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  • 1
    Electronic Resource
    Electronic Resource
    Molecular evolution of the nicotinic acetylcholine receptor: An example of multigene family in excitable cells (1995)
    Springer
    Journal of molecular evolution 40 (1995), S. 155-172 
    Details
    ISSN: 1432-1432
    Keywords: Nicotinic receptor ; Ligand-gated channel ; Multigene family ; Gene phylogeny
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract An extensive phylogenetic analysis of the nicotinic-acetylcholine-receptor subunit gene family has been performed by cladistic and phenetic methods. The conserved parts of amino acid sequences have been analyzed by CLUSTAL V and PHYLIP software. The structure of the genes was also taken in consideration. The results show that a first gene duplication may have occurred before the appearance of Bilateria. Three subfamilies then appeared: I-the neuronal α-bungarotoxin binding-site subunits (α7, α8); III-the neuronal nicotinic subunits (α2–α6, β2–β4), which also contain the muscle acetylcholine-binding subunit (α1); and IV—the muscle non-α subunits (β1, γ δ, ε). The Insecta subunits (subfamily II) could be orthologous to family III and IV. Several tissular switches of expression from neuron to muscle and the converse can be inferred from the extant expression of subunits and the reconstructed trees. The diversification of the neuronal nicotinic subfamily begins in the stem lineage of chordates, the last duplications occurring shortly before the onset of the mammalian lineage. Such evolution parallels the increase in complexity of the cholinergic systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00167110
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  • 2
    Electronic Resource
    Electronic Resource
    Assessment of nicotinic acetylcholine receptor subunit contributions to nicotine self-administration in mutant mice (1999)
    Springer
    Psychopharmacology 147 (1999), S. 25-26 
    Details
    ISSN: 1432-2072
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002130051135
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  • 3
    Electronic Resource
    Electronic Resource
    A kinetic mechanism for nicotinic acetylcholine receptors based on multiple allosteric transitions (1996)
    Springer
    Biological cybernetics 75 (1996), S. 361-379 
    Details
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract.  Nicotinic acetylcholine receptors are transmembrane oligomeric proteins that mediate interconversions between open and closed channel states under the control of neurotransmitters. Fast in vitro chemical kinetics and in vivo electrophysiological recordings are consistent with the following multi-step scheme. Upon binding of agonists, receptor molecules in the closed but activatable resting state (the Basal state, B) undergo rapid transitions to states of higher affinities with either open channels (the Active state, A) or closed channels (the initial Inactivatable and fully Desensitized states, I and D). In order to represent the functional properties of such receptors, we have developed a kinetic model that links conformational interconversion rates to agonist binding and extends the general principles of the Monod-Wyman-Changeux model of allosteric transitions. The crucial assumption is that the linkage is controlled by the position of the interconversion transition states on a hypothetical linear reaction coordinate. Application of the model to the peripheral nicotinic acetylcholine receptor (nAChR) accounts for the main properties of ligand-gating, including single-channel events, and several new relationships are predicted. Kinetic simulations reveal errors inherent in using the dose-response analysis, but justify its application under defined conditions. The model predicts that (in order to overcome the intrinsic stability of the B state and to produce the appropriate cooperativity) channel activation is driven by an A state with a Kd in the 50 nM range, hence some 140-fold stronger than the apparent affinity of the open state deduced previously. According to the model, recovery from the desensitized states may occur via rapid transit through the A state with minimal channel opening, thus without necessarily undergoing a distinct recovery pathway, as assumed in the standard ‘cyclic’ model. Transitions to the desensitized states by low concentration ‘pre-pulses’ are predicted to occur without significant channel opening, but equilibrium values of IC50 can be obtained only with long pre-pulse times. Predictions are also made concerning allosteric effectors and their possible role in coincidence detection. In terms of future developments, the analysis presented here provides a physical basis for constructing more biologically realistic models of synaptic modulation that may be applied to artificial neural networks.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004220050302
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  • 4
    Electronic Resource
    Electronic Resource
    Selective stabilization of muscle innervation during development: A mathematical model (1983)
    Springer
    Biological cybernetics 46 (1983), S. 207-215 
    Details
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract The biochemical model presented concerns a critical step of the development of skeletal muscle innervation. After invasion of the muscle by exploratory motor axons, several nerve terminals converge from different motoneurons onto each muscle fibre at a single endplate. During the folloing weeks the redundant innervation disappears: a single nerve ending per muscle fibre becomes stabilized. The model is based on the assumption that the numbers of motoneurons and of muscle fibres remain constant during this evolution and that the selective stabilization of the adult connectivity results from the competition of the active nerve terminals for a postsynaptic retrograde factor μ. At the peak of the multiple innervation, the synthesis of μ by the muscle fiber stops, possibly as a consequence of muscle electrical and/or mechanical activity. The stock of μ becomes limited; a retrograde trans-synaptic diffusion of μ from the muscle to the nerve endings takes place. Within each nerve ending, μ enters into a chemical autocatalytic reaction which results in the production of a presynaptic stabilization factor s. The nerve impulses reaching the nerve terminal initiate this reaction. Any given nerve terminal become stabilized when the concentration of s reaches a threshold value. The mathematical analysis of the model shows that there exists a unique solution which is physically acceptable. Its application and computer simulation predict that only one nerve terminal becomes stabilized per muscle fibre. The model accounts for the experimental observations that the reduction in size of the motor units is not necessarily accompanied by a reduction in the variability of their size. The model also accounts for the acceleration or delay in regression which follows modifications of the chronic activity of the nerve endings and for the variability of the pattern of innervation observed in isogenic organisms. Plausible biochemical hypotheses concerning the factors engaged in the “selective stabilization” of the nerve-endings are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00336802
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  • 5
    Electronic Resource
    Electronic Resource
    In Vitro excitation of purified membrane fragments by cholinergic agonists (1971)
    Springer
    The journal of membrane biology 6 (1971), S. 24-57 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The effect of carbamylcholine (Carb) on the release of22Na+ by excitable microsacs is reversible: Carb does not promote an irreversible lysis of the microsacs. The microsacs form closed vesicles which are more permeable to water than to solutes present in physiological media; their apparent volume is controlled by the osmotic pressure of the medium in which they are equilibrated; they behave like micro-osmometers. In the presence of Carb, the apparent volume of the microsacs does not change: Carb has no significant effect on water permeability. The time course of22Na+ release does not follow a simple exponential law and is fitted by a minimum of three exponentials. The complex kinetics are not due to electrical effects but presumably are caused by an heterogeneity both in size and in nature of the microsacs population. Microsacs at rest are permeable to45Ca+,42K+ and22Na+, and to36Cl− but slightly or not at all to32S−SO −2 4 . In the presence of Carb, the permeability to22Na+,42K+ and45Ca++ increases, whereas the permeability to14C-tetraethylammonium and14C-choline does not change. Carb has no effect on the permeability to negatively charged or uncharged permeants. The kinetics of22Na+ efflux is independent of the total concentration of Na+ either inside or outside the microsacs. The outward transport of Na+ thus varies only with the concentration of ion facing theinside of the membrane. Efflux of42K+ follows the same laws as the efflux of22Na+ except that it is blocked by+Na ions present in the outside medium.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01874113
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  • 6
    Electronic Resource
    Electronic Resource
    In vitro excitation of purified membrane fragments by cholinergic agonists (1971)
    Springer
    The journal of membrane biology 6 (1971), S. 58-80 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The reversible binding of14C-decamethonium (Deca) to excitable microsacs prepared from the electric tissue ofElectrophorus electricus is followed by an ultracentrifugal assay. α-Bungarotoxin, a snake venom toxin, blocks irreversibly the binding of14C-Deca. The displacement is partial. The fraction of14C-Deca displaced by α-bungarotoxin corresponds to molecules of Deca bound to the cholinergic receptor site, whereas the fraction of14C-Deca bound in the presence of α-bungarotoxin corresponds to molecules bound to the catalytic site of acetylcholinesterase (AcChE). The total number of cholinergic receptor sites is found to be close but not identical to the total number of catalytic sites of AcChE. On the same preparation of microsacs, the binding of14C-Deca and the permeability response corresponding to a given concentration of Deca are measured as a function of increased concentration of Deca. The dose-response curve and the binding curve superimpose almost exactly; in other words, the “apparent” affinity of Deca coincides with its “real” affinity. Displacement of14C-Deca byd-tubocurarine gives an “apparent” affinity ford-tubocurarine which coincides as well with its “real” affinity. The transport properties of the ionophore controlled by one Deca binding site are estimated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01874114
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  • 7
    Electronic Resource
    Electronic Resource
    In Vitro excitation of purified membrane fragments by cholinergic agonists (1971)
    Springer
    The journal of membrane biology 6 (1971), S. 1-23 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Excitation of membrane fragments by cholinergic agonists is measuredin vitro by a filtration technique. Membrane fragments which contain high levels of the enzyme acetylcholinesterase and presumably originate from the innervated excitable faces of electroplax are first purified from homogenates of electric organ ofElectrophorus electricus by centrifugation in a sucrose gradient. Then the fragments, which make closed vesicles or microsacs, are equilibrated overnight with a medium containing22Na+. After equilibration of the inside of the microsacs with the outside medium, the suspension is diluted into a nonradioactive medium. The22Na+ content of the microsacs as a function of time is then followed by rapid filtration on Millipore filters. In the presence of cholinergic agonists, the time course of22Na+ release changes: the rate of22Na+ release increases. This increase is blocked byd-tubocurarine and is absent with microsacs derived from the non-innervated inexcitable membrane of the electroplax. The response to cholinergic agonists is thus followed on a completely cell-free system, in a well-defined environment. The dose-response curves to cholinergic agents obtainedin vitro agree, quantitatively, with the dose-response curves recordedin vivo by electrophysiological methods. In particular, the dose-response curve to agonists is sigmoid, the antagonism betweend-tubocurarine and carbamylcholine competitive, and the antagonism between tetracaine and carbamylcholine noncompetitive. The effects of two different affinity labeling reagents on the response to agonists and on the catalytic activity of acetylcholinesterase are followed in parallel on the same microsac preparation. The effects of dithiothreitol and of gramicidin A on the microsacs are studied and are found to be similar to those observedin vivo with the isolated electroplax.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01874112
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  • 8
    Electronic Resource
    Electronic Resource
    In vitro excitation of purified membrane fragments by cholinergic agonists (1971)
    Springer
    The journal of membrane biology 6 (1971), S. 81-88 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Membrane fragments rich in acetylcholinesterase (AcChE) or in Na+−K+-ATPase are observed under the electron microscope on thin sections after fixation, after negative staining of unfixed material, and after freeze-etching. Both classes of membrane fragments make closed approximately spherical vesicles, or microsacs. The preparation appears to be free from mitochondria, nuclear envelopes or other cytoplasmic contamination. A subunit structure is seen with both kinds of microsacs by freeze-etching and negative staining, but the size, shape and arrangement of the subunits are different in the two classes of membrane fragments. On thin sections, globular repeating units are seen only with the AcChE-rich microsacs; the membrane of the ATPase-rich microsacs shows a classic triple-layered structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01874115
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  • 9
    Electronic Resource
    Electronic Resource
    Changes in extrinsic fluorescence intensity of the electroplax membrane during electrical excitation (1971)
    Springer
    The journal of membrane biology 5 (1971), S. 102-120 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The fluorescent dye 1-anilinonapthalene-8-sulfonic acid (ANS) has been used as a probe of changes in membrane conformation accompanying excitation of the electroplax ofElectrophorus electricus. ANS binds reversibly to the excitable membrane at rest. During generation of an action potential, an increase in ANS-fluorescence intensity is observed which resembles but does not strictly follow the membrane potential. Experiments using the current-clamp technique have demonstrated a linear relationship between the change in membrane potential and the change in ANS fluorescence intensity. The change in fluorescence intensity is not a consequence of binding to membrane sites of increased affinity nor of an electrophoretic concentration of ANS molecules at the membrane surface. It is not known whether the change in fluorescence intensity is due to a change in quantum yield of bound ANS or to an increase in the amount of bound ANS. In either case, the change in fluorescence intensity may be interpreted as a change in membrane conformation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01870827
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  • 10
    Electronic Resource
    Electronic Resource
    Membrane excitability and dissipative instabilities (1970)
    Springer
    The journal of membrane biology 2 (1970), S. 351-374 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Electrical excitation is interpreted in terms of a cooperative structural transition of membrane protomers coupled with the translocation of a permeant molecule in a non-equilibrium environment. Equations for flow of permeant and for membrane conformation are derived for the simple case of a single non-charged permeant. On the basis of a few simple physical assumptions, the theory predicts several important properties of electrically excitable membranes: the steepness of the relation between membrane conductance and potential, the presence of a negative conductance, and the occurrence of instabilities following rapid perturbations of membrane environment, giving rise to some simple cases of action potentials. Several experimental tests of the membrane with its changes of electrical properties are proposed. From a thermodynamic point of view, an electrically excitable membrane, in its resting state, lies beyond a dissipative instability and consequently is in a non-equilibrium state but with stable organization, a “dissipative structure” of Prigogine. Membrane excitation following a small perturbation of the environment would correspond to a jump from such an organization to another stable organization but close to thermodynamic equilibrium. It is shown how the cooperative molecular properties of the membrane are amplified by energy dissipation at the macroscopic level.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01869870
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