Abstract
In the last few years, we have studied the presence and involvement in synaptogenesis and mature transmitter release of the adenosine autoreceptors (AR) in the mammalian neuromuscular junction (NMJ). Here, we review and bring together the previously published data to emphasize the relevance of these receptors for developmental axonal competition, synaptic loss and mature NMJ functional modulation. However, in addition to AR, activity-dependent mediators originating from any of the three cells that make the synapse (nerve, muscle, and glial cells) cross the extracellular cleft to generate signals in target metabotropic receptors. Thus, the integrated interpretation of the complementary function of all these receptors is needed. We previously studied, in the NMJ, the links of AR with mAChR and the neurotrophin receptor TrkB in the control of synapse elimination and transmitter release. We conclude that AR cooperate with these receptors through synergistic and antagonistic effects in the developmental synapse elimination process. In the adult NMJ, this cooperation is manifested so as that the functional integrity of a given receptor group depends on the other receptors operating normally (i.e., the functional integrity of mAChR depends on AR operating normally). These observations underlie the relevance of AR in the NMJ function.
Introduction
In addition to the main neurotransmitter-receptor signal, several signaling pathways coordinate the pre- and postsynaptic cells and associated glia in the tripartite synapses in accordance with functional demands. In the NMJ, presynaptic mAChRs directly couple ACh secretion to the regulation of the release mechanism itself (; ; ; , ; ). Moreover, presynaptic nicotinic ACh autoreceptors (nAChRs) are also present at the NMJ (; ). Also, at the NMJ, the presynaptic neurotrophin and cytokine receptors can be influenced by target-derived signals (; ; ), and glutamate together with mGluR1 also mediate the signaling in this synapse (; ; ; ; ).
Studies in the early 1970s showed that adenosine and ATP modulate the presynaptic component through purinergic receptors (adenosine P1Rs and ATP P2Rs) (; ). The first authors describing adenosine effects at the mammalian NMJ were (, ; see also ). It is known that both nerve and muscle cell activity can contribute to the extracellular adenosine release (; ).
In the last few years, we have studied the presence and involvement of AR in the synaptogenesis and transmitter release in the developing and mature mammalian NMJ (; ; ; ; ,, ). Here, we review and bring together previously published data to contribute to emphasizing the relevance of these receptors in this synapse. Moreover, we also discuss our previous studies in relation with the interaction between AR, mAChR and the TrkB in the control of synapse elimination during development and transmitter release in the adult NMJ.
Our results indicate that during NMJ synaptogenesis, AR (A1R and A2AR subtypes) contribute to the developmental synapse elimination process, helping to define the winner of the competition between axon terminals. In the adult, AR help to modulate transmitter release by limiting spontaneous quantal leak of ACh and preserve synaptic function by reducing depression during repetitive activity. To realize these functions, several synergistic and antagonistic relations exist between AR and, at least, the mAChR subtypes (M1, M2, and M4) and the TrkB receptor. These observations underlie the relevance of AR in the NMJ function.
Adenosine Receptors Localization in the Nmj
Four subtypes of AR have been cloned (A1R, A2AR, A2BR, and A3R) and despite the fact that there was some uncertainty about how they were distributed in the cells of the paradigmatic NMJ (Lynge and Hellsten, 2000; Baxter et al., 2005), some of them have been localized in the mouse synapse with confocal immunohistochemistry (, , ; ). The confidence of protein localization by immunohistochemistry lies in the specificity of the antibodies used. Though the lack of signal in knockout animals was not investigated, we made an effort to characterize the commercially available antibodies by Western blotting. Our results have shown that the A1R receptor is more abundant in adult animals, whereas the A2AR receptor predominates in the newborn ones (). Moreover, the A2BR and A3R receptors are more expressed in the adult muscles than in the younger ones ().
Immunofluorescence staining coupled with confocal microscopy analysis was performed to determine the localization of A1R, A2AR (whose function has been further investigated) in P6 and P30 NMJ (Figure 1). Moreover, localization of A2BR, and A3R has been determined in P30 NMJ. By triple labeling of NMJ we stained each one of these protein receptors in green fluorescence together with Syntaxin or S-100 (blue fluorescence) and nAChR (red fluorescence) and we saw that the molecules were present. However, since the components of the NMJ (the nerve terminal, the Schwann cell and the muscle fiber) are juxtaposed, it is not always easy to locate the proteins with precision. Thus, in some cases, to better determine A1R and A2AR localization, we used plastic embedded semithin cross-sections as a tool for high-resolution together with the triple-labeling immunofluorescence analysis. Briefly, after conventional immunohistochemistry the samples were dehydrated with increasing concentrations of ethanol and acetone, the tissue fragments were embedded in Spurr’s resin in transverse orientation and sections (0.5–0.7 μm thick) were obtained (; ; ). Images (A–D) correspond to a single image made in a confocal microscope from a 0,5 μm muscle semithin cross section. Images (E–F) show localization of A2BR and A3R using single confocal Z planes from a projection image of at least 10 confocal Z planes obtained every 0,5 μm. As shown in Figure 1, these receptors are differentially located in the three cells that configure the NMJs (, ). To the purpose of the present review, it can be noted that the four subtypes of AR are present in the motor endings, which is understood to be a requisite to modulate presynaptic function. However, some AR are localized also in the other synaptic components (for instance A3R in the postsynaptic site) and the interpretation of the results here is not a full representation of the AR physiology.
FIGURE 1
Adenosine Receptors Role During Development
During the nervous system development there is an overwhelming production of synapses (that may promote connectiviy), followed by an activity-dependent reduction of them. Hebbian competition between axons refines connectivity and increases specificity (
The postsynaptic cell and the terminal Schwann cells may be intermediary in axonal competition. A decrement in polyneuronal innervation occurs at a time when relatively little loss of the nAChR postsynaptic receptors was observed (
At the time this process occurs, several signaling mechanisms coordinate the pre- and postsynaptic cell function. First of all, presynaptic mAChR receptors allow direct competitive interaction between nerve endings because of their different activity-dependent ACh secretion (
We evaluated the average number of axonal connections per NMJ from B6.Cg-Tg (Thy1-YFP) (from now YFP) and C57BL/J6 mice. To the flat and accessible mouse LAL muscle, we subcutaneously applied the unselective AR inhibitor 8-(p-sulfophenyl) theophylline (8SPT) and the agonist adenosine between P5-P15 (Figure 2). At P7, after the inhibition with 8SPT of the AR, we observed an acceleration of the axonal elimination on the NMJ indicating that, at this point of development, the role of AR is to delay axonal loss (
FIGURE 2

Involvement of AR in axonal elimination and in the morphological maturation of the postsynaptic apparatus. The (A) shows the percentage of the singly-, dually-, and triply- (or more) innervated NMJs in the YFP control mice exposed to PBS, and after 2 (P7 in a), 4 (P9, in b) and in some cases 10 (P15, in c) applications (one application every day after P5) of the AR pan-inhibitor 8SPT and the AR agonist adenosine (ADO). We also studied axonal elimination after selectively blocking A1R with the antagonist DPCPX and inhibiting A2AR with SCH-58261. The (B) shows the percentage of the MS1–MS4 maturation stages in the NMJ of the untreated YFP control mice (exposed to PBS), and after 2 (P7 in a), 4 (P9, in b), and in some cases 10 (P15, in c) applications of the 8SPT and ADO. Fisher’s test: ∗p < 0.05, ∗∗p < 0.01, ∗∗∗p < 0.005. This Figure has been adapted and redraw from the Figures 4, 8 in the original article “[Presynaptic muscarinic acetylcholine autoreceptors (M1, M2, and M4 subtypes), adenosine receptors (A1 and A2A) and tropomyosin-related kinase B receptor (TrkB) modulate the developmental synapse elimination process at the neuromuscular junction]” by [Nadal, L., N. Garcia, E. Hurtado, A. Simó, M. Tomàs, M. A. Lanuza, M. Santafé, and J. Tomàs].” Mol. Brain. 2016, 9: 67 (doi: 10.1186/s13041-016-0248-9). The original article is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Axonal elimination is accompanied by changes in the structure of the nicotinic ACh receptor (nAChR) clusters in the postsynaptic site (Figure 2B). Based on criteria from previous studies on developing mammalian NMJ (
In summary, AR are involved in the control of the competitive interactions between nerve endings, possibly helping to determine the winner or the losers but, thereafter, axon loss seems to occur with autonomy.
Adenosine Receptors Role in the Adult Neurotransmission
Once a NMJ becomes mature and monoinnervated, AR continue modulating neurotransmission. By measuring the activity-dependent efflux of radiolabelled ACh incorporated in nerve endings,
Because of these uncertainties, we induced muscle paralysis with μ-CgTx-GIIIB (
In summary, these findings confirmed that in basal conditions AR are not coupled to any immediate modulation of evoked neurotransmission. However, these receptors are still crucial to preserve resources by avoiding the leak of spontaneous quantal ACh, an action which is probably implicated in their protective role against synaptic depression after repetitive activity (
FIGURE 3

Diagrams showing an overall representation of the data. Between P6-P9 several nerve endings are eliminated and retract whereas one nerve terminal is stabilized. Green arrows indicate stimulation or promotion; red lines indicate inhibition. During the 1st week postnatal an A1R- and A2AR-mediated tonic acceleration of axonal retraction is observed (green arrows in the eliminated axon terminal). In addition to the role of AR, at least mAChR M1-, M2-, and M4-subtypes and the TrkB receptor are also involved. The downstream pathways integrate the signals related with the competitive interactions and accelerate axonal loss. In the adult NMJ, most experiments on transmitter release have been done in recording conditions that interfere with the synapse function to prevent muscle contraction. In these conditions A1R reduces it whereas A2AR increases it. Discontinuous lines in the mature axon terminal indicate these changes in conditions of safety factor reduction. In experiments with μ-CgTx-GIIIB only the voltage-dependent sodium channel of the muscle cells was shut-down, thus resulting in non-contractile muscles which have well preserved NMJ physiology and safety factor. In these conditions AR are not coupled to any immediate modulation of evoked neurotransmission. However, AR can restrict spontaneous quantal leak of ACh (A1R) and protect synaptic function by reducing the magnitude of depression during repetitive activity (an A1R- and A2AR-mediated effect).
Links of Ar With Other Metabotropic Receptors (Development and Adulthood)
Activity-dependent mediators derived from the three cells of the synapse cross the extracellular cleft in all directions to generate signals in target metabotropic receptors. In the NMJ, there are other purinergic receptors apart from AR (
In relation with the synapse elimination process during development, in addition to the role of AR subtypes (A1R and A2AR) described above, we investigated the involvement of individual mAChR M1-, M2-, and M4-subtypes and the TrkB receptor (
To study the collaboration of the AR with mAChR and TrkB, we applied two selective antagonists from two different receptors to see the additive or occlusive effects between them at P9 (
Thus, both AR are necessary to modulate synapse elimination and several synergistic and antagonistic links are observed between all receptors, which regulate axonal loss (Figure 4A). A1, M1, and TrkB are coupled to stimulate PKC whereas A2A, M2, and M4 inhibit PKA. We hypothesize that a membrane receptor-induced shifting in the protein kinases A and C activity in some nerve endings may play an important role in promoting developmental NMJ maturation.
FIGURE 4

Diagrams showing relations of AR with mAChR and TrkB receptors. In (A), synergistic and antagonistic roles of the AR, mAChR and TrkB receptor during development. AR are necessary to modulate axonal loss and several synergistic (green arrows) and antagonistic (red arrows) relations are clearly observed between all receptors, which affect synapse elimination. In (B), the diagram indicates that the functional integrity of the AR favors the normal operation of the mAChR and the TrkB pathways in the control of the neurotransmitter release in the adulthood.
In the adult NMJ, it is also known that A1R and A2AR as well as the M1-, M2-, and M4-subtypes of mAChR and the TrkB receptor play several roles in the regulation of transmitter release (
We also studied the consecutive incubation with two inhibitors affecting two different receptors to see their collaboration in neurotransmission. Adenosine outflow from nerve endings may, through A1R, reduce M2 effect on ACh release and adenylyl cyclase may be the shared intracellular node between both pathways (
Finally, it has also been reported that adenosine acting through A2A receptors is able to transactivate the TrkB receptor without the need of neurotrophin binding (
Concluding Remarks
Autoreceptors subtypes are present in the motor nerve endings in the NMJ. During development, AR modulate the competition between axon terminals, helping to define the winner and the losers (
Statements
Ethics statement
The mice were cared for in accordance with the guidelines of the European Community’s Council Directive of November 24, 1986 (86/609/EEC) for the humane treatment of laboratory animals. All experiments on animals have been reviewed and approved by the Animal Research Committee of the Universitat Rovira i Virgili (Reference number: 0233).
Author contributions
LN, EH, AS-O, VC-M, LJ-B, MS, and MT: data collection, quantitative analysis, literature search, data interpretation, and graphic design. NG and ML: statistics. JT, NG, and ML: conception and design, literature search, data interpretation, and manuscript preparation.
Funding
This work was supported by a grant from the Catalan Government (2014SGR344 and 2017SGR704) and a grant from MINECO (SAF2015-67143-P).
Acknowledgments
We thank the reviewers for their careful reading of our manuscript and their many insightful comments.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer CS and handling Editor declared their shared affiliation.
Abbreviations
- ACh
acetylcholine
- AR
adenosine autoreceptors
- ATP
adenosine triphosphate
- EPP
evoked endplate potentials
- LAL
Levator auris longus muscle
- mAChR
muscarinic acetylcholine receptor
- M1
M1-type muscarinic acetylcholine receptor
- M2
M2-type muscarinic acetylcholine receptor
- M4
M4-type muscarinic acetylcholine receptor
- NMJ
neuromuscular junction
- TrkB
tropomyosin-related kinase B receptor
- TrkB-Fc
inhibitor recombinant human TrkB-Fc Chimera
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Summary
Keywords
motor end-plate, postnatal synapse elimination, axonal competition, acetylcholine release, muscarinic acetylcholine receptors, adenosine receptors, neurotrophins, TrkB
Citation
Tomàs J, Garcia N, Lanuza MA, Santafé MM, Tomàs M, Nadal L, Hurtado E, Simó-Ollé A, Cilleros-Mañé V and Just-Borràs L (2018) Adenosine Receptors in Developing and Adult Mouse Neuromuscular Junctions and Functional Links With Other Metabotropic Receptor Pathways. Front. Pharmacol. 9:397. doi: 10.3389/fphar.2018.00397
Received
06 November 2017
Accepted
05 April 2018
Published
24 April 2018
Volume
9 - 2018
Edited by
Francisco Ciruela, Universitat de Barcelona, Spain
Reviewed by
Carles Solsona, Universitat de Barcelona, Spain; Susan B. Udin, University at Buffalo, United States
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© 2018 Tomàs, Garcia, Lanuza, Santafé, Tomàs, Nadal, Hurtado, Simó-Ollé, Cilleros-Mañé and Just-Borràs.
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*Correspondence: Josep Tomàs, josepmaria.tomas@urv.cat Neus Garcia, mariadelesneus.garcia@urv.cat Maria A. Lanuza, mariaangel.lanuza@urv.cat
†These authors have contributed equally to this work.
This article was submitted to Experimental Pharmacology and Drug Discovery, a section of the journal Frontiers in Pharmacology
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