Adenosine receptors and muscarinic receptors cooperate in acetylcholine release modulation in the neuromuscular synapse

Stress increases the spontaneous release of ACh and may be involved in the generation and maintenance of myofascial trigger points in mouse

An increase in spontaneous neurotransmission may be related to myofascial pain. Sympathetic neurons innervate most of the neuromuscular junction sand are involved in the modulation of synaptic transmission. Therefore, a direct action of stress on acetylcholine release is expected. For this reason, this study aims to evaluate the relationship between stress and spontaneous neurotransmission. Five acute stressors (immobilization, forced swimming, food and water deprivation, social isolation and ultrasound) were tested in 6 weeks adult Swiss male mice. Subsequently, these types of stress were combined to generate a model of chronic stress. The study of ACh release was evaluated before and after the application of stress by intracellular recording of spontaneous neurotransmission (mEPPs). In each one of the stressors, an increase in the frequency of mEPPs was obtained immediately after treatment, which remained elevated for 5 days and thereafter returned to control values after a week. With chronic stress, a much higher increase in the frequency of mEPPs was obtained and it was maintained for 15 days. In summary, stress, both in its acute and chronic forms, increased spontaneous neurotransmission significantly. There is a possibility that chronic stress is related with the genesis or maintenance of myofascial pain.

A New Unified Theory of Trigger Point Formation: Failure of Pre- and Post-Synaptic Feedback Control Mechanisms

The origin of the myofascial trigger point (TrP), an anomalous locus in muscle, has never been well-described. A new trigger point hypothesis (the new hypothesis) presented here addresses this lack. The new hypothesis is based on the concept that existing myoprotective feedback mechanisms that respond to muscle overactivity, low levels of adenosine triphosphate, (ATP) or a low pH, fail to protect muscle in certain circumstances, such as intense muscle activity, resulting in an abnormal accumulation of intracellular Ca²⁺, persistent actin-myosin cross bridging, and then activation of the nociceptive system, resulting in the formation of a trigger point. The relevant protective feedback mechanisms include pre- and postsynaptic sympathetic nervous system modulation, modulators of acetylcholine release at the neuromuscular junction, and mutations/variants or post-translational functional alterations in either of two ion channelopathies, the ryanodine receptor and the potassium-ATP ion channel, both of which exist in multiple mutation states that up- or downregulate ion channel function. The concepts that are central to the origin of at least some TrPs are the failure of protective feedback mechanisms and/or of certain ion channelopathies that are new concepts in relation to myofascial trigger points.

Muscarinic Receptors in Developmental Axonal Competition at the Neuromuscular Junction

In recent years, we have studied by immunohistochemistry, intracellular recording, and western blotting the role of the muscarinic acetylcholine receptors (mAChRs; M₁, M₂, and M₄ subtypes) in the mammalian neuromuscular junction (NMJ) during development and in the adult. Here, we evaluate our published data to emphasize the mAChRs' relevance in developmental synaptic elimination and their crosstalk with other metabotropic receptors, downstream kinases, and voltage-gated calcium channels (VGCCs). The presence of mAChRs in the presynaptic membrane of motor nerve terminals allows an autocrine mechanism in which the secreted acetylcholine influences the cell itself in feedback. mAChR subtypes are coupled to different downstream pathways, so their feedback can move in a broad range between positive and negative. Moreover, mAChRs allow direct activity-dependent interaction through ACh release between the multiple competing axons during development. Additional regulation from pre- and postsynaptic sites (including neurotrophic retrograde control), the agonistic and antagonistic contributions of adenosine receptors (AR; A₁ and A_2A), and the tropomyosin-related kinase B receptor (TrkB) cooperate with mAChRs in the axonal competitive interactions which lead to supernumerary synapse elimination that achieves the optimized monoinnervation of musculoskeletal cells. The metabotropic receptor-driven balance between downstream PKA and PKC activities, coupled to developmentally regulated VGCC, explains much of how nerve terminals with different activities finally progress to their withdrawal or strengthening.

GIRK channel as a versatile regulator of neurotransmitter release via L-type Ca2+ channel-dependent mechanism in the neuromuscular junction

G protein-gated inwardly rectifying potassium (GIRK) channels are one of the main regulators of neuronal excitability. Activation of GIRK channels in the CNS usually leads to postsynaptic inhibition. However, the function of GIRK channels in the presynaptic processes, notably neurotransmitter release form motor nerve terminals, is yet to be comprehensively understood. Here, using electrophysiological and fluorescent approaches, the role of GIRK channels in neurotransmitter release from frog motor nerve terminals was studied. We found that the inhibition of GIRK channels with nanomolar tertiapin-Q synchronized exocytosis events with action potential but suppressed spontaneous and evoked neurotransmitter release, as well as Ca²⁺ transient and membrane permeability for K⁺. The action of GIRK channel inhibition on evoked neurotransmission was prevented by selective antagonist of voltage-gated Ca²⁺ channels of L-type. Furthermore, the effects of muscarinic acetylcholine receptor activation on neurotransmitter release, Ca²⁺ transient and K⁺ channel activity were markedly modulated by inhibition of GIRK channels. Thus, at the motor nerve terminals GIRK channels can regulate timing of neurotransmitter release and be a positive modulator of synaptic vesicle exocytosis acting partially via L-type Ca²⁺ channels. In addition, GIRK channels are key players in a feedback control of neurotransmitter release by muscarinic acetylcholine receptors.

Acetylcholine exerts inhibitory and excitatory actions on mouse ileal pacemaker activity: role of muscarinic versus nicotinic receptors

The effect of acetylcholine (ACh) on pacemaking and spontaneous contractions in the gastrointestinal tract is not well characterized. The current study aims to profile the effect of several muscarinic and nicotinic receptor agonists and antagonists on pacemaker potentials in the ICR mouse ileum. Pacemaker potentials of whole thickness mouse ileal segments were recorded extracellularly using a 60-channel microelectrode array (MEA) platform. A spatiotemporal analysis integrated the frequency, amplitude, and velocity measurements of pacemaker currents. Comparative data were obtained by recording spontaneous smooth muscle tone in a conventional organ bath. On the MEA, ACh (0.3-300 μM) and bethanechol (0.3-300 μM) significantly reduced ileal pacemaker potentials. The inhibitory effect of ACh was mimicked by donepezil (300 μM) but not nicotine (0.3-7 mM). Atropine (300 μM), but not hexamethonium (300 μM), reversed the inhibitory actions of ACh and bethanechol and revealed excitatory properties manifested as increases in pacemaker frequency. A spatial analysis also revealed that atropine, but not hexamethonium, reversed the ACh-induced distortion of pacemaker propagation activity. Atropine (0.001-3 mM) and hexamethonium (0.3-7 mM) alone were inactive. In the organ bath, ACh (300 nM) and bethanechol (30 μM) induced ileal tonic contractions, while inhibiting basal spontaneous contractions at 300 μM. Atropine (1 μM), but not hexamethonium (1-300 μM), reversed both the tonic contractions and the inhibition of the spontaneous contractions of ACh and bethanechol and revealed an excitatory effect manifested as an increasing in the frequency of contractions. Muscarinic, but not nicotinic, receptors appear to mediate the inhibitory actions of ACh on mouse ileal pacemaker potentials.NEW & NOTEWORTHY The study discovered an acute action of acetylcholine on pacemaker potentials that is mediated by muscarinic receptors on the mouse ileum. Bethanechol, but not nicotine, mimicked the inhibitory actions of acetylcholine on pacemaker potentials. Atropine, but not hexamethonium, reversed the inhibitory actions of acetylcholine. When introduced after acetylcholine, atropine exhibited excitatory actions that increased the pacemaker frequency. Acetylcholine and bethanechol distorted the propagation activity and pattern, and this was also reversed by atropine. These actions of acetylcholine on pacemaker potentials may contribute to pathophysiology in bowel diseases.

Myasthenia Gravis: From the Viewpoint of Pathogenicity Focusing on Acetylcholine Receptor Clustering, Trans-Synaptic Homeostasis and Synaptic Stability

Myasthenia gravis (MG) is a disease of the postsynaptic neuromuscular junction (NMJ) where nicotinic acetylcholine (ACh) receptors (AChRs) are targeted by autoantibodies. Search for other pathogenic antigens has detected the antibodies against muscle-specific tyrosine kinase (MuSK) and low-density lipoprotein-related protein 4 (Lrp4), both causing pre- and post-synaptic impairments. Agrin is also suspected as a fourth pathogen. In a complex NMJ organization centering on MuSK: (1) the Wnt non-canonical pathway through the Wnt-Lrp4-MuSK cysteine-rich domain (CRD)-Dishevelled (Dvl, scaffold protein) signaling acts to form AChR prepatterning with axonal guidance; (2) the neural agrin-Lrp4-MuSK (Ig1/2 domains) signaling acts to form rapsyn-anchored AChR clusters at the innervated stage of muscle; (3) adaptor protein Dok-7 acts on MuSK activation for AChR clustering from “inside” and also on cytoskeleton to stabilize AChR clusters by the downstream effector Sorbs1/2; (4) the trans-synaptic retrograde signaling contributes to the presynaptic organization via: (i) Wnt-MuSK CRD-Dvl-β catenin-Slit 2 pathway; (ii) Lrp4; and (iii) laminins. The presynaptic Ca²⁺ homeostasis conditioning ACh release is modified by autoreceptors such as M1-type muscarinic AChR and A2A adenosine receptors. The post-synaptic structure is stabilized by: (i) laminin-network including the muscle-derived agrin; (ii) the extracellular matrix proteins (including collagen Q/perlecan and biglycan which link to MuSK Ig1 domain and CRD); and (iii) the dystrophin-associated glycoprotein complex. The study on MuSK ectodomains (Ig1/2 domains and CRD) recognized by antibodies suggested that the MuSK antibodies were pathologically heterogeneous due to their binding to multiple functional domains. Focussing one of the matrix proteins, biglycan which functions in the manner similar to collagen Q, our antibody assay showed the negative result in MG patients. However, the synaptic stability may be impaired by antibodies against MuSK ectodomains because of the linkage of biglycan with MuSK Ig1 domain and CRD. The pathogenic diversity of MG is discussed based on NMJ signaling molecules.

Nicotinic α7 receptor-induced adenosine release from perisynaptic Schwann cells controls acetylcholine spillover from motor endplates

Acetylcholine (ACh) spillover from motor endplates occurs after neuronal firing bursts being potentiated by cholinesterase inhibitors (e.g., neostigmine). Nicotinic α7 receptors (α7nAChR) on perisynaptic Schwann cells (PSCs) can control ACh spillover by unknown mechanisms. We hypothesized that adenosine might be the gliotransmitter underlying PSCs-nerve terminal communication. Rat isolated hemidiaphragm preparations were used to measure (1) the outflow of [³ H]ACh, (2) real-time transmitter exocytosis by video-microscopy with the FM4-64 fluorescent dye, and (3) skeletal muscle contractions during high-frequency (50 Hz) nerve stimulation bursts in the presence of a selective α7nAChR agonist, PNU 282987, or upon inhibition of cholinesterase activity with neostigmine. To confirm our prediction that α7nAChR-mediated effects require direct activation of PSCs, we used fluorescence video-microscopy in the real-time mode to measure PNU 282987-induced [Ca²⁺ ]_i transients from Fluo-4 NW loaded PSCs in non-stimulated preparations. The α7nAChR agonist, PNU 282987, decreased nerve-evoked diaphragm tetanic contractions. PNU 282987-induced inhibition was mimicked by neostigmine and results from the reduction of ACh exocytosis measured as decreases in [³ H]ACh release and FM4-64 fluorescent dye unloading. Methyllycaconitine blockage of α7nAChR and the fluoroacetate gliotoxin both prevented inhibition of nerve-evoked ACh release and PSCs [Ca²⁺ ]_i transients triggered by PNU 282987 and neostigmine. Adenosine deamination, inhibition of the ENT1 nucleoside outflow, and blockage of A₁ receptors prevented PNU 282987-induced inhibition of transmitter release. Data suggest that α7nAChR controls tetanic-induced ACh spillover from the neuromuscular synapse by promoting adenosine outflow from PSCs via ENT1 transporters and retrograde activation of presynaptic A₁ inhibitory receptors.

Opposed Actions of PKA Isozymes (RI and RII) and PKC Isoforms (cPKCβI and nPKCε) in Neuromuscular Developmental Synapse Elimination

Background: During neuromuscular junction (NMJ) development, synapses are produced in excess. By sensing the activity-dependent release of ACh, adenosine, and neurotrophins, presynaptic receptors prompt axonal competition and loss of the unnecessary axons. The receptor action is mediated by synergistic and antagonistic relations when they couple to downstream kinases (mainly protein kinases A and C (PKA and PKC)), which phosphorylate targets involved in axonal disconnection. Here, we directly investigated the involvement of PKA subunits and PKC isoforms in synapse elimination. Methods: Selective PKA and PKC peptide modulators were applied daily to the Levator auris longus (LAL) muscle surface of P5–P8 transgenic B6.Cg-Tg (Thy1-YFP) 16 Jrs/J (and also C57BL/6J) mice, and the number of axons and the postsynaptic receptor cluster morphology were evaluated in P9 NMJ. Results: PKA (PKA-I and PKA-II isozymes) acts at the pre- and postsynaptic sites to delay both axonal elimination and nAChR cluster differentiation, PKC activity promotes both axonal loss (a cPKCβI and nPKCε isoform action), and postsynaptic nAChR cluster maturation (a possible role for PKCθ). Moreover, PKC-induced changes in axon number indirectly influence postsynaptic maturation. Conclusions: PKC and PKA have opposed actions, which suggests that changes in the balance of these kinases may play a major role in the mechanism of developmental synapse elimination.

nPKCε Mediates SNAP-25 Phosphorylation of Ser-187 in Basal Conditions and After Synaptic Activity at the Neuromuscular Junction

Protein kinase C (PKC) and substrates like SNAP-25 regulate neurotransmission. At the neuromuscular junction (NMJ), PKC promotes neurotransmitter release during synaptic activity. Thirty minutes of muscle contraction enhances presynaptic PKC isoform levels, specifically cPKCβI and nPKCε, through retrograde BDNF/TrkB signaling. This establishes a larger pool of these PKC isoforms ready to promote neuromuscular transmission. The PKC phosphorylation site in SNAP-25 has been mapped to the serine 187 (Ser-187), which is known to enhance calcium-dependent neurotransmitter release in vitro. Here, we localize SNAP-25 at the NMJ and investigate whether cPKCβI and/or nPKCε regulate SNAP-25 phosphorylation. We also investigate whether nerve and muscle cell activities regulate differently SNAP-25 phosphorylation and the involvement of BDNF/TrkB signaling. Our results demonstrate that nPKCε isoform is essential to positively regulate SNAP-25 phosphorylation on Ser-187 and that muscle contraction prevents it. TrkB and cPKCβI do not regulate SNAP-25 protein level or its phosphorylation during neuromuscular activity. The results provide evidence that nerve terminals need both pre- and postsynaptic activities to modulate SNAP-25 phosphorylation and ensure an accurate neurotransmission process.

Mechanism of P2X7 receptor-dependent enhancement of neuromuscular transmission in pannexin 1 knockout mice

P2X7 receptors are present in presynaptic membranes of motor synapses, but their regulatory role in modulation of neurotransmitter release remains poorly understood. P2X7 receptors may interact with pannexin 1 channels to form a purinergic signaling unit. The potential mechanism of P2X7 receptor-dependent modulation of acetylcholine (ACh) release was investigated by recording miniature endplate potentials (MEPPs) and evoked endplate potentials (EPPs) in neuromuscular junctions of wild-type (WT) and pannexin 1 knockout (Panx1^-/-) mice. Modulation of P2X7 receptors with the selective inhibitor A740003 or the selective agonist BzATP did not alter the parameters of either spontaneous or evoked ACh release in WT mice. In Panx1^-/- mice, BzATP-induced activation of P2X7 receptors resulted in a uniformly increased quantal content of EPPs during a short stimulation train. This effect was accompanied by an increase in the size of the readily releasable pool, while the release probability did not change. Inhibition of calmodulin by W-7 or of calcium/calmodulin-dependent kinase II (CaMKII) by KN-93 completely prevented the potentiating effect of BzATP on the EPP quantal content. The blockade of L-type calcium channels also prevented BzATP action on evoked synaptic activity. Thus, the activation of presynaptic P2X7 receptors in mice lacking pannexin 1 resulted in enhanced evoked ACh release. Such enhanced release was provoked by triggering the calmodulin- and CaMKII-dependent signaling pathway, followed by activation of presynaptic L-type calcium channels. We suggest that in WT mice, this pathway is downregulated due to pannexin 1-dependent tonic activation of inhibitory presynaptic purinergic receptors, which overcomes P2X7-mediated effects.

Experimental myofascial trigger point creation in rodents

Myofascial pain syndrome is one of the most common forms of muscle pain. In this syndrome, pain is originated by the so-called trigger points, which consists of a set of palpable contraction knots in the muscle. It has been proposed that a high, spontaneous neurotransmission may be involved in the generation of these contraction knots. To confirm this hypothesis, we exposed mouse muscles to an anticholinesterasic agent to increase the neurotransmision in the synaptic cleft in two different conditions, in vivo and ex vivo experiments. Using intracellular recordings, a sharp increase in the spontaneous neurotransmission in the levator auris longus muscle and a lower increase in the diaphragm muscle could be seen. Likewise, electromyography recordings reveal an elevated endplate noise in gastrocnemius muscle of treated animals. These changes are associated with structural changes such as abundant neuromuscular contracted zones observed by rhodaminated α-bungarotoxin and the presence of abundant glycosaminoglycans around the contraction knots, as shown by Alcian PAS staining. In a second set of experiments, we aimed at demonstrating that the increases in the neurotransmission reproduced most of the clinical signs associated to a trigger point. We exposed rats to the anticholinesterase agent neostigmine, and 30 min afterward we observed the presence of palpable taut bands, the echocardiographic presence of contraction knots, and local twitch responses upon needle stimulation. In summary, we demonstrated that increased neurotransmission induced trigger points in both rats and mice, as evidenced by glycosaminoglycans around the contraction zones as a novel hallmark of this pathology. NEW & NOTEWORTHY In rodents, when neostigmine was injected subcutaneously, the neuromuscular neurotransmission increased, and several changes can be observed: an elevated endplate noise compared with normal endplate noise, as evidenced by electromyographyc recording; many muscular fibers with contraction knots (narrower sarcomeres and locally thickened muscle fiber) surrounded by infiltration of connective tissue like glycosaminoglycans molecules; and palpable taut bands and local twitch responses upon needle stimulation. Several of these signs are also observed in humans with muscle pain.

Effects of adenosine receptor agonist on the rocuroniuminduced neuromuscular block and sugammadex-induced recovery

Background

Several types of receptors are found at neuromuscular presynaptic membranes. Presynaptic inhibitory A₁ and facilitatory A_2A receptors mediate different modulatory functions on acetylcholine release. This study investigated whether adenosine A₁ receptor agonist contributes to the first twitch tension (T1) of train-of-four (TOF) stimulation depression and TOF fade during rocuronium-induced neuromuscular blockade, and sugammadex-induced recovery.

Methods

Phrenic nerve-diaphragm tissues were obtained from 30 adult Sprague-Dawley rats. Each tissue specimen was randomly allocated to either control group or 2-chloroadenosine (CADO, 10 μM) group. One hour of reaction time was allowed before initiating main experimental data collection. Loading and boost doses of rocuronium were sequentially administered until > 95% depression of the T1 was achieved. After confirming that there was no T1 twitch tension response, 15 min of resting time was allowed, after which sugammadex was administered. Recovery profiles (T1, TOF ratio [TOFR], and recovery index) were collected for 1 h and compared between groups.

Results

There were statistically significant differences on amount of rocuronium (actually used during experiment), TOFR changes during concentration-response of rocuronium (P = 0.04), and recovery profiles (P < 0.01) of CADO group comparing with the control group. However, at the initial phase of this experiment, dose-response of rocuronium in each group demonstrated no statistically significant differences (P = 0.12).

Conclusions

The adenosine A₁ receptor agonist (CADO) influenced the TOFR and the recovery profile. After activating adenosine receptor, sugammadex-induced recovery from rocuronium-induced neuromuscular block was delayed.

Adenosine Receptors in Developing and Adult Mouse Neuromuscular Junctions and Functional Links With Other Metabotropic Receptor Pathways

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.

Effects of presynaptic muscarinic cholinoreceptor blockade on neuromuscular transmission as assessed by the train-of-four and the tetanic fade response to rocuronium

This study investigated the effect of muscarinic M₁ and M₂ receptor antagonists on the rocuronium‐induced train of four (TOF) fade and tetanic fade, respectively. Ex‐vivo phrenic nerves and diaphragms were obtained from adult Sprague‐Dawley rats and stabilized in Krebs buffer; the nerve‐stimulated muscle TOF fade was observed at 20 s intervals. For the TOF study, phrenic nerves and diaphragms were incubated with pirenzepine (an M₁ blocker) at concentrations of 0 nmol L⁻¹ (control), 10 nmol L⁻¹ (PZP10), or 100 nmol L⁻¹ (PZP100). Rocuronium was then administered incrementally until the first twitch tension had depressed by >95% during TOF stimulation. The mean TOF ratios were compared when the first twitch tensions were depressed by 40%‐50%. For the tetanic fade study, 50 Hz/5 s tetani was applied initially, 30 min after the administration of a loading dose of rocuronium and methoctramine (an M₂ receptor blocker, loaded at 0 μmol L⁻¹ [control], 1 μmol L⁻¹ [MET1], or 10 μmol L⁻¹ [MET10]). The EC₉₅ of rocuronium was significantly lower in the PZP10 group than in the control group. In the PZP10 group, the TOF ratios at 50% and first twitch tension depression were significantly lower than those in the control group (P=.02). During tetanic stimulation, the tetanic fade was significantly enhanced in the MET10 group compared to the other groups. This study shows that antagonists of muscarinic M₁ and M₂ receptors affect the rocuronium‐induced neuromuscular block as demonstrated by the reduced EC₉₅ and TOF ratios (M₁ antagonist, pirenzepine) or the enhanced 50‐Hz tetanic fade (M₂ antagonist, methoctramine).

Presynaptic Membrane Receptors Modulate ACh Release, Axonal Competition and Synapse Elimination during Neuromuscular Junction Development

During the histogenesis of the nervous system a lush production of neurons, which establish an excessive number of synapses, is followed by a drop in both neurons and synaptic contacts as maturation proceeds. Hebbian competition between axons with different activities leads to the loss of roughly half of the neurons initially produced so connectivity is refined and specificity gained. The skeletal muscle fibers in the newborn neuromuscular junction (NMJ) are polyinnervated but by the end of the competition, 2 weeks later, the NMJ are innervated by only one axon. This peripheral synapse has long been used as a convenient model for synapse development. In the last few years, we have studied transmitter release and the local involvement of the presynaptic muscarinic acetylcholine autoreceptors (mAChR), adenosine autoreceptors (AR) and trophic factor receptors (TFR, for neurotrophins and trophic cytokines) during the development of NMJ and in the adult. This review article brings together previously published data and proposes a molecular background for developmental axonal competition and loss. At the end of the first week postnatal, these receptors modulate transmitter release in the various nerve terminals on polyinnervated NMJ and contribute to axonal competition and synapse elimination.

Presynaptic Muscarinic Acetylcholine Receptors and TrkB Receptor Cooperate in the Elimination of Redundant Motor Nerve Terminals during Development

The development of the nervous system involves the overproduction of synapses but connectivity is refined by Hebbian activity-dependent axonal competition. The newborn skeletal muscle fibers are polyinnervated but, at the end of the competition process, some days later, become innervated by a single axon. We used quantitative confocal imaging of the autofluorescent axons from transgenic B6.Cg-Tg (Thy1-YFP)16 Jrs/J mice to investigate the possible cooperation of the muscarinic autoreceptors (mAChR, M₁-, M₂- and M₄-subtypes) and the tyrosine kinase B (TrkB) receptor in the control of axonal elimination after the mice Levator auris longus (LAL) muscle had been exposed to several selective antagonist of the corresponding receptor pathways in vivo. Our previous results show that M₁, M₂ and TrkB signaling individually increase axonal loss rate around P9. Here we show that although the M₁ and TrkB receptors cooperate and add their respective individual effects to increase axonal elimination rate even more, the effect of the M₂ receptor is largely independent of both M₁ and TrkB receptors. Thus both, cooperative and non-cooperative signaling mechanisms contribute to developmental synapse elimination.

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

Background

The development of the nervous system involves an initially exuberant production of neurons that make an excessive number of synaptic contacts. The initial overproduction of synapses promotes connectivity. Hebbian competition between axons with different activities (the least active are punished) leads to the loss of roughly half of the overproduced elements and this refines connectivity and increases specificity. The neuromuscular junction is innervated by a single axon at the end of the synapse elimination process and, because of its relative simplicity, has long been used as a model for studying the general principles of synapse development. The involvement of the presynaptic muscarinic ACh autoreceptors may allow for the direct competitive interaction between nerve endings through differential activity-dependent acetylcholine release in the synaptic cleft. Then, the most active ending may directly punish the less active ones. Our previous results indicate the existence in the weakest axons on the polyinnervated neonatal NMJ of an ACh release inhibition mechanism based on mAChR coupled to protein kinase C and voltage-dependent calcium channels. We suggest that this mechanism plays a role in the elimination of redundant neonatal synapses.

Results

Here we used confocal microscopy and quantitative morphological analysis to count the number of brightly fluorescent axons per endplate in P7, P9 and P15 transgenic B6.Cg-Tg (Thy1-YFP)16 Jrs/J mice. We investigate the involvement of individual mAChR M₁-, M₂- and M₄-subtypes in the control of axonal elimination after the Levator auris longus muscle had been exposed to agonist and antagonist in vivo. We also analysed the role of adenosine receptor subtypes (A₁ and A_2A) and the tropomyosin-related kinase B receptor. The data show that postnatal axonal elimination is a regulated multireceptor mechanism that guaranteed the monoinnervation of the neuromuscular synapses.

Conclusion

The three receptor sets considered (mAChR, AR and TrkB receptors) intervene in modulating the conditions of the competition between nerve endings, possibly helping to determine the winner or the lossers but, thereafter, the final elimination would occur with some autonomy and independently of postsynaptic maturation.

Constitutively active PKA regulates neuronal acetylcholine release and contractility of guinea pig urinary bladder smooth muscle

Autonomic and somatic motor neurons that innervate the urinary bladder and urethra control the highly coordinated functions of the lower urinary tract, the storage, and the emptying of urine. ACh is the primary excitatory neurotransmitter in the bladder. Here, we aimed to determine whether PKA regulates neuronal ACh release and related nerve-evoked detrusor smooth muscle (DSM) contractions in the guinea pig urinary bladder. Isometric DSM tension recordings were used to measure spontaneous phasic and electrical field stimulation (EFS)- and carbachol-induced DSM contractions with a combination of pharmacological tools. The colorimetric method was used to measure ACh released by the parasympathetic nerves in DSM isolated strips. The pharmacological inhibition of PKA with H-89 (10 μM) increased the spontaneous phasic contractions, whereas it attenuated the EFS-induced DSM contractions. Intriguingly, H-89 (10 μM) attenuated the (primary) cholinergic component, whereas it simultaneously increased the (secondary) purinergic component of the nerve-evoked contractions in DSM isolated strips. The acetylcholinesterase inhibitor, eserine (10 μM), increased EFS-induced DSM contractions, and the subsequent addition of H-89 attenuated the contractions. H-89 (10 μM) significantly increased DSM phasic contractions induced by the cholinergic agonist carbachol. The inhibition of PKA decreased the neuronal release of ACh in DSM tissues. This study revealed that PKA-mediated signaling pathways differentially regulate nerve-evoked and spontaneous phasic contractions of guinea pig DSM. Constitutively active PKA in the bladder nerves controls synaptic ACh release, thus regulating the nerve-evoked DSM contractions, whereas PKA in DSM cells controls the spontaneous phasic contractility.

The novel protein kinase C epsilon isoform modulates acetylcholine release in the rat neuromuscular junction

Background

Various protein kinase C (PKC) isoforms contribute to the phosphorylating activity that modulates neurotransmitter release. In previous studies we showed that nPKCε is confined in the presynaptic site of the neuromuscular junction and its presynaptic function is activity-dependent. Furthermore, nPKCε regulates phorbol ester-induced acetylcholine release potentiation, which further indicates that nPKCε is involved in neurotransmission. The present study is designed to examine the nPKCε involvement in transmitter release at the neuromuscular junction.

Results

We use the specific nPKCε translocation inhibitor peptide εV1-2 and electrophysiological experiments to investigate the involvement of this isoform in acetylcholine release. We observed that nPKCε membrane translocation is key to the synaptic potentiation of NMJ, being involved in several conditions that upregulate PKC isoforms coupling to acetylcholine (ACh) release (incubation with high Ca²⁺, stimulation with phorbol esters and protein kinase A, stimulation with adenosine 3′,5′-cyclic monophosphorothioate, 8-Bromo-, Rp-isomer, sodium salt -Sp-8-BrcAMP-). In all these conditions, preincubation with the nPKCε translocation inhibitor peptide (εV1-2) impairs PKC coupling to acetylcholine release potentiation. In addition, the inhibition of nPKCε translocation and therefore its activity impedes that presynaptic muscarinic autoreceptors and adenosine autoreceptors modulate transmitter secretion.

Conclusions

Together, these results point to the importance of nPKCε isoform in the control of acetylcholine release in the neuromuscular junction.