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

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.

Impact of intravenous dexamethasone on the initiation and recovery of atracurium in children: A double-blinded randomized controlled trial

BACKGROUND

Chronic steroid intake has been associated with attenuation of neuromuscular block. Despite some promising animal and adult studies, the effect of a single dose of intravenous dexamethasone on neuromuscular blockers is not well established. Thus, the present study aimed to demonstrate the effect of dexamethasone given at the time of induction for the prevention of PONV on the action of neuromuscular blockers in children undergoing elective surgery.

METHOD

After obtaining approval from the Institute Ethics Committee and written informed parental consent, 100 ASA I and II children aged 4-15 years undergoing elective surgery randomized to receive either: 0.15 mg/kg (maximum of 5 mg) of dexamethasone diluted to a total volume of 2 ml with 0.9% saline (n = 50) or 2 ml of 0.9% saline (n = 50) at the time of induction. The time interval between application of atracurium and maximum T1 depression, 25% twitch height recovery of T1, amid 25% and 75% twitch height recovery of T1, amid the 25% twitch height recovery of T1 and recovery of the neuromuscular block to a TOF ratio of 0.9, and in between the initiation of atracurium injection till the recovery of the neuromuscular block to a TOF ratio of 0.9 was defined as onset time, clinical duration, recovery index, recovery time, and total recovery period, respectively, and recorded.

RESULTS

The onset time and recovery index time were lower (1.96 ± 0.39, 8.04 ± 2.14, respectively) with dexamethasone in comparison with saline (2.01 ± 0.51, 8.9 ± 3.4, respectively) but not statistically significant. The clinical duration, recovery time, and total recovery period were similar.

CONCLUSION

Application of a single bolus dose (0.15 mg/kg) of dexamethasone during induction does not attenuate atracurium-induced neuromuscular blockade in children.

Methylprednisolone alleviates cognitive functions through the regulation of neuroinflammation in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease and linked to abnormal deposition of amyloid-β (Aβ), neurofibrillary tangles (NFTs), synaptic dysfunction, and neuroinflammation. Despite significant progress in unravelling the pathogenesis of AD, currently main therapeutic interventions is limited to symptomatic alleviation. Methylprednisolone (MP), a synthetic glucocorticoid, is recognized for its extensive anti-inflammatory properties. Our study assessed the neuroprotective effect of MP (25 mg/kg) administration to an Aβ_1-42-induced AD mouse model. Our findings demonstrate that MP treatment can ameliorate cognitive impairment in Aβ_1-42-induced AD mice and suppress microglial activation in the cortex and hippocampus. RNA-Sequencing analysis reveals that MP ultimately rescues cognitive dysfunction through improving the synapse function and inhibiting the immune and inflammatory processes. Our study suggests that MP could be a promising drug alternative for the treatment of AD, either alone or in combination with other existing drugs.

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.

Non-genomic Actions of Methylprednisolone Differentially Influence GABA and Glutamate Release From Isolated Nerve Terminals of the Rat Hippocampus

Corticosteroids exert a dual role in eukaryotic cells through their action via (1) intracellular receptors (slow genomic responses), or (2) membrane-bound receptors (fast non-genomic responses). Highly vulnerable regions of the brain, like the hippocampus, express high amounts of corticosteroid receptors, yet their actions on ionic currents and neurotransmitters release are still undefined. Here, we investigated the effect of methylprednisolone (MP) on GABA and glutamate (Glu) release from isolated nerve terminals of the rat hippocampus. MP favored both spontaneous and depolarization-evoked [¹⁴C]Glu release from rat hippocampal nerve terminals, without affecting [³H]GABA outflow. Facilitation of [¹⁴C]Glu release by MP is mediated by a Na⁺-dependent Ca²⁺-independent non-genomic mechanism relying on the activation of membrane-bound glucocorticoid (GR) and mineralocorticoid (MR) receptors sensitive to their antagonists mifepristone and spironolactone, respectively. The involvement of Na⁺-dependent high-affinity EAAT transport reversal was inferred by blockage of MP-induced [¹⁴C]Glu release by DL-TBOA. Depolarization-evoked [³H]GABA release in the presence of MP was partially attenuated by the selective P2X7 receptor antagonist A-438079, but this compound did not affect the release of [¹⁴C]Glu. Data indicate that MP differentially affects GABA and glutamate release from rat hippocampal nerve terminals via fast non-genomic mechanisms putatively involving the activation of membrane-bound corticosteroid receptors. Facilitation of Glu release strengthen previous assumptions that MP may act as a cognitive enhancer in rats, while crosstalk with ATP-sensitive P2X7 receptors may promote a therapeutically desirable GABAergic inhibitory control during paroxysmal epileptic crisis that might be particularly relevant when extracellular Ca²⁺ levels decrease below the threshold required for transmitter release.

Delayed increase of acetylcholine quantal size induced by the activity-dependent release of endogenous CGRP but not ATP in neuromuscular junctions

In mouse motor synapses tetanic neuromuscular activity (30 Hz, 2 min) led to a delayed posttetanic potentiation of amplitude and duration of spontaneous miniature endplate potentials (MEPPs). Microelectrode recordings of MEPPs before and after nerve stimulation showed an increase in MEPP amplitude and time course by 30% and 15%, respectively, without changes in their frequency. Peak effect was detected 20 min after tetanic activity and progressively faded throughout the next 40 min of recording. The revealed potentiation of MEPPs was fully preserved in preparations from pannexin 1 knockout mice. It means, that myogenic ATP released via pannexin 1 channels from contracting muscle fibers is not likely to participate in the described phenomenon. But posttetanic potentiation of MEPPs was fully prevented by competitive antagonist of calcitonin gene-related peptide (CGRP) receptors CGRP_8-37 , ryanodine receptors inhibitor ryanodine and by vesicular acetylcholine transporter inhibitor vesamicol. It is suggested that the combination of intensive synaptic and contractile activity in neuromuscular junctions is required to induce Ca²⁺ -dependent exocytosis of endogenous CGRP. The accumulation of CGRP in the synaptic cleft and its presynaptic activity may induce posttetanic potentiation of MEPP amplitude due to CGRP-stimulated acetylcholine loading into vesicles and subsequent increase of quantal size.

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.

Adenosinergic signalling in chondrogenesis and cartilage homeostasis: Friend or foe?

Chondrocytes and their mesenchymal cell progenitors secrete a variety of bioactive molecules, including adenine nucleotides and nucleosides, but these molecules are not usually highlighted in review papers about the secretome of these cells. Ageing and inflammatory insults compromise chondrocytes ability to keep ATP/adenosine synthesis, release and turnover. Cartilage homeostasis depends on extracellular adenosine levels, which acting via four P1 purinoceptor subtypes modulates the release of pro-inflammatory mediators, including NO, PGE₂ and several cytokines. Native articular cartilage is challenged by synovial fluid flow during normal joint motion transiently increasing ATP release and adenosine formation in the joint microenvironment. Excessive joint motion and shockwave trauma are deleterious to cartilage homeostasis due to HIF-1α overexpression, resulting in disproportionate ecto-5'-nucleotidase/CD73 production, adenosine accumulation and superfluous A_2B receptors activation. Scarcity of data however exists on the putative interplay between coexistent high affinity (A_2A and A₃) and low affinity (A_2B) adenosine receptors activation affecting stem cells fate towards preferential chondrogenic or osteogenic lineages in the human cartilage. Hints gathered in this commentary result mainly from studies using human immortalized cell lines and animal (e.g. rodent, equine, bovine) tissue samples. The available data point towards adenosine A_2A and A₃ receptors having cartilage protective roles, while excessive adenosine accumulation may be detrimental via low affinity A_2B receptors activation, with little reference to the putative role of the adenosine forming enzyme ecto-5'-nucleotidase/CD73. Thus, emphasizing the multiple pathways responsible for controlling adenosine signalling in cartilage will certainly impact on the search for novel therapeutic targets for highly disabling articular disorders.

Dexamethasone concentration affecting rocuronium-induced neuromuscular blockade and sugammadex reversal in a rat phrenic nerve-hemidiaphragm model: An ex vivo study

BACKGROUND

The concentration range of dexamethasone that inhibits neuromuscular blockade (NMB) and sugammadex reversal remains unclear.

OBJECTIVE

To evaluate the effects of dexamethasone on rocuronium-induced NMB and sugammadex reversal.

DESIGN

Ex vivo study.

SETTING

Asan Institute for Life Sciences, Asan Medical Center, Korea, from July 2015 to November 2015.

ANIMALS

One hundred sixty male Sprague-Dawley rats.

INTERVENTIONS

We assessed the effect of four concentrations of dexamethasone [0, 0.5, 5 (clinical concentrations) and 50 μg ml (experimental concentration)] on partial NMB on 40 phrenic nerve-hemidiaphragm preparations (n=10 per concentration). Once the first twitch of train-of-four (TOF) had been depressed by 50% with rocuronium, dexamethasone was administered. To assess the effect of dexamethasone on sugammadex reversal, 120 phrenic nerve-hemidiaphragm preparations were used in three subexperiments (n=40 per experiment), using three administration regimens of rocuronium-equimolar sugammadex: a single dose, a split-dose (split and ) and a reduced split-dose (split and ). After complete NMB was achieved, dexamethasone and sugammadex were administered.

MAIN OUTCOME MEASURES

The change in the first twitch height, the recovery time to a TOF ratio at least 0.9, and the TOF ratio at 30 min were evaluated.

RESULTS

There were no significant differences in the first twitch height among groups (P = 0.532). With a single dose of sugammadex, dexamethasone did not affect the recovery time to a TOF ratio at least 0.9 (P = 0.070). After using a split-dose of sugammadex, the recovery time to a TOF ratio at least 0.9 was delayed only at a concentration of 50 μg ml of dexamethasone. With a reduced split-dose of sugammadex, the TOF ratio at 30 min was lowered only by a concentration of 50 μg ml of dexamethasone (P < 0.010).

CONCLUSION

Acute bolus administration of dexamethasone at clinical concentrations had no effect on NMB or on sugammadex reversal.

Effects of dexamethasone and hydrocortisone on rocuroniuminduced neuromuscular blockade and reversal by sugammadex in phrenic nerve-hemidiaphragm rat model

Background

The facilitator effects of steroids on neuromuscular transmission may cause resistance to neuromuscular blocking agents. Additionally, steroids may hinder sugammadex reversal of neuromuscular blockade, but these findings remain controversial. Therefore, we explored the effect of dexamethasone and hydrocortisone on rocuronium-induced neuromuscular blockade and their inhibitory effect on sugammadex.

Methods

We explored the effects of steroids, dexamethasone and hydrocortisone, in vitro using a phrenic nerve-hemidiaphragm rat model. In the first phase, an effective dose of rocuronium was calculated, and in the second phase, following sugammadex administration, the recovery of the train-of-four (TOF) ratio and T1 was evaluated for 30 minutes, and the recovery index was calculated in dexamethasone 0, 0.5, 5, and 50 μg/ml, or hydrocortisone 0, 1, 10, or 100 μg/ml.

Results

No significant effect of steroids on the effective dose of rocuronium was observed. The TOF ratios at 30 minutes after sugammadex administration were decreased significantly only at high experimental concentrations of steroids: dexamethasone 50 μg/ml and hydrocortisone 100 μg/ml (P < 0.001 and P = 0.042, respectively). There were no statistical significances in other concentrations. No differences were observed in T1. Recovery index was significantly different only in 100 μg/ml of hydrocortisone (P = 0.03).

Conclusions

Acute exposure to steroids did not resist the neuromuscular blockade caused by rocuronium. And inhibition of sugammadex reversal on rocuronium-induced neuromuscular blockade is unlikely at typical clinical doses of dexamethasone and also hydrocortisone. Conclusively, we can expect proper effects of rocuronium and sugammadex when dexamethasone or hydrocortisone is used during general anesthesia.

Adenosine A2A and histamine H3 receptors interact at the cAMP/PKA pathway to modulate depolarization-evoked [3H]-GABA release from rat striato-pallidal terminals

We previously reported that the activation of histamine H₃ receptors (H₃Rs) selectively counteracts the facilitatory action of adenosine A_2A receptors (A_2ARs) on GABA release from rat globus pallidus (GP) isolated nerve terminals (synaptosomes). In this work, we examined the mechanisms likely to underlie this functional interaction. Three possibilities were explored: (a) changes in receptor affinity for agonists induced by physical A_2AR/H₃R interaction, (b) opposite actions of A_2ARs and H₃Rs on depolarization-induced Ca²⁺ entry, and (c) an A_2AR/H₃R interaction at the level of adenosine 3',5'-cyclic monophosphate (cAMP) formation. In GP synaptosomal membranes, H₃R activation with immepip reduced A_2AR affinity for the agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride hydrate (CGS-21680) (K_i control 4.53 nM; + immepip 9.32 nM), whereas A_2AR activation increased H₃R affinity for immepip (K_i control 0.63 nM; + CGS-21680 0.26 nM). Neither A_2AR activation nor H₃R stimulation modified calcium entry through voltage-gated calcium channels in GP synaptosomes, as evaluated by microfluorometry. A_2AR-mediated facilitation of depolarization-evoked [2,3-³H]-γ-aminobutyric acid ([³H]-GABA) release from GP synaptosomes (130.4 ± 3.6% of control values) was prevented by the PKA inhibitor H-89 and mimicked by the adenylyl cyclase activator forskolin or by 8-Bromo-cAMP, a membrane permeant cAMP analogue (169.5 ± 17.3 and 149.5 ± 14.5% of controls). H₃R activation failed to reduce the facilitation of [³H]-GABA release induced by 8-Bromo-cAMP. In GP slices, A_2AR activation stimulated cAMP accumulation (290% of basal) and this effect was reduced (- 75%) by H₃R activation. These results indicate that in striato-pallidal nerve terminals, A_2ARs and H₃Rs interact at the level of cAMP formation to modulate PKA activity and thus GABA release.

Methylprednisolone as a memory enhancer in rats: Effects on aversive memory, long-term potentiation and calcium influx

It is well recognized that stress or glucocorticoids hormones treatment can modulate memory performance in both directions, either impairing or enhancing it. Despite the high number of studies aiming at explaining the effects of glucocorticoids on memory, this has not yet been completely elucidated. Here, we demonstrate that a low daily dose of methylprednisolone (MP, 5mg/kg, i.p.) administered for 10-days favors aversive memory persistence in adult rats, without any effect on the exploring behavior, locomotor activity, anxiety levels and pain perception. Enhanced performance on the inhibitory avoidance task was correlated with long-term potentiation (LTP), a phenomenon that was strengthen in hippocampal slices of rats injected with MP (5mg/kg) during 10days. Additionally, in vitro incubation with MP (30-300µM) concentration-dependently increased intracellular [Ca²⁺]_i in cultured hippocampal neurons depolarized by KCl (35mM). In conclusion, a low daily dose of MP for 10days may promote aversive memory persistence in rats.

Synaptic Homeostasis and Its Immunological Disturbance in Neuromuscular Junction Disorders

In the neuromuscular junction, postsynaptic nicotinic acetylcholine receptor (nAChR) clustering, trans-synaptic communication and synaptic stabilization are modulated by the molecular mechanisms underlying synaptic plasticity. The synaptic functions are based presynaptically on the active zone architecture, synaptic vesicle proteins, Ca²⁺ channels and synaptic vesicle recycling. Postsynaptically, they are based on rapsyn-anchored nAChR clusters, localized sensitivity to ACh, and synaptic stabilization via linkage to the extracellular matrix so as to be precisely opposed to the nerve terminal. Focusing on neural agrin, Wnts, muscle-specific tyrosine kinase (a mediator of agrin and Wnts signalings and regulator of trans-synaptic communication), low-density lipoprotein receptor-related protein 4 (the receptor of agrin and Wnts and participant in retrograde signaling), laminin-network (including muscle-derived agrin), extracellular matrix proteins (participating in the synaptic stabilization) and presynaptic receptors (including muscarinic and adenosine receptors), we review the functional structures of the synapse by making reference to immunological pathogenecities in postsynaptic disease, myasthenia gravis. The synapse-related proteins including cortactin, coronin-6, caveolin-3, doublecortin, R-spondin 2, amyloid precursor family proteins, glia cell-derived neurotrophic factor and neurexins are also discussed in terms of their possible contribution to efficient synaptic transmission at the neuromuscular junction.

Effect of dexamethasone on the onset time and recovery profiles of cisatracurium

BACKGROUND

The effect of dexamethasone injection on cisatracurium-induced neuromuscular block was compared according to different injection time points.

METHODS

One hundred seventeen patients were randomly assigned to three groups: 8 mg of dexamethasone injected intravenously 2-3 h before anesthesia (group A), just before anesthesia induction (group B), and at the end of surgery (control group). Three minutes after anesthesia induction, intubation was performed without neuromuscular blockers, and acceleromyography was initiated. All patients received 0.05 mg/kg cisatracurium; the onset time and recovery profiles were recorded.

RESULTS

Eighty patients were finally enrolled. The onset time (median [interquartile range], seconds) was significantly hastened in group A (520.0 [500.0-560.0], n = 30) compared to that in group B (562.5 [514.0-589.0], n = 22) (P = 0.008) and control group (586.5 [575.0-642.5], n = 28) (P < 0.001). The onset time in group B was faster than the control group (P = 0.015). The recovery time [mean (95% CI) minutes] was significantly hastened in group A [28.5 (27.3-29.6)] compared to that in group B [32.3 (31.0-33.6)] (P < 0.001) and control group [30.9 (29.9-31.8)] (P = 0.015). The total recovery time was significantly hastened more in group A [47.1 (45.5-48.6)] than group B [52.8 (51.6-54.0) minutes] (P < 0.001) and control group [50.5 (48.7-52.3) minutes] (P = 0.008).

CONCLUSIONS

A single dose of 8 mg of dexamethasone hastened the onset and total recovery times of cisatracurium-induced block by approximately 15 and 9%, respectively if administered 2-3 h prior to surgery.

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.