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

Inhibitory effect and underlying mechanism of essential oil of Prangos ferulacea Lindl (L.) on spontaneous and induced uterine contractions in non-pregnant rats

Evidence suggests the use of natural compounds as support in the management of uterine contractility disorders. We recently demonstrated that the essential oil of Apiacea Prangos ferulacea (L.) (Prangoil) modulates intestinal smooth muscle contractility. Thus, we aimed to evaluate if Prangoil could also affect the contractility of uterine muscle in non-pregnant rat and to investigate the related action mechanism/s. The effects of the aromatic monoterpenes, β-ocimene and carvacrol, constituents of Prangoil, were also evaluated. Spontaneous contractions and contraction-induced by K+-depolarization and oxytocin in rat uterus were recorded in vitro, using organ bath technique. Prangoil reduced the amplitude of spontaneous contractions as well as responses to KCl and oxytocin. β-ocimene and carvacrol matched oil inhibitory effects. Prangoil effects were not affected by nitrergic and adenylyl cyclase inhibitors or non-specific potassium channel blocker, but they were reduced by nifedipine, L-type calcium channel inhibitor, or 2-aminoethoxydiphenylborate (2-APB), membrane-permeant inositol 1,4,5-triphosphate receptor inhibitor. The response to β-ocimene was reduced by nifedipine and by 2-APB (20 μM), whilst carvacrol inhibitory effect was attenuated only by nifedipine. In conclusion, Prangoil, and its components, β-ocimene and carvacrol, reduced spontaneous and KCl or oxytocin-induced contractions of rat myometrium, mainly modulating extracellular Ca2+ influx through L-Type channels and Ca2+ release from the intracellular store. Further studies could contribute to evaluate the potential use of Prangoil against disorders characterized by abnormal uterine contractions.

Essential oil of Sicilian Prangos ferulacea (L.) Lindl. and its major component, β-ocimen, affect contractility in rat small and large intestine

ETHNOPHARMACOLOGICAL RELEVANCE

Prangos ferulacea (L.) Lindl is an Apiaceae plant, widely used in traditional medicine. Recently, chemical composition and biological activities of its essential oil (Prangroil) have been reported, but there are no studies on possible effects on intestinal contractility.

AIMS OF THE STUDY

We investigated the effects of essential oil Sicilian Prangoil on the contractility of rat small (duodenum) and large (colon) intestine and the related action mechanism.

MATERIALS AND METHODS

Responses to Prangoil and to its major component β-ocimen in intestinal segments were assessed in vitro as changes in isometric tension.

RESULTS

Prangoil, induced in duodenum, depending upon doses, contraction and/or muscular relaxation. Instead, in colon Prangoil only reduced the phasic contractions and induced muscular relaxation. β-ocimen, in both segments, produced only reduction of the spontaneous contractions without affecting basal tone. Prangoil contractile effects were abolished by ω-conotoxin, neural N-type Ca²⁺ channels blocker, atropine, muscarinic receptor antagonist, neostigmine, acetylcholinesterase (AChE) inhibitor, suggesting that Prangoil-induced contraction would be the result of an increase in neuronal cholinergic activity. Prangoil and β-ocimen inhibitory effects were unaffected by ω-conotoxin, L-NAME, blocker of the NO synthase, ODQ, soluble guanylate cyclase inhibitor, excluding involvement of neurotransmitter release or NO synthesis in the inhibitory effects. Potassium channel blocker did not affect Prangoil or β-οcimen inhibitory responses. Prangoil or β-οcimen inhibited the Ca²⁺ and high-KCl solution -induced contractions and the Carbachol-induced contractions in calcium free solution.

CONCLUSION

Prangoil affects the contractility of small and large intestine in rat, with regional differences, via potentiation of neural cholinergic activity, blockade of L-type voltage-gated calcium channel and reduction of Ca²⁺ release from the intracellular store. The Prangroil main components, β-ocimen, contributes to the inhibitory effects.

Opposite effects of dopamine on the mechanical activity of circular and longitudinal muscle of human colon

BACKGROUND

Because dopamine (DA) has gained increasing evidence as modulator of gut motility, we aimed to characterize dopaminergic response in human colon, evaluating function and distribution of dopamine receptors in circular vs longitudinal muscle strips.

METHODS

Mechanical responses to DA and dopaminergic agonists on slow phasic contractions and on basal tone were examined in vitro as changes in isometric tension. RT-PCR was used to reveal the distribution of dopaminergic receptors.

KEY RESULTS

In spontaneous active circular muscle, DA induced an increase in the amplitude of slow phasic contractions and of the basal tone, via activation of D1-like receptors. DA contractile responses were insensitive to neural blockers or to atropine and inhibited by phospholipase C (PLC) pathway inhibitors. In precontracted circular muscle strips, DA, at the higher concentrations tested, caused a relaxant response via activation of D2-like receptors. In the longitudinal muscle, DA caused only muscular relaxation due to activation of D2-like receptors. DA relaxant responses were insensitive to neural blockers or to nitric oxide synthase inhibitor and reduced by a wide-spectrum K⁺ channel blockers. Transcripts encoding for all the dopaminergic receptor subtypes was observed in both circular and longitudinal preparations.

CONCLUSIONS AND INFERENCES

Dopamine is able to modulate contractile activity of the human colon. In the circular muscle layer, DA induces mainly muscular contraction activating non-neural D1-like receptors, coupled to PLC/IP3 pathway. In the longitudinal muscle layer, DA induces muscular relaxation acting on non-neural D2-like receptors leading to the increase in K⁺ conductance.

Colonic motor dysfunctions in a mouse model of high-fat diet-induced obesity: an involvement of A2B adenosine receptors

Adenosine A_2B receptors (A_2BR) regulate several enteric functions. However, their implication in the pathophysiology of intestinal dysmotility associated with high-fat diet (HFD)-induced obesity has not been elucidated. We investigated the expression of A_2BR in mouse colon and their role in the mechanisms underlying the development of enteric dysmotility associated with obesity. Wild-type C57BL/6J mice were fed with HFD (60% kcal from fat) or normocaloric diet (NCD; 18% kcal from fat) for 8 weeks. Colonic A_2BR localization was examined by immunofluorescence. The role of A_2BR in the control of colonic motility was examined in functional experiments on longitudinal muscle preparations (LMPs). In NCD mice, A_2BR were predominantly located in myenteric neurons; in HFD animals, their expression increased throughout the neuromuscular layer. Functionally, the A_2BR antagonist MRS1754 enhanced electrically induced NK₁-mediated tachykininergic contractions in LMPs from HFD mice, while it was less effective in tissues from NCD mice. The A_2B receptor agonist BAY 60-6583 decreased colonic tachykininergic contractions in LMPs, with higher efficacy in preparations from obese mice. Both A_2BR ligands did not affect contractions elicited by exogenous substance P. Obesity is related with a condition of colonic inflammation, leading to an increase of A_2BR expression. A_2BR, modulating the activity of excitatory tachykininergic nerves, participate to the enteric dysmotility associated with obesity.

Impact of ectonucleotidases in autonomic nervous functions

Adenine and uracil nucleotides play key functions in the autonomic nervous system (ANS). For instance, ATP acts as a neurotransmitter, co-transmitter and neuromodulator in the ANS. The purinergic system encompasses (1) receptors that respond to extracellular purines, which are designated as P1 and P2 purinoceptors, (2) purine release and uptake, and (3) a cascade of enzymes that regulate the concentration of purines near the cell surface. Ectonucleotidases and adenosine deaminase (ADA) are enzymes responsible for the hydrolysis of ATP (and other nucleotides such as ADP, UTP, UDP, AMP) and adenosine, respectively. Accordingly, these enzymes are expected to play an important role in the control of neuro-effector transmission in tissues innervated by both the sympathetic and parasympathetic divisions of the ANS. Indeed, ectonucleotidases have the ability to either terminate P2 receptor responses initiated by nucleoside triphosphates (ATP and UTP), and/or to favor the activation of ADP (e.g. P2Y1,12,13) and UDP (e.g. P2Y6) and/or adenosine (P1) specific receptors. In addition, ectonucleotidases can also importantly protect some P2 receptors from desensitization (e.g. P2X1, P2Y1). In this review, we present the (putative) roles of ectonucleotidases and ADA in the ANS with a focus on their regulatory activity at neuro-effector junctions in the following tissues: heart, vas deferens, urinary bladder, salivary glands, blood vessels and the intestine. We also present their implication in nociceptive transmission.

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.

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.

Angiotensin II contractile effects in mouse colon: role for pre- and post-junctional AT(1A) receptors

AIM

This study investigates whether a local renin-angiotensin system (RAS) exists in mouse colon and whether angiotensin II (Ang II) may play a role in the regulation of the contractile activity.

METHODS

Isometric recordings were performed in vitro on the longitudinal muscle of mouse proximal and distal colon. Transcripts encoding for RAS components were investigated by RT-PCR.

RESULTS

Ang II caused, in both preparations, a concentration-dependent contractile effect, antagonized by losartan, AT(1) receptor antagonist, but not by PD123319, AT(2) receptor antagonist. The combination of losartan plus PD123319 caused no change on the Ang II-induced contraction than losartan alone. Tetrodotoxin, neural blocker, reduced the contractile response to Ang II in the proximal colon, whilst the response was abolished in the distal colon. In both preparations, atropine, muscarinic receptor antagonist, or SR140333, NK(1) receptor antagonist, reduced the Ang II responses. Ondansetron, 5-HT(3) receptor antagonist, SR48968, NK(2) receptor antagonist, or hexamethonium, nicotinic receptor antagonist, were ineffective. The joint application of atropine and SR140333 produced no additive effect. Atropine reduced NK(1) -induced contraction. Transcripts encoding RAS components were detected in the colon samples. However, just AT(1A) mRNA was expressed in both preparations, and AT(2) mRNA was expressed only in the distal colon.

CONCLUSION

In the murine colon, local RAS may play a significant role in the control of contractile activity. Ang II positively modulates the spontaneous contractile activity via activation of post-junctional and pre-junctional AT(1A) receptors, the latter located on the enteric neurones, modulating the release of tachykinins and acetylcholine.

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.

Pharmacological characterization of uracil nucleotide-preferring P2Y receptors modulating intestinal motility: a study on mouse ileum

We investigated the possible modulation of the intestinal contractility by uracil nucleotides (UTP and UDP), using as model the murine small intestine. Contractile activity of a mouse ileum longitudinal muscle was examined in vitro as changes in isometric tension. Transcripts encoding for uracil-sensitive receptors was investigated by RT-PCR. UDP induced muscular contractions, sensitive to PPADS, suramin, or MRS 2578, P2Y(6) receptor antagonist, and mimicked by PSB 0474, P2Y(6)-receptor agonist. UTP induced biphasic effects characterized by an early inhibition of the spontaneous contractile activity followed by muscular contraction. UTP excitatory effects were antagonized by PPADS, suramin, but not by MRS 2578, whilst the inhibitory effects were antagonized by PPADS but not by suramin or MRS 2578. UTPγS, P2Y(2)/(4) receptor agonist but not 2-thio-UTP, P2Y(2) receptor agonist, mimicked UTP effects. The inhibitory effects induced by UTP was abolished by ATP desensitization and increased by extracellular acidification. UDP or UTP responses were insensitive to TTX, atropine, or L-NAME antagonized by U-73122, inhibitor of phospholipase C (PLC) and preserved in the presence of nifedipine or low Ca(2+) solution. Transcripts encoding the uracil nucleotide-preferring receptors were expressed in mouse ileum. Functional postjunctional uracil-sensitive receptors are present in the longitudinal muscle of the mouse ileum. Activation of P2Y(6) receptors induces muscular contraction, whilst activation of P2Y(4) receptors leads to inhibition of the contractile activity. Indeed, the presence of atypical UTP-sensitive receptors leading to muscular contraction is suggested. All uracil-sensitive receptors are linked to the PLC pathway.