Role of cyclooxygenase isoforms in gastric mucosal defence.

A complex system of interacting mediators exists in the gastric mucosa to strengthen its resistance against injury. In this system prostaglandins play an important role. Prostaglandin biosynthesis is catalysed by the enzyme cyclooxygenase (COX), which exists in two isoforms, COX-1 and COX-2. Initially the concept was developed that COX-1 functions as housekeeping enzyme, whereas COX-2 yields prostaglandins involved in pathophysiological reactions such as inflammation. In the gastrointestinal tract, the maintenance of mucosal integrity was attributed exclusively to COX-1 without a contribution of COX-2 and ulcerogenic effects of non-steroidal anti-inflammatory drugs (NSAIDs) were believed to be the consequence of inhibition of COX-1. Recent findings, however, indicate that both COX-1 and COX-2 either alone or in concert contribute to gastric mucosal defence. Thus, in normal rat gastric mucosa specific inhibition of COX-1 does not elicit mucosal lesions despite near-maximal suppression of gastric prostaglandin formation. When a selective COX-2 inhibitor which is not ulcerogenic when given alone is added to the COX-1 inhibitor, severe gastric damage develops. In contrast to normal gastric mucosa which requires simultaneous inhibition of COX-1 and COX-2 for breakdown of mucosal resistance, in the acid-challenged rat stomach inhibition of COX-1 alone results in dose-dependent injury which is further increased by additional inhibition of COX-2 enzyme activity or prevention of acid-induced up-regulation of COX-2 expression by dexamethasone. COX-2 inhibitors do not damage the normal or acid-challenged gastric mucosa when given alone. However, when nitric oxide formation is suppressed or afferent nerves are defunctionalized, specific inhibition of COX-2 induces severe gastric damage. Ischemia-reperfusion of the gastric artery is associated with up-regulation of COX-2 but not COX-1 mRNA. COX-2 inhibitors or dexamethasone augment ischemia-reperfusion-induced gastric damage up to four-fold, an effect abolished by concurrent administration of 16,16-dimethyl-PGE(2). Selective inhibition of COX-1 is less effective. Furthermore, COX-2 inhibitors antagonize the protective effect of a mild irritant or intragastric peptone perfusion in the rat stomach, whereas the protection induced by chronic administration of endotoxin is mediated by COX-1. Finally, an important function of COX-2 is the acceleration of ulcer healing. COX-2 is up-regulated in chronic gastric ulcers and inhibitors of COX-2 impair the healing of ulcers to the same extent as non-selective NSAIDs. Taken together, these observations show that both COX isoenzymes are essential factors in mucosal defence with specific contributions in various physiological and pathophysiological situations.     

Activation of central opioid receptors determines the timing of hypotension during acute hemorrhage-induced hypovolemia in conscious sheep

After an initial compensatory phase, hemorrhage reduces blood pressure due to a widespread reduction of sympathetic nerve activity (decompensatory phase). Here, we investigate the influence of intracerebroventricular naloxone (opioid-receptor antagonist) and morphine (opioid-receptor agonist) on the two phases of hemorrhage, central and peripheral hemodynamics, and release of vasopressin and renin in chronically instrumented conscious sheep. Adult ewes were bled (0.7 ml x kg(-1) x min(-1)) from a jugular vein until mean arterial blood pressure (MAP) reached 50 mmHg. Starting 30 min before and continuing until 60 min after hemorrhage, either artificial cerebrospinal fluid (aCSF), naloxone, or morphine was infused intracerebroventricularly. Naloxone (200 microg/min but not 20 or 2.0 microg/min) significantly increased the hemorrhage volume compared with aCSF (19.5 +/- 3.2 vs. 13.9 +/- 1.1 ml/kg). Naloxone also increased heart rate and cardiac index. Morphine (2.0 microg/min) increased femoral blood flow and decreased hemorrhage volume needed to reduce MAP to 50 mmHg (8.9 +/- 1.5 vs. 13.9 +/- 1.1 ml/kg). The effects of morphine were abolished by naloxone at 20 microg/min. It is concluded that the commencement of the decompensatory phase of hemorrhage in conscious sheep involves endogenous activation of central opioid receptors. The effective dose of morphine most likely activated mu-opioid receptors, but they appear not to have been responsible for initiating decompensation as 1) naloxone only inhibited an endogenous mechanism at a dose much higher than the effective dose of morphine, and 2) the effects of morphine were blocked by a dose of naloxone, which, by itself, did not delay the decompensatory phase.     

Activation of angiotensin II receptors in brain potentiates the stimulating effect of endogenous opioid neurons on central sympathetic outflow

Endogenous opioid peptides appear to have neurotransmitter or neuromodulator functions in brain mediating a wide variety of effects. We have reported that intracisternal administration of synthetic human beta-endorphin increases plasma concentration of catecholamines, apparently by acting at unknown brain sites to increase sympathetic outflow to the adrenal medulla and sympathetic nerves. In the present study we examined the possibility that angiotensin II, acting in brain, modulates endorphin-induced catecholamine secretion. Simultaneous intracisternal administration of angiotensin II 1.0 nmol together with synthetic human beta-endorphin 1.45 nmol potentiated the plasma epinephrine, norepinephrine and dopamine responses to intracisternal beta-endorphin. In contrast, simultaneous intracisternal administration of the angiotensin II antagonist, [Sar1, Val5, Ala8]-angiotensin II (saralasin), 1.1 nmol together with beta-endorphin, blunted the plasma epinephrine, norepinephrine and dopamine responses to beta-endorphin. These data are consistent with the hypothesis that activation of angiotensin II receptors in brain potentiates the endorphin-induced stimulation of central sympathetic outflow. It remains to be demonstrated whether angiotensin II acting in brain to modulate activity of opioid neurons is synthesized in brain or is derived peripherally.     

Corticosteroid receptors in the central nervous system of the rat.

Corticosteroid receptors were demonstrated in the medial hypothalamus, the hippocampus and the parietal cortex of the rat while no such receptors were found in the hypophysis, the amygdala and the anterior hypothalamus. The findings suggest the role of extrahypothalamic regions in the perception of corticosteroid feedback as well as in the regulation of the hypothalamo-hypophysial-adrenal function and do not support the assumption that corticosteroids would inhibit corticotrophin secretion by acting directly on the hypophysis.     

Effects of ethanol on gastric acid secretion and gastric mucosal cyclic AMP in the rat.

The effect of ethanol on the secretion of gastric acid and the content of cyclic AMP of the gastric mucosa was studied in rats. Intravenously, ethanol (10 to 800 mg/kg) had no effect on the output of acid. Upon local application into the stomach, ethanol (1 to 10%) caused a concentration-dependent inhibition of the output of gastric acid. The effect was evident within 5 min. At the concentration of 1 %,ethanol decreased the rate of acid secretion maximally by about 30%. At the concentration of 3 %, the maximal inhibition was about 70 %. At the concentration of 10 %, ethanol caused a total cessation of the output of acid within 20 to 60 min.Five and 25 min after the administration of 10 % ethanol into the stomach, the gastric mucosal content of cyclic AMP was decreased by approximately 50 %. Also in vitro, the mucosal content of cyclic AMP was decreased by ethanol within 5 min. The decrease was about 30 % with 2.5 % ethanol, approximately 60 % with 10 % ethanol, and approximately 45 % with 20 % ethanol. Alcohol inhibited the activity of the cyclic AMP phosphodiesterase of the gastric mucosa in a competitive manner. The K_i-value was 0.16 M which would correspond to an alcohol concentration of 9.1 % (v/v). Ethanol caused a concentration-dependent inhibition of the activity of the gastric mucosal adenyl cyclase. By 0.166 M (9.4 %) alcohol the inhibition was nearly 100 %.It is concluded that the ethanol-induced decrease of cyclic AMP in the gastric mucosa is due to a decreased formation of the nucleotide. The accompanying inhibition of the output of acid by ethanol is consistent with the view that cyclic AMP is an intracellular regulator of the gastric acid secretion. In view of the role of cyclic AMP in the control of the integrity of the cells, it is suggested that the ethanol-induced damage of gastric mucosa might also be, at least partly, due to the decreased mucosal content of cyclic AMP.     

Trafficking of central opioid receptors and descending pain inhibition

The delta-opioid receptor (DOR) belongs to the superfamily of G-protein-coupled receptors (GPCRs) with seven transmembrane domains, and its membrane trafficking is regulated by intracellular sorting processes involving its C-tail motifs, intracellular sorting proteins, and several intracellular signaling pathways. In the quiescent state, DOR is generally located in the intracellular compartments in central neurons. However, chronic stimulation, such as chronic pain and sustained opioid exposure, may induce membrane trafficking of DOR and its translocation to surface membrane. The emerged functional DOR on cell membrane is actively involved in pain modulation and opioid analgesia. This article reviews current understanding of the mechanisms underlying GPCRs and DOR membrane trafficking, and the analgesic function of emerged DOR through membrane trafficking under certain pathophysiological circumstances.     

Analysis of the role of central and peripheral alpha2-adrenoceptor subtypes in gastric mucosal defense in the rat

The present study confirmed our previous assumption on the crucial role of central alpha2B-like adrenoceptor subtype in gastric mucosal defense. It was found that beside clonidine, rilmenidine, an alpha2/imidazoline receptor agonist and ST-91, an alpha2B-adrenoceptor preferring agonist inhibited the mucosal lesions induced by ethanol given intracerebroventricularly (i.c.v.). The ED50 values for clonidine, rilmenidine and ST-91 are 0.2, 0.01 and 16 nmol/rat i.c.v., respectively. The effect was reversed by the intracerebroventricularly injected alpha2B/2C-adrenoceptor antagonists prazosin and ARC-239, indicating the potential involvement of central alpha2B/2C-adrenoceptor subtype in the protective action. The gastroprotective effect of adrenoceptor stimulants was reversed by bilateral cervical vagotomy, suggesting that vagal nerve is likely to convey the central action to the periphery. In gastric mucosa both nitric oxide and prostaglandins may mediate the centrally-induced effect, since both indomethacin and N(G)-nitro-L-arginine reversed the protective effect of alpha2-adrenergic stimulants. Though expression of mRNA of alpha2B-, as well as alpha2A- and alpha2C-adrenoceptor subtypes was demonstrated in gastric mucosa of the rat, the hydrophilic ST-91, given peripherally (orally, subcutaneously), failed to exert mucosal protection, in contrast with clonidine and rilmenidine which were also effective. Consequently, while peripheral alpha2B-adrenoceptors are not likely to be involved in gastric mucosal protection, activation of central alpha2B-like adrenoceptor subtype may initiate a chain of events, which result in a vagal dependent gastroprotective action.     

Comparison of opioid receptor distributions in the rat central nervous system

The opioid receptors, mu, delta and kappa, conduct the major pharmacological effects of opioid drugs, and exhibit intriguing functional relationships and interactions in the CNS. Previously established hypotheses regarding the mechanisms underlying these phenomena specify theoretical patterns of relative cellular localisation for the different receptor types. In this study, we have used double-label immunohistochemistry to compare the cellular distributions of delta and kappa receptors with those of mu receptors in the rat CNS. Regions of established significance in opioid addiction were examined. Extensive mu/delta co-localisation was observed in neuron-like cells in several regions. mu and kappa receptors were also often co-localised in neuron-like cell bodies in several regions. However, intense kappa immunoreactivity (ir) also appeared in a separate, morphologically distinct population of cells that did not express mu receptors. These small, ovoid cells were often closely apposed against the larger, mu-ir cell bodies. Such cellular appositions were seen in several regions, but were particularly common in the medial thalamus, the periaqueductal grey and brainstem regions. These findings support proposals that functional similarities, synergy and cooperativity between mu and delta receptors arise from widespread co-expression by cells and intracellular molecular interactions. Although co-expression of mu and kappa receptors was also detected, the appearance of a separate population of kappa-expressing cells supports proposals that the contrasting and functionally antagonistic properties of mu and kappa receptors are due to expression in physiologically distinct cell types. Greater understanding of opioid receptor interaction mechanisms may provide possibilities for therapeutic intervention in opioid addiction and other conditions.     

Activation of delta opioid receptors induces receptor insertion and neuropeptide secretion

Here we describe a novel mechanism for plasma membrane insertion of the delta opioid receptor (DOR). In small dorsal root ganglion neurons, only low levels of DORs are present on the cell surface, in contrast to high levels of intracellular DORs mainly associated with vesicles containing calcitonin gene-related peptide (CGRP). Activation of surface DORs caused Ca(2+) release from IP(3)-sensitive stores and Ca(2+) entry, resulting in a slow and long-lasting exocytosis, DOR insertion, and CGRP release. In contrast, membrane depolarization or activation of vanilloid and P2Y(1) receptors induced a rapid DOR insertion. Thus, DOR activation induces a Ca(2+)-dependent insertion of DORs that is coupled to a release of excitatory neuropeptides, suggesting that treatment of inflammatory pain should include blockade of DORs.     

Activation of gastric mucosal calcium channels by epidermal growth factor

• 1.1. Gastric mucosal calcium channel complex was isolated from the solubilized epithelial cell membranes by affinity chromatography on wheat germ agglutinin.
• 2.2. The complex following labeling with [³H]PN200-110 was reconstituted into phosphatidylcholine vesicles which exhibited active ⁴⁵Ca²⁺ uptake into intravesicular space as evidenced by La³⁺ displacement and osmolarity measurements. The ⁴⁵Ca²⁺ uptake was independent of sodium and potassium gradients indicating the electroneutral nature of the process.
• 3.3. The gastric mucosal channels on epidermal growth factor binding in the presence of ATP responded by an increase in protein tyrosine phosphorylation of 55 and 170 kDa subunits of calcium channel.
• 4.4. The phosphorylated channels following reconstitution into vesicles displayed at 48% greater ⁴⁵Ca²⁺ uptake, thus indicating the tyrosine kinase involvement in EGF dependent activation of calcium channel.
• 5.5. The results point towards the importance of epidermal growth factor in the maintenance of gastric mucosal calcium homeostasis.

     

Activation of cytosol aldosterone receptors in rat kidney

Cytosolic aldosterone-protein complexes are isolated from rat kidney slices after incubation with [3H]aldosterone and dexamethasone. Activated and unactivated forms of the complex are characterized by gel electrophoresis and hydroxyapatite chromatography after incubation at 4 degrees C and 25 degrees C respectively. It is found that the activated form reaches a maximum after 30 min at 25 degrees C and can be separated as an homogeneous peak by electrophoresis. Intermediate forms can also be identified. In the presence of 10 mM ATP, activation immediately occurs at 4 degrees C and is almost complete. In the presence of 10 mM molybdate, the activation is strongly enhanced and the increase in activated form may be about fifteen-fold whether molybdate is added during kidney homogenization or just before incubation at 25 degrees C. On the other hand molybdate reduces to one third the binding of the aldosterone-receptor complexes to nuclei. In the presence of the steroid RU 26988 which is a pure glucocorticoid, experiments done on aldosterone-receptors complexes and their binding to nuclei are confirmed. This proves that aldosterone is specific for mineralocorticoid sites. The general pattern of the mineralocorticoid receptor activation is discussed and its resemblance to the case of other steroid hormones is emphasized.     

Kappa opioid receptors stimulate phosphoinositide turnover in rat brain

The effects of various subtype-selective opioid agonists and antagonists on the phosphoinositide (PI) turnover response were investigated in the rat brain. The kappa-agonists U-50,488H and ketocyclazocine produced a concentration-dependent increase in the accumulation of IP's in hippocampal slices. The other kappa-agonists Dynorphin-A (1-13) amide, and its protected analog D[Ala]2-dynorphin-A (1-13) amide also produced a significant increase in the formation of [3H]-IP's, whereas the mu-selective agonists [D-Ala2-N-Me-Phe4-Gly5-ol]-enkephalin and morphine and the delta-selective agonist [D-Pen2,5]-enkephalin were ineffective. The increase in IP's formation elicited by U-50,488H was partially antagonized by naloxone and more completely antagonized by the kappa-selective antagonists nor-binaltorphimine and MR 2266. The formation of IP's induced by U-50,488H varies with the regions of the brain used, being highest in hippocampus and amygdala, and lowest in striatum and pons-medulla. The results indicate that brain kappa- but neither mu- nor delta-receptors are coupled to the PI turnover response.     

Dietary nitrate inhibits stress-induced gastric mucosal injury in the rat

Dietary nitrate is reduced to nitrite by some oral bacteria and the resulting nitrite is converted to nitric oxide (NO) in acidic gastric juice. The aim of this study is to elucidate the pathophysiological role of dietary nitrate in the stomach. Intragastric administration of nitrate rapidly increased nitrate and NO in plasma and the gastric headspace, respectively. Water-immersion-restraint stress (WIRS) increased myeloperoxidase (MPO) activity in gastric mucosa and induced hemorrhagic erosions by a nitrate-inhibitable mechanism. In animals that had received either cardiac ligation or oral treatment with povidone-iodine, a potent bactericidal agent, administration of nitrate failed to increase gastric levels of NO and to inhibit WIRS-induced mucosal injury. WIRS decreased gastric mucosal blood flow by a mechanism which was inhibited by administration of nitrate. These data suggested that the enterosalivary cycle of nitrate and related metabolites consisted of gastrointestinal absorption and salivary secretion of nitrate, its conversion to nitrite by oral bacteria and then to NO in the stomach might play important roles in the protection of gastric mucosa from hazardous stress.     

Involvement of opioid receptors in the oxytocin-induced antinociception in the central nervous system of rats

Recent studies showed that oxytocin and opioid peptides play important roles in pain modulation at different levels in the central nervous system. The present study was performed to explore whether opioid system is involved in the oxytocin-induced antinociception in the brain of rats. The results showed that: (1) intracerebroventricular injection of oxytocin induced dose-dependent increases in hindpaw withdrawal latencies (HWL) to noxious thermal and mechanical stimulation in rats. (2) The antinociceptive effect of oxytocin was attenuated dose-dependently by intracerebroventricular injection of naloxone, indicating an involvement of opioid system in the oxytocin-induced antinociception. (3) It is interesting that the antinociceptive effect of oxytocin was attenuated by subsequent intracerebroventricular injection of the μ-opioid antagonist β-funaltrexamine (β-FNA) and the κ-opioid antagonist nor-binaltorphimine (nor-BNI), but not the δ-opioid antagonist naltrindole. The results indicate that oxytocin plays an antinociceptive role in the brain of rats; μ- and κ-opioid receptors, not δ-receptors, are involved in the oxytocin-induced antinociception in the central nervous system of rats.     

Enhancement of natural immunity seen after voluntary exercise in rats. Role of central opioid receptors

Chronic voluntary exercise in wheels for 5 weeks in spontaneously hypertensive rats (SHR) augments in vivo natural killer (NK) cell cytotoxicity. Endogenous beta-endorphin is increased in cerebrospinal fluid after voluntary exercise in rats and we have recently shown that beta-endorphin administered i.c.v. augments NK cell mediated cytotoxicity in vivo in a similar way as chronic voluntary exercise. We have now further investigated the involvement of central opioid systems in the exercise-induced augmentation in natural immunity. Exercise consisted of voluntary running in wheels for 5 weeks. In vivo cytotoxicity was measured as clearance of injected 51Cr-labeled YAC-1 lymphoma cells from the lungs. The clearance of YAC-1 cells in vivo was significantly increased in runners as compared to sedentary controls. Selective delta, kappa, or mu-opioid receptor antagonists were administered i.c.v. with osmotic minipumps during the last 6 days of the 5 weeks of running. The delta-receptor antagonist naltrindole (40-50 microg/day) significantly but not completely inhibited the enhanced NK-cell cytotoxicity seen after 5 weeks of exercise. Neither the kappa-receptor antagonist nor-BNI or the mu-receptor antagonist beta-FNA influenced the augmentation in NK cell cytotoxicity. Nor-BNI per se significantly augments in vivo cytotoxicity, indicating some inhibiting effect on natural immunity that could be mediated through the kappa-opioid receptor. Our data suggest the involvement of central delta-opioid receptors in the enhancement of natural cytotoxicity seen after chronic voluntary exercise.     

Protein biosynthesis by rat gastric mucosal cells in suspension

Suspensions of cells freshly isolated from rat gastric mucosa by the pronase technique did not synthesize protein unless washed. Synthetic activity proceeded for at least 3h in washed cells, but unless three or four washing cycles were given the synthesized protein was subsequently degraded. Supplementation of the incubation medium with 3% w/v bovine serum albumin markedly increased synthetic activity. Special consideration was given to the control of artifacts caused by bacterial activity, oxygen deficiency and adsorption of radioleucine.     

Muscarinic acetylcholine receptors in rat gastric mucosa

Summary The muscarinic cholinergic innervation of the rat gastric mucosa was investigated by localizing the muscarinic receptors using a tritiated muscarinic antagonist, pirenzepine. Radioautography was performed by freeze drying stomach tissue, which was then embedded in Epon and wet sectioned with ethylene glycol, and dry mounting on emulsion film by the wire-loop method to prevent loss of the labelled substance during fixation and the radioautographic procedure. Light and electron microscopy showed that the specific pirenzepine-binding sites were localized predominantly on parietal cells, chief cells and perivascular plexuses. Analysis of the grain distribution on parietal cells revealed that the silver grains corresponding to the pirenzepine-binding sites were mainly on the basolateral plasma membrane. On the other hand, the surface mucous or mucous neck cells had few pirenzepine-binding sites.     

Role of parasympathetic overactivity in water immersion stress-induced gastric mucosal lesion in rat.

Stress ulcer is clinically prevalent, but the underlying mechanisms are not well understood. This study aimed to investigate the role of sympathovagal imbalance in the development of water immersion restraint stress (WRS)-induced gastric mucosal lesion. Wistar rats were subjected to either restraint stress (RS) (n = 7) or WRS (n = 7) for 5 h. Linear parameters of heart rate variability and Poincaré plot were analyzed on the basis of the surface ECGs. Gastric mucosal lesion was evaluated by gross anatomy and histology. Mean R-R intervals significantly increased (P < 0.001) in a time-dependent manner in the WRS group but slightly decreased (P < 0.001) in the RS group. Root mean square of successive differences of R-R intervals and high-frequency norm (high-frequency power normalized by the total frequency power) were significantly higher in the WRS group than the RS group (P < 0.001). Low-frequency norm and low-to-high-frequency ratio increased significantly 1 h after stress and then declined to similar levels in both groups. The Poincaré plots of R-R intervals in the WRS group shifted right-upwardly and showed dispersed patterns compared with the RS group. Gastric mucosae showed serious hemorrhage, effusion, and structural collapse in the WRS group but remained normal in the RS group. Bilateral cervical vagotomy suppressed the increase of heart rate variability and prevented the gastric mucosal lesion induced by WRS. We conclude that parasympathetic overactivity is the predominant autonomic response to WRS and is most probably the leading mechanism of WRS-induced gastric mucosal lesion in rat.