Neuronal nicotinic receptors: from structure to pathology

Cloning and spatiotemporal expression of zebrafish neuronal nicotinic acetylcholine receptor alpha 6 and alpha 4 subunit RNAs

Acetylcholine plays an important role in regulation of nervous system development and function. We are developing zebrafish (Danio rerio) as a model system to study the role of specific neuronal nicotinic acetylcholine receptor (nAChR) subtypes in development and the effects of nicotine on the developing vertebrate nervous system. We previously characterized the expression of several zebrafish nAChR subunits. To further develop the zebrafish model, here we report a study on the molecular characterization of two additional nAChR subunit genes, designated chrna6 and chrna4. Both zebrafish nAChRs have a high degree of sequence identity to nAChRs expressed in a variety of mammalian species. Reverse transcription polymerase chain reaction was used to show that both nAChR subunit RNAs were expressed early in zebrafish development, with the chrna4 transcript present at 3 hours postfertilization (hpf) and the chrna6 RNA present at 10 hpf. In situ hybridization was used to localize chrna6 and chrna4 RNA expression in 24, 48, 72, and 96 hpf zebrafish. The chrna6 and chrna4 RNAs were each expressed in a unique pattern, which changed during development. At various ages, chrna6 was expressed in Rohon-Beard sensory neurons, trigeminal ganglion, retina, and the pineal gland. Most notably, chrna6 was expressed in catecholaminergic neurons in the midbrain, but was also present in noncatecholaminergic cells in both midbrain and hindbrain. The expression of chrna6 RNA in catecholaminergic cells supports the use of zebrafish as a valid model system to better understand the molecular basis of cholinergic regulation of dopaminergic signaling and the role of alpha6-containing nAChRs in Parkinson's disease. The most notable chrna4 expression was in neural crest cells at 24 hpf and reticulospinal neurons in hindbrain at 48 hpf. chrna4 RNA exhibited a widespread and robust expression pattern in the midbrain in 72 hpf and 96 hpf zebrafish.

Ric-3 chaperone-mediated stable cell-surface expression of the neuronal alpha7 nicotinic acetylcholine receptor in mammalian cells

AIM

Studies of the alpha7-type neuronal nicotinic acetylcholine receptor (AChR), one of the receptor forms involved in many physiologically relevant processes in the central nervous system, have been hampered by the inability of this homomeric protein to assemble in most heterologous expression systems. In a recent study, it was shown that the chaperone Ric-3 is necessary for the maturation and functional expression of alpha7-type AChRs(1). The current work aims at obtaining and characterizing a cell line with high functional expression of the human alpha7 AChR.

METHODS

Ric-3 cDNA was incorporated into SHE-P1-halpha7 cells expressing the alpha7-type AChR. Functional studies were undertaken using single-channel patch-clamp recordings. Equilibrium and kinetic [(125)I]alpha-bungarotoxin binding assays, as well as fluorescence microscopy using fluorescent alpha-bungarotoxin, anti-alpha7 antibody, and GFP-alpha7 were performed on the new clone.

RESULTS

The human alpha7-type AChR was stably expressed in a new cell line, which we coined SHE-P1-halpha7-Ric-3, by co-expression of the chaperone Ric-3. Cell-surface AChRs exhibited [(125)I]alphaBTX saturable binding with an apparent K(D) of about 55 nmol/L. Fluorescence microscopy revealed dispersed and micro-clustered AChR aggregates at the surface of SHE-P1-halpha7-Ric-3 cells. Larger micron-sized clusters were observed in the absence of receptor-clustering proteins or upon aggregation with anti-alpha7 antibodies. In contrast, chaperone-less SHE-P1-halpha7 cells expressed only intracellular alpha7 AChRs and failed to produce detectable single-channel currents.

CONCLUSION

The production of a stable and functional cell line of neuroepithelial lineage with robust cell-surface expression of neuronal alpha7-type AChR, as reported here, constitutes an important advance in the study of homomeric receptors in mammalian cells.

A role for Leu247 residue within transmembrane domain 2 in ginsenoside-mediated alpha7 nicotinic acetylcholine receptor regulation

Nicotinic acetylcholine receptors (nAChRs) play important roles in nervous system functions and are involved in a variety of diseases. We previously demonstrated that ginsenosides, the active ingredients of Panax ginseng, inhibit subsets of nAChR channel currents, but not alpha7, expressed in Xenopus laevis oocytes. Mutation of the highly conserved Leu247 to Thr247 in the transmembrane domain 2 (TM2) channel pore region of alpha7 nAChR induces alterations in channel gating properties and converts alpha7 nAChR antagonists into agonists. In the present study, we assessed how point mutations in the Leu247 residue leading to various amino acids affect 20(S)-ginsenoside Rg(3) (Rg(3)) activity against the alpha7 nAChR. Mutation of L247 to L247A, L247D, L247E, L247I, L247S, and L247T, but not L247K, rendered mutant receptors sensitive to Rg(3). We further characterized Rg(3) regulation of L247T receptors. We found that Rg(3) inhibition of mutant alpha7 nAChR channel currents was reversible and concentration-dependent. Rg(3) inhibition was strongly voltage-dependent and noncompetitive manner. These results indicate that the interaction between Rg(3) and mutant receptors might differ from its interaction with the wild-type receptor. To identify differences in Rg(3) interactions between wild-type and L247T receptors, we utilized docked modeling. This modeling revealed that Rg(3) forms hydrogen bonds with amino acids, such as Ser240 of subunit I and Thr244 of subunit II and V at the channel pore, whereas Rg(3) localizes at the interface of the two wild-type receptor subunits. These results indicate that mutation of Leu247 to Thr247 induces conformational changes in the wild-type receptor and provides a binding pocket for Rg(3) at the channel pore.

Structural and functional diversity of native brain neuronal nicotinic receptors

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of ligand-gated ion channels present in the central and peripheral nervous systems, that are permeable to mono- and divalent cations. They share a common basic structure but their pharmacological and functional properties arise from the wide range of different subunit combinations making up distinctive subtypes. nAChRs are involved in many physiological functions in the central and peripheral nervous systems, and are the targets of the widely used drug of abuse nicotine. In addition to tobacco dependence, changes in their number and/or function are associated with neuropsychiatric disorders, ranging from epilepsy to dementia. Although some of the neural circuits involved in the acute and chronic effects of nicotine have been identified, much less is known about which native nAChR subtypes are involved in specific physiological functions and pathophysiological conditions. We briefly review some recent findings concerning the structure and function of native nAChRs, focusing on the subtypes identified in the mesostriatal and habenulo-interpeduncular pathways, two systems involved in nicotine reinforcement and withdrawal. We also discuss recent findings concerning the effect of chronic nicotine on the expression of native subtypes.

Brain-targeting gene delivery and cellular internalization mechanisms for modified rabies virus glycoprotein RVG29 nanoparticles

A 29 amino-acid peptide derived from the rabies virus glycoprotein (RVG29) was exploited as a ligand for efficient brain-targeting gene delivery. RVG29 was modified on polyamidoamine dendrimers (PAMAM) through bifunctional PEG, then complexed with DNA, yielding PAMAM-PEG-RVG29/DNA nanoparticles (NPs). The NPs were observed to be uptaken by brain capillary endothelial cells (BCECs) through a clathrin and caveolae mediated energy-depending endocytosis. The specific cellular uptake can be inhibited by free RVG29 and GABA but not by nicotinic acetylcholine receptor (nAchR) agonists/antagonists, indicating RVG29 probably relates to the GABA(B) receptor besides nAchR reported previously. PAMAM-PEG-RVG29/DNA NPs showed higher blood-brain barrier (BBB)-crossing efficiency than PAMAM/DNA NPs in an in vitro BBB model. In vivo imaging showed that the NPs were preferably accumulated in brain. The report gene expression of the PAMAM-PEG-RVG29/DNA NPs was observed in brain, and significantly higher than unmodified NPs. Thus, PAMAM-PEG-RVG29 provides a safe and noninvasive approach for the gene delivery across the BBB.

Nicotinic actions on neuronal networks for cognition: general principles and long-term consequences

Nicotine enhances cognitive performance in humans and laboratory animals. The immediate positive actions of nicotine on learning, memory and attention are well-documented. Several brain areas involved in cognition, such as the prefrontal cortex, have been implicated. Besides acute effects on these brain areas and on brain function, a picture is emerging showing that long-term consequences of nicotine exposure during adolescence can be detrimental for cognitive performance. The majority of adult smokers started the habit during adolescence. Our knowledge on the types of nicotinic receptors in the brain areas that are candidates for mediating nicotine's effects is increasing. However, much less is known about the underlying cellular mechanisms. A series of recent studies have uncovered exciting features of the mechanisms by which nicotine alters prefrontal cortex neuronal activity, synaptic plasticity, gene expression and cognitive function, and how these changes may have a lasting effect on the developing brain. In this review, we discuss these exciting findings and identify several common principles by which nicotinic receptor activation modulates cortical circuits involved in cognition. Understanding how nicotine induces long-term changes in neuronal circuits and alters plasticity in the prefrontal cortex is essential to determining how these mechanisms interact to alter cognition.

An allosteric modulator of alpha7 nicotinic receptors, N-(5-Chloro-2,4-dimethoxyphenyl)-N'-(5-methyl-3-isoxazolyl)-urea (PNU-120596), causes conformational changes in the extracellular ligand binding domain similar to those caused by acetylcholine

Nicotinic acetylcholine receptors are implicated in several neuropsychiatric disorders, including nicotine addiction, Alzheimer's, schizophrenia, and depression. Therefore, they represent a critical molecular target for drug development and targeted therapeutic intervention. Understanding the molecular mechanisms by which allosteric modulators enhance activation of these receptors is crucial to the development of new drugs. We used the substituted cysteine accessibility method to study conformational changes induced by the positive allosteric modulator N-(5-chloro-2,4-dimethoxyphenyl)-N'-(5-methyl-3-isoxazolyl)-urea (PNU-120596) in the extracellular ligand binding domain of alpha7 nicotinic receptors carrying the L247T mutation. PNU-120596 caused changes in cysteine accessibility at the inner beta sheet, transition zone, and agonist binding site. These changes in accessibility are similar to but not identical to those caused by ACh alone. In particular, PNU-120596 induced changes in MTSEA accessibility at N170C (in the transition zone) that were substantially different from those evoked by acetylcholine (ACh). We found that PNU-120596 induced changes at position E172C in the absence of allosteric modulation. We identified a cysteine mutation of the agonist binding site (W148C) that exhibited an unexpected phenotype in which PNU-120596 acts as a full agonist. In this mutant, ACh-evoked currents were more sensitive to thiol modification than PNU-evoked currents, suggesting that PNU-120596 does not bind at unoccupied agonist-binding sites. Our results provide evidence that binding sites for PNU-120596 are not in the agonist-binding sites and demonstrate that positive allosteric modulators such as PNU-120596 enhance agonist-evoked gating of nicotinic receptors by eliciting conformational effects that are similar but nonidentical to the gating conformations promoted by ACh.

Genetic variation in the CHRNA5 gene affects mRNA levels and is associated with risk for alcohol dependence

Alcohol dependence frequently co-occurs with cigarette smoking, another common addictive behavior. Evidence from genetic studies demonstrates that alcohol dependence and smoking cluster in families and have shared genetic vulnerability. Recently a candidate gene study in nicotine dependent cases and nondependent smoking controls reported strong associations between a missense mutation (rs16969968) in exon 5 of the CHRNA5 gene and a variant in the 3'-UTR of the CHRNA3 gene and nicotine dependence. In this study we performed a comprehensive association analysis of the CHRNA5-CHRNA3-CHRNB4 gene cluster in the Collaborative Study on the Genetics of Alcoholism (COGA) families to investigate the role of genetic variants in risk for alcohol dependence. Using the family-based association test, we observed that a different group of polymorphisms, spanning CHRNA5-CHRNA3, demonstrate association with alcohol dependence defined by Diagnostic and Statistical Manual of Mental Disorders, 4th edn (DSM-IV) criteria. Using logistic regression we replicated this finding in an independent case-control series from the family study of cocaine dependence. These variants show low linkage disequilibrium with the SNPs previously reported to be associated with nicotine dependence and therefore represent an independent observation. Functional studies in human brain reveal that the variants associated with alcohol dependence are also associated with altered steady-state levels of CHRNA5 mRNA.

A biosocial developmental model of borderline personality: Elaborating and extending Linehan's theory

Over the past several decades, research has focused increasingly on developmental precursors to psychological disorders that were previously assumed to emerge only in adulthood. This change in focus follows from the recognition that complex transactions between biological vulnerabilities and psychosocial risk factors shape emotional and behavioral development beginning at conception. To date, however, empirical research on the development of borderline personality is extremely limited. Indeed, in the decade since M. M. Linehan initially proposed a biosocial model of the development of borderline personality disorder, there have been few attempts to test the model among at-risk youth. In this review, diverse literatures are reviewed that can inform understanding of the ontogenesis of borderline pathology, and testable hypotheses are proposed to guide future research with at-risk children and adolescents. One probable pathway is identified that leads to borderline personality disorder; it begins with early vulnerability, expressed initially as impulsivity and followed by heightened emotional sensitivity. These vulnerabilities are potentiated across development by environmental risk factors that give rise to more extreme emotional, behavioral, and cognitive dysregulation. (PsycINFO Database Record (c) 2009 APA, all rights reserved).

New conopeptides of the D-superfamily selectively inhibiting neuronal nicotinic acetylcholine receptors

The venom of cone snails (Conus spp.) is a rich source of peptides exhibiting a wide variety of biological activities. Several of these conopeptides are neuronal nicotinic acetylcholine receptor (nAChR) antagonists and belong to the A-, M-, S-, C and the recently described D-superfamily (alphaD-conopeptides). Here we describe the discovery and characterization of two alphaD-conopeptides isolated from the venom of Conus mustelinus and Conus capitaneus. Their primary structure was determined by Edman degradation, MS/MS analysis and by a PCR based approach. These peptides show close structural homology to the alphaD-VxXIIA, -B and -C conopeptides from the venom of Conus vexillum and are dimers (about 11kDa) of similar or identical peptides with 49 amino acid residues and a characteristic arrangement of ten conserved cysteine residues. These novel types of conopeptides specifically block neuronal nAChRs of the alpha7, alpha3beta2 and alpha4beta2 subtypes in nanomolar concentrations. Due to their high affinity, these new ligands may provide a tool to decipher the localisation and function of the various neuronal nAChRs.

Novel acetylcholine and carbamoylcholine analogues: development of a functionally selective alpha4beta2 nicotinic acetylcholine receptor agonist

A series of carbamoylcholine and acetylcholine analogues were synthesized and characterized pharmacologically at neuronal nicotinic acetylcholine receptors (nAChRs). Several of the compounds displayed low nanomolar binding affinities to the alpha4beta2 nAChR and pronounced selectivity for this subtype over alpha3beta4, alpha4beta4, and alpha7 nAChRs. The high nAChR activity of carbamoylcholine analogue 5d was found to reside in its R-enantiomer, a characteristic most likely true for all other compounds in the series. Interestingly, the pronounced alpha4beta2 selectivities exhibited by some of the compounds in the binding assays translated into functional selectivity. Compound 5a was a fairly potent partial alpha4beta2 nAChR agonist with negligible activities at the alpha3beta4 and alpha7 subtypes, thus being one of the few truly functionally selective alpha4beta2 nAChR agonists published to date. Ligand-protein docking experiments using homology models of the amino-terminal domains of alpha4beta2 and alpha3beta4 nAChRs identified residues Val111(beta2)/Ile113(beta4), Phe119(beta2)/Gln121(beta4), and Thr155(alpha4)/Ser150(alpha3) as possible key determinants of the alpha4beta2/alpha3beta4-selectivity displayed by the analogues.

Brain activation by short-term nicotine exposure in anesthetized wild-type and beta2-nicotinic receptors knockout mice: a BOLD fMRI study

RATIONALE

The behavioral effects of nicotine and the role of the beta2-containing nicotinic receptors in these behaviors are well documented. However, the behaviors altered by nicotine rely on the functioning on multiple brain circuits where the high-affinity beta2-containing nicotinic receptors (beta2*nAChRs) are located.

OBJECTIVES

We intend to see which brain circuits are activated when nicotine is given in animals naïve for nicotine and whether the beta2*nAChRs are needed for its activation of the blood oxygen level dependent (BOLD) signal in all brain areas.

MATERIALS AND METHODS

We used functional magnetic resonance imaging (fMRI) to measure the brain activation evoked by nicotine (1 mg/kg delivered at a slow rate for 45 min) in anesthetized C57BL/6J mice and beta2 knockout (KO) mice.

RESULTS

Acute nicotine injection results in a significant increased activation in anterior frontal, motor, and somatosensory cortices and in the ventral tegmental area and the substantia nigra. Anesthetized mice receiving no nicotine injection exhibited a major decreased activation in all cortical and subcortical structures, likely due to prolonged anesthesia. At a global level, beta2 KO mice were not rescued from the globally declining BOLD signal. However, nicotine still activated regions of a meso-cortico-limbic circuit likely via alpha7 nicotinic receptors.

CONCLUSIONS

Acute nicotine exposure compensates for the drop in brain activation due to anesthesia through the meso-cortico-limbic network via the action of nicotine on beta2*nAChRs. The developed fMRI method is suitable for comparing responses in wild-type and mutant mice.

Procognitive and neuroprotective activity of a novel alpha7 nicotinic acetylcholine receptor agonist for treatment of neurodegenerative and cognitive disorders

The alpha7 nicotinic acetylcholine receptor (nAChR) is a promising target for treatment of cognitive dysfunction associated with Alzheimer's disease and schizophrenia. Here, we report the pharmacological properties of 5-morpholin-4-yl-pentanoic acid (4-pyridin-3-yl-phenyl)-amide [SEN12333 (WAY-317538)], a novel selective agonist of alpha7 nAChR. SEN12333 shows high affinity for the rat alpha7 receptor expressed in GH4C1 cells (K(i) = 260 nM) and acts as full agonist in functional Ca(2+) flux studies (EC(50) = 1.6 microM). In whole-cell patch-clamp recordings, SEN12333 activated peak currents and maximal total charges similar to acetylcholine (EC(50) = 12 microM). The compound did not show agonist activity at other nicotinic receptors tested and acted as a weak antagonist at alpha3-containing receptors. SEN12333 treatment (3 mg/kg i.p.) improved episodic memory in a novel object recognition task in rats in conditions of spontaneous forgetting as well as cognitive disruptions induced via glutamatergic [5H-dibenzo[a,d]cyclohepten-5,10-imine (dizocilpine maleate); MK-801] or cholinergic (scopolamine) mechanisms. This improvement was blocked by the alpha7-selective antagonist methyllycaconitine, indicating that it is mediated by alpha7 activation. SEN12333 also prevented a scopolamine-induced deficit in a passive avoidance task. In models targeting other cognitive domains, including attention and perceptual processing, SEN12333 normalized the apomorphine-induced deficit of prepulse inhibition. Neuroprotection of SEN12333 was demonstrated in quisqualate-lesioned animals in which treatment with SEN12333 (3 mg/kg/day i.p.) resulted in a significant protection of choline acetyltransferase-positive neurons in the lesioned hemisphere. Cumulatively, our results demonstrate that the novel alpha7 nAChR agonist SEN12333 has procognitive and neuroprotective properties, further demonstrating utility of alpha7 agonists for treatment of neurodegenerative and cognitive disorders.

Angiogenic activity of nicotinic acetylcholine receptors: implications in tobacco-related vascular diseases

Cigarette smoking bears a strong etiological association with many neovascularization-related diseases like cancer, cardiovascular disease and macular degeneration. Although cigarette smoke is a complex mixture of many compounds, nicotine is the major active and addictive component of tobacco. Recent studies have shown that nicotine can enhance angiogenesis and arteriogenesis in several experimental systems and animal models. The pro-angiogenic activity of nicotine is mediated by nicotinic acetylcholine receptors, which have been found to be expressed on several types of cells in the vasculature like endothelial cells, smooth muscle cells and immune cells. The present review summarizes the pro-angiogenic activity of nicotine in neoplastic and non-neoplastic disease. The present article focuses on the role of nAChRs, particularly alpha7-nAChR in mediating the pro-angiogenic effects of nicotine. The expression patterns of nAChRs on various components of the vasculature are discussed. The complex signaling pathways underlying the angiogenic effect of nAChRs are described. The review also takes a look at the therapeutic potential of nAChR agonists and antagonists in angiogenesis-related diseases. More basic research as well as patient-oriented clinical studies is needed to firmly establish the clinical potential of nAChR ligands in angiogenesis-based therapies. Also the side effects of targeting nAChRs remain to be established in patients. The development of selective nAChR agonists and antagonists with improved specificity may represent novel therapeutic regimens in the treatment of angiogenesis-related diseases.

Role of non-neuronal nicotinic acetylcholine receptors in angiogenesis

Angiogenesis is a critical physiological process for cell survival and development. Endothelial cells, necessary for the course of angiogenesis, express several non-neuronal nicotinic acetylcholine receptors (AChRs). The most important functional non-neuronal AChRs are homomeric alpha7 AChRs and several heteromeric AChRs formed by a combination of alpha3, alpha5, beta2, and beta4 subunits, including alpha3beta4-containing AChRs. In endothelial cells, alpha7 AChR stimulation indirectly triggers the activation of the integrin alphavbeta3 receptor and an intracellular MAP kinase (ERK) pathway that mediates angiogenesis. Non-selective cholinergic agonists such as nicotine have been shown to induce angiogenesis, enhancing tumor progression. Moreover, alpha7 AChR selective antagonists such as alpha-bungarotoxin and methyllycaconitine as well as the non-specific antagonist mecamylamine have been shown to inhibit endothelial cell proliferation and ultimately blood vessel formation. Exploitation of such pharmacologic properties can lead to the discovery of new specific cholinergic antagonists as anti-cancer therapies. Conversely, the pro-angiogenic effect elicited by specific agonists can be used to treat diseases that respond to revascularization such as diabetic ischemia and atherosclerosis, as well as to accelerate wound healing. In this mini-review we discuss the pharmacological evidence supporting the importance of non-neuronal AChRs in angiogenesis. We also explore potential intracellular mechanisms by which alpha7 AChR activation mediates this vital cellular process.

Nicotinic modulation of synaptic transmission and plasticity in cortico-limbic circuits

Nicotine is the principle addictive agent delivered via cigarette smoking. The addictive activity of nicotine is due to potent interactions with nicotinic acetylcholine receptors (nAChRs) on neurons in the reinforcement and reward circuits of the brain. Beyond its addictive actions, nicotine is thought to have positive effects on performance in working memory and short-term attention-related tasks. The brain areas involved in such behaviors are part of an extensive cortico-limbic network that includes relays between prefrontal cortex (PFC) and cingulate cortex (CC), hippocampus, amygdala, ventral tegmental area (VTA) and the nucleus accumbens (nAcc). Nicotine activates a broad array of nAChRs subtypes that can be targeted to pre- as well as peri- and post-synaptic locations in these areas. Thereby, nicotine not only excites different types of neurons, but it also perturbs baseline neuronal communication, alters synaptic properties and modulates synaptic plasticity. In this review we focus on recent findings on nicotinic modulation of cortical circuits and their targets fields, which show that acute and transient activation of nicotinic receptors in cortico-limbic circuits triggers a series of events that affects cognitive performance in a long lasting manner. Understanding how nicotine induces long-term changes in synapses and alters plasticity in the cortico-limbic circuits is essential to determining how these areas interact in decoding fundamental aspects of cognition and reward.

Memantine for dementia in people with Down syndrome

BACKGROUND

Alzheimer's dementia (AD) is the most common form of dementia in people with Down Syndrome (DS). There is an understanding that an increase in L-glutamate contributes to the pathogenesis of cerebral ischemias and AD. Memantine acts as an antagonist of N-methyl-D-aspartate (NMDA) type receptors, which is thought to reduce abnormal activation of glutamate neurotransmission. It binds with a low affinity to the NMDA receptor and so should not prevent learning and the formation of memory. Memantine can improve cognitive function and slow the decline of AD in the general population over time, and is the subject of this review. It is important to note that people with DS tend to present with AD at a much younger age than the normal population as well as having subtle differences in physiology (e.g. metabolism and heart rate) and may therefore have different requirements from the general population.

OBJECTIVES

To determine the effectiveness and safety of memantine for people with DS who develop AD.

SEARCH STRATEGY

CENTRAL, MEDLINE, EMBASE, CINAHL, PsycINFO, BIOSIS, SCI, SSCI and the NRR were searched up to October 2008. We contacted the manufacturers of memantine, as well as experts in the field, to ask about reports of unpublished or ongoing trials.

SELECTION CRITERIA

Randomised controlled trials of participants with DS and AD in which treatment with memantine was administered compared with a placebo group.

DATA COLLECTION AND ANALYSIS

No study was identified which met the inclusion criteria for this review.

MAIN RESULTS

No study was identified which met inclusion criteria for this review, however there is an on-going randomised controlled study being conducted in the UK and data are expected in 2009.

AUTHORS' CONCLUSIONS

As there are no included trials, recommendations cannot be made about memantine for AD in DS. Well-designed, adequately powered studies are required.

Nicotine induces cell proliferation, invasion and epithelial-mesenchymal transition in a variety of human cancer cell lines

Cigarette smoking is strongly correlated with the onset of nonsmall cell lung cancer (NSCLC). Nicotine, an active component of cigarettes, has been found to induce proliferation of lung cancer cell lines. In addition, nicotine can induce angiogenesis and confer resistance to apoptosis. All these events are mediated through the nicotinic acetylcholine receptors (nAChRs) on lung cancer cells. In this study, we demonstrate that nicotine can promote anchorage-independent growth in NSCLCs. In addition, nicotine also induces morphological changes characteristic of a migratory, invasive phenotype in NSCLCs on collagen gel. These morphological changes were similar to those induced by the promigratory growth factor VEGF. The proinvasive effects of nicotine were mediated by alpha7-nAChRs on NSCLCs. RT-PCR analysis showed that the alpha7-nAChRs were also expressed on human breast cancer and pancreatic cancer cell lines. Nicotine was found to promote proliferation and invasion in human breast cancer. The proinvasive effects of nicotine were mediated via a nAChR, Src and calcium-dependent signaling pathway in breast cancer cells. In a similar fashion, nicotine could also induce proliferation and invasion of Aspc1 pancreatic cancer cells. Most importantly, nicotine could induce changes in gene expression consistent with epithelial to mesenchymal transition (EMT), characterized by reduction of epithelial markers like E-cadherin expression, ZO-1 staining and concomitant increase in levels of mesenchymal proteins like vimentin and fibronectin in human breast and lung cancer cells. Therefore, it is probable that the ability of nicotine to induce invasion and EMT may contribute to the progression of breast and lung cancers.

Mammalian nicotinic acetylcholine receptors: from structure to function

The classical studies of nicotine by Langley at the turn of the 20th century introduced the concept of a "receptive substance," from which the idea of a "receptor" came to light. Subsequent studies aided by the Torpedo electric organ, a rich source of muscle-type nicotinic receptors (nAChRs), and the discovery of alpha-bungarotoxin, a snake toxin that binds pseudo-irreversibly to the muscle nAChR, resulted in the muscle nAChR being the best characterized ligand-gated ion channel hitherto. With the advancement of functional and genetic studies in the late 1980s, the existence of nAChRs in the mammalian brain was confirmed and the realization that the numerous nAChR subtypes contribute to the psychoactive properties of nicotine and other drugs of abuse and to the neuropathology of various diseases, including Alzheimer's, Parkinson's, and schizophrenia, has since emerged. This review provides a comprehensive overview of these findings and the more recent revelations of the impact that the rich diversity in function and expression of this receptor family has on neuronal and nonneuronal cells throughout the body. Despite these numerous developments, our understanding of the contributions of specific neuronal nAChR subtypes to the many facets of physiology throughout the body remains in its infancy.

Involvement of nicotinic receptors in methamphetamine- and MDMA-induced neurotoxicity: pharmacological implications

During the last years, we have focused on the study of the neurotoxic effects of 3,4-methylenedioxymethamphetamine (MDMA) and methamphetamine (METH) on the central nervous system (CNS) and their pharmacological prevention methods. In the process of this research, we have used a semipurified synaptosomal preparation from striatum of mice or rats as a reliable in vitro model to study reactive oxygen species (ROS) production by these amphetamine derivatives, which is well-correlated with their dopaminergic injury in in vivo models. Using this preparation, we have demonstrated that blockade of alpha7 nicotinic receptors with methyllycaconitine (MLA) prevents ROS production induced by MDMA and METH. Consequently, in vivo, MLA significantly prevents MDMA- and METH-induced neurotoxicity at dopaminergic level (mouse striatum), without affecting hyperthermia induced by these amphetamines. Additionally, when neuroprotection was assayed with memantine (MEM), a dual antagonist of NMDA and alpha7 receptors, an effective neuroprotection was obtained also ahead of serotonergic injury induced by MDMA in rats. MEM also prevents MDMA effect on serotonin transporter functionality and METH effect on dopamine transporter (DAT), suggesting that behavioral effects of these psychostimulants can also be modulated by MEM. Finally, we have demonstrated that MEM prevents the impaired memory function induced by MDMA, and also, using binding studies with radioligands, we have characterized the interaction of these substances with nicotinic receptors. Studies at molecular level showed that both MDMA and METH displaced competitively the binding of radioligands with homomeric alpha7 and heteromeric nicotinic acetylcholine receptors (nAChRs), indicating that they can directly interact with them. In all the cases, MDMA displayed higher affinity than METH and it was higher for heteromeric than for alpha7 subtype. Pre-incubation of differentiated PC12 cells with MDMA or METH induces nAChR upregulation in a concentration- and time-dependent manner, as many nicotinic ligands do, supporting their functional interaction with nAChRs. Such interaction expands the pharmacological profile of amphetamines and can account for some of their effects.

Cough sensors. IV. Nicotinic membrane receptors on cough sensors

Cigarette smoke is undoubtedly one of the most common inhaled irritants in the human respiratory tract, and invariably evokes coughing in both smokers and nonsmokers. Results obtained from the studies in human volunteers and from single-fiber recording of vagal bronchopulmonary afferents in animals clearly indicate that nicotine is primarily responsible for the airway irritation and coughing caused by inhalation of cigarette smoke. Furthermore, both nicotine and acetylcholine can evoke inward current, membrane depolarization, and action potentials in isolated pulmonary sensory neurons, and these responses are blocked by hexamethonium. Taken together, these findings suggest that the tussive effect of nicotine is probably mediated through an activation of nicotinic acetylcholine receptors (nAChRs) expressed on the sensory terminals of cough receptors located in the airway mucosa. Indeed, the expressions of alpha4-alpha7 and beta2-beta4 subunits of nAChR transcripts in pulmonary sensory neurons have lent further support to this conclusion. The specific subtypes of the neuronal nAChRs and their subunit compositions expressed on the cough sensors remain to be determined.

Alpha7 nicotinic acetylcholine receptor agonists: prediction of their binding affinity through a molecular mechanics Poisson-Boltzmann surface area approach

A group of agonists for the alpha7 neuronal nicotinic acetylcholine receptors (nAChRs) was investigated, and their free energies of binding DeltaG(bind) were calculated by applying the molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) approach. This method, based on molecular dynamics simulations of fully solvated protein-ligand complexes, allowed us to estimate the contribution of both polar and nonpolar terms as well as the entropy to the overall free energy of binding. The calculated results were in a good agreement with the experimentally determined DeltaG(bind) values, thereby pointing to the MM-PBSA protocol as a valuable computational tool for the rational design of specific agents targeting the neuronal alpha7 nAChR subtypes.

Old and new pharmacology: positive allosteric modulation of the alpha7 nicotinic acetylcholine receptor by the 5-hydroxytryptamine(2B/C) receptor antagonist SB-206553 (3,5-dihydro-5-methyl-N-3-pyridinylbenzo[1,2-b:4,5-b']di pyrrole-1(2H)-carboxamide)

The alpha7 nicotinic acetylcholine receptor (nAChR) has been implicated in Alzheimer's disease and schizophrenia, leading to efforts targeted toward discovering agonists and positive allosteric modulators (PAMs) of this receptor. In a Ca2+ flux fluorometric imaging plate reader assay, SB-206553 (3,5-dihydro-5-methyl -N-3-pyridinylbenzo [1, 2-b:4,5 -b']-di pyrrole-1(2H)-carboxamide), a compound known as a 5-hydroxytryptamine(2B/2C) receptor antagonist, produced an 8-fold potentiation of the evoked calcium signal in the presence of an EC(20) concentration of nicotine and a corresponding EC(50) of 1.5 muM for potentiation of EC(20) nicotine responses in GH4C1 cells expressing the alpha7 receptor. SB-206553 was devoid of direct alpha7 receptor agonist activity and selective against other nicotinic receptors. Confirmation of the PAM activity of SB-206553 on the alpha7 nAChR was obtained in patch-clamp electrophysiological experiments in GH4C1 cells, where it failed to evoke any detectable currents when applied alone, yet dramatically potentiated the currents evoked by an EC(20) (17 microM) and EC(100) (124 microM) of acetylcholine (ACh). Native nicotinic receptors in CA1 stratum radiatum interneurons of rat hippocampal slices could also be activated by ACh (200 microM), an effect that was entirely blocked by the alpha7-selective antagonist methyllycaconitine (MLA). These ACh currents were potentiated by SB-206553, which increased the area of the current response significantly, resulting in a 40-fold enhancement at 100 microM. In behavioral experiments in rats, SB-206553 reversed an MK-801 (dizocilpine maleate)-induced deficit in the prepulse inhibition of acoustic startle response, an effect attenuated in the presence of MLA. This latter observation provides further evidence in support of the potential therapeutic utility of alpha7 nAChR PAMs in schizophrenia.

Radioligand binding characterization of neuronal nicotinic acetylcholine receptors

Described in this unit are radioligand binding protocols for three neuronal nicotinic acetylcholine receptor (nAChR) subtypes. Detailed descriptions of binding protocols are presented for the two predominant CNS subtypes of nAChRs, α4β2 and α7, as well as the ganglionic α3β4 nAChR. [(3)H]Cytisine is utilized for α4β2 nAChRs, while [(3)H]methyllycaconitine is utilized for α7 nAChRs, both in rat brain. α3β4 nAChRs in IMR-32 cells are labeled with [(3)H]epibatidine. Reference data are presented, as well as commentary on the current state of neuronal nicotinic receptor research.

Design and expression of human alpha7 nicotinic acetylcholine receptor extracellular domain mutants with enhanced solubility and ligand-binding properties

In order to facilitate structural studies of the extracellular domain (ECD) of human alpha7 nicotinic acetylcholine receptor (nAChR), we designed several mutants, since the wild-type-ECD forms large oligomers and microaggregates, and expressed them in the yeast Pichia pastoris. Mutant design was based on a 3D model of human alpha7-nAChR-ECD, constructed using as templates the X-ray crystal structure of the homologous acetylcholine-binding protein (AChBP) and the electron microscopy structure of the Torpedo alpha-nAChR-ECD. At least one mutant, mut10, carrying six single-point mutations (Phe3Tyr, Val69Thr, Cys116Ser, Ile165Thr, Val177Thr, Phe187Tyr) and the replacement of its Cys-loop with the corresponding and more hydrophilic AChBP Cys-loop, was expressed with a 4-fold higher expression yield (1.2 mg/L) than the wild-type alpha7-ECD, existing exclusively as a soluble oligomeric, probably pentameric, form, at concentrations up to at least 10 mg/mL, as judged by gel filtration and dynamic light scattering. This mutant displayed a significantly improved (125)I-alpha-bungarotoxin-binding affinity (K(d)=24 nM) compared to the wild-type-ECD (K(d)=70 nM), the binding being inhibited by unlabelled alpha-bungarotoxin, d-tubocurarine or nicotine (K(i) of 21.5 nM, 127 microM and 17.5 mM, respectively). Circular dichroism studies of mut10 revealed (a) a similar secondary structure composition ( approximately 5% alpha-helix, approximately 45% beta-sheet) to that of the AChBP, Torpedo alpha-nAChR-ECD, and mouse alpha1-nAChR-ECD, (b) a well-defined tertiary structure and (c) binding of small cholinergic ligands at micromolar concentrations. Furthermore, electron microscopy showed well-assembled, probably pentameric, particles of mut10. Finally, since deglycosylation did not alter its solubility or ligand-binding properties, mut10, in either its glycosylated or deglycosylated form, is a promising alpha7-ECD mutant for structural studies, useful for the rational drug design to treat alpha7-nAChR-related diseases.

Neuronal Ca2+ sensor VILIP-1 leads to the upregulation of functional alpha4beta2 nicotinic acetylcholine receptors in hippocampal neurons

The neuronal Ca2+-sensor protein VILIP-1, known to affect clathrin-dependent receptor trafficking, has been shown to interact with the cytoplasmic loop of the alpha4-subunit of the alpha4beta2 nicotinic acetylcholine receptor (nAChR), which is the most abundant nAChR subtype with high-affinity for nicotine in the brain. The alpha4beta2 nAChR is crucial for nicotine addiction and the beneficial effects of nicotine on cognition. Its dysfunction has been implicated in frontal lobe epilepsy, Alzheimer's disease and schizophrenia. Here we report that overexpression of VILIP-1 enhances ACh responsiveness, whereas siRNA against VILIP-1 reduces alpha4beta2 nAChR currents of hippocampal neurons. The underlying molecular mechanism likely involves enhanced constitutive exocytosis of alpha4beta2 nAChRs mediated by VILIP-1. The two interaction partners co-localize in a Ca2+-dependent manner with syntaxin-6, a Golgi-SNARE protein involved in trans-Golgi membrane trafficking. Thus, we speculate that regulation of VILIP-1-expression might modulate surface expression of ligand-gated ion channels, such as the alpha4beta2 nAChRs, possibly comprising a novel form of physiological up-regulation of ligand-gated ion channels.

Nicotinic acetylcholine receptor subunits in rhesus monkey retina

PURPOSE

The purpose of this study was to detect and establish the cellular localizations of nicotinic acetylcholine receptor (nAChR) subunits in Rhesus monkey retina.

METHODS

Retinas were dissected from the eyes of monkeys killed after unrelated experiments. RNA was extracted and analyzed by RT-PCR, using primers designed against human sequences of alpha3-alpha7, alpha9, and beta2-beta4 nAChR subunits. The RT-PCR products were separated by gel electrophoresis and sequenced. Frozen sections of postmortem fixed monkey eyes were immunolabeled with well-characterized and specific monoclonal antibodies against the alpha3, alpha4, alpha6, alpha7, beta2, or beta4 nAChR subunits and visualized with fluorescence labeling.

RESULTS

Products of the predicted size for the alpha3-alpha7, alpha9, and beta2-beta4 nAChR subunits were detected by RT-PCR in Rhesus monkey retina. Homology between transcripts from monkey retina and human nucleotide sequences ranged from 93 to 99%. Immunohistochemical studies demonstrated that neurons in various cell layers of monkey retina expressed alpha3, alpha4, alpha7, or beta2 nAChR subunits and cells with the morphology of microglia were immunoreactive for the alpha6 or beta4 nAChR subunits.

CONCLUSIONS

nAChR subunits are expressed in the monkey retina and localize to diverse retinal neurons as well as putative microglia. Besides mediating visual processing, retinal nAChRs may influence refractive development and ocular pathologies such as neovascularization.

Lactate dehydrogenase isoenzymes in sympathetic neurons and satellite gliocytes in normal conditions and in blockade of nicotinic cholinoreceptors

Integral cytophotometry was used to measure lactate dehydrogenase (LDH) and its H-and M-isoforms in neurons and satellite gliocytes in tissue sections from the cranial cervical sympathetic ganglion of the rabbit in normal conditions and after experimental partial and complete pharmacological blockade of nicotinic cholinoreceptors (n-CR). In normal conditions, both cell types showed both the H-and M-type isoforms, though the isoenzyme profiles differed--neurons showed a dominance of H-isoform activity while the M-isoform was more active in satellite gliocytes. In partial and complete blockade, the activity of LDH and its H-and M-isoforms decreased significantly in proportion to the number of blocked n-CR. In satellite gliocytes, increases in the extent of blockade were associated with decreases in the activity only of the M-isoform, while the activity of the H-isoform did not change. In partial blockade, the LDH isoenzyme profile of satellite gliocytes shifted towards the neuronal isoform, while in complete blockade there was no difference from the LDH isoenzyme profile of intact neurons. These data led to the suggestion that the formation of lactate in satellite gliocytes is induced by nicotinic cholinergic synapses directly involved in neuron-glial interactions and in controlling the activity of the LDH enzyme system in sympathetic neurons.

Functional expression of nicotinic acetylcholine receptors in rat neocortical layer 5 pyramidal cells

Neuronal nicotinic acetylcholine receptors (nAChRs) expressed by neurons of the neocortex are known to play a role in higher brain functions. Electrophysiological studies of neocortical neurons provided evidence that functional nAChRs are present on the axonal presynaptic terminals, on the somata and on dendrites of gamma-aminobutyric acid (GABA)ergic inhibitory interneurons. However, it is not clear if pyramidal neurons express functional postsynaptic nAChRs. Therefore, we investigated the action of locally applied acetylcholine (ACh) on layer 5 pyramidal neurons in the rat neocortex in vitro. In the presence of atropine, tetrodotoxin, glutamate receptor antagonists, and GABAA receptor antagonists, ACh induced membrane depolarizations which were generated by membrane inward currents consisting of a fast and a slow component. Analysis of the electrophysiological properties, the pharmacological characteristics, and the desensitization behavior of the 2 current components revealed that they were mediated by at least 2 different subtypes of the nAChR, most likely the alpha7-like and the alpha4beta2-like subtype. The expression of nAChRs in neocortical pyramidal cells raises the possibility that these neurons generate nicotinic excitatory postsynaptic potentials, thereby influencing cell excitability. Furthermore, because most nAChRs are permeable to calcium, they may modulate synaptic transmission and neuronal plasticity via a calcium-dependent postsynaptic mechanism.

Effects of the nicotinic receptor partial agonists varenicline and cytisine on the discriminative stimulus effects of nicotine in rats

The nicotinic partial agonist varenicline (VCL) is a recently approved medication for the treatment of tobacco dependence, yet very little preclinical research on this drug has been published. The present experiment examined the nicotinic partial agonist properties of VCL and its parent compound, cytisine (CYT), in a nicotine discrimination assay. Rats were trained to discriminate nicotine (0.4 mg/kg, s.c.) from saline using a two-lever discrimination procedure, followed by generalization and antagonism tests with VCL and CYT. Antagonism was examined across a range of nicotine doses. In generalization tests, VCL produced a maximum of 63% responding on the nicotine-appropriate lever, indicating partial generalization. In antagonism tests, VCL decreased the % responding on the nicotine-appropriate lever at 0.2 and 0.4 mg/kg nicotine, indicating antagonism of nicotine's discriminative stimulus effects. No dose of VCL produced significant effects on response rate. The two highest doses of CYT weakly substituted for nicotine, producing a maximum of 23% nicotine-appropriate responding. CYT produced a weak antagonism of the discrimination of moderate nicotine doses, but not of the training dose. These results demonstrate that VCL and CYT partially generalize to and partially antagonize nicotine's discriminative stimulus effects, consistent with a partial agonist mechanism of action.

Attenuation of ethanol-induced ataxia by alpha(4)beta(2) nicotinic acetylcholine receptor subtype in mouse cerebellum: a functional interaction

Many epidemiological studies support the notion that people who drink alcohol also smoke cigarettes and vice versa thereby suggesting a possible functional interaction between these two most widely used psychoactive substances. We have earlier demonstrated that direct intracerebellar (ICB) microinfusion of nicotine dose-dependently antagonizes ethanol-induced ataxia and further that this antagonism occurs in a glutamate-nitric oxide-cyclic guanylyl monophosphate (cGMP) sensitive manner. The present study was designed to determine the possible involvement of specific nicotinic acetylcholine receptor (nAChR) subtype alpha(4)beta(2) in nicotine-induced attenuation of ethanol ataxia. Using the Rotorod test and direct ICB microinfusion technique in stereotaxically cannulated CD-1 male mice, we performed the Rotorod test following ICB administration of the alpha(4)beta(2)-selective agonist, (E)-N-methyl-4-(3-pyridinyl)-3-buten-1-amine (RJR-2403; 31.25, 62.5, 125 ng) on ethanol (2 g/kg; i.p.) ataxia at 15, 30, 45, 60 min post-ethanol injection. RJR-2403 dose-dependently attenuated ethanol ataxia suggesting a role of alpha(4)beta(2) subtype in ameliorating ethanol-induced ataxia. Pretreatment with ICB dihydro-beta-erythroidine (DHbetaE: 125, 250, 500, 750 ng), a potent alpha(4)beta(2)-selective antagonist, significantly reduced RJR-2403's effect further supporting the alpha(4)beta(2) involvement. DHbetaE (ICB) also antagonized ICB nicotine-induced attenuation of ethanol ataxia again reinforcing the role of alpha(4)beta(2) subtype. Additional evidence for the role of alpha(4)beta(2) subtype was provided when ICB alpha(4)beta(2) antisense oligodeoxynucleotide treatment markedly antagonized RJR 2403-induced attenuation of ethanol ataxia compared with missense-treated animals. This was confirmed with an associated decrease in the expression of alpha(4)beta(2) subtypes indicated by immunoblot experiments. In conclusion, the results of the present investigation support an important role of alpha(4)beta(2) nAChR subtype in the expression of nicotine-induced attenuation of ethanol ataxia.

Diversity of vertebrate nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are pentameric neurotransmitter receptors. They are members of the Cys-loop family of ligand-gated ion channels which also include ionotropic receptors for 5-hydroxytryptamine (5-HT), gamma-aminobutyric acid (GABA) and glycine. Nicotinic receptors are expressed in both the nervous system and at the neuromuscular junction and have been implicated in several neurological and neuromuscular disorders. In vertebrates, seventeen nAChR subunits have been identified (alpha1-alpha10, beta1-beta4, gamma, delta and epsilon) which can co-assemble to generate a diverse family of nAChR subtypes. This review will focus on vertebrate nAChRs and will provide an overview of the extent of nAChR diversity based on studies of both native and recombinant nAChRs.