Arylacetamide kappa-opioid receptor agonists produce a tonic- and use-dependent block of tetrodotoxin-sensitive and -resistant sodium currents in colon sensory neurons

Nalbuphine May Be Superior to Sufentanil in Relieving Postcesarean Uterine Contraction Pain in Multiparas: A Retrospective Cohort Study

Purpose

Postcesarean pain remains a major complaint from puerperium women who have undergone cesarean section, especially uterine contraction induced visceral pain. The optimal opioid for pain relief after cesarean section (CS) is still unclear. The goal of this study was to compare the analgesic effect of Nalbuphine to Sufentanil in patients who underwent CS.

Patients and Methods

In this single-center retrospective cohort study, we included patients who received Nalbuphine or Sufentanil Patient-Controlled Intravenous Analgesia (PCIA) after CS between 1 January 2018 and 30 November 2020. Data on a Visual Analog Scale (VAS) at uterine contraction, at rest, and at movement, analgesic consumption, and side effects were collected. We performed logistic regression to identify predictors of severe uterine contraction pain.

Results

A total of 674 patients were identified in the unmatched cohort, and 612 patients in the matched one. Compared to the Sufentanil group, lower VAS-contraction was recorded in the Nalbuphine group in both the unmatched and matched cohorts, the mean difference (MD) on POD1 was 0.35 (95% CI: 0.17 to 0.54, p<0.001) and 0.28 (95% CI: 0.08 to 0.47, p<0.001), respectively, and the MD of POD2 was 0.12 (95% CI: 0.03 to 0.40, P=0.019) and 0.12 (95% CI: 0.03 to 0.41, P=0.026), respectively. On POD1 but not POD2, VAS-movement was lower in the Nalbuphine group as compared to the Sufentanil group. No difference was found between VAS-rest on POD1 and POD2 in both unmatched and matched cohorts. Less analgesic consumption, and side effects were recorded in the Nalbuphine group. Logistic regression indicated that multipara and analgesic consumption were risk factors for severe uterine contraction pain. In subgroup analysis, VAS-contraction was meaningfully reduced in the Nalbuphine group compared with the Sufentanil group in multipara patients, but not primiparas.

Conclusion

Compared to Sufentanil, Nalbuphine may provide better analgesia on uterine contraction pain. The superior analgesia may only exhibit in multiparas.

Replacement of current opioid drugs focusing on MOR-related strategies

The scarcity and limited risk/benefit ratio of painkillers available on the market, in addition to the opioid crisis, warrant reflection on new innovation strategies. The pharmacopoeia of analgesics is based on products that are often old and derived from clinical empiricism, with limited efficacy or spectrum of action, or resulting in an unsatisfactory tolerability profile. Although they are reference analgesics for nociceptive pain, opioids are subject to the same criticism. The use of opium as an analgesic is historical. Morphine was synthesized at the beginning of the 19th century. The efficacy of opioids is limited in certain painful contexts and these drugs can induce potentially serious and fatal adverse effects. The current North American opioid crisis, with an ever-rising number of deaths by opioid overdose, is a tragic illustration of this. It is therefore legitimate to develop research into molecules likely to maintain or increase opioid efficacy while improving their tolerability. Several avenues are being explored including targeting of the mu opioid receptor (MOR) splice variants, developing biased agonists or targeting of other receptors such as heteromers with MOR. Ion channels acting as MOR effectors, are also targeted in order to offer compounds without MOR-dependent adverse effects. Another route is to develop opioid analgesics with peripheral action or limited central nervous system (CNS) access. Finally, endogenous opioids used as drugs or compounds that modify the metabolism of endogenous opioids (Dual ENKephalinase Inhibitors) are being developed. The aim of the present review is to present these various targets/strategies with reference to current indications for opioids, concerns about their widespread use, particularly in chronic non-cancer pains, and ways of limiting the risk of opioid abuse and misuse.

Discovery and Computational Analyses of Novel Small Molecule Zika Virus Inhibitors

Zika virus (ZIKV), one of the flaviviruses, has attracted worldwide attention since its large epidemics around Brazil. Association of ZIKV infection with microcephaly and neurological problems such as Guillain–Barré syndrome has prompted intensive pathological investigations. However, there is still a long way to go on the discovery of effective anti-ZIKV therapeutics. In this study, an in silico screening of the National Cancer Institute (NCI) diversity set based on ZIKV NS3 helicase was performed using a molecular docking approach. Selected compounds with drug-like properties were subjected to cell-based antiviral assays resulting in the identification of two novel lead compounds (named Compounds 1 and 2). They inhibited ZIKV infection with IC₅₀ values at the micro-molar level (8.5 μM and 15.2 μM, respectively). Binding mode analysis, absolute binding free energy calculation, and structure–activity relationship studies of these two compounds revealed their possible interactions with ZIKV NS3 helicase, suggesting a mechanistic basis for further optimization. These two novel small molecules may represent new leads for the development of inhibitory drugs against ZIKV.

Opioidergic effects on enteric and sensory nerves in the lower GI tract: basic mechanisms and clinical implications

Opioids are one of the most prescribed drug classes for treating acute pain. However, chronic use is often associated with tolerance as well as debilitating side effects, including nausea and dependence, which are mediated by the central nervous system, as well as constipation emerging from effects on the enteric nervous system. These gastrointestinal (GI) side effects limit the usefulness of opioids in treating pain in many patients. Understanding the mechanism(s) of action of opioids on the nervous system that shows clinical benefit as well as those that have unwanted effects is critical for the improvement of opioid drugs. The opioidergic system comprises three classical receptors (μ, δ, κ) and a nonclassical receptor (nociceptin), and each of these receptors is expressed to varying extents by the enteric and intestinal extrinsic sensory afferent nerves. The purpose of this review is to discuss the role that the opioidergic system has on enteric and extrinsic afferent nerves in the lower GI tract in health and diseases of the lower GI tract, particularly inflammatory bowel disease and irritable bowel syndrome, and the implications of opioid treatment on clinical outcomes. Consideration is also given to emerging developments in our understanding of the immune system as a novel source of endogenous opioids and the mechanisms underlying opioid tolerance, including the potential influence of opioid receptor splice variants and heteromeric complexes.

Human native kappa opioid receptor functions not predicted by recombinant receptors: Implications for drug design

If activation of recombinant G protein-coupled receptors in host cells (by drugs or other ligands) has predictive value, similar data must be obtained with native receptors naturally expressed in tissues. Using mouse and human recombinant κ opioid receptors transfected into a host cell, two selectively-acting compounds (ICI204448, asimadoline) equi-effectively activated both receptors, assessed by measuring two different cell signalling pathways which were equally affected without evidence of bias. In mouse intestine, naturally expressing κ receptors within its nervous system, both compounds also equi-effectively activated the receptor, inhibiting nerve-mediated muscle contraction. However, whereas ICI204448 acted similarly in human intestine, where κ receptors are again expressed within its nervous system, asimadoline was inhibitory only at very high concentrations; instead, low concentrations of asimadoline reduced the activity of ICI204448. This demonstration of species-dependence in activation of native, not recombinant κ receptors may be explained by different mouse/human receptor structures affecting receptor expression and/or interactions with intracellular signalling pathways in native environments, to reveal differences in intrinsic efficacy between receptor agonists. These results have profound implications in drug design for κ and perhaps other receptors, in terms of recombinant-to-native receptor translation, species-dependency and possibly, a need to use human, therapeutically-relevant, not surrogate tissues.

The κ-opioid receptor agonist U-50488 blocks Ca2+ channels in a voltage- and G protein-independent manner in sensory neurons

BACKGROUND AND OBJECTIVES

κ-Opioid receptor (κ-OR) activation is known to play a role in analgesia and central sedation. The purpose of the present study was to examine the effect of the κ-OR agonist, U-50488 (an arylacetamide), on Ca channel currents and the signaling proteins involved in acutely isolated rat dorsal root ganglion (DRG) neurons expressing the putative promoter region of the tetrodotoxin-resistant Na channel (NaV 1.8) that is known to be involved in pain transmission.

METHODS

Acutely isolated rat DRG neurons were transfected with cDNA coding for enhanced green fluorescent protein (EGFP), whose expression is driven by the NaV 1.8 promoter region. Thereafter, the whole-cell variant of the patch-clamp technique was used to record Ca channel currents in neurons expressing EGFP.

RESULTS

Exposure of EGFP-expressing DRG neurons to U-50488 (0.3-40 μM) led to voltage-independent inhibition of the Ca channel currents. The modulation of the Ca currents did not appear to be mediated by the Gα protein subfamilies: Gαi/o, Gαs, Gαq/11, Gα14, and Gαz. Furthermore, dialysis of the hydrolysis-resistant GDP analog, GDP-β-S (1 mM), did not affect the U-50488-mediated blocking effect, ruling out involvement of other G protein subunits. Finally, U-50488 (20 μM) blocked Ca channels heterologously expressed in HeLa cells that do not express κ-OR.

CONCLUSION

These results suggest that the antinociceptive actions mediated by U-50488 are likely due to both a direct block of Ca channels in sensory neurons as well as G protein modulation of Ca currents via κ-OR-expressing neurons.

Early-in-life bladder inflammation alters U50,488H but not morphine-induced inhibition of visceromotor responses to urinary bladder distension

Previous research has suggested that early-in-life (EIL) exposure to bladder inflammation impairs the function of endogenous opioid inhibitory system(s) and may contribute to the development of chronic bladder pain. This study examined how acute adult and/or prior EIL exposure to bladder inflammation altered the inhibitory effects of systemic κ- and μ-opioid agonists on the visceromotor reflex (VMR) to urinary bladder distension (UBD). Female rats were exposed intravesically EIL (P14-P16) to either the inflammatory agent zymosan or anesthesia-alone, and then rechallenged as adults (12-17 weeks) with either anesthesia-alone or zymosan. The VMR to 60mmHg UBD was measured after cumulative intravenous (i.v.) administration of 1mg/kg and 4mg/kg of either the κ-opioid agonist U50,488H or the μ-opioid agonist morphine. Morphine produced dose-dependent inhibition of the VMR to UBD in all groups, and U50,488H produced dose-dependent inhibition of the VMR to UBD in all but one group. Animals that received bladder inflammation both EIL and as adults showed significantly augmented VMRs to UBD (>100% baseline values) following 1mg/kg of U50,488H and diminished inhibition of VMRs following 4mg/kg of U50,488H when compared with other groups. In contrast, neither EIL nor adult bladder inflammation markedly altered the inhibition of the VMR to UBD produced by either 1 or 4mg/kg of i.v. morphine. These data suggest EIL and adult exposure to bladder inflammation selectively decreases the inhibitory effects of κ-opioids and thereby may enhance bladder hypersensitivity in patients with painful bladder syndromes.

Asimadoline and its potential for the treatment of diarrhea-predominant irritable bowel syndrome: a review

Irritable bowel syndrome (IBS) is a multifactorial condition with principal symptoms of pain and altered bowel function. The kappa-opioid agonist asimadoline is being evaluated in Phase III as a potential treatment for IBS. Asimadoline, to date, has shown a good safety profile and the target Phase III population - diarrhea-predominant IBS patients with at least moderate pain - was iteratively determined in a prospective manner from a Phase II dose-ranging study. The clinical data in support of this population are reviewed in this article. Furthermore, the scientific rationale for the use of asimadoline in the treatment of IBS is reviewed. Considering the high patient and societal burdens of IBS, new treatments for IBS represent therapeutic advances.

Visceral pain: the neurophysiological mechanism

The mechanism of visceral pain is still less understood compared with that of somatic pain. This is primarily due to the diverse nature of visceral pain compounded by multiple factors such as sexual dimorphism, psychological stress, genetic trait, and the nature of predisposed disease. Due to multiple contributing factors there is an enormous challenge to develop animal models that ideally mimic the exact disease condition. In spite of that, it is well recognized that visceral hypersensitivity can occur due to (1) sensitization of primary sensory afferents innervating the viscera, (2) hyperexcitability of spinal ascending neurons (central sensitization) receiving synaptic input from the viscera, and (3) dysregulation of descending pathways that modulate spinal nociceptive transmission. Depending on the type of stimulus condition, different neural pathways are involved in chronic pain. In early-life psychological stress such as maternal separation, chronic pain occurs later in life due to dysregulation of the hypothalamic-pituitary-adrenal axis and significant increase in corticotrophin releasing factor (CRF) secretion. In contrast, in early-life inflammatory conditions such as colitis and cystitis, there is dysregulation of the descending opioidergic system that results excessive pain perception (i.e., visceral hyperalgesia). Functional bowel disorders and chronic pelvic pain represent unexplained pain that is not associated with identifiable organic diseases. Often pain overlaps between two organs and approximately 35% of patients with chronic pelvic pain showed significant improvement when treated for functional bowel disorders. Animal studies have documented that two main components such as (1) dichotomy of primary afferent fibers innervating two pelvic organs and (2) common convergence of two afferent fibers onto a spinal dorsal horn are contributing factors for organ-to-organ pain overlap. With reports emerging about the varieties of peptide molecules involved in the pathological conditions of visceral pain, it is expected that better therapy will be achieved relatively soon to manage chronic visceral pain.

Effect of a kappa-opioid agonist, i.v. JNJ-38488502, on sensation of colonic distensions in healthy male volunteers

Kappa-opioid receptors are located on visceral pain fibres. JNJ-38488502 is a highly selective tetrapeptide kappa-opioid agonist with little access to the central nervous system and low risk of central nervous system side effects. The aim of the study was to evaluate the effects of i.v. JNJ-38488502 on sensations, including pain, during colonic distension. In a single-centre study, 23 healthy adult males underwent a single-dose, randomized, double-blind crossover study of JNJ-38488502 (0.42 mg kg(-1) i.v. infusion) vs placebo on left colon compliance, sensory thresholds and ratings during standard distensions. One participant could not undergo sensation studies. In the other 22, JNJ-38488502 increased colonic compliance (pressure at half-maximum volume 17.9 +/- 0.8 mmHg) compared to placebo (21.6 +/- 0.9 mmHg, P = 0.007). There was no significant effect on sensory thresholds which, however, were not reached by 44 mmHg in >50% of participants in both treatment phases. There were no significant treatment effects on sensory ratings to distensions at 8, 16, 24, 32 and 36 mmHg above baseline operating pressure. JNJ-38488502 was associated with increased urine output and plasma prolactin, consistent with kappa-opioid receptor activation. This study concluded that i.v. JNJ-38488502 induced kappa-opioid effects, but did not attenuate colonic sensations following random order colonic distension. Further studies of effects on pain sensations in health and disease are required.

Novel pharmacology: asimadoline, a kappa-opioid agonist, and visceral sensation

Asimadoline is a potent kappa-opioid receptor agonist with a diaryl acetamide structure. It has high affinity for the kappa receptor, with IC(50) of 5.6 nmol L(-1) (guinea pig) and 1.2 nmol L(-1) (human recombinant), and high selectively with kappa : micro : delta binding ratios of 1 : 501 : 498 in human recombinant receptors. It acts as a complete agonist in in vitro assay. Asimadoline reduced sensation in response to colonic distension at subnoxious pressures in healthy volunteers and in irritable bowel syndrome (IBS) patients without alteration of colonic compliance. Asimadoline reduced satiation and enhanced the postprandial gastric volume (in female volunteers). However, there were no significant effects on gastrointestinal transit, colonic compliance, fasting or postprandial colonic tone. In a clinical trial in 40 patients with functional dyspepsia (Rome II), asimadoline did not significantly alter satiation or symptoms over 8 weeks. However, asimadoline, 0.5 mg, significantly decreased satiation in patients with higher postprandial fullness scores, and daily postprandial fullness severity (over 8 weeks); the asimadoline 1.0 mg group was borderline significant. In a clinical trial in patients with IBS, average pain 2 h post-on-demand treatment with asimadoline was not significantly reduced. Post hoc analyses suggest that asimadoline was effective in mixed IBS. In a 12-week study in 596 patients, chronic treatment with 0.5 mg and 1.0 mg asimadoline was associated with adequate relief of pain and discomfort, improvement in pain score and number of pain-free days in patients with IBS-D. The 1.0 mg dose was also efficacious in IBS-alternating. There were also weeks with significant reduction in bowel frequency and urgency. Asimadoline has been well tolerated in human trials to date.

Effects of kappa opioid receptor agonists on attention as assessed by a 5-choice serial reaction time task in rats

In humans, kappa opioid receptor agonists produce, among other effects, sedation and difficulty concentrating, suggesting that they may disrupt attention. The purpose of the present studies was therefore to evaluate the effects of kappa opioid receptor agonists on attention as assessed by a 5-choice serial reaction time task in rats. The kappa opioid receptor agonists (+)-U69,593 (0.1-0.56mg/kg), (+/-)-U50,488 (1.0-5.6mg/kg) and racemic GR89,696 (0.0003-0.01mg/kg) all produced dose-related decreases in the percentage of trials terminated by a correct or incorrect response and increases in the percentage of omissions. In contrast, the peripherally restricted opioid agonist ICI-204,448 was ineffective (1.0-10mg/kg). Moreover, the effects of GR89,696 were stereoselective in that (R)-GR89,696 was approximately equipotent to racemic GR89,696 and approximately 100-fold more potent than (S)-GR89,696. The opioid receptor antagonist naltrexone (0.3-3mg/kg) administered alone had no effects on performance. However, naltrexone, over the dose-range of 0.03-1.0mg/kg, produced a dose-related antagonism of the disruption produced by U69,593 (0.56mg/kg). In contrast, naltrexone, over the dose-range of 0.01-0.3mg/kg produced a dose-related antagonism of morphine (5.6mg/kg). Recent evidence has suggested that kappa opioid receptor agonists decrease dopaminergic and noradrenergic neurotransmission in prefrontal cortex and locus coeruleus. Together with previous findings, the present data indicate that kappa opioid receptor agonists disrupt performance of this attention task by decreasing the probability of responding by specific actions at central kappa opioid receptors, perhaps by decreasing dopaminergic and noradrenergic neurotransmission.

Dynorphin A, kappa opioid receptors and the antinociceptive efficacy of asimadoline in streptozotocin-induced diabetic rats

AIMS/HYPOTHESIS

We investigated spinal and peripheral kappa opioid systems in diabetic rats.

MATERIALS AND METHODS

Dynorphin A, N-methyl-D-aspartate (NMDA) and kappa opioid receptor (KOR) were measured in spinal cord, dorsal root ganglia, peripheral nerves and foot skin of control and streptozotocin-induced diabetic rats by immunoassay and Western blotting. Behavioural assessments of paw tactile sensitivity and formalin-evoked hyperalgesia were performed in normal and diabetic rats before and after treatment with asimadoline.

RESULTS

Dynorphin A protein levels were significantly increased in peripheral nerves and footpad skin of diabetic rats. Dynorphin A exhibits both anti- and pro-nociceptive properties depending on activation of either KOR or NMDA receptors. Spinal protein levels of these receptors were not changed by diabetes, while KOR levels in the sciatic and peroneal nerves were significantly increased. Exploiting the presence and elevated levels of KOR in the periphery, we investigated the effect of the peripheral KOR agonist asimadoline on formalin-evoked hyperalgesia and tactile allodynia in diabetic rats. Both formalin-evoked hyperalgesia and tactile allodynia in diabetic rats were acutely ameliorated by asimadoline. To confirm that the effect of asimadoline was related to its property as KOR agonist, diabetic rats were pretreated with the selective KOR antagonist nor-binaltorphimine. Intraplantar nor-binaltorphimine abolished the ability of asimadoline to alleviate tactile allodynia in diabetic rats. Systemic and intrathecal nor-binaltorphimine partially inhibited the effect of asimadoline against formalin-evoked hyperalgesia in diabetic rats.

CONCLUSIONS/INTERPRETATION

Using selective peripheral KOR agonists to take advantage of elevated peripheral KOR expression may provide a novel therapeutic approach for painful diabetic neuropathy.

Opioid mu receptor activation inhibits sodium currents in prefrontal cortical neurons via a protein kinase A- and C-dependent mechanism

Opioid transmission in the medial prefrontal cortex is involved in mood regulation and is altered by drug dependency. However, the mechanism by which ionic channels in cortical neurons are controlled by mu opioid receptors has not been elucidated. In this study, the effect of mu opioid receptor activation on voltage-dependent Na(+) currents was assessed in medial prefrontal cortical neurons. In 66 out of 98 nonpyramidal neurons, the application of 1 microM of DAMGO ([D-Ala(2), N-Me-Phe(4), Gly(5)-OL]-enkephalin), a specific mu receptor agonist, caused a decrease in the Na(+) current amplitude to approximately 79% of that observed in controls (half blocking concentration = 0.094 microM). Moreover, DAMGO decreased the maximum current activation rate, prolonged its time-dependent inactivation, and shifted the half inactivation voltage from -63.4 mV to -71.5 mV. DAMGO prolonged the time constant of recovery from inactivation from 5.4 ms to 7.4 ms. The DAMGO-evoked inhibition of Na(+) current was attenuated when GDP-beta-S (0.4 mM, Guanosine 5-[beta-thio]diphosphate trilithium salt) was included in the intracellular solution. Inhibitors of kinase A and C greatly attenuated the DAMGO-induced inhibition, while adenylyl cyclase and kinase C activators mirrored the DAMGO inhibitory effect. Na(+) currents in pyramidal neurons were insensitive to DAMGO. We conclude that the activation of mu opioid receptors inhibits the voltage-dependent Na(+) currents expressed in nonpyramidal neurons of the medial prefrontal cortex, and that kinases A and C are involved in this inhibitory pathway.

Characterisation of opioid receptors involved in modulating circular and longitudinal muscle contraction in the rat ileum

1. The aim of the present investigation was to characterise the opioid receptor subtypes present in the rat ileum using a method that detects drug action on the enteric nerves innervating the circular and longitudinal muscles. 2. Neurogenic contractions were reversibly inhibited by morphine (circular muscle pEC50, 6.43+/-0.17, Emax 81.7+/-5.0%; longitudinal muscle pEC50, 6.65+/-0.27, Emax 59.7+/-7.8%), the mu-opioid receptor-selective agonist, DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin acetate) (circular pEC50, 7.85+/-0.04, Emax 97.8+/-3.6%; longitudinal pEC50, 7.35+/-0.09, Emax 56.0+/-6.1%), the delta-selective agonist DADLE ([D-Ala2,D-Leu5]enkephalin acetate) (circular pEC50, 7.41+/-0.17, Emax, 93.3+/-8.4%; longitudinal pEC50, 6.31+/-0.07, Emax 66.5+/-5.2%) and the kappa-selective agonist U 50488H (trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide methanesulphonate) (circular pEC50, 5.91+/-0.41, Emax, 83.5+/-26.8%; longitudinal pEC50, 5.60+/-0.08, Emax 74.3+/-7.2%). Agonist potencies were generally within expected ranges for activity at the subtype for which they are selective, except for U 50488H, which was less potent than expected. 3. The mu and delta receptor-selective antagonists, CTAP (H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2) and naltrindole, caused progressive, parallel rightward shifts in the DAMGO and DADLE curves, respectively. Analysis indicated conformity to theoretical simple competitive antagonist behaviour. U 50488H effects were insensitive to the kappa-selective antagonist, n-BNI. A high concentration (1 microM) of naltrexone caused apparent potentiation of U 50488H effects. 4. CTAP pK(B) estimates were consistent with previously reported values for mu receptor antagonism (circular 7.84+/-0.17, longitudinal 7.64+/-0.35). However, the naltrindole pK(B) estimates indicated lower antagonist potency than expected (circular 8.22+/-0.23, longitudinal 8.53+/-0.35). 5. It is concluded that mu and possibly atypical delta receptors (but not kappa receptors) mediate inhibition of contraction in this model. Nonopioid actions of U 50488H are probably responsible for the inhibitory effects seen with this compound.

Basic and clinical aspects of visceral sensation: transmission in the CNS

Pain and discomfort are the leading cause for consultative visits to gastroenterologists. Acute pain should be considered a symptom of an underlying disease, thereby serving a physiologically important function. However, many patients experience chronic pain in the absence of potentially harmful stimuli or disorders, turning pain into the primary problem rather than a symptom. Vagal and spinal afferents both contribute to the sensory component of the gut-brain axis. Current evidence suggests that they convey different elements of the complex sensory experience. Spinal afferents play a key role in the discriminatory dimension, while vagal input primarily affects the strong emotional and autonomic reactions to noxious visceral stimuli. Drugs, surgical and non-pharmacological treatments can target these pathways and provide therapeutic options for patients with chronic visceral pain syndromes.

The kappa-opioid receptor agonist asimadoline inhibits epithelial transport in mouse trachea and colon

The potent kappa-opioid receptor agonist n-methyl-N-[(1S)-1-phenyl-2-((3S)-3-hydroxypyrrolidin-1-yl)-ethyl]-2,2-diphenyl-acetamide hydrochloride (asimadoline, EMD 61753) was initially developed for the treatment of chronic pain. Because opioids are well known to reduce secretion and to cause constipation, we investigated the effects on epithelial transport in murine trachea and colon. In Ussing chamber experiments, asimadoline (100 microM) decreased short-circuit currents in airways and colon epithelium. The inhibition of I(SC) was not blocked by naloxone (10 microM) or nor-binaltorphimine (10 microM), suggesting that the response was not mediated by kappa-opioid receptors. The effect of asimadoline on I(SC) was concentration-dependent with half-maximal inhibition of I SC at 23.7 (9.5-49.3) microM and was sensitive to the K+ channel blocker charybdotoxin (10 nM). The amiloride-sensitive Na+ current was reduced by asimadoline, but not in cAMP stimulated tissues. Asimadoline strongly inhibited transient Ca2+-dependent Cl- secretion, activated by the muscarinic receptor agonist carbachol (100 microM) or the purinergic agonist ATP (100 microM). Thus, asimadoline inhibits epithelial transport independent of kappa-opioid receptors, by inhibition of basolateral Ca2+-activated and charybdotoxin-sensitive K+ channels.

Preclinical studies of opioids and opioid antagonists on gastrointestinal function

Opioid receptors in the gastrointestinal (GI) tract mediate the effects of endogenous opioid peptides and exogenously administered opioid analgesics, on a variety of physiological functions associated with motility, secretion and visceral pain. The studies reviewed or reported here describe a range of in vivo activities of opioid receptor antagonists upon GI function in rodents, focusing on mu receptors. Naloxone, and the peripherally acting mu-opioid receptor antagonists alvimopan and methylnaltrexone, reverse morphine-induced inhibition of GI transit in mice and rats, and morphine- or loperamide-induced inhibition of castor oil-induced diarrhoea in mice. At doses producing maximal reversal of morphine-induced effects upon GI transit, only the central nervous system (CNS) penetrant antagonist naloxone was able to reverse morphine-induced analgesia. Both central and peripheral opioid antagonists may affect GI function and/or visceromotor sensitivity in the absence of exogenous opioid analgesics, suggesting a constitutive role for endogenous opioid peptides in the control of GI physiology. Furthermore, in contrast to naloxone, alvimopan does not produce hypersensitivity to the visceromotor response induced by nociceptive levels of colorectal distension in a rodent model of post-inflammatory colonic hypersensitivity, suggesting that in the periphery endogenous mu-opioid receptor-mediated mechanisms do not regulate colonic sensitivity. The data support the hypothesis that peripherally acting opioid antagonists may be able to selectively block opioid receptors in the GI tract, thereby preserving normal GI physiology, while not blocking the effects of endogenous opioid peptides or exogenous opioid analgesics in the CNS. These findings suggest that the primary sites of action of mu-opioid agonists with respect to inhibition of GI function are in the periphery, whereas analgesic activity resides primarily in the CNS.

Peripheral kappa-opioid agonists for visceral pain

Kappa (kappa)-opioid receptor agonists are particularly effective analgesics in experimental models of visceral pain. Their analgesic effects are mediated in the periphery. The molecular targets involved include peripherally located kappa-receptors and possibly, at least for some nonpeptidic kappa-agonists, additional nonopioid molecular targets such as sodium channels located on primary sensory afferents. Overall, these properties are expected to be of therapeutic interest in various visceral pain conditions, including abdominal surgery associated with postoperative pain and ileus, pancreatitis pain, dysmenorrhea, labor pain and functional disorders such as irritable bowel syndrome or dyspepsia. The first kappa-agonists to be developed were brain-penetrating organic small molecules. Their development was eventually discontinued due to central side effects such as sedation and dysphoria attributed to kappa-receptors located behind the blood-brain barrier. New drug discovery programs are now geared towards the design of peripherally-selective kappa-agonists. So far, most of the organic molecule-based peripheral kappa-agonists have achieved limited peripheral selectivity and a practically insufficient therapeutic window to justify full development. These compounds have been used in a small number of clinical pilot studies involving visceral pain. Although encouraging, the clinical data available so far with this class of compounds are too limited and fragmented to fully validate the therapeutic utility of kappa-agonists in visceral pain. Additional clinical studies with safer kappa-agonists (i.e. with higher peripheral selectivity) are still required. The most suitable tools to address this question in the future appear to be the newly discovered class of tetrapeptide-based kappa-agonists, which have shown unprecedented levels of peripheral selectivity.