs are directed at the extracellular surface, especially the main immunogenic region (MIR). Current treatments for MG can help many but not all patients. Antigen-specific immunosuppressive therapy for MG that specifically suppresses the autoimmune response without affecting the entire immune system and avoids side effects of general immunosuppression is currently unavailable. Early attempts at antigen-specific immunosuppression for EAMG using AChR extracellular domain sequences that form epitopes for pathological autoantibodies risked provoking autoimmunity rather than suppressing it. We discovered a novel approach to specific immunosuppression of EAMG with a therapeutic vaccine consisting of bacterially-expressed human AChR cytoplasmic domains, which has the potential to specifically suppress MG without danger of causing exacerbation. This approach prevents development of chronic EAMG when initiated immediately after the acute phase of EAMG, and rapidly reverses established chronic EAMG when started during the chronic phase of EAMG. Successfully treated rats exhibited long-term resistance to re-induction of EAMG. In this review we also discuss the current understanding of the mechanisms by which the therapy works. Vaccination with AChR cytoplasmic domains in adjuvant is promising as a safe, antigen-specific, potent, effective, rapidly acting, and long lasting approach to therapy of MG.

Mutations in the progranulin gene cause frontotemporal dementia (FTD), a debilitating neurodegenerative disease that involves atrophy of the frontal and temporal lobes and affects personality, behavior, and language. Progranulin-deficient mouse models of FTD exhibit deficits in compulsive and social behaviors reminiscent of patients with FTD, and develop excessive microgliosis and increased release of inflammatory cytokines. Activation of nicotinic acetylcholine receptors (nAChRs) by nicotine or specific alpha7 nAChR agonists reduces neuroinflammation. Here, we investigated whether activation of nAChRs by nicotine or alpha7 agonists improved the excessive inflammatory and behavioral phenotypes of a progranulin-deficient FTD mouse model. We found that treatment with selective alpha7 agonists, PHA-568487 or ABT-107, strongly suppressed the activation of NF-kappaB in progranulin-deficient cells. Treatment with ABT-107 also reduced microgliosis, decreased TNFalpha levels, and reduced compulsive behavior in progranulin-deficient mice. Collectively, these data suggest that targeting activation of the alpha7 nAChR pathway may be beneficial in decreasing neuroinflammation and reversing some of the behavioral deficits observed in progranulin-deficient FTD.

Subchronic oxidative stress and inflammation are being increasingly implicated in the pathogenesis of numerous diseases, such as Alzheimer's or Parkinson's disease. This study was designed to evaluate the potential protective role of alpha7 nicotinic receptor activation in an in vitro model of neurodegeneration based on subchronic oxidative stress. Rat organotypic hippocampal cultures (OHCs) were exposed for 4 days to low concentration of lipopolysaccharide (LPS) and the complex III mitochondrial blocker, antimycin-A. Antimycin-A (0.1muM) and lipopolysaccharide (1ng/ml) caused low neurotoxicity on their own, measured as propidium iodide fluorescence in CA1 and CA3 regions. However, their combination (LPS/AA) caused a greater detrimental effect, in addition to mitochondrial depolarization, overproduction of reactive oxygen species (ROS) and Nox4 overexpression. Antimycin-A per se increased ROS and mitochondrial depolarization, although these effects were significantly higher when combined with LPS. More interesting was the finding that exposure of OHCs to the combination of LPS/AA triggered aberrant protein aggregation, measured as thioflavin S immunofluorescence. The alpha7 nicotinic receptor agonist, PNU282987, prevented the neurotoxicity and the pathological hallmarks observed in the LPS/AA subchronic toxicity model (oxidative stress and protein aggregates); these effects were blocked by alpha-bungarotoxin and tin protoporphyrin, indicating the participation of alpha7 nAChRs and heme-oxygenase I induction. In conclusion, subchronic exposure of OHCs to low concentration of antimycin-A plus LPS reproduced pathological features of neurodegenerative disorders. alpha7 nAChR activation ameliorated these alterations by a mechanism involving heme-oxygenase I induction.

Studies in human populations consistently demonstrate an interaction between nicotine and ethanol use, each drug influencing the use of the other. Here we present data and review evidence from animal studies that nicotine influences operant self-administration of ethanol. The operant reinforcement paradigm has proven to be a behaviorally relevant and quantitative model for studying ethanol-seeking behavior. Exposure to nicotine can modify the reinforcing properties of ethanol during different phases of ethanol self-administration, including acquisition, maintenance, and reinstatement. Our data suggest that non-daily intermittent nicotine exposure can trigger a long-lasting increase in ethanol self-administration. The biological basis for interactions between nicotine and ethanol is not well understood but may involve the stress hormone systems and adaptations in the mesolimbic dopamine system. Future studies that combine operant self-administration with techniques for monitoring or manipulating in vivo neurophysiology may provide new insights into the neuronal mechanisms that link nicotine and alcohol use.

Accumulating evidence suggests that CNS alpha7 nicotinic acetylcholine receptors (nAChRs) are important targets for the development of therapeutic approaches for Parkinson's disease. This progressive neurodegenerative disorder is characterized by debilitating motor deficits, as well as autonomic problems, cognitive declines, changes in affect and sleep disturbances. Currently l-dopa is the gold standard treatment for Parkinson's disease motor problems, particularly in the early disease stages. However, it does not improve the other symptoms, nor does it reduce the inevitable disease progression. Novel therapeutic strategies for Parkinson's disease are therefore critical. Extensive pre-clinical work using a wide variety of experimental models shows that nicotine and nAChR agonists protect against damage to nigrostriatal and other neuronal cells. This observation suggests that nicotine and/or nAChR agonists may be useful as disease modifying agents. Additionally, studies in several parkinsonian animal models including nonhuman primates show that nicotine reduces l-dopa-induced dyskinesias, a side effect of l-dopa therapy that may be as incapacitating as Parkinson's disease itself. Work with subtype selective nAChR agonists indicate that alpha7 nAChRs are involved in mediating both the neuroprotective and antidyskinetic effects, thus offering a targeted strategy with optimal beneficial effects and minimal adverse responses. Here, we review studies demonstrating a role for alpha7 nAChRs in protection against neurodegenerative effects and for the reduction of l-dopa-induced dyskinesias. Altogether, this work suggests that alpha7 nAChRs may be useful targets for reducing Parkinson's disease progression and for the management of the dyskinesias that arise with l-dopa therapy.

Synchronization of neuronal network oscillations within the cortex and hippocampus has been closely linked to various cognitive domains, including attention, learning, and memory. The frequency, power, and connectivity of hippocampal oscillations provide quantitative measures for examining the modulation of network activity, which influences mnemonic functions and memory formation. The wide distribution of alpha7 nicotinic acetylcholine receptors (alpha7 nAChRs) throughout the hippocampus makes them well positioned to modulate neuronal network activity. Elicitation of hippocampal theta through high frequency stimulation of the brainstem nucleus pontis oralis (nPO) is shown to be sensitive to several agents exhibiting pharmacological effects on cognition, thus representing a suitable preclinical screening assay for such drugs, including alpha7 nAChR agonists. We hypothesize that increases in theta power and theta-phase gamma-amplitude coupling due to alpha7 nAChR agonists during elicited hippocampal oscillations could reflect changes in synchronous activity of pyramidal neurons which is a critical factor for hippocampal-dependent cognitive function. In this review, four major topics are discussed: neuronal network oscillations in the hippocampus, the characteristics and distribution of alpha7 nAChRs therein, the modulation of elicited hippocampal theta and gamma oscillations by alpha7 nAChR agonists, as well as potential intrinsic roles of alpha7 nAChRs in hippocampal oscillations using alpha7 nAChR knock-out mice.

Pharmacological activation of alpha7 nicotinic acetylcholine receptors (alpha7 nAChRs) may improve cognition in schizophrenia and Alzheimer's disease. The present studies describe an integrated pharmacological analysis of the effects of FRM-17874, an analogue of encenicline, on alpha7 nAChRs in vitro and in behavioral and neurophysiological assays relevant to cognitive function. FRM-17874 demonstrated high affinity binding to human alpha7 nAChRs, displacing [(3)H]-methyllacaconitine (Ki=4.3nM). In Xenopus laevis oocytes expressing human alpha7 nAChRs, FRM-17874 acted as an agonist, evoking inward currents with an EC50 of 0.42muM. Lower concentrations of FRM-17874 (0.01-3nM) elicited no detectable current, but primed receptors to respond to sub-maximal concentrations of acetylcholine. FRM-17874 improved novel object recognition in rats, and enhanced memory acquisition and reversal learning in the mouse water T-maze. Neurophysiological correlates of cognitive effects of drug treatment, such as synaptic transmission, long-term potentiation, and hippocampal theta oscillation were also evaluated. Modulation of synaptic transmission and plasticity was observed in rat hippocampal slices at concentrations of 3.2 and 5nM. FRM-17874 showed a dose-dependent facilitation of stimulation-induced hippocampal theta oscillation in mice and rats. The FRM-17874 unbound brain concentration-response relationship for increased theta oscillation power was similar in both species, exhibited a biphasic pattern peaking around 3nM, and overlapped with active doses and exposures observed in cognition assays. In summary, behavioral and neurophysiological assays indicate a bell-shaped effective concentration range and this report represents the first attempt to explain the concentration-response function of alpha7 nAChR-mediated pro-cognitive effects in terms of receptor pharmacology.

Methadone is a long-acting opioid agonist that is frequently prescribed as a treatment for opioid addiction. Almost all methadone maintenance patients are smokers, and there is a correlation between smoking habit and use of methadone. Methadone administration increases tobacco smoking, and heavy smokers use higher doses of methadone. Nevertheless, methadone maintenance patients are willing to quit smoking although their quit rates are low. Studies on nicotine-methadone interactions provide an example of the bedside-to-bench approach, i.e., observations in clinical settings have been studied experimentally in vivo and in vitro. In vivo studies have revealed the interplay between nicotine and the endogenous opioid system. At the receptor level, methadone has been shown to be an agonist of human alpha7 nAChRs and a non-competitive antagonist of human alpha4beta2 and alpha3* nAChRs. These drugs do not have significant interactions at the level of drug metabolism, and thus the interaction is most likely pharmacodynamic. The net effect of the interaction may depend on individual characteristics because pharmacogenetic factors influence the disposition of both methadone and nicotine.

Neuronal nicotinic acetylcholine receptors (nAChRs) containing the alpha3 subunit are known for their prominent role in normal ganglionic transmission while their involvement in the mechanisms underlying nicotine addiction and smoking-related disease has been emerging only in recent years. The amount of information available on the maturation and trafficking of alpha3-containing nAChRs is limited. We previously showed that UBXN2A is a p97 adaptor protein that facilitates the maturation and trafficking of alpha3-containing nAChRs. Further investigation of the mechanisms of UBXN2A actions revealed that the protein interacts with CHIP (carboxyl terminus of Hsc70 interacting protein), whose ubiquitin E3 ligase activity regulates the degradation of several disease-related proteins. We show that CHIP displays E3 ligase activity toward the alpha3 nAChR subunit and contributes to its ubiquitination and subsequent degradation. UBXN2A interferes with CHIP-mediated ubiquitination of alpha3 and protects the nicotinic receptor subunit from endoplasmic reticulum associated degradation (ERAD). UBXN2A also cross-talks with VCP/p97 and HSC70/HSP70 proteins in a complex where alpha3 is likely to be targeted by CHIP. Overall,we identify CHIP as an E3 ligase for alpha3 and UBXN2A as a protein that may efficiently regulate the stability of CHIP's client substrates.

The challenges associated with developing more effective treatments for neurologic and psychiatric illness such as Alzheimer's disease and schizophrenia are considerable. Both the symptoms and the pathophysiology of these conditions are complex and poorly understood and the clinical presentations across different patients can be very heterogeneous. Moreover, it has become apparent that the reductionist approach to drug discovery for these illnesses that has dominated the field for decades (i.e., the development of highly selective compounds or other treatment modalities focused on a very specific pathophysiologic target) has not been widely successful. Accordingly, a variety of new strategies have emerged including the development of "multitarget-directed ligands" (MTDLs), the development and/or identification of compounds that exhibit "multifunctional" activity (e.g., pro-cognitive plus neuroprotective, pro-cognitive plus antipsychotic activity), "repurposing" strategies for existing compounds that have other clinical indications, and novel "adjunctive" treatment strategies that might enhance the efficacy of the currently available treatments. Interestingly, a variety of ligands at nicotinic acetylcholine receptors (nAChRs) appear to have the potential to fulfill one or more of these desirable properties (i.e., multifunctional, repurposing, or adjunctive treatment potential). The purpose of this review (while not all-inclusive) is to provide an overview of a variety of nAChR ligands that demonstrate potential in these categories, particularly, "multifunctional" properties. Due to their densities in the mammalian brain and the amount of literature available, the review will focus on ligands of the high affinity alpha4beta2 nAChR and the low affinity alpha7 nAChR.

Neuronal nicotinic acetylcholine receptors (nAChRs) are the primary binding sites for nicotine within the brain. Using alpha(alpha)2 nAChR subunit-null mutant mice, the current study evaluates whether the absence of this gene product during mecamylamine-precipitated nicotine withdrawal eliminates neuronal activity within selective midbrain and limbic brain regions, as determined by the expression of the immediate early gene, cfos. Our results demonstrate that nicotine withdrawal enhances neuronal activity within the interpeduncular nucleus and dorsal hippocampus, which is absent in mice null for alpha2-containing nAChRs. In contrast, we observe that alpha2-null mutant mice exhibit a suppression of neuronal activity in the dentate gyrus in mice undergoing nicotine withdrawal. Interestingly, alpha2-null mutant mice display potentiated neuronal activity specifically within the stratum lacunosum moleculare layer of the hippocampus, independent of nicotine withdrawal. Overall, our findings demonstrate that alpha2-null mutant mice have altered cfos expression in distinct populations of neurons within selective midbrain and limbic brain structures that mediate baseline and nicotine withdrawal-induced neuronal activity.

Nicotine addiction is highly prevalent in current society and is often comorbid with other diseases. In the central nervous system, nicotine acts as an agonist for nicotinic acetylcholine receptors (nAChRs) and its effects depend on location and receptor composition. Although nicotinic receptors are found in most brain regions, many studies on addiction have focused on the mesolimbic system and its reported behavioral correlates such as reward processing and reinforcement learning. Profound modulatory cholinergic input from the pedunculopontine and laterodorsal tegmentum to dopaminergic midbrain nuclei as well as local cholinergic interneuron projections to dopamine neuron axons in the striatum may play a major role in the effects of nicotine. Moreover, an indirect mesocorticolimbic feedback loop involving the medial prefrontal cortex may be involved in behavioral characteristics of nicotine addiction. Therefore, this review will highlight current understanding of the effects of nicotine on the function of mesolimbic and mesocortical dopamine projections in the mesocorticolimbic circuit.