Inosine induces presynaptic inhibition of acetylcholine release by activation of A3 adenosine receptors at the mouse neuromuscular junction

Migraine signaling pathways: purine metabolites that regulate migraine and predispose migraineurs to headache

Migraine is a debilitating disorder that afflicts over 1 billion people worldwide, involving attacks that result in a throbbing and pulsating headache. Migraine is thought to be a neurovascular event associated with vasoconstriction, vasodilation, and neuronal activation. Understanding signaling in migraine pathology is central to the development of therapeutics for migraine prophylaxis and for mitigation of migraine in the prodrome phase before pain sets in. The fact that both vasoactivity and neural sensitization are involved in migraine indicates that agonists which promote these phenomena may very well be involved in migraine pathology. One such group of agonists is the purines, in particular, adenosine phosphates and their metabolites. This manuscript explores what is known about the relationship between these metabolites and migraine pathology and explores the potential for such relationships through their known signaling pathways. Reported receptor involvement in vasoaction and nociception.

Role of purines in brain development, from neuronal proliferation to synaptic refinement

The purinergic system includes P1 and P2 receptors, which are activated by ATP and its metabolites. They are expressed in adult neuronal and glial cells and are crucial in brain function, including neuromodulation and neuronal signaling. As P1 and P2 receptors are expressed throughout embryogenesis and development, purinergic signaling also has an important role in the development of the peripheral and central nervous system. In this review, we present the expression pattern and activity of purinergic receptors and of their signaling pathways during embryonic and postnatal development of the nervous system. In particular, we review the involvement of the purinergic signaling in all the crucial steps of brain development i.e. in neural stem cell proliferation, neuronal differentiation and migration as well as in astrogliogenesis and oligodendrogenesis. Then, we review data showing a crucial role of the ATP and adenosine signaling pathways in the formation of the peripheral neuromuscular junction and of central GABAergic and glutamatergic synapses. Finally, we examine the consequences of deregulation of the purinergic system during development and discuss the therapeutic potential of targeting it at adult stage in diseases with reactivation of the ATP and adenosine pathway.

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.

The impact of inosine on hippocampal synaptic transmission and plasticity involves the release of adenosine through equilibrative nucleoside transporters rather than the direct activation of adenosine receptors

Inosine has robust neuroprotective effects, but it is unclear if inosine acts as direct ligand of adenosine receptors or if it triggers metabolic effects indirectly modifying the activity of adenosine receptors. We now combined radioligand binding studies with electrophysiological recordings in hippocampal slices to test how inosine controls synaptic transmission and plasticity. Inosine was without effect at 30 μM and decreased field excitatory post-synaptic potentials by 14% and 33% at 100 and 300 μM, respectively. These effects were prevented by the adenosine A₁ receptor antagonist DPCPX. Inosine at 300 (but not 100) μM also decreased the magnitude of long-term potentiation (LTP), an effect prevented by DPCPX and by the adenosine A_2A receptor antagonist SCH58261. Inosine showed low affinity towards human and rat adenosine receptor subtypes with K_i values of > 300 µM; only at the human and rat A₁ receptor slightly higher affinities with K_i values of around 100 µM were observed. Affinity of inosine at the rat A₃ receptor was higher (K_i of 1.37 µM), while it showed no interaction with the human orthologue. Notably, the effects of inosine on synaptic transmission and plasticity were abrogated by adenosine deaminase and by inhibiting equilibrative nucleoside transporters (ENT) with dipyridamole and NBTI. This shows that the impact of inosine on hippocampal synaptic transmission and plasticity is not due to a direct activation of adenosine receptors but is instead due to an indirect modification of the tonic activation of these adenosine receptors through an ENT-mediated modification of the extracellular levels of adenosine.

Effect of endogenous purines on electrically evoked ACh release at the mouse neuromuscular junction

At the mouse neuromuscular junction, adenosine triphosphate (ATP), which is co-released with the neurotransmitter acetylcholine (ACh), and its metabolite adenosine, modulate neurotransmitter release by activating presynaptic inhibitory P2Y₁₃ receptors (a subtype of ATP/adenosine diphosphate [ADP] receptor), inhibitory A₁ and A₃ adenosine receptors, and excitatory A_2A adenosine receptors. To study the effect of endogenous purines, when phrenic-diaphragm preparations are depolarized by different nerve stimulation patterns, we analyzed the effect of the antagonists for P2Y₁₃ , A₁ , A₃ , and A_2A receptors (AR-C69931MX, 8-cyclopentyl-1,3-dipropylxanthine, MRS-1191, and SCH-58261, respectively) on the amplitude of the end-plate potentials of the trains, and contrasted these results with those obtained with the selective agonists of these receptors (2-methylthioadenosine 5'-diphosphate trisodium salt hydrate, 2-chloro-N⁶ -cyclopentyl-adenosine, inosine, and PSB-0777, respectively). During continuous 0.5-Hz stimulation, the amount of endogenous purines was not enough to activate purinergic receptors, while at continuous 5-Hz stimulation, an incipient action of endogenous purines on P2Y₁₃ , A₁ and A3 receptors might be evident just at the end of the trains. During continuous 50-Hz stimulation, the concentration of endogenous ATP/ADP and adenosine exerted an inhibitory action on ACh release after of the initial phase of the train, but when the nerve was stimulated at intermittent 50 Hz (5 bursts), this behavior was not observed. Excitatory A_2A receptors were only activated when continuous 100-Hz stimulation was applied. In conclusion, when motor nerve terminals are depolarized by repetitive stimulation of the phrenic nerve, endogenous ATP/ADP and adenosine are able to fine-tune neurosecretion depending on the frequency and pattern of stimulation.

L-type Ca2+ Channels at Low External Calcium Differentially Regulate Neurotransmitter Release in Proximal-Distal Compartments of the Frog Neuromuscular Junction

L-type Ca²⁺ channels (LTCCs) are key elements in electromechanical coupling in striated muscles and formation of neuromuscular junctions (NMJs). However, the significance of LTCCs in regulation of neurotransmitter release is still far from understanding. Here, we found that LTCCs can increase evoked neurotransmitter release (especially asynchronous component) and spontaneous exocytosis in two functionally different compartment of the frog NMJ, namely distal and proximal parts. The effects of LTCC blockage on evoked and spontaneous release as well as timing of exocytotic events were prevented by inhibition of either protein kinase C (PKC) or P2Y receptors (P2Y-Rs). Hence, endogenous signaling via P2Y-R/PKC axis can sustain LTCC activity. Application of ATP, a co-neurotransmitter able to activate P2Y-Rs, suppressed both evoked and spontaneous exocytosis in distal and proximal parts. Surprisingly, inhibition of LTCCs (but not PKC) decreased the negative action of exogenous ATP on evoked (only in distal part) and spontaneous exocytosis. Lipid raft disruption suppressed (1) action of LTCC antagonist on neurotransmitter release selectively in distal region and (2) contribution of LTCCs in depressant effect of ATP on evoked and spontaneous release. Thus, LTCCs can enhance and desynchronize neurotransmitter release at basal conditions (without ATP addition), but contribute to ATP-mediated decrease in the exocytosis. The former action of LTCCs relies on P2Y-R/PKC axis, whereas the latter is triggered by exogenous ATP and PKC-independent. Furthermore, relevance of lipid rafts for LTCC function as well as LTCCs for ATP effects is different in distal and proximal part of the NMJ.

Opposing Effects of Adenosine and Inosine in Human Subcutaneous Fibroblasts May Be Regulated by Third Party ADA Cell Providers

Human subcutaneous fibroblasts (HSCF) challenged with inflammatory mediators release huge amounts of ATP, which rapidly generates adenosine. Given the nucleoside’s putative relevance in wound healing, dermal fibrosis, and myofascial pain, we investigated the role of its precursor, AMP, and of its metabolite, inosine, in HSCF cells growth and collagen production. AMP (30 µM) was rapidly (t½ 3 ± 1 min) dephosphorylated into adenosine by CD73/ecto-5′-nucleotidase. Adenosine accumulation (t½ 158 ± 17 min) in the extracellular fluid reflected very low cellular adenosine deaminase (ADA) activity. HSCF stained positively against A_2A and A₃ receptors but were A₁ and A_2B negative. AMP and the A_2A receptor agonist, CGS21680C, increased collagen production without affecting cells growth. The A_2A receptor antagonist, SCH442416, prevented the effects of AMP and CGS21680C. Inosine and the A₃ receptor agonist, 2Cl-IB-MECA, decreased HSCF growth and collagen production in a MRS1191-sensitive manner, implicating the A₃ receptor in the anti-proliferative action of inosine. Incubation with ADA reproduced the inosine effect. In conclusion, adenosine originated from extracellular ATP hydrolysis favors normal collagen production by HSCF via A_2A receptors. Inhibition of unpredicted inosine formation by third party ADA cell providers (e.g., inflammatory cells) may be a novel therapeutic target to prevent inappropriate dermal remodeling via A₃ receptors activation.

Inosine protects against impairment of memory induced by experimental model of Alzheimer disease: a nucleoside with multitarget brain actions

RATIONALE

Inosine is a naturally occurring purine nucleoside formed by adenosine breakdown. This nucleoside is reported to exert potent effects on memory and learning, possibly through its antioxidant and anti-inflammatory actions.

OBJECTIVE

The objective is to evaluate the effects of inosine on the behavioral and neurochemical parameters in a rat model of Alzheimer's disease (AD) induced by streptozotocin (STZ).

METHODS

Adult male rats were divided into four groups: control (saline), STZ, STZ plus inosine (50 mg/kg), and STZ plus inosine (100 mg/kg). STZ (3 mg/kg) was administered by bilateral intracerebroventricular injection. The animals were treated intraperitoneally with inosine for 25 days. Memory, oxidative stress, ion pump activities, acetylcholinesterase (AChE), and choline acetyltransferase (ChAT) activities and expression were evaluated in the cerebral cortex and hippocampus.

RESULTS

The memory impairment induced by STZ was prevented by inosine. An increase in the Na⁺, K⁺-ATPase, and Mg-ATPase activities and a decrease in the Ca²⁺-ATPase activity were induced by STZ in the hippocampus and cerebral cortex, and inosine could prevent these alterations in ion pump activities. Inosine also prevented the increase in AChE activity and the alterations in AChE and ChAT expression induced by STZ. STZ increased the reactive oxygen species, nitrite levels, and superoxide dismutase activity and decreased the catalase and glutathione peroxidase activities. Inosine treatment conferred protection from these oxidative alterations in the brain.

CONCLUSIONS

Our findings demonstrate that inosine affects brain multiple targets suggesting that this molecule may have therapeutic potential against cognitive deficit and tissue damage in AD.

Inosine Accelerates the Regeneration and Anticipates the Functional Recovery after Sciatic Nerve Crush Injury in Mice

Trauma to the peripheral nervous system (PNS) results in loss of motor and sensory functions. After an injury, a complex series of events begins, allowing axonal regeneration and target reinnervation. However, this regenerative potential is limited by several factors such as age, distance from the lesion site to the target and severity of lesion. Many studies look for ways to overcome these limitations. Inosine, a purine nucleoside derived from adenosine, emerges as a potential treatment, due to its capacity to regulate axonal growth, neuroprotection and immunomodulation, contributing to motor recovery. However, no studies demonstrated their effects on PNS. C57/Black6 mice were submitted to sciatic nerve crush and received intraperitoneal injections of saline or inosine (70 mg/kg), one hour after injury and daily for one week. To evaluate axonal regeneration and functional recovery, electroneuromyography, Sciatic Function Index (SFI), rotarod and pinprick tests were performed. Our results showed that the inosine group presented a higher number of myelinated fibers and a large amount of fibers within the ideal G-ratio. In addition, the results of electroneuromyography showed greater amplitude of the compound muscle action potentials in the first and second weeks, suggesting anticipation of regeneration in the inosine group. We also observed in the inosine group, motor and sensory neurons survival, reduction in the number of macrophages and myelin ovoids in the sciatic nerves, and an early recovery of motor and sensory functions. Thus, we conclude that the use of inosine accelerates axonal regeneration promoting an early recovery of motor and sensory functions.

Guanine and inosine nucleotides/nucleosides suppress murine T cell activation

Damaged tissues and cells release intracellular purine nucleotides, which serve as intercellular signaling factors. We previously showed that exogenously added adenine nucleotide (250 μM ATP) suppressed the activation of murine splenic T lymphocytes. Here, we examined the effects of other purine nucleotides/nucleosides on mouse T cell activation. First, we found that pretreatment of mouse spleen T cells with 250 μM GTP, GDP, GMP, guanosine, ITP, IDP, IMP or inosine significantly reduced the release of stimulus-inducible cytokine IL-2. This suppression of IL-2 release was not caused by induction of cell death. Further studies with GTP, ITP, guanosine and inosine showed that pretreatment with these nucleotides/nucleosides also suppressed release of IL-6. However, these nucleotides/nucleosides did not suppress stimulus-induced phosphorylation of ERK1/2, suggesting that the suppression of the release of inflammatory cytokines does not involve inhibition of ERK1/2 signaling. In contrast to ATP pretreatment at the same concentration, guanine or inosine nucleotides/nucleosides did not attenuate the expression of CD25. Our findings indicate that exogenous guanine or inosine nucleotides/nucleosides can suppress inflammatory cytokine release from T cells, and may be promising candidates for use as supplementary agents in the treatment of T cell-mediated immune diseases.

Endogenous purines modulate K+ -evoked ACh secretion at the mouse neuromuscular junction

At the mouse neuromuscular junction, adenosine triphosphate (ATP) is co-released with the neurotransmitter acetylcholine (ACh), and once in the synaptic cleft, it is hydrolyzed to adenosine. Both ATP/adenosine diphosphate (ADP) and adenosine modulate ACh secretion by activating presynaptic P2Y₁₃ and A₁ , A_2A , and A₃ receptors, respectively. To elucidate the action of endogenous purines on K⁺ -dependent ACh release, we studied the effect of purinergic receptor antagonists on miniature end-plate potential (MEPP) frequency in phrenic diaphragm preparations. At 10 mM K⁺ , the P2Y₁₃ antagonist N-[2-(methylthio)ethyl]-2-[3,3,3-trifluoropropyl]thio-5'-adenylic acid, monoanhydride with (dichloromethylene)bis[phosphonic acid], tetrasodium salt (AR-C69931MX) increased asynchronous ACh secretion while the A₁ , A₃ , and A_2A antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), (3-Ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1, 4-(±)-dihydropyridine-3,5-, dicarboxylate (MRS-1191), and 2-(2-Furanyl)-7-(2-phenylethyl)-7H-pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-amine (SCH-58261) did not modify neurosecretion. The inhibition of equilibrative adenosine transporters by S-(p-nitrobenzyl)-6-thioinosine provoked a reduction of 10 mM K⁺ -evoked ACh release, suggesting that the adenosine generated from ATP is being removed from the synaptic space by the transporters. At 15 and 20 mM K⁺ , endogenous ATP/ADP and adenosine bind to inhibitory P2Y₁₃ and A₁ and A₃ receptors since AR-C69931MX, DPCPX, and MRS-1191 increased MEPP frequency. Similar results were obtained when the generation of adenosine was prevented by using the ecto-5'-nucleotidase inhibitor α,β-methyleneadenosine 5'-diphosphate sodium salt. SCH-58261 only reduced neurosecretion at 20 mM K⁺ , suggesting that more adenosine is needed to activate excitatory A_2A receptors. At high K⁺ concentration, the equilibrative transporters appear to be saturated allowing the accumulation of adenosine in the synaptic cleft. In conclusion, when motor nerve terminals are depolarized by increasing K⁺ concentrations, the ATP/ADP and adenosine endogenously generated are able to modulate ACh secretion by sequential activation of different purinergic receptors.

Effect of perinatal asphyxia on tuberomammillary nucleus neuronal density and object recognition memory: A possible role for histamine?

Perinatal asphyxia (PA) is associated with long-term neuronal damage and cognitive deficits in adulthood, such as learning and memory disabilities. After PA, specific brain regions are compromised, including neocortex, hippocampus, basal ganglia, and ascending neuromodulatory pathways, such as dopamine system, explaining some of the cognitive disabilities. We hypothesize that other neuromodulatory systems, such as histamine system from the tuberomammillary nucleus (TMN), which widely project to telencephalon, shown to be relevant for learning and memory, may be compromised by PA. We investigated here the effect of PA on (i) Density and neuronal activity of TMN neurons by double immunoreactivity for adenosine deaminase (ADA) and c-Fos, as marker for histaminergic neurons and neuronal activity respectively. (ii) Expression of the histamine-synthesizing enzyme, histidine decarboxylase (HDC) by western blot and (iii) thioperamide an H3 histamine receptor antagonist, on an object recognition memory task. Asphyxia-exposed rats showed a decrease of ADA density and c-Fos activity in TMN, and decrease of HDC expression in hypothalamus. Asphyxia-exposed rats also showed a low performance in object recognition memory compared to caesarean-delivered controls, which was reverted in a dose-dependent manner by the H3 antagonist thioperamide (5-10mg/kg, i.p.). The present results show that the histaminergic neuronal system of the TMN is involved in the long-term effects induced by PA, affecting learning and memory.

P2Y13 receptors mediate presynaptic inhibition of acetylcholine release induced by adenine nucleotides at the mouse neuromuscular junction

It is known that adenosine 5'-triphosphate (ATP) is released along with the neurotransmitter acetylcholine (ACh) from motor nerve terminals. At mammalian neuromuscular junctions (NMJs), we have previously demonstrated that ATP is able to decrease ACh secretion by activation of P2Y receptors coupled to pertussis toxin-sensitive Gi/o protein. In this group, the receptor subtypes activated by adenine nucleotides are P2Y12 and P2Y13. Here, we investigated, by means of pharmacological and immunohistochemical assays, the P2Y receptor subtype that mediates the modulation of spontaneous and evoked ACh release in mouse phrenic nerve-diaphragm preparations. First, we confirmed that the preferential agonist for P2Y12-13 receptors, 2-methylthioadenosine 5'-diphosphate trisodium salt hydrate (2-MeSADP), reduced MEPP frequency without affecting MEPP amplitude as well as the amplitude and quantal content of end-plate potentials (EPPs). The effect on spontaneous secretion disappeared after the application of the selective P2Y12-13 antagonists AR-C69931MX or 2-methylthioadenosine 5'-monophosphate triethylammonium salt hydrate (2-MeSAMP). 2-MeSADP was more potent than ADP and ATP in reducing MEPP frequency. Then we demonstrated that the selective P2Y13 antagonist MRS-2211 completely prevented the inhibitory effect of 2-MeSADP on MEPP frequency and EPP amplitude, whereas the P2Y12 antagonist MRS-2395 failed to do this. The preferential agonist for P2Y13 receptors inosine 5'-diphosphate sodium salt (IDP) reduced spontaneous and evoked ACh secretion and MRS-2211 abolished IDP-mediated modulation. Immunohistochemical studies confirmed the presence of P2Y13 but not P2Y12 receptors at the end-plate region. Disappearance of P2Y13 receptors after denervation suggests the presynaptic localization of the receptors. We conclude that, at motor nerve terminals, the Gi/o protein-coupled P2Y receptors implicated in presynaptic inhibition of spontaneous and evoked ACh release are of the subtype P2Y13. This study provides new insights into the types of purinergic receptors that contribute to the fine-tuning of cholinergic transmission at mammalian neuromuscular junction.

Adenosine enhances acetylcholine receptor channel openings and intracellular calcium 'spiking' in mouse skeletal myotubes

AIMS

The autocrine activity of the embryonic isoform of the nicotinic acetylcholine receptor is crucial for the correct differentiation and trophism of skeletal muscle cells before innervation. The functional activity of extracellular adenosine and adenosine receptor subtypes expressed in differentiating myotubes is still unknown. In this study, we performed a detailed analysis of the role of adenosine receptor-mediated effects on the autocrine-mediated nicotinic acetylcholine receptor channel openings and the associated spontaneous intracellular calcium 'spikes' generated in differentiating mouse myotubes in vitro.

METHODS

Cell-attached patch-clamp recordings and intracellular calcium imaging experiments were performed in contracting myotubes derived from mouse satellite cells.

RESULTS

The endogenous extracellular adenosine and the adenosine receptor-mediated activity modulated the properties of the embryonic isoform of the nicotinic acetylcholine receptor in myotubes in vitro, by increasing the mean open time and the open probability of the ion channel, and sustaining nicotinic acetylcholine receptor-driven intracellular [Ca(2+) ]i 'spikes'. The pharmacological characterization of the adenosine receptor-mediated effects suggested a prevalent involvement of the A2B adenosine receptor subtype.

CONCLUSION

We propose that the interplay between endogenous adenosine and nicotinic acetylcholine receptors represents a potential novel strategy to improve differentiation/regeneration of skeletal muscle.

The A3 adenosine receptor: history and perspectives

By general consensus, the omnipresent purine nucleoside adenosine is considered a major regulator of local tissue function, especially when energy supply fails to meet cellular energy demand. Adenosine mediation involves activation of a family of four G protein-coupled adenosine receptors (ARs): A(1), A(2)A, A(2)B, and A(3). The A(3) adenosine receptor (A(3)AR) is the only adenosine subtype to be overexpressed in inflammatory and cancer cells, thus making it a potential target for therapy. Originally isolated as an orphan receptor, A(3)AR presented a twofold nature under different pathophysiologic conditions: it appeared to be protective/harmful under ischemic conditions, pro/anti-inflammatory, and pro/antitumoral depending on the systems investigated. Until recently, the greatest and most intriguing challenge has been to understand whether, and in which cases, selective A(3) agonists or antagonists would be the best choice. Today, the choice has been made and A(3)AR agonists are now under clinical development for some disorders including rheumatoid arthritis, psoriasis, glaucoma, and hepatocellular carcinoma. More specifically, the interest and relevance of these new agents derives from clinical data demonstrating that A(3)AR agonists are both effective and safe. Thus, it will become apparent in the present review that purine scientists do seem to be getting closer to their goal: the incorporation of adenosine ligands into drugs with the ability to save lives and improve human health.

Direct visualisation of internalization of the adenosine A3 receptor and localization with arrestin3 using a fluorescent agonist

Fluorescence based probes provide a novel way to study the dynamic internalization process of G protein-coupled receptors (GPCRs). Recent advances in the rational design of fluorescent ligands for GPCRs have been used here to generate new fluorescent agonists containing tripeptide linkers for the adenosine A3 receptor. The fluorescent agonist BY630-X-(D)-A-(D)-A-G-ABEA was found to be a highly potent agonist at the adenosine A3 receptor in both reporter gene (pEC50 = 8.48 ± 0.09) and internalization assays (pEC50 = 7.47 ± 0.11). Confocal imaging studies showed that BY630-X-(D)-A-(D)-A-G-ABEA was internalized with A3 linked to yellow fluorescent protein, which was blocked by the competitive antagonist MRS1220. Internalization of untagged adenosine A3 could also be visualized with BY630-X-(D)-A-(D)-A-G-ABEA treatment. Further, BY630-X-(D)-A-(D)-A-G-ABEA stimulated the formation of receptor-arrestin3 complexes and was found to localize with these intracellular complexes. This highly potent agonist with excellent imaging properties should be a valuable tool to study receptor internalization. This article is part of the Special Issue entitled 'Fluorescent Tools in Neuropharmacology'.

Melatonin-mediated inhibition of Purkinje neuron P-type Ca²⁺ channels in vitro induces neuronal hyperexcitability through the phosphatidylinositol 3-kinase-dependent protein kinase C delta pathway

Although melatonin receptors are widely expressed in the mammalian central nervous system and peripheral tissues, there are limited data regarding the functions of melatonin in cerebellar Purkinje cells. Here, we identified a novel functional role of melatonin in modulating P-type Ca(2+) channels and action-potential firing in rat Purkinje neurons. Melatonin at 0.1 μm reversibly decreased peak currents (I(Ba)) by 32.9%. This effect was melatonin receptor 1 (MT(R1)) dependent and was associated with a hyperpolarizing shift in the voltage dependence of inactivation. Pertussis toxin pretreatment, intracellular application of QEHA peptide, and a selective antibody raised against the Gβ subunit prevented the inhibitory effects of melatonin. Pretreatment with phosphatidylinositol 3-kinase (PI3K) inhibitors abolished the melatonin-induced decrease in I(Ba). Surprisingly, melatonin responses were not regulated by Akt, a common downstream target of PI3K. Melatonin treatment significantly increased protein kinase C (PKC) activity 2.1-fold. Antagonists of PKC, but not of protein kinase A, abolished the melatonin-induced decrease in I(Ba). Melatonin application increased the membrane abundance of PKCδ, and PKCδ inhibition (either pharmacologically or genetically) abolished the melatonin-induced IBa response. Functionally, melatonin increased spontaneous action-potential firing by 53.0%; knockdown of MT(R1) and blockade of P-type channels abolished this effect. Thus, our results suggest that melatonin inhibits P-type channels through MT(R1) activation, which is coupled sequentially to the βγ subunits of G(i/o)-protein and to downstream PI3K-dependent PKCδ signaling. This likely contributes to its physiological functions, including spontaneous firing of cerebellar Purkinje neurons.

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

The mechanisms underlying improvement of neuromuscular transmission deficits by glucocorticoids are still a matter of debate despite these compounds have been used for decades in the treatment of autoimmune myasthenic syndromes. Besides their immunosuppressive action, corticosteroids may directly facilitate transmitter release during high-frequency motor nerve activity. This effect coincides with the predominant adenosine A2A receptor tonus, which coordinates the interplay with other receptors (e.g. muscarinic) on motor nerve endings to sustain acetylcholine (ACh) release that is required to overcome tetanic neuromuscular depression in myasthenics. Using myographic recordings, measurements of evoked [(3)H]ACh release and real-time video microscopy with the FM4-64 fluorescent dye, results show that tonic activation of facilitatory A2A receptors by endogenous adenosine accumulated during 50 Hz bursts delivered to the rat phrenic nerve is essential for methylprednisolone (0.3 mM)-induced transmitter release facilitation, because its effect was prevented by the A2A receptor antagonist, ZM 241385 (10 nM). Concurrent activation of the positive feedback loop operated by pirenzepine-sensitive muscarinic M1 autoreceptors may also play a role, whereas the corticosteroid action is restrained by the activation of co-expressed inhibitory M2 and A1 receptors blocked by methoctramine (0.1 μM) and DPCPX (2.5 nM), respectively. Inhibition of FM4-64 loading (endocytosis) by methylprednisolone following a brief tetanic stimulus (50 Hz for 5 s) suggests that it may negatively modulate synaptic vesicle turnover, thus increasing the release probability of newly recycled vesicles. Interestingly, bulk endocytosis was rehabilitated when methylprednisolone was co-applied with ZM241385. Data suggest that amplification of neuromuscular transmission by methylprednisolone may involve activation of presynaptic facilitatory adenosine A2A receptors by endogenous adenosine leading to synaptic vesicle redistribution.

Adenosine A2B and A3 receptor location at the mouse neuromuscular junction

To date, four subtypes of adenosine receptors have been cloned (A(1)R, A(2A)R, A(2B)R, and A(3)R). In a previous study we used confocal immunocytochemistry to identify A(1)R and A(2A)R receptors at mouse neuromuscular junctions (NMJs). The data shows that these receptors are localized differently in the three cells (muscle, nerve and glia) that configure the NMJs. A(1)R localizes in the terminal teloglial Schwann cell and nerve terminal, whereas A(2A)R localizes in the postsynaptic muscle and in the axon and nerve terminal. Here, we use Western blotting to investigate the presence of A(2B)R and A(3)R receptors in striated muscle and immunohistochemistry to localize them in the three cells of the adult neuromuscular synapse. The data show that A(2B)R and A(3)R receptors are present in the nerve terminal and muscle cells at the NMJs. Neither A(2B)R nor A(3)R receptors are localized in the Schwann cells. Thus, the four subtypes of adenosine receptors are present in the motor endings. The presence of these receptors in the neuromuscular synapse allows the receptors to be involved in the modulation of transmitter release.