ATP but not adenosine inhibits nonquantal acetylcholine release at the mouse neuromuscular junction

Presynaptic Purinergic Modulation of the Rat Neuro-Muscular Transmission

ATP, being a well-known universal high-energy compound, plays an important role as a signaling molecule and together with its metabolite adenosine they both attenuate the release of acetylcholine in the neuro-muscular synapse acting through membrane P2 and P1 receptors, respectively. In this work, using a mechanomyographic method, we analyzed the presynaptic mechanisms by which ATP and adenosine can modulate the transduction in the rat m. soleus and m. extensor digitorum longus. N-ethylmaleimide, a G-protein antagonist, prevents the modulating effects of both ATP and adenosine. The action of ATP is abolished by chelerythrin, a specific phospholipase C inhibitor, while the inhibitory effect of adenosine is slightly increased by Rp-cAMPS, an inhibitor of protein kinase A, and by nitrendipine, a blocker of L-type Ca2+ channels. The addition of DPCPX, an A1 receptor antagonist, fully prevents the inhibitory action of adenosine in both muscles. Our data indicate that the inhibitory action of ATP involves metabotropic P2Y receptors and is mediated by phospholipase C dependent processes in rat motor neuron terminals. We suggest that the presynaptic effect of adenosine consists of negative and positive actions. The negative action occurs by stimulation of adenosine A1 receptors while the positive action is associated with the stimulation of adenosine A2A receptors, activation of protein kinase A and opening of L-type calcium channels. The combined mechanism of the modulating action of ATP and adenosine provides fine tuning of the synapse to fast changing conditions in the skeletal muscles.

Purinergic Tuning of the Tripartite Neuromuscular Synapse

The vertebrate neuromuscular junction (NMJ) is a specialised chemical synapse involved in the transmission of bioelectric signals between a motor neuron and a skeletal muscle fiber, leading to muscle contraction. Typically, the NMJ is a tripartite synapse comprising (a) a presynaptic region represented by the motor nerve ending, (b) a postsynaptic skeletal motor endplate area, and (c) perisynaptic Schwann cells (PSCs) that shield the motor nerve terminal. Increasing evidence points towards the role of PSCs in the maintenance and control of neuromuscular integrity, transmission, and plasticity. Acetylcholine (ACh) is the main neurotransmitter at the vertebrate skeletal NMJ, and its role is fine-tuned by co-released purinergic neuromodulators, like adenosine 5'-triphosphate (ATP) and its metabolite adenosine (ADO). Adenine nucleotides modulate transmitter release and expression of postsynaptic ACh receptors at motor synapses via the activation of P2Y and P2X receptors. Endogenously generated ADO modulates ACh release by acting via co-localised inhibitory A₁ and facilitatory A_2A receptors on motor nerve terminals, whose tonic activation depends on the neuronal firing pattern and their interplay with cholinergic receptors and neuropeptides. Thus, the concerted action of adenine nucleotides, ADO, and ACh/neuropeptide co-transmitters is paramount to adapting the neuromuscular transmission to the working load under pathological conditions, like Myasthenia gravis. Unravelling these functional complexities prompted us to review our knowledge about the way purines orchestrate neuromuscular transmission and plasticity in light of the tripartite synapse concept, emphasising the often-forgotten role of PSCs in this context.

P2 Receptor Signaling in Motor Units in Muscular Dystrophy

The purine signaling system is represented by purine and pyrimidine nucleotides and nucleosides that exert their effects through the adenosine, P2X and P2Y receptor families. It is known that, under physiological conditions, P2 receptors play only a minor role in modulating the functions of cells and systems; however, their role significantly increases under some pathophysiological conditions, such as stress, ischemia or hypothermia, when they can play a dominant role as a signaling molecule. The diversity of P2 receptors and their wide distribution in the body make them very attractive as a target for the pharmacological action of drugs with a new mechanism of action. The review is devoted to the involvement of P2 signaling in the development of pathologies associated with a loss of muscle mass. The contribution of adenosine triphosphate (ATP) as a signal molecule in the pathogenesis of a number of muscular dystrophies (Duchenne, Becker and limb girdle muscular dystrophy 2B) is considered. To understand the processes involving the purinergic system, the role of the ATP and P2 receptors in several models associated with skeletal muscle degradation is also discussed.

Effect of endogenous purines on electrically evoked ACh release at the mouse neuromuscular junction

At the mouse neuromuscular junction, adenosine triphosphate (ATP), which is co-released with the neurotransmitter acetylcholine (ACh), and its metabolite adenosine, modulate neurotransmitter release by activating presynaptic inhibitory P2Y₁₃ receptors (a subtype of ATP/adenosine diphosphate [ADP] receptor), inhibitory A₁ and A₃ adenosine receptors, and excitatory A_2A adenosine receptors. To study the effect of endogenous purines, when phrenic-diaphragm preparations are depolarized by different nerve stimulation patterns, we analyzed the effect of the antagonists for P2Y₁₃ , A₁ , A₃ , and A_2A receptors (AR-C69931MX, 8-cyclopentyl-1,3-dipropylxanthine, MRS-1191, and SCH-58261, respectively) on the amplitude of the end-plate potentials of the trains, and contrasted these results with those obtained with the selective agonists of these receptors (2-methylthioadenosine 5'-diphosphate trisodium salt hydrate, 2-chloro-N⁶ -cyclopentyl-adenosine, inosine, and PSB-0777, respectively). During continuous 0.5-Hz stimulation, the amount of endogenous purines was not enough to activate purinergic receptors, while at continuous 5-Hz stimulation, an incipient action of endogenous purines on P2Y₁₃ , A₁ and A3 receptors might be evident just at the end of the trains. During continuous 50-Hz stimulation, the concentration of endogenous ATP/ADP and adenosine exerted an inhibitory action on ACh release after of the initial phase of the train, but when the nerve was stimulated at intermittent 50 Hz (5 bursts), this behavior was not observed. Excitatory A_2A receptors were only activated when continuous 100-Hz stimulation was applied. In conclusion, when motor nerve terminals are depolarized by repetitive stimulation of the phrenic nerve, endogenous ATP/ADP and adenosine are able to fine-tune neurosecretion depending on the frequency and pattern of stimulation.

L-type Ca2+ Channels at Low External Calcium Differentially Regulate Neurotransmitter Release in Proximal-Distal Compartments of the Frog Neuromuscular Junction

L-type Ca²⁺ channels (LTCCs) are key elements in electromechanical coupling in striated muscles and formation of neuromuscular junctions (NMJs). However, the significance of LTCCs in regulation of neurotransmitter release is still far from understanding. Here, we found that LTCCs can increase evoked neurotransmitter release (especially asynchronous component) and spontaneous exocytosis in two functionally different compartment of the frog NMJ, namely distal and proximal parts. The effects of LTCC blockage on evoked and spontaneous release as well as timing of exocytotic events were prevented by inhibition of either protein kinase C (PKC) or P2Y receptors (P2Y-Rs). Hence, endogenous signaling via P2Y-R/PKC axis can sustain LTCC activity. Application of ATP, a co-neurotransmitter able to activate P2Y-Rs, suppressed both evoked and spontaneous exocytosis in distal and proximal parts. Surprisingly, inhibition of LTCCs (but not PKC) decreased the negative action of exogenous ATP on evoked (only in distal part) and spontaneous exocytosis. Lipid raft disruption suppressed (1) action of LTCC antagonist on neurotransmitter release selectively in distal region and (2) contribution of LTCCs in depressant effect of ATP on evoked and spontaneous release. Thus, LTCCs can enhance and desynchronize neurotransmitter release at basal conditions (without ATP addition), but contribute to ATP-mediated decrease in the exocytosis. The former action of LTCCs relies on P2Y-R/PKC axis, whereas the latter is triggered by exogenous ATP and PKC-independent. Furthermore, relevance of lipid rafts for LTCC function as well as LTCCs for ATP effects is different in distal and proximal part of the NMJ.

Modulatory Roles of ATP and Adenosine in Cholinergic Neuromuscular Transmission

A review of the data on the modulatory action of adenosine 5’-triphosphate (ATP), the main co-transmitter with acetylcholine, and adenosine, the final ATP metabolite in the synaptic cleft, on neuromuscular transmission is presented. The effects of these endogenous modulators on pre- and post-synaptic processes are discussed. The contribution of purines to the processes of quantal and non-quantal secretion of acetylcholine into the synaptic cleft, as well as the influence of the postsynaptic effects of ATP and adenosine on the functioning of cholinergic receptors, are evaluated. As usual, the P2-receptor-mediated influence is minimal under physiological conditions, but it becomes very important in some pathophysiological situations such as hypothermia, stress, or ischemia. There are some data demonstrating the same in neuromuscular transmission. It is suggested that the role of endogenous purines is primarily to provide a safety factor for the efficiency of cholinergic neuromuscular transmission.

Extracellular ATP and β-NAD alter electrical properties and cholinergic effects in the rat heart in age-specific manner

Extracellular ATP and nicotinamide adenine dinucleotide (β-NAD) demonstrate properties of neurotransmitters and neuromodulators in peripheral and central nervous system. It has been shown previously that ATP and β-NAD affect cardiac functioning in adult mammals. Nevertheless, the modulation of cardiac activity by purine compounds in the early postnatal development is still not elucidated. Also, the potential influence of ATP and β-NAD on cholinergic neurotransmission in the heart has not been investigated previously. Age-dependence of electrophysiological effects produced by extracellular ATP and β-NAD was studied in the rat myocardium using sharp microelectrode technique. ATP and β-NAD could affect ventricular and supraventricular myocardium independent from autonomic influences. Both purines induced reduction of action potentials (APs) duration in tissue preparations of atrial, ventricular myocardium, and myocardial sleeves of pulmonary veins from early postnatal rats similarly to myocardium of adult animals. Both purine compounds demonstrated weak age-dependence of the effect. We have estimated the ability of ATP and β-NAD to alter cholinergic effects in the heart. Both purines suppressed inhibitory effects produced by stimulation of intracardiac parasympathetic nerve in right atria from adult animals, but not in preparations from neonates. Also, ATP and β-NAD suppressed rest and evoked release of acetylcholine (ACh) in adult animals. β-NAD suppressed effects of parasympathetic stimulation and ACh release stronger than ATP. In conclusion, ATP and β-NAD control the heart at the postsynaptic and presynaptic levels via affecting the cardiac myocytes APs and ACh release. Postsynaptic and presynaptic effects of purines may be antagonistic and the latter demonstrates age-dependence.

P2Y13 receptors mediate presynaptic inhibition of acetylcholine release induced by adenine nucleotides at the mouse neuromuscular junction

It is known that adenosine 5'-triphosphate (ATP) is released along with the neurotransmitter acetylcholine (ACh) from motor nerve terminals. At mammalian neuromuscular junctions (NMJs), we have previously demonstrated that ATP is able to decrease ACh secretion by activation of P2Y receptors coupled to pertussis toxin-sensitive Gi/o protein. In this group, the receptor subtypes activated by adenine nucleotides are P2Y12 and P2Y13. Here, we investigated, by means of pharmacological and immunohistochemical assays, the P2Y receptor subtype that mediates the modulation of spontaneous and evoked ACh release in mouse phrenic nerve-diaphragm preparations. First, we confirmed that the preferential agonist for P2Y12-13 receptors, 2-methylthioadenosine 5'-diphosphate trisodium salt hydrate (2-MeSADP), reduced MEPP frequency without affecting MEPP amplitude as well as the amplitude and quantal content of end-plate potentials (EPPs). The effect on spontaneous secretion disappeared after the application of the selective P2Y12-13 antagonists AR-C69931MX or 2-methylthioadenosine 5'-monophosphate triethylammonium salt hydrate (2-MeSAMP). 2-MeSADP was more potent than ADP and ATP in reducing MEPP frequency. Then we demonstrated that the selective P2Y13 antagonist MRS-2211 completely prevented the inhibitory effect of 2-MeSADP on MEPP frequency and EPP amplitude, whereas the P2Y12 antagonist MRS-2395 failed to do this. The preferential agonist for P2Y13 receptors inosine 5'-diphosphate sodium salt (IDP) reduced spontaneous and evoked ACh secretion and MRS-2211 abolished IDP-mediated modulation. Immunohistochemical studies confirmed the presence of P2Y13 but not P2Y12 receptors at the end-plate region. Disappearance of P2Y13 receptors after denervation suggests the presynaptic localization of the receptors. We conclude that, at motor nerve terminals, the Gi/o protein-coupled P2Y receptors implicated in presynaptic inhibition of spontaneous and evoked ACh release are of the subtype P2Y13. This study provides new insights into the types of purinergic receptors that contribute to the fine-tuning of cholinergic transmission at mammalian neuromuscular junction.

Amplification of neuromuscular transmission by methylprednisolone involves activation of presynaptic facilitatory adenosine A2A receptors and redistribution of synaptic vesicles

The mechanisms underlying improvement of neuromuscular transmission deficits by glucocorticoids are still a matter of debate despite these compounds have been used for decades in the treatment of autoimmune myasthenic syndromes. Besides their immunosuppressive action, corticosteroids may directly facilitate transmitter release during high-frequency motor nerve activity. This effect coincides with the predominant adenosine A2A receptor tonus, which coordinates the interplay with other receptors (e.g. muscarinic) on motor nerve endings to sustain acetylcholine (ACh) release that is required to overcome tetanic neuromuscular depression in myasthenics. Using myographic recordings, measurements of evoked [(3)H]ACh release and real-time video microscopy with the FM4-64 fluorescent dye, results show that tonic activation of facilitatory A2A receptors by endogenous adenosine accumulated during 50 Hz bursts delivered to the rat phrenic nerve is essential for methylprednisolone (0.3 mM)-induced transmitter release facilitation, because its effect was prevented by the A2A receptor antagonist, ZM 241385 (10 nM). Concurrent activation of the positive feedback loop operated by pirenzepine-sensitive muscarinic M1 autoreceptors may also play a role, whereas the corticosteroid action is restrained by the activation of co-expressed inhibitory M2 and A1 receptors blocked by methoctramine (0.1 μM) and DPCPX (2.5 nM), respectively. Inhibition of FM4-64 loading (endocytosis) by methylprednisolone following a brief tetanic stimulus (50 Hz for 5 s) suggests that it may negatively modulate synaptic vesicle turnover, thus increasing the release probability of newly recycled vesicles. Interestingly, bulk endocytosis was rehabilitated when methylprednisolone was co-applied with ZM241385. Data suggest that amplification of neuromuscular transmission by methylprednisolone may involve activation of presynaptic facilitatory adenosine A2A receptors by endogenous adenosine leading to synaptic vesicle redistribution.

Purinergic signalling in the musculoskeletal system

It is now widely recognised that extracellular nucleotides, signalling via purinergic receptors, participate in numerous biological processes in most tissues. It has become evident that extracellular nucleotides have significant regulatory effects in the musculoskeletal system. In early development, ATP released from motor nerves along with acetylcholine acts as a cotransmitter in neuromuscular transmission; in mature animals, ATP functions as a neuromodulator. Purinergic receptors expressed by skeletal muscle and satellite cells play important pathophysiological roles in their development or repair. In many cell types, expression of purinergic receptors is often dependent on differentiation. For example, sequential expression of P2X5, P2Y1 and P2X2 receptors occurs during muscle regeneration in the mdx model of muscular dystrophy. In bone and cartilage cells, the functional effects of purinergic signalling appear to be largely negative. ATP stimulates the formation and activation of osteoclasts, the bone-destroying cells. Another role appears to be as a potent local inhibitor of mineralisation. In osteoblasts, the bone-forming cells, ATP acts via P2 receptors to limit bone mineralisation by inhibiting alkaline phosphatase expression and activity. Extracellular ATP additionally exerts significant effects on mineralisation via its hydrolysis product, pyrophosphate. Evidence now suggests that purinergic signalling is potentially important in several bone and joint disorders including osteoporosis, rheumatoid arthritis and cancers. Strategies for future musculoskeletal therapies might involve modulation of purinergic receptor function or of the ecto-nucleotidases responsible for ATP breakdown or ATP transport inhibitors.

Purine P2Y receptors in ATP-mediated regulation of non-quantal acetylcholine release from motor nerve endings of rat diaphragm

We established the effect of ATP, which is released together with acetylcholine (ACh), on the non-quantal ACh release (NQR) in rat diaphragm endplates and checked what kind of purine receptors are involved. NQR was estimated by the amplitude of endplate hyperpolarization (the H-effect) following the blockade of postsynaptic nicotinic receptors and cholinesterase. 100 μM ATP reduced the H-effect to 66% of the control. The action of ATP remained unchanged after the inhibition of ionotropic P2X receptors by Evans blue and PPADS, but disappeared after the application of the broad spectrum P2 receptor antagonist suramin, metabotropic P2Y receptor blocker reactive blue 2 and U73122, an inhibitor of phospholipase C. P2Y-mediated regulation is not coupled to presynaptic voltage-dependent Ca(2+) channels. During the simultaneous application of ATP and glutamate (which is another ACh cotransmitter reducing non-quantal release), the additive depressant effect led to a disappearance of the H-effect. This can be explained by the independence of the action of ATP and glutamate. Unlike the effects of purines on the spontaneous quantal secretion of ACh, its non-quantal release is regulated via P2Y receptors coupled to G(q/11) and PLC. ATP thus regulates the neuromuscular synapse by two different pathways.

Possible mechanisms for the effect of protein sensitization on functional properties of the isolated m. soleus and m. EDL from mice

We studied possible involvement of ATP in the influence of protein sensitization on contractile function and non-quantum secretion in the end-plate of isolated skeletal muscles from mouse leg. The dynamic vector of muscle contraction force was shown to correlate with changes in non-quantum secretion of acetylcholine under various conditions of experimental pathology. However, the degree of these changes was lower in sensitized animals. It can be hypothesized that the ATP-induced variability in functional properties of slow muscles during protein sensitization reflects the development of resistance to external loads. The adaptive changes in fast muscles during protein sensitization are not associated with the ATP-mediated mechanisms of excitation.

Perisynaptic glia discriminate patterns of motor nerve activity and influence plasticity at the neuromuscular junction

In the nervous system, the induction of plasticity is coded by patterns of synaptic activity. Glial cells are now recognized as dynamic partners in a wide variety of brain functions, including the induction and modulation of various forms of synaptic plasticity. However, it appears that glial cells are usually activated by stereotyped, sustained neuronal activity, and little attention has been given to more subtle changes in the patterns of synaptic activation. To this end, we used the mouse neuromuscular junction as a simple and useful model to study glial modulation of synaptic plasticity. We used two patterns of motor nerve stimulation that mimic endogenous motor-neuronal activity. A continuous stimulation induced a post-tetanic potentiation and a phasic Ca(2+) response in perisynaptic Schwann cells (PSCs), glial cells at this synapse. A bursting pattern of activity induced a post-tetanic depression and oscillatory Ca(2+) responses in PSCs. The different Ca(2+) responses in PSCs indicate that they decode the pattern of synaptic activity. Furthermore, the chelation of glial Ca(2+) impaired the production of the sustained plasticity events indicating that PSCs govern the outcome of synaptic plasticity. The mechanisms involved were studied using direct photo-activation of PSCs with caged Ca(2+) that mimicked endogenous plasticity. Using specific pharmacology and transgenic knock-out animals for adenosine receptors, we showed that the sustained depression was mediated by A1 receptors while the sustained potentiation is mediated by A(2A) receptors. These results demonstrate that glial cells decode the pattern of synaptic activity and subsequently provide bidirectional feedback to synapses.

Interaction of hydrocortisone with ATP and adenosine on nerve-mediated contractions of frog skeletal muscle

The inhibitory effects of ATP and adenosine on the nerve-mediated contractile responses of isolated sartorius muscle of the frog, Rana ridibunda, evoked by electrical field stimulation (EFS) were studied using pharmacological organ-bath technique. The effects of hydrocortisone applied in vitro and in vivo on contractility of sartorius muscle were also examined. ATP (100 microM) significantly reduced the amplitude of contraction to EFS of sartorius muscle, while pyridoxalphosphate-6-azonphenyl-2',4'-disulfonic acid (PPADS; 10 microM), a P2 receptor antagonist, abolished inhibitory effect of ATP. A similar inhibitory effect of adenosine (100 microM) was fully antagonized by 8-(p-sulfophenyl)-theophylline (8-SPT, 100 microM), a P1 receptor antagonist. Incubation of the tissue with hydrocortisone (10 microM) caused a slight, but significant, decrease of muscle contractions. After incubation of muscle preparations with both hydrocortisone and ATP, no inhibition of muscle contractility was registered. A single injection of hydrocortisone (100 mg/kg) 12 h prior to experiments to frogs did not significantly change the nerve-mediated contractility of isolated sartorius muscle; however, it abolished the inhibitory action of ATP without changing inhibitory activity of adenosine. After treatment of frogs with hydrocortisone for 14 days (100 mg/kg/day), both ATP and adenosine retained their inhibitory action on EFS-induced contractions of the muscle, and their effects were antagonized by PPADS and 8-SPT, respectively. It is concluded that hydrocortisone has antagonistic actions against the inhibitory effects of ATP at the frog neuromuscular junction, although this effect is lost following long-term treatment with hydrocortisone.

Ovalbumin-induced sensitization affects non-quantal acetylcholine release from motor nerve terminals and alters contractility of skeletal muscles in mice

Skeletal muscles play key roles in the development of various pathologies, including bronchial asthma and several types of auto-immune disorders, e.g. polymyositis. Since most of these maladies have an immunological/allergic element, this paper is devoted to assessing the impact of immunobiological reorganization on the functional properties of isolated skeletal muscles in mice. A combination of two methods (myography and electrophysiology) was used to evaluate extensor digitorum longus (EDL) and diaphragmatic muscle (DM) in this regard. Conventional myographic technique showed that ovalbumin-induced sensitization (OS) produced different changes in the contractile properties of EDL and DM. The amplitudes of carbachol (CCh)-induced contractions increased in DM but decreased in EDL. Those changes were inversely related to OS-mediated changes of non-quantal acetylcholine (ACh) release intensity within the muscle endplate, as shown by the electrophysiologically measured H-effect. These results clearly show that OS-mediated changes of non-quantal ACh release alter the functional properties of postjunctional ACh receptors and therefore contribute to the disturbance of CCh-induced contractility of skeletal muscles. Other mechanisms of OS-mediated changes of skeletal muscle contractility are also proposed and discussed.

Role of protein kinase C in the effect of ATP on contractile function of the isolated strip from mouse diaphragm

We studied the effects of adenosine and ATP on contractile function of the isolated strip from mouse diaphragm. ATP significantly increased the strength of muscle contraction induced by carbachol. Adenosine had no effect on carbachol-induced muscle contraction. P2 receptor antagonist suramin abolished the effect of ATP. The positive chronotropic effect of ATP was not observed after treatment with specific protein kinase C inhibitor chelerythrine. Our results indicate that the effect of ATP on contractile function of mouse diaphragm is realized via protein kinase C.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Muscarinic M1 acetylcholine receptors regulate the non-quantal release of acetylcholine in the rat neuromuscular junctionviaNO-dependent mechanism

Nitric oxide (NO), previously demonstrated to participate in the regulation of the resting membrane potential in skeletal muscles via muscarinic receptors, also regulates non-quantal acetylcholine (ACh) secretion from rat motor nerve endings. Non-quantal ACh release was estimated by the amplitude of endplate hyperpolarization (H-effect) following a blockade of skeletal muscle post-synaptic nicotinic receptors by (+)-tubocurarine. The muscarinic agonists oxotremorine and muscarine lowered the H-effect and the M1 antagonist pirenzepine prevented this effect occurring at all. Another muscarinic agonist arecaidine but-2-ynyl ester tosylate (ABET), which is more selective for M2 receptors than for M1 receptors and 1,1-dimethyl-4-diphenylacetoxypiperidinium (DAMP), a specific antagonist of M3 cholinergic receptors had no significant effect on the H-effect. The oxotremorine-induced decrease in the H-effect was calcium and calmodulin-dependent. The decrease was negated when either NO synthase was inhibited by N(G)-nitro-L-arginine methyl ester or soluble guanylyl cyclase was inhibited by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. The target of muscle-derived NO is apparently nerve terminal guanylyl cyclase, because exogenous hemoglobin, acting as an NO scavenger, prevented the oxotremorine-induced drop in the H-effect. These results suggest that oxotremorine (and probably also non-quantal ACh) selectively inhibit the non-quantal secretion of ACh from motor nerve terminals acting on post-synaptic M1 receptors coupled to Ca(2+) channels in the sarcolemma to induce sarcoplasmic Ca(2+)-dependent synthesis and the release of NO. It seems that a substantial part of the H-effect can be physiologically regulated by this negative feedback loop, i.e., by NO from muscle fiber; there is apparently also Ca(2+)- and calmodulin-dependent regulation of ACh non-quantal release in the nerve terminal itself, as calmidazolium inhibition of the calmodulin led to a doubling of the resting H-effect.

Different effects of ATP on the contractile activity of mice diaphragmatic and skeletal muscles

Apart from acetyl-choline (Ach), adenosine-5'-trisphosphate (ATP) is thought to play a role in neuromuscular function, however little information is available on its cellular physiology. As such, effects of ATP and adenosine on contractility of mice diaphragmatic and skeletal muscles (m. extensor digitorum longa-MEDL) have been investigated in in vitro experiments. Application of carbacholine (CCh) in vitro in different concentrations led to pronounced muscle contractions, varying from 9.15+/-4.76 to 513.13+/-15.4 mg and from 44.65+/-5.01 to 101.46+/-9.11 mg for diaphragm and MEDL, respectively. Two hundred micromolars of CCh in both muscles caused the contraction with the 65% (diaphragm) to 75% (MEDL) of maximal contraction force-this concentration was thus used in further experiments. It was found that application of ATP (100 microM) increased the force of diaphragmatic contraction caused by CCh (200 microM) from 335.2+/-51.4 mg (n=21) in controls to 426.5+/-47.8 mg (n=10; P<0.05), but decreased the contractions of MEDL of CCh from 76.6+/-6.5mg (n=26) in control to 40.2+/-9.0mg (n=8; P<0.05). Application of adenosine (100 microM) had no effect on CCh-induced contractions of these muscles. Resting membrane potential (MP) measurements using sharp electrodes were done at 10, 20 and 30 min after the application of ATP and adenosine. Diaphragm showed depolarization from 75+/-0.6 down to 63.2+/-1.05, 57.2+/-0.96 and 53.6+/-1.1 mV after 10, 20 and 30 min of exposition, respectively (20 fibers from 4 muscles each, P<0.05 in all three cases). Adenosine showed no effect on diaphragmatic MP. Both agents were ineffective in case of MEDL. The effects of ATP in both tissues were abolished by suramin (100 microM), a P2-receptor antagonist, and chelerythrin (50 microM), a specific protein-kinase C (PKC) inhibitor, but were not affected by 1H-[1,2,4]-oxadiazolo-[4,3-alpha]-quinoxalin-1-one (ODQ, 1 microM), a guanylyl-cyclase inhibitor, or by adenosine-3,5-monophosphothioate (Rp-cAMP, 1 microM), a protein-kinase A (PKA) inhibitor. Besides the action on contractile activity, ATP (100 microM) led to a significant (P<0.001) depolarization of diaphragm muscle fibers from 74.5+/-2.3 down to 64+/-2.1, 58.2+/-2.2 and 54.3+/-2.4 mV after 10, 20 and 30 min of incubation, respectively. Incubation of MEDL with the same ATP concentration showed no significant change of MP. Denervation of the muscles for 28 days led to a decrease of CCh-induced contractions of diaphragm down to 171.1+/-34.5mg (n=11, P<0.05), but increased the contractile force of MEDL up to 723.9+/-82.3mg (n=9, P<0.01). Application of ATP elevated the contractility of denervated diaphragm caused by CCh up to normal values (311.1+/-79.7 mg, n=6, P>0.05 versus control), but did not significantly affect of contractility of MEDL, which became 848.1+/-62.7 mg (n=6). These results show that the effects of ATP on both diaphragmatic and skeletal muscles are mediated through P2Y receptors coupled to chelerytrin-sensitive protein-kinase C.

Presynaptic inhibition of spontaneous acetylcholine release mediated by P2Y receptors at the mouse neuromuscular junction

At the neuromuscular junction, ATP is co-released with the neurotransmitter acetylcholine (ACh) and once in the synaptic space, it is degraded to the presynaptically active metabolite adenosine. Intracellular recordings were performed on diaphragm fibers of CF1 mice to determine the action of extracellular ATP (100 muM) and the slowly hydrolysable ATP analog 5'-adenylylimidodiphosphate lithium (betagamma-imido ATP) (30 muM) on miniature end-plate potential (MEPP) frequency. We found that application of ATP and betagamma-imido ATP decreased spontaneous secretion by 45.3% and 55.9% respectively. 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective A(1) adenosine receptor antagonist and alpha,beta-methylene ADP sodium salt (alphabeta-MeADP), which is an inhibitor of ecto-5'-nucleotidase, did not prevent the inhibitory effect of ATP, demonstrating that the nucleotide is able to modulate spontaneous ACh release through a mechanism independent of the action of adenosine. Blockade of Ca(2+) channels by both, Cd(2+) or the combined application of nitrendipine and omega-conotoxin GVIA (omega-CgTx) (L-type and N-type Ca(2+) channel antagonists, respectively) prevented the effect of betagamma-imido ATP, indicating that the nucleotide modulates Ca(2+) influx through the voltage-dependent Ca(2+) channels related to spontaneous secretion. betagamma-Imido ATP-induced modulation was antagonized by the non-specific P2 receptor antagonist suramin and the P2Y receptor antagonist 1-amino-4-[[4-[[4-chloro-6-[[3(or4)-sulfophenyl] amino]-1,3,5-triazin-2-yl]amino]-3-sulfophenyl] amino]-9,10-dihydro-9,10-dioxo-2-anthracenesulfonic acid (reactive blue-2), but not by pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) tetrasodium salt (PPADS), which has a preferential antagonist effect on P2X receptors. Pertussis toxin and N-ethylmaleimide (NEM), which are blockers of G(i/o) proteins, prevented the action of the nucleotide, suggesting that the effect is mediated by P2Y receptors coupled to G(i/o) proteins. The protein kinase C (PKC) antagonist chelerythrine and the calmodulin antagonist N-(6-aminohexil)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) occluded the effect of betagamma-imido ATP, while the protein kinase A (PKA) antagonist KT-5720 and the inhibitor of the calcium/calmodulin-dependent protein kinase II (CAMKII) KN-62 failed to do so. betagamma-Imido ATP did not affect 10, 15 and 20 mM K(+)-evoked release and application of reactive blue-2 before incubation in high K(+) induced a higher asynchronous secretion. Thus, our results show that at mammalian neuromuscular junctions, ATP induces presynaptic inhibition of spontaneous ACh release due to the modulation of Ca(2+) channels related to tonic secretion through the activation of P2Y receptors coupled to G(i/o) proteins. We also demonstrated that at increasing degrees of membrane depolarization evoked by K(+), endogenously released ATP induces presynaptic inhibition as a means of preventing excessive neurotransmitter secretion.

Negative cross-talk between presynaptic adenosine and acetylcholine receptors

Functional interactions between presynaptic adenosine and acetylcholine (ACh) autoreceptors were studied at the frog neuromuscular junction by recording miniature end-plate potentials (MEPPs) during bath or local application of agonists. The frequency of MEPPs was reduced by adenosine acting on presynaptic adenosine A1 receptors (EC(50) = 1.1 microm) or by carbachol acting on muscarinic M2 receptors (EC(50) = 1.8 microm). However, carbachol did not produce the depressant effect when it was applied after the action of adenosine had reached its maximum. This phenomenon implied that the negative cross-talk (occlusion) had occurred between A1 and M2 receptors. Moreover, the occlusion was receptor-specific as ATP applied in the presence of adenosine continued to depress MEPP frequency. Muscarinic antagonists [atropine or 1-[[2-[(diethylamino)methyl)-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyrido [2,3-b][1,4]benzodiazepine-6-one) (AFDX-116)] had no effect on the inhibitory action of adenosine and adenosine antagonists [8-(p-sulfophenyl)theophylline (8-SPT) or 1,3-dipropyl-8-cyclopentylxanthine (DPCPX)] had no effect on the action of carbachol. These data suggested that membrane-delimited interactions did not occur between A1 and M2 receptors. Both carbachol and adenosine similarly inhibited quantal release triggered by high potassium, ionomycin or sucrose. These results indicated a convergence of intracellular pathways activated by M2 and A1 receptors to a common presynaptic effector located downstream of Ca(2+) influx. We propose that the negative cross-talk between two major autoreceptors could take place during intense synaptic activity and thereby attenuate the presynaptic inhibitory effects of ACh and adenosine.

Purinergic modulation of synaptic signalling at the neuromuscular junction

Purines have physiologically important functions throughout the nervous system. In both the central (CNS) and peripheral nervous systems (PNS), purines in the form of adenosine triphosphate and adenosine can play a number of roles in neuronal activation and inhibition. In addition, purines are known to be important for glial cell signaling in both the CNS and PNS. In the PNS, the neuromuscular junction (NMJ) is an excellent model for studying simple synaptic interactions. It is well suited to investigations of neuron-glia interactions because synaptic properties are well defined and perisynaptic Schwann cells (PSCs), glial cells at the NMJ, dynamically interact with the pre- and postsynaptic elements. At the NMJ, purines are critical for presynaptic modulation but also for neuron-glia interactions. Purines signal to PSCs through metabotropic and ionotropic receptors and activation of these receptors can have both modulatory and activating functions. This review will discuss recent developments in our understanding of purinergic modulation of the NMJ with an emphasis on the involvement of purines in neuron-glia interactions at this synapse.

The influence of hypothermia on P2 receptor-mediated responses of frog skeletal muscle

The contractile responses of isolated Rana ridibunda frog sartorius muscle contractions evoked by electrical field stimulation (EFS) were studied at three temperature conditions of 17, 22 and 27 degrees C. Temperature-dependent increase of muscle contractility was found. ATP (10-100 microM) concentration dependently inhibited the electrical field stimulation-evoked contractions of sartorius muscle at all three temperatures; this effect was significantly more prominent at a temperature of 17 degrees C than at other two temperatures. Adenosine (100 microM) also caused inhibition of electrical field stimulation-evoked contractions which was statistically identical at all three temperature conditions tested. A P2 receptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microM) reduced the inhibitory effect of ATP at all three temperatures but did not affect inhibitory action of adenosine. In contrast, 8-(p-sulfophenyl)theophylline (8-SPT, 100 microM), a nonselective P1 receptor antagonist, abolished inhibitory effects of adenosine at all three temperature conditions but did not antagonize inhibition caused by ATP. In electrophysiological experiments, ATP (100 microM) and adenosine (100 microM) temperature dependently reduced end-plate currents recorded in sartorius neuromuscular junction by voltage-clamp technique. The inhibitory effects of both agonists were enhanced with the decrease of temperature. 8-SPT (100 microM) abolished the inhibitory effect of adenosine but not ATP on end-plate currents. Suramin (100 microM), a nonselective P2 receptor antagonist, inhibited the action of ATP but not adenosine, while PPADS (10 microM) had no influence on the effects of either ATP or adenosine. It is concluded from this study that the effectiveness of P2 receptor-mediated inhibition of frog skeletal muscle contraction in contrast to that of adenosine is dependent on the temperature conditions.

Mechanisms of ATP action on motor nerve terminals at the frog neuromuscular junction

We have shown previously that ATP inhibits transmitter release at the neuromuscular junction through the action on metabotropic P2Y receptors coupled to specific second messenger cascades. In the present study we recorded K(+) or Ca(2+) currents in motor nerve endings or blocked K(+) or Ca(2+) channels in order to explore the nature of downstream presynaptic effectors. Endplate currents were presynaptically depressed by ATP. Blockers of Ca(2+)-activated K(+)-channels, such as iberiotoxin, apamin or tetraethylammonium, did not change the depressant action of ATP. By contrast, K(+) channel blocker 4-aminopyridine (4-AP) and raised extracellular Ca(2+) attenuated the effect of ATP. However, these effects of 4-AP and high Ca(2+) were reversed by Mg(2+), suggesting Ca(2+)-dependence of the ATP action. Ba(2+) promoted the depressant action of ATP as did glibenclamide, a blocker of ATP-sensitive K(+) channels, or mild depolarization produced by 7.5 mm K(+). None of the K(+) channel blockers affected the depressant action of adenosine. Focal recording revealed that neither ATP nor adenosine affected the fast K(+) currents of the motor nerve endings. However, unlike adenosine, ATP or UTP, an agonist of P2Y receptors, reversibly reduced the presynaptic Ca(2+)-current. This effect was abolished by suramin, an antagonist of P2 receptors. Depressant effect of ATP on the endplate and Ca(2+)-currents was mimicked by arachidonate, which precluded the action of ATP. ATP reduced acetylcholine release triggered by ionomycin or sucrose, suggesting inhibition of release machinery. Thus, the presynaptic depressant action of ATP is mediated by inhibition of Ca(2+) channels and by mechanism acting downstream of Ca(2+) entry.

Properties of presynaptic P2X7-like receptors at the neuromuscular junction

Adenosine triphosphate is released into the synaptic cleft of the neuromuscular junction during normal synaptic transmission, and in much greater quantities following injury and ischaemia. There is much data to suggest roles for presynaptic P2 receptors but little to demonstrate which specific receptor subunits are present. Here we show P2X7 receptor subunits on presynaptic motor nerve terminals from birth, but no evidence for P2X1, P2X2, P2X3, P2X4, P2X5 or P2X6 receptor subunits. Further, P2X receptor subunits are present as multimeric, membrane-inserted receptors. A selective agonist, 2'-3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate (BzATP: 100 microM), triggers vesicle release from motor nerve terminals, which is blocked by P2X7RS-specific concentrations of periodate oxidised ATP (OxATP: 100 microM) and brilliant blue G (BBG: 1 microM), but not by suramin (100 microM). Vesicle release is enhanced in the absence of extracellular divalent cations and occurs through activation of the ion channel and not any associated large pore, as we failed to label nerve terminals with large membrane-impermeant molecules after addition of BzATP. We conclude that a P2X7-like receptor is present at mouse motor nerve terminals, and that their activation promotes vesicle release.

Early postdenervation depolarization develops faster at endplates of hibernating golden hamsters where spontaneous quantal and non-quantal acetylcholine release is very small

The hyperpolarization produced by the application of curare to the postsynaptic membrane of the diaphragm neuromuscular synapse (H-effect) is a measure of non-quantal release (NQR) of acetylcholine (ACh) from the motor nerve ending. In mouse diaphragm, H-effect was 9.3 mV, significantly lower in awake hamsters (7.1 mV) and very small (1.1 mV) in hibernating hamsters. Also, the initial resting membrane potential (RMP) after dissection was highest in mouse (81.5 mV, inside negative), significantly smaller in awake hamsters (77.9 mV) and lowest in hibernating hamsters (75.1 mV). The early postdenervation depolarization of muscle fiber RMP to about 66-68 mV developed with half-decay time (T1/2) of 120 min in mouse, more rapidly in active hamsters (T1/2=60 min) and even faster in hibernating hamsters (T1/2=25 min) muscles. This reciprocal correlation between the H-effect and the rate of early depolarization indicates that non-quantal release is important for maintaining the resting membrane potential [Vyskocil, F. 2003. Early postdenervation depolarization is controlled by acetylcholine and glutamate via nitric oxide regulation of the chloride transporter. Neurochem. Res. 28, 575-585]. The amplitude of H-effect in mouse and hamster was proportional to the spontaneous quantal release. The frequency of miniature endplate potentials was highest in mouse (1.6 s-1), much smaller in awake hamsters (0.51 s-1) and very small in hibernating hamsters (0.08 s-1). This is in accordance with the idea that non-quantal release depends on the number of vesicles fused with the presynaptic membrane during quantal release [Edwards et al., 1985; Ferguson, S.M., Savchenko, V., Apparsundaram, S., Zwick, M., Wright J., Heilman, C.J., Yi, H., Levey, A.I., Blakely R.D. Vesicular localization and activity-dependent trafficking of presynaptic choline transporters. J. Neurosci. 23 (2003) 9697-9709].

Reactive oxygen species contribute to the presynaptic action of extracellular ATP at the frog neuromuscular junction

During normal cell metabolism the production of intracellular ATP is associated with the generation of reactive oxygen species (ROS), which appear to be important signalling molecules. Both ATP and ROS can be released extracellularly by skeletal muscle during intense activity. Using voltage clamp recording combined with imaging and biochemical assay of ROS, we tested the hypothesis that at the neuromuscular junction extracellular ATP generates ROS to inhibit transmitter release from motor nerve endings. We found that ATP produced the presynaptic inhibitory action on multiquantal end-plate currents. The inhibitory action of ATP (but not that of adenosine) was significantly reduced by several antioxidants or extracellular catalase, which breaks down H2O2. Consistent with these data, the depressant effect of ATP was dramatically potentiated by the pro-oxidant Fe2+. Exogenous H2O2 reproduced the depressant effects of ATP and showed similar sensitivity to anti- and pro-oxidants. While NO also inhibited synaptic transmission, inhibitors of the NO-producing cascade did not prevent the depressant action of ATP. The ferrous oxidation in xylenol orange assay showed the increase of ROS production by ATP and 2-MeSADP but not by adenosine. Suramin, a non-selective antagonist of P2 receptors, and pertussis toxin prevented the action of ATP on ROS production. Likewise, imaging with the ROS-sensitive dye carboxy-2',7'-dichlorodihydrofluorescein revealed increased production of ROS in the muscle treated with ATP or ADP while UTP or adenosine had no effect. Thus, generation of ROS contributed to the ATP-mediated negative feedback mechanism controlling quantal secretion of ACh from the motor nerve endings.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Loading and recycling of synaptic vesicles in the Torpedo electric organ and the vertebrate neuromuscular junction

In vertebrate motor nerve terminals and in the electromotor nerve terminals of Torpedo there are two major pools of synaptic vesicles: readily releasable and reserve. The electromotor terminals differ in that the reserve vesicles are twice the diameter of the readily releasable vesicles. The vesicles contain high concentrations of ACh and ATP. Part of the ACh is brought into the vesicle by the vesicular ACh transporter, VAChT, which exchanges two protons for each ACh, but a fraction of the ACh seems to be accumulated by different, unexplored mechanisms. Most of the vesicles in the terminals do not exchange ACh or ATP with the axoplasm, although ACh and ATP are free in the vesicle interior. The VAChT is controlled by a multifaceted regulatory complex, which includes the proteoglycans that characterize the cholinergic vesicles. The drug (-)-vesamicol binds to a site on the complex and blocks ACh exchange. Only 10-20% of the vesicles are in the readily releasable pool, which therefore is turned over fairly rapidly by spontaneous quantal release. The turnover can be followed by the incorporation of false transmitters into the recycling vesicles, and by the rate of uptake of FM dyes, which have some selectivity for the two recycling pathways. The amount of ACh loaded into recycling vesicles in the readily releasable pool decreases during stimulation. The ACh content of the vesicles can be varied over eight-fold range without changing vesicle size.

Distinct receptors and different transduction mechanisms for ATP and adenosine at the frog motor nerve endings

Corelease of ATP with ACh from motor endings suggests a physiological role for ATP in synaptic transmission. We previously showed that, on skeletal muscle, ATP directly inhibited ACh release via presynaptic P2 receptors. The receptor identification (P2X or P2Y) and its transduction mechanism remained, however, unknown. In the present study using the voltage-clamp technique we analyzed the properties of presynaptic ATP receptors and subsequent effector mechanisms. ATP or adenosine presynaptically depressed multiquantal end-plate currents, with longer latency for ATP action. ATPgammaS, agonist at P2X receptors, or Bz-ATP, agonist at P2X7 receptors, were ineffective. The action of ATP was prevented by suramin and unchanged by PPADS or TNP-ATP, antagonists of P2X receptors, or RB-2, a blocker of certain P2Y receptors. The depressant action of ATP was reproduced by UTP, metabotropic P2Y receptor agonist. Pertussis toxin (PTX), antagonist of Gi/o-proteins, and inhibitors of phosphatidylcholine specific PLC (D609) and PKC (staurosporine or chelerythrine) prevented the effect of ATP while blockers of PLA2 (OBAA) and COX (aspirin or indomethacin) attenuated it. Inhibitors of phosphatidylinositide-specific PLC (U73122), guanylylcyclase (ODQ), PKA (Rp-cAMPS) or PLD (1-butanol) did not affect the action of ATP. No inhibitor of second messengers (except PTX) changed the action of adenosine. Our data indicate, for motor nerve endings, the existence of inhibitory P2Y receptors coupled to multiple intracellular cascades including phosphatidylinositide-specific PLC/PKC/PLA2/COX. This divergent presynaptic P2 signalling (unlike the single effector mechanism for P1 receptors) could provide feedback inhibition of transmitter release and perhaps be involved in presynaptic plasticity.

Ecto-AMP deaminase blunts the ATP-derived adenosine A2A receptor facilitation of acetylcholine release at rat motor nerve endings

At synapses, ATP is released and metabolised through ecto-nucleotidases forming adenosine, which modulates neurotransmitter release through inhibitory A1 or facilitatory A2A receptors, according to the amounts of extracellular adenosine. Neuromuscular junctions possess an ecto-AMP deaminase that can dissociate extracellular ATP catabolism from adenosine formation. In this study we have investigated the pattern of ATP release and its conversion into adenosine, to probe the role of ecto-AMP deaminase in controlling acetylcholine release from rat phrenic nerve terminals. Nerve-evoked ATP release was 28 +/- 12 pmol (mg tissue)-1 at 1 Hz, 54 +/- 3 pmol (mg tissue)-1 at 5 Hz and disproportionally higher at 50 Hz (324 +/- 23 pmol (mg tissue)-1). Extracellular ATP (30 microM) was metabolised with a half time of 8 +/- 2 min, being converted into ADP then into AMP. AMP was either dephosphorylated into adenosine by ecto-5'-nucleotidase (inhibited by ATP and blocked by 200 microM alpha,beta-methylene ADP) or deaminated into IMP by ecto-AMP deaminase (inhibited by 200 microM deoxycoformycin, which increased adenosine formation). Dephosphorylation and deamination pathways also catabolised endogenously released adenine nucleotides, since the nerve-evoked extracellular AMP accumulation was increased by either alpha,beta-methylene ADP (200 microM) or deoxycoformycin (200 microM). In the presence of nitrobenzylthioinosine (30 microM) to inhibit adenosine transport, deoxycoformycin (200 microM) facilitated nerve-evoked [3H]acetylcholine release by 77 +/- 9 %, an effect prevented by the A2A receptor antagonist, ZM 241385 (10 nM). It is concluded that, while ecto-5'-nucleotidase is inhibited by released ATP, ecto-AMP deaminase activity transiently blunts adenosine formation, which would otherwise reach levels high enough to activate facilitatory A2A receptors on motor nerve terminals.

Lesions of rat skeletal muscle after local block of acetylcholinesterase and neuromuscular stimulation

In skeletal muscle, a local increase of acetylcholine (ACh) in a few end plates has been hypothesized to cause the formation of contraction knots that can be found in myofascial trigger points. To test this hypothesis in rats, small amounts of an acetylcholinesterase inhibitor [diisopropylfluorophosphate (DFP)] were injected into the proximal half of the gastrocnemius muscle, and the muscle nerve was electrically stimulated for 30-60 min for induction of muscle twitches. The distal half of the muscle, which performed the same contractions, served as a control to assess the effects of the twitches without DFP. Sections of the muscle were evaluated for morphological changes in relation to the location of blocked end plates. Compared with the distal half of the muscle, the DFP-injected proximal half exhibited significantly higher numbers of abnormally contracted fibers (local contractures), torn fibers, and longitudinal stripes. DFP-injected animals in which the muscle nerve was not stimulated and that were allowed to survive for 24 h exhibited the same lesions but in smaller numbers. The data indicate that an increased concentration of ACh in a few end plates causes damage to muscle fibers. The results support the assumption that a dysfunctional end plate exhibiting increased release of ACh may be the starting point for regional abnormal contractions, which are thought to be essential for the formation of myofascial trigger points.

Glutamate regulation of non-quantal release of acetylcholine in the rat neuromuscular junction

Glutamate, previously demonstrated to participate in regulation of the resting membrane potential in skeletal muscles, also regulates non-quantal acetylcholine (ACh) secretion from rat motor nerve endings. Non-quantal ACh secretion was estimated by the amplitude of endplate hyperpolarization (H-effect) following blockade of skeletal muscle post-synaptic nicotinic receptors by (+)-tubocurarine and cholinesterase by armin (diethoxy-p-nitrophenyl phosphate). Glutamate was shown to inhibit non-quantal release but not spontaneous and evoked quantal secretion of ACh. Glutamate-induced decrease of the H-effect was enhanced by glycine. Glycine alone also lowered the H-effect, probably due to potentiation of the effect of endogenous glutamate present in the synaptic cleft. Inhibition of N-methyl-d-aspartate (NMDA) receptors with (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine (MK801), dl-2-amino-5-phosphopentanoic acid (AP5) and 7-chlorokynurenic acid or the elimination of Ca2+ from the bathing solution prevented the glutamate-induced decrease of the H-effect with or without glycine. Inhibition of muscle nitric oxide synthase by NG-nitro-l-arginine methyl ester (l-NAME), soluble guanylyl cyclase by 1H[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and binding and inactivation of extracellular nitric oxide (NO) by haemoglobin removed the action of glutamate and glycine on the H-effect. The results suggest that glutamate, acting on post-synaptic NMDA receptors to induce sarcoplasmic synthesis and release of NO, selectively inhibits non-quantal secretion of ACh from motor nerve terminals. Non-quantal ACh is known to modulate the resting membrane potential of muscle membrane via control of activity of chloride transport and a decrease in secretion of non-quantal transmitter following muscle denervation triggers the early post-denervation depolarization of muscle fibres.

Early postdenervation depolarization is controlled by acetylcholine and glutamate via nitric oxide regulation of the chloride transporter

Resting non-quantal acetylcholine (ACh) and probably glutamate (Glu) release from nerve endings activates M1- and NMDA receptor-mediated Ca2+ entry into the sarcoplasm with following activation of NOS and production of NO. This is a trophic message from motoneurons, which keeps the Cl- transport inactive in the innervated sarcolemma. After denervation, the secretion of ACh and Glu at the neuromuscular junction is eliminated within 3-4 h and the production of NO in the sarcoplasm is lowered. As a result, the Cl- influx is probably activated by dephosphorylation of the Cl- transporter with subsequent elevation of intracellular Cl- concentration. The equilibrium Cl- potential becomes more positive and the muscle membrane becomes depolarized.

Modulatory role of adenosine receptors in insect motor nerve terminals

The effects of adenosine and ATP were studied on blowfly larvae Calliphora vicina neuromuscular preparation. Adenosine diminished (IC50 = 40 +/- 3 microM) the amplitude of nerve-evoked postsynaptic currents (EPSCs) and slightly decreased the frequency of spontaneous currents without affecting their amplitude. EPSCs were slightly reduced by ATP, and this effect was prevented by concanavalin A. Presynaptic inhibition by adenosine was temperature-dependent and insensitive to pertussis toxin. A1 agonists of vertebrate adenosine receptor CPA and NECA failed to reproduce the effect of adenosine, and 2-CADO enhanced the EPSCs. A1 antagonist DPCPX competitively inhibited adenosine action. A2 agonist DPMA potentiated EPSCs, and its effect was abolished by A2 antagonist DMPX. Adenosine and ATP failed to affect the nonquantal release of glutamate. The results show for the first time the presence of presynaptic adenosine receptors regulating transmitter release at insect motor nerve terminals and point to differences in pharmacological properties of adenosine receptor subtypes in insects and vertebrates.