A study of desensitization of acetylcholine receptors using nerve-released transmitter in the frog

Acute adaptation of central and peripheral motor unit features to exercise-induced fatigue differs with concentric and eccentric loading

NEW FINDINGS

What is the central question of this study? Conflicting evidence exists on motor unit (MU) firing rate in response to exercise-induced fatigue, possibly due to the contraction modality used: Do MU properties adapt similarly following concentric and eccentric loading? What is the main finding and its importance? MU firing rate increased following eccentric loading only despite a decline in absolute force. Force steadiness deteriorated following both loading methods. Central and peripheral MU features are altered in a contraction type-dependant manner, which is an important consideration for training interventions.

ABSTRACT

Force output of muscle is partly mediated by the adjustment of motor unit (MU) firing rate (FR). Disparities in MU features in response to fatigue may be influenced by contraction type, as concentric (CON) and eccentric (ECC) contractions demand variable amounts of neural input, which alters the response to fatigue. This study aimed to determine the effects of fatigue following CON and ECC loading on MU features of the vastus lateralis (VL). High-density surface (HD-sEMG) and intramuscular (iEMG) electromyography were used to record MU potentials (MUPs) from bilateral VLs of 12 young volunteers (six females) during sustained isometric contractions at 25% and 40% of the maximum voluntary contraction (MVC), before and after completing CON and ECC weighted stepping exercise. Multi-level mixed effects linear regression models were performed with significance assumed as P < 0.05. MVC decreased in both CON and ECC legs post-exercise (P < 0.0001), as did force steadiness at both 25% and 40% MVC (P < 0.004). MU FR increased in ECC at both contraction levels (P < 0.001) but did not change in CON. FR variability increased in both legs at 25% and 40% MVC following fatigue (P < 0.01). From iEMG measures at 25% MVC, MUP shape did not change (P > 0.1) but neuromuscular junction transmission instability increased in both legs (P < 0.04), and markers of fibre membrane excitability increased following CON only (P = 0.018). These data demonstrate that central and peripheral MU features are altered following exercise-induced fatigue and differ according to exercise modality. This is important when considering interventional strategies targeting MU function.

The Structure, Function, and Physiology of the Fetal and Adult Acetylcholine Receptor in Muscle

The neuromuscular junction (NMJ) is a highly developed synapse linking motor neuron activity with muscle contraction. A complex of molecular cascades together with the specialized NMJ architecture ensures that each action potential arriving at the motor nerve terminal is translated into an action potential in the muscle fiber. The muscle-type nicotinic acetylcholine receptor (AChR) is a key molecular component located at the postsynaptic muscle membrane responsible for the generation of the endplate potential (EPP), which usually exceeds the threshold potential necessary to activate voltage-gated sodium channels and triggers a muscle action potential. Two AChR isoforms are found in mammalian muscle. The fetal isoform is present in prenatal stages and is involved in the development of the neuromuscular system whereas the adult isoform prevails thereafter, except after denervation when the fetal form is re-expressed throughout the muscle. This review will summarize the structural and functional differences between the two isoforms and outline congenital and autoimmune myasthenic syndromes that involve the isoform specific AChR subunits.

Rapsyn facilitates recovery from desensitization in fetal and adult acetylcholine receptors expressed in a muscle cell line

Key points

The physiological significance of the developmental switch from fetal to adult acetylcholine receptors in muscle (AChRs) and the functional impact of AChR clustering by rapsyn are not well studied.

Using patch clamp experiments, we show that recovery from desensitization is faster in the adult AChR isoform.

Recovery from desensitization is determined by the AChR isoform‐specific cytoplasmic M3–M4 domain.

The co‐expression of rapsyn in muscle cells induced AChR clustering and facilitated recovery from desensitization in both fetal and adult AChRs. In fetal AChRs, facilitation of recovery kinetics by rapsyn was independent of AChR clustering.

These effects could be crucial adaptations to motor neuron firing rates, which, in rodents, have been shown to increase around the time of birth when AChRs cluster at the developing neuromuscular junctions.

Abstract

The neuromuscular junction (NMJ) is the site of a number of autoimmune and genetic disorders, many involving the muscle‐type nicotinic acetylcholine receptor (AChR), although there are aspects of normal NMJ development and function that need to be better understood. In particular, there are still questions regarding the implications of the developmental switch from fetal to adult AChRs, as well as how their functions might be modified by rapsyn that clusters the AChRs. Desensitization of human muscle AChRs was investigated using the patch clamp technique to measure whole‐cell currents in muscle‐type (TE671/CN21) and non‐muscle (HEK293) cell lines expressing either fetal or adult AChRs. Desensitization time constants were similar with both AChR isoforms but recovery time constants were shorter in cells expressing adult compared to fetal AChRs (P < 0.0001). Chimeric experiments showed that recovery from desensitization was determined by the M3–M4 cytoplasmic loops of the γ‐ and ε‐subunits. Expression of rapsyn in TE671/CN21 cells induced AChR aggregation and also, surprisingly, shortened recovery time constants in both fetal and adult AChRs. However, this was not dependent on clustering because rapsyn also facilitated recovery from desensitization in HEK293 cells expressing a δ‐R375H AChR mutant that did not form clusters in C2C12 myotubes. Thus, rapsyn interactions with AChRs lead not only to clustering, but also to a clustering independent faster recovery from desensitization. Both effects of rapsyn could be a necessary adjustment to the motor neuron firing rates that increase around the time of birth.

The physiological significance of the developmental switch from fetal to adult acetylcholine receptors in muscle (AChRs) and the functional impact of AChR clustering by rapsyn are not well studied.

Using patch clamp experiments, we show that recovery from desensitization is faster in the adult AChR isoform.

Recovery from desensitization is determined by the AChR isoform‐specific cytoplasmic M3–M4 domain.

The co‐expression of rapsyn in muscle cells induced AChR clustering and facilitated recovery from desensitization in both fetal and adult AChRs. In fetal AChRs, facilitation of recovery kinetics by rapsyn was independent of AChR clustering.

These effects could be crucial adaptations to motor neuron firing rates, which, in rodents, have been shown to increase around the time of birth when AChRs cluster at the developing neuromuscular junctions.

Desensitization mechanism in prokaryotic ligand-gated ion channel

Crystal structures of Gloeobacter violaceus ligand-gated ion channel (GLIC), a proton-gated prokaryotic homologue of pentameric ligand-gated ion channel (LGIC) from G. violaceus, have provided high-resolution models of the channel architecture and its role in selective ion conduction and drug binding. However, it is still unclear which functional states of the LGIC gating scheme these crystal structures represent. Much of this uncertainty arises from a lack of thorough understanding of the functional properties of these prokaryotic channels. To elucidate the molecular events that constitute gating, we have carried out an extensive characterization of GLIC function and dynamics in reconstituted proteoliposomes by patch clamp measurements and EPR spectroscopy. We find that GLIC channels show rapid activation upon jumps to acidic pH followed by a time-dependent loss of conductance because of desensitization. GLIC desensitization is strongly coupled to activation and is modulated by voltage, permeant ions, pore-blocking drugs, and membrane cholesterol. Many of these properties are parallel to functions observed in members of eukaryotic LGIC. Conformational changes in loop C, measured by site-directed spin labeling and EPR spectroscopy, reveal immobilization during desensitization analogous to changes in LGIC and acetylcholine binding protein. Together, our studies suggest conservation of mechanistic aspects of desensitization among LGICs of prokaryotic and eukaryotic origin.

Altered neurotransmitter release machinery in mice deficient for the deubiquitinating enzyme Usp14

Homozygous ataxic mice (ax(J)) express reduced levels of the deubiquitinating enzyme Usp14. They develop severe tremors by 2-3 wk of age, followed by hindlimb paralysis, and death by 6-8 wk. While changes in the ubiquitin proteasome system often result in the accumulation of ubiquitin protein aggregates and neuronal loss, these pathological markers are not observed in the ax(J) mice. Instead, defects in neurotransmission were observed in both the central and peripheral nervous systems of ax(J) mice. We have now identified several new alterations in peripheral neurotransmission in the ax(J) mice. Using the two-microelectrode voltage clamp technique on diaphragm muscles of ax(J) mice, we observed that under normal neurotransmitter release conditions ax(J) mice lacked paired-pulse facilitation and exhibited a frequency-dependent increase in rundown of the end plate current at high-frequency stimulation (HFS). Combined electrophysiology and styryl dye staining revealed a significant reduction in quantal content during the initial and plateau portions of the HFS train. In addition, uptake of styryl dyes (FM dye) during HFS demonstrated that the size of the readily releasable vesicle pool was significantly reduced. Destaining rates for styryl dyes suggested that ax(J) neuromuscular junctions are unable to mobilize a sufficient number of vesicles during times of intense activity. These results imply that ax(J) nerve terminals are unable to recruit a sufficient number of vesicles to keep pace with physiological rates of transmitter release. Therefore, ubiquitination of synaptic proteins appears to play an important role in the normal operation of the neurotransmitter release machinery and in regulating the size of pools of synaptic vesicles.

The number of components of enhancement contributing to short-term synaptic plasticity at the neuromuscular synapse during patterned nerve Stimulation progressively decreases as basal release probability is increased from low to normal levels by changing extracellular Ca2+

Presynaptic short-term plasticity (STP) dynamically modulates synaptic strength in a reversible manner on a timescale of milliseconds to minutes. For low basal vesicular release probability (prob0), four components of enhancement, F1 and F2 facilitation, augmentation (A), and potentiation (P), increase synaptic strength during repetitive nerve activity. For release rates that exceed the rate of replenishment of the readily releasable pool (RRP) of synaptic vesicles, depression of synaptic strength, observed as a rundown of postsynaptic potential amplitudes, can also develop. To understand the relationship between enhancement and depression at the frog (Rana pipiens) neuromuscular synapse, data obtained over a wide range of prob0 using patterned stimulation are analyzed with a hybrid model to reveal the components of STP. We find that F1, F2, A, P, and depletion of the RRP all contribute to STP during repetitive nerve activity at low prob0. As prob0 is increased by raising Ca(o)(2+) (extracellular Ca2+), specific components of enhancement no longer contribute, with first P, then A, and then F2 becoming undetectable, even though F1 continues to enhance release. For levels of prob0 that lead to appreciable depression, only F1 and depletion of the RRP contribute to STP during rundown, and for low stimulation rates, F2 can also contribute. These observations place prob0-dependent limitations on which components of enhancement contribute to STP and suggest some fundamental mechanistic differences among the components. The presented model can serve as a tool to readily characterize the components of STP over wide ranges of prob0.

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.

Neuromuscular fatigue in healthy muscle: underlying factors and adaptation mechanisms

OBJECTIVES

This review aims to define the concept of neuromuscular fatigue and to present the current knowledge of the central and peripheral factors at the origin of this phenomenon. This review also addresses the literature that focuses on the mechanisms responsible for the adaption to neuromuscular fatigue.

METHOD

One hundred and eighty-two articles indexed in PubMed (1954-2010) have been considered.

RESULTS

Neuromuscular fatigue has central and peripheral origins. Central fatigue, preponderant during long-duration, low-intensity exercises, may involve a drop in the central command (motor, cortex, motoneurons) elicited by the activity of cerebral neurotransmitters and muscular afferent fibers. Peripheral fatigue, associated with an impairment of the mechanisms from excitation to muscle contraction, may be induced by a perturbation of the calcium ion movements, an accumulation of phosphate, and/or a decrease of the adenosine triphosphate stores. To compensate for the consequent drop in force production, the organism develops several adaptation mechanisms notably implicating motor units.

CONCLUSION

Fatigue onset is associated with an alteration of the mechanisms involved in force production. Then, the interaction between central and peripheral mechanisms leads to a series of events that ultimately contribute to the observed decrease in force production.

Desensitization of neurotransmitter-gated ion channels during high-frequency stimulation: a comparative study of Cys-loop, AMPA and purinergic receptors

Changes in synaptic strength allow synapses to regulate the flow of information in the neural circuits in which they operate. In particular, changes lasting from milliseconds to minutes (‘short-term changes') underlie a variety of computational operations and, ultimately, behaviours. Most studies thus far have attributed the short-term type of plasticity to activity-dependent changes in the dynamics of neurotransmitter release (a presynaptic mechanism) while largely dismissing the role of the loss of responsiveness of postsynaptic receptor channels to neurotransmitter owing to entry into desensitization. To better define the response of the different neurotransmitter-gated ion channels (NGICs) to repetitive stimulation without interference from presynaptic variables, we studied eight representative members of all three known superfamilies of NGICs in fast-perfused outside-out patches of membrane. We found that the responsiveness of all tested channels (two nicotinic acetylcholine receptors, two glycine receptors, one GABA receptor, two AMPA-type glutamate receptors and one purinergic receptor) declines along trains of brief neurotransmitter pulses delivered at physiologically relevant frequencies to an extent that suggests that the role of desensitization in the synaptic control of action-potential transmission may be more general than previously thought. Furthermore, our results indicate that a sizable fraction (and, for some NGICs, most) of this desensitization occurs during the neurotransmitter-free interpulse intervals. Clearly, an incomplete clearance of neurotransmitter from the synaptic cleft between vesicle-fusion events need not be invoked to account for NGIC desensitization upon repetitive stimulation.

Structural basis of activation of cys-loop receptors: the extracellular-transmembrane interface as a coupling region

Cys-loop receptors mediate rapid transmission throughout the nervous system by converting a chemical signal into an electric one. They are pentameric proteins with an extracellular domain that carries the transmitter binding sites and a transmembrane region that forms the ion pore. Their essential function is to couple the binding of the agonist at the extracellular domain to the opening of the ion pore. How the structural changes elicited by agonist binding are propagated through a distance of 50 A to the gate is therefore central for the understanding of the receptor function. A step forward toward the identification of the structures involved in gating has been given by the recently elucidated high-resolution structures of Cys-loop receptors and related proteins. The extracellular-transmembrane interface has attracted attention because it is a structural transition zone where beta-sheets from the extracellular domain merge with alpha-helices from the transmembrane domain. Within this zone, several regions form a network that relays structural changes from the binding site toward the pore, and therefore, this interface controls the beginning and duration of a synaptic response. In this review, the most recent findings on residues and pairwise interactions underlying channel gating are discussed, the main focus being on the extracellular-transmembrane interface.

Fenitrothion, an organophosphate, affects running endurance but not aerobic capacity in fat-tailed dunnarts (Sminthopsis crassicaudata)

We measured aerobic metabolism during cold exposure and exercise performance (run duration and oxygen consumption while running at 1 m s(-1)) in the fat-tailed dunnart Sminthopsis crassicaudata, a dasyurid marsupial, before and after ingestion of 30 mg kg(-1) of fenitrothion, an organophosphate (OP) pesticide. Running endurance of OP-exposed animals was less than half that of control animals over the first 3 days after dosing and 55% of control animal endurance on day 5 post-dose. Despite these declines, peak metabolic rate at this running speed (9.3 times basal metabolic rate; BMR) was unaffected by OP exposure. Peak metabolic rate (PMR) and cumulative oxygen consumption during a 1-h exposure to conditions equivalent to -20 degrees C did not differ between OP-treated and control dunnarts, with PMR averaging 11 times BMR. We conclude that fenitrothion-induced exercise fatigue is not due to limitations in oxygen or substrate delivery to muscle or in their uptake per se, but more likely relates to decreased ability to sustain high-frequency neuromuscular function. The persistence of locomotor impairment following OP exposure in otherwise asymptomatic animals emphasizes the importance of using performance-based measures when characterising sublethal effects of pesticide exposure in an ecological context.

Making quantal analysis more convenient, fast, and accurate: user-friendly software QUANTAN

Quantal analysis of synaptic transmission is an important tool for understanding the mechanisms of synaptic plasticity and synaptic regulation. Although several custom-made and commercial algorithms have been created for the analysis of spontaneous synaptic activity, software for the analysis of action potential evoked release remains very limited. The present paper describes a user-friendly software package QUANTAN which has been created to analyze electrical recordings of postsynaptic responses. The program package is written using Borland C++ under Windows platform. QUANTAN employs and compares several algorithms to extract the average quantal content of synaptic responses, including direct quantal counts, the analysis of synaptic amplitudes, and the analysis of integrated current traces. The integration of several methods in one user-friendly program package makes quantal analysis of action potential evoked release more reliable and accurate. To evaluate the variability in quantal content, QUANTAN performs deconvolution of the distributions of amplitudes or areas of synaptic responses employing a ridge regression method. Other capabilities of QUANTAN include the analysis of the time-course and stationarity of quantal release. In summary, QUANTAN uses digital records of synaptic responses as an input and computes the distribution of quantal content and synaptic parameters. QUANTAN is freely available to other scholars over the internet.

Desensitization contributes to the synaptic response of gain-of-function mutants of the muscle nicotinic receptor

Although the muscle nicotinic receptor (AChR) desensitizes almost completely in the steady presence of high concentrations of acetylcholine (ACh), it is well established that AChRs do not accumulate in desensitized states under normal physiological conditions of neurotransmitter release and clearance. Quantitative considerations in the framework of plausible kinetic schemes, however, lead us to predict that mutations that speed up channel opening, slow down channel closure, and/or slow down the dissociation of neurotransmitter (i.e., gain-of-function mutations) increase the extent to which AChRs desensitize upon ACh removal. In this paper, we confirm this prediction by applying high-frequency trains of brief (∼1 ms) ACh pulses to outside-out membrane patches expressing either lab-engineered or naturally occurring (disease-causing) gain-of-function mutants. Entry into desensitization was evident in our experiments as a frequency-dependent depression in the peak value of succesive macroscopic current responses, in a manner that is remarkably consistent with the theoretical expectation. We conclude that the comparatively small depression of the macroscopic currents observed upon repetitive stimulation of the wild-type AChR is due, not to desensitization being exceedingly slow but, rather, to the particular balance between gating, entry into desensitization, and ACh dissociation rate constants. Disruption of this fine balance by, for example, mutations can lead to enhanced desensitization even if the kinetics of entry into, and recovery from, desensitization themselves are not affected. It follows that accounting for the (usually overlooked) desensitization phenomenon is essential for the correct interpretation of mutagenesis-driven structure–function relationships and for the understanding of pathological synaptic transmission at the vertebrate neuromuscular junction.

Macroscopic properties of spontaneous mutations in slow-channel syndrome: correlation by domain and disease severity

The slow-channel syndrome (SCS) is a neuromuscular disorder characterized by fatigability, progressive weakness, and degeneration of the neuromuscular junction. The SCS is caused by missense mutations in the four subunits of the skeletal muscle acetylcholine receptor (AChR), which leads to altered channel gating, prolonged neuromuscular postsynaptic currents, and impaired neuromuscular transmission. Although a diverse set of mutations in different functional domains of the AChR appear to be associated with symptoms of widely ranging severity, there is as yet no mutant channel property or combination that explains the variations in disease severity. By observing the recovery time of AChR from desensitization, the authors determined that this process is significantly enhanced in SCS channels. In addition, as expected, the authors found that SCS macroscopic decay currents in transfected HEK293 cells are slower than wild type currents. While slight differences in relative Ca(2+) permeability between some SCS mutations were identified, they did not correlate with apparent disease severity. These results suggest that of the different AChR kinetic features studied, only recovery from desensitization and slow postsynaptic currents correlate with the severity observed in the different mutations of this syndrome.

The control of neuromuscular transmission in health and disease

Multiple techniques are available to study failure of neuromuscular transmission. Electrophysiological techniques used in patients are well suited to detect failure of neuromuscular transmission; however, these methods offer little insight into the mechanisms underlying failure of transmission. More detailed techniques that are better suited for studying the underlying mechanisms can be performed in animal models of neuromuscular disease. However, it is often difficult to compare studies using different techniques to measure neuromuscular transmission. In this review, I discuss different techniques that are available to study failure of neuromuscular transmission. The strengths and weaknesses of various techniques are compared using several diseases as examples. The review concludes with a discussion of mechanisms that may contribute to failure of neuromuscular transmission during repetitive stimulation.

Kinetic evidence that desensitized nAChR may promote transitions of active nAChR to desensitized states during sustained exposure to agonists in skeletal muscle

During prolonged exposure of postjunctional nicotinic acetylcholine receptors (nAChR) of skeletal muscle to acetylcholine (ACh), agonist-activated nAChR (nAChRa) gradually fall into a refractory "desensitized" state (nAChRd), which no longer supports the high-conductance channel openings characteristic of the initially active nAChRa. In the present study, the possibility was examined that nAChRd, rather than simply constituting a passive "trap" for nAChRa, may actively promote further conversions of nAChRa to nAChRd in a formally autocatalytic manner. Single-ion whole-cell voltage-clamp currents (Na+ and Li+ in separate trials) were measured using two KCl-filled capillary electrodes (5-10 MOmega) implanted at the postjunctional locus of single frog skeletal muscle fibers (Rana pipiens) equilibrated in 30 mM K+ bath media to eliminate mechanical responses. Various nAChR agonists (carbamylcholine, acetylcholine, suberyldicholine) at different concentrations were delivered focally by positive pressure microjet. It was found that the decline of postmaximal agonist-induced currents under these different conditions (driven by the growth of the subpool of nAChRd) consistently followed an autocatalytic logistic rule modified for population growth of fixed units in a planar array: [Formula: see text] (where y represents the remaining agonist-induced current at time t, A=initial maximum current, and n is a constant). Some further experimental features that might result from a self-promoting growth of nAChRd were also tested, namely, (1) the effect of increased nAChRa and (2) the effect of increased nAChRd. Increase in agonist concentration of the superfusate, by increasing the planar density of active nAChRa at the outset, should enhance the probability of autocatalytic interactions with emerging nAChRd, hence, the rate of decline of agonist-induced current, and this was a consistent finding under all conditions tested. Raising the initial level of desensitized nAChRd by pretreatment of fibers with very low concentrations of agonist would be another way to increase autocatalytic interactions with active nAChRa, and this was also found to produce increased rates of decline of agonist-induced currents when tested in additional trials. It is concluded that several kinetic features of nAChR desensitization in skeletal muscle are consistent with an action of nAChRd to promote further transitions of nAChRa to desensitized forms. This could occur by a direct effect of nAChRd on contiguous nAChRa or perhaps through some intermediary membrane component or local intracellular pathway.

Augmentation increases vesicular release probability in the presence of masking depression at the frog neuromuscular junction

Synaptic augmentation is a short-term component of synaptic plasticity that increases transmitter release during repetitive stimulation and decays thereafter with a time constant of approximately 7 sec. Augmentation has typically been observed under conditions where there is little or no depression because of depletion of synaptic vesicles from the readily releasable pool (RRP) of transmitter. We now study augmentation under conditions of pronounced depression at the frog neuromuscular junction to gain additional insight into mechanism. If augmentation reflects an increase in the size of the RRP of transmitter, then augmentation should not be present with depression. Our findings using four different experimental approaches suggested that augmentation was still present in the presence of pronounced depression: mathematical extraction of augmentation from the changes in transmitter release after repetitive stimulation, identification of augmentation with Ba2+, correction of the data for the measured depletion of the RRP, and identification of an augmentation component of residual Ca2+. We conclude that the augmentation machinery still acts to increase transmitter release when depression reduces the RRP sufficiently to mask obvious augmentation. The masked augmentation was found to increase transmitter release by increasing the probability of releasing individual vesicles from the depressed RRP, countering the effects of depression. Because augmentation and depression have similar time courses, either process can mask the other, depending on their relative magnitudes. Consequently, the apparent absence of one of the processes does not exclude that it is still contributing to short-term synaptic plasticity.

Estimation of quantal parameters at the calyx of Held synapse

The calyx of Held has recently emerged as a convenient model system to study CNS synapses. In order to understand the mechanisms of synaptic transmission and short-term synaptic plasticity, quantal parameters and their changes should be estimated precisely. For this purpose, various methods have been applied to the calyx of Held synapse. The results confirm many aspects of the early findings on transmission at the neuromuscular junction. On the other hand, the simplest quantal hypothesis does not work at the calyx of Held, because of additional factors such as heterogeneous release probability of synaptic vesicles, intra- and intersite quantal variability, an overlap of facilitation and depression of transmitter release, changes in quantal sizes due to desensitization and saturation of postsynaptic receptors, and delayed clearance of transmitter from the synaptic cleft. These factors should always be taken into account for fully understanding the mechanisms of synaptic transmission and plasticity.

Analysis of a slow desensitized state of recombinant adult-type nicotinic acetylcholine receptor channels

A characteristic feature of the kinetics of nicotinic acetylcholine receptor (nAChR) channels is fast and nearly complete desensitization with a time course between 10 and 100 ms and recovery from desensitization in the range of some hundred ms. In the present study we used a piezo-driven system for ultra-fast solution exchange, analysed the recovery from the fast desensitized state of mouse recombinant adult-type nAChR channels and found no difference to that of embryonic-type channels. By double pulse experiments with application of pulses with a saturating concentration of 1 mm acetylcholine (ACh) with increasing duration of the first pulse and a constant interval between pulses we detected a second slow desensitized state which was entered with a time constant of 2835 ms. Recovery from the slow desensitized state proceeded with a single exponential with a time constant of 16134 ms. The experimental data were interpreted by the addition of a transition from the desensitized state with two bound ACh molecules to a slow desensitized state to the well known circular kinetic scheme of activation and desensitization of nAChR channels. This slow desensitized state might play a role in muscle fatigue or in pathological states like myasthenic syndromes.

Desensitization of diliganded mouse muscle nicotinic acetylcholine receptor channels

Nicotinic ACh receptor channels (AChRs) exposed to high concentrations of ACh adopt 'desensitized' conformations that have a high affinity for the transmitter and no measurable ion conductance. Single-channel currents elicited by 0.1 or 1 mM ACh were recorded from human embryonic kidney (HEK) cells that had been transiently transfected with mouse alpha, beta, delta, and epsilon subunits. On the time scale of approximately 0.1 ms to approximately 1 h, apparent open intervals are described by a single exponential component, and shut intervals associated with desensitization are described by the sum of four or five exponential components. The kinetic behaviour appeared to be stationary and homogeneous. Desensitization rate constants were estimated by kinetic modelling of currents from cell-attached and outside-out patches (where the number of channels in the patch was measured). A single AChR recovered from the longest-lived desensitized state only after approximately 5 min. The occupancy of an AChR for each of the desensitized states was calculated as a function of time after the continuous application of a pulse of saturating ACh. The longest-lived desensitized state accounted for 90 % of the total only after several seconds. The fractional recovery from desensitization (during a 200 ms wash period) decreased as the duration of the desensitizing pulse increased, suggesting that recovery is slower from the longer-lived desensitized states. The free energy landscape for the AChR desensitization reaction in cell-attached patches exhibited an initial destabilization, followed by a plateau region of gradually increasing stability, followed by a deep well.

Short-term synaptic plasticity

Synaptic transmission is a dynamic process. Postsynaptic responses wax and wane as presynaptic activity evolves. This prominent characteristic of chemical synaptic transmission is a crucial determinant of the response properties of synapses and, in turn, of the stimulus properties selected by neural networks and of the patterns of activity generated by those networks. This review focuses on synaptic changes that result from prior activity in the synapse under study, and is restricted to short-term effects that last for at most a few minutes. Forms of synaptic enhancement, such as facilitation, augmentation, and post-tetanic potentiation, are usually attributed to effects of a residual elevation in presynaptic [Ca(2+)]i, acting on one or more molecular targets that appear to be distinct from the secretory trigger responsible for fast exocytosis and phasic release of transmitter to single action potentials. We discuss the evidence for this hypothesis, and the origins of the different kinetic phases of synaptic enhancement, as well as the interpretation of statistical changes in transmitter release and roles played by other factors such as alterations in presynaptic Ca(2+) influx or postsynaptic levels of [Ca(2+)]i. Synaptic depression dominates enhancement at many synapses. Depression is usually attributed to depletion of some pool of readily releasable vesicles, and various forms of the depletion model are discussed. Depression can also arise from feedback activation of presynaptic receptors and from postsynaptic processes such as receptor desensitization. In addition, glial-neuronal interactions can contribute to short-term synaptic plasticity. Finally, we summarize the recent literature on putative molecular players in synaptic plasticity and the effects of genetic manipulations and other modulatory influences.

Long-lasting reconfiguration of two interacting networks by a cooperation of presynaptic and postsynaptic plasticity

The functional reconfiguration of central neuronal networks, a phenomenon by which neurons change their participation in network operation, is important for organizing adaptive behaviors. Such reconfiguration can be expressed in a long-lasting manner (hours, days) after a training paradigm. The present study shows that such a long-lasting network reconfiguration requires a cooperation of both presynaptic and postsynaptic modifications in a neuronal interaction between two functionally distinct networks. In isolated preparations of the lobster stomatogastric nervous system, the single ventral dilator (VD) neuron can switch its functional participation from one discrete network (the pyloric network) to another (the cardiac sac network). This switching capability can be long-lasting and can be induced by a sensitizing procedure. A persistent change that was associated with this neuronal switching was found in each of the two networks. First, the intrinsic membrane properties of the VD neuron that allow it to participate spontaneously in the pyloric network are altered. Second, bursting activity is strengthened in the inferior ventricular neurons that both drive cardiac sac network activity and monosynaptically excite the VD neuron in phase with this network activity. Importantly, these changes in intrinsic properties of both presynaptic and postsynaptic neurons are required to allow the VD neuron switching, because expression of either the presynaptic or the postsynaptic change alone did not permit VD neuron switching to occur. These results suggest that a cooperative modification of a discrete network interaction is able to persistently switch the output pattern of a motor neuron as a result of a sensitizing paradigm.

Synaptically Released Neurotransmitter Fails to Desensitize Postsynaptic GABAA Receptors in Cerebellar Cultures

GABA concentration jump experiments performed on membrane patches predict that postsynaptic GABAAreceptors will become desensitized following the release of the contents of a single GABA-containing synaptic vesicle. To examine this we used a single synaptic bouton stimulation technique to directly examine whether postsynaptic GABAA receptors in cultured cerebellar granule cells exhibit transmitter-induced desensitization. In a large number of recordings, no evidence was found for desensitization of postsynaptic GABAAreceptors by vesicularly released transmitter. This was the case even when as many as 40 vesicles were released from a single bouton within 1.5 s. In addition, postsynaptic depolarization and application of the benzodiazepine flunitrazepam, manipulations previously shown to enhance desensitization of GABAA receptors, failed to unmask transmitter-induced desensitization. In contrast, a single 2- to 3-s application of a high concentration of exogenous GABA was able to depress synaptic responsiveness for up to 70 s. Furthermore, pharmacological depletion of GABA eliminated inhibitory synaptic communication, suggesting that GABA is the transmitter and the desensitization-resistant inhibitory postsynaptic currents are not mediated by a “nondesensitizing” ligand such as β-alanine. Overall our data indicate that a specific desensitization-resistant population of GABAA receptors are present at postsynaptic sites on cultured cerebellar granule cells.

Long-lasting reconfiguration of two interacting networks by a cooperation of presynaptic and postsynaptic plasticity

The functional reconfiguration of central neuronal networks, a phenomenon by which neurons change their participation in network operation, is important for organizing adaptive behaviors. Such reconfiguration can be expressed in a long-lasting manner (hours, days) after a training paradigm. The present study shows that such a long-lasting network reconfiguration requires a cooperation of both presynaptic and postsynaptic modifications in a neuronal interaction between two functionally distinct networks. In isolated preparations of the lobster stomatogastric nervous system, the single ventral dilator (VD) neuron can switch its functional participation from one discrete network (the pyloric network) to another (the cardiac sac network). This switching capability can be long-lasting and can be induced by a sensitizing procedure. A persistent change that was associated with this neuronal switching was found in each of the two networks. First, the intrinsic membrane properties of the VD neuron that allow it to participate spontaneously in the pyloric network are altered. Second, bursting activity is strengthened in the inferior ventricular neurons that both drive cardiac sac network activity and monosynaptically excite the VD neuron in phase with this network activity. Importantly, these changes in intrinsic properties of both presynaptic and postsynaptic neurons are required to allow the VD neuron switching, because expression of either the presynaptic or the postsynaptic change alone did not permit VD neuron switching to occur. These results suggest that a cooperative modification of a discrete network interaction is able to persistently switch the output pattern of a motor neuron as a result of a sensitizing paradigm.

Quantitative relationship between transmitter release and calcium current at the calyx of held synapse

A newly developed deconvolution method (Neher and Sakaba, 2001) allowed us to resolve the time course of neurotransmitter release at the calyx of Held synapse and to quantify some basic aspects of transmitter release. First, we identified a readily releasable pool (RRP) of synaptic vesicles. We found that the size of the RRP, when tested with trains of strong stimuli, was constant regardless of the exact stimulus patterns, if stimuli were confined to a time interval of approximately 60 msec. For longer-lasting stimulus patterns, recruitment of new vesicles to the RRP made a substantial contribution to the total release. Second, the cooperativity of transmitter release as a function of Ca(2+) current was estimated to be 3-4, which confirmed previous results (Borst and Sakmann, 1999; Wu et al., 1999). Third, an initial small Ca(2+) influx increased the efficiency of Ca(2+) currents in subsequent transmitter release. This type of facilitation was blocked by a high concentration of EGTA (0.5 mm). Fourth, the release rates of synaptic vesicles at this synapse turned out to be heterogeneous: once a highly Ca(2+)-sensitive population of vesicles was consumed, the remaining vesicles released at lower rates. These components of release were more clearly separated in the presence of 0.5 mm EGTA, which prevented the buildup of residual Ca(2+). Conversely, raising the extracellular Ca(2+) concentration facilitated the slower population such that its release characteristics became more similar to those of the faster population under standard conditions. Heterogeneous release probabilities are expected to support the maintenance of synaptic transmission during high-frequency stimulation.

Combining deconvolution and noise analysis for the estimation of transmitter release rates at the calyx of held

The deconvolution method has been used in the past to estimate release rates of synaptic vesicles, but it cannot be applied to synapses where nonlinear interactions of quanta occur. We have extended this method to take into account a nonlinear current component resulting from the delayed clearance of glutamate from the synaptic cleft. We applied it to the calyx of Held and verified the important assumption of constant miniature EPSC (mEPSC) size by combining deconvolution with a variant of nonstationary fluctuation analysis. We found that amplitudes of mEPSCs decreased strongly after extended synaptic activity. Cyclothiazide (CTZ), an inhibitor of glutamate receptor desensitization, eliminated this reduction, suggesting that postsynaptic receptor desensitization occurs during strong synaptic activity at the calyx of Held. Constant mEPSC sizes could be obtained in the presence of CTZ and kynurenic acid (Kyn), a low-affinity blocker of AMPA-receptor channels. CTZ and Kyn prevented postsynaptic receptor desensitization and saturation and also minimized voltage-clamp errors. Therefore, we conclude that in the presence of these drugs, release rates at the calyx of Held can be reliably estimated over a wide range of conditions. Moreover, the method presented should provide a convenient way to study the kinetics of transmitter release at other synapses.

The role of desensitisation in decay time of miniature endplate currents in frogs Rana ridibunda and Rana temporaria

A new comparative characteristic of endplate microphysiology has been introduced. It is the feasibility of receptors to become desensitised as demonstrated on two frog species, Rana temporaria and Rana ridibunda: the decay times (tau(MEPC)) of single quantum miniature endplate currents (MEPCs) in the sartorius muscles of both species were about 1 ms and were not affected by the desensitisation-promoting agent proadifen when AChE was active. However, when the desensitisation was induced by anticholinesterase neostigmine and promoted by proadifen, the prolongation of tau(MEPC) from 1 ms was almost twice as great in Rana temporaria (tau(MEPC) = 4.4 ms) than in Rana ridibunda (tau(MEPC) = 3.1). This indicates that desensitisation reduces the number of available receptors and lowers the number of available ACh molecules for repetitive binding by trapping them by desensitised, high-affinity receptors significantly more in Rana ridibunda than in Rana temporaria. The application of proadifen, a promoter of desensitisation, decreased the prolongation of MEPCs in both species, but this shortening was more rapid in Rana ridibunda than in Rana temporaria. It is concluded that the desensitisation-induced reduction in the density, and the number of postsynaptic receptors is significantly higher at Rana ridibunda than in Rana temporaria endplates.

Acetylcholine receptor channel structure in the resting, open, and desensitized states probed with the substituted-cysteine-accessibility method

The nicotinic acetylcholine (ACh) receptors cycle among classes of nonconducting resting states, conducting open states, and nonconducting desensitized states. We previously probed the structure of the mouse-muscle ACh receptor channel in the resting state obtained in the absence of agonist and in the open states obtained after brief exposure to ACh. We now have probed the structure in the stable desensitized state obtained after many minutes of exposure to ACh. Muscle-type receptor has the subunit composition alpha(2)betagammadelta. Each subunit has four membrane-spanning segments, M1-M4. The channel lumen in the membrane domain is lined largely by M2 and to a lesser extent by M1 from each of the subunits. We determined the rates of reaction of a small, sulfhydryl-specific, charged reagent, 2-aminoethyl methanethiosulfonate with cysteines substituted for residues in alphaM2 and the alphaM1-M2 loop in the desensitized state and compared these rates to rates previously obtained in the resting and open states. The reaction rates of the substituted cysteines are different in the three functional states of the receptor, indicating significant structural differences. By comparing the rates of reaction of extracellularly and intracellularly added 2-aminoethyl methanethiosulfonate, we previously located the closed gate in the resting state between alphaG240 and alphaT244, in the predicted M1-M2 loop at the intracellular end of M2. Now, we have located the closed gate in the stable desensitized state between alphaG240 and alphaL251. The gate in the desensitized state includes the resting state gate and an extension further into M2.

Acetylcholine receptor channel structure in the resting, open, and desensitized states probed with the substituted-cysteine-accessibility method

The nicotinic acetylcholine (ACh) receptors cycle among classes of nonconducting resting states, conducting open states, and nonconducting desensitized states. We previously probed the structure of the mouse-muscle ACh receptor channel in the resting state obtained in the absence of agonist and in the open states obtained after brief exposure to ACh. We now have probed the structure in the stable desensitized state obtained after many minutes of exposure to ACh. Muscle-type receptor has the subunit composition alpha(2)betagammadelta. Each subunit has four membrane-spanning segments, M1-M4. The channel lumen in the membrane domain is lined largely by M2 and to a lesser extent by M1 from each of the subunits. We determined the rates of reaction of a small, sulfhydryl-specific, charged reagent, 2-aminoethyl methanethiosulfonate with cysteines substituted for residues in alphaM2 and the alphaM1-M2 loop in the desensitized state and compared these rates to rates previously obtained in the resting and open states. The reaction rates of the substituted cysteines are different in the three functional states of the receptor, indicating significant structural differences. By comparing the rates of reaction of extracellularly and intracellularly added 2-aminoethyl methanethiosulfonate, we previously located the closed gate in the resting state between alphaG240 and alphaT244, in the predicted M1-M2 loop at the intracellular end of M2. Now, we have located the closed gate in the stable desensitized state between alphaG240 and alphaL251. The gate in the desensitized state includes the resting state gate and an extension further into M2.

Quantitative relationship between transmitter release and calcium current at the calyx of held synapse

A newly developed deconvolution method (Neher and Sakaba, 2001) allowed us to resolve the time course of neurotransmitter release at the calyx of Held synapse and to quantify some basic aspects of transmitter release. First, we identified a readily releasable pool (RRP) of synaptic vesicles. We found that the size of the RRP, when tested with trains of strong stimuli, was constant regardless of the exact stimulus patterns, if stimuli were confined to a time interval of approximately 60 msec. For longer-lasting stimulus patterns, recruitment of new vesicles to the RRP made a substantial contribution to the total release. Second, the cooperativity of transmitter release as a function of Ca(2+) current was estimated to be 3-4, which confirmed previous results (Borst and Sakmann, 1999; Wu et al., 1999). Third, an initial small Ca(2+) influx increased the efficiency of Ca(2+) currents in subsequent transmitter release. This type of facilitation was blocked by a high concentration of EGTA (0.5 mm). Fourth, the release rates of synaptic vesicles at this synapse turned out to be heterogeneous: once a highly Ca(2+)-sensitive population of vesicles was consumed, the remaining vesicles released at lower rates. These components of release were more clearly separated in the presence of 0.5 mm EGTA, which prevented the buildup of residual Ca(2+). Conversely, raising the extracellular Ca(2+) concentration facilitated the slower population such that its release characteristics became more similar to those of the faster population under standard conditions. Heterogeneous release probabilities are expected to support the maintenance of synaptic transmission during high-frequency stimulation.

Combining deconvolution and noise analysis for the estimation of transmitter release rates at the calyx of held

The deconvolution method has been used in the past to estimate release rates of synaptic vesicles, but it cannot be applied to synapses where nonlinear interactions of quanta occur. We have extended this method to take into account a nonlinear current component resulting from the delayed clearance of glutamate from the synaptic cleft. We applied it to the calyx of Held and verified the important assumption of constant miniature EPSC (mEPSC) size by combining deconvolution with a variant of nonstationary fluctuation analysis. We found that amplitudes of mEPSCs decreased strongly after extended synaptic activity. Cyclothiazide (CTZ), an inhibitor of glutamate receptor desensitization, eliminated this reduction, suggesting that postsynaptic receptor desensitization occurs during strong synaptic activity at the calyx of Held. Constant mEPSC sizes could be obtained in the presence of CTZ and kynurenic acid (Kyn), a low-affinity blocker of AMPA-receptor channels. CTZ and Kyn prevented postsynaptic receptor desensitization and saturation and also minimized voltage-clamp errors. Therefore, we conclude that in the presence of these drugs, release rates at the calyx of Held can be reliably estimated over a wide range of conditions. Moreover, the method presented should provide a convenient way to study the kinetics of transmitter release at other synapses.

Disruption of actin impedes transmitter release in snake motor terminals

To investigate the role of actin in vertebrate nerve terminals, nerve-muscle preparations from garter snake (Thamnophis sirtalis) were treated with the actin-depolymerizing agent latrunculin A. Immunostaining revealed that actin filaments within presynaptic motor terminal boutons were disrupted by the drug. In preparations loaded with the optical probe FM1-43, destaining was reduced by latrunculin treatment, suggesting that transmitter release was partially blocked. Latrunculin treatment did not influence the amplitude or time course of spontaneous miniature endplate potentials (MEPPs). Similarly, endplate potentials (EPPs) evoked at low frequency were comparable in control and latrunculin-treated curarized preparations. Brief tetanic stimulation of the muscle nerve (25 Hz, 90 s) depressed EPP amplitudes in both control and latrunculin-treated preparations. After tetanus, EPPs elicited at 0. 2 Hz in control preparations recovered rapidly (0-5 min) and completely (usually potentiating to above pre-tetanus levels; 130 +/- 11 %, mean +/- s.e.m.). In contrast, EPPs evoked in latrunculin-treated preparations recovered slowly (8-10 min) and incompletely (84 +/- 8 %). The influence of latrunculin on post-tetanic EPPs depended on its concentration in the bath (KD = 3. 1 microM) and on time of incubation. These observations argue that actin filaments facilitate transmitter release rather than impede it. Specifically, actin may facilitate mobilization of vesicles towards the releasable pools.

Effect of phasic activation on endplate potential in rat diaphragm

Neuromuscular junction endplate potentials (EPPs) decrease quickly and to a large extent during continuous stimulation. The present study examined the hypothesis that EPP rundown recovers rapidly, thereby substantially preserving neurotransmission during intermittent compared with continuous stimulation. Studies were performed in vitro on rat diaphragm, using mu-conotoxin to allow recording of normal-sized EPPs from intact fibers. During continuous 5- to 100-Hz stimulation, EPP amplitude declined with a biphasic time course. The initial fast rate of decline was modulated substantially by stimulation frequency, whereas the subsequent slow rate of decline was relatively frequency independent. During intermittent 5- to 100-Hz stimulation (duty cycle 0.33), EPP amplitude declined rapidly during each train, but recovered substantially by the onset of the following train. The intra-train declines were substantially greater than the inter-train declines in EPP amplitude. Intra-train reductions in EPP amplitude were stimulation frequency dependent, based on both the total decline and rate constant of EPP decline. In contrast, the degree of recovery from train to train was independent of stimulation frequency, indicating low frequency dependence of inter-train rundown. The substantial recovery of EPP amplitude in between trains resulted in greater cumulative EPP size during intermittent compared with continuous stimulation. During continuous stimulation, EPP drop-out was only seen during 100-Hz stimulation; this was completed mitigated during intermittent stimulation. Miniature EPP size was unaffected by either continuous or intermittent stimulation. The pattern of rapid intra-train rundown and slow inter-train rundown of EPP size during intermittent stimulation is therefore due to rapid changes in the magnitude of neurotransmitter release rather than to axonal block or postsynaptic receptor desensitization. These findings indicate considerable rundown of EPP amplitudes within a stimulus train, with near complete recovery by the onset of the next train. This substantially attenuates the decrement in EPP amplitude during intermittent compared with continuous stimulation, thereby preserving the integrity of neurotransmission during phasic activation.

Fatigability of rat hindlimb muscles after acute irreversible acetylcholinesterase inhibition

The purpose of this study was to investigate the functional impact of acute irreversible inhibition of acetylcholinesterase (AChE) on the fatigability of medial gastrocnemius and plantaris muscles of Sprague-Dawley rats. After treatment with methanesulfonyl fluoride (a lipid-soluble anticholinesterase), which reduced their AChE activity by >90%, these muscles were subjected to an in situ indirect stimulation protocol, including a series of isolated twitch and tetanic contractions preceding a 3-min fatigue regimen (100-ms trains at 75 Hz applied every 1.5 s). During the first minute of the fatigue regimen, the effects of AChE inhibition were already near maximal, including marked reductions in peak tension and the force-time integral (area), as well as a decrement of compound muscle action potential amplitudes within a stimulus train. Neuromuscular transmission failure was the major contributor of the force decreases in the AChE-inhibited muscles. However, despite this neuromuscular transmission failure, muscles of which all AChE molecular forms were nearly completely inhibited were still able to function, although abnormally, during 3 min of intermittent high-frequency nerve stimulation.

Methyllycaconitine-sensitive neuronal nicotinic receptor-operated slow Ca2+ signal by local application or perfusion of ACh at the mouse neuromuscular junction

Local application of acetylcholine (ACh; 0.3 mM, 20 microl) elicited bi-phasic elevation of intracellular Ca2+ concentrations (contractile fast and non-contractile slow Ca2- signal measured as aequorin luminescence) in diaphragm muscle preparation. A neuronal nicotinic antagonist methyllycaconitine (MLA; 0.01-1 microM), which did not affect the fast Ca2+ transients and twitch tension, concentration-dependently depressed only the slow Ca2+ component. Ca2+ channel blockers, Cd2+ (200 microM), nitrendipine (1 microM), verapamil (1 microM) and diltiazem (1 microM), or a Na+ channel blocker tetrodotoxin (TTX; 0.1 microM) failed to prevent the generation of slow Ca2+ response. Perfusion of ACh (1 microM) to isolated single skeletal (flexor digitorum brevis) muscle cells pretreated with TTX (0.1 microM) also elicited a slow Ca2+ signal measured as confocal imaging with a fluorescent dye, fluo-3, at the endplate region. MLA (1 microM) antagonized against the ACh perfusion-elicited slow Ca2+ signal. Perfusion of choline (1 mM), a neuronal nicotinic agonist, also elicited the MLA-sensitive slow Ca2+ signal. These results strongly suggest that the ACh-induced slow Ca2+ signal reflects Ca2+ entry through a postsynaptic MLA-sensitive neuronal nicotinic ACh receptor subtype at the neuromuscular junction.

Heterologous desensitization of muscarinic receptors by P2Z purinoceptors in rat parotid acinar cells

We studied the heterologous desensitization of muscarinic receptors by ATP in fura-2-loaded rat parotid acinar cells. Exposure to ATP or 3'-o-(4-benzoyl) benzoyl-ATP shortened the duration and decreased the magnitude of acetylcholine-induced Ca2+ release from intracellular Ca2+ stores in a dose-dependent manner. The shortening was observed only in an early stage of desensitization (within 20 s), whereas the decrease in the magnitude of the response was dependent upon the time the cells were exposed to the nucleotides. Atropine induced a profound shortening during the progressive decrease in the magnitude of acetylcholine-induced Ca2+ release. 3'-o-(4-Benzoyl) benzoyl-ATP did not induce an increase in the cytosolic Ca2+ concentration when the cells were incubated in the Ca2+- and Na+-free medium, but it did induce a strong desensitization of muscarinic receptors. The specific protein kinase C inhibitor bisindoylmaleimide resensitized the 3'-o-(4-benzoyl) benzoyl-ATP-treated muscarinic receptors. Phorbol 12-myristate 13-acetate potentiated the desensitization of muscarinic receptors. Ceramides that prevent the activation of phospholipase D resensitized the 3'-o-(4-benzoyl) benzoyl-ATP-treated muscarinic receptors. These results suggest that ATP, acting through P2Z purinoceptor-mediated phospholipase D, may produce a Ca2+-independent protein kinase C. Heterologous desensitization of muscarinic receptors by protein kinase C may shorten the duration and decrease the magnitude of acetylcholine-induced Ca2+ release.

Long-term desensitization of nicotinic acetylcholine receptors is regulated via protein kinase A-mediated phosphorylation

During prolonged application of transmitter, ligand-gated ion channels enter a nonconducting desensitized state. Studies on Torpedo electroplax nicotinic acetylcholine (ACh) receptors have shown that entry into the desensitized state is accelerated by protein kinase A-dependent (PKA) receptor phosphorylation. To examine the effects of phosphorylation on desensitization of muscle-type ACh receptors, we expressed the frog embryonic receptor type in Xenopus oocytes. Treatment of embryonic muscle ACh receptors with 8-Br cAMP had no measurable effect on the rate of entry into a desensitized state, but it greatly accelerated the recovery from desensitization. Three complementary approaches to reduce the levels of receptor phosphorylation provided additional evidence for a role of PKA-dependent phosphorylation in rescuing receptors from long-term desensitization. Inactivation of the endogenous PKA activity by coexpression of an inhibitor protein, treatment of receptors with phosphatase, and removal of phosphorylation sites by site-specific subunit mutation all resulted in slowed recovery. Our findings point to the existence of two distinct desensitized states: one requiring several seconds for full recovery and a second state from which recovery requires minutes. Receptors lacking PKA phosphorylation sites exhibit a pronounced increase in the slowly recovering component of desensitization, suggesting that receptor phosphorylation speeds overall recovery by reducing the entry into a deep desensitized state. This newly described effect of phosphorylation on ACh receptor function may serve as an important modulator of postsynaptic receptor sensitivity.

Endurance training increases acetylcholine receptor quantity at neuromuscular junctions of adult rat skeletal muscle

The aim of the study was to test the hypothesis that a 16 week endurance training program would alter the abundance of endplate-associated nicotinic acetylcholine receptors (nAChR) in various rat skeletal muscles. We found a 20% increase in endplate-specific [125I]alpha-bungarotoxin binding in several muscles of trained rats, accompanied by equal susceptibility of toxin binding to the inhibitory effect of D-tubocurarine in sedentary and trained muscles. We conclude that the neuromuscular junction adaptations that occur with increased chronic activation include an increase in nAChR number. Results of experiments designed to determine nAChR turnover also suggest that this effect is mediated by an alteration in the receptor's metabolic state. The potential implications and mechanisms of this adaptation are discussed.

Long-term desensitization of nicotinic acetylcholine receptors is regulated via protein kinase A-mediated phosphorylation

During prolonged application of transmitter, ligand-gated ion channels enter a nonconducting desensitized state. Studies on Torpedo electroplax nicotinic acetylcholine (ACh) receptors have shown that entry into the desensitized state is accelerated by protein kinase A-dependent (PKA) receptor phosphorylation. To examine the effects of phosphorylation on desensitization of muscle-type ACh receptors, we expressed the frog embryonic receptor type in Xenopus oocytes. Treatment of embryonic muscle ACh receptors with 8-Br cAMP had no measurable effect on the rate of entry into a desensitized state, but it greatly accelerated the recovery from desensitization. Three complementary approaches to reduce the levels of receptor phosphorylation provided additional evidence for a role of PKA-dependent phosphorylation in rescuing receptors from long-term desensitization. Inactivation of the endogenous PKA activity by coexpression of an inhibitor protein, treatment of receptors with phosphatase, and removal of phosphorylation sites by site-specific subunit mutation all resulted in slowed recovery. Our findings point to the existence of two distinct desensitized states: one requiring several seconds for full recovery and a second state from which recovery requires minutes. Receptors lacking PKA phosphorylation sites exhibit a pronounced increase in the slowly recovering component of desensitization, suggesting that receptor phosphorylation speeds overall recovery by reducing the entry into a deep desensitized state. This newly described effect of phosphorylation on ACh receptor function may serve as an important modulator of postsynaptic receptor sensitivity.

Delimiting the binding site for quaternary ammonium lidocaine derivatives in the acetylcholine receptor channel

The triethylammonium QX-314 and the trimethylammonium QX-222 are lidocaine derivatives that act as open-channel blockers of the acetylcholine (ACh) receptor. When bound, these blockers should occlude some of the residues lining the channel. Eight residues in the second membrane-spanning segment (M2) of the mouse-muscle alpha subunit were mutated one at a time to cysteine and expressed together with wild-type beta, gamma, and delta subunits in Xenopus oocytes. The rate constant for the reaction of each substituted cysteine with 2-aminoethyl methanethiosulfonate (MTSEA) was determined from the time course of the irreversible effect of MTSEA on the ACh-induced current. The reactions were carried out in the presence and absence of ACh and in the presence and absence of QX-314 and QX-222. These blockers had no effect on the reactions in the absence of ACh. In the presence of ACh, both blockers retarded the reaction of extracellularly applied MTSEA with cysteine substituted for residues from alphaVal255, one third of the distance in from the extracellular end of M2, to alphaGlu241, flanking the intracellular end of M2, but not with cysteine substituted for alphaLeu258 or alphaGlu262, at the extracellular end of M2. The reactions of MTSEA with cysteines substituted for alphaLeu258 and alphaGlu262 were considerably faster in the presence of ACh than in its absence. That QX-314 and QX-222 did not protect alphaL258C and alphaE262C against reaction with MTSEA in the presence of ACh implies that protection of the other residues was due to occlusion of the channel and not to the promotion of a less reactive state from a remote site. Given the 12-A overall length of the blockers and the alpha-helical conformation of M2 in the open state, the binding site for both blockers extends from alphaVal255 down to alphaSer248.

Desensitization of mouse nicotinic acetylcholine receptor channels. A two-gate mechanism

The rate constants of acetylcholine receptor channels (AChR) desensitization and recovery were estimated from the durations and frequencies of clusters of single-channel currents. Diliganded-open AChR desensitize much faster than either unliganded- or diliganded-closed AChR, which indicates that the desensitization rate constant depends on the status of the activation gate rather than the occupancy of the transmitter binding sites. The desensitization rate constant does not change with the nature of the agonist, the membrane potential, the species of permeant cation, channel block by ACh, the subunit composition (epsilon or gamma), or several mutations that are near the transmitter binding sites. The results are discussed in terms of cyclic models of AChR activation, desensitization, and recovery. In particular, a mechanism by which activation and desensitization are mediated by two distinct, but interrelated, gates in the ion permeation pathway is proposed.

Desensitization of the post-synaptic membrane of neuromuscular synapses induced by spontaneous quantum secretion of mediator

Experiments on isolated frog neuromuscular junctions in voltage-clamped conditions demonstrated increases in the probability of spontaneous release of quanta of mediator, resulting in increases in the K+ ion concentration in the perfusion solution, and changes in the endplate miniature current amplitude induced by increased quantum generation were studied. Factors promoting the desensitization of the post-synaptic cholinergic membrane (inhibition of acetylcholinesterase and addition of proadifen) to levels greater than a certain (critical) frequency of endplate miniature current of the order of 50 spikes/sec were found to result in accumulation of activity and progressive reductions in the sensitivity of the post-synaptic membrane to the mediator acetylcholine.

Acetylcholine sensitivity of biphasic Ca2+ mobilization induced by nicotinic receptor activation at the mouse skeletal muscle endplate

1. Acetylcholine (ACh) was locally applied onto the endplate region in a mouse phrenic nerve-diaphragm muscle preparation to measure intracellular free calcium ([Ca2+]i) entry through nicotinic ACh receptors (AChRs) by use of Ca2+-aequorin luminescence. 2. ACh (0.1-3 mM, 20 microl) elicited biphasic elevation of [Ca2+]i (fast and slow Ca2+ mobilization) in muscle cells. The peak amplitude of the slow Ca2+ mobilization (not accompanied by twitch tension) was concentration-dependently increased by ACh, whereas that of the fast component (accompanied by twitch tension) reached a maximum response at a lower concentration (0.1 mM) of applied ACh. 3. A pulse of nicotinic agonists, (-)-nicotine (10 mM) and 1,1-dimethyl-4-phenyl-piperazinium (10 mM), but not a muscarinic agonist pilocarpine (10 mM), also elicited a biphasic Ca2+ signal. 4. Even though ACh release from motor nerve endings was blocked by botulinum toxin (5 microg, bolus i.p. before isolation of the tissue), the generation of both a fast and slow Ca2+ component caused by ACh application was observed. 5. These results strongly suggest that ACh locally applied onto the endplate region of skeletal muscle induces a slow Ca2+ signal reflecting Ca2+ entry through a postsynaptic nicotinic AChR, which has a low sensitivity to transmitter ACh.

Calcium-dependent desensitizing function of the postsynaptic neuronal-type nicotinic acetylcholine receptors at the neuromuscular junction

Several subunits that commonly have been regarded as neuronal-type nicotinic acetylcholine receptor (nAChR) subtypes, have been found in the postjunctional endplate membrane of adult skeletal muscle fibres. The postsynaptic function of these neuronal-type nAChR subtypes at the neuromuscular junction has been investigated by using aequorin luminescence and fluorescence confocal imaging. A biphasic elevation of intracellular Ca2+ is elicited by prolonged nicotinic action at the mouse muscle endplates. The fast and slow Ca2+ components are operated by a postsynaptic muscle- and colocalized neuronal-type nAChR, respectively. Neuromuscular functions may be regulated by a dual nAChR system to maintain the normal postsynaptic excitability. Certain neuronal-type nAChR may be endowed with the same functional role in the central nervous system also.

Desensitization of mutant acetylcholine receptors in transgenic mice reduces the amplitude of neuromuscular synaptic currents

While the slow onset of desensitization of nicotinic acetylcholine receptors (AChRs), relative to the rate of acetylcholine removal, excludes this kinetic state from shaping synaptic responses in normal neuromuscular transmission, its role in neuromuscular disorders has not been examined. The slow-channel congenital myasthenic syndrome (SCCMS) is a disorder caused by point mutations in the AChR subunit-encoding genes leading to kinetically abnormal (slow) channels, reduced miniature endplate current amplitudes (MEPCs), and degeneration of the postsynaptic membrane. Because of this complicated picture of kinetic and structural change in the neuromuscular junction, it is difficult to assess the importance of the multiple factors that may be responsible for the reduced endplate current amplitudes, and ultimately the clinical syndrome. In order to address this we have used a transgenic mouse model for the SCCMS that has slow AChR ion channels and reduced endplate responsiveness in the absence of any of the degenerative changes. We found that the reduction in MEPC amplitudes in these mice could not be explained by either reduced AChR number or by reduced AChR channel conductance. Rather, we found that the mutant AChRs in situ manifested an activity-dependent reduction in sensitivity that caused diminished MEPC and endplate current amplitude with nerve stimulation. This observation demonstrates that the basis for the reduction in MEPC amplitudes in the SCCMS may be multifactorial. Moreover, these findings demonstrate that, under conditions that alter their rate of desensitization, the kinetic properties of nicotinic AChRs can control the strength of synaptic responses.

Presynaptic depression at a calyx synapse: the small contribution of metabotropic glutamate receptors

Synaptic depression of evoked EPSCs was quantified with stimulation frequencies ranging from 0.2 to 100 Hz at the single CNS synapse formed by the calyx of Held in the rat brainstem. Half-maximal depression occurred at approximately 1 Hz, with 10 and 100 Hz stimulation frequencies reducing EPSC amplitudes to approximately 30% and approximately 10% of their initial magnitude, respectively. The time constant of recovery from depression elicited by 10 Hz afferent fiber stimulation was 4.2 sec. AMPA and NMDA receptor-mediated EPSCs depressed in parallel at 1-5 Hz stimulation frequencies, suggesting that depression was induced by presynaptic mechanism(s) that reduced glutamate release. To determine the contribution of autoreceptors to depression, we studied the inhibitory effects of the metabotropic glutamate receptor (mGluR) agonists (1S, 3S)-ACPD and L-AP4 and found them to be reversed in a dose-dependent manner by (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG), a novel and potent competitive antagonist of mGluRs. At 300 microM, CPPG completely reversed the effects of L-AP4 and (1S, 3S)-ACPD, but reduced 5-10 Hz elicited depression by only approximately 6%. CPPG-sensitive mGluRs, presumably activated by glutamate spillover during physiological synaptic transmission, thus contribute on the order of only 10% to short-term synaptic depression. We therefore suggest that the main mechanism contributing to the robust depression elicited by 5-10 Hz afferent fiber stimulation of the calyx of Held synapse is synaptic vesicle pool depletion.