Effect of selective mu 1, mu 2 and delta 2 opioid receptor agonists on gastric functions in the rat

Potential functional and pathological side effects related to off-target pharmacological activity

Most pharmaceutical companies test their discovery-stage proprietary molecules in a battery of in vitro pharmacology assays to try to determine off-target interactions. During all phases of drug discovery and development, various questions arise regarding potential side effects associated with such off-target pharmacological activity. Here we present a scientific literature curation effort undertaken to determine and summarize the most likely functional and pathological outcomes associated with interactions at 70 receptors, enzymes, ion channels and transporters with established links to adverse effects. To that end, the scientific literature was reviewed using an on-line database, and the most commonly reported effects were summarized in tabular format. The resultant table should serve as a practical guide for research scientists and clinical investigators for the prediction and interpretation of adverse side effects associated with molecules interacting with components of this screening battery.

Distribution and trafficking of the μ-opioid receptor in enteric neurons of the guinea pig

The μ-opioid receptor (MOR) is a major regulator of gastrointestinal motility and secretion and mediates opiate-induced bowel dysfunction. Although MOR is of physiological and therapeutic importance to gut function, the cellular and subcellular distribution and regulation of MOR within the enteric nervous system are largely undefined. Herein, we defined the neurochemical coding of MOR-expressing neurons in the guinea pig gut and examined the effects of opioids on MOR trafficking and regulation. MOR expression was restricted to subsets of enteric neurons. In the stomach MOR was mainly localized to nitrergic neurons (∼88%), with some overlap with neuropeptide Y (NPY) and no expression by cholinergic neurons. These neurons are likely to have inhibitory motor and secretomotor functions. MOR was restricted to noncholinergic secretomotor neurons (VIP-positive) of the ileum and distal colon submucosal plexus. MOR was mainly detected in nitrergic neurons of the colon (nitric oxide synthase positive, 87%), with some overlap with choline acetyltransferase (ChAT). No expression of MOR by intrinsic sensory neurons was detected. [d-Ala(2), MePhe(4), Gly(ol)(5)]enkephalin (DAMGO), morphiceptin, and loperamide induced MOR endocytosis in myenteric neurons. After stimulation with DAMGO and morphiceptin, MOR recycled, whereas MOR was retained within endosomes following loperamide treatment. Herkinorin or the δ-opioid receptor agonist [d-Ala(2), d-Leu(5)]enkephalin (DADLE) did not evoke MOR endocytosis. In summary, we have identified the neurochemical coding of MOR-positive enteric neurons and have demonstrated differential trafficking of MOR in these neurons in response to established and putative MOR agonists.

Expression of the nociceptin/orphanin FQ receptor in the intestinal mucosa of IBS patients

Nociceptin/orphanin FQ (N/OFQ) and the N/OFQ peptide (NOP) receptor play important roles in regulating gastrointestinal function. To assess whether the NOP receptor is implicated in the etiopathogenesis of irritable bowel syndrome (IBS), we measured the levels of NOP receptor mRNA and protein in the jejunal and colonic tissues of healthy subjects and patients with diarrhea-predominant IBS (D-IBS) and constipation-predominant IBS (C-IBS). Mucosal biopsies were obtained from the jejunum and colon of patients diagnosed with D-IBS and C-IBS by the Rome III criteria and from healthy control subjects. The expression of NOP receptor mRNA was measured quantitatively using quantitative PCR (qPCR) and NOP protein expression was assayed immunohistochemically using a rabbit monoclonal antibody to OFQ. NOP receptor mRNA was detected in the jejunum and colon of healthy subjects and was more highly expressed in the jejunum than in the colon. Expression was lower in the jejunum and colon of patients with D-IBS; however, it was similar in patients with C-IBS and healthy subjects. The numbers of OFQ-positive cells in the jejunum and colon were similar among the three groups. The NOP receptor may be involved in the regulation of intestinal movement in healthy individuals. Its involvement in the pathophysiology of IBS may depend on whether the IBS is constipation- or diarrhea-predominant.

Electrogastrography during and after cesarean delivery

PURPOSE

Although it has been generally believed that parturients have delayed gastric emptying during anesthesia, the most recent reports suggest that gastric emptying is not delayed during pregnancy except during labor. Electrical slow waves in the stomach determine the frequency and the peristaltic nature of gastric contractions. In this study we performed electrogastrography during and after elective cesarean section (CS) in an attempt to evaluate gastric motility.

METHODS

Sixteen American Society of Anesthesiologists physical status I or II term parturients undergoing elective CS were enrolled. Combined spinal-epidural anesthesia was initiated with 10 mg of bupivacaine plus 10 microg of fentanyl. Four-channel electrogastrography was obtained for 10 min prior to venous catheter insertion (baseline), 10 min following spinal injection of bupivacaine and fentanyl (Sp-1), 10 to 20 min following spinal injection (Sp-2), 10 min prior to the end of operation (end), and finally 10 min on the seventh postoperative day (POD 7).

RESULTS

The mean +/- SD values for dominant frequency of electrogastrography (DF) were determined as: 1.57 +/- 0.36 cpm (baseline), 1.81 +/- 0.32 cpm (Sp-1), 2.08 +/- 0.36 cpm (Sp-2), 1.96 +/- 0.36 cpm (end), and 3.02 +/- 0.28 cpm (POD 7). The DF of Sp-1, Sp-2, and end were significantly higher than that of baseline (P < 0.05). The DF of POD 7 was significantly higher than that of baseline, Sp-1, Sp-2, and end (P < 0.01).

CONCLUSION

Electrogastrography analysis suggests that the frequency of gastric contractions during CS was less than that in the postpartum period.

Central and peripheral role of the nociceptin/orphaninFQ system on normal and disturbed colonic motor function and faecal pellet output in the rat

In this study, seeking further information on the role of the nociceptin/orphanin FQ (N/OFQ)-ergic system in normal and disturbed colonic motor function in rats, we compared the colonic effects of UFP-112, a novel highly potent agonist, with those of N/OFQ. When injected intracerebroventricularly (i.c.v.) and intraperitoneally (i.p.), UFP-112 and N/OFQ increased bead expulsion time in a statistically significant and dose-related manner and reduced the percentage of rats with castor oil-induced diarrhoea. UFP-112 showed greater efficacy, higher potency and longer-lasting inhibitory effects than N/OFQ, and pretreatment with UFP-101, a selective antagonist, blocked the N/OFQ analogue-induced responses in both tests. When injected i.c.v., UFP-112 and N/OFQ inhibited corticotrophin releasing factor- and restrain stress-stimulated faecal pellet excretion significantly and in a dose-related manner. Conversely, when injected peripherally both peptides significantly inhibited colonic propulsive motility but did so in a non-dose-related manner. In conclusion, these findings indicate that, in the rat, the central and peripheral N/OFQ systems have an inhibitory role in modulating distal colonic propulsive motility under physiological and pathological conditions. UFP-112 therefore promises to be a useful pharmacological tool for investigating the role of the N/OFQ system in motor functions in the distal colonic tract under physiological and pathological conditions.

The gastric effects of UFP-112, a new nociceptin/orphanin receptor agonist, in physiological and pathological conditions

Nociceptin/orphanin FQ (N/OFQ), the endogenous NOP receptor ligand, centrally modulates gastric motor and secretory functions and prevents ethanol-induced gastric lesions in rats. A recently synthesized N/OFQ analog, [(pF)Phe(4)Aib(7)Arg(14)Lys(15)]N/OFQ-NH(2) (UFP-112), acts as a highly potent and selective peptide agonist for NOP receptors and produces longer-lasting in vitro and in vivo effects in mice than the natural ligand N/OFQ. In this study, we evaluated the effects of centrally (intracerebroventricularly/icv) and peripherally (intraperitoneally/ip) injected UFP-112 on gastric emptying and gastric acid secretion, and on the development of gastric mucosal lesions induced by 50% ethanol in the rat. When injected icv, it dose-dependently delayed gastric emptying of a phenol red meal (by up to 70%), decreased gastric secretion in water-loaded rats after 90 pylorus ligature, and reduced ethanol-induced gastric lesions (by up to 87%). In all three assays, UFP-112 was more effective than N/OFQ. The highly selective NOP receptor antagonist, UFP-101, decreased the efficacy of UFP-112, thus confirming that central NOP receptors mediate inhibitory control on these functional and pathological conditions in rats. Ip injected N/OFQ and UFP-112 induced non-dose-related gastric hypersecretory and antiulcer effects, which UFP-101 partially abolished. Ip N/OFQ appeared equiactive but about 30-100 times less potent than ip UFP-112 in stimulating gastric acid secretion and preventing lesion formation. When ip injected, both UFP-112 and N/OFQ left gastric emptying in rats unchanged, suggesting that peripheral NOP receptors have a role in mediating gastric hypersecretory and antiulcer effects but are not involved in regulating gastric motility. In addition, the inhibitory effects induced by this novel NOP receptor agonist lasted longer than those induced by N/OFQ. In conclusion, UFP-112 is a promising new pharmacological tool for studying the functional roles of the central and peripheral N/OFQ receptor system.

Methylnaltrexone mechanisms of action and effects on opioid bowel dysfunction and other opioid adverse effects

OBJECTIVE

To review the mechanisms of action of methylnaltrexone and its effects on opioid bowel dysfunction, as well as its effects on other opioid-induced adverse effects (ADEs), and its potential roles in clinical practice.

DATA SOURCES

A literature search using the MEDLINE and Cochrane Collaboration databases for articles published between 1966 and March 2007 was performed. Additional data sources were obtained from manual searches of recent journal articles, book chapters, and monographs. An updated literature search showed no additional publications.

STUDY SELECTION AND DATA EXTRACTION

Abstracts and original preclinical and clinical research reports published in the English language were identified for review. Review articles, commentaries, and news reports of this compound were excluded. Literature related to opioids, opioid receptors, opioid antagonists, methylnaltrexone, opioid-induced bowel dysfunction, constipation, nausea, and vomiting was evaluated and selected based on consideration of the support shown for the proof of concept, mechanistic findings, and timeliness. Fifty-eight original articles from preclinical studies and clinical trials using methylnaltrexone were identified. Pharmacologic action, benefits, and ADEs of methylnaltrexone were reviewed, with a focus on its effects on bowel dysfunction after opioids. Emphases were placed on its receptor binding activities and therapeutically relevant sites of action (peripheral vs central), in which peripheral opioid receptors in the body contribute to physiological and drug-induced effects.

DATA SYNTHESIS

Morphine and related opioids are associated with a number of limiting ADEs, including opioid-induced bowel dysfunction. Methylnaltrexone, a quaternary derivative of naltrexone, blocks peripheral effects of opioids while sparing central analgesic effects. It is currently under late-stage clinical investigation for the treatment of opioid-induced constipation in patients with advanced illness. Reported results showed the drug to be generally well-tolerated. The rapid reversal of constipation is very encouraging. Hastening postoperative discharge may also be possible.

CONCLUSIONS

Methylnaltrexone has the potential to prevent or treat opioid-induced peripherally mediated ADEs on bowel dysfunction without interfering with central analgesia. The study of methylnaltrexone leads to a greater understanding of the mechanisms of action of opioid pharmacology.

Opioid-induced gastrointestinal dysfunction

Use of opioid analgesics is associated with a number of side effects, especially opioid-induced gastrointestinal dysfunction. The extensive use of these compounds and the significant negative impact of the resulting gastrointestinal dysfunction on patients' quality of life make it an important clinical issue. In recent years our understanding of the mechanisms of opioid-induced gastrointestinal dysfunction has advanced greatly. This article reviews the underlying pathophysiological mechanisms of specific gastrointestinal adverse effects of opioids. The role of endogenous opioid peptides in certain gastrointestinal diseases is also discussed. A better understanding of the pathophysiological mechanisms of opioid-induced bowel dysfunction should lead to the development of newer opioid analgesics and improved regimens resulting in reduced gastrointestinal adverse effects.

Characterisation of alpha2-adrenoceptor subtypes involved in gastric emptying, gastric motility and gastric mucosal defence

The effect of clonidine on ethanol-induced gastric mucosal damage, gastric emptying and gastric motility was compared. The clonidine-induced gastroprotective effect (0.03-0.09 micromol/kg, s.c.) was antagonised by yohimbine (5 micromol/kg, s.c.), prazosin (0.23 micromol/kg; alpha2B-adrenoceptor antagonist) and naloxone (1.3 micromol/kg, s.c.). Clonidine also inhibited the gastric emptying of liquid meal (0.75-3.75 micromol/kg, s.c.) and gastric motor activity (0.75 micromol/kg, i.v.) stimulated by 2-deoxy-D-glucose (300 mg/kg, i.v.). Inhibition of gastric emptying and motility was reversed by yohimbine (5 and 10 micromol/kg, s.c., respectively), but not by prazosin (0.23 micromol/kg, s.c.) or naloxone (1.3 micromol/kg, s.c.). Oxymetazoline-an alpha2A-adrenoceptor agonist-inhibited both gastric emptying (0.67-6.8 micromol/kg, s.c.) and motility (0.185-3.4 micromol/kg, i.v.), whereas it failed to affect gastric mucosal lesions. The results indicate that in contrast to the gastroprotective effect, which is mediated by alpha2B-adrenoceptor subtype, alpha2A-adrenoceptor subtype may be responsible for inhibition of gastric emptying and motility. However, the site of action (central, peripheral, both) remains to be established.

The effects of [Arg14, Lys15] nociceptin/orphanin FQ, a highly potent agonist of the NOP receptor, on in vitro and in vivo gastrointestinal functions

Nociceptin/orphanin FQ (N/OFQ) administered into the lateral left cerebral ventricle of rats has been reported to inhibit in vivo gut motor and secretory functions. Recently, a novel N/OFQ analog, [Arg14, Lys15] N/OFQ, was synthesized and demonstrated to behave as a highly potent agonist at the human recombinant N/OFQ peptide (NOP) receptors and to produce long-lasting effects in vivo in mice compared with the natural ligand N/OFQ. In the present study, the pharmacological profile of [Arg14, Lys15] N/OFQ was further evaluated and compared with that of N/OFQ in vitro on guinea pig exocrine pancreas and in vivo on gastric emptying, colonic propulsion and gastric acid secretion in rats. [Arg14, Lys15] N/OFQ and N/OFQ significantly decreased the KCl-evoked amylase secretion from isolated pancreatic lobules of the guinea pig. In in vivo experiments, [Arg14, Lys15] N/OFQ mimicked the effects of N/OFQ, inducing, after intracerebroventricular injection, a delay (up to 70%) in the gastric emptying of a phenol red meal, an increase (about 40 times) of the mean bead colonic expulsion time and a decrease (up to 90%) of gastric acid secretion in water loaded rats after 90 min pylorus ligature. In all these assays, [Arg14, Lys15] N/OFQ was more effective than N/OFQ, and its effective doses were at least 10-fold lower than N/OFQ effective doses. The highly selective NOP receptor antagonist, UFP-101, decreased the efficacy of [Arg14, Lys15] N/OFQ in in vitro and in vivo assays above reported. These findings: (a) show that pancreatic NOP receptors mediate an in vitro inhibitory effect on stimulated guinea pig amylase secretion; (b) confirm that the stimulation of central NOP receptors exerts an inhibitory control on gastric emptying, colonic motility and gastric secretion in rats and (c) put in evidence that [Arg14, Lys15] N/OFQ, being more potent and effective than the natural ligand N/OFQ, represents a new pharmacological tool for the study of the physiological and pharmacological roles mediated by the N/OFQ-NOP receptor system.

Central effect of mu-opioid agonists on antral motility in conscious rats

Centrally applied opioids delay gastric emptying and inhibit intestinal transit. However, the mechanism of inhibitory effects of central opioids on gastric motility still remains unclear. It also remains unclear which opioid receptor (mu, delta, and kappa) stimulation affects gastric motility. We studied the central effect of opioids on antral motility in conscious rats. A strain gauge transducer was implanted on the gastric antrum to record the circular muscle contractions. The area under the curve of the antral motility, calculated as a motility index, was evaluated before and after the intracerebroventricular (icv) injection of various opioid agonists in each rat. [D-Ala2, N-Me-Phe4, Gly5-ol] enkephalin (DAMGO, 0.1-10 nmol), a mu-opioid selective agonist, significantly inhibited antral motility in a dose-dependent manner (n=4). The motility index was significantly decreased to 47.3+/-10.8% (n=4) of controls at 20 min after icv injection of DAMGO (1.0 nmol). In contrast, [D-pen2, L-Pen5] enkephalin (DADLE, 1.0 nmol), a delta-opioid selective agonist, and U50,488 (1.0 nmol), a kappa-opioid selective agonist, had no significant effects on antral motility. Pretreatment with subcutaneous guanethidine (5 mg/kg) and propranolol (1 mg/kg), but not phentolamine (1 mg/kg), significantly antagonized the inhibitory effect of DAMGO (1.0 nmol). Reduced motility index induced by DAMGO (1.0 nmol) was restored from 48.7+/-3.5% to 88.6+/-10.9% (n=5) and 80.4+/-2.2% (n=5) by guanethidine and propranolol, respectively. Our findings suggest that central mu-opioid receptor has major inhibitory effects on antral motility in conscious rats. The inhibitory effects of mu-opioid receptors are mediated via sympathetic pathways and beta-adrenoceptors.

Gastrointestinal effects of intracerebroventricularly injected nociceptin/orphaninFQ in rats

Nociceptin/orphanin FQ/(N/OFQ), a novel heptadecapeptide recently isolated from porcine and rat brain, is the endogenous ligand of the N/OFQ peptide receptor (NOP, previously known as ORL-1). In this study we examined the effects of intracerebroventricularly (icv) injected N/OFQ on gastric emptying, gastrointestinal transit, colonic propulsion and gastric acid secretion in rats. N/OFQ (0.01-10 nmol/rat) significantly delayed gastric emptying of a phenol red meal, inhibited transit of a non-absorbable charcoal marker through the small intestine and increased the mean colonic bead expulsion time. These N/OFQ-motor effects were abolished by the NOP receptor selective antagonist [NPhe(1)]N/OFQ(1-13)-NH(2) (50 nmol/rat), but were unaltered by the classical opioid receptor antagonist, naloxone (9.2 micromol/kg). Icv injected N/OFQ (10 nmol/rat) decreased gastric acid secretion in 2-h pylorus ligated rats in a naloxone sensitive manner. [NPhe(1)]N/OFQ(1-13)-NH(2) (100 nmol/rat) icv administered alone stimulated gastric acid secretion. These results indicate that N/OFQ activates via NOP receptor stimulation a central inhibitory pathway modulating gastrointestinal propulsive activity and gastric acid secretion in rats.

Effect of restraint and footshock stress and norepinephrine treatment on gastric emptying in rats

We investigated the effects of restraint and footshock stress and norepinephrine treatment on gastric emptying. The gastric emptying was significantly inhibited by restraint stress. beta(3)-adrenergic antagonist canceled the inhibition of gastric emptying caused by restraint stress, while beta(1)-, beta(2)-, alpha(1)-, and alpha(2)-adrenergic antagonists did not affect the inhibition. Norepinephrine treatment also inhibited gastric emptying. The inhibition by norepinephrine treatment was canceled by beta(1)-, beta(2)-, and beta(3)-adrenergic antagonists, but not by alpha(1)- and alpha(2)-adrenergic antagonists. On the other hand, footshock stress did not affect gastric emptying. These results suggest that beta(3)-adrenoceptors play an important role in the inhibition of gastric emptying caused by restraint stress, while nonspecific beta-adrenoceptors are involved in that caused by norepinephrine treatment.

In vitro and in vivo opioid activity of [DPro(6)]dermorphin, a new dermorphin analogue

To study the effects of inducing stereo-chemical modifications in the structure of dermorphin (DM) so as to improve its mu-opioid receptor affinity and its resistance to C-terminal enzymatic degradation, in the Institute of Molecular Genetics of Moscow, we synthesized a new DM analogue ([DPro(6)]DM) and analyzed the changes induced in the biological activities of DM by substituting the Pro(6) residue with DPro(6). We compared the activity of the new DM analogue and DM in in vitro assays and in in vivo tests of analgesia, thermoregulation, heart rate recordings, and gastrointestinal motility in rats. In the in vitro tests, guinea pig ileum (GPI) and mouse vas deferens (MVD), although the opioid activities of [DPro(6)]DM indicated that the peptide was always less potent than DM, its lower IC(50) ratios (mu/delta) showed that it had higher mu-opioid receptor selectivity. In the in vivo analgesic test, [DPro(6)]DM, when injected intraperitoneally (i.p.) (0.5-5 and 10mg/kg) in rats, had the same antinociceptive efficacy as DM and when injected intranasally (i.n.) (0.005 and 0.02 mg/kg) it induced a more stable and long-lasting analgesia than DM (the AUC was about 91% higher for [DPro(6)]DM than for DM). Moreover, these data confirm that the intranasal route is advantageous for peripheral drug administration. In the heart rate study, [DPro(6)]DM and DM (0.5mg/kg, i.p.), induced a similar, weak bradycardia. The only difference was that [DPro(6)]DM induced a longer-lasting effect than DM. Conversely, in body temperature regulation [DPro(6)]DM induced weaker inhibitory activity than DM (56% of the DM-induced response); it did so only in a cold environment and at the maximal used dose (0.5mg/kg, i.p.) without inducing vasomotor effects. In the gastrointestinal study, [DPro(6)]DM and DM (0.005, 0.05, and 0.5mg/kg, i.p.) significantly slowed upper gastrointestinal transit of a charcoal meal and inhibited colonic propulsion. Comparison of the ED(50) values of [DPro(6)]DM (0.03 mg/kg) and DM (0.009 mg/kg) showed that the DM analogue was about three times less potent than DM in slowing gastrointestinal and colonic transit. In conclusion, all these data overall suggest that structural maneuvering in the Pro(6)-residue of the DM molecule changes its affinity for mu-opioid receptor subtypes and confirms the usefulness of experimental studies involving structural modifications in obtaining new therapeutic agents.

Biochemical aspects in autism spectrum disorders: updating the opioid-excess theory and presenting new opportunities for biomedical intervention

Autism is a lifelong condition usually described as affecting social, cognitive and imaginative abilities. For many years, parents and some professionals have observed that in concordance with the behavioural and psychological symptoms of the condition, there are a number of physiological and biochemical correlates which may also be of relevance to the syndrome. One area of interest that encompasses many of these observations is the opioid-excess theory of autism. The main premise of this theory is that autism is the result of a metabolic disorder. Peptides with opioid activity derived from dietary sources, in particular foods that contain gluten and casein, pass through an abnormally permeable intestinal membrane and enter the central nervous system (CNS) to exert an effect on neurotransmission, as well as producing other physiologically-based symptoms. Numerous parents and professionals worldwide have found that removal of these exogenously derived compounds through exclusion diets can produce some amelioration in autistic and related behaviours. There is a surprisingly long history of research accompanying these ideas. The aim of this paper is to review the accompanying evidence in support of this theory and present new directions of intervention as a result of it.

Involvement of central K(ATP) channels in the gastric antisecretory action of alpha2-adrenoceptor agonists and beta-endorphin in rats

The intracerebroventricularly (i.c.v.) injected presynaptic alpha2-adrenoceptor agonists, clonidine and oxymetazoline, exerted a dose-dependent inhibition on the gastric acid secretion in pylorus-ligated rats; the ED50 values were 20 and 7.5 nmol/rat, respectively. Moreover, beta-endorphin, given i.c.v., also decreased acid secretion (ED50=0.25 nmol/rat i.c.v.). The antisecretory effect of these compounds was highly reduced by glibenclamide (10 nmol/rat i.c.v.), a selective blocker of K(ATP) channels. These results suggest that K(ATP) channels in the central nervous system are likely to be involved in the centrally initiated antisecretory action of both alpha2-adrenoceptor agonists and beta-endorphin.

Stimulatory effects of centrally injected kappa-opioid receptor agonists on gastric acid secretion in urethane-anesthetized rats

Gastric acid secretion has been proposed to be regulated by opioid receptors in the central nervous system (CNS). However, whether the effect of morphine is stimulatory or inhibitory, and the role of type specificity of opioid receptors have not been established. We investigated the effects of centrally injected opioid receptor agonists on gastric acid secretion in the perfused stomach of urethane-anesthetized rats. Injection of morphine (1-30 microg/rat, mu-opioid receptor agonist) into the fourth cerebroventricle inhibited the secretion stimulated by i.v. injection of 2-deoxy-D-glucose. Morphine itself did not show an inhibitory effect. In contrast, injection of kappa(1)-opioid receptor agonists such as (5alpha,7alpha,8beta)-(+)-N-methyl-N-(7-[1-pyrrolidinyl]-1-oxaspiro[4.5]dec-8-yl)benzeneacetamide (U59593, 0.3-3 microg) and (trans)-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl) benzeneacetamide hydrochloride (U50488H, 10 microg) and the kappa(2)-opioid receptor agonist, bremazocine (3 microg), into the lateral cerebroventricle markedly stimulated secretion. The effect of U59593 was inhibited by naloxone and norbinaltorphimine (an antagonist of kappa-opioid receptors) and in vagotomized rats. [D-Pen(2)-D-Pen(5)]enkephalin (10microg, delta-opioid receptor agonist) had no effect on secretion. The dual roles of the opioid system in the CNS in gastric acid secretion are discussed.

Involvement of the opioid system in the central antisecretory action of alpha-2 adrenoceptor agonists in rat

The aim of the present study was to analyse the role of the central alpha-2 adrenoceptors in the regulation of gastric acid secretion in pylorus ligated rats. It was found that the intracerebroventricularly (icv.) injected presynaptic alpha-2 adrenoceptor agonist clonidine and the alpha-2A adrenoceptor subtype selective stimulant oxymetazoline exerted a dose dependent inhibition on gastric acid secretion. The antisecretory ED(50) values for clonidine and oxymetazoline were 20 and 7.5 nmol/rat icv., respectively. The antisecretory effect of these compounds was antagonised by the presynaptic adrenoceptor antagonist yohimbine (50 nmol/rat icv.) indicating that the action is mediated through central presynaptic alpha-2 adrenoceptors. Moreover, naloxone (50 nmol/rat icv.)--non-selective opioid antagonist--and naltrindole (0.5 nmol/rat icv.)--delta-opioid receptor selective antagonist--also decreased the antisecretory effect of clonidine and oxymetazoline suggesting that the endogenous opioid system is likely to be involved in the central antisecretory action of alpha-2 adrenoceptor stimulants.

Activation of central opioid receptors may induce gastric mucosal defence in the rat

The effect of different opioid peptides on acidified ethanol- and indomethacin-induced gastric mucosal lesions was studied following intracerebroventricular (i.c.v.) administration. It was found that both the selective delta opioid receptor agonists--deltorphin II, [D-Ala(2), D-Leu(5)]-enkephalin (DADLE), [D-Pen(2), D-Pen(5)]-enkephalin (DPDPE)-, mu-opioid receptor agonist--[D-Ala(2), Phe(4), GlyT-ol]-enkephalin (DAGO)--as well as beta-endorphin inhibited the mucosal damage induced by both ethanol and indomethacin in pmolar dose range. In contrast, the gastric acid secretion was not influenced by DADLE in the dose of 16 nmol/rat and only a slight reduction (40%) was induced by DAGO in the dose of 1.9 nmol/rat. The protective effect was abolished in both ulcer models by bilateral cervical vagotomy. N(G)-nitro-L-arginine, an inhibitor of NO synthase, reduced the protective action in ethanol-induced, but not in indomethacin-induced gastric damage. The results suggest that activation of supraspinal delta and mu-opioid receptors resulted in inhibition of gastric mucosal lesions elicited by ethanol or indomethacin. The gastroprotective action is independent from the effect of opioids on acid secretion. Vagal nerve is involved in conveying the central action to the periphery. The mechanism of the gastroprotective effect of opioids is different in ethanol- and indomethacin-ulcer models: prostaglandins and nitric oxide are likely to be involved in the protective action of opioid peptides in ethanol-, but not in the indomethacin-ulcer model.

Pharmacology of amphibian opiate peptides

In 1980 the skin of certain frogs belonging to the genus Phyllomedusinae was found to contain two new peptides that proved to be selective mu-opioid agonists, and named dermorphins. Since 1987 deltorphins, a family of highly selective delta-opioid peptides were identified either by cloning of the cDNA from frog skins or isolation of the peptides. The distinctive feature of opioid peptides is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe. The discovery of the amphibian opiate peptides, provided new insights into the functional role of the mu- and delta-opiate systems. It also provided models for novel analgesics with enhanced therapeutic benefits and reduced toxicity.

Inhibitory effect of intracerebroventricularly-administered [D-Arg(2), beta-Ala(4)]-dermorphin (1-4) on gastrointestinal transit

The inhibitory effect of intracerebroventricularly-administered [D-Arg(2), beta-Ala(4)]-dermorphin (1-4) (TAPA), a highly selective mu(1)-opioid receptor agonist, on mouse gastrointestinal transit was compared with that of morphine and [D-Ala(2), N-methyl-Phe(4), Gly(5)-ol]-enkephalin (DAMGO). When administered intracerebroventricularly 5 min before the oral injection of charcoal meal, TAPA (10-100 pmol), morphine (0.25-4 nmol), and DAMGO (20-80 pmol) dose-dependently inhibited gastrointestinal transit of charcoal. The inhibitory effect of each mu-opioid receptor agonist was completely antagonized by naloxone, a nonselective opioid receptor antagonist. The inhibitory effects of morphine and DAMGO were significantly antagonized by both beta-funaltrexamine, a selective mu-opioid receptor antagonist, and naloxonazine, a selective mu(1)-opioid receptor antagonist. In contrast, the inhibitory effect of TAPA was not affected at all by beta-funaltrexamine, naloxonazine, nor-binaltorphimine (a selective kappa-opioid receptor antagonist), or naltrindole (a selective delta-opioid receptor antagonist). These results suggest that the inhibitory effect of TAPA on gastrointestinal transit may be mediated through an opioid receptor mechanism different from that of morphine and DAMGO.

What peptides these deltorphins be

The deltorphins are a class of highly selective delta-opioid heptapeptides from the skin of the Amazonian frogs Phyllomedusa sauvagei and P. bicolor. The first of these fascinating peptides came to light in 1987 by cloning of the cDNA of from frog skins, while the other members of this family were identified either by cDNA or isolation of the peptides. The distinctive feature of deltorphins is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe, comparable to dermorphin, which is the prototype of a group of mu-selective opioids from the same source. The D-amino acid and the anionic residues, either Glu or Asp, as well as their unique amino acid compositions are responsible for the remarkable biostability, high delta-receptor affinity, bioactivity and peptide conformation. This review summarizes a decade of research from many laboratories that defined which residues and substituents in the deltorphins interact with the delta-receptor and characterized pharmacological and physiological activities in vitro and in vivo. It begins with a historical description of the topic and presents general schema for the synthesis of peptide analogues of deltorphins A, B and C as a means to document the methods employed in producing a myriad of analogues. Structure activity studies of the peptides and their pharmacological activities in vitro are detailed in abundantly tabulated data. A brief compendium of the current level of knowledge of the delta-receptor assists the reader to appreciate the rationale for the design of these analogues. Discussion of the conformation of these peptides addresses how structure leads to further hypotheses regarding ligand receptor interaction. The review ends with a broad discussion of the potential applications of these peptides in clinical and therapeutic settings.

Central effect of SNC 80, a selective and systemically active delta-opioid receptor agonist, on gastrointestinal propulsion in the mouse

We investigated the effects of SNC 80 ((+)-4-[alphaR)-alpha-((2S,5R)-4-ally1-2,5-dimethyl-1-pipera zinyl)-3-methoxybenzyl]-N,N-diethylbenzamide), a new highly selective, non-peptidic and systemically active delta-opioid receptor agonist, on gastrointestinal and colonic propulsion in mice. Intraperitoneally (i.p.) SNC 80 (1, 10 and 30 mg/kg) significantly decreased gastrointestinal propulsion measured as transit of an orally administered charcoal meal. Pretreatment with the delta-opioid receptor antagonist, naltrindole (1 mg/kg) subcutaneously (s.c.), with the non-selective opioid antagonist, naloxone (5 mg/kg, s.c.) or the mu1-opioid receptor antagonist, naloxonazine (10 mg/kg, i.p.), significantly decreased the antitransit effect of SNC 80 but pretreatment with the non-selective opioid antagonist, naloxone methiodide (5 mg/kg, s.c.), a quaternary salt of naloxone that does not cross the blood-brain barrier, did not. SNC 80 (1, 5 and 10 mg/kg, i.p.), produced dose-related inhibition of colonic propulsion measured as the increase in mean expulsion time of a 3 mm glass bead placed in the distal colon. Naloxone (5 mg/kg, s.c.) and naltrindole (1 mg/kg, s.c.), completely antagonized the colonic antipropulsive effect of SNC 80. In contrast, naloxone methiodide (5 mg/kg, s.c.), left the inhibitory effect of i.p. SNC 80 on colonic function unchanged. These results suggest that peripherally injected SNC 80 inhibits gastrointestinal transit and colonic propulsion. It does so mainly through a central mechanism. Although the gastrointestinal antitransit effect of SNC 80 is naltrindole- and naloxonazine-sensitive, we cannot exclude an opioid-independent mechanism. The colonic antipropulsive effect of SNC 80 confirms the inhibitory role of the central delta-opioid receptor system on colonic motility.

Neuroinflammatory reactions in experimental gastric ulcer: Target for mucosal protection

The effect of different opioids receptor agonists-morphine, DAGO (mu-agonists), DADLE, DPDPE and deltorphin II (delta-agonists)-on gastric mucosal damage induced by either acidified ethanol or acidified aspirin was studied following subcutaneous (sc) administration of these agonists. The results indicate that both mu and delta receptors are involved in gastroprotection. Morphine, DAGO and DADLE, injected intracerebroventricularly, were also effective in both ulcer models. This suggests that gastric cytoprotection can be induced also be central action, since gastric acid secretion is not involved in the pathomechanism of mucosal damage induced by acidified ethanol. Interaction between the opioids and alpha(2)-adrenoceptors in gastroprotection is suggested by the findings that the gastroprotective effect of clonidine (0.09 mumol/kg orally) was antagonized by opioid antagonists. As both naloxone (1.38 mumol/kg sc) and naltrindole (12 mumol/kg sc) exerted antagonist effects, both mu and delta receptors are likely to be involved in presynaptic alpha(2)-receptor-mediated gastroprotection.

Analysis of the role of delta opioid receptors in gastroprotection in the rat

The effect of delta opioid agonists - [D-Ala2, D-Leu5]-enkephalin (DADLE), [D-Pen2, D-Pen5]-enkephalin (DPDPE) and deltorphin II - on acidified ethanol induced gastric mucosal lesions was studied in the rat compared with that of morphine. It was found that DADLE, DPDPE, deltorphin II and morphine exerted a dose-dependent inhibition on the mucosal lesions injected subcutaneously, their ID50 values were 0.037, 1.8, 3.5 and 0.35 micromoles/kg, respectively. Naltrindole (10 mg/kg sc.), the selective delta opioid receptor antagonist, inhibited the gastroprotective effect of DADLE, DPDPE and deltorphin II, but it failed to antagonise the effect of morphine. Our results suggest that 1. delta receptors are involved in opioid-mediated gastroprotection, 2. ethanol-induced gastric mucosal damage in the rat may be a quick, simple in vivo model for screening opioid delta receptor agonists and antagonists in the periphery.

The dermorphin peptide family

1. In 1980, the skin of certain frogs belonging to the genus Phyllomedusinae was found to contain two new peptides that proved to be selective mu-opioid agonists. Given the name dermorphins, these were the first members of a peptide family that in the past 15 years has grown to reach a total of seven naturally occurring peptides and nearly 30 synthetic analogs. 2. Dermorphin peptides are potent analgesics in rodents and primates, including man. Some dermorphins can enter the blood-brain barrier and produce central antinociception after peripheral administration. 3. The dermorphin family also includes mu 1-opioid receptor selective agonists that produce intense opioid analgesia, but stimulate pulmonary ventilation. 4. Experiments in rats and mice chronically exposed to dermorphins have shown that not only do they have higher antinociceptive efficacy and potency than morphine, but they are also less likely than morphine to produce tolerance, dependence and opiate side effects.

In vitro and in vivo biological activities of PG-KII, a novel kassinin-like peptide from the skin of the Australian frog, Pseudophryne güntheri

We compared the in vitro and in vivo biological activities of PG-KII (pGlu-Pro-Asn-Pro-Asp-Glu-Phe-Val-Gly-Leu-Met-NH2), a new peptide belonging to the tachykinin family, related to kassinin, isolated and sequenced from extracts of the skin of the Australian myobatrachid frog Pseudophryne güntheri, with those of the well-known tachykinins [substance P (SP), neurokinin A (NKA), neurokinin B (NKB), and kassinin (KASS)] to study its pharmacological and receptor profile. PG-KII always proved inactive in the in vitro and in vivo (gastric emptying) NK2 bioassays. It resulted equipotent to SP and more potent than KASS, NKA, and NKB in all in vitro smooth muscle preparations preferentially activated by the NK1-selective agonists. On an in vivo NK3 receptor-mediated function, gastric acid secretion, PG KII had a potency similar to that of NKB. In contracting guinea pig ileum, which contains NK1, NK2, NK3, and also new tachykinin receptor subtypes, PG-KII was more potent than SP, NKB, and NKA. The cholinergic antagonist, atropine, significantly reduced the guinea pig contractile activity of both PG-KII and NKB but not that of SP or NKA. Pretreatment with the NK1 selective antagonist, CP 96,345, and with the NK2-selective antagonist, MEN 10,376, modified neither the in vivo nor the in vitro effects of PG-KII. These findings indicate that PG-KII is neither an NK1 nor an NK2 receptor agonist but has a spectrum of biological actions close to that of the NK3 receptor agonists. PG-KII elicits strong contractile activity in guinea pig ileum. Administered centrally in the rat it regulates inhibition of gastric acid secretion.

Endogenous opiates: 1994

This article is the 17th installment of our annual review of research concerning the opiate system. It includes papers published during 1994 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics covered this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.