Activation of heteroliganded mouse muscle nicotinic receptors

Pharmacological manipulation of the olfactory bulb modulates beta oscillations: testing model predictions

The mammalian olfactory bulb (OB) generates gamma (40-100 Hz) and beta (15-30 Hz) local field potential (LFP) oscillations. Gamma oscillations arise at the peak of inhalation supported by dendrodendritic interactions between glutamatergic mitral cells (MCs) and GABAergic granule cells (GCs). Beta oscillations are induced by odorants in learning or odor sensitization paradigms, but their mechanism and function are still poorly understood. When centrifugal OB inputs are blocked, beta oscillations disappear, but gamma oscillations persist. Centrifugal inputs target primarily GABAergic interneurons in the GC layer (GCL) and regulate GC excitability, suggesting a causal link between beta oscillations and GC excitability. Our previous modeling work predicted that convergence of excitatory/inhibitory inputs onto MCs and centrifugal inputs onto GCs increase GC excitability sufficiently to produce beta oscillations primarily through voltage dependent calcium channel-mediated GABA release, independently of NMDA channels. We test some of the predictions of this model by examining the influence of NMDA and muscarinic acetylcholine (ACh) receptors, which affect GC excitability in different ways, on beta oscillations. A few minutes after intrabulbar infusion, scopolamine (muscarinic antagonist) suppressed odor-evoked beta in response to a strong stimulus but increased beta power in response to a weak stimulus, as predicted by our model. Pyriform cortex (PC) beta power was unchanged. Oxotremorine (muscarinic agonist) suppressed all oscillations, likely from overinhibition. APV, an NMDA receptor antagonist, suppressed gamma oscillations selectively (in OB and PC), lending support to the model's prediction that beta oscillations can be supported independently of NMDA receptors. NEW & NOTEWORTHY Olfactory bulb local field potential beta oscillations appear to be gated by GABAergic granule cell excitability. Reducing excitability with scopolamine reduces beta induced by strong odors but increases beta induced by weak odors. Beta oscillations rely on the same synapse as gamma oscillations but, unlike gamma, can persist in the absence of NMDA receptor activation. Pyriform cortex beta oscillations maintain power when olfactory bulb beta power is low, and the system maintains beta band coherence.

Estimating kinetic mechanisms with prior knowledge I: Linear parameter constraints

New mathematical tools are needed to incorporate existing knowledge into kinetic models of ion channels and other proteins. Salari et al. describe an algebraic transformation that can enforce linearly interdependent parameters into kinetic models in order to test new hypotheses.

To understand how ion channels and other proteins function at the molecular and cellular levels, one must decrypt their kinetic mechanisms. Sophisticated algorithms have been developed that can be used to extract kinetic parameters from a variety of experimental data types. However, formulating models that not only explain new data, but are also consistent with existing knowledge, remains a challenge. Here, we present a two-part study describing a mathematical and computational formalism that can be used to enforce prior knowledge into the model using constraints. In this first part, we focus on constraints that enforce explicit linear relationships involving rate constants or other model parameters. We develop a simple, linear algebra–based transformation that can be applied to enforce many types of model properties and assumptions, such as microscopic reversibility, allosteric gating, and equality and inequality parameter relationships. This transformation converts the set of linearly interdependent model parameters into a reduced set of independent parameters, which can be passed to an automated search engine for model optimization. In the companion article, we introduce a complementary method that can be used to enforce arbitrary parameter relationships and any constraints that quantify the behavior of the model under certain conditions. The procedures described in this study can, in principle, be coupled to any of the existing methods for solving molecular kinetics for ion channels or other proteins. These concepts can be used not only to enforce existing knowledge but also to formulate and test new hypotheses.

Role of acetylcholinesterase on the structure and function of cholinergic synapses: insights gained from studies on knockout mice

Electrophysiological and ultrastructural studies were performed on phrenic nerve-hemidiaphragm preparations isolated from wild-type and acetylcholinesterase (AChE) knockout (KO) mice to determine the compensatory mechanisms manifested by the neuromuscular junction to excess acetylcholine (ACh). The diaphragm was selected since it is the primary muscle of respiration, and it must adapt to allow for survival of the organism in the absence of AChE. Nerve-elicited muscle contractions, miniature endplate potentials (MEPPs) and evoked endplate potentials (EPPs) were recorded by conventional electrophysiological techniques from phrenic nerve-hemidiaphragm preparations isolated from 1.5- to 2-month-old wild-type (AChE(+/+)) or AChE KO (AChE(-/-)) mice. These recordings were chosen to provide a comprehensive assessment of functional alterations of the diaphragm muscle resulting from the absence of AChE. Tension measurements from AChE(-/-) mice revealed that the amplitude of twitch tensions was potentiated, but tetanic tensions underwent a use-dependent decline at frequencies below 70 Hz and above 100 Hz. MEPPs recorded from hemidiaphragms of AChE(-/-) mice showed a reduction in frequency and a prolongation in decay (37%) but no change in amplitude compared to values observed in age-matched wild-type littermates. In contrast, MEPPs recorded from hemidiaphragms of wild-type mice that were exposed for 30 min to the selective AChE inhibitor 5-bis(4-allyldimethyl-ammoniumphenyl)pentane-3-one (BW284C51) exhibited a pronounced increase in amplitude (42%) and a more marked prolongation in decay (76%). The difference between MEPP amplitudes and decays in AChE(-/-) hemidiaphragms and in wild-type hemidiaphragms treated with BW284C51 represents effective adaptation by the former to a high ACh environment. Electron microscopic examination revealed that diaphragm muscles of AChE(-/-) mice had smaller nerve terminals and diminished pre- and post-synaptic surface contacts relative to neuromuscular junctions of AChE(+/+) mice. The morphological changes are suggested to account, in part, for the ability of muscle from AChE(-/-) mice to function in the complete absence of AChE.

Activation and modulation of human α4β2 nicotinic acetylcholine receptors by the neonicotinoids clothianidin and imidacloprid

Neonicotinoids are synthetic, nicotine-derived insecticides used for agricultural and household pest control. Though highly effective at activating insect nicotinic receptors, many neonicotinoids are also capable of directly activating and/or modulating the activation of vertebrate nicotinic receptors. In this study, we have investigated the actions of the neonicotinoids clothianidin (CTD) and imidacloprid (IMI) on human neuronal α4β2 nicotinic acetylcholine receptors. The data demonstrate that the compounds are weak agonists of the human receptors with relative peak currents of 1-4% of the response to 1 mM acetylcholine (ACh). Coapplication of IMI strongly inhibited currents elicited by ACh. From Schild plot analysis, we estimate that the affinity of IMI for the human α4β2 receptor is 18 μM. The application of low concentrations of CTD potentiated responses to low concentrations of ACh, suggesting that receptors occupied by one ACh and one CTD molecule have a higher gating efficacy than receptors with one ACh bound. Interestingly, subunit stoichiometry affected inhibition by CTD, with (α4)(2) (β2)(3) receptors significantly more strongly inhibited than the (α4)(3) (β2)(2) receptors.

Responses of pigeon vestibular hair cells to cholinergic agonists and antagonists

Acetylcholine (ACh) is the major neurotransmitter released from vestibular efferent terminals onto hair cells and afferents. Previous studies indicate that the two classes of acetylcholine receptors, nicotinic (nAChRs) and muscarinic receptors (mAChRs), are expressed by vestibular hair cells (VHCs). To identify if both classes of receptors are present in VHCs, whole cell, voltage-clamp- and current-clamp-patch recordings were performed on isolated pigeon vestibular type I and type II HCs during the application of the cholinergic agonists, acetylcholine and carbachol, and the cholinergic antagonists, D-tubocurarine and atropine. By applying in different combinations, these compounds were used to selectively activate either nAChRs or mAChRs. The effects of nAChR and mAChR activation on HC currents and zero electrode current potential (V(z)) were monitored. It was found that presumed mAChR activation decreased both inward and outward currents in both type I and type II HCs, resulting in either a depolarization or hyperpolarization. Conversely, nAChR activation mainly increased both inward and outward currents in type II HCs, resulting in a hyperpolarization of their V(z). nAChR activation also increased outward currents in type I HCs resulting in either a depolarization or hyperpolarization of their V(z). The decrease of inward and outward currents and the depolarization of the V(z) in type I pigeon HCs by activation of mAChRs represents a new finding. Ion channel candidates in pigeon vestibular HCs that might underlie the modulation of the macroscopic ionic currents and V(z) by different AChR activation are discussed.

Agonist and blocking actions of choline and tetramethylammonium on human muscle acetylcholine receptors

Choline has been used widely as an agonist for the investigation of gain-of-function mutants of the nicotinic acetylcholine receptor. It is useful because it behaves like a partial agonist. The efficacy of choline is difficult to measure because choline blocks the channel at concentrations about four times lower than those that activate it. We have fitted activation mechanisms to single-channel activity elicited from HEK-expressed human recombinant muscle nicotinic receptors by choline and by tetramethylammonium (TMA). Channel block by the agonist was incorporated into the mechanisms that were fitted, and block was found not to be selective for the open state. The results also suggest that channel block is very fast and that the channel can shut almost as fast as normal when the blocker was bound. Single-channel data are compatible with a mechanism in which choline is actually a full agonist, its maximum response being limited only by channel block. However, they are also compatible with a mechanism incorporating a pre-opening conformation change ('flip') in which choline is a genuine partial agonist. The latter explanation is favoured by concentration jump experiments, and by the fact that only this mechanism fits the TMA data. We propose that choline, like TMA, is a partial agonist because it is very ineffective (approximately 600-fold less than acetylcholine) at eliciting the initial, pre-opening conformation change. Once flipping has occurred, all agonists, even choline, open the channel with similar efficiency.

Activation and block of the adult muscle-type nicotinic receptor by physostigmine: single-channel studies

The plant-derived acetylcholinesterase inhibitor physostigmine has previously been shown to act on the nicotinic acetylcholine receptor (nAChR) causing either direct activation or potentiation of currents elicited by low concentrations of nicotinic agonists, or, at higher concentrations, channel block. We examined mouse adult-type muscle nAChR activation by physostigmine and found that channel activation by physostigmine exhibits many characteristics common with channel activity elicited by nicotinic agonists. Single-channel conductance was indistinguishable, and mutants known to slow channel closing in the presence of nicotinic agonists had a similar effect in the presence of physostigmine. However, physostigmine is a very inefficacious agonist. The presence of physostigmine did not alter the effective opening rate for a subsaturating dosage of carbachol, suggesting that physostigmine does not interact with the nicotinic agonist binding site. Mutations to a residue (alphaLys125) previously identified as part of the putative binding site for physostigmine reduced the duration of openings elicited by physostigmine, but the effects were generally small and, in most cases, nonsignificant. At higher concentrations, physostigmine blocked channel activity. Block manifested as a reduction in the mean open time and the emergence of a closed state, with a mean duration of 3 to 7 ms. The properties of block were consistent with two equivalent blocking sites per receptor with microscopic binding and unbinding rate constants for physostigmine of 20 microM(-1) s(-1) and 450 s(-1) (K(D) = 23 microM). These observations indicate that physostigmine is able to activate muscle nAChR by interacting with a site other than the nicotinic ligand binding site.

Estimating binding affinities of the nicotinic receptor for low-efficacy ligands using mixtures of agonists and two-dimensional concentration-response relationships

The phenomenon of ligand-induced ion channel gating hinges upon the ability of a receptor channel to bind ligand molecules with conformation-specific affinities. However, our understanding of this fundamental phenomenon is notably limited, not only because the changes in binding site structure and ligand conformation that occur upon gating are largely unknown but, also, because the strength of these ligand–receptor interactions are experimentally elusive. Both high- and low-efficacy ligands pose a number of analytical and experimental challenges that can render the estimation of their conformation-specific binding affinities impossible. In this paper, we present a novel assay that overcomes some of the hurdles presented by weak agonists of the muscle nicotinic receptor and allows the estimation of their closed-state affinities. The method, which we have termed the “activation-competition” assay, consists of a single-channel concentration–response assay performed in the presence of a binary mixture of ligands of widely different efficacies. By plotting the channel response (i.e., the open probability) as a function of the concentration of each agonist in the mixture, interpreting the observed response in the framework of a plausible kinetic scheme, and fitting the open probability surface with the corresponding function, the affinities of the closed receptor for the two agonists can be simultaneously extracted as free parameters. Here, we applied this methodology to estimate the closed-state affinity of the muscle nicotinic receptor for choline (a very weak agonist) using acetylcholine (ACh) as the partner in the mixture. We estimated the dissociation equilibrium constant of choline (K_D) from the wild type's closed state to be 4.1 ± 0.5 mM (and that of ACh to be 106 ± 6 μM). We also discuss the use of accurate estimates of affinities for low-efficacy agonists as a tool to discriminate between binding and gating effects of mutations, and in the context of the rational design of therapeutic drugs.

Block of muscle nicotinic receptors by choline suggests that the activation and desensitization gates act as distinct molecular entities

Ion channel block in muscle acetylcholine nicotinic receptors (AChRs) is an extensively reported phenomenon. Yet, the mechanisms underlying the interruption of ion flow or the interaction of the blocker with the channel's gates remain incompletely characterized. In this paper, we studied fast channel block by choline, a quaternary-ammonium cation that is also an endogenous weak agonist of this receptor, and a valuable tool in structure-function studies. Analysis of the single-channel current amplitude as a function of both choline concentration and voltage revealed that extracellular choline binds to the open-channel pore with millimolar apparent affinity (K(B) congruent with 12 mM in the presence of approximately 155 mM monovalent and 3.5 mM divalent, inorganic cations), and that it permeates the channel faster than acetylcholine. This, together with its relatively small size ( approximately 5.5 A along its longest axis), suggests that the pore-blocking choline binding site is the selectivity filter itself, and that current blockages simply reflect the longer-lived sojourns of choline at this site. Kinetic analysis of single-channel traces indicated that increasing occupancy of the pore-blocking site by choline (as judged from the reduction of the single-channel current amplitude) is accompanied by the lengthening of (apparent) open interval durations. Consideration of a number of possible mechanisms firmly suggests that this prolongation results from the local effect of choline interfering with the operation of the activation gate (closure of blocked receptors is slower than that of unblocked receptors by a factor of approximately 13), whereas closure of the desensitization gate remains unaffected. Thus, we suggest that these two gates act as distinct molecular entities. Also, the detailed understanding gained here on how choline distorts the observed open-time durations can be used to compensate for this artifact during activation assays. This correction is necessary if we are to understand how choline binds to and gates the AChR.

Molecular determinants of pyrantel selectivity in nicotinic receptors

Nicotinic receptors (acetylcholine receptors, AChRs) play key roles in synaptic transmission throughout the nervous system. AChRs mediate neuromuscular transmission in nematodes, and they are targets for antiparasitic drugs. The anthelmintic agents levamisole and pyrantel, which are potent agonists of nematode muscle AChRs, are partial agonists of mammalian muscle AChRs. To further explore the structural basis of the differential activation of AChR subtypes by anthelmintics, we studied the activation of alpha7 AChRs using the high-conductance form of the alpha7-5-hydroxytryptamine-3A receptor, which is a good model for pharmacological studies involving the extracellular region of alpha7. Macroscopic and single-channel current recordings show that levamisole is a weak agonist of alpha7. It is interesting that pyrantel is a more potent agonist of alpha7 than acetylcholine (ACh). To identify determinants of this differential activation, we replaced residues of the complementary face of the binding site by the homologous residues in the muscle epsilon subunit and evaluated changes in activation. The mutation Q57G does not affect the activation by either ACh or levamisole. However, it increases EC50 values and decreases the maximal response to pyrantel. Kinetic analysis shows that gating of the mutant channel activated by pyrantel is profoundly impaired. The decreased sensitivity of alpha7-Q57G to pyrantel agrees with its weak action at muscle AChRs, indicating that when glycine occupies position 57, as in the mammalian muscle AChR, pyrantel behaves as a partial agonist. Thus, position 57 located at the complementary face of the binding site plays a key role in the selective activation of AChRs by pyrantel.

Maximum likelihood estimation of ion channel kinetics from macroscopic currents

We describe a maximum likelihood method for direct estimation of rate constants from macroscopic ion channel data for kinetic models of arbitrary size and topology. The number of channels in the preparation, and the mean and standard deviation of the unitary current can be estimated, and a priori constraints can be imposed on rate constants. The method allows for arbitrary stimulation protocols, including stimuli with finite rise time, trains of ligand or voltage steps, and global fitting across different experimental conditions. The initial state occupancies can be optimized from the fit kinetics. Utilizing arbitrary stimulation protocols and using the mean and the variance of the current reduce or eliminate problems of model identifiability (Kienker, 1989). The algorithm is faster than a recent method that uses the full autocovariance matrix (Celentano and Hawkes, 2004), in part due to the analytical calculation of the likelihood gradients. We tested the method with simulated data and with real macroscopic currents from acetylcholine receptors, elicited in response to brief pulses of carbachol. Given appropriate stimulation protocols, our method chose a reasonable model size and topology.