Molecular studies of the neuronal nicotinic acetylcholine receptor family

Structure and function meet at the nicotinic acetylcholine receptor-lipid interface

The nicotinic acetylcholine receptor (nAChR) is a transmembrane protein that mediates fast intercellular communication in response to the endogenous neurotransmitter acetylcholine. It is the best characterized and archetypal molecule in the superfamily of pentameric ligand-gated ion channels (pLGICs). As a typical transmembrane macromolecule, it interacts extensively with its vicinal lipid microenvironment. Experimental evidence provides a wealth of information on receptor-lipid crosstalk: the nAChR exerts influence on its immediate membrane environment and conversely, the lipid moiety modulates ligand binding, affinity state transitions and gating of ion translocation functions of the receptor protein. Recent cryogenic electron microscopy (cryo-EM) studies have unveiled the occurrence of sites for phospholipids and cholesterol on the lipid-exposed regions of neuronal and electroplax nAChRs, confirming early spectroscopic and affinity labeling studies demonstrating the close contact of lipid molecules with the receptor transmembrane segments. This new data provides structural support to the postulated "lipid sensor" ability displayed by the outer ring of M4 transmembrane domains and their modulatory role on nAChR function, as we postulated a decade ago. Borrowing from the best characterized nAChR, the electroplax (muscle-type) receptor, and exploiting new structural information on the neuronal nAChR, it is now possible to achieve an improved depiction of these sites. In combination with site-directed mutagenesis, single-channel electrophysiology, and molecular dynamics studies, the new structural information delivers a more comprehensive portrayal of these lipid-sensitive loci, providing mechanistic explanations for their ability to modulate nAChR properties and raising the possibility of targetting them in disease.

Autoimmune autonomic ganglionopathy: Ganglionic acetylcholine receptor autoantibodies

Autoimmune Autonomic Ganglionopathy (AAG) is a rare immune-mediated disease of the autonomic nervous system. The incidence of AAG is unknown and diagnosis is often difficult due to the multicompartmental nature of the autonomic nervous system - sympathetic, parasympathetic and enteric components - with variable severity and number of components affected. Diagnostic confidence is increased when ganglionic acetylcholine receptor (gnACHR) autoantibodies are detected. Three gnACHR autoantibody diagnostic assays have been described (two binding assays, one receptor immunomodulation assay), but cross-validation between assays is limited. The prevalence of gnACHR autoantibodies in AAG is not known, with application of different clinical and laboratory criteria in the few studies of AAG cohorts and large retrospective laboratory studies of positive gnACHR autoantibodies lacking adequate clinical characterisation. Furthermore, the rate of unexpected gnACHR autoantibody positivity in conditions without overt autonomic dysfunction (false positive results) adds to the complexity of their interpretation. We review the pathophysiology of gnACHR autoantibodies and assays for their detection, with immunomodulation and high titer radioimmunoprecipitation results likely offering better AAG disease identification.

Green Apple e-Cigarette Flavorant Farnesene Triggers Reward-Related Behavior by Promoting High-Sensitivity nAChRs in the Ventral Tegmental Area

While combustible cigarette smoking has declined, the use of electronic nicotine delivery systems (ENDS) has increased. ENDS are popular among adolescents, and chemical flavorants are an increasing concern because of the growing use of zero-nicotine flavored e-liquids. Despite this, little is known regarding the effects of ENDS flavorants on vaping-related behavior. Following previous studies demonstrating the green apple flavorant, farnesol, enhances nicotine reward and exhibits rewarding properties without nicotine, this work focuses on the green apple flavorant, farnesene, for its impact on vaping-related behaviors. Using adult C57BL/6J mice, genetically modified to contain fluorescent nicotinic acetylcholine receptors (nAChRs), and farnesene doses of 0.1, 1.0, and 10 mg/kg, we observed farnesene-alone produces reward-related behavior in both male and female mice. We then performed whole-cell patch-clamp electrophysiology and observed farnesene-induced inward currents in ventral tegmental area (VTA) putative dopamine (pDA) neurons that were blocked by the nAChR antagonist, DhβE. While the amplitudes of farnesene-induced currents are ∼30% of nicotine's efficacy, this indicates the potential for some ENDS flavorants to stimulate nAChR function. Additionally, farnesene enhances nicotine's potency for activating nAChRs on VTA dopamine neurons. This may be because of changes in nAChR stoichiometry as our data suggest a shift toward high-sensitivity α4β2 nAChRs. Consequently, these data show that the green apple flavorant, farnesene, causes reward-related behavior without nicotine through changes in nAChR stoichiometry that results in an enhanced effect of nicotine on VTA dopamine neurons. These results demonstrate the importance of future investigations into ENDS flavorants and their effects on vaping-related behaviors.

Nicotine induces aberrant hypermethylation of tumor suppressor genes in pancreatic epithelial ductal cells

Tobacco smoking is an independent risk factor for the initiation of pancreatic cancer (PC). Hypermethylation of tumor suppressor genes has been demonstrated to be associated with smoking. This study aimed to find the relationship between nicotine exposure and hypermethylation of tumor suppressor genes in normal pancreatic epithelial cells. Human pancreatic epithelial cells ware cultured exposing to nicotine and the methylation status of tumor suppressor genes were detected. Proenkephalin (PENK) was chosen as the target gene and methylation level of PENK promoter region was measured. Expression of DNA methyltransferase (DNMT), nicotine acetylcholine receptor (α7nAChR) and signaling pathway downstream were analyzed. Nicotine induces overexpression of DNMT3A and 3B, and methylated-inactivation of PENK gene in normal pancreatic epithelial cells. An activation of α7nAChR and MAPK signaling pathway has been detected in the nicotine-treated group. Demethylated drug, antagonist of α7nAChR and inhibitor of p38 MAPK is verified to attenuate the overexpression of DNMTs stimulated by nicotine as well as inhibit aberrant hypermethylation-related silence of PENK gene. Nicotine stimulation can induce aberrant hypermethylation of tumor suppressor genes by α7nAChR and MAPK signaling pathway-mediated up-regulation of DNMTs in pancreatic epithelial cells, thus we can provide epigenetic evidence of the mechanisms by which smoking causes pancreatic cancer and find new therapeutic target.

Nicotine/cigarette smoke promotes metastasis of pancreatic cancer through α7nAChR-mediated MUC4 upregulation

Despite evidence that long-term smoking is the leading risk factor for pancreatic malignancies, the underlying mechanism(s) for cigarette-smoke (CS)-induced pancreatic cancer (PC) pathogenesis has not been well-established. Our previous studies revealed an aberrant expression of the MUC4 mucin in PC as compared to the normal pancreas and its association with cancer progression and metastasis. Interestingly, here we explore a potential link between MUC4 expression and smoking-mediated PC pathogenesis and report that both cigarette-smoke-extract (CSE) and nicotine, which is the major component of CS, significantly up-regulates MUC4 in PC cells. This nicotine-mediated MUC4 overexpression was via α7 subunit of nicotinic acetylcholine receptor (nAChR) stimulation and subsequent activation of the JAK2/STAT3 downstream signaling cascade in cooperation with the MEK/ERK1/2 pathway; this effect was blocked by the α7nAChR antagonists, α-bungarotoxin and mecamylamine, and by specific siRNA-mediated STAT3 inhibition. Additionally, we demonstrated that nicotine-mediated MUC4 up-regulation promotes the PC cell migration through the activation of the downstream effectors such as HER2, c-Src and FAK; this effect was attenuated by shRNA-mediated MUC4 abrogation, further implying that these nicotine-mediated pathological effects on PC cells are MUC4 dependent. Furthermore, the in-vivo studies demonstrated a dramatic increase in the mean pancreatic tumor weight [low-dose (100 mg/m³ TSP), p=0.014; high-dose (247 mg/m³ TSP), p=0.02] and significant tumor metastasis to various distant organs in the CS-exposed-mice, orthotopically implanted with luciferase-transfected PC cells, as compared to the sham-controls. Moreover, the CS-exposed mice had elevated levels of serum cotinine [low-dose, 155.88±35.96 ng/ml; high-dose, 216.25±29.95 ng/ml] and increased MUC4, α7nAChR and pSTAT3 expression in the pancreatic tumor tissues. Altogether, our findings revealed for the first time that CS up-regulates the MUC4 mucin in PC via α7nAChR/JAK2/STAT3 downstream signaling cascade, thereby promoting metastasis of pancreatic cancer.

Stimulation of the alpha4beta2 nicotinic receptor by 5-I A-85380 improves auditory gating in DBA/2 mice

Abnormal auditory gating is a symptom of schizophrenia which has been proposed to be mediated through the alpha7 nicotinic acetylcholine receptor (nAChR). It has been shown that the non-selective nicotinic agonist nicotine has an influence on auditory gating in part by acting on the alpha4beta2 nAChR. The goal of this study was to determine the effect of 5-I A-85380, an agonist for the alpha4beta2 nAChR, in an inbred mouse model with a deficiency for auditory gating. Anesthetized DBA/2 mice were administered 5-I A-85380 alone and in combination with the alpha4beta2 nAChR antagonist, dihydro-beta-erythroidine, or the alpha7 nAChR antagonist, alpha-bungarotoxin. A recording electrode in the CA3 region of the hippocampus recorded P20-N40 waveforms in response to two auditory stimuli. The amplitudes of the response to the first and second clicks were used to determine TC ratios, the measure of auditory gating. 5-I A-85380 significantly decreased the TC ratios by selectively increasing the response amplitudes to the first click with no significant influence on the response amplitudes to the second click. The effect was blocked by dihydro-beta-erythroidine whereas alpha-bungarotoxin had no effect on response amplitude to either click. Although the alpha7 nAChR may mediate the hippocampal response of DBA/2 mice to the second click, the alpha4beta2 nAChR appears to modulate the response to the first click. Thus, the present study implicates the involvement of more than one subtype of nAChR in the auditory gating of DBA/2 mice, specifically the alpha4beta2 nAChR, and its role in the response amplitude to the first stimulus.

Structural and functional heterogeneity of nicotinic receptors

Three gene families of the ligand-gated ion channel gene superfamily encode proteins which bind cholinergic ligands: (1) nicotinic acetylcholine receptors (AChRs) from skeletal muscle, (2) AChRs from neurons, and (3) neuronal alpha-bungarotoxin-binding proteins (alpha BgtBPs). AChRs from muscles and nerves function as ACh-gated cation channels, but alpha BgtBPs do not appear to function in this way. A family of neuronal AChR subtypes has been characterized using monoclonal antibodies and cDNA probes. Neuronal AChRs exhibit sequence homologies with muscle AChRs, but differ in subunit composition, pharmacological and electrophysiological properties, and, in some cases, apparent functional roles. The genes that encode the subunits of the various purified AChR subtypes have been determined in several cases. Histological localization of AChR subunit mRNAs by in situ hybridization and of subunit proteins by immunohistochemistry is being conducted with increasing resolution. The subunit structure of alpha BgtBP is uncertain, but cDNAs have been identified for two subunits. Sequences of these cDNAs reveal that alpha BgtBPs are members of the ligand-gated ion channel gene family, and suggest that they could function as gated cation channels. Biochemical and molecular genetic approaches to studies of neuronal AChRs and related proteins are merging to provide a detailed description of a complex family of AChRs widely dispersed throughout the nervous system, which are probably important to many activities of the nervous system, but whose functional roles are not yet well characterized.

Presynaptic nicotinic receptors and the modulation of transmitter release

Nicotine is increasingly recognized to promote transmitter release in the brain by a direct action on presynaptic terminals. Pharmacological evidence indicates that this action is mediated by nicotinic receptors. From their sensitivity to mecamylamine, neosurugatoxin and neuronal bungarotoxin these presynaptic receptors can be distinguished from alpha-bungarotoxin-sensitive muscle-type nicotinic receptors, and can be correlated with [3H] nicotine binding sites in the brain. The release of many transmitters in different brain regions is susceptible to stimulation by nicotine, but this effect is not ubiquitous. However, lesioning and subcellular fractionation studies suggest that the majority of brain nicotine receptors are located presynaptically, so that a direct influence of nicotine on transmitter release assumes considerable importance. Although the sensitivity of presynaptic receptors is such that they are likely to be partially activated by doses of nicotine obtained by smoking, the desensitization-induced up-regulation of nicotinic binding sites that follows chronic nicotine treatment raises questions about their functional status during tobacco usage. Chronic administration of the agonist (+)anatoxin-a also up-regulated [3H] nicotine binding sites, and led to increased nicotine-evoked transmitter release in vitro. This could have implications for the involvement of these receptors during withdrawal.

Nicotinic receptor agonists as neuroprotective/neurotrophic drugs. Progress in molecular mechanisms

In the present work we reviewed recent advances concerning neuroprotective/neurotrophic effects of acute or chronic nicotine exposure, and the signalling pathways mediating these effects, including mechanisms implicated in nicotine addiction and nAChR desensitization. Experimental and clinical data largely indicate long-lasting effects of nicotine and nicotinic agonists that imply a neuroprotective/neurotrophic role of nAChR activation, involving mainly alpha7 and alpha4beta2 nAChR subtypes, as evidenced using selective nAChR agonists. Compounds interacting with neuronal nAChRs have the potential to be neuroprotective and treatment with nAChR agonists elicits long-lasting neurotrophic effects, e.g. improvement of cognitive performance in a variety of behavioural tests in rats, monkeys and humans. Nicotine addiction, which is mediated by interaction with nACh receptors, is believed to involve the modification of signalling cascades that modulate synaptic plasticity and gene expression. Desensitization, in addition to protecting cells from uncontrolled excitation, is recently considered as a form of signal plasticity. nAChR can generate these longe-lasting effects by elaboration of complex intracellular signals that mediate medium to long-term events crucial for neuronal maintenance, survival and regeneration. Although a comprehensive survey of the gene-based molecular mechanisms that underlie nicotine effects has yet not been performed a growing amount of data is beginning to improve our understanding of signalling mechanisms that lead to neurotrophic/neuroprotective responses. Evidence for an involvement of the fibroblast growth factor-2 gene in nAChR mechanisms mediating neuronal survival, trophism and plasticity has been obtained. However, more work is needed to establish the mechanisms involved in the effects of nicotinic receptor subtype activation from cognition-enhancing and neurotrophic effects to smoking behaviour and to determine more precisely the therapeutic objectives in potential nicotinic drug treatments of neurodegenerative diseases.

Genetic interaction between alpha4 and beta2 subunits of high affinity nicotinic receptor: analysis in schizophrenia

Cholinergic dysfunction is one of the hypotheses for the cognitive deficits of schizophrenia. Neurocognitive deficits, which are well-described clinical features of schizophrenia, may be remediated by nicotine; therefore investigations of nicotinic receptor subtypes is of considerable clinical interest. We typed polymorphisms in CHRNA4 and CHRNB2 genes controlling the expression of neuronal high-affinity nicotinic receptors in 117 Canadian families having at least one schizophrenic patient. Using a family-based association strategy, we performed allele, haplotype and interaction analysis of these two loci. In the families tested, the two cholinergic genes interact to affect schizophrenia in combination (P=0.010), while neither was sufficient alone to confer susceptibility. Our present study provided the first line of direct evidence suggesting that the CHRNA4 gene combined with CHRNB2 receptor gene may be linked to schizophrenia.

Nicotinic receptors regulate B lymphocyte activation and immune response

The presence of nicotinic acetylcholine receptors (nicotinic receptors) composed of either alpha7 or alpha4 and beta2 subunits is revealed in B lymphocytes by means of radioligand binding assay and Cell ELISA. Mouse B lymphocytes contained 12,200+/-3200 of epibatidine-binding sites and 3130+/-750 of alpha-Bungarotoxin-binding sites per cell. Mice lacking nicotinic receptor subunits alpha4, beta2 or alpha7 had less serum IgG and IgG-producing cells in the spleen, but showed stronger immune response to both protein antigen in vivo and CD40-specific antibody in vitro than wild-type mice. Anti-CD40-stimulated proliferation of B lymphocytes from beta2 knockout, but not wild-type mice was inhibited with nicotine. Our results indicate that signalling through nicotinic receptors affects both the pre-immune state and activation of B lymphocytes in the immune response, possibly via CD40-dependent pathway. This could contribute to immune depression found in tobacco smokers.

Desensitized nicotinic receptors in brain

Desensitization is an intriguing characteristic of ligand-gated channels, whereby a decrease or loss of biological response occurs following prolonged or repetitive stimulation. Nicotinic acetylcholine receptors (nAChRs), as a member of transmitter gated ion channels family, also can be desensitized by continuous or repeated exposure to agonist. Desensitization of nicotinic receptors can occur as a result of extended nicotine exposure during smoking or prolonged acetylcholine when treatment of Alzheimer's disease (AD) with cholinesterase inhibitors, or anticholinesterase agent poisoning. Studies from our lab have shown that nAChRs desensitization is not a nonfunctional state and we proposed that desensitized nAChRs could increase sensitivity of brain muscarinic receptor to its agonists. Here, we will review the regulation of nicotinic receptor desensitization and discuss the important biological function of desensitized nicotinic receptors in light of our previous studies. These studies provide the critical information for understanding the importance of nicotinic receptors desensitization in both normal physiological processing and in various disease states.

Modulation of cerebral microvascular permeability by endothelial nicotinic acetylcholine receptors

Nicotine increases the permeability of the blood-brain barrier in vivo. This implies a possible role for nicotinic acetylcholine receptors in the regulation of cerebral microvascular permeability. Expression of nicotinic acetylcholine receptor subunits in cerebral microvessels was investigated with immunofluorescence microscopy. Positive immunoreactivity was found for receptor subunits alpha3, alpha5, alpha7, and beta2, but not subunits alpha4, beta3, or beta4. Blood-brain barrier permeability was assessed via in situ brain perfusion with [14C]sucrose. Nicotine increased the rate of sucrose entry into the brain from 0.3 +/- 0.1 to 1.1 +/- 0.2 microl.g(-1).min(-1), as previously described. This nicotine-induced increase in blood-brain barrier permeability was significantly attenuated by both the blood-brain barrier-permeant nicotinic antagonist mecamylamine and the blood-brain barrier-impermeant nicotinic antagonist hexamethonium to 0.5 +/- 0.2 and 0.3 +/- 0.2 microl.g(-1).min(-1), respectively. These data suggest that nicotinic acetylcholine receptors expressed on the cerebral microvascular endothelium mediate nicotine-induced changes in blood-brain barrier permeability.

Expression of alpha 4 and alpha 7 nicotinic receptors in the brainstem of female rabbits after coitus

Coital signaling in the female rabbit involves sequential events in the brainstem and hypothalamus, resulting in a massive release of hypothalamic gonadotropin-releasing hormone (GnRH) that peaks within 1-2 h after mating. The neural connections between coitus and GnRH release involves norepinephrine (NE) and acetylcholine (ACh) since administration of antagonists against NE (dibenamine or phentolamine) or ACh (atropine, alpha-bungarotoxin (alpha-BTX) or scopolamine) blocks or attenuates ovulating events. Moreover, hypothalamic NE release and brainstem tyrosine hydroxylase (TH, the rate-limiting enzyme for NE synthesis) expression in the noradrenergic areas increase prior to, or in concert with, the preovulatory GnRH surge. How ACh is involved in the control of ovulation in the rabbit is lesser known. In the present study, the number of brainstem neurons expressing TH, alpha4 and alpha7 subunits of the nicotinic ACh receptor (nAChR) before and after coitus was determined by immunocytochemistry. Compared to non-mated female rabbits, the number of alpha4, alpha7 and TH single-labeled neurons as well as alpha4/TH and alpha7/TH double-labeled neurons increased in the A1, A2 and A6 brainstem noradrenergic areas at 1 h, but not 2 h, after coitus. The results suggest that the participation of ACh in the control of coitus-induced ovulation may include activation of alpha4beta2 and alpha7 nAChRs in neurons within or adjacent to the brainstem noradrenergic areas in female rabbits.

Functional Nicotinic Acetylcholine Receptors Are Expressed in B Lymphocyte-Derived Cell Lines

Nicotine has been shown to affect B lymphocyte immune response. In this study, we have explored the presence of nicotinic receptors in B lymphocyte-derived cell lines, myeloma X63-Ag8 and hybridoma 1D6. We found that myeloma expressed on average 10,170 +/- 1,100 [3H]epibatidine and 6,730 +/- 370 125I-alpha-bungarotoxin binding sites per cell, thus reflecting the presence of both homomeric and heteromeric nicotinic receptors. More specifically, the presence of alpha4- and alpha7-containing nicotinic receptor subunits was demonstrated in both myeloma and hybridoma cells with subunit-specific antibodies. It was significantly higher in dividing than in resting cells. Long-term exposure to nicotine, at physiological concentration found in smokers, resulted in up-regulation of both alpha4 and alpha7 subunits in hybridoma cells. Additionally, nicotine stimulated hybridoma cell proliferation, whereas it decreased antibody production. In contrast, alpha7-specific snake toxins inhibited cell proliferation but increased antibody production. It is concluded that myeloma and hybridoma cells express alpha4- and alpha7-containing nicotinic receptors, which participate in regulating cell proliferation and function.

The role of neuronal nicotinic acetylcholine receptor subunits in autonomic ganglia: lessons from knockout mice

Neuronal nicotinic acetylcholine receptors (nAChR), composed of 12 subunits (alpha2-alpha10, beta2-beta4), are expressed in autonomic ganglia, playing a central role in autonomic transmission. The repertoire of nicotinic subunits in autonomic ganglia includes alpha3, alpha5, alpha7, beta2 and beta4 subunits. In the last 10 years, heterologous expression studies have revealed much about the nature of neuronal nAChRs. However, there is only limited understanding of subunit actions in autonomic system. Functional deletions of subunit by gene knockout in animals could overcome these limitations. We review recent studies on nAChRs on autonomic ganglia for physiological and pharmacological properties and potential locations of the subunits.

Effects of P-Aminophenyl Dichloroarsine on Reduced High-affinity [3H]Nicotine Binding Sites from Chick Brain: A Covalent, Yet Reversible, Agent for Neuronal Nicotinic Receptors

Neuronal nicotinic acetylcholine receptor (nAChR) alpha-subunits contain a conserved disulphide that is essential for function. Here, we have examined the effects of sulphydryl redox reagents on [3H]nicotine binding to chick brain nAChR immunoisolated with the monoclonal antibody mAb35. The disulphide reducing agent, dithiothreitol (DTT), inhibited [3H]nicotine binding [50% inhibitory concentration (IC50)=146 microM] but this effect was reversed (93 +/- 1.5%) by subsequent reoxidation with 1 mM dithio-bis(nitrobenzoic acid) (DTNB). The trivalent arsenical, p-aminophenyl dichloroarsine (APA), which reacts with pairs of spatially close sulphydryls, was a potent inhibitor of reoxidation by DTNB (IC50=35 nM). However, application of the 'anti-arsenical', 2,3-dimercaptopropane sulphonic acid (DMPS), restored agonist binding after APA treatment (50% effective concentration=120 microM). Paradoxically, DMPS was also found to be a potent oxidizing agent of these receptors. Affinity alkylation of reduced nAChRs with bromoacetylcholine (BAC; 100 microM) irreversibly blocked nicotine binding (>90%). We propose (but have not proven) that APA interacts with the cysteines homologous to Cys192 - 193 in Torpedo AChRs, since APA pretreatment of reduced neuronal receptors protected against irreversible BAC alkylation, as shown by subsequent reversal of DMPS (2 mM; 20 min). This study illustrates the potent and reversible nature of the arsenical's covalent interaction with an isolated nAChR and suggests that modified arsenicals could be useful nAChR probes.

Pharmacological and Biochemical Properties of Nicotinic Receptors from Chick Retina

Previous work has established that functional nicotinic receptors in the chick retina are blocked by neuronal bungarotoxin (NBT), and that the binding of radio-iodinated NBT to retinal homogenates is displaced by nicotinic ligands. In the present study, we examined the desensitizing effects of agonists on nicotinically-mediated depolarizations recorded from chick retina. The concentrations of five agonists necessary to reduce the amplitude of these depolarizations by 50% were found to correlate closely with the concentrations of these same agonists previously found necessary to displace 50% of NBT binding. In addition, bromoacetylcholine (BAC), a selective affinity alkylating agent for the agonist binding site, irreversibly inactivated the functional responses of intact chick retina with an inhibiting concentration for 50% block (IC50) near 10-6 M, the same concentration of BAC that displaced 50% of labelled NBT binding from alkylated retinal homogenates. These data suggest that NBT acts at the receptor agonist binding site. Furthermore, this binding site has a relatively low affinity for agonists, in the micromolar range, even in the desensitized state. Multiple subtypes of nicotinic receptors are known to exist in neuronal tissue, and receptors that bind agonists in the nanomolar range have been detergent-solubilized and purified using monoclonal antibodies. Under similar conditions, detergent-solubilization of chick retinal homogenates interfere with the interaction between NBT and the low-affinity neuronal nicotinic receptors. These data suggest that the conditions used to purify high-affinity neuronal nicotinic receptors may denature the subtype(s) of neuronal receptors recognized by NBT.

Ion channel diseases of the central nervous system

In the last decade, advances in molecular genetics and cellular electrophysiology have increased our understanding of ion channel function. A number of diseases termed "channelopathies" have been discovered that are caused by ion channel dysfunction. Channelopathies can be caused by autoimmune, iatrogenic, toxic or genetic mechanisms. Mutations in genes encoding ion channel proteins that disrupt channel function are now the most commonly identified cause of channelopathies, perhaps because gene disruption is readily detected by the methods of molecular genetics. Ion channels are abundant in the central nervous system (CNS), but CNS channelopathies are rare; however, they overlap with some important neurological disorders, such as epilepsy, ataxia, migraine, schizophrenia, Alzheimer's disease and other neurodegenerative diseases. It is possible that more CNS channelopathies will be discovered when additional ion channels are characterized and the complex mechanisms of brain function are better understood. At present, increased knowledge of the identity, structure and function of ion channels is facilitating diagnosis and treatment of many channelopathies.

Nicotinic regulation of c-fos and osteopontin expression in human-derived osteoblast-like cells and human trabecular bone organ culture

Long-term in vivo studies have highlighted smoking as a risk factor in postmenopausal osteoporosis, bone fracture incidence, and increased nonunion rates. In contrast, there are few data postulating the effects of smoking at the cellular level in human skeletal tissue. In this study, we present novel evidence demonstrating that the nicotinic receptor alpha4 subunit is present in human primary bone cells by using reverse transcriptase-polymerase chain reaction (RT-PCR). In addition, we demonstrate direct cellular effects of nicotine on primary human bone cells and blockage of these effects with a nicotinic receptor antagonist, D-tubocurarine. Nicotine effects on cell proliferation were biphasic with toxic, antiproliferative effects at high levels of nicotine (>1 mmol/L) and stimulatory effects at very low levels (0.01-10 micromol/L) after 72 h. This nicotine-induced increase in cell proliferation was inhibited in a dose-dependent manner by the addition of D-tubocurarine. In addition, proliferation effects from low-level treatment correlated with an upregulation of expression of the AP-1 transcription factor, c-fos, within 1 h, which was blocked by incubation with D-tubocurarine. To determine in situ bone cell responses within their trabecular matrix, cores of human bone isolated from biopsies were perfused with 0.1 micromol/L nicotine for 24 h. Western analysis of proteins isolated from the cores highlighted an increase in osteopontin, a bone matrix protein implicated in regulating resorption, which was partially inhibited by the addition of D-tubocurarine. To conclude, our results suggest that nicotine has a direct effect on human bone cells in modulating proliferation, upregulation of the c-fos transcription factor, and the synthesis of the bone matrix protein, osteopontin.

Central nicotinic receptors, neurotrophic factors and neuroprotection

The multiple combinations of nAChR subunits identified in central nervous structures possess distinct pharmacological and physiological properties. A growing number of data have shown that compounds interacting with neuronal nAChRs have, both in vivo and in vitro, the potential to be neuroprotective and that treatment with nAChR agonists elicit long-lasting improving of cognitive performance in a variety of behavioural tests in rats, monkeys and humans. Epidemiological and clinical studies suggested also a potential neuroprotective/trophic role of (-)-nicotine in neurodegenerative disease, such as Alzheimer's and Parkinson's disease. Taken together experimental and clinical data largely indicate a neuroprotective/trophic role of nAChR activation involving mainly alpha7 and alpha4beta2 nAChR subtypes, as evidenced using selective nAChR antagonists, and by potent nAChR agonists recently found displaying efficacy and/or larger selective affinities than (-)-nicotine for neuronal nAChR subtypes. A neurotrophic factor gene regulation by nAChR signalling has been taken into consideration as possible mechanism involved in neuroprotective/trophic effects by nAChR activation and has evidenced an involvement of the fibroblast growth factor (FGF-2) gene as a target of nAChR signalling. These findings suggested that FGF-2 could be involved, according to the FGF-2 neurotrophic functions, in nAChR mechanisms mediating the neuronal survival, trophism and plasticity.

Neurotrophic effects of central nicotinic receptor activation

A growing number of data have shown that compounds interacting with neuronal nicotinic acetylcholine receptors (nAChRs) have, both in vivo and in vitro, the potential to be neuroprotective and that treatment with nAChR agonists elicit long-lasting improvement of cognitive performance in a variety of behavioural tests in rats, monkeys and humans. Epidemiological and clinical studies suggested also a potential neuroprotective/trophic role of (-)-nicotine in neurodegenerative disease, such as Alzheimer's disease and Parkinson's disease. This neuroprotective/trophic role of nAChR activation has been mainly mediated by alpha7 and alpha4beta2 nAChR subtypes, as evidenced using selective nAChR antagonists, and by potent nAChR agonists recently found displaying efficacy and/or larger selective affinities than (-)-nicotine for neuronal nAChR subtypes. A neurotrophic factor gene regulation by nAChR signalling has been taken into consideration as a possible mechanism involved in neuroprotective/trophic effects of nAChR activation and has given evidence that the fibroblast growth factor (FGF-2) gene is a target for nAChR signalling. These findings suggested that FGF-2 could be involved, in view of its neurotrophic functions, in nAChR mechanisms mediating neuronal survival, trophism and plasticity.

Subunit and region-specific decreases in nicotinic acetylcholine receptor mRNA in the aged rat brain

We have investigated possible changes in the mRNA levels for several alpha and beta subunits of the nicotinic acetylcholine receptor (nAChR) and the level of binding for nicotinic ligands in 7- to 32-month-old rats. Alpha4 and beta2, and to a lesser extent alpha6 and beta3, mRNA levels showed decreases between 20 and 30% at 29 months of age which in some areas reached 50% at 32 months of age. Alpha7 showed a small increase from 7 to 14 months and then a progressive decrease from 14 to 32 months down to the 7-month levels. 3H-epibatidine binding did not significantly change from 7 to 32 months of age in rat tel- and diencephalon. Binding in the substantia nigra was exceptional in that it showed a significant decrease starting from 23 months of age. 125I-alpha-bungarotoxin binding showed a pattern of change which roughly paralleled that of alpha7 mRNA. These findings show that an alteration in some steps of nAChR biosynthesis takes place during aging, which may be related to functional changes in nicotinic transmission.

Absence of Oligoclonal IgG Bands and Anti-Achr Antibodies in the Cerebrospinal Fluid of Patients with Myasthenia Gravis

Myasthenia gravis (MG) is a neuromuscular disorder in which antibodies are directed against the nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Some investigators describe the existence of oligoclonal IgG bands and anti-AChR antibodies in the cerebrospinal fluid (CSF) of MG patients while other refuse it. This study was performed in 15 patients with clinical and electrophysiological diagnosis of MG. Oligoclonal IgG bands (OCB) and antibodies to the AChR from human skeletal muscle were determined in the serum and the CSF of the above MG patients. The last one was done in order to investigate any possible central nervous system (CNS) involvement.

It was found that all the MG patients who had a high titre of anti- AChR antibodies in the serum (mean titre 29.2±24.3 nM, range 1.8 to 62 nM) did not present OCB and anti-AChR antibodies in their CSF. On the same time, in a group of 10 patients with a definite multiple sclerosis it was found that eight of them presented OCB in their CSF while the results were negative in another group of 10 patients without evidence of CNS disease. The last two groups served as control groups.

Our findings are in accordance with the concept that MG is a pure neuromuscular disorder.

Human neuronal nicotinic receptors

Nicotine is a very widely used drug of abuse, which exerts a number of neurovegetative, behavioural and psychological effects by interacting with neuronal nicotinic acetylcholine receptors (NAChRs). These receptors are distributed widely in human brain and ganglia, and form a family of ACh-gated ion channels of different subtypes, each of which has a specific pharmacology and physiology. As human NAChRs have been implicated in a number of human central nervous system disorders (including the neurodegenerative Alzheimer's disease, schizophrenia and epilepsy), they are suitable potential targets for rational drug therapy. Much of our current knowledge about the structure and function of NAChRs comes from studies carried out in other species, such as rodents and chicks, and information concerning human nicotinic receptors is still incomplete and scattered in the literature. Nevertheless, it is already evident that there are a number of differences in the anatomical distribution, physiology, pharmacology, and expression regulation of certain subtypes between the nicotinic systems of humans and other species. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining the molecular basis of their functional diversity viewed mainly from pharmacological and biochemical perspectives. It will also summarize our current knowledge concerning the structure and function of the NAChRs expressed by other species, and the newly discovered drugs used to classify their numerous subtypes. Finally, the role of NAChRs in behaviour and pathology will be considered.

The molecular biology of neuronal nicotinic acetylcholine receptors

The molecular cloning of genes encoding neuronal nicotinic acetylcholine receptors (nAChRs) has made possible a better understanding of the pharmacology and toxicology of cholinergic compounds. Neuronal nAChRs are related in structure to the nAChRs present at the neuromuscular junction. They are composed of multiple subunits designated either alpha and beta. Eight alpha and three beta subunit genes have been cloned. The alpha subunits contain the ligand binding sites, whereas beta subunits are structural subunits that contribute to the function of the receptor. A large number of nAChRs can be formed from different combinations of alpha and beta subunits. Different combinations of alpha and beta subunits can produce receptors in vitro with distinct ion conducting properties. Each subunit gene is expressed in a distinct pattern in the nervous system. The expression of at least some of the nAChR subunit genes is regulated during development and by cell-cell interactions. Each neuronal nAChR subtype has a distinct pharmacology. Both alpha and beta subunits contribute to the pharmacological properties of each subtype. The expression of multiple nAChR subtypes may allow for precise control of neurotransmission mediated by acetylcholine in diverse populations of neurons.

Heterogeneous levamisole receptors: a single-channel study of nicotinic acetylcholine receptors from Oesophagostomum dentatum

A muscle vesicle preparation from Oesophagostomum dentatum, a 5 mm parasitic nematode, was developed for single-channel recording. Properties of nicotinic acetylcholine receptors activated by the anthelmintic levamisole (10 microM) were investigated using cell-attached and isolated inside-out patches. The current-voltage relationships of the single-channel currents were linear with conductances in the range 24.6-57.7 pS (mean 39.5 pS). The distributions of open times were fitted with a single exponential and mean open times were in the range 0.98-4.43 ms (mean 2.2 ms). The distributions of conductances and open times of the channels showed that the receptors could not be described as a single homogeneous population. There were two main channel subtypes: one subtype, designated G35, had a mean conductance of 35.2 pS and mean open time of 1.6 ms: another subtype designated G45, had a mean conductance of 44.6 pS and mean open time of 2.7 ms. A channel with a conductance near 25 pS. designated G25, and a channel with a conductance near 55 pS. designated G55, were also observed. The designations were based on the mean conductances. G. of the channel subtypes. A model for the heterogeneous population of nicotinic acetylcholine channels predicting four subtypes of receptor separated by their conductance is discussed and related to the development of levamisole resistance.

Presumptive presynaptic nicotinic acetylcholine receptors in the chick tectum: effects of lesions of the lateral spiriform nucleus

There are indications that nicotinic acetylcholine receptor subunits in the superficial layers of the chick tectum (Cajal's layers 1-7) may be transported from the retina. However, nicotinic receptor subunits are detectable by immunohistochemistry in all layers of the optic tectum. In this study, we performed unilateral electrolytic lesions of the lateral spiriform nucleus, which projects to the deep layers of the tectum and contains high amounts of nicotinic receptors in its perikarya. Following lesions of the lateral spiriform nucleus, both the alpha 5 and the beta 2 subunits were markedly depleted in the neuropil of the deep layers of the ipsilateral optic tectum (layers 8-13). No changes were observed in somata that contain either subunit. The present results suggest that most of the nicotinic acetylcholine receptor subunits in the chick optic tectum occur in axonal systems and could then constitute presynaptic receptors.

Neurogenesis of cholinoceptive neurons in the chick retina

Immunocytochemistry and [3H]thymidine autoradiography were combined in this study to determine the neurogenesis of cholinoceptive cells in the chick retina. After injections of [3H]thymidine between embryonic days 1 and 11, the time of birth of retinal neurons containing either the alpha 3 or the alpha 8 subunit of the nicotinic acetylcholine receptors was determined. The results indicate that the alpha 3-positive neurons in the ganglion cell layer leave the cell cycle from E2 through E7, and those in the inner nuclear layer (amacrine and displaced ganglion cells) from E2 through E9. The alpha 8-positive cells in the ganglion cell layer were born from E1 through E7, and those in the inner nuclear layer (amacrine and bipolar cells) from E2 through E11. These data suggest that the time of birth of cholinoceptive neurons in the chick retina follows the general pattern of cell generation in the chick retina, and that alpha 8-positive cells in the ganglion cell layer start to leave the cell cycle almost one day earlier than the alpha 3-positive cells in the same layer.

Actions of nitromethylenes on an alpha-bungarotoxin-sensitive neuronal nicotinic acetylcholine receptor

Nine nitromethylene analogues were tested for their actions on insect neuronal nicotinic acetylcholine receptors (nAChRs). Microelectrode recordings were used to study the actions of nitromethylenes on the cell body of an identified cockroach (Periplaneta americana) motor neurone, the fast coxal depressor (Df) in the metathoracic ganglion. Six nitromethylenes showed potent nAChR agonist actions; others were without nAChR agonist actions. Five nitromethylenes competitively displaced bound [125I]-alpha-bungarotoxin from cockroach nervous system membranes. The rank orders of potency for the compounds determined by their depolarizing actions and their ability to displace [125I]-alpha-bungarotoxin binding were similar. These findings, together with toxicity data obtained on the insects, Nephotettix cinciteps and Nilaparvata lugens, support the hypothesis that insect nAChRs are molecular targets of nitromethylene insecticides. Structure-activity relationships of the nitromethylenes suggest that optimal activity at neuronal nAChRs requires the presence of an electron-withdrawing component in the region of the aryl substituent and an electron-donating component at the 3' position of the imidazolidine ring.

Actions of neurotoxins (bungarotoxins, neosurugatoxin and lophotoxins) on insect and nematode nicotinic acetylcholine receptors

Neurotoxins of natural origin have proved to be of considerable value in the isolation and characterization of vertebrate muscle and neuronal nicotinic acetylcholine receptors (nAChRs). To date, they have been used less extensively in studies of invertebrate nAChRs. Here we examine how a variety of neurotoxins (the snake toxins alpha-bungarotoxin, alpha-BGT, and kappa-bungarotoxin, kappa-BGT, the molluscan toxin, neosurugatoxin, and the soft coral toxins, lophotoxin and bipinnatin-B) can be used to characterize nAChRs in an insect, Periplaneta americana, and in a parasitic nematode, Ascaris suum. The agonist profiles of these nAChRs are distinct, but the most striking differences are in the actions of antagonists. Whereas the insect nAChR is blocked by both alpha- and kappa-bungarotoxins, the nematode receptor is only blocked by kappa-BGT. Neosurugatoxin blocks nAChRs in both species, but the lophotoxins which block all nAChRs investigated to date are much less effective on the Ascaris muscle receptor.

Effects of retinal lesions upon the distribution of nicotinic acetylcholine receptor subunits in the chick visual system

Immunohistochemistry was used in this study to evaluate the effects of retinal lesions upon the distribution of neuronal nicotinic acetylcholine receptor subunits in the chick visual system. Following unilateral retinal lesions, the neuropil staining with an antibody against the beta 2 receptor subunit, a major component of alpha-bungarotoxin-insensitive nicotinic receptors, was dramatically reduced or completely eliminated in all of the contralateral retinorecipient structures. On the other hand, neuropil staining with antibodies against two alpha-bungarotoxin-sensitive receptor subunits, alpha 7 and alpha 8, was only slightly affected after retinal lesions. Decreased neuropil staining for alpha 7-like immunoreactivity was only observed in the nucleus of the basal optic root and layers 2-4 and 7 of the optic tectum. For alpha 8-like immunoreactivity, slight reduction of neuropil staining could be observed in the ventral geniculate complex, griseum tecti, nucleus lateralis anterior, nucleus lentiformis mesencephali, layers 4 and 7 of the tectum, and nucleus suprachiasmaticus. Taken together with previous data on the localization of nicotinic receptors in the retina, the present results indicate that the beta 2 subunit is transported from retinal ganglion cells to their central targets, whereas the alpha 7 and alpha 8 subunit immunoreactivity appears to have a central origin. The source of these immunoreactivities could be, at least in part, the stained perikarya that were observed to contain alpha 7 and alpha 8 subunits in all retinorecipient areas. In agreement with this hypothesis, the beta 2 subunit of the nicotinic acetylcholine receptors was not frequently found in perikarya of the same areas.

Functional characterization of a nonclassical nicotine receptor associated with inositolphospholipid breakdown and mobilization of intracellular calcium pools

Classical nicotinic receptors are neurotransmitter-gated channels that, upon activation by acetylcholine, induce the opening of an intrinsic cationic channel. We have recently observed that, in frog pituitary melanotrophs, nicotine stimulates alpha-melanocyte-stimulating hormone (alpha-MSH) release through a noncholinergic mechanism. In the study reported here, we investigated the intracellular events that mediate the response of frog melanotrophs to nicotine. Nicotine was capable of stimulating alpha-MSH release in the absence of Ca2+ and/or Na+ in the extracellular medium. A short pulse of nicotine induced a rapid and transient increase of cytosolic free Ca2+ concentration ([Ca2+]i). The effect of nicotine on Ca2+ mobilization was not affected in the absence of Na+ and Ca2+ in the extracellular medium, indicating that the nicotine-evoked increase in [Ca2+]i did not result from Na+ or Ca2+ influx. Nicotine induced both an increase in inositol trisphosphate and a reduction in phosphaditylinositol bisphosphate concentrations but did not affect cAMP production. The present results indicate that nicotine-induced stimulation of alpha-MSH release in frog melanotrophs can be explained by activation of inositolphospholipid breakdown and mobilization of inositol triphosphate-dependent intracellular Ca2+ pools. These data provide evidence for the existence of an unusual type of noncholinergic nicotine receptor positively coupled to phospholipase C.

Expression of nicotinic acetylcholine receptors in the rat superior cervical ganglion on mRNA and protein level

The expression of nicotinic acetylcholine receptors (nAChR) in the rat superior cervical ganglion was investigated by Western blotting, immunohistochemistry and non-radioactive in situ hybridization applying probes for the alpha 4-1 and beta 2 subunit mRNA. Immunoblot analysis of homogenized ganglia using the anti-nAChRs antibody WF6 revealed a labeled protein band of apparent molecular weight of 40 kDa which is typical for the alpha subunit of nAChRs. Applying double-labeling immunofluorescence with antibodies against tyrosine hydroxylase, nAChR-like molecules were identified in most postganglionic neurons and in a subpopulation of small intensely fluorescent (SIF) cells. alpha 4-1 and beta 2 subunit mRNAs were detected in all perikarya of postganglionic sympathetic neurons but not in SIF cells. These results suggest that antibodies raised against purified Torpedo AChR bind to nAChR in sympathetic ganglia and indicate that alpha 4-1 and beta 2 subunits are constituents of nAChRs in sympathetic postganglionic neurons but not of SIF cells.

Light microscopic distribution of some cholinergic markers in the rat and rabbit locus coeruleus and the nucleus angularis grisea periventricularis of the domestic pig (Sus scrofa): a correlative electron microscopic investigation of cholinergic receptor proteins in the rabbit

Cholinergic modulation of locus coeruleus (LC) neurons evokes a variety of neuronal and behavioural effects. In an attempt to understand the LC cholinergic circuit, several markers has been investigated and compared. (Immuno)-histochemical and autoradiographic methods have been used on rat, rabbit, and pig tissue. To identify the boundaries of the LC in each of these species, sections through the entire brainstem have been stained for tyrosine hydroxylase. The results indicate that the pig does not possess a LC proper that conforms to the accepted features of this cell group. However, in this location fusiform cells reminiscent of LC interneurons are still present. This group of fusiform neurons has been named the nucleus angularis grisea periventricularis (NAGP). LC cells of the rat and rabbit show strong acetylcholinesterase (AChE) activity. In the pig the NAGP is markedly free from AChE staining. Muscarinic binding sites are densely distributed over the rabbit LC and adjacent region. The rat and rabbit LC neurons synthesise both muscarinic (mAChR) and nicotinic receptor protein (nAChR). In the pig NAGP region mAChR and nAChR positive cell bodies are almost absent, while some nAChR immunoreactive dendrites are present. The light microscopic data in the rabbit have been confirmed by electron microscopic analysis. It is concluded that the general concept of a noradrenergic LC that is present throughout mammals is questionable. At present, choline acetyltransferase immunoreactive terminals that closely correspond to the other cholinergic components in the rat or rabbit LC have not been observed. However, in these species the cholinergic sensitivity of LC cells is mediated via both muscarinic and nicotinic receptors on somata and dendrites.

Differential development of alpha-bungarotoxin-sensitive and alpha-bungarotoxin-insensitive nicotinic acetylcholine receptors in the chick retina

The development of cells containing neuronal nicotinic receptors (nAChRs) in the chick retina was investigated by means of immunohistochemical techniques with antibodies directed against the alpha 3 and alpha 8 nAChR subunits. The alpha 3 subunit is one of the major alpha-bungarotoxin-insensitive nicotinic receptor subunits in the chick retina, whereas alpha 8 appears to be the most common alpha-bungarotoxin-sensitive subunit in the same structure, alpha 3-like immunoreactivity (alpha 3-LI) was first detected in cells of the vitreal margin, on the embryonic day 4.5 (E4.5). alpha 8-LI was first detected in the same type of cell almost a day later. However, the processes of alpha 8-LI cells developed much faster than those of alpha 3-LI cells, generating visible stained laminae in the prospective inner plexiform layer as early as E7. alpha 3-LI was only clearly seen in laminae of the inner plexiform layer by E12. By this date, both alpha 3 and alpha 8-LI were seen in the same types of cells as in the adult retina, i.e., amacrines, displaced ganglion cells, and cells of the ganglion cell layer for alpha 3-LI; and amacrines, bipolar cells, and cells of the ganglion cell layer for alpha 8-LI. These results reveal different patterns of development of cells containing the alpha 3 and alpha 8 nAChR subunits in the chick retina and indicate that those nAChR subunits are expressed in the chick retina before choline acetyltransferase-positive cells can be detected and well before synaptogenesis. These data also suggest that nAChRs may have a developmental function in the retina.

Nicotinic acetylcholine receptors in the ground squirrel retina: localization of the beta 4 subunit by immunohistochemistry and in situ hybridization

Immunohistochemical and in situ hybridization techniques were used to localize the beta 4 subunit of the neuronal nicotinic acetylcholine receptors (nAChRs) in the ground squirrel retina. The beta 4 nAChR subunit was detected in both transverse and horizontal sections of the retina using a subunit-specific antiserum and the avidin-biotin complex technique. Two bands of labeled processes were seen in the inner plexiform layer, corresponding approximately to the laminae where the cholinergic cells arborize. Labeled cells were found in the ganglion cell layer and the inner third of the inner nuclear layer. The cells in the ganglion cell layer were medium- to large-sized and were frequently observed to give rise to axon-like processes. Most of the labeled neurons in the inner nuclear layer were small presumptive amacrine cells, but a few medium-to-large cells were also labeled. These could constitute a different class of amacrine cells or displaced ganglion cells. The latter possibility is supported by the existence of nAChR-containing displaced ganglion cells in the avian retina. In situ hybridization with a 35S-labeled cRNA probe revealed the expression of mRNA coding for the nAChR beta 4 subunit in the ganglion cell layer and the inner third of the inner nuclear layer. This finding confirmed the immunohistochemical data of the cellular localization of beta 4 nAChR subunit. These results indicate that the beta 4 nAChR subunit is expressed by specific subtypes of neurons on the ground squirrel retina.(ABSTRACT TRUNCATED AT 250 WORDS)