Possible transsynaptic cholinergic neuromodulation by ATP released from ileal longitudinal muscles of guinea pigs

Muscarinic M(3) facilitation of acetylcholine release from rat myenteric neurons depends on adenosine outflow leading to activation of excitatory A(2A) receptors

Acetylcholine (ACh) is a major excitatory neurotransmitter in the myenteric plexus, and it regulates its own release acting via muscarinic autoreceptors. Adenosine released from stimulated myenteric neurons modulates ACh release preferentially via facilitatory A(2A) receptors. In this study, we investigated how muscarinic and adenosine receptors interplay to regulate ACh from the longitudinal muscle-myenteric plexus of the rat ileum. Blockade of the muscarinic M(2) receptor with 11-[[2-1[(diethylamino) methyl-1-piperidinyl]- acetyl]]-5,11-dihydro-6H-pyrido [2,3-b][1,4] benzodiazepine-6-one (AF-DX 116), 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) and atropine facilitated [3H]ACh release evoked by short stimulation trains (5 Hz, 200 pulses). Prolonging stimulus train length (>750 pulses) shifted muscarinic autoinhibition towards facilitatory M(3) receptors activation, as predicted by blockade with J104129 (a selective M(3) antagonist), 4-DAMP and atropine, whereas the selective M(2) antagonist, AF-DX 116, was without of effect. Blockade of A(2A) receptors with ZM 241385, inhibition of adenosine transport with dipyridamole, and inhibition of ecto-5'-nucleotidase with concanavalin A, all attenuated release inhibition caused by 4-DAMP. J104129 and 4-DAMP, but not AF-DX 116, decreased ( approximately 60%) evoked adenosine outflow (5 Hz, 3000 pulses). Oxotremorine (300 micromol L(-1)) facilitated the release of [3H]ACh (34 +/- 4%, n = 5) and adenosine (57 +/- 3%, n = 6) from stimulated myenteric neurons. 4-DAMP, dipyridamole and concanavalin A prevented oxotremorine-induced facilitation. ZM 241385 blocked oxotremorine facilitation of [3H]ACh release, but kept adenosine outflow unchanged. Thus, ACh modulates its own release from myenteric neurons by activating inhibitory M(2) and facilitatory M(3) autoreceptors. While the M(2) inhibition is prevalent during brief stimulation periods, muscarinic M(3) facilitation is highlighted during sustained nerve activity as it depends on extracellular adenosine accumulation leading to activation of facilitatory A(2A) receptors.

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.

Fine-tuning modulation of myenteric motoneurons by endogenous adenosine: on the role of secreted adenosine deaminase

Besides the well-characterized inhibitory effect of adenosine in the gastrointestinal tract mediated by A1 receptors, we recently demonstrated that endogenously generated adenosine facilitates [3H]acetylcholine release from myenteric neurons through preferential activation of prejunctional A2A receptors. The co-existence of both receptor subtypes on cholinergic neurons prompted the question of how does adenosine discriminate between these receptors to regulate synaptic transmission in the longitudinal muscle-myenteric plexus (LM-MP) of the rat ileum. Electrical stimulation of the LM-MP increased the outflow of adenosine, inosine and hypoxanthine. Myenteric neurons seem to be the main source of endogenous adenosine, since blockade of action potentials with tetrodotoxin (1 microM) or omission of Ca2+ (plus EGTA, 1 mM) in the buffer essentially abolished nucleosides release, while adenosine outflow remained unchanged when smooth muscle contractions were prevented by nifedipine (1 microM). Inhibition of ecto-5'-nucleotidase by concanavalin A (0.1 mg ml-1) produced only a moderate decrease (approximately 25%) on adenosine accumulation in the LM-MP, indicating that the extracellular catabolism of released ATP might not be a major source of the nucleoside. Data using the acetylcholinesterase inhibitor, physiostigmine (10 microM), and several subtype-specific muscarinic receptor antagonists, 4-DAMP (100 nM), AF-DX 116 (10 microM) and muscarinic toxin-7 (1 nM), suggest that cholinergic motoneurons are endowed with muscarinic M3 autoreceptors facilitating the outflow of adenosine. Surprisingly, bath samples collected after stimulating the LM-MP exhibited a relatively high adenosine deaminase (ADA) activity (0.60+/-0.07 U ml-1), which increased in parallel with the accumulation of adenosine and its deamination products. Our findings are in keeping with the hypothesis that ADA secretion, along with a less-efficient dipyridamole-sensitive nucleoside transport system, may restrict endogenous adenosine actions to the synaptic region channelling to facilitatory A2A receptors activation. Such a local environment may also limit diffusion of exogenously added adenosine towards the active zones, as we showed that this constrain may be overcome by inhibiting ADA activity with erythro-9(2-hydroxy-3-nonyl) adenine (50 microM).

Dual effects of adenosine on acetylcholine release from myenteric motoneurons are mediated by junctional facilitatory A(2A) and extrajunctional inhibitory A(1) receptors

1. The coexistence of both inhibitory A(1) and facilitatory A(2) adenosine receptors in the rat myenteric plexus prompted the question of how adenosine activates each receptor subtype to regulate cholinergic neurotransmission. 2. Exogenously applied adenosine (0.3-300 microm) decreased electrically evoked [(3)H]acetylcholine ([(3)H]ACh) release. Blocking A(1) receptors with 1,3-dipropyl-8-cyclopentylxanthine (10 nm) transformed the inhibitory action of adenosine into a facilitatory effect. Adenosine-induced inhibition was mimicked by the A(1) receptor agonist R-N(6)-phenylisopropyladenosine (0.3 microm), but the A(2A) agonist CGS 21680C (0.003 microm) produced a contrasting facilitatory effect. 3. Increasing endogenous adenosine levels, by the addition of (1) the adenosine precursor AMP (30-100 microm), (2) the adenosine kinase inhibitor 5'-iodotubercidin (10 microm) or (3) inhibitors of adenosine uptake (dipyridamole, 0.5 microm) and of deamination (erythro-9(2-hydroxy-3-nonyl)adenine, 50 microm), enhanced electrically evoked [(3)H]ACh release (5 Hz for 40 s). Release facilitation was prevented by adenosine deaminase (ADA, 0.5 U ml(-1)) and by the A(2A) receptor antagonist ZM 241385 (50 nm); these compounds decreased [(3)H]ACh release by 31+/-6% (n=7) and 37+/-10% (n=6), respectively. 4. Although inhibition of ecto-5'-nucleotidase by alpha,beta-methylene ADP (200 microm) or by concanavalin A (0.1 mg ml(-1)) attenuated endogenous adenosine formation from AMP, analysed by HPLC, the corresponding reduction in [(3)H]ACh release only became evident when stimulation of the myenteric plexus was prolonged to over 250 s. 5. In summary, we found that endogenously generated adenosine plays a predominantly tonic facilitatory effect mediated by prejunctional A(2A) receptors. Extracellular deamination and cellular uptake may restrict endogenous adenosine actions to the neuro-effector region near the release/production sites.

Purinergic modulation of vascular sympathetic neurotransmission

It is generally agreed that the release of norepinephrine (NE) is inhibited by activation of prejunctional purinoceptor. We examined the pharmacological properties of purinoceptors on vascular sympathetic nerve terminals and the source of endogenous adenyl purines. Electrically (1 Hz) evoked NE-release was inhibited by not only P1-agonists but also P2-agonists. Although the inhibition induced by P2-agonists was blocked by P1-antagonists, P2-agonists-induced inhibition was not due to the breakdown to adenosine. Therefore, there may be a new class of purinoceptor that is activated by both P1- and P2-agonists and antagonized by P1-antagonists. Electrical stimulation at 8 Hz but not at 1 Hz evoked the release of adenyl purines such as ATP, ADP, AMP and adenosine, in addition to NE; and the purines-release was blocked by an alpha1-antagonist. Methoxamine, an alpha1-agonist, also evoked the release of purines. Electrically (1 Hz)-evoked NE-release was inhibited by methoxamine, and this inhibition was blocked by not only an alpha1-antagonist but also a P1-antagonist. Therefore, the activation of alpha1-adrenoceptor appeared to release purines, which in turn inhibited NE-release via prejunctional purinoceptors. From these results, it is suggested that the unique purinoceptor and the endogenous purines released from alpha1-adrenoceptor-sensitive sources participate in the antidromic transsynaptic modulation of vascular sympathetic neurotransmission.

Different receptors mediating the inhibitory action of exogenous ATP and endogenously released purines on guinea-pig intestinal peristalsis

1 Adenosine 5'-triphosphate (ATP) is an enteric neurotransmitter which acts at purine receptors on intestinal nerve and muscle. This study set out to shed light on the receptor mechanisms by which exogenous and endogenous ATP influences intestinal peristalsis. 2 Peristalsis in isolated segments of the guinea-pig small intestine was triggered by a perfusion-induced rise of the intraluminal pressure. Motor changes were quantified by alterations of the peristaltic pressure threshold (PPT) at which propulsive muscle contractions were elicited. 3 ATP (>/= 3 microM) increased PPT and abolished peristalsis at concentrations of 100-300 microM. Adenosine 5'-O-2-thiodiphosphate (ADPbetaS, 3-100 microM) was more potent, whereas alpha,beta-methylene ATP (alpha,beta-meATP, 3-100 microM) was less potent, than ATP in depressing peristalsis. 4 8-Phenyltheophylline (10 microM) attenuated the anti-peristaltic effect of 10 and 30 microM ATP but not that of higher ATP concentrations. Apamin (0.5 microM) counteracted the ability of ATP, ADPbetaS and alpha,beta-meATP to enhance PPT. Suramin (300 microM) and pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 150 microM) antagonized the inhibitory effect of alpha,beta-meATP on peristalsis but did not alter the effect of ATP and ADPbetaS. 5 PPADS (50-150 microM) reduced PPT by as much as 50%. This stimulant effect on peristalsis was prevented by suramin (300 microM) but left unaltered by apamin (0.5 microM) and NG-nitro-L-arginine methyl ester (300 microM). 6 These data show that exogenous and endogenous ATP inhibits intestinal peristalsis via different apamin-sensitive purinoceptor mechanisms. Exogenous ATP depresses peristalsis mostly via suramin- and PPADS-insensitive P2 receptors, whereas endogenous purines act via P2 receptors sensitive to both suramin and PPADS.

Cross desensitizations on contractions by P2-agonists of guinea pig ileum

The present study was designed to clarify the characteristics of contractions of guinea pig ileal longitudinal muscles evoked by alpha,beta-methylene ATP as compared with those by other P2-agonists. alpha,beta-Methylene ATP, ADP-beta-S and 2-methylthio ATP as P2-agonists produced remarkable phasic contractions of the segment in a suramin-sensitive- and reactive blue-2-insensitive manner. However, ADP-beta-S and 2-methylthio ATP, unlike alpha,beta-methylene ATP, showed a biphasic contraction accompanied by a second sustained phase. Their second sustained contractions were notably suppressed by 30 microM reactive blue-2, probably being a component mediated by P2Y-purinoceptor. The phasic contractile response to alpha,beta-methylene ATP, but not ADP-beta-S and 2-methylthio ATP, was largely reduced by tetrodotoxin and atropine, indicating that the contraction is due to acetylcholine released from the cholinergic nerves. At 100 microM, alpha,beta-methylene ATP inhibited the phasic contractions caused by a low concentration of itself, but not those induced by ADP-beta-S and 2-methylthio ATP, presumably serving as a desensitizer of the P2-receptor. Although beta,gamma-methylene ATP per se showed little contraction, it prevented the contraction evoked by alpha,beta-methylene ATP, but not those by ADP-beta-S and 2-methylthio ATP. The contraction evoked by 100 microM 2-methylthio ATP was attenuated in the presence of ADP-beta-S at 10 and 30 microM. From separate cross-interactions between two groups of P2-agonists, there seems to be different subtypes of P2X-purinoceptors in the pre- and postsynapse in producing phasic contractions, but not sustained contractions that are mediated by, presumably, the P2Y-purinoceptor of the ileum.

Connections between P2 purinoceptors and capsaicin-sensitive afferents in the intestine and other tissues

Relations between P2 purinoceptors and capsaicin-sensitive sensory neurons include an excitatory action of P2 purinoceptor agonists on spinal afferent neurons, as well as release of ATP from afferents at their central and peripheral endings, and a possible participation of ATP in nociception and/or in 'local efferent' responses mediated by sensory nerves at the periphery. The present paper briefly summarizes available evidence on these interrelations. Ample evidence shows that ATP and other P2 purinoceptor agonists can activate primary afferent neurons, through P2X3 receptors and probably other purinoceptors as well, but evidence for an involvement of P2 purinoceptors in nociception or in 'local efferent' responses due to activation of primary afferents is, at best, circumstantial. The possibility is also dealt with that P2 purinoceptor activation may cause small intestinal contraction with the mediation of capsaicin-sensitive sensory neurons and that the motor response to capsaicin in this tissue may involve the release of a P2 purinoceptor stimulant from sensory nerves. Our data show that cholinergic contractions of the guinea-pig ileum in response to the P2 purinoceptor agonist alpha,beta-methylene ATP (alpha,beta-meATP) are blocked by atropine, but not by in vitro capsaicin pretreatment (which completely blocks the contractile action of capsaicin). Cholinergic ileum contractions due to capsaicin (2 microM) are insensitive to suramin (a P2 purinoceptor antagonist; 100 microM). In the presence of antagonists acting at tachykinin NK1 and NK2 receptors, however, suramin (100 microM) causes a significant inhibition of the capsaicin-evoked contraction. These data indicate that capsaicin-sensitive nerves are not involved in the excitatory effect of alpha,beta-methylene ATP on myenteric neurons. On the other hand, ATP is probably involved in the 'non-tachykininergic' component of the capsaicin-induced excitatory response of the small intestine. ATP may originate from sensory neurons and probably acts as activator of myenteric nerves.

Dependence of gamma-aminobutyric acid modulation of cholinergic transmission on nitric oxide and purines in cat terminal ileum

The possible involvement of purines and/or nitric oxide (NO) in the gamma-aminobutyric acid (GABA)A receptor-mediated effects on the spontaneous activity of isolated preparations from longitudinal and circular muscles of cat terminal ileum was investigated. GABA had biphasic effects, which were neurogenic and muscarinic. ATP and adenosine dose dependently inhibited the activity of the muscles. A contractile response evoked by the nucleotide only was also observed. The effects of the purines were equipotent and resistant to Nomega-nitro-L-arginine (L-NNA), tetrodotoxin and to desensitization by alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP), except for the contractile effect of ATP, which was abolished by alpha,beta-meATP. Pretreatment of the preparations with ATP or adenosine produced: (i) desensitization to the effects of the respective purinoceptor agonist only; and (ii) suppression of the GABA-induced responses of longitudinal and circular muscles. Hemoglobin and L-NNA greatly reduced or completely blocked the GABA(A)-induced relaxation and decreased the GABA(A)-induced contraction. Our results indicate that purines and NO, to a different extent, mediate the relaxant phase of the GABA effects in both layers. Interactions between muscarinic cholinoceptors and GABA-nitrergic pathway and a concomitant activation of postjunctional P1 and P2y purinoceptors are suggested to explain the prejunctional biphasic effects of GABA.

Possible neuronal origin of ATP release evoked by forskolin and ouabain from guinea-pig atrial segments

The characteristics of ATP release evoked by forskolin and ouabain from atrial segments of guinea-pig were evaluated under electrical stimulation. Forskolin (1 microM) produced a massive release of ATP together with a positive inotropic response. Both 30 microM W-7 (N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide.HCI), a calmodulin antagonist, and 30 microM vinblastine, a mitotic inhibitor, markedly inhibited the evoked release of ATP without affecting the evoked contraction. However, 100 microM N-ethylmaleimide abolished completely the basal and drug-evoked ATP release and further the evoked contraction. Both the ATP release and contraction evoked by ouabain (3 microM) were similarly affected by W-7, vinblastine and n-ethylmaleimide. The release of ATP, but not the contraction, evoked by forskolin was strongly suppressed by 10 microM okadaic acid, a protein phosphatase inhibitor. The suppression by okadaic acid of the evoked release was thoroughly antagonized in the presence of 0.01 microM PMA (phorbol 12-myristate 13-acetate), but not 10 microM H-7 (1-(5-isoquinolinesulfonyl)-2-methylpiperazine). These results suggest that forskolin, like ouabain, may dominantly cause the neuronal release of ATP from cardiac adrenergic nerves, although the possible participation of release from muscular sources cannot be ignored.

Inhibitory effect of okadaic acid on noradrenaline exocytosis from guinea-pig vas deferens

The effects of okadaic acid (30 microM), a protein phosphatase inhibitor, on noradrenaline (NA) release evoked by 70 mM KCl and 100 microM ouabain were evaluated in guinea-pig vas deferens. Release of NA evoked by high KCl was inhibited by okadaic acid but this inhibition was antagonized by Bay K 8644. Furthermore, okadaic acid, like Ca(2+)-channel blockers, reduced NA release by ouabain. However, ATP-release induced by alpha,beta-methylene ATP was virtually unaffected by okadaic acid or Ca(2+)-free medium. These findings suggest that phosphatases may play an important role in Ca(2+)-channel activation and consequent NA exocytosis from adrenergic nerves.

Evidence for the presence of two types of P2 purinoceptor in the guinea-pig ileal longitudinal smooth muscle preparation

The effects of some agonists acting at P2 purinoceptors on guinea-pig isolated ileum longitudinal smooth muscle have been examined. The preparation contracted in response to ATP, alpha,beta-methylene ATP and 2-methylthio ATP, but not UTP. In this respect, alpha,beta-methylene ATP and 2-methylthio ATP were approximately equipotent and both were 10-50 times more active than ATP. Responses to alpha,beta-methylene ATP, but not 2-methylthio ATP or ATP, were antagonised by atropine and tetrodotoxin, suggesting that alpha,beta-methylene ATP activates cholinergic nerves in the ileum, whilst the other two compounds act on the smooth muscle. Two other purine nucleotide analogues, beta,gamma-methylene ATP and beta,gamma-imido ATP, did not cause contraction. However, both compounds antagonised responses to alpha,beta-methylene ATP, but not those to 2-methylthio ATP. Suramin antagonised responses to both alpha,beta-methylene ATP and 2-methylthio ATP, whilst Cibacron blue was without effect on responses to either agonist. We conclude that the purinoceptor on cholinergic nerves has some of the characteristics of the P2x purinoceptor, whilst the purinoceptor on ileal smooth muscle has some of the characteristics of the P2Y purinoceptor. However, further work will be necessary before definitive classification is possible.