Activation of the nicotinic acetylcholine receptor mobilizes calcium from caffeine-insensitive stores in C2C12 mouse myotubes

Zebrafish Models for Skeletal Muscle Senescence: Lessons from Cell Cultures and Rodent Models

Human life expectancy has markedly increased over the past hundred years. Consequently, the percentage of elderly people is increasing. Aging and sarcopenic changes in skeletal muscles not only reduce locomotor activities in elderly people but also increase the chance of trauma, such as bone fractures, and the incidence of other diseases, such as metabolic syndrome, due to reduced physical activity. Exercise therapy is currently the only treatment and prevention approach for skeletal muscle aging. In this review, we aimed to summarize the strategies for modeling skeletal muscle senescence in cell cultures and rodents and provide future perspectives based on zebrafish models. In cell cultures, in addition to myoblast proliferation and myotube differentiation, senescence induction into differentiated myotubes is also promising. In rodents, several models have been reported that reflect the skeletal muscle aging phenotype or parts of it, including the accelerated aging models. Although there are fewer models of skeletal muscle aging in zebrafish than in mice, various models have been reported in recent years with the development of CRISPR/Cas9 technology, and further advancements in the field using zebrafish models are expected in the future.

Nicotine inhibits potassium currents in Aplysia bag cell neurons

Acetylcholine and the archetypal cholinergic agonist, nicotine, are typically associated with the opening of ionotropic receptors. In the bag cell neurons, which govern the reproductive behavior of the marine snail, Aplysia californica, there are two cholinergic responses: a relatively large acetylcholine-induced current and a relatively small nicotine-induced current. Both currents are readily apparent at resting membrane potential and result from the opening of distinct ionotropic receptors. We now report a separate current response elicited by applying nicotine to cultured bag cell neurons under whole cell voltage-clamp. This current was ostensibly inward, best resolved at depolarized voltages, presented a noncooperative dose-response with a half-maximal concentration near 1.5 mM, and associated with a decrease in membrane conductance. The unique nicotine-evoked response was not altered by intracellular perfusion with the G protein blocker GDPβS or exposure to classical nicotinic antagonists but was occluded by replacing intracellular K(+) with Cs(+) Consistent with an underlying mechanism of direct inhibition of one or more K(+) channels, nicotine was found to rapidly reduce the fast-inactivating A-type K(+) current as well as both components of the delayed-rectifier K(+) current. Finally, nicotine increased bag cell neuron excitability, which manifested as reduction in spike threshold, greater action potential height and width, and markedly more spiking to continuous depolarizing current injection. In contrast to conventional transient activation of nicotinic ionotropic receptors, block of K(+) channels could represent a nonstandard means for nicotine to profoundly alter the electrical properties of neurons over prolonged periods of time.

Autophagic flux data in differentiated C2C12 myotubes following exposure to acetylcholine and caffeine

The C2C12 line of mouse myoblasts is a useful cell culture model in which to conduct in vitro analyses related to skeletal muscle. Here we present data regarding the autophagic response induced by two chemicals known to influence calcium release and contraction in skeletal muscles and C2C12 cells: acetylcholine and caffeine. More specifically, by concurrently administering acetylcholine or caffeine along with chloroquine to differentiated myotubes for various amounts of time and assessing the protein expression of LC3 and p62, we report data on the relative level of autophagic flux induced by these two calcium- and contraction-regulating chemicals.

CCDI: a new ligand that modulates mammalian type 1 ryanodine receptor (RyR1)

BACKGROUND AND PURPOSE

Ryanodine receptors (RyRs) are Ca(2+)-release channels on the sarco(endo)plasmic reticulum that modulate a wide array of physiological functions. Three RyR isoforms are present in cells: RyR1, RyR2 and RyR3. To date, there are no reports on ligands that modulate RyR in an isoform-selective manner. Such ligands are not only valuable research tools, but could serve as intermediates for development of therapeutics.

EXPERIMENTAL APPROACH

Pyrrole-2-carboxylic acid and 1,3-dicyclohexylcarbodiimide were allowed to react in carbon tetrachloride for 24 h at low temperatures and pressures. The chemical structures of the two products isolated were elucidated using NMR spectrometry, mass spectrometry and elemental analyses. [(3) H]-ryanodine binding, lipid bilayer and time-lapsed confocal imaging were used to determine their effects on RyR isoforms.

KEY RESULTS

The major product, 2-cyclohexyl-3-cyclohexylimino-2, 3, dihydro-pyrrolo[1,2-c]imidazol-1-one (CCDI) dose-dependently potentiated Ca(2+)-dependent binding of [(3)H]-ryanodine to RyR1, with no significant effects on [(3)H]-ryanodine binding to RyR2 or RyR3. CCDI also reversibly increased the open probability (P(o)) of RyR1 with minimal effects on RyR2 and RyR3. CCDI induced Ca(2+) transients in C2C12 skeletal myotubes, but not in rat ventricular myocytes. This effect was blocked by pretreating cells with ryanodine. The minor product 2-cyclohexyl-pyrrolo[1,2-c]imidazole-1,3-dione had no effect on either [(3)H]-ryanodine binding or P(o) of RyR1, RyR2 and RyR3.

CONCLUSIONS AND IMPLICATIONS

A new ligand that preferentially modulates RyR1 was identified. In addition to being an important research tool, the pharmacophore of this small molecule could serve as a template for the synthesis of other isoform-selective modulators of RyRs.

Boron nitride nanotube-mediated stimulation of cell co-culture on micro-engineered hydrogels

In this paper, we describe the effects of the combination of topographical, mechanical, chemical and intracellular electrical stimuli on a co-culture of fibroblasts and skeletal muscle cells. The co-culture was anisotropically grown onto an engineered micro-grooved (10 µm-wide grooves) polyacrylamide substrate, showing a precisely tuned Young’s modulus (∼ 14 kPa) and a small thickness (∼ 12 µm). We enhanced the co-culture properties through intracellular stimulation produced by piezoelectric nanostructures (i.e., boron nitride nanotubes) activated by ultrasounds, thus exploiting the ability of boron nitride nanotubes to convert outer mechanical waves (such as ultrasounds) in intracellular electrical stimuli, by exploiting the direct piezoelectric effect. We demonstrated that nanotubes were internalized by muscle cells and localized in both early and late endosomes, while they were not internalized by the underneath fibroblast layer. Muscle cell differentiation benefited from the synergic combination of topographical, mechanical, chemical and nanoparticle-based stimuli, showing good myotube development and alignment towards a preferential direction, as well as high expression of genes encoding key proteins for muscle contraction (i.e., actin and myosin). We also clarified the possible role of fibroblasts in this process, highlighting their response to the above mentioned physical stimuli in terms of gene expression and cytokine production. Finally, calcium imaging-based experiments demonstrated a higher functionality of the stimulated co-cultures.

Dust from hog confinement facilities impairs Ca2+ mobilization from sarco(endo)plasmic reticulum by inhibiting ryanodine receptors

Individuals working in commercial hog confinement facilities have elevated incidences of headaches, depression, nausea, skeletal muscle weakness, fatigue, gastrointestinal disorders, and cardiovascular diseases, and the molecular mechanisms for these nonrespiratory ailments remain incompletely undefined. A common element underlying these diverse pathophysiologies is perturbation of intracellular Ca(2+) homeostasis. This study assessed whether the dust generated inside hog confinement facilities contains compounds that alter Ca(2+) mobilization via ryanodine receptors (RyRs), key intracellular channels responsible for mobilizing Ca(2+) from internal stores to elicit an array of physiologic functions. Hog barn dust (HBD) was extracted with phosphate-buffered saline, sterile-filtered (0.22 μm), and size-separated using Sephadex G-100 resin. Fractions (F) 1 through 9 (Mw >10,000 Da) had no measurable effects on RyR isoforms. However, F10 through F17, which contained compounds of Mw ≤2,000 Da, modulated the [(3)H]ryanodine binding to RyR1, RyR2, and RyR3 in a biphasic (Gaussian) manner. The Ki values for F13, the most potent fraction, were 3.8 ± 0.2 μg/ml for RyR1, 0.2 ± 0.01 μg/ml and 19.1 ± 2.8 μg/ml for RyR2 (two binding sites), and 44.9 ± 2.8 μg/ml and 501.6 ± 9.2 μg/ml for RyR3 (two binding sites). In lipid bilayer assays, F13 dose-dependently decreased the open probabilities of RyR1, RyR2, and RyR3. Pretreating differentiated mouse skeletal myotubes (C2C12 cells) with F13 blunted the amplitudes of ryanodine- and K(+)-induced Ca(2+) transients. Because RyRs are present in many cell types, impairment in Ca(2+) mobilization from internal stores via these channels is a possible mechanism by which HBD may trigger these seemingly unrelated pathophysiologies.

Epigenetics: DNA demethylation promotes skeletal myotube maturation

Mesenchymal progenitor cells can be differentiated in vitro into myotubes that exhibit many characteristic features of primary mammalian skeletal muscle fibers. However, in general, they do not show the functional excitation-contraction coupling or the striated sarcomere arrangement typical of mature myofibers. Epigenetic modifications have been shown to play a key role in regulating the progressional changes in transcription necessary for muscle differentiation. In this study, we demonstrate that treatment of murine C2C12 mesenchymal progenitor cells with 10 μM of the DNA methylation inhibitor 5-azacytidine (5AC) promotes myogenesis, resulting in myotubes with enhanced maturity as compared to untreated myotubes. Specifically, 5AC treatment resulted in the up-regulation of muscle genes at the myoblast stage, while at later stages nearly 50% of the 5AC-treated myotubes displayed a mature, well-defined sarcomere organization, as well as spontaneous contractions that coincided with action potentials and intracellular calcium transients. Both the percentage of striated myotubes and their contractile activity could be inhibited by 20 nM TTX, 10 μM ryanodine, and 100 μM nifedipine, suggesting that action potential-induced calcium transients are responsible for these characteristics. Our data suggest that genomic demethylation induced by 5AC overcomes an epigenetic barrier that prevents untreated C2C12 myotubes from reaching full maturity.

Calcium-permeable ion channels involved in glutamate receptor-independent ischemic brain injury

Brain ischemia is a leading cause of death and long-term disabilities worldwide. Unfortunately, current treatment is limited to thrombolysis, which has limited success and a potential side effect of intracerebral hemorrhage. Searching for new cell injury mechanisms and therapeutic interventions has become a major challenge in the field. It has been recognized for many years that intracellular Ca(2+) overload in neurons is essential for neuronal injury associated with brain ischemia. However, the exact pathway(s) underlying the toxic Ca(2+) loading remained elusive. This review discusses the role of two Ca(2+)-permeable cation channels, TRPM7 and acid-sensing channels, in glutamate-independent Ca(2+) toxicity associated with brain ischemia.

Inhibition of mammalian muscle differentiation by regeneration blastema extract of Sternopygus macrurus

Tissue regeneration through stem cell activation and/or cell dedifferentiation is widely distributed across the animal kingdom. By comparison, regeneration in mammals is poor and this may reflect a limited dedifferentiation potential of mature cells. Because mammalian myotubes can dedifferentiate in the presence of newt blastema extract, the present study tested the dedifferentiation induction capability of the blastema from the teleost Sternopygus macrurus (SmBE). Our in vitro data showed that SmBE did not induce cell cycle reentry of myonuclei in myotubes. Instead, SmBE caused myotubes to detach and time-lapse imaging analyses characterized the cellular events before their detachment. Furthermore, SmBE enhanced myoblast proliferation and reversibly inhibited their differentiation. These data suggest the presence of protein factors in SmBE that regulate mammalian muscle physiology and differentiation, but do not support the conservation of a dedifferentiation induction capability by the blastema of S. macrurus.

Effects of nicotine on rat sternohyoid muscle contractile properties

Obstructive sleep apnoea (OSA) is a major clinical disorder characterised by recurring episodes of pharyngeal collapse during sleep. At present, there remains no satisfactory treatment for OSA. Pharmacological therapies as a potential treatment for the disorder are an attractive option and include agents that increase the contractility of the pharyngeal muscles. The aim of the present study was to examine the effects of nicotine on upper airway muscle contractile properties. In vitro isometric contractile properties were determined using strips of rat sternohyoid muscle in physiological salt solution containing nicotine (0-100 microg/ml) at 25 degrees C. Isometric twitch and tetanic tension, contraction time, half-relaxation time and tension-frequency relationship were determined by electrical field stimulation with platinum electrodes. Fatigue was induced by stimulation at 40 Hz with 300 ms trains at a frequency of 0.5 Hz for 5 min. Nicotine at a concentration of 1 microg/ml was associated with a significant increase in sternohyoid muscle specific tension compared to control data. Dose-dependent increases in contractile tension were not observed. Nicotine had effects on tension-frequency relationship and endurance properties of the sternohyoid muscle at some but not all doses. A leftward shift in the tension-frequency relationship was observed at low stimulus frequencies (20-30 Hz) for nicotine at a concentration of 1 and 5 microg/ml and a significant increase in fatigue resistance was observed with nicotine at a concentration of 10 microg/ml. As fatigue of the upper airway muscles has been implicated in obstructive airway conditions, a pharmacological agent that improves muscle endurance may prove useful as a potential treatment for such disorders. Therefore, further studies of the effects of nicotinic agonists on upper airway function are warranted.

Endocytosis function of a ligand-gated ion channel homolog in Caenorhabditis elegans

Ligand-gated ion channels are transmembrane proteins that respond to a variety of transmitters, including acetylcholine, gamma-aminobutyric acid (GABA), glycine, and glutamate [1 and 2]. These proteins play key roles in neurotransmission and are typically found in the nervous system and at neuromuscular junctions [3]. Recently, acetylcholine receptor family members also have been found in nonneuronal cells, including macrophages [4], keratinocytes [5], bronchial epithelial cells [5], and endothelial cells of arteries [6]. The function of these channels in nonneuronal cells in mammals remains to be elucidated, though it has been shown that the acetylcholine receptor alpha7 subunit is required for acetylcholine-mediated inhibition of tumor necrosis factor release by activated macrophages [4]. We show that cup-4, a gene required for efficient endocytosis of fluids by C. elegans coelomocytes, encodes a protein that is homologous to ligand-gated ion channels, with the highest degree of similarity to nicotinic acetylcholine receptors. Worms lacking CUP-4 have reduced phosphatidylinositol 4,5-bisphosphate levels at the plasma membrane, suggesting that CUP-4 regulates endocytosis through modulation of phospholipase C activity.

Ca2+ permeability of nicotinic acetylcholine receptors from rat dorsal root ganglion neurones

Ca2+ entry through neuronal nicotinic ACh receptors (nAChRs) modulates many biological processes in nervous tissue. In order to study the functional role of nAChRs in peripheral sensory signalling, we measured their Ca2+ permeability in rat dorsal root ganglion (DRG) neurones, and analysed the effects of nAChR-mediated Ca2+ influx on the function of the vanilloid receptor TRPV1. The fractional Ca2+ current (P(f), i.e. the percentage of current carried by Ca2+ ions) flowing through nAChR channels was measured by Ca2+ imaging fluorescence microscopy in combination with the patch-clamp technique. Functional nAChRs were expressed in a subset of adult DRG neurones (about 24% of the cells), typically with small to medium size as measured by their capacitance (40 +/- 3 pF). In most cells, ACh evoked slowly desensitizing currents, insensitive to methyllycaconitine (MLA, 10 nm), a potent antagonist of homomeric nAChRs. Fast decaying currents, probably mediated by alpha7*-nAChRs (i.e. native alpha7-containing nAChRs), were observed in 15% of ACh-responsive cells, in which slowly decaying currents, mediated by heteromeric nAChRs, were simultaneously present. The nAChRs of adult DRG neurones exhibited a P(f) value of 2.2 +/- 0.6% in the presence of MLA and 1.9 +/- 0.6% (P > 0.1) in the absence of MLA, indicating that homomeric MLA-sensitive nAChRs do not contribute to Ca2+ entry into adult DRG neurones. Conversely, 10% of neonatal DRG neurones showed ACh-evoked currents completely blocked by MLA. In these neurones, nAChRs showed a larger P(f) value (9.5 +/- 1.5%), indicating the expression of bona fide alpha7*-nAChRs. Finally, we report that Ca2+ influx through nAChRs in adult DRG neurones negatively modulated the TRPV1-mediated responses, representing a possible mechanism underlying the analgesic properties of nicotinic agonists on sensory neurones.

Characterization of specific GTP binding sites in C2C12 mouse skeletal muscle cells

Receptor sites, specific for guanosine 5'-triphosphate (GTP) were characterised in myoblasts and myotubes of C2C12 mouse skeletal muscle cells, using binding experiments and measurements of intracellular Ca2+ concentration ([Ca2+]i). We identified two GTP binding sites in myoblasts membranes: a high affinity site (Kd = 15.4 +/- 4.6 microM; Bmax = 1.7 +/- 0.5 nmol mg(-1) protein); and a low affinity site (Kd = 170 +/- 94.5 microM; Bmax = 14.2 +/- 3.9 nmol mg(-1) protein). In myotube membranes only a low affinity binding site for GTP (Kd = 169 +/- 39 microM; Bmax = 12.3 +/- 1.4 nmol mg(-1) protein) was detected. In myoblasts GTP binding was not displaced by ATP or UTP, even at high concentrations (up to of 1 mM), but it was affected by treatments with suramin or Reactive Blue 2 (RB2), the non-selective purine receptor antagonists. In contrast, in myotubes GTP binding was partially displaced by high concentrations of ATP, but treatments with the non-selective purine receptor antagonists, suramin or RB2, and with UTP had no effect on GTP binding. The addition of GTP to myoblasts, and to myotubes, resulted in elevations of [Ca2+]i. The patterns of Ca2+ response however, were different in the two cell phenotypes. In myoblasts the addition of GTP induced two types of Ca2+ responses: (1) a fast increase in [Ca2+]i, followed by a sustained [Ca2+]i elevation, and (2) a slow raising and steady prolonged increase in [Ca2+]i. In myotubes, however only fast Ca2+ responses were observed following the addition of 500 microM GTP. In the myoblasts and myotubes GTP-stimulated [Ca2+]i increases were abolished by treatments with suramin or RB2 at concentrations which had no effect on the ATP-induced Ca2+ responses. We conclude, that C2C12 cells express two distinct binding sites for GTP before differentiation, but only one after, the low affinity binding site. These results suggest a possible role of the high affinity GTP binding site in early stage of development of skeletal muscle.

Glutamate-induced calcium increase in myotubes depends on up-regulation of a sodium-dependent transporter

We report a study on the regulation by 2-chloro adenosine (2CA) of a glutamate (Glu) transporter in myogenic C2C12 cells. Long-term 2CA exposition significantly increased the V(max) of the Glu transporter. Moreover, 2CA-treated cells responded to Glu challenge by a rapid and transient increase in their intracellular calcium level. The above reported effects were totally abolished by treating C2C12 cells with the Na(+)-dependent Glu transporter inhibitors DL-threo-b-hydroxyaspartic acid and L-trans-pyrrolidine-2,4-dicarboxylic acid. We propose that the possible link between the Glu uptake increase and the Glu induction of calcium rise could be the depolarizing currents carried by Na(+) coupled with transporter activity.

Nitric oxide affects sarcoplasmic calcium release in skeletal myotubes

In the present study, we used real-time confocal microscopy to examine the effects of two nitric oxide (NO) donors on acetylcholine (ACh; 10 microM)- and caffeine (10 mM)-induced intracellular calcium concentration ([Ca2+]i) responses in C2C12 mouse skeletal myotubes. We hypothesized that NO reduces [Ca2+]i in activated skeletal myotubes through oxidation of thiols associated with the sarcoplasmic reticulum Ca2+-release channel. Exposure to diethylamine NONOate (DEA-NO) reversibly increased resting [Ca2+]i level and resulted in a dose-dependent reduction in the amplitude of ACh-induced [Ca2+]i responses (25 +/- 7% reduction with 10 microM DEA-NO and 78 +/- 14% reduction with 100 microM DEA-NO). These effects of DEA-NO were partly reversible after subsequent exposure to dithiothreitol (10 mM). Preexposure to DEA-NO (1, 10, and 50 microM) also reduced the amplitude of the caffeine-induced [Ca2+]i response. Similar data were obtained by using the chemically distinct NO donor S-nitroso-N-acetyl-penicillamine (100 microM). These results indicate that NO reduces sarcoplasmic reticulum Ca2+ release in skeletal myotubes, probably by a modification of hyperreactive thiols present on the ryanodine receptor channel.

Calcium increase in mouse skeletal muscles by triparanol: a drug to induce myotonic dystrophy-like clinical manifestations

Triparanol (Trp) is known to cause clinical features similar to those seen in myotonic dystrophy, including myotonia, cataract and baldness. To explore the pathophysiological mechanism of myotonic dystrophy, we examined the effect of Trp on intracellular calcium in cultured skeletal myoblasts and myotubes as well as cardiac myocytes by using a fluorescent indicator. Trp preferentially induced increase of intracellular calcium in myotubes of skeletal muscles. Since the increase of calcium was inhibited by thapsigargin pretreatment but not by extracellular calcium elimination, it appears that triparanol might act mostly on intracellular calcium stores. Trp also inhibited the increase of calcium in myotubes induced by acetylcholine. Trp might cause myotonia possibly through the increase of intracellular calcium from intracellular stores.

Retrograde Ca2+ signaling in C2C12 skeletal myocytes in response to mitochondrial genetic and metabolic stress: a novel mode of inter-organelle crosstalk

We have investigated the mechanism of mitochondrial-nuclear crosstalk during cellular stress in mouse C2C12 myocytes. For this purpose, we used cells with reduced mitochondrial DNA (mtDNA) contents by ethidium bromide treatment or myocytes treated with known mitochondrial metabolic inhibitors, including carbonyl cyanide m-chlorophenylhydrazone (CCCP), antimycin, valinomycin and azide. Both genetic and metabolic stresses similarly affected mitochondrial membrane potential (Deltapsim) and electron transport-coupled ATP synthesis, which was also accompanied by an elevated steady-state cytosolic Ca2+ level ([Ca2+]i). The mitochondrial stress resulted in: (i) an enhanced expression of the sarcoplasmic reticular ryanodine receptor-1 (RyR-1), hence potentiating the Ca2+ release in response to its modulator, caffeine; (ii) enhanced levels of Ca2+-responsive factors calineurin, calcineurin-dependent NFATc (cytosolic counterpart of activated T-cell-specific nuclear factor) and c-Jun N-terminal kinase (JNK)-dependent ATF2 (activated transcription factor 2); (iii) reduced levels of transcription factor, NF-kappaB; and (iv) enhanced transcription of cytochrome oxidase Vb (COX Vb) subunit gene. These cellular changes, including the steady-state [Ca2+]i were normalized in genetically reverted cells which contain near-normal mtDNA levels. We propose that the mitochondria-to-nucleus stress signaling occurs through cytosolic [Ca2+]i changes, which are likely to be due to reduced ATP and Ca2+ efflux. Our results indicate that the mitochondrial stress signal affects a variety of cellular processes, in addition to mitochondrial membrane biogenesis.

Presynaptic nicotinic receptors facilitate monoaminergic transmission

Nicotine is reported to increase arousal and attention and to elevate mood, effects that are most often associated with changes in the function of monoaminergic neuromodulatory systems (Feldman et al., 1997). Recent studies have shown a nicotinic receptor-mediated presynaptic enhancement of fast glutamatergic (McGehee et al., 1995; Gray et al., 1996) and GABAergic (Lena and Changeux, 1997) transmission. However, the mechanism of nicotinic effects on metabotropic-mediated transmission in general, and on monoaminergic transmission in particular, is less well understood. We have examined nicotinic effects on dorsal raphe neurons of rats using whole-cell current and voltage-clamp recording techniques in vitro. In the majority of these neurons, activation of presynaptic nicotinic receptors induced a depolarization mediated by norepinephrine acting on alpha1 receptors. Blockade of this response revealed a hyperpolarization mediated by serotonin acting on 5-HT1A receptors. Because the norepinephrine effect was sensitive to methyllycaconitine (100 nM), it is concluded that nicotinic receptors with an alpha7 subunit can facilitate release of norepinephrine to activate metabotropic receptors. In contrast, methyllycaconitine-insensitive nicotinic receptors can induce 5-HT release in the dorsal raphe nucleus.

Nicotine and nicotinic receptors in the circadian system

Considerable data support a role for cholinergic influences on the circadian system. The extent to which these influences are mediated by nicotinic acetylcholine receptors (nAChRs) has been controversial, as have the specific actions of nicotine and acetylcholine in the suprachiasmatic nucleus (SCN) of the hypothalamus. In this article we review the existing literature and present new data supporting an important role for nAChRs in both the developing and adult SCN. Specifically, we present data showing that nicotine is capable of causing phase shifts in the circadian rhythms of rats. Like light and carbachol, nicotine appears to cause phase delays in the early subjective night and phase advances in the late subjective night. In the isolated SCN slice, however, only phase advances are seen, and, surprisingly, nicotine appears to cause the inhibition rather than the excitation of neurons. Among nAChR subunit mRNAs, alpha 7 appears to be the most abundant subunit in the adult SCN, whereas in the perinatal period, the more typical nAChRs with higher affinity for nicotine predominate in the SCN. This developmental change in subunit expression may explain the dramatic sensitivity of the perinatal SCN to nicotine that we have previously observed. The effects of nicotine on the SCN may contribute to alterations caused by nicotine in other physiological systems. These effects might also contribute to the dependence properties of nicotine through influences on arousal.

Role of NMDA and non-NMDA ionotropic glutamate receptors in traumatic spinal cord axonal injury

We examined the role of glutamatergic mechanisms in acute injury to rat spinal cord white matter. Compound action potentials (CAPs) were recorded from isolated dorsal column segments in vitro. Under control conditions (Ringer's solution), the CAPs decreased to 71.4 +/- 2.0% of preinjury values after compression injury with a clip exerting a closing force of 2 g. The combination of the NMDA receptor blocker APV (50 microM) and the AMPA/kainate (KA) receptor blocker CNQX (10 microM) resulted in significantly improved recovery of CAP amplitude postinjury; however, the NMDA receptor antagonist APV alone did not enhance postinjury recovery, and infusion of NMDA (10 microM) did not affect recovery of the CAPs. In contrast, the AMPA/KA receptor blockers NBQX (10 microM) or CNQX (10 microM) significantly enhanced the recovery of CAP amplitude postinjury. The agonists AMPA (100 microM) or KA (100 microM) resulted in significant attenuation of CAP amplitude postinjury. Coapplication of AMPA/KA plus NBQX and CNQX was also associated with improved functional recovery. After incubation with AMPA and KA, Co(2+)-positive glia were visualized in spinal cord white matter. Similar results were seen after compressive injury but not in control cords. Immunohistochemistry and Western blot analysis demonstrated AMPA (GluR4)- and KA (GluR6/7 and KA2)-positive astrocytes in spinal cord white matter. In summary, non-NMDA ionotropic glutamate receptors seem to be involved in the pathophysiology of traumatic spinal cord injury. The presence of AMPA (GluR4) and KA (GluR6/7 and KA2) receptors on periaxonal astrocytes suggests a role for these cells in glutamatergic white matter injury.

Cholinergic stimulation of human microcytoma cell line H69

Experiments were addressed to investigate the mechanisms by which cholinergic stimulation is coupled to the enhancement of proliferation of small cell lung cancer cells H69. Muscarinic stimulation triggers the release of cytosolic Ca2+ and of inositol(1,4,5)trisphosphate with comparable time courses. The presence of alpha-bungarotoxin or the absence of Ca2+ in external medium suppresses enhancement of clonal growth induced by brief applications of nicotine. Here we suggest that Ca2+ mobilization represents a trigger for the enhancement of small cell lung cancer cell proliferation upon cholinergic stimulation.

Different mechanisms of Ca2(+)-handling following nicotinic acetylcholine receptor stimulation, P2U-purinoceptor stimulation and K(+)-induced depolarization in C2C12 myotubes

1. The increase in intracellular CA2+ on nicotinic acetylcholine receptor (nAChR) stimulation, P2U-purinoceptor stimulation and K(+)-induced depolarization was investigated in mouse C2C12 myotubes by use of fura-2 fluorescence to characterize the intracellular organisation of Ca2+ releasing stores and Ca(2+)-entry process. 2. Stimulation of nAChRs with carbachol induced a rapid rise in internal Ca2+ (EC50 = 0.85 +/- 0.09 microM), followed by a sustained phase. The Ca2+ response evoked by carbachol (10 microM) was completely blocked by the nAChR antagonist, pancuronium (3 microM), but was not affected by the muscarinic antagonist, atropine (3 microM), or under conditions when Ca2+ entry was blocked by La3+ (50 microM) or diltiazem (10 microM). Addition of pancuronium (3 microM) during the sustained phase of the carbachol-evoked response did not affect this phase. 3. Stimulation of P2U purinoceptors with ATP (1 mM) induced a somewhat higher biphasic Ca2+ response (EC50 of the rapid phase: 8.72 +/- 0.08 microM) than with carbachol. Pretreatment with La3+ abolished the sustained phase of the ATP-induced Ca2+ response, while the response was unaffected by diltiazem or pancuronium. 4. Stimulation of the cells with high K+ (60 mM), producing the same depolarization as with carbachol (10 microM), induced a rapid monophasic Ca2+ response, insensitive to diltiazem, pancuronium or La3+. 5. Under Ca(2+)-free conditions, the sustained phase of the carbachol- and ATP-evoked responses were abolished. Pre-emptying of depolarization-sensitive stores by high K+ under Ca(2+)-free conditions did not affect the carbachol- or ATP-evoked Ca2+ mobilization and vice versa. Preincubation of the cells with ATP in the absence of extracellular Ca2+ decreased the amplitude of the subsequent carbachol-induced Ca2+ response to 11%, while in the reverse procedure the ATP-induced response was decreased to 65%. Ca2+ mobilization evoked by simultaneous addition of optimal concentrations of carbachol and ATP was increased compared to levels obtained with either agonist. 6. Preincubation with high K+ under normal conditions abolished the sustained phase of the ATP-evoked Ca2+ response. The carbachol response consisted only of the sustained phase in the presence of high K+. 7. The carbachol-induced Ca2+ response was completely abolished under low Na+/Ca(2+)-free conditions, while under low Na+ conditions only a sustained Ca2+ response was observed. The ATP- and K(+)-induced responses were changed compared to Ca(2+)-free conditions. 8. ATP (300 microM) induced the formation of Ins(1,4,5)P3 under Ca(2+)-free conditions with a comparable time course to that found for the rise in internal Ca2+. In contrast to ATP, carbachol (10 microM) did not affect Ins(1,4,5)P3 levels under Ca(2+)-free conditions. 9. It is concluded that the Ca2+ release from discrete stores of C2C12 myotubes is induced by stimulation of nAChRs, P2U-purinoceptors and by high K+. Only the P2U-purinoceptor and nAChR activated stores show considerable overlap in releasable Ca2+. Sustained Ca(2+)-entry is activated by stimulation of nAChRs and P2U-purinoceptors via separate ion-channels, which are different from the skeletal muscle nAChR-coupled cation-channel.

Activation of nicotinic acetylcholine receptors increases the rate of fusion of cultured human myoblasts

1. Fusion of myogenic cells is important for muscle growth and repair. The aim of this study was to examine the possible involvement of nicotinic acetylcholine receptors (nAChR) in the fusion process of myoblasts derived from postnatal human satellite cells. 2. Acetylcholine-activated currents (ACh currents) were characterized in pure preparations of freshly isolated satellite cells, proliferating myoblasts, myoblasts triggered to fuse and myotubes, using whole-cell and single-channel voltage clamp recordings. Also, the effect of cholinergic agonists on myoblast fusion was tested. 3. No nAChR were observed in freshly isolated satellite cells. nAChR were first observed in proliferating myoblasts, but ACh current densities increased markedly only just before fusion. At that time most mononucleated myoblasts had ACh current densities similar to those of myotubes. ACh channels had similar properties at all stages of myoblast maturation. 4. The fraction of myoblasts that did not fuse under fusion-promoting conditions had no ACh current and thus resembled freshly isolated satellite cells. 5. The rate of myoblast fusion was increased by carbachol, an effect antagonized by alpha-bungarotoxin, curare and decamethonium, but not by atropine, indicating that nAChR were involved. Even though a prolonged exposure to carbachol led to desensitization, a residual ACh current persisted after several days of exposure to the nicotinic agonist. 6. Our observations suggest that nAChR play a role in myoblast fusion and that part of this role is mediated by the flow of ions through open ACh channels.

Acetylcholine contractures of skeletal muscles: inhibition by chlorpromazine and diltiazem

The maximum contractures evoked by 100 microM ACh of mouse soleus muscles denervated for 3-7 days are completely inhibited by a 10 min exposure to 100 microM chlorpromazine (CP). Recovery on washout of CP takes more than 1 hr to complete. ACh evoked contractures are also inhibited by diltiazem (D); washout of D is immediately followed by recovery. Electrically evoked twitches and K evoked contractures are largely unaffected by CP, caffeine evoked contractures are decreased but not abolished. Fast mouse and (non-denervated) frog tonic muscles behave similarly. Depolarization by ACh and ACh-evoked whole cell currents show enhanced desensitization at low, and block at high [CP] and [D]; more than 50% recovery is achieved by less than 1 min washout of CP and D. Currents carried by Na+ and Mg2+ ions behave similarly. It is concluded that activation of ACh evoked contractures may be blocked by CP independent of ionic currents through nicotinic ACh receptors and that it depends on intracellular processes linked to these receptors.

Acetylcholine-activated inward current induces cytosolic Ca2+ mobilization in mouse C2C12 myotubes

We examined the spatiotemporal pattern of intracellular Ca2+ liberation in mouse myotubes by means of fluorescence imaging of cytosolic free Ca2+ together with the simultaneous recording of membrane whole-cell currents. Acetylcholine (ACh) applications to C2C12 myotubes equilibrated in Ca(2+)-free medium and voltage clamped at -50 mV evoked localized fluorescence transients of variable amplitude with less than 0.5 s delay. Under the same experimental conditions, fluorescence transients were elicited by ACh also in mouse primary myotubes. Ca2+ transients were inhibited in myotubes clamped at depolarized potentials (-10 mV to +50 mV), or equilibrated in a Na+,Ca(2+)-free medium as well as in cells loaded with heparin, or with inositol (1,4,5) trisphosphate (InsP3). To investigate whether InsP3 could induce Ca2+ mobilization, [Ca2+]i determinations were carried out in myotubes loaded with InsP3 through the whole-cell patch-clamp recording pipette or by extracellular application in permeabilized cells. InsP3 diffusion into the myoplasm caused Ca2+ spikes with 5 +/- 1 s (mean +/- SEM) delay from the rupture of the membrane patch. Spikes were followed by sustained increases in fluorescence or by damped oscillations. In permeabilized myotubes, InsP3 induced the release of sequestered 45Ca2+ with a half-maximally effective concentration (EC50) of 0.28 +/- 0.05 microM, and Hill coefficient of 0.79 +/- 0.09. It is concluded that the ACh-activated inward current in mouse myotubes is coupled to cytosolic Ca2+ mobilization from internal InsP3-sensitive pools.

Activation of nicotinic receptors triggers exocytosis from bovine chromaffin cells in the absence of membrane depolarization

The traditional function of neurotransmitter-gated ion channels is to induce rapid changes in electrical activity. Channels that are Ca(2+)-permeable, such as N-methyl-D-aspartate receptors at depolarized membrane potentials, can have a broader repertoire of consequences, including changes in synaptic efficacy, developmental plasticity, and excitotoxicity. Neuronal nicotinic receptors for acetylcholine (nAChRs) are usually less Ca(2+)-permeable than N-methyl-D-aspartate receptors but have a significant Ca2+ permeability, which is greater at negative potentials. Here we report that in neuroendocrine cells, activation of nAChRs can trigger exocytosis at hyperpolarized potentials. We used whole-cell patch-clamp recordings to record currents and the capacitance detection technique to monitor exocytosis in isolated bovine chromaffin cells. Stimulation of nAChRs at hyperpolarized potentials (-60 or -90 mV) evokes a large current and a maximal capacitance increase corresponding to the fusion of approximately 200 large dense-core vesicles. The amount of exocytosis is controlled both by the Ca2+ influx through nAChRs and by a contribution from thapsigargin-sensitive Ca2+ sequestering stores. This is a form of neurotransmitter action in which activation of nAChRs triggers secretion through an additional coupling pathway that coexists with classical voltage-dependent Ca2+ entry.

Functional characterization of a nonclassical nicotine receptor associated with inositolphospholipid breakdown and mobilization of intracellular calcium pools

Classical nicotinic receptors are neurotransmitter-gated channels that, upon activation by acetylcholine, induce the opening of an intrinsic cationic channel. We have recently observed that, in frog pituitary melanotrophs, nicotine stimulates alpha-melanocyte-stimulating hormone (alpha-MSH) release through a noncholinergic mechanism. In the study reported here, we investigated the intracellular events that mediate the response of frog melanotrophs to nicotine. Nicotine was capable of stimulating alpha-MSH release in the absence of Ca2+ and/or Na+ in the extracellular medium. A short pulse of nicotine induced a rapid and transient increase of cytosolic free Ca2+ concentration ([Ca2+]i). The effect of nicotine on Ca2+ mobilization was not affected in the absence of Na+ and Ca2+ in the extracellular medium, indicating that the nicotine-evoked increase in [Ca2+]i did not result from Na+ or Ca2+ influx. Nicotine induced both an increase in inositol trisphosphate and a reduction in phosphaditylinositol bisphosphate concentrations but did not affect cAMP production. The present results indicate that nicotine-induced stimulation of alpha-MSH release in frog melanotrophs can be explained by activation of inositolphospholipid breakdown and mobilization of inositol triphosphate-dependent intracellular Ca2+ pools. These data provide evidence for the existence of an unusual type of noncholinergic nicotine receptor positively coupled to phospholipase C.

TNF-alpha increases the frequency of spontaneous miniature synaptic currents in cultured rat hippocampal neurons

Tumor necrosis factor-alpha (TNF-alpha) is a cytokine secreted by activated astrocytes and is known to alter evoked synaptic activity in slices of adult rat hippocampus. In this paper we show that TNF-alpha increases the frequency of spontaneous miniature synaptic currents in cultured hippocampal neurons, acting at nanomolar concentrations. In addition, we show that the mRNA for the 55 kDa TNF-alpha receptor (TNF-R1) is detected in embryonic rat hippocampal cultures, as well as in acutely dissected embryonic and adult rat hippocampi. Possible transduction pathways mediating the TNF-alpha effect are discussed.

Ca2+ signalling pathways activated by acetylcholine in mouse C2C12 myotubes

In mouse C2C12 myotubes acetylcholine (ACh) elevates the concentration of myoplasmic Ca2+ ([Ca2+]i) by inducing Ca2+ influx through transmitter-gated and voltage-gated channels, and by mobilizing Ca2+ from internal stores. The relative contribution of each of these ACh-activated sources to the global [Ca2+]i elevation was estimated. We found that Ca2+ entry through voltage- and ACh-gated channels accounts for roughly 80% of the total [Ca2+]i increment, while mobilization from internal caffeine-sensitive and inositoltrisphosphate- (InsP3-) sensitive stores contributes the remaining 20% to the maximal [Ca2+]i increment. Furthermore, we found that ACh-induced mobilization from InsP3-sensitive stores also develops in embryonic chick myotubes. The differential importance of the Ca2+ signalling pathways activated by ACh during myogenesis is discussed.

Independent regulation of activation and inactivation phases in non-contractile Ca2+ transients by nicotinic receptor at the mouse neuromuscular junction

Non-contractile Ca2+ mobilization (not accompanied by muscle contraction) occurs by the prolonged activation of nicotinic acetylcholine receptor in mouse diaphragm muscles treated with anticholinesterase. To elucidate the regulation properties of non-contractile Ca2+ mobilization by nicotinic receptor, the modes of action of competitive and depolarizing neuromuscular blockers were investigated. (+)-Tubocurarine (0.07-0.1 microM), pancuronium (0.05 microM) and alpha-bungarotoxin (0.03-0.06 microM) decreased decay time (T2, duration of inactivation phase) without changes in rise time (T1, duration of activation phase) of non-contractile Ca2+ transients. These competitive antagonists also suppressed their peak amplitude at higher concentrations than those affecting T2. Contractile Ca2+ transients were not inhibited by these antagonists at the concentrations used. Decamethonium (1 microM), a depolarizing blocker, suppressed the peak amplitude of non-contractile Ca2+ transients without affecting their duration. In contrast, succinylcholine (0.3 microM) suppressed both peak amplitude and T1 without changing T2, presumably via the receptor desensitization. Succinylcholine but not decamethonium inhibited contractile Ca2+ transients at the concentrations used. These results demonstrate that the activation and inactivation phases in non-contractile Ca2+ transients are independently regulated by nicotinic acetylcholine receptor.

Induction of Na+/K(+)-ATPase activity by long-term stimulation of nicotinic acetylcholine receptors in C2C12 myotubes

1. To investigate the role of long-term stimulation of nicotinic acetylcholine receptors (AChRs) on the regulation of membrane potential, non-contracting C2C12 myotubes were stimulated for 1-4 days with carbachol (10 microM) and membrane potentials were measured by the intracellular microelectrode technique after washing out of the drug. 2. The membrane potential (-45.7 mV) gradually increased by 10.1 mV to -55.8 mV during 4 days treatment, which was caused by enhanced electrogenic Na+/K(+)-pumping. 3. The concentration-dependent enhancement of Na+/K(+)-ATPase activity in long-term carbachol-treated myotubes (4 days, EC50 = 5.3 microM) was prevented by co-treatment with the competitive nicotinic AChR antagonist, pancuronium but not by the muscarinic antagonist, atropine. 4. Enhanced Na+/K(+)-ATPase activity still developed in carbachol-stimulated myotubes during co-treatment (4 days) with the nicotinic AChR-channel blocker, chlorpromazine (1 microM). Membrane depolarization as such, obtained by incubation in high K+ medium (40 mM, 4 days) did not enhance Na+/K(+)-ATPase activity. 5. Non-treated myotubes possessed a high-affinity ouabain binding site (Kd = 119 nM) in association with the low Na+/K(+)-pumping activity. Long-term stimulation of myotubes (4 days) with carbachol or with a combination of carbachol and chlorpromazine was accompanied by the development of an additional low-affinity ouabain binding site (Kd = 13 microM). 6. Binding of monoclonal antibodies directed against either alpha 1- or alpha 2-subunit of Na+/K(+)-ATPase were both increased in myotubes treated with carbachol (4 days). 7. These results support the concept that nicotinic AChRs regulate Na+/K(+)-ATPase activity, independent of the functionality of the receptor-operated ion-channel.

Cholinergic responses in cloned human TE671/RD tumour cells

The cholinergic responses of the human tumour cell line TE671/RD were examined using digital Ca2+ imaging fluorescence microscopy and patch-clamp measurements. In response to stimulation of the muscarinic acetylcholine (ACh) receptor (mAChR), the intracellular concentration of Ca2+ ([Ca2+]i) rose about two-fold, in parallel with inositol 1,4,5-trisphosphate accumulation, measured by chromatographic techniques. By contrast, there was no increment of [Ca2+]i upon stimulation of the nicotinic ACh receptor (nAChR), nor after caffeine application. Electrophysiological experiments showed that TE671/RD cells lack functional voltage-activated Ca2+ channels. The stimulation of the nAChR induced transient whole-cell currents (IACh). Little or no current was detected in isotonic extracellular Ca2+, with Cs+ in the patch pipette. Cell pretreatment with muscarine reduced IACh by about 20%, without consistent modifications of current kinetics. Muscarine applied to the extra-patch membrane under the cell-attached configuration had no obvious effect on ACh-evoked unitary events. In conclusion, in human TE671/RD cells, muscarinic stimulation increases [Ca2+]i, while nicotinic stimulation does not. In addition, the nAChR exhibits peculiar ion permeability properties and is not functionally regulated by the breakdown of phosphoinositides.