Inhibition Of Gastrointestinal Transit Research Articles

Role of mu opioid receptor (MOR) agonist efficacy as a determinant of opioid antinociception in a novel assay of pain-depressed behavior in female and male mice.

Intermediate efficacy mu opioid receptor (MOR) agonists have potential to retain analgesic effectiveness while improving safety, but the optimal MOR efficacy for effective and safe opioid analgesia is unknown. Preclinical assays of pain-depressed behavior can assess effects of opioids and other candidate analgesics on pain-related behavioral depression, which is a common manifestation of clinically relevant pain and target of pain treatment. Accordingly, the present study goal was to validate a novel assay of pain-depressed locomotor behavior in mice and evaluate the role of MOR efficacy as a determinant of opioid analgesic effects and related safety measures. Male and female ICR mice were tested in a locomotor chamber consisting of 2 compartments connected by a doorway that contained a 1-inch-tall barrier. Dependent measures during 15-min behavioral sessions included crosses between compartments (which required vertical activity to surmount the barrier) and total movement counts (which required horizontal activity to break photobeams in each compartment). Intraperitoneal injection of lactic acid (IP acid) produced a concentration- and time-dependent depression of both endpoints. Optimal blockade of IP acid-induced behavioral depression with minimal motor impairment was achieved with intermediate-efficacy MOR treatments that also produced less gastrointestinal-transit inhibition and respiratory depression than the high-efficacy MOR agonist fentanyl. Sex differences in treatment effects were rare. Overall, these findings validate a novel procedure for evaluating opioids and other candidate analgesic effects on pain-related behavioral depression in mice and support continued research with intermediate-efficacy MOR agonists as a strategy to retain opioid analgesic effectiveness with improved safety.

Mitochondrial-Derived Peptide MOTS-c Ameliorates Spared Nerve Injury-Induced Neuropathic Pain in Mice by Inhibiting Microglia Activation and Neuronal Oxidative Damage in the Spinal Cord via the AMPK Pathway.

MOTS-c, a recently discovered mitochondrial-derived peptide, plays an important role in many physiological and pathological functions via adenosine monophosphate-activated protein kinase (AMPK) activation. Numerous studies have demonstrated that AMPK is an emerging target for the modulation of neuropathic pain. Meanwhile, microglia-activation-evoked neuroinflammation is known to contribute to the development and progression of neuropathic pain. MOTS-c is also known to inhibit microglia activation, chemokine and cytokine expression, and innate immune responses. Accordingly, in this study, we evaluated the effects of MOTS-c on neuropathic pain and investigated the putative underlying mechanisms. We found that MOTS-c levels in plasma and spinal dorsal horn were significantly lower in mice with spared nerve injury (SNI)-induced neuropathic pain than in control animals. Accordingly, MOTS-c treatment produced pronounced dose-dependent antinociceptive effects in SNI mice; however, these effects were blocked by dorsomorphin, an AMPK inhibitor, but not naloxone, a nonselective opioid receptor antagonist. Moreover, intrathecal (i.t.) injection of MOTS-c significantly enhanced AMPKα1/2 phosphorylation in the lumbar spinal cord of SNI mice. MOTS-c also significantly inhibited proinflammatory cytokine production and microglia activation in the spinal cord. The antinociceptive effects of MOTS-c were retained even when microglia activation in the spinal cord was inhibited by minocycline pretreatment, indicating that spinal cord microglia are dispensable for the antiallodynic effects of MOTS-c. In the spinal dorsal horn, MOTS-c treatment inhibited c-Fos expression and oxidative damage mainly in neurons rather than microglia. Finally, in contrast to morphine, i.t. administration of MOTS-c resulted in limited side effects relating to antinociceptive tolerance, gastrointestinal transit inhibition, locomotor function, and motor coordination. Collectively, the present study is the first to provide evidence that MOTS-c may be a promising therapeutic target for neuropathic pain.

UHPLC-MS profiles and antidiarrheal activity of Quercus coccinea münchh. and Quercus robur L. employing in vivo technique.

Introduction: Quercus L. genus (Oak) belongs to the family Fagaceae and their galls are used commercially in leather tanning, dyeing, and ink preparation. Several Quercus species were traditionally used to manage wound healing, acute diarrhea, hemorrhoid, and inflammatory diseases. The present study aims to investigate the phenolic content of the 80% aqueous methanol extract (AME) of Q. coccinea and Q. robur leaves as well as to assess their anti-diarrheal activity. Methods: Polyphenolic content of Q. coccinea and Q. robur AME were investigated using UHPLC/MS. The antidiarrheal potential of the obtained extracts was evaluated by conducting a castor oil-induced diarrhea in-vivo model. Result and Discussion: Twenty-five and twenty-six polyphenolic compounds were tentatively identified in Q. coccinea and Q. robur AME, respectively. The identified compounds are related to quercetin, kaempferol, isorhamnetin, and apigenin glycosides and their aglycones. In addition, hydrolyzable tannins, phenolic acid, phenyl propanoides derivatives, and cucurbitacin F were also identified in both species AME of Q. coccinea (250, 500, and 1000mg/kg) exhibited a significant prolongation in the onset of diarrhea by 17.7 %, 42.6%, and 79.7% respectively while AME of Q. robur at the same doses significantly prolonged the onset of diarrhea by 38.6%, 77.3%, and 2.4 folds respectively as compared to the control. Moreover, the percentage of diarrheal inhibition of Q. coccinea was 23.8%, 28.57%, and 42,86% respectively, and for Q. robur 33.34%, 47.3%, and 57.14% respectively as compared to the control group. Both extracts significantly decreased the volume of intestinal fluid by 27%, 39.78%, and 50.1% for Q. coccinea respectively; and by 38.71%, 51.19%, and 60% for Q. robur respectively as compared to the control group. In addition, AME of Q. coccinea exhibited a peristaltic index of 53.48, 47.18, and 42.28 with significant inhibition of gastrointestinal transit by 18.98%, 28.53%, and 35.95 % respectively; while AME of Q. robur exhibited a peristaltic index of 47.71, 37, and 26.41 with significant inhibition of gastrointestinal transit by 27.72%, 43.89%, and 59.99% respectively as compared with the control group. Notably, Q. robur showed a better antidiarrheal effect in comparison with Q. coccinea and, the highest effect was observed for Q. robur at 1000mg/kg as it was nonsignificant from the loperamide standard group in all measured parameters.

A Novel Multi-Target Mu/Delta Opioid Receptor Agonist, HAGD, Produced Potent Peripheral Antinociception with Limited Side Effects in Mice and Minimal Impact on Human Sperm Motility In Vitro.

Pain is a common clinical symptom among patients. Although various opioid analgesics have been developed, their side effects hinder their application. This study aimed to develop a novel opioid analgesic, HAGD (H-Tyr-D-AIa-GIy-Phe-NH2), with limited side effects. In vivo studies on mouse models as well as in vitro studies on Chinese hamster ovary (CHO) cells expressing human mu, delta, or kappa opioid receptors (CHOhMOP, CHOhDOP, and CHOhKOP, respectively) and human sperm were conducted. Compared with subcutaneous morphine (10 mg/kg), subcutaneous HAGD (10 mg/kg) produced equipotent or even greater antinociception with a prolonged duration by activating mu/delta opioid receptors in preclinical mouse pain models. The analgesic tolerance, rewarding effects (i.e., conditioned place preference and acute hyperlocomotion), and gastrointestinal transit inhibition of HAGD were significantly reduced compared with those of morphine. Both HAGD and morphine exhibited a withdrawal response and had no impacts on motor coordination. In CHOhMOP and CHOhDOP, HAGD showed specific and efficient intracellular Ca2+ stimulation. HAGD had minimal impact on human sperm motility in vitro, whereas 1 × 10-7 and 1 × 10-8 mol/L of morphine significantly declined sperm motility at 3.5 h. Overall, HAGD may serve as a promising antinociceptive compound.

Benzo[b]thiophene-2-carboxamides as novel opioid receptor agonists with potent analgesic effect and reduced constipation

Currently, there is a significant unmet need for novel analgesics with fewer side effects. In this study, we carried out structural modification of a hit compound previously identified in an artificial-intelligence (AI) virtual screening and discovered the potent analgesic, benzo[b]thiophene-2-carboxamide analog (compound 25) with new structural scaffold. We investigated the signaling pathways of opioid receptors mediated by compound 25, and found this racemic compound activated mu-opioid receptor through the cyclic adenosine monophosphate (cAMP) and β-arrestin-2-mediated pathways with strong potency and efficacy, and accompanying nociceptin-orphanin FQ opioid peptide and delta-opioid receptors through the cAMP pathway with weak potencies. Compound 25 elicited potent antinociception in thermal-stimulated pain (ED50 value of 127.1 ± 34.65 μg/kg) and inflammatory-induced allodynia models with less gastrointestinal transit inhibition and antinociceptive tolerance than morphine. Overall, this study revealed a novel analgesic with reduced risks of side effects.

Oxidative Metabolism as a Modulator of Kratom’s Biological Actions

The leaves of Mitragyna speciosa (kratom), a plant native to Southeast Asia, are increasingly used as a pain reliever and for attenuation of opioid withdrawal symptoms. Using the tools of natural products chemistry, chemical synthesis, and pharmacology, we provide a detailed in vitro and in vivo pharmacological characterization of the alkaloids in kratom. We report that metabolism of kratom's major alkaloid, mitragynine, in mice leads to formation of (a) a potent mu opioid receptor agonist antinociceptive agent, 7-hydroxymitragynine, through a CYP3A-mediated pathway, which exhibits reinforcing properties, inhibition of gastrointestinal (GI) transit and reduced hyperlocomotion, (b) a multifunctional mu agonist/delta-kappa antagonist, mitragynine pseudoindoxyl, through a CYP3A-mediated skeletal rearrangement, displaying reduced hyperlocomotion, inhibition of GI transit and reinforcing properties, and (c) a potentially toxic metabolite, 3-dehydromitragynine, through a non-CYP oxidation pathway. Our results indicate that the oxidative metabolism of the mitragynine template beyond 7-hydroxymitragynine may have implications in its overall pharmacology in vivo.

Spinal DN-9, a Peptidic Multifunctional Opioid/Neuropeptide FF Agonist Produced Potent Nontolerance Forming Analgesia With Limited Side Effects

The development of multitarget opioid drugs has emerged as an attractive therapeutic strategy to eliminate opioid-related side effects. Our previous study developed a series of opioid and neuropeptide FF pharmacophore-containing chimeric peptides, including DN-9 (Tyr-D.Ala-Gly-NMe.Phe-Gly-Pro-Gln-Arg-Phe-NH2), which produced potent nontolerance forming analgesia at the supraspinal level. In the present study, the antinociceptive effects of DN-9 in a series of preclinical pain models and the potential side-effects were investigated at the spinal level in mice. In the tail-flick test, intrathecal injection of DN-9 produced potent analgesia with an ED50 value at 1.33 pmol, and the spinal antinociception of DN-9 was mainly mediated by μ- and κ-opioid receptors. In addition, DN-9-induced spinal antinociception was augmented by the neuropeptide FF receptors antagonist. Furthermore, DN-9 could decrease both the frequency and amplitude of sEPSCs in lamina IIo neurons of the spinal cord, which were mediated by opioid receptors. In contrast to morphine, chronic intrathecal treatments with DN-9 did not induce analgesic tolerance, c-Fos expression or microglial activation. Intrathecal injection of DN-9 showed potent analgesia with antinociceptive ED50 values between .66 and 55.04 pmol in different pain models, including the formalin test, acetic acid-induced writhing test, carrageenan-induced inflammatory pain and neuropathic pain. Moreover, DN-9 did not show side effects in locomotor function and coordination, gastrointestinal transit inhibition, the cardiovascular system, and body temperature regulation at antinociceptive doses. Taken together, the present study showed DN-9 produced effective, nontolerance forming analgesia with reduced side effects at the spinal level. DN-9 might be a promising compound for developing multifunctional opioid analgesics with limited adverse effects. PerspectiveThis article presents the potent and nontolerance forming analgesia effects of DN-9 in a series of preclinical pain models with less opioid related adverse effects at the spinal level in mice. This study also demonstrates that DN-9 has translational potential into an intrathecal analgesic.

Systemic administration of the bifunctional opioid/neuropeptide FF receptors agonist BN-9 produced peripheral antinociception in preclinical mouse models of pain

We recently characterized a novel bifunctional agonist for opioid and neuropeptide FF receptors, named BN-9, which exhibited potent analgesia in the mouse tail-flick test when given centrally. To further evaluate its potential therapeutic efficacy for translational-medical development, the current work was performed to explore the antinociceptive activities of intraperitoneal (i.p.) administration of BN-9 in mouse models of tail-flick assay, formalin pain, visceral pain and post-operative pain. In the tail-flick test, BN-9 induced a dose-related antinociceptive effect, which was fully blocked by systemic pretreatment with the peripheral acting opioid receptor antagonist naloxone methiodide, but not supraspinal naloxone methiodide, implying the involvement of the peripheral opioid receptors. In addition, the systemic antinociception of BN-9 was antagonized by the selective antagonists of the μ- and κ-opioid receptors, independently of the δ-opioid and neuropeptide FF receptors. Similarly, dose-dependent analgesia was also produced by systemic BN-9 in different pain models via the peripheral opioid receptors, independently of the neuropeptide FF receptors. Furthermore, the side-effects of systemic BN-9 on motor performance, tolerance development and gastrointestinal transit inhibition were also evaluated. Repeated systemic injection of BN-9 produced non-tolerance analgesia over 8 days. Compared with morphine, intraperitoneal administration of BN-9 exerted less inhibition of gastrointestinal transit. These data show that BN-9 induced systemic analgesia with reduced side-effects on tolerance and constipation. This article suggests that systemic injection of BN-9 causes effective antinociception in different preclinical pain models via the peripheral opioid receptors, providing an attractive approach to develop peripherally acting opioid analgesics with multiple targeting properties.

Pharmacologic Profile of Naloxegol, a Peripherally Acting µ-Opioid Receptor Antagonist, for the Treatment of Opioid-Induced Constipation

Opioid-induced constipation (OIC) is a common side effect of opioid pharmacotherapy for the management of pain because opioid agonists bind to µ-opioid receptors in the enteric nervous system (ENS). Naloxegol, a polyethylene glycol derivative of naloxol, which is a derivative of naloxone and a peripherally acting µ-opioid receptor antagonist, targets the physiologic mechanisms that cause OIC. Pharmacologic measures of opioid activity and pharmacokinetic measures of central nervous system (CNS) penetration were employed to characterize the mechanism of action of naloxegol. At the human µ-opioid receptor in vitro, naloxegol was a potent inhibitor of binding (Ki = 7.42 nM) and a neutral competitive antagonist (pA2 - 7.95); agonist effects were <10% up to 30 μM and identical to those of naloxone. The oral doses achieving 50% of the maximal effect in the rat for antagonism of morphine-induced inhibition of gastrointestinal transit and morphine-induced antinociception in the hot plate assay were 23.1 and 55.4 mg/kg for naloxegol and 0.69 and 1.14 mg/kg by for naloxone, respectively. In the human colon adenocarcinoma cell transport assay, naloxegol was a substrate for the P-glycoprotein transporter, with low apparent permeability in the apical to basolateral direction, and penetrated the CNS 15-fold slower than naloxone in a rat brain perfusion model. Naloxegol-derived radioactivity was poorly distributed throughout the rat CNS and was eliminated from most tissues within 24 hours. These findings corroborate phase 3 clinical studies demonstrating that naloxegol relieves OIC-associated symptoms in patients with chronic noncancer pain by antagonizing the µ-opioid receptor in the ENS while preserving CNS-mediated analgesia.

Mediation of buprenorphine analgesia by a combination of traditional and truncated mu opioid receptor splice variants.

Buprenorphine has long been classified as a mu analgesic, although its high affinity for other opioid receptor classes and the orphanin FQ/nociceptin ORL1 receptor may contribute to its other actions. The current studies confirmed a mu mechanism for buprenorphine analgesia, implicating several subsets of mu receptor splice variants. Buprenorphine analgesia depended on the expression of both exon 1-associated traditional full length 7 transmembrane (7TM) and exon 11-associated truncated 6 transmembrane (6TM) MOR-1 variants. In genetic models, disruption of delta, kappa1 or ORL1 receptors had no impact on buprenorphine analgesia, while loss of the traditional 7TM MOR-1 variants in an exon 1 knockout (KO) mouse markedly lowered buprenorphine analgesia. Loss of the truncated 6TM variants in an exon 11 KO mouse totally eliminated buprenorphine analgesia. In distinction to analgesia, the inhibition of gastrointestinal transit and stimulation of locomotor activity were independent of truncated 6TM variants. Restoring expression of a 6TM variant with a lentivirus rescued buprenorphine analgesia in an exon 11 KO mouse that still expressed the 7TM variants. Despite a potent and robust stimulation of (35) S-GTPγS binding in MOR-1 expressing CHO cells, buprenorphine failed to recruit β-arrestin-2 binding at doses as high as 10 µM. Buprenorphine was an antagonist in DOR-1 expressing cells and an inverse agonist in KOR-1 cells. Buprenorphine analgesia is complex and requires multiple mu receptor splice variant classes but other actions may involve alternative receptors.

BN‐9, a chimeric peptide with mixed opioid and neuropeptide FF receptor agonistic properties, produces nontolerance‐forming antinociception in mice

Neuropeptide FF (NPFF) behaves as an endogenous opioid-modulating peptide. In the present study, the opioid and NPFF pharmacophore-containing chimeric peptide BN-9 was synthesized and pharmacologically characterized. Agonist activities of BN-9 at opioid and NPFF receptors were characterized in in vitro cAMP assays. Antinociceptive activities of BN-9 were evaluated in the mouse tail-flick and formalin tests. Furthermore, its side effects were investigated in rotarod, antinociceptive tolerance, reward and gastrointestinal transit tests. BN-9 acted as a novel multifunctional agonist at μ, δ, κ, NPFF1 and NPFF2 receptors in cAMP assays. In the tail-flick test, BN-9 produced dose-related antinociception and was approximately equipotent to morphine; this antinociception was blocked by μ and κ receptor antagonists, but not by the δ receptor antagonist. In the formalin test, supraspinal administration of BN-9 produced significant analgesia. Notably, repeated administration of BN-9 produced analgesia without loss of potency over 8days. In contrast, repeated i.c.v. co-administration of BN-9 with the NPFF receptor antagonist RF9 produced significant antinociceptive tolerance. Furthermore, i.c.v. BN-9 induced conditioned place preference. When given by the same routes, BN-9 had a more than eightfold higher ED50 value for gastrointestinal transit inhibition compared with the ED50 values for antinociception. BN-9 produced a robust, nontolerance-forming analgesia with limited inhibition of gastrointestinal transit. As BN-9 is able to activate both opioid and NPFF systems, this provides an interesting approach for the development of novel analgesics with minimal side effects.

Neuropeptide VF Enhances Cannabinoid Agonist WIN55,212-2-Induced Antinociception in Mice.

Cannabinoids produce analgesia in several pain models, but the undesirable side effects from high doses of cannabinoid drugs limit their clinic use. Our recent results indicate that cannabinoid-induced antinociception was enhanced by neuropeptide VF (NPVF). Here, we investigate whether low-dose cannabinoid agonists combined with NPVF can produce effective antinociception with limited side effects. The in vivo properties of (R)-(-1)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-napthalenylmethanone (WIN55,212-2) given alone and its combination with NPVF were evaluated in nociceptive modulation, locomotor activity, gastrointestinal transit, and tolerance development assays after intracerebroventricular administration in mice. In the radiant tail-flick test, the antinociception of combination of WIN55,212-2 and NPVF was more potent than that of cannabinoid agonist given alone, with an ED50 shift from 3.51 to 0.69 nmol; 9 nmol WIN55,212-2 alone and 3 nmol WIN55,212-2 combined with NPVF induced equivalent antinociception after supraspinal administration. The cannabinoid-potentiating effects of NPVF were reduced by both the cannabinoid receptor type 1 and the neuropeptide FF receptor antagonists. In the formalin assay, WIN55,212-2 combined with NPVF also significantly reduced pain-related behaviors. However, the combination of WIN55,212-2 with NPVF exerted significant hypoactivity in a manner similar to high doses of WIN55,212-2. It was important to note that the combination of WIN55,212-2 with NPVF produced nontolerance-forming antinociception and weaker inhibition of gastrointestinal transit compared with high dose of WIN55,212-2. These data suggest that the cannabinoid agonist combined with NPVF produces effective antinociception-lacking tolerance via both cannabinoid receptor type 1 and neuropeptide FF receptors in the brain.

Diabetes attenuates the inhibitory effects of endomorphin-2, but not endomorphin-1 on gastrointestinal transit in mice

Diabetes affects the entire gastrointestinal tract from the esophagus to the anus. In the present study, the charcoal meal test was undertaken to evaluate and compare the effects of intracerebroventricular (i.c.v.) administration of endomorphins (EMs) on gastrointestinal transit in non-diabetic and diabetic mice. Significantly delayed gastrointestinal transit was found in both 4 and 8 weeks alloxan-induced diabetes compared to non-diabetes. Moreover, i.c.v. EM-1 and EM-2 dose-dependently delayed gastrointestinal transit in non-diabetes and diabetes. The EM-1-induced inhibitory effects of gastrointestinal transit in 4 weeks diabetes were qualitatively similar to those of non-diabetes. However, at higher doses, the EM-1-induced effects in 8 weeks diabetes were largely enhanced. Different to EM-1, the EM-2-induced inhibition of gastrointestinal transit in diabetic mice was significantly attenuated compared to non-diabetic mice. Moreover, these effects were further decreased in 8 weeks diabetes. The delayed gastrointestinal transit effects caused by EM-1 may be primarily mediated by μ2-opioid receptor in both non-diabetes and 4 weeks diabetes. Interestingly, in 8 weeks diabetes, these effects were mediated by μ2- and δ-receptors. However, the inhibitory effects of EM-2 were mediated by μ1-opioid receptor, which exerted a reduced function in diabetes. Also, poor blood glucose control might result in the attenuated effects of EM-2. Our present results demonstrated that diabetes attenuates the inhibitory effects of EM-2, but not EM-1 on gastrointestinal transit in mice. The different effects of EM-1 and EM-2 on gastrointestinal transit in diabetes may be due to changes of opioid receptor subtypes and their functional responses.

Lack of synergistic interaction between the two mechanisms of action of tapentadol in gastrointestinal transit

A multi-mechanistic approach offers potential enhancement of analgesic efficacy, but therapeutic gain could be offset by an increase in adverse events. The centrally acting analgesic tapentadol [(-)-(1R,2R)-3-(3-dimethylamino-1-ethyl-2-methyl-propyl)-phenol hydrochloride] combines μ-opioid receptor (MOR) agonism and neuronal noradrenaline reuptake inhibition (NRI), both of which contribute to its analgesic effects. Previously, isobolographic analysis of occupation-effect data and a theoretically equivalent methodology determining interactions from the effect scale demonstrated pronounced synergistic interaction between the two mechanisms of action of tapentadol in two models of antinociception (low-intensity tail-flick and spinal nerve ligation). The present study investigated the nature of interaction of the two mechanisms on a surrogate measure for gastrointestinal adverse effect (inhibition of gastrointestinal transit). Dose-response curves were generated in rats for tapentadol alone or in combination with the opioid receptor antagonist, naloxone, or the α2 -adrenoceptor antagonist, yohimbine, to reveal the effect of tapentadol based upon MOR agonism, NRI, and combined mechanisms. The dose-effect curve of tapentadol was shifted to the right by both antagonists, thereby providing data to distinguish between MOR agonism and NRI. Analysis revealed a simple additive interaction between the two mechanisms on this endpoint, in contrast to the synergistic interaction previously demonstrated for antinociception. We believe this is the first published evaluation of mechanistic interaction for a surrogate measure of adverse effect of a single compound with two mechanisms of action, and the results suggest that there is a greater separation between the analgesic and gastrointestinal effects of tapentadol than expected based upon its analgesic efficacy.

Mechanisms That Underlie μ-Opioid Receptor Agonist–Induced Constipation: Differential Involvement of μ-Opioid Receptor Sites and Responsible Regions

Reducing the side effects of pain treatment is one of the most important strategies for improving the quality of life of cancer patients. However, little is known about the mechanisms that underlie these side effects, especially constipation induced by opioid receptor agonists; i.e., do they involve naloxonazine-sensitive versus -insensitive sites or central-versus-peripheral μ-opioid receptors? The present study was designed to investigate the mechanisms of μ-opioid receptor agonist-induced constipation (i.e., the inhibition of gastrointestinal transit and colonic expulsion) that are antagonized by the peripherally restricted opioid receptor antagonist naloxone methiodide and naloxonazine in mice. Naloxonazine attenuated the fentanyl-induced inhibition of gastrointestinal transit more potently than the inhibition induced by morphine or oxycodone. Naloxone methiodide suppressed the oxycodone-induced inhibition of gastrointestinal transit more potently than the inhibition induced by morphine, indicating that μ-opioid receptor agonists induce the inhibition of gastrointestinal transit through different mechanisms. Furthermore, we found that the route of administration (intracerebroventricular, intrathecally, and/or intraperitoneally) of naloxone methiodide differentially influenced the suppressive effect on the inhibition of colorectal transit induced by morphine, oxycodone, and fentanyl. These results suggest that morphine, oxycodone, and fentanyl induce constipation through different mechanisms (naloxonazine-sensitive versus naloxonazine-insensitive sites and central versus peripheral opioid receptors), and these findings may help us to understand the characteristics of the constipation induced by each μ-opioid receptor agonist and improve the quality of life by reducing constipation in patients being treated for pain.

Sigma-1 receptor antagonism as opioid adjuvant strategy: Enhancement of opioid antinociception without increasing adverse effects

While opioids are potent analgesics widely used in the management of pain, a number of well-known adverse effects limit their use. The sigma-1 receptor is a ligand-regulated molecular chaperone involved in pain processing, including modulation of opioid antinociception. However, data supporting the potential use of sigma-1 receptor ligands as suitable opioid adjuvants are based on studies that use non selective ligands. Also, safety issues derived from combination therapy are poorly addressed. In this study we used the new selective sigma-1 receptor antagonist S1RA (E-52862) to characterize the effect of selective sigma-1 receptor blockade on opioid-induced efficacy- and safety-related outcomes in mice. S1RA (40mg/kg) had no effect in the tail-flick test but did enhance the antinociceptive potency of several opioids by a factor between 2 and 3.3. The potentiating effect of S1RA on morphine antinociception did not occur in sigma-1 receptor knockout mice, which supports the selective involvement of the sigma-1 receptor. Interestingly, S1RA co-administration restored morphine antinociception in tolerant mice and reverted the reward effects of morphine in the conditioned place preference paradigm. In addition, enhancement of antinociception was not accompanied by potentiation of other opioid-induced effects, such as the development of morphine analgesic tolerance, physical dependence, inhibition of gastrointestinal transit, or mydriasis. The use of sigma-1 receptor antagonists as opioid adjuvants could represent a promising pharmacological strategy to enhance opioid potency and, most importantly, to increase the safety margin of opioids. S1RA is currently in phase II clinical trials for the treatment of several pain conditions.

Potentiation of morphine-induced mechanical antinociception by σ1 receptor inhibition: Role of peripheral σ1 receptors

We studied the modulation of morphine-induced mechanical antinociception and side effects by σ1 receptor inhibition. Both wild-type (WT) and σ1 receptor knockout (σ1-KO) mice showed similar responses to paw pressure (100–600 g). The systemic (subcutaneous) or local (intraplantar) administration of σ1 antagonists (BD-1063, BD-1047, NE-100 and S1RA) was devoid of antinociceptive effects in WT mice. However, σ1-KO mice exhibited an enhanced mechanical antinociception in response to systemic morphine (1–16 mg/kg). Similarly, systemic treatment of WT mice with σ1 antagonists markedly potentiated morphine-induced antinociception, and its effects were reversed by the selective σ1 agonist PRE-084. Although the local administration of morphine (50–200 μg) was devoid of antinociceptive effects in WT mice, it induced dose-dependent antinociception in σ1-KO mice. This effect was limited to the injected paw. Enhancement of peripheral morphine antinociception was replicated in WT mice locally co-administered with σ1 antagonists and the opioid. None of the σ1 antagonists tested enhanced morphine-antinociception in σ1-KO mice, confirming a σ1-mediated action. Morphine-induced side-effects (hyperlocomotion and inhibition of gastrointestinal transit) were unaltered in σ1-KO mice. These results cannot be explained by a direct interaction of σ1 ligands with μ-opioid receptors or adaptive changes of μ-receptors in σ1-KO mice, given that [3H]DAMGO binding in forebrain, spinal cord, and hind-paw skin membranes was unaltered in mutant mice, and none of the σ1 drugs tested bound to μ-opioid receptors. These results show that σ1 receptor inhibition potentiates morphine-induced mechanical analgesia but not its acute side effects, and that this enhanced analgesia can be induced at peripheral level.

Synthesis and Evaluation of Aryl-Naloxamide Opiate Analgesics Targeting Truncated Exon 11-Associated μ Opioid Receptor (MOR-1) Splice Variants

3-Iodobenzoylnaltrexamide 1 (IBNtxA) is a potent analgesic acting through a novel receptor target that lack many side-effects of traditional opiates composed, in part, of exon 11-associated truncated six transmembrane domain MOR-1 (6TM/E11) splice variants. To better understand the SAR of this drug target, a number of 4,5-epoxymorphinan analogues were synthesized. Results show the importance of a free 3-phenolic group, a phenyl ring at the 6 position, an iodine at the 3'or 4' position of the phenyl ring, and an N-allyl or c-propylmethyl group to maintain high 6TM/E11 affinity and activity. 3-Iodobenzoylnaloxamide 15 (IBNalA) with a N-allyl group displayed lower δ opioid receptor affinity than its naltrexamine analogue, was 10-fold more potent an analgesic than morphine, elicited no respiratory depression or physical dependence, and only limited inhibition of gastrointestinal transit. Thus, the aryl-naloxamide scaffold can generate a potent analgesic acting through the 6TM/E11 sites with advantageous side-effect profile and greater selectivity.

Usefulness of Olanzapine as an Adjunct to Opioid Treatment and for the Treatment of Neuropathic Pain

The use of opioids for pain management is often associated with nausea and vomiting. Although conventional antipsychotics are often used to counter emesis, they can be associated with extrapyramidal symptoms. However, chronic pain can induce sleep disturbance. The authors investigated the effects of the atypical antipsychotic olanzapine on morphine-induced emesis and the sleep dysregulation associated with chronic pain. A receptor binding assay was performed using mouse whole brain tissue. The emetic response in ferrets was evaluated by counting retching and vomiting behaviors. Catalepsy in mice was evaluated by placing both of their forepaws over a horizontal bar. Released dopamine was measured by an in vivo microdialysis study. Sleep disturbance in mice in a neuropathic pain-like state was assayed by electroencephalogram and electromyogram recordings. Olanzapine showed high affinity for muscarinic M1 receptor in brain tissue. Olanzapine decreased morphine-induced nausea and vomiting in a dose-dependent manner. However, olanzapine at a dose that had an antiemetic effect (0.03 mg/kg) did not induce catalepsy or hyperglycemia. In addition, olanzapine at this dose had no effect on the morphine-induced release of dopamine or inhibition of gastrointestinal transit. Finally, olanzapine inhibited thermal hyperalgesia and completely alleviated the sleep disturbance induced by sciatic nerve ligation. These findings suggest that olanzapine may be useful for the treatment of morphine-induced emesis and as an adjunct for the treatment of neuropathic pain associated with sleep disturbance.

Synergistic Interaction Between Fentanyl and a Tramadol:Paracetamol Combination on the Inhibition of Nociception in Mice

Multimodal analgesic approaches to manage acute and chronic pain are commonly used in humans. Here, we attempted to characterize a synergistic interaction between fentanyl, tramadol, and paracetamol on the inhibition of nociception in a model of visceral pain in mice. The three-drug combined treatment displayed a potent synergistic antinociceptive effect, together with a significant reduction of gastrointestinal transit inhibition. Furthermore, selective μ- and κ-opioid receptor antagonists reversed these synergistic antinociceptive effects, thus suggesting a pivotal role of the opioid system. Overall, this study presents accurate pre-clinical data that might be useful to improve the clinical management of opioid-mediated analgesia.