αO-Conotoxin GeXIVA disulfide bond isomers exhibit differential sensitivity for various nicotinic acetylcholine receptors but retain potency and selectivity for the human α9α10 subtype

Nicotinic acetylcholine receptor (nAChR) subtypes exhibit distinct neuropharmacological properties that are involved in a range of neuropathological conditions, including pain, addiction, epilepsy, autism, schizophrenia, Tourette's syndrome, Alzheimer's and Parkinson's diseases, as well as many types of cancer. The α9α10 nAChR is a potential target in chronic pain, wound healing, the pathophysiology of the auditory system, and breast and lung cancers. αO-conotoxin GeXIVA is a potent antagonist of rat α9α10 nAChRs, with the 'bead' disulfide bond isomer displaying the lowest IC50 of the three possible isomers. In the rat chronic constriction injury model of neuropathic pain, this isomer reduced mechanical hyperalgesia without affecting motor performance. Here, we report the effects of the three disulfide bond isomers of GeXIVA on human α9α10 nAChRs, other human nAChR subtypes, various rat nAChR subtypes, and 10 rat α9α10 nAChR mutants. The three isomers displayed only ∼5-fold difference in potency on the human vs rat α9α10 receptors and had similar affinities at wild-type rat α9α10 nAChRs and all 10 α9α10 receptor mutants. From these findings, the binding site and mechanism of action of GeXIVA on rat and human α9α10 nAChR was deduced to be different from that of other conotoxins targeting this nAChR subtype. GeXIVA is therefore a unique ligand that might prove useful for further probing of binding sites on the α9α10 nAChR. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Neuronal Nicotinic Acetylcholine Receptors: Involvement in Alzheimer’s Disease and Schizophrenia

Nicotinic acetylcholine receptors (nAChRs) play a role in a variety of diseases of the central nervous system including Alzheimer's disease (AD) and schizophrenia. There is great interest in evaluating disease-related nAChR changes, and pharmacological treatment of nAChR deficits is a promising therapy. In AD, 7 nAChRs remain relatively stable, contrasting to 4 2 nAChRs that are lost in substantial numbers. -amyloid, a major neuropathology in AD, blocks 4 2 and 7 nAChRs. Agonists selective to 7 nAChRs are neuroprotective against amyloid. Paradoxically, 7 nAChRs may function as receptors for -amyloid. These results indicate 7 nAChR antagonists may be appropriate therapy in AD. In schizophrenia, 7 nAChRs are significantly reduced in hippocampus and neocortex. The exceptionally high rate of smoking in schizophrenics is likely a form of self-medication. Therapy with 7 nAChR agonists relieves some schizophrenic symptoms. Despite disparities in etiology and symptomatology, AD and schizophrenia share a target for therapeutic intervention— 7 nAChRs.

The subunit gene Ldα1 of nicotinic acetylcholine receptors plays important roles in the toxicity of imidacloprid and thiamethoxam against Leptinotarsa decemlineata

Nicotinic acetylcholine receptors (nAChRs) are pentameric ACh-gated ion channels. It is believed that nAChRs composed of different subunits may vary in their function and toxicological characteristics. Neonicotinoids are activators of nAChRs and important insecticides that are extensively used for crop protection and resistance has been developed by some pests. They are also major insecticides for the control of Leptinotarsa decemlineata, which is a destructive defoliator pest that invaded the Xinjiang region of China in the 1990s. However, little is known about the constitution or subunits of the target in this pest. In this study, the full-length cDNAs encoding four new nAChR subunits (named Ldα3, Ldα6, Ldα10, and Ldβ1) were cloned from L. decemlineata. These genes encode 822-, 753-, 672-, and 759-amino acid proteins, respectively, which share typical features of insect nAChRs subunits and closely resemble the corresponding subunits of the nAChRs from Tribolium castaneum. Temporal and spatial expression analyses showed that these genes, as well as the previously identified Ldα1, Ldα2, and Ldα8 genes, are widely expressed in all developmental stages, including eggs, larvae of various instars, pupae, and adults. All genes monitored were expressed at higher levels in the head than in the thorax and abdomen, except for Ldα10. Dietary ingestion of double-stranded RNA bacterially expressed for Ldα1 (dsLdα1) significantly reduced the mRNA level of Ldα1 in treated larvae and adults by 48.0% and 78.6%, respectively. Among the non-target genes, Ldα3, Ldα9, and Ldβ1 were significantly up-regulated in larvae. A toxicity bioassay showed that dsLdα1 treatment greatly decreased the sensitivity to imidacloprid and thiamethoxam in adults. The larval susceptibility to thiamethoxam but not to imidacloprid was also reduced because of the lower down-regulation of Ldα1. Thus, our results suggest that Ldα1 encodes a subunit of a functional nAChR that mediates the toxicity of imidacloprid and thiamethoxam against L. decemlineata and that the down-regulation of Ldα1 might be an important mechanism for resistance and/or tolerance of L. decemlineata to neonicotinoids.

Tribendimidine: mode of action and nAChR subtype selectivity in Ascaris and Oesophagostomum

The cholinergic class of anthelmintic drugs is used for the control of parasitic nematodes. One of this class of drugs, tribendimidine (a symmetrical diamidine derivative, of amidantel), was developed in China for use in humans in the mid-1980s. It has a broader-spectrum anthelmintic action against soil-transmitted helminthiasis than other cholinergic anthelmintics, and is effective against hookworm, pinworms, roundworms, and Strongyloides and flatworm of humans. Although molecular studies on C. elegans suggest that tribendimidine is a cholinergic agonist that is selective for the same nematode muscle nAChR as levamisole, no direct electrophysiological observations in nematode parasites have been made to test this hypothesis. Also the hypothesis that levamisole and tribendimine act on the same receptor, does not explain why tribendimidine is effective against some nematode parasites when levamisole is not. Here we examine the effects of tribendimidine on the electrophysiology and contraction of Ascaris suum body muscle and show that tribendimidine produces depolarization antagonized by the nicotinic antagonist mecamylamine, and that tribendimidine is an agonist of muscle nAChRs of parasitic nematodes. Further pharmacological characterization of the nAChRs activated by tribendimidine in our Ascaris muscle contraction assay shows that tribendimidine is not selective for the same receptor subtypes as levamisole, and that tribendimidine is more selective for the B-subtype than the L-subtype of nAChR. In addition, larval migration inhibition assays with levamisole-resistant Oesophagostomum dentatum isolates show that tribendimidine is as active on a levamisole-resistant isolate as on a levamisole-sensitive isolate, suggesting that the selectivity for levamisole and tribendimidine is not the same. It is concluded that tribendimidine can activate a different population of nematode parasite nAChRs than levamisole, and is more like bephenium. The different nAChR subtype selectivity of tribendimidine may explain why the spectrum of action of tribendimidine is different to that of other cholinergic anthelmintics like levamisole.

Nematode parasites are a plague on the human condition in many developing countries with limited health care and sanitation. The morbidity produced by these parasites limits human health, development and prosperity. Nematode parasites also adversely affect animal welfare and production. Vaccines are not effective, so anthelmintic drugs are necessary for prophylaxis and treatment. Most anthelmintics belong to one of three classes: the macrocyclic lactones (ivermectin, moxidectin); the nicotinic anthelmintics (levamisole, pyrantel, derquantel) or; the benzimidazoles (albendazole, mebendazole). With the limited number of drugs available, there is real concern about the development of resistance. Tribendimidine was developed in China in the mid-1980s as a broad spectrum anthelmintic against soil-transmitted nematodes. Its mode of action has been investigated molecularly in C. elegans and on expressed nAChRs but, its mode of action has not been investigated directly in parasitic nematodes. Here we describe its effects on muscle contraction and electrophysiology in the pig nematode parasite, A. suum, which is very similar or the same as the human parasite, A. lumbricoides. Here we show that tribendimidine is a B-subtype selective nicotinic anthelmintic agonist that activates muscle nAChRs that are pharmacologically different from other cholinergic anthelmintics. It is concluded that tribendimidine could be effective against nematode parasites resistant to another cholinergic anthelmintic.

[Nicotinic acetylcholine receptor in Alzheimer's disease]

Nicotinic acetylcholine receptor (nAChR) mediates pleiotropic actions in brain not only for nicotinic neurotransmission but also glutamatergic, dopaminergic, norepinephrinergic, GABAergic, and serotonergic transmissions, especially through allosteric potentiating ligand (APL) action. Because nAChR is rich in thalamus, the direct stimulation of nAChR through APL action is expected to show increasing function of cerebral cortex and limbic system through thalamic activation. In fact, a choline esterase inhibitor with this APL action such as galantamine exerts both cognitive and affective improvements which is called dual benefit for Alzheimer's disease patients.

Tunable pKa values and the basis of opposite charge selectivities in nicotinic-type receptors

Among ion channels, only the nicotinic-receptor superfamily has evolved to generate both cation- and anion-selective members. Although other, structurally unrelated, neurotransmitter-gated cation channels exist, no other type of neurotransmitter-gated anion channel, and thus no other source of fast synaptic inhibitory signals, has been described so far. In addition to the seemingly straightforward electrostatic effect of the presence (in the cation-selective members) or absence (in the anion-selective ones) of a ring of pore-facing carboxylates, mutational studies have identified other features of the amino-acid sequence near the intracellular end of the pore-lining transmembrane segments (M2) that are also required to achieve the high charge selectivity shown by native channels. However, the mechanism underlying this more subtle effect has remained elusive and a subject of speculation. Here we show, using single-channel electrophysiological recordings to estimate the protonation state of native ionizable side chains, that anion-selective-type sequences favour whereas cation-selective-type sequences prevent the protonation of the conserved, buried basic residues at the intracellular entrance of the pore (the M2 0' position). We conclude that the previously unrecognized tunable charge state of the 0' ring of buried basic side chains is an essential feature of these channels' versatile charge-selectivity filter.

Decreased nicotinic receptors and cognitive deficit in rats intracerebroventricularly injected with beta-amyloid peptide(1-42) and fed a high-cholesterol diet

To investigate whether the changes in nicotinic receptors (nAChRs) and in learning and memory associated with Alzheimer's disease (AD) are influenced by both beta-amyloid peptide (Abeta) and cholesterol in vivo, we examined the effects of intracerebroventricular injection of Abeta(1-42) and/or a high-cholesterol diet on brain levels of nAChRs and learning and memory in rats. The levels of nAChR subunit proteins and the corresponding mRNA were measured by Western blotting and RT-PCR, respectively; and learning and memory were evaluated with the Morris Water Maze examination. Injection of Abeta(1-42) resulted in deposition of this peptide, activation of astrocytes, decreased levels of the alpha7 and alpha4 protein subunits of the nAChR, and elevated expression of alpha7 mRNA, as well as impaired learning and spatial memory. A high-cholesterol diet activated astrocytes and, more importantly, potentiated the toxic effects of Abeta on nAChR subunit levels and on learning and memory. These findings may be highly relevant to the mechanisms underlying the cognitive deficits associated with AD.

Age-dependent changes in the functional expression of two nicotinic receptor subtypes in CA1 stratum radiatum interneurons in the rat hippocampus

Protein density measurements and mRNA analysis have provided valuable information on age-dependent changes in the distribution of different nicotinic receptor (nAChR) subtypes in various areas of the rat brain, including the hippocampus. However, very little is known regarding the functional expression of nAChRs in individual neuron types at various ages. Likewise, there is paucity of information regarding the functional and pharmacological profile of nAChRs in the mature rat hippocampus. To address these issues, we used the whole-cell patch-clamp technique to record nicotinic responses from CA1 stratum radiatum (SR) interneurons in hippocampal slices from rat pups (5-19 days old) and adult rats (2-5 months old). As previously observed in the hippocampus of rat pups, CA1 SR interneurons in the hippocampus of adult rats responded to choline (10mM, 12s) with whole-cell currents that decayed to the baseline within the agonist pulse, were sensitive to inhibition by methyllycaconitine (10nM) or alpha-bungarotoxin (50 nM), and were, therefore, mediated by alpha7*(1)[1] nAChRs. Likewise, as previously observed in the hippocampus of young rats, in the adult rat hippocampus excitatory postsynaptic currents (EPSCs) were recorded from SR interneurons in response to a pulse of ACh (0.1 mM, 12s) applied in the presence of the GABA(A) receptor antagonist bicuculline. ACh-triggered EPSCs were inhibited by mecamylamine (1 microM) or choline (1 mM) and were, therefore, likely to have resulted from activation of alpha3beta4beta2* nAChR. The magnitude of alpha7* nAChR-mediated responses increased with the age of the animals. In contrast, the magnitude of alpha3beta4beta2* nAChR-mediated responses was highest at the second postnatal week. The distinct age dependency of functional expression of alpha7* and alpha3beta4beta2* nAChRs strongly suggests that the excitability of CA1 SR interneurons is differentially regulated by the nicotinic cholinergic system in the hippocampus of rat pups and adult rats.

Structure-guided drug design: conferring selectivity among neuronal nicotinic receptor and acetylcholine-binding protein subtypes

Neuronal nicotinic receptors, encoded by nine genes of the alpha and three of the beta type of subunits, and whose gene products assemble in distinct permutations as pentameric molecules, constitute a fertile area for structure-guided drug design. Design strategies are augmented by a wide variety of peptide, alkaloid and terpenoid toxins from various marine and terrestrial species that interact with nicotinic receptors. Also, acetylcholine-binding proteins from mollusks, as structural surrogates of the receptor that mimic its extracellular domain, provide atomic resolution templates for analysis of structure and response. Herein, we describe a structure-guided approach to nicotinic ligand design that employs crystallography of this protein as the basic template, but also takes into consideration the dynamic properties of the receptor molecules in their biological media. We present the crystallographic structures of several complexes of various agonists and antagonists that associate with the agonist site and can competitively block the action of acetylcholine. In so far as the extracellular domain is involved, we identify additional non-competitive sites at those subunit interfaces where agonists do not preferentially bind. Ligand association at these interface sites may modulate receptor function. Ligand binding is also shown by solution-based spectroscopic and spectrometric methods to affect the dynamics of discrete domains of the receptor molecule. The surrogate receptor molecules can then be employed to design ligands selective for receptor subtype through the novel methods of freeze-frame, click chemistry that uses the very structure of the target molecule as a template for synthesis of the inhibitor.

The subtypes of nicotinic acetylcholine receptors on dopaminergic terminals of mouse striatum

This review summarizes studies that attempted to determine the subtypes of nicotinic acetylcholine receptors (nAChR) expressed in the dopaminergic nerve terminals in the mouse. A variety of experimental approaches has been necessary to reach current knowledge of these subtypes, including in situ hybridization, agonist and antagonist binding, function measured by neurotransmitter release from synaptosomal preparations, and immunoprecipitation by selective antibodies. Early developments that facilitated this effort include the radioactive labeling of selective binding agents, such as [(125)I]-alpha-bungarotoxin and [(3)H]-nicotine, advances in cloning the subunits, and expression and evaluation of function of combinations of subunits in Xenopus oocytes. The discovery of epibatidine and alpha-conotoxin MII (alpha-CtxMII), and the development of nAChR subunit null mutant mice have been invaluable in determining which nAChR subunits are important for expression and function in mice, as well as allowing validation of the specificity of subunit specific antibodies. These approaches have identified five nAChR subtypes of nAChR that are expressed on dopaminergic nerve terminals. Three of these contain the alpha6 subunit (alpha4alpha6beta2beta3, alpha6beta2beta3, alpha6beta2) and bind alpha-CtxMII with high affinity. One of these three subtypes (alpha4alpha6beta2beta3) also has the highest sensitivity to nicotine of any native nAChR that has been studied, to date. The two subtypes that do not have high affinity for alpha-CtxMII (alpha4beta2, alpha4alpha5beta2) are somewhat more numerous than the alpha6* subtypes, but do bind nicotine with high affinity. Given that our first studies detected readily measured differences in sensitivity to agonists and antagonists among these five nAChR subtypes, it seems likely that subtype selective compounds could be developed that would allow therapeutic manipulation of diverse nAChRs that have been implicated in a number of human conditions.

Nicotinic receptors as CNS targets for Parkinson's disease

Parkinson's disease is a debilitating neurodegenerative movement disorder characterized by damage to the nigrostriatal dopaminergic system. Current therapies are symptomatic only and may be accompanied by serious side effects. There is therefore a continual search for novel compounds for the treatment of Parkinson's disease symptoms, as well as to reduce or halt disease progression. Nicotine administration has been reported to improve motor deficits that arise with nigrostriatal damage in parkinsonian animals and in Parkinson's disease. In addition, nicotine protects against nigrostriatal damage in experimental models, findings that have led to the suggestion that the reduced incidence of Parkinson's disease in smokers may be due to the nicotine in tobacco. Altogether, these observations suggest that nicotine treatment may be beneficial in Parkinson's disease. Nicotine interacts with multiple nicotinic receptor (nAChR) subtypes in the peripheral and central nervous system, as well as in skeletal muscle. Work to identify the subtypes affected in Parkinson's disease is therefore critical for the development of targeted therapies. Results show that striatal alpha6beta2-containing nAChRs are particularly susceptible to nigrostriatal damage, with a decline in receptor levels that closely parallels losses in striatal dopamine. In contrast, alpha4beta2-containing nAChRs are decreased to a much smaller extent under the same conditions. These observations suggest that development of nAChR agonists or antagonists targeted to alpha6beta2-containing nAChRs may represent a particularly relevant target for Parkinson's disease therapeutics.

Central Nicotinic Receptors: Structure, Function, Ligands, and Therapeutic Potential

The growing interest in nicotinic receptors, because of their wide expression in neuronal and non-neuronal tissues and their involvement in several important CNS pathologies, has stimulated the synthesis of a high number of ligands able to modulate their function. These membrane proteins appear to be highly heterogeneous, and still only incomplete information is available on their structure, subunit composition, and stoichiometry. This is due to the lack of selective ligands to study the role of nAChR under physiological or pathological conditions; so far, only compounds showing selectivity between alpha4beta2 and alpha7 receptors have been obtained. The nicotinic receptor ligands have been designed starting from lead compounds from natural sources such as nicotine, cytisine, or epibatidine, and, more recently, through the high-throughput screening of chemical libraries. This review focuses on the structure of the new agonists, antagonists, and allosteric ligands of nicotinic receptors, it highlights the current knowledge on the binding site models as a molecular modeling approach to design new compounds, and it discusses the nAChR modulators which have entered clinical trials.

Heterogeneity and complexity of native brain nicotinic receptors

Neuronal cholinergic nicotinic receptors (nAChRs) are a heterogeneous class of cationic channels that are widely distributed in the nervous system that have specific functional and pharmacological properties. They consist of homologous subunits encoded by a large multigene family, and their opening is physiologically controlled by the acetylcholine neurotransmitter or exogenous ligands such as nicotine. Their biophysical and pharmacological properties depend on their subunit composition, which is therefore central to understanding receptor function in the nervous system and discovering new subtype-selective drugs. We will review rodent brain subtypes by discussing their subunit composition, pharmacology and localisation and, when possible, comparing them with the same subtypes present in the brain of other mammalian species or chick. In particular, we will focus on the nAChRs present in the visual pathway (retina, superior colliculus and nucleus geniculatus lateralis), in which neurons express most, if not all, nAChR subunits. In addition to the major alpha4beta2 and alpha7 nAChR subtypes, the visual pathway selectively expresses subtypes with a complex subunit composition. By means of ligand binding and immunoprecipitation and immunopurification experiments on tissues obtained from control and lesioned rats, and wild-type and nAChR subunit knockout mice, we have qualitatively and quantitatively identified, and pharmacologically characterised, the multiple complex native subtypes containing up to four different subunits.

Cholinergic modulation of Pavlovian fear conditioning in rats: differential effects of intrahippocampal infusion of mecamylamine and methyllycaconitine

The cholinergic system has consistently been implicated in Pavlovian fear conditioning. Considerable work has been done to localize specific nicotinic receptor subtypes in the hippocampus and determine their functional importance; however, the specific function of many of these subtypes has yet to be determined. An alpha7 nicotinic antagonist methyllycaconitine (MLA) (35 microg), and a broad spectrum non-alpha7 nicotinic antagonist mecamylamine (35 microg) was injected directly into the dorsal hippocampus or overlying cortex either 15 min pre-, 1 min post-, or 6h post-fear conditioning. One week after conditioning, retention of contextual and cue (tone) conditioning were assessed. A significant impairment in retention of contextual fear was observed when mecamylamine was injected 15 min pre- and 1 min post-conditioning. No significant impairment was observed when mecamylamine was injected 6h post-conditioning. Likewise, a significant impairment in retention of contextual fear was observed when MLA was injected 1 min post-conditioning; however, in contrast, MLA did not show any significant impairments when injected 15 min pre-conditioning, but did show a significant impairment when injected 6h post-conditioning. There were no significant impairments observed when either drug was injected into overlying cortex. No significant impairments were observed in cue conditioning for either drug. In general, specific temporal dynamics involved in nicotinic receptor function were found relative to time of receptor dysfunction. The results indicate that the greatest deficits in long-term retention (1 week) of contextual fear are produced by central infusion of MLA minutes to hours post-conditioning or mecamylamine within minutes of conditioning.

A prokaryotic proton-gated ion channel from the nicotinic acetylcholine receptor family

Ligand-gated ion channels (LGICs) mediate excitatory and inhibitory transmission in the nervous system. Among them, the pentameric or 'Cys-loop' receptors (pLGICs) compose a family that until recently was found in only eukaryotes. Yet a recent genome search identified putative homologues of these proteins in several bacterial species. Here we report the cloning, expression and functional identification of one of these putative homologues from the cyanobacterium Gloeobacter violaceus. It was expressed as a homo-oligomer in HEK 293 cells and Xenopus oocytes, generating a transmembrane cationic channel that is opened by extracellular protons and shows slow kinetics of activation, no desensitization and a single channel conductance of 8 pS. Electron microscopy and cross-linking experiments of the protein fused to the maltose-binding protein and expressed in Escherichia coli are consistent with a homo-pentameric organization. Sequence comparison shows that it possesses a compact structure, with the absence of the amino-terminal helix, the canonical disulphide bridge and the large cytoplasmic domain found in eukaryotic pLGICs. Therefore it embodies a minimal structure required for signal transduction. These data establish the prokaryotic origin of the family. Because Gloeobacter violaceus carries out photosynthesis and proton transport at the cytoplasmic membrane, this new proton-gated ion channel might contribute to adaptation to pH change.

Exploring the pharmacological properties of insect nicotinic acetylcholine receptors

Insect nicotinic acetylcholine (nACh) receptors are molecular targets of insecticides such as neonicotinoids that are used to control disease-carrying insects and agricultural pests. To date, several insect nACh receptor subunits have been identified, indicating different nACh receptor subtypes and pharmacological profiles. Because of the difficulty in expressing functional insect nACh receptors in heterologous systems, new research tools are needed. Studies on insects resistant to the insecticide imidacloprid and on laboratory-generated hybrid and chimaeric nACh receptors in vitro have provided information about the molecular basis of receptor diversity, neonicotinoid resistance and selectivity. Additionally, recent results indicate that the sensitivity of insect nACh receptors to imidacloprid can be modulated by intracellular phosphorylation mechanisms, which offers a new approach to studying insect nACh receptor pharmacology.

Differential nicotinic regulation of the nigrostriatal and mesolimbic dopaminergic pathways: implications for drug development

Neuronal nicotinic acetylcholine receptors (nAChRs) modulate dopaminergic function. Discovery of their multiplicity has lead to the search for subtype-selective nAChR agonists that might be therapeutically beneficial in diseases linked to brain dopaminergic pathways. The regulation and responses of the nigrostriatal and mesolimbic dopaminergic pathways are often similar, but some differences do exist. The cerebral distribution and characteristics of various nAChR subtypes differ between nigrostriatal and mesolimbic dopaminergic pathways. Comparison of nicotine and epibatidine, two nAChR agonists whose relative affinities for various nAChR subtypes differ, revealed differences in the nAChR-mediated regulation of dopaminergic activation between these dopamine systems. Nicotine preferentially stimulates the mesolimbic pathway, whereas epibatidine's stimulatory effect falls on the nigrostriatal pathway. Thus, it may be possible to stimulate the nigrostriatal pathway with selective nAChR agonists that do not significantly affect the mesolimbic pathway, and thus lack addictive properties. Furthermore, dopamine uptake inhibition revealed a novel inhibitory effect of epibatidine on accumbal dopamine release, which could form a basis for novel antipsychotics that could alleviate the elevated accumbal dopaminergic tone found in schizophrenia during the active psychotic state. Different regulation of nigrostriatal and mesolimbic dopaminergic pathways by nAChRs could be an important basis for developing novel drugs for treatment of Parkinson's disease and schizophrenia.

Ca2+Permeability of the (α4)3(β2)2Stoichiometry Greatly Exceeds That of (α4)2(β2)3Human Acetylcholine Receptors

Human alpha4beta2 nicotinic acetylcholine receptors (AChRs) expressed in Xenopus laevis oocytes or transfected cell lines are present as a mixture of two stoichiometries, (alpha4)2(beta2)3 and (alpha4)3(beta2)2, which differ depending on whether a beta2 or alpha4 subunit occupies the accessory subunit position corresponding to beta1 subunits of muscle AChRs. Pure populations of each stoichiometry can be expressed in oocytes by combining a linked pair of alpha4 and beta2 with free beta2 to produce the (alpha4)2(beta2)3 stoichiometry or with free alpha4 to produce the (alpha4)3(beta2)2 stoichiometry. We show that the (alpha4)3(beta2)2 stoichiometry and the (alpha4)2(beta2)2beta3 and (alpha4)2(beta2)2alpha5 subtypes in which beta3 or alpha5occupy the accessory positions have much higher permeability to Ca2+ than does (alpha4)2(beta2)3 and suggest that this could be physiologically significant in triggering signaling cascades if this stoichiometry or these subtypes were found in vivo. We show that Ca2+ permeability is determined by charged amino acids at the extracellular end of the M2 transmembrane domain, which could form a ring of amino acids at the outer end of the cation channel. Alpha4, alpha5, and beta3 subunits all have a homologous glutamate in M2 that contributes to high Ca2+ permeability, whereas beta2 has a lysine at this position. Subunit combinations or single amino acids changes at this ring that have all negative charges or a mixture of positive and negative charged amino acids are permeable to Ca2+. All positive charges in the ring prevent Ca2+ permeability. Increasing the proportion of negative charges is associated with increasing permeability to Ca2+.

Involvement of α-bungarotoxin-sensitive nicotinic receptors in long-term memory formation in the honeybee (Apis mellifera)

In the honeybee Apis mellifera, multiple-trial olfactory conditioning of the proboscis extension response specifically leads to long-term memory (LTM) which can be retrieved more than 24 h after learning. We studied the involvement of nicotinic acetylcholine receptors in the establishment of LTM by injecting the nicotinic antagonists mecamylamine (1 mM), alpha-bungarotoxin (alpha-BGT, 0.1 mM) or methyllycaconitine (MLA, 0.1 mM) into the brain through the median ocellus 20 min before or 20 min after multiple-trial learning. The retention tests were performed 1, 3, and 24 h after learning. Pre-training injections of mecamylamine induced a lower performance during conditioning but had no effect on LTM formation. Post-training injections of mecamylamine did not affect honeybees' performances. Pre-training injections of MLA or post-training injection of alpha-BGT specifically induced LTM impairment whereas acquisition as well as memory retrieval tested 1 or 3 h after learning was normal. This indicates that brain injections of alpha-BGT and MLA did not interfere with learning or medium-term memory. Rather, these blockers affect the LTM. To explain these results, we advance the hypothesis that honeybee alpha-BGT-sensitive acetylcholine receptors are also sensitive to MLA. These receptors could be essential for triggering intracellular mechanisms involved in LTM. By contrast, medium-term memory is not dependent upon these receptors but is affected by mecamylamine.

Distinct Pharmacologic Properties of Neuromuscular Blocking Agents on Human Neuronal Nicotinic Acetylcholine Receptors

Background

Nondepolarizing neuromuscular blocking agents (NMBAs) are extensively used in the practice of anesthesia and intensive care medicine. Their primary site of action is at the postsynaptic nicotinic acetylcholine receptor (nAChR) in the neuromuscular junction, but their action on neuronal nAChRs have not been fully evaluated. Furthermore, observed adverse effects of nondepolarizing NMBAs might originate from an interaction with neuronal nAChRs. The aim of this study was to examine the effect of clinically used nondepolarizing NMBAs on muscle and neuronal nAChR subtypes.

Methods

Xenopus laevis oocytes were injected with messenger RNA encoding for the subunits included in the human alpha1beta1epsilondelta, alpha3beta2, alpha3beta4, alpha4beta2, and alpha7 nAChR subtypes. The interactions between each of these nAChR subtypes and atracurium, cisatracurium, d-tubocurarine, mivacurium, pancuronium, rocuronium, and vecuronium were studied using an eight-channel two-electrode voltage clamp setup. Responses were measured as peak current and net charge.

Results

All nondepolarizing NMBAs inhibited both muscle and neuronal nAChRs. The neuronal nAChRs were reversibly and concentration-dependently inhibited in the low micromolar range. The mechanism (i.e., competitive vs. noncompetitive) of the block at the neuronal nAChRs was dependent both on subtype and the NMBA tested. The authors did not observe activation of the nAChR subtypes by any of the NMBAs tested.

Conclusions

The authors conclude that nondepolarizing NMBAs concentration-dependently inhibit human neuronal nAChRs. The inhibition of the presynaptic alpha3beta2 nAChR subtype expressed at the motor nerve ending provides a possible molecular explanation for the tetanic and train-of-four fade seen during a nondepolarizing neuromuscular block.

Brain nicotinic acetylcholine receptors: native subtypes and their relevance

Neuronal nicotinic acetylcholine receptors comprise a heterogeneous class of cationic channels that is present throughout the nervous system. These channels are involved both in physiological functions (including cognition, reward, motor activity and analgesia) and in pathological conditions such as Alzheimer's disease, Parkinson's disease, some forms of epilepsy, depression, autism and schizophrenia. They are also the targets of tobacco-smoking effects and addiction. Neuronal nicotinic acetylcholine receptors are pentamers of homomeric or heteromeric combinations of alpha (alpha2-alpha10) and beta (beta2-beta4) subunits, which have different pharmacological and biophysical properties and locations in the brain. The lack of subtype-specific ligands and the fact that many neuronal cells express multiple subtypes initially hampered the identification of the different native nicotinic acetylcholine receptor subtypes, but the increasing knowledge of subtype composition and roles will be of considerable interest for the development of new and clinically useful nicotinic acetylcholine receptor ligands.

Partial recovery of striatal nicotinic receptors in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys with chronic oral nicotine

Recent studies in nonhuman primates show that chronic nicotine treatment protects against nigrostriatal degeneration, with a partial restoration of neurochemical and functional measures in the striatum. The present studies were done to determine whether long-term nicotine treatment also protected against striatal nicotinic receptor (nAChR) losses after nigrostriatal damage. Monkeys were administered nicotine in the drinking water for 6 months and subsequently lesioned with the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) over several months while nicotine was continued. (125)I-Epibatidine, [(125)I]5-[(125)I]iodo-3(2(S)-azetidinylmethoxy)-pyridine (A85380), and (125)I-alpha-conotoxinMII autoradiography was performed to evaluate changes in alpha4beta2* and alpha3/alpha6beta2* nAChRs, the major striatal subtypes. Nicotine treatment increased alpha4beta2* nAChRs by > or =50% in striatum of both unlesioned and lesioned animals. This increase in alpha4beta2* nAChRs was significantly greater in lesioned compared with unlesioned monkey striatum. Chronic nicotine treatment led to a small decrease in alpha3/alpha6beta2* nAChR subtypes. The decline in alpha3/alpha6beta2* subtypes, defined using alpha-conotoxinMII-sensitive (125)I-epibatidine or [(125)I]A85380 binding, was significantly smaller in striatum of nicotine-treated lesioned monkeys compared with unlesioned monkeys. This difference was not observed for alpha3/alpha6beta2* nAChRs identified using (125)I-alpha-conotoxinMII. These data suggest that there are at least two striatal alpha3/alpha6beta2* subtypes that are differentially affected by chronic nicotine treatment in lesioned animals. In addition, the results showing an improvement in striatal alpha4beta2* and select alpha3/alpha6beta2* nAChR subtypes, combined with previous work, demonstrate that chronic nicotine treatment restores and/or protects against the loss of multiple molecular markers after nigrostriatal damage. Such findings suggest that nicotine or nicotinic agonists may be of therapeutic value in Parkinson's disease.

Nicotinic receptors mediate tumorigenic action of tobacco-derived nitrosamines on immortalized oral epithelial cells

Frequent users of smokeless tobacco (ST) have an increased risk for developing oral cancer. Nicotine and its derivatives may contribute to tumorigenesis through stimulation of nicotinic acetylcholine receptors (nAChRs) in target cells. Emerging evidence indicates that nAChRs can be stimulated by the nicotine-derived nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) that can induce oral cavity tumors in laboratory animals. This study was designed to elucidate the receptor-mediated mechanisms of the initiation and progression of NNK-, and NNN-induced oral cancers. We used Het-1A cells that were found to express alpha3, alpha5, alpha7, alpha9, beta2 and beta4 nAChR subunits. Both NNK and NNN competed with nicotinic radioligands for binding to Het-1A cells. NNK showed a higher than NNN affinity to the [3H]nicotine-labeled binding sites, and NNN-to the [3H]epibatidine-sensitive nAChRs. NNK and NNN increased proliferative potential of Het-1A cells and produced an anti-apoptotic effect, which was alleviated by antagonists. alpha-Bungarotoxin was most effective against NNK and mecamylamine against NNN. Treatment of Het-1A cells with either NNK or NNN led to acquisition of capability of anchorage independent growth and ability to produce tumors in nude mice, both of which can be by inhibited by antagonists. To elucidate the signaling mechanisms, we studied transcription of the genes encoding the cell cycle, apoptosis and signal transduction regulators at both the mRNA and protein levels. The Het-1A cells stimulated with nitrosamines showed multifold increases of the mRNA transcripts encoding PCNA and Bcl-2, and upregulated expression of the transcription factors GATA3, nuclear factor-kappaB, and STAT-1. The STAT-1 protein-binding activity induced by NNK and NNN correlated with elevated gene expression. The obtained results establish the role of specific nAChR subtypes in tobacco-related carcinogenesis and open a novel avenue for oral cancer chemoprevention.

Alpha7 mutants mimicking atypical motifs (YxxCC of loop-C, and E to H at -1' in TM2) in the C. elegans LEV-8 subunit affect nicotinic acetylcholine receptor function

The ACR-8-like group of C. elegans nicotinic acetylcholine receptor (nAChR) subunits contain unusual motifs in the ACh binding site and in the -1' position of transmembrane region two (TM2). Using site-directed mutagenesis (SDM) we have introduced these motifs into chicken alpha7 as it has not been possible to express C. elegans nAChR in vitro. Oocytes expressing alpha7 with the C. elegans binding motif show a reduced affinity and efficacy for both ACh and nicotine. The blocking action of the anthelmintic drug levamisole is reduced. The TM2 motif resulted in a non-functional receptor. We conclude that the TM2 motif profoundly restricts cation movement through the alpha7 channel but does not confer anion permeability. The altered form of the ACh binding motif is likely to result in a receptor with altered pharmacology, adding potential functional diversity at synapses in the nervous system and neuromuscular junctions of C. elegans.

Expression of the α7, α4 and α3 nicotinic receptor subtype in the brain and adrenal medulla of transgenic mice carrying genes coding for human AChE and β‐amyloid

Human AChE-enzyme (hAChE) enhances the over-expression of beta-amyloid (Abeta) containing plaques in the brain of transgenic mice (APP(SWE)/hAChE-Tg) carrying mutated genes for human amyloid precursor protein (APP(SWE)) and hAChE. In this study, we showed that interaction of hAChE with Abeta affects the plasticity of the alpha7 nicotinic acetylcholine receptors (nAChRs) both in the brain and adrenal medulla. An age-related increase in the (125)I-alphabungarotoxin ((125)I-alphaBTX) binding (specific to alpha7 nAChRs) was observed in the adrenal medulla of 3, 7 and 10 months old control mice. In contrast, a significant decrease in (125)I-alphaBTX binding was detected in the adrenal medulla of 10 months old APP(SWE)/hAChE-Tg. A significantly higher alpha7 nAChR mRNA level was observed in the brain of APP(SWE)/hAChE-Tg at 3 and 7 months of age and in the adrenal medulla at 3 and 10 months of age compared to those of the control mice. The alpha3 nAChR mRNA level was significantly higher in the brain of APP(SWE)/hAChE-Tg at 3 months of age and in the adrenal medulla at 10 months of age. The alpha4 nAChR mRNA level remained unchanged in the brain and adrenal medulla of APP(SWE)/hAChE-Tg for all age groups. Based on these observations, we conclude that a high load of Abeta and an over-expression of hAChE induce differences in the expression of the nAChR subtypes at various ages in the brain and in the adrenal medulla of hAChE/APP(SWE)Tg mice. The findings may have implications for a better understanding the underlying mechanism for AD-related pathogenesis.

Identification of molluscan nicotinic acetylcholine receptor (nAChR) subunits involved in formation of cation- and anion-selective nAChRs

Acetylcholine (ACh) is a neurotransmitter commonly found in all animal species. It was shown to mediate fast excitatory and inhibitory neurotransmission in the molluscan CNS. Since early intracellular recordings, it was shown that the receptors mediating these currents belong to the family of neuronal nicotinic acetylcholine receptors and that they can be distinguished on the basis of their pharmacology. We previously identified 12 Lymnaea cDNAs that were predicted to encode ion channel subunits of the family of the neuronal nicotinic acetylcholine receptors. These Lymnaea nAChRs can be subdivided in groups according to the residues supposedly contributing to the selectivity of ion conductance. Functional analysis in Xenopus oocytes revealed that two types of subunits with predicted distinct ion selectivities form homopentameric nicotinic ACh receptor (nAChR) subtypes conducting either cations or anions. Phylogenetic analysis of the nAChR gene sequences suggests that molluscan anionic nAChRs probably evolved from cationic ancestors through amino acid substitutions in the ion channel pore, a mechanism different from acetylcholine-gated channels in other invertebrates.

Succinylcholine-induced hyperkalemia in acquired pathologic states: etiologic factors and molecular mechanisms

Lethal hyperkalemic response to succinylcholine continues to be reported, but the molecular mechanisms for the hyperkalemia have not been completely elucidated. In the normal innervated mature muscle, the acetylcholine receptors (AChRs) are located only in the junctional area. In certain pathologic states, including upper or lower motor denervation, chemical denervation by muscle relaxants, drugs, or toxins, immobilization, infection, direct muscle trauma, muscle tumor, or muscle inflammation, and/or burn injury, there is up-regulation (increase) of AChRs spreading throughout the muscle membrane, with the additional expression of two new isoforms of AChRs. The depolarization of these AChRs that are spread throughout the muscle membrane by succinylcholine and its metabolites leads to potassium efflux from the muscle, leading to hyperkalemia. The nicotinic (neuronal) alpha7 acetylcholine receptors, recently described to be expressed in muscle also, can be depolarized not only by acetylcholine and succinylcholine but also by choline, persistently, and possibly play a critical role in the hyperkalemic response to succinylcholine in patients with up-regulated AChRs.

Brain Neuronal Nicotinic Receptors as New Targets for Drug Discovery

Neuronal nicotinic receptors (nAChRs) are a heterogeneous family of ion channels differently expressed in the nervous system where, by responding to the endogenous neurotransmitter acetylcholine, they contribute to a wide range of brain activities and influence a number of physiological functions. Over recent years, the application of newly developed molecular and cellular biological techniques has made it possible to correlate the subunit composition of nAChRs with specific nicotine-elicited behaviours, and refine some of the in vivo physiological functions of nAChR subtypes. The major new findings are the widespread expression of nAChRs, outside the nervous system, their specific and complex organisation, and their relevance to normal brain function. Moreover, the combination of clinical and basic research has better defined the involvement of nAChRs in a growing number of nervous pathologies other than degenerative diseases. However, there are still only a limited number of nicotinic-specific drugs and, although some nicotinic agonists have an interesting pharmacology, their clinical use is limited by undesirable side effects. Some selective nicotinic ligands have recently been developed and used to explore the complexity of nAChR subtype structure and function in the expectation that they will become rational therapeutic alternatives in a number of neurodegenerative, neuropsychiatric and neurological disorders. In this review, we will discuss the molecular basis of brain nAChR structural and functional diversity mainly in pharmacological and biochemical terms, and summarise current knowledge concerning the newly discovered drugs used to classify the numerous receptor subtypes and treat the brain diseases in which nAChRs are involved.

Nicotinic Acetylcholine Receptor Subtypes Involved in Facilitation of GABAergic Inhibition in Mouse Superficial Superior Colliculus

The superficial superior colliculus (sSC) is a key station in the sensory processing related to visual salience. The sSC receives cholinergic projections from the parabigeminal nucleus, and previous studies have revealed the presence of several different nicotinic acetylcholine receptor (nAChR) subunits in the sSC. In this study, to clarify the role of the cholinergic inputs to the sSC, we examined current responses induced by ACh in GABAergic and non-GABAergic sSC neurons using in vitro slice preparations obtained from glutamate decarboxylase 67-green fluorescent protein (GFP) knock-in mice in which GFP is specifically expressed in GABAergic neurons. Brief air pressure application of acetylcholine (ACh) elicited nicotinic inward current responses in both GABAergic and non-GABAergic neurons. The inward current responses in the GABAergic neurons were highly sensitive to a selective antagonist for α3β2- and α6β2-containing receptors, α-conotoxin MII (αCtxMII). A subset of these neurons exhibited a faster α-bungarotoxin-sensitive inward current component, indicating the expression of α7-containing nAChRs. We also found that the activation of presynaptic nAChRs induced release of GABA, which elicited a burst of miniature inhibitory postsynaptic currents mediated by GABAA receptors in non-GABAergic neurons. This ACh-induced GABA release was mediated mainly by αCtxMII-sensitive nAChRs and resulted from the activation of voltage-dependent calcium channels. Morphological analysis revealed that recorded GFP-positive neurons are interneurons and GFP-negative neurons include projection neurons. These findings suggest that nAChRs are involved in the regulation of GABAergic inhibition and modulate visual processing in the sSC.

Nicotinic cholinergic receptors in the rat cerebellum: multiple heteromeric subtypes

Nicotinic receptors (nAChRs) in the cerebellum have been implicated in the pathology of autism spectrum disorders (Lee et al., 2002; Martin-Ruiz et al., 2004). The subtypes of nAChRs in the cerebellum are not known in any detail, except that, in addition to the homomeric alpha7 subtype, there appears to be one or more heteromeric subtypes consisting of combinations of alpha and beta subunits. To begin to better understand the potential roles of these heteromeric nAChRs in cerebellar circuitry and their potential as targets for nicotinic drugs, we investigated their subunit composition. Using subunit-selective antibodies in sequential immunoprecipitation assays, we detected six structurally distinct heteromeric nAChR populations in the rat cerebellum. Among these were several subtypes that have not been encountered previously, including alpha3alpha4beta2 and alpha3alpha4beta4 nAChRs. This diversity suggests that nAChRs play multiple roles in cerebellar physiology.

Identification and functional expression of a family of nicotinic acetylcholine receptor subunits in the central nervous system of the mollusc Lymnaea stagnalis

We described a family of nicotinic acetylcholine receptor (nAChR) subunits underlying cholinergic transmission in the central nervous system (CNS) of the mollusc Lymnaea stagnalis. By using degenerate PCR cloning, we identified 12 subunits that display a high sequence similarity to nAChR subunits, of which 10 are of the alpha-type, 1 is of the beta-type, and 1 was not classified because of insufficient sequence information. Heterologous expression of identified subunits confirms their capacity to form functional receptors responding to acetylcholine. The alpha-type subunits can be divided into groups that appear to underlie cation-conducting (excitatory) and anion-conducting (inhibitory) channels involved in synaptic cholinergic transmission. The expression of the Lymnaea nAChR subunits, assessed by real time quantitative PCR and in situ hybridization, indicates that it is localized to neurons and widespread in the CNS, with the number and localization of expressing neurons differing considerably between subunit types. At least 10% of the CNS neurons showed detectable nAChR subunit expression. In addition, cholinergic neurons, as indicated by the expression of the vesicular ACh transporter, comprise approximately 10% of the neurons in all ganglia. Together, our data suggested a prominent role for fast cholinergic transmission in the Lymnaea CNS by using a number of neuronal nAChR subtypes comparable with vertebrate species but with a functional complexity that may be much higher.

Synaptically Released and Exogenous ACh Activates Different Nicotinic Receptors to Enhance Evoked Glutamatergic Transmission in the Lateral Geniculate Nucleus

The effects of activation of nicotinic acetylcholine receptors (nAChRs) on glutamatergic transmission in the ventral lateral geniculate nucleus (LGNv) were examined in chick brain slices. Whole cell recordings showed that monosynaptic postsynaptic currents (PSCs) evoked in LGNv neurons by optic tract stimulation were blocked by glutamate receptor antagonists. Exogenously applied nicotine (0.5 μM), choline (1 mM), or acetylcholine (ACh, 100 μM) markedly increased (>3-fold) these evoked PSCs. Potentiation by ACh was dose-dependent and did not desensitize during a 5-min application. In a second set of experiments, the effect of releasing endogenous ACh by stimulating the lateral portion of the LGNv through a separate conditioning electrode before optic tract stimulation was examined. Conditioning stimulation trains increased PSCs by an average of 5.2-fold, an effect dependent on both the intensity and number of conditioning pulses. This increase in PSC amplitude was most likely caused by released ACh activating α6- and/or α3-containing nAChRs because it was blocked by 100 nM α-conotoxin MII, 100 nM dihydro-β-erythroidine (DHβE), and 0.1–1.0 μM methyllycaconitine (MLA). In contrast, exogenously applied ACh increased PSC amplitude by activating a pharmacologically different population of nAChRs because this effect was inhibited by 100 nM α-bungarotoxin, 50 nM MLA, and a high concentration (30 μM) of DHβE, indicating that α7- and/or α8-containing receptors were involved. The results are consistent with a model whereby α6- and/or α3-containing nAChRs on retinal ganglion cell nerve terminals are located preferentially at cholinergic synapses, whereas α7- and/or α8-containing receptors are primarily extrasynaptic.

alpha-Conotoxins as selective probes for nicotinic acetylcholine receptor subclasses

alpha-Conotoxins are selective antagonists of neuromuscular or neuronal nicotinic acetylcholine receptors. Individual family members are often highly selective towards distinct receptor subclasses, most notably within neuronal nicotinic acetylcholine receptors. As such they are being used as tools to probe for the type and diversity of receptor subclasses in distinct parts of the central and peripheral nervous systems. Many new alpha-conotoxins are being identified every year, broadening the available armoury because small variations in their sequences and structures often confer altered selectivity towards receptor subunits and subclasses. Many neurological diseases are being associated wholly or in part with functional changes within specific subclasses of nicotinic acetylcholine receptors. Significantly, with more structures of alpha-conotoxins also becoming available this enables ready comparison of their similarities and, more notably, of their subtle differences, which dictate subclass selectivity. As such, alpha-conotoxins offer the potential to become templates for the creation, through rational drug design strategies, of pharmaceuticals highly selective for specific subclasses of nicotinic acetylcholine receptors.

RIC-3 Enhances Functional Expression of Multiple Nicotinic Acetylcholine Receptor Subtypes in Mammalian Cells

Recent studies have shown that RIC-3, originally identified in Caenorhabditis elegans as the protein encoded by the gene resistance to inhibitors of cholinesterase (ric-3), can enhance functional expression of alpha7 nicotinic acetylcholine receptors (nAChRs). In the present study, the influence of C. elegans and human RIC-3 upon multiple homomeric (alpha7, alpha8, and alpha9) and heteromeric (alpha3beta2, alpha3beta4, alpha4beta2, alpha4beta4, and alpha9alpha10) nAChR subtypes has been examined in transfected mammalian cells by radioligand binding and functional characterization. Coexpression of RIC-3 facilitates a dramatic enhancement of the ability of alpha7 (and the closely related alpha8 subunit) to generate functional nAChRs in otherwise nonpermissive mammalian cells. In contrast, coexpression of RIC-3 did not facilitate functional expression of either homomeric alpha9 or heteromeric alpha9alpha10 nAChRs in mammalian cell lines. It is noteworthy that whereas RIC-3 has been reported to cause a marked functional inhibition of heteromeric nAChRs such as alpha3beta4 and alpha4beta2 expressed in Xenopus laevis oocytes, RIC-3 significantly enhances levels of functional expression of these and other (alpha3beta2 and alpha4beta4) heteromeric nAChRs when expressed in mammalian cell lines. In addition, the interaction of multiple nAChR subunits (alpha3, alpha4, alpha7, beta2, and beta4) with RIC-3 has been demonstrated by coimmunoprecipitation from metabolically labeled transfected cells. It is significant that coimmunoprecipitation experiments have provided evidence that RIC-3 associates with unassembled nAChR subunits, a finding that is consistent with previous suggestions that RIC-3 may act by enhancing the maturation (subunit folding and assembly) of nAChRs. We conclude that RIC-3 is an nAChR-associated protein that can enhance functional expression of multiple nAChR subtypes in transfected mammalian cells.

A uniquely selective inhibitor of the mammalian fetal neuromuscular nicotinic acetylcholine receptor

We have purified and characterized a novel conotoxin from the venom of Conus obscurus, which has the unique property of selectively and potently inhibiting the fetal form of the mammalian neuromuscular nicotinic acetylcholine receptor (nAChR) (alpha1beta1gammadelta-subunits). Although this conotoxin, alphaA-conotoxin OIVB (alphaA-OIVB), is a high-affinity antagonist (IC50 of 56 nm) of the fetal muscle nAChR, it has >1800-fold lower affinity for the adult muscle nAChR (alpha1beta1epsilondelta-subunits) and virtually no inhibitory activity at a high concentration on various neuronal nAChRs (IC50 > 100 microm in all cases). The peptide (amino acid sequence, CCGVONAACPOCVCNKTCG), with three disulfide bonds, has been chemically synthesized in a biologically active form. Although the neuromuscular nAChRs are perhaps the most extensively characterized of the receptors/ion channels of the nervous system, the precise physiological roles of the fetal form of the muscle nAChR are essentially unknown.alphaA-OIVB is a potentially important tool for delineating the functional roles ofalpha1beta1gammadelta receptors in normal development, as well as in various adult tissues and in pathological states. In addition to its potential as a research tool, alphaA-OIVB may have some direct biomedical applications.

Heterogeneity and Selective Targeting of Neuronal Nicotinic Acetylcholine Receptor (nAChR) Subtypes Expressed on Retinal Afferents of the Superior Colliculus and Lateral Geniculate Nucleus: Identification of a New Native nAChR Subtype α3β2(α5 or β3) Enriched in Retinocollicular Afferents

The activation of neuronal nicotinic acetylcholine receptors (nAChRs) has been implicated in the activity-dependent development and plasticity of retina and the refinement of retinal projections. Pharmacological and functional studies have also indicated that different presynaptic nAChRs can have a modulatory function in retinotectal synapses. We biochemically and pharmacologically identified the multiple nAChR subtypes expressed on retinal afferents of the superior colliculus (SC) and lateral geniculate nucleus (LGN). We found that the alpha6beta2(*) and alpha4(nonalpha6)beta2(*) nAChRs are the major receptor populations expressed in both SC and LGN. In addition, the LGN contains two minor populations of alpha2alpha6beta2(*) and alpha3beta2(*) subtypes, whereas the SC contains a relatively large population of a new native subtype, the alpha3beta2(alpha5/beta3) nAChR. This subtype binds the alpha-conotoxin MII with an affinity 50 times lower than that of the native alpha6beta2(*) subtype. Studies of tissues obtained from eye-enucleated animals allowed the identification of nAChRs expressed by retinal afferents: in SC alpha6beta2(*), alpha4alpha6beta2(*), and alpha3beta2(*) (approximately 45, 35, and 20%, respectively), in LGN, alpha4alpha6beta2(*), alpha6beta2(*), alpha4beta2(*), alpha2alpha6beta2(*), and alpha3beta2(*) (approximately 40, 30, 20, 5, and 5%, respectively). In both regions, more than 50% of nAChRs were not expressed by retinal afferents and belonged to the alpha4beta2(*) (90%) or alpha4alpha5beta2(*) (10%) subtypes. Moreover, studies of the SC tissues obtained from wild-type and alpha4, alpha6, and beta3 knockout mice confirmed and extended the data obtained in rat tissue and allowed a comprehensive dissection of the composition of nAChR subtypes present in this retinorecipient area.

A conus peptide blocks nicotinic receptors of unmyelinated axons in human nerves

The novel alpha-conotoxin Vc1.1 is a potential analgesic for the treatment of painful neuropathic conditions. In the present study, the effects of Vc1.1 were tested on the nicotine-induced increase in excitability of unmyelinated C-fiber axons in isolated segments of peripheral human nerves. Vc1.1 in concentrations above 0.1 microM antagonized the increase in axonal excitability produced by nicotine; the maximal inhibition was observed with 10 microM. We also demonstrate immunoreactivity for alpha 3 and alpha 5 subunits of neuronal nicotinic receptors on unmyelinated peripheral human axons. Blockade of nicotinic receptors on unmyelinated peripheral nerve fibers may be helpful in painful neuropathies affecting unmyelinated sympathetic and/or sensory axons.

Alpha-7 and alpha-4 nicotinic receptor subunit immunoreactivity in genioglossus muscle motoneurons

In the present study, immunohistochemistry combined with retrograde labeling techniques were used to determine if hypoglossal motoneurons (HMNs), retrogradely labeled after cholera toxin B subunit (CTB) injection to the genioglossus muscle in rats, show immunoreactivity for alpha-7 and alpha-4 subunits of nicotinic acetylcholine receptors (nAChRs). CTB-positive HMNs projecting to the genioglossus muscle were consistently labeled throughout the rostrocaudal extent of the hypoglossal nuclei with the greatest labeling at and caudal to area postrema. Alpha-7 subunit immunoreactivity was found in 39.44+/-5.10% of 870 CTB-labeled motoneurons and the alpha-4 subunit in 51.01+/-3.71% of 983 CTB-positive neurons. Rostrally, the number of genioglossal motoneurons demonstrating immunoreactivity for the alpha-7 subunit was 45.85+/-10.04% compared to 34.96+/-5.11% at and caudal to area postrema (P>0.1). The number of genioglossal motoneurons that showed immunoreactivity for the alpha-4 subunit was 55.03+/-4.83% at and caudal to area postrema compared to 42.98+/-3.90% in rostral areas (P=0.074). These results demonstrate that nAChR immunoreactivity is present in genioglossal motoneurons and suggest a role for alpha-7 and alpha-4 subunits containing nAChRs in the regulation of upper airway patency.

Lower core body temperature and attenuated nicotine-induced hypothermic response in mice lacking the beta4 neuronal nicotinic acetylcholine receptor subunit

Diverse physiological and pathological effects of nicotine, including the alteration of body temperature, are presumably mediated by neuronal nicotinic acetylcholine receptors (nAChR). Previous studies have suggested the involvement of distinct nAChR subunits in nicotine-induced thermoregulation. We studied genetically manipulated knockout mice lacking the alpha7, alpha5 or beta4 subunit genes, in order to assess the effects of subunit deficiency on temperature regulation. Using a telemetry system, core body temperature was monitored continuously prior to and following nicotine administration in mutant mice and in wild-type littermates. Mice lacking in the beta4 nAChR subunit gene had significantly lower baseline core body temperature than all other mouse strains studied. beta4 null mice also demonstrated a reduced nicotine-induced hypothermic response and impaired desensitization following repeat nicotine exposure. These findings suggest the involvement of the beta4 nAChR subunit in both core body temperature homeostasis and nicotine-elicited thermo-alterations in mice.

Neuronal nicotinic receptors: from structure to pathology

Neuronal nicotinic receptors (NAChRs) form a heterogeneous family of ion channels that are differently expressed in many regions of the central nervous system (CNS) and peripheral nervous system. These different receptor subtypes, which have characteristic pharmacological and biophysical properties, have a pentameric structure consisting of the homomeric or heteromeric combination of 12 different subunits (alpha2-alpha10, beta2-beta4). By responding to the endogenous neurotransmitter acetylcholine, NAChRs contribute to a wide range of brain activities and influence a number of physiological functions. Furthermore, it is becoming evident that the perturbation of cholinergic nicotinic neurotransmission can lead to various diseases involving nAChR dysfunction during development, adulthood and ageing. In recent years, it has been discovered that NAChRs are present in a number of non-neuronal cells where they play a significant functional role and are the pathogenetic targets in several diseases. NAChRs are also the target of natural ligands and toxins including nicotine (Nic), the most widespread drug of abuse. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining their regional and cellular localisation and the molecular basis of their functional diversity mainly in pharmacological and biochemical terms. The recent availability of mice with the genetic ablation of single or double nicotinic subunits or point mutations have shed light on the role of nAChRs in major physiological functions, and we will here discuss recent data relating to their behavioural phenotypes. Finally, the role of NAChRs in disease will be considered in some details.

Edit, cut and paste in the nicotinic acetylcholine receptor gene family ofDrosophila melanogaster

Nicotinic acetylcholine receptors (nAChRs) are important for fast synaptic cholinergic transmission. They are targets of drugs/chemicals for human and animal health as well as for pest control. With the advent of genome sequencing, entire nAChR gene families have now been described for vertebrates and invertebrates. Mostly, these are extensive with a large number of distinct subunits, making possible many nAChR subtypes differing in transmitter affinity, channel conductance, ion selectivity, desensitization, modulation and pharmacology. The smallest nAChR gene family to date is that of the fruit fly, Drosophila melanogaster, with only 10 members. This apparently compact family belies its true diversity as 4 of the 10 subunits show alternative splicing. Also, using Drosophila, A-to-I pre-mRNA editing has been demonstrated for the first time in nAChRs. Such is the extent of this variation, that one subunit alone (Dalpha6) can potentially generate far more isoforms than seen in entire gene families from other species. We present here three-dimensional models constructed for insect nAChRs, which show that many variations introduced by alternative splicing and RNA editing may influence receptor function.

Expression and function of two nicotinic subunits in insect neurons

Nicotinic acetylcholine receptors (nAChRs) in insects are neuron-specific oligomeric proteins essential for the central transmission of sensory information. Little is known about their subunit composition because it is difficult to express functional insect nAChRs in heterologous systems. As an alternative approach we have examined the native expression of two subunits in neurons of the nicotinic-resistant, tobacco-feeding insect Manduca sexta. Both the alpha-subunit MARA1 and the beta-subunit MARB can be detected by in situ hybridization in the majority of cultured neurons with an overlapping, but not identical, distribution. Changes in intracellular Ca(2+) evoked by nicotinic stimulation are more strongly correlated to the expression of MARA1 than MARB and are independent of cell size. Unlike the previously reported critical role of MARA1 in mediating nicotinic Ca(2+) responses, down-regulation of MARB by RNA interference (RNAi) did not reduce the number of responding neurons or the size of evoked responses, suggesting that additional subunits remain to be identified in Manduca.

Different nicotinic acetylcholine receptor subtypes mediating striatal and prefrontal cortical [3H]dopamine release

Different nicotinic acetylcholine receptor subtypes appear to modulate dopamine release from the striatum and prefrontal cortex. In this study a combination of subtype-selective antagonists and agonists were used to extensively characterize the nAChRs involved in dopamine release from slice preparations of these two brain regions. alpha-conotoxin-MII inhibited nicotine-evoked [3H]dopamine (DA) release from striatum by 45%, but did not affect cortical dopamine release. Neither methyllycaconitine, alpha-bungarotoxin, nor alpha-conotoxin-ImI affected nicotine-evoked [3H]DA release from either striatum or prefrontal cortex. MG 624, a novel selective nAChR antagonist, inhibited cortical [3H]DA by 53%, but had no effect on striatal release. Compared to nicotine, (+/-)-UB-165 showed less efficacy with respect to dopamine release from striatum, and had no effect on cortical dopamine release. (+/-)-UB-165-evoked striatal dopamine release was completely blocked by mecamylamine, partially blocked (up to 55%) by alpha-conotoxin-MII, and unaffected by methyllycaconitine or alpha-conotoxin-ImI. alpha4beta2* and alpha6beta2beta3* nAChRs appear to play a role in striatal dopamine release, whereas alpha4beta2* nAChRs modulate release from prefrontal cortex. alpha7* nAChRs do not appear to play a role in nAChR-mediated dopamine release from either brain region.

The Molecular Pages of the mesotelencephalic dopamine consortium (DopaNet)

Background

DopaNet is a Systems Biology initiative that aims to investigate precisely and quantitatively all the aspects of neurotransmission in a specific neuronal system, the mesotelencephalic dopamine system. The project should lead to large-scale models of molecular and cellular processes involved in neuronal signaling. A prerequisite is the proper storage of knowledge coming from the literature.

Methods

DopaNet Molecular Pages are highly structured descriptions of quantitative parameters related to a specific molecular complex involved in neuronal signal processing. A Molecular Page is built by maintainers who are experts in the field, and responsible for the quality of the page content. Each piece of data is identified by a specific ontology code, annotated (method of acquisition, species, etc.) and linked to the relevant bibliography. The Molecular Pages are stored as XML files, and processed through the DopaNet Web Service, which provides functionalities to edit the Molecular Pages, to cross-link the Pages and generate the public display, and to search them.

Conclusions

DopaNet Molecular Pages are one of the core resources of the DopaNet project but should be of widespread utility in the field of Systems Neurobiology.

Identified spinal motoneurons of young rats possess nicotinic acetylcholine receptors of the heteromeric family

The aim of the present study was to determine whether, in young rats, spinal motoneurons possess functional nicotinic acetylcholine receptors. Motoneurons were identified either by retrograde labelling or by choline acetyltransferase immunohistochemistry. Whole-cell recordings were performed in spinal cord slices cut at the lumbar level. In voltage clamp, acetylcholine evoked a rapidly activating inward current. In current clamp, it depolarized the motoneuron membrane and induced action potential firing. The acetylcholine-evoked current was strongly reduced by d-tubocurarine or dihydro-beta-erythroidine, broad spectrum nicotinic antagonists, but was almost insensitive to methyllycaconitine, a nicotinic antagonist selective for receptors containing the alpha7 subunit. Moreover, exo-2-(2-pyridyl)-7-azabicyclo[2.2.1]heptane, an alpha7-specific agonist, was without effect. In young animals, light-microscopic autoradiography showed that in the central grey matter all laminae were intensely and equally labelled by [3H]epibatidine. A dense [125I]-alpha-bungarotoxin binding was also found in all laminae, with slightly lower levels in the superficial layers of the dorsal horns and in the ventral part of the grey matter. In adults, the density of [3H]epibatidine binding sites was much lower in the entire grey matter, except in layer 2 of the dorsal horn, and [125I]-alpha-bungarotoxin binding sites were present only in some selected areas. Our data indicate that spinal motoneurons possess functional nicotinic receptors of the heteromeric type and suggest that nicotinic cholinergic transmission may play a significant role in the developing spinal cord.

Nicotinic AChR in subclassified capsaicin-sensitive and -insensitive nociceptors of the rat DRG

Nociceptive cells of the dorsal root ganglion (DRG) were subclassified, in vitro, according to patterns of voltage-activated currents. The distribution and form of nicotinic ACh receptors (nAChRs) were determined. nAChRs were present on both capsaicin-sensitive and -insensitive nociceptors but were not universally present in unmyelinated nociceptors. In contrast, all A delta nociceptors (types 4, 6, and 9) expressed slowly decaying nAChR. Three major forms of nicotinic currents were identified. Specific agonists and antagonists were used to demonstrate the presence of alpha7 in two classes of capsaicin-sensitive, unmyelinated nociceptors (types 2 and 8). In type 2 cells, alpha7-mediated currents were found in isolation. Whereas alpha7 was co-expressed with other nAChR in type 8 cells. These were the only classes in which alpha7 was identified. Other nociceptive classes expressed slowly decaying currents with beta4 pharmacology. Based on concentration response curves formed by nicotinic agonists [ACh, nicotine, dimethyl phenyl piperazinium (DMPP), cytisine] evidence emerged of two distinct nAChR differentially expressed in type 4 (alpha3beta4) and types 5 and 8 (alpha3beta4 alpha5). Although identification could not be made with absolute certainty, patterns of potency (type 4: DMPP > cytisine > nicotine = ACh; type 5 and type 8: DMPP = cytisine > nicotine = ACh) and efficacy provided strong support for the presence of two distinct channels based on an alpha3beta4 platform. Studies conducted on one nonnociceptive class (type 3) failed to reveal any nAChR. After multiple injections of Di-I (1,1'-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) into the hairy skin of the hindlimb, we identified cell types 2, 4, 6, 8, and 9 as skin nociceptors that expressed nicotinic receptors. We conclude that at least three nicotinic AChR are diversely distributed into discrete subclasses of nociceptors that innervate hairy skin.

Neuronal Nicotinic Receptor Subtypes: DEFINING THERAPEUTIC TARGETS

Humans have appreciated the beneficial properties of the tobacco plant for thousands of years. These effects include alertness, reduced anxiety, muscle relaxation, and analgesia. Yet it has been less than two decades since the central actions of nicotine have been examined in earnest for potential therapeutic applications. In fact, the cholinergic systems, in comparison to other neurotransmitter systems of the body, have been relatively poorly exploited in terms of therapeutic agents, and the muscarinic cholinergic systems have been relegated mainly to the treatment of gastrointestinal disorders and glaucoma; for the nicotinic system, antagonists are used to induce muscle paralysis during certain surgical procedures. For both families of cholinergic receptors, widespread exploitation in terms of therapeutics has been limited by significant side effect profiles associated with available cholinergic drugs.

Nicotinic receptor alpha subunits in magnocellular neurons of rat hypothalamus

Mab35 is a monoclonal antibody against one specific immunogenic region in alpha1, alpha3, alpha5 subunits of nicotinic acetylcholine receptors (N-AChR) of a variety of species. It has previously been claimed that N-AChR-like immunoreactivity (-LI) identified by mab35 is present in vasopressin-containing magnocellular neurons. However, we show here by double immunofluorescence labelling that mab35 immunoreactivity is predominantly localized to oxytocinergic rather than vasopressinergic magnocellular neurons. We further infer that mab35 predominantly stained the alpha3 and/or alpha5 subunits in rat oxytocinergic neurons, and suggest that the unbalanced distribution of these subunits may contribute to some specific physiological properties of oxytocinergic neurons.