Anaid Shahbazian

Involvement of μ- and κ-, but not δ-, opioid receptors in the peristaltic motor depression caused by endogenous and exogenous opioids in the guinea-pig intestine

Opiates inhibit gastrointestinal propulsion, but it is not clear which opioid receptor types are involved in this action. For this reason, the effect of opioid receptor – selective agonists and antagonists on intestinal peristalsis was studied. Peristalsis in isolated segments of the guinea‐pig small intestine was triggered by a rise of the intraluminal pressure and recorded via the intraluminal pressure changes associated with the peristaltic waves. μ‐Opioid receptor agonists (DAMGO, morphine), κ‐opioid receptor agonists (ICI‐204,448 and BRL‐52,537) and a δ‐opioid receptor agonist (SNC‐80) inhibited peristalsis in a concentration‐related manner as deduced from a rise of the peristaltic pressure threshold (PPT) and a diminution of peristaltic effectiveness. Experiments with the δ‐opioid receptor antagonists naltrindole (30 nM) and HS‐378 (1 μM), the κ‐opioid receptor antagonist nor‐binaltorphimine (30 nM) and the μ‐opioid receptor antagonist cyprodime (10 μM) revealed that the antiperistaltic effect of ICI‐204,448 and BRL‐52,537 was mediated by κ‐opioid receptors and that of morphine and DAMGO by μ‐opioid receptors. In contrast, the peristaltic motor inhibition caused by SNC‐80 was unrelated to δ‐opioid receptor activation. Cyprodime and nor‐binaltorphimine, but not naltrindole and HS‐378, were per se able to stimulate intestinal peristalsis as deduced from a decrease in PPT. The results show that the neural circuits controlling peristalsis in the guinea‐pig small intestine are inhibited by endogenous and exogenous opioids acting viaμ‐ and κ‐, but not δ‐, opioid receptors.

Deletion of the acid-sensing ion channel ASIC3 prevents gastritis-induced acid hyperresponsiveness of the stomach-brainstem axis.

&NA; Gastric acid challenge of the rat and mouse stomach is signalled to the brainstem as revealed by expression of c‐Fos. The molecular sensors relevant to the detection of gastric mucosal acidosis are not known. Since the acid‐sensing ion channels ASIC2 and ASIC3 are expressed by primary afferent neurons, we examined whether knockout of the ASIC2 or ASIC3 gene modifies afferent signalling of a gastric acid insult in the normal and inflamed stomach. The stomach of conscious mice (C57BL/6) was challenged with intragastric HCl; two hours later the activation of neurons in the nucleus tractus solitarii (NTS) of the brainstem was visualized by c‐Fos immunocytochemistry. Mild gastritis was induced by addition of iodoacetamide (0.1%) to the drinking water for 7 days. Exposure of the gastric mucosa to HCl (0.25 M) caused a 3‐fold increase in the number of c‐Fos‐positive neurons in the NTS. This afferent input to the NTS remained unchanged by ASIC3 knockout, whereas ASIC2 knockout augmented the c‐Fos response to gastric HCl challenge by 33% (P < 0.01). Pretreatment of wild‐type mice with iodoacetamide induced mild gastritis, as revealed by increased myeloperoxidase activity, and enhanced the number of NTS neurons responding to gastric HCl challenge by 41% (P < 0.01). This gastric acid hyperresponsiveness was absent in ASIC3 knockout mice but fully preserved in ASIC2 knockout mice. The current data indicate that ASIC3 plays a major role in the acid hyperresponsiveness associated with experimental gastritis. In contrast, ASIC2 appears to dampen acid‐evoked input from the stomach to the NTS.

Chemo‐nociceptive signalling from the colon is enhanced by mild colitis and blocked by inhibition of transient receptor potential ankyrin 1 channels

Background and purpose:  Transient receptor potential ankyrin 1 (TRPA1) channels are expressed by primary afferent neurones and activated by irritant chemicals including allyl isothiocyanate (AITC). Here we investigated whether intracolonic AITC causes afferent input to the spinal cord and whether this response is modified by mild colitis, morphine or a TRPA1 channel blocker.

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

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. 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. ATP (3 μM) increased PPT and abolished peristalsis at concentrations of 100–300 μM. Adenosine 5′‐O‐2‐thiodiphosphate (ADPβS, 3–100 μM) was more potent, whereas α,β‐methylene ATP (α,β‐meATP, 3–100 μM) was less potent, than ATP in depressing peristalsis. 8‐Phenyltheophylline (10 μM) attenuated the anti‐peristaltic effect of 10 and 30 μM ATP but not that of higher ATP concentrations. Apamin (0.5 μM) counteracted the ability of ATP, ADPβS and α,β‐meATP to enhance PPT. Suramin (300 μM) and pyridoxal phosphate‐6‐azophenyl‐2′,4′‐disulphonic acid (PPADS, 150 μM) antagonized the inhibitory effect of α,β‐meATP on peristalsis but did not alter the effect of ATP and ADPβS. PPADS (50–150 μM) reduced PPT by as much as 50%. This stimulant effect on peristalsis was prevented by suramin (300 μM) but left unaltered by apamin (0.5 μM) and NG‐nitro‐L‐arginine methyl ester (300 μM). 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.

Experimental gastritis in mice enhances anxiety in a gender-related manner

There is a gender-related comorbidity of pain-related and inflammatory bowel diseases with psychiatric diseases. Since the impact of experimental gastrointestinal inflammation on the emotional-affective behavior is little known, we examined whether experimental gastritis modifies anxiety, stress coping and circulating corticosterone in male and female Him:OF1 mice. Gastritis was induced by adding iodoacetamide (0.1%) to the drinking water for at least 7 days. Inflammation was assessed by gastric histology and myeloperoxidase activity, circulating corticosterone determined by enzyme immunoassay, anxiety-related behavior evaluated with the elevated plus maze and stress-induced hyperthermia tests, and depression-like behavior estimated with the tail suspension test. Iodoacetamide-induced gastritis was associated with gastric mucosal surface damage and an increase in gastric myeloperoxidase activity, this increase being significantly larger in female mice than in male mice. The rectal temperature of male mice treated with iodoacetamide was enhanced, whereas that of female mice was diminished. The circulating levels of corticosterone were reduced by 65% in female mice treated with iodoacetamide but did not significantly change in male mice. On the behavioral level, iodoacetamide treatment caused a decrease in nocturnal home-cage activity, drinking and feeding. While depression-related behavior remained unaltered following induction of gastritis, behavioral indices of anxiety were significantly enhanced in female but not male mice. There was no correlation between the estrous cycle and anxiety as well as circulating corticosterone. Radiotracer experiments revealed that iodoacetamide did not readily enter the brain, the blood-brain ratio being 20:1. Collectively, these data show that iodoacetamide treatment causes gastritis in a gender-related manner, its severity being significantly greater in female than in male mice. The induction of gastritis in female mice is associated with a reduction of circulating corticosterone and an enforcement of behavioral indices of anxiety. Gastric inflammation thus has a distinct gender-dependent influence on emotional-affective behavior and its neuroendocrine control.

Involvement of endothelial NO in the dilator effect of VIP on rat isolated pulmonary artery

The endothelium and its interaction with smooth muscle play a central role in the local control of the pulmonary vasculature, and endothelial dysfunction is thought to contribute to pulmonary hypertension and chronic obstructive pulmonary disease. Vasoactive intestinal peptide (VIP), a 28-amino acid neuropeptide, relaxes the rat pulmonary artery, but there is controversy as to whether or not this action of VIP depends on the endothelium. The aim of this study, therefore, was to investigate the role of the endothelium and nitric oxide (NO), the major endothelium-derived relaxing factor, in the dilator action of VIP on the rat isolated pulmonary artery. Pulmonary artery preparations pre-contracted by the alpha(1)-adrenoceptor agonist L-phenylephrine were relaxed by VIP (0.003-1 microM) and acetylcholine (0.003-10 microM) in a concentration-dependent manner. Mechanical removal of the endothelium reduced the maximal response to VIP by about 50% and practically abolished the response to acetylcholine. Inhibition of NO synthesis by N(omega)-nitro-L-arginine methyl ester (0.5 mM) had a similar effect, abolishing the vasorelaxation caused by acetylcholine and attenuating the vasorelaxation caused by VIP by about 50%. From these data it is concluded that the relaxant action of VIP on the rat isolated pulmonary artery depends in part on the presence of the endothelium and that this part is mediated by endothelial NO.

Differential peristaltic motor effects of prostanoid (DP, EP, IP, TP) and leukotriene receptor agonists in the guinea-pig isolated small intestine.

Since the role of prostanoid receptors in intestinal peristalsis is largely unknown, the peristaltic motor effects of some prostaglandin (DP, EP, IP), thromboxane (TP) and leukotriene (LT) receptor agonists and antagonists were investigated. Propulsive peristalsis in fluid‐perfused segments from the guinea‐pig small intestine was triggered by a rise of the intraluminal pressure and recorded via the intraluminal pressure changes associated with the peristaltic waves. Alterations of distension sensitivity were deduced from alterations of the peristaltic pressure threshold and modifications of peristaltic performance were reflected by modifications of the amplitude, maximal acceleration and residual baseline pressure of the peristaltic waves. Four categories of peristaltic motor effects became apparent: a decrease in distension sensitivity and peristaltic performance as induced by the EP1/EP3 receptor agonist sulprostone and the TP receptor agonist U‐46,619 (1–1000 nM); a decrease in distension sensitivity without a major change in peristaltic performance as induced by PGD2 (3–300 nM) and LTD4 (10–100 nM); a decrease in peristaltic performance without a major change in distension sensitivity as induced by PGE1, PGE2 (1–1000 nM) and the EP1/IP receptor agonist iloprost (1–100 nM); and a decrease in peristaltic performance associated with an increase in distension sensitivity as induced by the EP2 receptor agonist butaprost (1–1000 nM). The DP receptor agonist BW‐245C (1–1000 nM) was without effect. The peristaltic motor action of sulprostone remained unchanged by the EP1 receptor antagonist SC‐51,089 (1 μM) and the DP/EP1/EP2 receptor antagonist AH‐6809 (30 μM), whereas that of U‐46,619 and LTD4 was prevented by the TP receptor antagonist SQ‐29,548 (10 μM) and the cysteinyl‐leukotriene1 (cysLT1) receptor antagonist tomelukast (10 μM), respectively. These observations and their pharmacological analysis indicate that activation of EP2, EP3, IP, TP and cysLT1 receptors, but not DP receptors, modulate intestinal peristalsis in a receptor‐selective manner, whereas activation of EP1 seems to be without influence on propulsive peristalsis. In a wider perspective it appears as if the effect of prostanoid receptor agonists to induce diarrhoea is due to their prosecretory but not peristaltic motor action.

Disturbance of peristalsis in the guinea-pig isolated small intestine by indomethacin, but not cyclo-oxygenase isoform-selective inhibitors

Since the cyclo‐oxygenase (COX) isoform‐nonselective inhibitor indomethacin is known to modify intestinal motility, we analysed the effects of COX‐1 and COX‐2 inhibition on intestinal peristalsis. Peristalsis in isolated segments of the guinea‐pig small intestine was triggered by a rise of the intraluminal pressure and recorded via the pressure changes associated with peristalsis. The COX‐1 inhibitor SC‐560, the COX‐2 inhibitor NS‐398 (both at 0.1  – 1 μM) and the isoform‐nonselective inhibitors flurbiprofen (0.01 – 10 μM) and piroxicam (0.1 – 50 μM) were without major influence on peristalsis, whereas indomethacin and etodolac (0.1 – 10 μM) disturbed the regularity of peristalsis by causing nonpropulsive circular muscle contractions. Radioimmunoassay measurements showed that SC‐560, NS‐398, indomethacin and etodolac (each at 1 μM) suppressed the release of 6‐keto‐prostaglandin F1α (6‐keto‐PGF1α) from the intestinal segments. Reverse transcription – polymerase chain reaction tests revealed that, relative to glyceraldehyde‐3 phosphate dehydrogenase ribonucleic acid, the expression of COX‐1 mRNA increased by a factor of 2.0 whereas that of COX‐2 mRNA rose by a factor of 7.9 during the 2 h experimental period. Pharmacological experiments indicated that the action of indomethacin to disturb intestinal peristalsis was unrelated to inhibition of L‐type calcium channels, adenosine triphosphate‐sensitive potassium channels or phosphodiesterase type IV. These results show that selective inhibition of COX‐1 and COX‐2 does not grossly alter peristaltic motor activity in the guinea‐pig isolated small intestine and that the effect of indomethacin to disturb the regular pattern of propulsive motility in this species is unrelated to COX inhibition.

PACAP-(6-38) inhibits the effects of vasoactive intestinal polypeptide, but not PACAP, on the small intestinal circular muscle

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating peptide-(1-38) (PACAP) have been found to stimulate distension-induced peristaltic motility in the guinea-pig isolated small intestine. In this study, we tested whether the putative VIP/PACAP receptor antagonist PACAP-(6-38) counteracts the properistaltic effect of VIP and PACAP in isolated segments of the guinea-pig small intestine. VIP (100 nM) and PACAP (30 nM) had a stimulatory effect, i.e., lowered the peristaltic pressure threshold at which peristaltic waves were triggered and enhanced the frequency of peristaltic waves. PACAP-(6-38) (3 microM) was per se without effect on peristalsis but prevented or reversed the peristaltic motor stimulation caused by VIP, when it was given before or after the agonist, respectively. PACAP-(6-38), however, failed to antagonize the properistaltic effect of PACAP. In ileal circular strips treated with tetrodotoxin (1 microM) and indomethacin (3 microM), spontaneous myogenic activity was inhibited by VIP (5-30 nM). This effect was significantly reduced by a pretreatment with PACAP-(6-38) (3 microM). A similar inhibition by PACAP-(1-38) (10-500 nM) was not influenced by the antagonist. It is concluded that PACAP-(6-38) is a VIP receptor antagonist, both in the peristaltic motor pathways and at the level of the circular muscle of the guinea-pig small intestine. The lack of a motor effect of PACAP-(6-38) on its own indicates that VIP acting on PACAP-(6-38)-sensitive receptors (located on neurons and/or the smooth muscle) is unlikely to participate in peristaltic motor regulation.

Increase in gastric acid-induced afferent input to the brainstem in mice with gastritis.

Acid challenge of the gastric mucosa is signaled to the brainstem. This study examined whether mild gastritis due to dextrane sulfate sodium (DSS) or iodoacetamide (IAA) enhances gastric acid-evoked input to the brainstem and whether this effect is related to gastric myeloperoxidase activity, gastric histology, gastric volume retention or cyclooxygenase stimulation. The stomach of conscious mice was challenged with NaCl (0.15 M) or HCl (0.15 and 0.25 M) administered via gastric gavage. Two hours later, activation of neurons in the nucleus tractus solitarii (NTS) was visualized by c-Fos immunocytochemistry. Gastritis was induced by DSS (molecular weight 8000; 5%) or IAA (0.1%) added to the drinking water for 7 days. Relative to NaCl, intragastric HCl increased the number of c-Fos protein-expressing cells in the NTS. Pretreatment with DSS or IAA for 1 week did not alter the c-Fos response to NaCl but significantly enhanced the response to HCl by 54 and 74%, respectively. Either pretreatment elevated gastric myeloperoxidase activity and induced histological injury of the mucosal surface. In addition, DSS caused dilation of the gastric glands and damage to the parietal cells. HCl-induced gastric volume retention was not altered by IAA but attenuated by DSS pretreatment. Indomethacin (5 mg/kg) failed to significantly alter HCl-evoked expression of c-Fos in the NTS of control, DSS-pretreated and IAA-pretreated mice. We conclude that the gastritis-evoked increase in the gastric acid-evoked c-Fos expression in the NTS is related to disruption of the gastric mucosal barrier, mucosal inflammation, mucosal acid influx and enhanced activation of the afferent stomach-NTS axis.