Facilitation of acetylcholine release and improvement in cognition by a selective M2 muscarinic antagonist, SCH 72788

Current treatment of Alzheimer's Disease (AD) requires acetylcholinesterase inhibition to increase acetylcholine (ACh) concentrations in the synaptic cleft. Another mechanism by which ACh levels can be increased is blockade of presynaptic M2 muscarinic autoreceptors that regulate ACh release. An antagonist designed for this purpose must be highly selective for M2 receptors to avoid blocking postsynaptic M1 receptors, which mediate the cognitive effects of ACh. Structure-activity studies of substituted methylpiperadines led to the synthesis of 4-[4-[1(S)-[4-[(1,3-benzodioxol-5-yl)sulfonyl]phenyl]ethyl]-3(R)-methyl-1-piperazinyl]-4-methyl-1-(propylsulfonyl)piperidine. This compound, SCH 72788, binds to cloned human M2 receptors expressed in CHO cells with an affinity of 0.5 nM, and its affinity at M1 receptors is 84-fold lower. SCH 72788 is a functional M2 antagonist that competitively inhibits the ability of the agonist oxotremorine-M to inhibit adenylyl cyclase activity. In an in vivo microdialysis paradigm, SCH 72788 increases ACh release from the striatum of conscious rats. The compound is also active in a rodent model of cognition, the young rat passive avoidance response paradigm. The effects of SCH 72788 suggest that M2 receptor antagonists may be useful for treating the cognitive decline observed in AD and other dementias.     

Retrograde Inhibition of Transmitter Release by ATP

Abstract: After labelling ACh tissue stores in Torpedo electric organ prisms with radioactive acetate, the release of ACh and ATP triggered by electrical stimulation or KCI depolarization was measured in the same perfusate samples. The luciferin-luciferase reaction for ATP was first counted, then the radioactive content of the sample determined. Further evidence showing that ATP release resulted from postsynaptic transmitter action was that carbachol could induce the release of ATP. A dose-response curve was obtained. Curare or α-bungarotoxin block the release of ATP elicited by carbachol. When triggered by KCI depolarization the increased efflux of ACh and ATP returned to low levels in spite of the maintained depolarization. After two successive KCI depolarizations, it was possible to dissociate the release of both substances. The efflux of ATP was exhausted while ACh release was maintained. If the second KCI depolarization was delayed ATP release recovered, but the release kinetics of ACh and ATP were sustained. The exhaustion of endogenous ATP release or the action of exogenous ATP had little or no effect on the release of ACh triggered by KCI depolarization. On the contrary, the release of ACh induced by electrical stimulation was sensitive to the action of adenine nucleotides, and a quantitative estimation of the inhibition of ACh release by ATP and adenosine could be made. At the onset of stimulation ATP release predominated, being gradually replaced by adenosine, which can be reuptaken. This would terminate the inhibitory action of the nucleotide. Carbachol inhibits evoked ACh release, while the effect of α-bungarotoxin was to increase spontaneous ACh release. These effects could be respectively mediated by an increased or a reduced release of ATP resulting from the postsynaptic action of ACh agonists or antagonists. However, a direct presynaptic effect of these substances is not excluded. It seems possible that the action of ATP on ACh release can be explained through its inhibition of the depolarization-evoked Ca²⁺ entry.     

Differential Effects of Bromocriptine on Dopamine and Acetylcholine Release Modulatory Receptors

Rabbit neostriatal slices were prelabeled with [3H]dopamine (DA) and [14C]choline and then superfused. The electrical stimulation-evoked release of DA and of acetylcholine (ACh) was abolished by 0.33 microM tetrodotoxin and by low calcium concentrations (0.13 mM). Bromocriptine, a selective D2-DA receptor agonist, inhibited in a concentration-dependent manner the evoked overflow of DA and ACh, without affecting the basal efflux of both transmitters. The effects of bromocriptine were antagonized by sulpiride, a specific antagonist of D2-DA receptors. With stimulation at 0.3 Hz and 120 pulses, bromocriptine was eight times more potent in inhibiting the evoked overflow of DA (IC50: 11 nM) than that of ACh (IC50: 83 nM). Stimulations at 3 Hz and 360 pulses markedly reduced the potency of bromocriptine in inhibiting DA and ACh release, and diminished its selectivity for presynaptic receptors. These results indicate that DA receptors that modulate the release of DA and ACh are of the D2 subtype. The greater potency of bromocriptine at pre- than at postsynaptic sites suggests that these receptors may be different in quantity and/or quality [D2-alpha (presynaptic) versus D2-beta (postsynaptic)]. Finally, marked differences in the potency and efficacy of DA agonist actions on DA and ACh release modulatory receptors are obtained, depending on the parameters of stimulation used.     

杀虫药剂的神经毒理学研究进展

大多数杀虫药剂都具有较强的神经毒性,它们对神经系统的作用靶标不同。有机磷类杀虫剂不仅抑制乙酰胆碱酯酶活性和乙酰胆碱受体功能,影响乙酰胆碱的释放,而且还具有非胆碱能毒性,有些有机磷杀虫剂还能引发迟发性神经毒性。新烟碱类杀虫剂作为烟碱型乙酰胆碱受体（nAChR）的激动剂,作用于该类受体的α亚基;它对昆虫的毒性比对哺乳动物的毒性大得多,乃是因为它对昆虫和哺乳动物nAChR的作用位点不同。拟除虫菊酯类杀虫剂主要作用于神经细胞钠通道,引起持续开放,导致传导阻滞;该类杀虫剂也可抑制钙通道。另外,这类杀虫剂还干扰谷氨酸递质和多巴胺神经元递质的释放。拟除虫菊酯类杀虫剂对昆虫的选择毒性很可能是因为昆虫神经元的钠通道结构与哺乳动物的不同。阿维菌素类杀虫剂主要作用于γ-氨基丁酸（GABA）受体,它能促进GABA的释放,增强GABA与GABA受体的结合,使氯离子内流增加,导致突触后膜超级化。由于这类杀虫剂难以穿透脊椎动物的血脑屏障而与中枢神经系统的GABA受体结合,故该类杀虫剂对脊椎动物的毒性远低于对昆虫的毒性。多杀菌素类杀虫剂可与中枢神经系统的nAChR作用,引起Ach长时间释放,此外,这类杀虫剂还可作用于昆虫的GABA受体,改变GABA门控氯通道的功能。     

The release of ATP triggered by transmitter action and its possible physiological significance: retrograde transmission.

1. When a slice of electric organ of Torpedo is stimulated and superfused with a solution containing a firefly lantern extract, it is possible to measure the release of ATP after each nerve impulse as a light emission. 2. The postsynaptic action of released ACh induces the release of ATP by the postsynaptic cell. Most of the released ATP is of postsynaptic origin. 3. Ion fluxes associated with depolarization, or depolarization itself, trigger the release of ATP from postsynaptic and presynaptic membranes (synaptosomes). 4. ATP is able to block ACh release; a postsynaptic "retrograde transmission" able to control presynaptic transmitter release is possible.     

Mechanisms of facilitation of synaptic glutamate release by nicotinic agonists in the nucleus of the solitary tract

The nucleus of the solitary tract (NTS) is the principal integrating relay in the processing of visceral sensory information. Functional nicotinic acetylcholine receptors (nAChRs) have been found on presynaptic glutamatergic terminals in subsets of caudal NTS neurons. Activation of these receptors has been shown to enhance synaptic release of glutamate and thus may modulate autonomic sensory-motor integration and visceral reflexes. However, the mechanisms of nAChR-mediated facilitation of synaptic glutamate release in the caudal NTS remain elusive. This study uses rat horizontal brainstem slices, patch-clamp electrophysiology, and fluorescent Ca(2+) imaging to test the hypothesis that a direct Ca(2+) entrance into glutamatergic terminals through active presynaptic non-α7- or α7-nAChR-mediated ion channels is sufficient to trigger synaptic glutamate release in subsets of caudal NTS neurons. The results of this study demonstrate that, in the continuous presence of 0.3 μM tetrodotoxin, a selective blocker of voltage-activated Na(+) ion channels, facilitation of synaptic glutamate release by activation of presynaptic nAChRs (detected as an increase in the frequency of miniature excitatory postsynaptic currents) requires external Ca(2+) but does not require activation of presynaptic Ca(2+) stores and presynaptic high- and low-threshold voltage-activated Ca(2+) ion channels. Expanding the knowledge of mechanisms and pharmacology of nAChRs in the caudal NTS should benefit therapeutic approaches aimed at restoring impaired autonomic homeostasis.     

突触前α7烟碱受体对海马神经元兴奋性突触传递的调控

采用盲法膜片钳技术观察突触前烟碱受体(nicotinic acetylcholinel receptors,nAChRs)对海马脑片CAl区锥体神经元兴奋性突触传递的调控作用。结果显示,nAChRs激动剂碘化二甲基苯基哌嗪(dimethylphenyl—piperazinium iodide,DMPP)不能在CAl区锥体神经元上诱发出烟碱电流。DMPP对CAl区锥体神经元自发兴奋性突触后电流(spontaneous excitatory postsynaptic current,sEPSC)具有明显的增频和增幅作用,并呈现明显的浓度依赖关系。DMPP对微小兴奋性突触后电流(miniature excitatory postsynaptic current,mEPSC)具有增频作用,但不具有增幅作用。上述DMPP增强突触传递的作用不能被nAChRs拮抗剂美加明、六烃季铵和双氢-β-刺桐丁所阻断,但可被α-银环蛇毒素阻断。上述结果提示,海马脑片CAl区锥体神经元兴奋性突触前nAChRs含有对α-银环蛇毒素敏感的胡亚单位,其激活可增强海马CAl区锥体神经元突触前递质谷氨酸的释放,从而对兴奋性突触传递发挥调控作用。     

The quantal release at a neuro-neuronal synapse is regulated by the content of acetylcholine in the presynaptic cell

Transmitter release was studied with respect to the presynaptic acetylcholine (ACh) content at a central identified inhibitory synapse (Cl- conductance) of Aplysia californica. Statistical analysis of the synaptic noise evoked by sustained depolarization of the presynaptic neuron allowed us to calculate the quantal parameters of the postsynaptic responses. Loading of the presynaptic neurone with injected ACh led to an increase in the postsynaptic responses whereas the calculated miniature postsynaptic current (MPSC) was unmodified. Destruction of choline by choline oxidase either applied extracellularly and coupled to intense stimulations of the presynaptic cell or injected into the presynaptic neuron induced a depression of the postsynaptic response although the amplitude of the calculated MPSC remained constant. As the size of the MPSC, i.e. the size of the quantum, did not change in these experiments, it was concluded that the presynaptic ACh content controls the number of quanta released by a given presynaptic depolarization. As additional evidence, effects of abrupt increase in tonicity of the external medium were studied. The observed transient enhancement of the quantal content of the postsynaptic response could be attributed to an increase in the presynaptic concentration of ACh, resulting from the reduction in cellular volume.     

8-[2-(2-pentyl-cyclopropylmethyl)-cyclopropyl]-octanoic acid stimulates GABA release from interneurons projecting to CA1 pyramidal neurons in the rat hippocampus via pre-synaptic alpha7 acetylcholine receptors

Nicotinic acetylcholine (ACh) receptors, such as alpha7, alpha3beta4 and alpha4beta2 receptors in the hippocampus, are suggested to modulate neurotransmitter release. 8-[2-(2-Pentyl-cyclopropylmethyl)-cyclopropyl]-octanoic acid (DCP-LA) (100 nM), a linoleic acid derivative, potentiated responses of alpha7, alpha3beta4 and alpha4beta2 ACh receptors expressed in Xenopus oocytes that are blocked by 3-(1-[dimethylaminopropyl] indol-3-yl)-4-[indol-3-yl] maleimide (GF109203X), a selective inhibitor of protein kinase C (PKC), except for alpha3beta4 ACh receptors. DCP-LA enhanced the nicotine-triggered release of GABA from rat hippocampal slices in the presence of tetrodotoxin in a bell-shaped dose-dependent manner at concentrations ranging from 10 nM to 10 microM, although DCP-LA by itself had no effect on GABA release. The DCP-LA action was inhibited by GF109203X or alpha-bungarotoxin, an inhibitor of alpha7 ACh receptors, but not by mecamylamine or dihydro-beta-erithroidine, an inhibitor of alpha3beta4 and alpha4beta2 ACh receptors. A similar effect on GABA release was obtained with 12-O-tetradecanoylphorbol 13-acetate, a PKC activator. DCP-LA (100 nM) also enhanced GABA release triggered by choline, an agonist of alpha7 ACh receptors, but not 3-[2(s)-azetidinylmethoxy] pyridine, an agonist of alpha4beta2 ACh receptors. In addition, DCP-LA (100 nM) increased the rate of nicotine-triggered GABA(A) receptor-mediated miniature inhibitory post-synaptic currents, monitored from CA1 pyramidal neurons of rat hippocampal slices, and the effect was also inhibited by GF109203X or alpha-bungarotoxin but not by mecamylamine. Thus, the results of the present study indicate that DCP-LA stimulates GABA release by enhancing activity of pre-synaptic alpha7 ACh receptors present on the GABAergic terminals of interneurons that transmit to CA1 pyramidal neurons via a PKC pathway.     

Endothelial-dependent relaxant effects of vaso-active intestinal polypeptide and arachidonic acid in rat aortic strips

The relaxant actions of vaso-active intestinal polypeptide (VIP), acetylcholine (ACh), histamine and papaverine have been compared using circular muscle strips of rat aorta contracted with noradrenaline (NA). Arachidonic acid (AA) in a low dose (6.7 X 10(-7M) also relaxed the aorta. The relaxant actions of all these substances except papaverine were abolished by removal of the endothelial cells. Higher doses of AA (6.7-13.4 X 10(-6M) contracted aortic strips in the absence of NA but the con tractile effect "faded" while AA was still present in the bathing fluid. De-endothelialisation abolished this "fade" portion of the response leaving a sustained contracture. Indomethacin inhibited the contractile effect of AA revealing a weak inhibitory effect. However, it did not affect the relaxations induced by VIP, ACh, histamine or papaverine. ETYA abolished the relaxant actions of all these substances except papaverine. The results are consistent with the hypothesis that VIP, ACh and histamine relax the rat aorta via an endothelial-dependent mechanism which may involve the synthesis of a lipoxygenase product.     

Receptor-mediated presynaptic facilitation of quantal release of acetylcholine induced by pralidoxime inAplysia

1. Possible interactions of contrathion (pralidoxime sulfomethylate), a reactivator of phosphorylated acetylcholinesterase (AChE), with the regulation of cholinergic transmission were investigated on an identified synapse in the buccal ganglion of Aplysia californica. 2. Transmitter release was evoked either by a presynaptic action potential or, under voltage clamp, by a long depolarization of the presynaptic cell. At concentrations higher than 10(-5) M, bath-applied contrathion decreased the amplitude of miniature postsynaptic currents and increased their decay time. At the same time, the quantal release of ACh was transiently facilitated. The facilitatory effect of contrathion was prevented by tubocurarine but not by atropine. Because in this preparation, these drugs block, respectively, the presynaptic nicotinic-like and muscarinic-like receptors involved in positive and negative feedback of ACh release, we proposed that contrathion activates presynaptic nicotinic-like receptors. 3. Differential desensitization of the presynaptic receptors is proposed to explain the transience of the facilitatory action of contrathion on ACh release. 4. The complexity of the synaptic action of contrathion raises the possibility that its therapeutic effects in AChE poisonings are not limited to AChE reactivation.     

Amphetamine effects on acetylcholine release and mammalian neuromuscular transmission

The mechanisms by which (+)-amphetamine biphasically modifies neuromuscular transmission were studied in the rat phrenic nervediaphragm preparation. Low to moderate amphetamine concentrations (30–300 μM) enhanced twitch height and potentiated the nerve stimulated release of acetylcholine (ACh) by up to 4.8-fold from the phrenic nerve. Higher amphetamine concentrations depressed muscle twitch and ACh release. Using a cannulated diaphragm preparation, amphetamine enhanced the twitch response to nerve stimulation but markedly depressed the contractions elicited by a pulsed injection of ACh. Amphetamine-induced enhancement of ACh release was prevented by pretreatment of animals with α-methyl-p-tyrosine, suggesting that amphetamine may be acting indirectly by releasing catecholamines. These results support the hypothesis that amphetamine enhancement results from a presynaptic increase in ACh release and the blocking actions are mediated by a postsynaptic inhibitory effect.     

A Novel Agonist Binding Site on Nicotinic Acetylcholine Receptors

Abstract

This report provides evidence that physostigmine (Phy) and benzoquinonium (BZQ) are able to activate nicotinic acetylcholine receptors (nAChRs) through binding site(s) distinct from those of the natural transmitter, ACh. Such findings are in agreement with a second pathway of activation of nAChRs. Receptor activation may be modulated through the novel site, and, consequently, physiological processes involving nicotinic synapses could be controlled. Using patch clamp techniques, single channel currents activated by ACh and anatoxin were recorded from frog interosseal muscle fibers under cell-attached condition and outside-out patches excised from cultured rat hippocampal neurons. Whole cell nicotinic currents were also studied in the cultured neurons. In most of the neurons, nicotinic responses were blocked by the nicotinic antagonists methyllycaconitine (MLA) and α-bungarotoxin (α-BGT). Evaluation of the effects of Phy and BZQ on the muscle and on the α-BGT- and MLA-sensitive neuronal nAChRs demonstrated that both compounds were open channel blockers at these receptors. Furthermore, at low micromolar concentrations, Phy and BZQ activated the nAChRs of all preparations tested, such an effect being unexpectedly resistant to α-BGT or MLA. Thus, the nAChRs could be activated via two distinct binding sites: one for ACh and the other for Phy and BZQ. These findings and previous biochemical results led us to suggest that a putative endogenous ligand could bind to the new site and thereby regulate the activation of nAChRs in nicotinic synapses.     

Direct evidence that release-stimulating alpha7* nicotinic cholinergic receptors are localized on human and rat brain glutamatergic axon terminals

The existence on glutamatergic nerve endings of nicotinic acetylcholine receptors (nAChRs) mediating enhancement of glutamate release has often been suggested but not demonstrated directly. Here, we study the effects of nAChR agonists on [3 H]-d-aspartate ([3 H]-d-ASP) release from synaptosomes superfused in conditions known to prevent indirect effects. Nicotinic receptor agonists, while unable to modify the basal [3 H]-d-ASP release from human neocortex or rat striatal synaptosomes, enhanced the Ca2+ -dependent exocytotic release evoked by K+ (12 mm) depolarization. Their rank order of potency were anatoxin-a > epibatidine > nicotine > ACh (+ atropine). The anatoxin-a effect, both in human and rat synaptosomes, was antagonized by mecamylamine, alpha-bungarotoxin or methyllycaconitine. The basal release of [3 H]ACh from human cortical synaptosomes was increased by (-)-nicotine (EC50 = 1.16 +/- 0.33 microm) or by ACh plus atropine (EC50 = 2.0 +/- 0.04 microm). The effect of ACh plus atropine was insensitive to alpha-bungarotoxin, methyllycaconitine or alpha-conotoxin MII, whereas it was totally antagonized by mecamylamine or dihydro-beta-erythroidine. To conclude, glutamatergic axon terminals in human neocortex and in rat striatum possess alpha7* nicotinic heteroreceptors mediating enhancement of glutamate release. Release-enhancing cholinergic autoreceptors in human neocortex are nAChRs with a pharmacological profile compatible with the alpha4beta2 subunit combination.     

Effect of N-acetylaspartylglutamate (NAAG) on non-quantal and spontaneous quantal release of acetylcholine at the neuromuscular synapse of rat

N-Acetylaspartylglutamate (NAAG), known to be present in rat motor neurons, may participate in neuronal modulation of non-quantal secretion of acetylcholine (ACh) from motor nerve terminals. Non-quantal release of ACh was estimated by the amplitude of the endplate membrane hyperpolarization (H-effect) caused by inhibition of nicotinic receptors by (+)-tubocurarine and acetylcholinesterase by armin (diethoxy-p-nitrophenyl phosphate). Application of exogenous NAAG decreased the H-effect in a dose-dependent manner. The reduction of the H-effect by NAAG was completely removed when N-acetyl-beta-aspartylglutamate (betaNAAG) or 2-(phosphonomethyl)-pentanedioic acid (2-PMPA) was used to inhibit glutamate carboxypeptidase II (GCP II), a presynaptic Schwann cell membrane-associated ectoenzyme that hydrolyzes NAAG to glutamate and N-acetylaspartate. Bath application of glutamate decreased the H-effect similarly to the action of NAAG but N-acetylaspartate was without effect. Inhibition of NMDA receptors by dl-2-amino-5-phosphopentanoic acid, (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine (MK801), and 7-chlorokynurenic acid or inhibition of muscle nitric oxide synthase (NO synthase) by N(G)-nitro-l-arginine methyl ester and 3-bromo-7-nitroindazole completely prevented the decrease of the H-effect by NAAG. These results suggest that glutamate, produced by enzymatic hydrolysis of bath-applied NAAG, can modulate non-quantal secretion of ACh from the presynaptic terminal of the neuromuscular synapse via activation of postsynaptic NMDA receptors and synthesis of nitric oxide (NO) in muscle fibers. NAAG also increased the frequency of miniature endplate potentials (mEPPs) generated by spontaneous quantal secretion of ACh, whereas the mean amplitude and time constants for rise time and for decay of mEPPs did not change.     

Retrograde modulation at developing neuromuscular synapses: involvement of G protein and arachidonic acid cascade.

Intracellular loading of nonhydrolyzable GTP analogs into innervated muscle cells in Xenopus cultures led to a marked increase in the frequency of spontaneous synaptic currents (SSCs), while extracellular application of the drugs at the same concentration was without effect. The increase in SSC frequency appeared to be unrelated to changes in the muscle membrane sensitivity toward acetylcholine (ACh), but resulted from an elevated spontaneous ACh secretion from the presynaptic nerve terminal. Postsynaptic loading of arachidonic acid (AA) produced a similar effect as the GTP analogs, and the potentiation effect of both GTP analogs and AA was reversed by an inhibitor of AA metabolism, AA861. Further studies indicate that a lipoxygenase metabolite, 5-HPETE, appears to be a likely candidate for the retrograde factor involved in modulating ACh secretion. These results suggest that G protein activation of the AA cascade in the postsynaptic cell could produce a retrograde signal to modulate transmitter secretion from the presynaptic nerve terminal at developing synapses.     

Inhibition by sodium bromide of acetylcholine release and synaptic transmission in the superior cervical ganglion of the rat

In the superior cervical ganglion (SCG) of rats, the interaction of sodium bromide (NaBr) with various drugs which interfere with the GABA system, such as 3-(4-chlorophenyl)-4-aminobutyrate [( + )baclofen, Bac], ( + )bicuculline (Bic), picrotoxin (Pic) and chlorpromazine (CPZ), and the effects of NaBr on the K⁺-induced release of [³H]acetylcholine ([³H]ACh) were studied in vitro. The effects on the evoked potentials induced by preganglionic stimulation were analysed in situ. The in vitro experiments revealed that 1 mM NaBr inhibits both the basal and the K⁺-induced release of [³H]ACh in a Ca²⁺-dependent manner. This NaBr effect was additive with the similar effect of the GABA agonist Bac, but it could not be blocked with any of the drugs applied. In vivo, 1 mM NaBr depressed the amplitude of the evoked potentials in the SCG. It is concluded that, in the SCG of rats, NaBr interacts with the presynaptic and postsynaptic membranes. The inhibitory effects of NaBr on both the [³H]ACh release and the potentials evoked by preganglionic stimulation cannot be attributed to a direct interference with GABA receptor complexes; some other binding site/s on the presynaptic and postsynaptic membranes might be responsible for the bromide-induced reduction of the synaptic transmission in the SCG of rats.     

硝苯吡啶对腹腔神经节突触传递的阻断作用

本文应用细胞内记录技术,观察了钙通道阻滞剂硝苯吡啶（nifedipine)对离体豚鼠腹腔神经节突触传递的影响,硝苯吡啶（0.1-10umol/L)不影响所检细胞的静息膜电位,膜电阻及细胞内刺激引起的动作电位,但能显著阻断N-型胆碱能的突触传递,并且这种作用可被低钙模拟、高钙拮抗,硝苯吡啶（10umol/L)也不影响突触后膜对乙酰胆碱（ACh)的敏感性;但在高钾克氏液中,能减少微小兴奋性突触后电位（mEPSPs)的频率;在低钙和高镁克氏液中,能减少量子含量,而对量子大小无影响。结果表明,治疗量的硝苯吡啶(0.1umol/L)通过阻滞突触前膜钙内流及ACh的量子性释放,产生突触阻断作用。这可能是硝苯吡啶降压机理的一个组成部分。     

Changes in the Parameters of Quantal Acetylcholine Release after Activation of PAR1-Type Thrombin Receptors at the Mouse Neuromuscular Junctions

In mature and newly formed neuromuscular synapses of mouse skeletal muscles, miniature endplate potentials (MEPPs) and multiquantal endplate potentials (EPPs) evoked by a single stimulation of the nerve were recorded using intracellular microelectrode technique. The mechanisms underlying the changes in spontaneous and evoked acetylcholine (ACh) release caused by the activation of PAR1-type muscle receptors induced by their peptide agonist TRAP6-NH₂ were studied. It has been shown for the first time that, in either mature or newly formed motor synapses, the activation of PAR1 that lack presynaptic localization causes a sustained increase in the MEPP amplitude due to the increase in the ACh quantal size at the presynaptic level. It was found that phospholipase C (PLC) participates in the signaling mechanism triggered by the PAR1 activation. Exogenously applied brain-derived neurotrophic factor (BDNF) mimics the effect of activation of PAR1 by TRAP6-NH₂. Moreover, an increase in the MEPP amplitude caused by the peptide PAR1 agonist was fully prevented by blocking the BDNF receptors–tropomyosin receptor kinases B (TrkB). Thus, it has been shown for the first time that the increase in ACh quantal size due to the activation of PAR1 in motor synapses is mediated by a complex signaling cascade that starts at the postsynaptic level of the motor synapse and ends at the presynaptic level. It is expected that the activation of PAR1 at the muscle fiber membrane followed by the PLC upregulation results in the release of neurotrophin BDNF as a retrograde signal. Its effect on the presynaptic TrkB receptors triggers the cascade leading to an increase in the quantal size of ACh.