Evaluation of intrinsic modulation of synaptic transmission by ATP in mouse fast twitch muscle

Modulation of dopamine-mediated facilitation at the neuromuscular junction of Wistar rats: A role for adenosine A1/A2A receptors and P2 purinoceptors

This study aims to understand how dopamine and the neuromodulators, adenosine and adenosine triphosphate (ATP) modulate neuromuscular transmission. Adenosine and ATP are well-recognized for their regulatory effects on dopamine in the central nervous system. However, if similar interactions occur at the neuromuscular junction is unknown. We hypothesize that the activation of adenosine A1/A2A and/or P2 purinoceptors may influence the action of dopamine on neuromuscular transmission. Using the rat phrenic nerve hemi-diaphragm, we assessed the influence of dopamine, adenosine and ATP on the height of nerve-evoked muscle twitches. We investigated how the selective blockade of adenosine A1 receptors (2.5nM DPCPX), adenosine A2A receptors (50nM CSC) and P2 purinoceptors (100μM suramin) modified the effects of dopamine. Dopamine alone increased indirect muscle contractions while adenosine and ATP either enhanced or depressed nerve-evoked muscle twitches in a concentration-dependent manner. The facilitatory effects of 256μM dopamine were significantly reduced to 29.62±2.79% or 53.69±5.45% in the presence of DPCPX or CSC, respectively, relative to 70.03±1.57% with dopamine alone. Alternatively, the action of 256μM dopamine was potentiated from 70.03±1.57, in the absence of suramin, to 86.83±4.36%, in the presence of suramin. It can be concluded that the activation of adenosine A1 and A2A receptors and P2 purinoceptors potentially play a central role in the regulation of dopamine effects at the neuromuscular junction. Clinically this study offers new insights for the indirect manipulation of neuromuscular transmission for the treatment of disorders characterized by motor dysfunction.

Presynaptic adenosine and P2Y receptors

Adenine-based purines, such as adenosine and ATP, are ubiquitous molecules that, in addition to their roles in metabolism, act as modulators of neurotransmitter release through activation of presynaptic P1 purinoceptors or adenosine receptors (activated by adenosine) and P2 receptors (activated by nucleotides). Of the latter, the P2Y receptors are G protein-coupled, whereas the P2X receptors are ligand-gated ion channels and not covered in this review.

P2 receptor-mediated modulation of neurotransmitter release-an update

Presynaptic nerve terminals are equipped with a number of presynaptic auto- and heteroreceptors, including ionotropic P2X and metabotropic P2Y receptors. P2 receptors serve as modulation sites of transmitter release by ATP and other nucleotides released by neuronal activity and pathological signals. A wide variety of P2X and P2Y receptors expressed at pre- and postsynaptic sites as well as in glial cells are involved directly or indirectly in the modulation of neurotransmitter release. Nucleotides are released from synaptic and nonsynaptic sites throughout the nervous system and might reach concentrations high enough to activate these receptors. By providing a fine-tuning mechanism these receptors also offer attractive sites for pharmacotherapy in nervous system diseases. Here we review the rapidly emerging data on the modulation of transmitter release by facilitatory and inhibitory P2 receptors and the receptor subtypes involved in these interactions.

Extracellular ATP in activity-dependent remodeling of the neuromuscular junction

Electrical activity during early development affects the development and maintenance of synapses (Spitzer [2006]: Nature 4447:707-712), but the intercellular signals regulating maintenance of synapses are not well identified. At the neuromuscular junction, adenosine 5-triphosphate (ATP) is coreleased with acetylcholine at activated nerve terminals to modulate synaptic function. Here we use cocultured mouse motor neurons and muscle cells in a three-compartment cell culture chamber to test whether endogenously released ATP plays a role in activity-dependent maintenance of neuromuscular synapses. The results suggest that ATP release at the synapse counters the negative effect of electrical activity, thus stabilizing activated synapses. Confirming our previous work (Li et al. [2001]: Nat Neurosci 4:871-872), we found that in doubly innervated muscles, electrical stimulation induced heterosynaptic downregulation of the nonstimulated convergent input to the muscle fiber with no or little change of the stimulated inputs. However, in preparations that were stimulated in the presence of apyrase, an enzyme that degrades extracellular ATP, synapse downregulation of stimulated inputs was substantial and significant, and end plate potentials were reduced. Apyrase treatment for 20 h in the absence of stimulation did result in moderate diminution, but this was prevented by blocking spontaneous neural activity with tetrodotoxin. The P2 receptor blocker, suramin, also induced activity-dependent synapse diminution. The decrease in synaptic efficacy produced by prolonged stimulation in the presence of apyrase persisted for greater than 20 h, consistent with a developmental time-course and distinct from the rapid neuromodulatory actions of ATP that have been demonstrated by others. We conclude that extracellular ATP promotes stabilization of the neuromuscular junction and may play a role in activity-dependent synaptic modification during development.

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.

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.

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.

The T-tubule membrane ATP-operated P2X4 receptor influences contractility of skeletal muscle

Evidence indicates that extracellular ATP may have relevant functions in skeletal muscle, even though the physiological role and distribution of specific signaling pathway elements are not well known. The present work shows that P2X4 receptor, an extracellular ATP-regulated cell membrane channel permeable to Ca2+, is expressed in several tissues of the rat, including skeletal muscle. A specific antibody detected a protein band of approximately 60 kDa. Immunofluorescence demonstrated that P2X4 has an intracellular localization, and confocal analysis revealed that the receptor colocalizes with the T-tubule membrane DHP receptor. Considering that the natural agonist of P2X4 is ATP, we explored if changes of extracellular ATP levels could occur in contracting skeletal muscle to regulate the channel. In vitro experiments showed that substantial ATP is released and rapidly hydrolyzed after electrical stimulation of rat muscle fibers. Results show that the presence of ATP-degrading enzymes (hexokinase/apyrase), inhibitors of P2X receptors or Ca2+-free conditions, all abolished the progressive twitch tension potentiation produced in soleus muscle by low-frequency (0.05 Hz) stimulation. These data reveal that ATP-mediated Ca2+ entry, most likely through P2X4 receptor, may play an important role in modulating the contractility of skeletal muscle.

Characterization of specific GTP binding sites in C2C12 mouse skeletal muscle cells

Receptor sites, specific for guanosine 5'-triphosphate (GTP) were characterised in myoblasts and myotubes of C2C12 mouse skeletal muscle cells, using binding experiments and measurements of intracellular Ca2+ concentration ([Ca2+]i). We identified two GTP binding sites in myoblasts membranes: a high affinity site (Kd = 15.4 +/- 4.6 microM; Bmax = 1.7 +/- 0.5 nmol mg(-1) protein); and a low affinity site (Kd = 170 +/- 94.5 microM; Bmax = 14.2 +/- 3.9 nmol mg(-1) protein). In myotube membranes only a low affinity binding site for GTP (Kd = 169 +/- 39 microM; Bmax = 12.3 +/- 1.4 nmol mg(-1) protein) was detected. In myoblasts GTP binding was not displaced by ATP or UTP, even at high concentrations (up to of 1 mM), but it was affected by treatments with suramin or Reactive Blue 2 (RB2), the non-selective purine receptor antagonists. In contrast, in myotubes GTP binding was partially displaced by high concentrations of ATP, but treatments with the non-selective purine receptor antagonists, suramin or RB2, and with UTP had no effect on GTP binding. The addition of GTP to myoblasts, and to myotubes, resulted in elevations of [Ca2+]i. The patterns of Ca2+ response however, were different in the two cell phenotypes. In myoblasts the addition of GTP induced two types of Ca2+ responses: (1) a fast increase in [Ca2+]i, followed by a sustained [Ca2+]i elevation, and (2) a slow raising and steady prolonged increase in [Ca2+]i. In myotubes, however only fast Ca2+ responses were observed following the addition of 500 microM GTP. In the myoblasts and myotubes GTP-stimulated [Ca2+]i increases were abolished by treatments with suramin or RB2 at concentrations which had no effect on the ATP-induced Ca2+ responses. We conclude, that C2C12 cells express two distinct binding sites for GTP before differentiation, but only one after, the low affinity binding site. These results suggest a possible role of the high affinity GTP binding site in early stage of development of skeletal muscle.

Safety factor at the neuromuscular junction

Reliable transmission of activity from nerve to muscle is necessary for the normal function of the body. The term 'safety factor' refers to the ability of neuromuscular transmission to remain effective under various physiological conditions and stresses. This is a result of the amount of transmitter released per nerve impulse being greater than that required to trigger an action potential in the muscle fibre. The safety factor is a measure of this excess of released transmitter. In this review we discuss the practical difficulties involved in estimating the safety factor in vitro. We then consider the factors that influence the safety factor in vivo. While presynaptic transmitter release may be modulated on a moment to moment basis, the postsynaptic features that determine the effect of released transmitter are not so readily altered to meet changing demands. Different strategies are used by different species to ensure reliable neuromuscular transmission. Some, like frogs, rely on releasing a large amount of transmitter while others, like man, rely on elaborate postsynaptic specialisations to enhance the response to transmitter. In normal adult mammals, the safety factor is generally 3-5. Both pre- and postsynaptic components change during development and may show plasticity in response to injury or disease. Thus, both acquired autoimmune and inherited congenital diseases of the neuromuscular junction (NMJ) can significantly reduce, or even transiently increase, safety factor.

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

The postsynaptic membrane of the neuromuscular synapse treated with antiacetylcholinesterase is depolarized due to nonquantal release of acetylcholine (ACh) from the motor nerve ending. This can be demonstrated by the hyperpolarization produced by the application of curare (H-effect). ATP (1 x 10-5 M) decreased the magnitude of the H-effect from 5 to 1.5 mV. The membrane input resistance and the ACh sensitivity were unchanged, and so changes in these cannot explain the ATP effect. Adenosine alone was without effect on the nonquantal release. On the other hand, both ATP and adenosine depressed the frequency of spontaneous miniature endplate potentials, to 56% and 43% respectively. The protein kinase A inhibitor Rp-cAMP or the guanylyl cyclase inhibitor 1H-[1,2,4]oxidiazolo[4,3-a]quinoxalin-1-one did not affect the inhibitory influence of ATP on the H-effect, whereas staurosporine, an inhibitor of protein kinase C, completely abolished the action of ATP. Suramin, an ATP antagonist, enhanced the H-effect to 8.6 mV and, like staurosporine, prevented the inhibitory effect of ATP. ATP thus suppresses the nonquantal release via a direct action on presynaptic metabotropic P2 receptors coupled to protein kinase C, whilst adenosine exerts its action mainly by affecting the mechanisms underlying quantal release. These data, together with earlier evidence, show that nonquantal release of ACh can be modulated by several distinct regulatory pathways, in particular by endogenous substances which may or may not be present in the synaptic cleft at rest or during activity.