Pharmacology and biophysical properties of alpha 7 and alpha 7-alpha 8 alpha-bungarotoxin receptor subtypes immunopurified from the chick optic lobe

Is the post-COVID-19 syndrome a severe impairment of acetylcholine-orchestrated neuromodulation that responds to nicotine administration?

Following a SARS-CoV-2 infection, many individuals suffer from post-COVID-19 syndrome. It makes them unable to proceed with common everyday activities due to weakness, memory lapses, pain, dyspnea and other unspecific physical complaints. Several investigators could demonstrate that the SARS-CoV-2 related spike glycoprotein (SGP) attaches not only to ACE-2 receptors but also shows DNA sections highly affine to nicotinic acetylcholine receptors (nAChRs). The nAChR is the principal structure of cholinergic neuromodulation and is responsible for coordinated neuronal network interaction. Non-intrinsic viral nAChR attachment compromises integrative interneuronal communication substantially. This explains the cognitive, neuromuscular and mood impairment, as well as the vegetative symptoms, characterizing post-COVID-19 syndrome. The agonist ligand nicotine shows an up to 30-fold higher affinity to nACHRs than acetylcholine (ACh). We therefore hypothesize that this molecule could displace the virus from nAChR attachment and pave the way for unimpaired cholinergic signal transmission. Treating several individuals suffering from post-COVID-19 syndrome with a nicotine patch application, we witnessed improvements ranging from immediate and substantial to complete remission in a matter of days.

Homomeric and Heteromeric α7 Nicotinic Acetylcholine Receptors in Health and Some Central Nervous System Diseases

Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels involved in the modulation of essential brain functions such as memory, learning, and attention. Homomeric α7 nAChR, formed exclusively by five identical α7 subunits, is involved in rapid synaptic transmission, whereas the heteromeric oligomers composed of α7 in combination with β subunits display metabotropic properties and operate in slower time frames. At the cellular level, the activation of nAChRs allows the entry of Na⁺ and Ca²⁺; the two cations depolarize the membrane and trigger diverse cellular signals, depending on the type of nAChR pentamer and neurons involved, the location of the intervening cells, and the networks of which these neuronal cells form part. These features make the α7 nAChR a central player in neurotransmission, metabolically associated Ca²⁺-mediated signaling, and modulation of diverse fundamental processes operated by other neurotransmitters in the brain. Due to its ubiquitous distribution and the multiple functions it displays in the brain, the α7 nAChR is associated with a variety of neurological and neuropsychiatric disorders whose exact etiopathogenic mechanisms are still elusive.

Therapeutic Targeting of α7 Nicotinic Acetylcholine Receptors

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

Conotoxin Interactions with α9α10-nAChRs: Is the α9α10-Nicotinic Acetylcholine Receptor an Important Therapeutic Target for Pain Management?

The α9α10-nicotinic acetylcholine receptor (nAChR) has been implicated in pain and has been proposed to be a novel target for analgesics. However, the evidence to support the involvement of the α9α10-nAChR in pain is conflicted. This receptor was first implicated in pain with the characterisation of conotoxin Vc1.1, which is highly selective for α9α10-nAChRs and is an efficacious analgesic in chronic pain models with restorative capacities and no reported side effects. Numerous other analgesic conotoxin and non-conotoxin molecules have been subsequently characterised that also inhibit α9α10-nAChRs. However, there is evidence that α9α10-nAChR inhibition is neither necessary nor sufficient for analgesia. α9α10-nAChR-inhibiting analogues of Vc1.1 have no analgesic effects. Genetically-modified α9-nAChR knockout mice have a phenotype that is markedly different from the analgesic profile of Vc1.1 and similar conotoxins, suggesting that the conotoxin effects are largely independent of α9α10-nAChRs. Furthermore, an alternative mechanism of analgesia by Vc1.1 and other similar conotoxins involving non-canonical coupling of GABAB receptors to voltage-gated calcium channels is known. Additional incongruities regarding α9α10-nAChRs in analgesia are discussed. A more comprehensive characterisation of the role of α9α10-nAChRs in pain is crucial for understanding the analgesic action of conotoxins and for improved drug design.

Structure of neuronal nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are a critical component of the brain's cholinergic neurotransmission system that modulates important physiological processes and whose dysfunction has been observed in patients with neurodegenerative diseases and mental illness. nAChRs are a heterogeneous family of receptor subtypes consisting of pentameric combinations of α and β subunits, and are widely expressed throughout the central and peripheral nervous system. nAChR subtypesnAChR subtypes share a common basic structure but their biophysical and pharmacological properties depend on their subunit compositionSubunit composition , which is therefore central to understanding receptor function in the nervous system and discovering new subtype-selective drugs. We briefly review some recent findings concerning the structure and function of nAChRs, particularly the native subtypes.

The novel α7β2-nicotinic acetylcholine receptor subtype is expressed in mouse and human basal forebrain: biochemical and pharmacological characterization

We examined α7β2-nicotinic acetylcholine receptor (α7β2-nAChR) expression in mammalian brain and compared pharmacological profiles of homomeric α7-nAChRs and α7β2-nAChRs. α-Bungarotoxin affinity purification or immunoprecipitation with anti-α7 subunit antibodies (Abs) was used to isolate nAChRs containing α7 subunits from mouse or human brain samples. α7β2-nAChRs were detected in forebrain, but not other tested regions, from both species, based on Western blot analysis of isolates using β2 subunit-specific Abs. Ab specificity was confirmed in control studies using subunit-null mutant mice or cell lines heterologously expressing specific human nAChR subtypes and subunits. Functional expression in Xenopus oocytes of concatenated pentameric (α7)5-, (α7)4(β2)1-, and (α7)3(β2)2-nAChRs was confirmed using two-electrode voltage clamp recording of responses to nicotinic ligands. Importantly, pharmacological profiles were indistinguishable for concatenated (α7)5-nAChRs or for homomeric α7-nAChRs constituted from unlinked α7 subunits. Pharmacological profiles were similar for (α7)5-, (α7)4(β2)1-, and (α7)3(β2)2-nAChRs except for diminished efficacy of nicotine (normalized to acetylcholine efficacy) at α7β2- versus α7-nAChRs. This study represents the first direct confirmation of α7β2-nAChR expression in human and mouse forebrain, supporting previous mouse studies that suggested relevance of α7β2-nAChRs in Alzheimer disease etiopathogenesis. These data also indicate that α7β2-nAChR subunit isoforms with different α7/β2 subunit ratios have similar pharmacological profiles to each other and to α7 homopentameric nAChRs. This supports the hypothesis that α7β2-nAChR agonist activation predominantly or entirely reflects binding to α7/α7 subunit interface sites.

Functional expression of human α9* nicotinic acetylcholine receptors in X. laevis oocytes is dependent on the α9 subunit 5' UTR

Nicotinic acetylcholine receptors (nAChRs) containing the α9 subunit are expressed in a wide variety of non-neuronal tissues ranging from immune cells to breast carcinomas. The α9 subunit is able to assemble into a functional homomeric nAChR and also co-assemble with the α10 subunit into functional heteromeric nAChRs. Despite the increasing awareness of the important roles of this subunit in vertebrates, the study of human α9-containing nAChRs has been severely limited by difficulties in its expression in heterologous systems. In Xenopus laevis oocytes, functional expression of human α9α10 nAChRs is very low compared to that of rat α9α10 nAChRs. When oocytes were co-injected with cRNA of α9 and α10 subunits of human versus those of rat, oocytes with the rat α9 human α10 combination had an ∼-fold higher level of acetylcholine-gated currents (I_ACh) than those with the human α9 rat α10 combination, suggesting difficulties with human α9 expression. When the ratio of injected human α9 cRNA to human α10 cRNA was increased from 1∶1 to 5∶1, I_ACh increased 36-fold (from 142±23 nA to 5171±748 nA). Functional expression of human α9-containing receptors in oocytes was markedly improved by appending the 5′-untranslated region of alfalfa mosaic virus RNA4 to the 5′-leader sequence of the α9 subunit cRNA. This increased the functional expression of homomeric human α9 receptors by 70-fold (from 7±1 nA to 475±158 nA) and of human α9α10 heteromeric receptors by 80-fold (from 113±62 nA to 9192±1137 nA). These findings indicate the importance of the composition of the 5′ untranslated leader sequence for expression of α9-containing nAChRs.

α7β2 nicotinic acetylcholine receptors assemble, function, and are activated primarily via their α7-α7 interfaces

We investigated assembly and function of nicotinic acetylcholine receptors (nAChRs) composed of α7 and β2 subunits. We measured optical and electrophysiological properties of wild-type and mutant subunits expressed in cell lines and Xenopus laevis oocytes. Laser scanning confocal microscopy indicated that fluorescently tagged α7 and β2 subunits colocalize. Förster resonance energy transfer between fluorescently tagged subunits strongly suggested that α7 and β2 subunits coassemble. Total internal reflection fluorescence microscopy revealed that assemblies localized to filopodia-like processes of SH-EP1 cells. Gain-of-function α7 and β2 subunits confirmed that these subunits coassemble within functional receptors. Moreover, α7β2 nAChRs composed of wild-type subunits or fluorescently tagged subunits had pharmacological properties similar to those of α7 nAChRs, although amplitudes of α7β2 nAChR-mediated, agonist-evoked currents were generally ~2-fold lower than those for α7 nAChRs. It is noteworthy that α7β2 nAChRs displayed sensitivity to low concentrations of the antagonist dihydro-β-erythroidine that was not observed for α7 nAChRs at comparable concentrations. In addition, cysteine mutants revealed that the α7-β2 subunit interface does not bind ligand in a functionally productive manner, partly explaining lower α7β2 nAChR current amplitudes and challenges in identifying the function of native α7β2 nAChRs. On the basis of our findings, we have constructed a model predicting receptor function that is based on stoichiometry and position of β2 subunits within the α7β2 nAChRs.

Chaperoning α7 neuronal nicotinic acetylcholine receptors

The α7 subtype of nicotinic acetylcholine receptors (AChRs) is one of the most abundant members of the Cys-loop family of receptors present in the central nervous system. It participates in various physiological processes and has received much attention as a potential therapeutic target for a variety of pathologies. The importance of understanding the mechanisms controlling AChR assembly and cell-surface delivery lies in the fact that these two processes are key to determining the functional pool of receptors actively engaged in synaptic transmission. Here we review recent studies showing that RIC-3, a protein originally identified in the worm Caenorhabditis elegans, modulates the expression of α7 AChRs in a subtype-specific manner. Potentiation of AChR expression by post-transcriptional events is also critically assessed.

Inhibitory Effects of Quercetin on Muscle-type of Nicotinic Acetylcholine Receptor-Mediated Ion Currents Expressed in Xenopus Oocytes

The flavonoid quercetin is a low molecular weight compound generally found in apple, gingko, tomato, onion and other red-colored fruits and vegetables. Like other flavonoids, quercetin has diverse pharmacological actions. However, relatively little is known about the influence of quercetin effects in the regulation of ligand-gated ion channels. Previously, we reported that quercetin regulates subsets of nicotinic acetylcholine receptors such as α3β4, α7 and α9α10. Presently, we investigated the effects of quercetin on muscle-type of nicotinic acetylcholine receptor channel activity expressed in Xenopus oocytes after injection of cRNA encoding human fetal or adult muscle-type of nicotinic acetylcholine receptor subunits. Acetylcholine treatment elicited an inward peak current (I(ACh)) in oocytes expressing both muscle-type of nicotinic acetylcholine receptors and co-treatment of quercetin with acetylcholine inhibited I(ACh). Pre-treatment of quercetin further inhibited I(ACh) in oocytes expressing adult and fetal muscle-type nicotinic acetylcholine receptors. The inhibition of I(ACh) by quercetin was reversible and concentration-dependent. The IC(50) of quercetin was 18.9±1.2 µM in oocytes expressing adult muscle-type nicotinic acetylcholine receptor. The inhibition of I(ACh) by quercetin was voltage-independent and non-competitive. These results indicate that quercetin might regulate human muscle-type nicotinic acetylcholine receptor channel activity and that quercetin-mediated regulation of muscle-type nicotinic acetylcholine receptor might be coupled to regulation of neuromuscular junction activity.

Quercetin enhances human α7 nicotinic acetylcholine receptor-mediated ion current through interactions with Ca(2+) binding sites

The flavonoid quercetin is a low molecular weight substance found in fruits and vegetables. Aside from its anti-oxidative effect, quercetin, like other flavonoids, has a wide range of neuropharmacological actions. The α7 nicotinic acetylcholine receptor (α7 nAChR) has a Ca(2+)-binding site, is highly permeable to the Ca(2+) ion, and plays important roles in Ca(2+)-related normal brain functions. Dysfunctions of α7 nAChR are associated with a variety of neurological disorders. In the present study, we investigated the effects of quercetin on the ACh-induced inward peak current (I(ACh)) in Xenopus oocytes that heterologously express human α7 nAChR. I(ACh) was measured with the two-electrode voltage clamp technique. In oocytes injected with α7 nAChR cRNA, the effects of the co-application of quercetin on I(ACh) were concentration-dependent and reversible. The ED(50) was 36.1 + 6.1 μM. Quercetin-mediated enhancement of I(ACh) caused more potentiation when quercetin was pre-applied. The degree of I(ACh) potentiation by quercetin pre-application was time-dependent and saturated after 1 min. Quercetin-mediated I(ACh) enhancement was not affected by ACh concentration and was voltage-independent. However, quercetin-mediated I(ACh) enhancement was dependent on extracellular Ca(2+) concentrations and was specific to the Ca(2+) ion, since the removal of extracellular Ca(2+) or the addition of Ba(2+) instead of Ca(2+) greatly diminished quercetin enhancement of I(ACh). The mutation of Glu195 to Gln195, in the Ca(2+)-binding site, almost completely diminished quercetin-mediated I(ACh) enhancement. These results indicate that quercetin-mediated I(ACh) enhancement human α7 nAChR heterologously expressed in Xenopus oocytes could be achieved through interactions with the Ca(2+)-binding site of the receptor.

Role of channel activation in cognitive enhancement mediated by alpha7 nicotinic acetylcholine receptors

BACKGROUND AND PURPOSE

Several agonists of the alpha7 nicotinic acetylcholine receptor (nAChR) have been developed for treatment of cognitive deficits. However, agonist efficacy in vivo is difficult to reconcile with rapid alpha7 nAChR desensitization in vitro; and furthermore, the correlation between in vitro receptor efficacy and in vivo behavioural efficacy is not well delineated. The possibility that agonists of this receptor actually function in vivo as inhibitors via desensitization has not been finally resolved.

EXPERIMENTAL APPROACH

Two structurally related alpha7 nAChR agonists were characterized and used to assess the degree of efficacy required in a behavioural paradigm.

KEY RESULTS

NS6784 activated human and rat alpha7 nAChR with EC(50)s of 0.72 and 0.88 microM, and apparent efficacies of 77 and 97% respectively. NS6740, in contrast, displayed little efficacy at alpha7 nAChR (<2% in oocytes, < or =8% in GH4C1 cells), although its agonist-like properties were revealed by adding a positive allosteric modulator of alpha7 nAChRs or using the slowly desensitizing alpha7V274T receptor. In mouse inhibitory avoidance (IA) memory retention, NS6784 enhanced performance as did the 60% partial agonist A-582941. In contrast, NS6740 did not enhance performance, but blocked effects of A-582941.

CONCLUSIONS AND IMPLICATIONS

Collectively, these findings suggest that a degree of alpha7 nAChR agonist efficacy is required for behavioural effects in the IA paradigm, and that such behavioural efficacy is not due to alpha7 nAChR desensitization. Also, a partial agonist of very low efficacy for this receptor could be used as an inhibitor, in the absence of alpha7 nAChR antagonists with favourable CNS penetration.

A review of experimental techniques used for the heterologous expression of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop family of neurotransmitter-gated ion channels, a family that also includes receptors for gamma-aminobutyric acid, glycine and 5-hydroxytryptamine. In humans, nAChRs have been implicated in several neurological and psychiatric disorders and are major targets for pharmaceutical drug discovery. In addition, nAChRs are important targets for neuroactive pesticides in insects and in other invertebrates. Historically, nAChRs have been one of the most intensively studied families of neurotransmitter receptors. They were the first neurotransmitter receptors to be biochemically purified and the first to be characterized by molecular cloning and heterologous expression. Although much has been learnt from studies of native nAChRs, the expression of recombinant nAChRs has provided dramatic advances in the characterization of these important receptors. This review will provide a brief history of the characterization of nAChRs by heterologous expression. It will focus, in particular, upon studies of recombinant nAChRs, work that has been conducted by many hundreds of scientists during a period of almost 30 years since the molecular cloning of nAChR subunits in the early 1980s.

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

AIM

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

METHODS

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

RESULTS

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

CONCLUSION

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

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

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

Cellular events in nicotine addiction

In the 25 years since the observation that chronic exposure to nicotine could regulate the number and function of high affinity nicotine binding sites in the brain there has been a major effort to link alterations in nicotinic acetylcholine receptors (nAChRs) to nicotine-induced behaviors that drive the addiction to tobacco products. Here we review the proposed roles of various nAChR subtypes in the addiction process, with emphasis on how they are regulated by nicotine and the implications for understanding the cellular neurobiology of addiction to this drug.

Diversity of vertebrate nicotinic acetylcholine receptors

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

Assembly and trafficking of nicotinic acetylcholine receptors (Review)

Nicotinic acetylcholine receptors (nAChRs) are members of an extensive super-family of neurotransmitter-gated ion channels. In humans, nAChRs are expressed within the nervous system and at the neuromuscular junction and are important targets for pharmaceutical drug discovery. They are also the site of action for neuroactive pesticides in insects and other invertebrates. Nicotinic receptors are complex pentameric transmembrane proteins which are assembled from a large family of subunits; seventeen nAChR subunits (alpha1-alpha10, beta1-beta4, gamma, delta and epsilon) have been identified in vertebrate species. This review will discuss nAChR subunit diversity and factors influencing receptor assembly and trafficking.

Synthesis and characterization of 125I-alpha-conotoxin ArIB[V11L;V16A], a selective alpha7 nicotinic acetylcholine receptor antagonist

The alpha7 nicotinic acetylcholine receptors (nAChRs) are widely expressed both in the central nervous system (CNS) and periphery. In the CNS, 125I-alpha-bungarotoxin is commonly used to identify alpha7 nAChRs specifically. However, alpha-bungarotoxin also interacts potently with alpha1* and alpha9alpha10 nAChRs, two receptor subtypes in peripheral tissues that are colocalized with the alpha7 subtype. [3H]Methyllycaconitine is also frequently used as an alpha7-selective antagonist, but it has significant affinity for alpha6* and alpha9alpha10 nAChR subtypes. In this study, we have developed a highly alpha7-selective alpha-conotoxin radioligand by iodination of a naturally occurring histidine. Both mono- and diiodo derivatives were generated and purified (specific activities were 2200 and 4400 Ci mmol(-1), respectively). The properties of the mono- and diiodo derivatives were very similar to each other, but the diiodo was less stable. For monoidodo peptide, saturation binding to mouse hippocampal membranes demonstrated a K(d) value of 1.15 +/- 0.13 nM, similar to that of 125I-alpha-bungarotoxin in the same preparations (0.52 +/- 0.16 nM). Association and dissociation kinetics were relatively rapid (k(obs) for association at 1 nM was 0.027 +/- 0.007 min(-1); k(off) = 0.020 +/- 0.001 min(-1)). Selectivity was confirmed with autoradiography using alpha7-null mutant tissue: specific binding was abolished in all regions of alpha7(-/-) brains, whereas wild-type mice expressed high levels of labeling and low nonspecific binding. 125I-alpha-conotoxin ArIB[V11L; V16A] should prove useful where alpha7 nAChRs are coexpressed with other subtypes that are also labeled by existing ligands. Furthermore, true equilibrium binding experiments could be performed on alpha7 nAChRs, something that is impossible with 125I-alpha-bungarotoxin.

Influence of the M3-M4 intracellular domain upon nicotinic acetylcholine receptor assembly, targeting and function

BACKGROUND AND PURPOSE

The aim of this study was to investigate the influence of the intracellular domain of nicotinic acetylcholine receptor (nAChR) subunits upon receptor assembly, targeting and functional properties.

EXPERIMENTAL APPROACH

Because most nAChR subunits form functional receptors only as heteromeric complexes, it can be difficult to examine the influence of individual subunits or subunit domains in isolation. A series of subunit chimaeras was constructed which contain the intracellular loop region (located between the M3 and M4 transmembrane domains) from nAChR subunits alpha1-alpha10 or beta1-beta4. All of these chimaeras contain common extracellular and transmembrane domains (from the nAChR alpha7 subunit and the 5-hydroxytryptamine receptor 5-HT(3A) subunit, respectively), thereby facilitating both homomeric receptor assembly and detection with radiolabelled or fluorescent alpha-bungarotoxin.

KEY RESULTS

The nAChR M3-M4 intracellular loop domain had no significant effect upon levels of total subunit protein detected in transfected cells but had a significant influence upon levels of both cell surface and intracellular assembled receptors. Comparisons of functional properties revealed a significant influence of the intracellular loop domain upon both single-channel conductance and receptor desensitization. In addition, studies conducted in polarized epithelial cells demonstrate that the nAChR loop can influence receptor targeting, resulting in either polarized (apical) or non-polarized distribution.

CONCLUSIONS AND IMPLICATIONS

Evidence has been obtained which demonstrates that the large intracellular loop domain of nAChR subunits can exert a profound influence upon receptor assembly, targeting and ion channel properties.

Actions of snake neurotoxins on an insect nicotinic cholinergic synapse

Here we examine the actions of six snake neurotoxins (alpha-cobratoxin from Naja naja siamensis, erabutoxin-a and b from Laticauda semifasciata; CM12 from N. haje annulifera, toxin III 4 from Notechis scutatus and a long toxin from N. haje) on nicotinic acetylcholine receptors in the cercal afferent, giant interneuron 2 synapse of the cockroach, Periplaneta americana. All toxins tested reduced responses to directly-applied ACh as well as EPSPs evoked by electrical stimulation of nerve XI with similar time courses, suggesting that their action is postsynaptic. Thus, these nicotinic receptors in a well-characterized insect synapse are sensitive to both long and short chain neurotoxins. This considerably expands the range of snake toxins that block insect nicotinic acetylcholine receptors and may enable further pharmacological distinctions between nAChR subtypes.

Discovery, Synthesis, and Structure Activity of a Highly Selective α7 Nicotinic Acetylcholine Receptor Antagonist

Nicotinic acetylcholine receptors (nAChRs) that contain an alpha7 subunit are widely distributed in neuronal and nonneuronal tissue. These receptors are implicated in the release of neurotransmitters such as glutamate and in functions ranging from thought processing to inflammation. Currently available ligands for alpha7 nAChRs have substantial affinity for one or more other nAChR subtypes, including those with an alpha1, alpha3, alpha6, and/or alpha9 subunit. An alpha-conotoxin gene was cloned from Conus arenatus. Predicted peptides were synthesized and found to potently block alpha3-, alpha6-, and alpha7-containing nAChRs. Structure-activity information regarding conotoxins from distantly related Conus species was employed to modify the C. arenatus derived toxin into a novel, highly selective alpha7 nAChR antagonist. This ligand, alpha-CtxArIB[V11L,V16D], has low nanomolar affinity for rat alpha7 homomers expressed in Xenopus laevis oocytes, and antagonism is slowly reversible. Kinetic analysis provided insight into the mechanism of antagonism. alpha-CtxArIB interacts with five ligand binding sites per alpha7 receptor, and occupation of a single site is sufficient to block function. The peptide was also shown to be highly selective in competition binding assays in rat brain membranes. alpha-CtxArIB[V11L,V16D] is the most selective ligand yet reported for alpha7 nAChRs.

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.

Stoichiometry of the alpha9alpha10 nicotinic cholinergic receptor

The alpha9 and alpha10 nicotinic cholinergic subunits assemble to form the receptor that mediates synaptic transmission between efferent olivocochlear fibers and hair cells of the cochlea. They are the latest vertebrate nicotinic cholinergic receptor (nAChR) subunits that have been cloned, and their identification has established a distant early divergent branch within the nAChR gene family. The alpha10 subunit serves as a "structural" component leading to heteromeric alpha9alpha10 nAChRs with distinct properties. We now have probed the stoichiometry of recombinant alpha9alpha10 nAChRs expressed in Xenopus oocytes. We have made use of the analysis of the population of receptors assembled from a wild-type subunit and its partner alpha9 or alpha10 subunit bearing a reporter mutation of a valine to threonine at position 13' of the second transmembrane domain (TM2). Because the mutation increased the sensitivity of the receptor for acetylcholine (ACh) but mutations at different subunits were not equivalent, the number of alpha9 and alpha10 subunits could be inferred from the number of components in compound concentration-response curves to ACh. The results were confirmed via the analysis of the effects of a mutation to threonine at position 17' of TM2. Because at this position the mutations at different subunits were equivalent, the stoichiometry was inferred directly from the shifts in the ACh EC50 values. We conclude that the recombinant alpha9alpha10 receptor is a pentamer with a (alpha9)2(alpha10)3 stoichiometry.

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 Novel α-Conotoxin, PeIA, Cloned from Conus pergrandis, Discriminates between Rat α9α10 and α7 Nicotinic Cholinergic Receptors

The alpha9 and alpha10 nicotinic cholinergic subunits assemble to form the receptor believed to mediate synaptic transmission between efferent olivocochlear fibers and hair cells of the cochlea, one of the few examples of postsynaptic function for a non-muscle nicotinic acetylcholine receptor (nAChR). However, it has been suggested that the expression profile of alpha9 and alpha10 overlaps with that of alpha7 in the cochlea and in sites such as dorsal root ganglion neurons, peripheral blood lymphocytes, developing thymocytes, and skin. We now report the cloning, total synthesis, and characterization of a novel toxin alpha-conotoxin PeIA that discriminates between alpha9alpha10 and alpha7 nAChRs. This is the first toxin to be identified from Conus pergrandis, a species found in deep waters of the Western Pacific. Alpha-conotoxin PeIA displayed a 260-fold higher selectivity for alpha-bungarotoxin-sensitive alpha9alpha10 nAChRs compared with alpha-bungarotoxin-sensitive alpha7 receptors. The IC50 of the toxin was 6.9 +/- 0.5 nM and 4.4 +/- 0.5 nM for recombinant alpha9alpha10 and wild-type hair cell nAChRs, respectively. Alpha-conotoxin PeIA bears high resemblance to alpha-conotoxins MII and GIC isolated from Conus magus and Conus geographus, respectively. However, neither alpha-conotoxin MII nor alpha-conotoxin GIC at concentrations of 10 microM blocked acetylcholine responses elicited in Xenopus oocytes injected with the alpha9 and alpha10 subunits. Among neuronal non-alpha-bungarotoxin-sensitive receptors, alpha-conotoxin PeIA was also active at alpha3beta2 receptors and chimeric alpha6/alpha3beta2beta3 receptors. Alpha-conotoxin PeIA represents a novel probe to differentiate responses mediated either through alpha9alpha10 or alpha7 nAChRs in those tissues where both receptors are expressed.

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.

Ca2+ permeability of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are expressed in muscle cells and neurons, as well as in an increasing number of other cell types. The nAChR channels are permeable to cations, including Ca(2+). Ca(2+) entry through nAChR channels has been shown to modulate several Ca(2+)-dependent cellular processes, such as neurotransmitter release, synaptic plasticity, and cell motility. The value of Ca(2+) permeability associated to a particular nAChR subtype thus represents an important indication for its physiological role. This review summarizes the quantitative data on Ca(2+) permeability obtained from several nAChR subtypes in native and heterologous systems. Different experimental approaches are compared, and the structural determinants of Ca(2+) permeability are discussed.

Nicotinic mechanisms in the autonomic control of organ systems

Most visceral organs are under the control of the autonomic nervous system (ANS). Information on the state and function of these organs is constantly relayed to the central nervous system (CNS) by sensory afferent fibers. The CNS integrates the sensory inputs and sends neural commands back to the organ through the ANS. The autonomic ganglia are the final site for the integration of the message traveling from the CNS. Nicotinic acetylcholine receptors (nAChRs) are the main mediators of fast synaptic transmission in ganglia, and therefore, are key molecules for the processing of neural information in the ANS. This review focuses on the role of nAChRs in the control of organ systems such as heart, gut, and bladder. The autonomic control of these organ systems is discussed in the light of the results obtained from the analysis of mice carrying mutations targeted to nAChR subunits expressed in the ANS.

Emerging structure of the nicotinic acetylcholine receptors

The conversion of acetylcholine binding into ion conduction across the membrane is becoming more clearly understood in terms of the structure of the receptor and its transitions. A high-resolution structure of a protein that is homologous to the extracellular domain of the receptor has revealed the binding sites and subunit interfaces in great detail. Although the structures of the membrane and cytoplasmic domains are less well determined, the channel lining and the determinants of selectivity have been mapped. The location and structure of the gates, and the coupling between binding sites and gates, remain to be established.

A novel human nicotinic receptor subunit, alpha10, that confers functionality to the alpha9-subunit

We present herein the cloning of the human nicotinic acetylcholine receptor alpha9-ortholog and the identification of a new alpha-like subunit (alpha10) that shares 58% identity with alpha9. Whereas alpha10 fails to produce functional receptors alone, it promoted robust acetylcholine-evoked currents when coinjected with alpha9. The presence of alpha10 modifies the physiological and pharmacological properties of the alpha9 receptor indicating that the two subunits coassemble in a single functional receptor. Fusing the N-terminal domain of alpha9 with the rest of the alpha10-cDNA yielded a functional alpha9:alpha10-chimera that displays the acetylcholine binding properties of alpha9 and ionic pore characteristics of alpha10-containing receptors. In addition, alpha9- and alpha10-subunit mRNAs show limited similar tissue distribution patterns and are expressed in cochlea, pituitary gland, and keratinocytes. These data suggest that, in vivo, alpha9-containing receptors coassemble with alpha10-subunit.

Neuronal nicotinic receptors: from protein structure to function

Neuronal nicotinic acetylcholine receptors are a prototype of ligand-gated channels that mediate transmission in the central and peripheral nervous system. Structure-function studies performed at the amino acid level are now unraveling the determinant residues either for the properties of the ligand-binding domain or the ionic pore. In this work we review, in the light of the latest finding, the structure-function relationship of these receptors and their implication in neurological diseases.

An α4β4 Nicotinic Receptor Subtype Is Present in Chick Retina: Identification, Characterization and Pharmacological Comparison with the Transfected α4β4 and α6β4 Subtypes

Retina from 1-day-old chicks is a valuable tissue model for studying neuronal nicotinic receptors because it expresses a large number of the developmentally regulated high affinity [(3)H]epibatidine labeled nicotinic receptors. Most of these receptors contain the beta4 subunit associated with different alpha subunits. Using a sequential immunodepletion procedure with anti-alpha6, anti-beta3, anti-beta2, and anti-beta4 antibodies, we purified an alpha4beta4 nicotinic receptor subtype that accounts for approximately 20 to 25% of the high affinity [(3)H]epibatidine labeled receptors present in retina at that developmental time. Immunoprecipitation and Western blotting experiments confirmed that the purified subtype contains only the alpha4 and beta4 subunits. This receptor binds a number of agonists and the antagonist dihydro-beta-erythroidine with nanomolar affinity, whereas it has micromolar affinity for the alpha-conotoxin MII and methyllycaconitine toxins and other nicotinic antagonists. Comparison of the pharmacological profile of this purified native subtype with that of the same subtype transiently expressed in human BOSC23 cells showed that they have very similar rank orders and absolute Ki values for several nicotinic drugs. Finally, because chick retina expresses an alpha6beta4-containing subtype with a high affinity for the alpha-conotoxin MII, we used native and transfected alpha4beta4 and alpha6beta4 subtypes to investigate the relative contributions of the alpha and beta subunits to this binding, and found that the alpha6 subunit determines the high affinity for this toxin.

Chick optic lobe contains a developmentally regulated alpha2alpha5beta2 nicotinic receptor subtype

The most widely expressed neuronal nicotinic acetylcholine receptor subtype in chick brain is that containing the alpha4 and beta2 subunits. However, immunoprecipitation and localization studies have shown that some brain areas also contain the alpha2 and/or alpha5 subunits, whose role in the definition of receptor properties is still intriguing. Using subunit-specific polyclonal antibodies, we found that the optic lobe is the chick central nervous system region that expresses the highest level of alpha2-containing receptors. Immunoprecipitation studies of these immunopurified alpha2-containing receptors labeled with the nicotinic agonist [(3)H]epibatidine showed that almost all of them contained the beta2 subunit and that more than 66% contained the alpha5 subunit. Western blot analyses of the purified receptors confirmed the presence of the alpha2, alpha5, and beta2 subunits and the absence of the alpha3, alpha4, alpha6, alpha7, alpha8, beta3, and beta4 subunits. The alpha2-containing receptors are developmentally regulated: their expression increases 25 times from embryonic day 7 to posthatching day 1 in the optic lobe, compared with an increase of only 5-fold in the forebrain. The alpha2-containing optic lobe receptors bind [(3)H]epibatidine (K(d) = 29 pM) and a number of other nicotinic agonists with very high affinity and have a pharmacological profile very similar to that of the alpha4beta2 subtype. They form functional cationic channels when reconstituted in lipid bilayers, with pharmacological and biophysical properties different from those of the alpha4beta2 subtype. These channels are activated by nicotinic agonists in a dose-dependent manner and are blocked by the nicotinic antagonist d-tubocurarine.

Drugs selective for nicotinic receptor subtypes: a real possibility or a dream?

Nicotine exerts a number of different effects on the nervous system by interacting with neuronal nicotinic acetylcholine receptors (nAChRs). These effects are mediated by its interaction with different nAChR subtypes, and this has led to the finding of subtype specific agonists and antagonists. In the search for subtype-selective drugs, we have synthesized some compounds derived from 4-oxystilbene, two of which (MG624 and F3) are selective ligands for the chick neuronal alphaBgtx receptors containing the alpha7 and/or alpha8 subunits. They have an antagonist action on oocyte-expressed chick and rat alpha7 subtypes. These compounds are selective toward the alpha7-containing receptors in chick, but, in mammals, although they still retain their potency toward alpha7-containing receptors, they are also active in non-alpha7-containing receptors.

Neuronal nicotinic acetylcholine receptors from Drosophila: two different types of alpha subunits coassemble within the same receptor complex

Although neuronal nicotinic acetylcholine receptors from insects have been reconstituted in vitro more than a decade ago, our knowledge about the subunit composition of native receptors as well as their functional properties still remains limited. Immunohistochemical evidence has suggested that two alpha subunits, alpha-like subunit (ALS) and Drosophila alpha2 subunit (Dalpha2), are colocalized in the synaptic neuropil of the Drosophila CNS and therefore may be subunits of the same receptor complex. To gain further understanding of the composition of these nicotinic receptors, we have examined the possibility that a receptor may imbed more than one alpha subunit using immunoprecipitations and electrophysiological investigations. Immunoprecipitation experiments of fly head extracts revealed that ALS-specific antibodies coprecipitate Dalpha2, and vice versa, and thereby suggest that these two alpha subunits must be contained within the same receptor complex, a result that is supported by investigations of reconstituted receptors in Xenopus oocytes. Discrimination between binary (ALS/beta2 or Dalpha2/beta2) and ternary (ALS/Dalpha2/beta2) receptor complexes was made on the basis of their dose-response curve to acetylcholine as well as their sensitivity to alpha-bungarotoxin or dihydro-beta-erythroidine. These data demonstrate that the presence of the two alpha subunits within a single receptor complex confers new receptor properties that cannot be predicted from knowledge of the binary receptor's properties.

beta3 subunit is present in different nicotinic receptor subtypes in chick retina

Although the neuronal nicotinic beta3 subunit was cloned several years ago, it has only recently been shown to form heteromeric channels when associated with other nicotinic subunits, and very little information is available concerning its assembly in the native nicotinic receptors of the nervous system. Using subunit-specific antibodies and immunoprecipitation experiments, we have identified the retina as being the chick central nervous system (CNS) area that expresses the highest level of the beta3 subunit. Sequential immunopurification experiments showed that there are at least two populations of beta3-containing receptors in chick retina: in one, the beta3 subunit is associated with the alpha6 and beta4 subunits; in the other more heterogeneous population, the beta3 subunit is associated with the alpha2, alpha3, alpha4, beta2 and beta4 subunits. Both of these receptor populations bind [3H]epibatidine and a number of nicotinic receptor agonists with high affinity (nM) and nicotinic receptor antagonists with a lower affinity (microM). The greatest pharmacological difference between the two populations is the affinity for the alpha-conotoxin MII, which inhibits binding to alpha6-containing receptors and not that to beta3-containing receptors. We also searched for the presence of the beta3 subunit associated with the alpha-bungarotoxin binding subunits alpha7 and/or alpha8 in retina and chick brain. Immunoprecipitation studies using anti-beta3 antibodies did not detect any specific alpha-bungarotoxin labeled receptors, thus, indicating that the beta3 subunit is not present in the alpha-bungarotoxin receptors of these areas.

Nicotine, brain nicotinic receptors, and neuropsychiatric disorders

Neuronal nicotinic acetylcholine receptors (nAChRs) represent a large family of ligand-gated cation channels with diverse structures and properties. In contrast to the muscular nAChRs, the physiological functions of neuronal nAChRs are not well defined to date. Behavioral studies indicate that brain nAChRs participate in complex functions such as attention, memory, and cognition, whereas clinical data suggest their involvement in the pathogenesis of certain neuropsychiatric disorders (Alzheimer's and Parkinson's diseases, Tourette's syndrome, schizophrenia, depression, etc.). For the majority of these disorders, the use of nAChRs' agonists may represent either a prophylactic (especially for Alzheimer's and Parkinson's diseases) or a symptomatic treatment. The possible mechanisms underlying these beneficial effects as well as the characteristics and potential therapeutic use of new, subtype-selective nAChRs agonists are presented.

Neuronal alpha-bungarotoxin receptors are alpha7 subunit homomers

Nicotinic acetylcholine receptors in the nervous system are heterogeneous with distinct pharmacological and functional properties resulting from differences in post-translational processing and subunit composition. Because of nicotinic receptor diversity, receptor purification and biochemical characterization have been difficult, and the precise subunit composition of each receptor subtype is poorly characterized. Evidence is presented that alpha-bungarotoxin (Bgt)-binding nicotinic receptors found in pheochromocytoma 12 (PC12) cells are pentamers composed solely of alpha7 subunits. Metabolically labeled, affinity-purified Bgt receptors (BgtRs) consisted of a single 55 kDa band on SDS gels, which was recognized by anti-alpha7 antibodies on immunoblots. Isoelectric focusing separated the 55 kDa band into multiple spots, all recognized by anti-alpha7 antibodies and, therefore, each a differentially processed alpha7 subunit. Cell-surface BgtR subunits, cross-linked to each other and (125)I-Bgt, migrated on gels as a ladder of five bands with each band a multiple of an alpha7 subunit monomer. Similar characteristics of BgtRs from rat brain suggest that they, like PC12 BgtRs, are alpha7 pentamers containing differentially processed alpha7 subunits.

Antibodies against neuronal nicotinic receptor subtypes in neurological disorders

Patients with myasthenia gravis (MG) have antibodies to the muscle nicotinic acetylcholine receptor (mAChR) which are responsible for their muscle weakness: but some patients with MG and other neuroimmunological disorders have autonomic symptoms. We characterised the neuronal forms of AChRs (nAChRs) into two neuroblastoma cell lines and developed immunoprecipitation assays to test for antibodies to the alpha7- and alpha3-containing nAChR subtypes, present in the autonomic ganglia. We then tested 70 sera samples from MG patients, 38 from subjects with other neurological diseases, and 30 from healthy individuals, for antibodies to these two forms of neuronal AChR subtypes. We used the alpha7 subtype extracted from the human neuroblastoma IMR32 cell line labeled with 125IalphaBungarotoxin (alphaBgtx), and the alpha3-containing subtype extracted from the human neuroblastoma SY5Y cell line labeled with 3H-Epibatidine (Epi). Nine subjects (five MG, one GBS, one CIPD and two LEMS) were positive for the alpha7 subtype; and four for the alpha3-containing subtype (two MG patients, one LEMS and the same GBS patient). None of the MG patients with undetectable levels of antibodies against muscle AChR were positive. The patients with serum antibodies to alpha7 or alpha3-containing neuronal AChRs showed a range of clinical features including autonomic symptoms and thymoma in two MG patients. These results indicate that patients with MG and other immune-mediated disorders can have antibodies to neuronal AChRs, and that these may contribute to the clinical characteristics of the diseases.

Agonist activation and alpha-bungarotoxin inhibition of wild type and mutant alpha7 nicotinic acetylcholine receptors

The properties of wild type and mutant rat nicotinic alpha7 receptors expressed in Xenopus oocytes were investigated using electrophysiology and site-directed mutagenesis. When compared at individual agonist concentrations, neither the normalised nicotinic, nor acetylcholine, responses of the wild type receptors were significantly different from the corresponding responses obtained from a first extracellular domain mutant, phenylalanine(189)tyrosine (P0.05). The dissociation constants (K(D)) of the wild type (4.7 nM) and Phe(189)Tyr mutant (5.2 nM) receptors for alpha-bungarotoxin were estimated by an electrophysiological approach. The similarity of the results suggests that the mutation did not lead to a widespread disruption of structure-function relationships, although a slight change in nicotine sensitivity may have occurred. In contrast, the mutations (Tyr(190)Gln, first extracellular domain), (Glu(261)Ala, M2 region) severely compromised receptor function. An additional mutation was made in a negatively charged motif of the second extracellular domain which is conserved in homomeric nicotinic receptors. This mutation, Asp(268)Ala, also caused a loss of function. Thus the structure-function relationships in nicotinic alpha7 receptors have parallels with heteromeric nicotinic receptors, but there may also be some marked differences.

The alpha7 nicotinic acetylcholine receptor in neuronal plasticity

A growing body of evidence indicates that neuronal nicotinic acetylcholine receptors (nAChRs), in addition to promoting fast cholinergic transmission, may modulate other neuronal activities within the central nervous system (CNS). In particular, the alpha7 nAChR is highly permeable to Ca2+ and may serve a distinct role in regulating neuronal plasticity. By elevating intracellular Ca2+ levels in discrete neuronal locations, these ligand-gated ion channels may influence numerous physiological processes in developing and adult CNS. In this article, we review evidence that both pre- and postsynaptic alpha7 nAChRs modulate transmitter release in the brain and periphery through Ca2+-dependent mechanisms. The possible role of alpha7 nAChRs in regulating neuronal growth and differentiation in developing CNS is also evaluated. We consider an interaction between cholinergic and glutamatergic transmission and propose a hypothesis on the possible coregulation of intracellular Ca2+ by N-methyl-D-aspartate (NMDA) receptors and alpha7 nAChRs. Finally, the clinical significance of alterations in the normal function of alpha7 nAChRs is discussed as it pertains to prenatal nicotine exposure, schizophrenia, and epilepsy.

Functional alpha6-containing nicotinic receptors are present in chick retina

Despite the fact that the neuronal chick alpha6 subunit was first cloned several years ago and recently has been shown to form acetylcholine (ACh)-activated channels in heterologous systems, no information is yet available concerning the structure and function of the alpha6-containing nicotinic receptors in neuronal tissues. Using subunit-specific antibodies directed against two different epitopes of the chick alpha6 subunit, we performed immunoprecipitation experiments on immunopurified alpha6-containing receptors radiolabeled with the nicotinic agonist [3H]epibatidine (Epi): almost all of the alpha6 receptors contained the beta4 subunit, 51% the beta3 subunit, 42% the alpha3 subunit, and 7.5% the beta2 subunit. Western blot analyses of the purified receptors confirmed the presence of the alpha3, beta3, beta2, and beta4 subunits, and the absence of the alpha4, alpha5, and alpha7 subunits. The alpha6-containing receptors bind [3H]Epi (Kd = 35 pM) and a number of other nicotinic agonists with very high affinity, the rank order being Epi >> cytisine > nicotine > 1, 1-dimethyl-4-phenylpiperazinium > acetylcholine > carbamylcholine. The alpha6 receptors also have a distinct antagonist pharmacological profile with a rank order of potency of alpha-conotoxin MII > methyllycaconitine > dihydro-beta-erythroydine > MG624 > d-tubocurarine > decamethonium > hexamethonium. When reconstituted in lipid bilayers, the alpha6-containing receptors form functional cationic channels with a main conductance state of 48 pS. These channels are activated by nicotinic agonists in a dose-dependent manner, and blocked by the nicotinic antagonist d-tubocurarine.

Characterisation of the binding of [3H]methyllycaconitine: a new radioligand for labelling alpha 7-type neuronal nicotinic acetylcholine receptors

Methyllycaconitine (MLA), a norditerpenoid alkaloid isolated from Delphinium seeds, is one of the most potent non-proteinacious ligands that is selective for alpha bungarotoxin-sensitive neuronal nicotinic acetylcholine receptors (nAChR). [3H]MLA bound to rat brain membranes with high affinity (Kd = 1.86 +/- 0.31 nM) with a good ratio of specific to non-specific binding. The binding of [3H]MLA was characterised by rapid association (t 1/2 = 2.3 min) and dissociation (t 1/2 = 12.6 min) kinetics. The radioligand binding displayed nicotinic pharmacology, consistent with an interaction with alpha bungarotoxin-sensitive nAChR. The snake alpha-toxins, alpha bungarotoxin and alpha cobratoxin, displaced [3H]MLA with high affinity (Ki = 1.8 +/- 0.5 and 5.5 +/- 0.9 nM, respectively), whereas nicotine was less potent (Ki = 6.1 +/- 1.1 microM). The distribution of [3H]MLA binding sites in crudely dissected rat brain regions was identical to that of [125I] alpha bungarotoxin binding sites, with a high binding site density in hippocampus and hypothalamus, but low density in striatum and cerebellum. [3H]MLA also labelled a sub-population of binding sites which are not sensitive to the snake alpha toxins, but which did not differ significantly from the major population with respect to their other pharmacological properties or regional distribution. [3H]MLA, therefore, is a novel radiolabel for characterising alpha 7-type nAChR. A good signal to noise ratio and rapid binding kinetics provide advantages over the use of radiolabelled alpha bungarotoxin for rapid and accurate equilibrium binding assays.

Synthesis and electrophysiological studies of a novel epibatidine analogue

The new epibatidine analogue exo-2-(2-pyridyl)-7-azabicyclo[2.2.1]heptane (2PABH) was synthesised. Separation of enantiomers was performed on chiral HPLC chromatography in polar-organic phase mode at 0 degree C. Enantiomeric purity was greater than 99.8%ee for the (-)- and 90.5%ee for the (+)-enantiomer respectively. Optical rotation was determined to be [alpha]23D = +/- 13 degrees. Electrophysiological studies of 2PABH were carried out on alpha 4 beta 2, alpha 3 beta 4 and alpha 7 nAChR subtypes cloned from rat and reconstituted in Xenopus oocytes. Both enantiomers could not significantly activate the heteromeric subtypes. The homomeric alpha 7 nAChR displays a high sensitivity only towards (-)-2PABH. The EC50 for (-)-2PABH and ACh were determined (32.5 +/- 9.5 microM, 137.3 +/- 16.5 microM). (-)-2PABH was shown to be a partial agonist (80% of ACh). Thus the efficacy of 2PABH differs markedly from that of epibatidine. The intramolecular N-N-distance and the spatial pyridine nitrogen orientation play a central role in nAChR recognition.

Selective effects of a 4-oxystilbene derivative on wild and mutant neuronal chick alpha7 nicotinic receptor

1. We assessed the pharmacological activity of triethyl-(beta-4-stilbenoxy-ethyl) ammonium (MG624), a drug that is active on neuronal nicotinic receptors (nicotinic AChR). Experiments on the major nicotinic AChR subtypes present in chick brain, showed that it inhibits the binding of [125I]-alphaBungarotoxin (alphaBgtx) to the alpha7 subtype, and that of [3H]-epibatidine (Epi) to the alpha4beta2 subtype, with Ki values of respectively 106 nM and 84 microM. 2. MG624 also inhibited ACh elicited currents (I(ACh)) in the oocyte-expressed alpha7 and alpha4beta2 chick subtypes with half-inhibitory concentrations (IC50) of respectively 109 nM and 3.2 microM. 3. When tested on muscle-type AChR, it inhibited [125I]-alphaBgtx binding with a Ki of 32 microM and ACh elicited currents (I(ACh)) in the oocyte-expressed alpha1beta1gammadelta chick subtype with an IC50 of 2.9 microM. 4. The interaction of MG624 with the alpha7 subtype was investigated using an alpha7 homomeric mutant receptor with a threonine-for-leucine 247 substitution (L247T alpha7). MG624 did not induce any current in oocytes expressing the wild type alpha7 receptor, but did induce large currents in the oocyte-expressed L247T alpha7 receptor. The MG624 elicited current (I(MG62)) has an EC50 of 0.2 nM and a Hill coefficient nH of 1.9, and is blocked by the nicotinic receptor antagonist methyllycaconitine (MLA). 5. These binding and electrophysiological studies show that MG624 is a potent antagonist of neuronal chick alpha7 nicotinic AChR, and becomes a competitive agonist following the mutation of the highly conserved leucine residue 247 located in the M2 channel domain.

Neuronal nicotinic acetylcholine receptors in rat trigeminal ganglia

The application of nicotine to the various epithelia served by the trigeminal nerve produces irritation and/or pain by activating neuronal nicotinic acetylcholine receptors (NnAChRS) in sensory neurons. In this study the NnAChRs were identified in rat trigeminal ganglia (TG) using RT-PCR and immunocytochemistry. With RT-PCR the subunits of NnAChRs in rat TG were determined, and with immunocytochemistry the localization of three prominent subunits (alpha 7, alpha 4 and beta 2) were localized in intact TG neurons. The relative abundance of the alpha and beta subunits were: alpha 7 approximately alpha 3 > alpha 6 > alpha 4 approximately alpha 5 > alpha 9 > or = alpha 2, and beta 2 approximately beta 3 > beta 4. This is the first report of the alpha 9 subunit in TG. Immunohistochemical studies revealed that almost all TG neurons contained alpha 7-LI and alpha 4-LI, and that 85% had beta 2-LI. For these three subunits much of the label was internalized. Immunocytochemical studies using antibodies raised against chick alpha 8 subunits did not specifically label rat TG. These data reveal that rat TG neurons contain the entire spectrum of mammalian NnAChR subunits.