Neurons assemble acetylcholine receptors with as many as three kinds of subunits while maintaining subunit segregation among receptor subtypes

Single-cell quantitative expression of nicotinic acetylcholine receptor mRNA in rat hippocampal interneurons

Hippocampal interneurons are a very diverse population of cells. Using single-cell quantitative PCR to analyze rat CA1 hippocampal interneurons, we quantified neuronal nicotinic acetylcholine receptor (nAChR) mRNA subunit expression and detailed possible nAChR subtype combinations for the α2, α3, α4, α5, α7, β2, β3, and β4 subunits. We also compared the expression detected in the stratum oriens and the stratum radiatum hippocampal layers. We show that the majority of interneurons in the CA1 of the rat hippocampus contain detectable levels of nAChR subunit mRNA. Our results highlight the complexity of the CA1 nAChR population. Interestingly, the α3 nAChR subunit is one of the highest expressed subunit mRNAs in this population, while the α4 is one of the least likely subunits to be detected in CA1 interneurons. The β2 nAChR subunit is the highest expressed beta subunit mRNA in these cells. In addition, Pearson's correlation coefficient values are calculated to identify significant differences between the nAChR subunit combinations expressed in the CA1 stratum oriens and the stratum radiatum. Statistical analysis also indicates that there are likely over 100 different nAChR subunit mRNA combinations expressed in rat CA1 interneurons. These results provide a valid avenue for identifying nAChR subtype targets that may be effective hippocampus-specific pharmacological targets.

Evaluation of commercially available antibodies and fluorescent conotoxins for the detection of surface ganglionic acetylcholine receptor on the neuroblastoma cell line, IMR-32 by flow cytometry

Commercially available antibodies that bind to the human muscle acetylcholine receptor (ACHR) have been validated previously for flow cytometric use (Keefe et al., 2009; Leite et al., 2008; Lozier et al., 2015). Despite a multitude of commercially available antibodies to other nicotinic ACHRs, validation in a wide variety of immunoassay formats is lacking; when studied, a large proportion of these antibodies have been deemed not fit for most research purposes (Garg and Loring, 2017). We have recently described a flow cytometric immunomodulation assay for the diagnosis of Autoimmune Autonomic Ganglionopathy (AAG) (Urriola et al., 2021) that utilises the monoclonal antibody mab35(Urriola et al., 2021) which is specific for ganglionic ACHR (gnACHR) that contain α3 subunits (Vernino et al., 1998). Other fluorescent ligands for α3-gnACHR have not been validated for flow cytometric use. We investigated 7 commercially sourced antibodies and 3 synthetic fluorescent novel conotoxins purported to specifically bind to the extracellular domains of the gnACHR, and compared the results to staining by mab35, using flow cytometry with the neuroblastoma cell line IMR-32. We also evaluated the degree of non-specific binding by depleting the cell membrane of the relevant acetylcholine receptor with a pre-incubation step involving the serum from a patient with Autoimmune Autonomic Ganglionopathy containing pathogenic antibodies to the ganglionic acetylcholine receptor. None of the assessed conotoxins, and only one antibody (mab35) was found to perform adequately in flow cytometric staining of the native ganglionic acetylcholine receptor.

Synaptic Components, Function and Modulation Characterized by GCaMP6f Ca2+ Imaging in Mouse Cholinergic Myenteric Ganglion Neurons

The peristaltic contraction and relaxation of intestinal circular and longitudinal smooth muscles is controlled by synaptic circuit elements that impinge upon phenotypically diverse neurons in the myenteric plexus. While electrophysiological studies provide useful information concerning the properties of such synaptic circuits, they typically involve tissue disruption and do not correlate circuit activity with biochemically defined neuronal phenotypes. To overcome these limitations, mice were engineered to express the sensitive, fast Ca²⁺ indicator GCaMP6f selectively in neurons that express the acetylcholine (ACh) biosynthetic enzyme choline acetyltransfarse (ChAT) thereby allowing rapid activity-driven changes in Ca²⁺ fluorescence to be observed without disrupting intrinsic connections, solely in cholinergic myenteric ganglion (MG) neurons. Experiments with selective receptor agonists and antagonists reveal that most mouse colonic cholinergic (i.e., GCaMP6f⁺/ChAT⁺) MG neurons express nicotinic ACh receptors (nAChRs), particularly the ganglionic subtype containing α3 and β4 subunits, and most express ionotropic serotonin receptors (5-HT₃Rs). Cholinergic MG neurons also display small, spontaneous Ca²⁺ transients occurring at ≈ 0.2 Hz. Experiments with inhibitors of Na⁺ channel dependent impulses, presynaptic Ca²⁺ channels and postsynaptic receptor function reveal that the Ca²⁺ transients arise from impulse-driven presynaptic activity and subsequent activation of postsynaptic nAChRs or 5-HT₃Rs. Electrical stimulation of axonal connectives to MG evoked Ca²⁺ responses in the neurons that similarly depended on nAChRs or/and 5-HT₃Rs. Responses to single connective shocks had peak amplitudes and rise and decay times that were indistinguishable from the spontaneous Ca²⁺ transients and the largest fraction had brief synaptic delays consistent with activation by monosynaptic inputs. These results indicate that the spontaneous Ca²⁺ transients and stimulus evoked Ca²⁺ responses in MG neurons originate in circuits involving fast chemical synaptic transmission mediated by nAChRs or/and 5-HT₃Rs. Experiments with an α7-nAChR agonist and antagonist, and with pituitary adenylate cyclase activating polypeptide (PACAP) reveal that the same synaptic circuits display extensive capacity for presynaptic modulation. Our use of non-invasive GCaMP6f/ChAT Ca²⁺ imaging in colon segments with intrinsic connections preserved, reveals an abundance of direct and modulatory synaptic influences on cholinergic MG neurons.

Progress in nicotinic receptor structural biology

Here we begin by briefly reviewing landmark structural studies on the nicotinic acetylcholine receptor. We highlight challenges that had to be overcome to push through resolution barriers, then focus on what has been gleaned in the past few years from crystallographic and single particle cryo-EM studies of different nicotinic receptor subunit assemblies and ligand complexes. We discuss insights into ligand recognition, ion permeation, and allosteric gating. We then highlight some foundational aspects of nicotinic receptor structural biology that remain unresolved and are areas ripe for future exploration. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

Blockade of nicotinic acetylcholine receptor enhances the responsiveness to bupropion in the mouse forced swim test

The objective of the present study is to investigate the role of α4, α5, α6 or β2 nAChR subunits in the antidepressant-like effect of bupropion. Adult male mice were treated with subcutaneous acute doses of bupropion (3 and 10 mg/kg) 30 min before the forced swim test (FST) in α4, α5, α6, or β2 nAChR subunit knockout (KO) and wild-type (WT) mice. In addition, the effects of β2* antagonist dihydro-β-erythroidine (DHβE, 3 mg/kg) on antidepressant-like effects of bupropion in C57BL/6 J mice were assessed. Our results showed that baseline immobility and climbing time did not differ between KO and corresponding WT mice except for β2 KO. Bupropion significantly decreased immobility time and increased climbing time in the α4, α6 and β2 nAChR KO mice in comparison to WT littermates, indicating that lack of these nAChR subunits enhanced antidepressant effects of bupropion. On the contrary, the α5 nAChR subunit deletion did not alter the FST behavior in the bupropion-treated mice. Not only in the transgenic mice, bupropion also showed antidepressant-like effects in the WT mice. In addition, DHβE pretreatment before bupropion administration resulted in decreased immobility time and increased climbing time. Taken together, the present study provides evidence on the involvement of α4*, α6*, and β2* (* indicates possible presence of other subunits) nAChRs in the antidepressant-like effects of bupropion in the FST.

The prototoxin LYPD6B modulates heteromeric α3β4-containing nicotinic acetylcholine receptors, but not α7 homomers

Prototoxins are a diverse family of membrane-tethered molecules expressed in the nervous system that modulate nicotinic cholinergic signaling, but their functions and specificity have yet to be completely explored. We tested the selectivity and efficacy of leukocyte antigen, PLAUR (plasminogen activator, urokinase receptor) domain-containing (LYPD)-6B on α3β4-, α3α5β4-, and α7-containing nicotinic acetylcholine receptors (nAChRs). To constrain stoichiometry, fusion proteins encoding concatemers of human α3, β4, and α5 (D and N variants) subunits were expressed in Xenopus laevis oocytes and tested with or without LYPD6B. We used the 2-electrode voltage-clamp method to quantify responses to acetylcholine (ACh): agonist sensitivity (EC50), maximal agonist-induced current (Imax), and time constant (τ) of desensitization. For β4-α3-α3-β4-α3 and β4-α3-β4-α3-α3, LYPD6B decreased EC50 from 631 to 79 μM, reduced Imax by at least 59%, and decreased τ. For β4-α3-α5D-β4-α3 and β4-α3-β4-α-α5D, LYPD6B decreased Imax by 63 and 32%, respectively. Thus, LYPD6B acted only on (α3)3(β4)2 and (α3)2(α5D)(β4)2 and did not affect the properties of (α3)2(β4)3, α7, or (α3)2(α5N)(β4)2 nAChRs. Therefore, LYPD6B acts as a mixed modulator that enhances the sensitivity of (α3)3(β4)2 nAChRs to ACh while reducing ACh-induced whole-cell currents. LYPD6B also negatively modulates α3β4 nAChRs that include the α5D common human variant, but not the N variant associated with nicotine dependence.

PACAP induces plasticity at autonomic synapses by nAChR-dependent NOS1 activation and AKAP-mediated PKA targeting

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic neuropeptide found at synapses throughout the central and autonomic nervous system. We previously found that PACAP engages a selective G-protein coupled receptor (PAC1R) on ciliary ganglion neurons to rapidly enhance quantal acetylcholine (ACh) release from presynaptic terminals via neuronal nitric oxide synthase (NOS1) and cyclic AMP/protein kinase A (PKA) dependent processes. Here, we examined how PACAP stimulates NO production and targets resultant outcomes to synapses. Scavenging extracellular NO blocked PACAP-induced plasticity supporting a retrograde (post- to presynaptic) NO action on ACh release. Live-cell imaging revealed that PACAP stimulates NO production by mechanisms requiring NOS1, PKA and Ca(2+) influx. Ca(2+)-permeable nicotinic ACh receptors composed of α7 subunits (α7-nAChRs) are potentiated by PKA-dependent PACAP/PAC1R signaling and were required for PACAP-induced NO production and synaptic plasticity since both outcomes were drastically reduced following their selective inhibition. Co-precipitation experiments showed that NOS1 associates with α7-nAChRs, many of which are perisynaptic, as well as with heteromeric α3*-nAChRs that generate the bulk of synaptic activity. NOS1-nAChR physical association could facilitate NO production at perisynaptic and adjacent postsynaptic sites to enhance focal ACh release from juxtaposed presynaptic terminals. The synaptic outcomes of PACAP/PAC1R signaling are localized by PKA anchoring proteins (AKAPs). PKA regulatory-subunit overlay assays identified five AKAPs in ganglion lysates, including a prominent neuronal subtype. Moreover, PACAP-induced synaptic plasticity was selectively blocked when PKA regulatory-subunit binding to AKAPs was inhibited. Taken together, our findings indicate that PACAP/PAC1R signaling coordinates nAChR, NOS1 and AKAP activities to induce targeted, retrograde plasticity at autonomic synapses. Such coordination has broad relevance for understanding the control of autonomic synapses and consequent visceral functions.

Feasibility of [18F]-2-Fluoro-A85380-PET imaging of human vascular nicotinic acetylcholine receptors in vivo

OBJECTIVES

The aim of this feasibility study was to evaluate [(18)F]-2-Fluoro-A85380 for in vivo imaging of arterial nicotinic acetylcholine receptors (nAChRs) in humans. Furthermore, potentially different vascular uptake patterns of this new tracer were evaluated in healthy volunteers and in patients with neurodegenerative disorders.

BACKGROUND

[(18)F]-2-Fluoro-A85380 was developed for in vivo positron emission tomography (PET) imaging of nAChR subunits in the human brain. These nAChRs are also found in arteries and seem to mediate the deleterious effects of nicotine as a part of tobacco smoke in the vasculature. It has been previously shown that uptake patterns of the radiotracer in the brain differs in patients with neurodegenerative disorders compared with healthy controls.

METHODS

[(18)F]-2-Fluoro-A85380 uptake was quantified in the ascending and descending aorta, the aortic arch, and the carotids in 5 healthy volunteers and in 6 patients with either Parkinson's disease or multiple system atrophy, respectively, as the maximum target-to-background ratio. The maximal standardized uptake value values, the single hottest segment, and the percent active segments of the [(18)F]-2-Fluoro-A85380 uptake in the arteries were also assessed.

RESULTS

[(18)F]-2-Fluoro-A85380 uptake was clearly visualized and maximum target-to-background ratio uptake values corrected for the background activity of the tracer showed specific tracer uptake in the arterial walls. Significantly higher uptake values were found in the descending aorta. Comparison between volunteers and patients revealed significant differences, with lower [(18)F]-2-Fluoro-A85380 uptake in the patient group when comparing single arterial territories but not when all arterial territories were pooled together.

CONCLUSIONS

[(18)F]-2-Fluoro-A85380 can provide specific information on the nAChR distribution in human arteries. Vascular nAChR density seems to be lower in patients with Parkinson's disease or multiple system atrophy. Once confirmed in larger study populations and in the experimental setting, this approach might provide insights into the pathogenic role of nAChRs in the human vasculature.

Function of human α3β4α5 nicotinic acetylcholine receptors is reduced by the α5(D398N) variant

Genome-wide studies have strongly associated a non-synonymous polymorphism (rs16969968) that changes the 398th amino acid in the nAChR α5 subunit from aspartic acid to asparagine (D398N), with greater risk for increased nicotine consumption. We have used a pentameric concatemer approach to express defined and consistent populations of α3β4α5 nAChR in Xenopus oocytes. α5(Asn-398; risk) variant incorporation reduces ACh-evoked function compared with inclusion of the common α5(Asp-398) variant without altering agonist or antagonist potencies. Unlinked α3, β4, and α5 subunits assemble to form a uniform nAChR population with pharmacological properties matching those of concatemeric α3β4* nAChRs. α5 subunit incorporation reduces α3β4* nAChR function after coinjection with unlinked α3 and β4 subunits but increases that of α3β4α5 versus α3β4-only concatemers. α5 subunit incorporation into α3β4* nAChR also alters the relative efficacies of competitive agonists and changes the potency of the non-competitive antagonist mecamylamine. Additional observations indicated that in the absence of α5 subunits, free α3 and β4 subunits form at least two further subtypes. The pharmacological profiles of these free subunit α3β4-only subtypes are dissimilar both to each other and to those of α3β4α5 nAChR. The α5 variant-induced change in α3β4α5 nAChR function may underlie some of the phenotypic changes associated with this polymorphism.

Impact of human D398N single nucleotide polymorphism on intracellular calcium response mediated by α3β4α5 nicotinic acetylcholine receptors

The human CHRNA5 D398N polymorphism (rs16969968) causes an aspartic acid to asparagine change in the nicotinic acetylcholine receptor (nAChR) α5 subunit gene. The N398 variant of CHRNA5 is linked to increased risk for nicotine dependence. In this study, we explored the effect of the CHRNA5 D398N polymorphism on the properties of human α3β4* nicotinic acetylcholine receptors in human embryonic kidney (HEK) cells. Addition of either D398 or N398 variant of α5 subunit in the α3β4* receptor did not affect total [(125)I]-epibatidine binding or surface expression of the receptor. However, addition of α5(D398) into α3β4* receptor decreased the maximal response to agonist without significantly affecting EC(50) in aequorin intracellular calcium assay. α3β4α5(N398) nAChRs showed further decreased maximal response. The differences in agonist efficacy between the receptor subtypes were found to be dependent upon the concentration of external calcium but independent of external sodium. Moreover, activation of α3β4α5 nAChRs led to significantly greater intracellular calcium release from IP(3) stores relative to α3β4 nAChRs although no effect of the α5 polymorphism was observed. Finally, inclusion of the α5 variant caused a small shift to the left in IC(50) for some of the antagonists tested, depending upon α5 variant but did not affect sensitivity of α3β4* receptors to desensitization in response to incubation with nicotine. In conclusion, addition of either variant of α5 into an α3β4α5 receptor similarly effects receptor pharmacology and function. However, the N398 variant exhibits a reduced response to agonists when extracellular calcium is high and it may lead to distinct downstream cellular signaling.

Generation of candidate ligands for nicotinic acetylcholine receptors via in situ click chemistry with a soluble acetylcholine binding protein template

Nicotinic acetylcholine receptors (nAChRs), which are responsible for mediating key physiological functions, are ubiquitous in the central and peripheral nervous systems. As members of the Cys loop ligand-gated ion channel family, neuronal nAChRs are pentameric, composed of various permutations of α (α2 to α10) and β (β2 to β4) subunits forming functional heteromeric or homomeric receptors. Diversity in nAChR subunit composition complicates the development of selective ligands for specific subtypes, since the five binding sites reside at the subunit interfaces. The acetylcholine binding protein (AChBP), a soluble extracellular domain homologue secreted by mollusks, serves as a general structural surrogate for the nAChRs. In this work, homomeric AChBPs from Lymnaea and Aplysia snails were used as in situ templates for the generation of novel and potent ligands that selectively bind to these proteins. The cycloaddition reaction between building-block azides and alkynes to form stable 1,2,3-triazoles was used to generate the leads. The extent of triazole formation on the AChBP template correlated with the affinity of the triazole product for the nicotinic ligand binding site. Instead of the in situ protein-templated azide-alkyne cycloaddition reaction occurring at a localized, sequestered enzyme active center as previously shown, we demonstrate that the in situ reaction can take place at the subunit interfaces of an oligomeric protein and can thus be used as a tool for identifying novel candidate nAChR ligands. The crystal structure of one of the in situ-formed triazole-AChBP complexes shows binding poses and molecular determinants of interactions predicted from structures of known agonists and antagonists. Hence, the click chemistry approach with an in situ template of a receptor provides a novel synthetic avenue for generating candidate agonists and antagonists for ligand-gated ion channels.

Functional characterization of the α5(Asn398) variant associated with risk for nicotine dependence in the α3β4α5 nicotinic receptor

Smoking is a major cause for premature death. Work aimed at identifying genetic factors that contribute to nicotine addiction has revealed several single nucleotide polymorphisms (SNPs) that are linked to smoking-related behaviors such as nicotine dependence and level of smoking. One of these SNPs leads to an aspartic acid-to-asparagine substitution in the nicotinic receptor α5 subunit at amino acid position 398 [rs16969968; α5(Asn398)]. The α5 subunit is expressed both in the brain and in the periphery. In the brain, it associates with the α4 and β2 subunits to form α4β2α5 receptors. In the periphery, the α5 subunit combines with the α3 and β4 subunits to form the major ganglionic postsynaptic nicotinic receptor subtype. The α3β4α5 receptor regulates a variety of autonomic responses such as control of cardiac rate, blood pressure, and perfusion. In this paradigm, the α5(Asn398) variant may act by regulating autonomic responses that may affect nicotine intake by humans. Here, we have investigated the effect of the α5(Asn398) variant on the function of the α3β4α5 receptor. The wild-type or variant α5 subunits were coexpressed with the α3 and β4 subunits in human embryonic kidney 293 cells. The properties of the receptors were studied using whole-cell and single-channel electrophysiology. The data indicate that the introduction of the α5(Asn398) mutation has little effect on the pharmacology of receptor activation, receptor desensitization, or single-channel properties. We propose that the effect of the α5(Asn398) variant on nicotine use is not mediated by an action on the physiological or pharmacological properties of the α3β4α5 subtype.

Naturally-expressed nicotinic acetylcholine receptor subtypes

Nicotinic acetylcholine receptors (nAChRs) warrant attention, as they play many critical roles in brain and body function and have been implicated in a number of neurological and psychiatric disorders, including nicotine dependence. nAChRs are composed as diverse subtypes containing specific combinations of genetically-distinct subunits and that have different functional properties, distributions, and pharmacological profiles. There had been confidence that the rules that define ranges of assembly partners for specific subunits were well-established, especially for the more prominent nAChR subtypes. However, we review here some newer findings indicating that nAChRs having largely the same, major subunits exist as isoforms with unexpectedly different properties. Moreover, we also summarize our own studies indicating that novel nAChR subtypes exist and/or have distributions not heretofore described. Importantly, the nAChRs that exist as new isoforms or subtypes or have interesting distributions require alteration in thinking about their roles in health and disease.

Desensitization of neurotransmitter-gated ion channels during high-frequency stimulation: a comparative study of Cys-loop, AMPA and purinergic receptors

Changes in synaptic strength allow synapses to regulate the flow of information in the neural circuits in which they operate. In particular, changes lasting from milliseconds to minutes (‘short-term changes') underlie a variety of computational operations and, ultimately, behaviours. Most studies thus far have attributed the short-term type of plasticity to activity-dependent changes in the dynamics of neurotransmitter release (a presynaptic mechanism) while largely dismissing the role of the loss of responsiveness of postsynaptic receptor channels to neurotransmitter owing to entry into desensitization. To better define the response of the different neurotransmitter-gated ion channels (NGICs) to repetitive stimulation without interference from presynaptic variables, we studied eight representative members of all three known superfamilies of NGICs in fast-perfused outside-out patches of membrane. We found that the responsiveness of all tested channels (two nicotinic acetylcholine receptors, two glycine receptors, one GABA receptor, two AMPA-type glutamate receptors and one purinergic receptor) declines along trains of brief neurotransmitter pulses delivered at physiologically relevant frequencies to an extent that suggests that the role of desensitization in the synaptic control of action-potential transmission may be more general than previously thought. Furthermore, our results indicate that a sizable fraction (and, for some NGICs, most) of this desensitization occurs during the neurotransmitter-free interpulse intervals. Clearly, an incomplete clearance of neurotransmitter from the synaptic cleft between vesicle-fusion events need not be invoked to account for NGIC desensitization upon repetitive stimulation.

A transcriptional regulatory element critical for CHRNB4 promoter activity in vivo

Genome-wide association studies have underscored the importance of the clustered neuronal nicotinic acetylcholine receptor (nAChR) subunit genes with respect to nicotine dependence as well as lung cancer susceptibility. CHRNB4, which encodes the nAChR β4 subunit, plays a major role in the molecular mechanisms that govern nicotine withdrawal. Thus, elucidating how expression of the β4 gene is regulated is critical for understanding the pathophysiology of nicotine addiction. We previously identified a CA box regulatory element, (5'-CCACCCCT-3') critical for β4 promoter activity in vitro. We further demonstrated that a 2.3-kb fragment of the β4 promoter region containing the 5'-CCACCCCT-3' regulatory element in the β4 gene promoter (CA box) is capable of directing cell-type specific expression of a reporter gene to a myriad of brain regions that endogenously express the β4 gene. To test the hypothesis that the CA box is critical for β4 promoter activity in vivo, transgenic animals expressing a mutant form of the β4 promoter were generated. Reporter gene expression was not detected in any tissue or cell type at embryonic day 18.5 (ED 18.5). Similarly, we observed drastically reduced reporter gene expression at postnatal day 30 (PD30) when compared to wild type (WT) transgenic animals. Finally, we demonstrated that CA box mutation results in decreased interaction of the transcription factor Sp1 with the mutant β4 promoter. Taken together these results demonstrate that the CA box is critical for β4 promoter activity in vivo.

Lateral mobility of nicotinic acetylcholine receptors on neurons is determined by receptor composition, local domain, and cell type

The lateral mobility of surface receptors can define the signaling properties of a synapse and rapidly change synaptic function. Here we use single-particle tracking with Quantum Dots to follow nicotinic acetylcholine receptors (nAChRs) on the surface of chick ciliary ganglion neurons in culture. We find that both heteropentameric alpha3-containing receptors (alpha3*-nAChRs) and homopentameric alpha7-containing receptors (alpha7-nAChRs) access synaptic domains by lateral diffusion. They have comparable mobilities and display Brownian motion in extrasynaptic space but are constrained and move more slowly in synaptic space. The two receptor types differ in the nature of their synaptic restraints. Disruption of lipid rafts, PDZ-containing scaffolds, and actin filaments each increase the mobility of alpha7-nAChRs in synaptic space while collapse of microtubules has no effect. The opposite is seen for alpha3*-nAChRs where synaptic mobility is increased only by microtubule collapse and not the other manipulations. Other differences are found for regulation of alpha3*-nAChR and alpha7-nAChR mobilities in extrasynaptic space. Most striking are effects on the immobile populations of alpha7-nAChRs and alpha3*-nAChRs. Disruption of either lipid rafts or PDZ scaffolds renders half of the immobile alpha3*-nAChRs mobile without changing the proportion of immobile alpha7-nAChRs. Similar results were obtained with chick sympathetic ganglion neurons, though regulation of receptor mobility differed in at least one respect from that seen with ciliary ganglion neurons. Control of nAChR lateral mobility, therefore, is determined by mechanisms that are domain specific, receptor subtype dependent, and cell-type constrained. The outcome is a system that could tailor nicotinic signaling capabilities to specific needs of individual locations.

The nicotinic acetylcholine receptor CHRNA5/A3/B4 gene cluster: dual role in nicotine addiction and lung cancer

More than 1 billion people around the world smoke, with 10 million cigarettes sold every minute. Cigarettes contain thousands of harmful chemicals including the psychoactive compound, nicotine. Nicotine addiction is initiated by the binding of nicotine to nicotinic acetylcholine receptors, ligand-gated cation channels activated by the endogenous neurotransmitter, acetylcholine. These receptors serve as prototypes for all ligand-gated ion channels and have been extensively studied in an attempt to elucidate their role in nicotine addiction. Many of these studies have focused on heteromeric nicotinic acetylcholine receptors containing α4 and β2 subunits and homomeric nicotinic acetylcholine receptors containing the α7 subunit, two of the most abundant subtypes expressed in the brain. Recently however, a series of linkage analyses, candidate-gene analyses and genome-wide association studies have brought attention to three other members of the nicotinic acetylcholine receptor family: the α5, α3 and β4 subunits. The genes encoding these subunits lie in a genomic cluster that contains variants associated with increased risk for several diseases including nicotine dependence and lung cancer. The underlying mechanisms for these associations have not yet been elucidated but decades of research on the nicotinic receptor gene family as well as emerging data provide insight on how these receptors may function in pathological states. Here, we review this body of work, focusing on the clustered nicotinic acetylcholine receptor genes and evaluating their role in nicotine addiction and lung cancer.

PACAP/PAC1R signaling modulates acetylcholine release at neuronal nicotinic synapses

Neuropeptides collaborate with conventional neurotransmitters to regulate synaptic output. Pituitary adenylate cyclase-activating polypeptide (PACAP) co-localizes with acetylcholine in presynaptic nerve terminals, is released by stimulation, and enhances nicotinic acetylcholine receptor- (nAChR-) mediated responses. Such findings implicate PACAP in modulating nicotinic neurotransmission, but relevant synaptic mechanisms have not been explored. We show here that PACAP acts via selective high-affinity G-protein coupled receptors (PAC(1)Rs) to enhance transmission at nicotinic synapses on parasympathetic ciliary ganglion (CG) neurons by rapidly and persistently increasing the frequency and amplitude of spontaneous, impulse-dependent nicotinic excitatory postsynaptic currents (sEPSCs). Of the canonical adenylate cyclase (AC) and phospholipase-C (PLC) transduction cascades stimulated by PACAP/PAC(1)R signaling, only AC-generated signals are critical for synaptic modulation since the increases in sEPSC frequency and amplitude were mimicked by 8-Bromo-cAMP, blocked by inhibiting AC or cAMP-dependent protein kinase (PKA), and unaffected by inhibiting PLC. Despite its ability to increase agonist-induced nAChR currents, PACAP failed to influence nAChR-mediated impulse-independent miniature EPSC amplitudes (quantal size). Instead, evoked transmission assays reveal that PACAP/PAC(1)R signaling increased quantal content, indicating that it modulates synaptic function by increasing vesicular ACh release from presynaptic terminals. Lastly, signals generated by the retrograde messenger, nitric oxide- (NO-) are critical for the synaptic modulation since the PACAP-induced increases in spontaneous EPSC frequency, amplitude and quantal content were mimicked by NO donor and absent after inhibiting NO synthase (NOS). These results indicate that PACAP/PAC(1)R activation recruits AC-dependent signaling that stimulates NOS to increase NO production and control presynaptic transmitter output at neuronal nicotinic synapses.

Nicotinic receptors concentrated in the subsynaptic membrane do not contribute significantly to synaptic currents at an embryonic synapse in the chicken ciliary ganglion

Rapid synaptic transmission at the calyciform synapse in the embryonic chicken ciliary ganglion is mediated by two classes of nicotinic receptors: those containing alpha3 subunits [alpha3-nicotinic ACh receptors (nAChRs)] and those containing alpha7 subunits (alpha7-nAChRs). alpha3-nAChRs and alpha7-nAChRs are differentially distributed on the cell surface; alpha3-nAChRs are concentrated at postsynaptic densities, whereas both alpha7-nAChRs and alpha3-nAChRs are found extrasynaptically on somatic spines. I explored the contribution of alpha3-nAChRs and alpha7-nAChRs to uniquantal responses, measured as mEPSCs, or as evoked responses under low release probability conditions. The contribution that each nAChR makes to uniquantal response shape was determined by blocking one nAChR type; pharmacologically isolated alpha7-nAChR responses were kinetically fast (rise time, 0.32 +/- 0.02 ms; decay time, 1.66 +/- 0.18 ms; mean +/- SD; n = 6 cells), whereas pharmacologically isolated alpha3-nAChR responses were slow (rise time, 1.28 +/- 0.35 ms; decay time, 6.71 +/- 1.46 ms; n = 8 cells). In the absence of antagonists, most cells (11 of 14) showed heterogeneity in the kinetics of uniquantal responses, with approximately 25% of events exhibiting fast, alpha7-nAChR-like kinetics and approximately 75% of events exhibiting the kinetics expected of coactivation of alpha7-nAChRs and alpha3-nAChRs. Cells rarely showed significant numbers of uniquantal responses with slow, alpha3-nAChR-like kinetics, which was unexpected given that alpha3-nAChRs alone are concentrated at postsynaptic densities. The only site where ACh quanta can activate both alpha3-nAChRs and alpha7-nAChRs readily is on the somatic spines, where alpha7-nAChRs and alpha3-nAChRs are present extrasynaptically. At the calyciform synapse, rapid synaptic transmission is mediated apparently without participation of ionotropic receptors concentrated at postsynaptic densities.

Nicotinic acetylcholine receptor subunit mRNA expression in adult and developing rat medullary catecholamine neurons

Nicotinic acetylcholine receptors (nAChRs) mediate numerous visceral functions via medullary catecholamine (CA) neurons found in the nucleus tractus solitarius (NTS), dorsal motor nucleus of the vagus (DMV), and ventrolateral medulla (VLM). However, the nAChR subtypes involved are not known. We have therefore characterized expression of nine nAChR subunit mRNAs in adult and developing rat medullary CA nuclei using combined isotopic/nonisotopic in situ hybridization. Tyrosine hydroxylase (TH) mRNA, the CA-synthesizing enzyme, was used as a marker for CA neurons, because these nuclei consist of heterogeneous populations of cells. Subunit mRNA expression varied within and between nuclei, along the rostrocaudal axis, between cell types, and across development. All CA neurons expressed beta2 mRNA, whereas alpha2 mRNA was completely absent. alpha6 And beta3 mRNA expression were restricted mainly to the VLM. alpha4, alpha5, And alpha7 mRNA expression was significantly greater in the rostral than in the caudal VLM. alpha3 And beta4 mRNAs were highly expressed in the dorsal region of the NTS, whereas dense alpha7 mRNA expression was restricted to the DMV and ventral NTS. The remaining subunit mRNAs were detected to some degree in both DMV and NTS. Except for alpha4 mRNA, which peaked prenatally, expression levels of subunit transcripts in the NTS and DMV were lower during development compared with adults. In the VLM, alpha3, alpha4, and alpha5 mRNAs expression peaked perinatally, whereas alpha6 and beta3 levels increased with age. These variations in nAChR subunit mRNA expression suggest that different receptor subtypes may produce function-specific regulation of medullary CA systems.

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.

Cognitive deficits in schizophrenia: focus on neuronal nicotinic acetylcholine receptors and smoking

Patients with schizophrenia present with deficits in specific areas of cognition. These are quantifiable by neuropsychological testing and can be clinically observable as negative signs. Concomitantly, they self-administer nicotine in the form of cigarette smoking. Nicotine dependence is more prevalent in this patient population when compared to other psychiatric conditions or to non-mentally ill people. The target for nicotine is the neuronal nicotinic acetylcholine receptor (nAChR). There is ample evidence that these receptors are involved in normal cognitive operations within the brain. This review describes neuronal nAChR structure and function, focusing on both cholinergic agonist-induced nAChR desensitization and nAChR up-regulation. The several mechanisms proposed for the nAChR up-regulation are examined in detail. Desensitization and up-regulation of nAChRs may be relevant to the physiopathology of schizophrenia. The participation of several subtypes of neuronal nAChRs in the cognitive processing of non-mentally ill persons and schizophrenic patients is reviewed. The role of smoking is then examined as a possible cognitive remediator in this psychiatric condition. Finally, pharmacological strategies focused on neuronal nAChRs are discussed as possible therapeutic avenues that may ameliorate the cognitive deficits of schizophrenia.

T cells express alpha7-nicotinic acetylcholine receptor subunits that require a functional TCR and leukocyte-specific protein tyrosine kinase for nicotine-induced Ca2+ response

Acute and chronic effects of nicotine on the immune system are usually opposite; acute treatment stimulates while chronic nicotine suppresses immune and inflammatory responses. Nicotine acutely raises intracellular calcium ([Ca(2+)](i)) in T cells, but the mechanism of this response is unclear. Nicotinic acetylcholine receptors (nAChRs) are present on neuronal and non-neuronal cells, but while in neurons, nAChRs are cation channels that participate in neurotransmission; their structure and function in nonexcitable cells are not well-defined. In this communication, we present evidence that T cells express alpha7-nAChRs that are critical in increasing [Ca(2+)](i) in response to nicotine. Cloning and sequencing of the receptor from human T cells showed a full-length transcript essentially identical to the neuronal alpha7-nAChR subunit (>99.6% homology). These receptors are up-regulated and tyrosine phosphorylated by treatment with nicotine, anti-TCR Abs, or Con A. Furthermore, knockdown of the alpha7-nAChR subunit mRNA by RNA interference reduced the nicotine-induced Ca(2+) response, but unlike the neuronal receptor, alpha-bungarotoxin and methyllycaconitine not only failed to block, but also actually raised [Ca(2+)](i) in T cells. The nicotine-induced release of Ca(2+) from intracellular stores in T cells did not require extracellular Ca(2+), but, similar to the TCR-mediated Ca(2+) response, required activation of protein tyrosine kinases, a functional TCR/CD3 complex, and leukocyte-specific tyrosine kinase. Moreover, CD3zeta and alpha7-nAChR co-immunoprecipitated with anti-CD3zeta or anti-alpha7-nAChR Abs. These results suggest that in T cells, alpha7-nAChR, despite its close sequence homology with neuronal alpha7-nAChR, fails to form a ligand-gated Ca(2+) channel, and that the nicotine-induced rise in [Ca(2+)](i) in T cells requires functional TCR/CD3 and leukocyte-specific tyrosine kinase.

Nicotinic alpha5 subunit deletion locally reduces high-affinity agonist activation without altering nicotinic receptor numbers

Neuronal nicotinic acetylcholine receptor subunit alpha5 mRNA is widely expressed in the CNS. An alpha5 gene polymorphism has been implicated in behavioral differences between mouse strains, and alpha5-null mutation induces profound changes in mouse acute responses to nicotine. In this study, we have examined the distribution and prevalence of alpha5* nicotinic acetylcholine receptor in mouse brain, and quantified the effects of alpha5-null mutation on pre-synaptic nicotinic acetylcholine receptor function (measured using synaptosomal (86)Rb(+) efflux) and overall [(125)I]epibatidine binding site expression. alpha5* nicotinic acetylcholine receptor expression was found in nine of fifteen regions examined, although < 20% of the total nicotinic acetylcholine receptor population in any region contained alpha5. Deletion of the alpha5 subunit gene resulted in localized loss of function (thalamus, striatum), which was itself confined to the DHbetaE-sensitive receptor population. No changes in receptor expression were seen. Consequently, functional changes must occur as a result of altered function per unit of receptor. The selective depletion of high agonist activation affinity sites results in overall nicotinic function being reduced, and increases the overall agonist activation affinity. Together, these results describe the receptor-level changes underlying altered behavioral responses to nicotine in nicotinic acetylcholine receptor alpha5 subunit-null mutants.

Transcriptional Repression by a Conserved Intronic Sequence in the Nicotinic Receptor α3 Subunit Gene

The genes encoding the nicotinic acetylcholine receptor alpha3, alpha5, and beta4 subunits are genomically clustered. These genes are co-expressed in a variety of cells in the peripheral and central nervous systems. Their gene products assemble in a number of stoichiometries to generate several nicotinic receptor subtypes that have distinct pharmacological and physiological properties. Signaling through these receptors is critical for a variety of fundamental biological processes. Despite their importance, the transcriptional mechanisms underlying their coordinated expression remain to be completely elucidated. By using a bioinformatics approach, we identified a highly conserved intronic sequence within the fifth intron of the alpha3 subunit gene. Reporter gene analysis demonstrated that this sequence, termed "alpha3 intron 5," inhibits the transcriptional activities of the alpha3 and beta4 subunit gene promoters. This repressive activity is position- and orientation-independent. Importantly, repression occurs in a cell type-specific manner, being present in cells that do not express the receptor genes or expresses them at very low levels. Electrophoretic mobility shift assays demonstrate that nuclear proteins specifically interact with alpha3 intron 5 at two distinct sites. We propose that this intronic repressor element is important for the restricted expression patterns of the nicotinic receptor alpha3 and beta4 subunit genes.

Reversible inhibition of GABAA receptors by alpha7-containing nicotinic receptors on the vertebrate postsynaptic neurons

Nicotinic acetylcholine receptors (nAChRs) are expressed throughout the central nervous system and influence a variety of higher order functions including learning and memory. While the effects of presynaptic nAChRs on transmitter release have been well documented, little is known about possible postsynaptic actions. A major species of neuronal nAChRs contains the alpha7 gene product and has a high relative permeability to calcium. Both on rodent hippocampal interneurons and on chick ciliary ganglion neurons these alpha7-nAChRs are often closely juxtaposed to GABAA receptors. We show here that in both cases activation of alpha7-nAChRs on the postsynaptic neuron acutely down-regulates GABA-induced currents. Nicotine application to dissociated ciliary ganglion neurons diminished subsequent GABAA receptor responses to GABA. The effect was blocked by alpha7-nAChR antagonists, by chelation of intracellular Ca2+ with BAPTA, and by inhibition of both Ca2+-calmodulin-dependent protein kinase II and mitogen-activated protein kinase. A similar outcome was obtained in the hippocampus where electrical stimulation to activate cholinergic fibres reduced the amplitude of subsequent GABAA receptor-mediated inhibitory postsynaptic currents. The reduction showed the same calcium and kinase dependence seen in ciliary ganglion neurons and was absent in hippocampal slices from alpha7-nAChR knockout mice. Moreover, alpha7-nAChR blockade in hippocampal slices reduced rundown of GABAA receptor-mediated whole-cell responses, indicating ongoing endogenous modulation. The results demonstrate regulation of GABAA receptors by alpha7-nAChRs on the postsynaptic neuron and identify a new mechanism by which nicotinic cholinergic signalling influences nervous system function.

Heterogeneity of nicotinic cholinergic receptors in rat superior cervical and nodose Ganglia

Nicotinic cholinergic receptors (nAChRs) are present in ganglia in the peripheral nervous system. In autonomic ganglia, they are responsible for fast synaptic transmission, whereas in the sensory ganglia and sensory neurons, they may be involved in modulation of neurotransmission. The present study measured nAChRs in several rat autonomic ganglia: the superior cervical ganglia (SCG), sensory nodose ganglia, stellate ganglia, and pelvic ganglia. The densities of the heteromeric nAChRs determined by receptor binding assay in those four ganglia are 481, 45, 9, and 11 fmol/mg protein, respectively. Immunoprecipitation studies with subunit-specific antibodies showed that a majority of the nAChRs in the SCG and nodose ganglia contain the alpha3 and beta4 subunits, but a significant percentage of the nAChRs in these ganglia also contain alpha5 and beta2 subunits. A small percentage of the nAChRs in nodose ganglia also contain alpha2 and alpha4 subunits. Sequential immunoprecipitation assays indicated that in the SCG, all alpha5 subunits are associated with alpha3 and beta4 subunits, forming the mixed heteromeric alpha3beta4alpha5 subtype. A receptor composed of alpha3, beta2, and beta4 subunits in the SCG was also detected. In rat SCG, we found the following distribution of nAChRs subtypes: 55 to 60% simple alpha3beta4 subtype, 25 to 30% alpha3beta4alpha5 subtype, and 10 to 15% alpha3beta4beta2 subtype. These findings indicate that the nAChRs in SCG and nodose ganglia are heterogeneous, which suggests that different receptor subtypes may play different roles in these ganglia or may be activated under different conditions.

Ca2+ flux and signaling implications by nicotinic acetylcholine receptors in rat medial habenula

The fraction of inward current carried by Ca(2+) (FCa(2+)) through nicotinic acetylcholine receptors (nAChRs) on acutely isolated rat medial habenula (MHb) neurons was calculated from experiments that simultaneously monitored agonist-induced membrane currents and intracellular [Ca(2+)], measured with patch-clamp and indo-1 fluorescence, respectively. In physiological concentrations of extracellular Ca(2+) (2 mM) at -50 mV, the percentage of current carried by Ca(2+) was determined to be roughly 3-4%, which is in close agreement with measurements from other heteromeric nicotinic receptors expressed in peripheral tissue. Among factors that may have affected this measurement, such as Ca(2+) influx through voltage-gated Ca(2+) channels, the concentration of intracellular Ca(2+) buffer, and Ca(2+) sequestration and release from intracellular stores, only Ca(2+) uptake by mitochondria was shown to confound the analysis. Furthermore, we find that because of the high density of nAChRs on MHb cells, low concentrations of ACh (10 microM) and its hydrolysis product, choline (1 mM), can significantly elevate intracellular Ca(2+). Moreover, during persistent activation of nAChRs, the level of intracellular Ca(2+) is proportional to its extracellular concentration in the physiological range. Together, these findings support the suggestion that nAChRs may be capable of sensing low concentrations of diffusely released neurotransmitter and, in addition, transfer information about ongoing local synaptic activity by changes in extracellular Ca(2+).

Pre- and postsynaptic actions of L1-CAM in nicotinic pathways

Cell adhesion molecules (CAMs) have long been known to guide axon outgrowth and pathfinding. More recent evidence indicates they contribute to synapse formation as well. The L1 family of IgCAMs has been implicated in long-term potentiation, learning, and some features of synaptic structure. We show here that L1 is localized in nicotinic pathways at both pre- and postsynaptic sites. In the chick ciliary ganglion, postsynaptic L1 is associated with nicotinic receptors and potentiates their downstream signaling. Postsynaptic L1 is also important for aligning presynaptic structures over the postsynaptic cell. Dominant negative experiments suggest this latter effect depends on homophilic interactions with presynaptic L1. At the neuromuscular junction L1 is also found presynaptically where dominant negative experiments again indicate a role in aligning presynaptic structures over postsynaptic receptors, both in culture and in vivo. These findings identify new roles for L1 at nicotinic synapses and underscore the multipotency of L1-CAMs.

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.

Feasibility of 2-deoxy-2-[18F]fluoro-D-glucose- A85380-PET for imaging of human cardiac nicotinic acetylcholine receptors in vivo

Nicotinic acetylcholine receptors mediate the parasympathetic autonomic control of cardiac function. Aim of this study was the assessment of cardiac nicotinic acetylcholine receptor distribution with a novel (alpha4beta2) nicotinic acetylcholine receptor PET ligand (2-deoxy-2- [18F]fluoro-D-glucose-A85380) in humans. Five healthy volunteers without cardiac disease and six patients with either Parkinson's disease or multiple system atrophy without additional overt cardiac disease were evaluated with 2-deoxy-2-[18F]fluoro-D-glucose-A85380 PET-imaging to assess the cardiac parasympathetic innervation and the putative impact of both disorders. 2-deoxy-2- [18F]fluoro-D-glucose-A85380 whole body PET-scans were performed on a Siemens PET/CT biograph(TM) 75.4 min +/- 6.7 after i.v. injection of 371.2 +/- 58.1 MBq. Average count rate density of left ventricle ROI's and a standard ROI in the right lung were measured within three consecutive slices of 10.0 mm thickness. Heart-to-lung ratios were calculated in each volunteer and patient. Tracer uptake in the left ventricle could be measured in all of the five volunteers and the six patients. Heart-to-lung ratios in the volunteer group were not different from patients suffering from Parkinson's disease or MSA (3.2 +/- 0.5 vs 3.2 +/- 0.8 and 2.96+/-0.7, mean +/- SD), respectively. Human cardiac nicotinic acetylcholine receptors can be visualized and measured by 2-deoxy-2- [18F]fluoro-D-glucose-A85380 PET scans both in cardiac-healthy subjects and patients suffering from Parkinson's disease or multiple system atrophy. The heart- as well as the lung-tracer uptake was almost constant throughout all subjects leading to a good target-to-background ratio. These first results suggest no impact of either PD or MSA on cardiac nicotinic acetylcholine receptors.

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.

Neuronal nicotinic receptors as targets for novel analgesics

The potential use of nicotinic acetylcholine receptor agonists has been the subject of a number of recent reviews. Despite the promises of better things to come, few new compounds have been identified that circumvent the issues hindering the widespread use of the previously described nicotinic analgesics, mainly a narrow therapeutic window between analgesic efficacy and toxicity, and a lack of knowledge of native nicotinic acetylcholine receptor expression. However, several recent developments have potentially opened new windows of opportunity in the use of nicotinic agents for analgesia. A small number of laboratories have reported that peripheral nerve injury alters the pharmacology of nicotinic receptors, resulting in a leftward shift of analgesic potency but not of toxicity. Another important development in the pathophysiology of neuropathic pain is the reliance of nerve injury-induced behavioural hypersensitivity on both peripheral and central neural immune interactions. Finally, the reported neuroprotective effects of nicotine following spinal cord injury may provide an opportunity for the development of selective nicotinic agonists that are capable of attenuating chronic pain. The current review will attempt to highlight these recent developments and outline key findings that demonstrate further opportunity for the development of nicotinic agonists as novel analgesics.

The effects of subunit composition on the inhibition of nicotinic receptors by the amphipathic blocker 2,2,6,6-tetramethylpiperidin-4-yl heptanoate

The therapeutic targeting of nicotinic receptors in the brain will benefit from the identification of drugs that may be selective for their ability to activate or inhibit a limited range of nicotine acetylcholine receptor subtypes. In the present study, we describe the effects of 2,2,6,6-tetramethylpiperidin-4-yl heptanoate (TMPH), a novel compound that is a potent inhibitor of neuronal nicotinic receptors. Evaluation of nicotinic acetylcholine receptor (nAChR) subunits expressed in Xenopus laevis oocytes indicated that TMPH can produce a potent and long-lasting inhibition of neuronal nAChR formed by the pairwise combination of the most abundant neuronal alpha (i.e., alpha3 and alpha4) and beta subunits (beta2 and beta4), with relatively little effect, because of rapid reversibility of inhibition, on muscle-type (alpha1beta1gammadelta) or alpha7 receptors. However, the inhibition of neuronal beta subunit-containing receptors was also decreased if any of the nonessential subunits alpha5, alpha6, or beta3 were coexpressed. This decrease in inhibition is shown to be associated with a single amino acid present in the second transmembrane domain of these subunits. Our data indicate great potential utility for TMPH to help relate the diverse central nervous system effects to specific nAChR subtypes.

Nicotinic acetylcholine receptors in rat and human placenta

Smoking during pregnancy causes low birth weight, premature delivery, neonatal morbidity, and mortality. Nicotine is a main pathogenic compound of cigarette smoke, and depresses active amino-acid uptake by human placental villi. It binds to the acetylcholine binding site of the alpha-subunits of nicotinic acetylcholine receptors (nAChR). Eight different neuronal nAChR alpha-subunits have been identified in mammals. Here, we investigated their localisation and distribution in the human and rat placenta by RT-PCR and immunofluorescence. The mRNAs of all alpha-subunits are expressed in the human and rat placenta. Immunohistochemically, subunits alpha2-5, alpha7, alpha9 and alpha10 are localised in different combinations in rat cytotrophoblast, human and rat syncytiotrophoblast, vascular smooth muscle cells, endothelial cells, Hofbauer cells, human amnion epithelium and rat visceral yolk sac epithelium. Thus, all human and rat placental cell types exhibit receptor subunits with binding sites for the endogenous ligand ACh and nicotine. ACh is suggested to be an important placental signalling molecule that, through stimulation of nAChR, controls the uptake of nutrients, blood flow and fluid volume in placental vessels, and the vascularisation during placental development. Chronic stimulation of nAChR by nicotine might result in unbalanced receptor activation or functional desensitisation followed by the known pathological effects of smoking.

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.

Brain-derived neurotrophic factor and trkB signaling in parasympathetic neurons: relevance to regulating alpha7-containing nicotinic receptors and synaptic function

Parasympathetic neurons do not require neurotrophins for survival and are thought to lack high-affinity neurotrophin receptors (i.e., trks). We report here, however, that mRNAs encoding both brain-derived neurotrophic factor (BDNF) and its high-affinity receptor tropomyosin-related kinase B (trkB) are expressed in the parasympathetic chick ciliary ganglion (CG) and that BDNF-like protein is present in the ganglion and in the iris, an important peripheral target of ciliary neurons. Moreover, CG neurons express surface trkB and exogenous BDNF not only initiates trk-dependent signaling, but also alters nicotinic acetylcholine receptor (nAChR) expression and synaptic transmission. In particular, BDNF applied to CG neurons rapidly activates cAMP-dependent response element-binding protein (CREB), and over the long-term selectively upregulates expression of alpha7-subunit-containing, homomeric nAChRs (alpha7-nAChRs), increasing alpha7-subunit mRNA levels, alpha7-nAChR surface sites, and alpha7-nAChR-mediated whole-cell currents. At nicotinic synapses formed on CG neurons in culture, brief and long-term BDNF treatments also increase the frequency of spontaneous EPSCs, most of which are mediated by heteromeric nAChRs containing alpha3, alpha5, beta4, and beta2 subunits (alpha3*-nAChRs) with a minor contribution from alpha7-nAChRs. Our findings demonstrate unexpected roles for BDNF-induced, trk-dependent signaling in CG neurons, both in regulating expression of alpha7-nAChRs and in enhancing transmission at alpha3*-nAChR-mediated synapses. The presence of BDNF-like protein in CG and iris target coupled with that of functional trkB on CG neurons raise the possibility that signals generated by endogenous BDNF similarly influence alpha7-nAChRs and nicotinic synapses in vivo.

The alpha7 nicotinic acetylcholine receptor subunit exists in two isoforms that contribute to functional ligand-gated ion channels

Fast synaptic transmission in mammalian autonomic ganglia is mediated primarily by nicotinic receptors, and one of the most abundant nicotinic acetylcholine receptor subtypes in these neurons contains the alpha7 subunit (alpha7-nAChRs). Unlike alpha7-nAChRs expressed in other cells, the predominant alpha7-nAChR subtype found in rat intracardiac and superior cervical ganglion neurons exhibits a slow rate of desensitization and is reversibly blocked by alpha-bungarotoxin (alphaBgt). We report here the identification of an alpha7 subunit sequence variant in rat autonomic neurons that incorporates a novel 87-base pair cassette exon in the N terminus of the receptor and preserves the reading frame of the transcript. This alpha7 isoform was detected using reverse transcriptase-polymerase chain reaction techniques in neonatal rat brain and intracardiac and superior cervical ganglion neurons. Immunoblot experiments using a polyclonal antibody directed against the deduced amino acid sequence of the alpha7-2 insert showed a pattern of expression consistent with alpha7-2 subunit mRNA distribution. Moreover, the alpha7-2 subunit could be immunodepleted from protein extracts by solid-phase immunoprecipitation techniques using the anti-alpha7 monoclonal antibody 319. The alpha7-2 subunit was shown to form functional homomeric ion channels that were activated by acetylcholine and blocked by alpha-bungarotoxin when expressed in Xenopus laevis oocytes. This alpha7 isoform exhibited a slow rate of desensitization, and inhibition of these channels by alphaBgt reversed rapidly after washout. Taken together, these data indicate that the alpha7-2 subunit is capable of forming functional alphaBgt-sensitive acetylcholine receptors that resemble the alpha7-nAChRs previously identified in rat autonomic neurons. Furthermore, the distribution of the alpha7-2 isoform is not limited to peripheral neurons.

The beta-subunit composition of nicotinic acetylcholine receptors in the neurons of the guinea pig inferior mesenteric ganglion

The antibodies against synthetic (183-192) fragments of beta2- and beta4-subunits of rat neuronal nicotinic acetylcholine receptor were used to study a beta-subunit composition of nicotinic receptors in the inferior mesenteric ganglion of the guinea pig by both immunocytochemical staining and blocking of excitatory postsynaptic potentials induced by electric stimulation of the pre-ganglionic nerve (intermesenteric trunk). The beta4-specific antibody stained 59.8 +/- 7.5% of neurons and inhibited the synaptic responses in all (n = 10) neurons studied by 25.5 +/- 1.8%. The beta2-specific antibody did not stain ganglionic neurons and did not affect the synaptic transmission. Taking into account the previously obtained data on the alpha-subunits found in this ganglion, it is concluded that the neurons of inferior mesenteric ganglion contain nicotinic receptors of alpha3(alpha5)beta4 subtypes involved in synaptic transmission through the intermesenteric tract.

Alpha-protocadherins are presynaptic and axonal in nicotinic pathways

The protocadherin families pcdh-alpha, beta, and gamma have been proposed to mediate synaptic specificity via homophilic interactions. Here we report isolation of two pcdh-alpha family members from chick. We find pcdh-alpha mRNA in multiple regions of chick CNS including cerebellum, tectum, olfactory bulb, and forebrain, and in the autonomic nervous system. Immunoblots identify major components of 120 and 140 kDa both in brain and ciliary ganglion extracts. Immunohistochemistry reveals pcdh-alphas in axons and perisynaptically in preganglionic terminals, adjacent to transmitter release sites. Pcdh-alphas appear to be absent from postsynaptic sites: They are nonoverlapping with postsynaptic receptor clusters in the ganglion and are rapidly lost after ganglionic denervation. Similar pcdh-alpha patterns are found in motor axons and at neuromuscular junctions of birds and mammals, and persist into adulthood. The results indicate that pcdh-alphas are widely expressed in nicotinic cholinergic pathways and may engage in heterophilic interactions at synapses and on axons.

Knockout and knockin mice to investigate the role of nicotinic receptors in the central nervous system

The recent use of genetically engineered knockout (Ko) and knockin (Kin) animals for neurotransmitter receptor genes, in particular, nicotinic acetylcholine receptors (nAChRs) in the brain, has provided a powerful alternative to the classical pharmacological approach. These animal models are not only useful in order to reexamine and refine the results derived from pharmacological studies, but they do also provide a unique opportunity to determine the subunit composition of the nicotinic receptors which modulate various brain functions. Ultimately, this knowledge will be valuable in the process of designing new drugs that will mimic the effects of nicotine on several important pathologies or on smoking cessation therapies. In this review, we present recent data obtained from the studies of mutant animals that contributed to our understanding of the role and composition of nAChRs in the central nervous system (CNS). The advantages and pitfalls of Ko animal models will also be discussed.

Nicotinic acetylcholine receptors: from structure to brain function

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels and can be divided into two groups: muscle receptors, which are found at the skeletal neuromuscular junction where they mediate neuromuscular transmission, and neuronal receptors, which are found throughout the peripheral and central nervous system where they are involved in fast synaptic transmission. nAChRs are pentameric structures that are made up of combinations of individual subunits. Twelve neuronal nAChR subunits have been described, alpha2-alpha10 and beta2-beta4; these are differentially expressed throughout the nervous system and combine to form nAChRs with a wide range of physiological and pharmacological profiles. The nAChR has been proposed as a model of an allosteric protein in which effects arising from the binding of a ligand to a site on the protein can lead to changes in another part of the molecule. A great deal is known about the structure of the pentameric receptor. The extracellular domain contains binding sites for numerous ligands, which alter receptor behavior through allosteric mechanisms. Functional studies have revealed that nAChRs contribute to the control of resting membrane potential, modulation of synaptic transmission and mediation of fast excitatory transmission. To date, ten genes have been identified in the human genome coding for the nAChRs. nAChRs have been demonstrated to be involved in cognitive processes such as learning and memory and control of movement in normal subjects. Recent data from knockout animals has extended the understanding of nAChR function. Dysfunction of nAChR has been linked to a number of human diseases such as schizophrenia, Alzheimer's and Parkinson's diseases. nAChRs also play a significant role in nicotine addiction, which is a major public health concern. A genetically transmissible epilepsy, ADNFLE, has been associated with specific mutations in the gene coding for the alpha4 or beta2 subunits, which leads to altered receptor properties.