Characterization of adenosine receptors on rat ileum, ileal longitudinal muscle and muscularis mucosae

Neurotransmitters responsible for purinergic motor neurotransmission and regulation of GI motility

Classical concepts of peripheral neurotransmission were insufficient to explain enteric inhibitory neurotransmission. Geoffrey Burnstock and colleagues developed the idea that ATP or a related purine satisfies the criteria for a neurotransmitter and serves as an enteric inhibitory neurotransmitter in GI muscles. Cloning of purinergic receptors and development of specific drugs and transgenic mice have shown that enteric inhibitory responses depend upon P2Y₁ receptors in post-junctional cells. The post-junctional cells that transduce purinergic neurotransmitters in the GI tract are PDGFRα⁺ cells and not smooth muscle cells (SMCs). PDGFRα⁺ cells express P2Y₁ receptors, are activated by enteric inhibitory nerve stimulation and generate Ca²⁺ oscillations, express small-conductance Ca²⁺-activated K⁺ channels (SK3), and generate outward currents when exposed to P2Y₁ agonists. These properties are consistent with post-junctional purinergic responses, and similar responses and effectors are not functional in SMCs. Refinements in methodologies to measure purines in tissue superfusates, such as high-performance liquid chromatography (HPLC) coupled with etheno-derivatization of purines and fluorescence detection, revealed that multiple purines are released during stimulation of intrinsic nerves. β-NAD⁺ and other purines, better satisfy criteria for the purinergic neurotransmitter than ATP. HPLC has also allowed better detection of purine metabolites, and coupled with isolation of specific types of post-junctional cells, has provided new concepts about deactivation of purine neurotransmitters. In spite of steady progress, many unknowns about purinergic neurotransmission remain and require additional investigation to understand this important regulatory mechanism in GI motility.

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.

Adenosine-mediated enteric neuromuscular function is affected during herpes simplex virus type 1 infection of rat enteric nervous system

Adenosine plays an important role in regulating intestinal motility and inflammatory processes. Previous studies in rodent models have demonstrated that adenosine metabolism and signalling are altered during chronic intestinal inflammatory diseases. However, the involvement of the adenosinergic system in the pathophysiology of gut dysmotility associated to a primary neurodysfunction is still unclear. Recently, we showed that the neurotropic Herpes simplex virus type-1 (HSV-1), orally inoculated to rodents, infects the rat enteric nervous system (ENS) and affects gut motor function without signs of systemic infection. In this study we examined whether changes in purinergic metabolism and signaling occur during permanent HSV-1 infection of rat ENS. Using isolated organ bath assays, we found that contraction mediated by adenosine engagement of A₁ or A_2A receptors was impaired at 1 and 6 weeks post-viral administration. Immunofluorescence studies revealed that viral infection of ENS led to a marked redistribution of adenosine receptors: A₁ and A_2B receptors were confined to the muscle layers whereas A_2A and A₃ receptors were expressed mainly in the myenteric plexus. Viral-induced ENS neurodysfunction influenced adenosine metabolism by increasing adenosine deaminase and CD73 levels in longitudinal muscle-myenteric plexus with no sign of frank inflammation. This study provides the first evidence for involvement of the adenosinergic system during HSV-1 infection of the ENS. As such, this may represent a valid therapeutic target for modulating gut contractility associated to a primary neurodysfunction.

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.

Adenosine negatively regulates duodenal motility in mice: role of A(1) and A(2A) receptors

BACKGROUND AND PURPOSE

Adenosine is considered to be an important modulator of intestinal motility. This study was undertaken to investigate the role of adenosine in the modulation of contractility in the mouse duodenum and to characterize the adenosine receptor subtypes involved.

EXPERIMENTAL APPROACH

RT-PCR was used to investigate the expression of mRNA encoding for A(1), A(2A), A(2B) and A(3) receptors. Contractile activity was examined in vitro as changes in isometric tension.

KEY RESULTS

In mouse duodenum, all four classes of adenosine receptors were expressed, with the A(2B) receptor subtype being confined to the mucosal layer. Adenosine caused relaxation of mouse longitudinal duodenal muscle; this was antagonized by the A(1) receptor antagonist and mimicked by N(6) -cyclopentyladenosine (CPA), selective A(1) agonist. The relaxation induced by A(1) receptor activation was insensitive to tetrodotoxin (TTX) or N(ω) -nitro-l-arginine methyl ester (l-NAME). Adenosine also inhibited cholinergic contractions evoked by neural stimulation, effect reversed by the A(1) receptor antagonist, but not myogenic contractions induced by carbachol. CPA and 2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride hydrate (CGS-21680), A(2A) receptor agonist, both inhibited the nerve-evoked cholinergic contractions. l-NAME prevented only the CGS-21680-induced effects. S-(4-Nitrobenzyl)-6-thioinosine, a nucleoside uptake inhibitor, reduced the amplitude of nerve-evoked cholinergic contractions, an effect reversed by an A(2A) receptor antagonist or l-NAME.

CONCLUSIONS AND IMPLICATIONS

Adenosine can negatively regulate mouse duodenal motility either by activating A(1) inhibitory receptors located post-junctionally or controlling neurotransmitter release via A(1) or A(2A) receptors. Both receptors are available for pharmacological recruitment, even if only A(2A) receptors appear to be preferentially stimulated by endogenous adenosine.

Localization and function of adenosine receptor subtypes at the longitudinal muscle--myenteric plexus of the rat ileum

Adenosine plays a dual role on acetylcholine (ACh) release from myenteric motoneurons via the activation of high-affinity inhibitory A₁ and facilitatory A(2A) receptors. The therapeutic potential of adenosine-related compounds for controlling intestinal motility and inflammation, prompted us to investigate further the role of low-affinity adenosine receptors, A(2B) and A₃, on electrically-evoked (5 Hz, 200 pulses) [³H]ACh release from myenteric neurons. Immunolocalization studies showed that A(2B) receptors exhibit a pattern of distribution similar to the glial cell marker, GFAP. Regarding A₁ and A₃ receptors, they are mainly distributed to cell bodies of ganglionic myenteric neurons, whereas A(2A) receptors are localized predominantly on cholinergic nerve terminals. Using selective antagonists (DPCPX, ZM241385 and MRS1191), data indicate that modulation of evoked [³H]ACh release is balanced through tonic activation of inhibitory (A₁) and facilitatory (A(2A) and A₃) receptors by endogenous adenosine. The selective A(2B) receptor antagonist, PSB603, alone was devoid of effect and failed to modify the inhibitory effect of NECA. The A₃ receptor agonist, 2-Cl-IB MECA (1-10 nM), concentration-dependently increased the release of [³H]ACh. The effect of 2-Cl-IB MECA was attenuated by MRS1191 and by ZM241385, which selectively block respectively A₃ and A(2A) receptors. In contrast to 2-Cl-IB MECA, activation of A(2A) receptors with CGS21680C attenuated nicotinic facilitation of ACh release induced by focal depolarization of myenteric nerve terminals in the presence of tetrodotoxin. Tandem localization of excitatory A₃ and A(2A) receptors along myenteric neurons explains why stimulation of A₃ receptors (with 2-Cl-IB MECA) on nerve cell bodies acts cooperatively with prejunctional facilitatory A(2A) receptors to up-regulate acetylcholine release. The results presented herein consolidate and expand the current understanding of adenosine receptor distribution and function in the myenteric plexus of the rat ileum, and should be taken into consideration for data interpretation regarding the pathophysiological implications of adenosine on intestinal motility disorders.

A1 and A2a receptors mediate inhibitory effects of adenosine on the motor activity of human colon

Experimental evidence in animal models suggests that adenosine is involved in the regulation of digestive functions. This study examines the influence of adenosine on the contractile activity of human colon. Reverse transcription-polymerase chain reaction revealed A(1) and A(2a) receptor expression in colonic neuromuscular layers. Circular muscle preparations were connected to isotonic transducers to determine the effects of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; A(1) receptor antagonist), ZM 241385 (A(2a) receptor antagonist), CCPA (A(1) receptor agonist) and 2-[(p-2-carboxyethyl)-phenethylamino]-5'-N-ethyl-carboxamide-adenosine (CGS 21680; A(2a) receptor agonist) on motor responses evoked by electrical stimulation or carbachol. Electrically evoked contractions were enhanced by DPCPX and ZM 241385, and reduced by CCPA and CGS 21680. Similar effects were observed when colonic preparations were incubated with guanethidine (noradrenergic blocker), L-732,138, GR-159897 and SB-218795 (NK receptor antagonists). However, in the presence of guanethidine, NK receptor antagonists and N(omega)-propyl-L-arginine (NPA; neuronal nitric oxide synthase inhibitor), the effects of DPCPX and CCPA were still evident, while those of ZM 241385 and CGS 21680 no longer occurred. Carbachol-induced contractions were unaffected by A(2a) receptor ligands, but they were enhanced or reduced by DPCPX and CCPA, respectively. When colonic preparations were incubated with guanethidine, NK antagonists and atropine, electrically induced relaxations were partly reduced by ZM 241385 or NPA, but unaffected by DPCPX. Dipyridamole or application of exogenous adenosine reduced electrically and carbachol-evoked contractions, whereas adenosine deaminase enhanced such motor responses. In conclusion, adenosine exerts an inhibitory control on human colonic motility. A(1) receptors mediate direct modulating actions on smooth muscle, whereas A(2a) receptors operate through inhibitory nitrergic nerve pathways.

Regulation of enteric functions by adenosine: pathophysiological and pharmacological implications

The wide distribution of ATP and adenosine receptors as well as enzymes for purine metabolism in different gut regions suggests a complex role for these mediators in the regulation of gastrointestinal functions. Studies in rodents have shown a significant involvement of adenosine in the control of intestinal secretion, motility and sensation, via activation of A1, A2A, A2B or A3 purinergic receptors, as well as the participation of ATP in the regulation of enteric functions, through the recruitment of P2X and P2Y receptors. Increasing interest is being focused on the involvement of ATP and adenosine in the pathophysiology of intestinal disorders, with particular regard for inflammatory bowel diseases (IBDs), intestinal ischemia, post-operative ileus and related dysfunctions, such as gut dysmotility, diarrhoea and abdominal discomfort/pain. Current knowledge suggests that adenosine contributes to the modulation of enteric immune and inflammatory responses, leading to anti-inflammatory actions. There is evidence supporting a role of adenosine in the alterations of enteric motor and secretory activity associated with bowel inflammation. In particular, several studies have highlighted the importance of adenosine in diarrhoea, since this nucleoside participates actively in the cross-talk between immune and epithelial cells in the presence of diarrhoeogenic stimuli. In addition, adenosine exerts complex regulatory actions on pain transmission at peripheral and spinal sites. The present review illustrates current information on the role played by adenosine in the regulation of enteric functions, under normal or pathological conditions, and discusses pharmacological interventions on adenosine pathways as novel therapeutic options for the management of gut disorders and related abdominal symptoms.

Purinergic mechanisms in the control of gastrointestinal motility

For many years, ATP and adenosine have been implicated in movement regulation of the gastrointestinal tract. They act through three major receptor subtypes: adenosine or P1 receptors, P2X receptors and P2Y receptors. Each of these major receptor types can be subdivided into several different classes and is widely distributed amongst various neurons, muscle types, glia and interstitial cells that regulate intestinal functions. Several key roles for the different receptors and their endogenous ligands have been identified in physiological and pharmacological studies. For example, adenosine acting at A(1) receptors appears to inhibit intestinal motility in various pathological conditions. Similarly, ATP acting at P2Y receptors is an important component of inhibitory neuromuscular transmission, acting as a cotransmitter with nitric oxide. ATP acting at P2X and P2Y(1) receptors is important for synaptic transmission in simple descending excitatory and inhibitory reflex pathways. Some P2Y receptor subtypes prefer uridine nucleotides over purine nucleotides. Thus, roles for UTP and UDP as enteric transmitters in place of ATP cannot be excluded. ATP also appears to be important for sensory transduction, especially in chemosensitive pathways that initiate local inhibitory reflexes. Despite this evidence, data are lacking about the roles of either adenosine or ATP in more complex motility patterns such as segmentation or the interdigestive migrating motor complex. Clarification of roles for purinergic transmission in these common, but understudied, motility patterns will depend on the use of subtype-specific antagonists that in some cases have not yet been developed.

Pretreatment with adenosine and adenosine A1 receptor agonist protects against intestinal ischemia-reperfusion injury in rat

AIM: To examine the effects of adenosine and A1 receptor activation on reperfusion-induced small intestinal injury.

METHODS: Rats were randomized into groups with sham operation, ischemia and reperfusion, and systemic treatments with either adenosine or 2-chloro-N⁶-cyclopentyladenosine, A1 receptor agonist or 8-cyclopentyl-1,3-dipropylxanthine, A1 receptor antagonist, plus adenosine before ischemia. Following reperfusion, contractions of ileum segments in response to KCl, carbachol and substance P were recorded. Tissue myeloperoxidase, malondialdehyde, and reduced glutathione levels were measured.

RESULTS: Ischemia significantly decreased both contraction and reduced glutathione level which were ameliorated by adenosine and agonist administration. Treatment also decreased neutrophil infiltration and membrane lipid peroxidation. Beneficial effects of adenosine were abolished by pretreatment with A1 receptor antagonist.

CONCLUSION: The data suggest that adenosine and A1 receptor stimulation attenuate ischemic intestinal injury via decreasing oxidative stress, lowering neutrophil infiltration, and increasing reduced glutathione content.

The bidirectional communication between neurons and mast cells within the gastrointestinal tract

Normal or disordered behaviour of the gastrointestinal tract is determined by a complex interplay between the epithelial barrier, immune cells, blood vessels, smooth muscle and intramurally located nerve elements. Mucosal mast cells (MMCs), which are able to detect noxious and antigenic threats and to generate or amplify signals to the other cells, are assigned a rather central position in this complex network. Signal input from MMCs to intrinsic enteric neurons is particularly crucial, because the enteric nervous system fulfils a pivotal role in the control of gastrointestinal functions. Activated enteric neurons are able to generate an alarm program involving alterations in motility and secretion. MMC signalling to extrinsic nerve fibres takes part in pathways generating visceral pain or extrinsic reflexes contributing to the disturbed motor and secretory function. Morphological and functional studies, especially studies concerning physiological stress, have provided evidence that, apart from the interaction between the enteric nervous system and MMCs, there is also a functional communication between the central nervous system and these mast cells. Psychological factors trigger neuronal pathways, which directly or indirectly affect MMCs. Further basic and clinical research will be needed to clarify in more detail whether basic patterns of this type of interactions are conserved between species including humans.

Inhibitory responses to exogenous adenosine in murine proximal and distal colon

The aims of the present study were firstly, to characterize pharmacologically the subtypes of P(1) purinoreceptors involved in the inhibitory effects induced by exogenous adenosine in longitudinal smooth muscle of mouse colon, and secondly, to examine differences in the function and distribution of these receptors between proximal and distal colon. Adenosine (100 microM-3 mM) caused a concentration-dependent reduction of the amplitude of spontaneous contractions in the proximal colon, and muscular relaxation in the distal colon. In the proximal colon, adenosine effects were antagonized by a selective A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 10 nM), but were not modified by 3,7-dimethyl-1-propargylxanthine (DMPX, 10 microM) or by 9-chloro-2-(2-furanyl)-5-((phenylacetyl)amino)- [1,2,4]triazolo[1,5-c]quinazoline (MRS 1220, 0.1 microM), selective A(2) and A(3) receptor antagonists, respectively. In the distal colon, adenosine effects were antagonized by DPCPX, DMPX, and by a selective A(2B) receptor antagonist, 8-[4-[((4-cyanophenyl)carbamoylmethyl)oxy]phenyl]-1,3-di(n-propyl) xanthine (MRS 1754, 10 microM), but not by 8-(3-chlorostyryl)-caffeine (CSC, 10 microM), a selective A(2A) receptor antagonist, or by MRS 1220. Tetrodotoxin (TTX 1 microM), the nitric oxide (NO) synthase inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME, 100 microM), or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10 microM), an inhibitor of soluble guanylyl cyclase, reduced adenosine effects only in distal colon. In addition, L-NAME induced a further reduction of adenosine relaxation in the presence of DPCPX, but not in the presence of MRS 1754. From these results we conclude that, in the murine proximal colon, adenosine induces inhibitory effects via TTX-insensitive activation of A(1) receptor. In the distal colon, adenosine activates both A(1) and A(2B) receptors, the latter located on enteric inhibitory neurons releasing NO.

Pharmacological characterisation of the adenosine receptor mediating increased ion transport in the mouse isolated trachea and the effect of allergen challenge

The effect of adenosine on transepithelial ion transport was investigated in isolated preparations of murine trachea mounted in Ussing chambers. The possible regulation of adenosine receptors in an established model of allergic airway inflammation was also investigated. Mucosally applied adenosine caused increases in short-circuit current (I(SC)) that corresponded to approximately 50% of the response to the most efficacious secretogogue, ATP (delta I(SC) 69.5 +/- 6.7 microA cm2). In contrast, submucosally applied adenosine caused only small (<20%) increases in I(SC), which were not investigated further. The A1-selective (N6-cyclopentyladenosine, CPA, 1 nM-10 microM), A2A-selective (2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxoamido adenosine; CGS 21680; 0.1-100 microM) and A3-selective (1-deoxy-1-[6-[[(3-iodophenyl)-methyl]amino]-9H-purin-9-yl]-N-methyl-beta-D-ribofuranuronamide; IB-MECA; 30 nM-100 microM) adenosine receptor agonists were either equipotent or less potent than adenosine, suggesting that these receptors do not mediate the response to adenosine. The A1 receptor selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 10 nM-1 microM) caused a rightward shift of the adenosine concentration-effect curve only at 1 microM. The mixed A2A/A2B receptor antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385) also caused rightward shift of the adenosine concentration-effect curve, again only at micromolar concentrations, suggestive of the involvement of A2B receptors. In preparations from animals sensitised to ovalbumin and challenged over 3 days with aerosol ovalbumin, a decrease in baseline I(SC) was observed and responses to ATP were diminished. Similarly, the amplitude of responses to adenosine were attenuated although there was no change in potency. These results suggest that the A2B receptor mediates the I(SC) response to adenosine in the mouse trachea. This receptor does not appear to be regulated in a standard asthma model.

Microvascular responses to adenosine help explain functional and pathologic differences between intestinal segments

BACKGROUND

Many physiologic (post-prandial hyperemia), pathologic (inflammatory bowel disease), and clinical (enteral feeding) phenomena involve changes in microvascular blood flow to the intestine. Adenosine (Ado) derived from energy metabolism causes vasodilation and appears to be involved in some of these events. The Ado-mediated control mechanisms appear to vary with the diameter of the microvessels and the function of the tissue. This suggests the possibility that Ado-based microvascular control varies between anatomic intestinal segments and microvascular levels in those intestinal segments.

METHODS

In vivo digital intravital microscopy was used to measure the responses of larger distributing (A1) and smaller premucosal (A3) vessels to Ado in intact neurovascular loops of jejunum and terminal ileum of the rat. Dose-response curves to Ado were determined.

RESULTS

Microvascular dilation and augmented blood flow to Ado were significantly greater in the jejunum than in the terminal ileum. Ado-induced dilation was greater in the smaller A3 than in the larger A1 microvessels.

DISCUSSION

These data indicate (1) different vasodilator mechanisms for the jejunum and the terminal ileum, (2) a greater role for Ado-related microvascular control in the jejunum compared with the ileum, and (3) a greater Ado-related control in the premucosal (A3) vessels. These findings suggest that Ado-mediated microvascular effects could explain why some clinical phenomena vary in intensity in selective intestinal segments, and are likely to involve different microvascular control mechanisms in the different segments. Knowledge of these Ado mechanisms could be beneficial in certain clinical scenarios to control blood flow during pathologic conditions.

Streptozotocin-induced diabetes modulates presynaptic and postsynaptic function in the rat ileum

Altered gastrointestinal motility frequently occurs in diabetic patients and also in animal models of diabetes but the underlying causes are not clear. In the present study, contractile responses to agonists and electrical field stimulation (EFS) and the inhibitory actions of an adenosine A(1) receptor agonist were investigated on ilea from 8-week streptozotocin (STZ)-induced diabetic rats. Contractile responses to carbachol, prostaglandin F(2 alpha) (PGF(2 alpha)), the calcium ionophore A23187 and to EFS were increased in diabetic tissues compared to controls. In contrast, the inhibitory effects of a potent and selective adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) on electrical field stimulation-evoked contractions were decreased in diabetic tissues compared to controls but its ability to relax carbachol-contracted tissues was unaltered. These results suggest that diabetes may cause alterations at both pre- and postsynaptic sites and this may lead in turn to the gastrointestinal complications seen in diabetic patients.

Alteration of the purinergic modulation of enteric neurotransmission in the mouse ileum during chronic intestinal inflammation

1. The effect of chronic intestinal inflammation on the purinergic modulation of cholinergic neurotransmission was studied in the mouse ileum. Chronic intestinal inflammation was induced by infection of mice with the parasite Schistosoma mansoni during 16 weeks. 2. S. mansoni infection induced a chronic inflammatory response in the small intestine, which was characterised by intestinal granuloma formation, increased intestinal wall thickness, blunted mucosal villi and an enhanced activity of myeloperoxidase. 3. In control ileum and in chronically inflamed ileum, electrical field stimulation (EFS) of longitudinal muscle strips induced frequency-dependent contractions that were abolished by tetrodotoxin (TTX) and atropine. Carbachol induced dose-dependent contractions that were not affected by TTX but abolished by atropine. 4. In control ileum, adenosine and ATP dose-dependently inhibited the contractions to EFS. Theophylline and 8-phenyltheophylline, P(1) and A(1) receptor antagonists respectively, prevented this inhibitory effect of adenosine and ATP. PPADS, DMPX and MRS 1220, antagonists of P(2), A(2) and A(3) receptors, respectively, did not prevent this inhibitory effect of adenosine and ATP. Adenosine and ATP did not affect the contractions to carbachol. 5. The inhibitory effect of adenosine and ATP on contractions to EFS in control ileum was mimicked by the stable adenosine analogue methyladenosine and by the A(1)-receptor agonist N(6)-cyclohexyladenosine, but not by the A3 receptor agonist 2-Cl IB-MECA or by the ATP analogues alphabeta-methylene-ATP and ADPbetaS. The inhibitory effect of adenosine on contractions to EFS was lost after prolonged (90 min) treatment of control ileum with methyladenosine (100 micro M). 6. In chronically inflamed ileum, adenosine, methyladenosine, N(6)-cyclohexyladenosine and ATP all failed to inhibit the cholinergic nerve-mediated contractions to EFS. Also theophylline, 8-phenyltheophylline, PPADS, DMPX and MRS 1220 had no effect on the contractions to EFS and carbachol. The loss of effect of adenosine and ATP was still evident after 52 weeks of infection. 7. These results indicate that in physiological conditions neuronal adenosine A(1) receptors modulate cholinergic nerve activity in the mouse ileum. However, during chronic intestinal inflammation, this purinergic modulation of cholinergic nerve activity is impaired. This suggests that chronic intestinal inflammation leads to a dysfunction of specific neuronal regulatory mechanisms in the enteric nervous system.

Modulatory effect of adenosine receptors on the ascending and descending neural reflex responses of rat ileum

BACKGROUND

Adenosine is known to act as a neuromodulator by suppressing synaptic transmission in the central and peripheral nervous system. Both the release of adenosine within the small intestine and the presence of adenosine receptors on enteric neurons have been demonstrated. The aim of the present study was to characterize a possible involvement of adenosine receptors in the modulation of the myenteric reflex. The experiments were carried out on ileum segments 10 cm in length incubated in an single chambered organ bath, and the reflex response was initiated by electrical stimulation (ES).

RESULTS

ES caused an ascending contraction and a descending relaxation followed by a contraction. All motility responses to ES were completely blocked by tetrodotoxin, indicating that they are mediated by neural mechanisms. Atropine blocked the contractile effects, whereas the descending relaxation was significantly increased. The A1 receptor agonist N6-cyclopentyladenosine increased the ascending contraction, whereas the ascending contraction was reduced by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine. Activation of the A1 receptor further reduced the descending relaxation and the latency of the peristaltic reflex. The A2B receptor antagonist alloxazine increased ascending contraction, whereas descending relaxation remained unchanged. For A2A and A3 receptors, we found contradictory effects of the agonists and antagonists, thus there is no clear physiological role for these receptors at this time.

CONCLUSIONS

This study suggests that the myenteric ascending and descending reflex response of the rat small intestine is modulated by release of endogenous adenosine via A1 receptors.

Modulation of the release of endogenous adenosine by cannabinoids in the myenteric plexus-longitudinal muscle preparation of the guinea-pig ileum

1. Interactions between the cannabinoid system and the adenosine system were investigated in the myenteric plexus-longitudinal muscle (MPLM) of the guinea-pig ileum. 2. Electrically-evoked contractions of the MPLM were inhibited in a concentration dependent manner by exogenous adenosine and the adenosine receptor agonist 2-chloroadenosine. These inhibitory effects were reversed by the selective A(1) receptor antagonist DPCPX (20 nM). 3. Preincubation of the MPLM with the cannabinoid receptor agonist CP55,940 (1 nM) or the endogenous cannabinoid ligand anandamide caused a significant leftward shift in the concentration-effect curves to adenosine and 2-chloroadenosine. 4. Electrically-evoked contractions of the MPLM were inhibited in a concentration dependent manner by the adenosine uptake inhibitor dipyridamole. This inhibition was reversed by DPCPX (20 nM). 5. Pretreatment with CP55,940 (1 nM) or anandamide (10 microM) significantly reduced the inhibition produced by dipyridamole, an effect which was completely reversed by the selective CB(1) receptor ligand SR141716 (100 nM). 6. Electrically evoked adenosine release, measured in real time by means of adenosine-specific biosensors, was inhibited by CP55,940 (10 nM). This inhibition was blocked when CP55,940 was applied in the presence of SR141716 (100 nM). 7. These results confirm the presence of presynaptic CB(1) and A(1) receptors in the guinea-pig MPLM, and suggest that CB(1) receptor stimulation reduces electrically-evoked adenosine release. Overall the data raise the possibility that the cannabinoid system plays a role in the modulation of adenosine transmission in the MPLM.

Smooth muscle does not have a common P2x receptor phenotype: expression, ontogeny and function of P2x1 receptors in mouse ileum, bladder and reproductive systems

The distribution, ontogeny and role of P2x1 receptors were examined in the smooth muscle of the mouse intestine, bladder, and male and female reproductive tracts using P2x1 receptor subtype selective antibodies and contraction studies. P2x1 receptor immunoreactivity showed a heterogeneous distribution in smooth muscle with high levels expressed in adult vas deferens, bladder, arteries and male reproductive organs. In contrast, P2x1, receptors were below the level of detection in the smooth muscle of the ileum and female reproductive tract. P2x1 receptor immunoreactivity was detected at adult levels from birth in the bladder. However, in the vas deferens, immunoreactivity was only detected from 10 days after birth and reached adult levels by approximately 1 month old. A similar pattern of expression was seen in the vesicular seminalis, epididymis, gland of the vas deferens and coagulating gland. Sensitivity to the P2x1 receptor agonist alpha,beta-methylene ATP (alpha,beta-meATP) and P2x1 receptor-deficient mice were used in functional studies to determine the role of P2x1 receptors in the control of smooth muscle. alpha,beta-meATP (100 microM) failed to evoke contractions of the epididymis, or seminal vesicle and P2x1 receptors did not contribute to the control of uterine smooth muscle. In the ileum, alpha,beta-meATP (100 microM) evoked a transient relaxation followed by a contraction. These responses were abolished by the P2 receptor antagonist iso-pyridoxalphosphate-6-azophenyl-2'-5'-disulphonate (iso-PPADS) (30 microM). Relaxant responses were abolished by the adenosine A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) (1 microM). Contractile responses were reduced by > 80% in the ileum from P2x1 receptor-deficient mice. alpha,beta-meATP-evoked contractions were reduced by approximately 35% by TTX (1 microM) and were unaffected by atropine (10 microM). These studies indicate that P2x1 receptors are not expressed throughout all smooth muscles and that their expression is developmentally regulated. In addition, they provide evidence to suggest that P2x1 receptors are present on pre-synaptic nerve terminals in the enteric nervous system.

Molecular identification and pharmacological characterization of adenosine receptors in the guinea-pig colon

The aim of this study is to elucidate the role of adenosine in the motor function of the guinea-pig distal colon.2 To determine whether adenosine A(1) receptors and A(2B) receptors are expressed in the guinea-pig colon, we employed the reverse transcription-polymerase chain reaction (RT - PCR). The gene expression of A(1) receptor and A(2B) receptor was found for the first time in the guinea-pig proximal and distal colon.3 Adenosine A(1) agonist N(6)-cyclopentyladenosine (CPA), and A(1)/A(2) agonist 5'-N-ethylcarboxamidoadenosine (NECA) concentration-dependently inhibited neurogenic responses to electrical field stimulation (EC(50)=1.07x10(-8) and 2.12x10(-8) M) in the longitudinal muscle, but A(2A) agonist 2-p-(2-carboxyethyl)phenylethylamino-5'-N-ethycarboxamido-ad enosine (CGS21680) had only a slight inhibitory effect (25.9%, 1 microM). A(1) antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 10 nM: A(1) selective concentration) antagonized responses to CPA and NECA. Furthermore, the affinity order of antagonists at inhibiting the effect NECA was: DPCPX>8-phenyltheophylline (8-PT: A(1)/A(2) antagonist).3 In the presence of tetrodotoxin (TTX, 0.3 microM), CPA and NECA relaxed myogenic precontraction induced by KCl (50 mM) (EC(50)=1.26x10(-5) and 1.04x10(-5) M, respectively), but CGS21680 (1 microM) did not cause any relaxation. DPCPX did not affect responses to CPA and NECA at a concentration of 10 nM, but a higher concentration (1 microM) of DPCPX and 10 microM of 8-PT antagonized those responses.5 These data lead us to the hypothesis that adenosine may mediate relaxation through two different inhibitory receptor subtypes; A(1) receptors on the enteric neuron and A(2B) receptor on the smooth muscle in the guinea-pig distal colon.

Characterization and tissue location of the neural adenosine receptor in the rat ileum

1. The aim of the present investigation was to characterize and determine the tissue location of the adenosine receptors present in the rat ileum using a method that detects drug action on the cholinergic nerves innervating the longitudinal and circular muscles. 2. The non-selective adenosine agonist, NECA (10 and 100 nM) caused significant concentration-related reductions in the circular muscle responses to transmural stimulation over the frequency range of 2.5-40 Hz, but did not affect the responses of the longitudinal muscle, nor did it reduce the muscle responses of the guinea-pig ileum. 3. The affinity order of antagonists at inhibiting the effect of NECA on the circular muscle was: CPDPX>8-PT>DMPX with apparent pA2 values of 9.31, 7.54 and 5.63 respectively. CPDPX (10-100 nM) caused parallel displacements of the concentration-effect curves to CPA with a pKb value of 9.15 and Schild slope of 1.03. 4. The agonists previously tested in the rat jejunum peristaltic reflex preparation were also shown to inhibit responses of the rat ileum in the following decreasing order of potency: CPA>NECA>2-CADO>R-PIA>S-PIA>>PAA. In addition, CHA and CCPA were also potent agonists. NECA (100 nM) and CPA (32 nM) did not inhibit carbachol (1 microM)-induced tone of tissues pre-treated with TTX (1 microM). 5. In conclusion, the rat ileum contains inhibitory A1 adenosine receptors situated on cholinergic nerve endings innervating the circular muscle.

Characterization and tissue location of the neural adenosine receptor in the rat ileum

1. The aim of the present investigation was to characterize and determine the tissue location of the adenosine receptors present in the rat ileum using a method that detects drug action on the cholinergic nerves innervating the longitudinal and circular muscles. 2. The non-selective adenosine agonist, NECA (10 and 100 nM) caused significant concentration-related reductions in the circular muscle responses to transmural stimulation over the frequency range of 2.5-40 Hz, but did not affect the responses of the longitudinal muscle, nor did it reduce the muscle responses of the guinea-pig ileum. 3. The affinity order of antagonists at inhibiting the effect of NECA on the circular muscle was: CPDPX>8-PT>DMPX with apparent pA2 values of 9.31, 7.54 and 5.63 respectively. CPDPX (10-100 nM) caused parallel displacements of the concentration-effect curves to CPA with a pKb value of 9.15 and Schild slope of 1.03. 4. The agonists previously tested in the rat jejunum peristaltic reflex preparation were also shown to inhibit responses of the rat ileum in the following decreasing order of potency: CPA>NECA>2-CADO>R-PIA>S-PIA>>PAA. In addition, CHA and CCPA were also potent agonists. NECA (100 nM) and CPA (32 nM) did not inhibit carbachol (1 microM)-induced tone of tissues pre-treated with TTX (1 microM). 5. In conclusion, the rat ileum contains inhibitory A1 adenosine receptors situated on cholinergic nerve endings innervating the circular muscle.

Characterization of adenosine receptors evoking excitation of mesenteric afferents in the rat

We examined the effects of adenosine receptor agonists and antagonists on the discharge of mesenteric afferent nerves supplying the jejunum in pentobarbitone sodium-anaesthetized rats. Adenosine (0.03-10 mg kg(-1), i.v.), NECA (0.3-300 microg kg(-1), i.v.) and the A1 receptor agonist, GR79236 (0.3-1000 microg kg(-1), i.v.), each induced dose-dependent increases in afferent nerve activity and intrajejunal pressure, hypotension and bradycardia. The A1 receptor antagonist, DPCPX (3 mg kg(-1), i.v.), antagonized all the effects of GR79236 but only the haemodynamic effects of adenosine and NECA. The A2A receptor antagonist, ZM241385 (3 mg kg(-1), i.v.), antagonized the hypotensive effect of NECA but none of the effects of GR79236. The A2A receptor agonist, CGS21680 (0.3-300 microg kg(-1), i.v.), and the A3 receptor agonist, IB-MECA (0.3-300 microg kg(-1), i.v.), each induced only a dose-dependent hypotension. Subsequent administration of adenosine (3 mg kg(-1), i.v.) induced increases in afferent nerve activity and intrajejunal pressure and bradycardia. ZM241385 (3 mg kg(-1), i.v.) antagonized the hypotensive effect of CGS21680 but not the effects of adenosine. Bethanechol (300 microg kg(-1), i.v.) evoked increases in afferent nerve activity and intrajejunal pressure, hypotension and bradycardia. However, adenosine (3 mg kg(-1), i.v.) evoked greater increases in afferent nerve activity than bethanechol despite inducing smaller increases in intrajejunal pressure. In summary, A1 and A2B and/or A2B-like receptors evoke adenosine-induced increases in mesenteric afferent nerve activity and intrajejunal pressure in the anaesthetized rat. Furthermore, elevations in intrajejunal pressure do not wholly account for adenosine-evoked excitation of mesenteric afferent nerves.