SciCombinator

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Concept: Acetylcholine

256

Chronic supplementation with creatine monohydrate has been shown to promote increases in total intramuscular creatine, phosphocreatine, skeletal muscle mass, lean body mass and muscle fiber size. Furthermore, there is robust evidence that muscular strength and power will also increase after supplementing with creatine. However, it is not known if the timing of creatine supplementation will affect the adaptive response to exercise. Thus, the purpose of this investigation was to determine the difference between pre versus post exercise supplementation of creatine on measures of body composition and strength.

Concepts: Muscle, Physical exercise, Cardiac muscle, Glycogen, Myosin, Muscular system, Acetylcholine, Exercise physiology

219

Nicotine cigarette smoke is a large public health burden worldwide, contributing to various types of disease. Anti-tobacco media campaigns and control programs have significantly reduced smoking in the United States, yet trends for menthol cigarette smoking have not been as promising. Menthol cigarette smoking is particularly prevalent among young adults and African Americans, with implications for long-term impacts on health care. Continuing high rates of menthol cigarette addiction call into question the role of menthol in nicotine addiction. To date, a biological basis for the high rate of addiction and relapse among menthol cigarette smokers has not been defined. Studies have demonstrated a role for menthol in the metabolism of nicotine in the body. More recent findings now reveal an interaction between menthol and the nicotinic acetylcholine (nACh) receptor in cells. This receptor is central to the actions of nicotine in the brain, and plays an important role in nicotine addiction. The newly discovered effect of menthol on nACh receptors may begin to explain the unique addictive properties of menthol cigarettes.

Concepts: Smoking, Tobacco smoking, Cigarette, Nicotine, Addiction, Acetylcholine, Muscarinic acetylcholine receptor, Nicotinic acetylcholine receptor

202

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

191

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

185

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

178

Extracellular plaques of amyloid-β and intraneuronal neurofibrillary tangles made from tau are the histopathological signatures of Alzheimer’s disease. Plaques comprise amyloid-β fibrils that assemble from monomeric and oligomeric intermediates, and are prognostic indicators of Alzheimer’s disease. Despite the importance of plaques to Alzheimer’s disease, oligomers are considered to be the principal toxic forms of amyloid-β. Interestingly, many adverse responses to amyloid-β, such as cytotoxicity, microtubule loss, impaired memory and learning, and neuritic degeneration, are greatly amplified by tau expression. Amino-terminally truncated, pyroglutamylated (pE) forms of amyloid-β are strongly associated with Alzheimer’s disease, are more toxic than amyloid-β, residues 1-42 (Aβ(1-42)) and Aβ(1-40), and have been proposed as initiators of Alzheimer’s disease pathogenesis. Here we report a mechanism by which pE-Aβ may trigger Alzheimer’s disease. Aβ(3(pE)-42) co-oligomerizes with excess Aβ(1-42) to form metastable low-n oligomers (LNOs) that are structurally distinct and far more cytotoxic to cultured neurons than comparable LNOs made from Aβ(1-42) alone. Tau is required for cytotoxicity, and LNOs comprising 5% Aβ(3(pE)-42) plus 95% Aβ(1-42) (5% pE-Aβ) seed new cytotoxic LNOs through multiple serial dilutions into Aβ(1-42) monomers in the absence of additional Aβ(3(pE)-42). LNOs isolated from human Alzheimer’s disease brain contained Aβ(3(pE)-42), and enhanced Aβ(3(pE)-42) formation in mice triggered neuron loss and gliosis at 3 months, but not in a tau-null background. We conclude that Aβ(3(pE)-42) confers tau-dependent neuronal death and causes template-induced misfolding of Aβ(1-42) into structurally distinct LNOs that propagate by a prion-like mechanism. Our results raise the possibility that Aβ(3(pE)-42) acts similarly at a primary step in Alzheimer’s disease pathogenesis.

Concepts: Alzheimer's disease, Neuron, Polymer chemistry, Monomer, Acetylcholine, Toxicity, Oligomer, Neurofibrillary tangle

176

Skeletal muscle is a plastic organ that is maintained by multiple pathways regulating cell and protein turnover. During muscle atrophy, proteolytic systems are activated, and contractile proteins and organelles are removed, resulting in the shrinkage of muscle fibers. Excessive loss of muscle mass is associated with poor prognosis in several diseases, including myopathies and muscular dystrophies, as well as in systemic disorders such as cancer, diabetes, sepsis and heart failure. Muscle loss also occurs during aging. In this paper, we review the key mechanisms that regulate the turnover of contractile proteins and organelles in muscle tissue, and discuss how impairments in these mechanisms can contribute to muscle atrophy. We also discuss how protein synthesis and degradation are coordinately regulated by signaling pathways that are influenced by mechanical stress, physical activity, and the availability of nutrients and growth factors. Understanding how these pathways regulate muscle mass will provide new therapeutic targets for the prevention and treatment of muscle atrophy in metabolic and neuromuscular diseases.

Concepts: Metabolism, Muscle, Glycogen, Actin, Myosin, Muscular system, Acetylcholine, Neuromuscular disease

171

BACKGROUND: Sympathetic nervous activity contributes to the maintenance of muscle oxygenation. However, patients with chronic pain may suffer from autonomic dysfunction. Furthermore, insufficient muscle oxygenation is observed among workers with chronic neck and shoulder pain. The aim of our study was to investigate how muscle load tasks affect sympathetic nervous activity and changes in oxygenation of the trapezius muscles in subjects with chronic neck and shoulder pain. METHODS: Thirty females were assigned to two groups: a pain group consisting of subjects with chronic neck and shoulder pain and a control group consisting of asymptomatic subjects. The participants performed three sets of isometric exercise in an upright position; they contracted their trapezius muscles with maximum effort and let the muscles relax (Relax). Autonomic nervous activity and oxygenation of the trapezius muscles were measured by heart rate variability (HRV) and Near-Infrared Spectroscopy. RESULTS: Oxyhemoglobin and total hemoglobin of the trapezius muscles in the pain group were lower during the Relax period compared with the control group. In addition, the low frequency / high frequency (LF/HF) ratio of HRV significantly increased during isometric exercise in the control group, whereas there were no significant changes in the pain group. CONCLUSIONS: Subjects with neck and shoulder pain showed lower oxygenation and blood flow of the trapezius muscles responding to isometric exercise, compared with asymptomatic subjects. Subjects with neck and shoulder pain also showed no significant changes in the LF/HF ratio of HRV responding to isometric exercise, which would imply a reduction in sympathetic nervous activity.

Concepts: Energy, Muscle, Muscle contraction, Acetylcholine, Autonomic nervous system, Isometric exercise, Trapezius muscle, Clavicle

168

Alzheimer’s dementia (AD) is increasingly being recognized as one of the most important medical and social problems in older people in industrialized and non-industrialized nations. To date, only symptomatic treatments exist for this disease, all trying to counterbalance the neurotransmitter disturbance. Three cholinesterase inhibitors (CIs) are currently available and have been approved for the treatment of mild to moderate AD. A further therapeutic option available for moderate to severe AD is memantine, an N-methyl-D-aspartate receptor noncompetitive antagonist. Treatments capable of stopping or at least effectively modifying the course of AD, referred to as ‘disease-modifying’ drugs, are still under extensive research. To block the progression of the disease they have to interfere with the pathogenic steps responsible for the clinical symptoms, including the deposition of extracellular amyloid β plaques and intracellular neurofibrillary tangle formation, inflammation, oxidative damage, iron deregulation and cholesterol metabolism. In this review we discuss current symptomatic treatments and new potential disease-modifying therapies for AD that are currently being studied in phase I-III trials.

Concepts: Alzheimer's disease, Medicine, Neurology, Dementia, Acetylcholine, Acetylcholinesterase inhibitor, Memantine, Donepezil

167

Sarcopenia, or senile muscle atrophy, is the slow and progressive loss of muscle mass with advancing age that constitutes the most prevalent form of muscle atrophy. The effects of ageing on skeletal muscle have been extensively studied in humans and laboratory animals (mice), while the few reports on wild animals are based on short-lived mammals. The present study describes the age-related changes in cetacean muscles regarding the three factors that determine muscle mass: fibre size, fibre number, and fibre type. We show that the skeletal muscle fibres in cetaceans change with advancing age, evolving towards a slower muscle phenotype. We suggest that this physiological evolution constitutes an adaptation that allows these marine mammals to perform prolonged, deep dives.

Concepts: Muscle, Cardiac muscle, Glycogen, Muscle atrophy, Myosin, Muscular system, Acetylcholine, Atrophy