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

Inhibition of uterine contractility by guanine-based purines in non-pregnant rats

Growing evidence pointed out that guanine-based purines are able to modulate smooth muscle contractile activity of blood vessels and gastrointestinal tract. Since, so far, possible guanine-based purine modulation of uterine musculature is unknown, the aim of the present study was to investigate in vitro, using organ bath technique, guanosine and guanine effects on spontaneous uterine contraction, and uterine contraction induced by K⁺-depolarization and oxytocin in a non-pregnant rat. Guanosine, but not guanine, reduced the amplitude of spontaneous contraction of the uterine muscle in a dose-dependent manner. The inhibitory response was antagonized by S-(4-nitrobenzyl)-6-thioinosine (NBTI), a membrane nucleoside transporter inhibitor, but persisted in the presence of theophylline, a nonselective adenosine receptor antagonist, or propanolol, β₁/β₂ adrenoreceptor antagonist or blockers of a nitrergic pathway. In addition, potassium channel blockers did not influence guanosine-induced effects. Guanosine was able to inhibit the external calcium (Ca²⁺) influx-induced contraction, but it did not affect the contraction induced by high-KCl solution, indicating that guanosine does not interact with L-type voltage-gated calcium channel. Guanosine prevented/reduced uterine contractions induced by oxytocin, even in the absence of external calcium. In conclusion, guanosine is able to reduce both spontaneous and oxytocin-induced contractions of rat myometrium, likely subsequently to its intracellular intake. The blockade of extracellular Ca²⁺ influx and reduction of Ca²⁺ release from the intracellular store are the mechanisms involved in the guanosine-induced tocolytic effects.

Altered gastrointestinal motility in an animal model of Lesch-Nyhan disease

Mutations in the HGPRT1 gene, which encodes hypoxanthine-guanine phosphoribosyltransferase (HGprt), housekeeping enzyme responsible for recycling purines, lead to Lesch-Nyhan disease (LND). Clinical expression of LND indicates that HGprt deficiency has adverse effects on gastrointestinal motility. Therefore, we aimed to evaluate intestinal motility in HGprt knockout mice (HGprt¯). Spontaneous and neurally evoked mechanical activity was recorded in vitro as changes in isometric tension in circular muscle strips of distal colon. HGprt¯ tissues showed a lower in amplitude spontaneous activity and atropine-sensitivity neural contraction compared to control mice. The responses to carbachol and to high KCl were reduced, demonstrating a widespread impairment of contractility. L-NAME was not able in the HGprt¯ tissues to restore the large amplitude contractile activity typical of control. In HGprt¯ colon, a reduced expression of dopaminergic D1 receptor was observed together with the loss of its tonic inhibitory activity present in control-mice. The analysis of inflammatory and oxidative stress in colonic tissue of HGprt¯ mice revealed a significant increase of lipid peroxidation associated with over production of oxygen free radicals. In conclusion, HGprt deficiency in mice is associated with a decrease in colon contractility, not dependent upon reduction of acetylcholine release from the myenteric plexus or hyperactivity of inhibitory signalling. By contrast the increased levels of oxidative stress could partially explain the reduced colon motility in HGprt¯ mice. Colonic dysmotility observed in HGprt¯ mice may mimic the gastrointestinal dysfunctions symptoms of human syndrome, providing a useful animal model to elucidate the pathophysiology of this problem in the LND.

Dopamine induces inhibitory effects on the circular muscle contractility of mouse distal colon via D1- and D2-like receptors

Dopamine (DA) acts as gut motility modulator, via D1- and D2-like receptors, but its effective role is far from being clear. Since alterations of the dopaminergic system could lead to gastrointestinal dysfunctions, a characterization of the enteric dopaminergic system is mandatory. In this study, we investigated the role of DA and D1- and D2-like receptors in the contractility of the circular muscle of mouse distal colon by organ-bath technique. DA caused relaxation in carbachol-precontracted circular muscle strips, sensitive to domperidone, D2-like receptor antagonist, and mimicked by bromocriptine, D2-like receptor agonist. 7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH-23390), D1-like receptor antagonist, neural toxins, L-NAME (nitric oxide (NO) synthase inhibitor), 2'-deoxy-N₆-methyl adenosine 3',5'-diphosphate diammonium salt (MRS 2179), purinergic P2Y1 antagonist, or adrenergic antagonists were ineffective. DA also reduced the amplitude of neurally evoked cholinergic contractions. The effect was mimicked by (±)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrobromide (SKF-38393), D1-like receptor agonist and antagonized by SCH-23390, MRS 2179, or L-NAME. Western blotting analysis determined the expression of DA receptor proteins in mouse distal colon. Notably, SCH-23390 per se induced an increase in amplitude of spontaneous and neurally evoked cholinergic contractions, unaffected by neural blockers, L-NAME, MRS 2179, muscarinic, adrenergic, or D2-like receptor antagonists. Indeed, SCH-23390-induced effects were antagonized by an adenylyl cyclase blocker. In conclusion, DA inhibits colonic motility in mice via D2- and D1-like receptors, the latter reducing acetylcholine release from enteric neurons, involving nitrergic and purinergic systems. Whether constitutively active D1-like receptors, linked to adenylyl cyclase pathway, are involved in a tonic inhibitory control of colonic contractility is questioned.

The Guanine-Based Purinergic System: The Tale of An Orphan Neuromodulation

Guanine-based purines (GBPs) have been recently proposed to be not only metabolic agents but also extracellular signaling molecules that regulate important functions in the central nervous system. In such way, GBPs-mediated neuroprotection, behavioral responses and neuronal plasticity have been broadly described in the literature. However, while a number of these functions (i.e., GBPs neurothophic effects) have been well-established, the molecular mechanisms behind these GBPs-dependent effects are still unknown. Furthermore, no plasma membrane receptors for GBPs have been described so far, thus GBPs are still considered orphan neuromodulators. Interestingly, an intricate and controversial functional interplay between GBPs effects and adenosine receptors activity has been recently described, thus triggering the hypothesis that GBPs mechanism of action might somehow involve adenosine receptors. Here, we review recent data describing the GBPs role in the brain. We focus on the involvement of GBPs regulating neuronal plasticity, and on the new hypothesis based on putative GBPs receptors. Overall, we expect to shed some light on the GBPs world since although these molecules might represent excellent candidates for certain neurological diseases management, the lack of putative GBPs receptors precludes any high throughput screening intent for the search of effective GBPs-based drugs.

Opposite role played by GABAA and GABAB receptors in the modulation of peristaltic activity in mouse distal colon

We investigated the role of GABA on intestinal motility using as model the murine distal colon. Effects induced by GABA receptors recruitment were examined in whole colonic segments and isolated circular muscle preparations to analyze their influence on peristaltic reflex and on spontaneous and neurally-evoked contractions. Using a modified Trendelenburg set-up, rhythmic peristaltic contractions were evoked by gradual distension of the colonic segments. Spontaneous and neurally-evoked mechanical activity of circular muscle strips were recorded in vitro as changes in isometric tension. GABA, at low concentrations (10-50 µM), potentiated peristaltic activity and the neural cholinergic contractions, whilst it, at higher concentrations (500 µM-1mM), had inhibitory effects. GABA excitatory effects were mimicked by muscimol, GABAA-receptor agonist, and prevented by bicuculline, GABAA-receptor antagonist, which per se reduced peristaltic activity and the cholinergic contractile responses. Inhibitory effects were mimicked by baclofen, GABAB-receptor agonist, and antagonized by phaclofen, GABAB-receptor antagonist and by hexamethonium, neural nicotinic receptor antagonist. Guanethidine was ineffective on GABA effects. Non-cholinergic responses were not affected by GABA agents. All drugs failed to affect the response to carbachol. Lastly, GABAC receptor agonist/antagonist had any effect on colonic motility. In conclusion, GABA in mouse distal colon is a modulator of peristaltic activity via the regulation of acetylcholine release from cholinergic neurons through interaction with GABAA or GABAB receptors. GABAA receptors are recruited at low GABA concentrations, increasing acetylcholine release and propulsive activity. At high GABA concentrations the activation of GABAB receptors overrides GABAA receptor effects, decreasing acetylcholine release and peristaltic activity.

Guanosine negatively modulates the gastric motor function in mouse

The aim of the present study was to evaluate if guanine-based purines may affect the gastric motor function in mouse. Thus, the influence of guanosine on the gastric emptying rate in vivo was determined and its effects on spontaneous gastric mechanical activity, detected as changes of the intraluminal pressure, were analyzed in vitro before and after different treatments. Gastric gavage of guanosine (1.75-10 mg/kg) delayed the gastric emptying. Guanosine (30 μM-1 mM) induced a concentration-dependent relaxation of isolated stomach, which was not affected by the inhibition of the purine nucleoside phosphorylase enzyme by 4'-deaza-1'-aza-2'-deoxy-1'-(9-methylene)-immucillin-H. The inhibitory response was antagonized by S-(4-nitrobenzyl)-6-thioinosine, a membrane nucleoside transporter inhibitor, but not affected by 9-chloro-2-(2-furanyl)-[1,2,4]triazolo[1,5-c]quinazolin-5-amine, a nonselective adenosine receptor antagonist, or by tetrodotoxin, a blocker of neuronal voltage-dependent Na(+) channels. Moreover, guanosine-induced effects persisted in the presence of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, an inhibitor of soluble guanylyl cyclase or tetraethylammonium, a nonselective potassium channel blocker, but they were progressively reduced by increasing concentrations of 2'5'dideoxyadenosine, an adenylyl cyclase inhibitor. Lastly, the levels of cyclic adenosine monophosphate (cAMP), measured by ELISA, in gastric full thickness preparations were increased by guanosine. In conclusion, our data indicate that, in mouse, guanosine is able to modulate negatively the gastric motor function, reducing gastric emptying and inducing muscular relaxation. The latter is dependent by its cellular uptake and involves adenylyl cyclase activation and increase in cAMP intracellular levels, while it is independent on neural action potentials, adenosine receptors, and K(+) channel activation.