The antagonism between tubocurarine and substances which depolarize the motor end-plate

Nicotinic acetylcholine receptors in cancer: Limitations and prospects

Nicotinic acetylcholine receptors (nAChRs) have long been considered to solely mediate neurotransmission. However, their widespread distribution in the human body suggests a more diverse physiological role. Additionally, the expression of nAChRs is increased in certain cancers, such as lung cancer, and has been associated with cell proliferation, epithelial-to-mesenchymal cell transition, angiogenesis and apoptosis prevention. Several compounds that interact with these receptors have been identified as potential therapeutic agents. They have been tested as drugs for treating nicotine addiction, alcoholism, depression, pain and Alzheimer's disease. This review focuses on nAChR-mediated signalling in cancer, presenting opportunities for the development of innovative nAChR-based anticancer drugs. It displays the differences in expression of each nAChR subunit between normal and cancer cells for selected cancer types, highlighting their possible involvement in specific cases. Antagonists of nAChRs that could complement existing cancer therapies are summarised and critically discussed. We hope that this review will stimulate further research on the role of nAChRs in cancer potentially leading to innovative cancer therapies.

Structural mechanism of muscle nicotinic receptor desensitization and block by curare

Binding of the neurotransmitter acetylcholine to its receptors on muscle fibers depolarizes the membrane and thereby triggers muscle contraction. We sought to understand at the level of three-dimensional structure how agonists and antagonists alter nicotinic acetylcholine receptor conformation. We used the muscle-type receptor from the Torpedo ray to first define the structure of the receptor in a resting, activatable state. We then determined the receptor structure bound to the agonist carbachol, which stabilizes an asymmetric, closed channel desensitized state. We find conformational changes in a peripheral membrane helix are tied to recovery from desensitization. To probe mechanisms of antagonism, we obtained receptor structures with the active component of curare, a poison arrow toxin and precursor to modern muscle relaxants. d-Tubocurarine stabilizes the receptor in a desensitized-like state in the presence and absence of agonist. These findings define the transitions between resting and desensitized states and reveal divergent means by which antagonists block channel activity of the muscle-type nicotinic receptor.

Predictive factors of unacceptable movement and motor-evoked potentials during intraoperative neurophysiological monitoring in adult patients undergoing brain surgery: A retrospective study

Motor-evoked potential (MEP) monitoring is an essential monitoring for clinicians to improve outcomes. Although unacceptable movement during MEP is a rare complication but it can lead to terrible results. The aim of this study was to evaluate the risk factors associated with unacceptable movements in patients undergoing brain surgery with MEP monitoring.We performed a retrospective observational study of patients who underwent brain surgery with MEP monitoring under general anesthesia while using a partial neuromuscular blocker in a tertiary care hospital from January 2014 to August 2017. Unacceptable movement was defined as a condition in which MEP stimulation induced vigorous movement of patient hindered the smooth progress of the operation. We compared the baseline patient characteristics and laboratory results according to unacceptable movements during surgery to identify factors associated with unacceptable movement during MEP monitoring.768 patients were included in this analysis, and unacceptable movements were observed in 278 patients (36.2%). A multivariate logistic regression analysis revealed that an increase in ionized calcium was associated with the most strongly unpredictable movement during surgery [odds ratio (OR): 1.79, 95% confidence interval (CI): 1.37-2.36, P < .001]. In addition, age (OR, 0.98; 95% CI, 0.96-0.99; P = .001), male sex (OR, 1.59; 95% CI, 1.09-2.33; P = .017), and body mass index (OR, 0.90; 95% CI, 0.86-0.95; P <0.0010) were also associated with unacceptable movement. Serum ionized calcium concentration was the best predictor associated with unacceptable movement with MEP monitoring under general anesthesia.Serum ionized calcium concentration was the best predictor associated with unacceptable movement with MEP monitoring under general anesthesia.

Neurobiology and therapeutic utility of neurotoxins targeting postsynaptic mechanisms of neuromuscular transmission

The neuromuscular junction (NMJ) is the principal site for the translation of motor neurochemical signals to muscle activity. Therefore, the release and sensing machinery of acetylcholine (ACh) along with muscle contraction are two of the main targets of natural toxins and pathogens, causing paralysis. Given pharmacology and medical advances, the active ingredients of toxins that target postsynaptic mechanisms have become of major interest, showing promise as drug leads. Herein, we review key facets of prevalent toxins modulating the mechanisms of ACh sensing and generation of the postsynaptic response, with muscle contraction. We consider the correlation between their outstanding selectivity and potency plus effects on motor function, and discuss emerging data advocating their usage for the development of therapies alleviating neuromuscular dysfunction.

Stem cell derived phenotypic human neuromuscular junction model for dose response evaluation of therapeutics

There are currently no functional neuromuscular junction (hNMJ) systems composed of human cells that could be used for drug evaluations or toxicity testing in vitro. These systems are needed to evaluate NMJs for diseases such as amyotrophic lateral sclerosis, spinal muscular atrophy or other neurodegenerative diseases or injury states. There are certainly no model systems, animal or human, that allows for isolated treatment of motoneurons or muscle capable of generating dose response curves to evaluate pharmacological activity of these highly specialized functional units. A system was developed in which human myotubes and motoneurons derived from stem cells were cultured in a serum-free medium in a BioMEMS construct. The system is composed of two chambers linked by microtunnels to enable axonal outgrowth to the muscle chamber that allows separate stimulation of each component and physiological NMJ function and MN stimulated tetanus. The muscle's contractions, induced by motoneuron activation or direct electrical stimulation, were monitored by image subtraction video recording for both frequency and amplitude. Bungarotoxin, BOTOX^® and curare dose response curves were generated to demonstrate pharmacological relevance of the phenotypic screening device. This quantifiable functional hNMJ system establishes a platform for generating patient-specific NMJ models by including patient-derived iPSCs.

End-plate acetylcholine receptor: structure, mechanism, pharmacology, and disease

The synapse is a localized neurohumoral contact between a neuron and an effector cell and may be considered the quantum of fast intercellular communication. Analogously, the postsynaptic neurotransmitter receptor may be considered the quantum of fast chemical to electrical transduction. Our understanding of postsynaptic receptors began to develop about a hundred years ago with the demonstration that electrical stimulation of the vagus nerve released acetylcholine and slowed the heart beat. During the past 50 years, advances in understanding postsynaptic receptors increased at a rapid pace, owing largely to studies of the acetylcholine receptor (AChR) at the motor endplate. The endplate AChR belongs to a large superfamily of neurotransmitter receptors, called Cys-loop receptors, and has served as an exemplar receptor for probing fundamental structures and mechanisms that underlie fast synaptic transmission in the central and peripheral nervous systems. Recent studies provide an increasingly detailed picture of the structure of the AChR and the symphony of molecular motions that underpin its remarkably fast and efficient chemoelectrical transduction.

Depolarization-induced Ca2+ entry preferentially evokes release of large quanta in the developing Xenopus neuromuscular junction

The amplitude histogram of spontaneously occurring miniature synaptic currents (mSCs) is skewed positively at developing Xenopus neuromuscular synapses formed in culture. To test whether the quantal size of nerve-evoked quanta (eSCs) distributes similarly, we compared the amplitude histogram of single quantum eSCs in low external Ca(2+) with that of mSCs and found that nerve stimulation preferentially released large quanta. Depolarization of presynaptic terminals by elevating [K(+)] in the external solution or by direct injection of current through a patch pipette increased the mSC frequency and preferentially, but not exclusively, evoked the release of large quanta, resulting in a second broad peak in the amplitude histogram. Formation of the second peak under these conditions was blocked by the N-type Ca(2+) channel blocker, ω-conotoxin GVIA. In contrast, when the mSC frequency was elevated by thapsigargin- or caffeine-induced mobilization of internal Ca(2+), formation of the second peak did not occur. We conclude that the second peak in the amplitude histogram is generated by Ca(2+) influx through N-type Ca(2+) channels, causing a local elevation of internal Ca(2+). The mSC amplitude in the positively skewed portion of the histogram varied over a wide range. A competitive blocker of acetylcholine (ACh) receptors, d-tubocurarine, reduced the amplitude of smaller mSCs in this range relatively more than that of larger mSCs, suggesting that this variation in the mSC amplitude is due to variable amounts of ACh released from synaptic vesicles. We suggest that Ca(2+) influx through N-type Ca(2+) channels preferentially induces release of vesicles with large ACh content.

Mapping spatial relationships between residues in the ligand-binding domain of the 5-HT3 receptor using a molecular ruler

The serotonin 5-HT(3) receptor (5-HT(3)R) is a member of the Cys-loop ligand-gated ion channel family. We used a combination of site-directed mutagenesis, homology modeling, and ligand-docking simulations to analyze antagonist-receptor interactions. Mutation of E236, which is near loop C of the binding site, to aspartate prevents expression of the receptor on the cell surface, and no specific ligand binding can be detected. On the other hand, mutation to glutamine, asparagine, or alanine produces receptors that are expressed on the cell surface, but decreases receptor affinity for the competitive antagonist d-tubocurarine (dTC) 5-35-fold. The results of a double-mutant cycle analysis employing a panel of dTC analogs to identify specific points of interactions between the dTC analogs and E236 are consistent with E236 making a direct physical interaction with the 12 -OH of dTC. dTC is a rigid molecule of known three-dimensional structure. Together with previous studies linking other regions of dTC to specific residues in the binding site, these data allow us to define the relative spatial arrangement of three different residues in the ligand-binding site: R92 (loop D), N128 (loop A), and E236 (near loop C). Molecular modeling employing these distance constraints followed by molecular-dynamics simulations produced a dTC/receptor complex consistent with the experimental data. The use of the rigid ligands as molecular rulers in conjunction with double-mutant cycle analysis provides a means of mapping the relative positions of various residues in the ligand-binding site of any ligand-receptor complex, and thus is a useful tool for delineating the architecture of the binding site.

Ultrastructural visualization of the transmembranous and cytomatrix-related part of nicotinic acetylcholine receptor of frog motor endplate by means of an immunochemical avidity of IgG for d-tubocurarine

In the present study, a fine ultrastructural localization of nicotinic acetylcholine receptor (nAChR) was attempted, using d-tubocurarine (d-TC), a quaternary ammonium compound binding to nAChR. The localization was based on the binding avidity of immunoglobulin G (IgG) for acetylcholine (ACh) and other quaternary ammonium compounds, such as d-TC. d-TC was applied to the frog neuromuscular preparation and caused a blockade of neuromuscular transmission. Then, d-TC was rendered insoluble in situ by silicotungstic acid (STA), a precipitating agent of soluble proteins and quaternary ammonium compounds. After tissue fixation, a normal rabbit serum was applied to the fine precipitate of the insoluble salt of d-TC silicotungstate (quaternary ammonium radical of d-TC) to form the immunochemical complex d-TC- rabbit IgG at ACh binding sites. The IgG of the complex was revealed by means of the conventional immunoperoxidase procedure used for ultrastructural localization. Under the electron microscope, fine diaminobenzidine (DAB) precipitates appeared as regular rod-like structures oriented to cytoplasmic side of the horizontal part (crest) of the postsynaptic membrane (between the junctional folds) which is known to be endowed with nAChR. The rod-like precipitates were not observed in the postsynaptic junctional folds which are devoid of nAChR. The distance separating the rods each other was rather constant (12 - 15 nm), while the length of the rods was variable and exceeded the usual length of nAChR. The present work indicates that the rod-like structures, already observed in association with sarcoplasmic side of the postsynaptic membrane, did correspond to the intramembranous and intracytoplasmic part of nAChR and related proteins. These cytochemical results confirm that d-TC binds to ACh binding sites in the pore of nAChR, and raise the question of DAB staining of cytoskeletal proteins related to the nAChR complex.

The kinetics of inhibition of rat recombinant heteromeric alpha1beta glycine receptors by the low-affinity antagonist SR-95531

The GABA(A) antagonist SR-95531 (gabazine) is known to block glycine receptors, albeit with low affinity. We have studied the effect of SR-95531 on rat recombinant alpha1beta glycine receptors expressed in human embryonic kidney (HEK293) cells by recording macroscopic currents elicited by rapid glycine application to outside-out patches. SR-95531 has a fast unbinding rate (k(offSR), about 3000 s(-1)) and this means that the time course of its unbinding is comparable to the expected time course of the transmitter in the cleft. We also found that equilibrium applications of SR-95531 reduced the response to brief glycine applications by an amount inversely proportional to the duration of glycine application. The fast unbinding rate of SR-95531 from the glycine receptor will make it useful for establishing the time course of glycine concentration at glycinergic synapses.

Neuromuscular block

Descriptions of the South American arrow poisons known as curares were reported by explorers in the 16th century, and their site of action in producing neuromuscular block was determined by Claude Bernard in the mid-19th century. Tubocurarine, the most important curare alkaloid, played a large part in experiments to determine the role of acetylcholine in neuromuscular transmission, but it was not until after 1943 that neuromuscular blocking drugs became established as muscle relaxants for use during surgical anaesthesia. Tubocurarine causes a number of unwanted effects, and there have been many attempts to replace it. The available drugs fall into two main categories: the depolarising blocking drugs and the nondepolarising blocking drugs. The former act by complex mixed actions and are now obsolete with the exception of suxamethonium, the rapid onset and brief duration of action of which remain useful for intubation at the start of surgical anaesthesia. The nondepolarising blocking drugs are reversible acetylcholine receptor antagonists. The main ones are the atracurium group, which possess a built-in self-destruct mechanism that makes them specially useful in kidney or liver failure, and the vecuronium group, which are specially free from unwanted side effects. Of this latter group, the compound rocuronium is of special interest because its rapid onset of action allows it to be used for intubation, and there is promise that its duration of action may be rapidly terminated by a novel antagonist, a particular cyclodextrin, that chelates the drug, thereby removing it from the acetylcholine receptors.

Mapping residues in the ligand-binding domain of the 5-HT(3) receptor onto d-tubocurarine structure

The serotonin 5-HT(3) receptor (5-HT(3)R) is a member of the cys-loop ligand-gated ion channel family. We have used the combination of site-directed mutagenesis, homology modeling of the 5-HT(3)R extracellular domain, and ligand docking simulations as a way to map the architecture of the 5-HT(3)R ligand binding domain. Mutation of Phe226 in loop C of the binding site to tyrosine (F226Y) has no effect on the apparent affinity of the competitive antagonist d-tubocurarine (dTC) for the receptor. On the other hand, replacement of Asn128 in loop A of the binding site with alanine (N128A) increases the apparent affinity of dTC by approximately 10-fold. Double-mutant cycle analysis employing a panel of dTC analogs with substitutions at various positions to identify specific points of interactions between the dTC analogs and Asn128 suggests that Asn128 makes a direct interaction with the 2'N of dTC. Molecular modeling of the 5-HT(3)R extracellular domain using the antagonist-bound conformation of the Aplysia californica acetylcholine binding protein as a template followed by ligand docking simulations produces two classes of structures of the 5-HT(3)R/dTC complex; only one of these has the 2'N of dTC positioned at Asn128 and thus is consistent with the data from this study and previously published data. The use of the rigid dTC analogs as "molecular rulers" in conjunction with double-mutant cycle analysis of mutant receptors can allow the spatial mapping of the position of various residues in the ligand-binding site.

The receptor concept: pharmacology's big idea

Chemical signalling is the main mechanism by which biological function is controlled at all levels, from the single cell to the whole organism. Chemical recognition is the function of receptors, which, in addition to recognising endogenous chemical signals, are also the target of many important experimental and therapeutic drugs. Receptors, therefore, lie at the heart of pharmacology. This article describes the way in which the receptor concept originated early in the 20th century, and evolved through a highly innovative stage of quantitative theory based on chemical kinetics, to the point where receptors were first isolated and later cloned, until we now have a virtually complete catalogue of all the receptors present in the genome. Studies on signal transduction are revealing great complexity in the events linking ligand binding to the physiological or therapeutic response. Though some simple quantitative rules of 'receptor theory' are still useful, the current emphasis is on unravelling the pathways that link receptors to responses, and it will be some time before we know enough about them to embark on the next phase of 'receptor theory'.

The acetylcholinesterase inhibitor BW284c51 is a potent blocker of Torpedo nicotinic AchRs incorporated into the Xenopus oocyte membrane

This work was aimed to determine if 1,5-bis(4-allyldimethylammoniumphenyl)pentan-3-one dibromide (BW284c51), the most selective acetylcholinesterase inhibitor (AchEI), affects the nicotinic acetylcholine (Ach) receptor (AchR) function. Purified Torpedo nicotinic AchRs were injected into Xenopus laevis oocytes and BW284c51 effects on Ach- and carbamylcholine (Cch)-elicited currents were assessed using the voltage-clamp technique.BW284c51 (up to 1 mM) did not evoke any change in the oocyte membrane conductance. When BW284c51 (10 pM-100 microM) and Ach were co-applied, Ach-evoked currents (I(Ach)) were reversibly inhibited in a concentration-dependent manner (Hill coefficient, 1; IC(50), 0.2-0.5 muM for 0.1-1000 microM Ach). Cch-elicited currents showed a similar inhibition by BW284c51.I(Ach) blockade by BW284c51 showed a strong voltage dependence, being only apparent at hyperpolarising potentials. BW284c51 also enhanced I(Ach) desensitisation.BW284c51 changed the Ach concentration-dependence curve of Torpedo AchR response from two-site to single-site kinetics, without noticeably affecting the EC(50) value. The BW284c51 blocking effect was highly selective for nicotinic over muscarinic receptors. BW284c51 inhibition potency was stronger than that of tacrine, and similar to that of d-tubocurarine (d-TC). Coapplication of BW284c51 with either tacrine or d-TC revealed synergistic inhibitory effects. Our results indicate that BW284c51 antagonises nicotinic AchRs in a noncompetitive way by blocking the receptor channel, and possibly by other, yet unknown, mechanisms. Therefore, besides acting as a selective AchEI, BW284c51 constitutes a powerful and reversible blocker of nicotinic AchRs that might be used as a valuable tool for understanding their function.

Open channel and competitive block of nicotinic receptors by pancuronium and atracurium

Mouse myotubes were used to investigate effects of the nondepolarizing neuromuscular blocking drugs pancuronium and atracurium on embryonic-type nicotinic acetylcholine receptor channels. Experiments were performed using patch-clamp techniques in combination with devices for ultra-fast solution exchange at outside--out patches. Application of 0.1 mM acetylcholine resulted in a fast current transient. When the peak amplitude was achieved, the current decayed monoexponentially due to desensitization. After application of drugs (pancuronium or atracurium), two different mechanisms of block were observed: (1) open channel block of embryonic-type nicotinic acetylcholine receptor channels after coapplication of blocker and acetylcholine, characterized by decrease of the time constant of current decay; (2) competitive block of embryonic-type nicotinic acetylcholine receptor channels by pancuronium or atracurium after preincubation of outside-out patches with the respective blocker. Different affinities of pancuronium (K(B) approximately 0.01 microM) and atracurium (K(B) approximately 1 microM) to embryonic-type nicotinic acetylcholine receptor channels were observed.

An algorithmic method for determining the kinetic system of receptor-channel complexes

The mathematical study of receptor-channel kinetics involving numerous sites and conformations of the channel calls for specific analytic methods generally based on stochastic formulation in terms of Markov processes. These methods allow the determination of the number of states from the experimental data. When the number of states is known, it is necessary to try numerous kinetic diagrams to find the best one. The construction of the kinetic diagram and the corresponding kinetic system are based on physiological hypotheses. When the number of states is large, the kinetic schema becomes difficult to establish. We present a method that uses an algorithmic scheme to deduce a kinetic system directly from physiological hypotheses. This method takes into account any number of ligands and sites. The set of all the states given by the combination of site occupation and channel conformations is reduced by using two types of hypothesis: (1) molecular constraints that specify the transitions physically possible between states and (2) kinetic considerations related to the assumed physiology of the system, which gives the conditions necessary for a transition between two states. These hypotheses are expressed in terms of rules operating on the initial states of transitions. The expression of rules does not ensure their coherence (i.e., the fact that each kinetic transition is defined by one and only one rule). A mathematical condition has been found that ensures the coherence of rules. When coherence has been established, the corresponding dynamic system can be automatically generated. Because the rules are established in a systematic way and their coherence can be mathematically established, the computer implementation of this method makes it easy to test various kinetic hypotheses for problems where the number of states is large.

Hippocampal interneurons are excited via serotonin-gated ion channels

Hippocampal interneurons are excited via serotonin-gated ion channels. J. Neurophysiol. 78: 2493-2502, 1997. Serotonergic neurons of the median raphe nucleus heavily innervate hippocampal GABAergic interneurons located in stratum radiatum of area CA1, suggesting that this strong subcortical projection may modulate interneuron excitability. Using whole cell patch-clamp recording from interneurons in brain slices, we tested the effects of serotonin (5-HT) on the physiological properties of these interneurons. Serotonin produces a rapid inward current that persists when synaptic transmission is blocked by tetrodotoxin and cobalt, and is unaffected by ionotropic glutamate and gamma-aminobutyric acid (GABA) receptor antagonists. The 5-HT-induced current was independent of G-protein activation. Pharmacological evidence indicates that 5-HT directly excites these interneurons through activation of 5-HT3 receptors. At membrane potentials negative to -55 mV, the current-voltage (I-V) relationship of the 5-HT current displays a region of negative slope conductance. Therefore the response of interneurons to 5-HT strongly depends on membrane potential and increases greatly as cells are depolarized. Removal of extracellular calcium, but not magnesium, increases the amplitude of 5-HT-induced currents and removes the region of negative slope conductance, thereby linearizing the I-V relationship. The axons of 5-HT-responsive interneurons ramify widely within CA1; some of these interneurons also project to and arborize extensively in the dentate gyrus. The organization of these inhibitory connections strongly suggests that these cells regulate excitability of both CA1 pyramidal cells and dentate granule cells. As our results indicate that 5-HT may mediate fast excitatory synaptic transmission onto these interneurons, serotonergic inputs can simultaneously modulate the output of both hippocampus and dentate gyrus.

Local neurogenic regulation of rat hindlimb circulation: role of calcitonin gene-related peptide in vasodilatation after skeletal muscle contraction

1. The mechanism of neurogenic regulation of skeletal muscle circulation was studied in the hindlimb of anaesthetized rats in vivo. Regional blood flow (RBF) of the hindlimb was recorded with a pulsed Doppler flow probe positioned in the iliac artery. 2. A short period (1 min) of sciatic nerve stimulation at 10 Hz caused a sustained increase in RBF (from 2.0 +/- 0.2 to 3.7 +/- 0.2 kHz at the peak), but no appreciable change in either MBP or HR, suggesting that the nerve stimulation produced local vasodilatation of the peripheral vasculature. The hyperaemic response reached a peak within 15 s and characteristically remained above the basal level for more than 5 min after the cessation of nerve stimulation. The response was regarded as a secondary response brought about by the contraction of skeletal muscles since (+)-tubocurarine (0.73 micromol kg(-1), i.a.) almost abolished it. 3. Lignocaine (43 micromol kg(-1), i.a.) and capsaicin (0.33 micromol kg(-1), i.a.) significantly suppressed the hyperaemic response to skeletal muscle contraction, suggesting that capsaicin-sensitive sensory nerves contribute to the hyperaemia. In contrast, an inhibitor of NO synthase, N(omega)-nitro-L-arginine methyl ester (1 micromol kg(-1) min(-1), i.v.), did not affect the hyperaemic response. 4. Serum levels of calcitonin gene-related peptide (CGRP) in iliac venous effluent significantly increased from 51 +/- 4 to 77 +/- 5 fmol ml(-1) during the hyperaemic response to skeletal muscle contraction. A bolus injection of CGRP (300 pmol kg(-1), i.a.) induced a long-lasting increase in RBF of the hindlimb. Moreover, CGRP(8-37) (100 nmol kg(-1) min(-1), i.v.), a specific CGRP1 receptor antagonist, significantly suppressed the hyperaemic response, especially the sustained phase of the response which was almost abolished by this antagonist. 5. These results suggest that CGRP, which is released from peripheral endings of capsaicin-sensitive sensory nerves, partly mediates the hyperaemia evoked by skeletal muscle contraction of the rat hindlimb.

OBSERVATIONS ON THE MODE OF ACTION OF SOME CENTRAL DEPRESSANT DRUGS ON TRANSMISSION THROUGH THE CAT SUPERIOR CERVICAL GANGLION

Methylpentynol, paraldehyde, amylobarbitone and procainamide blocked transmission through the cat superior cervical ganglion, and antagonized the ganglion-stimulating actions of acetylcholine and carbachol injected intra-arterially to the ganglion. Comparison with the effects of tetraethylammonium indicated that the impaired response to acetylcholine could not wholly account for the failure of transmission, which suggested that an impaired release of transmitter substance was a contributory factor. Methylpentynol, paraldehyde and procainamide also blocked the ganglion-stimulating action of potassium chloride. In contrast, amylobarbitone and pentobarbitone did not block the stimulating action of potassium chloride, but antagonized specifically the actions of acetylcholine and carbachol. The anti-acetylcholine activities of the two barbiturate drugs at this site accord with their relative ganglion-blocking activities. It is concluded that the ganglion-blocking action of methylpentynol, paraldehyde and procainamide arises from a nonspecific depression of both presynaptic and postsynaptic elements in the ganglion, but that barbiturate compounds act more specifically on the acetylcholine receptor.

Thermodynamics of the interaction of d-tubocurarine with nicotinic receptors of mammalian skeletal muscle in vitro

Thermodynamic study is important for defining drug receptor interactions, and denervated rat hemidiaphragm is a unique preparation for such a study on nicotinic receptors. As a continuation of our earlier study with acetylthiocholine on the same preparation, we now report on the characteristics of temperature-dependent binding of d-tubocurarine, a reversible antagonist. The O. Arunlakshana and H.O. Schild (1959, Br. J. Pharmacol. 14, 48) equation, as improved by D.R. Woud and R.B. Parker (1971, J. Pharmacol. Exp. Ther. 177, 13), was used to calculate the dissociation constant of d-tubocurarine at various temperatures (10-37 degrees C) from the parallel shift of the acetylcholine dose-response curve to the right by effective doses of d-tubocurarine. It was observed that the values of the dissociation constant increased with a decrease in temperature. Both the enthalpy (delta H degree) and entropy (delta S degree) changes as evaluated from the van't Hoff plot (In Kd vs. 1/T) were found to be positive and their relative value (delta H degree - T delta S degree) produced a negative free energy change which characterises the binding of d-tubocurarine as an entropy-controlled process. This finding is in agreement with the neurotoxin binding reported earlier. The present finding and earlier observations with acetylthiocholine reveal that agonist and antagonist binding to the nicotinic receptor may differ depending on the experimental conditions.

Open channel and competitive block of the embryonic form of the nicotinic receptor of mouse myotubes by (+)-tubocurarine

1. Embryonic-like nicotinic channels were studied in mouse myotubes. Channel currents were measured by patch clamping outside-out excised patches to which pulses of agonists and drugs could be applied by a liquid filament switch. The holding potential of the patches was generally around-10 to-40 mV. 2. Pulses of 100 microM or 1 mM acetylcholine (ACh) elicited average channel currents which reached a maximum open probability of 0.93 within 0.5-1.0 ms, decayed with a time constant of desensitization of 20-80 ms, and fell rapidly to zero at the end of the pulse. When such pulses together with increasing concentrations of (+)-tubocurarine (TC) were applied to outside-out patches, the time constant of current decay, tau, decreased beginning at concentrations of TC added to the test solution of > 10 microM, and the peak amplitude of the current decreased markedly at concentrations of TC of > 30 microM due to an open channel block of nicotinic channels by TC. 3. When the outside-out patches were pre-incubated with TC, the peak current elicited by pulses of 100 microM ACh or 1 mM ACh + TC decreased markedly, beginning with concentrations of TC > 30 nM due to a competitive block. 4. The results could be quantitatively modelled by computer calculations based on a circular reaction scheme containing desensitization. TC blocked the open state as well as the unliganded closed state of the embryonic-like nicotinic receptors of mouse myotubes. Also the blocked open channel was subject to desensitization. 5. The rates of block and unblock of the open channel were 3 x 10(6) M-1 S-1 and 0.8 S-1, respectively, and those of the competitive block were 0.5 x 10(6) M-1 S-1 and 0.1 S-1, respectively (at 20 degrees C).

Use of an antagonist for estimating the degree of agonist stimulation during physiological release

Antagonist activity can be measured either as the percentage reduction in the effect of an agonist or as a dose ratio. In this article, Dick Barlow explains how these are connected and how the relationship between the concentration of antagonist producing 50% inhibition (IC50) and the antagonist equilibrium constant (Ki) involves the degree of agonist stimulation ([A]/[A50]), which is the concentration of agonist expressed as a multiple of the concentration producing a half-maximal response. If Ki is known it is possible to use the IC50 value of the antagonist to estimate the degree of agonist stimulation in physiological experiments where there is neuronal or hormonal release of the agonist.

The inhibitory effects of nicotinic antagonists on currents elicited by GABA in rat hippocampal neurons

The nicotinic antagonists d-tubocurarine and trimethaphan camsylate competitively inhibit GABA-induced currents. Hexamethonium, mecamylamine and dihydro-beta-erythroidine, other nicotinic antagonists, do not affect GABA-elicited currents. The trimethaphan effect is completely reversed by a putative convulsant receptor antagonist, alpha-isopropyl-alpha-methyl-gamma-butyrolactone, which implies that the trimethaphan binding site may be closely associated with the convulsant site. However, nicotine was ineffective in competing for either the d-tubocurarine or trimethaphan effect at the GABAA receptor. From these observations, we propose that the nicotinic and GABAA receptor ionophore complexes share similar configurational patterns that accommodate some of the same molecules. Possible mechanisms for the trimethaphan and d-tubocurarine blockades are discussed.

Snake venom toxins, unlike smaller antagonists, appear to stabilize a resting state conformation of the nicotinic acetylcholine receptor

Previous studies have shown that the pattern and degree of 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine ([125I]TID) photoincorporation into the nicotinic acetylcholine receptor (nAChR) can be used as a sensitive measure of nAChR conformation. Upon desensitization by prolonged exposure to agonists, certain drugs and detergents, or reconstitution into desensitizing lipids, the levels of [125I]TID incorporation into the subunits of the nAChR are dramatically reduced. In this study, we characterized the effects of the snake venom proteins alpha-bungarotoxin and alpha-cobrotoxin, as well as the smaller antagonists tubocurarine and gallamine, on [125I]TID incorporation into the subunits of both partially-purified nAChR in native lipids, or affinity-purified nAChR reconstituted into different combinations of lipids. Unlike all other compounds previously tested, alpha-bungarotoxin and alpha-cobrotoxin reproducibly increased the level of [125I]TID incorporation into all four subunits of nAChR reconstituted into dioleoylphosphatidylcholine, dioleoylphosphatidic acid and cholesterol. Gallamine had little or no effect on [125I]TID incorporation at any concentration tested (0.1 microM-5 mM). Tubocurarine had no effect on [125I]TID incorporation at low concentrations, but at higher concentrations reduced the level of [125I]TID labeling. The snake venom proteins may shift the population of nAChR, which exists as a mixture of resting state and desensitized conformations, entirely to the resting state. However, the binding of the snake venom toxins does not appear sufficient to induce the resting state conformation in nAChR which have been desensitized by other means, such as solubilization in desensitizing detergents or reconstitution in densitizing lipids.

Changes in MEPP frequency during depression of evoked release at the frog neuromuscular junction

1. Endplate potentials (EPPs) and miniature endplate potentials (MEPPs) were recorded from frog neuromuscular junctions bathed in Ringer solutions containing normal (1.8 mM) or high (3.6 mM) Ca2+. The peptide toxin mu-conotoxin GIIIA was added to the Ringer solution to prevent muscle action potentials and contraction. 2. The nerve was stimulated with conditioning trains of 200-4800 impulses applied at 20 impulses s-1 to characterize the effects of repetitive stimulation on changes in EPP amplitude and MEPP frequency under high quantal conditions. 3. MEPP frequency was dramatically increased during and immediately following repetitive stimulation under high quantal conditions, whereas EPP amplitude was greatly depressed. There was no effect of repetitive stimulation on MEPP amplitude. 4. Following the conditioning stimulation the increase in MEPP frequency decayed back to the control level with a time course that could be described by four exponentials. The time constants of these exponentials were very similar to those that describe the components of stimulation-induced increases in EPP amplitude and MEPP frequency observed under low quantal conditions when depression is absent. 5. The results of this study indicate that depression and the components of stimulation-induced increases in release (facilitation, augmentation and potentiation) can be present at the same time, suggesting that the mechanism of depression involves different underlying factors from the mechanism(s) responsible for increases in release. They also indicate either that depression selectively affects only those quanta destined to be released in direct response to the nerve action potential, which would suggest that EPPs and MEPPs arise from different pools of transmitter, or that depression in some way affects a step in the release process involved only in evoked release, and not asynchronous (spontaneous) release.

Characterization of d-tubocurarine binding site of Torpedo acetylcholine receptor

d-Tubocurarine (curare) is a well-characterized competitive antagonist of nicotinic acetylcholine receptors (AChRs), and it is usually assumed that curare and agonists share a common binding site. We have examined the role of several highly conserved residues of the alpha-, gamma-, and delta-subunits in the interaction of curare with the Torpedo acetylcholine receptor (AChR). Curare inhibition of wild-type receptors is consistent with curare binding to a single high-affinity binding site [inhibitor constant (Ki) = 20 nM]. Phenylalanine substitutions for two tyrosine residues implicated as being in the ligand binding site (alpha Y93F, alpha Y190F) reduce curare affinity, indicating that these residues are also important for high-affinity curare binding. Phenylalanine substitution for alpha Y198 [alpha Y198F (notation used here: subunit/amino acid in wild-type/residue number/substitution)] causes a 10-fold increase in curare affinity (Ki = 3.1 nM), and measurement of the recovery from curare inhibition indicates that this increase in affinity is due to a reduction in the rate of curare dissociation from the receptor. In addition to the alpha-subunits, portions of the ligand binding sites also reside on the gamma- and delta-subunits, and photoaffinity studies have implicated two residues (gamma W55 and delta W57) as forming part of the curare sites. The gamma W55L mutation results in an eightfold decrease in curare affinity (Ki = 170 nM), whereas the delta W57L mutation has no effect. These data support the notion that the high-affinity curare binding site is formed by segments of the alpha- and gamma-subunits.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of glycerol treatment on transmitter release at the frog neuromuscular junction

Exposure of frog skeletal muscle to a Ringer solution made hyperosmotic with glycerol, followed by return of the preparation to a normal Ringer, results in an irreversible uncoupling of the mechanical activity of the muscle. This technique for preventing muscle contraction has often been used to study neuromuscular transmission under normal or high levels of release without interference from muscle contraction. Little was known, however, about the effects of glycerol treatment on the process of transmitter release. In this paper we report the results of a study in which we examined the effects of glycerol treatment on transmitter release during repetitive stimulation at the frog sartorius neuromuscular junction. We found that glycerol treatment altered the stimulation-induced changes in end-plate potential (EPP) amplitude normally observed during repetitive stimulation at this synapse. This effect, which increased progressively with time for up to 8 h following removal of glycerol, could be accounted for by presynaptic changes in the amount of transmitter released.

Use of mu-conotoxin GIIIA for the study of synaptic transmission at the frog neuromuscular junction

We have looked at the effect of synthetic mu-conotoxin GIIIA, a selective blocker of muscle Na channels, on various parameters of synaptic transmission at the frog sartorius nerve-muscle preparation. We found that 5 microM mu-conotoxin consistently blocked muscle action potentials, but had no effect on nerve action potentials. The toxin also had no effect on the amplitude or frequency of miniature endplate potentials (MEPPs), on the amplitude or time course of endplate potentials (EPPs), or on stimulation-induced changes in EPP amplitude. The lack of an effect of synthetic mu-conotoxin GIIIA on transmitter release makes this toxin an invaluable tool in the study of neuromuscular transmission under conditions of normal levels of release.

Differential potentiation by calcium antagonists of neuromuscular blockade induced by pancuronium and succinylcholine in cats in vivo

The effects of several calcium antagonists (verapamil, nicardipine and two diltiazem isomers, d-cis and l-cis diltiazem) alone and associated to non-depolarizing (pancuronium) and depolarizing (succinylcholine) neuromuscular blockers, were evaluated on sciatic nerve-tibialis anterior muscle preparations from cats in vivo. The calcium antagonists used (at 0.1 and 0.5 mg/kg iv) did not modify the height of muscular twitches elicited indirectly. However, these agents potentiated in a dose-dependent way the neuromuscular blockade induced by iv pancuronium (2-40 micrograms/kg) and succinylcholine (6-200 micrograms/kg). The order of potency in increasing the effects of pancuronium was nicardipine much greater than d-cis diltiazem greater than or equal to verapamil, whereas the order of potency in enhancing succinylcholine effects was d-cis diltiazem greater than or equal to verapamil much greater than nicardipine. The effects of diltiazem were stereoselective, thus the potentiation induced by d-cis diltiazem was significantly greater in all cases than that induced by l-cis diltiazem, which suggests that calcium channel blockade plays a role in these interactions. However, other mechanisms such as calcium antagonists-induced nicotinic receptor desensitization may also be involved.

Kinetics of (+)-tubocurarine blockade at the neuromuscular junction

1. Although (+)-tubocurarine (Tc) is classically considered to be a competitive antagonist at the neuromuscular junction, kinetic details of the interaction remain unclear. 2. We studied the competitive action of Tc on the nicotinic receptor at the frog neuromuscular junction using a quantitative analysis of the generation phase of miniature endplate currents (m.e.p.cs) recorded in Ringer solution (20 degrees C) under voltage clamp (-90 mV) in the absence or presence of 1-5 microM Tc. Under control conditions four neurotransmission parameters were estimated by non-linear regression using a mathematical model of synaptic transmission incorporating transmitter release, diffusion, hydrolysis, receptor binding and channel gating. These parameters were then used in a further regression to estimate binding rate constants for Tc at the same endplate. Allowance was made for open channel block by Tc, which under the conditions of this study was only a small component of total blockade. 3. The results suggest that Tc binds to the two agonist recognition sites on the nicotinic receptor with equal affinity (stoichiometric KDs of 2.2 and 8.8 microM), and that most of the functional blockade at concentrations up to 5 microM is due to occupancy of only one site. 4. The association rate constant for Tc binding to sites on the nicotinic acetylcholine receptor appears to be very fast (k+D = 8.9 x 10(8) M-1 s-1) and comparable to that for acetylcholine (ACh). 5. In the brief time during which an m.e.p.c. is generated (approximately 200 microseconds, reversal of Tc blockade by transiently high concentrations of ACh seems to be kinetically limited.

The effects of manganese and barium on the cardiac pacemaker current, if, in rabbit sino-atrial node myocytes

The isolation of ionic fluxes contributing to electric currents through cell membranes often requires block of other undesired components which can be achieved, among others, by divalent cations. Mn2+ and Ba2+ are often used, for example, to block Ca and K currents. Here we have investigated the effects of these two cations on the properties of the hyperpolarization-activated pacemaker current if, in rabbit sino-atrial node myocytes, as obtained by voltage clamp analysis. We find that 2 mM Mn2+ shifts the if activation curve by 3.2 +/- 0.3 mV towards more positive values. However, when 1 mM Ba2+ is also added, the positive shift is more than halved (1.3 +/- 0.2 mV). We find, too, that in the absence of blocking cations the ACh-induced if inhibition is slightly higher than in their presence. These results indicate that the alteration of if kinetic properties by Ba2+ plus Mn(2+)-containing solutions is minimal.

5-HT3 receptors mediate rapid responses in cultured hippocampus and a clonal cell line

Serotonin (5-HT) induces a large inward current accompanied by a conductance increase when applied focally to either neurons of mouse hippocampal cultures or cells of the NG108-15 clonal cell line. In both systems, the response is blocked by ICS 205-930, curare, and metoclopramide, while 2-methyl-5-HT is an agonist. The actions of ICS 205-930 and 2-methyl-5-HT indicate that the response is mediated by the 5-HT3 receptor. In NG108-15 cells the response activated in as little as 35 ms. The rapidity of the response suggests a direct coupling between the 5-HT3 receptor and a channel, which are probably both part of a single membrane protein. In both cell types, prolonged application of 5-HT resulted in desensitization; the rates of desensitization were accelerated by the adenylate cyclase activator forskolin. The 5-HT3 receptor has much in common with the nicotinic receptor and is probably involved in rapid synaptic transmission in the mammalian brain. Since this response is modulated by manipulations that elevate intracellular cAMP levels, the central synapses in which this receptor operates may exhibit plasticity.

Presynaptic effects of d-tubocurarine on neurotransmitter release at the neuromuscular junction of the frog

1. Presynaptic effects of d-tubocurarine on neurotransmitter release were examined at the frog neuromuscular junction, using intracellular and extracellular recording techniques. 2. d-Tubocurarine in concentrations of 10(-7)-10(-6) M decreased the quantal content (m) measured by the coefficient of variation and failure methods. 3. d-Tubocurarine produced a shift to the right of the curve relating log quantal content to log [Ca2+]o without changing the slope. 4. The duration of twin-impulse facilitation was not affected by 5 x 10(-7) M-d-tubocurarine. Early facilitation was higher in d-tubocurarine. 5. d-Tubocurarine altered the synaptic delay histogram. The peak of the histogram was shifted to longer delays. Prolongation of the minimal delay was seen in most but not all experiments. 6. These results suggest that d-tubocurarine inhibits release of neurotransmitter by affecting a stage in the process of release, which occurs after the entry of Ca2+ ions.

Activation of ion channels in the frog end-plate by high concentrations of acetylcholine

1. The equilibrium relationship between acetylcholine (ACh) concentration and response (fraction of channels open), corrected for the effects of desensitization, has been estimated by single-ion-channel recording at the adult frog skeletal neuromuscular junction. At high ACh concentration channel openings occur in well-defined clusters separated by long desensitized intervals. The response, po, was estimated as the proportion of time for which a single channel was open during a cluster. 2. At negative membrane potential (-120 mV) po reached a maximum value of 0.9 at 100 microM-ACh and was half-maximum at 15 microM with a Hill slope of 1.6 at this point. At concentrations higher than 200 microM-ACh, po declined as a result of open-channel block by free ACh itself. 3. At positive membrane potentials (+100 mV) there was little channel block by ACh; po reached a maximum value of 0.41 at 500 microM-ACh, with half-maximum activation at 50 microM and Hill slope of 1.2 at this point. 4. Particular mechanisms for channel activation by ACh were fitted to the data by the method of least squares. Fits were fully determinate only if the two binding sites for ACh were assumed to be equivalent with no co-operativity in the ACh binding reactions. At negative potential the microscopic equilibrium constant for binding was K1 = K2 = 77 microM and the equilibrium constant for channel opening (opening/closing rates, beta/alpha) was 32. At positive potential the affinity was slightly higher, K = 32 microM, which confirms the view that the binding sites for ACh are outside the membrane electric field. The equilibrium constant for channel opening was reduced to 0.7 mainly as a result of the much shorter open lifetime (increased closing rate alpha) at positive potentials. 5. The data were also fitted well by very high values of beta/alpha together with a high degree of negative co-operativity or non-equivalence in ACh binding affinity (K2 much greater than K1). A good fit could also be obtained with moderate positive co-operativity combined with non-equivalence of the binding sites. 6. A mechanism that postulates a receptor with two independent gating subunits provided a poor fit to the data at negative potential. 7. The rate constants for channel opening and ACh dissociation were estimated by constraining the fitted parameters so that the burst length for channel opening was equal to its observed value at low concentrations of ACh.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of T cell specificity on the heterogeneity and disease-causing capability of antibody against the acetylcholine receptor

Adoptive secondary anti-acetylcholine receptor (AChR) antibody responses were examined in rats to evaluate the influence of helper T cell specificity on the nature and disease-causing potential of antibody produced. Mixtures of B cells reactive with the intact AChR plus T cells reactive with purified AChR subunits (alpha, beta, gamma, delta) were transferred and antigen-challenged in immunologically naive recipient rats; the serum anti-AChR antibody produced was assessed by radioimmunoassay for differences in titers and by isoelectric focusing for differences in clonal heterogeneity as a function of the subunit specificity of T cells transferred. In addition, rats receiving different sources of AChR or AChR subunit-reactive T cells were examined for AChR-dependent muscle dysfunction. The results indicated a clear reduction in anti-AChR antibody concentrations and clonal heterogeneity in recipient rats receiving T cells of specificities restricted to individual subunits. However, except for a clear relationship between serum anti-AChR antibody concentration and disease induction, no particular AChR subunit-reactive helper T cell specificity appeared to preferentially cause muscle dysfunction. We conclude that if such relationships exists, T cells with specificities more restricted than those described here will have to be used.

Ruthenium red reduces acetylcholine sensitivity and increases desensitization at the frog neuromuscular junction

The actions of Ruthenium Red on the synaptic membrane were studied at the neuromuscular junction of the frog Rana pipiens. Ruthenium Red blocks mitochondrial calcium transport and thus is expected to elevate the intracellular calcium level [Alnaes and Rahamimoff (1975) J. Physiol., Lond. 248, 285-306]. The postjunctional membrane sensitivity was reduced by Ruthenium Red as determined by (a) the amplitude of the miniature endplate potentials, (b) iontophoretic application of carbachol, (c) microperfusion of carbachol. Desensitization was assayed by measuring the decline in the amplitude of the postsynaptic depolarization produced during repetitive iontophoretic application of carbachol. Following Ruthenium Red treatment desensitization was increased. This action of Ruthenium Red was enhanced by raising the extracellular calcium concentration from 1.8 to 10 mM.

Analysis of contractile properties of muscles from rats immunized with purified acetylcholine receptor

Experimental autoimmune myasthenia gravis (EAMG) was induced in rats by injection of purified acetylcholine receptor (AChR). In addition to detecting elevated serum titers of anti-AChR antibodies, we observed decreased twitch-tension at submaximal stimulation voltages and increased curare sensitivity by muscles obtained from immunized rats when compared to muscles obtained from nonimmune control rats. Furthermore, antibody-induced neuromuscular impairment was expressed to differing extents dependent on whether the diaphragm, soleus, or extensor digitorum longus muscle was examined. Thus, we conclude that potential antibody perturbation of AChR function will depend not only on the nature of the antibody, but also on the complex structure-function relationships that exist in individual muscles. This may partially explain the variable impairment of different muscle groups in patients with myasthenia.

Examination of the mechanisms involved in tetanic fade produced by vecuronium and related analogues in the rat diaphragm

The effects of vecuronium (Org NC45), Org 7678 and Org 7684 were examined on twitches and tetani recorded from rat isolated diaphragms. Org 7678 and Org 7684 exhibited approximately one tenth of the neuromuscular blocking potency of vecuronium. At concentrations producing equivalent amounts of twitch block, Org 7684 produced significantly less tetanic fade than did vecuronium or Org 7678. In cut muscles both vecuronium and Org 7684 reduced the endplate current (e.p.c.) amplitude (Ie.p.c.), reduced e.p.c. decay time constant (tau e.p.c.), and increased the e.p.c. train rundown. The effects of vecuronium were not voltage-dependent and vecuronium did not change tau noise. The effect of Org 7684 on Ie.p.c. and tau e.p.c. became greater with hyperpolarization, but the effect on e.p.c. train rundown was not voltage-dependent. It is concluded that both vecuronium and Org 7684 produce e.p.c. train rundown and tetanic fade by a prejunctional mechanism. However, whereas postjunctionally vecuronium blocks only the acetylcholine receptor, Org 7684 blocks both the receptor and its associated ion channel.

Acetylcholine receptor inhibition by d-tubocurarine involves both a competitive and a noncompetitive binding site as determined by stopped-flow measurements of receptor-controlled ion flux in membrane vesicles

The issue of whether d-tubocurarine, the classical acetylcholine receptor inhibitor, inhibits the receptor by a competitive or noncompetitive mechanism has long been controversial. d-Tubocurarine, in this study, has been found to be both a competitive (KC = 120 nM) and a noncompetitive (KNC = 4 microM) inhibitor of receptor-mediated ion flux at zero transmembrane voltage in membrane vesicles prepared from Electrophorus electricus electroplax. A spectrophotometric stopped-flow method, based on fluorescence quenching of entrapped anthracene-1,5-disulfonic acid by Cs+, was used to measure both the rate coefficient of ion flux prior to receptor inactivation (desensitization) and the rate coefficient of the rapid inactivation process. Inhibition by d-tubocurarine of the initial rate of ion flux decreased with increasing acetylcholine concentration, consistent with competitive inhibition, but the inhibition by micromolar concentrations of d-tubocurarine could not be overcome with saturating concentrations of acetylcholine, consistent with noncompetitive inhibition. A minimum mechanism is proposed in which d-tubocurarine competes for one of the two acetylcholine activating sites and also binds to a noncompetitive site. The present data do not distinguish between one or two competitive sites, although one successfully accounts for all of the data. By variation of the acetylcholine concentration, the two types of sites could be studied in isolation. Binding of d-tubocurarine to the noncompetitive site does not change the rate of rapid receptor inactivation, whereas binding of d-tubocurarine to the competitive site decreases the rate of rapid inactivation by displacing acetylcholine, in agreement with the observation that d-tubocurarine does not inactivate (desensitize) the E. electricus receptor by itself.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptors on individual neurones

Membrane potential or ionic conductance of neurones of the mammalian central or peripheral nervous system maintained in vitro can be measured over periods of several hours. Drugs or transmitters which change potential or conductance can be applied repeatedly under equilibrium conditions, and pharmacological null methods used to characterize the receptors with which they interact. The method offers an advantage over ligand binding studies on nervous tissue because both agonist and antagonist affinities can be estimated on individual functioning cells. The results to date suggest the hypothesis that a given receptor subtype is always associated with the same change in ion conductance, and the corollary that distinct ion conductances affected by the same transmitter result from interactions with different receptor subtypes.