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Concept: Acetylcholine receptor

334

Opioids are first-line drugs for moderate to severe acute pain and cancer pain. However, these medications are associated with severe side effects, and whether they are efficacious in treatment of chronic nonmalignant pain remains controversial. Medications that act through alternative molecular mechanisms are critically needed. Antagonists of α9α10 nicotinic acetylcholine receptors (nAChRs) have been proposed as an important nonopioid mechanism based on studies demonstrating prevention of neuropathology after trauma-induced nerve injury. However, the key α9α10 ligands characterized to date are at least two orders of magnitude less potent on human vs. rodent nAChRs, limiting their translational application. Furthermore, an alternative proposal that these ligands achieve their beneficial effects by acting as agonists of GABAB receptors has caused confusion over whether blockade of α9α10 nAChRs is the fundamental underlying mechanism. To address these issues definitively, we developed RgIA4, a peptide that exhibits high potency for both human and rodent α9α10 nAChRs, and was at least 1,000-fold more selective for α9α10 nAChRs vs. all other molecular targets tested, including opioid and GABAB receptors. A daily s.c. dose of RgIA4 prevented chemotherapy-induced neuropathic pain in rats. In wild-type mice, oxaliplatin treatment produced cold allodynia that could be prevented by RgIA4. Additionally, in α9 KO mice, chemotherapy-induced development of cold allodynia was attenuated and the milder, temporary cold allodynia was not relieved by RgIA4. These findings establish blockade of α9-containing nAChRs as the basis for the efficacy of RgIA4, and that α9-containing nAChRs are a critical target for prevention of chronic cancer chemotherapy-induced neuropathic pain.

Concepts: Nicotine, Pain, Acetylcholine, Myasthenia gravis, Muscarinic acetylcholine receptor, Nicotinic acetylcholine receptor, Acetylcholine receptor, Dextromethorphan

208

Habitual chewing of “betel nut” preparations constitutes the fourth most common human self-administration of a psychoactive substance after alcohol, caffeine, and nicotine. The primary active ingredient in these preparations is arecoline, which comes from the areca nut, the key component of all such preparations. Arecoline is known to be a relatively non-selective muscarinic partial agonist, accounting for many of the overt peripheral and central nervous system effects, but not likely to account for the addictive properties of the drug. We report that arecoline has activity on select nicotinic acetylcholine receptor (nAChR) subtypes, including the two classes of nAChR most related to the addictive properties of nicotine: receptors containing α4 and β2 subunits and those which also contain α6 and β3 subunits. Arecoline is a partial agonist with about 6-10% efficacy for the α4* and α6* receptors expressed in Xenopus oocytes. Additionally, arecoline is a silent agonist of α7 nAChR; while it does not activate α7 receptors when applied alone, it produces substantial activation when co-applied with the positive allosteric modulator PNU-120696. Some α7 silent agonists are effective inhibitors of inflammation, which might account for anti-inflammatory effects of arecoline. Arecoline’s activity on nAChR associated with addiction may account for the habitual use of areca nut preparations in spite of the well-documented risk to personal health associated with oral diseases and cancer. The common link between betel and tobacco suggests that partial agonist therapies with cytisine or the related compound varenicline may also be used to aid betel cessation attempts.

Concepts: Nicotine, Acetylcholine, Muscarinic acetylcholine receptor, Nicotinic acetylcholine receptor, Acetylcholine receptor, Ibogaine, Cytisine, Areca nut

186

The parasympathetic branch of the autonomic nervous system regulates the activity of multiple organ systems. Muscarinic receptors are G-protein-coupled receptors that mediate the response to acetylcholine released from parasympathetic nerves. Their role in the unconscious regulation of organ and central nervous system function makes them potential therapeutic targets for a broad spectrum of diseases. The M2 muscarinic acetylcholine receptor (M2 receptor) is essential for the physiological control of cardiovascular function through activation of G-protein-coupled inwardly rectifying potassium channels, and is of particular interest because of its extensive pharmacological characterization with both orthosteric and allosteric ligands. Here we report the structure of the antagonist-bound human M2 receptor, the first human acetylcholine receptor to be characterized structurally, to our knowledge. The antagonist 3-quinuclidinyl-benzilate binds in the middle of a long aqueous channel extending approximately two-thirds through the membrane. The orthosteric binding pocket is formed by amino acids that are identical in all five muscarinic receptor subtypes, and shares structural homology with other functionally unrelated acetylcholine binding proteins from different species. A layer of tyrosine residues forms an aromatic cap restricting dissociation of the bound ligand. A binding site for allosteric ligands has been mapped to residues at the entrance to the binding pocket near this aromatic cap. The structure of the M2 receptor provides insights into the challenges of developing subtype-selective ligands for muscarinic receptors and their propensity for allosteric regulation.

Concepts: Nervous system, Receptor, Neurotransmitter, Acetylcholine, Muscarinic acetylcholine receptor, Nicotinic acetylcholine receptor, Acetylcholine receptor, Atropine

167

Although the activity of the nicotinic acetylcholine receptor (nAChR) is exquisitely sensitive to its membrane environment, the underlying mechanisms remain poorly defined. The homologous prokaryotic pentameric ligand gated ion channel, GLIC, represents an excellent model for probing the molecular basis of nAChR sensitivity due to its high structural homology, ease of expression, and amenability to crystallographic analysis. We show here that membrane-reconstituted GLIC exhibits structural and biophysical properties similar to those of membrane-reconstituted nAChR, although GLIC is substantially more thermally stable. GLIC, however, does not possess the same exquisite lipid sensitivity. In particular, GLIC does not exhibit the same propensity to adopt an uncoupled conformation where agonist binding is uncoupled from channel gating. Structural comparisons provide insight into the chemical features that may predispose the nAChR to the formation of an uncoupled state.

Concepts: Receptor, Acetylcholine, Ion channels, Myasthenia gravis, Muscarinic acetylcholine receptor, Nicotinic acetylcholine receptor, Acetylcholine receptor, Ligand-gated ion channel

155

Alterations in expression patterns of alpha4beta2 nicotinic acetylcholine receptors (nAChR) have been demonstrated to alter cholinergic neurotransmission and implicated in neurological disorders including Autism, nicotine addiction, Alzheimer’s and Parkinson’s disease. Positive allosteric modulators (PAMs) represent promising new leads in the development of therapeutic agents for the treatment of these disorders. This study investigates the involvement of the beta2 containing subunit interfaces of alpha4beta2 receptors in the modulation of acetylcholine- (ACh) induced responses by the PAM desformylflustrabromine (dFBr). Eight amino acids on the principal face of the beta2 subunit were mutated to alanine in order to explore the involvement of this region in potentiation of ACh-induced currents by dFBr. ACh-induced responses obtained from wild type and mutant alpha4beta2 receptors expressed in Xenopus laevis oocytes were recorded in the presence and absence of dFBr using two-electrode voltage clamp electrophysiology. Wild type and mutant receptors were expressed in both high- (HS) and low- (LS) ACh sensitivity isoforms by using biased injection ratios of 1:5 or 5:1 alpha4: beta2 cRNA. Mutations were made in the B, C and A loops of the principal face of the beta2 subunit; regions not involved in binding of ACh. Mutant β2(Y120A) significantly eliminated dFBr potency in both isoform preparations. Several other mutations altered dFBr potentiation levels in both preparations. Our findings support the involvement of the principal face of the beta2 subunit in dFBr modulation of ACh-induced responses. Findings from this study will aid in the improved design of dFBr-like PAMs for potential therapeutic use.

Concepts: Nicotine, Neurotransmitter, Acetylcholine, Myasthenia gravis, Muscarinic acetylcholine receptor, Nicotinic acetylcholine receptor, Acetylcholine receptor, Alpha-4 beta-2 nicotinic receptor

72

The current frontline symptomatic treatment for Alzheimer’s disease (AD) is whole-body upregulation of cholinergic transmission via inhibition of acetylcholinesterase. This approach leads to profound dose-related adverse effects. An alternative strategy is to selectively target muscarinic acetylcholine receptors, particularly the M1 muscarinic acetylcholine receptor (M1 mAChR), which was previously shown to have procognitive activity. However, developing M1 mAChR-selective orthosteric ligands has proven challenging. Here, we have shown that mouse prion disease shows many of the hallmarks of human AD, including progressive terminal neurodegeneration and memory deficits due to a disruption of hippocampal cholinergic innervation. The fact that we also show that muscarinic signaling is maintained in both AD and mouse prion disease points to the latter as an excellent model for testing the efficacy of muscarinic pharmacological entities. The memory deficits we observed in mouse prion disease were completely restored by treatment with benzyl quinolone carboxylic acid (BQCA) and benzoquinazoline-12 (BQZ-12), two highly selective positive allosteric modulators (PAMs) of M1 mAChRs. Furthermore, prolonged exposure to BQCA markedly extended the lifespan of diseased mice. Thus, enhancing hippocampal muscarinic signaling using M1 mAChR PAMs restored memory loss and slowed the progression of mouse prion disease, indicating that this ligand type may have clinical benefit in diseases showing defective cholinergic transmission, such as AD.

Concepts: Receptor, Neurotransmitter, Acetylcholine, Muscarinic acetylcholine receptor, Nicotinic acetylcholine receptor, Acetylcholine receptor, Atropine, Oxybutynin

57

Chronic exposure to nicotine upregulates nicotinic acetylcholine receptors (nAChRs), and such upregulation is critical for the development of nicotine dependence in humans and animal models. However, how nicotine upregulates nAChRs is not well understood. Here, we identify a key role for microRNA in regulating nicotine-dependent behavior by modulating nAChR expression in C. elegans. We show that the nAChR gene acr-19 and alg-1, a key Argonaute-family member in the microRNA machinery, are specifically required for nicotine withdrawal response following chronic nicotine treatment. Chronic exposure to nicotine downregulates alg-1, leading to upregulation of acr-19. This effect is mediated by the microRNA miR-238 that recognizes the 3' UTR of acr-19 transcript. Our results unveil a previously unrecognized role for microRNA in nicotine signaling, providing insights into how chronic nicotine administration leads to upregulation of nAChR and ultimately nicotine dependence.

Concepts: Nicotine, Acetylcholine, Myasthenia gravis, Muscarinic acetylcholine receptor, Nicotinic acetylcholine receptor, Acetylcholine receptor, Neuromuscular junction, Alpha-4 beta-2 nicotinic receptor

45

Possible roles of brain-derived signals in the regulation of embryogenesis are unknown. Here we use an amputation assay in Xenopus laevis to show that absence of brain alters subsequent muscle and peripheral nerve patterning during early development. The muscle phenotype can be rescued by an antagonist of muscarinic acetylcholine receptors. The observed defects occur at considerable distances from the head, suggesting that the brain provides long-range cues for other tissue systems during development. The presence of brain also protects embryos from otherwise-teratogenic agents. Overexpression of a hyperpolarization-activated cyclic nucleotide-gated ion channel rescues the muscle phenotype and the neural mispatterning that occur in brainless embryos, even when expressed far from the muscle or neural cells that mispattern. We identify a previously undescribed developmental role for the brain and reveal a non-local input into the control of early morphogenesis that is mediated by neurotransmitters and ion channel activity.Functions of the embryonic brain prior to regulating behavior are unclear. Here, the authors use an amputation assay in Xenopus laevis to demonstrate that removal of the brain early in development alters muscle and peripheral nerve patterning, which can be rescued by modulating bioelectric signals.

Concepts: Neuron, Developmental biology, Receptor, Neurotransmitter, Acetylcholine, Muscarinic acetylcholine receptor, Nicotinic acetylcholine receptor, Acetylcholine receptor

30

Due to intensive agriculture honeybees are threatened by various pesticides. The use of one group of them, the neonicotinoids, was recently restricted by the European Union. These chemicals bind to the nicotinic acetylcholine receptor (nAchR) in the honeybee brain. Recently, Bayer AG released a new pesticide by the name of “Sivanto” against sucking insects. It is assumed to be harmless for honeybees, although its active ingredient, flupyradifurone, binds nAchR similar to the neonicotinoids. We investigated if this pesticide affects the taste for sugar and cognitive performance in honeybee foragers. These bees are directly exposed to the pesticide while foraging for pollen or nectar. Our results demonstrate that flupyradifurone can reduce taste and appetitive learning performance in honeybees foraging for pollen and nectar, although only the highest concentration had significant effects. Most likely, honeybee foragers will not be exposed to these high concentrations. Therefore, the appropriate use of this pesticide is considered safe for honeybees, at least with respect to the behaviors studied here.

Concepts: Insect, Nicotine, Acetylcholine, Myasthenia gravis, Muscarinic acetylcholine receptor, Nicotinic acetylcholine receptor, Acetylcholine receptor, Ligand-gated ion channel

29

In this multicenter, double-blind, placebo-controlled, randomized, four way cross-over proof-of-mechanism study, we tested the effect of the positive allosteric α7 nicotinic acetylcholine receptor (nAChR) modulator JNJ-39393406 in a key translational assay (sensory P50 gating) in 39 regularly smoking male patients with schizophrenia. All patients were clinically stable and JNJ-39393406 was administered as an adjunct treatment to antipsychotics. No indication was found that JNJ-39393406 has the potential to reverse basic deficits of information processing in schizophrenia (sensory P50 gating) or has a significant effect on other tested electrophysiological markers (MMN, P300 and quantitative resting EEG). Sensitivity analyses including severity of disease, baseline P50 gating, medication and gene variants of the CHRNA7 gene did not reveal any subgroups with consistent significant effects. It is discussed that potential positive effects in subgroups not present or not large enough in the current study or upon chronic dosing are possible, but unlikely to be developed. This article is part of a Special Issue entitled ‘Cognitive Enhancers’.

Concepts: Nicotine, Acetylcholine, Myasthenia gravis, Muscarinic acetylcholine receptor, Nicotinic acetylcholine receptor, Acetylcholine receptor, Alpha-7 nicotinic receptor, CHRNA7