Acetylcholine receptors

Biomolecular recognition of antagonists by α7 nicotinic acetylcholine receptor: Antagonistic mechanism and structure-activity relationships studies

As the key constituent of ligand-gated ion channels in the central nervous system, nicotinic acetylcholine receptors (nAChRs) and neurodegenerative diseases are strongly coupled in the human species. In recently years the developments of selective agonists by using nAChRs as the drug target have made a large progress, but the studies of selective antagonists are severely lacked. Currently these antagonists rest mainly on the extraction of partly natural products from some animals and plants; however, the production of these crude substances is quite restricted, and artificial synthesis of nAChR antagonists is still one of the completely new research fields. In the context of this manuscript, our primary objective was to comprehensively analyze the recognition patterns and the critical interaction descriptors between target α7 nAChR and a series of the novel compounds with potentially antagonistic activity by means of virtual screening, molecular docking and molecular dynamics simulation, and meanwhile these recognition reactions were also compared with the biointeraction of α7 nAChR with a commercially natural antagonist - methyllycaconitine. The results suggested clearly that there are relatively obvious differences of molecular structures between synthetic antagonists and methyllycaconitine, while the two systems have similar recognition modes on the whole. The interaction energy and the crucially noncovalent forces of the α7 nAChR-antagonists are ascertained according to the method of Molecular Mechanics/Generalized Born Surface Area. Several amino acid residues, such as B/Tyr-93, B/Lys-143, B/Trp-147, B/Tyr-188, B/Tyr-195, A/Trp-55 and A/Leu-118 played a major role in the α7 nAChR-antagonist recognition processes, in particular, residues B/Tyr-93, B/Trp-147 and B/Tyr-188 are the most important. These outcomes tally satisfactorily with the discussions of amino acid mutations. Based on the explorations of three-dimensional quantitative structure-activity relationships, the structure-antagonistic activity relationships of antagonists and the characteristics of α7 nAChR-ligand recognitions were received a reasonable summary as well. These attempts emerged herein would not only provide helpful guidance for the design of α7 nAChR antagonists, but shed new light on the subsequent researches in antagonistic mechanism.

Anti-inflammatory effects of Neurotoxin-Nna, a peptide separated from the venom of Naja naja atra

Background

Neurotoxin-Nna (NT), an analgesic peptide separated from the venom of Naja naja atra, has reported to have an exceptional specificity to block transmission of the nerve impulse by binding to the α- subunit of the nicotinic acetylcholine receptor in the membrane. However, little information is available on the anti-inflammatory effects of NT. Therefore, the anti-inflammatory activity of Neurotoxin-Nna was investigated in this study.

Methods

The anti-inflammatory effects of NT were evaluated by measuring its influence on several crucial factors in inflammatory pathways, including total antioxidant activity, antinociceptive effects in vivo, nuclear factor kappa B (NF-κB), polymorphonuclear cells (PMN), inducible nitric oxide synthase (iNOS), adhesion molecule (ICAM-1) and tactile hyperalgesia.

Results

NT treatment decreased the levels of tumor necrosis factor alpha (TNF-α) and interleukin 1 beta (IL-1β). NT treatment decreased the total antioxidant status (TAOS) and reduced CFA-induced tactile hyperalgesia in a dose-dependent manner. NT significantly inhibited regulation of NF-kappaB activation and the production of IL-1β, TNF-α, iNOS and CAM-1. Moreover, NT suppressed infiltration of PMN.

Conclusions

Our results showed that NT reduced CFA-induced tactile hyperalgesia through inhibition inflammatory pathways in experimental inflammatory rats.

Antinociceptive properties of nasal delivery of neurotoxin-loaded nanoparticles coated with polysorbate-80

Neurotoxin-1 (NT) is an analgesic peptide which is endowed an exceptional specificity of action that blocks transmission of the nerve impulse. The aim of this study was to evaluate the potential application of nanoparticles technology as drug carrier system for the nasal delivery of NT. Mice were administered intranasally (i.n.) with NT (NT-P-NP), free NT solution (F-NT) and intravenously (i.v.) with NT (IV-NT) respectively. The NT levels in animal brain and antinociceptive activity of NT were analyzed. The result on brain transport showed that nanoparticles could exert enhanced delivery of NT into the brain significantly after i.n. administration. The results of antinociceptive activity showed that NT-P-NP increased immobility in the open-field test, both phases of formalin test were significantly inhibited by the NT-P-NP and NT-P-NP significantly inhibited the reaction time to thermal stimuli at 60 and 90 min. Both NT-P-NP and IV-NT were able to inhibit constrictions in acetic acid-induced writhing reaction. These data suggest that NT-loaded nanoparticles coated with polysorbate-80 could generate a significant improvement of drug levels in the brain. Intranasal administration of Neurotoxin-1 entrapped in nanoparticles coated with polysorbate-80 is an attractive alternative to intravenous administration.

Evaluation of the number of agonist molecules needed to activate a ligand-gated channel from the current rising phase

We propose a new method for calculating the number of agonist binding sites (n) in ligand-gated receptor channels from the initial phase of the current. This method is based on the fact that the relation between the current (I) and its first-time derivative (I') at the beginning of the current reflects the number of transitions that lead to channel opening. We show that, for constant agonist concentration, the above relationship at t --> 0 provides the number of steps leading to channel opening. When the agonist concentration is not constant but rather increases linearly with time, the corresponding value can be obtained using a slightly modified procedure. The analytical results were compared with computer simulations and a good match between the two was obtained. The theoretical procedure was then validated experimentally using the nicotinic receptor, because, for this receptor, the number of binding sites is well established. Indeed, the expected number of two binding sites was obtained. The method was then tested for the quisqualate-type glutamate receptor channel from the opener muscle of crayfish. The number of this receptor's binding sites is not fully resolved. Our results suggest that, for this glutamate receptor as well, two binding sites must be occupied to open the channel.

Cholinergic receptors and catecholamine secretion from adrenal chromaffin cells of the toad

1. The effects of cholinergic drugs on catecholamine (CA) secretion from adrenal chromaffin tissue of the toad were studied. 2. CA secretion was induced by ACh or nicotine, but not by muscarine. 3. Hexamethonium inhibited the CA release evoked by ACh or nicotine, while d-tubocurarine only affected the nicotinic response. Atropine did not prevent the secretory response. 4. Muscarine abolished the secretion induced by the agonists, this effect being prevented by atropine or gallamine, but not by pirenzepine. 5. In conclusion, CA secretion in the toad is stimulated by activation of nicotinic receptors. Inhibitory muscarinic receptors are present, most likely of type M2, which may play a regulatory function.

1H nuclear-magnetic-resonance studies of the three-dimensional structure of the cardiotoxin CTXIIb from Naja mossambica mossambica in aqueous solution and comparison with the crystal structures of homologous toxins

Using the previously reported sequence-specific 1H-NMR assignments, structural constraints for the cardiotoxin CTXIIb from Naja mossambica mossambica were collected. These include distance constraints from nuclear Overhauser enhancement measurements both in the laboratory and in the rotating frame, dihedral angle constraints derived from spin-spin coupling constants, and constraints from hydrogen bonds and disulfide bridges. Structure calculations with the distance geometry program DISMAN confirmed the presence of the previously identified antiparallel beta-sheets formed by residues 1-5 and 10-14, and by 20-27, 35-39 and 49-55, and established the nature of the connections between the individual beta-strands. These include a right-handed crossover between the two peripheral strands in the triple-stranded beta-sheet, and a type I tight turn immediately preceding the beta-strand 49-55. The spatial arrangement of the polypeptide backbone in the solution structure of CTXIIb is closely similar to that in the crystal structure of the homologous cardiotoxin VII4 from the same species. In an Appendix the origin of the large pH dependence of two amide proton chemical shifts in CTXIIb is explained.

Crystal structure of a snake venom cardiotoxin

Cardiotoxin VII4 from Naja mossambica mossambica crystallizes in space group P61 (a = b = 73.9 A; c = 59.0 A) with two molecules of toxin (molecular mass = 6715 Da) in the asymmetric unit. The structure was solved by using a combination of multiple isomorphous replacement and density modification methods. Model building and least-squares refinement led to an agreement factor of 27% for a data set to 3-A resolution prior to any inclusion of solvent molecules. The topology of the molecule is similar to that found in short and long snake neurotoxins, which block the nicotinic acetylcholine receptor. Major differences occur in the conformation of the central loop, resulting in a change in the concavity of the molecule. Hydrophobic residues are clustered in two distinct areas. The existence of stable dimeric entities in the crystalline state, with the formation of a six-stranded antiparallel beta sheet, may be functionally relevant.

The role of the sodium pump during prolonged end-plate currents in guinea-pig diaphragm

1. Depolarization caused by carbachol or decamethonium is followed by spontaneous recovery of membrane potential in the presence of the drug. The involvement of the Na pump in this recovery has been investigated in guinea-pig diaphragm at 37 degrees C. 2. Restoration of potassium ions (K+) to the bathing solution gives a rapid recovery of membrane potential which is compatible with a component of recovery of potential being attributable to an electrogenic ion pump and from which a Na pump current of over 60 nA has been estimated. 3. The maintenance of membrane potential in the presence of depolarizing drugs is interpreted in terms of a residual rate of channel opening at a time when the membrane potential is restored, balanced by Na pump action producing tubular depletion of K+. To account for these results a Na pump conductance has been added to a model circuit of drug action. 4. The peak end-plate current produced by carbachol (80 microM) is 100 nA (n = 11) as recorded by the voltage clamp technique; similar estimates may be obtained from measurements of input resistance which falls to 31% of the initial value (n = 5). In muscles desensitized by carbachol for 30 min the end-plate current is 11 nA. 5. In normal muscle removal of K+ from the bathing solution produces a reversible hyperpolarization. In muscles where the membrane potential has recovered in the continued presence of the drug, a hyperpolarization is also found on removal of K+. Withdrawal of K+ during the early stage of spontaneous recovery of potential produces a depolarization or an arrest of the spontaneous repolarization. These results are interpreted in terms of the Na pump producing different effects during the course of spontaneous repolarization. 6. Indirect evidence for K+ depletion in the transverse tubules by the Na pump is provided by an increased resistance to inward current following brief exposure to carbachol or decamethonium. A similar mechanism is used to interpret both the observed change in end-plate revérsal potential to a more negative value and the marked diminution in the amplitude of the action potential at the end-plate during drug action.

Relaxation studies on the interaction of hexamethonium with acetylcholine-receptor channels in Aplysia neurons

The mode of action of the cholinergic antagonist hexamethonium on the excitatory responses of voltage-clamped Aplysia neurons to acetylcholine (ACh) has been examined by voltage- and concentration-jump relaxation analysis. At steady-state concentrations of ACh hyperpolarizing command steps induced inward current relaxations to a new steady-state level (Iss). The time constants of these inward relaxations, tau f, which approximate the mean single-channel lifetime, were increased both by increasing the membrane potential and by lowering the bath temperature (Q10 = 3) but were not affected by increasing the ACh concentration over the dose range employed. In the presence of hexamethonium hyperpolarizing command steps produced biphasic relaxations of the agonist-induced current. tau f was reduced in a voltage-dependent manner, the degree of reduction increasing with hyperpolarization. Slow, inverse relaxations were also triggered in the presence of hexamethonium. The time constant of this relaxation was reduced by increasing membrane potential and hexamethonium concentration. Both the estimated association (kf = 5 X 10(4) M-1 . sec-1) and the estimated dissociation (kb = 0.24-0.29 sec-1) rate constants derived from a three-state sequential model for block by hexamethonium were independent of the membrane potential. Similar rate constants were estimated from experiments with the concentration-jump technique, which were also independent of the membrane potential over the range -50 to -110 mV. It is suggested that the voltage-dependent actions of hexamethonium may originate either from an alteration of the channel opening and closing rate constants through an allosteric interaction with the ACh receptor, rather than through an influence of the transmembrane electric field on the rate of drug binding, or through a fast reaction which is rate-limited by voltage-independent diffusion.

Acetylcholine responses of identified neurons in Helix pomatia--III. Ionic composition of the depolarizing currents induced by acetylcholine

The ionic composition of the currents underlying the acetylcholine (ACh) depolarizations in the identified neurons B1 and B3 of the buccal ganglia of Helix pomatia was analysed. The equilibrium potential of the ACh responses was -2.8 +/- 0.6 mV (N = 49) and -4.0 +/- 0.7 mV (N = 79; mean +/- SEM) in the neurons B1 and B3, respectively. Replacement of NaCl in the bath solution by sucrose shifted the ACh equilibrium potential into the negative direction. A similar but less pronounced shift occurred when Ca2+ was substituted for Na+. Substitution of Cl- in the bath solution by propionate or an increase of the intracellular Cl- concentration did not affect the ACh equilibrium potential. Changes of K+ concentration in the bath between 1 and 50 mmol/l left the ACh equilibrium potential nearly unaffected when the Na+ concentration was at the control level. With a simultaneous reduction of extracellular Na+ an increase of K+ concentration shifted the ACh equilibrium potential towards more positive potentials. The findings are compatible with calculated K+ permeabilities if a K+ redistribution across the cell membrane is considered. In the neurons B1 and B3, channels operated by ACh are permeable for K+, Na+ and Ca2+, with the relative permeabilities 1.6:1.0:0.1.

A study of the cholinolytic actions of strychnine using the technique of concentration jump relaxation analysis

The blocking actions of strychnine on excitatory acetylcholine (ACh) responses in isolated, voltage clamped Aplysia neuronal cell bodies has been studied using a rapid drug application technique. Rapid microperfusion of strychnine (10-50 microM) produced a reduction of the steady-state ACh-induced inward current in Aplysia neurons which decayed exponentially with a highly dose-dependent time constant. At the cessation of strychnine perfusion the ACh-induced current recovered to its original value with an exponential time course which was not sensitive to the dose of strychnine previously applied. The calculated association (k1) and dissociation (k-1) constants for a pseudo-first-order reaction between strychnine and its binding site were k1 = 1.2 X 10(4) M-1. sec-1 and k-1 = 0.12 sec-1 (KD = 1 X 10(-5) M-1). These results demonstrate that concentration jump relaxation experiments can be performed on isolated neurons for the study of voltage-independent antagonists by the use of rapid microperfusion systems and provide the first direct estimates to date of the rate constants of the cholinolytic effect of strychnine.

Two-component desensitization of nicotinic receptors induced by acetylcholine agonists in Lymnaea stagnalis neurones

The kinetics of desensitization induced by different agonists of acetylcholine (ACh) as well as the kinetics of recovery from densensitization, have been studied using the voltage-clamp technique in isolated, identified Lymnaea stagnalis neurones. Desensitization follows the sum of two exponentials: one fast and one slow. The time constant of the fast desensitization component (tau Ids) under ACh application is in the range of seconds at room temperature (18-23 degrees C). It increases upon cooling (Q10 = 2.8 +/- 0.9), decreases with increasing ACh concentration and is independent of membrane voltage. The time constant of the slow component of densensitization (tau Ids) is in the range of tens of seconds. It decreases with increasing drug concentration and is weakly dependent upon temperature (Q10 = 1.3 +/- 0.4). The relative amplitude of the fast component, estimated by back extrapolation to the position of the peak current, increases with agonist concentration and decreases upon cooling. Recovery from desensitization follows the sum of two exponentials with time constants (tau Ir and tau IIr) of the order of seconds and minutes, respectively. Cooling prolongs the slow component (Q10 of tau IIr is approx. 3) and reduces its contribution during recovery. A comparison of the desensitization induced by various agonists indicates that for the small monoquaternary agonists the onset and recovery of desensitization resemble the onset and recovery observed with ACh. For more bulky agonists, like ethoxysebacylcholine, sebacylcholine and suberylcholine, the decay of the response during prolonged application of the agonist may involve an additional blocking process.

Kinetics of acetylcholine-activated cation channel blockade by the calcium antagonist D-600 in Aplysia neurons

The effects of the calcium channel blocker D-600 on the cation channels activated by acetylcholine (ACh) was studied in voltage-clamped Aplysia neurons by voltage-jump relaxation analysis. D-600 blocked the steady-state ACh current in a highly voltage-dependent manner, the degree of antagonism increasing with membrane hyperpolarization. In the presence of D-600 the current relaxations following hyperpolarizing command steps became biphasic. The time constants of ACh-induced current relaxations (tau f), which approximate the mean channel lifetime, were reduced in a voltage-dependent manner, the degree of reduction of tau f increasing with increasing membrane potential. In addition to the acceleration of tau f, a slow, inverse kinetic component (tau s) of the relaxation appeared in the presence of D-600. The rate of this inverse kinetic component was accelerated either by increasing the agonist or antagonist dose or by increasing the membrane potential. These results suggest that D-600 acts to antagonize the acetylcholine response through a blockade of the open state of the transmitter-activated cation channel. Possible kinetic schemes for this interaction are discussed.

Mouse macrophage Fc receptor for IgG gamma 2b/gamma 1 in artificial and plasma membrane vesicles functions as a ligand-dependent ionophore

We tested the effect of specific ligands to the mouse macrophage IgG gamma 2b/gamma 1 Fc fragment receptor (FcR) on ion permeability of plasma membrane vesicles prepared from J774 macrophages by nitrogen cavitation. The monoclonal antibody directed against IgG gamma 2b/gamma 1 FcR (gamma 2b/gamma 1 FcR), 2.4G2 IgG, and soluble and immobilized immunocomplexes induces a dramatic cation flow through plasma membrane vesicles, as measured by [3H]tetraphenylphosphonium+ accumulation. Challenge with the monovalent 2.4G2 Fab also produces an ion flow but the effect is smaller by a factor of 2, and three other monoclonal antibodies directed against major surface antigens of mouse macrophages produce no net ion flow. Membrane vesicles incubated with FcR ligands do not discriminate between Na+ and K+ and show low permeability to Ca2+. gamma 2b/gamma 1 FcR was purified by using monoclonal 2.4G2 and reconstituted into proteoliposomes. Under these circumstances, the purified receptor increased the cation permeability of the proteoliposomes in the presence of specific ligands. The data indicate that the gamma 2b/gamma 1 FcR of J774 macrophages functions as a ligand-dependent ionophore. The ion influx into macrophages mediated by the FcR may play an important role as a signal for internalization of membranes and stimulus-secretion coupling.

Sequential individual resonance assignments in the 1H nuclear-magnetic-resonance spectrum of cardiotoxin VII2 from Naja mossambica mossambica

The assignment of the 1H nuclear magnetic resonance (NMR) spectrum of cardiotoxin VII2 from Naja mossambica mossambica is described and documented. The assignments are based entirely on the amino acid sequence and on two-dimensional NMR experiments at 500 MHz. Individual assignments were obtained at 45 degrees C for the backbone protons of 56 out of the total of 60 amino acid residues, the exceptions being the N-terminal dipeptide segment Leu-1--Lys-2--, Pro-8 and Pro-15. Complete assignments of the non-labile hydrogen atoms of the side chains were obtained for 37 residues, and for Asn-4 and Asn-19 the delta amide protons were also identified. For 19 long side chains the individual assignments include only the backbone and C-beta proton resonances; these are Gln-5, Pro-9, Pro-33, Pro-43, Leu-47, all three methionines, two arginines and nine lysines. The chemical shifts for the assigned resonances at 45 degrees C are listed for an aqueous solution at pH 3.6. A preliminary interpretation of the sequential connectivity patterns indicates that approximately 30 out of the total of 60 amino acid residues in cardiotoxin VII2 are in extended, beta-type secondary structures, and there is no indication for the formation of alpha-helical structure.

'Steady/equilibrium approximation' in relaxation and fluctuation. I. Procedure to simplify first-order reaction

A general procedure to simplify a complex first-order reaction by two approximations, the principle of fast equilibration and the steady-state approximation, is presented. Rate constants are classified into two groups: those of the order of unity and those of the order of epsilon (much less than 1) or less, and are represented in the schemes by thick and thin arrows, respectively. The fast and the slow components are defined: from the fast component at least one thick arrow originates and from the slow component no thick arrow originates. Fast components are divided into several groups. In a group, the fast components are connected by thick arrows in both directions in each reaction step. When at least one thick arrow originates from the components in a group G and terminates on a component not belonging to group G (group G is open), then the steady-state approximation or principle of fast equilibration holds on each component in group G after an induction period To. When no thick arrow originating from group G is directed to components not belonging to group G (group G is closed), the principle of fast equilibration holds on the fast components in group G after To. The induction period To is less than the order of 1/epsilon.

Effects of extracellular sodium concentration on null potential, conductance and open time of endplate channels

(i) Effects of extracellular sodium concentration, [Na]o, on endplate channel characteristics were investigated in voltage-clamped, glycerol-treated toad sartorius fibres. (ii) The relation between [Na]o (and [K]o) and acetylcholine null potential could be reasonably well fitted by the Goldman-Hodgkin-Katz type of equation, except when [Na]o was higher than normal. Anions had no significant effect on the null potential. (iii) Endplate channel open time (phi), whether measured from miniature endplate currents or from current fluctuations induced by iontophoresis of acetylcholine, varied inversely with [Na]o. The relation between phi-1(= alpha) and [Na]0 could be fitted by alpha = alpha max [Na]o/(Km + [Na]o) with a Km of 92 mM. (iv) Endplate conductance, measured at the peak of endplate currents or at the peak of spontaneous miniature endplate currents, increased nonlinearly with [Na]o. (v) Single channel conductance, gamma, also increased nonlinearly with [Na]o. Experimental observations at -90 mV could be fitted by the relation gamma = gamma max [Na]o/(Km + [Na]o), giving values for gamma max and Km of 47 pS and 146 mM respectively. Correcting channel conductance for the contribution from potassium ions gave values of gamma max and Km of 78 pS and 423 mM respectively. (vi) The results are consistent with the hypothesis that binding sites for Na ions can modulate both channel lifetime and conductance and that these sites become saturated at higher sodium concentrations.

Voltage clamp analysis of the effect of cationic substitution on the conductance of end-plate channels

The effect of two commonly used sodium substitutes, tris and glucosamine, on the amplitude and kinetics of miniature end-plate currents (MEPCs), acetylcholine (ACh) induced end-plate currents (EPC) and EPC fluctuations was studied in voltage clamped single muscle fibres from a monolayer preparation of the cutaneous pectoris muscle. Total replacement of sodium with each substitute shifted the reversal potential from -4.7 mV (normal sodium solution) to -3.6 mV (tris) and -49.0 mV (glucosamine). In tris and glucosamine substituted solutions the current (MEPC or EPC) - voltage relation became markedly nonlinear, with peak current decreasing with membrane hyperpolarization. Peak current at +40 mV, was unaltered in tris solutions and reduced in glucosamine substituted solutions. MEPCs decayed with a single exponential time course and the EPC fluctuation spectra were characterized by single Lorentzian functions in both normal sodium solution and each substituted solution. Analysis of EPC fluctuations demonstrated that both tris and glucosamine decrease single channel conductance and increase channel lifetime. Both effects were enhanced by either membrane hyperpolarization or by increasing the concentration of each substitute. In the presence of each cationic substitute, single channel conductance increased and mean channel lifetime decreased with membrane depolarization. Analysis of the data according to the constant field assumptions (Goldman, Hodgkin, Katz equation) provided an inadequate description of experimental currents obtained at hyperpolarized membrane potentials with total ion substitution. Shifts in reversal potential with partial substitution were, however, adequately predicted by the GHK equation. These results suggest that tris and glucosamine ions interact with end-plate channels to reduce cation permeability and decrease channel closing rates. The dependence of this block on the level of membrane potential suggests that these cations bind to site(s) within open end-plate channels.

A newly recognized congenital myasthenic syndrome attributed to a prolonged open time of the acetylcholine-induced ion channel

Five familial cases (in two families) and one sporadic case of a new congenital myasthenic syndrome were investigated. Symptoms arise in infancy or later life. Typically, one finds selective involvement of cervical, scapular, and finger extensor muscles, ophthalmoparesis, and variable involvement of other muscles. There is a repetitive muscle action potential to single nerve stimulus in all muscles and a decremental response at 2 to 3 Hz stimulation in clinical affected muscles. Microelectrode studies reveal markedly prolonged end-plate potential (epp), miniature end-plate potential (mepp), and miniature end-plate current; normal quantum content of the epp; and a smaller than normal or low-normal mepp amplitude. Light microscopy demonstrates predominance of type I fibers, small groups of atrophic fibers, tubular aggregates and vacuoles near end-plates, abnormal end-plate configuration, and nonspecific myopathic changes. Abundant acetylcholinesterase activity is present at all end-plates, and the activity and kinetic properties of this enzyme in muscle are normal. Calcium accumulated at the end-plate in one patient. Quantitative electron microscopy shows decrease in the size of nerve terminals, increase in the density of synaptic vesicles, and reduction in the length of postsynaptic membranes. There is focal degeneration of junctional folds with corresponding loss of acetylcholine receptor, most marked in cases with the lowest mepp amplitude. There are no immune complexes at the end-plate. Fiber regions near end-plates display dilation, proliferation, and degeneration of the sarcoplasmic reticulum; nuclear, mitochondrial, and myofibrillar degeneration; and vacuoles resembling those found in periodic paralysis. A prolonged open time of the acetylcholine-induced ion channel is considered to be the basic abnormality and may account for the physiological, morphological, and clinical alterations.

Effects of divalent cations on toad end-plate channels

Miniature end-plate currents (MEPCs) and acetylcholine-induced current fluctuations were recorded in voltage-clamped, glycerol-treated toad sartorius muscle fibers in control solution and in solutions with added divalent cations. In isosmotic solutions containing 20 mM Ca or Mg, MEPCs had time constants of decay (tau D) which were about 30% slower than normal. In isotonic Ca solutions (Na-free), greater increases in both tau D and channel lifetime were seen; the null potential was -34 mV, and single-channel conductance decreased to approximately 5 pS. Zn or Ni, at concentrations of 0.1-5 mM, were much more effective in increasing tau D than Ca or Mg, although they did not greatly affect channel conductance. The normal temperature and voltage sensitivity of tau was not significantly altered by any of the added divalent cations. Surface potential shifts arising from screening of membrane fixed charge by divalent cations cannot entirely explain the observed increases in tau, especially when taken together with changes in channel conductance.

Cholinergic modulation of the release of [3H]acetylcholine from synaptosomes of the myenteric plexus

The purpose of these experiments was to determine if cholinergic agents affected the release of acetylcholine (ACh) from a synaptosomal preparation of the guinea pig ileum myenteric plexus. The synaptosomal preparation was first incubated with the precursor [3H]choline; subsequently, release of the stored [3H]ACh was measured. The release was decreased by oxotremorine or exogenous ACh plus hexamethonium and increased by exogenous ACh plus evoked release that was inhibited by nicotinic antagonists or muscarinic agonists. Release was stimulated half-maximally by approximately 2 microM- and maximally by 10 microM-DMPP. Either in the absence of calcium or at 0 degrees C, DMPP was without effect. The effect of 10 microM-DMPP was brief, a significant stimulation occurring only within the first 2 min at 37 degrees C. Tetrodotoxin also inhibited excitation by DMPP but not completely. Thus, the release of [3H]ACh appears to be presynaptically modulated, negatively by muscarinic agonists and positively by nicotinic agonists.

Differential accessibility to two insect neurones does not account for differences in sensitivity to alpha-bungarotoxin

The nicotinic acetylcholine receptor probe alpha-bungarotoxin (1.0 x 10(-7) M) blocks the depolarising response to ionophoretic application of acetylcholine onto the cell body membrane of the fast coxal depressor motoneurone (Df) of desheathed cockroach (Periplaneta americana) metathoracic ganglia, but at the same concentration is completely ineffective in blocking the depolarising action af acetylcholine on dorsal unpaired median (DUM) neurones in the same ganglion. The possibility that this is due to differences in accessibility of the toxin to the neurones has been tested by a combination of ionophoretic injection of horseradish peroxidase into single neurones with a study of the distribution of the exogenous tracer lanthanum, which is of similar effective size to alpha-bungarotoxin. The peripherally located cell body membranes and the fine axonal processes of Df and DUM neurones of desheathed metathoracic ganglia are equally accessible to lanthanum. Differential accessibility to the two cell types does not account therefore for the differences in sensitivity to alpha-bungarotoxin.

5-Hydroxytryptamine receptors of visceral primary afferent neurones on rabbit nodose ganglia

1. The electrophysiological characteristics of 5-hydroxytryptamine (5-HT) receptors distributed on visceral primary afferent neurones (the nodose ganglion cells of the vagus) in rabbits were investigated with intracellular recording and voltage-clamp techniques.2. In response to 5-HT applied by superfusion (>/= 10 mum) or by ionophoresis (>/= 5 nA, 50 msec), the majority of type C neurones (mean axonal conduction velocity: 0.83+/-0.25 m/sec) showed a rapid depolarization of 20-30 mV in amplitude which was followed by a hyperpolarization of a few millivolts. Both the initial depolarization and afterhyperpolarization were associated with a reduction in membrane resistance.3. Type A neurones (mean axonal conduction velocity: 7.7+/-0.4 m/sec) did not show any significant alterations in membrane potential and resistance during or after application of 5-HT.4. The initial depolarization induced by 5-HT was abolished by Na(+)-free Krebs solution and showed a reduction of a few millivolts in K(+)-free or Ca(2+)-free Krebs solution. The response in normal Krebs solution was reversed at a membrane potential level of +7.3+/-1.1 mV.5. The afterhyperpolarization disappeared in Na(+)-free or Ca(2+)-free Krebs solution, while it was markedly enhanced in K(+)-free Krebs solution. The response in normal Krebs solution reversed at a membrane potential of -88.7+/-0.8 mV, and was abolished at membrane potentials more positive than -20 mV.6. Unlike 5-HT voltage responses, which were biphasic in the majority of neurones examined, 5-HT induced currents were usually monophasic when recorded at holding membrane levels ranging from -80 to +50 mV. The reversal potential of the inward current was +7.5+/-0.8 mV which was in good agreement with the reversal level for 5-HT-induced depolarizations. The reversal potentials for inward currents which were obtained at various concentrations of Na(+) or K(+) corresponded to the theoretical values calculated by the equivalent circuit equation.7. These results suggest that the initial depolarization induced by 5-HT is due mainly to simultaneous increases in Na(+) and K(+) conductances, while the afterhyperpolarization is brought about by an increase of K(+) conductance which is triggered by a voltage-dependent influx of Na(+) and Ca(2+).8. The mean value for the ;limiting slope' of conductance change vs. 5-HT concentration and the slope of 5-HT current vs. 5-HT concentration obtained by superfusion of 5-HT, were in good agreement, 1.84+/-0.26 and 1.88+/-0.31, respectively. On the other hand, the mean Hill coefficient obtained from the dose-response curves for the inward current induced by ionophoresis was 2.51+/-0.14.9. Tetrodotoxin (0.2 mum) blocked the soma action potential completely, but did not show any effect on 5-HT-induced responses.10. (+)-Lysergic acid diethylamide and methysergide (1-100 mum) had no depressant effect on the 5-HT-induced depolarization.11. (+)-Tubocurarine at low concentrations (1-5 mum) inhibited the 5-HT induced inward current competitively. The mode of its inhibitory action became noncompetitive at higher concentrations (10-20 mum).

The three-ion test for the identity of the ionophores mediating the hyperpolarizing response to cholinomimetics in neurones of Helix aspersa

1. The hyperpolarizing response investigated is mediated by Cl- ions. Additional anions NO-3 and BrO-3 were introduced into the perfusate to produce a system with 3 permeant anions. 2. Under such circumstances the reversal potential (Er) becomes a means of discriminating between the molecular mechanisms invoked by different agonists that produce an increase in anionic conductance. 3. A statistically significant difference of 0.49 mV was detected between the Er for acetylcholine and those for acetyl-beta-methyl choline and tetramethylammonium (acetylcholine more negative). 4. This difference was not significantly affected by changing to a different 3-ion solution. 5. It is argued from theoretical grounds that a change in this difference is to be expected when molecular mechanisms differ; alternative explanations for the difference are suggested.

1H nuclear-magnetic-resonance studies of the conformation of cardiotoxin VII2 from Naja mossambica mossambica

The membrane toxin VII2 from the venom of Naja mossambica mossambica was investigated in aqueous solution by one-dimensional and two-dimensional high-resolution nuclear magnetic resonance (NMR) techniques at 360 MHz. The spectral characterization included identification of the complete spin systems for several amino acid residues, nuclear Overhauser effect measurements, the use of chemically induced dynamic nuclear polarization and studies of the pH dependence of the NMR spectrum. Data from homologous toxins, in particular direct lytic factor 12B from Haemachatus haemachatus, were used to establish assignments of aromatic and methyl proton resonances. From these experiments a short, triple-stranded fragment of antiparallel beta structure could be determined, which includes the residues 23-27, 43-46 and 60-62. Furthermore, the nuclear Overhauser effect measurements indicate close proximity in the protein conformation of the aromatic rings of Trp-14, Tyr-25 and Tyr-59, and the side chain of Ile-46.

Characterization of acetylcholine receptor isolated from Torpedo californica electroplax through the use of an easily removable detergent, beta-D-octylglucopyranoside

Non-ionic detergents used for the solubilization and purification of acetylcholine receptor from Torpedo californica electroplax may remain tightly bound to this protein. The presence of detergent greatly hinders spectrophotometric and hydrodynamic studies of the receptor protein. beta-D-Octylglucopyranoside, however, is found to be effective in solubilizing the receptor from electroplax membranes with minimal interference in the characterization of the protein. The acetylcholine receptor purified from either octylglucopyranoside- or Triton X-100-solubilized extracts exhibits identical amino acid compositions, alpha-Bungarotoxin and (+)-tubocurarine binding parameters, and subunit distributions in SDS-polyacrylamide gels. The use of octylglucopyranoside allows for the assignment of a molar absorptivity for the purified receptor at 280 nm of approx. 530000 M-1 . cm-1. Additionally, successful reconstitution of octylglucopyranoside-extracted acetylcholine receptor into functional membrane vesicles has recently been achieved (Gonzales-Ros, J.M., Paraschos, A. and Martinez-Carrion, M. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 1796--1799). Removal of octylglucopyranoside by dialysis does not alter the specific toxin and antagonist binding ability of the receptor or its solubility at low protein concentrations. Sedimentation profiles of the purified acetylcholine receptor in sucrose density gradients reveal several components. Sedimentation coefficients obtained for the slowest sedimenting species agree with previously reported molecular weight values. Additionally, the different sedimenting forms exhibit distinctive behavior in isoelectric focusing gels. Our results suggest that both the concentration and type of detergent greatly influence the physicochemical behavior of the receptor protein.

The characteristics of synaptic currents and responses to acetylcholine of rat submandibular ganglion cells

1. Synaptic currents and responses to acetylcholine (ACh) have been recorded at 20 degrees C from rat submandibular ganglion cells by a two micro-electrode volatage-clamp technique.2. The peak amplitude (a(p)) of excitatory synaptic currents (e.s.c.s) was linearly related to membrane potential (E(m)), with a reversal potential close to - 10 mV. E.s.c.s decayed with a bi-exponential time course, the fast phase comprising just over half the total amplitude. The time constant (tau(f)) of the fast phase was 5-9 msec, while that of the slow phase (tau(s)) was 27-45 msec. The relative amplitudes of the two components remained constant at different membrane potentials, showing that the reversal potential was the same for both.3. Both tau(f) and tau(s) increased as the cell was hyperpolarized, the ratio tau(-80)/tau(-40) being about 1.6 for both fast and slow components.4. Increasing the calcium concentration from 2.5 to 7.5 mm increased the amplitude of both components by about 40% and also prolonged the synaptic currents 30-50%, its effect being slightly greater on tau(s) than on tau(f).5. In contrast to e.s.c.s, spontaneous or potassium-evoked miniature synaptic currents (m.s.c.s) showed a simple exponential decay with a time constant (tau(m.s.c.)) very similar to tau(f). tau(m.s.c.) showed the same sensitivity to membrane potential and calcium concentration as tau(f).6. In the presence of neostigmine (10 mum) e.s.c.s were prolonged, tau(f) about 3.5-fold and tau(s) about 2.5-fold. The decay remained bi-exponential, with little change in the relative amplitude or voltage-dependence of the two components. M.s.c.s were prolonged to a lesser extent (1.5-2-fold) and the voltage dependence of tau(m.s.c.) was unaffected by neostigmine.7. Reduction of the quantal content of the e.s.c. by low calcium-high magnesium solution did not affect the time course. The relative amplitudes, and the time constants of the two components were unchanged even with a 90% reduction of a(p).8. Voltage-jump studies, in which the cell was abruptly hyperpolarized by 20-40 mV during a response to ionophoretically applied ACh, showed a relaxation pattern consisting of two distinct exponential components, whose relative amplitudes varied considerably in different cells. The two rate constants tau(f.rel) and tau(s.rel) were somewhat shorter than tau(f) and tau(s) for e.s.c.s, the difference being generally less than two-fold.9. Measurements of ACh noise also revealed two kinetic components, the time constants of which corresponded closely to tau(f) and tau(s) for e.s.c.s. On the assumption that the two components represent channels of equal conductance, the single channel conductance, gamma, was calculated to be 31+/-3 pS, similar to that of endplate channels.10. It is concluded that the two kinetic components of e.s.c.s and ACh responses probably represent two distinct classes of ACh-operated ionic channels, whose mean lifetime differs about fivefold. The two types of channel show the same ionic selectivity and their mean lifetime varies in the same way with the membrane potential. The absence of a slow component in m.s.c.s suggests that the two types of channel are spatially separate in the membrane.

Prolonged exposure to acetylcholine: noise analysis and channel inactivation in cat tenuissimus muscle

1. Micro-electrodes were used to record membrane potential and associated noise at the end-plate region of cat tenuissimus muscle (37 degrees C), during applications of acetylcholine (ACh) in continuously flowing Krebs solution containing eserine and tetrodotoxin. 2. Densensitization was assessed from the frequency of channel opening calculated from the noise variance. 3. At higher concentrations of ACh (10-50 microM), desensitization occurred with an exponential fall to a plateau. 4. At low concentrations of ACh (1-2 microM) only slight desensitization occurred and at a much lower rate. Frequency of channel opening decreased at the rate of 0.045 +/- 0.024 min-1. Maximum frequency was (33 +/- 9) X 10(7)/sec while maximum depolarization was 20.5 +/- 1.4 mV (n = 11 cats). Depolarization was well maintained. 5. This slow rate of desensitization at low concentrations of ACh was confirmed in experiments where voltage clamped current, its associated noise, and miniature end-plate current amplitude were measured. 6. At low concentrations of ACh (1-2 microM) in the presence of eserine there was sustained block in neuromuscular transmission when twitch tension was measured. 8. It is concluded that the mechanism of neuromuscular block by ACh at around 1 microM concentration is by depolarization itself, not desensitization.

125I-Labeled alpha-bungarotoxin and [3H]quinuclidinylbenzilate binding to rat brain membranes. Effects of physical, chemical and enzymatic treatments

The effects of various physical, chemical and enzymatic treatments of rat brain membranes were investigated with respect to 125I-labeled alpha-bungarotoxin ([125I]alpha BuTx) and [3H]quinuclidinylbenzilate ([3H]QNB) binding. Binding appeared relatively stable to autolysis, mechanical shearing, freeze-thawing, and divalent cation addition (Sr2+) or removal (EGTA, EDTA). Binding for [125I]-alpha BuTx was slightly reduced by trypsin digestion of the membranes while both [125I]alpha BuTx and [3H]QNB binding were reduced by phospholipase A2 digestion (Crotalus adamantus phospholipase A2 and beta-bungarotoxin). Treatment of the membranes with the disulfide reducing agent, dithiothreitol, resulted in additional [125I]alpha BuTx binding but showed little effect on [3H]QNB binding. Binding of the cholinergic agonists, nicotine and carbamylcholine, was studied by observing their concentration-dependent ability to inhibit [125I]alpha BuTx and [3H]QNB binding, respectively. Membrane sulfhydryl group reduction and endogenous cation removal by EGTA or EDTA resulted in a lowered affinity for nicotine bindng. Alkylation of membranes with N-ethylmaleimide resulted in an increase in carbamylcholine affinity. Other treatments had little or no effect on nicotine or carbamylcholine binding.

End-plate channel opening and the kinetics of quinacrine (mepacrine) block

1. The effects of quinacrine on the relaxation of the agonist-induced currents in response to a voltage step were investigated at voltage clamped frog end-plates. A fast perfusion technique allowed the application of known concentrations of the agonist acetylcholine (ACh) or carbachol to end-plate viewed with Nomarski optics. 2. In the presence of quinacrine, and in response to a hyperpolarizing voltage jump, an agonist-induced current shows a fast initial relaxational increase and then relaxes slowly back to a new equilibrium level. 3. The slow relaxation can be described by a single exponential with a time constant tau s . tau s gets smaller at increasing quinacrine concentrations (0--2 microM) and the decay rate constant, 1/tau s, increases linearly with quinacrine concentration. Increasing agonist concentration reduces tau s, in a manner dependent on the nature of the agonist. Tau s is markedly lengthened at more hyperpolarized potential, but this voltage effect gets less at higher concentrations of agonist. 4. These data suggest a slow voltage dependent blockage of open end-plate channels by quinacrine. The binding rate constant of quinacrine is estimated as 10(8) M-1 s-1, and the voltage dependent, backward rate constant, as 5 s-1 at -60 mV and 1 s-1 at -140 mV. These values are in fair agreement with those obtained from the analysis presented in the preceding paper. 5. The agonist concentration dependence of the blocking kinetics is compatible with a simple model for channel opening. In this model, independent sequential binding of two agonist molecules leads to an isomerization of the receptor. The intrinsic binding constant of ACh is estimated to be around 20 microM, and for carbachol around 200 microM. Distinct isomerization constants could lead to a maximal activation of 70% of the available channels by ACh, and only 40% by carbachol. 6. An example of a possible interaction in between quinacrine block and desensitization is shown. At the break of an hyperpolarizing jump which has increased quinacrine blockade, a transient increase in the synaptic current is observed with apparently a temporary reduction of the desensitization.

Voltage dependence of desensitization. Influence of calcium and activation kinetics

The voltage dependence of carbachol-induced desensitization has been analyzed in potassium-depolarized frog sartorius muscle preparations with voltage clamp techniques over a wide voltage range (-120 to +40 mV). Desensitization developed exponentially at all voltages with tau, the time constant of desensitization onset, varying as a logarithmic function of membrane voltage. The voltage dependence of tau remained in calcium-deficient solutions and was not altered by elevating either the level of extracellular or intracellular calcium. We have analyzed our results according to a simple sequential kinetic scheme in which the rate-limiting step in the development of desensitization is a transition of the receptor channel complex from the activated conducting state to a desensitized, nonconducting state. We conclude (a) that the observed voltage sensitivity of desensitization primarily resides in the voltage dependence of this transition, and (b) the kinetics of activation appear to have a greater influence on the observed rate of desensitization than on its voltage dependence. The magnitude of the voltage dependence suggests that a greater change in free energy is required for the transition to the desensitized state than for the transition between the open and closed states of the receptor channel complex.

Reconstitution of purified acetylcholine receptors with functional ion channels in planar lipid bilayers

Acetylcholine receptor, solubilized and purified from Torpedo californica electric organ under conditions that preserve the activity of its ion channel, was reconstituted into vesicles of soybean lipid by the cholate-dialysis technique. The reconstituted vesicles were then spread into monolayers at an air-water interface and planar bilayers were subsequently formed by apposition of two monolayers. Addition of carbamoylcholine caused an increase in membrane conductance that was transient and relaxed spontaneously to the base level (i.e., became desensitized). The response to carbamoylcholine was dose dependent and competitively inhibited by curare. Fluctuations of membrane conductance corresponding to the opening and closing of receptor channels were observed. Fluctuation analysis indicated a single-channel conductance of 16 +/- 3 pS (in 0.1 M NaCl) with a mean channel open time estimated to be 35 +/- 5 ms. Thus, purified acetylcholine receptor reconstituted into lipid bilayers exhibited the pharmacological specificity, activation, and desensitization properties expected of this receptor in native membranes.