Prejunctional autoinhibition of purinergic transmission in circular muscle of guinea-pig ileum; a mechanism distinct from P1-purinoceptor activation

P2X3 receptors participate in purinergic inhibition of gastrointestinal smooth muscle

The ATP analogue α,β-meATP is a potent relaxant of gastrointestinal smooth muscle, but its molecular target is uncertain inside the gut. α,β-meATP relaxed the carbachol-precontracted guinea-pig taenia coli in a concentration-dependent manner (EC₅₀, 2.0 ± 0.1 μM). A luciferase-based assay confirmed that α,β-meATP solutions were minimally contaminated with ATP. α,β-meATP-evoked relaxations were inhibited by the competitive P2Y1 antagonist MRS2179 (pA₂ = 5.36), but also by the competitive P2X3 antagonist, A-317491 (pA₂ = 5.51). When MRS2179 and A-317491 were applied together, residual α,β-meATP responses converted from brief to prolonged relaxations. Sodium nitroprusside (a nitric oxide donor) also caused prolonged relaxations. Immunohistochemistry revealed that P2X3 receptors were present in myenteric ganglion cells and their varicose nerve terminals. The amplitude of α,β-meATP responses was not inhibited by TTX (NaV channel blocker) and ωCgTx (N-type CaV channel blocker). However, responses to α,β-meATP were inhibited by TEA (non-selective K⁺-channel blocker), indicating that relaxations involved opening K⁺-channels. The findings of this study are consistent with the conclusion that α,β-meATP stimulates Ca²⁺-permeable P2X3 receptors on varicose nerve terminals to release inhibitory nucleotides: 1) ATP and β-NAD release results in P2Y1-mediated brief relaxations; 2) another released transmitter (possibly NO) results in prolonged relaxations. Prejunctional P2X3 receptors represent a purinergic feed-forward mechanism to augment the action of inhibitory nerves on gut motility. This positive feed-forward mechanism may counter-balance the known negative feedback mechanism caused by adenosine and prejunctional A1 receptors on inhibitory motor nerves.

Inverse gradient of nitrergic and purinergic inhibitory cotransmission in the mouse colon

AIM

Gastrointestinal smooth muscle relaxation is accomplished by the neural corelease of ATP or a related purine and nitric oxide. Contractions are triggered by acetylcholine and tachykinins. The aim of this work was to study whether regional differences in neurotransmission could partially explain the varied physiological roles of each colonic area.

METHODS

We used electrophysiological and myography techniques to evaluate purinergic (L-NNA 1 mm incubated tissue), nitrergic (MRS2500 0.3 μm incubated tissue) and cholinergic neurotransmission (L-NNA 1 mm and MRS2500 0.3 μm incubated tissue) in the proximal, mid and distal colon of CD1 mice (n = 42).

RESULTS

Purinergic electrophysiological responses elicited by single pulses (28 V) were greater in the distal (IJPfMAX = -35.3 ± 2.2 mV), followed by the mid (IJPfMAX = -30.6 ± 1.0 mV) and proximal (IJPfMAX = -11.7 ± 1.1 mV) colon. In contrast, nitrergic responses decreased from the proximal colon (IJPsMAX = -11.4 ± 1.1 mV) to the mid (IJPsMAX = -9.1 ± 0.4 mV), followed by the distal colon (IJPsMAX = -1.8 ± 0.3 mV). A similar rank of order was observed in neural mediated inhibitory mechanical responses including electrical field stimulation-mediated responses and neural tone. ADPβs concentration-response curve was shifted to the left in the distal colon. In contrast, NaNP responses did not differ between regions. Cholinergic neurotransmission elicited contractions of a similar amplitude throughout the colon.

CONCLUSION

An inverse gradient of purinergic and nitrergic neurotransmission exists through the mouse colon. The proximal and mid colon have a predominant nitrergic neurotransmission probably due to the fact that their storage function requires sustained relaxations. The distal colon, in contrast, has mainly purinergic neurotransmission responsible for the phasic relaxations needed to propel dehydrated faeces.

Differential functional role of purinergic and nitrergic inhibitory cotransmitters in human colonic relaxation

AIM

ATP and nitric oxide (NO) are released from enteric inhibitory motor neurones and are responsible for colonic smooth muscle relaxation. However, how frequency of neural stimulation affects this cotransmission process and the post-junctional responses has not been systematically characterized in the human colon.

METHODS

The dynamics of inhibitory cotransmission were studied using different protocols of electrical field stimulation (EFS) to characterize the inhibitory junction potentials (IJP) and the corresponding relaxation in colonic strips obtained from 36 patients.

RESULTS

Single pulses elicited a fast IJP (IJPf(MAX) = -27.6 ± 1.6 mV), sensitive to the P2Y1 antagonist MRS2500 1 μm, that ran down with frequency increase leaving a residual hyperpolarization at high frequencies (IJPf∞ = -3.7 ± 0.6 mV). Accordingly, low frequencies of EFS caused purinergic transient relaxations that cannot be maintained at high frequencies. Addition of the P2Y1 agonist MRS2365 10 μm during the purinergic rundown did not cause any hyperpolarization. Protein kinase C (PKC), a putative P2Y1 desensitizator, was able to reduce the amplitude of the IJPf when activated, but the rundown was not modified by PKC inhibitors. Frequencies higher than 0.60 ± 0.15 Hz were needed to evoke a sustained nitrergic hyperpolarization that progressively increased reaching IJPs∞ = -13 ± 0.4 mV at high frequencies and leading to a sustained inhibition of spontaneous motility.

CONCLUSION

Changes in frequency of stimulation possibly mimicking neuronal firing will post-junctionally determine purinergic vs. nitrergic responses underlying different functional roles. NO will be responsible for sustained relaxations needed in physiological processes such as storage, while purinergic neurotransmission evoking sharp transient relaxations will be dominant in processes such as propulsion.

Purinergic neuromuscular transmission in the gastrointestinal tract; functional basis for future clinical and pharmacological studies

Nerve-mediated relaxation is necessary for the correct accomplishment of gastrointestinal (GI) motility. In the GI tract, NO and a purine are probably released by the same inhibitory motor neuron as inhibitory co-transmitters. The P2Y1 receptor has been recently identified as the receptor responsible for purinergic smooth muscle hyperpolarization and relaxation in the human gut. This finding has been confirmed in P2Y1 -deficient mice where purinergic neurotransmission is absent and transit time impaired. However, the mechanisms responsible for nerve-mediated relaxation, including the identification of the purinergic neurotransmitter(s) itself, are still debatable. Possibly different mechanisms of nerve-mediated relaxation are present in the GI tract. Functional demonstration of purinergic neuromuscular transmission has not been correlated with structural studies. Labelling of purinergic neurons is still experimental and is not performed in routine pathology studies from human samples, even when possible neuromuscular impairment is suspected. Accordingly, the contribution of purinergic neurotransmission in neuromuscular diseases affecting GI motility is not known. In this review, we have focused on the physiological mechanisms responsible for nerve-mediated purinergic relaxation providing the functional basis for possible future clinical and pharmacological studies on GI motility targeting purine receptors.

Specific and complementary roles for nitric oxide and ATP in the inhibitory motor pathways to rat internal anal sphincter

BACKGROUND

The neurotransmitters mediating inhibitory pathways to internal anal sphincter (IAS) have not been fully characterized. Our aim was to assess the putative release of nitric oxide, purines and vasoactive intestinal peptide (VIP) from inhibitory motor neurons (MNs) and their role in the myogenic tone, resting membrane potential (RMP) of smooth muscle cells (SMC), spontaneous inhibitory junction potentials (sIJP), mechanical relaxation, and IJP induced by electrical field stimulation (EFS) or nicotine.

METHODS

Rat IAS strips were studied using organ baths, microelectrodes, and immunohistochemistry.

KEY RESULTS

Internal anal sphincter strips developed active myogenic tone (0.31 g), enhanced and stabilized by prostaglandin F(2α) (PGF2α). L-NNA (1 mmol L(-1)) depolarized SMC and increased tone but did not modify sIJP. In contrast, the specific P2Y(1) receptor antagonist MRS2500 (1 μmol L(-1)) did not modify the RMP or the basal tone but abolished sIJP. Electrical field stimulation and nicotine (10 μmol L(-1)) caused IAS relaxation (-45.9%VS-52.2%), partially antagonized by L-NNA (35%-45%, P ≤ 0.05) and fully abolished by MRS2500 (P ≤ 0.001). Electrical field stimulation induced a biphasic inhibitory junction potential (IJP), the initial fast component was selectively blocked by MRS2500 and the sustained slow component was blocked by L-NNA. Vasoactive intestinal peptide 6-28 (0.1 μmol L(-1)) or α-chymotrypsin (10 U mL(-1)) did not modify the RMP, sIJP, EFS-induced IJP, or relaxation. P2Y(1) receptors were immunolocalized in the circular SMC of IAS.

CONCLUSIONS & INFERENCES

The effects of inhibitory MNs on rat IAS are mediated by a functional co-transmission process involving nitrergic and purinergic pathways through P2Y(1) receptors with specific and complementary roles on the control of tone, sIJP, and hyperpolarization and relaxation of IAS following stimulation of inhibitory MNs.

Purinergic and nitrergic junction potential in the human colon

The aim of the present work is to investigate a putative junction transmission [nitric oxide (NO) and ATP] in the human colon and to characterize the electrophysiological and mechanical responses that might explain different functions from both neurotransmitters. Muscle bath and microelectrode techniques were performed on human colonic circular muscle strips. The NO donor sodium nitroprusside (10 microM), but not the P2Y receptor agonist adenosine 5'-O-2-thiodiphosphate (10 microM), was able to cause a sustained relaxation. NG-nitro-L-arginine (L-NNA) (1 mM), a NO synthase inhibitor, but not 2'-deoxy-N6-methyl adenosine 3',5'-diphosphate tetraammonium salt (MRS 2179) (10 microM), a P2Y antagonist, increased spontaneous motility. Electrical field stimulation (EFS) at 1 Hz caused fast inhibitory junction potentials (fIJPs) and a relaxation sensitive to MRS 2179 (10 microM). EFS at higher frequencies (5 Hz) showed biphasic IJP with fast hyperpolarization sensitive to MRS 2179 followed by sustained hyperpolarization sensitive to L-NNA; both drugs were needed to fully block the EFS relaxation at 2 and 5 Hz. Two consecutive single pulses induced MRS 2179-sensitive fIJPs that showed a rundown. The rundown mechanism was not dependent on the degree of hyperpolarization and was present after incubation with L-NNA (1 mM), hexamethonium (100 microM), MRS 2179 (1 microM), and NF023 (10 microM). We concluded that single pulses elicit ATP release from enteric motor neurons that cause a fIJP and a transient relaxation that is difficult to maintain over time; also, NO is released at higher frequencies causing a sustained hyperpolarization and relaxation. These differences might be responsible for complementary mechanisms of relaxation being phasic (ATP) and tonic (NO).

Involvement of P2Y1 and P2Y11 purinoceptors in parasympathetic inhibition of colonic smooth muscle

Purinergic signaling was first recognized in the guinea pig (Cavia porcellus) taenia coli, where relaxation of smooth muscle by nerve-released ATP may involve the activation of P2Y(1) and P2Y(11) receptors, and where transcripts for both genes have been found. A partial sequence for P2Y(11) protein was identified; the full-length P2Y(1) sequence has already been described. P2Y(1) and P2Y(11) proteins were localized by immunohistochemistry in smooth muscle cells. P2X(2) and P2X(3) proteins were also localized in motoneurons of the myenteric plexus. alphabeta-Methylene-ATP (alphabetameATP) and dibenzoyl-ATP (BzATP) evoked fast relaxations in the taenia, and they were inhibited by the P2Y(1) receptor antagonist 2'-deoxy-N(6)-methyladenosine 3',5'-bisphosphate (MRS2179). However, alphabetameATP and BzATP may stimulate neuronal P2X receptors to release ATP, which then acts on P2Y(1) receptors. In accordance, fast relaxations evoked by alphabetameATP and BzATP were inhibited by the P2X(3) and P2X(2/3) receptor antagonist 5-({[3-phenoxybenzyl][(1S)-1,2,3,4-tetrahydro-1-naphthalenyl] amino} carbonyl)-1,2,4-benzene-tricarboxylic acid (A317491). When P2Y(1), P2X(3), and P2X(2/3) receptors were blocked and adenosine was removed enzymatically, alphabetameATP and BzATP evoked slow relaxations that were inhibited by Reactive Red. Fast and slow relaxations involve small and large conductance calcium-activated potassium channels; the latter are dependent on intracellular cyclic AMP levels, which altered the duration and amplitude of relaxations. alphabetameATP and BzATP were confirmed as agonists, and Reactive Red as an antagonist, of human P2Y(11) receptors. In summary, G(q)-coupled P2Y(1) receptors are involved mainly in fast relaxations, whereas G(q)and G(s)-coupled P2Y(11) receptors are involved in both fast and slow relaxations. These P2Y receptor subtypes, plus neuronal P2X receptors, may explain the phenomenon of parasympathetic inhibition first described by Langley (1898).

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.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Caffeine reduces the efficacy of electroreceptor cell synapses: an electrophysiological single-unit in vivo study

Ampullary electroreceptor organs of catfish, Ictalurus melas, were exposed apically to caffeine solutions at concentrations of 0, 5, 7.5, 10, and 15 mM. Recording sinusoidally-modulated activity of single-unit afferents reveals a dose-dependent decrease in mean afferent activity and sensitivity. A rebound effect of average activity occurs after caffeine is washed out. After 25 min exposure to 15 mM caffeine the peak of the gain curve shifts from 8 Hz to 4 Hz. The corresponding phase characteristic shows an increased phase lag with a maximum shift of 35 degrees at 20 Hz. The latency between stimulus and response increases from 12 to 19 ms; the recovery time after onset of the pulse decreases with 60 ms. The most probable explanation for the recorded effects is that caffeine reduces the availability of intracellular Ca2+ by blocking of the inositol triphosphate receptors in the endoplasmic reticulum. This in turn would affect many intracellular properties and processes. The unavailability of Ca2+ could reduce the synaptic efficacy and increase latency by suppressing fusion of synaptic vesicles with the presynaptic membrane and by depressing vesicle transport. The change in frequency response corresponds in part to reduction of the apical membrane surface area of the receptor cells, and in part to the increased latency. Accumulation of glutamate-containing vesicles could account for the higher mean activity and modulation amplitude in the lower frequency range after caffeine is washed out. Caffeine might act postsynaptically by inducing hyperpolarization of the terminals of the primary afferents.