Evidence that ATP or a related purine is an excitatory neurotransmitter in the longitudinal muscle of mouse distal colon

ATP-Induced Contractile Response of Esophageal Smooth Muscle in Mice

The tunica muscularis of mammalian esophagi is composed of striated muscle and smooth muscle. Contraction of the esophageal striated muscle portion is mainly controlled by cholinergic neurons. On the other hand, smooth muscle contraction and relaxation are controlled not only by cholinergic components but also by non-cholinergic components in the esophagus. Adenosine triphosphate (ATP) is known to regulate smooth muscle contraction and relaxation in the gastrointestinal tract via purinergic receptors. However, the precise mechanism of purinergic regulation in the esophagus is still unclear. Therefore, the aim of the present study was to clarify the effects of ATP on the mechanical responses of the esophageal muscle in mice. An isolated segment of the mouse esophagus was placed in a Magnus's tube and longitudinal mechanical responses were recorded. Exogenous application of ATP induced contractile responses in the esophageal preparations. Tetrodotoxin, a blocker of voltage-dependent sodium channels in neurons and striated muscle, did not affect the ATP-induced contraction. The ATP-evoked contraction was blocked by pretreatment with suramin, a purinergic receptor antagonist. RT-PCR revealed the expression of mRNA of purinergic receptor genes in the mouse esophageal tissue. The findings suggest that purinergic signaling might regulate the motor activity of mouse esophageal smooth muscle.

Emerging Role of Alarmins in Food Allergy: An Update on Pathophysiological Insights, Potential Use as Disease Biomarkers, and Therapeutic Implications

Food allergies are immuno-mediated adverse reactions to ingestion or contact with foods, representing a widespread health problem. The immune response can be IgE-mediated, non-IgE-mediated, or with a mixed mechanism. The role of innate immunity and alarmins in the pathogenesis of diseases such as asthma and atopic dermatitis is well known. Some authors have investigated the correlation between alarmins and food allergies, often obtaining interesting results. We analyzed articles published in English from the last 22 years present on PubMed concerning the role of alarmins in the pathogenesis of food allergies and their potential use as disease biomarkers, response biomarkers to therapy, or potential therapeutic targets. Nuclear alarmins (TSLP, IL-33, IL-25) appear to have a critical role in IgE-mediated allergies but are also implicated in entities such as eosinophilic esophagitis. Calprotectin and defensins may play a role as disease biomarkers and could help predict response to therapy, although results in the literature are often conflicting. Despite the promising results, more studies on humans still need to be conducted. Deepening our knowledge regarding alarmins and their involvement in food allergies could lead to the development of new biological therapies, significantly impacting patients' quality of life.

Nitrergic and Purinergic Nerves in the Small Intestinal Myenteric Plexus and Circular Muscle of Mice and Guinea Pigs

ATP is an excitatory and inhibitory neurotransmitter, while nitric oxide (NO) is an inhibitory neurotransmitter in the enteric nervous system (ENS). We used a vesicular nucleotide transporter (SLC17A9, VNUT) antibody and a nitric oxide synthase (NOS) antibody to identify purinergic and nitrergic nerves in mouse and guinea ileum. Mouse: VNUT-immunoreactivity (ir) was detected in nerve fibers in myenteric ganglia and circular muscle. VNUT-ir fibers surrounded choline acetyltransferase (ChAT), nitric oxide synthase (nNOS), and calretinin-ir neurons. VNUT-ir nerve cell bodies were not detected. Tyrosine hydroxylase (TH)-ir nerves were detected in myenteric ganglia and the tertiary plexus. Guinea pig: VNUT-ir was detected in neurons and nerves fibers and did not overlap with NOS-ir nerve fibers. VNUT-ir was detected in nerve fibers in ganglia but not nerve cell bodies. VNUT-ir nerve fibers surrounded NOS-ir and NOS^- neurons. NOS-ir and VNUT-ir nerve fibers did not overlap in myenteric ganglia or circular muscle. VNUT-ir nerves surrounded some ChAT-ir neurons. VNUT-ir and ChAT-ir were detected in separate nerves in the CM. VNUT-ir nerve fibers surrounded calretinin-ir neurons.Conclusions: VNUT-ir neurons likely mediate purinergic signaling in small intestinal myenteric ganglia and circular muscle. ATP and NO are likely released from different inhibitory motorneurons. VNUT-ir and ChAT-ir interneurons mediate cholinergic and purinergic synaptic transmission in the myenteric plexus.

Ibogaine-Mediated ROS/Antioxidant Elevation in Isolated Rat Uterus Is β-Adrenergic Receptors and KATP Channels Mediated

Ibogaine effects are mediated by cellular receptors, ATP depletion followed by ROS production and antioxidant enzyme activity elevation in a dose and time dependent manner. Since the role of K_ATP channels and β-adrenoceptors in ROS cellular circuit was established here we explored their role in ibogaine pro-antioxidant effectiveness. Single dose of ibogaine (10 mg/L i.e., 28.8 μmol/L) was applied to isolated rat uterus (spontaneous and Ca²⁺-stimulated) and contractility and antioxidant enzymes activity were monitored during 4 h. Ibogaine increased amplitude and frequency of spontaneous active uteri immediately after addition that was prevented by propranolol (β₁ and β₂ adrenoceptors selective antagonists) and glibenclamide (K_ATP sensitive channels inhibitor; only frequency) pre-treatment. In Ca²⁺-stimulated uteri, ibogaine decreased both amplitude and frequency after 4 h. Pre-treatment with propranolol abolished ibogaine induced amplitude lowering, while glibenclamide had no effect. In both types of active uterus, ibogaine induced a decrease in SOD1 and an increase in CAT activity after 2 h. In Ca²⁺-stimulated uterus, there was also a decrease of SOD2 activity after 2 h. After 4 h, SOD1 activity returned to the baseline level, but GSH-Px activity increased. Pre-treatment with both propranolol and glibenclamide abolished observed changes of antioxidant enzymes activity suggesting that ibogaine pro-antioxidative effectiveness is β-adrenergic receptors and K_ATP channels mediated.

Functional atropine sensitive purinergic responses in the healthy rat bladder

While acetylcholine is regarded to be the main directly contractile transmitter substance in the urinary bladder, interactions with other transmitters likely occur. Presently, the interplay between purinergic and cholinergic signalling was investigated to unravel the involvement of the urothelium and efferent neurons in the functionally important purinergically evoked release of acetylcholine in vitro. Functional characterization of receptor subtypes involved in this interplay was also performed. In vitro organ bath experiments with electrical field stimulation (EFS) or administration of agonist were performed in the absence and presence of the neurotoxin tetrodotoxin (TTX; 5 × 10^-7 M) and/or receptor antagonists, in intact and urothelium-denuded full thickness rat bladder strip preparations. Interestingly, functional contractions to ATP (10^-6-10^-3 M) remained unaffected by TTX, but were significantly lowered in the presence of the muscarinic antagonist atropine (10^-6 M). However, in urothelium-denuded strip preparations, this latter phenomenon was not present and the ATP response remained unaltered. To rule out purinergic interference caused by break-down of ATP, experiments were performed in which the stable ATP-analogue αβMeATP (10^-7-10^-5 M) gave rise to functional atropine-sensitive contractions. Furthermore, contractions to ATP were not affected by P2Y6 purinoceptor blockade (by MRS2578; 10^-7, 10^-5 M), nor were relaxatory responses to ATP sensitive to atropine, PPADS (3 × 10^-5 M) or αβMeATP. Lastly, relaxations to ADP (10^-6-10^-3 M) or NECA (10^-8-10^-5 M) were unaltered by the presence of atropine. To conclude, purinergic functional contractile, but not relaxatory, responses are supported by the cholinergic transmitter system in vitro, through non-neuronal mechanisms in the urothelium. Involved purinoceptors are of the P2X-subtype, most likely P2X1 and/or P2X3.

Laxative Activity of the Hot-Water Extract Mixture of Hovenia dulcis Thunb. and Phyllostachys pubescens Mazel in Chronic Constipation Model SD Rats

This study examined the laxative effects of hot-water extracts of Hovenia dulcis Thunb. (HD), Phyllostachys pubescens Mazel (PM), and a 2:8 mixture of both (HP) in two chronic constipation models. For the loperamide-induced constipation model, animals were divided into an untreated group, negative control group (loperamide 4 mg/kg), positive control group (bisacodyl 4 mg/kg) group, and six treatment groups (HP 100 or 400, HD 50 or 100, and PM 100 or 400 mg/kg). For the lowfiber diet-induced constipation model, animals were divided into an untreated group (normal diet), negative control group (low-fiber diet), positive control group (Agio granule, 620 mg/kg), and the same treatment groups. Fecal number, weight, fecal water content, and intestinal transit ratio were higher in the groups treated with HP, HD, and PM than in the groups treated with loperamide or lowfiber diet. Thickness of colon mucosa and muscle layers were increased in the treated groups. Colon tension increased in the HP groups, and [Ca2+]i measurements using fura-2 as an indicator showed that HP inhibits ATP-mediated Ca2+ influx in IEC-18 cells. These results showed that the HP mixture has laxative activity by increased mucin secretion and inducing contractile activity and relaxation. It may be a useful therapeutic strategy for ameliorating in chronic constipation.

The asymmetric innervation of the circular and longitudinal muscle of the mouse colon differently modulates myogenic slow phasic contractions

BACKGROUND

Neuromuscular transmission has been extensively studied in the circular layer of the mouse colon where a co-transmission of purines acting on P2Y₁ receptors and NO has been previously described. However, the corresponding mechanisms in the longitudinal layer are less known.

METHODS

Electrophysiological and myography techniques were used to evaluate spontaneous phasic contractions (SPC) and neural-mediated responses in the proximal, mid, and distal colon devoid of CD1 mice. Immunohistochemistry against c-kit and PDGFRα was performed in each colonic segment.

KEY RESULTS

SPC were recorded in both muscle layers at a similar frequency being about four contractions per minute (c.p.m.) in the proximal and distal colon compared to the mid colon (2 c.p.m.). In non-adrenergic, non-cholinergic conditions, L-NNA (1 mmol/L) increased contractility in the circular but not in the longitudinal layer. In the longitudinal muscle, both electrophysiological and mechanical neural-mediated inhibitory responses were L-NNA and ODQ (10 µmol/L) sensitive. NaNP (1 µmol/L) caused cessation of SPC and the response was blocked by ODQ. Neither ADPßS (10 µmol/L) nor CYPPA (10 µmol/L), which both targeted the purinergic pathway, altered longitudinal contractions. PDGFRα + cells were located in both muscle layers and were more numerous compared with cKit + cells, which both formed a heterologous cellular network. A decreasing gradient of the PDGFRα labeling was observed along the colon.

CONCLUSION

An inhibitory neural tone was absent in the longitudinal layer and neuronal inhibitory responses were mainly nitrergic. Despite the presence of PDGFRα + cells, purinergic responses were absent. Post-junctional pathways located in different cell types might be responsible for neurotransmitter transduction.

Inhibitory action of oxytocin on spontaneous contraction of rat distal colon by nitrergic mechanism: involvement of cyclic GMP and apamin-sensitive K+ channels

AIM

The mechanisms underlying the inhibitory effects of oxytocin (OT) on colon tone are not totally understood. We explore the mechanisms of OT on spontaneous contractility in rat distal colon and identify the mediators involved in this action.

METHODS

In rat distal colon strips, mechanical activity was analysed and the production of nitric oxide (NO) in tissue loaded with the fluorochrome DAF-FM was visualized by confocal microscopy. OT receptor (OTR) expression was determined by Western blotting and immunofluorescence.

RESULTS

In rat distal colon, OT produced a concentration-dependent reduction in the spontaneous contraction, which was abolished by the OTR antagonist atosiban, the neural blocker tetrodotoxin and the inhibitor of neuronal nitric oxide synthase (nNOS) NPLA. The inhibitory effects of OT were not affected by propranolol, atropine, the nicotinic cholinoceptor blocker hexamethonium, the vasoactive intestinal peptide receptor antagonist VIPHyb, the P₂ purinoceptor antagonist PPADS, the adenosine A1 receptors antagonist DPCPX and the prostacyclin receptor antagonist Ro1138452. The soluble guanylyl cyclase (sGC) inhibitor ODQ and the small conductance Ca²⁺ -activated K⁺ (_Ca K⁺ ) channels blocker apamin significantly reduced the relaxation induced by OT, nicotine, sodium nitroprusside and the sGC activator BAY 41-2272. The neural release of NO elicited by OT was prevented by NPLA, tetrodotoxin and atosiban. The presence of the OTR and its co-localization with nNOS was detected by immunohistochemistry and Western blotting experiments.

CONCLUSION

These results demonstrate the NO release from enteric neurones induced by activation of OTR mediates distal colon relaxation. sGC and small conductance _Ca K⁺ channels are involved in this relaxation.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

The role of purinergic pathways in the pathophysiology of gut diseases: pharmacological modulation and potential therapeutic applications

Gut homeostasis results from complex neuro-immune interactions aimed at triggering stereotypical and specific programs of coordinated mucosal secretion and powerful motor propulsion. A prominent role in the regulation of this highly integrated network, comprising a variety of immune/inflammatory cells and the enteric nervous system, is played by purinergic mediators. The cells of the digestive tract are literally plunged into a "biological sea" of functionally active nucleotides and nucleosides, which carry out the critical task of driving regulatory interventions on cellular functions through the activation of P1 and P2 receptors. Intensive research efforts are being made to achieve an integrated view of the purinergic system, since it is emerging that the various components of purinergic pathways (i.e., enzymes, transporters, mediators and receptors) are mutually linked entities, deputed to finely modulating the magnitude and the duration of purinergic signaling, and that alterations occurring in this balanced network could be intimately involved in the pathophysiology of several gut disorders. This review article intends to provide a critical appraisal of current knowledge on the purinergic system role in the regulation of gastrointestinal functions, considering these pathways as a whole integrated network, which is capable of finely controlling the levels of bioactive nucleotides and nucleosides in the biophase of their respective receptors. Special attention is paid to the mechanisms through which alterations in the various compartments of the purinergic system could contribute to the pathophysiology of gut disorders, and to the possibility of counteracting such dysfunctions by means of pharmacological interventions on purinergic molecular targets.

Satellite glial cell P2Y12 receptor in the trigeminal ganglion is involved in lingual neuropathic pain mechanisms in rats

Background

It has been reported that the P2Y₁₂ receptor (P2Y₁₂R) is involved in satellite glial cells (SGCs) activation, indicating that P2Y₁₂R expressed in SGCs may play functional roles in orofacial neuropathic pain mechanisms. However, the involvement of P2Y₁₂R in orofacial neuropathic pain mechanisms is still unknown. We therefore studied the reflex to noxious mechanical or heat stimulation of the tongue, P2Y₁₂R and glial fibrillary acidic protein (GFAP) immunohistochemistries in the trigeminal ganglion (TG) in a rat model of unilateral lingual nerve crush (LNC) to evaluate role of P2Y₁₂R in SGC in lingual neuropathic pain.

Results

The head-withdrawal reflex thresholds to mechanical and heat stimulation of the lateral tongue were significantly decreased in LNC-rats compared to sham-rats. These nocifensive effects were apparent on day 1 after LNC and lasted for 17 days. On days 3, 9, 15 and 21 after LNC, the mean relative number of TG neurons encircled with GFAP-immunoreactive (IR) cells significantly increased in the ophthalmic, maxillary and mandibular branch regions of TG. On day 3 after LNC, P2Y₁₂R expression occurred in GFAP-IR cells but not neuronal nuclei (NeuN)-IR cells (i.e. neurons) in TG. After 3 days of successive administration of the P2Y₁₂R antagonist MRS2395 into TG in LNC-rats, the mean relative number of TG neurons encircled with GFAP-IR cells was significantly decreased coincident with a significant reversal of the lowered head-withdrawal reflex thresholds to mechanical and heat stimulation of the tongue compared to vehicle-injected rats. Furthermore, after 3 days of successive administration of the P2YR agonist 2-MeSADP into the TG in naïve rats, the mean relative number of TG neurons encircled with GFAP-IR cells was significantly increased and head-withdrawal reflex thresholds to mechanical and heat stimulation of the tongue were significantly decreased in a dose-dependent manner compared to vehicle-injected rats.

Conclusions

The present findings provide the first evidence that the activation of P2Y₁₂R in SGCs of TG following lingual nerve injury is involved in the enhancement of TG neuron activity and nocifensive reflex behavior, resulting in neuropathic pain in the tongue.

A holistic view of adenosine in the control of intestinal neuromuscular functions: the enteric 'purinome' concept

Adenosine is involved in the modulation of enteric neuromuscular functions, operating a fine tuning of smooth muscle contractility, peristaltic reflex and transit. In this issue of the BJP, Zizzo et al. report novel findings on the expression of adenosine receptors in mouse duodenum, extending our knowledge of their involvement in the control of spontaneous and neurogenic intestinal motility. In this study, particular attention was paid to the differential activation of adenosine receptors, as a result of their interplay with regulatory systems, modulating the availability of endogenous adenosine in a compartmentalised manner. This evidence will contribute to the holistic evaluation of the role played by adenosine in the regulation of intestinal motility, in accordance with the novel concept of the enteric 'purinome'. This commentary discusses the role of the 'purinome' in the modulation of enteric neuromuscular activity, pointing out its involvement in the intestinal neuroplasticity associated with bowel dysmotility.

Can guanine-based purines be considered modulators of intestinal motility in rodents?

Adenine-based purines play a pivotal role in the control of gastrointestinal motility in rodents. Recently, guanine-based purines have been also shown to exert extracellular effects in the central nervous system raising the possibility of the existence of distinct receptors for guanine-based purines. Thus, it seems likely to speculate that also guanine-based purines may play a role in the modulation of the intestinal contractility. Spontaneous and neurally-evoked mechanical activity was recorded in vitro as changes in isometric tension in circular muscle strips from mouse distal colon. Guanosine up to 3mM or guanine up to 1mM failed to affect the spontaneous mechanical activity, but reduced the amplitude of the electrical field stimulation (EFS)-induced cholinergic contractions, without affecting the early nitrergic relaxation. Both compounds failed to affect the direct contractile responses evoked by carbachol. No desensitization of the response was observed. Guanine-based purine effects were not altered by theophylline, P1 purinoceptor antagonist, by PPADS or suramin, P2 purinoceptor antagonists, by ODQ, guanilyl cyclase inhibitor, or by DDA, adenylyl cyclase inhibitor. Nucleoside uptake inhibitors, dipyridamole or 6-[(4-Nitrobenzyl)thio]-9-β-D-ribofuranosylpurine (NBTI), antagonized the inhibitory effects induced by guanosine without interfering with guanine. On the contrary, adenine, a competitive inhibitor of nucleobase uptake, antagonized guanine-induced effects. In conclusion, our data indicate that guanosine and guanine are able to modulate negatively the excitatory cholinergic neurotransmission in the circular muscle layer of mouse colon. Guanine-based purines appear to interfere with prejunctional acethylcoline release. Their effects are dependent by their cellular uptake, and independent by adenine-based purine receptors.

Inhibitory effect of caffeic acid phenethyl ester, a plant-derived polyphenolic compound, on rat intestinal contractility

Caffeic acid phenethyl ester (CAPE) exerts pharmacological actions (e.g. anti-inflammatory, chemopreventive) which are relevant for potential clinical application in the digestive tract. However, no study has been published on its possible effects on intestinal motility, to date. In the present study, we investigated the effect of this plant-derived polyphenolic compound on the spontaneous contractions of the rat isolated ileum. CAPE reduced (in a tetrodotoxin-insensitive manner) spontaneous ileal contractions and this effect was reduced by the L-type Ca2+ channel blocker nifedipine and the chelant of calcium ethylenediaminetetraacetic acid. However, the effect of CAPE was not modified by a number of inhibitors/antagonists such as of phentolamine plus propranolol, atropine, tetrodotoxin, cyclopiazonic acid, omega-conotoxin, apamin, NG-nitro-L-arginine methyl ester, 3-isobutyl-1-methylxanthine, 9-(tetrahydro-2-furanyl)-9H-purin-6-amine, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one or a combination of SR 140333, SR48968 and SR142801. In conclusion our study shows that (i) CAPE relaxed myogenic contractions of rat ileum and that (ii) this effect occurs, at least in part, throughout a mechanism involving L-type Ca2+ channels.

Deletion of P2X2 and P2X3 receptor subunits does not alter motility of the mouse colon

Purinergic P2X receptors contribute to neurotransmission in the gut. P2X receptors are ligand-gated cation channels that mediate synaptic excitation in subsets of enteric neurons. The present study evaluated colonic motility in vitro and in vivo in wild type (WT) and P2X2 and P2X3 subunit knockout (KO) mice. The muscarinic receptor agonist, bethanechol (0.3–3 μM), caused similar contractions of the longitudinal muscle in colon segments from WT, P2X2 and P2X3 subunit KO mice. Nicotine (1–300 μM), acting at neuronal nicotinic receptors, caused similar longitudinal muscle relaxations in colonic segments from WT and P2X2 and P2X3 subunit KO mice. Nicotine-induced relaxations were inhibited by nitro-l-arginine (NLA, 100 μM) and apamin (0.1 μM) which block inhibitory neuromuscular transmission. ATP (1–1000 μM) caused contractions only in the presence of NLA and apamin. ATP-induced contractions were similar in colon segments from WT, P2X2 and P2X3 KO mice. The mouse colon generates spontaneous migrating motor complexes (MMCs) in vitro. The MMC frequency was higher in P2X2 KO compared to WT tissues; other parameters of the MMC were similar in colon segments from WT, P2X2 and P2X3 KO mice. 5-Hydroxytryptophan-induced fecal output was similar in WT, P2X2 and P2X3 KO mice. These data indicate that nicotinic receptors are located predominately on inhibitory motor neurons supplying the longitudinal muscle in the mouse colon. P2X2 or P2X3 subunit containing receptors are not localized to motor neurons supplying the longitudinal muscle. Synaptic transmission mediated by P2X2 or P2X3 subunit containing receptors is not required for propulsive motility in the mouse colon.

A comparative analysis of the activity of ligands acting at P2X and P2Y receptor subtypes in models of neuropathic, acute and inflammatory pain

BACKGROUND AND PURPOSE

This study was undertaken to compare the analgesic activity of antagonists acting at P2X1, P2X7, and P2Y12 receptors and agonists acting at P2Y1, P2Y2, P2Y4, and P2Y6 receptors in neuropathic, acute, and inflammatory pain.

EXPERIMENTAL APPROACH

The effect of the wide spectrum P2 receptor antagonist PPADS, the selective P2X7 receptor antagonist Brilliant Blue G (BBG), the P2X1 receptor antagonist (4,4',4'',4-[carbonylbis(imino-5,1,3-benzenetriyl-bis(carbonylimino))]tetrakis-1,3-benzenedisulfonic acid, octasodium salt (NF449) and (8,8'-[carbonylbis(imino-3,1-phenylenecarbonylimino)]bis-1,3,5-naphthalene-trisulphonic acid, hexasodium salt (NF023), the P2Y12 receptor antagonist (2,2-dimethyl-propionic acid 3-(2-chloro-6-methylaminopurin-9-yl)-2-(2,2-dimethyl-propionyloxymethyl)-propylester (MRS2395), the selective P2Y1 receptor agonist ([[(1R,2R,3S,4R,5S)-4-[6-amino-2-(methylthio)-9H-purin-9-yl]-2,3-dihydroxybicyclo[3.1.0]hex-1-yl]methyl] diphosphoric acid mono ester trisodium salt (MRS2365), the P2Y2/P2Y4 agonist uridine-5'-triphosphate (UTP), and the P2Y4/P2Y6 agonist uridine-5'-diphosphate (UDP) were examined on mechanical allodynia in the Seltzer model of neuropathic pain, on acute thermal nociception, and on the inflammatory pain and oedema induced by complete Freund's adjuvant (CFA).

KEY RESULTS

MRS2365, MRS2395 and UTP, but not the other compounds, significantly alleviated mechanical allodynia in the neuropathic pain model, with the following rank order of minimal effective dose (mED) values: MRS2365 > MRS2395 > UTP. All compounds had a dose-dependent analgesic action in acute pain except BBG, which elicited hyperalgesia at a single dose. The rank order of mED values in acute pain was the following: MRS2365 > MRS2395 > NF449 > NF023 > UDP = UTP > PPADS. MRS2365 and MRS2395 had a profound, while BBG had a mild effect on inflammatory pain, with a following rank order of mED values: MRS2395 > MRS2365 > BBG. None of the tested compounds had significant action on oedema evoked by intraplantar injection of CFA.

CONCLUSIONS AND IMPLICATIONS

Our results show that antagonism at P2X1, P2Y12, and P2X7 receptors and agonism at P2Y1 receptors define promising therapeutic strategies in acute, neuropathic, and inflammatory pain respectively.

Nitric oxide and carbon monoxide act as inhibitory neurotransmitters in the longitudinal muscle of C57BL/6J mouse distal colon

The present study was designed to identify the inhibitory neurotransmitters mediating nonadrenergic noncholinergic relaxation in the longitudinal muscle of C57/BL mouse distal colon. Relaxation induced by electrical field stimulation (EFS) was recorded isotonically in the presence of atropine and guanethidine. Cyclic guanosine-3',5'-monophosphate (cyclic GMP) content was measured by radioimmunoassay. EFS-induced relaxation was inhibited by nitro-L-arginine (L-NNA) and Sn (IV) protoporphyrin dichloride IX (SnPP-IX), a nitric oxide (NO) and carbon monoxide (CO) synthase inhibitor, respectively. A combination of both inhibitors produced an additive effect. ODQ, a soluble guanylate cyclase inhibitor, inhibited EFS-induced relaxation. NOR-1, a NO donor, and carbon monoxide-releasing molecule-2 (CORM-2), a CO donor, treatment relaxed the distal colon and increased cyclic GMP content. The effects of NOR-1 and CORM-2 were inhibited by ODQ. KT5823, a cyclic GMP-dependent protein kinase inhibitor, inhibited EFS-induced relaxation. EFS-induced relaxation in the presence of KT5823 was further inhibited by L-NNA, but not by SnPP-IX. In addition, KT5823 inhibited CORM-2-induced relaxation, but not NOR-1-induced relaxation. H89, a cyclic AMP-dependent protein kinase inhibitor, inhibited EFS-induced relaxation, and EFS-induced relaxation in the presence of H89 was further inhibited by L-NNA. These results suggested that NO and CO function as inhibitory neurotransmitters in the longitudinal muscle of C57BL mouse distal colon.

Relative contribution of ecto-ATPase and ecto-ATPDase pathways to the biphasic effect of ATP on acetylcholine release from myenteric motoneurons

BACKGROUND AND PURPOSE

The relative contribution of distinct ecto-nucleotidases to the modulation of purinergic signalling may depend on differential tissue distribution and substrate preference.

EXPERIMENTAL APPROACH

Extracellular ATP catabolism (assessed by high-performance liquid chromatography) and its influence on [(3)H]acetylcholine ([(3)H]ACh) release were investigated in the myenteric plexus of rat ileum in vitro.

KEY RESULTS

ATP was primarily metabolized via ecto-ATPDase (adenosine 5'-triphosphate diphosphohydrolase) into AMP, which was then dephosphorylated into adenosine by ecto-5'-nucleotidase. Alternative conversion of ATP into ADP by ecto-ATPase (adenosine 5'-triphosphatase) was more relevant at high ATP concentrations. ATP transiently increased basal [(3)H]ACh outflow in a 2',3'-O-(2,4,6-trinitrophenyl)adenosine-5'-triphosphate (TNP-ATP)-dependent, tetrodotoxin-independent manner. ATP and ATPgammaS (adenosine 5'-[gamma-thio]triphosphate), but not alpha,beta-methyleneATP, decreased [(3)H]ACh release induced by electrical stimulation. ADP and ADPbetaS (adenosine 5'[beta-thio]diphosphate) only decreased evoked [(3)H]ACh release. Inhibition by ADPbetaS was prevented by MRS 2179 (2'-deoxy-N(6)-methyl adenosine 3',5'-diphosphate diammonium salt, a selective P2Y(1) antagonist); blockade of ADP inhibition required co-application of MRS 2179 plus adenosine deaminase (which inactivates endogenous adenosine). Blockade of adenosine A(1) receptors with 1,3-dipropyl-8-cyclopentyl xanthine enhanced ADPbetaS inhibition, indicating that P2Y(1) stimulation is cut short by tonic adenosine A(1) receptor activation. MRS 2179 facilitated evoked [(3)H]ACh release, an effect reversed by the ecto-ATPase inhibitor, ARL67156, which delayed ATP conversion into ADP without affecting adenosine levels.

CONCLUSIONS AND IMPLICATIONS

ATP transiently facilitated [(3)H]ACh release from non-stimulated nerve terminals via prejunctional P2X (probably P2X(2)) receptors. Hydrolysis of ATP directly into AMP by ecto-ATPDase and subsequent formation of adenosine by ecto-5'-nucleotidase reduced [(3)H]ACh release via inhibitory adenosine A(1) receptors. Stimulation of inhibitory P2Y(1) receptors by ADP generated alternatively via ecto-ATPase might be relevant in restraining ACh exocytosis when ATP saturates ecto-ATPDase activity.

Alteration of cholinergic, purinergic and sensory neurotransmission in the mouse colon of food allergy model

It is well known that intestinal anaphylaxis results in a disturbed intestinal motility. It is hypothesized that the chronic intestinal anaphylaxis-induced changes in the enteric neuronal circuitry cause intestinal motor malfunctions. However, detailed mechanisms largely remain unclear. The aim of this study was to investigate the pathophysiological role of ATP, which acts as a non-cholinergic neurotransmitter and a neuroimmune modulator, in a disturbed intestinal motility of food allergy (FA). The FA mice developed allergic diarrhea accompanied with chronic inflammation and mast cell hyperplasia in the colon. The excised proximal colons (PCs) were suspended in the longitudinal direction in organ baths. In the PCs precontracted by KCl (50 mM), contractile responses to exogenous ATP (1 mM) were significantly (P < 0.01) higher in FA mice (34.2% of KCl-induced precontractions) as compared to control mice (17.2%). Pretreatment with P2 purinoceptor antagonists [suramin and PPADs] significantly (P < 0.01) reduced the ATP-evoked contractions to 7.7% and 1.5% in FA and control PCs, respectively. Furthermore, in the presence of inhibitors of cholinergic nerves and capsaicin-sensitive sensory nerves the electrical field stimulation (EFS; 10Hz)-evoked contractions were significantly (P < 0.05) higher in FA mice (65.8% of EFS-evoked maximum contractions, n = 6) than those in control mice (47.9%, n = 6). In addition, cumulative application of suramin and PPADs further inhibited EFS-induced contractions by 21.7% in FA mice (n = 6, P < 0.01) and 8.7% in control mice (n = 6, P < 0.05). Thus, the present study suggests that the sustained alteration in cholinergic, purinergic and sensory neurotransmission contribute to the disturbed motility during the chronic intestinal anaphylaxis.

Activation of P2Y receptors by ATP and by its analogue, ADPbetaS, triggers two calcium signal pathways in the longitudinal muscle of mouse distal colon

Our previous research showed that ATP and adenosine 5'-O-2-thiodiphosphate (ADPbetaS) induce contractile effects in the longitudinal muscle of mouse distal colon via activation of P2Y receptors which are not P2Y(1) or P2Y(12) subtypes. This study investigated the nature of the P2Y receptor subtype(s) and the mechanisms leading to the intracellular calcium concentration increase necessary to trigger muscular contraction. Motor responses of mouse colonic longitudinal muscle to P2Y receptor agonists were examined in vitro as changes in isometric tension. ATP or ADPbetaS induced muscular contraction, which was not affected by P2Y(11) or P2Y(13) selective antagonists. Calcium-free solution or the calcium channel blocker, nifedipine, failed to modify the contractile responses to ATP or ADPbetaS, which were virtually abolished by depletion of calcium intracellular stores after repetitive addition of carbachol in calcium-free medium with addition of cyclopiazonic acid. Neomycin or U-73122, phospholipase C inhibitors, or 2-aminoethoxy-diphenylborate (2-APB), membrane-permeant IP(3) receptor inhibitor reduced the response to ATP, whilst ryanodine or ruthenium red, inhibiting calcium release from ryanodine-sensitive stores, abolished the response to ADPbetaS. Responses to maximally effective concentrations of ATP and ADPbetaS were not fully additive. Desensitisation with ADPbetaS antagonized the contractile effects of ATP, as desensitisation with ATP antagonized the response to ADPbetaS. In the longitudinal muscle of mouse distal colon, ATP and ADPbetaS induce muscular contraction via a P2Y receptor, coupled to differential signal pathways leading to intracellular calcium increase.

Lysophosphatidic acid-induced membrane ruffling and brain-derived neurotrophic factor gene expression are mediated by ATP release in primary microglia

We examined the effects of lysophosphatidic acid (LPA) on microglia, which may play an important role in the development and maintenance of neuropathic pain. LPA caused membrane ruffling as detected by scanning electron microscopy, and increased the expression of brain-derived neurotrophic factor (BDNF) in a primary culture of rat microglia, which express LPA(3), but not LPA(1) or LPA(2) receptors. These actions were inhibited by a Galpha(q/11)-antisense oligodeoxynucleotide (AS-ODN), U73122, an inhibitor of phospholipase C (PLC), and apyrase, which specifically degrades ATP and ADP. When ATP release was measured using a luciferin-luciferase bioluminescence assay, LPA was shown to increase it in an LPA(3) and PLC inhibitor-reversible manner. However, LPA-induced ATP release was also blocked by the Galpha(q/11) AS-ODN, but not by pertussis toxin. These results suggest that LPA induces the release of ATP from rat primary cultured microglia via the LPA(3) receptor, Galpha(q/11) and PLC, and that the released ATP or ectopically converted ADP may in turn cause membrane ruffling via P2Y(12) receptors and Galpha(i/o) activation, and BDNF expression via activation of P2X(4) receptors.

Inhibitory purinergic transmission in mouse caecum: role for P2Y1 receptors as prejunctional modulators of ATP release

Using conventional microelectrode recording techniques, we investigated, in the circular muscle of the mouse caecum, the neurotransmitter(s) involved in the neurally-evoked inhibitory junction potentials (IJPs) and the existence of possible prejunctional mechanisms controlling neurotransmitter release. Electrical field stimulation with single pulses elicited IJPs, consisting only of a "fast" hyperpolarization, while using train stimuli (30-50 Hz) the initial fast hyperpolarization was followed by a slower hyperpolarization. The fast and the slow component were selectively antagonized by apamin, a blocker of calcium-activated potassium channels, and N(omega)-nitro-l-arginine methyl ester (l-NAME), a nitric oxide synthase inhibitor, respectively. Fast IJPs were antagonized also by P2 purinoceptor antagonists, suramin or 4-[[4-formyl-5-hydroxy-6-methyl-3-[(phosphonooxy)methyl]-2-pyridinyl]azo]-1,3-benzenedisulfonic acid tetrasodium salt (PPADS), P2Y purinoceptor desensitization by adenosine 5'-O-2-thiodiphosphate (ADPbetaS). 2'-Deoxy-N(6)-methyl ADP diammonium salt (MRS 2179), P2Y1 purinoceptor antagonist, at the concentration of 1 microM increased the amplitude of the fast IJP, while at the concentration of 10 microM induced a reduction. 8,8'-[Carbonylbis[imino-3,1-phenylenecarbonylimino (4-fluoro-3,1-phenylene) carbonylimino]] bis-1,3,5-naphthalenetrisulfonic acid hexasodium salt (NF 157) and 2,2-dimethyl-propionic acid 3-(2-chloro-6-methylaminopurin-9-yl)-2-(2,2-dimethyl-propionyl-oxymethyl)-propyl ester (MRS 2395), P2Y11 and P2Y12 purinoceptor antagonist, were without any effect. ATP-induced hyperpolarization was affected by apamin and by P2Y purinoceptor desensitization, but not by MRS 2179. 2-(Methylthio)ATP tetrasodium salt hydrate (2-MeSATP), P2Y1 purinoceptor agonist, at a concentration which did not cause changes in the membrane potential, reduced the amplitude of the fast IJPs. This effect was prevented by MRS 2179. Paired nerve stimulation, either using single pulses or train stimuli, did not cause any alteration of the second-evoked IJP. In conclusion, in the circular muscle of the mouse caecum, ATP is responsible for the fast IJP while nitric oxide is responsible for the slow IJP. ATP-mediated response is dependent on ADPbetaS-sensitive P2Y receptors, which are in part P2Y1, but not P2Y11 or P2Y12 receptor subtypes. In addition, the most substantial finding of this study is the functional demonstration that ATP released by nerve stimulation activates P2Y1 receptors, located prejunctionally, limiting its release by motoneurons.